WorldWideScience

Sample records for halide exciton bands

  1. Exciton-relaxation dynamics in lead halides

    OpenAIRE

    Iwanaga, Masanobu; Hayashi, Tetsusuke

    2002-01-01

    We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of s...

  2. Luminescence of mono halide excitons and intraband luminescence in alkali-halide crystals

    International Nuclear Information System (INIS)

    Nanosecond electron beam pulses excite in RbCl, RbBr and KBr crystals the continuum of temperature of two-halide auto localized excitons, as well as weak rapid luminescence interference as luminescence of metastable monohalide auto localized excitons luminescence was performed for KBr crystal, excited by 6.6-7.7 and 14-20 eV photons

  3. Lattice defect formation via exciton dissociation in ammonium halide crystals

    International Nuclear Information System (INIS)

    Peculiarities of lattice defect formation via exciton dissociations in ammonium halide crystals are investigated by means of optical and thermoactivation spectroscopy. Crystals were activated by ytterbium and europium bivalent ions and X-ray irradiated at liquid nitorgen temperature. Present experimental results and published data prove the formation diagram of lattice defects at exciton nonradiating dissociation-formation of stable Frenkel defects where Vk-center an NH30-center considered as electron center are initial ones. This formation diagram is suggested for alkali-halide crystals

  4. Temperature-dependent excitonic photoluminescence of hybrid organometal halide perovskite films

    KAUST Repository

    Wu, Kewei

    2014-01-01

    Organometal halide perovskites have recently attracted tremendous attention due to their potential for photovoltaic applications, and they are also considered as promising materials in light emitting and lasing devices. In this work, we investigated in detail the cryogenic steady state photoluminescence properties of a prototypical hybrid perovskite CH3NH3PbI3-xClx. The evolution of the characteristics of two excitonic peaks coincides with the structural phase transition around 160 K. Our results further revealed an exciton binding energy of 62.3 ± 8.9 meV and an optical phonon energy of 25.3 ± 5.2 meV, along with an abnormal blue-shift of the band gap in the high-temperature tetragonal phase. This journal is

  5. Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking, and Exciton Fine Structure.

    Science.gov (United States)

    Rain, Gabriele; Nedelcu, Georgian; Protesescu, Loredana; Bodnarchuk, Maryna I; Kovalenko, Maksym V; Mahrt, Rainer F; Stferle, Thilo

    2016-02-23

    Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics. PMID:26771336

  6. Excitons versus free charges in organo-lead tri-halide perovskites

    Science.gov (United States)

    D'Innocenzo, Valerio; Grancini, Giulia; Alcocer, Marcelo J. P.; Kandada, Ajay Ram Srimath; Stranks, Samuel D.; Lee, Michael M.; Lanzani, Guglielmo; Snaith, Henry J.; Petrozza, Annamaria

    2014-04-01

    Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50?meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.

  7. Polaronic exciton binding energy in iodide and bromide organic-inorganic lead halide perovskites

    Science.gov (United States)

    Soufiani, Arman Mahboubi; Huang, Fuzhi; Reece, Peter; Sheng, Rui; Ho-Baillie, Anita; Green, Martin A.

    2015-12-01

    The last 4 years have seen the rapid emergence of a new solar cell technology based on organic-inorganic lead halide perovskites, primarily CH3NH3PbI3 and related halides involving Cl and Br. Debate continues on the role of excitons and free carriers in these materials. Recent studies report values of exciton binding energy for the iodide ranging from 0.7 meV to 200 meV, with vastly different implications for device operation and design. In the present work, previously neglected polarons are shown likely to have a major impact in determining excitonic properties. Polaronic exciton binding energies calculated using effective longitudinal optical phonon energies, deduced from permittivity measurements, are shown consistent with experimental energies for good quality samples of CH3NH3PbI3 and CH3NH3PbBr3, as determined over a large temperature range from optical absorption data. Bandgaps determined simultaneously show a discontinuity at the orthorhombic to tetragonal phase transition for the iodide, but not for the bromide.

  8. Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies

    OpenAIRE

    P. G. Baranov; Romanov, N. G.; Poluektov, O. G.; Schmidt, J.

    2010-01-01

    Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4? complex as a result of the JahnTeller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means...

  9. Plasmonic band gap engineering of plasmon-exciton coupling.

    Science.gov (United States)

    Karademir, Ertugrul; Balci, Sinan; Kocabas, Coskun; Aydinli, Atilla

    2014-10-01

    Controlling plasmon-exciton coupling through band gap engineering of plasmonic crystals is demonstrated in the Kretschmann configuration. When the flat metal surface is textured with a sinusoidal grating only in one direction, using laser interference lithography, it exhibits a plasmonic band gap because of the Bragg scattering of surface plasmon polaritons on the plasmonic crystals. The contrast of the grating profile determines the observed width of the plasmonic band gap and hence allows engineering of the plasmonic band gap. In this work, resonant coupling between the molecular resonance of a J-aggregate dye and the plasmonic resonance of a textured metal film is extensively studied through plasmonic band gap engineering. Polarization dependent spectroscopic reflection measurements probe the spectral overlap occurring between the molecular resonance and the plasmonic resonance. The results indicate that plasmon-exciton interaction is attenuated in the band gap region along the grating direction. PMID:25360962

  10. Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

    International Nuclear Information System (INIS)

    Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C12H25NH3)2PbI4(1?y)Br4y (y?=?0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510?nm to 350?nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices

  11. Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides

    Energy Technology Data Exchange (ETDEWEB)

    Ahmad, Shahab; Vijaya Prakash, G., E-mail: prakash@physics.iitd.ac.in [Nanophotonics Lab, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016 (India); Baumberg, Jeremy J. [Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE (United Kingdom)

    2013-12-21

    Room-temperature tunable excitonic photoluminescence is demonstrated in alloy-tuned layered Inorganic-Organic (IO) hybrids, (C{sub 12}H{sub 25}NH{sub 3}){sub 2}PbI{sub 4(1?y)}Br{sub 4y} (y?=?0 to 1). These perovskite IO hybrids adopt structures with alternating stacks of low-dimensional inorganic and organic layers, considered to be naturally self-assembled multiple quantum wells. These systems resemble stacked monolayer 2D semiconductors since no interlayer coupling exists. Thin films of IO hybrids exhibit sharp and strong photoluminescence (PL) at room-temperature due to stable excitons formed within the low-dimensional inorganic layers. Systematic variation in the observed exciton PL from 510?nm to 350?nm as the alloy composition is changed, is attributed to the structural readjustment of crystal packing upon increase of the Br content in the Pb-I inorganic network. The energy separation between exciton absorption and PL is attributed to the modified exciton density of states and diffusion of excitons from relatively higher energy states corresponding to bromine rich sites towards the lower energy iodine sites. Apart from compositional fluctuations, these excitons show remarkable reversible flips at temperature-induced phase transitions. All the results are successfully correlated with thermal and structural studies. Such structural engineering flexibility in these hybrids allows selective tuning of desirable exciton properties within suitable operating temperature ranges. Such wide-range PL tunability and reversible exciton switching in these novel IO hybrids paves the way to potential applications in new generation of optoelectronic devices.

  12. Temperature Behavior of Exciton Absorption Bands in PbI2 Layer Crystals

    Directory of Open Access Journals (Sweden)

    V. Kramar

    2002-06-01

    Full Text Available The influence of bending waves on the warm-up behavior of exciton absorption bands in layer crystals has been investigated. The effective mass of the current carriers in the layer semiconductor PbJ2 has been computed and used to obtain the values of the exciton-phonon interaction function by pseudopotential method energy spectra calculations. It was shown that the different signs of the warm-up dynamics of an exciton absorption peak shift and existence of inversion points is related with the concurrent influence of two exciton energy relaxation mechanisms on both the bending waves and the lattice phonons.

  13. Picosecond spin relaxations of acceptor-bound exciton and A-band free exciton in wurtzite GaN

    Energy Technology Data Exchange (ETDEWEB)

    Tackeuchi, A.; Otake, H.; Fujita, T.; Kuroda, T. [Department of Applied Physics, Waseda University, Tokyo 169-8555 (Japan); Chinone, T.; Liang, J.H.; Kajikawa, M. [Stanley Electric Company, Ltd., Edanishi 1-3-1, Aoba, Yokohama 225-0014 (Japan)

    2006-07-01

    The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. These spin relaxation times are slightly longer than those of the A-band free excitons of 0.47-0.25 ps in GaN at 150-225 K. The spin relaxation time is found to be proportional to T{sup -0.175}, where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  14. Nature of the narrow optical band in H*-aggregates: Dozy-chaosexciton coupling

    International Nuclear Information System (INIS)

    Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brnsted relations for proton-transfer reactions; the temperature-dependent electron transfer in LangmuirBlodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H*-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaosexciton coupling effect. It is emphasized that in the H*-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H*-aggregates. A similar enhancement in the H*-effect caused by the strengthening of the exciton coupling in H*-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted

  15. Band gaps and structural properties of graphene halides and their derivates: A hybrid functional study with localized orbital basis sets

    CERN Document Server

    Karlick, Frantiek; Otyepka, Michal; 10.1063/1.4736998

    2012-01-01

    DFT calculations of the electronic structure of graphane and stoichiometrically halogenated graphene derivatives (fluorographene and other analogous graphene halides) show (i) localized orbital basis sets can be successfully and effectively used for such 2D materials; (ii) several functionals predict that the band gap of graphane is greater than that of fluorographene, whereas HSE06 gives the opposite trend; (iii) HSE06 functional predicts quite good values of band gaps w.r.t benchmark theoretical and experimental data; (iv) the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides; (v) the stability of graphene halides decreases sharply with increasing size of the halogen atom - fluorographene is stable, whereas graphene iodide spontaneously decomposes. In terms of band gap and stability, the C2FBr, and C2HBr derivatives seem to be promising materials, e.g., for (opto)electronics applications, because their band gaps a...

  16. Linear Scaling of the Exciton Binding Energy versus the Band Gap of Two-Dimensional Materials

    Science.gov (United States)

    Choi, Jin-Ho; Cui, Ping; Lan, Haiping; Zhang, Zhenyu

    2015-08-01

    The exciton is one of the most crucial physical entities in the performance of optoelectronic and photonic devices, and widely varying exciton binding energies have been reported in different classes of materials. Using first-principles calculations within the G W -Bethe-Salpeter equation approach, here we investigate the excitonic properties of two recently discovered layered materials: phosphorene and graphene fluoride. We first confirm large exciton binding energies of, respectively, 0.85 and 2.03 eV in these systems. Next, by comparing these systems with several other representative two-dimensional materials, we discover a striking linear relationship between the exciton binding energy and the band gap and interpret the existence of the linear scaling law within a simple hydrogenic picture. The broad applicability of this novel scaling law is further demonstrated by using strained graphene fluoride. These findings are expected to stimulate related studies in higher and lower dimensions, potentially resulting in a deeper understanding of excitonic effects in materials of all dimensionalities.

  17. Switchable S = 1/2 and J = 1/2 Rashba bands in ferroelectric halide perovskites.

    Science.gov (United States)

    Kim, Minsung; Im, Jino; Freeman, Arthur J; Ihm, Jisoon; Jin, Hosub

    2014-05-13

    The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1/2 and J = 1/2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices. PMID:24785294

  18. Band-Gap Renormalization and Excitonic Effects in Tunneling in Asymmetric Double Quantum Wells

    Science.gov (United States)

    Tackeuchi, Atsushi; Heberle, Albert; Rhle, Wolfgang; Khler, Klaus; Muto, Shunichi

    1995-05-01

    Tunneling in asymmetric double quantum wells is studied using time-resolved photoluminescence. The photoluminescence lineshape and peak position of the narrow quantum well are strongly influenced by band-gap renormalization caused by the tunneling carriers. Tunneling is quenched in a field regime of 10 kV/cm around the ground-state resonance due to excitonic effects.

  19. Decay of Wannier-Mott excitons interacting with acoustic phonon in semiconductors with a degenerate valence band

    International Nuclear Information System (INIS)

    Decay probabilities of light and heavy excitons interacting with acoustic phonons in cubic semiconductors with a degenerate valence band are calculated. The numerical results for GaAs showed that the decay probability of the light exciton is much greater than that of the heavy one. (author). 10 refs, 1 fig

  20. Direct measurement of the exciton binding energy and effective masses for charge carriers in organic-inorganic tri-halide perovskites

    Science.gov (United States)

    Miyata, Atsuhiko; Mitioglu, Anatolie; Plochocka, Paulina; Portugall, Oliver; Wang, Jacob Tse-Wei; Stranks, Samuel D.; Snaith, Henry J.; Nicholas, Robin J.

    2015-07-01

    Solar cells based on the organic-inorganic tri-halide perovskite family of materials have shown significant progress recently, offering the prospect of low-cost solar energy from devices that are very simple to process. Fundamental to understanding the operation of these devices is the exciton binding energy, which has proved both difficult to measure directly and controversial. We demonstrate that by using very high magnetic fields it is possible to make an accurate and direct spectroscopic measurement of the exciton binding energy, which we find to be only 16 meV at low temperatures, over three times smaller than has been previously assumed. In the room-temperature phase we show that the binding energy falls to even smaller values of only a few millielectronvolts, which explains their excellent device performance as being due to spontaneous free-carrier generation following light absorption. Additionally, we determine the excitonic reduced effective mass to be 0.104me (where me is the electron mass), significantly smaller than previously estimated experimentally but in good agreement with recent calculations. Our work provides crucial information about the photophysics of these materials, which will in turn allow improved optoelectronic device operation and better understanding of their electronic properties.

  1. Exciton spectra and energy band structure of CuAlS2 crystals

    International Nuclear Information System (INIS)

    Three exciton series are investigated in the reflectivity spectra of CuAlS2 crystals at the temperature of 10 K. The nA=1 (?T=3.543 eV, ?L= 3.546 eV) and nA=2 (3.565 eV) lines of the ?4 (A-series) excitons are observed in the E-parallel c polarization. In the E-perpendicular c polarization, the ?5 excitons (nB=1 at 3.668 eV and nB=2 at 3.686 eV) of the B-series, and nC=1 at 3.813 eV of the C-series are observed. The parameters of the excitons and the more exact values of the ?7 - ?6, ?6 - ?6, and ?7 - ?6 energy gaps are determined. The crystal field and spin-orbit splitting of the valence band is calculated. The electron (mc1*) and hole (mV1*, mV2*, and mV3*) effective masses have been estimated.

  2. State Counting for Excited Bands of the Fractional Quantum Hall Effect: Exclusion Rules for Bound Excitons

    Science.gov (United States)

    Coimbatore Balram, Ajit; Wjs, Arkadiusz; Jain, Jainendra

    2014-03-01

    Exact diagonalization studies have revealed that the energy spectrum of interacting electrons in the lowest Landau level splits, non-perturbatively, into bands. The theory of nearly free composite fermions (CFs) has been shown to be valid for the lowest band, and thus to capture the low temperature physics, but it over-predicts the number of states for the excited bands. We explain the state counting of higher bands in terms of composite fermions with an infinitely strong short range interaction between a CF particle and a CF hole. This interaction, the form of which we derive from the microscopic CF theory, eliminates configurations containing certain tightly bound CF excitons. With this modification, the CF theory reproduces, for all well-defined excited bands, an exact counting for ? > 1 / 3 , and an almost exact counting for ? Marie Curie Grant PCIG09-GA-2011-294186, Research Computing and Cyberinfrastructure, PSU and Wroclaw Centre for Networking and Supercomputing

  3. Excitonic effect on optical response in one-dimensional two-band Hubbard model

    CERN Document Server

    Matsueda, H; Maekawa, S

    2004-01-01

    Motivated by the gigantic nonlinear optical response in the halogen-bridged Ni-compounds, the underlying electronic states of the compounds are examined in the one-dimensional two-band Hubbard model, by studying the current-current correlation function and the charge density in the ground state. The dynamical density matrix renormalization group method is employed. We find that the low-energy peak of the correlation function consists of a single Lorentzian component for a parameter range appropriate to the compounds. This is due to an excitonic state induced by the intersite Coulomb repulsion between holes on the metal and halogen ions. This is consistent with the optical absorption spectra of the compounds. We suggest that the localization of holes on the metal ions in the ground state brings about the formation of the excitonic state.

  4. High Photoluminescence Quantum Yield in Band Gap Tunable Bromide Containing Mixed Halide Perovskites.

    Science.gov (United States)

    Sutter-Fella, Carolin M; Li, Yanbo; Amani, Matin; Ager, Joel W; Toma, Francesca M; Yablonovitch, Eli; Sharp, Ian D; Javey, Ali

    2016-01-13

    Hybrid organic-inorganic halide perovskite based semiconductor materials are attractive for use in a wide range of optoelectronic devices because they combine the advantages of suitable optoelectronic attributes and simultaneously low-cost solution processability. Here, we present a two-step low-pressure vapor-assisted solution process to grow high quality homogeneous CH3NH3PbI3-xBrx perovskite films over the full band gap range of 1.6-2.3 eV. Photoluminescence light-in versus light-out characterization techniques are used to provide new insights into the optoelectronic properties of Br-containing hybrid organic-inorganic perovskites as a function of optical carrier injection by employing pump-powers over a 6 orders of magnitude dynamic range. The internal luminescence quantum yield of wide band gap perovskites reaches impressive values up to 30%. This high quantum yield translates into substantial quasi-Fermi level splitting and high "luminescence or optically implied" open-circuit voltage. Most importantly, both attributes, high internal quantum yield and high optically implied open-circuit voltage, are demonstrated over the entire band gap range (1.6 eV ? Eg ? 2.3 eV). These results establish the versatility of Br-containing perovskite semiconductors for a variety of applications and especially for the use as high-quality top cell in tandem photovoltaic devices in combination with industry dominant Si bottom cells. PMID:26691065

  5. Degradation of wide band-gap electrolumienscent materials by exciton-polaron interactions (Presentation Recording)

    Science.gov (United States)

    Aziz, Hany; Wang, Qi

    2015-10-01

    The limited performance stability and gradual loss in the electroluminescence efficiency of OLEDs utilizing wide band-gap materials, such as blue-emitting phosphorescent and fluorescent devices, continues to be a challenge for wider technology adoption. We recently found that interactions between excitons and polarons play an important role in the aging behavior of electroluminescent materials, and that a correlation exists between the susceptibility of these materials to this aging mode and their band-gap. This degradation mode is also found to be often associated with the emergence of new bands - at longer wavelength - in the electroluminescence spectra of the materials, that can often be detected after prolonged electrical driving. Such bands contribute to the increased spectral broadening and color purity loss often observed in these devices over time. Exciton-polaron interactions, and the associated degradation, are also found to occur most significantly in the vicinity of device inter-layer interfaces such as at the interface between the emitter layer and the electron or hole transport layers. New results obtained from investigations of these phenomena in a wide range of commonly used host and guest OLED materials will be presented.

  6. Direct determination of indium antimonide energy band parameters from diamagnetic exciton spectra

    International Nuclear Information System (INIS)

    Diamagnetic exciton spectra of InSb crystals derived from transmission measurements carried out on thin unstressed samples at 1.8 K and in magnetic fields up to 80 kOe are used for a direct determination of parameters of the c-and v-bands. Detailed identification and precise binding energy calculations permit one to derive the cyclotron frequencies of the electron and the holes, and the spin splittings. The m*/sub c/(epsilon/sub c/) and g*/sub c/(epsilon/sub c/) relationships thus obtained exhibit good agreement with Kane's model up to energies close to epsilon/sub c/ approximately epsilon/sub g/. Light holes also reveal a strong nonparabolicity characterized in the linear-in-k4 approximation by the coefficient p/sub lh/ = 0.47 meV-1. An analysis of the splitting of two light-hole long-wavelength transitions including the diamagnetic exciton binding energies and the data on the c-band obtained permits one to calculate the v-band parameter set. (author)

  7. Principles of Chemical Bonding and Band Gap Engineering in Hybrid OrganicInorganic Halide Perovskites

    Science.gov (United States)

    2015-01-01

    The performance of solar cells based on hybrid halide perovskites has seen an unparalleled rate of progress, while our understanding of the underlying physical chemistry of these materials trails behind. Superficially, CH3NH3PbI3 is similar to other thin-film photovoltaic materials: a semiconductor with an optical band gap in the optimal region of the electromagnetic spectrum. Microscopically, the material is more unconventional. Progress in our understanding of the local and long-range chemical bonding of hybrid perovskites is discussed here, drawing from a series of computational studies involving electronic structure, molecular dynamics, and Monte Carlo simulation techniques. The orientational freedom of the dipolar methylammonium ion gives rise to temperature-dependent dielectric screening and the possibility for the formation of polar (ferroelectric) domains. The ability to independently substitute on the A, B, and X lattice sites provides the means to tune the optoelectronic properties. Finally, ten critical challenges and opportunities for physical chemists are highlighted. PMID:25838846

  8. Excitonic spectra and band structure of CdGa2Se4 birefractive crystals

    International Nuclear Information System (INIS)

    We report on the intersection of spectral dependences of refractive indices no and ne at the wavelengths 546 nm (?0) and 450 nm (?01) in CdGa2Se4 single crystals. The value of difference ?n=ne?no is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa2Se4 exhibit a transmission band at the wavelength of ?0=546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa2Se4 crystals, and other parameters of excitons and bands were determined. In the ? point of Brillouin zone the effective mass of electrons mc is equal to 0.14m0, and the effective masses of holes mv2 and mv3 are equal to 0.76m0 and 0.94m0, respectively. The hole mass mv1 depends upon the direction of wave vector k: at polarization E?c, k?a the mass mv1=1.15m0, and at polarization E?c, k?b mv1=0.84m0. The values of valence bands splitting in the center of Brillouin zone by the crystal field (?cf=49 meV) and spinorbital interaction (?so=351 meV) were determined. The optical functions n, k, ?1 and ?2 in polarizations E?c and E?c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by KramersKronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa2Se4 crystals

  9. Study of the point defect creation and of the excitonic luminescence in alkali halides irradiated by swift heavy ions

    International Nuclear Information System (INIS)

    The aim of this experimental thesis is to study the excitonic mechanisms and of the defect creation, in NaCl and KBr, under dense electronic excitations induced by swift heavy ion irradiations. In the first part, we present the main features of the interaction of swift heavy ions with solid targets, and after we review the well known radiolytic processes of the defect creation during X-ray irradiation. In the second chapter, we describe our experimental set-up. In the chapter III, we present our results of the in-situ optical absorption measurements. This results show that defect creation is less sensitive to the temperature than during a classical irradiation. Besides, we observe new mechanisms concerning the defect aggregation. In the chapter IV, we present the results of excitonic luminescence induced by swift by swift heavy ions. We observe that the luminescence yields only change with the highest electronic stopping power. In the chapter V, we perform thermal spike and luminescence yields calculations and we compare the numerical results to the experiments presented in the chapter IV. (author). 121 refs., 65 figs., 30 tabs

  10. Quasiparticle Band Gaps, Excitonic Effects, and Anisotropic Optical Properties of Monolayer Distorted 1-T Diamond-chain Structures

    CERN Document Server

    Zhong, Hong-Xia; Shi, Jun-Jie; Yang, Li

    2015-01-01

    We report many-body perturbation theory calculations of excited-state properties of distorted 1-T diamond-chain monolayer rhenium disulfide (ReS2) and diselenide (ReSe2). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound excitons. Unlike hexagonal structures, the lowest-energy bright exciton of distorted 1-T ReS2 exhibits a perfect figure-8 shape polarization dependence but those of ReSe2 only exhibit a partial polarization dependence, which results from two nearly-degenerated bright excitons whose polarization preferences are not aligned. Our first-principles calculations are in agreement with experiments and pave the way for optoelectronic applications.

  11. Exciton band structure in layered MoSe2: from a monolayer to the bulk limit

    Science.gov (United States)

    Arora, Ashish; Nogajewski, Karol; Molas, Maciej; Koperski, Maciej; Potemski, Marek

    2015-12-01

    We present the micro-photoluminescence (?PL) and micro-reflectance contrast (?RC) spectroscopy studies on thin films of MoSe2 with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe2 is tentatively attributed to the appearance of a Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe2 with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe2.

  12. Exciton band structure in layered MoSe2: from a monolayer to the bulk limit.

    Science.gov (United States)

    Arora, Ashish; Nogajewski, Karol; Molas, Maciej; Koperski, Maciej; Potemski, Marek

    2015-12-28

    We present the micro-photoluminescence (?PL) and micro-reflectance contrast (?RC) spectroscopy studies on thin films of MoSe2 with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe2 is tentatively attributed to the appearance of a Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe2 with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe2. PMID:26603094

  13. Band-edge Exciton States in a Single-walled Carbon Nanotube Revealed by Magneto-optical Spectroscopy in Ultra-high Magnetic Field

    OpenAIRE

    Zhou, Weihang; Sasaki, Tatsuya; Daisuke NAKAMURA; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

    2013-01-01

    We report high field magneto-optical study on the first and second sub-band transitions of single-chirality single-walled carbon nanotubes. The ordering and relative energy splitting between bright and dark excitonic states were found to be inverse between the first and second subbands. We verified that the zero-momentum dark singlet exciton lies below the bright exciton for the first subband transitions, while for the second sub-band transitions, it was found to have higher energy than the b...

  14. Influence of vibronic coupling on band structure and exciton self-trapping in ?-perylene.

    Science.gov (United States)

    West, Brantley A; Womick, Jordan M; McNeil, L E; Tan, Ke Jie; Moran, Andrew M

    2011-05-12

    Exciton sizes influence transport processes and spectroscopic phenomena in molecular aggregates and crystals. Thermally driven nuclear motion generally localizes electronic states in equilibrium systems. Exciton sizes also undergo dynamic changes caused by nonequilibrium relaxation in the lattice structure local to the photoexcitations (i.e., self-trapping). The ?-phase of crystalline perylene is particularly well-suited for fundamental studies of exciton self-trapping mechanisms. It is generally agreed that a subpicosecond self-trapping process in ?-perylene localizes photoexcited excitons onto pairs of closely spaced molecules (i.e., dimers), which then relax through excimer emission. Here, electronic relaxation dynamics in ?-perylene single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Linear absorption and photon echo spectroscopies suggest that excitons are delocalized over less than four unit cells (16 molecules) at 78 K prior to self-trapping. Stimulated Raman spectroscopies conducted on and off electronic resonance reveal significant vibronic coupling in a mode at 104 cm(-1), which corresponds to the displacement between perylene molecules comprising a dimer. Strong vibronic coupling in this mode suggests that motion along the interdimer axis is instrumental in driving the self-trapping process. The results are discussed in the context of our recent study of tetracene and rubrene single crystals in which similar experiments and models were employed. PMID:20806944

  15. New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps.

    Science.gov (United States)

    Zhou, Liujiang; Kou, Liangzhi; Sun, Yan; Felser, Claudia; Hu, Feiming; Shan, Guangcun; Smith, Sean C; Yan, Binghai; Frauenheim, Thomas

    2015-12-01

    Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized two-dimensional (2D) TIs, quantum spin Hall (QSH) insulators, suffer from ultrahigh vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. On the basis of the first-principles calculations, we predict a novel family of 2D QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers, especially, which is the first case based on transition-metal halide-based QSH insulators. MX family has the large nontrivial gaps of 0.12-0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV), and larger than ZrTe5 (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30-70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d-d band inversion, significantly different from conventional band inversion between s-p, p-p, or d-p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials. PMID:26524118

  16. The excitonic photoluminescence mechanism and lasing action in band-gap-tunable CdS1-xSex nanostructures.

    Science.gov (United States)

    Dai, Jun; Zhou, Pengxia; Lu, Junfeng; Zheng, Hongge; Guo, Jiyuan; Wang, Fang; Gu, Ning; Xu, Chunxiang

    2015-12-23

    Bandgap tunable semiconductor materials have wide application in integrated-optoelectronic and communication devices. The CdS1-xSex ternary semiconductor materials covering green-red bands have been reported previously, but their basic band-gap and optical properties crucial to the performance of the CdS1-xSex-based optoelectronic devices have not been deeply understood. In this paper, we theoretically simulated and discussed the feasibility of bandgap-tunable CdS1-xSex nanomaterials for designing wavelength tunable microlasers. Then we fabricated the CdS1-xSex nanobelts with their band gap ranging from 2.4 to 1.74 eV by adjusting the composition ratio x in the vapor-phase-transport growth process. The temperature-dependent photoluminescence and exciton-related optical constants of the CdS1-xSex nanobelts were carefully demonstrated. Finally, the wavelength-tunable Fabry-Perot lasing in CdS1-xSex nanobelts was obtained, and the Fabry-Perot lasing mechanism was numerically simulated by the FDTD method. The systematic results on the mechanism of the tunable band gap, exciton properties and lasing of the CdS1-xSex nanostructure help us deeply understand the intrinsic optical properties of this material, and will build a strong foundation for future application of green-red wavelength-tunable CdS1-xSex microlasers. PMID:26488436

  17. Excitonic spectra and energy band structure of ZnAl2Se4 crystals

    Science.gov (United States)

    Syrbu, N. N.; Zalamai, V. V.; Tiron, A. V.; Tiginyanu, I. M.

    2015-11-01

    Absorption, reflection and wavelength modulated reflection spectra were investigated in ZnAl2Se4 crystals. The energy positions of ground and excited states for three excitonic series (A, B and C) were determined. The main parameters of excitons and more precise values of energy intervals V1(?7)-C1(?6), V2(?6)-C1(?6), and V3(?7)-C1(?6) were estimated. Values of splitting due to crystal field and spin-orbital interaction were calculated. Effective masses of electrons (mC1?) and holes (mV1?, mV2?, mV3?) were estimated. Reflection spectra contours in excitonic region were calculated using dispersion equations. Optical functions for E > Eg from measured reflection spectra were assigned on the base of Kramers-Kronig relations.

  18. Excitonic instability at the spin-state transition in the two-band Hubbard model.

    Czech Academy of Sciences Publication Activity Database

    Kune, Jan; Augustinsk, Pavel

    2014-01-01

    Ro?. 89, ?. 11 (2014), "115134-1"-"115134-8". ISSN 1098-0121 R&D Projects: GA ?R GA13-25251S Institutional support: RVO:68378271 Keywords : excitonic condensation * spin -state transition * dynamical mean-field theory Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.736, year: 2014

  19. Research Update: Physical and electrical characteristics of lead halide perovskites for solar cell applications

    Directory of Open Access Journals (Sweden)

    Simon A. Bretschneider

    2014-04-01

    Full Text Available The field of thin-film photovoltaics has been recently enriched by the introduction of lead halide perovskites as absorber materials, which allow low-cost synthesis of solar cells with efficiencies exceeding 16%. The exact impact of the perovskite crystal structure and composition on the optoelectronic properties of the material are not fully understood. Our progress report highlights the knowledge gained about lead halide perovskites with a focus on physical and optoelectronic properties. We discuss the crystal and band structure of perovskite materials currently implemented in solar cells and the impact of the crystal properties on ferroelectricity, ambipolarity, and the properties of excitons.

  20. Synthesis of tunable-band-gap "Open-Box" halide perovskites by use of anion exchange and internal dissolution procedures.

    Science.gov (United States)

    Wu, Zhengcui; Wang, Baohua; He, Jian; Chen, Tao

    2016-01-01

    We demonstrate the synthesis of cuboid MAPbBr3 (MA=CH3NH3) microcrystals and subsequent conversion into open-box-like MAPb(Br1-xIx)3 (0?x?1) microcrystals by anion exchange in MAI solution. During the substitution of Br(-) with I(-), the initial cuboid framework of MAPbBr3 crystals is retained. The preferential internal dissolution of MAPbBr3 due to the surface coverage and protection of MAPb(Br1-xIx)3 induces voids inside the cuboid crystals, finally leading to open-box-like iodide-rich MAPb(Br1-xIx)3. By controlling the degree of anion exchange, the intense light absorption of the product is able to be tuned in specific wavelengths throughout the visible range. This solution-phase anion exchange approach provides a synthetic strategy in designing sophisticated organolead halide perovskites structures as well as tuning the band gaps for further applications across a range of possible domains. PMID:26397923

  1. Excitonic emissions and above-band-gap luminescence in the single-crystal perovskite semiconductors CsPbB r3 and CsPbC l3

    Science.gov (United States)

    Sebastian, M.; Peters, J. A.; Stoumpos, C. C.; Im, J.; Kostina, S. S.; Liu, Z.; Kanatzidis, M. G.; Freeman, A. J.; Wessels, B. W.

    2015-12-01

    The ternary compounds CsPb X3 (X =Br or Cl) have perovskite structures that are being considered for optical and electronic applications such as lasing and gamma-ray detection. An above-band-gap excitonic photoluminescence (PL) band is seen in both CsPb X3 compounds. An excitonic emission peak centered at 2.98 eV, 0.1 eV above the room-temperature band gap, is observed for CsPbC l3 . The thermal quenching of the excitonic luminescence is well described by a two-step quenching model, yielding activation energies of 0.057 and 0.0076 eV for high- and low-temperature regimes, respectively. CsPbB r3 exhibits bound excitonic luminescence peaks located at 2.29 and 2.33 eV that are attributed to recombination involving Br vacancy centers. Activation energies for thermal quenching of the excitonic luminescence of 0.017 and 0.0007 eV were calculated for CsPbB r3 . Temperature-dependent PL experiments reveal unexpected blueshifts for all excitonic emission peaks in CsPb X3 compounds. A phonon-assisted step-up process leads to the blueshift in CsPbB r3 emission, while there is a contribution from band-gap widening in CsPbC l3 . The absence of significant deep level defect luminescence in these compounds makes them attractive candidates for high-resolution, room-temperature radiation detection.

  2. New band of bound exciton emission in crystals ZnSe and multi-plasmon optical transitions

    International Nuclear Information System (INIS)

    Studies on the cathodoluminescence in the ZnSe crystals, initial and annealed ones in the Bi melt at 1200 K during 120 h with subsequent hardening, are carried out. New series of IiS-nLO-mPl, consisting of a radiation line of the IiS bound excitons with the wave length ? 455.9 nm and its plasmon and LO-phonon reiterations IiS - LO (?1 461.3 nm), IiS - 2LO (?2 = 466.8 nm) , IiS - 3LO (?3 472.4 nm) and IiS - 4LO (?4 = 478.3 nm), is identified in the 450-480 nm wave length area. The average number of the released LO-phonons NLO = 2.2 0.1 for a photon, is determined. It is shown that the more finer band structure observed may be conditioned by multi-plasmon optical transitions

  3. Excitonic spectra and band structure of CdGa{sub 2}Se{sub 4} birefractive crystals

    Energy Technology Data Exchange (ETDEWEB)

    Syrbu, N.N., E-mail: sirbunn@yahoo.com [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Stamov, I.G. [T.G. Shevchenko State University of Pridnestrovie, 25 Oktyabrya Street 107, 3300 Tiraspol, Republic of Moldova (Moldova, Republic of); Parvan, V.I. [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of); Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of); Tiginyanu, I.M. [Institute of Electronic Engineering and Nanotechnologies, Academy of Sciences of Moldova, 3/3 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of)

    2013-11-15

    We report on the intersection of spectral dependences of refractive indices n{sub o} and n{sub e} at the wavelengths 546 nm (?{sub 0}) and 450 nm (?{sub 01}) in CdGa{sub 2}Se{sub 4} single crystals. The value of difference ?n=n{sub e}?n{sub o} is equal to zero at the wavelengths involved. When placed between two crossed polarizers, the crystals of CdGa{sub 2}Se{sub 4} exhibit a transmission band at the wavelength of ?{sub 0}=546 nm (300 K). The ground and excited states of three excitonic series (A, B and C) were found out at 13 K in CdGa{sub 2}Se{sub 4} crystals, and other parameters of excitons and bands were determined. In the ? point of Brillouin zone the effective mass of electrons m{sub c} is equal to 0.14m{sub 0}, and the effective masses of holes m{sub v2} and m{sub v3} are equal to 0.76m{sub 0} and 0.94m{sub 0}, respectively. The hole mass m{sub v1} depends upon the direction of wave vector k: at polarization E?c, k?a the mass m{sub v1}=1.15m{sub 0}, and at polarization E?c, k?b m{sub v1}=0.84m{sub 0}. The values of valence bands splitting in the center of Brillouin zone by the crystal field (?{sub cf}=49 meV) and spinorbital interaction (?{sub so}=351 meV) were determined. The optical functions n, k, ?{sub 1} and ?{sub 2} in polarizations E?c and E?c for the energy diapason from 3 to 6 eV were calculated from the reflectivity spectra by KramersKronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa{sub 2}Se{sub 4} crystals.

  4. Band gaps and optical spectra from single- and double-layer fluorographene to graphite fluoride: many-body effects and excitonic states

    International Nuclear Information System (INIS)

    First-principle band gaps and optical absorption spectra of single- and double-layer fluorographene with bulk graphite fluoride are compared. The electronic properties are calculated using the many-body GW approximation and the optical spectra using the Bethe-Salpeter equation (BSE). The inclusion of electron-hole interactions is crucial for predicting low energy excitonic absorption peaks. The position of the first exciton peak is identical in single-, double- and multilayer fluorographene, which may indicate that the onset of the absorption spectra does not differ in these materials. (copyright 2014 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  5. Quasiparticle band gaps, excitonic effects, and anisotropic optical properties of the monolayer distorted 1 T diamond-chain structures ReS2 and ReSe2

    Science.gov (United States)

    Zhong, Hong-Xia; Gao, Shiyuan; Shi, Jun-Jie; Yang, Li

    2015-09-01

    We report many-body perturbation theory calculations of excited-state properties of distorted 1 T diamond-chain monolayer rhenium disulfide (ReS2) and diselenide (ReSe2). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound excitons. Unlike hexagonal structures, the lowest-energy bright exciton of distorted 1 T ReS2 exhibits a perfect figure-eight shape polarization dependence but those of ReSe2 only exhibit a partial polarization dependence, which results from two nearly degenerated bright excitons whose polarization preferences are not aligned. Our first-principles calculations are in excellent agreement with experiments and pave the way for optoelectronic applications.

  6. Spectroscopy and dynamics of charge transfer excitons in type-II band aligned quantum confined heterostructures

    International Nuclear Information System (INIS)

    We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ?5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ?597 to ?746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ?51 ns as compared to ?6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons

  7. Spectroscopy and dynamics of charge transfer excitons in type-II band aligned quantum confined heterostructures

    Energy Technology Data Exchange (ETDEWEB)

    Kushavah, Dushyant [Centre for Research in Nanotechnology and Science, IIT Bombay-400076, Mumbai (India); Mohapatra, P. K.; Vasa, P.; Singh, B. P., E-mail: bhanups@iitb.ac.in [Department of physics, IIT Bombay, Mumbai-400076 (India); Rustagi, K. C. [Indian Institute of Science Education and Research Bhopal-462066, Bhopal (India); Bahadur, D. [Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai-400076 (India)

    2015-05-15

    We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ?5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ?597 to ?746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ?51 ns as compared to ?6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons.

  8. Emergent U(1) gauge theory with fractionalized boson/fermion from the bose condensation of exciton in multi-band insulator

    OpenAIRE

    Lee, Sung-Sik; Lee, Patrick A.

    2005-01-01

    Fractionalized phases are studied in a low energy theory of exciton bose condensate in a multi-band insulator. It is shown that U(1) gauge theory with either fractionalized boson or fermion can emerge out of a single model depending on the coupling constants. Both the statistics and spin of the fractionalized particles are dynamically determined, satisfying the spin-statistics theorem in the continuum limit. We present two mutually consistent descriptions for the fractionali...

  9. Investigation of doped cuprous halides for photovoltaic and display applications

    OpenAIRE

    Vijayaraghavan, Rajani

    2011-01-01

    The thesis mainly focuses on the growth and optoelectronic characterisation of the doped cuprous halides (CuX) with high UV/blue emission properties and the light harvesting in the CuBr/Si based heterojunction photovoltaic (PV) cells. Since cuprous halides are short wavelength emitters with high excitonic binding energies, growth of lower resistivity, highly luminescent p and n-type films are essential for the development of the future excitonic based light emitting devices with these materia...

  10. Band-structure calculations of noble-gas and alkali halide solids using accurate Kohn-Sham potentials with self-interaction correction

    International Nuclear Information System (INIS)

    The optimized-effective-potential (OEP) method and a method developed recently by Krieger, Li, and Iafrate (KLI) are applied to the band-structure calculations of noble-gas and alkali halide solids employing the self-interaction-corrected (SIC) local-spin-density (LSD) approximation for the exchange-correlation energy functional. The resulting band gaps from both calculations are found to be in fair agreement with the experimental values. The discrepancies are typically within a few percent with results that are nearly the same as those of previously published orbital-dependent multipotential SIC calculations, whereas the LSD results underestimate the band gaps by as much as 40%. As in the LSD---and it is believed to be the case even for the exact Kohn-Sham potential---both the OEP and KLI predict valence-band widths which are narrower than those of experiment. In all cases, the KLI method yields essentially the same results as the OEP

  11. Temperature dependence of energies and broadening parameters of the band-edge excitons of ReS2 and ReSe2

    Science.gov (United States)

    Ho, C. H.; Liao, P. C.; Huang, Y. S.; Tiong, K. K.

    1997-06-01

    We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps Edg of ReS2 and ReSe2 in the temperature range between 25 and 450 K using piezoreflectance (PzR). From a detailed line-shape fit to the PzR spectra we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated.

  12. Structural, optical, and electronic studies of wide-bandgap lead halide perovskites

    KAUST Repository

    Comin, Riccardo

    2015-01-01

    © The Royal Society of Chemistry 2015. We investigate the family of mixed Br/Cl organolead halide perovskites which enable light emission in the blue-violet region of the visible spectrum. We report the structural, optical and electronic properties of this air-stable family of perovskites, demonstrating full bandgap tunability in the 400-550 nm range and enhanced exciton strength upon Cl substitution. We complement this study by tracking the evolution of the band levels across the gap, thereby providing a foundational framework for future optoelectronic applications of these materials.

  13. Ab initio many-body effects in TiSe2: A possible excitonic insulator scenario from GW band-shape renormalization

    Science.gov (United States)

    Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, C.; Aebi, P.; Onida, G.; Olevano, V.

    2012-05-01

    We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ?0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of k2 parabolic dispersion toward a k4 shape. In particular, GW moves the top of valence moved toward a circle of points away from ?, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.

  14. Quasiparticle Band Gaps, Excitonic Effects, and Anisotropic Optical Properties of Monolayer Distorted 1-T Diamond-chain Structures

    OpenAIRE

    Zhong, Hong-xia; Gao, Shiyuan; Shi, Jun-Jie; Yang, Li

    2015-01-01

    We report many-body perturbation theory calculations of excited-state properties of distorted 1-T diamond-chain monolayer rhenium disulfide (ReS2) and diselenide (ReSe2). Electronic self-energy substantially enhances their quasiparticle band gaps and, surprisingly, converts monolayer ReSe2 to a direct-gap semiconductor, which was, however, regarded to be an indirect one by density-functional-theory calculations. Their optical absorption spectra are dictated by strongly bound...

  15. Contrastive analysis of multiple exciton generation theories

    Science.gov (United States)

    Tan, Hengyu; Chang, Qing

    2015-10-01

    Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.

  16. Spatially indirect excitons in coupled quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Lai, Chih-Wei Eddy

    2004-03-01

    Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

  17. Spatially indirect excitons in coupled quantum wells

    Science.gov (United States)

    Lai, Chih-Wei Eddy

    2004-12-01

    Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under a cross-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time---a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect exciton such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 mum)2 was observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

  18. Excitons in narrow-gap carbon nanotubes

    OpenAIRE

    Hartmann, R. R.; Shelykh, I.A.; Portnoi, M. E.

    2010-01-01

    We calculate the exciton binding energy in single-walled carbon nanotubes with narrow band gaps, accounting for the quasi-relativistic dispersion of electrons and holes. Exact analytical solutions of the quantum relativistic two-body problem are obtain for several limiting cases. We show that the binding energy scales with the band gap, and conclude on the basis of the data available for semiconductor nanotubes that there is no transition to an excitonic insulator in quasi-metallic nanotubes ...

  19. Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

    CERN Document Server

    Khn, Oliver

    2011-01-01

    The theoretical description and the properties of Frenkel excitons in non-covalently bonded molecular aggregates are reviewed from a multi-exciton perspective of dissipative quantum dynamics. First, the photophysical and quantum chemical characterization of the monomeric dye building blocks is discussed, including the important aspect of electron-vibrational coupling within the Huang-Rhys model. Supplementing the model by the Coulombic interactions between monomers, the description of aggregates in terms of excitonic or vibrational-excitonic bands follows. Besides of giving rise to complex absorption and emission line shapes, exciton-vibrational interaction is responsible for energy and phase relaxation and thereby limits the size of coherent excitations in larger aggregates. Throughout, emphasis is put on the electronic three-level model as a minimum requirement to describe nonlinear spectroscopies including effects of two-exciton states such as excited state absorption and exciton-exciton annihilation. The ...

  20. Band gap bowing and exciton localization in strained cubic InxGa1-xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

    Science.gov (United States)

    Chichibu, S. F.; Sugiyama, M.; Kuroda, T.; Tackeuchi, A.; Kitamura, T.; Nakanishi, H.; Sota, T.; DenBaars, S. P.; Nakamura, S.; Ishida, Y.; Okumura, H.

    2001-11-01

    Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1-xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1-xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1-xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material's inhomogeneity is much enhanced compared to that of the hexagonal polytype.

  1. Bright and dark excitons in semiconductor carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Tretiak, Sergei [Los Alamos National Laboratory

    2008-01-01

    We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.

  2. Long-time Luminescence Kinetics of Localized excitons and conduction Band Edges Smearing in ZnSe(1-c)Tec Solid Solutions

    DEFF Research Database (Denmark)

    Klochikhin, O.; Ogloblin, S. G.; Permogorov, S.; Reznitsky, A.; Klingshirn, C.; Vadim, Lyssenko; Hvam, Jrn Mrcher

    2000-01-01

    It is shown that the integrated luminescence intensity of localized excitons in solid solutions ZnSe(1 - c)Tec has a component slowly decaying with time. After the excitation above the mobility threshold, the long-time intensity decreases exponentially, with a fractional exponent changing from a value corresponding to the critical index of anomalous diffusion to the index of normal diffusion as the temperature increases from 5 to 80 K. This change allows estimation of the energy scale for the fl...

  3. Photofragmentation of metal halides

    International Nuclear Information System (INIS)

    The author deals with photodissociation of molecules of alkali halides. It is shown that the total absorption cross section consists of two contributions arising from transitions to excited states of total electronic angular momentum ?=0+ and ?=1. From the inversion of the absorption continua potential energy curves of the excited states can be constructed in the Franck-Condon region. It is found that for all alkali halides the 0+ state is higher in energy than the ?=1 state. Extensive studies are reported on three thallium halides, TlI, TlBr and TlCl at various wavelengths covering the near ultraviolet region. (Auth.)

  4. Effects of various halide ions and probe molecules on inelastic Mie scattering from surface enhanced Raman scattering active surfaces: Determination of particle size distributions from band shapes simulation

    Science.gov (United States)

    Felidj, Nordin; Aubard, Jean; Lvi, Georges

    1996-06-01

    Extensive Raman experiments at very low frequencies arising from the scattering of mechanical vibrations of surface roughness features and colloidal particles (inelastic Mie scattering of localized acoustic vibrations) have been carried out at various laser excitation wavelengths. These low frequency bands have been studied either on electrochemically roughened silver electrodes or in colloidal silver sols, in the presence of polarizable molecules (pyridine, benzoic acid, acridine) or only with various salts. A simple model has been built and allows to account satisfactorily for the experimental band shapes. From the fit between experimental and calculated curves, we can approach the size distribution for resonant particles. These distributions display a shift of their maximum toward large size particles and a broadening of their widths when the laser excitation wavelength is turned from violet to red. An unexpected intensity enhancement of these acoustic modes, detected when excitation takes place in the red, cannot be explained as originating solely from electric dipolar plasmon resonance. Likewise, surface enhanced Raman scattering (SERS) spectra, molecular mechanisms via charge transfer complexes and/or dipolar magnetic scattering are invoked in an attempt of explanation.

  5. Binary technetium halides

    Science.gov (United States)

    Johnstone, Erik Vaughan

    In this work, the synthetic and coordination chemistry as well as the physico-chemical properties of binary technetium (Tc) chlorides, bromides, and iodides were investigated. Resulting from these studies was the discovery of five new binary Tc halide phases: alpha/beta-TcCl3, alpha/beta-TcCl 2, and TcI3, and the reinvestigation of the chemistries of TcBr3 and TcX4 (X = Cl, Br). Prior to 2009, the chemistry of binary Tc halides was poorly studied and defined by only three compounds, i.e., TcF6, TcF5, and TcCl4. Today, ten phases are known (i.e., TcF6, TcF5, TcCl4, TcBr 4, TcBr3, TcI3, alpha/beta-TcCl3 and alpha/beta-TcCl2) making the binary halide system of Tc comparable to those of its neighboring elements. Technetium binary halides were synthesized using three methods: reactions of the elements in sealed tubes, reactions of flowing HX(g) (X = Cl, Br, and I) with Tc2(O2CCH3)4Cl2, and thermal decompositions of TcX4 (X = Cl, Br) and alpha-TcCl 3 in sealed tubes under vacuum. Binary Tc halides can be found in various dimensionalities such as molecular solids (TcF6), extended chains (TcF5, TcCl4, alpha/beta-TcCl2, TcBr 3, TcI3), infinite layers (beta-TcCl3), and bidimensional networks of clusters (alpha-TcCl3); eight structure-types with varying degrees of metal-metal interactions are now known. The coordination chemistry of Tc binary halides can resemble that of the adjacent elements: molybdenum and ruthenium (beta-TcCl3, TcBr3, TcI 3), rhenium (TcF5, alpha-TcCl3), platinum (TcCl 4, TcBr4), or can be unique (alpha-TcCl2 and beta-TcCl 2) in respect to other known transition metal binary halides. Technetium binary halides display a range of interesting physical properties that are manifested from their electronic and structural configurations. The thermochemistry of binary Tc halides is extensive. These compounds can selectively volatilize, decompose, disproportionate, or convert to other phases. Ultimately, binary Tc halides may find application in the nuclear fuel cycle and as precursors in inorganic and organometallic chemistry.

  6. Excitons in nanoscale systems

    Science.gov (United States)

    Scholes, Gregory D.; Rumbles, Garry

    2006-09-01

    Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.

  7. Coherent excitonic molecules

    Science.gov (United States)

    Keldysh, L. V.

    1992-10-01

    A model which allows to consider self-consistently many-body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by intense electromagnetic field. In the presence of a pump the normal electromagnetic wave spectrum in the vicinity of the nonlinear biexciton resonance is shown to be composed by four polariton branches, each being as a matter of fact the superposition of two photons and two excitons. In the self-consistent field approximation the structure of the interconnected strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at strong enough pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron-hole "liquid". Appearance and temporary evolution of an exciton-biexciton system after the switching on of a pump is also considered.

  8. THEORY OF QUANTUM WELL EXCITONS IN ELECTRIC AND MAGNETIC FIELDS

    OpenAIRE

    Bauer, G.; Ando, T

    1987-01-01

    The effects of external electric and magnetic fields on excitons in GaAs/AlGaAs quantum wells have been calculated in the effective mass approximation. Recent experiments by Via et al. which identify exciton couplings induced by an electric field are satisfactorily explained by the effects of valence band mixing. The properties of magneto-excitons in quantum wells are also found to be characteristically affected by electric fields.

  9. Calculation of intraband electron and band-to-band hole luminescence spectra of some alkali halide crystals under pulse excitation by dense electron and laser beams of nano- and picosecond duration

    International Nuclear Information System (INIS)

    Theoretical calculation of spectra of two new types of fundamental radioluminescence of dielectrics - intraband electron and band-to-band hole ones - was conducted. These types of radioluminescence were revealed and studied experimentally due to the use of two high-power nano- and picosecond radiation sources for dielectric excitation - high-current electron accelerators and lasers with generators of ultraviolet harmonics. Satisfactory agreement of calculated spectra with ones, measured experimentally, was achieved. Correlation of calculation results with experimental data enabled to determine some important parameters of electron spectrum and specifically the upper boundary of passive region for conduction band ionization for CsI crystals. 27 refs., 8 figs

  10. Purcell effect in an organic-inorganic halide perovskite semiconductor microcavity system

    Science.gov (United States)

    Wang, Jun; Cao, Runan; Da, Peimei; Wang, Yafeng; Hu, Tao; Wu, Lin; Lu, Jian; Shen, Xuechu; Xu, Fei; Zheng, Gengfeng; Chen, Zhanghai

    2016-01-01

    Organic-inorganic halide perovskite semiconductors with the attractive physics properties, including strong photoluminescence (PL), huge oscillator strengths, and low nonradiative recombination losses, are ideal candidates for studying the light-matter interaction in nanostructures. Here, we demonstrate the coupling of the exciton state and the cavity mode in the lead halide perovskite microcavity system at room temperature. The Purcell effect in the coupling system is clearly observed by using angle-resolved photoluminescence spectra. Kinetic analysis based on time-resolved PL reveals that the spontaneous emission rate of the halide perovskite semiconductor is significantly enhanced at resonance of the exciton energy and the cavity mode. Our results provide the way for developing electrically driven organic polariton lasers, optical devices, and on-chip coherent quantum light sources.

  11. Quantum confinement of excitons in wurtzite InP nanowires

    International Nuclear Information System (INIS)

    Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands

  12. Quantum confinement of excitons in wurtzite InP nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Pemasiri, K.; Jackson, H. E.; Smith, L. M. [Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221 (United States); Wong, B. M. [Department of Chemical and Environmental Engineering and Materials Science, University of California, Riverside, California 92521 (United States); Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C. [Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200 (Australia)

    2015-05-21

    Exciton resonances are observed in photocurrent spectra of 80?nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30?nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.

  13. Is actinometry reliable for monitoring Si and silicone halides produced in silicon etching plasmas? A comparison with their absolute densities measured by UV broad band absorption

    Science.gov (United States)

    Kogelschatz, M.; Cunge, G.; Sadeghi, N.

    2006-03-01

    SiCl{x} radicals, the silicon etching by-products, are playing a major role in silicon gate etching processes because their redeposition on the wafer leads to the formation of a SiOCl{x} passivation layer on the feature sidewalls, which controls the final shape of the etching profile. These radicals are also the precursors to the formation of a similar layer on the reactor walls, leading to process drifts. As a result, the understanding and modelling of these processes rely on the knowledge of their densities in the plasma. Actinometry technique, based on optical emission, is often used to measure relative variations of the density of the above mentioned radicals, even if it is well known that the results obtained with this technique might not always be reliable. To determine the validity domain of actinometry in industrial silicon-etching high density plasmas, we measure the RF source power and pressure dependences of the absolute densities of SiCl{x} (x=0{-}2), SiF and SiBr radicals, deduced from UV broad band absorption spectroscopy. These results are compared to the evolution of the corresponding actinometry signals from these radicals. It is shown that actinometry predicts the global trends of the species density variations when the RF power is changed at constant pressure (that is to say when only the electron density changes) but it completely fails if the gas pressure, hence the electron temperature, changes.

  14. Excitons in type-II quantum dots finite offsets

    CERN Document Server

    Laheld, U E H; Hemmer, P C; Laheld, U E H; Pedersen, F B; Hemmer, P C

    1995-01-01

    Quantum size effects for an exciton attached to a spherical quantum dot are calculated by a variational approach. The band line-ups are assumed to be type-II with finite offsets. The dependence of the exciton binding energy upon the dot radius and the offsets is studied for different sets of electron and hole effective masses.

  15. Exciton Dynamics in Hexagonal InP Nanowires

    Science.gov (United States)

    Kaveh-Baghbadorani, Masoud; Langbein, Wolfgang; Gao, Qiang; Jagadish, Chennupati; Wagner, Hans-Peter

    2013-03-01

    We study the exciton dynamics in InP nanowire ensembles by intensity- and temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP nanowires were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at a temperature of 450 C resulting in nearly wurtzite type nanowires. The PL measurements at 15 K show a strong emission band at 837 nm and two weak side bands at nearly 820 and 860 nm. The bands are tentatively attributed to trapped, free and zinc-blende related exciton transitions, respectively. With increasing temperature the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC measurements show an increasing PL decay rate of all emission bands with increasing temperature most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these nanowires. The support of the Australian Research Council is kindly acknowledged.

  16. Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

    International Nuclear Information System (INIS)

    Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 1016 cm?3 to 6 1017 cm?3. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ?3.47?eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 12 1017 cm?3 is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission

  17. Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials

    KAUST Repository

    Tizei, Luiz H. G.

    2015-03-01

    Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.

  18. Optics of excitonic molecules in semiconductors and semiconductor microstructures

    Science.gov (United States)

    Ivanov, A. L.; Haug, H.; Keldysh, L. V.

    1998-03-01

    A bipolariton concept of an excitonic molecule in direct-band-gap semiconductors is reviewed. The bipolariton model interprets a molecule as two quasi-bound polaritons provided that both the exciton-exciton Coulombic interaction and the exciton-photon coupling (polariton effect) are treated simultaneously and beyond a low-order perturbation theory. The molecule-mediated optics is reformulated within the bipolariton concept for bulk semiconductors (e.g., CuCl and CdS) and semiconductor microstructures (e.g., GaAs quantum wells and wires). We discuss such phenomena as two-photon absorption and four-wave-mixing due to excitonic molecules, molecule-mediated optical Stark effect and other coherent processes which effectively involve excitonic molecules, and polariton solitons coupled through the molecule state.

  19. Excitonic polarons in low-dimensional transition metal dichalcogenides

    Science.gov (United States)

    Thilagam, A.

    2015-05-01

    We examine the excitonic polaron properties of common monolayer transition metal dichalcogenides (MoS2, MoSe2, WS2 and WSe2). The excitonic polaron is formed when excitons interact with acoustic or optical phonons via coupling to the deformation potentials associated with the conduction and valence bands. A unitary transformation which performs an approximate diagonalization of the exciton-phonon operator is used to evaluate the ground state energy of the excitonic polaron. We derive analytical expressions of the changes in the excitonic polaron energy and mass at small exciton wavevectors involving the deformation potential due to optical phonons. The polaronic effect of the monolayer transition metal dichalcogenides is examined by comparing changes in the energy gap shift and effective masses based on known deformation potential constants for carrier-phonon interactions. Our results indicate the occurrence of comparable energy shifts when the ground state exciton interacts with optical or acoustic phonons. We extend our calculations to explore the influence of exciton-lattice interactions on the binding energies and the self-trapping of excitons in two-dimensional layers of transition metal dichalcogenides.

  20. Confined exciton spectroscopy

    International Nuclear Information System (INIS)

    Full text: In this work, the exciton is considered as a sensor of the electronic and optical properties of materials such as semiconductors, which have size compared to the exciton De Broglie wavelength, approximately 20 nm, depending on the semiconductor. Examples of electron-phonon, electron-electron, photon-electron, exciton-polariton, phonon-plasmon, are presented, under different confinement conditions such as quantum wells, superlattices

  1. Excitons in cuprous oxide

    OpenAIRE

    Fishman, Dmitry,

    2008-01-01

    This work revisits the excitonic properties of Cu2O as observed with optical techniques, making use of current days experimental capabilities providing very precise measurements, with high energy and/or time resolution, using a variety of high power coherent pulsed light sources, intense magnetic fields, and cryogenic temperatures. A short overview of the theory of excitons and the excitonic Bose-Einstein condensation is presented. We discuss the ideas and results of the experiments aimed at ...

  2. Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds

    CERN Document Server

    Ohara, J; Ohara, Jun; Yamamoto, Shoji

    2006-01-01

    In order to reveal the relaxation mechanism of photogenerated charge-transfer excitations in quasi-one-dimensional halogen-bridged diplatinum complexes, we calculate the low-lying adiabatic potential energy surfaces of a one-dimensional extended Peierls-Hubbard model. High-energy excitations above the electron-hole continuum may relax into polarons, while excitons pumped within the optical gap are self-localized and then either decay by luminescence or divide into solitons. Neutral solitons, charged solitons, and polarons may be simultaneously photogenerated in a diplatinum-halide chain, which has never been observed in any conventional platinum-halide chain. Optical conductivity is also simulated along the decay paths for experimental verification.

  3. Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes

    Science.gov (United States)

    Chmeliov, Jevgenij; Narkeliunas, Jonas; Graham, Matt W.; Fleming, Graham R.; Valkunas, Leonas

    2016-01-01

    We present a thorough analysis of one- and two-color transient absorption measurements performed on single- and double-walled semiconducting carbon nanotubes. By combining the currently existing models describing exciton-exciton annihilation--the coherent and the diffusion-limited ones--we are able to simultaneously reproduce excitation kinetics following both E11 and E22 pump conditions. Our simulations revealed the fundamental photophysical behavior of one-dimensional coherent excitons and non-trivial excitation relaxation pathways. In particular, we found that after non-linear annihilation a doubly-excited exciton relaxes directly to its E11 state bypassing the intermediate E22 manifold, so that after excitation resonant with the E11 transition, the E22 state remains unpopulated. A quantitative explanation for the observed much faster excitation kinetics probed at E22 manifold, comparing to those probed at the E11 band, is also provided.

  4. Ultraviolet optical absorption of alkali cyanides and alkali halide cyanides

    International Nuclear Information System (INIS)

    The ultraviolet absorption spectra of alkali cyanide and mixed alkali halide cyanide crystals were measured at temperatures ranging from 300K down to 4.2K. A set of small absorption peaks was observed at energies near 6 eV and assigned to parity forbidden X1?+?a'3?+ transitions of the CN- molecular ions. It was observed that the peak position depends on the alkali atom while the absorption cross section strongly depends on the halogen and on the CN- concentration of the mixed crystals. These effects are explained in terms of an interaction between the triplet molecular excitons and charge transfer excitons. The experimental data were fit with a coupling energy of a few meV. The coupling mechanism is discussed and it is found to be due to the overlap between the wave functions of the two excitations. (Author)

  5. Multiscale photosynthetic exciton transfer

    OpenAIRE

    Ringsmuth, A. K.; Milburn, G J; Stace, T. M.

    2012-01-01

    Photosynthetic light harvesting provides a natural blueprint for bioengineered and biomimetic solar energy and light detection technologies. Recent evidence suggests some individual light harvesting protein complexes (LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction centers (RCs) via an interplay between excitonic quantum coherence, resonant protein vibrations, and thermal decoherence. The role of coherence in vivo is unclear however, where exci...

  6. Cu halide nanoparticle formation by diffusion of copper in alkali halide crystals

    Scientific Electronic Library Online (English)

    A., Prez-Rodrguez; M., Flores-Acosta; R., Rodrguez-Mijangos; R., Prez-Salas.

    2006-04-01

    Full Text Available Atomos de cobre han sido introducidos por difusin en cristales de NaCl, KCl y KBr a 500C. Los cristales han sido analizados pticamente con medidas de fotoluminiscencia y por microscopa electrnica de barrido. Los espectros de emisin y excitacin, medidos a baja temperatura muestran el efecto de [...] confinamiento de exciton, indicando la formacin de nanopartculas de CuX (X=Cl, Br), lo cual ha sido confirmado por imgenes de microscopa electrnica. Este mtodo es propuesto como un mtodo alternativo para obtener nanopartculas de CuX en cristales halogenuros alcalinos. Abstract in english Copper atoms have been introduced by diffusion in NaCl, KCl and KBr crystals at 500C. The crystals have been optically analyzed with photoluminescence measurements and by scanning electron microscopy. The emission and excitation spectra measured at low temperatures show the exciton confinement effe [...] ct, indicating the formation of CuX (X=Cl, Br) nanoparticles, which has been confirmed by electron microscopy images. This is proposed as an alternative method to obtain CuX nanoparticles in alkali halides crystals.

  7. Halide laser glasses

    International Nuclear Information System (INIS)

    Energy storage and energy extraction are of prime importance for efficient laser action and are affected by the line strengths and linewidths of optical transitions, excited-state lifetimes, nonradiative decay processes, spectroscopic inhomogeneities, nonlinear refractive index, and damage threshold. These properties are all host dependent. To illustrate this, the spectroscopic properties of Nd3+ have been measured in numerous oxide, oxyhalide, and halide glasses. A table summarizes the reported ranges of stimulated emission cross sections, peak wavelengths, linewidths, and radiative lifetimes associated with the 4F/sub 3/2/ ? 4I/sub 11/2/ lasing transition

  8. Quasienergy Spectroscopy of Excitons

    DEFF Research Database (Denmark)

    Johnsen, Kristinn; Jauho, Antti-Pekka

    1999-01-01

    We theoretically study nonlinear optics of excitons under intense THz irradiation. In particular, the linear near-infrared absorption and resonantly enhanced nonlinear sideband generation are described. We predict a rich structure in the spectra which an be interpreted in terms of the quasienergy spectrum of the exciton, via a remarkably transparent expression for the susceptibility, and show that the effects of strongly avoided quasienergy crossings manifest themselves directly, both in the abs...

  9. Cuprous halides semiconductors as a new means for highly efficient light-emitting diodes.

    Science.gov (United States)

    Ahn, Doyeol; Park, Seoung-Hwan

    2016-01-01

    In group-III nitrides in use for white light-emitting diodes (LEDs), optical gain, measure of luminous efficiency, is very low owing to the built-in electrostatic fields, low exciton binding energy, and high-density misfit dislocations due to lattice-mismatched substrates. Cuprous halides I-VII semiconductors, on the other hand, have negligible built-in field, large exciton binding energies and close lattice matched to silicon substrates. Recent experimental studies have shown that the luminescence of I-VII CuCl grown on Si is three orders larger than that of GaN at room temperature. Here we report yet unexplored potential of cuprous halides systems by investigating the optical gain of CuCl/CuI quantum wells. It is found that the optical gain and the luminescence are much larger than that of group III-nitrides due to large exciton binding energy and vanishing electrostatic fields. We expect that these findings will open up the way toward highly efficient cuprous halides based LEDs compatible to Si technology. PMID:26880097

  10. Exciton-exciton annihilation in MoSe2 monolayers

    CERN Document Server

    Kumar, Nardeep; Ceballos, Frank; He, Dawei; Wang, Yongsheng; Zhao, Hui

    2013-01-01

    We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption microscopy. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in the range of 790 - 820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in the bulk under comparable conditions. We also observe the saturated absorption induced by exciton phase-space filling in both monolayers and the bulk, which indicates their potential applications as saturable absorbers.

  11. Making and Breaking of Lead Halide Perovskites.

    Science.gov (United States)

    Manser, Joseph S; Saidaminov, Makhsud I; Christians, Jeffrey A; Bakr, Osman M; Kamat, Prashant V

    2016-02-16

    A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80-150 °C) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions. This Account provides insight into the lifecycle of organic-inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices. PMID:26789596

  12. Making and Breaking of Lead Halide Perovskites

    KAUST Repository

    Manser, Joseph S.

    2016-02-16

    A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80–150 °C) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions. This Account provides insight into the lifecycle of organic–inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices.

  13. Second harmonic generation spectroscopy of excitons in ZnO

    OpenAIRE

    Lafrentz, M.; Brunne, D.; Rodina, A. V.; Pavlov, V. V.; R. V. Pisarev; Yakovlev, D.R.; Bakin, A; Bayer, M.

    2013-01-01

    Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in...

  14. Probing Excitonic Dark States in Single-layer Tungsten Disulfide

    CERN Document Server

    Ye, Ziliang; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

    2014-01-01

    Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, exhibiting a number of intriguing optical phenomena such as valley selective circular dichroism, doping dependent charged excitons, and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of exc...

  15. Multiple Exciton Generation in Colloidal Nanocrystals

    Directory of Open Access Journals (Sweden)

    Charles Smith

    2013-12-01

    Full Text Available In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG, can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

  16. Excitonic gap formation in neutral bilayer structures

    Science.gov (United States)

    Apinyan, V.; Kopeć, T. K.

    2015-08-01

    We consider the pairing between conduction band electrons, and the valence band holes in the neutral bilayer-type structures. By employing the bilayer Hubbard model, we show the possibility of the inter-plane exciton formation in the system without applied external field. The in-plane and inter-plane Coulomb interaction effects on the pairing mechanism are considered, and the role of the in-plane particle hopping asymmetry on the gap behavior is analyzed in the paper. We show that both Frenkel-type pairing channel and Wannier-Mott-type excitonic pairings are present in the considered system. We analyze also the structure of the chemical potential in the bilayer system. The temperature effects, and the tunable inter-plane electron hopping effects are discussed. For the Frenkel channel, we have shown a particular behavior of the chemical potential at very low temperatures, which is related to the degenerated Frenkel-gap.

  17. Actinide halide complexes

    Energy Technology Data Exchange (ETDEWEB)

    Avens, L.R.; Zwick, B.D.; Sattelberger, A.P.; Clark, D.L.; Watkin, J.G.

    1991-02-07

    A compound of the formula MX{sub n}L{sub m} wherein M = Th, Pu, Np,or Am thorium, X = a halide atom, n = 3 or 4, L is a coordinating ligand selected from the group consisting of aprotic Lewis bases having an oxygen-, nitrogen-, sulfur-, or phosphorus-donor, and m is 3 or 4 for monodentate ligands or is 2 for bidentate ligands, where n + m = 7 or 8 for monodentate ligands or 5 or 6 for bidentate ligands, a compound of the formula MX{sub n} wherein M, X, and n are as previously defined, and a process of preparing such actinide metal compounds including admixing the actinide metal in an aprotic Lewis base as a coordinating solvent in the presence of a halogen-containing oxidant, are provided.

  18. Role of microstructure in the electron-hole interaction of hybrid lead halide perovskites

    Science.gov (United States)

    Grancini, Giulia; Srimath Kandada, Ajay Ram; Frost, Jarvist M.; Barker, Alex J.; de Bastiani, Michele; Gandini, Marina; Marras, Sergio; Lanzani, Guglielmo; Walsh, Aron; Petrozza, Annamaria

    2015-10-01

    Organic-inorganic metal halide perovskites have demonstrated high power conversion efficiencies in solar cells and promising performance in a wide range of optoelectronic devices. The existence and stability of bound electron-hole pairs in these materials and their role in the operation of devices with different architectures remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modelling as a function of the degree of polycrystallinity and temperature, that the electron-hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycrystalline disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphology strongly influence perovskite behaviour, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.

  19. Structural and electronic properties of organo-halide hybrid perovskites from ab initio molecular dynamics.

    Science.gov (United States)

    Quarti, Claudio; Mosconi, Edoardo; De Angelis, Filippo

    2015-04-14

    The last two years have seen the unprecedentedly rapid emergence of a new class of solar cells, based on hybrid organic-inorganic halide perovskites. The success of this class of materials is due to their outstanding photoelectrochemical properties coupled to their low cost, mainly solution-based, fabrication techniques. Solution processed materials are however often characterized by an inherent flexible structure, which is hardly mapped into a single local minimum energy structure. In this perspective, we report on the interplay between structural and electronic properties of hybrid lead iodide perovskites investigated using ab initio molecular dynamics (AIMD) simulations, which allow the dynamical simulation of disordered systems at finite temperature. We compare the prototypical MAPbI3 (MA = methylammonium) perovskite in its cubic and tetragonal structure with the trigonal phase of FAPbI3 (FA = formamidinium), investigating different starting arrangements of the organic cations. Despite the relatively short time scale amenable to AIMD, typically a few tens of ps, this analysis demonstrates the sizable structural flexibility of this class of materials, showing that the instantaneous structure could significantly differ from the time and thermal averaged structure. We also highlight the importance of the organic-inorganic interactions in determining the fluxional properties of this class of materials. A peculiar spatial localization of the valence and conduction band edges is also found, with a dynamics in the range of 0.1 ps, which is associated with the positional dynamics of the organic cations within the cubo-octahedral perovskite cage. This asymmetry in the spatial localization of the band edges is expected to ease exciton dissociation and assist the initial stages of charge separation, possibly constituting one of the key factors for the impressive photovoltaic performances of hybrid lead-iodide perovskites. PMID:25766785

  20. STRAIN INDUCED EFFECTS OF DISLOCATIONS IN CdS:SHIFT OF EXCITONIC LINES, PHOTOVOLTAIC EFFECT

    OpenAIRE

    Vyvenko, O.

    1983-01-01

    The calculation of the energy displacement of excitonic lines due to elastic strain of DSB have been performed for wurzite type crystals using the deformation potential theory. It is shown that nonlinear dependence of the position of valence bands on lattice strain gives rise to a different behavior of excitonic lines for various types of dislocation. The results of calculations are applied for the explanation of the characteristics of the free exciton reflection spectra and the spectra of ph...

  1. Dynamical process of exciton-exciton scattering in CuI thin films

    International Nuclear Information System (INIS)

    We have investigated the time-resolved photoluminescence (PL) spectra of CuI thin films under intense excitation conditions in the time region up to 25 ps. In the time-integrated PL spectra, we have clearly observed the PL band originated from the inelastic scattering of excitons, so-called P emission. The time-resolved PL spectra obtained by using an optical Kerr gating method exhibit that the peak energy of the P-emission band temporally changes in a picosecond region, which reflects the variation of the effective temperature of excitonic system. In the time profile of the P-emission band, we found the following two characteristic properties. One is that the decay time hardly depends on the excitation power. The other is that the rise time becomes considerably faster with increasing excitation power. Moreover, we have found that the inverse of the rise time of the P emission exhibits an almost quadratic dependence on the excitation power

  2. Multiscale photosynthetic exciton transfer

    CERN Document Server

    Ringsmuth, A K; Stace, T M; 10.1038/nphys2332

    2012-01-01

    Photosynthetic light harvesting provides a natural blueprint for bioengineered and biomimetic solar energy and light detection technologies. Recent evidence suggests some individual light harvesting protein complexes (LHCs) and LHC subunits efficiently transfer excitons towards chemical reaction centers (RCs) via an interplay between excitonic quantum coherence, resonant protein vibrations, and thermal decoherence. The role of coherence in vivo is unclear however, where excitons are transferred through multi-LHC/RC aggregates over distances typically large compared with intra-LHC scales. Here we assess the possibility of long-range coherent transfer in a simple chromophore network with disordered site and transfer coupling energies. Through renormalization we find that, surprisingly, decoherence is diminished at larger scales, and long-range coherence is facilitated by chromophoric clustering. Conversely, static disorder in the site energies grows with length scale, forcing localization. Our results suggest s...

  3. Machine Learning Exciton Dynamics

    CERN Document Server

    Hse, Florian; Pyzer-Knapp, Edward; Aspuru-Guzik, Aln

    2015-01-01

    Obtaining the exciton dynamics of large photosynthetic complexes by using mixed quantum mechanics/molecular mechanics (QM/MM) is computationally demanding. We propose a machine learning technique, multi-layer perceptrons, as a tool to reduce the time required to compute excited state energies. With this approach we predict time-dependent density functional theory (TDDFT) excited state energies of bacteriochlorophylls in the Fenna-Matthews-Olson (FMO) complex. Additionally we compute spectral densities and exciton populations from the predictions. Different methods to determine multi-layer perceptron training sets are introduced, leading to several initial data selections. In addition, we compute spectral densities and exciton populations. Once multi-layer perceptrons are trained, predicting excited state energies was found to be significantly faster than the corresponding QM/MM calculations. We showed that multi-layer perceptrons can successfully reproduce the energies of QM/MM calculations to a high degree o...

  4. Excitons in motion in II-VI semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Davies, J.J.; Smith, L.C.; Wolverson, D. [Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom); Kochereshko, V.P. [A.F. Ioffe Physico-Technical Institute, RAS, 194021 St. Petersburg (Russian Federation); Cibert, J.; Mariette, H.; Boukari, H. [Institut Neel, CNRS-Universite Joseph Fourier, 38042 Grenoble Cedex 9 (France); Wiater, M.; Karczewski, G.; Wojtowicz, T. [Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw (Poland); Gust, A.; Kruse, C.; Hommel, D. [Institute for Solid State Physics, Semiconductor Epitaxy Group, University of Bremen, 28334 Bremen (Germany)

    2010-06-15

    We have shown recently that the magnetic properties of excitons change significantly as the excitons acquire kinetic energy. In particular, the exciton magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectroscopic studies of excitons confined in quantum wells of large width (greater than five times the exciton Bohr radius) and these motion-induced changes in the magnetic properties have now been observed for CdTe, ZnSe, ZnTe and GaAs. The present paper summarises these phenomena, with particular focus on CdTe and ZnSe, and shows that the changes can be accounted for by motion-induced mixing between the exciton ground and higher lying states. The mixing is caused by the {gamma}{sub 3} term in the Luttinger Hamiltonian which describes the dispersion curves for the valence band and, as a result, the form of the exciton wavefunction becomes motion-dependent. For both materials, excellent agreement is obtained between experiment and the results predicted by this mechanism. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

  5. Excitonic properties of ZnS quantum wells

    Science.gov (United States)

    Urbaszek, B.; Townsley, C. M.; Tang, X.; Morhain, C.; Balocchi, A.; Prior, K. A.; Nicholas, R. J.; Cavenett, B. C.

    2001-10-01

    The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by molecular-beam epitaxy on GaP substrates has allowed the observation of heavy- and light-hole exciton transitions with values for the full width at half maximum as narrow as 5 meV. The 2s state of the heavy-hole exciton is identified and exciton binding energies of as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light- and heavy-hole exciton transitions appear in magnetoreflectivity spectra in magnetic fields up to 54 T. Large light-hole exciton g values of the order of 4 for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile-strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

  6. Theory of two-dimensional spatially indirect equilibrium exciton condensates

    Science.gov (United States)

    Wu, Feng-Cheng; Xue, Fei; MacDonald, A. H.

    2015-10-01

    We present a theory of bilayer two-dimensional electron systems that host a spatially indirect exciton condensate when in thermal equilibrium. Equilibrium bilayer exciton condensates (BXCs) are expected to form when two nearby semiconductor layers are electrically isolated, and when the conduction band of one layer is brought close to degeneracy with the valence band of a nearby layer by varying bias or gate voltages. BXCs are characterized by spontaneous interlayer phase coherence and counterflow superfluidity. The bilayer system we consider is composed of two transition metal dichalcogenide monolayers separated and surrounded by hexagonal boron nitride. We use mean-field theory and a bosonic weakly interacting exciton model to explore the BXC phase diagram, and time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. We find that a phase transition occurs between states containing one and two condensate components as the layer separation and the exciton density are varied, and derive simple approximate expressions for the exciton-exciton interaction strength which we show can be measured capacitively.

  7. Excitonic condensation in spatially separated one-dimensional systems

    International Nuclear Information System (INIS)

    We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field

  8. Excitonic condensation in spatially separated one-dimensional systems

    Energy Technology Data Exchange (ETDEWEB)

    Abergel, D. S. L. [Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm (Sweden); Center for Quantum Materials, KTH and Nordita, Roslagstullsbacken 17, SE-106 91 Stockholm (Sweden)

    2015-05-25

    We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field.

  9. Determination of the Exciton Binding Energy in CdSe Quantum Dots

    Energy Technology Data Exchange (ETDEWEB)

    Meulenberg, R; Lee, J; Wolcott, A; Zhang, J; Terminello, L; van Buuren, T

    2009-10-27

    The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using x-ray spectroscopy. Using x-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e. without and exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

  10. Actinide halide complexes

    Science.gov (United States)

    Avens, Larry R. (Los Alamos, NM); Zwick, Bill D. (Santa Fe, NM); Sattelberger, Alfred P. (Los Alamos, NM); Clark, David L. (Los Alamos, NM); Watkin, John G. (Los Alamos, NM)

    1992-01-01

    A compound of the formula MX.sub.n L.sub.m wherein M is a metal atom selected from the group consisting of thorium, plutonium, neptunium or americium, X is a halide atom, n is an integer selected from the group of three or four, L is a coordinating ligand selected from the group consisting of aprotic Lewis bases having an oxygen-, nitrogen-, sulfur-, or phosphorus-donor, and m is an integer selected from the group of three or four for monodentate ligands or is the integer two for bidentate ligands, where the sum of n+m equals seven or eight for monodentate ligands or five or six for bidentate ligands, a compound of the formula MX.sub.n wherein M, X, and n are as previously defined, and a process of preparing such actinide metal compounds including admixing the actinide metal in an aprotic Lewis base as a coordinating solvent in the presence of a halogen-containing oxidant, are provided.

  11. Nanosecond excitonic spin relaxation in cubic GaN

    Science.gov (United States)

    Tackeuchi, Atsushi; Otake, Hirotaka; Ogawa, Yusuke; Ushiyama, Takafumi; Fujita, Taisuke; Takano, Fumiyoshi; Akinaga, Hiro

    2007-04-01

    The excitonic spin relaxation in cubic GaN is observed by spin-dependent pump and probe reflectance measurements with sub-picosecond time resolution. The spin relaxation times at 15 - 75 K are found to be longer than 5 ns and short spin relaxation times on the picosecond order are not present. Although these long spin relaxation times are in striking contrast to the sub-picosecond spin relaxation of A-band free excitons in hexagonal GaN, they are consistent with the dependence that spin relaxation time becomes longer for wider-band-gap zincblende semiconductors.

  12. Triplet exciton dynamics

    International Nuclear Information System (INIS)

    Results are presented of electron spin echo experiments combined with laser flash excitation on triplet states of aromatic molecules. Some of the theoretical and experimental aspects of the photoexcited triplet state are discussed in detail and the electron spin echo spectrometers and laser systems are described. All the experiments described in this thesis were performed at liquid helium temperatures. An account is given of the ESE experiments performed on the photoexcited, non-radiative, triplet state of pentacene in napthalene. This is an example of the ESE technique being used to ascertain the zero-field splitting parameters, the populating and depopulating rates, and the orientation of the pentacene molecules in the naphthalene host. A combination of high resolution laser flash excitation and electron-spin echoes in zero-magnetic field allowed the author to observe directly k(vector)?k(vector)' exciton scattering processes in the one-dimensional triplet excitons in tetrachlorobenzene for the first time. Additional experimental data about exciton scattering is provided and a study of the orientational dependence of the spin-lattice relaxation of the triplet excitons in an external magnetic field is described. (Auth.)

  13. Excitons in a mirror: Formation of optical bilayers using MoS2 monolayers on gold substrates

    OpenAIRE

    Mertens, Jan; Shi, Yumeng; Molina-Sanchez, Alejandro; Wirtz, Ludger; Ying Yang, Hui; Baumberg, Jeremy J.

    2014-01-01

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a mirror b...

  14. Exciton Transport in Nanostructured Solids

    Science.gov (United States)

    Bulovic, Vladimir

    2015-03-01

    Transport of nanoscale energy in the form of excitons is at the core of operation of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. The talk will present recent advancements in directly visualizing exciton transport, with spatial, temporal and spectral evolution recorded for molecular crystals, disordered thin films, and colloidal quantum dot solids. Our measurements demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology and the design of nanoscale building blocks. In addition, the talk will show that the excitonic energy landscape can be directly manipulated in solid-state thin films using dipole -dipole interactions, which can be increased under mechanical pressure, or molecular doping with polar molecules, leading to dramatic shifts in the exciton energy structure.

  15. Bright Interlayer Exciton Dynamics in MoSe2-WSe2 Heterostructures

    Science.gov (United States)

    Rivera, Pasqual; Seyler, Kyle; Ross, Jason; Schaibley, John; Yu, Hongyi; Ell, Jon; Scott, Marie; Yan, Jiaqiang; Mandrus, David; Yao, Wang; Xu, Xiaodong

    2015-03-01

    Monolayer transition metal dichalcogenide heterostructures have recently demonstrated type-II band alignment, prompting great interest in characterizing the properties of this new material system. In the monolayer MoSe2-WSe2 heterostructure, bright spatially indirect excitons with dramatically extended lifetimes have been demonstrated. Since the interlayer excitons are permanent electrical dipoles, they allow for electrical and optical control. Here, we report on the investigation of interlayer exciton emission energy, lifetime, and in-plane spatial diffusion, as a function of electric field and exciton density, in the MoSe2-WSe2 heterostructure.

  16. Singlet exciton fission photovoltaics.

    Science.gov (United States)

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses long- and short-wavelength donors and an acceptor and a simpler, two-layer combination of a singlet-fission donor and a long-wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared-absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption. PMID:23611026

  17. Study of the point defect creation and of the excitonic luminescence in alkali halides irradiated by swift heavy ions; Etude de la creation de defauts ponctuels et de la luminescence excitonique d`halogenures d`alcalins irradies par les ions lourds de grande vitesse

    Energy Technology Data Exchange (ETDEWEB)

    Protin, L.

    1994-10-05

    The aim of this experimental thesis is to study the excitonic mechanisms and of the defect creation, in NaCl and KBr, under dense electronic excitations induced by swift heavy ion irradiations. In the first part, we present the main features of the interaction of swift heavy ions with solid targets, and after we review the well known radiolytic processes of the defect creation during X-ray irradiation. In the second chapter, we describe our experimental set-up. In the chapter III, we present our results of the in-situ optical absorption measurements. This results show that defect creation is less sensitive to the temperature than during a classical irradiation. Besides, we observe new mechanisms concerning the defect aggregation. In the chapter IV, we present the results of excitonic luminescence induced by swift by swift heavy ions. We observe that the luminescence yields only change with the highest electronic stopping power. In the chapter V, we perform thermal spike and luminescence yields calculations and we compare the numerical results to the experiments presented in the chapter IV. (author). 121 refs., 65 figs., 30 tabs.

  18. Dielectric screening of excitons and trions in single-layer MoS2.

    Science.gov (United States)

    Lin, Yuxuan; Ling, Xi; Yu, Lili; Huang, Shengxi; Hsu, Allen L; Lee, Yi-Hsien; Kong, Jing; Dresselhaus, Mildred S; Palacios, Toms

    2014-10-01

    Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics. PMID:25216267

  19. Many-body effects and excitonic features in 2D biphenylene carbon.

    Science.gov (United States)

    Lder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-14

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future. PMID:26772582

  20. Many-body effects and excitonic features in 2D biphenylene carbon

    Science.gov (United States)

    Lder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-01

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  1. Topological insulator phase in halide perovskite structures

    Science.gov (United States)

    Jin, Hosub; Im, Jino; Freeman, Arthur J.

    2012-09-01

    Topological insulators are a novel quantum state of matter that reveals their properties and shows exotic phenomena when combined with other phases. Hence, priority has been given to making a good quality topological insulator interface with other compounds. From the applications point of view, the topological insulator phase in perovskite structures could be important to provide the various heterostructure interfaces with multifunctional properties. Here, by performing a tight-binding analysis and first-principles calculations, we predict that cubic-based CsPbI3 and CsSnI3 perovskite compounds under reasonable hydrostatic pressure are feasible candidates for three-dimensional topological insulators. Combined with cubic symmetry, the spin and total angular momentum doublets forming the valence and conduction bands result in a prototype of a continuum model, representing three-dimensional isotropic Dirac fermions, and govern the topological phase transition in halide perovskite materials.

  2. Helium irradiation of alkali halides

    International Nuclear Information System (INIS)

    An investigation has been made of radiation damage in alkali halide crystals induced by heavy bombardment of 1 MeV helium ions. The channeling technique has been employed and three processes have been monitored simultaneously to explain the apparent reduction of damage at high doses. It is shown that the initial rise and fall of the back-scattered yield from different alkali halides can be related to the Pooley mechanism coupled with the aggregation of interstitials to form dislocation loops through the intermediate stage of clusters

  3. Exciton formation stimulated by optically pumped electron - hole plasma in semiconductor nanostructures

    International Nuclear Information System (INIS)

    A mechanism to increase the exciton binding energy in wide band-gap semiconductors due to the presence of optically pumped electron-hole plasma is proposed. These exciton states with high binding energy (>150 MeV) can exist at room temperature when the dielectric permittivity of a semiconductor approaches zero in the infrared region. Calculations for CdS show that the density of electron-hole plasma should be above 1019 cm-3 to form such excitons. There are large numbers of closely spaced energy levels of exciton states with a high binding energy in the forbidden band gap of semiconductors. We believe these excitons are involved in lasing process observed in optically pumped semiconductor nanocrystals

  4. Optically programmable excitonic traps

    OpenAIRE

    Alloing, M.; A. Lema\\xeetre; Galopin, E.; Dubin, F.

    2013-01-01

    With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable ...

  5. Excitons in disordered polymers

    CERN Document Server

    Avgin, I

    2005-01-01

    We investigate the effects of disorder on Frenkel excitons in disordered conjugated polymers with allowed rotations about single bonds. In these materials, the principal effect of the disorder is to modify the transfer integrals appearing in the exciton Hamiltonian by changing the angle of rotation between single-bonded segments in a random manner. It is assumed the integrals have the form Jk,k+1 cos(qk-qk+1) where Jk,k+1 is taken to be a constant and qk denotes the orientation angle of the kth planar segment. Two types of disorder can be present: segmental disorder which is characterized by infrequent, large fluctuations in qk-qk+1 and a worm-like disorder marked by repeated, small fluctuations in the angular difference that are governed by a Gaussian distribution. We calculate the density of states and the localization lengths for the exciton modes in chains of 5x10^6 segments by mode counting techniques and the optical absorption spectra by direct matrix diagonalization. Both types of disorder are investig...

  6. Monolayer excitonic laser

    Science.gov (United States)

    Ye, Yu; Wong, Zi Jing; Lu, Xiufang; Ni, Xingjie; Zhu, Hanyu; Chen, Xianhui; Wang, Yuan; Zhang, Xiang

    2015-11-01

    Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals and distributed Bragg reflector microcavities, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications.

  7. Exciton-polariton transition induced by elastic exciton-exciton collisions in ultrahigh quality AIGaAs alloys

    International Nuclear Information System (INIS)

    The stationary and time-resolved polariton radiation in ultrahigh quality AIGaAs layers have been studied. It has been found that elastic exciton-exciton collisions lead to the appearance of a low-energy line of polariton radiation. We show that the rate of exciton-to-polariton transitions caused by elastic exciton-exciton collisions is determined not only by the density of the excitonic gas, but also by its temperature; this is in accordance with existing theoretical predictions

  8. Transient excitons at metal surfaces

    OpenAIRE

    Cui, Xuefeng; Wang, Cong; Argondizzo, Adam; Garrett-Roe, Sean; Gumhalter, Branko; Petek, Hrvoje

    2014-01-01

    Excitons, electron-hole pairs bound by the Coulomb potential, are fundamental quasiparticles of coherent light-matter interaction energizing processes from photosynthesis to optoelectronics. Excitons are observed in semiconductors, and their existence is implicit in the quantum theory of metals, yet their appearance is tenuous due to the screening of the Coulomb interaction on few femtosecond timescale. Here we present direct evidence for the dominant transient excitonic res...

  9. Competition between excitonic charge and spin density waves: Influence of electron-phonon and Hund's rule couplings

    Science.gov (United States)

    Kaneko, Tatsuya; Zenker, Bernd; Fehske, Holger; Ohta, Yukinori

    2015-09-01

    We analyze the stability of excitonic ground states in the two-band Hubbard model with additional electron-phonon and Hund's rule couplings using a combination of mean-field and variational cluster approaches. We show that both the interband Coulomb interaction and the electron-phonon interaction will cooperatively stabilize a charge density wave (CDW) state which typifies an "excitonic" CDW if predominantly triggered by the effective interorbital electron-hole attraction or a "phononic" CDW if mostly caused by the coupling to the lattice degrees of freedom. By contrast, the Hund's rule coupling promotes an excitonic spin density wave. We determine the transition between excitonic charge and spin density waves and comment on a fixation of the phase of the excitonic order parameter that would prevent the formation of a superfluid condensate of excitons. The implications for exciton condensation in several material classes with strongly correlated electrons are discussed.

  10. Influences of Exciton Diffusion and Exciton-Exciton Annihilation on Photon Emission Statistics of Carbon Nanotubes.

    Science.gov (United States)

    Ma, Xuedan; Roslyak, Oleskiy; Duque, Juan G; Pang, Xiaoying; Doorn, Stephen K; Piryatinski, Andrei; Dunlap, David H; Htoon, Han

    2015-07-01

    Pump-dependent photoluminescence imaging and second-order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature. These studies enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these parameters is attributed to the effect of environmental disorder in setting the exciton mean free path and capture-limited Auger recombination at this length scale. A suppression of photon antibunching is attributed to the creation of multiple spatially nonoverlapping excitons in SWCNTs, whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of the exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization. PMID:26182119

  11. Luminescence of excitons in mesoscopic ZnO particles

    International Nuclear Information System (INIS)

    Luminescence and time-resolved luminescence spectra of ZnO particles with the size of ∼20 nm have been investigated under band-to-band excitation. In the ultraviolet spectral region, there appear broad luminescence bands at 367.6, 368.8, 374.5 and 381.9 nm at 10 K. Decay profiles at 369 and 374 nm exhibit two decay components. The decay times are ∼20 and ∼100 ps for 369 nm, and ∼40 and ∼400 ps for 374 nm. These luminescence bands are attributable to the radiative recombination of free and bound excitons. Based on the present results, the effects of surface states and defects on excitons in mesoscopic ZnO particles have been discussed

  12. Excitonic complexes in single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy

    International Nuclear Information System (INIS)

    We study by microphotoluminescence the optical properties of single zinc-blende GaN/AlN quantum dots grown by droplet epitaxy. We show evidences of both excitonic and multiexcitonic recombinations in individual quantum dots with radiative lifetimes shorter than 287 ± 8 ps. Owing to large band offsets and a large exciton binding energy, the excitonic recombinations of single zinc-blende GaN/AlN quantum dots can be observed up to 300 K.

  13. Development of Halide and Oxy-Halides for Isotopic Separations

    Energy Technology Data Exchange (ETDEWEB)

    Leigh R. Martin; Aaron T. Johnson; Jana Pfeiffer; Martha R. Finck

    2014-10-01

    The goal of this project was to synthesize a volatile form of Np for introduction into mass spectrometers at INL. Volatile solids of the 5f elements are typically those of the halides (e.g. UF6), however fluorine is highly corrosive to the sensitive internal components of the mass separator, and the other volatile halides exist as several different stable isotopes in nature. However, iodide is both mono-isotopic and volatile, and as such presents an avenue for creation of a form of Np suitable for introduction into the mass separator. To accomplish this goal, the technical work in the project sought to establish a novel synthetic route for the conversion NpO2+ (dissolved in nitric acid) to NpI3 and NpI4.

  14. Excitonic signatures in the optical response of single-wall carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Voisin, Christophe; Berger, Sebastien; Cassabois, Guillaume; Roussignol, Philippe [Laboratoire Pierre Aigrain, Ecole Normale Superieure, CNRS UMR8551, UPMC, Universite Paris Diderot, 24 rue Lhomond, 75005 Paris (France); Berciaud, Stephane [IPCMS, UMR 7504, CNRS Universite de Strasbourg, 23 rue du Loess, 67034 Strasbourg (France); Yan, Hugen; Hone, James; Heinz, Tony F. [Physics, Mechanical Engineering and Electrical Engineering Departments, Columbia University, New York, NY (United States); Lauret, Jean-Sebastien [Laboratoire de Photonique Quantique et Moleculaire, Ecole Normale Superieure de Cachan, CNRS UMR 8537, Institut Dalembert, 61 Avenue Wilson, Cachan (France)

    2012-05-15

    The optical properties of single-wall carbon nanotubes (SWNTs) are dominated by the excitonic character of the transitions even at room temperature. The very peculiar properties of these excitons arise from both the one-dimensional (1D) nature of carbon nanotubes and from the electronic properties of graphene from which nanotubes are made. We first propose a brief qualitative review of the structure of the excitonic manifold and emphasize the role of dark states. We describe recent experimental investigations of this excitonic structure by means of temperature dependent PL measurements. We investigate the case of upper sub-bands and show that high-order optical transitions remain excitonic for large diameter nanotubes. A careful investigation of Rayleigh scattering spectra at the single nanotube level reveals clear exciton-phonon side-bands and Lorentzian line profiles for all semi-conducting nanotubes. In contrast, metallic nanotubes show an ambivalent behavior which is related to the reduced excitonic binding energy. Schematic of the exciton manifold in single-wall carbon nanotubes. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  15. Exciton Formation in Disordered Semiconductors

    DEFF Research Database (Denmark)

    Klochikhin, A.; Reznitsky, A.; Permogorov, S.; Lyssenko, V. G.; Breitkopf, T.; Klingshirn, Claus; Hvam, Jrn Mrcher

    1999-01-01

    Stationary luminescence spectra of disordered solid solutions can be accounted by the model of localized excitons. Detailed analysis of the long time decay kinetics of luminescence shows that exciton formation in these systems is in great extent due to the bimolecular reaction of separated carrie...

  16. Microstructure and enhanced exciton-phonon coupling in Fe doped ZnO nanoparticles.

    Science.gov (United States)

    Pandiyarajan, T; Udayabhaskar, R; Karthikeyan, B

    2013-02-15

    We report the microstructure and exciton-phonon coupling properties of Fe doped ZnO nanoparticles. Particles are prepared through sol-gel method at room temperature. Doping of Fe(3+) induces strain in the host lattice. Microstructural properties are analysed through Williamson-Hall analysis. Optical absorption studies show strong free excitonic absorption band at 369 nm. The photoluminescence (PL) studies reveal that ultraviolet, blue and green emission bands are located at 380, 445 and 500 nm respectively. Fe doped ZnO nanoparticles exhibits only ultraviolet and blue emission bands. Increase of Fe concentration makes green emission gradually disappeared. Gaussian fitted photoluminescence spectra show the emission is composed of free exciton (FX) recombination and its higher orders of longitudinal optical (LO) phonon replicas. Doping induced blue shift in FX peak and also increases the exciton-phonon coupling. PMID:23261610

  17. Excitons and interconfigurational transitions in CaF2:Yb2+ crystals

    International Nuclear Information System (INIS)

    A time-resolved VUV spectroscopic study of emission and excitation spectra of CaF2:Yb2+ has been performed to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in CaF2. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons have been discussed. The change in free exciton excitation peak position with increasing lattice temperature has been measured and is well approximated by Via's expression for the temperature shift of a semiconductor band gap. The 4f14?4f135d CaF2:Yb2+ absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation. - Highlights: We present VUV emission and excitation spectra of CaF2:Yb2+. Formation of free excitons leads to emission from intrinsic and extrinsic excitons. Temperature shifts of semiconductor band gaps apply to the intrinsic exciton peak. 4f14?4f135dYb2+ absorption is modeled by a semi-empirical Hamiltonian

  18. Excitons and interconfigurational transitions in CaF{sub 2}:Yb{sup 2+} crystals

    Energy Technology Data Exchange (ETDEWEB)

    Hughes-Currie, Rosa B.; Salkeld, Alexander J. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Ivanovskikh, Konstantin V. [ANK Service Ltd., PB 58, Novouralsk 624131, Sverdlovsk region (Russian Federation); Ural Federal University, 19 Mira st., Ekaterinburg 620002 (Russian Federation); Reid, Michael F., E-mail: mike.reid@canterbury.ac.nz [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Wells, Jon-Paul R. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Reeves, Roger J. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand)

    2015-02-15

    A time-resolved VUV spectroscopic study of emission and excitation spectra of CaF{sub 2}:Yb{sup 2+} has been performed to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in CaF{sub 2}. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons have been discussed. The change in free exciton excitation peak position with increasing lattice temperature has been measured and is well approximated by Via's expression for the temperature shift of a semiconductor band gap. The 4f{sup 14}?4f{sup 13}5d CaF{sub 2}:Yb{sup 2+} absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation. - Highlights: We present VUV emission and excitation spectra of CaF{sub 2}:Yb{sup 2+}. Formation of free excitons leads to emission from intrinsic and extrinsic excitons. Temperature shifts of semiconductor band gaps apply to the intrinsic exciton peak. 4f{sup 14}?4f{sup 13}5dYb{sup 2+} absorption is modeled by a semi-empirical Hamiltonian.

  19. Exciton interaction with impurity in barium fluoride crystals

    International Nuclear Information System (INIS)

    Optical absorption excitation and emission of alkaline-earth fluoride crystals doped with impurities, which did not introduce a new absorption band up to 8 eV, were investigated. Comparison of emission of BaF2-LaF3, BaF2-KF and doubly doped BaF2-LaF3, KF shows that interstitial fluorines are not the main exciton suppressors as thought before.The two stages of exciton suppression on the plot of exciton intensity against impurity concentration were observed. The first stage has the interaction length near 130 A, while the interaction lengths of second stage are 10-30 A, depending on impurity

  20. Exciton mediated superconductivity in PrOs4Sb12

    International Nuclear Information System (INIS)

    The most important character of the exotic superconductor PrOs4Sb12 is the existence of low-lying excitations (excitons) with a finite energy gap and it appears as the magnetic field-induced order above 4.5 T. We focus on the au conduction band, which hybridizes with a Pr4f2 state strongly, coupled to the excitons. It results in an attractive interaction between the conduction electrons. The symmetry of the superconducting order parameter is determined by dispersion relation of the exciton. For the bcc system PrOs4Sb12, a d-wave state (kxky + ?kykz + ?2kzkx, ?=ei2?/3) is stabilized with broken time reversal symmetry. (author)

  1. DEVELOPMENT AND EVALUATION OF METHODS FOR TOTAL ORGANIC HALIDE AND PURGEABLE ORGANIC HALIDE IN WASTEWATER

    Science.gov (United States)

    This report describes a series of studies involving the use of 'surrogate' methods for the determination of total organic halides (TOX), purgeable organic halides (POX), and solvent extractable organic halides (EOX), in wastewater and solid wastes. A pyrolysis/microcoulometric sy...

  2. Roles of Hund's rule coupling in excitonic density-wave states

    Science.gov (United States)

    Kaneko, Tatsuya; Ohta, Yukinori

    2014-12-01

    Excitonic density-wave states realized by the quantum condensation of electron-hole pairs (or excitons) are studied in the two-band Hubbard model with Hund's rule coupling and the pair hopping term. Using the variational cluster approximation, we calculate the grand potential of the system and demonstrate that Hund's rule coupling always stabilizes the excitonic spin-density-wave state and destabilizes the excitonic charge-density-wave state and that the pair hopping term enhances these effects. The characteristics of these excitonic density-wave states are discussed using the calculated single-particle spectral function, density of states, condensation amplitude, and pair coherence length. Implications of our results in the materials' aspects are also discussed.

  3. Stark effect of excitons in corrugated lateral surface superlattices: effect of centre-of-mass quantization

    International Nuclear Information System (INIS)

    The quantum confined Stark effect (QCSE) of excitons in GaAs/AlAs corrugated lateral surface superlattices (CLSSLs) is calculated. Blue and red shifts in the exciton energies are predicted for the heavy- and light-excitons in the CLSSLs, respectively, comparing with those in the unmodulated quantum well due to the different effective hole masses in the parallel direction. Sensitive dependence of the QCSE on the hole effective mass in the parallel direction is expected because of the ''centre-of-mass'' quantization (CMQ) induced by the periodic corrugated interfaces of the CLSSLs. The effect of the CMQ on the exciton mini-bands and the localization of the excitons in the CLSSLs is discussed. (author)

  4. In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

    International Nuclear Information System (INIS)

    The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

  5. Excitons and excess electrons in nanometer size molecular polyoxotitanate clusters: electronic spectra, exciton dynamics, and surface states.

    Science.gov (United States)

    Bao, Jianhua; Yu, Zhihao; Gundlach, Lars; Benedict, Jason B; Coppens, Philip; Chen, Hung Cheng; Miller, John R; Piotrowiak, Piotr

    2013-04-25

    The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(?4-O)4(?3-O)16(?2-O)4(OPr(i))20 (in short Ti17) was studied using femtosecond pump-probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17(-), are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17(-) indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ?1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17*. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with ? > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons (3)Ti17*. PMID:23113586

  6. Exciton condensation in quantum wells. Exciton hydrodynamics. The effect of localized states

    Directory of Open Access Journals (Sweden)

    V.I. Sugakov

    2014-09-01

    Full Text Available The hydrodynamic equations for indirect excitons in the double quantum wells are studied taking into account 1 a possibility of an exciton condensed phase formation, 2 the presence of pumping, 3 finite value of the exciton lifetime, 4 exciton scattering by defects. The threshold pumping emergence of the periodical exciton density distribution is found. The role of localized and free exciton states is analyzed in the formation of emission spectra.

  7. Dehydrated rare earth halides and production process

    International Nuclear Information System (INIS)

    Rare earth chlorides, bromides or iodides containing less than 1 wt% water and less than 3 wt% oxyhalide are dehydrated by a gas flow of hydrogen halide through the halide bed. Structural water can interfer in some applications for instance metal preparation by chemical or electrochemical reduction

  8. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers.

    Science.gov (United States)

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-01-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25?eV at 300?K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10?K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications. PMID:26506907

  9. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers

    Science.gov (United States)

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-01-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25?eV at 300?K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10?K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications. PMID:26506907

  10. Excitonic effects in the optical properties of alkaline earth chalcogenides from first-principles calculations

    Science.gov (United States)

    Nejatipour, Hajar; Dadsetani, Mehrdad

    2015-08-01

    This paper studies excitonic effects in the optical properties of alkaline earth chalcogenides (AECs) by solving the equation of motion of the two-particle Green function, the Bethe-Salpeter equation (BSE). On the basis of quasi-particle states obtained by the GW approximation, (BSE + GW), the solution of BSE improves agreement with experiments. In these compounds, the main excitonic structures were reproduced appropriately. In the optical absorption spectra of AECs, the main excitonic structures originate in the direct transitions at X and ? symmetry points, as confirmed by the experiments. In addition to real and imaginary parts of the dielectric functions, excitonic effects were studied in the electron energy loss functions of AECs. Moreover, the G0W0 approximation was used in order to determine the energy band gaps of AECs. This showed that except for MgO and BaO, the other AECs under study have indirect band gaps from ? to X.

  11. Sub-picosecond exciton spin-relaxation in GaN

    Science.gov (United States)

    Tackeuchi, Atsushi; Kuroda, Takamasa; Otake, Hirotaka; Taniguchi, Kazuyoshi; Chinone, Takako; Liang, Ji-Hao; Kajikawa, Masataka; Horio, Naochika

    2006-02-01

    Exciton spin relaxations in bulk GaN were directly observed with sub-picosecond's time resolution. The obtained spin relaxation times of A-band free exciton are 0.47 ps - 0.25 ps at 150 K - 225 K. The spin relaxation time of the acceptor bound exciton at 15K is measured to be 1.1 ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin relaxation time of A-band free exciton is found to be proportional to T -1.4, where T is the temperature. The fact that the spin relaxation time in GaN is shorter than that in GaAs, in spite of the small spin-orbit splitting, suggests that the spin relaxation is dominated by the defect-assisted Elliot-Yafet process.

  12. Modeling of Lead Halide Perovskites for Photovoltaic Applications

    OpenAIRE

    Jishi, Radi A.; Ta, Oliver B.; Sharif, Adel A.

    2014-01-01

    We report first-principles calculations, using the full potential linear augmented plane wave method, on six lead halide semiconductors, namely, CH3NH3PbI3, CH3NH3PbBr3, CsPbX3 (X=Cl, Br, I), and RbPbI3. Exchange is modeled using the modified Becke-Johnson potential. With an appropriate choice of the parameter that defines this potential, an excellent agreement is obtained between calculated and experimental band gaps of the six compounds. We comment on the possibility that ...

  13. Excitons with large binding energies in MgS/ZnSe/MgS and ZnMgS/ZnS/ZnMgS quantum wells

    Science.gov (United States)

    Urbaszek, B.; Morhain, C.; Bradford, C.; O'Donnell, C. B.; Telfer, S. A.; Tang, X.; Balocchi, A.; Prior, K. A.; Cavenett, B. C.; Townsley, C. M.; Nicholas, R. J.

    2001-03-01

    The wide bandgap II-VI semiconductors have unique properties which allow the possibility of suppressing the exciton-phonon scattering up to room temperature in quantum well structures designed so that the exciton excitation E1s?2s>h?LO. In particular, magnetic field and temperature dependent measurements are used to study the exciton binding energies and to investigate the exciton-LO phonon scattering processes of high quality ZnSe quantum wells in MgS grown by MBE. The small inhomogeneous broadening of the exciton transitions in these samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS the exciton binding energy in a 5 nm well is found to be 43.9 meV, which is the largest reported for this material system. The FWHM of the heavy hole absorption transitions measured as a function of temperature shows that the scattering of the excitons by the LO phonons is partially suppressed. These results are compared with ZnS quantum wells where the exciton g-values have been measured and the exciton binding energies have been deduced from the exciton diamagnetic shifts. The results show the possibility of suppressing exciton-LO phonon scattering in these structures.

  14. Low-energy exciton pocket at finite momentum in tetracene molecular solids

    Science.gov (United States)

    Roth, Friedrich; Nohr, Markus; Hampel, Silke; Knupfer, Martin

    2015-11-01

    The excited state dynamics in organic semiconductors plays an important role for many processes associated with light absorption and emission. We have studied the momentum dependence of the lowest singlet excitons in tetracene molecular solids, an archetype system for other organic semiconductors. Our results reveal an anisotropic band structure of these excitons with an energy minimum at finite momentum, i.e., a low-energy exciton pocket. The existence of such low-energy states might have important consequences for the photophysical behavior, also in view of applications in, e.g., organic solar cells. Our studies stress the importance of momentum-dependent considerations in organic systems.

  15. Collective $d-$wave Excitonic Modes in the Fe-Superconductors

    OpenAIRE

    Scalapino, D.J.; Devereaux, T. P.

    2009-01-01

    Calculations of the pairing interaction in multi-band models of the Fe superconductors show that it is attractive in both the $A_{1g}$ (s-wave) and $B_{1g}$ (d-wave) channels. This raises the possibility that these materials may have collective excitonic modes. Here, assuming an s-wave groundstate, we investigate the d-wave collective excitonic mode and its coupling to the Raman scattering.

  16. Thermal effects in exciton harvesting in biased one-dimensional systems

    OpenAIRE

    Vlaming, S. M.; Malyshev, V. A.; Knoester, J.

    2007-01-01

    The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Previously, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (slope of the energy gradient towards...

  17. Sharp bound and free exciton lines from homoepitaxial AlN

    Energy Technology Data Exchange (ETDEWEB)

    Feneberg, Martin [Institut fuer Experimentelle Physik, Abteilung Materialphysik, Otto-von-Guericke-Universitaet Magdeburg (Germany); Institut fuer Quantenmaterie/Gruppe Halbleiterphysik, Universitaet Ulm (Germany); Neuschl, Benjamin; Thonke, Klaus [Institut fuer Quantenmaterie/Gruppe Halbleiterphysik, Universitaet Ulm (Germany); Collazo, Ramon; Rice, Anthony [Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC (United States); Sitar, Zlatko [Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC (United States); HexaTech, Inc., Morrisville, NC (United States); Dalmau, Rafael; Xie, Jinqiao; Mita, Seiji [HexaTech, Inc., Morrisville, NC (United States); Goldhahn, Ruediger [Institut fuer Experimentelle Physik, Abteilung Materialphysik, Otto-von-Guericke-Universitaet Magdeburg (Germany)

    2011-07-15

    MOCVD AlN layers grown in c-direction on PVT bulk AlN substrates are investigated by high-resolution photoluminescence in the energy region of the band gap. The experiments allow for observation of different donor bound exciton lines with a full width at half maximum below 0.5 meV. Assignments are suggested for the substitutional shallow donors silicon and oxygen. The visibility of an excited state of the free exciton with a hole from the highest valence band allows to determine the exciton binding energy to 52.4 meV by use of a simple hydrogen model. The resulting band gap energy is 6.094 eV at 10 K. Comparison with earlier reports on homoepitaxial AlN leads to a new identification of the bands reported there. (Copyright copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  18. The electronic structure of the F-center in alkali-halides-The Bethe cluster - lattice

    International Nuclear Information System (INIS)

    The electronic structure of the F-center in alkali-halides with the NaCl structure has been studied using the Bethe Cluster lattice method. The central cluster has been taken as constituted by the vacancy and the nearest- and second-neighbors to it, respectively cations and anions. The optical transitions have been calculated and compared to experimental data on the location of the peak of the F-absorption band. The agreement obtained indicates that this method may be used to study properties of this defect in alkali halides. (Author)

  19. Spin-Orbit Interactions in Bilayer Exciton-Condensate Ferromagnets

    OpenAIRE

    Shim, Y. -P.; MacDonald, A. H.

    2009-01-01

    Bilayer electron-hole systems with unequal electron and hole densities are expected to have exciton condensate ground states with spontaneous spin-polarization in both conduction and valence bands. In the absence of spin-orbit and electron-hole exchange interactions there is no coupling between the spin-orientations in the two quantum wells. In this article we show that Rashba spin-orbit interactions lead to unconventional magnetic anisotropies, whose strength we estimate, and to ordered stat...

  20. Exciton states in narrow-gap carbon nanotubes

    Science.gov (United States)

    Hartmann, R. R.; Portnoi, M. E.

    2016-02-01

    Quasi-exact solutions to the quantum relativistic two-body problem are obtained for a one-dimensional Woods-Saxon-like potential. The quantised positive energy spectrum is obtained in the square well potential limit in terms of a set of simple transcendental equations. This potential is used to calculate excitonic states in narrow-gap single-walled carbon nanotubes and the binding energy is shown to scale with the band gap.

  1. Linear and nonlinear optical properties of free excitons in CdS

    International Nuclear Information System (INIS)

    The authors report here an accurate measurement done at cryogenic temperature of both the linear and nonlinear transmission characteristics of a very high-quality optical platelet of CdS. From an expression for the contribution of the various excitons to the dielectric constant of the material, the index of refraction and the absorption coefficient can be extracted. From a fit to the experimental transmission data, the free exciton dipole dephasing time, sample thickness, background absorption coefficient, transverse exciton frequency, and impurity concentration can all be extracted. Knowledge of these parameters is very important to understand pulse propagation near the band gap and to optimize the performance of bistable devices. The authors obtain a very good fit to the experimental data. The transverse A exciton frequency is measured to be (20589 +. 1)cm/sup -1/. A study of the temperature dependence of the broadening of the A free exciton reveals that the A free exciton dephasing time below the resonance is temperature independent from 2 to 50 K and is of the order of 50 ps. This time is directly related to polariton-impurity and polariton-defect scattering. Acoustic or LO phonons do not contribute to the polariton dephasing time at these temperatures. This is a direct manifestation of the exciton-polariton bottleneck. Above the A exciton resonance, the authors observe a temperature dependence for the polariton dephasing time which they attribute to phonon absorption and emission. The measurements indicate that even 50-60 cm/sup -1/ below the free exciton resonance, the major contribution to the absorption coefficient comes from free exciton tail absorption

  2. Exciton-photon interaction and the exciton absorption lineshape in the spatially imhomogeneous wave model

    Energy Technology Data Exchange (ETDEWEB)

    Robinette, S.L.; Stevenson, S.; Small, G.J.

    1979-01-01

    The polariton and spatially inhomogeneous wave models are used to analyze the exciton-photon coupling contribution to the exciton absorption lineshape. This contribution must be known for determination of the exciton scattering time. The polariton model is shown to be inappropriate for the a-singlet exciton of naphthalene.

  3. Oxygen-related band gap state in single crystal rubrene

    OpenAIRE

    Mitrofanov, O; Lang, D. V.; Kloc, C.; Wikberg, J. M.; Siegrist, T.; So, W. Y.; Sergent, M. A.; Ramirez, A. P.

    2006-01-01

    A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton tran...

  4. Fullerenes doped with metal halides

    International Nuclear Information System (INIS)

    The cage-like structure of fullerenes is a challenge to every experimental to put something inside - to dope the fullerenes. In fact, the research team that first identified C60 as a football-like molecule quickly succeeded in trapping metal atoms inside and in shrinking the cage around this atom by photofragmentation. In this paper we report the results of ''shrink-wrapping'' the fullerenes around metal halide molecules. Of special interest is the critical size (the minimum number of carbon atoms) that can still enclose the dopant. A rough model for the space available inside a carbon cage gives good agreement with the measured shrinking limits. (author). 8 refs, 6 figs

  5. Aging of alkali halide windows

    Science.gov (United States)

    Kennedy, Michael; Trung, D.; Meyer, Franz; Buth, T.; Ristau, Detlev; Schmidt, Holger; Korth, Joerg U.; Hamburg, K.

    1995-07-01

    The environmental effect on the aging behavior of NaCl and KCl windows was studied. Laser windows were aged at different relative humidities in a controlled climate-chamber. Degradation is monitored with a microscope inspection system equipped with a computer controlled image processing board. The temporal development of surface defect density under different atmospheric conditions was investigated with respect to optical absorption and damage thresholds of the windows at 10.6 micrometers . Laser windows coated with single layers of NaF deposited by an adapted IAD-technique were analyzed. The performance of the coated and uncoated laser windows is discussed under consideration of typical applications. In comparison to the bare samples, the coated windows show an improved resistivity against environmental influences. Accelerated testing theory is employed to model the aging behavior of the samples. An approach to deduce a qualified acceleration factor is made in order to extrapolate the lifetime of alkali halide laser window under normal conditions.

  6. Optical generation of excitonic valley coherence in monolayer WSe2

    Science.gov (United States)

    Jones, Aaron M.; Yu, Hongyi; Ghimire, Nirmal J.; Wu, Sanfeng; Aivazian, Grant; Ross, Jason S.; Zhao, Bo; Yan, Jiaqiang; Mandrus, David G.; Xiao, Di; Yao, Wang; Xu, Xiaodong

    2013-09-01

    As a consequence of degeneracies arising from crystal symmetries, it is possible for electron states at band-edges (`valleys') to have additional spin-like quantum numbers. An important question is whether coherent manipulation can be performed on such valley pseudospins, analogous to that implemented using true spin, in the quest for quantum technologies. Here, we show that valley coherence can be generated and detected. Because excitons in a single valley emit circularly polarized photons, linear polarization can only be generated through recombination of an exciton in a coherent superposition of the two valley states. Using monolayer semiconductor WSe2 devices, we first establish the circularly polarized optical selection rules for addressing individual valley excitons and trions. We then demonstrate coherence between valley excitons through the observation of linearly polarized luminescence, whose orientation coincides with that of the linearly polarized excitation, for any given polarization angle. In contrast, the corresponding photoluminescence from trions is not observed to be linearly polarized, consistent with the expectation that the emitted photon polarization is entangled with valley pseudospin. The ability to address coherence, in addition to valley polarization, is a step forward towards achieving quantum manipulation of the valley index necessary for coherent valleytronics.

  7. Landau levels of the C-exciton in CuInSe{sub 2} studied by magneto-transmission

    Energy Technology Data Exchange (ETDEWEB)

    Yakushev, M. V., E-mail: michael.yakushev@strath.ac.uk [Department of Physics, SUPA, Strathclyde University, Glasgow G4 0NG (United Kingdom); Ural Federal University and Ural Branch of RAS, Ekaterinburg 620002 (Russian Federation); Rodina, A. V. [A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021 (Russian Federation); Shuchalin, G. M. [A.F. Ioffe Physico-Technical Institute, St. Petersburg 194021 (Russian Federation); St. Petersburg State Polytechnical University, St. Petersburg 195251 (Russian Federation); Seisian, R. P. [St. Petersburg State Polytechnical University, St. Petersburg 195251 (Russian Federation); Abdullaev, M. A. [Institute of Physics, Academy of Science of Russia, Makhachkala (Russian Federation); Rockett, A. [University of Illinois, Urbana, Illinois 61801 (United States); Zhivulko, V. D. [Scientific-Practical Material Research Centre of the National Academy of Science of Belarus, 19 P. Brovki, Minsk 220072 (Belarus); Mudryi, A. V. [Department of Physics, SUPA, Strathclyde University, Glasgow G4 0NG (United Kingdom); Scientific-Practical Material Research Centre of the National Academy of Science of Belarus, 19 P. Brovki, Minsk 220072 (Belarus); Faugeras, C. [LNCMI, 25 avenue des Martyrs, BP 166, Grenoble Cedex 938042 (France); Martin, R. W., E-mail: r.w.martin@strath.ac.uk [Department of Physics, SUPA, Strathclyde University, Glasgow G4 0NG (United Kingdom)

    2014-10-06

    The electronic structure of the solar cell absorber CuInSe{sub 2} is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29?T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7?T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0?T (8.5?meV), band gap (1.2828?eV), and hole effective mass m{sub so}?=?(0.31??0.12)m{sub 0} for the C valence sub-band.

  8. Landau levels of the C-exciton in CuInSe2 studied by magneto-transmission

    International Nuclear Information System (INIS)

    The electronic structure of the solar cell absorber CuInSe2 is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29?T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7?T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0?T (8.5?meV), band gap (1.2828?eV), and hole effective mass mso?=?(0.31??0.12)m0 for the C valence sub-band.

  9. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

    KAUST Repository

    Moody, Galan

    2015-09-18

    The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors.

  10. Low-temperature dynamics of weakly localized Frenkel excitons in disordered linear chains.

    Science.gov (United States)

    Bednarz, M; Malyshev, V A; Knoester, J

    2004-02-22

    We calculate the temperature dependence of the fluorescence Stokes shift and the fluorescence decay time in linear Frenkel exciton systems resulting from the thermal redistribution of exciton population over the band states. The following factors, relevant to common experimental conditions, are accounted for in our kinetic model: (weak) localization of the exciton states by static disorder, coupling of the localized excitons to vibrations in the host medium, a possible nonequilibrium of the subsystem of localized Frenkel excitons on the time scale of the emission process, and different excitation conditions (resonant or nonresonant). A Pauli master equation, with microscopically calculated transition rates, is used to describe the redistribution of the exciton population over the manifold of localized exciton states. We find a counterintuitive nonmonotonic temperature dependence of the Stokes shift. In addition, we show that depending on experimental conditions, the observed fluorescence decay time may be determined by vibration-induced intraband relaxation, rather than radiative relaxation to the ground state. The model considered has relevance to a wide variety of materials, such as linear molecular aggregates, conjugated polymers, and polysilanes. PMID:15268548

  11. Cerium doped lanthanum halides: fast scintillators for medical imaging

    International Nuclear Information System (INIS)

    This work is dedicated to two recently discovered scintillating crystals: cerium doped lanthanum halides (LaCl3:Ce3+ and LaBr3:Ce3+).These scintillators exhibit interesting properties for gamma detection, more particularly in the field of medical imaging: a short decay time, a high light yield and an excellent energy resolution. The strong hygroscopicity of these materials requires adapting the usual experimental methods for determining physico-chemical properties. Once determined, these can be used for the development of the industrial manufacturing process of the crystals. A proper comprehension of the scintillation mechanism and of the effect of defects within the material lead to new possible ways for optimizing the scintillator performance. Therefore, different techniques are used (EPR, radioluminescence, laser excitation, thermally stimulated luminescence). Alongside Ce3+ ions, self-trapped excitons are involved in the scintillation mechanism. Their nature and their role are detailed. The knowledge of the different processes involved in the scintillation mechanism leads to the prediction of the effect of temperature and doping level on the performance of the scintillator. A mechanism is proposed to explain the thermally stimulated luminescence processes that cause slow components in the light emission and a loss of light yield. Eventually the study of afterglow reveals a charge transfer to deep traps involved in the high temperature thermally stimulated luminescence. (author)

  12. H-point exciton transitions in bulk MoS{sub 2}

    Energy Technology Data Exchange (ETDEWEB)

    Saigal, Nihit; Ghosh, Sandip, E-mail: sangho10@tifr.res.in [Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005 (India)

    2015-05-04

    Reflectance and photoreflectance spectrum of bulk MoS{sub 2} around its direct bandgap energy have been measured at 12?K. Apart from spectral features due to the A and B ground state exciton transitions at the K-point of the Brillouin zone, one observes additional features at nearby energies. Through lineshape analysis the character of two prominent additional features are shown to be quite different from that of A and B. By comparing with reported electronic band structure calculations, these two additional features are identified as ground state exciton transitions at the H-point of the Brillouin zone involving two spin-orbit split valance bands. The excitonic energy gap at the H-point is 1.965?eV with a valance bands splitting of 185?meV. While at the K-point, the corresponding values are 1.920?eV and 205?meV, respectively.

  13. Quantum process of exciton dissociation at organic semiconductor interfaces: Effects of interface roughness and hot exciton

    Science.gov (United States)

    Iizuka, Hideyuki; Nakayama, Takashi

    2016-02-01

    Quantum processes of bound exciton dissociation at organic donor/acceptor interfaces were studied by the time-evolution simulation of a wave packet, focusing on the effects of interface roughness and hot excitons. It is shown that the dissociation is remarkably enhanced by the interface roughness that is comparable to the exciton diameter. Moreover, the dissociation probability increases for the hot excitons compared with the ground-state exciton owing to their small binding energies and large diameters.

  14. Exciton Polarization in Carbon Nanotubes

    Science.gov (United States)

    Konobeeva, N. N.; Belonenko, M. B.

    2015-09-01

    The nonlinear process of propagation of optical pulses in the spectral region inside the polarization gap in carbon nanotubes has been investigated. A simultaneous solution of the Maxwell equation and the equation of motion for exciton polarization has been carried out. The dynamics of an electromagnetic pulse has been examined as a function of the parameters of the problem. It is shown that taking exciton polarization into account does not have a substantial effect on the propagation process, but alters the shape of the optical pulse.

  15. Electron scattering from alkali halide molecules

    International Nuclear Information System (INIS)

    The low energy electron scattering from molecules belonging to alkali halide matrix is studied. The modified Born-Eikonal Series method is employed to calculate rotational excitation cross sections. The curves are plotted for the incident electron energy versus total cross section for each column of alkali halide matrix. The conclusion is drawn regarding dependence of the total cross section with the electron-negativity difference of the molecules. (author)

  16. Excitonic emission of CuInS2 crystals using confocal microscopy system

    International Nuclear Information System (INIS)

    Photoluminescence (PL) spectra in the band-edge region on bulk single-crystals of CuInS2 grown by the traveling heater method have been investigated using a confocal microscopy system. The observed PL spectra are separated into two Lorentzian peaks which are assigned to be A and B free excitons, by the analysis of the excitation intensity dependence of the emissions. Consequently, we present the behaviour of B free exciton within a wide range of temperatures. The time-resolved emissions of A free exciton have also been examined. The decay of the emissions is analyzed using a double exponential curve. Fast and slow components are attributed to nonradiative relaxation and radiative recombination, respectively. The decay-time constant of the slow component corresponds to the radiative lifetime of A free exciton and is obtained over the wide temperature region until 300 K. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  17. Exciton-polariton behaviour in bulk and polycrystalline ZnO

    International Nuclear Information System (INIS)

    We report detailed reflectance studies of the exciton-polariton structure of thin film polycrystalline ZnO and comparison with bulk crystal behaviour. Near-normal incidence reflectance spectra of these samples are fitted using a two-band dielectric response function. Our data show that the reflectance data in polycrystalline ZnO differ substantially from the bulk material, with Fabry-Perot oscillations at energies below the transverse A exciton and above the longitudinal B exciton in the films. In the strong interaction regime between these energies no evidence is seen of the normally rapid oscillations associated with the anomalous waves. We demonstrate that the strong interaction of the damped exciton with the photon leads to polaritons in this region with substantial damping such that the Fabry-Perot modes are eliminated. Good qualitative agreement is achieved between the model and data. The importance of the polariton model in understanding the reflectance data of polycrystalline material is clearly seen

  18. Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor–Acceptor Conjugated Polymers

    KAUST Repository

    Ayzner, Alexander L.

    2015-12-30

    © 2015 American Chemical Society. Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

  19. Expanded fluid mercury, a ferroelectric excitonic insulator

    International Nuclear Information System (INIS)

    We relate the metal-nonmetal transition in expanded liquid mercury to the dieletric transition in dense mercury vapor. We propose that the high-temperature, high-pressure fluid is an excitonic insulator phase. Condensation of Frenkel excitons is shown to occur at the dieletric transition. The phase diagram of excitonic insulator explains naturally both the metal-nonmetal and the dieletric transitions. The permanent dipole moments of the condensed excitons are estimated to order as a ferroelectric phase. The critical exponents of the liquid-gas transition are therefore classical within the ferroelectric excitonic insulator phase, crossing over 3d Ising at the ferroelectric-paraelectric transition. (The authors)

  20. Co-existence of free and self-trapped excitons in J-aggregates

    International Nuclear Information System (INIS)

    Nature of excited electronic states of amphi-PIC J-aggregates, which are the source of the self-trapping states, have been investigated using low-temperature site-selective, time-resolved spectroscopy techniques. The self-trapping states are shown to evolve from the delocalized exciton states within the J-band. The strongly localized electronic states located on the low-frequency edge of the J-band, are not able to form polaronic states and, hence, the polaronic relaxation process is particularly collective one. The exciton self-trapping is more effective in J-aggregates with strong disorder, requires overcoming a self-trapping barrier

  1. Exciton size and quantum transport in nanoplatelets

    Science.gov (United States)

    Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.

    2015-12-01

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  2. Excitons in a mirror: Formation of optical bilayers using MoS{sub 2} monolayers on gold substrates

    Energy Technology Data Exchange (ETDEWEB)

    Mertens, Jan; Baumberg, Jeremy J., E-mail: jjb12@cam.ac.uk [Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE (United Kingdom); Shi, Yumeng; Yang, Hui Ying, E-mail: yanghuiying@sutd.edu.sg [Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore 138682 (Singapore); Molina-Snchez, Alejandro; Wirtz, Ludger [Physics and Materials Science Research Unit, University of Luxembourg, L-1511 Luxembourg (Luxembourg)

    2014-05-12

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a mirror biexciton with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS{sub 2} on gold thus resembles a bilayer of MoS{sub 2} which has been created by optical coupling. Additional top-mirrors produce an optical bulk..

  3. Excitons in a mirror: Formation of "optical bilayers" using MoS2 monolayers on gold substrates

    Science.gov (United States)

    Mertens, Jan; Shi, Yumeng; Molina-Snchez, Alejandro; Wirtz, Ludger; Yang, Hui Ying; Baumberg, Jeremy J.

    2014-05-01

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a "mirror biexciton" with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS2 on gold thus resembles a bilayer of MoS2 which has been created by optical coupling. Additional top-mirrors produce an "optical bulk."

  4. Excitons in a mirror: Formation of optical bilayers using MoS2 monolayers on gold substrates

    International Nuclear Information System (INIS)

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a mirror biexciton with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS2 on gold thus resembles a bilayer of MoS2 which has been created by optical coupling. Additional top-mirrors produce an optical bulk.

  5. Energy change of exciton in ultrafine particle due to a dipole at the surface of the particle

    International Nuclear Information System (INIS)

    The Hamiltonian of the exciton in an ultrafine particle (UFP) with a dipole at its surface is given. The exciton energy is calculated as a function of the dielectric constant of the medium outside the UFP and the strength of the dipole. The results show that, for materials with small exciton reduced mass, the change of exciton binding energy caused by the dipole, up to a several meV is largely affected by the dielectric constant of the medium of the UFP, and that of the medium outside the UFP may change the exciton binding energy even more. These two factors shift the band edge of the absorption spectra toward the red end. (author). 18 refs, 6 figs, 4 tabs

  6. Theoretical study of excitonic complexes in semiconductors quantum wells

    International Nuclear Information System (INIS)

    A physical system where indistinguishable particles interact with each other creates the possibility of studying correlation and exchange effect. The simplest system is that one with only two indistinguishable particles. In condensed matter physics, these complexes are represented by charged excitons, donors and acceptors. In quantum wells, the valence band is not parabolic, therefore, the negatively charged excitons and donors are theoretically described in a simpler way. Despite the fact that the stability of charged excitons (trions) is known since the late 50s, the first experimental observation occurred only at the early 90s in quantum well samples, where their binding energies are one order of magnitude larger due to the one dimensional carriers confinement. After this, these complexes became the subject of an intense research because the intrinsic screening of electrical interactions in semiconductor materials allows that magnetic fields that are usual in laboratories have strong effects on the trion binding energy. Another rich possibility is the study of trions as an intermediate state between the neutral exciton and the Fermi edge singularity when the excess of doping carriers is increased. In this thesis, we present a theoretical study of charged excitons and negatively charged donors in GaAs/Al0.3Ga0.7As quantum wells considering the effects of external electric and magnetic fields. We use a simple, accurate and physically clear method to describe these systems in contrast with the few and complex treatments s available in the literature. Our results show that the QW interface defects have an important role in the trion dynamics. This is in agreement with some experimental works, but it disagrees with other ones. (author)

  7. Exciton correlations and input-out relations in non-equilibrium exciton superfluids

    CERN Document Server

    Ye, Jinwu; Yu, Yi-Xiang; Yu, Xiaolu; Liu, Wuming

    2011-01-01

    In this paper, we calculate various exciton correlation functions in putative exciton superfluids in electron-hole bilayer systems. These correlation functions include both normal and anomalous Greater, Lesser, Advanced, Retarded and time-ordered exciton Green functions, the corresponding normal and anomalous spectral weights and also various two exciton correlation functions. We stress the important roles of the violations of the fluctuation and dissipation theorem among these various exciton correlation function in the non-equilibrium exciton superfluid systems. We also explore the input-output relations between various exciton correlation functions and those of emitting photons. These relations bring out interesting and important connections between various important properties of exciton superfluids to be probed and Photoluminescent measurements on emitted photons from the exciton superfluids such as angle resolved photon power spectrum, phase sensitive homodyne measurements and HanburyBrown-Twiss type of...

  8. Optical properties of alkali halide crystals from all-electron hybrid TD-DFT calculations

    International Nuclear Information System (INIS)

    We present a study of the electronic and optical properties of a series of alkali halide crystals AX, with A = Li, Na, K, Rb and X = F, Cl, Br based on a recent implementation of hybrid-exchange time-dependent density functional theory (TD-DFT) (TD-B3LYP) in the all-electron Gaussian basis set code CRYSTAL. We examine, in particular, the impact of basis set size and quality on the prediction of the optical gap and exciton binding energy. The formation of bound excitons by photoexcitation is observed in all the studied systems and this is shown to be correlated to specific features of the Hartree-Fock exchange component of the TD-DFT response kernel. All computed optical gaps and exciton binding energies are however markedly below estimated experimental and, where available, 2-particle Greens function (GW-Bethe-Salpeter equation, GW-BSE) values. We attribute this reduced exciton binding to the incorrect asymptotics of the B3LYP exchange correlation ground state functional and of the TD-B3LYP response kernel, which lead to a large underestimation of the Coulomb interaction between the excited electron and hole wavefunctions. Considering LiF as an example, we correlate the asymptotic behaviour of the TD-B3LYP kernel to the fraction of Fock exchange admixed in the ground state functional cHF and show that there exists one value of cHF (?0.32) that reproduces at least semi-quantitatively the optical gap of this material

  9. Temperature-dependent relaxation of excitons in tubular molecular aggregates: Fluorescence decay and stokes shift.

    Science.gov (United States)

    Pugzlys, A; Augulis, R; van Loosdrecht, P H M; Didraga, C; Malyshev, V A; Knoester, J

    2006-10-19

    We report temperature-dependent steady-state and time-resolved fluorescence studies to probe the exciton dynamics in double-wall tubular J-aggregates formed by self-assembly of the dye 3,3'-bis(3-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine. We focus on the lowest energy fluorescence band, originating from the inner cylindrical wall. At low temperatures, the experiments reveal a nonexponential decay of the fluorescence, with a typical time scale that depends on the emission wavelength. At these temperatures we also find a dynamic Stokes shift of the fluorescence spectrum and its nonmonotonic dependence on temperature under steady-state conditions. All these data indicate that below about 20 K the excitons in the lowest fluorescence band do not reach thermal equilibrium before emission occurs, while above about 60 K thermalization on this time scale is complete. By comparing the two lowest fluorescence bands, we also find indications for fast energy transfer from the outer to the inner wall. We show that the Frenkel exciton model with diagonal disorder, which previously has been proposed to explain the absorption and linear dichroism spectra of these aggregates, yields a quantitative explanation to the observed dynamics. To this end, we extend the model to account for weak phonon-induced scattering of the localized exciton states; the spectral dynamics are then described by solving a Pauli master equation for the exciton populations. PMID:17034206

  10. Exciton-photon interaction in a quantum dot embedded in a photonic microcavity

    CERN Document Server

    Sodagar, Majid; Eftekharian, Amin; Khorasani, Sina

    2008-01-01

    We present a detailed analysis of exciton-photon interaction in a microcavity made out of a photonic crystal slab. Here we have analyzed a disk-like quantum dot where an exciton is formed. Excitonic eigen-functions in addition to their eigen-energies are found through direct matrix diagonalization, while wave functions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wave functions precisely, we have used Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to conduction band for electron states. After analyzing Excitonic states, a photonic crystal based microcavity with a relatively high quality factor mode has been proposed and its lattice constant has been adjusted to obtain the prescribed resonant frequency. We use finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for exciton-photon interaction both where intra-band and inter-ban...

  11. Halide Ion Enhancement of Nitrate Ion Photolysis

    Science.gov (United States)

    Richards, N. K.; Wingen, L. M.; Callahan, K. M.; Tobias, D. J.; Finlayson-Pitts, B. J.

    2009-12-01

    Nitrate ion photochemistry is an important source of NOx in the polar regions. It is uncertain whether coexisting ions such as halides play a role in nitrate photochemistry. The effect of halides on NO3 photolysis was investigated using photolysis experiments in 230 L Teflon chambers that contain deliquesced aerosols of NaBr:NaNO3, KBr:KNO3 and ternary mixtures of NaCl:NaBr:NaNO3. Gas phase NO2 and gaseous halogen products were measured as a function of photolysis time using long path FTIR, NOx chemiluminescence and API-MS (atmospheric pressure ionization mass spectrometry). Experiments were conducted with NO3- held at a constant 0.5 M and with the amount of total halide concentration varying from 0.25 M to 4 M. Studies on NaBr:NaNO3 mixtures suggest that as the bromide ion to nitrate ion ratio increases, there is an enhancement in the rate of production of NO2 in the nitrate-bromide mixtures over that formed in the photolysis of NaNO3. Molecular dynamic (MD) simulations provide molecular level insight into the ions near the air-water interface in the aqueous halide-nitrate mixtures. These studies suggest that the presence of sodium halides at the air-water interface may encourage some nitrate ions to approach the top layers of water, allowing for more efficient escape of photoproducts than is seen in the absence of halides. Experiments on mixtures of KBr:KNO3 are being conducted to determine potential cation effects. In addition, ternary mixtures of NaCl:NaBr:NaNO3 are being examined to determine the effects of mixtures of halides on production of NO2 and gaseous halogen products. The implications of this photochemistry for tropospheric chemistry will be discussed.

  12. G W quasiparticle band gap of the hybrid organic-inorganic perovskite CH3NH3PbI3 : Effect of spin-orbit interaction, semicore electrons, and self-consistency

    Science.gov (United States)

    Filip, Marina R.; Giustino, Feliciano

    2014-12-01

    We study the quasiparticle band gap of the hybrid organic-inorganic lead halide perovskite CH3NH3PbI3 , using many-body perturbation theory based on the G W approximation. We perform a systematic analysis of the band gap sensitivity to relativistic spin-orbit effects, to the description of semicore Pb-5 d and I-4 d electrons, and to the starting Kohn-Sham eigenvalues. We find that the inclusion of semicore states increases the calculated band gap by 0.2 eV, and self-consistency on the quasiparticle eigenvalues using a scissor correction increases the band gap by 0.5 eV with respect to the G0W0 result. These findings allow us to resolve an inconsistency between previously reported G W calculations for CH3NH3PbI3 . Our most accurate band gap is 1.72 eV, and is in good agreement with the measured optical gap after considering a small excitonic shift as determined in experiments.

  13. Exciton-related nonlinear optical response and photoluminescence in dilute nitrogen InxGa1?xNyAs1?y/GaAs cylindrically shaped quantum dots

    International Nuclear Information System (INIS)

    An investigation of the effects of the dilute nitrogen contents in the exciton states of cylindrical InxGa1?xNyAs1?y/GaAs quantum dots is presented. The exciton states in the system are obtained within the effective mass theory and the band anti-crossing model. Exciton-related nonlinear optical absorption and refractive index change, as well as excitonic photoluminescence are studied with the help of the calculated exciton states. - Highlights: Theoretical study of excitons in cylindrical InxGa1?xNyAs1?y/GaAs quantum dots. Calculations of binding energy for different configurations of electron-hole pairs. Nonlinear optical absorption and refractive index changes. Dependence of photoluminescence energy transitions with several inputs

  14. Localized surface plasmon and exciton interaction in silver-coated cadmium sulphide quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Ghosh, P.; Rustagi, K. C.; Vasa, P.; Singh, B. P., E-mail: bhanuprs@gmail.com [Department of Physics, Indian Institute of Technology Bombay, Mumbai- 400076 (India)

    2015-05-15

    Localized surface plasmon and exciton coupling has been investigated on colloidal solutions of silver-coated CdS nanoparticles (NPs), synthesized by gamma irradiation. Two broad photoluminescence (PL) bands (blue/red) corresponding to band to band and defect state transitions have been observed for the bare and coated samples. In case of bare CdS NPs, the intensity of the red PL peak is about ten times higher than the blue PL peak intensity. However, on coating the CdS NPs with silver, the peak intensity of the blue PL band gets enhanced and becomes equal to that of the red PL band. High-resolution transmission electron microscopic (HRTEM) images adequately demonstrate size distribution of these metal/semiconductor nanocomposites. UV-Vis absorption studies show quantum confinement effect in these semiconductor quantum dot (SQD) systems. Absorption spectrum of silver-coated SQDs shows signature of surface plasmon-exciton coupling which has been theoretically verified.

  15. Localized surface plasmon and exciton interaction in silver-coated cadmium sulphide quantum dots

    Science.gov (United States)

    Ghosh, P.; Rustagi, K. C.; Vasa, P.; Singh, B. P.

    2015-05-01

    Localized surface plasmon and exciton coupling has been investigated on colloidal solutions of silver-coated CdS nanoparticles (NPs), synthesized by gamma irradiation. Two broad photoluminescence (PL) bands (blue/red) corresponding to band to band and defect state transitions have been observed for the bare and coated samples. In case of bare CdS NPs, the intensity of the red PL peak is about ten times higher than the blue PL peak intensity. However, on coating the CdS NPs with silver, the peak intensity of the blue PL band gets enhanced and becomes equal to that of the red PL band. High-resolution transmission electron microscopic (HRTEM) images adequately demonstrate size distribution of these metal/semiconductor nanocomposites. UV-Vis absorption studies show quantum confinement effect in these semiconductor quantum dot (SQD) systems. Absorption spectrum of silver-coated SQDs shows signature of surface plasmon-exciton coupling which has been theoretically verified.

  16. Microscopic Investigation of Grain Boundaries in Organolead Halide Perovskite Solar Cells.

    Science.gov (United States)

    Li, Jiang-Jun; Ma, Jing-Yuan; Ge, Qian-Qing; Hu, Jin-Song; Wang, Dong; Wan, Li-Jun

    2015-12-30

    Grain boundaries (GBs) play an important role in organic-inorganic halide perovskite solar cells, which have generally been recognized as a new class of materials for photovoltaic applications. To definitely understand the electrical structure and behavior of GBs, here we present Kelvin probe force microscopy and conductive atomic force microscopy (c-AFM) measurements of both typical and inverted planar organolead halide perovskite solar cells. By comparing the contact potential difference (CPD) of these two devices in the dark and under illumination, we found that a downward band bending exists in GBs that predominantly attract photoinduced electrons. The c-AFM measurements observed that higher photocurrents flow through GBs when a low bias overcomes the barrier created by the band bending, indicating that GBs act as effective charge dissociation interfaces and photocurrent transduction pathways rather than recombination sites. PMID:26633192

  17. Luminescence of bound excitons in ZnO:Zn phosphor powders

    International Nuclear Information System (INIS)

    Luminescence and time-resolved luminescence spectra of ZnO:Zn phosphor powders have been investigated at low temperatures. ZnO:Zn phosphor powders under the band-to-band excitation exhibit two prominent luminescence lines attributed to the radiative recombination of bound excitons at 3.360 and 3.365eV. The luminescence line at 3.360eV originates from an exciton bound to an intrinsic defect serving as a neutral donor. It is found that the luminescence line at 3.365eV has a short decay time of ∼80ps, and shows a long tail toward the lower energy side of the spectral peak. The origin of the luminescence line at 3.365eV is discussed in terms of bound excitons associated with surface defect states in ZnO:Zn phosphor

  18. Binding Energy and Lifetime of Excitons in InxGa1-xAs/GaAs Quantum Wells

    DEFF Research Database (Denmark)

    Orani, D.; Polimeni, A.; Patane, A.; Capizzi, M.; Martelli, F.; D'Andrea, A.; Tommassini, N.; Borri, Paola; Gurioli, M.; Colocci, M.

    1997-01-01

    We report a systematic study of exciton binding energies and lifetimes in InGaAs/GaAs quantum wells. The experimental binding energies have been deduced from photoluminescence excitation measurements taking into account the contribution of the 2s state of the exciton and the line broadening. The ...... experimental results have been compared with accurate calculations in a four-band model, where exciton energies take into account the polaron correction. The theory accounts for all the experimental observations and provides a good quantitative agreement with the experimental values....

  19. Excitation of surface-radiation exciton-polariton modes at strong attenuations of excitons

    International Nuclear Information System (INIS)

    Formation of a photoluminescence spectrum of surface-radiation exciton-polariton modes is analysed at strong exciton attenuations when criteria of applicability of Boltzmann kinetic equation are broken for function of polariton distribution. (author)

  20. Spectroscopy of the two Lowest Exciton Zero-Phonon Lines in Single CdSe/ZnS Nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Louyer, Y; Biadala, L; Tamarat, Ph; Lounis, B, E-mail: y.louyer@cpmoh.u-bordeaux1.f [Centre de Physique Moleculaire Optique et Hertzienne, University of Bordeaux and CNRS, 351 cours de la Liberation, Talence, F-33405 (France)

    2010-09-01

    We study the optical properties of the lowest-energy exciton states in highly photostable individual CdSe/ZnS nanocrystals at low temperatures. We observe two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest-energy levels of the band-edge exciton fine structure. By using resonant laser excitation on these two lines we measure a spectral broadening of 10 {mu}eV over integration times of 100 ms.

  1. Efficiency of primary radiation-induced defects in alkali halide crystals

    International Nuclear Information System (INIS)

    Temperature dependences of F centers formation efficiency (FE) are measured by the method of pulse absorption spectrometry with nanosecond resolution with the aim of determination of the relative yield of decay reaction of electron excitations into primary pairs of structural defects in alkali halide crystals (LiF, NaF, NaCl, NaBr, KCl, KBr, KI, RbBr). It is shown that for all studied compounds FE of F centers increases in the temperature range from 100 up to 300 K. Activation energies of the primary defects storage process are determined. It is supposed that FE of primary defects is determined mainly by the ratio of exciton energy parameters of predissociated and predefect state as well as of energy, stored on defect

  2. Harmonic dynamical behaviour of thallous halides

    Indian Academy of Sciences (India)

    Sarvesh K Tiwari; L J Shukla; K S Upadhyaya

    2010-05-01

    Harmonic dynamical behaviour of thallous halides (TlCl and TlBr) have been studied using the new van der Waals three-body force shell model (VTSM), which incorporates the effects of the van der Waals interaction along with long-range Coulomb interactions, three-body interactions and short-range second neighbour interactions in the framework of rigid shell model (RSM). Phonon dispersion curves (PDC), variations of Debye temperature with absolute temperature and phonon density of state (PDS) curves have been reported for thallous halides using VTSM. Comparison of experimental values with those of VTSM and TSM are also reported in the paper and a good agreement between experimental and VTSM values has been found, from which it may be inferred that the incorporation of van der Waals interactions is essential for the complete harmonic dynamical behaviour of thallous halides.

  3. A minimal model for excitons within time-dependent density-functional theory.

    Science.gov (United States)

    Yang, Zeng-hui; Li, Yonghui; Ullrich, Carsten A

    2012-07-01

    The accurate description of the optical spectra of insulators and semiconductors remains an important challenge for time-dependent density-functional theory (TDDFT). Evidence has been given in the literature that TDDFT can produce bound as well as continuum excitons for specific systems, but there are still many unresolved basic questions concerning the role of dynamical exchange and correlation (xc). In particular, the roles of the long spatial range and the frequency dependence of the xc kernel f(xc) for excitonic binding are still not very well explored. We present a minimal model for excitons in TDDFT, consisting of two bands from a one-dimensional (1D) Kronig-Penney model and simple approximate xc kernels, providing an easily accessible model system for studying excitonic effects in TDDFT. For the 1D model system, it is found that adiabatic xc kernels can produce at most two bound excitons, confirming that the long spatial range of f(xc) is not a necessary condition. It is shown how the Wannier model, featuring an effective electron-hole interaction, emerges from TDDFT. The collective, many-body nature of excitons is explicitly demonstrated. PMID:22779671

  4. Complexing in binary molten halide systems with a common anion, containing sodium halides

    International Nuclear Information System (INIS)

    Analysis of meltability diagrams of binary halide systems with a common anion published in literature permitted studying the interaction in sodium halide melts NaX (X = F-I) and halides of alkali, alkaline-earth, transition and rare earth elements, as well as uranium and thorium. It was ascertained that in most systems enhancement of polarizing effect of cation in the melt involves intensification of complexing. Transition from ideal to non-ideal eutectic systems and further to systems with incongruently and congruently melting compounds was pointed out, as well

  5. Exciton Seebeck effect in molecular systems.

    Science.gov (United States)

    Yan, Yun-An; Cai, Shaohong

    2014-08-01

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect. PMID:25106568

  6. Exciton Seebeck effect in molecular systems

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Yun-An, E-mail: yunan@nano.gznc.edu.cn [Guizhou Provincial Key Laboratory of Computational Nanomaterial Science, Guizhou Normal College, Guiyang, Guizhou 550018 (China); Cai, Shaohong [Guizhou Key Laboratory of Economic System Simulation, Guizhou University of Finance and Economics, Guiyang 550004 (China)

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  7. Reference spectroscopic data for hydrogen halides, Part II: The line lists

    International Nuclear Information System (INIS)

    Accurate spectroscopic parameters for the hydrogen halides, namely HF, HCl, HBr, and HI, together with their deuterated isotopologues, are crucial for the quantitative study of terrestrial and planetary atmospheres, astrophysical objects, and chemical lasers. A thorough evaluation of all the hydrogen halide line parameters in previous HITRAN editions has been carried out. A new set of line lists was generated for the HITRAN2012 edition using methods described here. In total, 131,798 entries were generated for numerous pure-rotational and ro-vibrational transitions (fundamental, overtone, and hot bands) for hydrogen halides and their deuterated species in a standard HITRAN 160-character format. Data for the deuterated isotopologues have been entered into HITRAN for the first time. The calculations employ the recently developed semi-empirical dipole moment functions [Li G, et al. J Quant Spectrosc Radiat Transfer 2013;121:7890] and very accurate analytical potential energy functions and associated functions characterizing BornOppenheimer breakdown effects. Line-shape parameters have also been updated using the most recent available experimental and theoretical studies. Comparison with the previous HITRAN compilation has shown significant improvements. -- Highlights: Significant improvements of line positions and intensities of hydrogen halides. Extended ro-vibrational range for non-LTE applications. First time inclusion of the deuterated species, DF, DCl, DBr and DI in HITRAN. Updated line-shape parameters

  8. Localized Excitons in Carbon Nanotubes.

    Science.gov (United States)

    Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-03-01

    It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.

  9. Versatile, isotopically specific hydrogen halide TEA pin laser

    International Nuclear Information System (INIS)

    A practical, easily constructed design for a laboratory hydrogen/deuterium halide chemical TEA laser is presented. Typical output energies in excess of 50 mJ/pulse broadband and 5 mJ/pulse on single lines are easily obtained. Isotopically specific oscillation on the v=1?0 band of single isotopes of H35Cl--H37Cl, D35Cl--D37Cl, H79Br--H81Br, and D79Br--D81Br is demonstrated. The ease of conversion from one laser species to the next and the flexible design provide a highly versatile device for laboratory problems in chemical and physical dynamics. Major advances in the development of laboratory HCl and HBr chemical TEA lasers are also reviewed

  10. New ternary transplutonium compounds: chalcogenide halides

    International Nuclear Information System (INIS)

    Although chalcogenide halides have been identified in almost all groups of the Periodic Table, they were previously unknown in the case of the actinide elements. Based on the known lanthanide chalcogenide halides, they focused their investigation on the sulfur-based compounds. Consistent with the scarcity of transplutonium elements, they developed an original microscale preparation of the ternary compounds AnSI (An = 242Pu, 243Am, 248Cm). The reaction scheme is: An2O3 (or AnO2) ? AnBr3 ?AnI3 ? AnSI. All products are examined by X-ray powder diffraction and solid state spectroscopy

  11. Topologically protected excitons in porphyrin thin films

    OpenAIRE

    Yuen-Zhou, Joel; Saikin, Semion S.; Yao, Norman Y.; Aspuru-Guzik, Aln

    2014-01-01

    The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyze a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. T...

  12. Extraordinary exciton conductance induced by strong coupling

    OpenAIRE

    Feist, Johannes; Francisco J. Garcia-Vidal

    2014-01-01

    We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transpo...

  13. Extraordinary exciton conductance induced by strong coupling

    CERN Document Server

    Feist, Johannes

    2014-01-01

    We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role.

  14. Nonlinear optical response of interacting excitons

    Science.gov (United States)

    Keldysh, Leonid V.

    1995-02-01

    In the vicinity of the direct intrinsic absorption threshold in semiconductors nonlinearity of the optical response is dominated by manybody interaction effects in the polarization cloud accompanying the propagation of the electromagnetic field. This polarization is essentially the system of electron-hole pairs. Its evolution with increasing density (radiation intensity) is analyzed in the framework of simplified model substituting effective short range pseudopotential instead of real Coulomb potential. At moderate densities excitonic resonance is essentially modified by the formation of excitonic molecules. Multistable behavior arise. Transient processes are discussed including gradual formation of bound state-excitons and excitonic molecules. The latter manifest themselves in anomalously large four-wave mixing amplitude.

  15. Comment on 'Topological stability of the half-vortices in spinor exciton-polariton condensates'

    International Nuclear Information System (INIS)

    We show that the conclusions of recent paper by Flayac et al. [Phys. Rev. B 81, 045318 (2010)] concerning the stability of half-quantum vortices are misleading. We demonstrate the existence of static half-quantum vortices in exciton-polariton condensates and calculate the warping of their texture produced by transverse-electric-transverse-magnetic splitting of polariton band.

  16. The nature of singlet excitons in oligoacene molecular crystals

    KAUST Repository

    Yamagata, H.

    2011-01-01

    A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32cm-1, far smaller than the measured value of 631cm-1 and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601cm-1 and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport. © 2011 American Institute of Physics.

  17. The influence of Halide and pseudo-Halide antioxidants in Fenton-like reaction systems

    OpenAIRE

    Malesic, J.; Kolar, J.; Strlic, M.; Polanc, S.

    2006-01-01

    An application of the N,N'-(5-nitro-1,3-phenylene) bisglutaramide (NPG) hydroxylation assay for spectrophotometric determination of the rate of oxidising species generation in Fenton-like systems in the presence of halide and pseudo-halide antioxidants was evaluated. Using ion chromatography it was demonstrated that the concentration of antioxidants did not decrease during the course of the experiment. Although the stoichiometry was not constant, determination of the rate constant of NPG hydr...

  18. Subpicosecond exciton spin relaxation in GaN

    Science.gov (United States)

    Kuroda, T.; Yabushita, T.; Kosuge, T.; Tackeuchi, A.; Taniguchi, K.; Chinone, T.; Horio, N.

    2004-10-01

    The spin-relaxation process of A-band exciton in GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The spin-relaxation times at 150-225K are 0.47-0.25ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin-relaxation time ?s is found to be proportional to T-1.4, where T is the temperature.

  19. Interferometric Rayleigh Scattering by Excitons in a Single Quantum Dot

    CERN Document Server

    Alen, B; Kroner, M; Seidl, S; Karrai, K; Warburton, R J; Badolato, A; Medeiros-Ribeiro, G; Petroff, P M

    2005-01-01

    We have determined both the real and imaginary parts of the dielectric susceptibility of a single quantum dot. The experiment is based on the observation and the manipulation of Rayleigh scattering at photon frequencies near the resonance of an optical exciton transition in single self-assembled InAs and InGaAs quantum dots. The weak electromagnetic field coherently scattered by the quantum dot is made to interfere with the narrow-band reference laser field in a Fabry-Perot setup operated at cryogenic temperature.

  20. Formation of structured nanophases in halide crystals.

    Czech Academy of Sciences Publication Activity Database

    Kulveit, Jan; Demo, Pavel; Polk, Karel; Sveshnikov, Alexey; Koek, Zden?k

    2013-01-01

    Ro?. 5, ?. 6 (2013), s. 561-564. ISSN 2164-6627 R&D Projects: GA ?R GAP108/12/0891 Institutional support: RVO:68378271 Keywords : halide crystals * nucleation Subject RIV: BM - Solid Matter Physics ; Magnetism http://www.aspbs.com/asem.html#v5n6

  1. Formation of structured nanophases in halide crystals.

    Czech Academy of Sciences Publication Activity Database

    Kulveit, J.; Demo, Pavel; Polk, Karel; Sveshnikov, Alexey; Koek, Zden?k

    2011-01-01

    Ro?. 134, ?. 14 (2011), 144504/1-144504/7. ISSN 0021-9606 R&D Projects: GA AV ?R IAA100100806 Institutional research plan: CEZ:AV0Z10100521 Keywords : nanophase * halides * nucleation * spectroscopy Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.333, year: 2011

  2. Reactivity of perfluoroalkyl halides towards nucleophiles

    International Nuclear Information System (INIS)

    The method of cyclic voltammetry was employed for the study and comparative evaluation of reactivity of perfluoroalkyl iodides and arylperfluoroalkyliodonium salts towards nucleophiles in various solvents. The method suggested can be used for predicting the reactivity of perfluoroalkyl halides in reactions proceeding according to the SRN1 mechanism. 16 refs., 3 figs., 4 tabs

  3. Unraveling halide hydration: A high dilution approach

    International Nuclear Information System (INIS)

    The hydration properties of halide aqua ions have been investigated combining classical Molecular Dynamics (MD) with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Three halide-water interaction potentials recently developed [M. M. Reif and P. H. Hnenberger, J. Chem. Phys. 134, 144104 (2011)], along with three plausible choices for the value of the absolute hydration free energy of the proton (?Ghyd?[H+]), have been checked for their capability to properly describe the structural properties of halide aqueous solutions, by comparing the MD structural results with EXAFS experimental data. A very good agreement between theory and experiment has been obtained with one parameter set, namely LE, thus strengthening preliminary evidences for a ?Ghyd?[H+] value of ?1100 kJ mol?1 [M. M. Reif and P. H. Hnenberger, J. Chem. Phys. 134, 144104 (2011)]. The Cl?, Br?, and I? ions have been found to form an unstructured and disordered first hydration shell in aqueous solution, with a broad distribution of instantaneous coordination numbers. Conversely, the F? ion shows more ordered and defined first solvation shell, with only two statistically relevant coordination geometries (six and sevenfold complexes). Our thorough investigation on the effect of halide ions on the microscopic structure of water highlights that the perturbation induced by the Cl?, Br?, and I? ions does not extend beyond the ion first hydration shell, and the structure of water in the F? second shell is also substantially unaffected by the ion

  4. Novel Ultra-Violet/Blue Optoelectronic Materials and Devices Based on Copper Halides (CuHa)

    OpenAIRE

    Cowley, A.J.

    2011-01-01

    Considerable research is being carried out in the area of wide band gap semiconductor materials for light emission applications in the UV/Blue (300-400 nm) spectral range. This project explores the novel use of the Copper Halides (CuHa), specifically γ-CuCl and γ-CuBr, I–VII wide band gap mixed ionic–electronic semiconducting materials with light emitting properties suitable for novel UV/blue light applications. This project details novel research carried out towards achieving single crystal...

  5. Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd1-xMnxTe: Froehlich and deformation potential exciton-phonon interaction

    International Nuclear Information System (INIS)

    A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd1-xMnxTe as an example. The incident radiation frequency ?l is taken above the fundamental absorption region. The photoexcited electron and hole make real transitions through the LO phonon, when one considers Froehlich (F) and deformation potential (DP) interactions. The strong exchange interaction, typical of these materials, leads to a large spin splitting of the exciton states in the magnetic field. Neglecting Landau quantization, this Zeeman splitting gives rise to the formation of eight bands (two conduction and six valence ones) and ten different exciton states according to the polarization of the incident light. Explicit expressions for the MPRRS intensity of second and third order, the indirect creation and annihilation probabilities, the exciton lifetime, and the probabilities of transition between different exciton states and different types of exciton as a function of ?l and the external magnetic field are presented. The selection rules for all hot exciton transitions via exciton-photon interaction and F and DP exciton-phonon interactions are investigated. The exciton energies, as a function of B, the Mn concentration x, and the temperature T, are compared to a theoretical expression. Graphics for creation and annihilation probabilities, lifetime, and Raman intensity of second and third order are discussed

  6. Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd{sub 1-x}Mn{sub x}Te: Froehlich and deformation potential exciton-phonon interaction

    Energy Technology Data Exchange (ETDEWEB)

    Riera, R [Departamento de Fisica, Universidad de Sonora, Apartado Postal 1626, 83000 Hermosillo, Sonora (Mexico); Rosas, R [Departamento de Fisica, Universidad de Sonora, Apartado Postal 1626, 83000 Hermosillo, Sonora (Mexico); Marin, J L [Centro de Investigacion en Fisica, Universidad de Sonora, Apartado Postal 5-088, 83190 Hermosillo, Sonora (Mexico); Bergues, J M [Centro de Investigacion en Fisica, Universidad de Sonora, Apartado Postal 5-088, 83190 Hermosillo, Sonora (Mexico); Campoy, G [Centro de Investigacion en Fisica, Universidad de Sonora, Apartado Postal 5-088, 83190 Hermosillo, Sonora (Mexico)

    2003-05-21

    A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd{sub 1-x}Mn{sub x}Te as an example. The incident radiation frequency {omega}{sub l} is taken above the fundamental absorption region. The photoexcited electron and hole make real transitions through the LO phonon, when one considers Froehlich (F) and deformation potential (DP) interactions. The strong exchange interaction, typical of these materials, leads to a large spin splitting of the exciton states in the magnetic field. Neglecting Landau quantization, this Zeeman splitting gives rise to the formation of eight bands (two conduction and six valence ones) and ten different exciton states according to the polarization of the incident light. Explicit expressions for the MPRRS intensity of second and third order, the indirect creation and annihilation probabilities, the exciton lifetime, and the probabilities of transition between different exciton states and different types of exciton as a function of {omega}{sub l} and the external magnetic field are presented. The selection rules for all hot exciton transitions via exciton-photon interaction and F and DP exciton-phonon interactions are investigated. The exciton energies, as a function of B, the Mn concentration x, and the temperature T, are compared to a theoretical expression. Graphics for creation and annihilation probabilities, lifetime, and Raman intensity of second and third order are discussed.

  7. Picosecond spin relaxation of acceptor-bound exciton in wurtzite GaN

    Science.gov (United States)

    Otake, Hirotaka; Kuroda, Takamasa; Tackeuchi, Atsushi; Taniguchi, Kazuyoshi; Chinone, Takako; Liang, Ji-Hao; Kajikawa, Masataka; Horio, Naochika

    2007-04-01

    The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to spin relaxation times of 1.40 - 1.14 ps. The spin relaxation time, ?s, is found to be proportional to T-0.175, where T is the temperature. This temperature dependence is quite weak compared with that of A-band free excitons showing ?s ? T-1.41 at 150 - 225K.

  8. Selective optical pumping of charged excitons in unintentionally doped InAs quantum dots

    International Nuclear Information System (INIS)

    We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm

  9. Radiative recombination of excitons in amorphous semiconductors

    International Nuclear Information System (INIS)

    A theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities of excitonic radiative recombination are considered and the corresponding rates are derived at thermal equilibrium. The radiative lifetime is calculated from the inverse of the maximum rate for all the four possibilities. Results agree very well with experiments

  10. Exciton dimensional quantization in CdTe

    International Nuclear Information System (INIS)

    An increase in the binding energy of a free exciton is found thin CdTe films. It is caused by decrease in screening of the Coulomb interaction in electron-hole pairs. The observed change in the exciton binding energy is shown to be in a satisfactory agreement with the results of theoretical calculations

  11. Exciton dimensional quantization in CdTe

    Energy Technology Data Exchange (ETDEWEB)

    Babaev, N.A.; Bagaev, V.S.; Garin, F.V.; Kochemasov, A.V.; Paramonov, L.V.; Poyarkov, A.G.; Salashchenko, N.N.; Stopachinskij, V.B.

    1984-09-10

    An increase in the binding energy of a free exciton is found thin CdTe films. It is caused by decrease in screening of the Coulomb interaction in electron-hole pairs. The observed change in the exciton binding energy is shown to be in a satisfactory agreement with the results of theoretical calculations.

  12. DNA-mediated excitonic upconversion FRET switching

    Science.gov (United States)

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-01

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Frster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  13. Mid-infrared emission properties of Nd-doped lead halides for photonic applications

    International Nuclear Information System (INIS)

    The development of rare earth host materials with low maximum phonon energies remains of current interest for infrared (IR) light source development. In this work, we present results of the material preparation and mid-IR emission properties of Nd-doped lead halides PbX2 (X = Cl, Br, and I). Lead halides are characterized by maximum phonon energies of less than ?200 cm-1, which leads to small non-radiative decay rates for intra-4f rare earth transitions. Nd-doped lead halides were synthesized from purified starting materials and were grown by vertical Bridgman technique. Under diode-laser excitation at ?808 nm, all investigated samples exhibited broad Nd3+ emission bands centered at ?2.6 and ?5.2 ?m with decay times in the millisecond range. The dominant contributions of these emission bands were assigned to the transitions 4I13/2 ? 4I9/2 (2.6 ?m) and 4I11/2 ? 4I9/2 (5.2 ?m), respectively. The temperature dependence of the 5.2 ?m emission lifetime revealed significant difference between the samples and showed a 80, 11, and 17% reduction between 77 K and room-temperature for Nd:PbCl2, Nd:PbBr2, and Nd:PbI2, respectively

  14. Quantum-confined Stark effect on spatially indirect excitons in CdTe/Cdx Zn1-x Te quantum wells

    Science.gov (United States)

    Haas, H.; Magnea, N.; Dang, Le Si

    1997-01-01

    The quantum-confined Stark effect is studied in the mixed type-I/type-II CdTe/Cdx Zn1-x Te strained heterostructures. The type-II nature of the light-hole excitons is unambiguously confirmed by the blueshift observed under increasing electric field, in good agreement with calculations. On the other hand, the heavy-hole excitons are redshifted as expected for type-I excitons. The peculiar valence-band alignment, resulting from the sign reversal of the strain between the wells and the barriers, is used to detect the electric-field induced mixing of LH1 and HH2 confined hole states. An accurate value for the long-disputed chemical valence-band offset of CdTe/ZnTe system is extracted as ?EV=(11+/-3)% of the band-gap difference between unstrained CdTe and ZnTe materials.

  15. Energetics and dynamics in organic-inorganic halide perovskite photovoltaics and light emitters.

    Science.gov (United States)

    Sum, Tze Chien; Chen, Shi; Xing, Guichuan; Liu, Xinfeng; Wu, Bo

    2015-08-28

    The rapid transcendence of organic-inorganic metal halide perovskite solar cells to above the 20% efficiency mark has captivated the broad photovoltaic community. As the efficiency race continues unabated, it is essential that fundamental studies keep pace with these developments. Further gains in device efficiencies are expected to be increasingly arduous and harder to come by. The key to driving the perovskite solar cell efficiencies towards their Shockley-Queisser limit is through a clear understanding of the interfacial energetics and dynamics between perovskites and other functional materials in nanostructured- and heterojunction-type devices. In this review, we focus on the current progress in basic characterization studies to elucidate the interfacial energetics (energy-level alignment and band bending) and dynamical processes (from the ultrafast to the ultraslow) in organic-inorganic metal halide perovskite photovoltaics and light emitters. Major findings from these studies will be distilled. Open questions and scientific challenges will also be highlighted. PMID:26234397

  16. Giant enhancement of the optical second-harmonic emission of WSe(2) monolayers by laser excitation at exciton resonances.

    Science.gov (United States)

    Wang, G; Marie, X; Gerber, I; Amand, T; Lagarde, D; Bouet, L; Vidal, M; Balocchi, A; Urbaszek, B

    2015-03-01

    We show that the light-matter interaction in monolayer WSe_{2} is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T=4??K. At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k-valley coherence and polarization. PMID:25793850

  17. Effect of uniaxial stress on free and bismuth-bound excitons in InP

    International Nuclear Information System (INIS)

    The reduction of the shear deformation potentials of holes bound to the isoelectronic impurity Bi in InP is determined by piezoluminescence. It is compared with the corresponding reduction for holes bound to the Coulomb-type acceptors C and Zn. The theory for an effective mass acceptor describes well the cases of C and Zn. However, additional effects as local strain and Stark fields must be involved in the case of Bi leading to an extremely large reduction of the deformation potentials. No change in binding energy with applied stress as well as no exchange splitting of the Bi-bound exciton can be detected within experimental accuracy. The stress dependence of the free exciton reflectance reveals values for the band deformation potentials and a value of 0.07 meV for the exchange splitting of the free exciton in InP. (author)

  18. Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV

    Energy Technology Data Exchange (ETDEWEB)

    Consani, Cristina; Koch, Federico [Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg (Germany); Panzer, Fabian; Unger, Thomas; Köhler, Anna [Lehrstuhl Experimentalphysik II, Universität Bayreuth, Universitätsstraße 30, 95440 Bayreuth (Germany); Brixner, Tobias, E-mail: brixner@phys-chemie.uni-wuerzburg.de [Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg (Germany); Center for Nanosystems Chemistry (CNC), Universität Würzburg, Am Hubland, 97074 Würzburg (Germany)

    2015-06-07

    Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution. By combining the results of the two techniques, we differentiate between an initial exciton relaxation, which is not characterized by significant exciton mobility, and energy transport between different chromophores in the aggregate.

  19. Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV

    International Nuclear Information System (INIS)

    Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution. By combining the results of the two techniques, we differentiate between an initial exciton relaxation, which is not characterized by significant exciton mobility, and energy transport between different chromophores in the aggregate

  20. Magneto-Optical Properties of Bound Excitons in ZnO

    CERN Document Server

    Rodina, A V; Dworzak, M; Haboeck, U; Hoffmann, A; Zeuner, A; Alves, H R; Hofmann, D M; Meyer, B K

    2003-01-01

    We present results of magneto-optical measurements and theoretical analysis of shallow bound exciton complexes in bulk ZnO. Polarization and angular dependencies of magneto-photoluminescence spectra at 5 T suggest that the upper valence band has $\\Gamma_7$ symmetry. Nitrogen doping leads to the formation of an acceptor center that compensates shallow donors. This is confirmed by the observation of excitons bound to ionized donors in nitrogen doped ZnO. The strongest transition in the ZnO:N ($I_9$ transition) is associated with a donor bound exciton. This conclusion is based on its thermalization behavior in temperature-dependent magneto-transmission measurements and is supported by comparison of the thermalization properties of the $I_9$ and $I_4$ emission lines in temperature-dependent magneto-photoluminescence investigations.

  1. Exciton absorption spectrum of thin Ag2ZnI4

    International Nuclear Information System (INIS)

    In Ag2ZnI4 compound thin films one investigated into the electron spectrum of absorption within 3-6 eV photon energy range. The boundary of interband absorption is determined to correspond to the direct permitted transitions with Eg = 3.7 eV forbidden gap width. A strong exciton band at E = 3.625 eV (80 K) ? half width temperature run of which within 80-390 K range is governed by exciton-phonon interaction typical for quasi-single-dimensional excitons, is adjacent to the absorption boundary. At T ? 390 K one observes a bend in E(T) and ?(T) dependences associated with generation of the Frenkel defects and followed by transfer of Ag ions to the interstices and vacancies of the compound crystalline lattice

  2. The transformation of exciton photoluminescence of CdTe:Cl single crystals caused by technological treatments

    International Nuclear Information System (INIS)

    The effect of microwave radiation and thermal annealing on the transformation of exciton-impurity complexes in CdTe:Cl single crystals is studied using low-temperature photoluminescence (PL) measurements. It is shown that microwave radiation activates ClTe centers. The improvement of crystal structure perfection is obtained at optimal regime of microwave treatment (30 s). Changes in the excitonic bands due to thermal annealing at temperature <180C is not observed. This testifies about thermal stability of studied material. But the restructurization of exciton-impurity complexes due to annealing at temperature ?180C is observed, which is related with 'evaporation' of the chlorine atoms from the surface of the crystals. (authors)

  3. Exciton states in monolayer MoSe2: impact on interband transitions

    Science.gov (United States)

    Wang, G.; Gerber, I. C.; Bouet, L.; Lagarde, D.; Balocchi, A.; Vidal, M.; Amand, T.; Marie, X.; Urbaszek, B.

    2015-12-01

    We combine linear and nonlinear optical spectroscopy at 4 K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In one-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV, in good agreement with our calculations. In two-photon PLE we detect for the A- and B-exciton the 2p state 180 meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than two orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-density functional theory calculations show in the conduction band along the K? direction a local minimum at the ?-point that is energetically and in k-space close to the global minimum at the K-point. The influence of this local minimum on exciton transitions is discussed.

  4. Observation of long-lived interlayer excitons in monolayer MoSe2-WSe2 heterostructures.

    Science.gov (United States)

    Rivera, Pasqual; Schaibley, John R; Jones, Aaron M; Ross, Jason S; Wu, Sanfeng; Aivazian, Grant; Klement, Philip; Seyler, Kyle; Clark, Genevieve; Ghimire, Nirmal J; Yan, Jiaqiang; Mandrus, D G; Yao, Wang; Xu, Xiaodong

    2015-01-01

    Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8?ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices. PMID:25708612

  5. Exciton-phonon complexes and optical properties in CdSe nanocrystals

    International Nuclear Information System (INIS)

    A method to calculate the quantum states of exciton-phonon complexes in semiconductor nanocrystals is presented. The exciton-phonon complexes are built from a basis set made of products of phonon states and electron-hole pairs, which are coupled through the electron-phonon Froehlich interaction, and the electron-hole Coulomb and exchange interactions. In CdSe nanocrystals, the conduction band electrons are described by the effective mass equation, while the holes are represented by the spherical 4 x 4 Baldereschi-Lipari Hamiltonian. It is shown that a flexible and complete electron-hole basis, not limited to the 1s-1S3/2 octet, is essential to obtain converged eigenvalues and the correct polaron shift to the exciton energy. A study of the spectral properties is presented; in particular, the spectral region which involves the lowest exciton-phonon complex eigenstates is analysed in details. Specifically, the non-adiabatic nature of the exciton-phonon dynamics in the nanocrystals examined is clearly shown by the vibron eigenstates that were obtained

  6. Computational Screening of Mixed Metal Halide Ammines

    DEFF Research Database (Denmark)

    Jensen, Peter Bjerre; Lysgaard, Steen

    Metal halide ammines, e.g. Mg(NH3)6Cl2 and Sr(NH3)8Cl2, can reversibly store ammonia, with high volumetric hydrogen storage capacities. In this project we are searching for improved mixed materials with optimal desorption temperature and kinetics. We apply DFT calculations on mixed compounds selected by a Genetic Algorithm (GA), relying on biological principles of natural selection. The GA is evolving from an initial (random) population and selecting those with highest fitness, e.g. stability, release temperature and storage capacity. The search space includes all alkaline, alkaline earth, 3d and 4d metals and the four lightest halides, giving in total almost two million combinations.

  7. Can Disorder Enhance Incoherent Exciton Diffusion?

    Science.gov (United States)

    Lee, Elizabeth M Y; Tisdale, William A; Willard, Adam P

    2015-07-30

    Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we introduce a general model, based upon Frster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates, which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific hopping rates is broadened in a manner that results in a decrease in average exciton diffusivity relative to that in a perfectly ordered film. Second, since excitons prefer to make transitions that are downhill in energy, the steady state distribution of exciton energies is biased toward low-energy molecular subunits, those that exhibit reduced diffusivity relative to a perfectly ordered film. These effects combine to reduce the net diffusivity in a manner that is time dependent and grows more pronounced as disorder is increased. Notably, however, we demonstrate that the presence of energetic disorder can give rise to a population of molecular subunits with exciton transfer rates exceeding those of subunits in an energetically uniform material. Such enhancements may play an important role in processes that are sensitive to molecular-scale fluctuations in exciton density field. PMID:26106811

  8. Unraveling halide hydration: A high dilution approach

    Energy Technology Data Exchange (ETDEWEB)

    Migliorati, Valentina, E-mail: valentina.migliorati@uniroma1.it; Sessa, Francesco; DAngelo, Paola, E-mail: p.dangelo@uniroma1.it [Dipartimento di Chimica, Universit di Roma La Sapienza, P. le A. Moro 5, 00185 Roma (Italy); Aquilanti, Giuliana [Sincrotrone Trieste S.C.p.A s.s. 14, km 163.5, I-34149 Basovizza, Trieste (Italy)

    2014-07-28

    The hydration properties of halide aqua ions have been investigated combining classical Molecular Dynamics (MD) with Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. Three halide-water interaction potentials recently developed [M. M. Reif and P. H. Hnenberger, J. Chem. Phys. 134, 144104 (2011)], along with three plausible choices for the value of the absolute hydration free energy of the proton (?G{sub hyd}{sup ?}[H{sup +}]), have been checked for their capability to properly describe the structural properties of halide aqueous solutions, by comparing the MD structural results with EXAFS experimental data. A very good agreement between theory and experiment has been obtained with one parameter set, namely L{sub E}, thus strengthening preliminary evidences for a ?G{sub hyd}{sup ?}[H{sup +}] value of ?1100 kJ mol{sup ?1} [M. M. Reif and P. H. Hnenberger, J. Chem. Phys. 134, 144104 (2011)]. The Cl{sup ?}, Br{sup ?}, and I{sup ?} ions have been found to form an unstructured and disordered first hydration shell in aqueous solution, with a broad distribution of instantaneous coordination numbers. Conversely, the F{sup ?} ion shows more ordered and defined first solvation shell, with only two statistically relevant coordination geometries (six and sevenfold complexes). Our thorough investigation on the effect of halide ions on the microscopic structure of water highlights that the perturbation induced by the Cl{sup ?}, Br{sup ?}, and I{sup ?} ions does not extend beyond the ion first hydration shell, and the structure of water in the F{sup ?} second shell is also substantially unaffected by the ion.

  9. Electrical conductivity of supercooled halide melts.

    Czech Academy of Sciences Publication Activity Database

    Sveshnikov, Alexey; Demo, Pavel; Koek, Zden?k; Nitsch, Karel; Rodov, Miroslava

    Praha : MAXDORF s.r.o., 2002 - (Nitsch, K.; Rodov, M.), s. 51-52 ISBN 80-85912-85-8. [Joint Seminar "Development of Materials Science in Research and Education " /12./. Praha (CZ), 10.09.2002-12.09.2002] R&D Projects: GA AV ?R IAA1010010; GA ?R GA203/00/1423 Institutional research plan: CEZ:AV0Z1010914 Keywords : supercooled halide metals * electrical conductivity Subject RIV: BM - Solid Matter Physics ; Magnetism

  10. Raman Spectroscopy of Organic-Inorganic Halide Perovskites.

    Science.gov (United States)

    Ledinsk, Martin; Lper, Philipp; Niesen, Bjoern; Holovsk, Jakub; Moon, Soo-Jin; Yum, Jun-Ho; De Wolf, Stefaan; Fejfar, Antonn; Ballif, Christophe

    2015-02-01

    Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide-bromide (CH3NH3PbI3-xBrx) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3-xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio. PMID:26261955

  11. Computational screening of mixed metal halide ammines

    DEFF Research Database (Denmark)

    Jensen, Peter Bjerre; Lysgaard, Steen

    Metal halide ammines, e.g. Mg(NH3)6Cl2 and Sr(NH3)8Cl2, can reversibly store ammonia, with high volumetric hydrogen storage capacities. The storage in the halide ammines is very safe, and the salts are therefore highly relevant as a carbon-free energy carrier in future transportation infrastructure. In this project we are searching for improved mixed materials with optimal desorption temperatures and kinetics, optimally releasing all ammonia in one step. We apply Density Functional Theory, DFT, calculations on mixed compounds selected by a Genetic Algorithm (GA), relying on biological principles of natural selection. The GA is evolving from an initial (random) population and selecting those with highest fitness, a function based on e.g. stability, release temperature and storage capacity. The search space includes all alkaline, alkaline earth, 3d and 4d metals and the four lightest halides. In total the search spaces consists of millions combinations, which makes a GA ideal, to reduce the number of necessarycalculations. We are screening for a one step release from either a hexa or octa ammine, and we have found promising candidates, which will be further investigated ? both computationally and experimentally.

  12. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter

    2011-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is...

  13. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, Jørn Märcher; Stobbe, Søren; Lodahl, Peter

    2010-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have...... determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is...

  14. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, Jrn Mrcher Technical University of Denmark,

    2010-01-01

    The interaction between light and quantum-dot (QD) excitons is strongly influenced by the environment in which the QD is placed. We have investigated the interaction by measuring the time-resolved spontaneous-emission rate of QD excitons in different nanostructured environments. Thereby, we have determined the oscillator strength, quantum efficiency and spin-flip rates of QD excitons as well as their dependencies on emission wavelength and QD size. Enhancement and inhibition of QD spontaneous emission in photonic crystal membranes (PCMs) is observed. Efficient coupling to PCM waveguides is demonstrated and the influence of disorder is discussed. The findings have a strong bearing on future nanophotonic devices.

  15. Structure vs. excitonic transitions in self-assembled porphyrin nanotubes and their effect on light absorption and scattering.

    Science.gov (United States)

    Arteaga, Oriol; Canillas, Adolf; El-Hachemi, Zoubir; Crusats, Joaquim; Rib, Josep M

    2015-12-01

    The optical properties of diprotonated meso-tetrakis(4-sulphonatophenyl)porphyrin (TPPS4) J-aggregates of elongated thin particles (nanotubes in solution and ribbons when deposited on solid interfaces) are studied by different polarimetric techniques. The selective light extinction in these structures, which depends on the alignment of the nanoparticle with respect to the polarization of light, is contributed by excitonic absorption bands and by resonance light scattering. The optical response as a function of the polarization of light is complex because, although the quasi-one-dimensional structure confines the local fields along the nanotube axis, there are two orthogonal excitonic bands, of H- and J-character, that can work in favor of or against the field confinement. Results suggest that resonance light scattering is the dominant effect in solid state preparations, i.e. in collective groups (bundles) of ribbons but in diluted solutions, i.e. with isolated nanotubes, the absorption at the excitonic transitions remains dominant and linear dichroism spectra can be a direct probe of the exciton orientations. Therefore, by analyzing scattering and absorption data we can determine the alignment of the excitonic bands within the nanoparticle, i.e. of the orientation of the basic 2D porphyrin architecture in the nanoparticle. This is a necessary first step for understanding the directions of energy transport, charge polarization and non-linear optical properties in these materials. PMID:26584333

  16. Excitons in poly(para phenylene vinylene): a quantum-chemical perspective based on high-level ab initio calculations.

    Science.gov (United States)

    Mewes, Stefanie A; Mewes, Jan-Michael; Dreuw, Andreas; Plasser, Felix

    2016-01-20

    Excitonic effects play a fundamental role in the photophysics of organic semiconductors such as poly(para phenylene vinylene) (PPV). The emergence of these effects is examined for PPV oligomers based on high level ab initio excited-state calculations. The computed many-body wavefunctions are subjected to our recently developed exciton analysis protocols to provide a qualitative and quantitative characterization of excitonic effects. The discussion is started by providing high-level benchmark calculations using the algebraic-diagrammatic construction for the polarization propagator in third order of perturbation theory (ADC(3)). These calculations support the general adequacy of the computationally more efficient ADC(2) method in the case of singly excited states but also reveal the existence of low-energy doubly excited states. In a next step, a series of oligomers with chains of two to eight phenyl rings is studied at the ADC(2) level showing that the confinement effects are dominant for small oligomers, while delocalized exciton bands emerge for larger systems. In the case of the largest oligomer, the first twenty singlet and triplet excited states are computed and a detailed analysis in terms of the Wannier and Frenkel models is presented. The presence of different Wannier bands becomes apparent, showing a general trend that exciton sizes are lowered with increasing quasi-momentum within the bands. PMID:26700493

  17. Frenkel Excitons--Charge-Transfer Excitons--Phonons Coupling in One-Component Molecular Crystals

    OpenAIRE

    Lalov, I. J.; Zhelyazkov, I.

    2011-01-01

    In this paper, we simulate the linear absorption spectra of the MePTCDI and PTCDA crystals. The basic Hamiltonian describes the Frenkel excitons and charge-transfer excitons mixing in the molecular stack (point group C_i) and their linear coupling with one vibrational mode of an intramolecular vibration. Using the vibronic approach, we calculate the linear optical susceptibility in the excitonic and one-phonon vibronic regions of the molecular stack and of a crystal with two...

  18. Short-arc metal halide lamp suitable for projector application

    Science.gov (United States)

    Kawai, Kouji; Matsumoto, Masayuki

    1995-04-01

    Recently, progress has been made in the development of projectors used for large screen displays. Projection systems employing high luminous efficiency Metal Halide Lamps as a light source demonstrate unique and desirable features. For example, compared to conventional lamps, a Metal Halide Lamp has longer life, higher luminous efficiency, the shortest arc length possible, and higher color-rendering characteristics, resulting in an overall superior performance. Several lamps, including Metal Halide, Xenon, and Halogen lamps, are used in HDTV (High Definition Television). Recently, Metal Halide Lamps have become accepted as the light source of choice. The reason for this is the high brightness, color balance, and long life of the Metal Halide Lamp. The fundamental operating principal for Metal Halide Lamps is almost the same as that of Mercury lamps. A Metal Halide Lamp has very different characteristics in that, the spectrum of illumination can be varied by varying the mixture of Metal Halide generated in the lamp. For these lamps, we have improved the electrode, the chemical composition of the Metal Halide, and the glass envelope. We have achieved a longer average life (greater than 2500 hours) for a compact, single-bulb, projection-type lamp.

  19. Lanthanide doped strontium-barium cesium halide scintillators

    Science.gov (United States)

    Bizarri, Gregory; Bourret-Courchesne, Edith; Derenzo, Stephen E.; Borade, Ramesh B.; Gundiah, Gautam; Yan, Zewu; Hanrahan, Stephen M.; Chaudhry, Anurag; Canning, Andrew

    2015-06-09

    The present invention provides for a composition comprising an inorganic scintillator comprising an optionally lanthanide-doped strontium-barium, optionally cesium, halide, useful for detecting nuclear material.

  20. Optical nutation in the exciton range of spectrum

    Energy Technology Data Exchange (ETDEWEB)

    Khadzhi, P. I. [Academy of Sciences of Moldova, Institute of Applied Physics (Moldova, Republic of); Vasiliev, V. V., E-mail: vasscorp@mail.ru [Taras Shevchenko Transnistria State University (Moldova, Republic of)

    2013-08-15

    Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

  1. Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

    OpenAIRE

    Aichmayr, G.; M. Jetter; Vina, L.; Dickerson, J.; Camino, F.; Mendez, E. E.

    1999-01-01

    We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\\sim 4$ meV to zero, at a field of $\\sim $35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separat...

  2. Exciton-related nonlinear optical response and photoluminescence in dilute nitrogen In{sub x}Ga{sub 1?x}N{sub y}As{sub 1?y}/GaAs cylindrically shaped quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Duque, C.M.; Morales, A.L. [Grupo de Materia Condensada-UdeA, Instituto de Fsica, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medelln (Colombia); Mora-Ramos, M.E. [Grupo de Materia Condensada-UdeA, Instituto de Fsica, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medelln (Colombia); Facultad de Ciencias, Universidad Autnoma del Estado de Morelos, Av. Universidad 1001, CP 62209 Cuernavaca, Morelos (Mexico); Duque, C.A. [Grupo de Materia Condensada-UdeA, Instituto de Fsica, Facultad de Ciencias Exactas y Naturales, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medelln (Colombia)

    2014-10-15

    An investigation of the effects of the dilute nitrogen contents in the exciton states of cylindrical In{sub x}Ga{sub 1?x}N{sub y}As{sub 1?y}/GaAs quantum dots is presented. The exciton states in the system are obtained within the effective mass theory and the band anti-crossing model. Exciton-related nonlinear optical absorption and refractive index change, as well as excitonic photoluminescence are studied with the help of the calculated exciton states. - Highlights: Theoretical study of excitons in cylindrical In{sub x}Ga{sub 1?x}N{sub y}As{sub 1?y}/GaAs quantum dots. Calculations of binding energy for different configurations of electron-hole pairs. Nonlinear optical absorption and refractive index changes. Dependence of photoluminescence energy transitions with several inputs.

  3. Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films.

    Science.gov (United States)

    Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E; Van de Walle, Chris G; Nguyen, Nhan V; Zhang, Yong-Wei; Cao, Linyou

    2015-01-01

    We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075

  4. Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

    Science.gov (United States)

    Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; Van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou

    2015-01-01

    We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 57 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 57 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters. PMID:26598075

  5. Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

    CERN Document Server

    Yu, Yiling; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E; Van de Walle, Chris G; Nguyen, Nhan V; Zhang, Yong-Wei; Cao, Linyou

    2015-01-01

    We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectri...

  6. Impurity trapped exciton states related to rare earth ions in crystals under high hydrostatic pressure

    International Nuclear Information System (INIS)

    Emission related to rare earth ions in solids takes place usually due to 4fn ? 4fn and 4fn?15d1 ? 4fn internal transitions. In the case of band to band excitation the effective energy transfer from the host to optically active impurity is required. Among other processes one of the possibilities is capturing of the electron at excited state and hole at the ground state of impurity. Localization of electron or hole at the dopand site creates a long range Coulomb potential that attracts the second carrier which then occupies the localized Rydberg-like states. Such a system can be considered as impurity trapped exciton. Usually impurity trapped exciton is a short living phenomenon which decays non-radiatively leaving the impurity ion in the excited state. However, in several compounds doped with Eu2+ the impurity trapped exciton states become stable and contribute to the radiative processes though anomalous luminescence that appears apart of the 4f7 ? 4f7 and 4f75d1 ? 5f7 emission. In this contribution pressure effect on energies of the 4fn?15d1?5fn transitions in Ln doped oxides and fluorides as well as influence of pressure on the energy of impurity trapped exciton states is discussed. The latest results on high pressure investigations of luminescence related to Pr3+, and Eu2+ in different lattices are reviewed.

  7. Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

    Science.gov (United States)

    Yu, Yiling; Yu, Yifei; Cai, Yongqing; Li, Wei; Gurarslan, Alper; Peelaers, Hartwin; Aspnes, David E.; van de Walle, Chris G.; Nguyen, Nhan V.; Zhang, Yong-Wei; Cao, Linyou

    2015-11-01

    We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 57 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 57 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

  8. Exciton pumping across type-I gallium chalcogenide heterojunctions.

    Science.gov (United States)

    Cai, Hui; Kang, Jun; Sahin, Hasan; Chen, Bin; Suslu, Aslihan; Wu, Kedi; Peeters, Francois; Meng, Xiuqing; Tongay, Sefaattin

    2016-02-12

    Quasi-two-dimensional gallium chalcogenide heterostructures are created by transferring exfoliated few-layer GaSe onto bulk GaTe sheets. Luminescence spectroscopy measurements reveal that the light emission from underlying GaTe layers drastically increases on heterojunction regions where GaSe layers make contact with the GaTe. Density functional theory (DFT) and band offset calculations show that conduction band minimum (CBM) (valance band maximum (VBM)) values of GaSe are higher (lower) in energy compared to GaTe, forming type-I band alignment at the interface. Consequently, GaSe layers provide photo-excited electrons and holes to GaTe sheets through relatively large built-in potential at the interface, increasing overall exciton population and light emission from GaTe. Observed results are not specific to the GaSe/GaTe system but observed on GaS/GaSe heterolayers with type-I band alignment. Observed experimental findings and theoretical studies provide unique insights into interface effects across dissimilar gallium chalcogenides and offer new ways to boost optical performance by simple epitaxial coating. PMID:26759069

  9. Can disorder enhance incoherent exciton diffusion?

    CERN Document Server

    Lee, Elizabeth M Y; Willard, Adam P

    2015-01-01

    Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we present a general model, based upon F\\"orster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific diffusivity is broadened in a manner that results i...

  10. Exciton in closed and opened quantum dot

    Directory of Open Access Journals (Sweden)

    M.V.Tkach

    2007-01-01

    Full Text Available The theory of exciton spectrum in spherically symmetric states for the three- shell closed spherical quantum dot is proposed. The evolution of the exciton spectrum while varying the outer well thickness from zero (stationary spectrum of single closed spherical quantum dot to infinity (quasistationary spectrum of a single open spherical quantum dot is investigated. The mechanism of damping (semiwidth of quasistationary states due to the redistribution over the energy levels of probability of exciton location in the space of two inner shells of nanosystem is studied. It is shown that the three shell closed spherical quantum dot of a rather big thickness of the outer well quite sufficiently and exactly reflects the basic properties of the quasistationary exciton spectrum in a single open spherical quantum dot.

  11. Cavity-Enhanced Transport of Excitons

    Science.gov (United States)

    Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido

    2015-05-01

    We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.

  12. Triplet excitons: Bringing dark states to light

    Science.gov (United States)

    Bardeen, Christopher J.

    2014-11-01

    Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.

  13. Topologically protected excitons in porphyrin thin films

    Science.gov (United States)

    Yuen-Zhou, Joel; Saikin, Semion K.; Yao, Norman Y.; Aspuru-Guzik, Aln

    2014-11-01

    The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.

  14. Exciton in type-II quantum dot

    Energy Technology Data Exchange (ETDEWEB)

    Sierra-Ortega, J; Escorcia, R A [Universidad del Magdalena, A. A. 731, Santa Marta (Colombia); Mikhailov, I D, E-mail: jsierraortega@gmail.co [Universidad Industrial de Santander, A. A. 678, Bucaramanga (Colombia)

    2009-05-01

    We study the quantum-size effect and the influence of the external magnetic field on the exciton ground state energy in the type-II InP quantum disk, lens and pyramid deposited on a wetting layer and embedded in a GaInP matrix. We show that the charge distribution over and below quantum dot and wetting layer induced by trapped exciton strongly depends on the quantum dot morphology and the strength of the magnetic field.

  15. Exciton in closed and opened quantum dot

    OpenAIRE

    M.V. Tkach; Ju.O. Seti

    2007-01-01

    The theory of exciton spectrum in spherically symmetric states for the three- shell closed spherical quantum dot is proposed. The evolution of the exciton spectrum while varying the outer well thickness from zero (stationary spectrum of single closed spherical quantum dot) to infinity (quasistationary spectrum of a single open spherical quantum dot) is investigated. The mechanism of damping (semiwidth) of quasistationary states due to the redistribution over the energy levels of probability o...

  16. Reference spectroscopic data for hydrogen halides, Part II: The line lists

    Science.gov (United States)

    Li, Gang; Gordon, Iouli E.; Hajigeorgiou, Photos G.; Coxon, John A.; Rothman, Laurence S.

    2013-11-01

    Accurate spectroscopic parameters for the hydrogen halides, namely HF, HCl, HBr, and HI, together with their deuterated isotopologues, are crucial for the quantitative study of terrestrial and planetary atmospheres, astrophysical objects, and chemical lasers. A thorough evaluation of all the hydrogen halide line parameters in previous HITRAN editions has been carried out. A new set of line lists was generated for the HITRAN2012 edition using methods described here. In total, 131,798 entries were generated for numerous pure-rotational and ro-vibrational transitions (fundamental, overtone, and hot bands) for hydrogen halides and their deuterated species in a standard HITRAN 160-character format. Data for the deuterated isotopologues have been entered into HITRAN for the first time. The calculations employ the recently developed semi-empirical dipole moment functions [Li G, et al. J Quant Spectrosc Radiat Transfer 2013;121:78-90] and very accurate analytical potential energy functions and associated functions characterizing Born-Oppenheimer breakdown effects. Line-shape parameters have also been updated using the most recent available experimental and theoretical studies. Comparison with the previous HITRAN compilation has shown significant improvements.

  17. Exciton hopping probed by picosecond time-resolved cathodoluminescence

    Science.gov (United States)

    Shahmohammadi, Mehran; Jacopin, Gwnol; Fu, Xuewen; Ganire, Jean-Daniel; Yu, Dapeng; Deveaud, Benot

    2015-10-01

    The exciton transport is studied in high quality ZnO microwires using time resolved cathodoluminescence. Owing to the available picosecond temporal and nanometer spatial resolution, a direct estimation of the exciton average speed has been measured. When raising the temperature, a strong decrease of the effective exciton mobility (hopping speed of donor-bound excitons) has been observed in the absence of any remarkable change in the effective lifetime of excitons. Additionally, the exciton hopping speed was observed to be independent of the strain gradient value, revealing the hopping nature of exciton movement. These experimental results are in good agreement with the behavior predicted for impurity-bound excitons in our previously published theoretical model based on Monte-Carlo simulations, suggesting the hopping process as the main transport mechanism of impurity-bound excitons at low temperatures.

  18. Excitonic nonlinearities in single-wall carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Nguyen, D.T.; Voisin, C.; Roussignol, P. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Roquelet, C.; Lauret, J.S. [Laboratoire de Photonique Quantique et Moleculaire de l' Ecole Normale Superieure de Cachan (France); Cassabois, G. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Laboratoire Charles Coulomb, UMR5221, Universite Montpellier 2, Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, Montpellier (France)

    2012-05-15

    Excitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the exciton-exciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single-photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient exciton-exciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral-hole burning experiments bring evidence of pure dephasing induced by exciton-exciton scattering (EES) in the k-space. This mechanism controls the exciton collision-induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  19. Observation of structural relaxation during exciton self-trapping via excited-state resonant impulsive stimulated Raman spectroscopy

    International Nuclear Information System (INIS)

    We detect the change in vibrational frequency associated with the transition from a delocalized to a localized electronic state using femtosecond vibrational wavepacket techniques. The experiments are carried out in the mixed-valence linear chain material [Pt(en)2][Pt(en)2Cl2]⋅(ClO4)4 (en = ethylenediamine, C2H8N2), a quasi-one-dimensional system with strong electron-phonon coupling. Vibrational spectroscopy of the equilibrated self-trapped exciton is carried out using a multiple pulse excitation technique: an initial pump pulse creates a population of delocalized excitons that self-trap and equilibrate, and a time-delayed second pump pulse tuned to the red-shifted absorption band of the self-trapped exciton impulsively excites vibrational wavepacket oscillations at the characteristic vibrational frequencies of the equilibrated self-trapped exciton state by the resonant impulsive stimulated Raman mechanism, acting on the excited state. The measurements yield oscillations at a frequency of 160 cm−1 corresponding to a Raman-active mode of the equilibrated self-trapped exciton with Pt-Cl stretching character. The 160 cm−1 frequency is shifted from the previously observed wavepacket frequency of 185 cm−1 associated with the initially generated exciton and from the 312 cm−1 Raman-active symmetric stretching mode of the ground electronic state. We relate the frequency shifts to the changes in charge distribution and local structure that create the potential that stabilizes the self-trapped state

  20. Generation of multiple excitons in Ag2S quantum dots: Single high-energy versus multiple-photon excitation

    KAUST Repository

    Sun, Jingya

    2014-02-20

    We explored biexciton generation via carrier multiplication (or multiple-exciton generation) by high-energy photons and by multiple-photon absorption in Ag2S quantum dots (QDs) using femtosecond broad-band transient absorption spectroscopy. Irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S QDs, two distinct characteristic time constants of 9.6-10.2 and 135-175 ps are obtained for the nonradiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely, tightly bound and weakly bound excitons. More importantly, the lifetimes of multiple excitons in Ag 2S QDs were about 1 and 2 orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices. © 2014 American Chemical Society.

  1. Resonance Raman spectra of metal halide vapor complexes

    International Nuclear Information System (INIS)

    Resonance Raman spectra of complex vapor phase compounds formed by reacting ''acidic'' gases (A2X6 = Al2Cl6, Al2Br6, In2Cl6) with metal halides have been measured. Spectra obtained from equilibrium vapor mixtures of A2X6 over solid MX2 (= PdCl2, PdBr2, CuCl2, CoBr2, TiCl2, FeCl2, NiCl2, PtCl2) were a superposition of the A2X6-AX3 bands and in few cases of new resonance-enhanced polarized bands due to MA2X8 and/or MAX5 complexes. At temperatures above 8000K, characteristic bands due to MX2(g) (M = Fe, Co, Ni, Cu, Zn) and M2X4(g) (M = Cu) were observed. The predominant features of the PdAl2Cl8, CuAl2Cl8, and PdAl2Br6 spectra were three high-intensity, polarized bands which were attributed to the vibrational modes of the complex coupled to the electronic state of the central atom. The spectra of CuAlCl5(g), CuInCl5(g) and Cu2Cl4(g) species showed resonance enhancement of selective fundamentals which were attributed to vibrational modes of trigonally coordinated Cu(II). Resonance Raman spectra of U2Cl10(g) and UCl5.AlCl3(g) were characterized by the presence of a strong band attributed to the U-Cl/sub t/ stretching frequency. Raman band intensity measurements were carried out for the iron(III) chloride vapors and for the vapor complexes of CuAl2Cl8, CuInCl5 and UCl5.AlCl3 using different laser powers and frequencies. The measurements suggested increasing spectroscopic temperatures and decomposition of the vapor complexes. The data are discussed in terms of the distribution of vibrational modes and the structure of the vapor species. 22 figs

  2. Gastric Banding

    Science.gov (United States)

    ... Radiation-Emitting Products Vaccines, Blood & Biologics Animal & Veterinary Cosmetics Tobacco ... Banding Benefits of Gastric Banding Lifestyle Changes after Gastric Banding Surgery Gastric banding is a weight loss option for ...

  3. Metal halide reduction with molten sodium/potassium alloy

    International Nuclear Information System (INIS)

    A method of obtaining a desired metal, selected from the group consisting of titanium, aluminium, iron, manganese, hafnium, zirconium, tantalum, vanadium, uranium and tungsten, which comprises reacting a halide of the desired metal with an alkali metal reducing agent at temperature at which the reducing agent is molten, in order to produce the desired metal and halide of the metal reducing agent

  4. The coacervation of aqueous solutions of tetraalkylammonium halides

    International Nuclear Information System (INIS)

    The coacervation of aqueous solutions of tatraalkylammonium halides in the presence of not of inorganic halides and acids has been studied, considering thermodynamic and spectroscopic aspects. The importance of dispersion forces as well as forces resulting from hydrophobic hydration has been assessed. The analogy between these systems and anionic ion exchange resins has been shown especially for Uranium VI extraction

  5. Double-Diffusive Convection During Growth of Halides and Selenides

    Science.gov (United States)

    Singh, N. B.; Su, Ching-Hua; Duval, Walter M. B.

    2015-01-01

    Heavy metal halides and selenides have unique properties which make them excellent materials for chemical, biological and radiological sensors. Recently it has been shown that selenohalides are even better materials than halides or selenides for gamma-ray detection. These materials also meet the strong needs of a wide band imaging technology to cover ultra-violet (UV), midwave infrared wavelength (MWIR) to very long wavelength infrared (VLWIR) region for hyperspectral imager components such as etalon filters and acousto-optic tunable filters (AO). In fact AOTF based imagers based on these materials have some superiority than imagers based on liquid crystals, FTIR, Fabry-Perot, grating, etalon, electro-optic modulation, piezoelectric and several other concepts. For example, broadband spectral and imagers have problems of processing large amount of information during real-time observation. Acousto-Optic Tunable Filter (AOTF) imagers are being developed to fill the need of reducing processing time of data, low cost operation and key to achieving the goal of covering long-wave infrared (LWIR). At the present time spectral imaging systems are based on the use of diffraction gratings are typically used in a pushbroom or whiskbroom mode. They are mostly used in systems and acquire large amounts of hyperspectral data that is processed off-line later. In contrast, acousto-optic tunable filter spectral imagers require very little image processing, providing new strategies for object recognition and tracking. They are ideally suited for tactical situations requiring immediate real-time image processing. But the performance of these imagers depends on the quality and homogeneity of acousto-optic materials. In addition for many systems requirements are so demanding that crystals up to sizes of 10 cm length are desired. We have studied several selenides and halide crystals for laser and AO imagers for MWIR and LWIR wavelength regions. We have grown and fabricated crystals of several materials such as mercurous chloride, mercurous bromide, mercurous iodide, lead chloride lead bromide, lead iodide, thallium arsenic selenide, gallium selenide, zince sulfide zinc selenide and several crystals into devices. We have used both Bridgman and physical vapor transport (PVT) crystal growth methods. In the past have examined PVT growth numerically for conditions where the boundary of the enclosure is subjected to a nonlinear thermal profile. Since past few months we have been working on binary and ternary materials such as selenoiodides, doped zinc sulfides and mercurous chloro bromide and mercurous bromoiodides. In the doped and ternary materials thermal and solutal convection play extremely important role during the growth. Very commonly striations and banding is observed. Our experiments have indicated that even in highly purified source materials, homogeneity in 1-g environment is very difficult. Some of our previous numerical studies have indicated that gravity level less than 10-4 (?-g) helps in controlling the thermosolutal convection. We will discuss the ground based growth results of HgClxBr(1-x) and ZnSe growth results for the mm thick to large cm size crystals. These results will be compared with our microgravity experiments performed with this class of materials. For both HgCl-HgBr and ZnS-ZnSe the lattice parameters of the mixtures obey Vagard's law in the studied composition range. The study demonstrates that properties are very anisotropic with crystal orientation, and performance achievement requires extremely careful fabrication to utilize highest figure of merit. In addition, some parameters such as crystal growth fabrication, processing time, resolution, field of view and efficiency will be described based on novel solid solution materials. It was predicted that very similar to the pure compounds solid solutions also have very large anisotropy, and very precise oriented and homogeneous bulk and thin film crystals is required to achieve maximum performance of laser or imagers. Some of the parameters controlling the homogeneity su

  6. Calcium manganate: A promising candidate as buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems

    Energy Technology Data Exchange (ETDEWEB)

    Zhao, Pengjun; Wang, Hongguang; Kong, Wenwen [Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011 (China); University of Chinese Academy of Sciences, Beijing 100049 (China); Xu, Jinbao, E-mail: xujb@ms.xjb.ac.cn; Wang, Lei; Ren, Wei; Bian, Liang; Chang, Aimin [Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011 (China)

    2014-11-21

    We have systematically studied the feasibility of CaMnO{sub 3} thin film, an n-type perovskite, to be utilized as the buffer layer for hybrid halide perovskite photovoltaic-thermoelectric device. Locations of the conduction band and the valence band, spontaneous polarization performance, and optical properties were investigated. Results indicate the energy band of CaMnO{sub 3} can match up well with that of CH{sub 3}NH{sub 3}PbI{sub 3} on separating electron-hole pairs. In addition, the consistent polarization angle helps enlarge the open circuit voltage of the composite system. Besides, CaMnO{sub 3} film shows large absorption coefficient and low extinction coefficient under visible irradiation, demonstrating high carrier concentration, which is beneficial to the current density. More importantly, benign thermoelectric properties enable CaMnO{sub 3} film to assimilate phonon vibration from CH{sub 3}NH3PbI{sub 3}. All the above features lead to a bright future of CaMnO{sub 3} film, which can be a promising candidate as a buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems.

  7. Calcium manganate: A promising candidate as buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems

    International Nuclear Information System (INIS)

    We have systematically studied the feasibility of CaMnO3 thin film, an n-type perovskite, to be utilized as the buffer layer for hybrid halide perovskite photovoltaic-thermoelectric device. Locations of the conduction band and the valence band, spontaneous polarization performance, and optical properties were investigated. Results indicate the energy band of CaMnO3 can match up well with that of CH3NH3PbI3 on separating electron-hole pairs. In addition, the consistent polarization angle helps enlarge the open circuit voltage of the composite system. Besides, CaMnO3 film shows large absorption coefficient and low extinction coefficient under visible irradiation, demonstrating high carrier concentration, which is beneficial to the current density. More importantly, benign thermoelectric properties enable CaMnO3 film to assimilate phonon vibration from CH3NH3PbI3. All the above features lead to a bright future of CaMnO3 film, which can be a promising candidate as a buffer layer for hybrid halide perovskite photovoltaic-thermoelectric systems

  8. Excitonic polariton-polariton scattering in CdS crystals

    International Nuclear Information System (INIS)

    Photoluminescence (PL) of US single crystals, its dependence on the excitation power density, and anti-Stokes PL from the ground and excited free-exciton states are studied at 1.6 K. Investigations of PL from the excited exciton states, anti-Stokes PL of free excitons, depolarization of a line of the free A(1s)-exciton in the PL spectrum, half widths of free excitons, and the transfer of the PL intensity to higher excited states with increasing excitation power density suggest that excitonic polariton-polariton scattering occurs in US crystals. 8 refs., 5 figs

  9. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jrn Mrcher; Bacher, G.; Kmmell, T.; Forchel, A.

    1998-01-01

    The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering within the wire due to the electron trapping in surface states and exciton localization. The exciton-exciton scattering efficiency, determined by the density dependence of the exciton dephasing, is foun...

  10. Exciton correlations and input-output relations in non-equilibrium exciton superfluids

    Energy Technology Data Exchange (ETDEWEB)

    Ye, Jinwu, E-mail: jy306@ccs.msstate.edu [Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University, Beijing 100048 (China); Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Sun, Fadi; Yu, Yi-Xiang [Department of Physics and Astronomy, Mississippi State University, MS 39762 (United States); Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China); Liu, Wuming [Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 (China)

    2013-02-15

    The photoluminescence (PL) measurements on photons and the transport measurements on excitons are the two types of independent and complementary detection tools to search for possible exciton superfluids in electron-hole semi-conductor bilayer systems. In fact, it was believed that the transport measurements can provide more direct evidences on superfluids than the spectroscopic measurements. It is important to establish the relations between the two kinds of measurements. In this paper, using quantum Heisenberg-Langevin equations, we establish such a connection by calculating various exciton correlation functions in the putative exciton superfluids. These correlation functions include both normal and anomalous greater, lesser, advanced, retarded, and time-ordered exciton Green functions and also various two exciton correlation functions. We also evaluate the corresponding normal and anomalous spectral weights and the Keldysh distribution functions. We stress the violations of the fluctuation and dissipation theorem among these various exciton correlation functions in the non-equilibrium exciton superfluids. We also explore the input-output relations between various exciton correlation functions and those of emitted photons such as the angle resolved photon power spectrum, phase sensitive two mode squeezing spectrum and two photon correlations. Applications to possible superfluids in the exciton-polariton systems are also mentioned. For a comparison, using conventional imaginary time formalism, we also calculate all the exciton correlation functions in an equilibrium dissipative exciton superfluid in the electron-electron coupled semi-conductor bilayers at the quantum Hall regime at the total filling factor {nu}{sub T}=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: Black-Right-Pointing-Pointer Establish the relations between photoluminescence and transport measurements. Black-Right-Pointing-Pointer Stress the violations of the fluctuation-dissipation theorem in non-equilibrium systems. Black-Right-Pointing-Pointer Explore the input-output relations between excitons and photons. Black-Right-Pointing-Pointer Discuss differences between non-equilibrium systems and dissipative ones.

  11. Color separation in metal halide lamps

    Science.gov (United States)

    Stoffels, W. W.; Nimalasuriya, T.; Flikweert, A. J.; Brok, W. J. M.; Mullen, J. J. A. M.; Kroesen, G. M. W.; Haverlag, M.

    2006-10-01

    Metal halide discharge lamps are efficient lighting sources. However their widespread application is hindered by several problems. One problem is color separation. This is caused by a non-homogeneous distribution of radiating species within the lamp. It is believed to be the result of a complex interplay between diffusion and convection processes. In this contribution convection in the lamp is varied by placing the lamp in a rotating centrifuge. The resulting centrifugal force of up to ten times the normal gravitational force enhances the convection within the lamp and allows studying its effect on the color separation.

  12. Ground state properties of heavy alkali halides

    OpenAIRE

    Doll, Klaus; Stoll, Hermann

    1998-01-01

    We extend previous work on alkali halides by calculations for the heavy-atom species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic effects are included by means of energy-consistent pseudopotentials, correlations are treated at the coupled-cluster level. A striking deficiency of the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from experimental values which is reduced to a maximum error of 2.4 % by taking into account electron ...

  13. Low absorption state of phycocyanin from Acaryochloris marina antenna system: On the interplay between ionic strength and excitonic coupling

    Science.gov (United States)

    Nganou, Collins

    2013-07-01

    This paper studies the excitonic factor in the excited state energy transfer of phycobilisome (PBS) by using a polarized time-resolved pump-probe and by changing the ionic strength of the cofactors' medium in the PBS of Acaryochloris marina (A. marina). As a result, the interplay between the surrounding medium and the closely excited adjacent cofactors is shown to be a negligible factor of the excitonic decay kinetics at 618 nm of the phycocyanin (PC), while it appears as a driving factor of an increase in excitonic delocalization at 630 nm. The obtained anisotropy values are consistent with the contribution of ionic strength in the excitonic mechanism in PBS. These values were 0.38 in high ionic strength and 0.4 in low ionic strength at 618 nm, and 0.52 in high ionic strength and 0.4 in low ionic strength at 630-635 nm. The anisotropy value of 0.52 in high phosphate is similar at 630 nm and 635 nm, which is consistent with an excitonic delocalization band at 635 nm. The 635 nm band is suggested to show the true low energy level of PC in A. marina PBS. The anisotropy decay kinetic at 630 nm suggests that the excited state population of PC is not all equilibrated in 3 ps because of the existence of the 10 ps decay kinetic component. The presence of the slow kinetic decay component in high, and low ionic strength, is consistent with a 10 and 14 ps energy transfer pathway, while the 450 fs kinetic decay component is consistent with the presence of an additional excitation energy transfer pathway between adjacent ?84 and ?84. Furthermore, the 450 fs decay kinetic is suggested to be trapped in the trimer, while the 400 fs decay kinetic rules out an excitonic flow from low energy level PC to allophycoyanin. This excitonic flow may occur between ?84 in adjacent trimers, towards the low energy state of the PBS rod.

  14. Exciton interference revealed by energy dependent exciton transfer rate for ring-structured molecular systems.

    Science.gov (United States)

    Yan, Yun-An

    2016-01-14

    The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today's nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored. PMID:26772569

  15. Exciton effects in the index of refraction of multiple quantum wells and superlattices

    Science.gov (United States)

    Kahen, K. B.; Leburton, J. P.

    1986-01-01

    Theoretical calculations of the index of refraction of multiple quantum wells and superlattices are presented. The model incorporates both the bound and continuum exciton contributions for the gamma region transitions. In addition, the electronic band structure model has both superlattice and bulk alloy properties. The results indicate that large light-hole masses, i.e., of about 0.23, produced by band mixing effects, are required to account for the experimental data. Furthermore, it is shown that superlattice effects rapidly decrease for energies greater than the confining potential barriers. Overall, the theoretical results are in very good agreement with the experimental data and show the importance of including exciton effects in the index of refraction.

  16. Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

    Science.gov (United States)

    Tran, Vy; Fei, Ruixiang; Yang, Li

    2015-12-01

    We report first-principles GWBetheSalpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G0W0) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlettriplet splitting, and electronhole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electronhole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.

  17. Manipulation of exciton and photon lasing in a membrane-type ZnO microcavity

    Energy Technology Data Exchange (ETDEWEB)

    Lai, Ying-Yu; Chen, Jee-Wei; Chang, Tsu-Chi; Lu, Tien-Chang, E-mail: timtclu@mail.nctu.edu.tw [Department of Photonics, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan (China); Chou, Yu-Hsun [Department of Photonics, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan (China); Institute of Lighting and Energy Photonics, National Chiao Tung University, 301 Gaofa 3rd Road, Guiren District, Tainan 71150, Taiwan (China)

    2015-03-30

    We report on the fabrication and characterization of a membrane-type ZnO microcavity (MC). The ZnO membrane was cut from a single crystalline ZnO substrate by using focused ion beam milling, and was then placed onto a SiO{sub 2} substrate by using glass microtweezers. Through changing the pumping regime, manipulation of P-band exciton lasing and whispering-gallery mode (WGM) photon lasing could be easily achieved. P-band exciton lasing was observed only when the pumping laser was focused at the center of the ZnO MC with a small pumping size because of the innate ring-shaped WGM distribution. Furthermore, the lasing threshold of the ZnO MC could be reduced to an order lower by using a larger pumping spot because of the more favorable spatial overlap between the optical gain and WGM.

  18. Ultrafast Exciton Fine Structure Relaxation Dynamics in Lead Chalcogenide Nanocrystals

    International Nuclear Information System (INIS)

    The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes

  19. Ultrafast exciton fine structure relaxation dynamics in lead chalcogenide nanocrystals.

    Science.gov (United States)

    Johnson, Justin C; Gerth, Kathrine A; Song, Qing; Murphy, James E; Nozik, Arthur J; Scholes, Gregory D

    2008-05-01

    The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes. PMID:18376866

  20. Ultrafast Exciton Fine Structure Relaxation Dynamics in Lead Chalcogenide Nanocrystals

    Energy Technology Data Exchange (ETDEWEB)

    Johnson, J. C.; Gerth, K. A.; Song, Q.; Murphy, J. E.; Nozik, A. J.; Scholes, G. D.

    2008-01-01

    The rates of fine structure relaxation in PbS, PbSe, and PbTe nanocrystals were measured on a femtosecond time scale as a function of temperature with no applied magnetic field by cross-polarized transient grating spectroscopy (CPTG) and circularly polarized pump-probe spectroscopy. The relaxation rates among exciton fine structure states follow trends with nanocrystal composition and size that are consistent with the expected influence of material dependent spin-orbit coupling, confinement enhanced electron-hole exchange interaction, and splitting between L valleys that are degenerate in the bulk. The size dependence of the fine structure relaxation rate is considerably different from what is observed for small CdSe nanocrystals, which appears to result from the unique material properties of the highly confined lead chalcogenide quantum dots. Modeling and qualitative considerations lead to conclusions about the fine structure of the lowest exciton absorption band, which has a potentially significant bearing on photophysical processes that make these materials attractive for practical purposes.

  1. Femtosecond dynamics of exciton localization: self-trapping from the small to the large polaron limit

    International Nuclear Information System (INIS)

    We use femtosecond vibrational wavepacket techniques to time-resolve the coupled electronic and vibrational dynamics of exciton self-trapping in a series of materials in which the relative strength of the electronphonon coupling can be compositionally tuned from the small to the large polaron limit. Transient absorption experiments are carried out in the quasi-one-dimensional halide-bridged mixed-valence transition metal linear chain complexes [Pt(en)2][Pt(en)2X2]?(ClO4)4 (en=ethylenediamine, C2H8N2) with X=Cl, Br and I. In each complex, we detect the formation of the self-trapped exciton through the appearance of its characteristic red-shifted optical absorption, and find that self-trapping occurs on a time scale of the order of a single vibrational period of the optical phonon mode that dominates the self-trapping dynamics. The associated optical phonon response, detected as wavepacket oscillations that modulate the exciton absorption, shows a significant softening of the optical phonon frequency compared to that of the unexcited system. The degree of softening is found to vary significantly with coupling strength, ranging from more than 40% in the strongly coupled chloride-bridged complex to less than 20% in the weakly coupled iodide-bridged complex. We relate these results to the extent of electronic delocalization by comparison with the electronic properties of the ground states of the materials and with the properties of their equilibrated self-trapped electronic states predicted by theoretical modeling. (paper)

  2. Stacking in colloidal nanoplatelets: tuning excitonic properties.

    Science.gov (United States)

    Guzelturk, Burak; Erdem, Onur; Olutas, Murat; Kelestemur, Yusuf; Demir, Hilmi Volkan

    2014-12-23

    Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield, while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultraefficient Frster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultralong range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultraefficient, reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Frster radius of ?13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in nonemissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. The rate-equation-based model, which considers the exciton transfer and the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in colloidal nanocrystals due to the absence of inhomogeneous broadening. PMID:25469555

  3. Unexpectedly Slow Two Particle Decay of Ultra-Dense Excitons in Cuprous Oxide

    CERN Document Server

    Frazer, Laszlo; Ketterson, J B

    2013-01-01

    For an ultra-dense exciton gas in cuprous oxide (Cu$_2$O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

  4. Temperature Effect on Exciton Absorption of CuBr Nanocrystals in Potassium-Alumina-Borate Glass

    Directory of Open Access Journals (Sweden)

    Babkina A.N.

    2013-09-01

    Full Text Available The paper describes the research of temperature effect of potassium-alumina-borate (PAB glass with CuBr nanocrystals, which was obtained in optical wavelength region for the first time. Temperature dependence of optical density at different wavelengths was examined. Melting and crystallization temperatures were evaluated for different nanocrystals sizes. Great changes of exciton absorption band influenced by temperatures below 100C were recorded. Propositions for studied glass application as tunable filters in electro-optic circuit were suggested.

  5. Communication: Strong excitonic and vibronic effects determine the optical properties of Li?O?

    DEFF Research Database (Denmark)

    Garca Lastra, Juan Maria; Bass, J. D.; Thygesen, Kristian Sommer

    2011-01-01

    The band structure and optical absorption spectrum of lithium peroxide (Li2O2) is calculated from first-principles using the G0W0 approximation and the Bethe-Salpeter equation, respectively. A strongly localized (Frenkel type) exciton corresponding to the ?*??* transition on the O2 ?2 peroxide io...... of the high potential losses and low current densities, which are presently limiting the performance of Li-air batteries....

  6. Charge tuning of non-resonant magneto-exciton phonon interactions in graphene

    OpenAIRE

    Rmi, Sebastian; Goldberg, Bennett B; Swan, Anna K.

    2013-01-01

    Far from resonance, the coupling of the G-band phonon to magneto-excitons in single layer graphene displays kinks and splittings versus filling factor that are well described by Pauli blocking and unblocking of inter- and intra- Landau level transitions. We explore the non-resonant electron-phonon coupling by high-magnetic field Raman scattering while electrostatic tuning of the carrier density controls the filling factor. We show qualitative and quantitative agreement betwe...

  7. Exciton magnetic polarons in Cd1-xMnxTe quantum wells

    International Nuclear Information System (INIS)

    We study the exciton magnetic polaron system in semimagnetic quantum wells. The exact numerical solution of a non-linear Wannier equation describing this system allow us to analyse its stability as a function of temperature, well width and band offset. We find a decrease in the polaron energy with increasing temperature and/or well width. The calculated polaron properties are in good agreement with recent experimental results. (author)

  8. Strong localization induced anomalous temperature dependence exciton emission above 300?K from SnO2 quantum dots

    International Nuclear Information System (INIS)

    SnO2 quantum dots (QDs) are potential materials for deep ultraviolet (DUV) light emitting devices. In this study, we report the temperature and excitation power-dependent exciton luminescence from SnO2 QDs. The exciton emission exhibits anomalous blue shift, accompanied with band width reduction with increasing temperature and excitation power above 300?K. The anomalous temperature dependences of the peak energy and band width are well interpreted by the strongly localized carrier thermal hopping process and Gaussian shape of band tails states, respectively. The localized wells and band tails at conduction minimum are considered to be induced by the surface oxygen defects and local potential fluctuation in SnO2 QDs

  9. Engineering excitonic dynamics and environmental stability of post-transition metal chalcogenides by pyridine functionalization technique

    Science.gov (United States)

    Meng, Xiuqing; Pant, Anupum; Cai, Hui; Kang, Jun; Sahin, Hasan; Chen, Bin; Wu, Kedi; Yang, Sijie; Suslu, Aslihan; Peeters, F. M.; Tongay, Sefaattin

    2015-10-01

    Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations, we attribute these to formation of bound exciton complexes at the trap sites introduced by chemical reaction between pyridine and GaSe. On the contrary, pyridine functionalization does not impact the optical properties of GaTe, instead treats GaTe surface to prevent oxidization of tellurium atoms. Overall, results suggest novel ways to control properties of gallium chalcogenides on demand and unleash their full potential for a range of applications in photonics and optoelectronics.Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations, we attribute these to formation of bound exciton complexes at the trap sites introduced by chemical reaction between pyridine and GaSe. On the contrary, pyridine functionalization does not impact the optical properties of GaTe, instead treats GaTe surface to prevent oxidization of tellurium atoms. Overall, results suggest novel ways to control properties of gallium chalcogenides on demand and unleash their full potential for a range of applications in photonics and optoelectronics. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr04879f

  10. Electron transport mechanism of bathocuproine exciton blocking layer in organic photovoltaics.

    Science.gov (United States)

    Lee, Jeihyun; Park, Soohyung; Lee, Younjoo; Kim, Hyein; Shin, Dongguen; Jeong, Junkyeong; Jeong, Kwangho; Cho, Sang Wan; Lee, Hyunbok; Yi, Yeonjin

    2016-02-10

    Efficient exciton management is a key issue to improve the power conversion efficiency of organic photovoltaics (OPVs). It is well known that the insertion of an exciton blocking layer (ExBL) having a large band gap promotes the efficient dissociation of photogenerated excitons at the donor-acceptor interface. However, the large band gap induces an energy barrier which disrupts the charge transport. Therefore, building an adequate strategy based on the knowledge of the true charge transport mechanism is necessary. In this study, the true electron transport mechanism of a bathocuproine (BCP) ExBL in OPVs is comprehensively investigated by in situ ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, density functional theory calculation, and impedance spectroscopy. The chemical interaction between deposited Al and BCP induces new states within the band gap of BCP, so that electrons can transport through these new energy levels. Localized trap states are also formed upon the Al-BCP interaction. The activation energy of these traps is estimated with temperature-dependent conductance measurements to be 0.20 eV. The Al-BCP interaction induces both transport and trap levels in the energy gap of BCP and their interplay results in the electron transport observed. PMID:26821701

  11. Bound excitons in GaN

    International Nuclear Information System (INIS)

    The electronic structure of bound excitons in GaN is discussed, with reference to available optical data. Emphasis is given to the neutral-donor and neutral-acceptor spectra, which are the most prominent ones in the experimental photoluminescence data. Two dominant donor bound excitons are observed with photoluminescence lines just above 3.47 eV at 2 K in unstrained samples, tentatively associated with Si and O shallow donors. Several acceptor bound excitons are present; the most prominent one with a photoluminescence line at about 3.466 eV is tentatively assigned to the Mg acceptor. We attempt an explanation of the available data from magneto-optical experiments on this line in terms of a spin-like acceptor hole, as observed in independent magnetic resonance data. Characteristic deep emissions related to P and As doping are reported; they may be interpreted in terms of isoelectronic bound excitons. Excitons bound to structural defects in GaN are also briefly discussed. (author)

  12. Transport of Indirect Excitons in Coupled Quantum Wells

    Science.gov (United States)

    Kuznetsova, Yuliya Yevgenyevna

    This dissertation explores studies of transport of indirect excitons (bosonic quasiparticles composed of bound pairs of an electron and a hole confined to spatially separated layers) in GaAs coupled quantum wells. The small mass and long lifetime of indirect excitons result in a relatively high quantum degeneracy temperature and efficient cooling, making indirect excitons a model system for studies of physics of cold bosons. In addition, indirect excitons are optically active, electronically controllable and have long enough lifetimes that their transport distances can be accommodated by lithography. These properties make indirect excitons a promising system for creating excitonic devices. The direction of the research presented in this dissertation is thus twofold: studying fundamental physics of excitons, including transport, thermalization, coherence, spin currents, and properties in high magnetic fields, and realization of optical excitonic devices, such as traps and transistors.

  13. Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides

    Science.gov (United States)

    Hao, Kai; Moody, Galan; Dass, Chandriker; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berguser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2015-03-01

    Excitons in monolayer transition metal dichalcogenides (TMDCs) exhibit exceptionally large binding energy, strong optical absorption, and spin valley coupling. These characteristics make TMDCs a promising system for optoelectronics and valleytronics. An important yet unknown property of excitons in TMDCs is the intrinsic homogeneous linewidth, which reflect radiative recombination and irreversible dissipative decay. Here, we use optical coherent two-dimensional spectroscopy to reveal the exciton homogeneous linewidth in monolayer CVD grown Tungsten Diselenide (WSe2). With excitation density and temperature dependent measurements, exciton-exciton interaction and exciton-phonon interactions are quantitatively evaluated. Extrapolating to zero density and temperature, we obtain a residual homogeneous linewidth of ~ 1.5 meV, which places a lower bound of 0.2 ps on the exciton radiative lifetime. This result is consistent with microscopic calculations, which suggest that fast radiative decay of delocalized excitons arises from their large oscillator strength. We acknowledge AFOSR and NSF for funding.

  14. Dynamics of indirect exciton transport by moving acoustic fields

    International Nuclear Information System (INIS)

    We report on the modulation of indirect excitons (IXs) as well as their transport by moving periodic potentials produced by surface acoustic waves (SAWs). The potential modulation induced by the SAW strain modifies both the band gap and the electrostatic field in the quantum wells confining the IXs, leading to changes in their energy. In addition, this potential captures and transports IXs over several hundreds of ?m. While the IX packets keep to a great extent their spatial shape during transport by the moving potential, the effective transport velocity is lower than the SAW group velocity and increases with the SAW amplitude. This behavior is attributed to the capture of IXs by traps along the transport path, thereby increasing the IX transit time. The experimental results are well-reproduced by an analytical model for the interaction between trapping centers and IXs during transport. (paper)

  15. Excitons in Time-Dependent Density-Functional Theory.

    Science.gov (United States)

    Ullrich, Carsten A; Yang, Zeng-Hui

    2016-01-01

    This chapter gives an overview of the description of the optical and dielectric properties of bulk insulators and semiconductors in time-dependent density-functional theory (TDDFT), with an emphasis on excitons. We review the linear-response formalism for periodic solids, discuss excitonic exchange-correlation kernels, calculate exciton binding energies for various materials, and compare the treatment of excitons with TDDFT and with the Bethe-Salpeter equation. PMID:25805143

  16. Exciton Mott transition in Si Revealed by Terahertz Spectroscopy

    OpenAIRE

    Suzuki, Takeshi; Shimano, Ryo

    2012-01-01

    Exciton Mott transition in Si is investigated by using terahertz time-domain spectroscopy. The excitonic correlation as manifested by the 1s-2p resonance is observed above the Mott density. The scattering rate of charge carriers is prominently enhanced at the proximity of Mott density, which is attributed to the non-vanishing exciton correlation in the metallic electron-hole plasma. Concomitantly, the signature of plasmon-exciton coupling is observed in the loss function spe...

  17. How bilayer excitons can greatly enhance thermoelectric efficiency

    OpenAIRE

    Wu, Kai; Rademaker, Louk; Zaanen, Jan

    2014-01-01

    Currently, one of the major nanotechnological challenges is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons. Bilayer exciton systems are shown to have an improved thermopower and an enhanced electric counterflow and thermal conductivity, with respect to regular semiconductor-based thermoelectrics. Here we present a roadmap towards experimental realization of a bilayer exciton thermocouple. A bilayer exciton h...

  18. Ultrafast exciton relaxation in quasi-one-dimensional perylene derivatives

    OpenAIRE

    Engel, Egbert

    2006-01-01

    This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addresse...

  19. Probing Bose-Einstein Condensation of Excitons with Electromagnetic Radiation

    OpenAIRE

    Johnsen, K.; Kavoulakis, G. M.

    2000-01-01

    We examine the absorption spectrum of electromagnetic radiation from excitons, where an exciton in the $1s$ state absorbs a photon and makes a transition to the $2p$ state. We demonstrate that the absorption spectrum depends strongly on the quantum degeneracy of the exciton gas, and that it will generally manifest many-body effects. Based on our results we propose that absorption of infrared radiation could resolve recent contradictory experimental results on excitons in Cu$...

  20. Optical microcavities enhance the exciton coherence length and eliminate vibronic coupling in J-aggregates

    International Nuclear Information System (INIS)

    The properties of polaritons in J-aggregate microcavities are explored using a Hamiltonian which treats exciton-vibrational coupling and exciton-photon coupling on equal footing. When the cavity mode is resonant with the lowest-energy (0-0) transition in the J-aggregate, two polaritons are formed, the lowest-energy polariton (LP) and its higher-energy partner (P1), separated by the Rabi splitting. Strong coupling between the material and cavity modes leads to a decoupling of the exciton and vibrational degrees of freedom and an overall reduction of disorder within the LP. Such effects lead to an expanded material coherence length in the LP which leads to enhanced radiative decay rates. Additional spectral signatures include an amplification of the 0-0 peak coincident with a reduction in the 0-1 peak in the photoluminescence spectrum. It is also shown that the same cavity photon responsible for the LP/P1 splitting causes comparable splittings in the higher vibronic bands due to additional resonances between vibrationally excited states in the electronic ground state manifold and higher energy vibronic excitons

  1. Polarization analysis of excitons in monolayer and bilayer transition-metal dichalcogenides

    Science.gov (United States)

    Dery, Hanan; Song, Yang

    2015-09-01

    The polarization analysis of optical transitions in monolayer and bilayer transition-metal dichalcogenides provides invaluable information on the spin and valley (pseudospin) degrees of freedom. To explain optical properties of a given monolayer transition-metal dichalcogenide, one should consider (i) the order of its spin-split conduction bands, (ii) whether intervalley scattering is prone to phonon bottleneck, (iii) and whether valley mixing by electron-hole exchange can take place. Using these principles, we present a consistent physical picture that elucidates a variety of features in the optical spectra of these materials. We explain the differences between optical transitions in monolayer MoSe2 and monolayer WSe2, finding that indirect excitons in the latter correspond to several low-energy optical transitions that so far were attributed to excitons bound to impurities. A possible mechanism that can explain the vanishing polarization in MoSe2 is discussed. Finally, we consider the effect of an out-of-plane electric field, showing that it can reduce the initial polarization of bright excitons due to a Rashba-type coupling with dark excitons.

  2. Optical microcavities enhance the exciton coherence length and eliminate vibronic coupling in J-aggregates

    Energy Technology Data Exchange (ETDEWEB)

    Spano, F. C. [Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 (United States)

    2015-05-14

    The properties of polaritons in J-aggregate microcavities are explored using a Hamiltonian which treats exciton-vibrational coupling and exciton-photon coupling on equal footing. When the cavity mode is resonant with the lowest-energy (0-0) transition in the J-aggregate, two polaritons are formed, the lowest-energy polariton (LP) and its higher-energy partner (P{sub 1}), separated by the Rabi splitting. Strong coupling between the material and cavity modes leads to a decoupling of the exciton and vibrational degrees of freedom and an overall reduction of disorder within the LP. Such effects lead to an expanded material coherence length in the LP which leads to enhanced radiative decay rates. Additional spectral signatures include an amplification of the 0-0 peak coincident with a reduction in the 0-1 peak in the photoluminescence spectrum. It is also shown that the same cavity photon responsible for the LP/P{sub 1} splitting causes comparable splittings in the higher vibronic bands due to additional resonances between vibrationally excited states in the electronic ground state manifold and higher energy vibronic excitons.

  3. Density of states and excitonic condensation in the double layer correlated systems

    Science.gov (United States)

    Apinyan, V.; Kope?, T. K.

    2016-01-01

    We consider the single-particle density of states (DOS) in the strongly correlated double layer (DL) system, without applied external fields. We demonstrate an unusual collapse effect in the spectrum of the normal single-particle spectral function at the particular high-symmetry point corresponding to the specific bunching-point solution of the chemical potential in the Frenkel channel. We show that at the low-temperature limit the anomalous spectral function obeys a concave like structure, which is directly related to the interlayer pair formation and condensation. We calculate the normal DOS functions, and we find their temperature dependence for different values of the interlayer Coulomb interaction parameter. We show that the normal electron and hole DOS functions demonstrate typical condensates double peak structures on the background of the excitonic pair formation quasiparticle spectra and we have found the evidence of the hybridization gap in the case of high-temperature limit, and small interlayer coupling parameter. Meanwhile, we show a possible crossover from the excitonic condensate regime into the band insulator state. The structure of the normal DOS spectra, in the Frenkel channel and for the strong interlayer coupling regime, is found gapless for all temperature limits, which clearly indicates the strong coherence effects in the DL structure, and the excitonic condensates therein. We have shown that the excitonic pair formation and pair condensation occur simultaneously in the DL system, in contrast with the purely three-dimensional (3D) or two-dimensional cases (2D), discussed previously.

  4. Excited-state photoelectron spectroscopy of excitons in C60 and photopolymerized C60 films

    Science.gov (United States)

    Long, J. P.; Chase, S. J.; Kabler, M. N.

    2001-11-01

    Laser-excited states in films of pristine C60 and photopolymerized C60 (pp-C60) prepared in ultrahigh vacuum have been characterized in situ with pump-probe photoelectron spectroscopy using both synchrotron radiation and picosecond laser sources. Photoelectron spectra of singlet (S1) and triplet (T1) excitons overlap because of vibrational broadening in the photoemission final state. The spectra have been individually isolated in pp-C60 with time-resolved methods and are split by 0.33 eV. Signals from pristine C60 are weaker but are spectroscopically similar. The origin for exciton transitions for both C60 and pp-C60 is found to be properly located near the maximum of the highest occupied molecular orbital. Increasing excitation density favors T1 production over S1 such that, at high exciton concentrations, T1 states predominate, even at times much shorter than the unimolecular intersystem crossing time, measured to be ~2.5 ns for pp-C60 at 81 K. A weaker photoemission band located ~0.5 eV above S1 is also observed and is attributed to charge carriers and/or charge-transfer excitons.

  5. Resonant Transfer of Excitons and Quantum Computation

    CERN Document Server

    Lovett, B; Nazir, A; Kothari, B; Briggs, A; Lovett, Brendon; Reina, John H.; Nazir, Ahsan; Kothari, Beeneet; Briggs, Andrew

    2003-01-01

    The excitation-energy transfer--the so-called Forster resonant energy transfer--plays a key role in light harvesting processes in photosynthetic organisms in nature. Here we give two methods for performing quantum logic operations by tailoring this interaction. The first implementation uses a coupled quantum dot molecule where the exciton-exciton interaction and the Forster coupling are controlled by means of the dot size, interdot separation, material composition, confinement potential and applied electric field to obtain high fidelity logic. The second proposes the use of biological systems for embodying qubits where, as a result of a stronger Forster interaction, extended exciton states are expected. These states are likely to be more immune to decoherence.

  6. Exciton-exciton interaction engineering in coupled GaN quantum dots

    CERN Document Server

    De Rinaldis, S; Rossi, F; Rinaldis, Sergio De; Amico, Irene D'; Rossi, Fausto

    2002-01-01

    We present a fully three-dimensional study of the multiexciton optical response of vertically coupled GaN-based quantum dots via a direct-diagonalization approach. The proposed analysis is crucial in understanding the fundamental properties of few-particle/exciton interactions and, more important, may play an essential role in the design/optimization of semiconductor-based quantum information processing schemes. In particular, we focus on the interdot exciton-exciton coupling, key ingredient in recently proposed all-optical quantum processors. Our analysis demonstrates that there is a large window of realistic parameters for which both biexcitonic shift and oscillator strength are compatible with such implementation schemes.

  7. Thermodynamic model of the macroscopically ordered exciton state

    OpenAIRE

    Andreev, Sergey

    2012-01-01

    We explain the experimentally observed instability of cold exciton gases and formation of a macroscopically ordered exciton state (MOES) in terms of a thermodynamic model accounting for the phase fluctuations of the condensate. We show that the temperature dependence of the exciton energy exhibits fundamental scaling behavior with the signature of the second order phase transition.

  8. Instantaneous Rayleigh scattering from excitons localized in monolayer islands

    DEFF Research Database (Denmark)

    Langbein, Wolfgang; Leosson, Kristjan; Jensen, Jacob Riis; Hvam, Jrn Mrcher; Zimmermann, R.

    2000-01-01

    resonance is observed. Instead, when exciting only a subsystem of the exciton resonance, in our case excitons localized in quantum well regions of a specific monolayer thickness, the rise has an instantaneous component. This is due to the spatial nonuniformity of the initially excited exciton polarization...

  9. Exciton cascade model for fast neutron reactions

    International Nuclear Information System (INIS)

    A more sophisticated version of the exciton cascade model, treating equilibrium and pre-equilibrium particle emissions in a unique way has been developed and applied to the description of neutron induced reactions, using realistic input data. The master equation describing the nuclear relaxation process has been solved by Monte-Carlo method. The role of Pauli's exclusion principle and different estimates of the transition matrix elements between different exciton configurations are discussed. The model is free of any adjustable parameter. Good agreement of the results of calculations with experimental data has been found for some medium and heavy nuclei. (author)

  10. Detecting an exciton crystal by statistical means

    Energy Technology Data Exchange (ETDEWEB)

    Breyel, D. [Institut fr Theoretische Physik, Ruprecht-Karls-Universitt Heidelberg, Philosophenweg 19, D-69120 Heidelberg (Germany); Soller, H., E-mail: hsoller@googlemail.com [Institut fr Theoretische Physik, Ruprecht-Karls-Universitt Heidelberg, Philosophenweg 19, D-69120 Heidelberg (Germany); Schmidt, T.L. [Departement Physik, Universitt Basel, Klingelbergstrasse 82, 5056 Basel (Switzerland); Komnik, A. [Institut fr Theoretische Physik, Ruprecht-Karls-Universitt Heidelberg, Philosophenweg 19, D-69120 Heidelberg (Germany)

    2014-05-15

    We investigate an ensemble of excitons in a coupled quantum well excited via an applied laser field. Using an effective disordered quantum Ising model, we perform a numerical simulation of the experimental procedure and calculate the probability distribution function P(M) to create M excitons as well as their correlation function. It shows clear evidence of the existence of two phases corresponding to a liquid and a crystal phase. We demonstrate that not only the correlation function but also the distribution P(M) is very well suited to monitor this transition.

  11. Heat pumping with optically driven excitons

    CERN Document Server

    Gauger, Erik M

    2010-01-01

    We present a theoretical study showing that an optically driven excitonic two-level system in a solid state environment acts as a heat pump by means of repeated phonon emission or absorption events. We derive a master equation for the combined phonon bath and two-level system dynamics and analyze the direction and rate of energy transfer as a function of the externally accessible driving parameters. We discover that if the driving laser is detuned from the exciton transition, cooling the phonon environment becomes possible.

  12. Excitonic and photonic processes in materials

    CERN Document Server

    Williams, Richard

    2015-01-01

    This book is expected to present state-of-the-art understanding of a selection of excitonic and photonic processes in useful materials from semiconductors to insulators to metal/insulator nanocomposites, both inorganic and organic. Among the featured applications are components of solar cells, detectors, light-emitting devices, scintillators, and materials with novel optical properties. Excitonic properties are particularly important in organic photovoltaics and light emitting devices, as also in questions of the ultimate resolution and efficiency of new-generation scintillators for medical diagnostics, border security, and nuclear nonproliferation. Novel photonic and optoelectronic applications benefit from new material combinations and structures to be discussed.

  13. Exciton dynamics in perturbed vibronic molecular aggregates.

    Science.gov (United States)

    Brning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V

    2016-07-01

    A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840

  14. An investigation of exciton behavior in type-II self-assembled GaSb/GaAs quantum dots

    Science.gov (United States)

    Qiu, Feng; Qiu, Weiyang; Li, Yulian; Wang, Xingjun; Zhang, Yun; Zhou, Xiaohao; Lv, Yingfei; Sun, Yan; Deng, Huiyong; Hu, Shuhong; Dai, Ning; Wang, Chong; Yang, Yu; Zhuang, Qiandong; Hayne, Manus; Krier, A.

    2016-02-01

    We report the investigation of exciton dynamics in type-II self-assembled GaSb/GaAs quantum dots. The GaSb/GaAs quantum dots (QDs) were grown using a modified liquid phase epitaxy technique. Statistical size distributions of the uncapped QDs were investigated experimentally by field-emission scanning electron microscopy (SEM) and atomic force microscopy (AFM), and theoretically by an eight-band k p calculation, which demonstrated a dissolution effect. Furthermore, the low-temperature luminescence spectra of type-II GaSb/GaAs QDs with a thick capping layer exhibit well-resolved emission bands and LO-phonon-assisted transitions in the GaSb wetting layer. However, the luminescence lines quench at temperatures above 250 K, which is attributed to the weak quantum confinement of electrons participating in indirect exciton recombination. It was demonstrated that the room temperature stability of the excitons in type-II GaSb/GaAs QDs could be achieved by growing thin a capping layer, which provides strong quantum confinement in the conduction band and enhances the electronhole Coulomb interaction, stabilizing the excitons.

  15. A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells.

    Science.gov (United States)

    McMeekin, David P; Sadoughi, Golnaz; Rehman, Waqaas; Eperon, Giles E; Saliba, Michael; Hrantner, Maximilian T; Haghighirad, Amir; Sakai, Nobuya; Korte, Lars; Rech, Bernd; Johnston, Michael B; Herz, Laura M; Snaith, Henry J

    2016-01-01

    Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ?20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2](0.83)Cs(0.17)Pb(I(0.6)Br(0.4))3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells. PMID:26744401

  16. Thermoluminescence of alkali halides and its implications

    Energy Technology Data Exchange (ETDEWEB)

    Gartia, R.K., E-mail: rkgartia02@yahoo.in [Physics Department, Manipur University, Imphal 795003 (India); Rey, L. [Aerial-CRT-parc d' Innovation, B.P. 40443, F-67412 Illkirch Cedex (France); Tejkumar Singh, Th. [Physics Department, Manipur University, Imphal 795003 (India); Basanta Singh, Th. [Luminescence Dating Laboratory, Manipur University, Imphal 795003 (India)

    2012-03-01

    Trapping levels present in some alkali halides namely NaCl, KCl, KBr, and KI are determined by deconvolution of the thermoluminescence (TL) curves. Unlike most of the studies undertaken over the last few decades, we have presented a comprehensive picture of the phenomenon of TL as an analytical technique capable of revealing the position of the trapping levels present in the materials. We show that for all practical purposes, TL can be described involving only the three key trapping parameters, namely, the activation energy (E), the frequency factor (s), and the order of kinetics (b) even for complex glow curves having a number of TL peaks. Finally, based on these, we logically infer the importance of TL in development and characterization of materials used in dosimetry, dating and scintillation.

  17. Thermoluminescence of alkali halides and its implications

    International Nuclear Information System (INIS)

    Trapping levels present in some alkali halides namely NaCl, KCl, KBr, and KI are determined by deconvolution of the thermoluminescence (TL) curves. Unlike most of the studies undertaken over the last few decades, we have presented a comprehensive picture of the phenomenon of TL as an analytical technique capable of revealing the position of the trapping levels present in the materials. We show that for all practical purposes, TL can be described involving only the three key trapping parameters, namely, the activation energy (E), the frequency factor (s), and the order of kinetics (b) even for complex glow curves having a number of TL peaks. Finally, based on these, we logically infer the importance of TL in development and characterization of materials used in dosimetry, dating and scintillation.

  18. Multiphonon resonant Raman scattering in the semimagnetic semiconductor Cd sub 1 sub - sub x Mn sub x Te: Froehlich and deformation potential exciton-phonon interaction

    CERN Document Server

    Riera, R; Marin, J L; Bergues, J M; Campoy, G

    2003-01-01

    A theory describing multiphonon resonant Raman scattering (MPRRS) processes in wide-gap diluted magnetic semiconductors is presented, with Cd sub 1 sub - sub x Mn sub x Te as an example. The incident radiation frequency omega sub l is taken above the fundamental absorption region. The photoexcited electron and hole make real transitions through the LO phonon, when one considers Froehlich (F) and deformation potential (DP) interactions. The strong exchange interaction, typical of these materials, leads to a large spin splitting of the exciton states in the magnetic field. Neglecting Landau quantization, this Zeeman splitting gives rise to the formation of eight bands (two conduction and six valence ones) and ten different exciton states according to the polarization of the incident light. Explicit expressions for the MPRRS intensity of second and third order, the indirect creation and annihilation probabilities, the exciton lifetime, and the probabilities of transition between different exciton states and diff...

  19. Scattering process between polaron and exciton in conjugated polymers

    OpenAIRE

    Zhen SUN; Liu, Desheng; Stafstrm, Sven; An, Zhong

    2011-01-01

    Scattering process between a negative polaron and an exciton in a polymer chain is investigated by using the Su-Schrieffer-Heeger model modified to include electron-electron interactions, the Brazovskii-Kirova symmetry breaking term, and an external electric field. It is found that the scattering process is spin dependent. If the polaron and the exciton have parallel spins, the polaron can easily pass through the exciton as if it "do not see" the exciton. If the polaron and the exciton have a...

  20. Theoretical limit of power conversion efficiency for organic and hybrid halide perovskite photovoltaics

    Science.gov (United States)

    Seki, Kazuhiko; Furube, Akihiro; Yoshida, Yuji

    2015-08-01

    We calculated the maximum power conversion efficiency as a function of the optical band gap for organic photovoltaic (PV) cells by assuming that charge separation is accompanied by the energy loss required to dissociate strongly bound charge pairs. The dissociation energy can be estimated from the relationship between the open circuit voltage (VOC) and the optical band gap (Eg). By analyzing the published data on VOC and Eg, the dissociation energy can be estimated. The result could be used as a guide for selecting donor and acceptor materials. We also studied the theoretical limit of power conversion efficiency of hybrid halide perovskite by taking into account the energy loss involved in the carrier transfer from the perovskite phase to the metal oxide charge transport layer.

  1. Exploiting exciton-exciton interactions in semiconductor quantum dots for quantum-information processing

    OpenAIRE

    Troiani, Filippo; Hohenester, Ulrich; Molinari, Elisa

    2000-01-01

    We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

  2. Optimized spatial frequency response in silver halide sensitized gelatin

    OpenAIRE

    Fimia Gil, Antonio; Pascual Villalobos, Inmaculada; Belndez Vzquez, Augusto

    1991-01-01

    Silver halide sensitized gelatin processing is optimized to increase the spatial frequency response in Agfa-Gevaert 8E75 HD emulsion; therefore a diffraction efficiency of 55% in reflection gratings of 5000 lines/mm is achieved.

  3. Exciton Transfer Integrals Between Polymer Chains

    CERN Document Server

    Barford, W

    2007-01-01

    The line-dipole approximation for the evaluation of the exciton transfer integral, $J$, between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wavefunction, it is shown analytically that $J \\sim L$ when the chain lengths are smaller than the separation between them, or $J\\sim L^{-1}$ when the chain lengths are larger than their separation, where $L$ is the polymer length. Scaling relations are also obtained numerically for the more realistic standing-wave approximation for the exciton center-of-mass wavefunction, where it is found that for chain lengths larger than their separation $J \\sim L^{-1.8}$ or $J \\sim L^{-2}$, for parallel or collinear chains, respectively. These results have important implications for the photo-physics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the F\\"orster transfer rate for exciton migration decreases with chain lengths l...

  4. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre

    2007-01-01

    We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with opposite charges and a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum of their relative motion is well described by a one-dimensional effective Hamiltonian which is exactly solvable. Udgivelsesdato: FEB

  5. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    2007-01-01

    We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with opposite charges and a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum of their relative motion is well described...

  6. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    We analyse the low lying spectrum of a model of excitons in carbon nanotubes. Consider two particles with a Coulomb self-interaction, placed on an infinitely long cylinder. If the cylinder radius becomes small, the low lying spectrum is well described by a one-dimensional effective Hamiltonian...

  7. Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

    Energy Technology Data Exchange (ETDEWEB)

    Yu, Yifei [North Carolina State University, Raleigh; Hu, Shi [North Carolina State University, Raleigh; Su, Liqin [University of North Carolina, Charlotte; Huang, Lujun [North Carolina State University, Raleigh; Liu, Yi [North Carolina State University, Raleigh; Jin, Zhenghe [North Carolina State University, Raleigh; Puretzky, Alexander A [ORNL; Geohegan, David B [ORNL; Kim, Ki Wook [North Carolina State University, Raleigh; Zhang, Yong [University of North Carolina, Charlotte; Cao, Linyou [North Carolina State University

    2014-01-01

    Semiconductor heterostructurs provide a powerful platform for the engineering of excitons. Here we report on the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and non-epitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and an extra absorption peak at low energy region of both heterostructures. The MoS2/WS2 heterostructures show weak interlayer coupling and essentially act as an atomic-scale heterojunction with the intrinsic band structures of the two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise to provide capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.

  8. Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

    OpenAIRE

    Satoshi Kamiguchi; Sayoko Nagashima; Teiji Chihara

    2014-01-01

    Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metalmetal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150250 C, catalytically active sites develop, and the clu...

  9. Exciton photoluminescence in strained and unstrained ZnSe under hydrostatic pressure

    Science.gov (United States)

    Tuchman, Judah A.; Kim, Sangsig; Sui, Zhifeng; Herman, Irving P.

    1992-11-01

    Near-band-gap photoluminescence (PL) from exciton recombination in bulk crystalline ZnSe and in strained and strain-relaxed ZnSe epilayers grown on GaAs substrates is examined for pressures up to 80 kbar at 9 K in a diamond-anvil cell. The small sublinear dependence of PL energy with pressure for bulk ZnSe is attributed to the pressure dependence of the bulk modulus. In the strained ZnSe film, the change in biaxial strain with pressure is seen by the changing separation of the heavy- and light-hole exciton peaks. The heavy-hole exciton energy goes from being 12.5 meV below that of the light hole (1 bar), to the same energy as the light hole (~29 kbar), to relatively higher enegy (>29 kbar) as pressure is increased, which shows that the strain in the ZnSe film has been tuned from being compressive to tensile. In contrast, PL suggests that strain-relaxed films slip when pressure is applied. The hydrostatic deformation potential for near-band-gap transitions in ZnSe is a=-4.37 eV, while ||a|| is unexpectedly larger for transitions associated with deep levels. There is evidence that the tetragonal deformation potential b is a function of either volume deformation or strain.

  10. Excitonpolariton condensates near the Dirac point in a triangular lattice

    International Nuclear Information System (INIS)

    Dirac particles, massless relativistic entities, obey linear energy dispersions and hold important implications in particle physics. The recent discovery of Dirac fermions in condensed matter systems including graphene and topological insulators has generated a great deal of interest in exploring the relativistic properties associated with Dirac physics in solid-state materials. In addition, there are stimulating research activities to engineer Dirac particles, elucidating their exotic physical properties in a controllable setting. One of the successful platforms is the ultracold atomoptical lattice system, whose dynamics can be manipulated and probed in a clean environment. A microcavity excitonpolaritonlattice system offers the advantage of forming high-orbital condensation in non-equilibrium conditions, which enables one to explore novel quantum orbital order in two dimensions. In this paper, we experimentally construct the band structures near Dirac points, the vertices of the first hexagonal Brillouin zone with excitonpolariton condensates trapped in a triangular lattice. Due to the finite spectral linewidth, the direct map of band structures at Dirac points is elusive; however, we identify the linear part above Dirac points and its associated velocity value is ?0.92 108 cm s?1, consistent with the theoretical estimate 1 108 cm s?1 with a 2 ?m lattice constant. We envision that the exciton polariton condensates in lattices would be a promising solid-state platform, where the system order parameter can be accessed in both real and momentum spaces. (paper)

  11. Optical study of the temperature dependent scattering of the a exciton (polariton) of naphthalene

    International Nuclear Information System (INIS)

    High resolution (approx.0.1 cm-1) one-photon absorption profiles of the a band of naphthalene at 31 475 cm-1 from pure strain-free mounted (PSF) crystals have been measured between approx.2 and 40 K. Low temperature linewidths (FWHM) as narrow as 0.4 cm-1 have been observed. The a band profile exhibits an asymmetry which increases with the temperature. The high E side is close to Lorenztian at all temperatures (particularly for T> or approx. =8 K) and tails more steeply than the low E side. Arguments are presented for the asymmetric line shape being homogeneous so that the T-dependent FWHM are a direct measure of the transverse optical dephasing (T2) of the a exciton. The data and theory are consistent with the exciton--phonon scattering contribution to T2 being due to low frequency (?/sub eff/approx.15 cm-1) acoustical phonons. However, existing theories on asymmetric but homogenous exciton line shapes fail to account for the observed asymmetries

  12. Optical study of the temperature dependent scattering of the a exciton (polariton) of naphthalene

    Energy Technology Data Exchange (ETDEWEB)

    Robinette, S.L.; Small, G.J.; Stevenson, S.

    1978-06-01

    High resolution (approx.0.1 cm/sup -/1) one-photon absorption profiles of the a band of naphthalene at 31 475 cm/sup -/1 from pure strain-free mounted (PSF) crystals have been measured between approx.2 and 40 K. Low temperature linewidths (FWHM) as narrow as 0.4 cm/sup -/1 have been observed. The a band profile exhibits an asymmetry which increases with the temperature. The high E side is close to Lorenztian at all temperatures (particularly for T> or approx. =8 K) and tails more steeply than the low E side. Arguments are presented for the asymmetric line shape being homogeneous so that the T-dependent FWHM are a direct measure of the transverse optical dephasing (T/sub 2/) of the a exciton. The data and theory are consistent with the exciton--phonon scattering contribution to T/sub 2/ being due to low frequency (..omega../sub eff/approx.15 cm/sup -/1) acoustical phonons. However, existing theories on asymmetric but homogenous exciton line shapes fail to account for the observed asymmetries.

  13. Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide

    Energy Technology Data Exchange (ETDEWEB)

    Bory, Benjamin F.; Wang, Jingxin; Janssen, Ren A. J.; Meskers, Stefan C. J., E-mail: s.c.j.meskers@tue.nl [Molecular Materials and Nanosystems and Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven (Netherlands); Gomes, Henrique L. [Instituto de Telecomunicaes, Av. Rovisco, Pais 1, 1049-001 Lisboa, Portugal and Universidade do Algarve, Campus de Gambelas, 8005-139 Faro (Portugal); De Leeuw, Dago M. [Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany and King Abdulaziz University, Jeddah (Saudi Arabia)

    2014-12-08

    Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electronhole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 10{sup 25}/m{sup 3}. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics.

  14. Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide

    International Nuclear Information System (INIS)

    Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electronhole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 1025/m3. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics

  15. Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halide

    Science.gov (United States)

    Bory, Benjamin F.; Wang, Jingxin; Gomes, Henrique L.; Janssen, Ren A. J.; De Leeuw, Dago M.; Meskers, Stefan C. J.

    2014-12-01

    Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 1025/m3. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics.

  16. Evaluation of approximations in molecular exciton theory. 1. Applications to dimeric systems of interest in photosynthesis

    Energy Technology Data Exchange (ETDEWEB)

    LaLonde, D.E.; Petke, J.D.; Maggiora, G.M.

    1988-08-11

    Molecular exciton theory has been applied in an investigation of spectral shifts of the Q/sub y/ band of bacteriochlorin and methyl bacteriopheophorbide a dimers. Exciton matrix elements were evaluated by using both the point-dipole approximation and the Hall and Shipman distributed point-charge approximations of monomeric electron densities obtained from floating Gaussian-based SCF-MO-CI wave functions. Spectral shifts computed at selected dimer geometries using the point-dipole model were not found to be satisfactory approximations of those obtained from the more rigorous point-charge-based calculations. Judging from the experimental spectra of a number of model chlorophyll dimers, the present calculations using either point-charge model appear to account for approximately 60-85% of the magnitude of spectral shifts.

  17. Observations of exciton and carrier spin relaxation in Be doped p-type GaAs

    International Nuclear Information System (INIS)

    We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100?K. Two PL peaks were observed at 1.511?eV (peak 1) and 1.497?eV (peak 2) at 10?K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.33.1?ns at 10100?K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 1077?K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism

  18. Observations of exciton and carrier spin relaxation in Be doped p-type GaAs

    Science.gov (United States)

    Asaka, Naohiro; Harasawa, Ryo; Lu, Shulong; Dai, Pan; Tackeuchi, Atsushi

    2014-03-01

    We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100 K. Two PL peaks were observed at 1.511 eV (peak 1) and 1.497 eV (peak 2) at 10 K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.3-3.1 ns at 10-100 K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 10-77 K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism.

  19. On the equivalence of two approaches in the exciton-polariton theory

    International Nuclear Information System (INIS)

    The polariton effect in the optical processes involving photons with energies near that of an exciton is investigated by the Bogolubov diagonalization and the Green function approaches in a simple model of the direct band gap semiconductor with the electrical dipole allowed transition. To take into account the non-resonant terms of the interaction Hamiltonian of the photon-exciton system the Green function approach derived by Nguyen Van Hieu is presented with the use of Green's function matrix technique analogous to that suggested by Nambu in the theory of superconductivity. It is shown that with the suitable choice of the phase factors the renormalization constants are equal to the diagonalization coefficients. The disperson of polaritons and the matrix elements of processes with the participation of polaritons are identically calculated by both methods. However the Green function approach has an advantage in including the damping effect of polaritons. (author)

  20. Observations of exciton and carrier spin relaxation in Be doped p-type GaAs

    Energy Technology Data Exchange (ETDEWEB)

    Asaka, Naohiro; Harasawa, Ryo; Tackeuchi, Atsushi, E-mail: atacke@waseda.jp [Department of Applied Physics, Waseda University, Shinjuku, Tokyo 169-8555 (Japan); Lu, Shulong; Dai, Pan [Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Dushu Lake Higher Education Town, Ruoshui Road 398, Suzhou Industrial Park, Suzhou 215028 (China)

    2014-03-17

    We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100?K. Two PL peaks were observed at 1.511?eV (peak 1) and 1.497?eV (peak 2) at 10?K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.33.1?ns at 10100?K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 1077?K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism.

  1. Excitonic spin-splitting in quantum wells with a tilted magnetic field

    Science.gov (United States)

    Fernandes dos Santos, L.; Castelano, L. K.; Padilha, J. X.; Pusep, Y.; Marques, G. E.; Smirnov, D.; Bakarov, A. K.; Toropov, A. I.; Lopez-Richard, V.

    2016-02-01

    This work aims to investigate the effects of magnetic field strength and direction on the electronic properties and optical response of GaAs/AlGaAs-based heterostructures. An investigation of the excitonic spin-splitting of a disordered multiple quantum well embedded in a wide parabolic quantum well is presented. The results for polarization-resolved photoluminescence show that the magnetic field dependencies of the excitonic spin-splitting and photoluminescence linewidth are crucially sensitive to magnetic field orientation. Our experimental results are in good agreement with the calculated Zeeman splitting obtained by the Luttinger model, which predicts a hybridization of the spin character of states in the valence band under tilted magnetic fields.

  2. Thermal Delocalization of Excitons in GaAs/AlGaAs Quantum Well Tube Nanowires.

    Science.gov (United States)

    Shi, Teng; Jackson, Howard E; Smith, Leigh M; Jiang, Nian; Tan, H Hoe; Jagadish, Chennupati

    2016-02-10

    We use temperature-dependent photoluminescence (PL), photoluminescence imaging, and time-resolved photoluminescence measurements to gain insights into the localization of excitons in single 2 nm GaAs/AlGaAs quantum well tube nanowires. PL spectra reveal the coexistence of localized and delocalized states at low temperatures, with narrow quantum dot-like emission lines on the high energy side of a broad emission band, and delocalized states on the low energy side. We find that the high energy QD-like emissions are metastable, disappearing at higher temperatures with only delocalized states (quantum well tube ground states) surviving. By comparing temperature- and time-dependent PL, we develop a theoretical model which provides insights into the confinement potentials and relaxation dynamics which localize the excitons in these quantum well tube nanowires. PMID:26784952

  3. Highly efficient excitonic emission of CBD grown ZnO micropods (Presentation Recording)

    Science.gov (United States)

    Aad, Roy; Gokarna, Anisha; Nomenyo, Komla; Miska, Patrice; Geng, Wei; Couteau, Christophe; Lrondel, Gilles

    2015-10-01

    Due to its wide direct band gap and large exciton binding energy allowing for efficient excitonic emission at room temperature, ZnO has attracted attention as a luminescent material in various applications such as UV-light emitting diodes, chemical sensors and solar cells. While low-cost growth techniques, such as chemical bath deposition (CBD), of ZnO thin films and nanostructures have been already reported; nevertheless, ZnO thin films and nanostructures grown by costly techniques, such as metalorganic vapour phase epitaxy, still present the most interesting properties in terms of crystallinity and internal quantum efficiency. In this work, we report on highly efficient and highly crystalline ZnO micropods grown by CBD at a low temperature (costly and more complex growth techniques. These results are of great interest demonstrating that high quality ZnO microstructures can be obtained at low temperatures using a low-cost CBD growth technique.

  4. Excitonic luminescence of ZnSe single crystals doped with Au

    International Nuclear Information System (INIS)

    Photoluminescence spectra of ZnSe single crystals doped with Au during a long high-temperature treatment of as-grown crystals in Zn+Au or Se+Au melts are investigated in the temperature range from 83 to 300 K. The Au-doping from Zn+Au melt leads to the formation of both simple defects (Aui donors and AuZn acceptors) and acceptor associates (AuZn-Aui). The edge luminescence is attributed to radiative annihilation of Aui and VSe donor-bound excitons. The edge spectra of the crystals doped with Au from Se+Au melt contain the band ascribed to radiative annihilation of AuZn acceptor-bound excitons

  5. Structure vs. excitonic transitions in self-assembled porphyrin nanotubes and their effect on light absorption and scattering

    Science.gov (United States)

    Arteaga, Oriol; Canillas, Adolf; El-Hachemi, Zoubir; Crusats, Joaquim; Ribó, Josep M.

    2015-12-01

    The optical properties of diprotonated meso-tetrakis(4-sulphonatophenyl)porphyrin (TPPS4) J-aggregates of elongated thin particles (nanotubes in solution and ribbons when deposited on solid interfaces) are studied by different polarimetric techniques. The selective light extinction in these structures, which depends on the alignment of the nanoparticle with respect to the polarization of light, is contributed by excitonic absorption bands and by resonance light scattering. The optical response as a function of the polarization of light is complex because, although the quasi-one-dimensional structure confines the local fields along the nanotube axis, there are two orthogonal excitonic bands, of H- and J-character, that can work in favor of or against the field confinement. Results suggest that resonance light scattering is the dominant effect in solid state preparations, i.e. in collective groups (bundles) of ribbons but in diluted solutions, i.e. with isolated nanotubes, the absorption at the excitonic transitions remains dominant and linear dichroism spectra can be a direct probe of the exciton orientations. Therefore, by analyzing scattering and absorption data we can determine the alignment of the excitonic bands within the nanoparticle, i.e. of the orientation of the basic 2D porphyrin architecture in the nanoparticle. This is a necessary first step for understanding the directions of energy transport, charge polarization and non-linear optical properties in these materials.The optical properties of diprotonated meso-tetrakis(4-sulphonatophenyl)porphyrin (TPPS4) J-aggregates of elongated thin particles (nanotubes in solution and ribbons when deposited on solid interfaces) are studied by different polarimetric techniques. The selective light extinction in these structures, which depends on the alignment of the nanoparticle with respect to the polarization of light, is contributed by excitonic absorption bands and by resonance light scattering. The optical response as a function of the polarization of light is complex because, although the quasi-one-dimensional structure confines the local fields along the nanotube axis, there are two orthogonal excitonic bands, of H- and J-character, that can work in favor of or against the field confinement. Results suggest that resonance light scattering is the dominant effect in solid state preparations, i.e. in collective groups (bundles) of ribbons but in diluted solutions, i.e. with isolated nanotubes, the absorption at the excitonic transitions remains dominant and linear dichroism spectra can be a direct probe of the exciton orientations. Therefore, by analyzing scattering and absorption data we can determine the alignment of the excitonic bands within the nanoparticle, i.e. of the orientation of the basic 2D porphyrin architecture in the nanoparticle. This is a necessary first step for understanding the directions of energy transport, charge polarization and non-linear optical properties in these materials. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr05633k

  6. High Pressure Electrochemistry: Application to silver halides

    Science.gov (United States)

    Havens, K.; Kavner, A.

    2007-12-01

    Electron and ion charge transfer processes help govern electrical conductivity and diffusive mass and heat transport properties in deep Earth minerals. In an attempt to understand how pressure influences charge transfer behavior, the halide silver bromide (AgBr) was studied under the influence of an electric potential difference applied across two electrodes in a diamond anvil cell. This study follows our previous work on AgI, which was found to dissociate to molecular iodine and silver metal due to pressure and voltage influences. We performed two sets of experiments on AgBr at high pressure in a diamond anvil cell: electrochemical dissociation and electrical resistance measurements. In our study, we were able to electrochemically dissociate AgBr at pressures of 0.25-1.6 GPa by applying a voltage across the electrodes in the diamond cell sample chamber. Ag metal grew visibly on the negatively-charged electrode when voltages varying from 0.1 V to 5 V were applied. Additionally, a dark blue color appeared in low pressure areas of the diamond cell and grew darker from both voltage application and light exposure, indicating photochemical effects. We found that the reaction area and growth rate of both metal and dark blue color strongly increased as voltage increased, but tended to decrease with greater pressure. The resistance across the cell was observed to be influenced by both pressure and light exposure. As the AgBr sample was exposed to visible light, the resistance dropped instantaneously, and after the light was turned off, the resistance increased on a timescale of 10's of seconds to minutes. Notably, at higher pressures, the AgBr showed less photosensitivity. Exploration of these metal halide systems has many potential applications. First, these experiments explore the pressure-dependence of photochemical and photovoltaic processes, and may spur development of pressure-tuned microscale electronic devices. Second, these experimental results can be used to constrain thermodynamic models of pressure-dependent electrochemical behavior of materials, which may then be applied to the high temperature, high pressure mineral phases of the deep Earth and planets.

  7. Electric field controlled exciton transport in a conjugated polymer chain

    International Nuclear Information System (INIS)

    The control of exciton transport in conjugated polymers is of fundamental importance for luminescence and photovoltaic properties of polymeric optoelectronic devices. We have investigated the elastic scattering processes of a negative polaron and a neutral exciton in a conjugated polymer chain in the presence of an external electric field. We demonstrate that the exciton can be pushed or pulled to transport in a given direction by the polaron, or migrate by exchanging positions with the polaron in particular range of electric field strength. The exciton can preserve quantum coherence in these field induced transfer processes. The manner of exciton motion depends on the spin configuration of the exciton and the polaron as well as the electric field strength. This knowledge will serve to understand the dynamics of intrachain energy transport in conjugated polymers. -- Highlights: ? Neutral exciton can be controlled to move by electric fields by interacting with a polaron. ? Triplet excitons can be pushed or pulled by moving polarons. ? Singlet excitons can only be pulled by moving polarons. ? Both triplet and singlet excitons can perform a hopping like migration by exchanging positions with a moving polaron.

  8. Photoluminescence dynamics originating from exciton-exciton and exciton-electron scattering in a GaN thin film

    Energy Technology Data Exchange (ETDEWEB)

    Wakaiki, Shuji [Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Tokumaru, Hitoshi; Kim, DaeGwi; Nakayama, Masaaki [Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585 (Japan); Ichida, Hideki [Venture Business Laboratory, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Mizoguchi, Kohji [Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen, Naka-ku, Sakai, Osaka 599-8531 (Japan); Kanematsu, Yasuo [Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan); Venture Business Laboratory, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan)

    2012-12-15

    We have investigated the photoluminescence (PL) dynamics of a GaN thin film under intense excitation conditions using an optical-Kerr-gating method. It has been found that the PL originating from the exciton-exciton scattering (P emission) appears at the temperatures lower than around 100 K, while at higher temperatures P emission vanishes and the PL originating from exciton-electron scattering (H emission) dominates. The ultrashort gating time of 0.6 ps enabled us to obtain precise information of the temporal profiles of the peak energy, the bandwidth, and the intensity of the P emission and H emission. We have found that the decay time of the H emission is shorter than that of the P emission. By taking into account the strong correlation between the decay time and the peak-energy position of the emissions, we conclude that the difference in the decay times is attributed to a change of the photon-like characteristics of the lower polariton branch that is the final state of the scattering process; namely, the photon-like component of the H emission is larger than that of the P emission. (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  9. Femtosecond time-resolved photodissociation dynamics of methyl halide molecules on ultrathin gold films

    Directory of Open Access Journals (Sweden)

    Mihai E. Vaida

    2011-09-01

    Full Text Available The photodissociation of small organic molecules, namely methyl iodide, methyl bromide, and methyl chloride, adsorbed on a metal surface was investigated in real time by means of femtosecond-laser pumpprobe mass spectrometry. A weakly interacting gold surface was employed as substrate because the intact adsorption of the methyl halide molecules was desired prior to photoexcitation. The gold surface was prepared as an ultrathin film on Mo(100. The molecular adsorption behavior was characterized by coverage dependent temperature programmed desorption spectroscopy. Submonolayer preparations were irradiated with UV light of 266 nm wavelength and the subsequently emerging methyl fragments were probed by photoionization and mass spectrometric detection. A strong dependence of the excitation mechanism and the light-induced dynamics on the type of molecule was observed. Possible photoexcitation mechanisms included direct photoexcitation to the dissociative A-band of the methyl halide molecules as well as the attachment of surface-emitted electrons with transient negative ion formation and subsequent molecular fragmentation. Both reaction pathways were energetically possible in the case of methyl iodide, yet, no methyl fragments were observed. As a likely explanation, the rapid quenching of the excited states prior to fragmentation is proposed. This quenching mechanism could be prevented by modification of the gold surface through pre-adsorption of iodine atoms. In contrast, the A-band of methyl bromide was not energetically directly accessible through 266 nm excitation. Nevertheless, the one-photon-induced dissociation was observed in the case of methyl bromide. This was interpreted as being due to a considerable energetic down-shift of the electronic A-band states of methyl bromide by about 1.5 eV through interaction with the gold substrate. Finally, for methyl chloride no photofragmentation could be detected at all.

  10. GaN nanotubes grown by halide vapor phase epitaxy

    International Nuclear Information System (INIS)

    Full text: Wide-band gap GaN nanostructures such as quantum dots, nanorods, nanowires, nano columns and nanotubes have a strong potential within areas of biochemical sensing, nanofluidics, and optoelectronics. GaN nanotubes play a role of the building blocks for several applications such as solution-based transistors and highly sensitive nanotube molecular sensors. We have studied non-catalytic and Au-assisted growth of GaN nanotubes using halide vapor phase epitaxy (HVPE) technique. The growth was performed in the temperature range 480 degrees Celsius to 520 degrees Celsius using pure N2 as a carrier gas at atmospheric pressure. The nanotubes size, shape, density and the selectivity of growth have been studied depending on V/III ratio, growth temperature and substrate material. By increasing the GaCl partial pressure, the structure changed from dot-like to nanotubes. The nanotubes were about 1 ?m long with a diameter of typically 200 nm. In addition, it was observed that the nanostructures were spontaneously nucleated at droplets of Ga or, when using Au-coated Al2O3, on droplets of Au/Ga alloy. By varying the growth temperature, the inner diameter of the nanotubes could be controlled. A growth model is suggested, where the nanotubes are nucleated at droplets of Ga or an Au/Ga alloy. Our experimental results suggest that the approach with pre-patterned Au-coated Al2O3 substrates has the potential for fabrication of well-organized nanotubes with a high density. Nanostructures were characterized using electron microscopy methods and by low temperature time-resolved photoluminescence (TRPL). Studies were performed on samples with different wall thickness in the range of 35-75 nm. Two recombination processes with different dynamics contribute to the emission spectra of the GaN nanotubes. The photoluminescence peak shifts rapidly to the higher energy from 3.47 eV to 3.75 eV within a very short time of 30 ps. The origin of the emission having a short lifetime is related to the near band gap luminescence in GaN, while the higher energy luminescence with a relatively slow decay of 2 ns is associated with the interface underlayer formed on the substrate during the growth. (authors)

  11. Mott transition of excitons in GaAs-GaAlAs quantum wells

    International Nuclear Information System (INIS)

    We investigate the breakup of bound electronhole pairs, known as Mott transition of excitons, in GaAs-GaAlAs quantum wells with increasing excitation, comparing two different theoretical approaches. Firstly, a thermodynamic approach is used to investigate the ionization equilibrium between electrons, holes and excitons, where the abrupt jump of the degree of ionization from 0 to 1 indicates the Mott density. It is extended to a self-consistent quasi-particle approximation (QPA) for the carrier properties, including dynamical screening of the Coulomb interaction between carriers. Secondly, a spectral approach based on the semiconductor Bloch equations within linear optical response is used, considering the quasi-particle (QP) properties of carriers and the dynamical screening between electronhole pairs. While the first is effectively a one-particle approach, in the second the whole two-particle spectrum is analyzed. Within the thermodynamic approach, a simple criterion for the Mott transition can be given: namely, if the sum of chemical potentials of carriers, reflecting the effective shrinkage of the band edge, crosses the exciton energy with increasing excitation. We demonstrate that this simple picture cannot be maintained in the two-particle approach. Here, a compact quantity, which describes the behavior of the band edge, does not exist. In fact, the behavior of the single states in the spectrum is generated by the interplay of dynamical screening in the interband self-energy and the effective interaction of the electronhole pairs. Moreover, the band edge cannot be clearly resolved, since it is merged with excited exciton states (e.g. 2s state), which show up only for densities far below the Mott density. Instead of a Mott density, only a density range can be given, where the Mott transition appears. We demonstrate that a small damping as a prerequisite for the validation of the extended QPA in the thermodynamic approach breaks down, analyzing (i) the dephasing processes with increasing excitation, (ii) the strong increase of the excitonic linewidth and (iii) comparing with the lifetime of carriers in the QP description. (paper)

  12. Microcavity controlled coupling of excitonic qubits

    CERN Document Server

    Albert, F; Kasprzak, J; Strau, M; Schneider, C; Hfling, S; Kamp, M; Forchel, A; Reitzenstein, S; Muljarov, E A; Langbein, W

    2012-01-01

    Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. The most relevant mechanism of coherent coupling of distant qubits is coupling via the electromagnetic field. Here, we demonstrate the controlled coherent coupling of spatially separated excitonic qubits via the photon mode of a solid state microresonator. This is revealed by two-dimensional spectroscopy of the sample's coherent response, a sensitive and selective probe of the coherent coupling. The experimental results are quantitatively described by a rigorous theory of the cavity mediated coupling within a cluster of quantum dots excitons. Having demonstrated this mechanism, it can be used in extended coupling channels - sculptured, for instance, in photonic crystal cavities - to enable a long-range, non-local wiring up of individual emitters in solids.

  13. Dynamical crossovers in Markovian exciton transport

    Science.gov (United States)

    Hossein-Nejad, Hoda; Olaya-Castro, Alexandra; Fassioli, Francesca; Scholes, Gregory D.

    2013-08-01

    The large deviation theory has recently been applied to open quantum systems to uncover dynamical crossovers in the space of quantum trajectories associated to Markovian evolutions. Such dynamical crossovers are characterized by qualitative changes in the fluctuations of rare quantum jump trajectories, and have been observed in the statistics of coherently driven quantum systems. In this paper we investigate the counting statistics of the rare quantum trajectories of an undriven system undergoing a pure relaxation process, namely, Markovian exciton transport. We find that dynamical crossovers occur in systems with a minimum of three interacting molecules, and are strongly activated by exciton delocalization and interference. Our results illustrate how quantum features of the underlying system Hamiltonian can influence the statistical properties of energy transfer processes in a non-trivial manner.

  14. Frenkel-exciton-polaritons in organic microcavities

    Scientific Electronic Library Online (English)

    Hashem, Zoubi; G. C., La Rocca.

    2006-06-01

    Full Text Available We study organic microcavities in the strong coupling regime. The cavity exciton-polariton dispersion relations and quantum states are derived in using a microscopic theory. We consider the two cases of anisotropic organic crystals with one and two molecules per unit cell. In general, the cavity exc [...] iton-polaritons are a coherent superposition of both cavity mode polarizations and of both Davydov exciton branches. The obtained polarization mixing is in contrast to the case of typical inorganic semiconductor cavities in which no mixing between the TM and TE polarizations occurs. By applying the quasi-mode approximation, we derive the transmission, reflection, and absorption coefficients for high quality organic cavities.

  15. Exciton Polaritons in Microcavities New Frontiers

    CERN Document Server

    Sanvitto, Daniele

    2012-01-01

    In the past decade, there has been a burst of new and fascinating physics associated to the unique properties of two-dimensional exciton polaritons, their recent demonstration of condensation under non-equilibrium conditions and all the related quantum phenomena, which have stimulated extensive research work. This monograph summarizes the current state of the art of research on exciton polaritons in microcavities: their interactions, fast dynamics, spin-dependent phenomena, temporal and spatial coherence, condensation under non-equilibrium conditions, related collective quantum phenomena and most advanced applications. The monograph is written by the most active authors who have strongly contributed to the advances in this area. It is of great interests to both physicists approaching this subject for the first time, as well as a wide audience of experts in other disciplines who want to be updated on this fast moving field.

  16. Photosynthetic light harvesting: excitons and coherence

    OpenAIRE

    Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C.; Scholes, Gregory D.

    2014-01-01

    Photosynthesis begins with light harvesting, where specialized pigmentprotein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state...

  17. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    OpenAIRE

    Olina, Jan; Dijkstra, Arend G.; Chen WANG; Cao, Jianshu

    2014-01-01

    Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined...

  18. Single exciton spectroscopy of semimagnetic quantum dots

    OpenAIRE

    Fernndez Rossier, Joaqun

    2005-01-01

    A photoexcited II-VI semiconductor quantum dots doped with a few Mn spins is considered. The effects of spin-exciton interactions and the resulting multispin correlations on the photoluminescence are calculated by numerical diagonalization of the Hamiltonian, including exchange interaction between electrons, holes, and Mn spins, as well as spin-orbit interaction. The results provide a unified description of recent experiments on the photoluminesnce of dots with one and many Mn atoms as well a...

  19. Observation of a Dark Condensate of Excitons

    OpenAIRE

    Beian, Mussie; Alloing, Mathieu; Anankine, Romain; Cambril, Edmond; Carbonell, Carmen Gomez; Lemaitre, Aristide; Dubin, Francois

    2015-01-01

    Starting with a couple of alkali atoms, the family of ultra-cold gases has rapidly expanded in the need of degrees of freedom required to study complex quantum phases. Besides more versatile but more complicated atomic systems, semiconductor excitons are attractive candidates to explore strongly interacting many-body quantum systems. They can combine unique physical properties such as a four-component internal "spin" structure with a giant electric dipole moment. This richness, however, comes...

  20. Reference spectroscopic data for hydrogen halides. Part I: Construction and validation of the ro-vibrational dipole moment functions

    International Nuclear Information System (INIS)

    Knowledge of the infrared transition moments of hydrogen halides, namely HF, HCl, HBr, and HI, is essential for atmospheric, astrophysical, and laser applications. Recently, a new polynomial empirical dipole moment function (DMF) for HCl has been constructed using an efficient approach that involves a direct fit of experimental ro-vibrational intensities [Li et al. J Quant Spectrosc Radiat Transfer 2011;112:154350]. In the present study, this method was extended to the use of Pad approximation representations of the DMF and applied to all four hydrogen halides. To carry out the fits, the best available experimental data were collected and critically evaluated. Combining dipole moment functions with the wavefunctions obtained from highly-accurate empirical potential energy curves, line intensities were computed numerically for numerous ro-vibrational bands, and compared with the experimental values and with intensities calculated using the most recent ab initio dipole moment functions. Results obtained in this work form basis for calculating intensities of spectral lines of hydrogen halides and their isotopologues in the HITRAN 2012 database. -- Highlights: ? New semi-empirical dipole moment functions (DMF) of HF, HCl, HBr and HI were derived. ? Fit employed experimental ro-vibrational intensities with their uncertainties. ? New DMFs can predict intensities over extensive range of internuclear distances

  1. Reference spectroscopic data for hydrogen halides. Part I: Construction and validation of the ro-vibrational dipole moment functions

    Science.gov (United States)

    Li, Gang; Gordon, Iouli E.; Le Roy, Robert J.; Hajigeorgiou, Photos G.; Coxon, John A.; Bernath, Peter F.; Rothman, Laurence S.

    2013-05-01

    Knowledge of the infrared transition moments of hydrogen halides, namely HF, HCl, HBr, and HI, is essential for atmospheric, astrophysical, and laser applications. Recently, a new polynomial empirical dipole moment function (DMF) for HCl has been constructed using an efficient approach that involves a direct fit of experimental ro-vibrational intensities [Li et al. J Quant Spectrosc Radiat Transfer 2011;112:1543-50]. In the present study, this method was extended to the use of Pad approximation representations of the DMF and applied to all four hydrogen halides. To carry out the fits, the best available experimental data were collected and critically evaluated. Combining dipole moment functions with the wavefunctions obtained from highly-accurate empirical potential energy curves, line intensities were computed numerically for numerous ro-vibrational bands, and compared with the experimental values and with intensities calculated using the most recent ab initio dipole moment functions. Results obtained in this work form basis for calculating intensities of spectral lines of hydrogen halides and their isotopologues in the HITRAN 2012 database.

  2. Modulational instability and solitons in excitonic semiconductor waveguides

    CERN Document Server

    Smyrnov, Oleksii A; Malzer, Stefan

    2011-01-01

    Nonlinear light propagation in a single-mode micron-size waveguide made of semiconducting excitonic material has been theoretically studied in terms of exciton-polaritons by using an analysis based on macroscopic fields. When a light pulse is spectrally centered in the vicinity of the ground-state Wannier exciton resonance, it interacts with the medium nonlinearly. This optical cubic nonlinearity is caused by the repulsive exciton-exciton interactions in the semiconductor, and at resonance it is orders of magnitude larger than the Kerr nonlinearity (e.g., in silica). We demonstrate that a very strong and unconventional modulational instability takes place, which has not been previously reported. After reducing the problem to a single nonlinear Schr\\"odinger-like equation, we also explore the formation of solitary waves both inside and outside the polaritonic gap and find evidence of spectral broadening. A realistic physical model of the excitonic waveguide structure is proposed.

  3. Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers

    Energy Technology Data Exchange (ETDEWEB)

    Ying, L.; Sie, X.S. [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.

    1998-12-10

    The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.

  4. Fractional Solitons in Excitonic Josephson Junctions.

    Science.gov (United States)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ?0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q?=??0/2? is the lowest energy state starting from zero ?0 - until ?0?>?? - then the alternative group of solitons with Q?=??0/2??-?1 takes place and switches the polarity of CPR. PMID:26511770

  5. Exciton dynamics at oligothiophene / Au(111) interfaces

    Energy Technology Data Exchange (ETDEWEB)

    Varene, Erwan; Martin, Isabel; Bronner, Christopher; Bogner, Lea; Tegeder, Petra [Freie Universitaet Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin (Germany)

    2011-07-01

    Semiconducting materials based on organic molecules or polymers are promising candidates for applications in electronic devices such as organic photovoltaic cells. The electronic structure and carrier dynamics at interfaces between organic semiconductors and inorganic substrates are the most fundamental issues in order to understand the functionalities of organic films. Time-resolved two-photon photoemission (2PPE) spectroscopy is employed to determine the electronic structure, charge carrier dynamics and energetics at the sexithiophene (6T)/Au(111) interface. We found the HOMO and HOMO-1 to be located at -0.8 and -1.5 eV, respectively and two 6T-derived unoccupied states at 2.1 eV and 3.0 eV with respect to the Fermi level which we assign to the LUMO and LUMO+1. In addition an exciton state possessing a binding energy of 0.9 eV is observed. The timescale for the exciton breakup exhibits a strong 6T coverage dependency, i.e., the exciton lifetime increases with increasing coverage. For instance at a coverage 12 ML the decay times are around 700 fs for the fast component and 5 ps for the slow component.

  6. Fractional Solitons in Excitonic Josephson Junctions

    Science.gov (United States)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ?0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q?=??0/2? is the lowest energy state starting from zero ?0 until ?0?>?? then the alternative group of solitons with Q?=??0/2????1 takes place and switches the polarity of CPR. PMID:26511770

  7. Excitonic effects in organic semi-conductors

    International Nuclear Information System (INIS)

    Full text: The main objective of the present work is to investigate the role of electron-hole (e-h) correlations on the optical absorption spectra of novel organic materials by means of ab-initio techniques. This is achieved by solving the equation of motion for the two-particle e-h Green's function, the so-called Bethe-Salpeter equation. Electron-hole correlations are taken into account in terms of a repulsive exchange interaction, and expressed in terms of one-electron wave functions, which are expanded in a linearized augmented plane wave (LAPW) basis. Bulk Si and LiF serve as test cases for the present method. We find that the inclusion of electron-hole correlations considerably improves the agreement with experimental absorption spectra, both for Si as well as for LiF. As a main result, we discuss the excitonic structure of isolated as well as crystalline polymers. We notice that inter-chain interactions crucially alter the excitonic states. We find strongly bound excitation states for the isolated chains with exciton binding energies in the order of 0.5 eV. On the other hand, a crystalline arrangement of the polymer chains drastically reduces the strength of the electron-hole interaction. (author)

  8. Surface exciton polariton spectrum in semiconductor superlattices

    International Nuclear Information System (INIS)

    It is reported a surface exciton polarization spectrum in superlattices made up of two alternating media A and B, where one of the layers shows spatial dispersion in its frequency dependent dielectric function. We have used a theoretical approach based on the effective medium theory to determine the surface exciton polariton dispersion relation. The inclusion of an external magnetic field parallel to the interfaces of the superlattice is also considered, and it leads to the so-called magneto plasmon-polariton excitations. It is shown that, in the absence of the external magnetic field, the surface modes are doubly degenerate, corresponding to propagation in opposite directions for a given magnitude of the wavenumber Kx. However, in the presence of the external magnetic field, this degeneracy is removed. We have also calculated the so-called Attenuated Total Reflectivity (ATR) to probe the surface exciton polariton modes. The effective-medium description has been widely used in ATR spectroscopy of semiconductor superlattices, and it generally gives a good account of the data together with a clear physical interpretation of the various spectral features. (author). 22 refs., 2 figs

  9. Process for oxidation of hydrogen halides to elemental halogens

    Science.gov (United States)

    Lyke, Stephen E. (Middleton, WI)

    1992-01-01

    An improved process for generating an elemental halogen selected from chlorine, bromine or iodine, from a corresponding hydrogen halide by absorbing a molten salt mixture, which includes sulfur, alkali metals and oxygen with a sulfur to metal molar ratio between 0.9 and 1.1 and includes a dissolved oxygen compound capable of reacting with hydrogen halide to produce elemental halogen, into a porous, relatively inert substrate to produce a substrate-supported salt mixture. Thereafter, the substrate-supported salt mixture is contacted (stage 1) with a hydrogen halide while maintaining the substrate-supported salt mixture during the contacting at an elevated temperature sufficient to sustain a reaction between the oxygen compound and the hydrogen halide to produce a gaseous elemental halogen product. This is followed by purging the substrate-supported salt mixture with steam (stage 2) thereby recovering any unreacted hydrogen halide and additional elemental halogen for recycle to stage 1. The dissolved oxygen compound is regenerated in a high temperature (stage 3) and an optical intermediate temperature stage (stage 4) by contacting the substrate-supported salt mixture with a gas containing oxygen whereby the dissolved oxygen compound in the substrate-supported salt mixture is regenerated by being oxidized to a higher valence state.

  10. Genetic Control of Methyl Halide Production in Arabidopsis

    Science.gov (United States)

    Rhew, R. C.; Ostergaard, L.; Saltzman, E. S.; Yanofsky, M. F.

    2003-12-01

    Methyl chloride and methyl bromide are the primary carriers of natural chlorine and bromine to the stratosphere where they catalyze the destruction of ozone, whereas methyl iodide influences aerosol formation and ozone loss in the troposphere. Methyl bromide is also an agricultural fumigant whose use is scheduled to be phased out by international agreement. Despite the economic and environmental importance of these methyl halides, their natural sources and biological production mechanisms are poorly understood. Currently identified sources include oceans, biomass burning, industrial and agricultural use, fuel combustion, salt marshes, wetlands, rice paddies, certain terrestrial plants and fungi, and abiotic processes. We demonstrate that the model plant Arabidopsis thaliana produces and emits methyl halides and that the enzyme primarily responsible for the production is encoded by the HARMLESS TO OZONE LAYER (HOL) gene located on chromosome II. In mutant plants that have a disruption of the HOL gene, methyl halide production is largely eliminated. A phylogenetic analysis using the HOL gene suggests that the ability to produce methyl halides is widespread among vascular plants. This approach provides a genetic basis for understanding and predicting patterns of methyl halide production by plants.

  11. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.

    Science.gov (United States)

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-01-01

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281

  12. Long-range excitons in conjugated polymers with ring torsions

    OpenAIRE

    Harigaya, Kikuo

    1997-01-01

    Ring torsion effects on optical excitation properties in poly(para-phenylene) (PPP) and polyaniline (PAN) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We find that ring torsions affect the long-range excitons in PAN more easily than in PPP, due to the larger torsion angle of PAN and the large number of bonds whose hopping integrals are modulated by torsions.

  13. Exciton-polaritons by two-photon absorption in semiconductors

    International Nuclear Information System (INIS)

    The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semiclassical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment. (author)

  14. Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides

    OpenAIRE

    Moody, Galan; Dass, Chandriker Kavir; Hao, Kai; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Bergauser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2014-01-01

    Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. These unique attributes have been leveraged for electrical and optical control of excitons for atomically-thin optoelectronics and valleytronics. The development of such technologies relies on understanding and quantifying the fundamental properties of the exciton. A key parameter is the intrin...

  15. Excitonic Properties in GaAs Parabolic Quantum Dots

    OpenAIRE

    Jaziri, S.; Bennaceur, R.

    1995-01-01

    Certain classes of semiconductor quantum dots being actually fabricated exhibit a nearly parabolic confinement for both the electron and the hole. In undoped quantum dots, excitonic effects are important. In this work, first we present theoretical results on exciton properties in parabolic quantum dots: resonance energy, binding energy and oscillator strength. Then, we investigate the effects of external electric and magnetic fields on exciton in quantum dots.

  16. Exciton resonances quench the photoluminescence of zigzag carbon nanotubes

    OpenAIRE

    Reich, Stephanie; Thomsen, Christian; Robertson, John

    2005-01-01

    We show that the photoluminescence intensity of single-walled carbon nanotubes is much stronger in tubes with large chiral angles - armchair tubes - because exciton resonances make the luminescence of zigzag tubes intrinsically weak. This exciton-exciton resonance depends on the electronic structure of the tubes and is found more often in nanotubes of the +1 family. Armchair tubes do not necessarily grow preferentially with present growth techniques; they just have stronger ...

  17. Photoluminescence of high-density exciton-polariton condensates

    OpenAIRE

    Ishida, Natsuko; Byrnes, Tim; Horikiri, Tomoyuki; Nori, Franco; Yamamoto, Yoshihisa

    2013-01-01

    We examine the photoluminescence of highly-excited exciton-polariton condensates in semiconductor microcavities. Under strong pumping, exciton-polariton condensates have been observed to undergo a lasing transition where strong coupling between the excitons and photons is lost. We discuss an alternative high-density scenario, where the strong coupling is maintained. We find that the photoluminescence smoothly transitions between the lower polariton energy to the cavity photo...

  18. Synergetics in multiple exciton generation effect in quantum dots

    OpenAIRE

    Turaeva, N. N.; Oksengendler, B. L.; Uralov, I.

    2011-01-01

    We present detailed analysis of the non-Poissonian population of excitons produced by MEG effect in quantum dots on the base of statistic theory of MEG and synergetic approach for chemical reactions. From the analysis we can conclude that a non-Poissonian distribution of exciton population is evidence of non-linear and non-equilibrium character of the process of multiple generation of excitons in quantum dots at a single photon absorption

  19. Exciton Dynamics on Rubrene (001) Crystal Surfaces with Microstructure Confinement

    OpenAIRE

    Stoehr, R. J.; Beirne, G. J.; P. Michler; Scholz, R; Wrachtrup, J; Pflaum, J.

    2008-01-01

    The exciton dynamics on flat (001) rubrene crystal surfaces have been compared with those under confined pyramidal geometry by time-resolved photoluminescence with micrometer spatial resolution. The luminescence spectra can be interpreted in terms of generation of a free and a self-trapped exciton. Their ratio depends significantly on the structural size which we explain by the optical absorption profile of the pyramids in combination with the exciton diffusion constant. For...

  20. Lattice exciton-polaron problem by quantum Monte Carlo simulations

    OpenAIRE

    Hohenadler, Martin; Littlewood, Peter B; Fehske, Holger

    2007-01-01

    Exciton-polaron formation in one-dimensional lattice models with short or long-range carrier-phonon interaction is studied by quantum Monte Carlo simulations. Depending on the relative sign of electron and hole-phonon coupling, the exciton-polaron size increases or decreases with increasing interaction strength. Quantum phonon fluctuations determine the (exciton-)polaron size and yield translation-invariant states at all finite couplings.

  1. Surface states and band-to-band non-radiative transitions in silicon single crystal investigated by piezoelectric photothermal spectroscopy

    International Nuclear Information System (INIS)

    The effectiveness of piezoelectric photothermal spectroscopy (PPTS) to investigate surface states and bulk properties of single crystal silicon was demonstrated. PPTS measurements were conducted on p- and n-type, single crystal silicon. A broad peaked signal around 1.180.01 eV at room temperature showed the characteristics of slow states present on silicon surface. Another signal bearing a peak around 1.070.005 eV at room temperature was due to bulk effect. In the indirect band gap of silicon, the excitation of electrons from valence band to conduction band (so called band-to-band excitation) is not possible without phonon assistance. The PPTS measurements conducted at various temperatures revealed band-to-band and valence band-to-excitons states transition with phonon assistance. The measurements at 4.2 and 110 K resolved four types of phonon participation. A good agreement between theoretical expressions and experimental data substantiated the phonon participation in band-to-band and valence band-to-excitons states transitions

  2. Surface states and band-to-band non-radiative transitions in silicon single crystal investigated by piezoelectric photothermal spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Memon, Aftab A.; Fukuyama, Atsuhiko; Sato, Syoichiro; Ikari, Tetsuo

    2003-09-15

    The effectiveness of piezoelectric photothermal spectroscopy (PPTS) to investigate surface states and bulk properties of single crystal silicon was demonstrated. PPTS measurements were conducted on p- and n-type, single crystal silicon. A broad peaked signal around 1.18{+-}0.01 eV at room temperature showed the characteristics of slow states present on silicon surface. Another signal bearing a peak around 1.07{+-}0.005 eV at room temperature was due to bulk effect. In the indirect band gap of silicon, the excitation of electrons from valence band to conduction band (so called band-to-band excitation) is not possible without phonon assistance. The PPTS measurements conducted at various temperatures revealed band-to-band and valence band-to-excitons states transition with phonon assistance. The measurements at 4.2 and 110 K resolved four types of phonon participation. A good agreement between theoretical expressions and experimental data substantiated the phonon participation in band-to-band and valence band-to-excitons states transitions.

  3. Internal field induced exciton binding energy and the optical transition in a strained Mg based IIVI quantum well

    Energy Technology Data Exchange (ETDEWEB)

    Elangovan, P. [Department of Physics, Maamallan Institute of Technology, Chennai 602105 (India); John Peter, A., E-mail: a.john.peter@gmail.com [Department of Physics, Government Arts College, Melur 625 106. Madurai (India); Kyoo Yoo, Chang [Center for Environmental Studies/Green Energy Center, Deptartment of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of)

    2013-11-15

    Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated. The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. The oscillator strength and the exciton lifetime for radiative recombination are computed. The internal field induced transition energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated. The results show that the nonlinear optical properties strongly depend on Mg composition.

  4. Cu halide nanoparticle formation by diffusion of copper in alkali halide crystals

    Directory of Open Access Journals (Sweden)

    A. Prez-Rodrguez

    2006-01-01

    Full Text Available Atomos de cobre han sido introducidos por difusin en cristales de NaCl, KCl y KBr a 500C. Los cristales han sido analizados ptimamente con medidas de fotoluminiscencia y por microscopa electrnica de barrido. Los espectros de emisin y excitacin, medidos a baja temperatura muestran el efecto de confinamiento de exciton, indicando la formacin de nanopartculas de CuX (X=Cl, Br, lo cual ha sido confirmado por imgenes de microscopa electrnica. Este mtodo es propuesto como un mtodo alternativo para obtener nanopartculas de CuX en cristales halogenuros alcalinos.

  5. Unconventional Superconductivity in electron-doped layered metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I)

    OpenAIRE

    Kasahara, Yuichi; Kuroki, Kazuhiko; Yamanaka, Shoji; Taguchi, Yasujiro

    2014-01-01

    In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate th...

  6. Polarization-dependent exciton dynamics in tetracene single crystals

    International Nuclear Information System (INIS)

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors

  7. Localization length scales of triplet excitons in singlet fission materials

    Science.gov (United States)

    Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, Hlne; Greenham, Neil C.; Chepelianskii, Alexei D.

    2015-09-01

    We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.

  8. Exciton management in organic photovoltaic multidonor energy cascades.

    Science.gov (United States)

    Griffith, Olga L; Forrest, Stephen R

    2014-05-14

    Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Frster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468

  9. Polarization-dependent exciton dynamics in tetracene single crystals

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Bo; Zhang, Chunfeng, E-mail: cfzhang@nju.edu.cn; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong [National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093 (China); Xiao, Min, E-mail: mxiao@uark.edu [National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093 (China); Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States)

    2014-12-28

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

  10. Recombination dynamics of exciton and exciton complexes in single quantum dots

    International Nuclear Information System (INIS)

    We have studied the relaxation dynamics of excitons and exciton complexes in a single quantum dot using time-resolved photoluminescence. To avoid any interference of carrier diffusion and trapping we have excited the dot with energies below that of the wetting layer. The times extracted from a quantitative analysis are thus related only to the relaxation of carriers inside the dot. Since the dot is rather large it is possible to observe the recombination of carriers from different atomic shells. We observe a retardation of the carrier relaxation increasing excitation power due to the large number of charges filling the dot.

  11. When Halides Come to Lithium Niobate Nanopowders Purity and Morphology Assistance.

    Science.gov (United States)

    Lamouroux, Emmanuel; Badie, Laurent; Miska, Patrice; Fort, Yves

    2016-03-01

    The preparation of pure lithium niobate nanopowders was carried out by a matrix-mediated synthesis approach. Lithium hydroxide and niobium pentachloride were used as precursors. The influence of the chemical environment was studied by adding lithium halide (LiCl or LiBr). After thermal treatment of the precursor mixture at 550 C for 30 min, the morphology of the products was obtained from transmission electron microscopy and dynamic light scattering, whereas the crystallinity and phase purity were characterized by X-ray diffraction and UV-visible and Raman spectroscopies. Our results point out that the chemical environment during lithium niobate formation at 550 C influences the final morphology. Moreover, direct and indirect band-gap energies have been determined from UV-visible spectroscopy. Their values for the direct-band-gap energies range from 3.97 to 4.36 eV with a slight dependence on the Li/Nb ratio, whereas for the indirect-band-gap energies, the value appears to be independent of this ratio and is 3.64 eV. No dependence of the band-gap energies on the average crystallite and nanoparticle sizes is observed. PMID:26859157

  12. Luminescent unit computerization to research spectral characteristics of fine film alkali halide crystal

    International Nuclear Information System (INIS)

    The fundamental optical absorption of ion crystals characterizes the creation of different free low energetic electronic excitation (the excitons and electron-hole pairs), but their straight registration is not possible because of incommensurable big absorption factor of alkali halide monocrystals. So to registration the spectrums of alkali halide monocrystal very fine layers are necessary. We have received fine films of Nal and KCl in system of KCl-Nal-KCl, KCl-KI-KCl on the base of universal vacuum post VUP-4, VUP-5 by thermal evaporation. A unique spectral unit has been created For this on the basic the SDL-2 complex. Complex consists of radiator, systems of condensers, monochromators MDR-12 and MDR-23, receivers of radiation, controller by unit. Connect and control of monochromators by means of IBM-compatible computer has been created. Kinematics schemes of monochromators provide consequent removing on output slot of monochromatic radiation in operating range of each diffraction lattice and indication its wavelength. The tumbling diffraction lattices is done the crossbar engines SHDR-711. For this special plate of control and block of reinforcement for crossbar engines in monochromators MDR-12 and MDR-23 are designed and constructed. Created controller of monochromators consists of double cascade preamplifier on transistors n-p-n type (815G) and logical scheme, constructed on summers and K555 series triggers. The preamplifier is used for reinforcement of signal to available amplifier on transistors KT837D. The logical scheme reduces the number of used categories of bidirectional port and enables unhooking the feeding to the windings of crossbar engine at conservation of previous combination of signals. The connection controller of monochromators is done through controller of port of computer with use the parallel interface. For installing computerized system of collection and data processing is provided marketed by means of modern object-oriented programming languages (Visual C++ 6.0, Visual Basic 6.0). As the result of the experiments it has been noted that the burning effect of own luminescence is in its maximum in the iodides of alkaline metals, for instance, in KI, NaI, RbI, and CsI crystals. From the point of the experimental equipment in order to registration the fundamental optical absorption of these crystals there is no necessity in such rare vacuum monochromators

  13. The ultraviolet photodissociation dynamics of the hydrogen halides

    International Nuclear Information System (INIS)

    The first electronic absorption bands of hydrogen halide (HX) molecules - HI, HBr, HCl and DCl - have been studied using two experimental, laser-based techniques. Each band of HX consists of absorption of ultraviolet (UV) radiation and leads to fragmentation into two sets of products: H(2S) + X(2Pj); j=3/2, 1/2. Numerous measurements that cover most of the UV absorption bands (??200-300 nm) of HI and HBr were made using H Rydberg atom photofragment translational spectroscopy (HRPTS). The results comprise two important properties: the branching between the accessible product channels and the spatial distributions of the recoil velocities of the photofragments. These extensive observations afford a detailed interpretation of the featureless absorption bands in terms of the relative influences of transitions and dissociations involving multiple electronic states: 3?(1), 3?(0+), 1?(1) and 3?+(1). The photolysis of HCl between 201 and 210 nm was also studied by HRPTS to provide a number of direct determinations of the relative product yield for comparison with the best available theoretical predictions based on ab initio electronic structure calculations and a time-independent treatment of the photodissociation dynamics. The good agreement between the calculations and observations provided encouraging support for the theoretical description. The second experimental method combined resonance enhanced multiphoton ionization (REMPI) and a time-of-flight mass spectrometer (TOF MS) to quantify the relative production of die spin-orbit components of the halogen photoproducts from HI, DCl and vibrationally-excited HCl. Relating the observed REMPI yields for each quantum state to the nascent populations required a scaling factor to account for the different ionization probabilities. This was derived for the ground state of the chlorine atom by comparing the Cl(2Pj) REMPI signal recorded following photolysis of HCl at ?=205.5 nm with a direct determination obtained by HRPTS. The calibration factor was subsequently used to convert chlorine REMPI yields into the relative branching between Cl(2Pj) atoms in two studies. The first set of experiments photodissociated DCl molecules at 5 wavelengths between 200 and 220 nm and the results showed reasonable agreement with the calculations employing the aforementioned time-independent theory. The second study used infrared absorption to create HCl molecules in selected rotational states of vibrational levels v=1-3 prior to photolysis. These results were compared against values generated by time-dependent wave propagation using the existing ab initio data and highlighted some inadequacies of the theory. The REMPI-TOF MS technique was further applied to detect the iodine atoms from the photolysis of HI. The results confirmed a slight partiality to the HRPTS results that had been previously considered in a Monte Carlo simulation of the HRPTS experiment. Guided by the I-atom REMPI observations, the effect of the detection bias was successfully accounted for and the former determinations corrected. (author)

  14. On water sp3sp2 cross-couplings between benzylic and alkenyl halides

    OpenAIRE

    Krasovskaya, Valeria; Krasovskiy, Arkady; Bhattacharjya, Anish; Lipshutz, Bruce H.

    2011-01-01

    Organic-solvent-free cross-couplings between benzylic and alkenyl halides have been developed. Various alkenyl halides can be effciently benzylated by combining the precursor halides in the presence of Zn dust and a Pd catalyst at room temperature, in water as the only medium.

  15. Metal-halide mixtures for latent heat energy storage

    Science.gov (United States)

    Chen, K.; Manvi, R.

    1981-01-01

    Alkali metal and alkali halide mixtures are identified which may be suitable for thermal energy storage at temperatures above 600 C. The use of metal-halides is appropriate because of their tendency to form two immiscible melts with a density difference, which reduces scale formation and solidification on heat transfer surfaces. Also, the accumulation of phase change material along the melt interface is avoided by the self-dispersing characteristic of some metal-halides, in particular Sr-SrCl2, Ba-BaCl2, and Ba-BaBr2 mixtures. Further advantages lie in their high thermal conductivities, ability to cope with thermal shock, corrosion inhibition, and possibly higher energy densities.

  16. Chemistry of gaseous lower-valent actinide halides

    International Nuclear Information System (INIS)

    The objective of this program is to provide accurate thermochemical information for the uranium and thorium halide and oxyhalide systems, two of the key actinide systems, so that the basic factors underlying the chemical bonding and chemical reactivity in these systems can be elucidated in a systematic way. A complete characterization of the uranium and thorium systems will serve as a reliable base for modeling the behavior of the heavier actinides, which will be much more difficult to study because of sample size and safety restrictions. Our principal focus is on the the gaseous halides, mainly the lower-valent halides, because these molecular species largely will define the high-temperature chemistry in the nuclear applications of interest. 2 refs., 1 tab

  17. Built-in dipole moments of InGaN/GaN single quantum dot excitons

    Energy Technology Data Exchange (ETDEWEB)

    Ostapenko, Irina A.; Kindel, Christian; Hoenig, Gerald; Rodt, Sven; Strittmatter, Andre; Hoffmann, Axel; Bimberg, Dieter [Institut fuer Festkoerperphysik, TU Berlin, Hardenbergstr 36, 10623 Berlin (Germany)

    2011-07-01

    We report on direct determination of intrinsic dipole moments of excitonic complexes in InGaN/GaN quantum dots from cathodoluminescence experiments. Single nitride-based QDs show large potential as sources of entangled photon pairs at room temperature for quantum information processing and cryptography applications. The built-in piezoelectric and pyroelectric fields tremendously affect electro-optical properties of nitride heterostructures. The insight into interplay between confined charge carriers and electric fields is crucial for improvement and control of nitride QD-based devices. Only in cathodoluminescence we observe a re-occurring characteristic pattern in temporal traces of emission lines and explain this feature with a model of interaction between an exciton in a quantum dot and a gradually changing electric field of a charge carrier, moving through the material. We derive the magnitude of the built-in excitonic dipole moments as 0.7-7{sup *}10{sup -28}C{sup *}m{sup *}0.3-3 e{sup *}nm. These values are in good agreement with calculations based on 8-band k{sup *}p, extended by a self-consistent Hartree formalism.

  18. Evaluation of approximations in molecular exciton theory. 2. Applications to oligomeric systems of interest in photosynthesis

    Energy Technology Data Exchange (ETDEWEB)

    LaLonde, D.E.; Petke, J.D.; Maggiora, G.M. (Univ. of Kansas, Lawrence (USA))

    1989-01-26

    Molecular exciton calculations have been performed on bacteriochlorin and methyl bacteriopheophorbide a (Me-BPheo-a) oligomers in one-dimensional stacked geometries, in an investigation of spectral shifts and oscillator strengths of the X{sub y} band. Exciton matrix elements were evaluated by using both the point dipole and Shipman distributed point-charge approximations of monomeric electron densities, obtained from floating-Gaussian-based SCF-MO-CI monomer wave functions. Calculations performed with the point-dipole approximation produced unacceptable errors in both Q{sub y} spectral shifts and oscillator strengths, relative to those obtained with the more rigorous point-charge approximation. For both approximations, calculations in which only nearest-neighbor intermolecular interactions were considered significantly underestimated the spectral shifts obtained with the inclusion of all interactions. However, neglect of interactions between molecules separated by >20 {angstrom} did not significantly affect the values of calculated spectral shifts. The present point-charge-based calculations on Me-BPheo-a oligomers indicate that the method appears to account for approximately 50-60% of the Q{sub y} red shift observed in crystals. Thus, additional factors such as the use of higher quality monomeric wave functions in the exciton treatment, as well as consideration of charge-transfer and charge redistribution effects, appear necessary to obtain accurate spectral shifts.

  19. Exciton trapping in vibrationally excited organic molecules near a ZnO surface

    CERN Document Server

    Foglia, Laura; Wolf, Martin; Stähler, Julia

    2014-01-01

    We present a systematic study of the exciton population dynamics at the interface of the spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2',7'-ditertbutyl-9.9'-spirobifluorene (SP6) and the non-polar (10-10) surface of ZnO, using time-resolved excited state optical transmission spectroscopy. The photoexcited dye first undergoes intramolecular vibrational relaxation in the S1 state on a 2 to 9 ps timescale. Subsequently, the excited state transmission reveals transitions from two distinct vibrational levels of S1, with a lifetime of the vibrationally excited state that is comparable to the one of the vibrational ground state (vGS). The electronic population relaxes by (i) decay to the electronic ground state (ii) transfer to a long-lived dark state that remains populated for longer than 5 microseconds, and (iii) diffusion-limited charge transfer to the ZnO conduction band. Remarkably, the lifetime of the vibratioanlly trapped excition (exciton-vibron) and vGS exciton are not equally affected by a change of s...

  20. Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

    International Nuclear Information System (INIS)

    In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron–phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors

  1. Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

    Science.gov (United States)

    Kasahara, Yuichi; Kuroki, Kazuhiko; Yamanaka, Shoji; Taguchi, Yasujiro

    2015-07-01

    In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors.

  2. Stimulation of excitonic and defect-related luminescence in porous SiC

    Energy Technology Data Exchange (ETDEWEB)

    Torchynska, T.V. [National Polytechnic Institute, Mexico D.F. 07738 (Mexico)]. E-mail: ttorch@esfm.ipn.mx; Diaz Cano, A. [National Polytechnic Institute, Mexico D.F. 07738 (Mexico); Dybic, M. [University of South Florida, Tampa, FL 33620 (United States); Ostapenko, S. [University of South Florida, Tampa, FL 33620 (United States); Mynbaeva, M. [Ioffe Institute, St. Petersburg 194021 (Russian Federation)

    2006-04-01

    This paper presents the results of porous SiC (PsiC) characterization using photoluminescence (PL) and scanning electronic microscopy. It is shown that the intensities of visible PL bands increase monotonically with the thickness rise of PSiC layers. Intensity enhancement for excitonic PL bands (2.86, 3.05 and 3.28 eV) is attributed to the recombination rate increasing (recombination time decreasing) due to electron-hole confinement effect in SiC nano-crystallites. The intensity increasing for defect-related PL bands (1.82, 2.10, 2.30 and 2.58 eV) is assigned to grow up of defect concentrations on the PSiC surface at the etching process.

  3. Interaction driven subgap spin exciton in the Kondo insulator SmB6.

    Science.gov (United States)

    Fuhrman, W T; Leiner, J; Nikoli?, P; Granroth, G E; Stone, M B; Lumsden, M D; DeBeer-Schmitt, L; Alekseev, P A; Mignot, J-M; Koohpayeh, S M; Cottingham, P; Phelan, W Adam; Schoop, L; McQueen, T M; Broholm, C

    2015-01-23

    Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. We find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion. PMID:25659009

  4. Halide-Dependent Electronic Structure of Organolead Perovskite Materials

    KAUST Repository

    Buin, Andrei

    2015-06-23

    © 2015 American Chemical Society. Organometal halide perovskites have recently attracted tremendous attention both at the experimental and theoretical levels. These materials, in particular methylammonium triiodide, are still limited by poor chemical and structural stability under ambient conditions. Today this represents one of the major challenges for polycrystalline perovskite-based photovoltaic technology. In addition to this, the performance of perovskite-based devices is degraded by deep localized states, or traps. To achieve better-performing devices, it is necessary to understand the nature of these states and the mechanisms that lead to their formation. Here we show that the major sources of deep traps in the different halide systems have different origin and character. Halide vacancies are shallow donors in I-based perovskites, whereas they evolve into a major source of traps in Cl-based perovskites. Lead interstitials, which can form lead dimers, are the dominant source of defects in Br-based perovskites, in line with recent experimental data. As a result, the optimal growth conditions are also different for the distinct halide perovskites: growth should be halide-rich for Br and Cl, and halide-poor for I-based perovskites. We discuss stability in relation to the reaction enthalpies of mixtures of bulk precursors with respect to final perovskite product. Methylammonium lead triiodide is characterized by the lowest reaction enthalpy, explaining its low stability. At the opposite end, the highest stability was found for the methylammonium lead trichloride, also consistent with our experimental findings which show no observable structural variations over an extended period of time.

  5. Strong localization induced anomalous temperature dependence exciton emission above 300 K from SnO{sub 2} quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Pan, S. S., E-mail: sspan@issp.ac.cn, E-mail: ghli@issp.ac.cn; Li, F. D.; Liu, Q. W.; Xu, S. C.; Luo, Y. Y. [Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031 (China); Li, G. H., E-mail: sspan@issp.ac.cn, E-mail: ghli@issp.ac.cn [Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031 (China); School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026 (China)

    2015-05-07

    SnO{sub 2} quantum dots (QDs) are potential materials for deep ultraviolet (DUV) light emitting devices. In this study, we report the temperature and excitation power-dependent exciton luminescence from SnO{sub 2} QDs. The exciton emission exhibits anomalous blue shift, accompanied with band width reduction with increasing temperature and excitation power above 300 K. The anomalous temperature dependences of the peak energy and band width are well interpreted by the strongly localized carrier thermal hopping process and Gaussian shape of band tails states, respectively. The localized wells and band tails at conduction minimum are considered to be induced by the surface oxygen defects and local potential fluctuation in SnO{sub 2} QDs.

  6. Two-photon absorption and multi-exciton generation in lead salt quantum dots

    Science.gov (United States)

    Padilha, Lazaro A.; Nootz, Gero; Webster, Scott; Hagan, David J.; Van Stryland, Eric W.; Levina, Larissa; Sukhovatkin, Vlad; Sargent, Edward H.

    2010-02-01

    Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate two-photon absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with h?> 3 Eg (where Eg is the bandgap energy).

  7. Minimal Electronic Model for a Layered Nitride Halide Superconductor ?-ZrNCl

    Science.gov (United States)

    Tanaka, Hiroshi; Suzuki, Katsuhiro; Usui, Hidetomo; Kuroki, Kazuhiko

    2015-12-01

    We construct tight-binding models of the layered nitride halide superconductor LixZrNCl by first-principles calculation. 14-, 10-, 8-, and 4-orbital models are constructed, all of which reproduce the Fermi surface by the first-principles calculation, and the band structure away from the Fermi level at various levels. On the basis of the tight-binding parameters in real space, it is found that bilayer coupling is relatively strong, suggesting that a single-layer approximation may not be valid. Taking into account the on-site and nearest-neighbor electronelectron interactions, and calculating the spin and charge susceptiblities for each model, we conclude that the 8 (= text{Zr}dx2 - y2,dxy,text{N}px,py bilayer)-orbital model is the minimal model for describing low-energy physics.

  8. A study of ion-photon emission in alkali-halide crystals

    International Nuclear Information System (INIS)

    Optical emission under bombardment of the NaCl, NaI, CsCl, and CsI crystals by Ar+ ions with an energy of 25 keV was studied, and the emission quantum yield of the sputtered alkaline-metal atoms is found to be higher for chlorides than for iodides. At the same time, the crystal luminescence reveals an inverse dependence. The assumption is made that population of the excited states of sputtered atoms is caused by electrons localized at the defect levels in the forbidden band. Studies of the energy distribution of the sputtered excited particles revealed the existence of a group of slow atoms emitting upon transitions from the highly-excited states. This fact testifies to the absence of substantial metallization of the surface under ionic bombardment of alkali-halide crystals. 20 refs., 3 figs., 1 tab

  9. Optical modelling data for room temperature optical properties of organicinorganic lead halide perovskites

    Directory of Open Access Journals (Sweden)

    Yajie Jiang

    2015-06-01

    Full Text Available The optical properties of perovskites at ambient temperatures are important both to the design of optimised solar cells as well as in other areas such as the refinement of electronic band structure calculations. Limited previous information on the optical modelling has been published. The experimental fitting parameters for optical constants of CH3NH3PbI3?xClx and CH3NH3PbI3 perovskite films are reported at 297K as determined by detailed analysis of reflectance and transmittance data. The data in this study is related to the research article Room temperature optical properties of organicinorganic lead halide perovskites in Solar Energy Materials & Solar Cells [1].

  10. Modified Becke-Johnson exchange potential: improved modeling of lead halides for solar cell applications

    Directory of Open Access Journals (Sweden)

    Radi A. Jishi

    2016-01-01

    Full Text Available We report first-principles calculations, within density functional theory, on the lead halide compounds PbCl2, PbBr2, and CH3NH3PbBr3?xClx, taking into account spin-orbit coupling. We show that, when the modified Becke-Johnson exchange potential is used with a suitable choice of defining parameters, excellent agreement between calculations and experiment is obtained. The computational model is then used to study the effect of replacing the methylammonium cation in CH3NH3PbI3 and CH3NH3PbBr3 with either N2H5+or N2H3+, which have slightly smaller ionic radii than methylammonium. We predict that a considerable downshift in the values of the band gaps occurs with this replacement. The resulting compounds would extend optical absorption down to the near-infrared region, creating excellent light harvesters for solar cells.

  11. An optical criterion to obtain miscible mixed crystals in alkali halides

    Scientific Electronic Library Online (English)

    R, Rodrguez-Mijngos; G, Vzquez-Polo; J.J., Palafox; R, Prez-Salas.

    2008-11-01

    Full Text Available Este trabajo ofrece un nuevo criterio para predecir la formacin de soluciones slidas en halogenuros alcalinos cristalinos y discute algunos resultados obtenidos en el desarrollo de mezclas dielctricas cristalinas miscibles de halogenuros alcalinos ternarias y cuaternarias. Estas mezclas son misci [...] bles en cualquier concentracin de sus componentes. Tener el resultado de estas mezclas cristalinas est relacionado al centro F a travs del comportamiento observado en la banda F de absorcin en funcin de la constante de red de los halogenuros alcalinos donde el defecto fue formado (centro F). Dando un vistazo a la grfica de Energa de banda F versus constante de red (ley de Mollwo-Ivey), se observa un conjunto de puntos, que dan la pauta (tal como KCl, KBr, RbCl), de posibles mezclas de materiales correspondientes a puntos adyacentes y una solucin slida podra formarse, significando un cristal de una sola fase, que dan por resultado cristales ternarios y cuaternarios. As, la banda F de absorcin nos permite tener un criterio numrico, basado en el porcentaje de cambio de la energa de la banda F que permite obtener soluciones slidas. Encontramos informacin experimental, usando difractogramas de las mezclas cristalinas, se obtienen las constantes de red y se comparan con la obtenida tericamente a travs de la generalizacin de la Regla de Vegard, finalmente se discute la posibilidad de crecer cristales partiendo de cinco componentes, tomando cinco puntos consecutivos en la grafica de la Ley de Mollwo-Ivey. Abstract in english This work gives a novel criterion to predict the formation of alkali halide solid solutions and discusses some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. These mixtures are miscible in any concentration of their component [...] s. The miscibility of these mixed crystals is quite related to the F center through the behavior observed in the spectral position of the optical absorption F band as a function of the lattice constant of the alkali halide where the F center was formed. By inspection of an energy graph of the F band energy versus lattice constant (Mollwo-Ivey law), a set of points is observed corresponding to several pure alkali halides (such as KCl, KBr, RbCl), which gives a notion of possible mixed materials that would correspond to adjacent points and a solid solution could be obtained, meaning a single phase crystal, which result in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows have a numerical criterion, based on the percentage respective of the F band energy, in order to predict possible solid solutions. We obtained experimental information using diffractograms of the mixed crystals, from which the lattice constant was obtained and compared with a theoretical calculus using a generalization of Vegard's law, finally it is discussed the case of a crystal growing, starting from five components, picking up five consecutive dots from the graph of Mollwo-Ivey's law.

  12. Thallous and cesium halide materials for use in cryogenic applications

    International Nuclear Information System (INIS)

    Certain thallous and cesium halides, either used alone or in combination with other ceramic materials, are provided in cryogenic applications such as heat exchange material for the regenerator section of a closed-cycle cryogenic refrigeration section, as stabilizing coatings for superconducting wires, and as dielectric insulating materials. The thallous and cesium halides possess unusually large specific heats at low temperatures, have large thermal conductivities, are nonmagnetic, and are nonconductors of electricity. They can be formed into a variety of shapes such as spheres, bars, rods, or the like and can be coated or extruded onto substrates or wires. (author)

  13. Ultrafast time-resolved spectroscopy of lead halide perovskite films

    Science.gov (United States)

    Idowu, Mopelola A.; Yau, Sung H.; Varnavski, Oleg; Goodson, Theodore

    2015-09-01

    Recently, lead halide perovskites which are organic-inorganic hybrid structures, have been discovered to be highly efficient as light absorbers. Herein, we show the investigation of the excited state dynamics and emission properties of non-stoichiometric precursor formed lead halide perovskites grown by interdiffusion method using steady-state and time-resolved spectroscopic measurements. The influence of the different ratios of the non-stoichiometric precursor solution was examined. The observed photoluminescence properties were correlated with the femtosecond transient absorption measurements.

  14. Layered structures of organic/inorganic hybrid halide perovskites

    OpenAIRE

    Huan, Tran Doan; Tuoc, Vu Ngoc; Minh, Nguyen Viet

    2015-01-01

    Organic/inorganic hybrid halide perovskites, formed by substituting the cations A of ABX$_3$ halide perovskites with certain organic cations, may be used for solar thermoelectric applications. In this work, we systematically study three lead-free hybrid perovskites, i.e., methylammonium tin iodide CH$_3$NH$_3$SnI$_3$, ammonium tin iodide NH$_4$SnI$_3$, and formamidnium tin iodide HC(NH$_2$)$_2$SnI$_3$, by first-principles calculations. We find that in addition to the commonl...

  15. Reactivity of halide and pseudohalide ligands in transition metal complexes

    International Nuclear Information System (INIS)

    The review generalizes experimental material on reactions of coordinated halide ligands, as well as cyanide, azide, thiocyanate and cyanate ligands in transition metal (Mo, W, Ru, etc.) complexes. It is shown that transformation of the intrasphere pseudohalide ligands is a very important method of directed synthesis of coordination compounds. The tendency of halide and pseudohalide ions to the formation of low-soluble salts or strong complexes can be widely used in preparation coordination chemistry for the ''forced'' introduction of solvent molecules into the complexes

  16. Facile Preparation of Silver Halide Nanoparticles as Visible Light Photocatalysts

    OpenAIRE

    Cui, Linfan; Jiao, Tifeng; Zhang, Qingrui; Zhou, Jingxin; Peng, Qiuming

    2015-01-01

    In this study, highly efficient silver halide (AgX)-based photocatalysts were successfully fabricated using a facile and template-free direct-precipitation method. AgX nanoparticles, which included silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI), were synthesized using different potassium halides and silver acetate as reactive sources. The size distribution of the AgX nanopar? ticles was determined by the reaction time and ratio of the reagents, which were monitored by U...

  17. Excitons or spaced-charged-perturbed fields in irradiated SiO2

    International Nuclear Information System (INIS)

    Flat-band voltage shifts in MOS capacitors irradiated by a kilovolt electron beam have been measured. The relatively low dose conditions employed resulted in the data points being obtained under non-equilibrium conditions and therefore sensitive to the exact manner in which the bias voltage across the sample was cycled. All the results obtained can be explained by considering the transient growth and decay of hole space-charges at the oxide interfaces with the metal electrode and the silicon substrate. No evidence was found to support the role of excitons in generating hole space charge at the oxide/silicon interface as postulated by previous authors. (author)

  18. Strongly driven exciton resonances in quantum wells: light-induced dressing versus coulomb scattering

    Science.gov (United States)

    Ciuti; Piermarocchi; Savona; Selbmann; Schwendimann; Quattropani

    2000-02-21

    The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra. PMID:11017617

  19. Fundamental optical spectra and excitons of crystalline and amorphous GeO2

    International Nuclear Information System (INIS)

    Full text : The experimental GeO2 reflectivity spectra are adversed, the model of the photoemission spectra similarity between GeO2 and SiO2 are unknown and the results of the many known theoretical energy structure calculations are very unfull. It was calculated the spectra of the four optical functions complexes using these experimental spectra. The maxima of the decomposed spectra of crystals are caused by the direct interband transitions of metastable excitons. The energy of the theoretically possible transitions and their localization are obtained from the known theoretical bands

  20. Exciton States of Dynamic DNA Double Helices: Alternating dCdG Sequences

    OpenAIRE

    Emanuele, E; Zakrzewska, K; Markovitsi, D.; Lavery, R; Millie, P.

    2005-01-01

    The present communication deals with the excited states of the alternating DNA oligomer (dCdG)5(dCdG)5 which correspond to the UV absorption band around 260 nm. Their properties are studied in the frame of the exciton theory, combining molecular dynamics simulations and quantum chemistry data. It is shown that the dipolar coupling undergoes important variations with the site and the helix geometry. In contrast, the energy of the monomer transitions within the double helix is not sensitive to...

  1. Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films

    Directory of Open Access Journals (Sweden)

    Christian Wehrenfennig

    2014-08-01

    Full Text Available The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3?xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL emission of vapor-deposited CH3NH3PbI3?xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.

  2. Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films

    Science.gov (United States)

    Wehrenfennig, Christian; Liu, Mingzhen; Snaith, Henry J.; Johnston, Michael B.; Herz, Laura M.

    2014-08-01

    The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3-xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3-xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.

  3. Contribution of the exciton--photon interaction to the exciton absorption line shape in the spatially inhomogeneous wave model: Application to the a-singlet exciton of naphthalene

    International Nuclear Information System (INIS)

    The problem of determining the contribution of the exciton--photon coupling to the absorption line shape or width of an exciton is addressed. In this regard, both the spatially homogeneous (polariton) and inhomogeneous wave models are considered. For the latter model, the theory is complex although a line shape expression is derived for the case of high exciton velocity and low oscillator strength. The related problem of determining the conditions under which the strong exciton--photon coupling limit is attained is also discussed. It is in this limit that light attenuation is independent of the oscillator strength. The theory plus new experimental data on the a exciton of naphthalene argue for the inadequacy of the polariton model. On the other hand, reasonable agreement between the data and the spatially inhomogeneous wave theory developed here is obtained. Thus, it is possible to estimate that the exciton--photon coupling contribution to the a-exciton linewidth is 0.26 cm-1. This is the major portion of the 2 K linewidth of 0.3 cm-1

  4. Contribution of the exciton--photon interaction to the exciton absorption line shape in the spatially inhomogeneous wave model: Application to the a-singlet exciton of naphthalene

    Energy Technology Data Exchange (ETDEWEB)

    Robinette, S.L.; Stevenson, S.H.; Small, G.J.

    1978-12-15

    The problem of determining the contribution of the exciton--photon coupling to the absorption line shape or width of an exciton is addressed. In this regard, both the spatially homogeneous (polariton) and inhomogeneous wave models are considered. For the latter model, the theory is complex although a line shape expression is derived for the case of high exciton velocity and low oscillator strength. The related problem of determining the conditions under which the strong exciton--photon coupling limit is attained is also discussed. It is in this limit that light attenuation is independent of the oscillator strength. The theory plus new experimental data on the a exciton of naphthalene argue for the inadequacy of the polariton model. On the other hand, reasonable agreement between the data and the spatially inhomogeneous wave theory developed here is obtained. Thus, it is possible to estimate that the exciton--photon coupling contribution to the a-exciton linewidth is 0.26 cm/sup -1/. This is the major portion of the 2 K linewidth of 0.3 cm/sup -1/.

  5. Excitonvibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

    International Nuclear Information System (INIS)

    The influence of excitonvibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these proteinpigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of excitonvibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of excitonvibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of excitonvibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the excitonvibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real systembath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be given in terms of a stochastic decoupling ansatz. This method has become the standard in excitonvibrational theory and illustrative examples will be presented as well as a comparison with ML-MCTDH. Applications will be shown for generic model systems as well as for small aggregates mimicking those formed by perylene bisimide dyes. Further, photosynthetic antenna complexes will be discussed, including spectral densities and the role of excitonvibrational coupling in two-dimensional electronic spectroscopy

  6. Magnetic excitons in singlet-ground-state ferromagnets

    DEFF Research Database (Denmark)

    Birgeneau, R.J.; Als-Nielsen, Jens Aage

    1971-01-01

    The authors report measurements of the dispersion of singlet-triplet magnetic excitons as a function of temperature in the singlet-ground-state ferromagnets fcc Pr and Pr3Tl. Well-defined excitons are observed in both the ferromagnetic and paramagnetic regions, but with energies which are nearly temperature independent, in disagreement with existing theory

  7. Dynamic Fano Resonance of Quasienergy Excitons in Superlattices

    CERN Document Server

    Liu, R B; Liu, Ren-Bao; Zhu, Bang-fen

    2000-01-01

    The dynamic Fano resonance (DFR) between discrete quasienergy excitons and sidebands of their ionization continua is predicted and investigated in dc- and ac-driven semiconductor superlattices. This DFR, well controlled by the ac field, delocalizes the excitons and opens an intrinsic decay channel in nonlinear four-wave mixing signals.

  8. Angular momentum dependence of transition rates in exciton model

    International Nuclear Information System (INIS)

    Based on statistical approach, simple analytical expressions have been derived for the spin part of the intermediate-state transition rates in the exciton model. The master-equation coefficients are related to those of exciton model without account of angular momentum conservation. The effect of spin conservation on the establishment of the thermodynamic equilibrium is discussed. (author). 13 refs., 6 figs

  9. Band gap tuning of ZnO nanoparticles via Mg doping by femtosecond laser ablation in liquid environment

    International Nuclear Information System (INIS)

    Highlights: ? Femtosecond laser ablation synthesis of Mg doped ZnO nanoparticles. ? Electronic properties of ZnO are modified by Mg. ? Band gap and exciton energy shifts to the blue. ? The exciton energy shift is saturated at Mg content of about 20%. ? Phase separation at Mg content is at more than 25%. ? Mechanism of exciton pinning recombination via new surface states. - Abstract: We use multiphoton IR femtosecond laser ablation to induce non-thermal non-equilibrium conditions of the nanoparticle growth in liquids. Modifications of the electronic properties of ZnO NP were achieved by Mg ion doping of targets prepared from mixtures of Zn and Mg acetylacetonates. The nanoparticle sizes were 320 nm depending on the ablation conditions. X-ray fluorescence indicates that stoichiometric ablation and incorporation of Mg in nanocrystalline ZnO occurs. HRTEM observations show that nanoparticles retain their wurtzite structure, while at high Mg concentrations we detect the MgO rich domains. Exciton emissions exhibit relatively narrow bands with progressive and controlled blue shifts up to 184 meV. The exciton energy correlates to band edge absorption indicating strong modification of the NP band gaps. Stabilisation of the exciton blue shift is observed at high Mg concentration. It is accompanied by the formation of structure defects and ZnO/MgO phase separation within the nanoparticles.

  10. Optical diode based on exciton-polaritons

    Science.gov (United States)

    Espinosa-Ortega, T.; Liew, T. C. H.; Shelykh, I. A.

    2013-11-01

    We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.

  11. Spatio--temporal dynamics of quantum--well excitons

    CERN Document Server

    Zhao, H; Moehl, S; Kalt, H; Zhao, Hui; Moehl, Sebastian; Kalt, Heinz

    2003-01-01

    We investigate the lateral transport of excitons in a ZnSe quantum well by using time-resolved micro-photoluminescence enhanced by solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effect, in consistence with our previous observations. The coupled transport-relaxation process of hot excitons is modeled by Monte Carlo simulation. We prove that two basic assumptions generally accepted in optical investigations on excitonic transport, namely (i) the classical diffusion model as well as (ii) the equivalence between the temporal and spatial evolution of the exciton population and of the measured differential absorption/photoluminescence, are not valid. Since the models used for the analysis of experimental data were based on unreasonable assumptions, the values of diffusivity, mobility and diffusion length deduced from these investigations have to be re-consi...

  12. Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton states.

    Science.gov (United States)

    Iwamura, Munechiyo; Akizuki, Naoto; Miyauchi, Yuhei; Mouri, Shinichiro; Shaver, Jonah; Gao, Zhenghong; Cognet, Laurent; Lounis, Brahim; Matsuda, Kazunari

    2014-11-25

    We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics. PMID:25331628

  13. Exciton complexes in low dimensional transition metal dichalcogenides

    Energy Technology Data Exchange (ETDEWEB)

    Thilagam, A., E-mail: thilaphys@gmail.com [Information Technology, Engineering and Environment, University of South Australia, Adelaide 5095 (Australia)

    2014-08-07

    We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter ?, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of ? is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.

  14. Calculating exciton downconversion rates in Coulombically coupled chromophores

    Science.gov (United States)

    Chapman, Craig; Schatz, George

    2015-03-01

    Exciton downconversion is a second order energy transfer process that splits a high energy exciton in a donor chromophore into multiple lower energy excitons in acceptor chromophores. Downconversion has been seen in a variety of materials including rare-earth doped glassy matrices, organic crystals, and semiconductor nanocrystals, and has the potential to efficiently convert a single high energy photon into a broad distribution of lower energy excitons. A comprehensive mechanistic understanding of the energy conversion process will allow for the rational engineering of materials that can control the flow of energy in a guided fashion. To this end we formulate and implement a method for calculating multi-chromophore Frster-like exciton transfer rates using transition charges obtained from time-dependent density functional theory.

  15. Dynamics of a single exciton in strongly correlated bilayers

    Science.gov (United States)

    Rademaker, Louk; Wu, Kai; Zaanen, Jan

    2012-08-01

    We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit when the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin-wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling ? = J?/Jz. In the disordered phase at large ?, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Nel phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.

  16. Methyl halide emission estimates from domestic biomass burning in Africa

    Science.gov (United States)

    Mead, M. I.; Khan, M. A. H.; White, I. R.; Nickless, G.; Shallcross, D. E.

    Inventories of methyl halide emissions from domestic burning of biomass in Africa, from 1950 to the present day and projected to 2030, have been constructed. By combining emission factors from Andreae and Merlet [2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochemical Cycles 15, 955-966], the biomass burning estimates from Yevich and Logan [2003. An assessment of biofuel use and burning of agricultural waste in the developing world. Global Biogeochemical Cycles 17(4), 1095, doi:10.1029/2002GB001952] and the population data from the UN population division, the emission of methyl halides from domestic biomass usage in Africa has been estimated. Data from this study suggest that methyl halide emissions from domestic biomass burning have increased by a factor of 4-5 from 1950 to 2005 and based on the expected population growth could double over the next 25 years. This estimated change has a non-negligible impact on the atmospheric budgets of methyl halides.

  17. Halide glass containing trivalent uranium ions and its fabrication process

    International Nuclear Information System (INIS)

    This halide glass, showing an optical attenuation -1 in the near infrared from 2.2 to 304 micrometers, is prepared with a glass containing uranium ions as U4+ and/or U5+ reduced by ionizing radiations in U3+. Application is made to the fabrication of optical fibers and lasers doped with trivalent uranium

  18. Method for calcining nuclear waste solutions containing zirconium and halides

    International Nuclear Information System (INIS)

    A reduction in the quantity of gelatinous solids which are formed in aqueous zirconium-fluoride nuclear reprocessing waste solutions by calcium nitrate added to suppress halide volatility during calcination of the solution while further suppressing chloride volatility is achieved by increasing the aluminum to fluoride mole ratio in the waste solution prior to adding the calcium nitrate

  19. Taming excitons in II–VI semiconductor nanowires and nanobelts

    International Nuclear Information System (INIS)

    Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II–VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II–VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II–VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton–exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton–phonon interaction, which has interesting applications for laser cooling; and (3) exciton–plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II–VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology. (paper)

  20. The search for BoseEinstein condensation of excitons in Cu2O: exciton-Auger recombination versus biexciton formation

    International Nuclear Information System (INIS)

    Excitons in high-purity crystals of Cu2O undergo a density-dependent lifetime that opposes BoseEinstein condensation (BEC). This rapid decay rate of excitons at a density n has generally been attributed to Auger recombination having the form dn/dt=?An2, where A is an exciton-Auger constant. Various measurements of A, however, have reported values that are orders-of-magnitude larger than the existing theory. In response to this conundrum, recent work has suggested that excitons bind into excitonic molecules, or biexcitons, which are short-lived and expected to be optically inactive. Of particular interest is the case of excitons confined to a parabolic strain wella method that has recently achieved exciton densities approaching BEC. In this paper we report time- and space-resolved luminescence data that supports the existence of short-lived biexcitons in a strain well, implying an exciton loss rate of the form dn/dt=?2Cn2 with a biexciton capture coefficient C(T) proportional to 1/T, as predicted by basic thermodynamics. This alternate theory will be considered in relation to recent experiments on the subject. (paper)

  1. Slow propagation of photon-like polaritons generated by exciton-exciton scattering in ZnO thin films

    OpenAIRE

    Kim D.; Mizoguchi K.; Kawase T; Wakaiki S.; Ichida H.; Nakayama M.; Kanematsu Y.

    2013-01-01

    We report on the first observation of the thickness-dependent photoluminescence-decay time of exciton-exciton scattering in ZnO thin films, which indicates the slower propagation of photon-like polaritons compared to that in bulk by two orders magnitude.

  2. Exciton mechanisms and modeling of the ionoluminescence in silica

    Science.gov (United States)

    Bachiller-Perea, D.; Jiménez-Rey, D.; Muñoz-Martín, A.; Agulló-López, F.

    2016-03-01

    A theoretical model is presented in order to discuss detailed kinetic data describing the evolution of the two main ionoluminescence bands at 650 nm (1.9 eV) and 460 nm (2.7 eV) in silica as a function of the irradiation fluence at room temperature. The model is based on the generation of self-trapped excitons (STEs), their hopping migration through the silica network and their recombination at non-bridging oxygen hole and Type II oxygen-deficient centers to produce the red and blue emission bands, respectively. For heavy ions, which have a high electronic stopping power, the two emission yields experience a fast initial growth with fluence up to a maximum value and then decrease at a fairly comparable rate. The fluence for such a maximum strongly increases on decreasing ion mass and stopping power and, finally, for H and He it is not observed for any of the investigated energy and fluence range. This kinetic behavior is explained in terms of the strong structural distortions (compaction) induced by the heavy-ion irradiations. In particular, it is proposed that these strong structural distortions cause a significant decrease in the STE migration length and, consequently, in the recombination rates at the two active recombination centers. The model offers a good quantitative accordance with detailed infrared spectroscopy reporting on the changes in the frequency ω 4 of a first-order vibrational mode in the SiO2 network as a function of irradiation fluence.

  3. Exciton dynamics in semiconductor quantum dot structures

    Energy Technology Data Exchange (ETDEWEB)

    Schwab, M.

    2005-07-01

    The aim of this thesis is to contribute a small piece of new information to this vast field of research, and propose ways of controlling the radiative recombination of excitons in quantum dots. A review of growth techniques and optical properties in connection with the theoretical background of optical processes in quantum dots is presented in Chapter 2. The working principle of a streak camera, used for most of the experiments will be given before we turn our point of interest to quantum dot molecules in Chapter 3. This chapter is split to the investigation of exciton dephasing and time resolved photoluminescence, which are combined to extract new insight to quantum dot molecules. The effects of applying external electric and magnetic fields in order to tailor radiative recombination processes in quantum dots are topic of Chapter 4. A new technique for lateral electric fields is presented. Magnetic fields are applied to influence the QD confinement and energy levels. Single photon devices are of key importance for quantum cryptography. In this context, quantum dots embedded in a microresonator are under discussion, as an increased light field results in an increase of the spontaneous emission rate, thus making photons on demand possible. Measurements on micropillars and photonic crystals will be resumed in Chapter 5. The results will be summarized in Chapter 6 before final remarks and an outlook will conclude this thesis in Chapter 7.

  4. PbSe Nanocrystal Excitonic Solar Cells

    KAUST Repository

    Choi, Joshua J.

    2009-11-11

    We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that Is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date. © 2009 American Chemical Society.

  5. Exciton-Polariton Bose-Einstein Condensates

    Science.gov (United States)

    Deveaud, Benot

    2015-03-01

    Exciton-polaritons, mixed light-matter quasiparticles in semiconductors, have recently shown evidence for Bose-Einstein condensation. Some of the properties of condensates of exciton-polaritons are reviewed in this article. We first discuss the spontaneous appearance of long-range order and the way this can be easily accessed in the case of polariton fluids. We show that the Penrose-Onsager criterion is valid even for such a very special case of condensate. We then describe the experiments that allow observation of topological defects in the fluid: quantized vortices, half vortices, and hyperbolic spin vortices. We demonstrate through the comparison with the gross Pitaevskii equation that the appearance and stability of such vortices are linked with the dissipative nature of the condensate together with the presence of disorder. We then briefly summarize the experiments on superfluid behavior of the polaritons at large-enough densities and expand somewhat more on the dynamical behavior of turbulence in the wake of an obstacle, with the appearance of vortex streets. We finally show that the Bogoliubov transformation has been revealed through four-wave mixing experiments.

  6. Exciton coherence in semiconductor quantum dots

    International Nuclear Information System (INIS)

    The coherent dynamics of excitons in InAs quantum dots (QDs) was investigated in the telecommunication wavelength range using a transient four-wave mixing technique. The sample was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to fabricate a 150-layer stacked QD structure for obtaining a high S/N in the four-wave mixing measurements, although no high-sensitive heterodyne detection was carried out. The dephasing time and transition dipole moment were precisely estimated from the polarization dependence of signals, taking into account their anisotropic properties. The population lifetimes of the excitons were also measured by using a polarization-dependent pumpprobe technique. A quantitative comparison of these anisotropies demonstrates that in our QDs, non-radiative population relaxation, polarization relaxation and pure dephasing are considerably smaller than the radiative relaxation. A comparison of the results of the four-wave mixing and pump-probe measurements revealed that the pure dephasing could be directly estimated with an accuracy of greater than 0.1 meV by comparing the results of four-wave mixing and pump-probe measurements. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  7. Coherent dynamics of mixed Frenkel and Charge Transfer Excitons in Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

    CERN Document Server

    Fujita, Takatoshi; Sawaya, Nicolas P D; Aspuru-Guzik, Alan

    2016-01-01

    Charge transfer states in organic semiconductors play crucial roles in processes such as singlet fission and exciton dissociation at donor/acceptor interfaces. Recently, a time-resolved spectroscopy study of dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene (DNTT) thin films provided evidence for the formation of mixed Frenkel and charge-transfer excitons after the photoexcitation. Here we investigate optical properties and excitation dynamics of the DNTT thin films by combining ab initio calculations and a stochastic Schrodinger equation. Our theory predicts that the low-energy Frenkel exciton band consists of 8 to 47% CT character. The quantum dynamics simulations show coherent dynamics of Frenkel and CT states in 50 fs after the optical excitation. We demonstrate the role of charge delocalization and localization in the mixing of CT states with Frenkel excitons as well as the role of their decoherence.

  8. Effects of inter-molecular charge-transfer excitons on the external quantum efficiency of zinc-porphyrin/C60 heterojunction photovoltaic cells

    International Nuclear Information System (INIS)

    We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C60/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons.

  9. Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods.

    Science.gov (United States)

    Wu, Kaifeng; Hill, Lawrence J; Chen, Jinquan; McBride, James R; Pavlopolous, Nicholas G; Richey, Nathaniel E; Pyun, Jeffrey; Lian, Tianquan

    2015-04-28

    A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications. PMID:25803834

  10. Combined effects of intense laser field and applied electric field on exciton states in GaAs quantum wells: Transition from the single to double quantum well

    Energy Technology Data Exchange (ETDEWEB)

    Duque, C.A. [Instituto de Fisica, Universidad de Antioquia, Medellin (Colombia); Mora-Ramos, M.E. [Facultad de Ciencias, Universidad Autonoma del Estado de Morelos, Cuernavaca, Morelos (Mexico); Kasapoglu, E.; Sari, H. [Physics Department, Cumhuriyet University, Sivas (Turkey); Soekmen, I. [Physics Department, Dokuz Eyluel University, Izmir (Turkey)

    2012-01-15

    The effects of intense laser radiation on the exciton states in GaAs-Ga{sub 1-x}Al{sub x}As quantum wells are studied with the inclusion of applied dc electric fields oriented along the growth direction of the system. The calculations are made within the effective mass and parabolic band approximations. The intense laser effects have been included along the lines of the Floquet method, modifying the confinement potential associated to the heterostructure. The results for the exciton binding energy, the energy of the exciton-related photoluminescence peak, and the carriers overlap integral are presented for several configurations of the quantum well size, the strength of the applied electric fields, and the incident laser radiation. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  11. Excitons in coupled type-II double quantum wells under electric and magnetic fields: InAs/AlSb/GaSb

    Science.gov (United States)

    Lyo, S. K.; Pan, W.

    2015-11-01

    We calculate the wave functions and the energy levels of an exciton in double quantum wells under electric (F) and magnetic (B) fields along the growth axis. The result is employed to study the energy levels, the binding energy, and the boundary on the F-B plane of the phase between the indirect exciton ground state and the semiconductor ground state for several typical structures of the type-II quasi-two-dimensional quantum wells such as InAs/AlSb/GaSb. The inter-well inter-band radiative transition rates are calculated for exciton creation and recombination. We find that the rates are modulated over several orders of magnitude by the electric and magnetic fields.

  12. Observation of oscillations in energy of Aharonov-Bohm excitons in type-II ZnTe/ZnSe stacked submonolayer quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Roy, Bidisha; Ji, Haojie; Dhomkar, Siddharth; Kuskovsky, Igor L. [Department of Physics, Queens College of CUNY, 65-30 Kissena Blvd., Queens, NY 11367 (United States); The Graduate Center of CUNY, 365 Fifth Ave, New York, NY 10016 (United States); Tamargo, Maria C. [The Graduate Center of CUNY, 365 Fifth Ave, New York, NY 10016 (United States); Department of Chemistry, City College of CUNY, 160 Convent Ave, New York, NY 10031 (United States)

    2014-07-15

    We report our recent experiments with type-II ZnTe/ZnSe stacked submonolayer quantum dots that show the dependence of photoluminescence (PL) emission energy on the magnetic field. This is explained as due to the excitonic Aharonov-Bohm (AB) effect. The AB oscillations in energy are observed for each of the contributing emission bands and are correlated with the changes in the magneto-PL emission intensity. We propose that interplay of magneto-excitonic behavior, the Coulomb interaction and mixing of angular momentum states are responsible for the observed behavior of the AB excitons depending on the magnetic field ranges. (copyright 2014 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  13. Theory of exciton transfer and diffusion in conjugated polymers

    Energy Technology Data Exchange (ETDEWEB)

    Barford, William, E-mail: william.barford@chem.ox.ac.uk [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); Tozer, Oliver Robert [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); University College, University of Oxford, Oxford OX1 4BH (United Kingdom)

    2014-10-28

    We describe a theory of Frster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.

  14. Theory of exciton transfer and diffusion in conjugated polymers

    International Nuclear Information System (INIS)

    We describe a theory of Frster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations

  15. Luminescence, autolocalization and decay of excitons on defects in ionic crystals (CsBr)

    International Nuclear Information System (INIS)

    The review is made of investigations on specific features of excitons in alkali haloid crystals: existence of free and autolocalized excitons, autolocalizing excitons with defects production, defects recombination with luminescent exciton production, cubic crystals (mainly CsBr) taken as an exsample

  16. On the theory of phonoriton in cubic semiconductors with a degenerate valence band

    International Nuclear Information System (INIS)

    The ''phonoriton'' is an elementary excitation constructed from an exciton polariton and phonon in semiconductors under intense excitation by an electromagnetic wave near the exciton resonance (L.V. Keldysh and A.L. Ivanov, 1982). In this paper we develop a theory of phonoriton in direct band gap cubic semiconductor with a degenerate valence band using the simple model of J.L. Birman and B.S. Wang (1990). In addition to experimental proofs of the existence of phonoriton we propose an experiment to measure its flight time. (author). 33 refs

  17. Two-Dimensional Valley Electrons and Excitons in Noncentrosymmetric 3 R -MoS2

    Science.gov (United States)

    Akashi, Ryosuke; Ochi, Masayuki; Bordcs, Sndor; Suzuki, Ryuji; Tokura, Yoshinori; Iwasa, Yoshihiro; Arita, Ryotaro

    2015-07-01

    We find that the motion of the valley electronselectronic states close to the K and K' points of the Brillouin zoneis confined into two dimensions when the layers of MoS2 form the 3 R stacking, while in the 2 H polytype, the bands have dispersion in all three dimensions. According to our first-principles band-structure calculations, the valley states have no interlayer hopping in 3 R -MoS2 , which is proven to be the consequence of the rotational symmetry of the Bloch functions. By measuring the reflectivity spectra and analyzing an anisotropic hydrogen-atom model, we confirm that the valley excitons in 3 R -MoS2 have two-dimensional hydrogenlike spectral series, and the spreads of the wave function are smaller than the interlayer distance. In contrast, the valley excitons in 2 H -MoS2 are well described by the three-dimensional model and, thus, not confined in a single layer. Our results indicate that the dimensionality of the valley degree of freedom can be controlled simply by the stacking geometry, which can be utilized in future valleytronics.

  18. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites.

    Science.gov (United States)

    Lee, Michael M; Teuscher, Jol; Miyasaka, Tsutomu; Murakami, Takurou N; Snaith, Henry J

    2012-11-01

    The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite. PMID:23042296

  19. Theoretical study of excitonic complexes in semiconductors quantum wells; Estudo teorico de complexos excitonicos em pocos quanticos de semicondutores

    Energy Technology Data Exchange (ETDEWEB)

    Dacal, Luis Carlos Ogando

    2001-08-01

    A physical system where indistinguishable particles interact with each other creates the possibility of studying correlation and exchange effect. The simplest system is that one with only two indistinguishable particles. In condensed matter physics, these complexes are represented by charged excitons, donors and acceptors. In quantum wells, the valence band is not parabolic, therefore, the negatively charged excitons and donors are theoretically described in a simpler way. Despite the fact that the stability of charged excitons (trions) is known since the late 50s, the first experimental observation occurred only at the early 90s in quantum well samples, where their binding energies are one order of magnitude larger due to the one dimensional carriers confinement. After this, these complexes became the subject of an intense research because the intrinsic screening of electrical interactions in semiconductor materials allows that magnetic fields that are usual in laboratories have strong effects on the trion binding energy. Another rich possibility is the study of trions as an intermediate state between the neutral exciton and the Fermi edge singularity when the excess of doping carriers is increased. In this thesis, we present a theoretical study of charged excitons and negatively charged donors in GaAs/Al{sub 0.3}Ga{sub 0.7}As quantum wells considering the effects of external electric and magnetic fields. We use a simple, accurate and physically clear method to describe these systems in contrast with the few and complex treatments s available in the literature. Our results show that the QW interface defects have an important role in the trion dynamics. This is in agreement with some experimental works, but it disagrees with other ones. (author)

  20. Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

    KAUST Repository

    Andernach, Rolf

    2015-07-22

    We report the synthesis of a novel polythiophene-based host-guest copolymer incorporating a Pt-porphyrin complex (TTP-Pt) into the backbone for efficient singlet to triplet polymer exciton sensitization. We elucidated the exciton dynamics in thin films of the material by means of Transient Absorption Spectrosopcy (TAS) on multiple timescales and investigated the mechanism of triplet exciton formation. During sensitization, single exciton diffusion is followed by exciton transfer from the polymer backbone to the complex where it undergoes intersystem crossing to the triplet state of the complex. We directly monitored the triplet exciton back transfer from the Pt-porphyrin to the polymer and find that 60% of the complex triplet excitons are transferred with a time constant of 1087 ps. We propose an equilibrium between polymer and porphyrin triplet states as a result of the low triplet diffusion length in the polymer backbone and hence an increased local triplet population resulting in increased triplet-triplet annihilation. This novel system has significant implications for the design of novel materials for triplet sensitized solar cells and up-conversion layers.

  1. Thermodynamic Origin of Photoinstability in the CH3NH3Pb(I1-xBrx)3 Hybrid Halide Perovskite Alloy.

    Science.gov (United States)

    Brivio, Federico; Caetano, Clovis; Walsh, Aron

    2016-03-17

    The formation of solid-solutions of iodide, bromide, and chloride provides the means to control the structure, band gap, and stability of hybrid halide perovskite semiconductors for photovoltaic applications. We report a computational investigation of the CH3NH3PbI3/CH3NH3PbBr3 alloy from density functional theory with a thermodynamic analysis performed within the generalized quasi-chemical approximation. We construct the phase diagram and identify a large miscibility gap, with a critical temperature of 343 K. The observed photoinstability in some mixed-halide solar cells is explained by the thermodynamics of alloy formation, where an initially homogeneous solution is subject to spinodal decomposition with I and Br-rich phases, which is further complicated by a wide metastable region defined by the binodal line. PMID:26952337

  2. Enhancement of Exciton Emission from Zno Nanocrystalline Films by Pulsed Laser Annealing

    CERN Document Server

    Ozerov, I; Safarov, V I; Marine, W; Giorgio, S; Sentis, M; Ozerov, Igor; Arab, Madjid; Safarov, Viatcheslav I.; Marine, Wladimir; Giorgio, Suzanne; Sentis, Marc; ccsd-00000831, ccsd

    2004-01-01

    Pulsed ArF laser annealing in air and in hydrogen atmosphere improves the optical properties of ZnO nanostructured films. Independently on the ambient atmosphere, laser annealing produces two major effects on the photoluminescence (PL) spectra: first, the efficiency of the exciton PL increases due to decrease of the number of non-radiative recombination centers; second, the intensity of the defect-related orange band decreases because of the removing of excessive oxygen trapped into the films during deposition. However, annealing in the ambient air also increases the intensity of the green band related to oxygen vacancies. We show that the combination of laser annealing and passivation of oxygen vacancies by hydrogen results in films free of defect-related emission and keeps intact their nanostructural character.

  3. Matrix isolation infrared spectra of hydrogen halide and halogen complexes with nitrosyl halides

    Science.gov (United States)

    Allamandola, Louis J.; Lucas, Donald; Pimentel, George C.

    1982-01-01

    Matrix isolation infrared spectra of nitrosyl halide (XNO) complexes with HX and X2 (X = Cl, Br) are presented. The relative frequency shifts of the HX mode are modest (ClNO H-Cl, delta-nu/nu = -0.045; BrNO H-Br, delta-nu/nu = -0.026), indicating weak hydrogen bonds 1-3 kcal/mol. These shifts are accompanied by significant shifts to higher frequencies in the XN-O stretching mode (CIN-O HCl, delta-nu/nu = +0.016; BrN-O HBr, delta-nu/nu = +0.011). Similar shifts were observed for the XN-O X2 complexes (ClN-O Cl2, delta-nu/nu = +0.009; BrN-O-Br2, delta-nu/nu = +0.013). In all four complexes, the X-NO stretching mode relative shift is opposite in sign and about 1.6 times that of the NO stretching mode. These four complexes are considered to be similar in structure and charge distribution. The XN-O frequency shift suggests that complex formation is accompanied by charge withdrawal from the NO bond ranging from about .04 to .07 electron charges. The HX and X2 molecules act as electron acceptors, drawing electrons out of the antibonding orbital of NO and strengthening the XN-O bond. The implications of the pattern of vibrational shifts concerning the structure of the complexes are discussed.

  4. Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

    OpenAIRE

    Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, Bernhard; Wassner, T. A.; Eickhoff, Martin; Feneberg, M.; Goldhahn, R.

    2011-01-01

    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg(x)Zn(1-x)O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced fea...

  5. Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

    CERN Document Server

    Yu, Yiling; Xu, Chao; Barrette, Andy; Gundogdu, Kenan; Cao, Linyou

    2015-01-01

    We quantitatively evaluate the exciton-exciton annihilation (EEA) and its effect on light emission properties in monolayer TMDC materials, including WS2, MoS2, and WSe2. The EEA rate is found to be 0.3 cm2/s and 0.1 cm2/s for suspended WS2 and MoS2 monolayers, respectively, and subject to the influence from substrates, being 0.1 cm2/s and 0.05 cm2/s for the supported WS2 and MoS2 on sapphire substrates. It can substantially affect the luminescence efficiency of suspended monolayers even at an exciton concentration as low as 109 cm-2, but plays a milder role for supported monolayers due to the effect of the substrate. However, regardless the presence of substrates or not, the lasing threshold of the monolayer is always predominantly determined by the EEA, which is estimated to be 12-18 MW/cm2 if using 532 nm as the pumping wavelength.

  6. Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

    KAUST Repository

    Choi, Joshua J.

    2010-05-12

    Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices. © 2010 American Chemical Society.

  7. Evidence of Excitonic Optical Tamm States using Molecular Materials

    CERN Document Server

    Nez-Snchez, S; Murshidy, M M; Abdel-Hady, A G; Serry, M Y; Adawi, A M; Rarity, J G; Oulton, R; Barnes, W L

    2015-01-01

    We report the first experimental observation of an Excitonic Optical Tamm State supported at the interface between a periodic multilayer dielectric structure and an organic dye-doped polymer layer. The existence of such states is enabled by the metal-like optical properties of the excitonic layer based on aggregated dye molecules. Experimentally determined dispersion curves, together with simulated data, including field profiles, allow us to identify the nature of these new modes. Our results demonstrate the potential of organic excitonic materials as a powerful means to control light at the nanoscale, offering the prospect of a new alternative type of nanophotonics based on molecular materials.

  8. Dark and Bright Excitonic States in Nitride Quantum Dots

    OpenAIRE

    Bagga, Anjana; Chattopadhyay, P.K.; Ghosh, Subhasis

    2004-01-01

    Formation of excitonic states in quantum dots of nitride based III-V semiconductors GaN and AlN including coulomb and exchange interactions are investigated. Dark exciton formation is found to occur for both GaN quantum dots(QDs) with wurtzite structure having positive crystal field splitting and GaN and AlN QDs with zero crystal field splitting with a transition from dark to bright exciton at about 40{\\AA}. In wurtzite AlN QDs with negative crystal field splitting the splitting between the d...

  9. Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals

    Science.gov (United States)

    Lyu, ByungGook

    This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton fission and fusion that successfully explained photoluminescence dynamics. The only additional assumption that is required to do so is that triplet excitons be able dissociate and release free holes by direct interaction with a defect state. The 100 microsecond build-up time of the impulsively induced photocurrent then corresponds to the triplet lifetime.

  10. Spin-excitons in heavy-fermion semimetals

    Science.gov (United States)

    Riseborough, Peter S.; Magalhaes, S. G.

    2016-02-01

    Spin-excitons are sharp and dispersive magnetic fluctuations in paramagnetic semiconductors where the dispersion relation lies within the semiconducting gap. Spin-excitons are found in the vicinity of magnetic quantum critical points in semiconductors, much the same as antiparamagnons are precursor fluctuations for quantum critical points in metals. Here we show that this concept of spin-exciton excitations can be extended to heavy-fermion semimetals and provides a natural explanation of the magnetic modes found by inelastic neutron scattering experiments on paramagnetic CeFe2Al10.

  11. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    Energy Technology Data Exchange (ETDEWEB)

    Repp, J. [Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universitt Mnchen, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Mnchen (Germany); Center for NanoScience and Fakultt fr Physik, Ludwig-Maximilians-Universitt, Geschwister-Scholl-Platz 1, 80539 Mnchen (Germany); Schinner, G. J.; Schubert, E. [Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Mnchen (Germany); Center for NanoScience and Fakultt fr Physik, Ludwig-Maximilians-Universitt, Geschwister-Scholl-Platz 1, 80539 Mnchen (Germany); Rai, A. K.; Wieck, A. D. [Angewandte Festkrperphysik, Ruhr-Universitt Bochum, Universittsstrae 150, 44780 Bochum (Germany); Reuter, D. [Angewandte Festkrperphysik, Ruhr-Universitt Bochum, Universittsstrae 150, 44780 Bochum (Germany); Department Physik, Universitt Paderborn, 33098 Paderborn (Germany); Wurstbauer, U.; Holleitner, A. W. [Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universitt Mnchen, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Mnchen (Germany); and others

    2014-12-15

    We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.

  12. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    International Nuclear Information System (INIS)

    We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK

  13. Charged two-exciton emission from a single semiconductor nanocrystal

    International Nuclear Information System (INIS)

    Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the on and off levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate grey level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state

  14. Charged two-exciton emission from a single semiconductor nanocrystal

    Energy Technology Data Exchange (ETDEWEB)

    Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong, E-mail: wxiaoyong@nju.edu.cn [National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093 (China); Xiao, Min, E-mail: mxiao@uark.edu [National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093 (China); Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States)

    2015-03-30

    Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the on and off levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate grey level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.

  15. Theory for electric dipole superconductivity with an application for bilayer excitons

    OpenAIRE

    Qing-Dong Jiang; Zhi-qiang Bao; Qing-Feng Sun; Xie, X. C.

    2015-01-01

    Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we provide a general theory for the electric dipole superconductivity, and derive the London-...

  16. Theory for electric dipole superconductivity with an application for bilayer excitons

    OpenAIRE

    Jiang, Qing-Dong; Bao, Zhi-qiang; Sun, Qing-Feng; Xie, X. C.

    2014-01-01

    Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we provide a general theory for the electric dipole superconductivity, and ...

  17. Refractive index modulation based on excitonic effects in GaInAs-InP coupled asymmetric quantum wells

    DEFF Research Database (Denmark)

    Thirstrup, Carsten

    1995-01-01

    The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm3/J at a wavelength of 1.55 ?m. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials

  18. Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

    Energy Technology Data Exchange (ETDEWEB)

    Senanayake, Pubudu S.; Hughes-Currie, Rosa B. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8410 (New Zealand); Wells, Jon-Paul R., E-mail: jon-paul.wells@canterbury.ac.nz [The Dodd-Walls Centre for Photonic and Quantum Technologies and Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Reid, Michael F. [MacDiarmid Institute for Advanced Materials and Nanotechnology and Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Berden, Giel [Radboud University Nijmegen, Institute for Molecules and Materials, FELIX Facility, Toernooiveld 7, 6525 ED Nijmegen (Netherlands); Reeves, Roger J. [Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8410 (New Zealand); MacDiarmid Institute for Advanced Materials and Nanotechnology and Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8140 (New Zealand); Meijerink, Andries [Debye Institute for NanoMaterials Science, University of Utrecht, P.O. Box 80000, TA 3508 Utrecht (Netherlands)

    2015-04-07

    We model the dynamic behaviour observed for impurity-trapped excitons in SrF{sub 2}:Yb{sup 2+} using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF{sub 2}. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states.

  19. Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

    International Nuclear Information System (INIS)

    We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states

  20. Multi-excitons in self-assembled InAs/GaAs quantum dots A pseudopotential, many-body approach

    CERN Document Server

    Williamson, A J; Zunger, A

    2000-01-01

    We present many-band, many-body, pseudopotential calculations of the energy of multiple excitons in lens shaped, InAs/GaAs quantum dots. We discuss the effects of (i) The direct Coulomb energies, including the differences of electron and hole wavefunctions, (ii) exchange energies, (iii) correlation energies given by a full configuration interaction calculation and (iv) the spin-orbit interaction. We predict multiple transition energies associated with the fundamental e_0-h_0 recombination, due to the presence of spectator excitons. We find that Coulomb interactions produce a blue shift of N->N-1 decays with respect to the fundamental 1->0, while correlation effects introduce an even larger red shift which, for example, yields a biexciton binding energy of 3 meV.