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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. 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

  4. 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.

  5. Time-resolved spectroscopic study on the type I self-trapped excitons in alkali halide crystals. 2. Excitation spectra and relaxation processes

    International Nuclear Information System (INIS)

    Excitation spectra for the fluorescent and phosphorescent components of type I bands in seven alkali halides, NaCl, NaBr, KBr, RbBr, NaI, KI and RbI, are measured in both energy ranges of the free exciton absorption and the band-to-band transition. Using SR pulses as the excitation light source, the two components are resolved by the method of simultaneous photon-counting through two independent time-windows. Excitation spectra for the phosphorescent bands of type II or III are also measured for comparison. From these spectra, total luminescence yield and the fraction of the type I band are determined as a function of excitation energy. On the basis of these results, the relaxation processes of free electron-hole pairs and free excitons, especially on the difference between them, are discussed. (author)

  6. 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 Jahn–Teller 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...

  7. 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

  8. 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.

  9. 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

  10. 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.

  11. Efficient Excitonic Photoluminescence in Direct and Indirect Band Gap Monolayer MoS2.

    Science.gov (United States)

    Steinhoff, A; Kim, J-H; Jahnke, F; Rösner, M; Kim, D-S; Lee, C; Han, G H; Jeong, M S; Wehling, T O; Gies, C

    2015-10-14

    We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory. The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra. For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure. Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands. The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap. The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect. For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission. The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap). PMID:26322814

  12. 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

  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. 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.

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

    Science.gov (United States)

    Tackeuchi, Atsushi; Heberle, Albert; Rühle, Wolfgang; Köhler, 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.

  16. 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.

  17. 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

  18. Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic–Inorganic 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

  19. Transient reflectivity changes of ?-ZnP2 exciton bands by pump-probe spectroscopy using a femtosecond laser

    International Nuclear Information System (INIS)

    Ultrafast reflectivity changes of exciton bands in ?-ZnP2 have been investigated by pump-probe spectroscopy using a femtosecond laser. The spectrum of the pump-probe signal shows a significant negative reflectivity change in the energy range between ET and EL of the 1s exciton polariton, where ET and EL are the transverse and longitudinal exciton energies, respectively. The reflection relaxation time is 1.1 ps near EL and 1.7 ps near ET. These relaxation times are related to acoustic-phonon scattering of exciton polaritons. Transient reflectivity changes with a dispersion-type spectral shape are also observed at the energy where the excitonic molecule luminescence appears

  20. 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.

  1. 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 spin–orbital 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 Kramers–Kronig analysis. The evidenced features are discussed on the basis of recent theoretical calculations of the band structure of CdGa2Se4 crystals

  2. 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

  3. 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.

  4. Exciton spectra and energy band structure of Cu{sub 2}ZnSiSe{sub 4}

    Energy Technology Data Exchange (ETDEWEB)

    Guc, M., E-mail: gmax@phys.asm.md [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of); Levcenko, S. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Dermenji, L. [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of); Gurieva, G. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Schorr, S. [Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin (Germany); Free University Berlin, Institute of Geological Sciences, Malteserstr. 74-100, Berlin (Germany); Syrbu, N.N. [Technical University of Moldova, Chisinau MD-2004, Republic of Moldova (Moldova, Republic of); Arushanov, E. [Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, Chisinau MD 2028, Republic of Moldova (Moldova, Republic of)

    2014-02-25

    Highlights: • Reflection spectra of Cu{sub 2}ZnSiSe{sub 4} were studied for E ? c and E || c light polarizations. • Four excitonic series are revealed in the reflection spectra at 10 K. • Model of exciton dispersion and the presence of a dead-layer. • Exciton Rydberg energies and free carriers effective masses were calculated. • Reflectivity for E ? c and E || c were analyzed in the region 3–6 eV at 300 K. -- Abstract: Exciton spectra are studied in Cu{sub 2}ZnSiSe{sub 4} single crystals at 10 and 300 K by means of reflection spectroscopy. The exciton parameters, dielectric constant and free carriers effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The structure found in the reflectivity was analyzed and related to the theoretical electronic band structure of close related Cu{sub 2}ZnSiS{sub 4} semiconductor.

  5. 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

  6. 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.

  7. Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

    International Nuclear Information System (INIS)

    The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) N0 dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons (SE's) and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-1/2 magnetic resonance conditions reduce the total polaron population. Since the sublinear N0 dependences of the positive (PL-enhancing) spin-1/2 PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent ?-conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures

  8. 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

  9. 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

  10. 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

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

    Czech Academy of Sciences Publication Activity Database

    Kuneš, Jan; Augustinský, 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

  12. On the interpretation of luminescence of lead halide crystals.

    Czech Academy of Sciences Publication Activity Database

    Babin, V.; Krasnikov, A.; Nikl, Martin; Stolovits, A.; Zazubovich, S.

    2002-01-01

    Ro?. 229, ?. 3 (2002), s. 1295-1304. ISSN 0370-1972 Institutional research plan: CEZ:AV0Z1010914 Keywords : luminescence * lead halide * exciton Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 0.930, year: 2002

  13. 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...

  14. 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)

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

    Energy Technology Data Exchange (ETDEWEB)

    Karlicky, Frantisek; Otyepka, Michal [Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Olomouc (Czech Republic)

    2014-10-15

    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)

  16. 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 spin–orbital 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 Kramers–Kronig 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.

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. 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...

  2. Spin-flip Raman scattering on ?-X mixed excitons in indirect band-gap (In,Al)As/AlAs quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Kudlacik, Dennis; Debus, J.; Dunker, D.; Shamirzaev, T.S.; Bayer, M. [Experimentelle Physik 2, Technische Universitaet Dortmund, 44227 Dortmund (Germany); Sapega, V.F.; Ivchenko, E.L. [Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation); Yakovlev, D.R. [Experimentelle Physik 2, Technische Universitaet Dortmund, 44227 Dortmund (Germany); Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation)

    2013-07-01

    We studied the fine structure of the indirect exciton in self-assembled (In,Al)As/AlAs quantum dots (QDs) by means of spin-flip Raman scattering (SFRS). The QDs are characterized by a type-I band alignment, wherein, dependent on the dot size, a crossover between the energetically lowest conduction-band states of the ?- and X-valley occurs. This ?-X mixing of the electron levels is used to optically study the indirect in momentum-space exciton. It has a long recombination lifetime and longitudinal spin relaxation time of up to several milliseconds. Using the resonant SFRS the g-factor tensors of the indirect exciton, ?-valley heavy-hole, and X-valley electron are determined. The spin-flip scattering mechanisms are based on acoustic phonon interaction in tilted magnetic field geometries. The efficiencies of the electron and heavy-hole spin scattering strongly depend on the excitation energy across the inhomogeneously broadened QD ensemble. The ?-valley electron cannot be observed because of its short lifetime and the broad dispersion of its g-factor corresponding to the strong variation in the QD sizes, which is evidenced in experiment and theory.

  3. 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.

  4. Triangular lattice exciton model

    OpenAIRE

    Gunlycke, Daniel; Tseng, Frank

    2015-01-01

    We present a minimalistic equilateral triangular lattice model, from which we derive electron and exciton band structures for semiconducting transition-metal dichalcogenides. With explicit consideration of the exchange interaction, this model is appropriate across the spectrum from Wannier to Frenkel excitons. The single-particle contributions are obtained from a nearest-neighbor tight-binding model parameterized using the effective mass and spin-orbit coupling. The solution...

  5. Ab initio modeling of excitonic and charge-transfer states in organic semiconductors: the PTB1/PCBM low band gap system.

    Science.gov (United States)

    Borges, Itamar; Aquino, Adélia J A; Köhn, Andreas; Nieman, Reed; Hase, William L; Chen, Lin X; Lischka, Hans

    2013-12-11

    A detailed quantum chemical simulation of the excitonic and charge-transfer (CT) states of a bulk heterojunction model containing poly(thieno[3,4-b]thiophene benzodithiophene) (PTB1)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. The largest molecular model contains two stacked PTB1 trimer chains interacting with C60 positioned on top of and lateral to the (PTB1)3 stack. The calculations were performed using the algebraic diagrammatic construction method to second order (ADC(2)). One main result of the calculations is that the CT states are located below the bright inter-chain excitonic state, directly accessible via internal conversion processes. The other important aspects of the calculations are the formation of discrete bands of CT states originating from the lateral C60's and the importance of inter-chain charge delocalization for the stability of the CT states. A simple model for the charge separation step is also given, revealing the energetic feasibility of the overall photovoltaic process. PMID:24215627

  6. 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...

  7. 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.

  8. 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.)

  9. 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.

  10. 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.

  11. Optical second harmonic generation from Wannier excitons

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm; Cornean, Horia

    2007-01-01

    Excitonic effects in the linear optical response of semiconductors are well known and the subject of countless experimental and theoretical studies. For the technologically important second-order nonlinear response, however, description of excitonic effects has proved to be difficult. In this work, a simplified three-band Wannier exciton model of cubic semiconductors is applied and a closed form expression for the complex second harmonic response function including broadening is derived. Our calculated spectra are found to be in excellent agreement with the measured response near the band edge. In addition, a very substantial enhancement of the nonlinear response is predicted for the transparency region.

  12. Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

    CERN Document Server

    Kühn, 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 ...

  13. 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.

  14. 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.

  15. 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.

  16. 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; Lévi, 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.

  17. Manifestations of Vibronic Coupling Effects in Molecular Spectroscopy: from the Quenching of Excitonic Energy Splittings to the Clements Bands of SO2

    Science.gov (United States)

    Koppel, Horst

    2013-06-01

    We investigate the excitation of vibrational modes and its impact on the excitonic energy splittings in doubly hydrogen-bonded molecular dimers. The experimental analysis, performed in collaboration by S. Leutwyler and coworkers (Univ. Bern), is based on high-resolution resonant two-photon ionization spectroscopy. The potential energy surfaces underlying the theoretical investigation are obtained at the RICC2/aug-cc-pVTZ level and are used for the dynamical analysis in the framework of a well-established vibronic coupling approach. The vertical electronic Davydov splitting of the S_1 and S_2 excited states exceeds the observed excitonic splitting by a factor of 10--40. This discrepancy can be understood by considering the quenching of the excitonic splitting by the excitation of vibrational modes in the electronic transition. Two different approaches have been employed and found to reconcile theory and experiment. The analysis of the vibronic structure of the S_2 ? S_0 excitation spectrum focusses on the ortho-cyanophenol dimer as a representative example. Most of the observed spectral features can be reproduced by the calculations, although some deviations remain. In the second part, new results on the UV absorption spectrum of SO_2 will be presented. This is complementary to the excitonic systems in that higher vibrational energies are involved and a conical intersection is accessible to the nuclear motion. Using the concept of regularized diabatic states in combination with high-accuracy MRCI potential energy surfaces, semi-quantitative agreement with the complex experimental (low-resolution) spectrum has been achieved for the first time. P. Ottiger, S. Leutwyler and H. Köppel, J. Chem. Phys. 136, 174308 (2012). S. Kopec, P. Ottiger, S. Leutwyler and H. Köppel, J. Chem. Phys. 137, 184312 (2012). H. Köppel and B. Schubert, Mol. Phys. 104, 1069 (2006). C. Leveque, A. Komainda, R. Taieb and H. Köppel, J. Chem. Phys. 138, 044320 (2013).

  18. 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

  19. 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 1–2 × 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

  20. 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.

  1. Polarizable Optical Bistability of Frenkel Excitons

    Directory of Open Access Journals (Sweden)

    O. Derevyanchuk

    2003-06-01

    Full Text Available Optical spectra of molecular crystals with two molecules in their elementary cell are investigated. Depending on the polarization angle one or two exciton excitation bands can be realized in the spectrum. The bands are separated by the terminal frequency interval. The exciton absorption bands genesis and the peculiarities of the function of their form at the change of the laser radiation polarization have been analyzed and the regularity of arising bistable states in organic benzol-type crystals has been established.

  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, Jřrn Märcher

    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. 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 Vińa 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.

  4. 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

  5. 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.

  6. 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.

  7. Excitons in Electrostatic Traps

    OpenAIRE

    Hammack, A. T.; Gippius, N. A.; Andreev, G. O.; Butov, L. V.; Hanson, M; Gossard, A. C.

    2005-01-01

    We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively confine excitons in the electrostatic traps. We present calculations for various classes...

  8. 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.

  9. 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

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

    Scientific Electronic Library Online (English)

    A., Pérez-Rodríguez; M., Flores-Acosta; R., Rodríguez-Mijangos; R., Pérez-Salas.

    2006-04-01

    Full Text Available Atomos de cobre han sido introducidos por difusión en cristales de NaCl, KCl y KBr a 500°C. Los cristales han sido analizados ópticamente con medidas de fotoluminiscencia y por microscopía electrónica de barrido. Los espectros de emisión y excitación, medidos a baja temperatura muestran el efecto de [...] confinamiento de exciton, indicando la formación de nanopartículas de CuX (X=Cl, Br), lo cual ha sido confirmado por imágenes de microscopía electrónica. Este método es propuesto como un método alternativo para obtener nanopartículas de CuX en cristales halogenuros alcalinos. Abstract in english Copper atoms have been introduced by diffusion in NaCl, KCl and KBr crystals at 500°C. 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.

  11. 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.

  12. Intrachain exciton dynamics in conjugated polymer chains in solution

    Science.gov (United States)

    Tozer, Oliver Robert; Barford, William

    2015-08-01

    We investigate exciton dynamics on a polymer chain in solution induced by the Brownian rotational motion of the monomers. Poly(para-phenylene) is chosen as the model system and excitons are modeled via the Frenkel exciton Hamiltonian. The Brownian fluctuations of the torsional modes were modeled via the Langevin equation. The rotation of monomers in polymer chains in solution has a number of important consequences for the excited state properties. First, the dihedral angles assume a thermal equilibrium which causes off-diagonal disorder in the Frenkel Hamiltonian. This disorder Anderson localizes the Frenkel exciton center-of-mass wavefunctions into super-localized local exciton ground states (LEGSs) and higher-energy more delocalized quasi-extended exciton states (QEESs). LEGSs correspond to chromophores on polymer chains. The second consequence of rotations—that are low-frequency—is that their coupling to the exciton wavefunction causes local planarization and the formation of an exciton-polaron. This torsional relaxation causes additional self-localization. Finally, and crucially, the torsional dynamics cause the Frenkel Hamiltonian to be time-dependent, leading to exciton dynamics. We identify two distinct types of dynamics. At low temperatures, the torsional fluctuations act as a perturbation on the polaronic nature of the exciton state. Thus, the exciton dynamics at low temperatures is a small-displacement diffusive adiabatic motion of the exciton-polaron as a whole. The temperature dependence of the diffusion constant has a linear dependence, indicating an activationless process. As the temperature increases, however, the diffusion constant increases at a faster than linear rate, indicating a second non-adiabatic dynamics mechanism begins to dominate. Excitons are thermally activated into higher energy more delocalized exciton states (i.e., LEGSs and QEESs). These states are not self-localized by local torsional planarization. During the exciton's temporary occupation of a LEGS—and particularly a quasi-band QEES—its motion is semi-ballistic with a large group velocity. After a short period of rapid transport, the exciton wavefunction collapses again into an exciton-polaron state. We present a simple model for the activated dynamics which is in agreement with the data.

  13. 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

  14. 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

  15. 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.

  16. 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...

  17. Exciton laser rate equations

    Directory of Open Access Journals (Sweden)

    Garkavenko A. S.

    2011-08-01

    Full Text Available The rate equations of the exciton laser in the system of interacting excitons have been obtained and the inverted population conditions and generation have been derived. The possibility of creating radically new gamma-ray laser has been shown.

  18. Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy

    OpenAIRE

    Karki, Khadga J.; Ma, Fei; Zheng, Kaibo; Zidek, Karel; Mousa, Abdelrazek; Mohamed A. Abdellah; Messing, Maria E.; Wallenberg, L. Reine; Yartsev, Arkadi; Pullerits, Tőnu

    2013-01-01

    Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usu- ally done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that ...

  19. 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.

  20. 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...

  1. 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...

  2. Physical model of the vapor-liquid (insulator-metal) transition in an exciton gas

    Science.gov (United States)

    Khomkin, A. L.; Shumikhin, A. S.

    2015-04-01

    We propose a simple physical model describing the transition of an exciton gas to a conducting exciton liquid. The transition occurs due to cohesive coupling of excitons in the vicinity of the critical point, which is associated with transformation of the exciton ground state to the conduction band and the emergence of conduction electrons. We calculate the cohesion binding energy for the exciton gas and, using it, derive the equations of state, critical parameters, and binodal. The computational method is analogous to that used by us earlier [5] for predicting the vapor-liquid (insulator-metal) phase transition in atomic (hypothetical, free of molecules) hydrogen and alkali metal vapors. The similarity of the methods used for hydrogen and excitons makes it possible to clarify the physical nature of the transition in the exciton gas and to predict more confidently the existence of a new phase transition in atomic hydrogen.

  3. 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.

  4. 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

  5. Excitonic gap formation in neutral bilayer structures

    Science.gov (United States)

    Apinyan, V.; Kope?, 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.

  6. 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.

  7. 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.

  8. 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...

  9. 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...

  10. Optical response and excitons in gapped graphene

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm; Jauho, Antti-Pekka; Pedersen, K.

    2009-01-01

    Graphene can be rendered semiconducting via energy gaps introduced in a variety of ways, e.g., coupling to substrates, electrical biasing, or nanostructuring. To describe and compare different realizations of gapped graphene we propose a simple two-band model in which a "mass" term is responsible for the gap. The optical conductivity predicted for this model is obtained as a simple closed-form expression. In addition, analytical estimates for the binding energy of excitons are derived and the im...

  11. Anharmonic properties of rubidium halides

    International Nuclear Information System (INIS)

    The anharmonic properties of rubidium halides are investigated using a three-body interaction (TBI) potential. This includes the prediction of second-, third- and fourth-order elastic constants, the pressure dependence of second- and third-order elastic constants. The agreement between experimental and theoretical results is, generally, good. The inclusion of anharmonic terms in the potential might lead to further improvements. (author)

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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

  17. 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.

  18. 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.

  19. 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...

  20. Machine Learning Exciton Dynamics

    CERN Document Server

    Häse, Florian; Pyzer-Knapp, Edward; Aspuru-Guzik, Alán

    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...

  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. Exciton diffusion, end quenching, and exciton-exciton annihilation in individual air-suspended carbon nanotubes

    OpenAIRE

    Ishii, A; M. Yoshida; Kato, Y. K.

    2014-01-01

    Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotu...

  3. 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, Tomás

    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

  4. 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.

  5. 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

  6. 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.)

  7. 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.

  8. Excitonic effects in the luminescence of quantum wells

    International Nuclear Information System (INIS)

    We report on the origin of the excitonic luminescence in quantum wells. This study is carried out by time-resolved photoluminescence experiments performed on a very high-quality InGaAs quantum well sample in which the photoluminescence contributions at the energy of the exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. This allows us to compare the two conflicting theoretical approaches to the question of the origin of the excitonic luminescence in quantum wells: the model of the exciton population and the model of the Coulomb correlated plasma. We measure the exciton formation time and we show the fast exciton formation and its dependence with carrier density. We are also able to give the boundaries of the Mott transition in our system, and to show the absence of observable renormalization of the gap below the onset of this transition. We detail the characteristics of the trion formation and evidence the possible formation of both positive and negative trions in the absence of any resident free carrier populations

  9. 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 ...

  10. 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

  11. 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)

  12. 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

  13. Freeze Enhanced Halate Halide Reactions

    Science.gov (United States)

    Newberg, J. T.; Weaver, K.; Broderick, A.

    2014-12-01

    Relatively little is known about halate ion species (XO3-; X = I, Br, Cl) in atmospheric condensed phases. It was initial thought that iodate was a terminal stable species upon iodide oxidation. However, it is becoming increasingly recognized that reactions involving iodate can lead to reactive iodine, and this chemistry is accelerated under acidic conditions. The environmental concentrations and chemistry of bromate and chlorate are largely unexplored in environmental ices. We present results from a series of aqueous phase halate ion reactions with halides under acidic conditions, showing that the kinetics are strongly enhanced upon freezing. The products of these reactions are reactive halogens, which have important implications to marine boundary layer chemistry.

  14. 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.

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

    Energy Technology Data Exchange (ETDEWEB)

    Sergent, S. [Institute for Nano Quantum Information Electronics, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan); Kako, S. [Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan); Bürger, M.; Schupp, T.; As, D. J. [Department Physik, Universität Paderborn, 33095 Paderborn (Germany); Arakawa, Y. [Institute for Nano Quantum Information Electronics, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan); Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8505 (Japan)

    2014-10-06

    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.

  16. 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)

  17. 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.

  18. Dynamics of Excitons in Bare and Organic/Metal coated InP Nanowires

    Science.gov (United States)

    Kaveh, Masoud; Gao, Qiang; Jagadish, Chaennupati; Duscher, Gerd; Wagner, Hans-Peter

    2014-03-01

    We investigate the exciton dynamics in bare and organic/metal coated wurzite/zincblende (WZ/ZB) InP nanowires (NW) by temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). Aluminum quinoline (Alq3) as well as Alq3/Mg:Ag covered NW heterostructures are fabricated by organic molecular beam deposition. PL measurements on bare InP nanowires at 15 K reveal two emission bands at 1.45, and 1.48 eV originating from indirect WZ/ZB and point-defect (PD) trapped excitons, respectively. TR PL traces show an approximately single exponential decay for PD trapped excitons with a lifetime of 2 ns and biexponential decay for indirect WZ/ZB excitons with lifetimes of 3.3 ns and 14 ns. In Alq3 covered NWs we observe a stronger emission from both exciton transitions and longer decay times for indirect excitons indicating surface state passivation at the Alq3/NW interface. In Alq3/Mg:Ag NWs the PD trapped exciton emission is notably reduced which is attributed to a fast energy-transfer from free excitons in the WZ segments to plasmon oscillations in the metal film. The emission from indirect excitons is still comparable to the PL yield of bare NWs. The support of URC at UC and of the Australian Research Council is kindly acknowledged.

  19. 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

  20. 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...

  1. Critical Velocities in Exciton Superfluidity

    International Nuclear Information System (INIS)

    The presence of exciton-phonon interactions is shown to play a key role in the exciton superfluidity. We apply the Landau criterion for an exciton-phonon condensate moving uniformly at zero temperature. It turns out that there are essentially two critical velocities in the theory. Within the range of these velocities the condensate can exist only as a bright soliton. The excitation spectrum and differential equations for the wave function of this condensate are derived. copyright 1997 The American Physical Society

  2. 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 ...

  3. 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.

  4. 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 Vińa'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

  5. 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 Vińa'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.

  6. 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

  7. 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, ?=e±i2?/3) is stabilized with broken time reversal symmetry. (author)

  8. 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.

  9. 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)

  10. Exciton Formation in Disordered Semiconductors

    DEFF Research Database (Denmark)

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

    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 carrier pairs.

  11. Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes

    CERN Document Server

    Harigaya, Kikuo

    2007-01-01

    Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the onsite interaction U=2t with the hopping integral t=1.2 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of the nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiral index.

  12. 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)

  13. 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)

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. Excitons into one-axis crystals of zinc phosphide (Zn3P2

    Directory of Open Access Journals (Sweden)

    D.M. Stepanchikov

    2009-01-01

    Full Text Available Theoretical study of excitons spectra is offered in this report as for Zn3P2 crystals. Spectra are got in the zero approach of the theory of perturbations with consideration of both the anisotropy of the dispersion law and the selection rules. The existence of two exciton series was found, which corresponds to two valence bands (hh, lh and the conductivity band (c. It is noteworthy that anisotropy of the dispersion law plus the existence of crystalline packets (layers normal to the main optical axis, both will permit the consideration of two-dimensional excitons too. The high temperature displaying of these 2D-exciton effects is not eliminated even into bulk crystals. The calculated values of the binding energies as well as the oscillator's strength for the optical transitions are given for a volume (3D and for two-dimensional (2D excitons. The model of energy exciton transitions and four-level scheme of stimulated exciton radiation for receiving laser effect are offered.

  1. 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.

  2. 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...

  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. 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.

  5. Exciton dynamics in atomically thin MoS2: inter-excitonic interaction and broadening kinetics

    OpenAIRE

    Sim, Sangwan; Park, Jusang; Song, Jeong-Gyu; In, Chihun; Lee, Yun-Shik; Kim, Hyungjun; Choi, Hyunyong

    2013-01-01

    We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS2. Spectrally- and temporally-resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated inter-excitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observe...

  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. Exciton emission from hybrid organic and plasmonic polytype InP nanowire heterostructures

    Science.gov (United States)

    Kaveh, M.; Dyck, O.; Duscher, G.; Gao, Q.; Jagadish, C.; Wagner, H. P.

    2015-04-01

    We investigate the emission of excitons in bare, hybrid organic, and metal coated polytype wurtzite/zincblende (WZ/ZB) InP nanowire (NW) heterostructures by intensity- and temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). TI PL measurements at 20 K reveal two strong emission bands at ?1.48 and ?1.44 eV that are attributed to non-thermalized weakly and deeply localized indirect WZ/ZB excitons due to randomly distributed short WZ and ZB segments. The PL yield of both bands increases when the NWs are covered with an Alq3 layer which is attributed to surface charge passivation. In metal coated NWs the weakly localized indirect WZ/ZB exciton emission is significantly reduced while the strongly localized indirect WZ/ZB band is less affected. The observed PL quenching is attributed to radiationless Förster energy-transfer from NW excitons to plasmon oscillations in the deposited metal. TR PL investigations support this interpretation revealing enhanced PL lifetimes in Alq3 coated NWs compared to uncovered NWs. The lifetime of weakly trapped indirect excitons is shortest in metal coated NWs due to Förster energy-transfer while the dynamics of strongly localized indirect WZ/ZB excitons is less affected because of the small dipole-moment of these transitions.

  8. Deformation Bleaching of Coloration in Alkali Halide Crystals

    Science.gov (United States)

    Chandra, B. P.; Vishwakarma, H. L.; Khare, P. K.

    1997-12-01

    The present paper reports the phenomenological theory of deformation bleaching of coloration in alkali halide crystals. Expressions are derived for the dislocation capture probability of F-centre electrons in alkali halide crystals, and for the rate of generation of electrons in the dislocation band. On the basis of the rate of generation of dislocation electrons and their subsequent recombination with hole centres, deep traps and other compatible traps (including deformation generated traps), an expression is derived for the deformation bleaching, which indicates the exponential decrease of the density of F-centres with deformation of the crystal. A good agreement is found between the theoretical and experimental results. The value of the effective radius re of the interaction of dislocations with F-centres, is estimated and it is found to be 1.95b, 2.00b, 1.33b and 1.70b for KCl, KBr, NaCl and NaBr crystals, respectively (where b is the Burgers vector)

  9. 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

  10. Structural effects in molecular metal halides.

    Science.gov (United States)

    Hargittai, Magdolna

    2009-03-17

    Metal halides are a relatively large class of inorganic compounds that participate in many industrial processes, from halogen metallurgy to the production of semiconductors. Because most metal halides are ionic crystals at ambient conditions, the term "molecular metal halides" usually refers to vapor-phase species. These gas-phase molecules have a special place in basic research because they exhibit the widest range of chemical bonding from the purely ionic to mostly covalent bonding through to weakly interacting systems. Although our focus is basic research, knowledge of the structural and thermodynamic properties of gas-phase metal halides is also important in industrial processes. In this Account, we review our most recent work on metal halide molecular structures. Our studies are based on electron diffraction and vibrational spectroscopy, and increasingly, we have augmented our experimental work with quantum chemical computations. Using both experimental and computational techniques has enabled us to determine intriguing structural effects with better accuracy than using either technique alone. We loosely group our discussion based on structural effects including "floppiness", relativistic effects, vibronic interactions, and finally, undiscovered molecules with computational thermodynamic stability. Floppiness, or serious "nonrigidity", is a typical characteristic of metal halides and makes their study challenging for both experimentalists and theoreticians. Relativistic effects are mostly responsible for the unique structure of gold and mercury halides. These molecules have shorter-than-expected bonds and often have unusual geometrical configurations. The gold monohalide and mercury dihalide dimers and the molecular-type crystal structure of HgCl(2) are examples. We also examined spin-orbit coupling and the possible effect of the 4f electrons on the structure of lanthanide trihalides. Unexpectedly, we found that the geometry of their dimers depends on the f electron configuration. Metal halides are unique in exhibiting strong vibronic interactions such as the Jahn-Teller effect and the related Renner-Teller effect. Some metal trihalide molecules have an almost T-shape due to static Jahn-Teller distortions. The nonlinear structure with a 150 degree bond angle of the chromium dichloride molecule demonstrates the Renner-Teller effect. Finally, we present a few examples of unknown structures that appear to be thermodynamically stable, including gold and silver triiodides and all silver subhalides. The combination of experimental and computational techniques has brought new insights to the structural chemistry of metal halides. We expect that the continuing progress in computational chemistry will shed further light on the intricate details of these and other molecular structures. PMID:19170522

  11. Li 1s near-edge spectra in six lithium halides

    Energy Technology Data Exchange (ETDEWEB)

    Shirley, Eric L. E-mail: eric.shirley@nist.gov

    2004-07-01

    Theoretical results are presented for Li 1s near-edge absorption spectra in LiH, LiF, LiCl, LiBr, LiI, and LiAt. These results are obtained using a realistic band structure and wave functions, and include effects of the electron-core hole attraction in detail. The spectra are compared and trends through the family of lithium halides are discussed. Appropriate improvements to be implemented in future theoretical work are also discussed.

  12. Li 1s near-edge spectra in six lithium halides

    International Nuclear Information System (INIS)

    Theoretical results are presented for Li 1s near-edge absorption spectra in LiH, LiF, LiCl, LiBr, LiI, and LiAt. These results are obtained using a realistic band structure and wave functions, and include effects of the electron-core hole attraction in detail. The spectra are compared and trends through the family of lithium halides are discussed. Appropriate improvements to be implemented in future theoretical work are also discussed

  13. 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

  14. Models of coherent exciton condensation

    OpenAIRE

    EASTHAM, PAUL

    2004-01-01

    That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefun...

  15. 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.

  16. 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;121:78–90] 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. -- 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

  17. 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.

  18. 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

  19. 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.

  20. 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.

  1. 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

  2. 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

  3. 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

  4. 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.

  5. 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.

  6. 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.)

  7. 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

  8. 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

  9. Formation of structured nanophases in halide crystals.

    Czech Academy of Sciences Publication Activity Database

    Kulveit, Jan; Demo, Pavel; Polák, Karel; Sveshnikov, Alexey; Kožíšek, 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

  10. Formation of structured nanophases in halide crystals.

    Czech Academy of Sciences Publication Activity Database

    Kulveit, J.; Demo, Pavel; Polák, Karel; Sveshnikov, Alexey; Kožíšek, 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

  11. 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. Hünenberger, 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. Hünenberger, 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

  12. 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-Sánchez, 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.”.

  13. 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

  14. Theory of coherent transient spectroscopy in molecular aggregates: The effects of interacting excitons

    Science.gov (United States)

    Spano, Frank C.; Manas, Eric S.

    1995-10-01

    Using a density matrix formalism we derive a general expression for the resonant response of an ensemble of molecular aggregates to two optical pulses. The aggregates, which can be of arbitrary geometry, are described by a Frenkel exciton Hamiltonian including exciton-exciton interactions. A hierarchy of resonant coherent transient signals is generated having wave vectors sk1+(1-s)k2, where ki is the wave vector of the ith pulse and s is an integer. General expressions for the differential pump-probe absorption spectrum D(?;tD) (s=0) and the photon echo Iecho(tD) (s=-1) are presented for arbitrary pulse intensities. Applications are made to a linear chain of interacting Frenkel excitons with nearest neighbor exciton-exciton interaction A and exciton transfer J. Analytical expressions for D(?;tD) and Iecho(tD) are obtained which are valid to third order in the aggregate-field interaction. The influence of the optically active red (blue) biexciton which detaches from the bottom (top) of the two-exciton band when A<-2?J? (A?2?J?) is investigated. Biexcitons appear as extra peaks in D(?;tD) and as oscillations in Iecho(tD). Finite size effects are responsible for quantum beat oscillations in D(?;tD) and long time oscillations in the echo decay. Quantum beats arise predominantly from the difference between the lowest one-exciton frequencies. Our theory successfully describes the salient spectral features in the pump-probe spectra of J-aggregates.

  15. 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...

  16. 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

  17. 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)

  18. 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.

  19. 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.

  20. 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.

  1. 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)

  2. 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.

  3. All conjugated copolymer excitonic multiferroics.

    Science.gov (United States)

    Lohrman, Jessica; Liu, Yueying; Duan, Shaofeng; Zhao, Xiaoyong; Wuttig, Manfred; Ren, Shenqiang

    2013-02-01

    A substantial magnetoelectric coupling effect of an excitonic all-conjugated block copolymer multiferroics consisting of electronically distinct polythiophene derivatives is reported. The observations open new avenues for the multifunctional all-conjugated block copolymer synthesis and electric field tunable multiferroic devices. PMID:23172730

  4. Low temperature exciton-exciton annihilation in amphi-PIPE J-aggregates

    Directory of Open Access Journals (Sweden)

    S. Daehne

    2006-11-01

    Full Text Available The mobility of optically excited excitons on J-aggregates can be demonstrated by the phenomena of exciton-exciton annihilation. In this intensity-dependent process the collision of two excitons results in their annihilation and hence in a shortening of the mean excitation lifetime. By measuring the intensity-dependent fluorescent lifetime in contrast to the predicted immobilization of the excitons at low temperature we could prove the excellent mobility of the excitons at a temperature (4K, which is far below their expected freezing point.

  5. Recombination of free and bound excitons in GaN

    Energy Technology Data Exchange (ETDEWEB)

    Monemar, B.; Paskov, P.P.; Bergman, J.P. [Department of Physics, Chemistry and Biology, Linkoeping University (Sweden); Toropov, A.A.; Shubina, T.V. [Ioffe Physico-Technical Institute, St. Petersburg (Russian Federation); Malinauskas, T. [Institute of Materials Science and Applied Research, Vilnius University (Lithuania); Usui, A. [R and D Division, Furukawa Co., Ltd. Tsukuba, Ibaraki (Japan)

    2008-09-15

    We report on recent optical investigations of free and bound exciton properties in bulk GaN. In order to obtain reliable data it is important to use low defect density samples of low doping. We have used thick GaN layers (of the order of 1 mm) grown by halide vapour phase epitaxy (HVPE) with a residual doping down to <10{sup 16} cm{sup -3} in this work. With such samples all polarisation geometries could also easily be exploited. The influence of the surface states on the photoluminescence (PL) experiments is analysed, it is concluded that surface recombination plays an important role for the free exciton (FE) recombination. The electronic structure of the FEs is discussed in detail, including the influence of spin-exchange and polariton effects, and compared with polarised PL spectra at 2 K. The detailed structure of excited states from the PL spectra is discussed, but further data are needed to fully explain all the peaks observed. The polarized FE spectra at room temperature allow a determination of the bandgap as 3.437 eV at 290 K, assuming an exciton binding energy of 25 meV. The PL transient of the A FE is very short (about 100 ps) for the no-phonon (NP) line interpreted as dominated by nonradiative surface recombination. The longitudinal-optical (LO) phonon replicas of the A FE exhibit a longer decay of about 1.4 ns at 2 K, suggested to represent the bulk lifetime of the FE. The corresponding decay time at 290 K is 9 ns in our samples, a value that might be affected by nonradiative recombination. The Si and O donor bound exciton (DBE) spectra with sharp NP lines at 3.4723 eV and 3.4714 eV respectively, are well resolved together with the so-called two-electron transitions (TETs) and several optical phonon replicas. The electronic structure of the DBE states including excited rotational states is discussed and compared with experiment. The well-resolved TET lines allow an accurate determination of the ground state binding energy of the Si donor as 30.4 meV and 33.2 meV for the O donor. The PL transients of the DBEs reveal a non-exponential decay for the NP lines. The DBE NP transient lineshape is assumed to be influenced by optical dispersion and scattering in the vicinity of exciton resonances, as well as by surface effects. The DBE decay time can most properly be deduced from the PL decay of the respective TETs and LO replicas, leading to values in the range of 1.1-1.8 ns. These values differ significantly from previous theoretical predictions, where values about two orders of magnitude shorter were obtained. A tentative discussion of the main observed features of acceptor bound excitons (ABEs), which are much less studied in GaN, is given. A decay time of about 0.9 ns for the shallowest 3.466 eV ABE is estimated, i.e. shorter than that for the shallow donor BEs. (copyright 2008 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  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. Raman Spectroscopy of Organic-Inorganic Halide Perovskites.

    Science.gov (United States)

    Ledinský, Martin; Löper, Philipp; Niesen, Bjoern; Holovský, Jakub; Moon, Soo-Jin; Yum, Jun-Ho; De Wolf, Stefaan; Fejfar, Antonín; 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

  8. Coordination Networks of Mercury(II) Halides and Polyether Ligand

    OpenAIRE

    Crochet, Aurélien; Fromm, Katharina M.

    2012-01-01

    New crystalline mercury halide adducts with polyetheras ligands were isolated, characterized, and identified as trans-[HgIICl?(diox)?]n (1) and trans-[HgIII?(diox)]n (2). The compounds were obtained from the metal halide salts in solution of 1,4-dioxane and can be considered as “cutting-out” products from the metal halide as they show similarity in the arrangement with starting compounds.

  9. The Neutrino-Exciton Analogy

    Science.gov (United States)

    Simhony, M.

    1997-04-01

    The 'freeing' of a conduction electron and hole within a solid requires absorption of their binding energy E_g. Excitons are formed with the absorption of certain allowed energies Ea smaller than E_g, or when the 'free' electron and hole emit the energy E_n=E_g-E_a, which is their binding energy in the exciton. The exciton disappears on absorption of the En energy, freeing the electron and hole, or on emission of the Ea energy, with the recombination of the pair. In the electron-positron lattice (epola) space, (M.Simhony,Invitation to the Natural Physics of Matter, Space, and Radiation, World Scientific, 1994 (292pp).) neutrinos of energy En may form when a pair of host particles, bound in the lattice by the energy _bE=1.02 MeV, absorbs an energy E_a=_bE-E_n, or when a free electron and positron emit the energy En and fall into the neutrino bonds. The neutrino disappears on absorption of the En energy, freeing the two particles, or on emission of the Ea energy, when they fall into the lattice bonds. For the creation and anihilation of excitons in solids and of neutrinos in the epola, the lattices must contain irregularities able to provide En (or E_a) energy levels. Such irregularities are abundant in solids, with their close-packed, overlapping, thermally vibrating host and impurity atoms and ions, thus excitons are quite common. In our epola, the temperature is 3K only, and the hosts are 50 particle radii away from one another. Epola excitation, able to create or annihilate neutrinos, are scarce. Hence the few neutrinos, created here in (or near to) nuclear reactions, as well as solar neutrinos, have a long lifetime in our epola.

  10. Electrical conductivity of supercooled halide melts.

    Czech Academy of Sciences Publication Activity Database

    Sveshnikov, Alexey; Demo, Pavel; Kožíšek, Zden?k; Nitsch, Karel; Rodová, Miroslava

    Praha : MAXDORF s.r.o., 2002 - (Nitsch, K.; Rodová, 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

  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. Lanthanide doped strontium-barium cesium halide scintillators

    Energy Technology Data Exchange (ETDEWEB)

    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.

  13. 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.

  14. Photonic Crystal Architecture for Room-Temperature Equilibrium Bose-Einstein Condensation of Exciton Polaritons

    Science.gov (United States)

    Jiang, Jian-Hua; John, Sajeev

    2014-07-01

    We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense 10-?m-1-cm- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of Cd0.6 Mg0.4Te with a 3D PBG of 9.2% (gap-to-central-frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as ??=55 meV (i.e., a vacuum Rabi splitting of 2??=110 meV). The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For N =106 QWs embedded in the cavity, the collective exciton-photon coupling per QW (??/?N =5.4 meV) is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately 10-5m0, where m0 is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to (11aB)-2, where aB?3.5 nm is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton lifetime to beyond 150 ps at room temperature, sufficient for thermal equilibrium BEC.

  15. Exciton trapping in a periodically modulated magnetic field

    Scientific Electronic Library Online (English)

    J.A.K., Freire; V.N., Freire; G.A., Farias; F.M., Peeters.

    2002-06-01

    Full Text Available The behavior of excitons in spatially modulated magnetic fields is described taking into account theb exciton spin contribution. The results show that the exciton trapping in periodic magnetic fields is possible and dependent on the modulation profile. [...

  16. 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

  17. 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.

  18. 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...

  19. Topologically protected excitons in porphyrin thin films

    OpenAIRE

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

    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...

  20. 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.

  1. Excitons in the rare gas solids

    International Nuclear Information System (INIS)

    Excitons play a prominent role in the chemistry and physics of condensed matter. Excitons in the rare gas solids, the prototypical van der Waals insulators, will be the focus of the remainder of this report. The goal here is to investigate the controversies surrounding the description of excitons in insulators and, therefore the simplest class of these solids, namely the rare gas solids, is chosen as the exemplary system. Specific problems associated with molecular crystals are, therefore, avoided and only the salient features of excitons are thus considered. 47 refs., 9 figs., 4 tabs

  2. 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.

  3. 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

  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. 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

  6. 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.

  7. 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

  8. Longitudinal-transverse splitting of individual excitonic polaritons in ZnO derived from ellipsometry

    International Nuclear Information System (INIS)

    The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands ?7,?9,?7. The transverse-longitudinal splittings which separate the upper polariton branch from the lower branch are deduced precisely for each subband and for different strain levels. Mainly for E parallel c, additional absorption peaks are observed at the longitudinal B-exciton and closely above. One is considered to be a mixed-mode and the other is seen as a consequence of interference effects in an exciton free surface layer which is also visible in Reflectance Anisotropy Spectroscopy (RAS). Furthermore, the effect of d-level-hybridization on valence-band-symmetries is evaluated by the effective number of electrons neff in high energy data between 3 and 32 eV. CdS polaritonic spectra were also measured as a reference and further support the inversed ordering of valence bands (negative spin-orbit splitting) in ZnO.

  9. 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 ...

  10. Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy

    CERN Document Server

    Karki, Khadga J; Zheng, Kaibo; Zidek, Karel; Mousa, Abdelrazek; Abdellah, Mohamed A; Messing, Maria; Wallenberg, L Reine; Yartsev, Arkadi; Pullerits, Tonu

    2013-01-01

    Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usu- ally done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods.

  11. 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

  12. 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)

  13. 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.

  14. Exciton effects in optical absorption spectra of boron-nitride (BN) nanotubes

    CERN Document Server

    Harigaya, Kikuo

    2008-01-01

    Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the onsite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes which show metallic and semiconducting properties depending on the chiralities.

  15. 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.

  16. THEORY OF THE EXCITONIC STARK SHIFT

    OpenAIRE

    Combescot, M.; Combescot, R.

    1988-01-01

    The excitonic Stark shift is due to a coupling between the exciton and all the biexcitonic states. This coupling induces a blue shift at large detuning which goes into a red shift at small detuning, when the biexcitonic molecule is stable.

  17. 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 Förster 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.

  18. 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

  19. 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.

  20. 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

  1. 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

  2. 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. PMID:26584333

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

  4. Charged Excitons in Self-assembled Quantum Dots

    OpenAIRE

    Warburton, Richard J.; B. Urbaszek; McGhee, E. J.; Schulhauser, C.; Högele, A.; Karrai, Khaled; Govorov, Alexander O.; García, Jorge M.; B. D. Gerardot; Pierre M. Petroff

    2003-01-01

    We have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons.

  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. 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

  7. Efficient exciton funneling in cascaded PbS quantum dot superstructures.

    Science.gov (United States)

    Xu, Fan; Ma, Xin; Haughn, Chelsea R; Benavides, Jamie; Doty, Matthew F; Cloutier, Sylvain G

    2011-12-27

    Benzenedithiol (BDT) and ethanedithiol (EDT) ligand-exchange treatments can be used to cross-link colloidal PbS quantum dots into nanocrystalline film structures with distinct optoelectronic properties. Such structures can provide a unique platform to study the energy transfer between layers of quantum dots with different sizes. In this report, efficient exciton funneling and recycling of surface state-bound excitons is observed in cascaded PbS quantum dot-based multilayered superstructures, where the excitons transfer from the larger band gap or donor layers to the smallest band gap or acceptor layers. In this system, both the BDT- and EDT-treated cascaded structures exhibit dramatically enhanced photoluminescence from the acceptor layers. As we show, the energy transfer mechanisms involved and their efficiencies are significantly different depending on the ligand-exchange treatment. In the future, we believe these efficient exciton recycling and funneling mechanisms could be used to improve significantly the photocurrent, charge-transport, and conversion efficiencies in low-cost nanocrystalline and hybrid solar cells and the emission efficiencies in hybrid light-emitting devices. PMID:22085035

  8. Exciton localization and drift in tailored-potential quantum nanowires

    Energy Technology Data Exchange (ETDEWEB)

    Szeszko, J., E-mail: justyna.szeszko@epfl.ch; Rudra, A.; Kapon, E. [Laboratory of Physics of Nanostructures, École Polytechnique Fédérale de Lausanne, 1015 Lausanne (Switzerland); Belykh, V. V.; Sibeldin, N. N. [P. N. Lebedev Physical Institute, Russian Academy of Sciences, Moscow 119991 (Russian Federation)

    2014-06-30

    Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).

  9. 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.

  10. Optimized spatial frequency response in silver halide sensitized gelatin

    OpenAIRE

    Fimia Gil, Antonio; Pascual Villalobos, Inmaculada; Beléndez Vázquez, 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.

  11. 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...

  12. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, JŘrn Märcher 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.

  13. 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.

  14. Transition metal-catalyzed nucleophilic substitution at thiophene-halides

    OpenAIRE

    Potratz, Stefanie

    2009-01-01

    Task of this thesis was nucleophilic substitution at thiophene-halides with O- and N-nucleophiles using copper and palladium catalysts. It was shown that phenols could be coupled under mild reactions conditions using modern copper catalysts. However, reaction conditions were not universal and conversion of diols was not possible. The poisoning of the catalyst due to stable copper-dialcoholate complexes was discussed. For amination of thiophene halides palladium catalysis was most effect...

  15. 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 metal–metal 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 150–250 °C, catalytically active sites develop, and the clu...

  16. Recent advances in new holographic silver halide materials

    OpenAIRE

    Neipp López, Cristian; Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto

    2000-01-01

    Photographic emulsions are still one of the most widely used recording materials, due to the high energetic and spectral sensitivity and ease of processing. In order to obtain holograms of high quality on photographic emulsions, the different chemical procedures applied to the emulsions must be optimized. In this work we study two particular procedures, silver halide sensitized gelatin and fixation-free rehalogenating bleaching applied to two new commercial silver halide emulsions: the BB-64...

  17. 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...

  18. Halide and proton binding kinetics of yellow fluorescent protein variants.

    Science.gov (United States)

    Seward, Harriet E; Basran, Jaswir; Denton, Roanne; Pfuhl, Mark; Muskett, Frederick W; Bagshaw, Clive R

    2013-04-01

    A T203Y substitution in green fluorescent protein causes a red shift in emission to yield a class of mutants known as yellow fluorescent protein (YFP). Many of these YFP mutants bind halides with affinities in the millimolar range, which often results in the chromophore pK values being shifted into the physiological range. While such sensitivities may be exploited for halide and pH sensors, it is desirable to reduce such environmental sensitivities in other studies, such as in Förster resonance energy transfer probes to measure conformational changes within fusion proteins. Venus and Citrine are two such variants that have been developed with much reduced halide sensitivities. Here we compare the kinetics of halide binding, and the coupled protonation reaction, for several YFP variants and detect slow kinetics (dissociation rate constants in the range of 0.1-1 s(-1)), indicative of binding to an internal site, in all cases. The effective halide affinity for Venus and Citrine is much reduced compared with that of the original YFP 10C construct, primarily through a reduced association rate constant. Nuclear magnetic resonance studies of YFP 10C confirm halide binding occurs on a slow time scale (chemical shift occur throughout the sequence and structure. PMID:23514090

  19. 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, René 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 Telecomunicaçőes, 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 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 10{sup 25}/m{sup 3}. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics.

  20. 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 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

  1. 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, René 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.

  2. 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.

  3. 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 GW–Bethe–Salpeter-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 singlet–triplet splitting, and electron–hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron–hole 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.

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

    International Nuclear Information System (INIS)

    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 SiO2 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

  5. Giant Rydberg Excitons in Cuprous Oxide

    CERN Document Server

    Kazimierczuk, Tomasz; Scheel, Stefan; Stolz, Heinrich; Bayer, Manfred

    2014-01-01

    Highly excited atoms with an electron moved into a level with large principal quantum number are fascinating hydrogen-like objects. The giant extension of these Rydberg atoms leads to huge interaction effects. Monitoring these interactions has provided novel insights into molecular and condensed matter physics problems on a single quantum level. Excitons, the fundamental optical excitations in semiconductors consisting of a negatively charged electron and a positively charged hole, are the condensed matter analogues of hydrogen. Highly excited excitons with extensions similar to Rydberg atoms are attractive because they may be placed and moved in a crystal with high precision using microscopic potential landscapes. Their interaction may allow formation of ordered exciton phases or sensing of elementary excitations in the surrounding, also on a quantum level. Here we demonstrate the existence of Rydberg excitons in cuprous oxide, Cu2O, with principal quantum numbers as large as n=25 . These states have giant w...

  6. 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.

  7. 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...

  8. 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.

  9. 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.

  10. Topologically protected excitons in porphyrin thin films

    Science.gov (United States)

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

    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.

  11. 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.

  12. Anisotropic observation of absorption and fluorescence transition dipoles in exciton-polariton properties of PIC J-aggregates

    International Nuclear Information System (INIS)

    Fibril-shaped J-aggregates of pseudoisocyanine dyes are prepared in thin-film matrices of polyvinyl sulfate and their microscopic fluorescence and reflectance imaging are investigated at room temperatures. Improved experiment, both in sample preparations and in microscope optics, allows us to resolve the higher-energy bands in the spectrum and to determine the directions of local absorption and fluorescence transition dipoles more precisely. Based on the anisotropies of the local transition dipoles, we have assigned the 540 nm middle band to be the upper exciton, Davidov split, transition and the higher 495 nm to its vibronic sub-band, while the J-band at 572 nm to be the lower Frenkel exciton transition. Assuming linear chain directions of individual J's to be parallel in average to the long axis of the fibril, the result also implies that a zigzag-type molecular conformation is most suitable for the model of PIC-J aggregates

  13. 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

  14. Exciton hopping probed by picosecond time-resolved cathodoluminescence

    Science.gov (United States)

    Shahmohammadi, Mehran; Jacopin, Gwénolé; Fu, Xuewen; Ganičre, Jean-Daniel; Yu, Dapeng; Deveaud, Benoît

    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.

  15. 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)

  16. 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...

  17. 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.

  18. 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)

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

    OpenAIRE

    Rémi, 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...

  20. 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 100°C were recorded. Propositions for studied glass application as tunable filters in electro-optic circuit were suggested.

  1. Exciton quasicondensation in one-dimensional systems

    Science.gov (United States)

    Werman, Yochai; Berg, Erez

    2015-06-01

    Two Luttinger liquids, with an equal density and opposite sign of charge carriers, may exhibit enhanced excitonic correlations. We term such a system an exciton quasicondensate, with a possible realization being two parallel oppositely doped quantum wires, coupled by repulsive Coulomb interactions. We show that this quasiexciton condensate can be stabilized in an extended range of parameters, in both spinless and spinful systems. We calculate the interwire tunneling current-voltage characteristic, and find that a negative differential conductance is a signature of excitonic correlations. For spinful electrons, the excitonic regime is shown to be distinct from the usual quasi-long-range ordered Wigner crystal phase characterized by power-law density wave correlations. The two phases can be clearly distinguished through their interwire tunneling current-voltage characteristics. In the quasiexciton condensate regime the tunneling conductivity diverges at low temperatures and voltages, whereas in the Wigner crystal it is strongly suppressed. Both the Wigner crystal and the excitonic regime are characterized by a divergent Coulomb drag at low temperature. Finally, metallic carbon nanotubes are considered as a special case of such a one-dimensional setup, and it is shown that exciton condensation is favorable due to the additional valley degree of freedom.

  2. Structure and energetics of trivalent metal halides

    International Nuclear Information System (INIS)

    Metal trihalide (MX3) systems represent a stern challenge in terms of constructing transferable potential models. Starting from a previously published set of potentials, 'extended' ionic models are developed which, at the outset, include only anion polarization. Deficiencies in these models, particularly for smaller (highly polarizing) cations, axe shown to be significant. For example, crystal structures different to those observed experimentally axe adopted. The potentials axe improved upon by reference to ab initio information available for alkali halides with the 'constraint' that the parameters transfer systematically in a physically transparent manner, for example, in terms of ion radii. The possible influence of anion compression ('breathing') and the relative abundance of anion-anion interactions are considered. Simulation techniques axe developed to allow for the effective simulation of any system symmetry and for the study of transitions between different crystals (constant stress). The developed models are fully tested for a large range of metal trichloride (MCl3) systems. Particular attention is paid to the comparison with recent neutron and X-ray diffraction data on the liquid state. Polarization effects axe shown to be vital in reproducing strong experimental features. The excellent agreement between simulation and experiment allows for differences in experimental procedures to be highlighted. The transferability is further tested by modelling mixtures of the lanthanides with alkali halides with potentials unchanged from the pure systems. The complex evolution of the melt structure is highlighted as the concentration of MCl3 increases. The effectiveness of the models is tested by reference to dynamical properties. Particular attention is paid to the comparison with Raman scattering data available for a wide range of systems and mixture concentrations. The simulated spectra are generated both by a simple molecular picture of the underlying vibrations and by a more Complex (fluctuating polarizability) model in which the spectra are broken down into contributions from different mechanisms. This comparison allows for the validity of treating network-like systems as a series of 'isolated' molecules to be assessed. The transferability of the potentials is pushed to the limits by modelling metal tribromides, in which the parameters are obtained from the trichlorides by the same simple scaling arguments. (author)

  3. 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

  4. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jřrn Märcher; Bacher, G.; Kümmell, 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...

  5. 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 pump–probe 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.

  6. 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 Padé 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.

  7. 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)

  8. 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.

  9. 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...

  10. 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 Förster 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 Förster 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

  11. Improved value for the silicon free exciton binding energy

    Directory of Open Access Journals (Sweden)

    Martin A. Green

    2013-11-01

    Full Text Available The free exciton binding energy is a key parameter in silicon material and device physics. In particular, it provides the necessary link between the energy threshold for valence to conduction band optical absorption and the bandgap determining electronic properties. The long accepted low temperature binding energy value of 14.7 ± 0.4 meV is reassessed taking advantage of developments subsequent to its original determination, leading to the conclusion that this value is definitely an underestimate. Using three largely independent experimental data sets, an improved low temperature value of 15.01 ± 0.06 meV is deduced, in good agreement with the most comprehensive theoretical calculations to date.

  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; Bergäuser, 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. Observation of Two-Exciton States in Perylene Bisimide Aggregates

    Directory of Open Access Journals (Sweden)

    Lochbrunner S.

    2013-03-01

    Full Text Available The behavior of excitons on perylene bisimide aggregates is investigated at high excitation densities by femtosecond absorption spectroscopy. Indications for a significant population in the two-exciton manifold are found.

  15. 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...

  16. 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.

  17. “On water” sp3–sp2 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.

  18. 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...

  19. 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...

  20. 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...

  1. 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

  2. Disorder-enhanced exciton delocalization in an extended dendrimer

    Science.gov (United States)

    Pouthier, Vincent

    2014-08-01

    The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory.

  3. 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$...

  4. 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)

  5. 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)

  6. 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.

  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 electron–electron 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 organic–inorganic 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 297 K 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 organic–inorganic lead halide perovskites” in Solar Energy Materials & Solar Cells [1].

  10. An optical criterion to obtain miscible mixed crystals in alkali halides

    Scientific Electronic Library Online (English)

    R, Rodríguez-Mijángos; G, Vázquez-Polo; J.J., Palafox; R, Pérez-Salas.

    2008-11-01

    Full Text Available Este trabajo ofrece un nuevo criterio para predecir la formación de soluciones sólidas en halogenuros alcalinos cristalinos y discute algunos resultados obtenidos en el desarrollo de mezclas dieléctricas cristalinas miscibles de halogenuros alcalinos ternarias y cuaternarias. Estas mezclas son misci [...] bles en cualquier concentración de sus componentes. Tener el resultado de estas mezclas cristalinas está relacionado al centro F a través del comportamiento observado en la banda F de absorción en función de la constante de red de los halogenuros alcalinos donde el defecto fue formado (centro F). Dando un vistazo a la gráfica de Energía 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 solución sólida podría formarse, significando un cristal de una sola fase, que dan por resultado cristales ternarios y cuaternarios. Así, la banda F de absorción nos permite tener un criterio numérico, basado en el porcentaje de cambio de la energía de la banda F que permite obtener soluciones sólidas. Encontramos información experimental, usando difractogramas de las mezclas cristalinas, se obtienen las constantes de red y se comparan con la obtenida teóricamente a través de la generalización 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.

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

    Directory of Open Access Journals (Sweden)

    A. Pérez-Rodríguez

    2006-01-01

    Full Text Available Atomos de cobre han sido introducidos por difusión en cristales de NaCl, KCl y KBr a 500±C. Los cristales han sido analizados óptimamente con medidas de fotoluminiscencia y por microscopía electrónica de barrido. Los espectros de emisión y excitación, medidos a baja temperatura muestran el efecto de confinamiento de exciton, indicando la formación de nanopartículas de CuX (X=Cl, Br, lo cual ha sido confirmado por imágenes de microscopía electrónica. Este método es propuesto como un método alternativo para obtener nanopartículas de CuX en cristales halogenuros alcalinos.

  12. 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.

  13. 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)

  14. 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.

  15. 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...

  16. 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.

  17. Nanocrystal skins with exciton funneling for photosensing.

    Science.gov (United States)

    Akhavan, Shahab; Cihan, Ahmet Fatih; Bozok, Berkay; Demir, Hilmi Volkan

    2014-06-25

    Highly photosensitive nanocrystal (NC) skins based on exciton funneling are proposed and demonstrated using a graded bandgap profile across which no external bias is applied in operation for light-sensing. Four types of gradient NC skin devices (GNS) made of NC monolayers of distinct sizes with photovoltage readout are fabricated and comparatively studied. In all structures, polyelectrolyte polymers separating CdTe NC monolayers set the interparticle distances between the monolayers of ligand-free NCs to <1 nm. In this photosensitive GNS platform, excitons funnel along the gradually decreasing bandgap gradient of cascaded NC monolayers, and are finally captured by the NC monolayer with the smallest bandgap interfacing the metal electrode. Time-resolved measurements of the cascaded NC skins are conducted at the donor and acceptor wavelengths, and the exciton transfer process is confirmed in these active structures. These findings are expected to enable large-area GNS-based photosensing with highly efficient full-spectrum conversion. PMID:24599603

  18. 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.

  19. 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.

  20. 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.

  1. 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. PMID:26419224

  2. 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.

  3. Detecting an exciton crystal by statistical means

    Energy Technology Data Exchange (ETDEWEB)

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

  4. 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)

  5. 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.

  6. 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 1.18±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

  7. 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.

  8. 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 1.3–3.1?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

  9. 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.

  10. 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)

  11. 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 1.3–3.1?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.

  12. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner

    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 found to increase with decreasing win width. This is assigned to the reduced phase space in a quasi-one-dimensional system, enhancing the repulsive interaction between excitons due to Pauli blocking.

  13. 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.

  14. Dielectric measurements of adsorbed krypton on lamellar halides

    OpenAIRE

    Laheurte, J.P.; Noiray, J.C.; Obadia, M.; Romagnan, J.P.

    1981-01-01

    We present simultaneous measurements of adsorption isotherms and dielectric isotherms for krypton on lamellar halides. The comparison between the two isotherms shows that dielectric method is well adapted to characterize the multilayer formation. The evolution of dielectric properties from a two-dimensional system towards the bulk is also obtained.

  15. 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

  16. 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...

  17. 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

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

    International Nuclear Information System (INIS)

    We investigate the breakup of bound electron–hole 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 electron–hole 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 electron–hole 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)

  19. Ultrafast exciton dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films.

    Science.gov (United States)

    Ishino, Yuuta; Miyata, Kiyoshi; Sugimoto, Toshiki; Watanabe, Kazuya; Matsumoto, Yoshiyasu; Uemura, Takafumi; Takeya, Jun

    2014-04-28

    Ultrafast dynamics of excitons in organic semiconductors is essential for a deep understanding of the working mechanism of plastic opto-electronic devices. In this work, excited state dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films has been studied with femtosecond transient absorption and time-resolved photoluminescence spectroscopy. Upon the excitation with a femtosecond pulse at 400 nm, a broad positive absorption band at 1.5-2.4 eV is observed that contains two components: one decays with a time constant of a few ps and the other with 67 ± 7 ps. Because the decay curve of the latter coincides with that of photoluminescence, the slow decay component is ascribed to the lowest singlet exciton. The former fast decay component is ascribed to mixed states between charge transfer (CT) and Frenkel excitons, because it is accompanied by a feature due to the Stark effect caused by transient charged species: a combination of bleach and positive absorption at h?probe > 2.4 eV which looks like derivative modulations of the ground state absorption spectrum. A pronounced polarization dependence is observed for the derivative-like features; this is due to anisotropic distributions of the dipole moments formed by the CT excitons. The derivative-like feature changes its shape after the decay of the mixed Frenkel-CT exciton and grows with a pump-probe delay time of up to 1 ns due to a thermal effect. The decay rate of the mixed Frenkel-CT exciton strongly depends on its density because of exciton-exciton annihilation at high density. PMID:24626573

  20. 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.

  1. 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)

  2. Internal field induced exciton binding energy and the optical transition in a strained Mg based II–VI 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.

  3. 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 3–20 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.

  4. 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.

  5. 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.

  6. 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

  7. Molecular Weight Dependence of Exciton Diffusion in Poly(3-hexylthiophene)

    DEFF Research Database (Denmark)

    Masri, Zarifi; Ruseckas, Arvydas

    2013-01-01

    A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial ?–? aggregates of polymer chromophores and about 100 times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions.

  8. Molecular weight dependence of exciton diffusion in poly(3-hexylthiophene)

    DEFF Research Database (Denmark)

    Masri, Zarifi; Ruseckas, Arvydas

    2013-01-01

    A joint experimental and theoretical study of singlet exciton diffusion in spin-coated poly(3-hexylthiophene) (P3HT) films and its dependence on molecular weight is presented. The results show that exciton diffusion is fast along the co-facial ?–? aggregates of polymer chromophores and about 100 times slower in the lateral direction between aggregates. Exciton hopping between aggregates is found to show a subtle dependence on interchain coupling, aggregate size, and Boltzmann statistics. Additionally, a clear correlation is observed between the effective exciton diffusion coefficient, the degree of aggregation of chromophores, and exciton delocalization along the polymer chain, which suggests that exciton diffusion length can be enhanced by tailored synthesis and processing conditions.

  9. Electrolytic systems and methods for making metal halides and refining metals

    Energy Technology Data Exchange (ETDEWEB)

    Holland, Justin M.; Cecala, David M.

    2015-05-26

    Disclosed are electrochemical cells and methods for producing a halide of a non-alkali metal and for electrorefining the halide. The systems typically involve an electrochemical cell having a cathode structure configured for dissolving a hydrogen halide that forms the halide into a molten salt of the halogen and an alkali metal. Typically a direct current voltage is applied across the cathode and an anode that is fabricated with the non-alkali metal such that the halide of the non-alkali metal is formed adjacent the anode. Electrorefining cells and methods involve applying a direct current voltage across the anode where the halide of the non-alkali metal is formed and the cathode where the non-alkali metal is electro-deposited. In a representative embodiment the halogen is chlorine, the alkali metal is lithium and the non-alkali metal is uranium.

  10. Basic properties of the F-type centers in halides, oxides and perovskites

    International Nuclear Information System (INIS)

    We present a short survey of the optical properties of primary radiation-induced point defects in alkali halides, simple oxides and some ABO3 perovskites. We discuss in details the optical properties of single electron F and F+ centers in rock-salt (f.c.c.) alkali halides and oxides and show that the Mollwo-Ivey law well-known for the F-type centers in alkali halides may be extended for other rock-salt structure insulators. We also discuss the major differences in point defect production mechanisms in halides and oxides. We show that the Rabin-Klick diagram may be generalized for a whole family of alkali halides. The F-type center migration and aggregation into metal colloids in alkali halides and oxides is also discussed.

  11. 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...

  12. 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.

  13. Alkali halide microstructured optical fiber for X-ray detection

    International Nuclear Information System (INIS)

    Microstructured optical fibers containing alkali halide scintillation materials of CsI(Na), CsI(Tl), and NaI(Tl) are presented. The scintillation materials are grown inside the microstructured fibers using a modified Bridgman-Stockbarger technique. The x-ray photon counts of these fibers, with and without an aluminum film coating are compared to the output of a collimated CdTe solid state detector over an energy range from 10 to 40 keV. The photon count results show significant variations in the fiber output based on the materials. The alkali halide fiber output can exceed that of the CdTe detector, dependent upon photon counter efficiency and fiber configuration. The results and associated materials difference are discussed

  14. Temperature dependent dynamic ESD processes in alkali halides

    International Nuclear Information System (INIS)

    The effect of the sample temperature on angular-resolved kinetic-energy distributions of alkali and halogen atoms, electronically desorbed from single crystal alkali halides, has been measured. It was found that while the emission of particles with thermal energies increased by about a factor of 40 in the temperature range 90-300degC, the nonthermal halogen atom intensity decreased by about a factor of 3. From these temperature dependent measurements the activation energies for thermally assisted defect migration processes have been estimated. The results will be compared with the data available in the literature and the predictions of a recently proposed model for electron-stimulated desorption (ESD) of alkali halides. (orig.)

  15. Alkali halide microstructured optical fiber for X-ray detection

    Energy Technology Data Exchange (ETDEWEB)

    DeHaven, S. L., E-mail: stanton.l.dehaven@nasa.gov, E-mail: russel.a.wincheski@nasa.gov; Wincheski, R. A., E-mail: stanton.l.dehaven@nasa.gov, E-mail: russel.a.wincheski@nasa.gov [NASA Langley Research Center, Hampton, VA 23681 (United States); Albin, S., E-mail: salbin@nsu.edu [Norfolk State University, Norfolk, VA 23504 (United States)

    2015-03-31

    Microstructured optical fibers containing alkali halide scintillation materials of CsI(Na), CsI(Tl), and NaI(Tl) are presented. The scintillation materials are grown inside the microstructured fibers using a modified Bridgman-Stockbarger technique. The x-ray photon counts of these fibers, with and without an aluminum film coating are compared to the output of a collimated CdTe solid state detector over an energy range from 10 to 40 keV. The photon count results show significant variations in the fiber output based on the materials. The alkali halide fiber output can exceed that of the CdTe detector, dependent upon photon counter efficiency and fiber configuration. The results and associated materials difference are discussed.

  16. Stability diagrams for complexes in molten mixtures of halide salts

    International Nuclear Information System (INIS)

    The stability of local fourfold coordination for divalent and trivalent metal ions in liquid mixtures of polyvalent metal halides and alkali halides is classified by means of structural coordinates obtained from properties of the elements. In parallel with earlier classifications of compound crystal structures and molecular shapes, the elemental properties are taken from first-principles calculations of valence electron orbitals in atoms, in the form of (i) the nodal radii of Andreoni, Baldereschi and Guizzetti or (ii) the pseudo-potential radii of Zunger and Cohen. As a third alternative we also consider a classification based on Pettifor's phenomenological chemical scale of the elements. The alternative structural classification schemes that are developed from these elemental properties are very successful in distinguishing molten mixtures in which the available experimental evidence indicates stability of ''complexes'', i.e. long-lived fourfold coordination of polyvalent metal ions. (author). 55 refs, 3 figs

  17. Optical radiation from electron sputtering of alkali halides

    International Nuclear Information System (INIS)

    Electron-surface collisions have been observed to result in the emission of optical radiation from excited atoms and molecules desorbed from alkali halide single crystal surfaces. The detected radiation included Na and Li resonance lines, hydrogen Balmer emission, and OH molecular radiation. Much previous experimental and theoretical effort has been devoted to electron stimulated desorption (ESD) of ground state neutrals and ions, and in particular from alkali halides. The few studies concerned with excited particles have dealt primarily with ejected metastable neutrals. In one case involving electron bombardment fluorescence of ice, a tentative identification has been made of OH molecular radiation. The results reported here include a) the first definitive work on optical radiation from electron bombardment induced emission of excited free substrate particles, and b) the first observations of characteristic radiation from previously adsorbed free atoms and molecules. (orig.)

  18. Solution-Phase Synthesis of Cesium Lead Halide Perovskite Nanowires.

    Science.gov (United States)

    Zhang, Dandan; Eaton, Samuel W; Yu, Yi; Dou, Letian; Yang, Peidong

    2015-07-29

    Halide perovskites have attracted much attention over the past 5 years as a promising class of materials for optoelectronic applications. However, compared to hybrid organic-inorganic perovskites, the study of their pure inorganic counterparts, like cesium lead halides (CsPbX3), lags far behind. Here, a catalyst-free, solution-phase synthesis of CsPbX3 nanowires (NWs) is reported. These NWs are single-crystalline, with uniform growth direction, and crystallize in the orthorhombic phase. Both CsPbBr3 and CsPbI3 are photoluminescence active, with composition-dependent temperature and self-trapping behavior. These NWs with a well-defined morphology could serve as an ideal platform for the investigation of fundamental properties and the development of future applications in nanoscale optoelectronic devices based on all-inorganic perovskites. PMID:26181343

  19. Electron-beam production of colour centres on alkali halide crystals and films

    International Nuclear Information System (INIS)

    In this article we analyze the production of localized colour centres on the surface of alkali halide crystals and films produced by electron beam radiations in the 10-30 keV range. It was found that the colour centre production is restricted to electron penetration (3-10 ?m depending on the material and electron energy) with a very intense local optical density (above 104 cm-1). As the refractive index variation calculated using the Kramers-Kroenig relations from the observed absorption bands should be enough for light confinement (10-3-10-2), electron beam generated colour centres should allow the production of light wave guides for several applications. Optical absorption bands can be created in ionic crystals anywhere from the ultraviolet to the infrared, preserving transparent regions of the spectrum. With an appropriate computer aided design (CAD) program and a microcomputer interface to the adapted micropositioning mechanical devices and shutter, it was possible to control an electron microprobe sample holder in order to draw precise patterns and have various controlled exposure times. (orig.)

  20. Hot-carrier cooling and photoinduced refractive index changes in organic-inorganic lead halide perovskites

    Science.gov (United States)

    Price, Michael B.; Butkus, Justinas; Jellicoe, Tom C.; Sadhanala, Aditya; Briane, Anouk; Halpert, Jonathan E.; Broch, Katharina; Hodgkiss, Justin M.; Friend, Richard H.; Deschler, Felix

    2015-09-01

    Metal-halide perovskites are at the frontier of optoelectronic research due to solution processability and excellent semiconductor properties. Here we use transient absorption spectroscopy to study hot-carrier distributions in CH3NH3PbI3 and quantify key semiconductor parameters. Above bandgap, non-resonant excitation creates quasi-thermalized carrier distributions within 100 fs. During carrier cooling, a sub-bandgap transient absorption signal arises at ~1.6 eV, which is explained by the interplay of bandgap renormalization and hot-carrier distributions. At higher excitation densities, a `phonon bottleneck' substantially slows carrier cooling. This effect indicates a low contribution from inelastic carrier-impurity or phonon-impurity scattering in these polycrystalline materials, which supports high charge-carrier mobilities. Photoinduced reflectivity changes distort the shape of transient absorption spectra and must be included to extract physical constants. Using a simple band-filling model that accounts for these changes, we determine a small effective mass of mr=0.14 mo, which agrees with band structure calculations and high photovoltaic performance.

  1. 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 ...

  2. 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...

  3. 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.

  4. 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.

  5. 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...

  6. Josephson oscillations between exciton condensates in electrostatic traps

    OpenAIRE

    Rontani, Massimo; Sham, L. J.

    2009-01-01

    Technological advances allow for tunable lateral confinement of cold dipolar excitons in coupled quantum wells. We consider theoretically the Josephson effect between exciton condensates in two traps separated by a weak link. The flow of the exciton supercurrent is driven by the dipole energy difference between the traps. The Josephson oscillations may be observed after ensemble average of the time correlation of photons separately emitted from the two traps. The fringe visi...

  7. 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

  8. 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...

  9. Multiparticle Exciton Ionization in Shallow Doped Carbon Nanotubes.

    Science.gov (United States)

    Sau, Jay D; Crochet, Jared J; Doorn, Stephen K; Cohen, Marvin L

    2013-03-21

    Shallow hole doping in small-diameter semiconducting carbon nanotubes with a valley degeneracy is predicted to result in the resonant ionization of excitons into free electron-hole pairs. This mechanism, which relies on the chirality of the electronic states, causes excitons to decay with high efficiencies where the rate scales as the square of the dopant density. Moreover, multiparticle exciton ionization can account for delocalized fluorescence quenching when a few holes per micrometer of tube length are present. PMID:26291364

  10. 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).

  11. 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.

  12. 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.

  13. 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.

  14. Influence of the Print Run on Silver Halide Printing Plates

    Directory of Open Access Journals (Sweden)

    Tomislav Cigula

    2010-09-01

    Full Text Available The most common printing technique today is lithography. The difference between printing and nonprinting areason a printing plate is accomplished by opposite physical and chemical properties of those areas (MacPhee, 1998.The printing areas are made of photoactive layer that attracts oil and chemical substances with oil solvent – printinginks. The nonprinting areas are made of aluminium-oxide which attracts water based substances – the fountainsolution.There are many of various types of photoactive layer which are used for production of offset printing plates, amongothers is silver halide layer. The usage of the silver halide technology in the graphic reproduction is not a novelty.The filmmaking phase is based on the usage of the silver halide as the photographically active ingredient, for instance,AgBr (silver bromide. The new, digital plate making technology (Computer to Plate, CtP eliminates thefilmmaking phase and therefore enables control of the printing plate’s exposure made by computer. CtP technologyeliminates the filmmaking phase, but it also results with the reduction of needed material quantities and requiredtime for the production (Limburg, 1994; Seydel, 1996.In this paper the basis of the graphic reproduction by using the silver halide digital printing plates was described.The changes of the AgX copying layer and the surface of the aluminium base in the printing process have beenobserved. The surface characteristics were determined by measuring the relevant surface roughness parameters. Inaddition, measurements of coverage values on the prints, detailed at smaller print run, were conducted.Results showed that surface changes on the printing plate are changing during printing process and that thesechanges influence transfer of the printing ink on the printing substrate. These measurements proved to be of greatinterest in the graphic reproduction as they enable us to determine consistency of the printing plates during theprinting process, to predict the endurance as well as to define the print run which will result with optimal qualityprints.

  15. Energy determination from electron range in alkali-halide crystals

    International Nuclear Information System (INIS)

    The coloration of alkali-halide crystals was used to determine the electron energies <10 MeV. The coloration depth in a crystal - the visual range, was measured by using a lighted transparent ruler and an empirical relation was used to determine the extrapolated range. The sharpest edge of coloration was obtained in KBr crystals. The electron energy was calculated from a semi-empirical relation between the extrapolated range of electrons and their energy

  16. Silver halide sensitized gelatin as a holographic storage medium

    OpenAIRE

    Fimia Gil, Antonio; Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto

    1988-01-01

    Silver halide sensitized gelatin is one of the most promising techniques for the manufacturing of transmission holographic optical elements. These techniques combine the relatively high sensitivity of photographic material with the low scattering and high light-stability of dichromated gelatin. The influences of the developer and the bleaching in the diffraction efficiency and noise is analyzed starting with Agfa 8E75 HD plates.

  17. Reflection holographic optical elements in silver-halide sensitized gelatin

    OpenAIRE

    Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto; Fimia Gil, Antonio

    1991-01-01

    Silver halide sensitized gelatin has proven to be an alternative to dichromated gelatin as a recording material in the production of transmission holographic optical elements (HOEs). In this paper we discuss the possible applications of this process to the production of reflection HOEs as well as the possible use of one of them in the construction of a hybrid refraction- diffraction system that could be used to copy transmission HOEs using partially coherent light.

  18. Silver halide sensitized gelatin derived from BB-640 holographic emulsion

    OpenAIRE

    Neipp lópez, Cristian; Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto

    1998-01-01

    Silver halide sensitized gelatin (SHSG) is one of the most interesting techniques for the production of holographic optical elements, achieving relatively high sensitivity of photographic material with a low scattering of dichromated gelatin. Here we present experimental results for SHSG derived from the novel BB-640, a red-sensitive ultra-fine-grain emulsion from Holographic Recording Technologies (Steinau, Germany). The material is characterized before recording and after processing, and in...

  19. PLENARY SESSIONThermoluminescence and lattice defects in alkali halides

    OpenAIRE

    Alvarez Rivas, J.

    1980-01-01

    Recent developments in the study of the thermoluminescence of irradiated alkali halides indicate that many of the observed glow peaks are due to thermoluminescent processes related to the recombination of radiation induced lattice defects. There is a large amount of experimental results which supports that the F+H recombination is a rather common thermoluminescent process. Under some conditions it has been observed that the I + F, I + VK and H + ? recombinations can indirectly induce light em...

  20. Study of methyl halide fluxes in temperate and tropical ecosystems

    OpenAIRE

    Blei, Emanuel

    2010-01-01

    CH3Br and CH3Cl (methyl halides) are the most abundant natural vectors of bromine and chlorine into the stratosphere and play an important role in stratospheric ozone destruction. The current knowledge of their respective natural sources is incomplete leading to large uncertainties in their global budgets. Beside the issue of quantification, characterisation of possible sources is needed to assist modelling of future environmental change impacts on these sources and hence the s...

  1. Na+ and Rb+ tracer diffusion in alkali halides

    International Nuclear Information System (INIS)

    We have undertaken a fundamental study of heterodiffusion of foreign ions in pure single crystals. The present work describes measurements of the diffusion coefficient of monovalent cations in some alkali halides, namely Na+ and Rb+ into KCl, KBr, NaI and KI. Priority is given to the super-accuracy of the experimental data. The target is to test the validity of the existing theories for calculating the enthalpy and entropy of migration. (author)

  2. Organic-inorganic hybrid photovoltaics based on organometal halide perovskites

    OpenAIRE

    Lee, Michael M.; Henry J. Snaith

    2013-01-01

    This thesis details the development of a novel photovoltaic device based on organometal halide perovskites. The initial focus of this thesis begins with the study of lighttrapping strategies in solid-state dye-sensitised solar cells (detailed in chapter 3). While I report enhancement in device performance through the application of near and far-?eld light-trapping techniques, I ?nd that improvements remain step-wise due to fundamental limitations currently employed in dye-sensitised ...

  3. Improved spatial frequency response in silver halide sensitized gelatin holograms

    OpenAIRE

    Beléndez Vázquez, Augusto; Neipp López, Cristian; Pascual Villalobos, Inmaculada

    1998-01-01

    We report what we believe to be the best results obtained to date with regard to the spatial frequency response of silver halide sensitized gelatin (SHSG) holograms. A very high diffraction efficiency, as high as 91% (after allowing for reflection), and an almost flat spatial frequency response between 800 lines/mm and 2800 lines/mm have been achieved using the new BB-640 plates manufactured by Holographic Recording Technologies. The results are compared with those for gratings recorded in th...

  4. Silver halide sensitized gelatin holograms using Slavich PFG-01 emulsion

    OpenAIRE

    Pascual Villalobos, Inmaculada; Neipp lópez, Cristian; Beléndez Vázquez, Augusto

    1999-01-01

    Silver halide sensitized gelatin (SHSG) is an interesting technique for the production of holographic optical elements. It combines the high sensitivity of photographic emulsions with the well-known low scattering and high diffraction efficiency corresponding to dichromated gelatin. In this paper we present a comparative study of SHSG holograms recorded on both Slavich PFG-01 and Agfa 8E75 HD emulsions. We will show that real high diffraction efficiencies can be obtained (as high as 93% allo...

  5. Silver halide sensitized gelatin as a holographic recording material

    OpenAIRE

    Fimia Gil, Antonio; Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto

    1994-01-01

    Silver halide sensitized gelatin (SHSG) has proven to be a good alternative to dichromated gelatin and bleached emulsions in the production of volume phase holographic optical elements. In the present paper, the processing procedure for SHSG derived from Agfa-Gevaert holographic plates is reviewed and the holographic characteristics of the processed SHSG are experimentally measured and analysed. Several important aspects related to the influence of the developer and the bleach bath, the noise...

  6. 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 pigment–protein 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...

  7. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    OpenAIRE

    Olšina, 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...

  8. Single exciton spectroscopy of semimagnetic quantum dots

    OpenAIRE

    Fernández Rossier, Joaquín

    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...

  9. Exciton Hierarchies in Gapped Carbon Nanotubes

    OpenAIRE

    Konik, Robert M.

    2010-01-01

    We present evidence that the strong electron-electron interactions in gapped carbon nanotubes lead to finite hierarchies of excitons within a given nanotube subband. We study these hierarchies by employing a field theoretic reduction of the gapped carbon nanotube permitting electron-electron interactions to be treated exactly. We analyze this reduction by employing a Wilsonian-like numerical renormalization group. We are so able to determine the gap ratios of the one-photon ...

  10. 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

  11. 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)

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

    DEFF Research Database (Denmark)

    García 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 ion gives rise to a narrow absorption peak around 1.2 eV below the calculated bandgap of 4.8 eV. In the excited state, the internal O2 ?2 bond is significantly weakened due to the population of the ?* orbi...

  13. 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...

  14. 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

  15. 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 Förster 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

  16. Polarization-dependent exciton dynamics in tetracene single crystals

    CERN Document Server

    Zhang, Bo; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min

    2014-01-01

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission 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.

  17. Theory of the late stage of radiolysis of alkali halides

    International Nuclear Information System (INIS)

    Recent results on heavily irradiated natural and synthetic NaCl crystals give evidence for the formation of large vacancy voids, which were not addressed by the conventional Jain-Lidiard model of radiation damage in alkali halides. This model was constructed to describe metal colloids and dislocation loops formed in alkali halides during earlier stages of irradiation. We present a theory based on a new mechanism of dislocation climb, which involves the production of VF centers (self-trapped hole neighboring a cation vacancy) as a result of the absorption of excess H centers. Voids are shown to arise due to the reaction between F and VF centers at the surface of halogen bubbles. Critical parameters associated with the bubble-to-void transition are evaluated. Voids can grow to sizes exceeding the mean distance between colloids and bubbles, eventually absorbing them, and, hence, igniting a back reaction between the halogen gas and metal. The amount of radiation damage in alkali halides should be evaluated with account of void formation, which strongly affects the radiation stability of material

  18. Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

    Directory of Open Access Journals (Sweden)

    Satoshi Kamiguchi

    2014-04-01

    Full Text Available 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 metal–metal 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 150–250 °C, catalytically active sites develop, and the cluster framework is retained up to 350–450 °C. One of the active sites is a Brřnsted acid resulting from a hydroxo ligand that is produced by the elimination of hydrogen halide from the halogen and aqua ligands. The other active site is a coordinatively unsaturated metal, which can be isoelectronic with the platinum group metals by taking two or more electrons from the halogen ligands. In the case of the rhenium chloride cluster Re3Cl9, the cluster framework is stable at least up to 300 °C under inert atmosphere; however, it is reduced to metallic rhenium at 250–300 °C under hydrogen. The activated clusters are characterized by X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure analysis, thermogravimetry–differential thermal analysis, infrared spectrometry, acid titration with Hammett indicators, and elemental analyses.

  19. 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)

  20. 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

  1. 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

  2. Transient magneto-photoinduced absorption study of singlet fission in low band gap copolymers

    Science.gov (United States)

    Huynh, Uyen; Vardeny, Z. Valy

    2015-03-01

    We have observed the existence of singlet fission in thin films of low band gap (LBG) copolymers, PDTP-DFBT and PTB7, using the ultrafast optical pump/probe spectroscopy, probed at the energy range from IR to MIR. The singlet fission is the dissociation of a singlet exciton into two triplets through an intermediate triplet pair state (TT pair) in an overall singlet configuration; in the studied copolymers, it was observed to be very fast, in femtosecond time domain. The intermediate TT state, which dissociates into two separated triplets at later time, or recombines to the ground state appears instantaneously with the singlet exciton formation using our laser system that has ~ 150 fs time resolution. The interplay between the rate of singlet fission into sTT pairs, triplet fusion back to singlet excitons and relaxation between the TT spin sublevels explains the obtained opposite pattern of the transient magnetic field response on the dynamics of singlet excitons and TT pairs.

  3. Excitonic properties of strained wurtzite and zinc-blende GaN/Al(x)Ga(1-x)N quantum dots

    OpenAIRE

    Fonoberov, Vladimir A.; Balandin, Alexander A.

    2003-01-01

    We investigate exciton states theoretically in strained GaN/AlN quantum dots with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies the conduction and valence band edges of GaN quantum dots. The piezoelectric field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN quantum...

  4. 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; Lérondel, 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 (photoluminescence (PL) investigations on as-grown ZnO micropods revealed a highly crystalline ZnO structure and a strong UV excitonic emission with internal quantum efficiency (IQE) of 10% at room temperature. Thermal annealing at 900°C of the as-grown ZnO micropods leads to further enhancement in their structural and optical properties. Low-temperature PL measurements on annealed ZnO micropods showed the presence of phonon replicas, which was not the case for as-grown samples. The appearance of phonon replicas provides a strong proof of the improved crystal quality of annealed ZnO micropods. Most importantly, low-temperature PL reveals an improved IQE of 15% in the excitonic emission of ZnO micropods. The ZnO micropods IQE reported here are comparable to IQEs reported on ZnO structures obtained by 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.

  5. 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

  6. Quantum phases and transitions of excitons, metastable excitonic supersolid and its internal photon detection in electron-hole bilayer systems

    OpenAIRE

    Ye, Jinwu

    2007-01-01

    We construct a quantum Ginsburg-Landau theory to study the quantum phases and transitions in electron hole bilayer system. We propose that in the dilute limit as distance is increased, there is a first order transition from the excitonic superfluid (ESF) to the excitonic supersolid (ESS) driven by the collapsing of a roton minimum, then a 2nd order transition from the ESS to excitonic normal solid. We show the latter transition is in the same universality class of superfluid...

  7. Exciton-phonon bound complex in single-walled carbon nanotubes revealed by high-field magneto-optical spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Zhou, Weihang; Nakamura, Daisuke; Takeyama, Shojiro, E-mail: takeyama@issp.u-tokyo.ac.jp [Institute for Solid State Physics, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581 (Japan); Sasaki, Tatsuya; Saito, Hiroaki [Institute for Solid State Physics, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581 (Japan); Department of Applied Physics, University of Tokyo, Hongo 113-8656 (Japan); Liu, Huaping [Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Kataura, Hiromichi [Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562 (Japan)

    2013-12-02

    High-field magneto-optical spectroscopy was performed on highly enriched (6,5) single-walled carbon nanotubes. Spectra of phonon sidebands in both 1st and 2nd sub-bands were unchanged by an external magnetic field up to 52?T. The dark K-momentum singlet (D-K-S) exciton, which plays an important role for the external quantum efficiency of the system for both sub-bands in the near-infrared and the visible light region, respectively, was clarified to be the origin of the phonon sidebands.

  8. Involvement of S-adenosylmethionine-dependent halide/thiol methyltransferase (HTMT) in methyl halide emissions from agricultural plants: isolation and characterization of an HTMT-coding gene from Raphanus sativus (daikon radish)

    OpenAIRE

    Taniguchi Tomokazu; Negishi Takashi; Matsuda Michiko; Toda Hiroshi; Itoh Nobuya; Ohsawa Noboru

    2009-01-01

    Abstract Background Biogenic emissions of methyl halides (CH3Cl, CH3Br and CH3I) are the major source of these compounds in the atmosphere; however, there are few reports about the halide profiles and strengths of these emissions. Halide ion methyltransferase (HMT) and halide/thiol methyltransferase (HTMT) enzymes concerning these emissions have been purified and characterized from several organisms including marine algae, fungi, and higher plants; however, the correlation between emission pr...

  9. 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.

  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. 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.)

  12. Gastric Banding

    Science.gov (United States)

    ... gastric banding before deciding to have the procedure. Advertisements for a device or procedure may not include ... demonstrated benefits for people who have not been successful using non-surgical weight loss methods. This surgical ...

  13. 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.

  14. 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

  15. 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

  16. Exciton-mediated quantum search on a star graph

    Science.gov (United States)

    Pouthier, Vincent

    2015-09-01

    A fast and efficient quantum search algorithm is established by using the ability of an exciton to propagate along a star graph that exhibits two identical energetic defects. The first defect lies on the well-defined input site where the exciton is initially created, whereas the second defect occupies the target site whose unknown position must be determined. It is shown that when the energetic defects are judiciously chosen, specific quantum interferences arise so that the probability to observe the exciton on the target site becomes close to unity at a very short time . Consequently, a measurement of the exciton quantum state at time will reveal the identity of the position of the target site. The key point is that is the shortest time independent on the size of the graph that is physically accessible to the exciton to tunnel.

  17. 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...

  18. 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.

  19. Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH3NH3PbI3-xClx

    OpenAIRE

    Wehrenfennig, C.; Liu, M.; Snaith, HJ; Johnston, MB; Herz, LM (Laura)

    2014-01-01

    The organic-inorganic hybrid perovskites methylammonium lead iodide (CH3NH3PbI3) and the partially chlorine-substituted mixed halide CH3NH3PbI 3-xClx emit strong and broad photoluminescence (PL) around their band gap energy of ?1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH3NH3PbI 3-xClx using time-...

  20. The search for Bose–Einstein 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 Bose–Einstein 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 well—a 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. Two-Dimensional Valley Electrons and Excitons in Noncentrosymmetric 3 R -MoS2

    Science.gov (United States)

    Akashi, Ryosuke; Ochi, Masayuki; Bordács, Sándor; Suzuki, Ryuji; Tokura, Yoshinori; Iwasa, Yoshihiro; Arita, Ryotaro

    2015-07-01

    We find that the motion of the valley electrons—electronic states close to the K and K' points of the Brillouin zone—is 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.

  3. 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)

  4. Impact of the organic halide salt on final perovskite composition for photovoltaic applications

    Directory of Open Access Journals (Sweden)

    David T. Moore

    2014-08-01

    Full Text Available The methylammonium lead halide perovskites have shown significant promise as a low-cost, second generation, photovoltaic material. Despite recent advances, however, there are still a number of fundamental aspects of their formation as well as their physical and electronic behavior that are not well understood. In this letter we explore the mechanism by which these materials crystallize by testing the outcome of each of the reagent halide salts. We find that components of both salts, lead halide and methylammonium halide, are relatively mobile and can be readily exchanged during the crystallization process when the reaction is carried out in solution or in the solid state. We exploit this fact by showing that the perovskite structure is formed even when the lead salt's anion is a non-halide, leading to lower annealing temperature and time requirements for film formation. Studies into these behaviors may ultimately lead to improved processing conditions for photovoltaic films.

  5. Thermal evolution of the band edges of 6H-SiC: X-ray methods compared to the optical band gap

    Energy Technology Data Exchange (ETDEWEB)

    Miedema, P.S., E-mail: piter.miedema@helmholtz-berlin.de [Institute for Methods and Instrumentation in Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin (Germany); Beye, M.; Könnecke, R.; Schiwietz, G. [Institute for Methods and Instrumentation in Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin (Germany); Föhlisch, A. [Institute for Methods and Instrumentation in Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin (Germany); Fakultät für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam (Germany)

    2014-12-15

    Highlights: • Conduction band minima (CBM) of 6H-SiC are estimated with Si 2p XAS. • Valence band maxima (VBM) of 6H-SiC are estimated with non-resonant Si 2p XES. • Temperature-dependent VBM and CBM of 6H-SiC show asymmetric band gap closing. • XAS, XES and RIXS band gap estimates are compared with the optical band gap. • XAS + XES versus optical band gap provides core-excitonic screening energies. - Abstract: The band gap of semiconductors like silicon and silicon carbide (SiC) is the key for their device properties. In this research, the band gap of 6H-SiC and its temperature dependence were analyzed with silicon 2p X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) allowing for a separate analysis of the conduction-band minimum (CBM) and valence-band maximum (VBM) components of the band gap. The temperature-dependent asymmetric band gap shrinking of 6H-SiC was determined with a valence-band slope of +2.45 × 10{sup ?4} eV/K and a conduction-band slope of ?1.334 × 10{sup ?4} eV/K. The apparent asymmetry, e.g., that two thirds of the band-gap shrinking with increasing temperature is due to the VBM evolution in 6H-SiC, is similar to the asymmetry obtained for pure silicon before. The overall band gap temperature-dependence determined with XAS and non-resonant XES is compared to temperature-dependent optical studies. The core-excitonic binding energy appearing in the Si 2p XAS is extracted as the main difference. In addition, the energy loss of the onset of the first band in RIXS yields to values similar to the optical band gap over the tested temperature range.

  6. Thermal evolution of the band edges of 6H-SiC: X-ray methods compared to the optical band gap

    International Nuclear Information System (INIS)

    Highlights: • Conduction band minima (CBM) of 6H-SiC are estimated with Si 2p XAS. • Valence band maxima (VBM) of 6H-SiC are estimated with non-resonant Si 2p XES. • Temperature-dependent VBM and CBM of 6H-SiC show asymmetric band gap closing. • XAS, XES and RIXS band gap estimates are compared with the optical band gap. • XAS + XES versus optical band gap provides core-excitonic screening energies. - Abstract: The band gap of semiconductors like silicon and silicon carbide (SiC) is the key for their device properties. In this research, the band gap of 6H-SiC and its temperature dependence were analyzed with silicon 2p X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) allowing for a separate analysis of the conduction-band minimum (CBM) and valence-band maximum (VBM) components of the band gap. The temperature-dependent asymmetric band gap shrinking of 6H-SiC was determined with a valence-band slope of +2.45 × 10?4 eV/K and a conduction-band slope of ?1.334 × 10?4 eV/K. The apparent asymmetry, e.g., that two thirds of the band-gap shrinking with increasing temperature is due to the VBM evolution in 6H-SiC, is similar to the asymmetry obtained for pure silicon before. The overall band gap temperature-dependence determined with XAS and non-resonant XES is compared to temperature-dependent optical studies. The core-excitonic binding energy appearing in the Si 2p XAS is extracted as the main difference. In addition, the energy loss of the onset of the first band in RIXS yields to values similar to the optical band gap over the tested temperature range

  7. 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

  8. Mobile interstitial model and mobile electron model of mechano-induced luminescence in coloured alkali halide crystals

    International Nuclear Information System (INIS)

    A theoretical study is made on the mobile interstitial and mobile electron models of mechano-induced luminescence in coloured alkali halide crystals. Equations derived indicate that the mechanoluminescence intensity should depend on several factors like strain rate, applied stress, temperature, density of F-centres and volume of crystal. The equations also involve the efficiency and decay time of mechanoluminescence. Results of mobile interstitial and mobile electron models are compared with the experimental observations, which indicated that the latter is more suitable as compared to the former. From the temperature dependence of ML, the energy gaps between the dislocation band and ground state of F-centre is calculated which are 0.08, 0.072 and 0.09 eV for KCl, KBr and NaCl crystals, respectively. The theory predicts that the decay of ML intensity is related to the process of stress relaxation in crystals. (author). 33 refs., 5 figs., 1 tab

  9. Revealing the role of organic cations in hybrid halide perovskite CH3NH3PbI3

    Science.gov (United States)

    Motta, Carlo; El-Mellouhi, Fedwa; Kais, Sabre; Tabet, Nouar; Alharbi, Fahhad; Sanvito, Stefano

    2015-04-01

    The hybrid halide perovskite CH3NH3PbI3 has enabled solar cells to reach an efficiency of about 20%, demonstrating a pace for improvements with no precedents in the solar energy arena. Despite such explosive progress, the microscopic origin behind the success of such material is still debated, with the role played by the organic cations in the light-harvesting process remaining unclear. Here van der Waals-corrected density functional theory calculations reveal that the orientation of the organic molecules plays a fundamental role in determining the material electronic properties. For instance, if CH3NH3 orients along a (011)-like direction, the PbI6 octahedral cage will distort and the bandgap will become indirect. Our results suggest that molecular rotations, with the consequent dynamical change of the band structure, might be at the origin of the slow carrier recombination and the superior conversion efficiency of CH3NH3PbI3.

  10. 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.

  11. Evaluation of field test equipment for halide and DOP testing

    International Nuclear Information System (INIS)

    The Nucon Testing Services Department, field testing at power reactor sites, has performed tests using R-11, R-12, and R-112 in conjunction with gas chromatographs and direct reading halide detectors. The field operational experience with these detector systems, thus sensitivity, precision, and manner of field calibration, are presented. Laboratory experiments regarding 3H-tagged methyl iodide for in place leak testing of adsorber systems indicate a low hazard, high reliability process for leak testing in facilities where atmospheric cross contamination occurs. (U.S.)

  12. Inelastic low-energy electron collisions with hydrogen halides

    Science.gov (United States)

    Horá?ek, J.

    2000-02-01

    Inelastic low-energy electron collisions with hydrogen halides HCl, HBr, DBr and HI are studied theoretically on the basis of the nonlocal resonance model of Domcke and Mündel. The model takes account of the dependence of the dipole-modified threshold exponent on the internuclear distance and of the precise form of the long-range part of the negative ion potential. Cross sections for vibrational excitation, dissociative attachment and associative detachment have been calculated. For all three collision processes, the cross sections calculated are in better agreement with experiment than previous calculations.

  13. The Oxidation State of Europium in Halide Glasses

    OpenAIRE

    Weber, J. K. R.; Vu, M.; Paßlick, C.; Schweizer, S.; Brown , D E; C. E. Johnson; Johnson, J. A.

    2011-01-01

    The luminescent properties of divalent europium ions can be exploited to produce storage phosphors for x-ray imaging applications. The relatively high cost and limited availability of divalent europium halides makes it desirable to synthesize them from the readily available trivalent salts. In this work, samples of pure EuCl3 and fluoride glass melts doped with EuCl3 were processed at 700-800 °C in an inert atmosphere furnace. The Eu oxidation state in the resulting materials was determined u...

  14. Effect of chromone-substituted benzothiazolium halides on photosynthetic processes

    International Nuclear Information System (INIS)

    The effects of 3-R2-2[2-(6-R1-chromone-3-yl)ethenyl]benzothiazolium halides (CBH) on photosynthetic electron transport in spinach chloroplasts and in the legal suspension of Chlorella vulgaris were investigated. Using EPR spectroscopy it was confirmed that these compounds containing in their molecules two heterocyclic skeletons, namely benzothiazole and chromone, interact with the intermediate D+, corresponding to the tyrosine radical TyrD situated in D2 protein on the donor side of photosystem 2. Consequently, higher concentrations of CBH inhibited oxygen evolution rate in Chlorella vulgaris and the inhibitory effectiveness depended on the lipophilicity of the of the compound. (authors)

  15. Thermal conductivity of halide solid solutions: Measurement and prediction

    Science.gov (United States)

    Gheribi, Aďmen E.; Poncsák, Sándor; St-Pierre, Rémi; Kiss, László I.; Chartrand, Patrice

    2014-09-01

    The composition dependence of the lattice thermal conductivity in NaCl-KCl solid solutions has been measured as a function of composition and temperature. Samples with systematically varied compositions were prepared and the laser flash technique was used to determine the thermal diffusivity from 373 K to 823 K. A theoretical model, based on the Debye approximation of phonon density of state (which contains no adjustable parameters) was used to predict the thermal conductivity of both stoichiometric compounds and fully disordered solid solutions. The predictions obtained with the model agree very well with our measurement. A general method for predicting the thermal conductivity of different halide systems is discussed.

  16. 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 Förster-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.

  17. Theory of exciton transfer and diffusion in conjugated polymers

    International Nuclear Information System (INIS)

    We describe a theory of Förster-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

  18. 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

  19. 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.

  20. 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

  1. 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.

  2. Electronic and Optical Properties of Luminescent Centers in Halides and Oxides

    Science.gov (United States)

    Du, Mao-Hua

    2014-03-01

    Luminescent materials, such as phosphors and scintillators, are widely used for fluorescent lighting, laser, medical imaging, nuclear material detection, etc. . The luminescence is usually activated by impurities (or activators), which act as luminescence centers. The activators are typically multi-valent ions that insert multiple electronic states in the band gap of the host material. In this talk, first-principles calculations of electronic structure and optical transitions are shown for a wide range of activators, including rare-earth ions (e.g., Ce3+, Eu2+) , ns2 ions (the ions that have outer electronic configurations of ns2, such as Tl+, Pb2+, Bi3+) , and transition-metal ions (e.g., Mn4+) , in a large number of halides and oxides. The results reveal how the activator-ligand hybridization affects the emission energy and the luminescence mechanism. New phosphors and scintillators are proposed based on the chemical trends emerging from the calculations of a large number of materials.

  3. Ab initio investigation of hybrid organic-inorganic perovskites based on tin halides

    Science.gov (United States)

    Borriello, Ivo; Cantele, Giovanni; Ninno, Domenico

    2008-06-01

    The structural and electronic properties of both inorganic and hybrid organic-inorganic perovskites based on tin halides are investigated from the first principles. In particular, we contrast the inorganic CsSnCl3 and CsSnI3 to their hybrid counterparts (CH3NH3)SnCl3 , (CH3NH3)SnI3 , and (NH2CH=NH2)SnI3 , which were obtained by substituting the inorganic Cs cation with the methylammonium CH3NH3 and the formamidinium NH2CH=NH2 cations. The comparison between the hybrid perovskites and the inorganic counterparts sheds light on the effects of the filling molecule on the structural and electronic properties of the compound. We show that the stability against the distortion of the perovskitic cage strongly depends on the embedded cation. The electronic properties (in particular, the band gap) can be tuned by a suitable choice of the organic molecule, and, in particular, of its size.

  4. Nanoscale Charge Transport in Excitonic Solar Cells

    Energy Technology Data Exchange (ETDEWEB)

    Venkat Bommisetty, South Dakota State University

    2011-06-23

    Excitonic solar cells, including all-organic, hybrid organic-inorganic and dye-sensitized solar cells (DSSCs), offer strong potential for inexpensive and large-area solar energy conversion. Unlike traditional inorganic semiconductor solar cells, where all the charge generation and collection processes are well understood, these excitonic solar cells contain extremely disordered structures with complex interfaces which results in large variations in nanoscale electronic properties and has a strong influence on carrier generation, transport, dissociation and collection. Detailed understanding of these processes is important for fabrication of highly efficient solar cells. Efforts to improve efficiency are underway at a large number of research groups throughout the world focused on inorganic and organic semiconductors, photonics, photophysics, charge transport, nanoscience, ultrafast spectroscopy, photonics, semiconductor processing, device physics, device structures, interface structure etc. Rapid progress in this multidisciplinary area requires strong synergetic efforts among researchers from diverse backgrounds. Such effort can lead to novel methods for development of new materials with improved photon harvesting and interfacial treatments for improved carrier transport, process optimization to yield ordered nanoscale morphologies with well defined electronic structures.

  5. 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.

  6. Exciton-Polariton Bose-Einstein Condensates

    Science.gov (United States)

    Deveaud, Benoît

    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.

  7. 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.)

  8. Excitonic emission of hybrid nanosystem ''spherical semiconductor quantum dot + spherical metal nanoparticle''

    International Nuclear Information System (INIS)

    The hybrid nanosystem composed of a spherical metal nanoparticle (NP) and a spherical semiconductor quantum dot (QD) of a direct-band semiconductor with a cubic lattice structure and a fourfold degenerate valence band G8 has been studied. The excitonic emission of the system is considered as a sum of contributions from point dipoles located at the QD lattice sites. The description of the QD + NP nanosystem, nonspherical as a whole, is based on using three spherical coordinate systems and finding the relations between the coefficients of multipole expansions of electromagnetic (EM) fields in those systems. The origins of two of them are fixed at the centers of NP and QD, and their polar axes are directed along the line connecting the centers. The orientation of the third coordinate system with the origin in the QD is determined by the orientation of the QD crystal lattice. It is shown that, unlike the electric scalar potential, which is induced by the exciton state in the QD and looks like a point-dipole potential, the EM field of the QD excitonic emission cannot be represented as that of a point dipole emission, because it contains only dipole, quadrupole, and octupole components. The multiple scattering, between the NP and the QD, of the EM field emitted by the QD is taken into account. The dependences of the excitonic emission efficiency on the separation distance between the QD and the NP surfaces are calculated in a particular case of the CdTe QD and a silver or gold NP for various QD and NP sizes

  9. 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 ...

  10. Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers.

    Science.gov (United States)

    Andernach, Rolf; Utzat, Hendrik; Dimitrov, Stoichko D; McCulloch, Iain; Heeney, Martin; Durrant, James R; Bronstein, Hugo

    2015-08-19

    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 time scales and investigated the mechanism of triplet exciton formation. During sensitization, singlet 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 found that 60% of the complex triplet excitons were 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 upconversion layers. PMID:26200595

  11. 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.

  12. Band Together!

    Science.gov (United States)

    Olson, Cathy Applefeld

    2011-01-01

    After nearly a decade as band director at St. James High School in St. James, Missouri, Derek Limback knows that the key to building a successful program is putting the program itself above everything else. Limback strives to augment not only his students' musical prowess, but also their leadership skills. Key to his philosophy is instilling a…

  13. Dislocation unpinning model of acoustic emission from alkali halide crystals

    Indian Academy of Sciences (India)

    B P Chandra; Anubha S Gour; Vivek K Chandra; Yuvraj Patil

    2004-06-01

    The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constant $\\tau_{\\text{s}}$ for surface annihilation of dislocations and the pinning time $\\tau_{\\text{p}}$ of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.

  14. Epitaxial growth of alkali halides on stepped metal surfaces

    International Nuclear Information System (INIS)

    We present a growth study of the alkali halides NaCl and KCl on the stepped metal surfaces Cu(311) and Cu(221) using high resolution low energy electron diffraction. For all systems studied it was found that the alkali halide deposit forms (100)-terminated epitaxial layers which have essential structural features in common: The interfacial arrangement between the ionic adlayer and the stepped metal substrate is characterized by an alignment of the polar in-plane Cl ion rows parallel and perpendicular to the intrinsic Cu steps. For low coverages (?1 ML) the Cl ions are in registry with the intrinsic substrate steps causing uniaxial strain in the direction perpendicular to the steps. In contrast, parallel to the steps the Cl ions are free to adjust their optimum Cl-Cl spacing. For coverages >3 ML, the strain perpendicular to the intrinsic Cu steps is accommodated which suggests the incorporation of monoatomic Cu defect steps at the interface in this regime of higher coverage

  15. 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.

  16. Evidence of Excitonic Optical Tamm States using Molecular Materials

    CERN Document Server

    Núńez-Sánchez, 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.

  17. 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.

  18. 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

  19. 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 Universität München, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (Germany); Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München (Germany); Schinner, G. J.; Schubert, E. [Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (Germany); Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister-Scholl-Platz 1, 80539 München (Germany); Rai, A. K.; Wieck, A. D. [Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum (Germany); Reuter, D. [Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum (Germany); Department Physik, Universität Paderborn, 33098 Paderborn (Germany); Wurstbauer, U.; Holleitner, A. W. [Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universität München, D-85748 Garching (Germany); Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München (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.

  20. Ordered Dissipative Structures in Exciton Systems in Semiconductor Quantum Wells

    Directory of Open Access Journals (Sweden)

    Andrey A. Chernyuk

    2006-02-01

    Full Text Available A phenomenological theory of exciton condensation in conditions of inhomogeneous excitation is proposed. The theory is applied to the study of the development of an exciton luminescence ring and the ring fragmentation at macroscopical distances from the central excitation spot in coupled quantum wells. The transition between the fragmented and the continuous ring is considered. With assumption of a defect in the structure, a possibility of a localized island of the condensed phase in a fixed position is shown. Exciton density distribution is also analyzed in the case of two spatially separated spots of the laser excitation.

  1. 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.

  2. The origin of a 650 nm photoluminescence band in rubrene.

    Science.gov (United States)

    Chen, Yuanzhen; Lee, Bumsu; Fu, Danni; Podzorov, Vitaly

    2011-12-01

    Commonly observed variations in photoluminescence (PL) spectra of crystalline organic semiconductors, including the appearance or enhancement of certain PL bands, are shown to originate from a small amount of structural disorder (e.g., amorphous inclusions embedded in a crystal), rather than be necessarily related to chemical impurities or material oxidation. For instance, in rubrene, a minute amount of such disorder can lead to the appearance of a dominant PL band at 650 nm as a result of triplet excitons captured and fused at these sites, with a subsequent emission from the amorphous phase. PMID:22002212

  3. Interface Energetics in Organo-Metallic Halide Perovskite-based Photovoltaic Cells

    Science.gov (United States)

    Schulz, Philip

    2015-03-01

    In my presentation I will talk about the most recent findings on the electronic structure of methylammonium lead tri-halide (MAPbX3, X =I, Br) perovskite films and their interfaces to adjacent transport layers. Intricate knowledge of the electronic alignment at the contact interfaces in perovskite solar cells is essential for the understanding of the working principle as well as improving design and thus performance of respective devices. In our studies we employ ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS, IPES) to directly determine valence and conduction band offsets. In this way we are able to report a direct measurement of the electronic band gap as well as ionization energy and electron affinity found for perovskite surfaces. Furthermore, our findings indicate that the electronic energy level alignment of adjacent organic hole transport layers, such as spiro-MeOTAD, can limit the maximum attainable open circuit voltage (Voc) in solar cells if the highest occupied molecular orbital of the hole transport material is not well aligned to the valence band maximum of the perovskite layer. Using better suited hole transporters, like CBP, values for Voc larger than 1.5 V could be achieved in the case of MAPbBr3 based devices. More recently, inverted perovskite solar cells based on nickel oxide bottom anodes have been reported to yield viable power conversion efficiencies and stability. We find that the interface between the p-doped NiO surface and the MAPbI3 layer on top lead to p-type perovskite filsm while the same material deposited on TiO2 in the conventional cell geometry turns out to be n-type. A further investigation of a C60 layer deposited on top of p-type perovskite films reveals an ideal alignment between the lowest unoccupied molecular orbital of the organic electron transport materials and the conduction band minimum of the perovskite film underneath. These results explain why the inverted solar cell structure could achieve similar successes as the conventional structure and highlight the versatility of perovskite sub-cells in potential tandem cell architectures.

  4. Energetics of the ruthenium-halide bond in olefin metathesis (pre)catalysts.

    Science.gov (United States)

    Falivene, Laura; Poater, Albert; Cazin, Catherine S J; Slugovc, Christian; Cavallo, Luigi

    2013-05-28

    A DFT analysis of the strength of the Ru-halide bond in a series of typical olefin metathesis (pre)catalysts is presented. The calculated Ru-halide bond energies span the rather broad window of 25-43 kcal mol(-1). This indicates that in many systems dissociation of the Ru-halide bond is possible and is actually competitive with dissociation of the labile ligand generating the 14e active species. Consequently, formation of cationic Ru species in solution should be considered as a possible event. PMID:23235534

  5. Energetics of the ruthenium-halide bond in olefin metathesis (pre)catalysts

    OpenAIRE

    Falivene, Laura; Poater Teixidor, Albert; Cazin, Catherine S. J.; Slugovc, Christian; Cavallo, Luigi

    2013-01-01

    A DFT analysis of the strength of the Ru-halide bond in a series of typical olefin metathesis (pre)catalysts is presented. The calculated Ru-halide bond energies span the rather broad window of 25-43 kcal mol(-1). This indicates that in many systems dissociation of the Ru-halide bond is possible and is actually competitive with dissociation of the labile ligand generating the 14e active species. Consequently, formation of cationic Ru species in solution should be considered as a possible event

  6. Energetics of the ruthenium-halide bond in olefin metathesis (pre)catalysts

    KAUST Repository

    Falivene, Laura

    2013-01-01

    A DFT analysis of the strength of the Ru-halide bond in a series of typical olefin metathesis (pre)catalysts is presented. The calculated Ru-halide bond energies span the rather broad window of 25-43 kcal mol-1. This indicates that in many systems dissociation of the Ru-halide bond is possible and is actually competitive with dissociation of the labile ligand generating the 14e active species. Consequently, formation of cationic Ru species in solution should be considered as a possible event. © 2013 The Royal Society of Chemistry.

  7. Photophysical behavior and fluorescence quenching by halides of quinidine dication: Steady state and time resolved study

    Energy Technology Data Exchange (ETDEWEB)

    Joshi, Neeraj Kumar; Tewari, Neeraj; Arora, Priyanka; Rautela, Ranjana; Pant, Sanjay [Photophysics Laboratory, Department of Physics, DSB Campus, Kumaun University, Nainital 263002, Uttarakhand (India); Joshi, Hem Chandra, E-mail: hem_sup@yahoo.co.uk [Institute for Plasma Research, Laser Diagnostics Division, Bhat, Near Indira Bridge, Gandhinagar 382428, Gujarat (India)

    2015-02-15

    The fluorescence quenching of quinidine in acidified aqueous solution by various halides (Cl{sup ?}, Br{sup ?} and I{sup ?}) was studied using steady state and time resolved fluorescence techniques. The quenching process was characterized by Stern–Volmer (S–V) plots. Possibility of conformers (one is not quenched by halide and the other is quenched) is invoked to explain the observed results. - Highlights: • Fluorescence quenching of quinidine in acidified aqueous solution by halides. • Various quenching parameters have been estimated. • Possibility of conformers is invoked to explain the observed results.

  8. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    CERN Document Server

    Kenny, Elise P

    2015-01-01

    Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. We also caution ...

  9. Synthesis of zinc oxide microrods and nano-fibers with dominant exciton emission at room temperature

    International Nuclear Information System (INIS)

    Employing a simple chemical synthesis method, hexagonal-shaped zinc oxide microrods and zinc oxide nano-fibers were deposited on pyrex-glass and aluminum substrates, respectively. Both kinds of deposits showed zincite crystalline phase with lattice parameters: a=3.2498 A and c=5.2066 A. Microrods showed very uniform wide and large sizes of around 1 and 10 ?m, respectively. Both deposits were homogeneous over all substrate surfaces. Microrods and nano-fibers resulted with good optical quality and with preferential crystalline growth in [1 0 1 0] and [0 0 0 1] directions. The principal optical characteristics for both microrods and nano-fibers were: a) room-temperature photo and cathodo-luminescent spectra with strong exciton emission centered around 390 nm and with FWHMs around 125 and 160 meV, respectively, b) poor photo and cathode-luminescent emissions in the visible region of the electromagnetic spectrum, c) energy band gap of 3.32 eV, d) good emission efficiency supported by the not-required high energy densities to obtain strong exciton emission and e) good ZnO stoichiometry endorsed by photoluminescent results. These characteristics make of these microrods and nano-fibers good for potential photonic applications. - Research highlights: ? Microrods and nano-fibers resulted with good optical quality and with preferential crystalline growth in [1 0 1 0] and [0 0 0 1] directions. ? Microrods and nano-fibers resulted with good emission efficiency supported by the not-required high energy densities to obtain strong exciton emission. ? The wet chemical method is appropriated for deposition of microrods and nano-fibers with the desired optical properties for its possible application in photonics.

  10. How bilayer excitons can greatly enhance thermoelectric efficiency

    Science.gov (United States)

    Wu, Kai; Rademaker, Louk; Zaanen, Jan

    2015-03-01

    Presently, a major nanotechnological challenge is to design thermoelectric devices that have a high figure of merit. To that end, we propose to use bilayer excitons in two-dimensional nanostructures. 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. We suggest an experimental realization of a bilayer exciton thermocouple. Based on current experimental parameters, a bilayer exciton heterostructures of p- and n-doped Bi2Te3 can enhance the figure of merit an order of magnitude compared to bulk Bi2Te3. Another material suggestion is to make a bilayer out of electron-doped SrTiO3 and hole-doped Ca3Co4O9.

  11. Excitonic condensation in systems of strongly correlated electrons

    Science.gov (United States)

    Kuneš, Jan

    2015-08-01

    The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a ‘dictionary’ that would allow the reader to efficiently combine results obtained in these different fields.

  12. Organic-Inorganic Composites of Semiconductor Nanocrystals for Efficient Excitonics.

    Science.gov (United States)

    Guzelturk, Burak; Demir, Hilmi Volkan

    2015-06-18

    Nanocomposites of colloidal semiconductor nanocrystals integrated into conjugated polymers are the key to soft-material hybrid optoelectronics, combining advantages of both plastics and particles. Synergic combination of the favorable properties in the hybrids of colloidal nanocrystals and conjugated polymers offers enhanced performance and new functionalities in light-generation and light-harvesting applications, where controlling and mastering the excitonic interactions at the nanoscale are essential. In this Perspective, we highlight and critically consider the excitonic interactions in the organic-inorganic nanocomposites to achieve highly efficient exciton transfer through rational design of the nanocomposites. The use of strong excitonic interactions in optoelectronic devices can trigger efficiency breakthroughs in hybrid optoelectronics. PMID:26266593

  13. Singlet exciton yield in conjugated polymers: Influence of the Pauli exclusion principle

    CERN Document Server

    Thilagam, A

    2014-01-01

    Optical devices fabricated using conjugated polymer systems give rise to singlet exciton yields which are high compared to the statistically predicted estimate of 25% obtained using simple recombination schemes. In this study we evaluate the singlet exciton yield in conjugated polymers systems by fitting to a model that incorporates the Pauli exclusion principle. The rate equations which describe the exciton dynamics include quantum informative components (both density and spin-dependent) which appear during the spin-allowed conversion of composite intra-molecular excitons into loosely bound charge-transfer (CT) electron-hole pairs. We incorporate a crucial mechanism by which singlet excitons are increased at the expense of triplet excitons. Triplet excitons, if present at high densities, are rerouted via the Pauli exclusion principle to form loosely bound CT states which subsequently convert to singlet excitons. Our derived expression for the yield in singlet exciton incorporates the purity measure, and prov...

  14. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy

    Science.gov (United States)

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-10-01

    Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200?fs time resolution and 50?nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

  15. Optical properties of localized excitons in semiconductor nanostructures

    DEFF Research Database (Denmark)

    Leosson, Kristjan; Hvam, JŘrn Märcher

    2002-01-01

    Denne afhandling beskriver optiske undersřgelser af lokaliserede excitoner i III-V halvleder nanostrukturer. Det drejer sig isćr om tredimensional lokalisering af excitoner i to typer af selvorganiserede systemer, nemlig kvantebrřnde med fluktuerende lagtykkelse og sĺkaldte selv-dannede kvantepunkter. Optiske spektre af GaAs/AlAs kvantebrřnde med grćnseflader, der er ru pĺ forskellige lćngdeskalaer, er blevet undersřgt for indvirkningen af exciton lokalisering i planen. Exciton lokalisering er ogsĺ blevet studeret med stor rumlig oplřsning ved hjćlp af mikro-fotoluminescens. Rumligt lokaliserede og spektralt smalle emissionslinier er blevet observeret, hvilket antyder, at de stammer kvantepunkt-lignende exciton tilstande. Mĺlinger af de spektrale liniebredder i fotoluminescens fra individuelle tilstande er blevet suppleret af speckle analyse af den sekundćre emission fra de lokaliserede excitoner for at samle information om exciton overgangenes homogene liniebredde. Excitoner, der er indesluttet i selv-dannede kvantepunkter er ogsĺ blevet studeret. Mikro-fotoluminescens spektroskopi pĺ smĺ ensembler af kvantepunkter afslřrer en fordeling af fotoluminescens liniebredder, som er tilskrevet spektrale fluktuationer af de enkelte linier pĺ grund af ladningsuktuationer i kvantepunkternes omegn. Den homogene liniebredde af selv-dannede kvantepunkter er ogsĺ mĺlt ved hjćlp af rebřlge-blanding, hvilket afslřrer, at den homogene liniebredder kun er en brřkdel af den observerede fotolu minescens liniebredde ved lav temperatur. Ved stigende temperatur vokser den homogene liniebredde imidlertid hastigt, og en analyse af temperaturafhćngigheden antyder, at exciton LO-fonon vekselvirkning i kvantepunkterne er den dominerende spredningsmekanisme ved hřjere temperaturer i den type kvantepunkter, der her er studeret. Det eksperimentelle apparatur til mĺling af fotoluminescens ved lav temperatur med hřj rumlig og spektral oplřsning er blevet konstrueret som en del af projektet, ligesom de nřdvendige proces teknikker for at passere diffraktionsgrćnsen for den rumlige oplřsning i den optiske detektion er blevet udviklet. Denne afhandling demonstrerer at mikro-fotoluminescens med stor rumlig oplřsning er et nyttigt vćrktřj til optisk karakterisering af lokaliserede excitoner i halvleder nanostrukturer. Hřj spektral oplřsning er imidlertid i almindelighed ikke nok til at bestemme den naturlige liniebredde af skarpe emissionslinier fra lokaliserede excitoner. Dette skyldes at energiniveauerne for kvantepunkter er meget fřlsomme overfor variationer i de lokale omgivelser.

  16. Evaluation of defects in cuprous oxide through exciton luminescence imaging

    OpenAIRE

    Frazer, Laszlo; Lenferink, Erik J.; Kelvin B. Chang; Poeppelmeier, Kenneth R.; Stern, Nathaniel P.; Ketterson, John B.

    2014-01-01

    The various decay mechanisms of excitons in cuprous oxide (Cu2O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton luminescence from cuprous oxide crystals grown via the floating zone method showing the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation ...

  17. Coherent transport and manipulation of spins in indirect exciton nanostructures

    OpenAIRE

    Violante, Adriano; Hey, Rudolf; Santos, Paulo Ventura

    2014-01-01

    We report on the coherent control and transport of indirect exciton (IX) spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was investigated by optically generating spins using a focused, circularly polarized light spot and by probing their spatial distribution using spatially and polarization resolved photoluminescence spectroscopy. Optically injected exciton spins precess while diffusing over distances exceeding 20 {\\mu}m from the excitation spot wit...

  18. Nonlinear optical response and exciton dephasing in quantum dots

    OpenAIRE

    Muljarov, E. A.; Zimmermann, R(Physikalisches Institut, University of Bonn, Bonn, Germany)

    2006-01-01

    The full time-dependent four-wave mixing polarization in quantum dots is microscopically calculated, taking into account acoustic phonon-assisted transitions between different exciton states of the dot. It is shown that quite different dephasing times of higher exciton states in pancake anisotropic InGaAs quantum dots are responsible for the experimentally observed [1] double-exponential decay in the photon echo signal.

  19. Excitonic fine structure splitting in type-II quantum dots

    OpenAIRE

    K?ápek, Vlastimil; Klenovský, Petr; Šikola, Tomáš

    2015-01-01

    Excitonic fine structure splitting in quantum dots is closely related to the lateral shape of the wave functions. We have studied theoretically the fine structure splitting in InAs quantum dots with a type-II confinement imposed by a GaAsSb capping layer. We show that very small values of the fine structure splitting comparable with the natural linewidth of the excitonic transitions are achievable for realistic quantum dot morphologies despite the structural elongation and t...

  20. Quantum and nanoscale modelling of exciton dynamics in polymeric systems

    OpenAIRE

    Barbosa, Helder M. C.; Correia, Helena M. G.; Ramos, Marta M. D.

    2010-01-01

    One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its orphology at nanoscale. To get a deep unde...

  1. Long-range exciton dissociation in organic solar cells

    OpenAIRE

    Caruso D; Troisi A

    2012-01-01

    It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range ...

  2. Exciton spectrum in multi-shell hexagonal semiconductor nanotube

    OpenAIRE

    O.M. Makhanets; Gutsul, V. I.; Tsiupak, N. R.; O.M. Voitsekhivska

    2012-01-01

    The theory of exciton spectrum in multi-shell hexagonal semiconductor nanotube is developed within the effective masses and rectangular potentials approximations using the method of effective potential. It is shown that the exciton binding energy for all states non-monotonously depends on the inner wire diameter, approaching several minimal and maximal magnitudes. The obtained theoretical results explain well the experimental positions of luminescence peaks for GaAs/Al0.4Ga0.6As nanotubes.

  3. Quantum condensation from a tailored exciton population in a microcavity

    OpenAIRE

    EASTHAM, PAUL

    2009-01-01

    An experiment is proposed on the coherent quantum dynamics of a semiconductor microcavity containing quantum dots. Modeling the experiment using a generalized Dicke model, we show that a tailored excitation pulse can create an energy-dependent population of excitons, which subsequently evolves to a quantum condensate of excitons and photons. The population is created by a generalization of adiabatic rapid passage and then condenses due to a dynamical analog of the BCS instability.

  4. Strong-field terahertz-optical mixing in excitons

    OpenAIRE

    Su, M. Y.; CARTER, S.; Sherwin, M. S.; Huntington, A.; Coldren, L. A.

    2002-01-01

    Driving a double-quantum-well excitonic intersubband resonance with a terahertz (THz) electric field of frequency \\omega_{THz} generated terahertz optical sidebands \\omega=\\omega_{THz}+\\omega_{NIR} on a weak NIR probe. At high THz intensities, the intersubband dipole energy which coupled two excitons was comparable to the THz photon energy. In this strong-field regime the sideband intensity displayed a non-monotonic dependence on the THz field strength. The oscillating refra...

  5. Exciton spectrum in multi-shell hexagonal semiconductor nanotube

    Directory of Open Access Journals (Sweden)

    O.M. Makhanets

    2012-10-01

    Full Text Available The theory of exciton spectrum in multi-shell hexagonal semiconductor nanotube is developed within the effective masses and rectangular potentials approximations using the method of effective potential. It is shown that the exciton binding energy for all states non-monotonously depends on the inner wire diameter, approaching several minimal and maximal magnitudes. The obtained theoretical results explain well the experimental positions of luminescence peaks for GaAs/Al0.4Ga0.6As nanotubes.

  6. Magnetic-Field-Induced Exciton Tunneling in Shallow Quantum Wells

    CERN Document Server

    Getter, A

    1999-01-01

    We study the effect of the magnetic field orientation on the electroabsorption spectra of excitons confined in extremely shallow quantum wells. When the applied electric field is parallel to the quantum well plane, we demonstrate that, for in-plane magnetic field orientation, the discrete confined exciton peak undergoes a transition into a continuum resonance. In contrast, for perpendicular magnetic fields, the exciton peak exhibits the usual Stark red-shift. We show that such a dramatic dependence on the magnetic field orientation originates from a resonant coupling between the confined and the bulk-like excitons. Such coupling is caused by the interplay between the quantum-well potential and a velocity-dependent two-body interaction between the exciton center-of-mass and relative motion degrees of freedom induced by the in-plane magnetic field. As a result, the exciton tunnels out of the quantum well as a whole without being ionized. We discuss possible experimental applications of this effect.

  7. Gas phase chromatography of halides of elements 104 and 105

    International Nuclear Information System (INIS)

    On-line isothermal gas phase chromatography was used to study halides of 261104 (T1/2 = 65 s) and 262,263105 (T1/2 = 34 s and 27 s) produced an atom-at-a time via the reactions 248Cm(18O, 5n) and 249Bk(18O, 5n, 4n), respectively. Using HBr and HCl gas as halogenating agents, we were able to produce volatile bromides and chlorides of the above mentioned elements and study their behavior compared to their lighter homologs in Groups 4 or 5 of the periodic table. Element 104 formed more volatile bromides than its homolog Hf. In contrast, element 105 bromides were found to be less volatile than the bromides of the group 5 elements Nb and Ta. Both 104 and Hf chlorides were observed to be more volatile than their respective bromides. 31 refs., 8 figs

  8. Silver Halide Sensitized Gelatin Derived from BB-640 Holographic Emulsion

    Science.gov (United States)

    Neipp, Cristian; Pascual, Inmaculada; Beléndez, Augusto

    1999-03-01

    Silver halide sensitized gelatin (SHSG) is one of the most interesting techniques for the production of holographic optical elements, achieving relatively high sensitivity of photographic material with a low scattering of dichromated gelatin. Here we present experimental results for SHSG derived from the novel BB-640, a red-sensitive ultra-fine-grain emulsion from Holographic Recording Technologies (Steinau, Germany). The material is characterized before recording and after processing, and information about the thickness, absorption, and refractive-index modulation of the final holograms is obtained. The influence of the developer is analyzed, and diffraction efficiencies as great as 96.2% (after allowing for reflections) with a transmission of 1% and absorption and scatter losses of 2.8% are obtained with AAC developer. Our investigations reveal that high-quality SHSG transmission holograms may be obtained with the new BB-640 plates.

  9. Theory of Hydrogen Migration in Organic–Inorganic Halide Perovskites**

    Science.gov (United States)

    Egger, David A; Kronik, Leeor; Rappe, Andrew M

    2015-01-01

    Solar cells based on organic–inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current–voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites—interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin–Corbett mechanism. Our analysis highlights the structural flexibility of organic–inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells. PMID:26073061

  10. Two-photon pumped lead halide perovskite nanowire lasers

    CERN Document Server

    Gu, Zhiyuan; Sun, Wenzhao; Li, Jinakai; Liu, Shuai; Song, Qinghai; Xiao, Shumin

    2015-01-01

    Solution-processed lead halide perovskites have shown very bright future in both solar cells and microlasers. Very recently, the nonlinearity of perovskites started to attract considerable research attention. Second harmonic generation and two-photon absorption have been successfully demonstrated. However, the nonlinearity based perovskite devices such as micro- & nano- lasers are still absent. Here we demonstrate the two-photon pumped nanolasers from perovskite nanowires. The CH3NH3PbBr3 perovskite nanowires were synthesized with one-step solution self-assembly method and dispersed on glass substrate. Under the optical excitation at 800 nm, two-photon pumped lasing actions with periodic peaks have been successfully observed at around 546 nm. The obtained quality (Q) factors of two-photon pumped nanolasers are around 960, and the corresponding thresholds are about 674?J=cm2. Both the Q factors and thresholds are comparable to conventional whispering gallery modes in two-dimensional polygon microplates. Ou...

  11. Fabrication of alkali halide UV photocathodes by pulsed laser deposition

    Science.gov (United States)

    Brendel', V. M.; Bukin, V. V.; Garnov, Sergei V.; Bagdasarov, V. Kh; Denisov, N. N.; Garanin, Sergey G.; Terekhin, V. A.; Trutnev, Yurii A.

    2012-12-01

    A technique has been proposed for the fabrication of atmospheric corrosion resistant alkali halide UV photocathodes by pulsed laser deposition. We produced photocathodes with a highly homogeneous photoemissive layer well-adherent to the substrate. The photocathodes were mounted in a vacuum photodiode, and a tungsten grid was used as an anode. Using pulsed UV lasers, we carried out experiments aimed at evaluating the quantum efficiency of the photocathodes. With a dc voltage applied between the photocathode and anode grid, we measured a shunt signal proportional to the total charge emitted by the cathode exposed to UV laser light. The proposed deposition technique enables one to produce photocathodes with photoemissive layers highly uniform in quantum efficiency, which is its main advantage over thin film growth by resistive evaporation.

  12. Fabrication of alkali halide UV photocathodes by pulsed laser deposition

    International Nuclear Information System (INIS)

    A technique has been proposed for the fabrication of atmospheric corrosion resistant alkali halide UV photocathodes by pulsed laser deposition. We produced photocathodes with a highly homogeneous photoemissive layer well-adherent to the substrate. The photocathodes were mounted in a vacuum photodiode, and a tungsten grid was used as an anode. Using pulsed UV lasers, we carried out experiments aimed at evaluating the quantum efficiency of the photocathodes. With a dc voltage applied between the photocathode and anode grid, we measured a shunt signal proportional to the total charge emitted by the cathode exposed to UV laser light. The proposed deposition technique enables one to produce photocathodes with photoemissive layers highly uniform in quantum efficiency, which is its main advantage over thin film growth by resistive evaporation. (laser technologies)

  13. Irradiation-induced defects in alkali halide crystals

    International Nuclear Information System (INIS)

    Models are presented for the successive stages of aggregation and recombination of radiation-induced defects in alkali halides. The construction of the present models is assisted by the calculation of defect energies. Three main areas are discussed: firstly, initial aggregation of H centres is considered, where the formation of a weakly bound di-H centre which collapses into a neutral halogen molecule occupying an interstitial site is proposed. Secondly, the thermodynamics of formation of the interstitial loops detected in the electron microscope by Hobbs et al (Proc. R. Soc; A332; 167 (1973)) are considered. The previously proposed mechanism by which the loops are created is confirmed; this involves the displacement of lattice ions to dislocation loops by halogen molecules which occupy the vacancy pairs created. Thirdly, the mechanism of annealing of metal colloids is considered, and it is suggested that the vacancy-molecule complexes, which are created together with the dislocation loops, may play an important role. (author)

  14. Thermal conductivity of molten alkali halides and their mixtures

    International Nuclear Information System (INIS)

    Thermal conductivity of molten alkali halides and their mixtures with common anions is measured as a function of temperature. Use is made of the most reliable method of coaxial cylinders made of platinum, with radiation heat transfer being taken into account. It is found to vary directly with temperature. Expressions for its temperature dependence are derived from the experimental observations. Linear relations of the thermal conductivity to the molar volume are established for molten alkali fluorides, chlorides, bromides, iodides and their mixtures with common anions. Deviations from the additive quantities towards smaller values are observed in the case of the mixtures. They are in a good accordance with inverse deviations of their adiabatic compressibility. The heat conduction of ionic melts is semiquantitatively examined. Elementary quantities of heat transferred from cations to anions and vice versa are estimated. (author)

  15. Games people play with interstitials (in alkali halides)

    International Nuclear Information System (INIS)

    A survey is given of the various ways in which interstitial halogen atoms produced by ionising radiation can be trapped in alkali halides. First, the fundamental interstitial halogen atom center, the H-center, is discussed. Then, interstitial centers trapped by, or in the neighbourhood of, various impurities are presented. Particular attention is given to trapping by the following impurities: foreign halogen ions, foreign alkali ions or pairs of both. The discussion is limited to a description of the production and the models of these H-type centers and little is said about their sometimes interesting physical properties. A few speculations are offered why certain interstitial centers have not yet been observed. The models of a few paramagnetic diinterstitial centers are also presented

  16. Theory of Hydrogen Migration in Organic-Inorganic Halide Perovskites.

    Science.gov (United States)

    Egger, David A; Kronik, Leeor; Rappe, Andrew M

    2015-10-12

    Solar cells based on organic-inorganic halide perovskites have recently been proven to be remarkably efficient. However, they exhibit hysteresis in their current-voltage curves, and their stability in the presence of water is problematic. Both issues are possibly related to a diffusion of defects in the perovskite material. By using first-principles calculations based on density functional theory, we study the properties of an important defect in hybrid perovskites-interstitial hydrogen. We show that differently charged defects occupy different crystal sites, which may allow for ionization-enhanced defect migration following the Bourgoin-Corbett mechanism. Our analysis highlights the structural flexibility of organic-inorganic perovskites: successive iodide displacements, combined with hydrogen bonding, enable proton diffusion with low migration barriers. These findings indicate that hydrogen defects can be mobile and thus highly relevant for the performance of perovskite solar cells. PMID:26073061

  17. Electric field effects on alanine tripeptide in sodium halide solutions.

    Science.gov (United States)

    Astrakas, Loukas G; Gousias, Christos; Tzaphlidou, Margaret

    2015-12-01

    The electric field effects on conformational properties of trialanine in different halide solutions were explored with long-scale molecular dynamics simulations. NaF, NaCl, NaBr and NaI solutions of low (0.2?M) and high (2?M) concentrations were exposed to a constant electric field of 1000?V/m. Generally, the electric field does not disturb trialanine's structure. Large structural changes appear only in the case of the supersaturated 2.0?M NaF solution containing NaF crystals. Although the electric field affects in a complex way, all the ions-water-peptide interactions, it predominantly affects the electroselectivity effect, which describes specific interactions such as the ion-pair formation. PMID:25006865

  18. Interionic interactions in alcoholic solutions of hydrogen halides

    International Nuclear Information System (INIS)

    Based on experimental data of ionization constants the effective short-range potential dij for HCl, HBr and HI in n-alcohols (methanol-ethanol) has been calculated and analyzed at 5-55 Deg C. Established regularities in changing values dij (as well as values ?ijG*=NAdij) depend on temperature, solvent, anion have been explained on the basis of the conception of Samojlov about near solvation. Changes in values ?ijG* are demonstrated to be conditioned by the correlation of contributions from ion-molecular and intermolecular non-coulomb interactions that in turn to be determined by structural state of solvent and solvation of electrolyte particles in hydrogen halide solution

  19. Material Innovation in Advancing Organometal Halide Perovskite Functionality.

    Science.gov (United States)

    Zheng, Fan; Saldana-Greco, Diomedes; Liu, Shi; Rappe, Andrew M

    2015-12-01

    Organometal halide perovskites (OMHPs) have garnered much attention recently for their unprecedented rate of increasing power conversion efficiency (PCE), positioning them as a promising basis for the next-generation photovoltaic devices. However, the gap between the rapid increasing PCE and the incomplete understanding of the structure-property-performance relationship prevents the realization of the true potential of OMHPs. This Perspective aims to provide a concise overview of the current status of OMHP research, highlighting the unique properties of OMHPs that are critical for solar applications but still not adequately explained. Stability and performance challenges of OMHP solar cells are discussed, calling upon combined experimental and theoretical efforts to address these challenges for pioneering commercialization of OMHP solar cells. Various material innovation strategies for improving the performance and stability of OMHPs are surveyed, showing that the OMHP architecture can serve as a promising and robust platform for the design and optimization of materials with desired functionalities. PMID:26631361

  20. Mechanical properties of silver halide core/clad IR fibers

    Science.gov (United States)

    Shalem, Shaul; German, Alla; Moser, Frank; Katzir, Abraham

    1996-04-01

    We have developed core/clad polycrystalline silver halide optical fibers with a loss of roughly 0.3 dB/m at 10.6 micrometers. Such fibers, with core diameters 0.3 - 0.6 mm and lengths of 1 to 2 meters are capable of continuously delivering output power densities as high as 14 KW/cm2. The fibers were repetitively bent in the plastic and elastic regimes and the optical transmission monitored during bending. The mechanical properties of the core/clad fibers and of the core only fibers are similar. It was also demonstrated that the 'bending' properties of the core/clad fibers are determined by the cladding material. Our investigations suggest that proper design of the core/clad structure may give significant improvement in mechanical properties such as more cycles to optical failure. This will be very important especially for endoscopic laser surgery and other medical applications.

  1. Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

    Energy Technology Data Exchange (ETDEWEB)

    Yang, S.; Liu, W.-R. [Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan (China); Hsu, H. C., E-mail: hsuhc@mail.ncku.edu.tw, E-mail: wfhsieh@mail.nctu.edu.tw [Department of Photonics and Advanced Optoelectronic Technology Center, National Cheng Kung University, 701 Tainan, Taiwan (China); Lin, B. H.; Hsu, C.-H. [Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan (China); Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (China); Kuo, C. C.; Hsieh, W. F., E-mail: hsuhc@mail.ncku.edu.tw, E-mail: wfhsieh@mail.nctu.edu.tw [Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (China); Eriksson, M. O.; Holtz, P. O. [Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping (Sweden)

    2014-07-07

    We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.

  2. Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination

    Directory of Open Access Journals (Sweden)

    Alves Eduardo

    2011-01-01

    Full Text Available Abstract Undoped self-assembled GaN quantum dots (QD stacked in superlattices (SL with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

  3. Photoelectron spectroscopy of alkali halide clusters with excess electrons

    International Nuclear Information System (INIS)

    We have studied excess electrons in alkali halide clusters, using a combination of a laser vaporization source, time-of-flight mass spectrometer, and magnetic bottle photoelectron electron spectrometer. We have obtained photoelectron spectra of (NaCl)n-, (NaCl)nNa-, (NaCl)nNam- and (KI)3Km- cluster anions contain either a single excess electron, two excess electrons, or several excess electrons. In the spectra of (NaCl)n, our measured electron vertical binding energies are in excellent agreement with the calculated values. We found single excess electrons in the (NaCl)n- cluster anions are mostly localized, either occupying an anion vacancy as an F-center in solids, or neutralizing a Na+ ion. In a few case, electrons are delocalized on the surface of clusters. In the spectra of (NaCl)nNam-, we observed three distinct types of spectra, corresponding to three different electron accommodation modes. We suggest that in certain clusters, the excess electrons are spin paired, localizing in a single anion vacancy as an F'-center in solids or on a Na+ cation to form a Na-. In other clusters, the electrons separately occupy a pair of anion vacancies as a double F-center. In the spectra of (NaCl)3Nam- and (KI)3Km-, we found features of the spectra that resemble those seen in the spectra of pure alkali metal anions after m ? 2. We suggest that the metallic and ionic parts in these clusters are separated, analogous to the metal and nonmetal phase separation in alkali-alkali halide metals

  4. Halide based MBE of crystalline metals and oxides

    Energy Technology Data Exchange (ETDEWEB)

    Greenlee, Jordan D.; Calley, W. Laws; Henderson, Walter; Doolittle, W. Alan [Georgia Institute of Technology, School of Electrical and Computer Engineering, Atlanta, Georgia (United States)

    2012-02-15

    A halide based growth chemistry has been demonstrated which can deliver a range of transition metals using low to moderate effusion cell temperatures (30-700 C) even for high melting point metals. Previously, growth with transition metal species required difficult to control electron beam or impurity inducing metal organic sources. Both crystalline oxide and metal films exhibiting excellent crystal quality are grown using this halide-based growth chemistry. Films are grown using a plasma assisted Molecular Beam Epitaxy (MBE) system with metal-chloride precursors. Crystalline niobium, cobalt, iron, and nickel were grown using this chemistry but the technology can be generalized to almost any metal for which a chloride precursor is available. Additionally, the oxides LiNbO{sub 3} and LiNbO{sub 2} were grown with films exhibiting X-ray diffraction (XRD) rocking curve full-widths at half maximum of 150 and 190 arcseconds respectively. LiNbO{sub 2} films demonstrate a memristive response due to the rapid movement of lithium in the layered crystal structure. The rapid movement of lithium ions in LiNbO{sub 2} memristors is characterized using impedance spectroscopy measurements. The impedance spectroscopy measurements suggest an ionic current of.1 mA for a small drive voltage of 5 mV AC or equivalently an ionic current density of {proportional_to}87 A/cm{sup 2}. This high ionic current density coupled with low charge transfer resistance of {proportional_to}16.5 {omega} and a high relaxation frequency (6.6 MHz) makes this single crystal material appealing for battery applications in addition to memristors. (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  5. Bright light-emitting diodes based on organometal halide perovskite

    Science.gov (United States)

    Tan, Zhi-Kuang; Moghaddam, Reza Saberi; Lai, May Ling; Docampo, Pablo; Higler, Ruben; Deschler, Felix; Price, Michael; Sadhanala, Aditya; Pazos, Luis M.; Credgington, Dan; Hanusch, Fabian; Bein, Thomas; Snaith, Henry J.; Friend, Richard H.

    2014-09-01

    Solid-state light-emitting devices based on direct-bandgap semiconductors have, over the past two decades, been utilized as energy-efficient sources of lighting. However, fabrication of these devices typically relies on expensive high-temperature and high-vacuum processes, rendering them uneconomical for use in large-area displays. Here, we report high-brightness light-emitting diodes based on solution-processed organometal halide perovskites. We demonstrate electroluminescence in the near-infrared, green and red by tuning the halide compositions in the perovskite. In our infrared device, a thin 15?nm layer of CH3NH3PbI3-xClx perovskite emitter is sandwiched between larger-bandgap titanium dioxide (TiO2) and poly(9,9?-dioctylfluorene) (F8) layers, effectively confining electrons and holes in the perovskite layer for radiative recombination. We report an infrared radiance of 13.2?W?sr-1?m-2 at a current density of 363?mA?cm-2, with highest external and internal quantum efficiencies of 0.76% and 3.4%, respectively. In our green light-emitting device with an ITO/PEDOT:PSS/CH3NH3PbBr3/F8/Ca/Ag structure, we achieved a luminance of 364?cd?m-2 at a current density of 123?mA?cm-2, giving external and internal quantum efficiencies of 0.1% and 0.4%, respectively. We show, using photoluminescence studies, that radiative bimolecular recombination is dominant at higher excitation densities. Hence, the quantum efficiencies of the perovskite light-emitting diodes increase at higher current densities. This demonstration of effective perovskite electroluminescence offers scope for developing this unique class of materials into efficient and colour-tunable light emitters for low-cost display, lighting and optical communication applications.

  6. Universal properties of quasi-one-dimensional excitons in semiconducting single-walled carbon nanotubes and $\\pi$-conjugated polymers

    CERN Document Server

    Zhao, H; Sheng, C X; Vardeny, Z V

    2006-01-01

    The nature of the primary photoexcitations in semiconducting single-walled carbon nanotubes (S-SWCNTs) is of strong current interest. We have studied the emission spectra of S-SWCNTs and two different $\\pi$-conjugated polymers in solutions and films, and have also performed ultrafast pump-probe spectroscopy on these systems. The emission spectra relative to the absorption bands are very similar in S-SWCNTs and polymers, with redshifted photoluminescence in films showing exciton migration. The transient photoinduced absorptions (PAs) in SWCNTs and $\\pi$-conjugated polymers are also remarkably similar, with a low energy PA$_1$ and a higher energy PA$_2$ in all cases. Theoretical calculations of excited state absorptions within a correlated $\\pi$-electron Hamiltonian find the same excitonic energy spectrum for S-SWCNTs and $\\pi$-conjugated polymers, illustrating the universal features of quasi-one-dimensional excitons in carbon-based $\\pi$-conjugated systems. In both cases PA$_1$ is an excited state absorption f...

  7. Effect of exciton polaritons of absorption edge of GaTe

    International Nuclear Information System (INIS)

    The experimental results, pointing to the dependence of spectral and integral coefficients of exciton absorption as well as to the exciton relaxation parameter ?sub(0) over the exciton zone on the sample thickness, are presented. It is tried to explain the inverse dependences of absorption intensity in the maximum of ?sub(max) and ?sub(0) exciton line within the limits of polariton theory. The values of polariton free path length in GaTe at various temperatures, as well as the volume ?sub(vol.) and surface ?sub(surf.) parameters of exciton relaxation over the exciton zone are discussed

  8. Probing charge transfer excitons in a MoSe2-WS2 van der Waals heterostructure

    Science.gov (United States)

    Ceballos, Frank; Bellus, Matthew Z.; Chiu, Hsin-Ying; Zhao, Hui

    2015-10-01

    We show that the van der Waals heterostructure formed by MoSe2 and WS2 provides a unique system with near degenerate interlayer and intralayer excitonic states. Photoluminescence measurements indicate that the charge transfer exciton states are approximately 50 meV below the MoSe2 exciton states, with a significant spectral overlap. The transient absorption of a femtosecond pulse was used to study the dynamics of the charge transfer excitons at room temperature. We found a lifetime of approximately 80 ps for the charge transfer excitons. A diffusion coefficient of about 14 cm2 s-1 was deduced, which is comparable to individual excitons in transition metal dichalcogenides.

  9. Bose–Einstein condensation of excitons in Cu2O: progress over 30 years

    International Nuclear Information System (INIS)

    Experiments on Bose–Einstein condensation (BEC) of excitons in the semiconductor Cu2O started over 30 years ago, as one of the first serious attempts at exciton BEC. Early claims were based on spectroscopic signatures and transport data which have since been reinterpreted, in large part because the Auger recombination process for excitons was not well understood. Understanding of the Auger process has advanced, and recent experiments have made significant progress toward exciton BEC. We review the history of experiments on exciton BEC in Cu2O, the Auger recombination process, and the prospects for observing exciton BEC in this system in the near future. (review article)

  10. Efficiency of energy transfer from ?-irradiated ammonium halides in aqueous iodide and nitrate solutions

    International Nuclear Information System (INIS)

    It is well known that ammonium halide (NH4X) crystals, on ?-exposure, store energy in the form of primary and secondary radiolytic products. Such crystals on dissolution in aqueous iodide and nitrate solutions result in oxidation of iodide and reduction of nitrate, respectively. The yields of iodine and nitrite are determined by chemical methods under varying conditions of the amount, dose and particle size of the irradiated ammonium halide salts. The maximum values of the efficiency of energy transfer for oxidation and reduction processes for ammonium halide salts correspond to 40% and 10%, respectively. At low doses, an empirical relation proposed between the percent efficiency of energy transfer and the absorbed dose is valid. The concentrations of inherent oxidizing and reducing species initially present are 7.0*1018 and 1.0*1018 per mol of ammonium halide, respectively. (author) 21 refs.; 7 figs.; 2 tabs

  11. Extraction of halide and thiocyanate complexes of cadmium by tributylphosphate from sulfuric acid solutions

    International Nuclear Information System (INIS)

    Extraction of the halide and thiocyanate complexes of cadmium from H2SO4-KX solutions, where X=Cl, Br, I, or SCN, by solutions of tributyl phosphate (TBP) in benzene has been examined as a function of the acidity of the medium, halide ion and TBP concentration, and also of the temperature. Optimal conditions have been established for extraction. Cadmium is extracted as CdX2.2TBP at low halide-ion concentrations (less than or equal to 0.1 g-ion I-/liter, and less than or equal to 1 g-ion Br-, Cl-, and SCN-/liter). The extraction constants and the thermodynamic extraction characteristics of the cadmium complexes have been determined. In order of their extractability by tributyl phosphate the cadmium halide complexes follow the order: iodide > bromide > chloride > thiocyanate. Extraction of cadmium as its iodide complex by tributyl phosphate enables cadmium to be separated from indium and zinc

  12. NEW THIO S2- ADDUCTS WITH ANTIMONY (III AND V) HALIDE: SYNTHESIS AND INFRARED STUDY

    OpenAIRE

    HASSAN ALLOUCH; LIBASSE DIOP

    2013-01-01

    Five new S2- adducts with SbIII and SbV halides have been synthesized and studied by infrared. Discrete structures have been suggested, the environment around the antimony being tetrahedral, trigonal bipyramidal or octahedral.

  13. NEW THIO S2- ADDUCTS WITH ANTIMONY (III AND V HALIDE: SYNTHESIS AND INFRARED STUDY

    Directory of Open Access Journals (Sweden)

    HASSAN ALLOUCH

    2013-12-01

    Full Text Available Five new S2- adducts with SbIII and SbV halides have been synthesized and studied by infrared. Discrete structures have been suggested, the environment around the antimony being tetrahedral, trigonal bipyramidal or octahedral.

  14. Enhanced efficiency in single-host white organic light-emitting diode by triplet exciton conversion

    International Nuclear Information System (INIS)

    The authors observe that the external quantum efficiency (EQE) of the Iridium (III) bis(4-phenylthieno [3,2-c]pyridinato-N,C2?)acetylacetonate (PO-01) based yellow organic light-emitting diode (OLED) is significantly increased by uniformly co-doping Iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C2?] (FIrpic) and PO-01 into the same wide band-gap host of N,N?-dicarbazolyl-3, 5-benzene (mCP). Detailed investigation indicates that the efficiency enhancement is ascribed to effective triplet exciton gathering by FIrpic, followed by energy transfer to PO-01. Compared to the control device, which has maximum EQE of 10.5%, an improved maximum EQE of 13.2% is obtained in the optimization white device based on FIrpic and PO-01 emission according to this principle. This work makes it easier for a single host white OLED to simultaneously harvest high efficiency in both blue and yellow units. Comprehensive experimental results show that this phenomenon can also be found and utilized in other popular hosts to realize more efficient white devices. -- Highlights: • This work makes easier for a single host white OLED to harvest high efficiency in both blue and yellow units. • Efficiency enhancement is ascribed to effective triplet exciton gathering by FIrpic, followed by energy transfer to PO-01. • This phenomenon can also be found and utilized in other popular hosts to realize more efficient white devices

  15. Strain induced optical properties of exciton in a CdTe/ZnTe quantum dot

    Science.gov (United States)

    Sangeetha, R.; John Peter, A.; Lee, Chang Woo

    2014-02-01

    The influence of strain on the binding energies of heavy and light hole is obtained in a ZnxCd1-xTe/ZnTe quantum dot taking into account the phonon confinement effect. The band offsets are calculated using model-solid approach. The dielectric mismatch effect is introduced between the dielectric constants of dot and the barrier. The strain induced energies of excitonic transitions in a CdZnTe quantum dot with ZnTe barrier are brought out considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been obtained using Bessel function as an orthonormal basis for different confinement potentials of barrier height, strain induced linear and third-order nonlinear optical absorption coefficients and the changes of refractive index with the incident photon energy are observed. Our results show that the exciton binding energy is enhanced with the inclusion of potential taking into account the effects of dielectric mismatch and the geometry of quantum dot with various Zinc alloy content has a great influence on the optical properties of the dot.

  16. Communication: Strong excitonic and vibronic effects determine the optical properties of Li2O2

    Science.gov (United States)

    Garcia-Lastra, J. M.; Bass, J. D.; Thygesen, K. S.

    2011-09-01

    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 ion gives rise to a narrow absorption peak around 1.2 eV below the calculated bandgap of 4.8 eV. In the excited state, the internal O2-2 bond is significantly weakened due to the population of the ?* orbital. As a consequence, the bond is elongated by almost 0.5 Ĺ leading to an extreme Stokes shift of 2.6 eV. The strong vibronic coupling entails significant broadening of the excitonic absorption peak in good agreement with diffuse reflectance data on Li2O2 which shows a rather featureless spectrum with an absorption onset around 3.0 eV. These results should be important for understanding the origin of the high potential losses and low current densities, which are presently limiting the performance of Li-air batteries.

  17. Enhanced efficiency in single-host white organic light-emitting diode by triplet exciton conversion

    Energy Technology Data Exchange (ETDEWEB)

    Wu, Qingyang, E-mail: wqy1527@163.com [State Key laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012 (China); Zhang, Shiming [State Key laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012 (China); Département of Chemical Engineering, École Polytechnique de Montréal, Montréal, Québec, Canada H3C3J7 (Canada); Yue, Shouzhen; Zhang, Zhensong [State Key laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012 (China); Xie, Guohua [Institut für Angewandte Photophysik, Technische Universtität Dresden, Dresden 01062 (Germany); Zhao, Yi; Liu, Shiyong [State Key laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012 (China)

    2013-11-15

    The authors observe that the external quantum efficiency (EQE) of the Iridium (III) bis(4-phenylthieno [3,2-c]pyridinato-N,C{sup 2?})acetylacetonate (PO-01) based yellow organic light-emitting diode (OLED) is significantly increased by uniformly co-doping Iridium (III)bis[(4,6-difluorophenyl)-pyridinato-N,C{sup 2?}] (FIrpic) and PO-01 into the same wide band-gap host of N,N{sup ?}-dicarbazolyl-3, 5-benzene (mCP). Detailed investigation indicates that the efficiency enhancement is ascribed to effective triplet exciton gathering by FIrpic, followed by energy transfer to PO-01. Compared to the control device, which has maximum EQE of 10.5%, an improved maximum EQE of 13.2% is obtained in the optimization white device based on FIrpic and PO-01 emission according to this principle. This work makes it easier for a single host white OLED to simultaneously harvest high efficiency in both blue and yellow units. Comprehensive experimental results show that this phenomenon can also be found and utilized in other popular hosts to realize more efficient white devices. -- Highlights: • This work makes easier for a single host white OLED to harvest high efficiency in both blue and yellow units. • Efficiency enhancement is ascribed to effective triplet exciton gathering by FIrpic, followed by energy transfer to PO-01. • This phenomenon can also be found and utilized in other popular hosts to realize more efficient white devices.

  18. Complete study of excitonic fine-structure splitting in GaN/AlN quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Hoenig, Gerald; Winkelnkemper, Momme; Schliwa, Andrei; Hoffmann, Axel; Bimberg, Dieter [Institut fuer Festkoerperphysik, Technische Universitaet Berlin (Germany); Kindel, Christian [Institut fuer Festkoerperphysik, Technische Universitaet Berlin (Germany); Research Center for Advanced Science and Technology, University of Tokyo (Japan); Kako, Satoshi [Institute for Nano Quantum Information Electronics, University of Tokyo (Japan); Kawano, Takeshi; Oishi, Hiroaki [Research Center for Advanced Science and Technology, University of Tokyo (Japan); Arakawa, Yasuhiko [Research Center for Advanced Science and Technology, University of Tokyo (Japan); Institute for Nano Quantum Information Electronics, University of Tokyo (Japan); Institute of Industrial Science, University of Tokyo, Komaba, Meguro, Tokyo (Japan)

    2010-07-01

    A detailed understanding of the excitonic fine structure in quantum dots (QDs) is indispensable for their use in quantum cryptography devices. While the fine structure in As-based QDs has been studied extensively, there is a lack of such investigations for N-based QDs, which might operate at room temperature. We present the first complete study of excitonic fine-structure splitting (FSS) in GaN/AlN QDs. Our experimental studies reveal a huge FSS of up to 7 meV with a strong dependence on the emission energy inverse to that in As-based QDs. Our theoretical studies, performed with a configuration-interaction method based on realistic 8-band-k.p Hartree-Fock states, confirm the experimental results and identify the origin of FSS as lattice strain induced. Based on our results it is possible to induce a strain gradient (by micro mechanic techniques or structuring methods), which will reduce the FSS to zero for the emission of entangled photon pairs.

  19. Excitonic effects in GeC hybrid: Many-body Green's function calculations

    Science.gov (United States)

    Drissi, L. B.; Ramadan, F. Z.

    2015-11-01

    Many-body effects on the electronic and optical absorption properties of a GeC sheet are studied by means of first principle many-body Green's function and Bethe-Salpeter equation formalism. The absence of soft modes in the phonon-spectrum indicates the stability of the system. The inclusion of quasiparticle corrections increases significantly the band gap. The local field effects induce significant change in the absorption spectra for the out-plane polarization rendering the GeC monolayer transparent below 7 eV. The excitonic effects are significant on the optical absorption properties. A detailed analysis of the spectrum shows a strong binding energy of 1.82 eV assigned to the lowest-energy bound excitons that is characterized by an effective mass of 1.68 m0 and a Bohr radius of 2 Ĺ. The results of this study hold the promise for potential applications of the GeC hybrid in optoelectronics.

  20. Tunable exciton g-factor in height and composition engineered quantum dots

    International Nuclear Information System (INIS)

    We present experimental and theoretical investigations of the influence of static electric and magnetic fields on the exciton g-factor (gex,zz) of self-assembled InGaAs-GaAs quantum dots. The use of a novel growth procedure allows us to precisely control the dot height (h=2-6 nm) by partially capping the dots with GaAs before introducing a growth interruption and annealing step to remove Indium from the growth surface (''In-flush'' method). By performing single quantum dot photoluminescence and photocurrent absorption measurements with magnetic fields up to 15 T applied parallel to the quantum dot growth axis we show that the gex,zz can be tuned from 0.4 to -0.4 by applying static electric fields?70 kV/cm. Microscopically, the effect is caused by pushing the electron and hole components of the exciton wavefunction into different regions of the dot, with differing local In-Ga composition. For the tallest dots (h=6 nm) we find that vertical stroke gex,zz vertical stroke is also influenced by the static magnetic field. Our experimental findings are in good qualitative agreement with detailed 3D eight-band k.p calculations that incorporates the magnetic field in a fully gauge invariant manner.