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Sample records for halide exciton bands

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

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

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

    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

  3. Spectral and Dynamical Properties of Single Excitons, Biexcitons, and Trions in Cesium-Lead-Halide Perovskite Quantum Dots.

    Makarov, Nikolay S; Guo, Shaojun; Isaienko, Oleksandr; Liu, Wenyong; Robel, István; Klimov, Victor I

    2016-04-13

    Organic-inorganic lead-halide perovskites have been the subject of recent intense interest due to their unusually strong photovoltaic performance. A new addition to the perovskite family is all-inorganic Cs-Pb-halide perovskite nanocrystals, or quantum dots, fabricated via a moderate-temperature colloidal synthesis. While being only recently introduced to the research community, these nanomaterials have already shown promise for a range of applications from color-converting phosphors and light-emitting diodes to lasers, and even room-temperature single-photon sources. Knowledge of the optical properties of perovskite quantum dots still remains vastly incomplete. Here we apply various time-resolved spectroscopic techniques to conduct a comprehensive study of spectral and dynamical characteristics of single- and multiexciton states in CsPbX3 nanocrystals with X being either Br, I, or their mixture. Specifically, we measure exciton radiative lifetimes, absorption cross-sections, and derive the degeneracies of the band-edge electron and hole states. We also characterize the rates of intraband cooling and nonradiative Auger recombination and evaluate the strength of exciton-exciton coupling. The overall conclusion of this work is that spectroscopic properties of Cs-Pb-halide quantum dots are largely similar to those of quantum dots of more traditional semiconductors such as CdSe and PbSe. At the same time, we observe some distinctions including, for example, an appreciable effect of the halide identity on radiative lifetimes, considerably shorter biexciton Auger lifetimes, and apparent deviation of their size dependence from the "universal volume scaling" previously observed for many traditional nanocrystal systems. The high efficiency of Auger decay in perovskite quantum dots is detrimental to their prospective applications in light-emitting devices and lasers. This points toward the need for the development of approaches for effective suppression of Auger recombination in these nanomaterials, using perhaps insights gained from previous studies of II-VI nanocrystals. PMID:26882294

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

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

    2016-02-23

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

  5. Stable geometries of the self-trapped exciton in alkali halide crystals

    According to recent works, the self-trapped excitons (STE) in alkali halides are grouped in three different types. In some (e.g. RbI), a population transfer is observed among coexisting types as temperature is raised. In others (NaBr and NaI) extra type emission bands have been observed under dilational strain. We studied the adiabatic potential energy surface (APES) of the STE under the effect of expanded or compressed lattices, and of rotation of the Vk core in several directions, based on both extended-ion and ab initio Hartree-Fock methods. A critical study of the zero field splitting parameter D of the spin Hamiltonian is made in assessing the effect of rotation. It is shown that: (a) the potential barrier separating the adjacent local minima on the APES becomes smaller as the lattice is dilated; (b) the APES associated with rotation of the halogen molecule-ion axis from [110] toward [001], upto about 30 , is flat; Rotations in other directions are stiff, however; (c) the observed anomaly of the D parameter in NaCl is compatible with the rotation described in (b) above. The geometries of the three types are discussed on the basis of this study. ((orig.))

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

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

    2015-12-01

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

  7. The experimental estimation of screen barrier of self trapped excitons' relaxation in alkali halide crystals

    Full text: Earlier the effect of intrinsic luminescence ignition of alkali halide crystals (AHC) at lattice symmetry lowering was found. This is probably connected with the efficiency decrease of nonradiative channel of exciton decay into initial radiation defects. It is well known that at self trapped exciton (STE) luminescence quenching the radiation defects creation efficiency increases. Experimental method for the estimation of nonradiative STE transfer activation energy in AHC at low temperature uniaxial stress is proposed. On the basis of values of STE luminescence quenching activation energy for the range of AHC obtained by temperature dependence of X-ray luminescence for both in absentia and at low temperature uniaxial stress, we can mark out the main rule: the activation energy increase at elastic uniaxial stress is observed in the range KBr→NaCl →KI→Rbl→CsBr and explained by the increase of potential barrier of STE nonradiative decay into radiation defects. It is represented the values of STE luminescence quenching activation energy for the range of AHC for both in absentia and at low temperature uniaxial stress. Thus, it becomes obvious that radiation defects' creation decrease in KI and Rbl crystals depends on elastic stress degree because of the increase of potential barrier sharing radiative and nonradiative STE decay channels

  8. Theory of two-photon absorption by exciton states in cubic semiconductors with degenerate valence bands

    The coefficient of the absorption of two polarized photons is calculated for direct band gap semiconductors with degenerate valence bands. Wannier-Mott exciton states are included in both the intermediate and final states. Numerical calculations are performed for ZnSe and are compared with Sondergeld's experimental and theoretical results. (author). 11 refs, 2 tabs

  9. Nature of the narrow optical band in H*-aggregates: Dozy-chaos–exciton coupling

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

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

    Karlick, Frantiek; Otyepka, Michal; 10.1063/1.4736998

    2012-01-01

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

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

    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.

  12. Exciton spectra and energy band structure of CuAlS2 crystals

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

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

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

    2014-03-01

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

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

    Matsueda, H; Maekawa, S

    2004-01-01

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

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

    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.

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

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

    2016-01-13

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

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

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

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

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

    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.

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

    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 π-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

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

    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

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

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

    2015-12-01

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

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

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

    2013-01-01

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

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

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

    2015-12-01

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

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

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

    2015-12-23

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

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

    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

  7. Excitonic spectra and energy band structure of ZnAl2Se4 crystals

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

    2015-11-01

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

  8. Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content

    Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers

  9. Alternation of band gap and localization of excitons in InGaNAs nanostructures with low nitrogen content

    Gholami, M; Haratizadeh, H; Esmaeili, M; Amiri, R [Physics Department, Shahrood University of Technology, 3619995161, PO Box 316, Shahrood (Iran, Islamic Republic of); Holtz, P O [Department of Physics, Chemistry and Biology, Linkoping University, SE-581 581 83 Linkoping (Sweden); Hammar, M [Department of Microelectronics and Applied Physics, School of Information and Communication Technology, Royal Institute of Technology (KTH), Electrum 229, 16440 Kista-Stocholm (Sweden)], E-mail: mgholamim@gmail.com

    2008-08-06

    Continuous wave photoluminescence (cw PL) spectroscopy has been used to study the optical properties of a set of InGaNAs epilayers and single quantum wells with nitrogen concentration less than a few per cent at different temperatures and different excitation powers. We found that nitrogen has a critical role on the emission light of InGaNAs nanostructures and the recombination mechanism. The incorporation of a few per cent of nitrogen leads to shrinkage of the InGaNAs band gap. The physical origin of such band gap reduction has been investigated both experimentally and theoretically by using a band anticrossing model. We have found that localization of excitons that have been caused by incorporation of a few per cent of nitrogen in these structures is the main explanation of such anomalous behavior observed in the low-temperature photoluminescence spectra of these nanostructures. The localization energies of carriers have been evaluated by studying the variation of the quantum well (QW) emission versus temperature, and it was found that the localization energy increases with increasing nitrogen composition. Our data also show that, with increasing excitation intensity, the PL peak position moves to higher energies (blue shift) due to the filling of localized states and capture centers for excitons by photo-generated carriers.

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

    Kuneš, Jan; Augustinský, Pavel

    2014-01-01

    Roč. 89, č. 11 (2014), "115134-1"-"115134-8". ISSN 1098-0121 R&D Projects: GA ČR 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

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

    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.

  12. A correlation between the spectral location of the absorption bands due to the localized centers and the lattice constant in alkali halide crystals

    For four anionic impurity centers with s2-configuration (H-, Cu-, Ag-, and Au- centers) and the F center in alkali halides, a correlation between the peak energy, E, of the absorption band concerned and the lattice constant, d is extensively investigated. All the results indicate that the Smakula (STSN) relation, E = ad/sup -n/, holds for every alkali family (a and n are adjustable parameters). The values of exponent, n, derived from the analysis for all centers treated show a systematic variation with the defect center, the absorption band, and the alkali ion. It seems that the factor n expresses a degree of the lattice relaxation. In fact, a relation between the n-value and the Stokes-shift ΔE concerned, is very simple. An empirical relation, ΔE = bn/sup k/, roughly holds and the constants b and k depend only on the crystal structures of the alkali halides. (author)

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

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

    2015-12-01

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

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

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

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

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

    2013-11-15

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

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

    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

  17. Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states

    Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

  18. Exciton localization in solution-processed organolead trihalide perovskites

    He, Haiping; Yu, Qianqian; Li, Hui; Li, Jing; Si, Junjie; Jin, Yizheng; Wang, Nana; Wang, Jianpu; He, Jingwen; Wang, Xinke; Zhang, Yan; Ye, Zhizhen

    2016-01-01

    Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium–lead–halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices. PMID:26996605

  19. Exciton localization in solution-processed organolead trihalide perovskites

    He, Haiping; Yu, Qianqian; Li, Hui; Li, Jing; Si, Junjie; Jin, Yizheng; Wang, Nana; Wang, Jianpu; He, Jingwen; Wang, Xinke; Zhang, Yan; Ye, Zhizhen

    2016-03-01

    Organolead trihalide perovskites have attracted great attention due to the stunning advances in both photovoltaic and light-emitting devices. However, the photophysical properties, especially the recombination dynamics of photogenerated carriers, of this class of materials are controversial. Here we report that under an excitation level close to the working regime of solar cells, the recombination of photogenerated carriers in solution-processed methylammonium-lead-halide films is dominated by excitons weakly localized in band tail states. This scenario is evidenced by experiments of spectral-dependent luminescence decay, excitation density-dependent luminescence and frequency-dependent terahertz photoconductivity. The exciton localization effect is found to be general for several solution-processed hybrid perovskite films prepared by different methods. Our results provide insights into the charge transport and recombination mechanism in perovskite films and help to unravel their potential for high-performance optoelectronic devices.

  20. Band-Edge Exciton Fine Structure and Recombination Dynamics in InP/ZnS Colloidal Nanocrystals.

    Biadala, Louis; Siebers, Benjamin; Beyazit, Yasin; Tessier, Mickaël D; Dupont, Dorian; Hens, Zeger; Yakovlev, Dmitri R; Bayer, Manfred

    2016-03-22

    We report on a temperature-, time-, and spectrally resolved study of the photoluminescence of type-I InP/ZnS colloidal nanocrystals with varying core size. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. In addition to the typical bright-dark doublet, the photoluminescence stems from an upper bright state in spite of its large energy splitting (∼100 meV). This striking observation results from dramatically lengthened thermalization processes among the fine structure levels and points to optical-phonon bottleneck effects in InP/ZnS nanocrystals. Furthermore, our data show that the radiative recombination of the dark exciton scales linearly with the bright-dark energy splitting for CdSe and InP nanocrystals. This finding strongly suggests a universal dangling bonds-assisted recombination of the dark exciton in colloidal nanostructures. PMID:26889780

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

    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 Γ- and X-valley occurs. This Γ-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, Γ-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 Γ-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.

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

    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...

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

    Comin, Riccardo

    2015-01-01

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

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

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

    2012-05-01

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

  5. Strong interlayer coupling mediated giant two-photon absorption in MoS e2 /graphene oxide heterostructure: Quenching of exciton bands

    Sharma, Rituraj; Aneesh, J.; Yadav, Rajesh Kumar; Sanda, Suresh; Barik, A. R.; Mishra, Ashish Kumar; Maji, Tuhin Kumar; Karmakar, Debjani; Adarsh, K. V.

    2016-04-01

    A complex few-layer MoS e2 /graphene oxide (GO) heterostructure with strong interlayer coupling was prepared by a facile hydrothermal method. In this strongly coupled heterostructure, we demonstrate a giant enhancement of two-photon absorption that is in stark contrast to the reverse saturable absorption of a weakly coupled MoS e2 /GO heterostructure and saturable absorption of isolated MoS e2 . Spectroscopic evidence of our study indicates that the optical signatures of isolated MoS e2 and GO domains are significantly modified in the heterostructure, displaying a direct coupling of both domains. Furthermore, our first-principles calculations indicate that strong interlayer coupling between the layers dramatically suppresses the MoS e2 excitonic bands. We envision that our findings provide a powerful tool to explore different optical functionalities as a function of interlayer coupling, which may be essential for the development of device technologies.

  6. Contrastive analysis of multiple exciton generation theories

    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.

  7. Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites

    Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.

    2016-03-01

    UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.

  8. Spatially indirect excitons in coupled quantum wells

    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.

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

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

  10. Excitons in narrow-gap carbon nanotubes

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

    2010-01-01

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

  11. Terahertz absorption by excitonic polaritons

    Vera-Ciro, Carlos Andres; Delgado, Alain; Gonzalez, Augusto

    2010-01-01

    We use linear response theory in order to compute the light absorption spectrum, in the terahertz band, of a polariton system composed by excitons in a quantum dot very strongly coupled to the lowest photon mode of a thin micropillar. In a thermalized (Bose condensed) system at low temperatures, the spectral function shows a peak (GDR) associated to a 1s - 2p exciton transition, enhanced by polariton effects. On the other hand, in a non-equilibrium system absorption is peaked at low energies....

  12. Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates

    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. Polynuclear technetium halide clusters

    Development of chemistry of polynuclear technetium halide clusters in works devoted to synthesis, structure and investigation of their chemical and physical properties is considered. The role of academician V.I. Spitsyn as an initiator of investigation of polynuclear technetium halide clusters in the Institute of Physical Chemistry of Academy of Science of USSR is noted. Reactions and stability of cluster halides, their molecular and electronic structures are analyzed. Prospects of development of polynuclear technetium halide clusters chemistry as a direction being on the junction of cluster chemistry and theory of metal-metal multiple bonds are appreciated

  14. Nature of the band gap of halide perovskites ABX3 (A = CH3NH3, Cs; B = Sn, Pb; X = Cl, Br, I): First-principles calculations

    Yuan, Ye; Xu, Run; Xu, Hai-Tao; Hong, Feng; Xu, Fei; Wang, Lin-Jun

    2015-11-01

    The electronic structures of cubic structure of ABX3(A=CH3NH3, Cs; B=Sn, Pb; X=Cl, Br, I) are analyzed by density functional theory using the Perdew-Burke-Ernzerhof exchange-correlation functional and using the Heyd-Scuseria-Ernzerhof hybrid functional. The valence band maximum (VBM) is found to be made up by an antibonding hybridization of B s and X p states, whereas bands made up by the π antibonding of B p and X p states dominates the conduction band minimum (CBM). The changes of VBM, CBM, and band gap with ion B and X are then systematically summarized. The natural band offsets of ABX3 are partly given. We also found for all the ABX3 perovskite materials in this study, the bandgap increases with an increasing lattice parameter. This phenomenon has good consistency with the experimental results. Project supported by the National Natural Science Foundation of China (Grant No. 11375112).

  15. Refractive index of the alkali halides. II. Effect of pressure on the refractive index of 11 alkali halides

    Johannsen, P. G.; Reiß, G.; Bohle, U.; Magiera, J.; Müautller, R.; Spiekermann, H.; Holzapfel, W. B.

    1997-03-01

    A recently developed comparative interferometric method for the determination of the refractive index at high pressures is applied to NaF, NaBr, NaI, KCl, KBr, KI, RbCl, RbI, CsCl, CsBr, and CsI. In the studied pressure range up to 12 GPa, the potassium and rubidium halides show a polymorphic phase transition from NaCl- to CsCl-type structure, accompanied by a discontinuous increase of the refractive index. The pressure data of the sodium and cesium halides are converted from pressure to density dependences by the help of ultrasonic equations of state. The refractive index of the sodium halides shows an almost linear density dependence, while the cesium halides exhibit strong nonlinear behavior. The constant joint-density-of-states (CJDOS) model, proposed in the first paper of this series, is used for the further analysis of the data. In the CJDOS model the density dependence of the dielectric function is related to the different behaviors of s and d conduction bands with density. While the almost linear behavior of the sodium halides can be understood by a competition of an increasing contribution of transitions to the d bands, and a decreasing contribution of transitions to the s conduction band, the nonlinear behavior of the cesium halides is predominantly caused by the closure of the band gap, with a d-band character of the lower conduction-band states.

  16. Polarizable Optical Bistability of Frenkel Excitons

    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.

  17. Photofragmentation of metal halides

    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 Ω=0+ and Ω=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 Ω=1 state. Extensive studies are reported on three thallium halides, TlI, TlBr and TlCl at various wavelengths covering the near ultraviolet region. (Auth.)

  18. Excitons in nanoscale systems

    Scholes, Gregory D.; Rumbles, Garry

    2006-09-01

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

  19. Coherent excitonic molecules

    Keldysh, L. V.

    1992-10-01

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

  20. Exciton circular dichroism in channelrhodopsin.

    Pescitelli, Gennaro; Kato, Hideaki E; Oishi, Satomi; Ito, Jumpei; Maturana, Andrés Daniel; Nureki, Osamu; Woody, Robert W

    2014-10-16

    Channelrhodopsins (ChRs) are of great interest currently because of their important applications in optogenetics, the photostimulation of neurons. The absorption and circular dichroism (CD) spectra of C1C2, a chimera of ChR1 and ChR2 of Chlamydomonas reinhardtii, have been studied experimentally and theoretically. The visible absorption spectrum of C1C2 shows vibronic fine structure in the 470 nm band, consistent with the relatively nonpolar binding site. The CD spectrum has a negative band at 492 nm (Δε(max) = -6.17 M(-1) cm(-1)) and a positive band at 434 nm (Δε(max) = +6.65 M(-1) cm(-1)), indicating exciton coupling within the C1C2 dimer. Time-dependent density functional theory (TDDFT) calculations are reported for three models of the C1C2 chromophore: (1) the isolated protonated retinal Schiff base (retPSB); (2) an ion pair, including the retPSB chromophore, two carboxylate side chains (Asp 292, Glu 162), modeled by acetate, and a water molecule; and (3) a hybrid quantum mechanical/molecular mechanical (QM/MM) model depicting the binding pocket, in which the QM part consists of the same ion pair as that in (2) and the MM part consists of the protein residues surrounding the ion pair within 10 Å. For each of these models, the CD of both the monomer and the dimer was calculated with TDDFT. For the dimer, DeVoe polarizability theory and exciton calculations were also performed. The exciton calculations were supplemented by calculations of the coupling of the retinal transition with aromatic and peptide group transitions. For the dimer, all three methods and three models give a long-wavelength C2-axis-polarized band, negative in CD, and a short-wavelength band polarized perpendicular to the C2 axis with positive CD, differing in wavelength by 1-5 nm. Only the retPSB model gives an exciton couplet that agrees qualitatively with experiment. The other two models give a predominantly or solely positive band. We further analyze an N-terminal truncated mutant because it was assumed that the N-terminal domain has a crucial role in the dimerization of ChRs. However, the CD spectrum of this mutant has an exciton couplet comparable to that of the wild-type, demonstrating that it is dimeric. Patch-clamp experiments suggest that the N-terminal domain is involved in protein stabilization and channel kinetics rather than dimerization or channel activity. PMID:25247388

  1. Mechanoluminescence of γ-irradiated alkali halide crystals

    The mechanoluminescence (ML) spectra of γ-irradiated alkali halide crystals consist of a narrow band of different ranges towards the ultraviolet or violet region. The ML spectra of LiF and NaCl consist of two bands. The ML spectra obtained during the release of pressure are similar to the ML spectra obtained during the application of pressure. The ML spectra of γ-irradiated alkali halide crystals both at the release and at the application of pressure are similar to that of X-irradiated alkali halide crystals. The ML spectra are compared with the other types of luminescence spectra and it is found that the ML spectra of coloured alkali halide crystals resemble the high energy spectra and the thermoluminescence spectra to a large extent. The wavelength corresponding to the peak of the ML spectra is found to be proportional to the square of the lattice constant of the crystals. (author)

  2. Quantum confinement of excitons in wurtzite InP nanowires

    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.

  3. Quantum confinement of excitons in wurtzite InP nanowires

    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

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

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

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

    Wang, Jun; Wang, Yafeng; Hu, Tao; Wu, Lin; Shen, Xuechu; Chen, Zhanghai, E-mail: lujian@fudan.edu.cn, E-mail: zhanghai@fudan.edu.cn [State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Fudan University, Shanghai 200433 (China); Cao, Runan; Xu, Fei [Department of Physics, Shanghai University, Shanghai 200444 (China); Da, Peimei; Zheng, Gengfeng [Laboratory of Advanced Materials, Department of Chemistry, Fudan University, Shanghai 200433 (China); Lu, Jian, E-mail: lujian@fudan.edu.cn, E-mail: zhanghai@fudan.edu.cn [State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Fudan University, Shanghai 200433 (China); Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210 (China)

    2016-01-11

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

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

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

    2016-01-01

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

  7. Surface photovoltage in exciton absorption range in CdS

    Morawski, A.; Banisch, R.; Lagowski, J.

    1977-01-01

    The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120-160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.

  8. Exciton Dynamics in Hexagonal InP Nanowires

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

    2013-03-01

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

  9. Phase coherence and spectral functions in the two-dimensional excitonic systems

    The nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose–Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered

  10. Phase coherence and spectral functions in the two-dimensional excitonic systems

    Apinyan, V., E-mail: V.Apinyan@int.pan.wroc.pl; Kopeć, T.K.

    2015-09-15

    The nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose–Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered.

  11. Photophysics of Hybrid Lead Halide Perovskites: The Role of Microstructure.

    Srimath Kandada, Ajay Ram; Petrozza, Annamaria

    2016-03-15

    Since the first reports on high efficiency, solution processed solar cells based on hybrid lead halide perovskites, there has been an explosion of activities on these materials. Researchers with interests spanning the full range from conventional inorganic to emerging organic and hybrid optoelectronic technologies have been contributing to the prolific research output. This has led to solar cell power conversion efficiencies now exceeding 20% and the demonstration of proofs of concept for electroluminescent and lasing devices. Hybrid perovskites can be self-assembled by a simple chemical deposition of the constituent units, with the possibility of integrating the useful properties of organic and inorganic compounds at the molecular scale within a single crystalline material, thus enabling a fine-tuning of the electronic properties. Tellingly, the fundamental properties of these materials may make us think of a new, solution processable, GaAs-like semiconductor. While this can be true to a first approximation, hybrid perovskites are intrinsically complex materials, where the presence of various types of interactions and structural disorder may strongly affect their properties. In particular, a clear understanding and control of the relative interactions between the organic and inorganic moieties is of paramount importance to properly disentangle their innate physics. In this Account we review our recent studies which aim to clarify the relationship between structural and electronic properties from a molecular to mesoscopic level. First we identify the markers for local disorder at the molecular level by using Raman spectroscopy as a probe. Then, we exploit such a tool to explore the role of microstructure on the absorption and luminescence properties of the semiconductor. Finally we address the controversy surrounding electron-hole interactions and excitonic effects. We show that in hybrid lead-halide perovskites dielectric screening also depends on the local microstructure of the hybrid crystals and not only on its chemical composition. This leads to the possibility of band gap engineering and the consequent control of the elementary photoexcitation dynamics that determine the perovskites' performances in different optoelectronic devices. PMID:26883988

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

    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.

  13. Wannier-Mott excitons in semiconductors with a superlattice

    The effect of the motion of a Wannier-Mott exciton in semiconductors with a superlattice formed by heterojunctions on the exciton binding energy and wave function is analyzed. This effect arises as a result of the fact that the dispersion laws of the electron and hole that form an exciton in a superlattice differ from the quadratic law. The investigated one-dimensional superlattice consists of alternating semiconductor layers with different energy positions of the conduction and valence bands, i.e., with one-dimensional wells and barriers. The exciton state in a superlattice consisting of quantum dots is analyzed. It is demonstrated that the closer the electron and hole effective masses, the greater the dependence of the binding energy on the exciton quasi-momentum. The possibility of replacing the tunneling excitation transfer between superlattice cells with the dipole-dipole one at certain exciton quasi-wave vector values is investigated

  14. Excitonic properties of ZnSe/ZnSeS superlattices

    Cingolani, R.; Lomascolo, M.; Lovergine, N.; Dabbicco, M.; Ferrara, M.; Suemune, I.

    1994-05-01

    We report an optical investigation of the excitons in ZnSe/ZnSeS superlattices of well widths ranging between 2 and 15 nm. An almost constant exciton binding energy is found. The exciton confinement is found to be dominated by the hole quantization, consistent with the expectation of negligible conduction-band discontinuity in these heterostructures. The effect of strain has been included to properly reproduce the well width dependence of the light-hole energies. Finally strong evidence of hot exciton photogeneration is obtained from the oscillatory behavior of the photoluminescence excitation spectra. This is consistent with the strong exciton-phonon coupling deduced from the temperature dependence of the excitonic linewidth measured by transmission experiments.

  15. Optics of excitonic molecules in semiconductors and semiconductor microstructures

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

    1998-03-01

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

  16. Excitonic polarons in low-dimensional transition metal dichalcogenides

    Thilagam, A.

    2015-05-01

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

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

    Kaneko, Tatsuya; Ohta, Yukinori

    2014-01-01

    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 effec...

  18. Excitons in cuprous oxide

    Fishman, Dmitry,

    2008-01-01

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

  19. Confined exciton spectroscopy

    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

  20. Excitonic photoconductivity and structural defects in Cu2O crystals

    HAYDAR, A.; Coret, A.

    1980-01-01

    The photoconductivity spectrum of Cu2O at 4 K is studied in crystals submitted to different treatments. The role of strained zones near inclusions in the dissociation process of excitons in free carriers is pointed out. A direct correlation exists between the density of inclusions and the intensity of the photocurrent. The band to band photocurrent is submitted to the same variations as the excitonic photocurrent.

  1. Exciton optical transitions in a hexagonal boron nitride single crystal

    Museur, L. [Laboratoire de Physique des Lasers - LPL, CNRS UMR 7538, Institut Galilee, Universite Paris 13, 93430 Villetaneuse (France); Brasse, G.; Maine, S.; Ducastelle, F.; Loiseau, A. [ONERA - Laboratoire d' Etude des Microstructures - LEM, ONERA-CNRS, UMR 104, BP 72, 92322 Chatillon Cedex (France); Pierret, A. [ONERA - Laboratoire d' Etude des Microstructures - LEM, ONERA-CNRS, UMR 104, BP 72, 92322 Chatillon Cedex (France); CEA-CNRS, Institut Neel/CNRS, Universite J. Fourier, CEA/INAC/SP2M, 17 rue des Martyrs, 38 054 Grenoble Cedex 9 (France); Attal-Tretout, B. [ONERA - Departement Mesures Physiques - DMPh, 27 Chemin de la Huniere, 91761 Palaiseau Cedex (France); Barjon, J. [GEMaC, Universite de Versailles St Quentin, CNRS Bellevue, 1 Place Aristide Briand, 92195 Meudon Cedex (France); Watanabe, K.; Taniguchi, T. [National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044 (Japan); Kanaev, A. [Laboratoire des Sciences des Procedes et des Materiaux - LSPM, CNRS UPR 3407, Universite Paris 13, 93430 Villetaneuse (France)

    2011-06-15

    Near band gap photoluminescence (PL) of a hexagonal boron nitride single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by trapped-exciton optical transitions, while the photoluminescence excitation (PLE) spectra show features assigned to free excitons. Complementary photoconductivity and PLE measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

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

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

    2016-01-01

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

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

    Ohara, J; Ohara, Jun; Yamamoto, Shoji

    2006-01-01

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

  4. Exciton laser rate equations

    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.

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

    The ultraviolet absorption spectra of alkali cyanide and mixed alkali halide cyanide crystals were measured at temperatures ranging from 300K down to 4.2K. A set of small absorption peaks was observed at energies near 6 eV and assigned to parity forbidden X1Σ+→a'3Σ+ transitions of the CN- molecular ions. It was observed that the peak position depends on the alkali atom while the absorption cross section strongly depends on the halogen and on the CN- concentration of the mixed crystals. These effects are explained in terms of an interaction between the triplet molecular excitons and charge transfer excitons. The experimental data were fit with a coupling energy of a few meV. The coupling mechanism is discussed and it is found to be due to the overlap between the wave functions of the two excitations. (Author)

  6. Auger decay of degenerate and Bose-condensed excitons in Cu$_2$O

    Kavoulakis, G. M.; Baym, Gordon

    1996-01-01

    We study the non-radiative Auger decay of excitons in Cu$_2$O, in which two excitons scatter to an excited electron and hole. The exciton decay rate for the direct and the phonon-assisted processes is calculated from first principles; incorporating the band structure of the material leads to a relatively shorter lifetime of the triplet state ortho excitons. We compare our results with the Auger decay rate extracted from data on highly degenerate triplet excitons and Bose-condensed singlet exc...

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

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

    2006-04-01

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

  8. Exciton-exciton annihilation in MoSe2 monolayers

    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.

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

    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...

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

    Ahn, Doyeol; Park, Seoung-Hwan

    2016-01-01

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

  11. Interwell excitons in GaAs superlattices

    Birkedal, Dan; Sayed, Karim El; Sanders, G.; Spiegelberg, C.; Vadim, Lyssenko; Stanton, C.; Hvam, Jørn Märcher; Timofeev, V. B.; Bayer, M.

    1996-01-01

    The formation of spatially indirect excitons in superlattices with narrow minibands is theoretically and experimentally investigated. We identify the experimental conditions for the observation of interwell excitons and find a distinct excitonic state energetically located between the Is exciton ...

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

    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

  13. Excitonic gap formation in neutral bilayer structures

    Apinyan, V.; Kopeć, T. K.

    2015-08-01

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

  14. Making and Breaking of Lead Halide Perovskites

    Manser, Joseph S.

    2016-02-16

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

  15. Making and Breaking of Lead Halide Perovskites.

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

    2016-02-16

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

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

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

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

    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.

  18. Actinide halide complexes

    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.

  19. Machine Learning Exciton Dynamics

    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...

  20. Multiscale photosynthetic exciton transfer

    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...

  1. On luminescence polarization of free and localized excitons in NaI, KI and CsBr

    The results of investigations on the polarization of the luminescence of excitons induced in cubic alkali haloid crystals on irradiation in exciton absorption bands are considered. On inducing excitons with I=1 in NaI by linear-polarized light of 5.6 eV the luminescence of excitons with I=2 at 80 K shows partial negative polarization. The observation of polarized luminescence of single-haloid excitons suggests that single-haloid excitons in NaI are free and can be optically aligned. The luminescence of self-captured two-haloid excitons on inducing single-haloid excitons by linear-polarized light is not polarized. The KCl-I, NaBr-I systems are studied

  2. Excitons in motion in II-VI semiconductors

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

    2010-06-15

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

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

    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

  4. Halide-Substituted Electronic Properties of Organometal Halide Perovskite Films: Direct and Inverse Photoemission Studies.

    Li, Chi; Wei, Jian; Sato, Mikio; Koike, Harunobu; Xie, Zhong-Zhi; Li, Yan-Qing; Kanai, Kaname; Kera, Satoshi; Ueno, Nobuo; Tang, Jian-Xin

    2016-05-11

    Solution-processed perovskite solar cells are attracting increasing interest due to their potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics is of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions of various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on the electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of the electronic properties, with a qualitatively blue shift along the I → Br → Cl series and showing the increase in ionization potentials from 5.96 to 7.04 eV and the change of transport band gaps in the range from 1.70 to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes. PMID:27101940

  5. Excitonic condensation in spatially separated one-dimensional systems

    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

  6. Excitonic condensation in spatially separated one-dimensional systems

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

    2015-05-25

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

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

    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.

  8. Triplet exciton dynamics

    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.)

  9. Quasienergy Spectroscopy of Excitons

    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...

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

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

    2014-01-01

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

  11. Bright Interlayer Exciton Dynamics in MoSe2-WSe2 Heterostructures

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

    2015-03-01

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

  12. Singlet exciton fission photovoltaics.

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

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

    2016-01-01

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

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

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

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

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

    2016-01-14

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

  16. Bleaching in the region of exciton resonance of layered GaSe crystals

    Kyazym-zade, A. G.; Salmanov, V. M.; Guseinov, A. G.; Salmanova, A. A.; Mamedov, R. M.; Dzhavadzade, A. A.

    2014-09-01

    Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd3+ laser emitting 30-ps light pulses and a dye laser with a pulse width of ˜3 ns tunable within the range 594-643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.

  17. Anion exchange extraction of halide cadmium complexes

    Anion exchange extraction of halide cadmium complexes, by trinonyloctadecyl ammonium salts has been studied by the method of intermediate exchange. Conventional and thermodynamic constants of exchange of halide cadmium complexes for a series of mineral ions have been calculated

  18. Theory of exciton-exciton correlation in nonlinear optical response

    Oestreich, Th.; Schoenhammer, K.; Sham, L. J.

    1998-01-01

    We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of phase-space filling correction, mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory-effects beyond the...

  19. Triplet exciton dissociation in singlet exciton fission photovoltaics

    Jadhav, Priya J.; Mohanty, Aseema; Bulovic, Vladimir; Baldo, Marc A. [Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Brown, Patrick R. [Department of Physics, Massachusetts Institute of Technology, Cambridge, MA (United States); Thompson, Nicholas [Department of Materials Science, Massachusetts Institute of Technology, Cambridge, MA (United States); Wunsch, Benjamin [School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA (United States); Yost, Shane R.; Hontz, Eric; Van Voorhis, Troy; Bawendi, Moungi G. [Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA (United States)

    2012-12-04

    Triplet exciton dissociation in singlet exciton fission devices with three classes of acceptors are characterized: fullerenes, perylene diimides, and PbS and PbSe colloidal nanocrystals. Using photocurrent spectroscopy and a magnetic field probe it is found that colloidal PbSe nanocrystals are the most promising acceptors, capable of efficient triplet exciton dissociation and long wavelength absorption. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  20. Excitons in disordered polymers

    Avgin, I

    2005-01-01

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

  1. Monolayer excitonic laser

    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.

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

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

  3. Heat capacity of shocked alkali halides

    Boness, David A.; Brown, J. Michael

    1996-05-01

    The rarefaction-overtake method, combined with optical-pyrometry temperature measurements, allows the determination of the heat capacity for shocked transparent materials. We measured heat capacity for shocked single-crystal CsBr, KBr, KCl, and NaCl, and compared these results with previously-reported values for CsI. For these alkali halides as a suite, temperature strongly correlates linearly with compression, and the heat capacities rise with temperature from a classical 3R value to more than twice that for two-fold compressions and eV-level temperature. Electronic structure and thermalization of electrons likely account for the observed correlation between heat capacity and material band gap.

  4. Exciton-polariton mediated superconductivity

    Laussy, Fabrice P.; Kavokin, Alexey V; Shelykh, Ivan A

    2010-01-01

    We revisit the exciton mechanism of superconductivity in the framework of microcavity physics, replacing virtual excitons as a binding agent of Cooper pairs by excitations of an exciton-polariton Bose-Einstein condensate. We consider a model microcavity where a quantum well with a two-dimensional electron gas is sandwiched between two undoped quantum wells, where a polariton condensate is formed. We show that the critical temperature for superconductivity dramatically increases with the conde...

  5. Electronic conduction in molten halides

    Heus, R.J.; Egan, J.J.

    1976-01-01

    Methods of measuring electronic conductivity in molten halides are reviewed. These include increase of total conductivity with addition of metal, polarization techniques, chronopotentiometry, and motion of colored subhalides in a potential gradient. The applicability of the Nernst-Einstein equation and the role of convection are considered. Results are presented for several halide melts. Applications of these results are elucidated, including self-discharge rate of molten salt batteries, measurement of alloy thermodynamics using molten salt electrolytes, and kinetics of tarnishing reactions with formation of liquid films.

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

    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.

  7. Direct and indirect two-photon excitonic processes in solids

    The theory of direct and indirect two-photon transitions to exciton states has been theoretically investigated in semiconductors. The effects of the nonparabolic bands and the degeneracy of the valence band have been taken into account. Expressions for the absorption coefficient through the most energetic band models are calculated. The numerical applications to a number of semiconductors show that: (i) For the direct process, the three-band model, where the third band is the spin-orbit splitting valence band gives the largest contribution to the absorption coefficient. The transition mechanism through this model leads to an s-like state. (ii) In the indirect process, the four-band model dominates the other models which leads also to a final s-exciton state. An exciton peak appears at photon energy which is close to that recently observed in CdI2. The nonparabolic effect enhances the absorption by a factor of two in the case of the direct process and by two order of magnitude in the indirect one. (author). 9 refs, 2 figs, 2 tabs

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

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

    2015-07-01

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

  9. Luminescence of excitons in mesoscopic ZnO particles

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

  10. Self-trapped excitons in quartz

    Triplet-state electronic excitations in quartz were studied using density functional theory (DFT). By using periodic boundary conditions, the lattice response and electronic structure relaxations can be determined in the bulk. Several self-trapped exciton (STE) states have been discovered, in addition to the oxygen-distorted state, which was originally predicted 10 years ago. One of these states is a silicon-distorted state that lies energetically close to the oxygen-distorted state. The results reveal that these two major STE states are likely responsible for two distinct luminescence bands. The luminescence energies for STE states of impurities and intrinsic defects were also determined

  11. Excitons in atomically thin black phosphorus

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.

    2016-03-01

    Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.

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

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

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

    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)

  14. Excitons in semiconductor nano structures

    A formation of excitons is studied in semiconductor nano structures where electrons and holes are spatially separated by a potential barrier. The disorder present within electron-hole interface is due to structural imperfections which are unavoidable in the course of fabrication. The exciton density is calculated as a function of the disorder and of the hopping integral value. (Author)

  15. Exciton Formation in Disordered Semiconductors

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

    1999-01-01

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

  16. Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots

    It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot)–(insulating matrix) interface) in a nanosystem containing insulating Al2O3 quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al2O3 single crystal. It is established that, in the band gap of an Al2O3 nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al2O3 nanoparticles at room temperature from the absorption spectrum of the nanosystem

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

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

    2015-02-15

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

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

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

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

    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.

  20. Multiple exciton generation and ultrafast exciton dynamics in HgTe colloidal quantum dots.

    Al-Otaify, Ali; Kershaw, Stephen V; Gupta, Shuchi; Rogach, Andrey L; Allan, Guy; Delerue, Christophe; Binks, David J

    2013-10-21

    The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model. PMID:23999734

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

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

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

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

    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.

  4. Creation of free excitons in solid krypton investigated by time-resolved luminescence spectroscopy

    The creation and relaxation of secondary excitons in solid Kr was investigated using energy-and time-resolved luminescence spectroscopy in the vacuum ultraviolet region. The spectrally selected emission of the free exciton (FE) was used as a probe for an investigation of the different exciton creation processes. Delayed FE creation via electron-hole recombination and 'prompt' (in terms of the time-resolution of the experiment) creation of excitons were separated. The 'prompt' creation of a FE appears in the region above threshold energy Eth, which is equal to the sum of the band gap energy and the free exciton energy. 'Prompt' creation of excitons above Eth is ascribed to a superposition of two processes: (i) creation of the electronic polaron complex (one-step process) and (ii) inelastic scattering of photoelectrons described in the framework of the multiple-parabolic-branch band model (two-step process). In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton (STE) emission was analysed. The behaviour of the ratio spectrum is a proof that electron-hole recombination leads to STE states through FE states as precursors

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

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

    2015-10-01

    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.

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

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

    2015-11-01

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

  7. Synthesis and Spectroscopic studies on cadmium halide complexes of isonicotinic acid

    In this study infrared spectra (4000-400cm-1) are reported for the cadmium(II) halide isonicotinic acid complexes. Vibrational assignments are given for all observed bands. Some structure spectra correlations and frequency shifts were found. It's found the frequency shifts depends on the halogen for a given metal. Certain chemical formulas were determined using elemental analysis results

  8. Cohesive Energy-Lattice Constant and Bulk Modulus-Lattice Constant Relationships: Alkali Halides, Ag Halides, Tl Halides

    Schlosser, Herbert

    1992-01-01

    In this note we present two expressions relating the cohesive energy, E(sub coh), and the zero pressure isothermal bulk modulus, B(sub 0), of the alkali halides. Ag halides and TI halides, with the nearest neighbor distances, d(sub nn). First, we show that the product E(sub coh)d(sub 0) within families of halide crystals with common crystal structure is to a good approximation constant, with maximum rms deviation of plus or minus 2%. Secondly, we demonstrate that within families of halide crystals with a common cation and common crystal structure the product B(sub 0)d(sup 3.5)(sub nn) is a good approximation constant, with maximum rms deviation of plus or minus 1.36%.

  9. Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals

    Bawendi M.G.; Strasfeld D.; Roitblat A.; Sachs H.; Gdor I.; Ruhman S.

    2013-01-01

    Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times tha...

  10. Optical properties of one-dimensional exciton systems: Beyond the Heitler-London approximation

    Bakalis, LD; Knoester, J.

    1997-01-01

    We study the properties of one-dimensional exciton systems in which the commonly made Heitler-London approximation (HLA) is relaxed. The nonresonant interaction terms which then exist, mix the multi-exciton bands of the HLA. Our approach is based on the exact diagonalization of the Hamiltonian, which is possible using the Jordan-Wigner and Bogoliubov transformations. Exact expressions for transition dipoles between multi-particle states are given. Results of our exact theory for the ground st...

  11. Sharp bound and free exciton lines from homoepitaxial AlN

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

    2011-07-15

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

  12. Deviations of the exciton level spectrum in Cu2O from the hydrogen series

    Schöne, F.; Krüger, S.-O.; Grünwald, P.; Stolz, H.; Scheel, S.; Aßmann, M.; Heckötter, J.; Thewes, J.; Fröhlich, D.; Bayer, M.

    2016-02-01

    Recent high-resolution absorption spectroscopy on excited excitons in cuprous oxide [Nature (London) 514, 343 (2014), 10.1038/nature13832] has revealed significant deviations of their spectrum from that of the ideal hydrogen-like series. Here we show that the complex band dispersion of the crystal, which determines the kinetic energy of electrons and holes, strongly affects the exciton binding energy. Specifically, we show that the nonparabolicity of the band dispersion is the main cause of the deviation from the hydrogen series. Experimental data collected from high-resolution absorption spectroscopy in electric fields validate the assignment of the deviation to the nonparabolicity of the band dispersion.

  13. The electronic structure of thorium halides predicted by HSE and GW

    Ellis, Jason; Wen, Xiaodong; Martin, Richard

    2014-03-01

    Recently, there has been a significant experimental push to measuring the VUV nuclear excitation of 229Th using optical spectroscopy. Large band gap Thorium halides such as ThF4 and Na2ThF6 have been suggested as candidate materials for studying this nuclear transition, as they are transparent to the relevant optical frequencies. In this work, we compare the many body GW approach, hybrid density functional theory, and local density approximation calculations of the electronic structure of these materials, as well as the rest of the binary thorium halides (ThX4, X=Cl,Br,I).

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

    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)

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

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

    2009-01-01

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

  16. Exciton states in narrow-gap carbon nanotubes

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

    2016-02-01

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

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

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

    1979-01-01

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

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

    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.

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

    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.

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

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

    2004-02-22

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

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

    Iizuka, Hideyuki; Nakayama, Takashi

    2016-02-01

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

  2. Couple molecular excitons to surface plasmon polaritons in an organic-dye-doped nanostructured cavity

    Zhang, Kun; Shi, Wen-Bo; Wang, Di; Xu, Yue; Peng, Ru-Wen; Fan, Ren-Hao; Wang, Qian-Jin; Wang, Mu

    2016-05-01

    In this work, we demonstrate experimentally the hybrid coupling among molecular excitons, surface plasmon polaritons (SPPs), and Fabry-Perot (FP) mode in a nanostructured cavity, where a J-aggregates doped PVA (polyvinyl alcohol) layer is inserted between a silver grating and a thick silver film. By tuning the thickness of the doped PVA layer, the FP cavity mode efficiently couples with the molecular excitons, forming two nearly dispersion-free modes. The dispersive SPPs interact with these two modes while increasing the incident angle, leading to the formation of three hybrid polariton bands. By retrieving the mixing fractions of the polariton band components from the measured angular reflection spectra, we find all these three bands result from the strong coupling among SPPs, FP mode, and excitons. This work may inspire related studies on hybrid light-matter interactions, and achieve potential applications on multimode polariton lasers and optical spectroscopy.

  3. Excitons and electron-hole plasma in InSe

    Cingolani, A.; Cingolani, R.; Ferrara, M.; Lugarà, M.

    1985-09-01

    The spontaneous emission from either thin or thick samples of layered indirect-gap InSe semiconductor under dye laser excitation has been investigated either above or below the critical Mott density. The observed spectra consist of five bands in the near infrared region; four of them are localized in the energy range between the direct and the indirect gap and one only lies below the indirect energy gap. These bands can be associated with cooperative indirect and direct excitonic transitions and with electronhole plasma recombinations.

  4. Fullerenes doped with metal halides

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

  5. Aging of alkali halide windows

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

    1995-07-01

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

  6. Exciton-polariton behaviour in bulk and polycrystalline ZnO

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

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

    Ayzner, Alexander L.

    2015-12-30

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

  8. Excitonic resonances in thin films of WSe2: from monolayer to bulk material

    Arora, Ashish; Koperski, Maciej; Nogajewski, Karol; Marcus, Jacques; Faugeras, Clément; Potemski, Marek

    2015-06-01

    We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interactions as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated.

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

    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)

  10. Multiphoton emission induced by exciton Bose condensate

    Lozovik, Yu E

    2002-01-01

    The processes of the coherent recombination of several (N) excitons from the Bose condensate with simultaneous N photon generation are considered. While the excitons from the condensate have the zero pulse, the generated photons have the summary zero pulse. It is shown that when these processes are stimulated by the N - 1 external laser beams, than in such a case this should lead to origination of the undirected radiation from the exciton system. The numerical estimates of the 3- and 4-exciton coherent recombination intensity in the exciton system in the Cu sub 2 O are presented

  11. Expanded fluid mercury, a ferroelectric excitonic insulator

    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)

  12. Toxicity of organometal halide perovskite solar cells

    Babayigit, Aslihan; Ethirajan, Anitha; Muller, Marc; Conings, Bert

    2016-03-01

    In the last few years, the advent of metal halide perovskite solar cells has revolutionized the prospects of next-generation photovoltaics. As this technology is maturing at an exceptional rate, research on its environmental impact is becoming increasingly relevant.

  13. Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals

    Bawendi M.G.

    2013-03-01

    Full Text Available Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times that of the band gap.

  14. Magnetic exciton dispersion in praseodymium

    Rainford, B. D.; Houmann, Jens Christian Gylden

    1971-01-01

    Measurements of the dispersion of magnetic excitons have been made in a single crystal of praseodymium metal using inelastic neutron scattering. A preliminary analysis of the data yields the first detailed information about the exchange interactions and the crystal field splittings in the light...

  15. Exciton size and quantum transport in nanoplatelets

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

    2015-12-01

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

  16. Exciton size and quantum transport in nanoplatelets

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

  17. Oxidation of hydrogen halides to elemental halogens

    Rohrmann, Charles A.; Fullam, Harold T.

    1985-01-01

    A process for oxidizing hydrogen halides having substantially no sulfur impurities by means of a catalytically active molten salt is disclosed. A mixture of the subject hydrogen halide and an oxygen bearing gas is contacted with a molten salt containing an oxidizing catalyst and alkali metal normal sulfates and pyrosulfates to produce an effluent gas stream rich in the elemental halogen and substantially free of sulfur oxide gases.

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

    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

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

    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.”.

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

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

    2014-05-01

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

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

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

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

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

    2011-01-01

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

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

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

    2006-10-19

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

  4. Experimental and theoretical optical properties of methylammonium lead halide perovskites

    Leguy, Aurélien M. A.; Azarhoosh, Pooya; Alonso, M. Isabel; Campoy-Quiles, Mariano; Weber, Oliver J.; Yao, Jizhong; Bryant, Daniel; Weller, Mark T.; Nelson, Jenny; Walsh, Aron; van Schilfgaarde, Mark; Barnes, Piers R. F.

    2016-03-01

    The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3]+ cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS). Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr05435d

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

    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.

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

    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.

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

    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.

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

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

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

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

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

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

    1997-01-01

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

  11. Dimming of metal halide lamps

    Schurer, Kees

    1994-01-01

    We ran some tests on the effect of dimming of metal halide (MH) lamps upon the stability and the spectral quality of the light output. Lamps used were a new Philips lamp HPI-T 250W, a similar Philips lamp with a few thousand burning hours and a new Osram lamp HQI-T 250W/D. The ballast was a BBC type DJ 250/2KS, the starter a BAS TORGI type MZN 250 SE and the dimmer an Elstrom Control System type ERHQ-T 250. Power was derived from a Philips stabilizer, type PE 1602. Lamp output was monitored with a PAR meter. Spectra were taken at 100% and at 50% output as measured with the PAR meter. Lamps were allowed to stabilize at any setting for 30 minutes before measurements were made. Lamp manufacturers advise against dimming for fear of poor stability and intolerable changes of the spectrum. However, none of the lamps showed a decrease in stability, no flicker or wandering of the discharge, and the changes of the spectrum were not negligible, but certainly not dramatic. Lamps of either manufacture retain their white color, relative peak heights of spectral lines did shift, but no gaps in the spectrum occurred. Spectra taken at 50% with 30 minutes intervals coincided. Differences between the new and the older Philips lamp were noticeable, but not really significant.

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

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

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

    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

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

    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.

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

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

    2012-07-01

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

  16. Exciton Seebeck effect in molecular systems

    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.

  17. Palladium-catalyzed asymmetric reductive heck reaction of aryl halides.

    Yue, Guizhou; Lei, Kaining; Hirao, Hajime; Zhou, Jianrong Steve

    2015-05-26

    Asymmetric reductive Heck reaction of aryl halides is realized in high stereoselectivity. Hydrogen-bond donors, trialkylammonium salts in a glycol solvent, were used to promote halide dissociation from neutral arylpalladium complexes to access cationic, stereoselective pathways. PMID:25867113

  18. Exciton Binding Energy of Monolayer WS2

    Bairen Zhu; Xi Chen; Xiaodong Cui

    2015-01-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach toward measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71eV around K valley in the Brillouin zone. The trion binding energy of 34meV, two-photon absorption cross s...

  19. EXCITON DYNAMICS IN ORGANIC MOLECULAR CRYSTALS

    A. Matsui; Mizuno, K.; Kobayashi, M.

    1985-01-01

    Dynamical behavior of Frenkel excitons in aromatic hydrocarbon crystals, pyrene, α-perylene, β-perylene, and tetracene are overviewed based on the published references and in terms of the self-trap depth. Then pressure-induced instability in exciton states (the change in the self-trap depth) in α-perylene and anthracene is demonstrated and discussed. Finally a quasi-free exciton state is suggested to be the origin of the luminescence in anthracene at room temperature.

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

    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

  1. Experimental and theoretical optical properties of methylammonium lead halide perovskites.

    Leguy, Aurélien M A; Azarhoosh, Pooya; Alonso, M Isabel; Campoy-Quiles, Mariano; Weber, Oliver J; Yao, Jizhong; Bryant, Daniel; Weller, Mark T; Nelson, Jenny; Walsh, Aron; van Schilfgaarde, Mark; Barnes, Piers R F

    2016-03-17

    The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS). PMID:26477295

  2. Plasmon and Exciton Coupling and Purcell Enhancement

    Rice, Quinton; Rigo, Maria Veronica; Fudala, Rafal; Cho, Hyoyeong; Kim, Wan-Joong; Rich, Ryan; Tabibi, Bagher; Gryczynski, Zygmunt; Gryczynski, Ignacy; Yu, William; Seo, Jaetae

    2014-05-01

    The photoluminescence from plasmon-coupled exciton is of great interest for optoelectronic applications, because of the large quantum yield with localized field enhancement and reduced nonradiative transition. The Coulomb interaction through plasmon-exciton coupling results in the Purcell enhancement of quantum dots (QDs) in the vicinity of metal nanoparticles (MNPs). With plasmon-exciton coupling, the radiative and non-radiative decay rates and the coupling rates compete with each other. The coupling rate is closely related to the coupling distance between QDs and MNPs. The optimized coupling distance scales the re-excitation density of localized fields and the plasmon-exciton coupling rates. If the plasmon-exciton coupling rate is much faster than the radiative and non-radiative transitions of excitons, the re-excitations of excitons by the localized plasmonic field and the reduction of non-radiative transitions may occur. This presentation includes plasmon-exciton coupling dynamics, large enhancement and temporal properties of PL, and dipole-PL polarization fidelity of hybrid optical materials of plasmonic nanometals and excitonic semiconductor QDs. The work at Hampton University was supported by the National Science Foundation (NSF HRD-1137747), and Army Research Office (ARO W911NF-11-1-0177). The work at University of North Texas was supported by National Institutes of Health (NIH R01EB12003, and 5R21CA14897 (Z.G.)).

  3. Extraordinary exciton conductance induced by strong coupling

    Feist, Johannes

    2014-01-01

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

  4. Nonlinear optical response of interacting excitons

    Keldysh, Leonid V.

    1995-02-01

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

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

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

  6. Pair spectra and exciton emission of sodium implanted ZnSe:Li

    The luminescence of ZnSe:Li was investigated after sodium implantation as a function of the doses and annealing conditions. At low temperature we observed emission due to bound excitons and donor-acceptors pair recombination. The spectra show a strong Na dependence similar for both Isup(Y1) and P-series band. (orig.)

  7. Method for recovering hydrocarbons from molten metal halides

    Pell, Melvyn B.

    1979-01-01

    In a process for hydrocracking heavy carbonaceous materials by contacting such carbonaceous materials with hydrogen in the presence of a molten metal halide catalyst to produce hydrocarbons having lower molecular weights and thereafter recovering the hydrocarbons so produced from the molten metal halide, an improvement comprising injecting into the spent molten metal halide, a liquid low-boiling hydrocarbon stream is disclosed.

  8. Harmonic dynamical behaviour of thallous halides

    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.

  9. Exciton-polariton emission from organic semiconductor optical waveguides

    Ellenbogen, Tal; Crozier, Kenneth B.

    2011-01-01

    We photo-excite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Non-resonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symm...

  10. The nature of singlet excitons in oligoacene molecular crystals

    Yamagata, H.

    2011-01-01

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

  11. Computational screening of mixed metal halide ammines

    Jensen, Peter Bjerre; Lysgaard, Steen; Quaade, Ulrich; Vegge, Tejs

    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...

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

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

    2005-01-01

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

  13. A quantitative analysis of excitonic superhyperchromism in porphyrin J-/H-aggregates

    Yıldırım, Hasan; İşeri, Erkut İnan; Gülen, Demet

    2004-06-01

    We argue that the mixing between energetically well-separated bands should have a magnified importance in the aggregates exhibiting an excitonic structure with a collective superstate, for which the J-/H-aggregates of porphyrins represent excellent examples. We present a quantitative account of the excitonic intensity borrowing in such systems and reveal a remarkably simple explanation of its enhancement: superhyperchromism is manifested by the mediation of the interband (Q-B) coupling between the two superstates (in Q and B) of such aggregates. The significance of the superhyperchromism is conveyed through its effects on the absorption spectrum of a natural `porphyrin J-aggregate' - the chlorosome.

  14. Quantum Memory for Photons in Case of Many Close Lying Exciton Resonances in Solids

    Gazazyan, A. D.; Gazazyan, E. A.; Margaryan, A. G.

    2008-01-01

    The possibility of storage of quantum information with photons is studied in the case of resonant transitions via many close lying exciton levels in a solid with impurity Lambda-atoms. The upper levels of the impurity atom form resonant Fano states, similar to the autoionization atomic states, due to the configuration interaction with the continuum of the exciton band. In this case slowing of light pulses is shown to be realistic, in the presence of the control field, down to the group veloci...

  15. Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

    Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B., E-mail: benito.alen@csic.es [IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid (Spain); Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A. [Institute for Systems based on Optoelectronics and Microtechnology (ISOM), Universidad Politécnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid (Spain)

    2015-11-02

    External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

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

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

  17. Exciton line broadening in ZnSexTe1-x/GaAs

    Photoreflection (PR) measurements on ZnSexTe1-x epilayers were performed in the concentration range x≤0.5 and 0.90 band gap on the composition leads to a bowing parameter of b 1.37±0.03 eV. Additional photoluminescence (PL) measurements were used to investigate the localization energy of excitons caused by compositional fluctuations. It is shown that the large broadening of excitons in the PR spectra can be explained by localization effects. (author)

  18. Excitons in organic multiple quantum well structures

    Agranovich, Vladimir M.; Atanasov, Rosen D.; Bassani, Giuseppe F.

    1992-11-01

    Multiple quantum well structures consisting of alternating layers of two crystalline organic semiconductors PTCDA and NTCDA are considered. It is shown that the blue-shift of the lowest energy PTCDA singlet exciton line with decreasing layer thickness is described quantitatively within the model of "small radius" excitons.

  19. Radiative recombination of excitons in amorphous semiconductors

    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

  20. DNA-mediated excitonic upconversion FRET switching

    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.

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

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

    2006-01-01

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

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

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

    1997-01-01

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

  3. Coherent spontaneous radiation of Frenkel excitons

    Andrianov, S.N.; Samartsev, V.V.; Sheibut, Yu.E.

    1988-02-01

    We have constructed a mathematical model that describes the coherent spontaneous radiation of a macroscopically filled exciton mode. We have demonstrated qualitatively ways in which a coherent subsystem of excitons can be formed. When this occurs under the influence of an external coherent source, exciton free induction occurs, while in the case of a spontaneous transition of the excitons to a coherent state through freezing of the reservoir of intermolecular interactions superradiance occurs. We have concluded that superradiance in the system of Frenkel excitons is possible under the influence of the non-Dicke mechanisms of self-induction of correlations, when the dipole transition moments of the individual atoms are coupled into a single macroscopic effective spin through their electrostatic interaction, this being analogous to the ordering of magnetic dipole moments in a ferromagnet.

  4. Vibration-Resistant Support for Halide Lamps

    Kiss, J.

    1987-01-01

    Lamp envelope protected against breakage. Old and new mounts for halide arc lamp sealed in housing with parabolic refector and quartz window. New version supports lamp with compliant garters instead of rigid brazed joint at top and dimensionally unstable finger stock at bottom.

  5. Structural study of praseodymium oxalate halide trihydrates

    The praseodymium oxalate halide trihydrates, PrC2O4X.3H2O, X = Cl and Br, crystallize in the monoclinic system with P21/n as the space group (Z = 4). The overall structure consists of layers parallel to the ac plane. The layers are formed by a network of two nearly perpendicular chains of Pr atoms linked by oxalate groups. The water molecules and halide ions reside between the layers and in the voids formed by the two Pr-oxalate chains. The main interaction between adjacent layers occurs by hydrogen bonding between water molecules and halide ions. The praseodymium atoms are coordinated to five oxygens from the two non-equivalent oxalate groups, to three oxygens from water molecules and to one halide. The coordination polyhedron is strongly distorted tricapped trigonal prism. One of the two oxalate groups acts as a hexadentate ligand while the other is only tetradentate. The thermal decomposition proceeds via anhydrous salt and corresponding oxyhalide PrOX to praseodymium oxide, Pr6O11. Several intermediate phases were observed but not identified with certainty. The relation between the thermal decomposition and the crystal structure was discussed

  6. Computational Screening of Mixed Metal Halide Ammines

    Jensen, Peter Bjerre; Lysgaard, Steen; Quaade, Ulrich; Vegge, Tejs

    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...

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

    Cowley, A.J.

    2011-01-01

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

  8. Dominant factors limiting the optical gain in layered two-dimensional halide perovskite thin films.

    Chong, Wee Kiang; Thirumal, Krishnamoorthy; Giovanni, David; Goh, Teck Wee; Liu, Xinfeng; Mathews, Nripan; Mhaisalkar, Subodh; Sum, Tze Chien

    2016-05-25

    Semiconductors are ubiquitous gain media for coherent light sources. Solution-processed three-dimensional (3D) halide perovskites (e.g., CH3NH3PbI3) with their outstanding room temperature optical gain properties are the latest members of this family. Their two-dimensional (2D) layered perovskite counterparts with natural multiple quantum well structures exhibit strong light-matter interactions and intense excitonic luminescence. However, despite such promising traits, there have been no reports on room temperature optical gain in 2D layered perovskites. Herein, we reveal the challenges towards achieving amplified spontaneous emission (ASE) in the archetypal (C6H5C2H4NH3)2PbI4 (or PEPI) system. Temperature-dependent transient spectroscopy uncovers the dominant free exciton trapping and bound biexciton formation pathways that compete effectively with biexcitonic gain. Phenomenological rate equation modeling predicts a large biexciton ASE threshold of ∼1.4 mJ cm(-2), which is beyond the damage threshold of these materials. Importantly, these findings would rationalize the difficulties in achieving optical gain in 2D perovskites and provide new insights and suggestions for overcoming these challenges. PMID:27184073

  9. Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots

    Pokutnyi, S. I., E-mail: pokutnyi-sergey@inbox.ru [National Academy of Sciences of Ukraine, Chuiko Institute of Surface Chemistry (Ukraine); Kulchin, Yu. N.; Dzyuba, V. P. [Russian Academy of Sciences, Institute of Automation and Control Processes, Far East Branch (Russian Federation)

    2015-10-15

    It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot)–(insulating matrix) interface) in a nanosystem containing insulating Al{sub 2}O{sub 3} quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al{sub 2}O{sub 3} single crystal. It is established that, in the band gap of an Al{sub 2}O{sub 3} nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al{sub 2}O{sub 3} nanoparticles at room temperature from the absorption spectrum of the nanosystem.

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

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

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

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

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

  13. Theoretical studies of excitons in type II CdSe/CdTe quantum dots

    We present a method for calculating exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k · p Hamiltonian of the zinc- blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

  14. Exciton absorption spectrum of thin Ag2ZnI4

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

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

    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.

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

    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

  17. Ultrafast Exciton Dynamics in CdxHg(1-x)Te alloy Quantum Dots

    Leontiadou, Marina A.; Al-Otaify, Ali; Kershaw, Stephen V.; Zhovtiuk, Olga; Kalytchuk, Sergii; Mott, Derrick; Maenosono, Shinya; Rogach, Andrey L.; Binks, David J.

    2016-05-01

    Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in CdxHg(1-x)Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling, the mono-exponential decay of which had a characteristic lifetime of 91 ± 1 ps and was attributed to biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between 1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for different pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be determined, and was 115 ± 1% for pump photons with energy equivalent to 2.6 times the band gap.

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

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

    2015-08-28

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

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

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

    2015-12-01

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

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

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

    2015-01-01

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

  1. Can Disorder Enhance Incoherent Exciton Diffusion?

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

    2015-07-30

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

  2. Quantum-dot excitons in nanostructured environments

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

    2011-01-01

    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......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...

  3. Quantum-dot excitons in nanostructured environments

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

    2010-01-01

    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......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...

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

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

    2016-01-28

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

  5. On Bose condensation of excitons in quasi-two-dimensional semiconductor heterostructures

    The review concerns on two semiconductor systems which exhibit the Bose condensation of excitons, namely, quantum wells with spatially indirect dipolar excitons and excitonic polaritons, in semiconductor microresonators.

  6. Optical nutation in the exciton range of spectrum

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

    2013-08-15

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

  7. Flame inhibition by hydrogen halides - Some spectroscopic measurements

    Lerner, N. R.; Cagliostro, D. E.

    1973-01-01

    The far-ultraviolet absorption spectrum of an air-propane diffusion flame inhibited with hydrogen halides has been studied. Plots of the absorption of light by hydrogen halides as a function of position in the flame and also as a function of the amount of hydrogen halide added to the flame have been obtained. The hydrogen halides are shown to be more stable on the fuel side of the reaction zone than they are on the air side. Thermal diffusion is seen to be important in determining the concentration distribution of the heavier hydrogen halides in diffusion flames. The relationship between the concentration distribution of the hydrogen halides in the flame and the flame inhibition mechanism is discussed.

  8. 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

    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.

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

    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

  10. Unraveling halide hydration: A high dilution approach

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

    2014-07-28

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

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

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

    2015-11-01

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

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

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

    2015-01-01

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

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

    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...

  14. Resonance Raman scattering and excitonic spectra in TlInS{sub 2} crystals

    Zalamai, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 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); 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); Ursaki, V.V. [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy Street, 2028 Chisinau, Republic of Moldova (Moldova, Republic of); Dorogan, V. [Technical University of Moldova, 168 Stefan cel Mare Avenue, 2004 Chisinau, Republic of Moldova (Moldova, Republic of)

    2015-04-15

    The excitons ground and excited states for E∥a and E∥b polarizations in absorption and reflection spectra of TlInS{sub 2} crystals were detected. The fundamental parameters of excitons and bands were determined at k=0. The resonance Raman spectra were investigated in the region of excitons transitions. The resonance Raman scattering spectra with participation of optical phonons that are active at the center of Brillouin zone were identified. The Raman scattering in Y(YX)Z and Y(ZX)Z geometries at 10 K with excitation by He–Ne laser was researched. Energies of phonons with A{sub g} and B{sub g} symmetries were determined. It was shown that the number of modes at 10 K was two times lower than expected according to theoretical calculations. - Highlights: • The resonance Raman scattering in geometry Y(YX)Z and Y(ZX)Z at 10 K was investigated. • Energies of phonons with A{sub g} and B{sub g} symmetries were determined. • The experimental and theoretical calculations completely conform if crystals are described by symmetry group D{sub 4h}{sup 15}. • The main parameters of excitons and bands were determined. • The model of electron transitions in k=0 was suggested.

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

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

  16. Resonance Raman scattering and excitonic spectra in TlInS2 crystals

    The excitons ground and excited states for E∥a and E∥b polarizations in absorption and reflection spectra of TlInS2 crystals were detected. The fundamental parameters of excitons and bands were determined at k=0. The resonance Raman spectra were investigated in the region of excitons transitions. The resonance Raman scattering spectra with participation of optical phonons that are active at the center of Brillouin zone were identified. The Raman scattering in Y(YX)Z and Y(ZX)Z geometries at 10 K with excitation by He–Ne laser was researched. Energies of phonons with Ag and Bg symmetries were determined. It was shown that the number of modes at 10 K was two times lower than expected according to theoretical calculations. - Highlights: • The resonance Raman scattering in geometry Y(YX)Z and Y(ZX)Z at 10 K was investigated. • Energies of phonons with Ag and Bg symmetries were determined. • The experimental and theoretical calculations completely conform if crystals are described by symmetry group D4h15. • The main parameters of excitons and bands were determined. • The model of electron transitions in k=0 was suggested

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

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

    2016-02-12

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

  18. Exciton entanglement in two coupled semiconductor microcrystallites

    Entanglement of the excitonic states in the system of two coupled semiconductor microcrystallites, whose sizes are much larger than the Bohr radius of the exciton in the bulk semiconductor but smaller than the relevant optical wavelength, is quantified in terms of the entropy of entanglement. It is observed that the nonlinear interaction between excitons increases the maximum values of the entropy of entanglement more than that of the linear coupling model. Therefore, a system of two coupled microcrystallites can be used as a good source of entanglement with fixed exciton number. The relationship between the entropy of entanglement and the population imbalance of two microcrystallites is numerically shown and the uppermost envelope function for them is estimated by applying the Jaynes principle

  19. Can disorder enhance incoherent exciton diffusion?

    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...

  20. Giant Rydberg Excitons in Cuprous Oxide

    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...

  1. Excitonic molecules in type-II superlattices

    Tsuchiya, T.; Katayama, S.; Ando, T.

    1998-01-01

    Excitonic molecules in GaAs/AlAs type-II superlattices are numerically investigated. In spite of large difference of electronic structures between type-II and type-I superlattices, variational calculations show that the configuration of particles is similar to that in type-I superlattices. This is because the layer width is smaller than the extent of excitonic wavefunctions in the direction parallel to the layers in the present superlattices.

  2. The Optical Activity of the Dark Exciton

    Don, Y.; Zielinski, M.; Gershoni, D.

    2016-01-01

    We present a phenomenological model to consider the effect of shape symmetry breaking on the optical properties of self-assembled quantum dots. We compare between quantum dots with two-fold rotational and two reflections ($C_{2v}$) symmetry and quantum dots in which this symmetry is reduced by perturbation to one reflection only ($C_{s}$). We show that this symmetry reduction drastically affects the optical activity of the dark exciton. In symmetric quantum dots, one of the dark exciton eigen...

  3. Ballistic spin transport in exciton gases

    Kavokin A.V.; Vladimirova M.; Jouault B.; Liew T.C.H.; Leonard J.R.; Butov L.V.

    2013-01-01

    Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They c...

  4. Exciton in closed and opened quantum dot

    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...

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

    Kawai, Kouji; Matsumoto, Masayuki

    1995-04-01

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

  6. Lanthanide doped strontium-barium cesium halide scintillators

    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.

  7. Excitonic nonlinearities in single-wall carbon nanotubes

    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)

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

    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.

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

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

  10. Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals.

    Volonakis, George; Filip, Marina R; Haghighirad, Amir Abbas; Sakai, Nobuya; Wenger, Bernard; Snaith, Henry J; Giustino, Feliciano

    2016-04-01

    Lead-based halide perovskites are emerging as the most promising class of materials for next-generation optoelectronics; however, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, perform structural refinement using single-crystal X-ray diffraction, and characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm3̅m space group and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-vis and photoluminescence measurements we obtain an indirect gap of 2.2 eV. PMID:26982118

  11. Exciton dephasing in ZnSe quantum wires

    Wagner, Hans Peter; Langbein, Wolfgang Werner; Hvam, Jørn Märcher; Bacher, G.; Kümmell, T.; Forchel, A.

    1998-01-01

    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......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...

  12. Excitonic polariton-polariton scattering in CdS crystals

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

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

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

    2013-02-15

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

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

    Yan, Yun-An

    2016-01-14

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

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

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

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

    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.

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

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

    1986-01-01

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

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

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

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

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

    2008-05-01

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

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

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

    2008-01-01

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

  1. Resonance Raman spectra of metal halide vapor complexes

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

  2. Nonlinear exciton transfer in protein helices

    We study the transfer of vibronic excitation energy in helical forms of proteins. The steric structure of the helix protein is modelled by a three-dimensional network of oscillators representing peptide groups. The covalent and hydrogen bonds between the peptide groups are described by pair interaction potentials. Each peptide group possesses one internal vibrational (excitonic) degree of freedom embodying the amide-I mode. The transfer dynamics of an amide-I exciton along the helix is expressed in terms of a tight-binding system. In the first part of this paper we study a reduced system arising when the vibrations of the covalent bonds are neglected. For the resulting system consisting of the exciton coupled to the hydrogen bond vibrations oriented along the helix axis we construct polaron solutions. Subsequently we investigate the mobility of the polarons within the complete protein matrix including deformations of the covalent bonds too. In particular we show that, during a phase of adaptation going along with internal energy exchange between the exciton and the bond vibrations, a relaxation into a new steady regime takes place. The newly reached equilibrium state is characterized by a localized exciton breather and is attributed local deformations of the steric peptide cage in the form of phonobreathers. Finally, coherent motion of an exciton breather is initiated through suitable injection of kinetic energy. In this way the long-range transfer of vibronic amide-I energy in the steric protein cage is provided. Interestingly, the α-helix possesses better facilities in supporting mobile localized excitons compared to the 3-10-helix form of proteins

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

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

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

    Frazer, Laszlo; Ketterson, J B

    2013-01-01

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

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

    Babkina A.N.

    2013-09-01

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

  6. Exciton magnetic polarons in Cd1-xMnxTe quantum wells

    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)

  7. Optical gain by self-trapped excitons in CdI2

    Cingolani, A.; Ferrara, M.; Lugará, M.; Avlijas, T.

    1984-08-01

    The stimulated emission in CdI2 at temperatures between 10 K and room temperature has been studied under N2-laser and dye-laser optical pumping. The observed spectra consist of only one broad band due to a strong saturation effect, but the unsaturated optical gain spectrum shows that the stimulated emission is due to the overlapping of four main amplified transitions. These transitions are highly consistent with the model of self-trapped excitons in ionic crystals.

  8. Excitonic Exchange Splitting and Radiative Lifetime in PbSe Quantum Dots

    An exciton evolving from an m-fold degenerate hole level and an n-fold degenerate electron level has a nominal m x n degeneracy, which is often removed by electron-hole interactions. In PbSe quantum dots, the degeneracy of the lowest-energy exciton is m x n = 64 because both the valence-band maximum and the conduction-band minimum originate from the 4-fold degenerate (8-fold including spin) L valleys in the Brillouin zone of bulk PbSe. Using a many-particle configuration-interaction approach based on atomistic single-particle wave functions, we have computed the fine structure of the lowest-energy excitonic manifold of two nearly spherical PbSe quantum dots of radius R = 15.3 and 30.6 (angstrom). We identify two main energy splittings, both of which are accessible to experimental probe: (i) The intervalley splitting is the energy difference between the two near-edge peaks of the absorption spectrum. We find (delta) = 80 meV for R = 15.3 (angstrom) and (delta) = 18 meV for R = 30.6 (angstrom). (ii) The exchange splitting Δx is the energy difference between the lowest-energy optically dark exciton state and the first optically bright exciton state. We find that Δx ranges between 17 meV for R = 15.3 (angstrom), and 2 meV for R = 30.6 (angstrom). We also find that the room-temperature radiative lifetime is τR ∼ 100 ns, considerably longer than the ∼10 ns radiative lifetime of CdSe dots, in quantitative agreement with experiment

  9. The coacervation of aqueous solutions of tetraalkylammonium halides

    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

  10. Metal halide reduction with molten sodium/potassium alloy

    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

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

    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

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

    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

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

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

    2014-11-21

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

  14. Radiation damage in the alkali halide crystals

    A general review is given of the experimental data on radiation damage in the alkali halide crystals. A report is presented of an experimental investigation of irradiation produced interstitial dislocation loops in NaCl. These loops are found to exhibit the usual growth and coarsening behaviour during thermal annealing which operates by a glide and self-climb mechanism. It is shown that the recombination of defects in these crystals is a two stage process, and that the loss of interstitials stabilized at the loops is caused by extrinsic vacancies. The theoretical techniques used in simulating point defects in ionic crystals are described. Shell model potentials are derived for all the alkali halide crystals by fitting to bulk crystal data. The fitting is supplemented by calculations of the repulsive second neighbour interactions using methods based on the simple electron gas model. The properties of intrinsic and substitutional impurity defects are calculated. The HADES computer program is used in all the defect calculations. Finally the report returns to the problems of irradiation produced interstitial defects. The properties of H centres are discussed; their structure, formation energies, trapping at impurities and dimerization. The structure, formation energies and mobility of the intermediate and final molecular defects are then discussed. The thermodynamics of interstitial loop formation is considered for all the alklai halide crystals. The nucleation of interstitial loops in NaCl and NaBr is discussed, and the recombination of interstitial and vacancy defects. The models are found to account for all the main features of the experimental data. (author)

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

    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

  16. Color separation in metal halide lamps

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

    2006-10-01

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

  17. Structural Characterization of Methanol Substituted Lanthanum Halides

    Boyle, Timothy J.; Ottley, Leigh Anna M.; Alam, Todd M.; Rodriguez, Mark A.; Yang, Pin; McIntyre, Sarah K.

    2010-01-01

    The first study into the alcohol solvation of lanthanum halide [LaX3] derivatives as a means to lower the processing temperature for the production of the LaBr3 scintillators was undertaken using methanol (MeOH). Initially the de-hydration of {[La(µ-Br)(H2O)7](Br)2}2 (1) was investigated through the simple room temperature dissolution of 1 in MeOH. The mixed solvate monomeric [La(H2O)7(MeOH)2](Br)3 (2) compound was isolated where the La metal center retains its original 9-coordination through...

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

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

    2016-02-10

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

  19. Absorption spectrum and excitons in Ag2CdI4 ionic conductor

    One studied electron spectrum of absorption of Ag2CdI4 thin films of stoichiometric composition prepared by thermal deposition in vacuum at the substrate quartz. The spectrum type is sensitive to the preparation procedure (temperature of substrate, rate of deposition). In case of the optimal preparation conditions, the films are free from AgI and CdI2 impurity and the boundary of the inherent absorption is near 3.28 eV. Long-wave exciton A-band at 3.31 eV (90 K) is associated with exciton excitation in the compound AgI-sublattice. When studying temperature behavior of the spectral position and A-band half width within 90-430 K range one has detected that at T ≤ 360 K the exciton-phonon interaction contributes mainly into the band widening, while at higher T values the contribution associated with generation of the Frenkel defects with U = 0.200 ± 0.025 eV activation energy

  20. Observation of Two-Exciton States in Perylene Bisimide Aggregates

    Lochbrunner S.; Seidel M.; Würthner F.; Wolter S.

    2013-01-01

    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.

  1. Observation of Two-Exciton States in Perylene Bisimide Aggregates

    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.

  2. Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides

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

    2015-03-01

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

  3. Dynamics of indirect exciton transport by moving acoustic fields

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

  4. Improved value for the silicon free exciton binding energy

    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.

  5. Electron-phonon coupling in hybrid lead halide perovskites.

    Wright, Adam D; Verdi, Carla; Milot, Rebecca L; Eperon, Giles E; Pérez-Osorio, Miguel A; Snaith, Henry J; Giustino, Feliciano; Johnston, Michael B; Herz, Laura M

    2016-01-01

    Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)2PbI3, HC(NH2)2PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Fröhlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Fröhlich coupling constants of ∼40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites. PMID:27225329

  6. A new fundamental hydrogen defect in alkali halides

    Atom hydrogen in neutral (H0) and negative (H-) form on substitutional and interstitial lattice sites gives rise to well characterized model defects in alkali-halides (U,U1,U2,U3 centers), which have been extensively investigated in the past. When studying the photo-decomposition of OH- defects, a new configuration of atomic charged hidrogen was discovered, which can be produced in large quantities in the crystal and is apparently not connected to any other impurity. This new hidrogen defect does not show any pronounced electronic absorption, but displays a single sharp local mode band (at 1114cm-1 in KCl) with a perfect isotope shift. The defect can be produced by various UV or X-ray techniques in crystais doped with OH-, Sh- or H- defects. A detailed study of its formation kinetics at low temperature shows that it is primarily formed by the reaction of a mobile CI2- crowdion (H-center) with hidrogen defects

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

    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$_2$O.

  8. Exciton states in GaAs δ-doped systems under magnetic fields and hydrostatic pressure

    Excitons in GaAs n-type δ-doped quantum wells are studied taking into account the effects of externally applied magnetic fields as well as of hydrostatic pressure. The one-dimensional potential profile in both the conduction and valence bands is described including Hartree effects via a Thomas–Fermi-based local density approximation. The allowed uncorrelated energy levels are calculated within the effective mass and envelope function approximations by means of an expansion over an orthogonal set of infinite well eigenfunctions and a variational method is used to obtain the exciton states. The results are presented as functions of the two-dimensional doping concentration and the magnetic field strength for zero and finite values of the hydrostatic pressure. In general, it is found that the exciton binding energy is a decreasing function of the doping-density and an increasing function of the magnetic field intensity. A comparison with recent experiments on exciton-related photoluminescence in n-type δ-doped GaAs is made

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

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

    2015-05-14

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

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

    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.

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

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

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

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

    2001-11-01

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

  13. Theory for resonant X-ray emission of core excitons with lattice relaxation: changeover from shallow to deep level

    A simple model is proposed for the X-ray emission of core excitons which is accompanied by a strong lattice relaxation. The effect of electron itinerancy in the conduction band is taken into account in addition to that of a local attractive potential and the lattice deformation. The X-ray absorption and the emission spectra are calculated exactly. It is shown that, as the lattice deformation goes on, the wave function of the exciton shrinks and the effective electron-phonon interaction changes from the weak coupling regime to the strong coupling regime

  14. Topological excitonic superfluids in three dimensions

    We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current.

  15. Self-trapped excitons in circular cacteriochlorophyll antenna complexes

    Fluorescence line narrowing and hole-burning spectroscopic studies of excitons in the LH2 pigment-protein complex, which is a part of the light harvesting system of purple bacteria, are combined with straightforward numerical simulations of the emission spectra based on exciton polaron model. The analysis provides evidence for self-trapping of all the excitons, except the lowest one

  16. Instantaneous Rayleigh scattering from excitons localized in monolayer islands

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

    2000-01-01

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

  17. Optical properties of localized excitons in semiconductor nanostructures

    Leosson, Kristjan; Hvam, Jørn Märcher; Langbein, Wolfgang Werner; Østergaard, John Erland

    2002-01-01

    excitoner for at samle information om exciton overgangenes homogene liniebredde. Excitoner, der er indesluttet i selv-dannede kvantepunkter er også blevet studeret. Mikro-fotoluminescens spektroskopi på små ensembler af kvantepunkter afslører en fordeling af fotoluminescens liniebredder, som er tilskrevet...

  18. Exciton dynamics in perturbed vibronic molecular aggregates.

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

    2016-07-01

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

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

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

    2016-02-01

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

  20. Core exciton migration in Rb{sub 0.82}Cs{sub 0.18}Cl under excitation with synchrotron radiation and laser light

    Tsujibayashi, Toru [Department of Physics, Osaka Dental University, 8-1 Kuzuha-hanazono, Hirakata, Osaka 573-1121 (Japan)]. E-mail: toru-t@cc.osaka-dent.ac.jp; Azuma, Junpei [Synchrotron Light Application Center, Saga University, Saga 840-8502 (Japan); Inabe, Yoshiyuki [Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553 (Japan); Toyoda, Koichi [Department of Physics, Osaka Dental University, 8-1 Kuzuha-hanazono, Hirakata, Osaka 573-1121 (Japan); Kamada, Masao [Synchrotron Light Application Center, Saga University, Saga 840-8502 (Japan); Itoh, Minoru [Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553 (Japan)

    2007-01-15

    The migration of the core exciton in a mixed crystal of Rb{sub 0.82}Cs{sub 0.18}Cl is investigated through time-resolved measurement under excitation with synchrotron radiation (SR) and laser. The photon energy of SR is tuned to the absorption band due to the exciton composed of a conduction electron and the hole originated from the outermost core state of the Rb ion (Rb-core exciton). The time-integrated intensity of Auger-free luminescence (AFL) due to the outermost core state of the Cs ion is increased by the laser irradiation. The lifetime of the laser-induced AFL depends on the photon energy of SR. The experimental result suggests the difference between the migration length of the Rb-core exciton and that of the Cs-core hole.

  1. Core exciton migration in Rb0.82Cs0.18Cl under excitation with synchrotron radiation and laser light

    The migration of the core exciton in a mixed crystal of Rb0.82Cs0.18Cl is investigated through time-resolved measurement under excitation with synchrotron radiation (SR) and laser. The photon energy of SR is tuned to the absorption band due to the exciton composed of a conduction electron and the hole originated from the outermost core state of the Rb ion (Rb-core exciton). The time-integrated intensity of Auger-free luminescence (AFL) due to the outermost core state of the Cs ion is increased by the laser irradiation. The lifetime of the laser-induced AFL depends on the photon energy of SR. The experimental result suggests the difference between the migration length of the Rb-core exciton and that of the Cs-core hole

  2. Compensation of the exciton-ion exchange interaction in a quantum dot by application of a magnetic field

    Malinowski, F. K.; Smoleński, T.; Goryca, M.; Koperski, M.; Wojnar, P.; Kossacki, P.

    2014-08-01

    The exchange interaction between band carriers and electrons from the d-shell of a single magnetic ion embedded in a semiconductor quantum dot leads to the splitting of exciton-ion states which allows to optically read the ion spin state. The influence of this interaction on ion states can be described in terms of an effective magnetic field induced by the exciton. It means that the s,p\\text{-}d exchange interaction can be partially compensated by the application of a magnetic field along the growth axis. We study this effect for CdTe/ZnTe quantum dots with a single Mn2+ ion. The in-plane component added to the perpendicular magnetic field induces mixing of different ion spin states, resulting in multiple anticrossings of the excitonic photoluminescence lines of the dot. This enables detailed investigation of the compensation.

  3. Exciton decay dynamics controlled by impurity occupation in strongly Mn-doped and partially compensated bulk GaAs

    Münzhuber, F.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.; Gieseking, B.; Astakhov, G. V.; Dyakonov, V.

    2014-09-01

    We report on a pronounced prolongation of the exciton decay in strongly p-doped and partially compensated direct band-gap semiconductor GaAs:Mn with increasing optical excitation power. Using time-resolved photoluminescence we show that the intricate interplay of excitons, shallow and deep impurity centers in GaAs:Mn results in a complex recombination behavior that cannot be characterized in terms of simple rates. The decay can be precisely described by a model based on Shockley-Read-Hall recombination, which shows that the observed dynamics arise from a varying neutralization of shallow and deep recombination centers. This enables the investigation of the carrier dynamics in the impurity system by measuring only the exciton decay time.

  4. Mechanoluminescence response to the plastic flow of coloured alkali halide crystals

    Chandra, B.P., E-mail: bpchandra4@yahoo.co.i [Shri Shankaracharya College of Engineering and Technology, Junwani, Bhilai (C.G.) 490020 (India); Bagri, A.K. [Department of Postgraduate Studies and Research in Physics, Rani Durgawati University, Jabalpur 482001 (India); Chandra, V.K. [Department of Electrical and Electronics Engineering, Chhatrapati Shivaji Institute of Technology, Shivaji Nagar, Kolihapuri, Durg (C.G.) 491001 (India)

    2010-02-15

    The present paper reports the luminescence induced by plastic deformation of coloured alkali halide crystals using pressure steps. When pressure is applied onto a gamma-irradiated alkali halide crystal, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value and later on it decreases with time. The ML of diminished intensity also appears during the release of applied pressure. The intensity I{sub m} corresponding to the peak of ML intensity versus time curve and the total ML intensity I{sub T} increase with increase in value of the applied pressure. The time t{sub m} corresponding to the ML peak slightly decreases with the applied pressure. After t{sub m}, initially the ML intensity decreases at a fast rate and later on it decreases at a slow rate. The decay time of the fast decrease in the ML intensity is equal to the pinning time of dislocations and the decay time for the slow decrease of ML intensity is equal to the diffusion time of holes towards the F-centres. The ML intensity increases with the density of F-centres and it is optimum for a particular temperature of the crystals. The ML spectra of coloured alkali halide crystals are similar to the thermoluminescence and afterglow spectra. The peak ML intensity and the total ML intensity increase drastically with the applied pressure following power law, whereby the pressure dependence of the ML intensity is related to the work-hardening exponent of the crystals. The ML also appears during the release of the applied pressure because of the movement of dislocation segments and movements of dislocation lines blocked under pressed condition. On the basis of the model based on the mechanical interaction between dislocation and F-centres, expressions are derived for the ML intensity, which are able to explain different characteristics of the ML. From the measurements of the plastico ML induced by the application of loads on gamma-irradiated alkali halide crystals, the pinning time of dislocations, diffusion time of holes towards F-centres, the energy gap E{sub a} between the bottom of acceptor dislocation band and the energy level of interacting F-centres, and work-hardening exponent of the crystals can be determined. As in the elastic region the strain increases linearly with stress, the ML intensity also increases linearly with stress, however, as in the plastic region, the strain increases drastically with stress and follows power law, the ML intensity also increases drastically with stress and follows power law. Thus, the ML is intimately related to the plastic flow of alkali halide crystals.

  5. Scattering process between polaron and exciton in conjugated polymers

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

    2011-01-01

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

  6. Nonboson treatment of excitonic nonlinearity in optically excited media

    The present article shortly reviews some recent results in the study of excitonic nonlinearity in optically excited media using a nonboson treatment for many-exciton systems. After a brief discussion of the exciton nonbosonity the closed commutation relations are given for exciton operators which hold for any exciton density and type. The nonboson treatment is then applied to the problems of intrinsic optical bistability and nonlinear polariton yielding quite interesting and new effects, e.g. new shapes of hysteresis loops of intrinsic optical bistability or anomalies of polariton dispersion. (author). 71 refs, 4 figs

  7. Inversion of exciton level splitting in quantum dots

    Young, R J; Shields, A J; Atkinson, P; Cooper, K; Ritchie, D A; Groom, K M; Tartakovskii, A I; Skolnick, M S

    2005-01-01

    The demonstration of degeneracy of the exciton spin states is an important step towards the production of entangled photons pairs from the biexciton cascade. We measure the fine structure of exciton and biexciton states for a large number of single InAs quantum dots in a GaAs matrix; the energetic splitting of the horizontally and vertically polarised components of the exciton doublet is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Thermal annealing is shown to reduce the exciton confinement, thereby increasing the number of dots with splitting close to zero.

  8. Anion exchange extraction of cadmium halide complexes by triphenylnonylphosphonium salts

    Using the method of intermediate exchange, anion exchange extraction of cadmium halide complexes by triphenylnonylphosphonium salts has been studied. At 293 K and different ratios of components thermodynamic exchange constants of cadmium halide complexes for anion of certain inorganic acids are calculated. The highest values of the constants are observed in case of hydrophilic gegenions, the lowest ones - in case of hydrophobic gegenions. A high affinity of cadmium halide complexes to anionite can be used to develop methods of cadmium isolation, concentration and determination

  9. Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

    We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films

  10. Excitons in van der Waals heterostructures

    Latini, Simone; Olsen, Thomas; Thygesen, Kristian Sommer

    2015-01-01

    -dimensional (2D) excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model, which assumes a dielectric function of the form epsilon(q) = 1 + 2 pi alpha q, and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons...... calculate exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model, and both are in good agreement with ab initio many-body calculations. On the other hand, for more complex...... structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function, and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless...

  11. Effective models for excitons in carbon nanotubes

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

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

  12. Effective models for excitons in carbon nanotubes

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    2007-01-01

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

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

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

    2014-01-01

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

  14. Coupled exciton-photon Bose condensate in path integral formalism

    Elistratov, A. A.; Lozovik, Yu. E.

    2016-03-01

    We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use a two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of transformation from the exciton-photon basis to the lower-upper polariton basis, and obtain the exciton and photon occupation numbers of the lower and upper polariton branches for nonzero temperatures. For an inhomogeneous system, the set of coupled equations of the Bogoliubov-de Gennes type is derived. The equations govern the spectra and spatial distributions of noncondensate photons and excitons.

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

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

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

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

    1978-06-01

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

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

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

    1992-11-01

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

  18. Study of Ammonium Halides by Neutron Spectrometry

    The scattering of slow neutrons from the ammonium halides NH4Cl, ND4Cl, and NH4Br has been studied at several temperatures with a view to elucidate the nature of the specific heat anomaly associated with these substances. According to Pauling the anomaly is due to the onset of free rotation of the ammonium ion, whereas according to Frenkel the anomaly is due to an order-disorder process. The present measurements rule out the possibility of free rotation thus confirming the Frenkel hypothesis. Values of the torsional frequencies and barrier heights for rotation have been derived and the relation of neutron scattering data to infra-red absorption. Raman scattering and NMR results are discussed. (author)

  19. Thermoluminescence of alkali halides and its implications

    Gartia, R. K.; Rey, L.; Tejkumar Singh, Th.; Basanta Singh, Th.

    2012-03-01

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

  20. Thermoluminescence of alkali halides and its implications

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

  1. Optical property of the near band-edge transitions in rhenium disulfide and diselenide

    An optical study of the band-edge property of ReX2 (X=S, Se) has been carried out using the techniques of transmittance, photoreflectance (PR), and piezoreflectance (PzR). The polarized transmittance measurements indicate that the absorption edge of E parallel b polarization presents a significant red-shift behavior with respect to that of the E perpendicular b polarization. Band-edge excitons E1ex and E2ex for ReX2 are characterized using polarized PzR measurements. The polarization dependence of E1ex and E2ex provides conclusive evidence that the band-edge excitons are interband excitonic transitions originated from different origins. In addition higher energy excitonic series in ReX2 layered compounds is also studied using low-temperature PR measurements at 25 K. Prominent and enlarged excitonic features positioned at higher energy side with respect to the E1ex and E2ex are observed in the PR spectra. The observed excitonic sequence has been shown to be corresponding to the Rydberg series starting with principal quantum number n=2. From the experimental results together with the theoretical density-of-states calculations, the near band-edge property of rhenium disulfide (ReS2) and diselenide (ReSe2) are characterized and discussed

  2. High temperature reactions between molybdenum and metal halides

    Good colour rendering properties, high intensity and efficacy are of vital importance for high-end lighting applications. These requirements can be achieved by high intensity discharge lamps doped with different metal halide additives (metal halide lamps). To improve their reliability, it is very important to understand the different failure processes of the lamps. In this paper, the corrosion reactions between different metal halides and the molybdenum electrical feed-through electrode are discussed. The reactions were studied in the feed-through of real lamps and on model samples too. X-ray photoelectron spectroscopy (XPS) was used to establish the chemical states. In case of the model samples we have also used atomic absorption spectroscopy (AAS) to measure the reaction product amounts. Based on the measurement results we were able to determine the most corrosive metal halide components and to understand the mechanism of the reactions

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

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

    2015-10-01

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

  4. Excitonic luminescence of ZnSe single crystals doped with Au

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

  5. Theoretical study of exciton dissociation through hot states at donor-acceptor interface in organic photocell.

    Shimazaki, Tomomi; Nakajima, Takahito

    2015-05-21

    We theoretically study the dissociation of geminate electron-hole pairs (i.e., excitons) through vibrational hot states at the donor-acceptor interface of organic photocells. To conduct this, we modify the formalism of Rubel et al. [Phys. Rev. Lett., 2008, 100, 196602], and use the theoretical concepts of Arkhipov et al. [Phys. Rev. Lett., 1999, 82, 1321] and Knights et al. [J. Phys. Chem. Sol., 1974, 35, 543] to consider vibrational hot states. The effects of band-offset energy and the dissipation of excess energy are discussed on the basis of calculations of the escape probability for hot electrons. Furthermore, we show that vibrational hot state and delocalization of excitons lead to an increased probability to separate geminate electron-hole pairs. PMID:25898910

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

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

    2014-03-17

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

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

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

    1988-08-11

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

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

    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)

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

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

    2016-02-10

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

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

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

    2016-02-01

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

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

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

    2015-12-01

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

  12. The role of the fano resonance in multiple exciton generation in quantum dots

    Oksengendler, B. L.; Marasulov, M. B.; Nikiforov, V. N.

    2016-02-01

    The phenomenon of interference between two pathways of electron transfer from the valence to the conduction band at a quantum dot is considered. The first way is the conventional "valence band-conduction band" transition, while the second is the transition via a virtual two-electron state on the Tamm level in a quantum dot (QD) followed by the Auger effect, which ejects one electron from the Tamm level to the conduction band. In the case of a coherent addition of these ionization pathways, the Fano resonance can take place, this leading to an increase in the coefficient of photon absorption. This results in increasing internal efficiency of light conversion and can provide a basis for increasing the efficiency of solar cells due to the phenomenon of multiple exciton generation.

  13. Quasiparticle energies, excitonic effects, and dielectric screening in transparent conducting oxides

    Schleife, André

    Using the power of high-performance super computers, computational materials scientists nowadays employ highly accurate quantum-mechanical approaches to reliably predict materials properties. In particular, many-body perturbation theory is an excellent framework for performing theoretical spectroscopy on novel materials including transparent conducting oxides, since this framework accurately describes quasiparticle and excitonic effects.We recently used hybrid exchange-correlation functionals and an efficient implementation of the Bethe-Salpeter approach to investigate several important transparent conducting oxides. Despite their exceptional potential for applications in photovoltaics and optoelectronics their optical properties oftentimes remain poorly understood: Our calculations explain the optical spectrum of bixbyite indium oxide over a very large photon energy range, which allows us to discuss the importance of quasiparticle and excitonic effects at low photon energies around the absorption onset, but also for excitations up to 40 eV. We show that in this regime the energy dependence of the electronic self energy cannot be neglected. Furthermore, we investigated the influence of excitonic effects on optical absorption for lanthanum-aluminum oxide and hafnium oxide. Their complicated conduction band structures require an accurate description of quasiparticle energies and we find that for these strongly polar materials, a contribution of the lattice polarizability to dielectric screening needs to be taken into account. We discuss how this affects the electron-hole interaction and find a strong influence on excitonic effects.The deep understanding of electronic excitations that can be obtained using these modern first-principles techniques, eventually will allow for computational materials design, e.g. of band gaps, densities of states, and optical properties of transparent conducting oxides and other materials with societally important applications.

  14. Coherent two-dimensional electronic spectroscopy in the Soret band of a chiral porphyrin dimer

    Using coherent two-dimensional (2D) electronic spectroscopy in fully noncollinear geometry, we observe the excitonic coupling of β,β′-linked bis[tetraphenylporphyrinato-zinc(II)] on an ultrafast timescale in the excited state. The results for two states in the Soret band originating from an excitonic splitting are explained by population transfer with approximately 100 fs from the energetically higher to the lower excitonic state. This interpretation is consistent with exemplary calculations of 2D spectra for a model four-level system with coupling. (paper)

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

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

    2012-12-15

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

  16. Excitonic and photonic processes in materials

    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.

  17. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    Olšina, Jan; Wang, Chen; 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 by the non-canonical thermal distribution resulting from the entanglement with the phonon bath. The decay of the excited state population to the common ground state establishes a non-equilibrium steady-state flux driven by the sunlight, and it defines a time window to observe the transition from dynamical to static coherence. For the parameters relevant to photosynthetic systems, the exciton dynamics initiated by the sunlight exhibits a non-negligible amount of dynamical coherence (quantum beats) on the sub-picosecond timescale; however, ...

  18. Microcavity controlled coupling of excitonic qubits

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

    2012-01-01

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

  19. High Pressure Electrochemistry: Application to silver halides

    Havens, K.; Kavner, A.

    2007-12-01

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

  20. Can Natural Sunlight Induce Coherent Exciton Dynamics?

    Olšina, Jan; Dijkstra, Arend G.; Wang, Chen; 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 by the non-canoni...

  1. Decoherence in Weakly Coupled Excitonic Complexes

    Mancal, Tomas; Balevicius Jr., Vytautas; Valkunas, Leonas

    2010-01-01

    Equations of motion for weakly coupled excitonic complexes are derived. The description allows to treat the system in the basis of electronic states localized on individual chromophores, while at the same time accounting for experimentally observable delocalization effects in optical spectra. The equations are show to be related to the well-known Foerster type energy transfer rate equations, but unlike Foerster equations, they provide a description of the decoherence processes leading to supp...

  2. Observation of a Dark Condensate of Excitons

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

    2015-01-01

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

  3. Properties of Excitons Bound to Ionized Donors

    Skettrup, Torben; Suffczynski, M.; Gorzkowski, W.

    1971-01-01

    Binding energies, interparticle distances, oscillator strengths, and exchange corrections are calculated for the three-particle complex corresponding to an exciton bound to an ionized donor. The results are given as functions of the mass ratio of the electron and hole. Binding of the complex is o...... results are applied to CdS and ZnO and compared with experimental results on these....

  4. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    Kenny, Elise P.; Kassal, Ivan

    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 ...

  5. Excitonic ring formation in ultrapure bulk GaAs

    Bieker, S.; Henn, T.; Kiessling, T.; Ossau, W.; Molenkamp, L. W.

    2014-11-01

    We report on spatially resolved low-temperature photoluminescence (PL) measurements of excitons in ultrapure bulk GaAs. At moderate excitation densities we observe butterfly-shaped luminescence images in the wavelength-radial distance plane with a pronounced quench of the exciton PL intensity at the excitation center. The shapes of the PL images show a delicate dependence on excitation wavelength and pump power. We present a model that quantitatively explains the PL intensity quench by a localized overheating of the exciton ensemble due to nonresonant optical excitation. Our model allows us to extract absolute exciton temperatures and to trace the influence of excitation excess energy on the spatial dependence of the exciton energy relaxation. We observe temperature gradients in the exciton system which persist over distances ≥10 μ m away from the excitation spot.

  6. Singlet Exciton Fission in Nanostructured Organic Solar Cells

    Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc

    2011-04-13

    Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.

  7. Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers

    Ying, L.; Sie, X.S. [Pacific Northwest National Lab., Richland, WA (United States). William R. Wiley Environmental Molecular Sciences Lab.

    1998-12-10

    The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.

  8. Surface influence on the exciton characteristics of semiconductors

    Litovchenko, P G; Korbutyak, D V; Sarikov, A V

    2002-01-01

    An investigation of the influence of surface treatment on basic exciton characteristics in the sub-surface region of the semiconductor (for GaAs) and spatial distribution of the basic exciton characteristics (for CdS) is carried out. On the grounds of an analysis of the experimental data, it has been shown that deposition of dielectric layers with smaller permittivity on the semiconductor surface leads to the enhancement of the exciton-photon coupling and to the exciton binding energy rise. Occurrence on surface of a defect-enriched layer, of which the imperfection increases after certain surface treatments, results in decrease of the exciton binding energy alongside with the exciton-phonon coupling factor

  9. Modulational instability and solitons in excitonic semiconductor waveguides

    Smyrnov, Oleksii A; Malzer, Stefan

    2011-01-01

    Nonlinear light propagation in a single-mode micron-size waveguide made of semiconducting excitonic material has been theoretically studied in terms of exciton-polaritons by using an analysis based on macroscopic fields. When a light pulse is spectrally centered in the vicinity of the ground-state Wannier exciton resonance, it interacts with the medium nonlinearly. This optical cubic nonlinearity is caused by the repulsive exciton-exciton interactions in the semiconductor, and at resonance it is orders of magnitude larger than the Kerr nonlinearity (e.g., in silica). We demonstrate that a very strong and unconventional modulational instability takes place, which has not been previously reported. After reducing the problem to a single nonlinear Schr\\"odinger-like equation, we also explore the formation of solitary waves both inside and outside the polaritonic gap and find evidence of spectral broadening. A realistic physical model of the excitonic waveguide structure is proposed.

  10. Surface exciton polariton spectrum in semiconductor superlattices

    It is reported a surface exciton polarization spectrum in superlattices made up of two alternating media A and B, where one of the layers shows spatial dispersion in its frequency dependent dielectric function. We have used a theoretical approach based on the effective medium theory to determine the surface exciton polariton dispersion relation. The inclusion of an external magnetic field parallel to the interfaces of the superlattice is also considered, and it leads to the so-called magneto plasmon-polariton excitations. It is shown that, in the absence of the external magnetic field, the surface modes are doubly degenerate, corresponding to propagation in opposite directions for a given magnitude of the wavenumber Kx. However, in the presence of the external magnetic field, this degeneracy is removed. We have also calculated the so-called Attenuated Total Reflectivity (ATR) to probe the surface exciton polariton modes. The effective-medium description has been widely used in ATR spectroscopy of semiconductor superlattices, and it generally gives a good account of the data together with a clear physical interpretation of the various spectral features. (author). 22 refs., 2 figs

  11. Fractional Solitons in Excitonic Josephson Junctions.

    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 ϕ0 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 = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770

  12. Fractional Solitons in Excitonic Josephson Junctions

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-10-01

    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 ϕ0 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 = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.

  13. Exciton dynamics at oligothiophene / Au(111) interfaces

    Varene, Erwan; Martin, Isabel; Bronner, Christopher; Bogner, Lea; Tegeder, Petra [Freie Universitaet Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin (Germany)

    2011-07-01

    Semiconducting materials based on organic molecules or polymers are promising candidates for applications in electronic devices such as organic photovoltaic cells. The electronic structure and carrier dynamics at interfaces between organic semiconductors and inorganic substrates are the most fundamental issues in order to understand the functionalities of organic films. Time-resolved two-photon photoemission (2PPE) spectroscopy is employed to determine the electronic structure, charge carrier dynamics and energetics at the sexithiophene (6T)/Au(111) interface. We found the HOMO and HOMO-1 to be located at -0.8 and -1.5 eV, respectively and two 6T-derived unoccupied states at 2.1 eV and 3.0 eV with respect to the Fermi level which we assign to the LUMO and LUMO+1. In addition an exciton state possessing a binding energy of 0.9 eV is observed. The timescale for the exciton breakup exhibits a strong 6T coverage dependency, i.e., the exciton lifetime increases with increasing coverage. For instance at a coverage 12 ML the decay times are around 700 fs for the fast component and 5 ps for the slow component.

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

    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.

  15. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-01-01

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281

  16. Lattice exciton-polaron problem by quantum Monte Carlo simulations

    Hohenadler, Martin; Littlewood, Peter B; Fehske, Holger

    2007-01-01

    Exciton-polaron formation in one-dimensional lattice models with short or long-range carrier-phonon interaction is studied by quantum Monte Carlo simulations. Depending on the relative sign of electron and hole-phonon coupling, the exciton-polaron size increases or decreases with increasing interaction strength. Quantum phonon fluctuations determine the (exciton-)polaron size and yield translation-invariant states at all finite couplings.

  17. Photoluminescence of high-density exciton-polariton condensates

    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 photon energy. An intui...

  18. Exciton resonances quench the photoluminescence of zigzag carbon nanotubes

    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 luminescence. Our ...

  19. Polarization dynamics in quantum dots: The role of dark excitons

    Tsitsishvili, E.; Kalt, H.

    2014-01-01

    We study an impact of the fine structure of the heavy--hole ground state exciton confined in semiconductor quantum dots on the photoluminescence polarization dynamics solving the relevant system of the rate equations. The presence of the dark excitons is usually ignored and the polarization decay is assumed to be caused by direct transitions within the radiative doublet. We demonstrate that in strongly confined quantum dots the dark excitons, which are energetically well below the bright exci...

  20. One-dimensional excitons in V-shaped quantum wires

    Rinaldi, R.; Cingolani, R.; Lepore, M.; Ferrara, M.; Catalano, I. M.; Marti, U.; Martin, D.; Morier-Gemoud, U.; Ruterana, P.; Reinhart, F. K.

    We report a detailed study of one-dimensional excitons in a planar array of single V-shaped GaAs quantum wires. Two-photon absorption, magnetoluminescence and linear photoluminescence spectroscopy have been used to measure the exciton binding energy, the excited 2 p states of the excitons, the higher index transitions and the extension of the confined wavefunctions in the V-shaped region of the quantum wires.

  1. Density Functional Study of Tetraphenylporphyrin Long-Range Exciton Coupling

    Moore, Barry; Autschbach, Jochen

    2012-01-01

    The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intens...

  2. Excitonic Properties in GaAs Parabolic Quantum Dots

    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.

  3. Exciton-polaritons by two-photon absorption in semiconductors

    The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semiclassical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment. (author)

  4. Polarization-dependent exciton dynamics in tetracene single crystals

    Zhang, Bo; Zhang, Chunfeng, E-mail: cfzhang@nju.edu.cn; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong [National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093 (China); Xiao, Min, E-mail: mxiao@uark.edu [National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093 (China); Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 (United States)

    2014-12-28

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

  5. Revealing and Characterizing Dark Excitons Through Coherent Multidimensional Spectroscopy

    Tollerud, Jonathan O; Davis, Jeffrey A

    2016-01-01

    Dark excitons are of fundamental importance in a broad range of contexts, but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify different types of dark excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms and coupling strengths. The observations of coherent coupling between bright and dark excitons hint at a role for a multi-step process by which excitons in the barrier can relax into the quantum wells.

  6. Free exciton luminescence in cubic silicon carbide crystals

    Photoluminescence (PL) spectra of cubic silicon carbide crystals are investigated in the 4.2-300 K temperature range. The free exciton PL with absorption of one and emission of two or three phonons is found. The selection rules for exciton-phonon transitions with participation of one or the two phonons are calculated. Th interpretation of the two-phonon PL spectrum of free excitons is given. The structure in the free excitons luminesce spectrum related to spin-orbital splitting of the hole states is found

  7. Surface trapping of excitons in rare-gas solids

    Ejected-atom kinetic energy spectra from electronically excited Ar, Kr, and Xe have been interpreted in terms of two mechanisms: the first due to exciton decay in the surface layer and the second due to exciton decay in the subsurface layers. We use the absolute ejection yields for the surface component to determine the efficiency of the surface layer in trapping excitons produced by incident energetic ions and electrons. Estimates of the ratio of surface-to-bulk exciton trapping efficiencies are 4.7, 6.5, and 3.9 for Ar, Kr and Xe, respectively, at 15 K

  8. Localization length scales of triplet excitons in singlet fission materials

    Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, Hlne; Greenham, Neil C.; Chepelianskii, Alexei D.

    2015-09-01

    We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.

  9. Excitonic fine structure splitting in ZnTe/ZnX (X = S and Se) core/shell nanocrystals: Atomistic tight-binding theory

    Sukkabot, Worasak

    2016-03-01

    Implementing the atomistic tight-binding theory in the conjunction with a configuration interaction method of coulomb and exchange description, the excitonic fine structure splitting (FSS) in core/shell semiconductor nanocrystals is usually caused by the intrinsic electron-hole exchange interaction. I demonstrate the control of the excitonic fine structure splitting by suitably engineering the type of the band alignments and the thickness of the growth shell. ZnTe/ZnS and ZnTe/ZnSe core/shell nanocrystals exhibiting the type-I and type-II band profile are used to be the simulated candidates with various growth shell thicknesses. The detailed calculations, consisting of single-particle spectra, optical band gaps, ground-state wave function overlaps, ground-state oscillation strengths, ground-state coulomb energies, ground-state exchange energies and excitonic splitting energies, are all sensitive with the type and dimension of the coated shells. The simulations highlight that ZnTe/ZnSe type-II core/shell nanocrystals with the thick growth shell can offer an intensely reduced excitonic splitting as defined by an associated reduction of electron-hole exchange interaction. This insight is important for the theoretical understanding and practical control by the type of the band alignments and sizes in the growth shell for the quantum state of the emitted light from a biexciton cascade recombination process which will be implemented to the entangled photon source in the novel application of quantum information processing.

  10. Recombination dynamics of exciton and exciton complexes in single quantum dots

    We have studied the relaxation dynamics of excitons and exciton complexes in a single quantum dot using time-resolved photoluminescence. To avoid any interference of carrier diffusion and trapping we have excited the dot with energies below that of the wetting layer. The times extracted from a quantitative analysis are thus related only to the relaxation of carriers inside the dot. Since the dot is rather large it is possible to observe the recombination of carriers from different atomic shells. We observe a retardation of the carrier relaxation increasing excitation power due to the large number of charges filling the dot.

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

    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. -- Highlights: ► New semi-empirical dipole moment functions (DMF) of HF, HCl, HBr and HI were derived. ► Fit employed experimental ro-vibrational intensities with their uncertainties. ► New DMFs can predict intensities over extensive range of internuclear distances

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

    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 such as thermos-solutal convection driven forces can be controlled in microgravity environments to utilize the benefits of these unique materials.

  13. Evaluation of approximations in molecular exciton theory. 2. Applications to oligomeric systems of interest in photosynthesis

    LaLonde, D.E.; Petke, J.D.; Maggiora, G.M. (Univ. of Kansas, Lawrence (USA))

    1989-01-26

    Molecular exciton calculations have been performed on bacteriochlorin and methyl bacteriopheophorbide a (Me-BPheo-a) oligomers in one-dimensional stacked geometries, in an investigation of spectral shifts and oscillator strengths of the X{sub y} band. Exciton matrix elements were evaluated by using both the point dipole and Shipman distributed point-charge approximations of monomeric electron densities, obtained from floating-Gaussian-based SCF-MO-CI monomer wave functions. Calculations performed with the point-dipole approximation produced unacceptable errors in both Q{sub y} spectral shifts and oscillator strengths, relative to those obtained with the more rigorous point-charge approximation. For both approximations, calculations in which only nearest-neighbor intermolecular interactions were considered significantly underestimated the spectral shifts obtained with the inclusion of all interactions. However, neglect of interactions between molecules separated by >20 {angstrom} did not significantly affect the values of calculated spectral shifts. The present point-charge-based calculations on Me-BPheo-a oligomers indicate that the method appears to account for approximately 50-60% of the Q{sub y} red shift observed in crystals. Thus, additional factors such as the use of higher quality monomeric wave functions in the exciton treatment, as well as consideration of charge-transfer and charge redistribution effects, appear necessary to obtain accurate spectral shifts.

  14. Exciton trapping in vibrationally excited organic molecules near a ZnO surface

    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...

  15. Process for oxidation of hydrogen halides to elemental halogens

    Lyke, Stephen E.

    1992-01-01

    An improved process for generating an elemental halogen selected from chlorine, bromine or iodine, from a corresponding hydrogen halide by absorbing a molten salt mixture, which includes sulfur, alkali metals and oxygen with a sulfur to metal molar ratio between 0.9 and 1.1 and includes a dissolved oxygen compound capable of reacting with hydrogen halide to produce elemental halogen, into a porous, relatively inert substrate to produce a substrate-supported salt mixture. Thereafter, the substrate-supported salt mixture is contacted (stage 1) with a hydrogen halide while maintaining the substrate-supported salt mixture during the contacting at an elevated temperature sufficient to sustain a reaction between the oxygen compound and the hydrogen halide to produce a gaseous elemental halogen product. This is followed by purging the substrate-supported salt mixture with steam (stage 2) thereby recovering any unreacted hydrogen halide and additional elemental halogen for recycle to stage 1. The dissolved oxygen compound is regenerated in a high temperature (stage 3) and an optical intermediate temperature stage (stage 4) by contacting the substrate-supported salt mixture with a gas containing oxygen whereby the dissolved oxygen compound in the substrate-supported salt mixture is regenerated by being oxidized to a higher valence state.

  16. Genetic Control of Methyl Halide Production in Arabidopsis

    Rhew, R. C.; Ostergaard, L.; Saltzman, E. S.; Yanofsky, M. F.

    2003-12-01

    Methyl chloride and methyl bromide are the primary carriers of natural chlorine and bromine to the stratosphere where they catalyze the destruction of ozone, whereas methyl iodide influences aerosol formation and ozone loss in the troposphere. Methyl bromide is also an agricultural fumigant whose use is scheduled to be phased out by international agreement. Despite the economic and environmental importance of these methyl halides, their natural sources and biological production mechanisms are poorly understood. Currently identified sources include oceans, biomass burning, industrial and agricultural use, fuel combustion, salt marshes, wetlands, rice paddies, certain terrestrial plants and fungi, and abiotic processes. We demonstrate that the model plant Arabidopsis thaliana produces and emits methyl halides and that the enzyme primarily responsible for the production is encoded by the HARMLESS TO OZONE LAYER (HOL) gene located on chromosome II. In mutant plants that have a disruption of the HOL gene, methyl halide production is largely eliminated. A phylogenetic analysis using the HOL gene suggests that the ability to produce methyl halides is widespread among vascular plants. This approach provides a genetic basis for understanding and predicting patterns of methyl halide production by plants.

  17. Design and construction of a copper-halide laser

    The considerations necessary for the design and construction of an inexpensive, reliable copper halide laser are presented. A broad range of parameters which govern the output of the double-pulse copper-halide laser was studied. The laser has an optimum operating temperature which is different for each halide uCl, CuBr, CuI), but corresponds to a halide vapor pressure of about 0.1 torr for all three halides. At low repetition rates the highest laser output is achieved with helium as buffer gas. The laser output increases linearly with laser tube cross-section. As the tube diameter is increased, the dissociation energy must be increased, but not the excitation energy. A thyratron bias circuit has been developed which permits both dissociation and excitation pulses to be obtained from the same capacitor (with a small delay) without thyratron latch. Use of the single thyratron reduces the laser cost considerably. A laser tube with an operating life of many hundreds of hours has been designed. The description of a laser with an output of 0.5 mJ per pulse at repetition rates of up to 120 pulses per second is given. The design and construction of various components of this laser are given in detail. (authors)

  18. Collective state transitions of exciton-polaritons loaded into a periodic potential

    Winkler, K.; Egorov, O. A.; Savenko, I. G.; Ma, X.; Estrecho, E.; Gao, T.; Müller, S.; Kamp, M.; Liew, T. C. H.; Ostrovskaya, E. A.; Höfling, S.; Schneider, C.

    2016-03-01

    We study the loading of a nonequilibrium, dissipative system of composite bosons—exciton polaritons—into a one-dimensional periodic lattice potential. Utilizing momentum resolved photoluminescence spectroscopy, we observe a transition between an incoherent Bose gas and a polariton condensate, which undergoes further transitions between different energy states in the band-gap spectrum of the periodic potential with increasing pumping power. We demonstrate controlled loading into distinct energy bands by modifying the size and shape of the excitation beam. The observed effects are comprehensively described in the framework of a nonequilibrium model of polariton condensation. In particular, we implement a stochastic treatment of quantum and thermal fluctuations in the system and conclude that polariton-phonon scattering is a plausible energy relaxation mechanism enabling transitions from the highly nonequilibrium polariton condensate in the gap to the ground band condensation for large pump powers.

  19. Exciton absorption bleaching studies in ordered GaxIn1-xP

    Using time-resolved small-signal exciton absorption bleaching at low temperature as a spectroscopic technique, the optical transition energies from all three valence bands in spontaneously ordered GaxIn1-xP have been measured with high accuracy. The origin of the bleaching signal and the contributions of reflection, strain, and binding energy are discussed. With three measured energies from each sample, all parameters in the quasicubic perturbation model can be fitted. Good agreement is obtained with a spin-orbit-splitting parameter of 103 meV, nearly independent of the degree of ordering. The ratio of band-gap reduction to crystal-field-splitting parameter is found to be 2.7, slightly higher than previous works. This difference is attributed to a more accurate determination of light-hole-like band-gap energy. copyright 1997 The American Physical Society

  20. Strong localization induced anomalous temperature dependence exciton emission above 300 K from SnO{sub 2} quantum dots

    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.

  1. When Halides Come to Lithium Niobate Nanopowders Purity and Morphology Assistance.

    Lamouroux, Emmanuel; Badie, Laurent; Miska, Patrice; Fort, Yves

    2016-03-01

    The preparation of pure lithium niobate nanopowders was carried out by a matrix-mediated synthesis approach. Lithium hydroxide and niobium pentachloride were used as precursors. The influence of the chemical environment was studied by adding lithium halide (LiCl or LiBr). After thermal treatment of the precursor mixture at 550 C for 30 min, the morphology of the products was obtained from transmission electron microscopy and dynamic light scattering, whereas the crystallinity and phase purity were characterized by X-ray diffraction and UV-visible and Raman spectroscopies. Our results point out that the chemical environment during lithium niobate formation at 550 C influences the final morphology. Moreover, direct and indirect band-gap energies have been determined from UV-visible spectroscopy. Their values for the direct-band-gap energies range from 3.97 to 4.36 eV with a slight dependence on the Li/Nb ratio, whereas for the indirect-band-gap energies, the value appears to be independent of this ratio and is 3.64 eV. No dependence of the band-gap energies on the average crystallite and nanoparticle sizes is observed. PMID:26859157

  2. Luminescent unit computerization to research spectral characteristics of fine film alkali halide crystal

    The fundamental optical absorption of ion crystals characterizes the creation of different free low energetic electronic excitation (the excitons and electron-hole pairs), but their straight registration is not possible because of incommensurable big absorption factor of alkali halide monocrystals. So to registration the spectrums of alkali halide monocrystal very fine layers are necessary. We have received fine films of Nal and KCl in system of KCl-Nal-KCl, KCl-KI-KCl on the base of universal vacuum post VUP-4, VUP-5 by thermal evaporation. A unique spectral unit has been created For this on the basic the SDL-2 complex. Complex consists of radiator, systems of condensers, monochromators MDR-12 and MDR-23, receivers of radiation, controller by unit. Connect and control of monochromators by means of IBM-compatible computer has been created. Kinematics schemes of monochromators provide consequent removing on output slot of monochromatic radiation in operating range of each diffraction lattice and indication its wavelength. The tumbling diffraction lattices is done the crossbar engines SHDR-711. For this special plate of control and block of reinforcement for crossbar engines in monochromators MDR-12 and MDR-23 are designed and constructed. Created controller of monochromators consists of double cascade preamplifier on transistors n-p-n type (815G) and logical scheme, constructed on summers and K555 series triggers. The preamplifier is used for reinforcement of signal to available amplifier on transistors KT837D. The logical scheme reduces the number of used categories of bidirectional port and enables unhooking the feeding to the windings of crossbar engine at conservation of previous combination of signals. The connection controller of monochromators is done through controller of port of computer with use the parallel interface. For installing computerized system of collection and data processing is provided marketed by means of modern object-oriented programming languages (Visual C++ 6.0, Visual Basic 6.0). As the result of the experiments it has been noted that the burning effect of own luminescence is in its maximum in the iodides of alkaline metals, for instance, in KI, NaI, RbI, and CsI crystals. From the point of the experimental equipment in order to registration the fundamental optical absorption of these crystals there is no necessity in such rare vacuum monochromators

  3. Strongly driven exciton resonances in quantum wells: light-induced dressing versus coulomb scattering

    Ciuti; Piermarocchi; Savona; Selbmann; Schwendimann; Quattropani

    2000-02-21

    The nonequilibrium dynamics of a two-dimensional electron-hole gas is studied in the regime of strong and resonant pumping of the exciton resonance. The Coulomb collision rates are consistently determined by taking into account the light-induced coherence of the two-band system that leads to a dressing of the carrier spectral functions. The light dressing dramatically reduces the Coulomb scattering efficiency. Results are presented for Rabi oscillations in the time domain and dynamical Stark splitting in the pump-probe absorption spectra. PMID:11017617

  4. Structure of spectrum of excitons coupled with neutral boron atoms in silicon carbide

    Absorption spectra related to excitation of excitons localized on neutral boron in 6H and 15R polytype silicon carbide crystals are investigated. The spectra are obtained at temperatures of 86 and 6 K and for different light polarization with respect to the crystal axis c. The studies have made it possible to detect band series related to nonequivalent positions of substitution impurity in a crystal lattice. The model of the energy spectrum of the excitations in question is proposed which takes account of valley-orbit interaction and interprets polarized properties of the spectra

  5. Fundamental optical spectra and excitons of crystalline and amorphous GeO2

    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

  6. The role of excitons and interfaces in molecular organic devices

    Bulovic, Vladimir

    Operating characteristics of molecular organic devices are largely governed by the formation of excitons, and exciton interactions with interfaces between dissimilar materials. The study of these excitonic processes and their implementation in practical optoelectronic applications is the focus of this work. We demonstrate a number of novel molecular organic devices by utilizing unique optical and electronic properties of this class of solids. Excitons dominate the fundamental processes in organic materials determining their absorption, photoconduction, luminescence, and lasing characteristics. The spatial extent of the exciton governs its dynamics and depends on the strength of intermolecular interaction. Different types of excitons are illustrated in our spectroscopic study of the archetype molecular compound 3,4,9,10-perylineteracarboxylic dianhydride. Through our discussion of exciton self-trapping, line broadening, diffusion, and inter-level transitions in this archetype molecular organic crystal, we access a rich array of excitonic processes. The exciton-interface interaction influences the luminescence and photogeneration efficiency, energy quenching, exciton quantum confinement, and exciton lifetime. After examining these phenomena, we tailor our molecular organic structures to demonstrate photovoltaic cells and efficient organic light emitting devices (OLEDs). We demonstrate transparent OLEDs that can be used in lightweight, conformable, head-up displays, and inverted OLEDs that can be integrated with conventional electronics. We also demonstrate a stacked OLED that integrates three transparent OLEDs to generate a color-tunable, true color device. Bright and efficient electroluminescence (EL) is a general property of many organic thin films. It is generated by radiative recombination of an exciton formed by electrically injected carriers. We investigate the formation of excitons in the EL process by analyzing our measurements in terms of trap-limited conduction in amorphous materials. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum. We also show that spectral emission can be modified when the luminescent center is in the vicinity of a strong electric dipole, where by adjusting the strength of the dipole field the EL spectrum can be altered. The radiative recombination of Frenkel excitons in luminescent devices is also influenced by the presence of multilayer structures which introduce microcavity effects. We develop a comprehensive theoretical description of microcavity effects in OLEDs which accounts for the spectral shape and intensity as a function of the emission angle, treating both radiative and waveguided modes. We finally show that optically excited organic material in a microcavity can undergo population inversion and lase.

  7. Contribution of the exciton--photon interaction to the exciton absorption line shape in the spatially inhomogeneous wave model: Application to the a-singlet exciton of naphthalene

    The problem of determining the contribution of the exciton--photon coupling to the absorption line shape or width of an exciton is addressed. In this regard, both the spatially homogeneous (polariton) and inhomogeneous wave models are considered. For the latter model, the theory is complex although a line shape expression is derived for the case of high exciton velocity and low oscillator strength. The related problem of determining the conditions under which the strong exciton--photon coupling limit is attained is also discussed. It is in this limit that light attenuation is independent of the oscillator strength. The theory plus new experimental data on the a exciton of naphthalene argue for the inadequacy of the polariton model. On the other hand, reasonable agreement between the data and the spatially inhomogeneous wave theory developed here is obtained. Thus, it is possible to estimate that the exciton--photon coupling contribution to the a-exciton linewidth is 0.26 cm-1. This is the major portion of the 2 K linewidth of 0.3 cm-1

  8. Contribution of the exciton--photon interaction to the exciton absorption line shape in the spatially inhomogeneous wave model: Application to the a-singlet exciton of naphthalene

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

    1978-12-15

    The problem of determining the contribution of the exciton--photon coupling to the absorption line shape or width of an exciton is addressed. In this regard, both the spatially homogeneous (polariton) and inhomogeneous wave models are considered. For the latter model, the theory is complex although a line shape expression is derived for the case of high exciton velocity and low oscillator strength. The related problem of determining the conditions under which the strong exciton--photon coupling limit is attained is also discussed. It is in this limit that light attenuation is independent of the oscillator strength. The theory plus new experimental data on the a exciton of naphthalene argue for the inadequacy of the polariton model. On the other hand, reasonable agreement between the data and the spatially inhomogeneous wave theory developed here is obtained. Thus, it is possible to estimate that the exciton--photon coupling contribution to the a-exciton linewidth is 0.26 cm/sup -1/. This is the major portion of the 2 K linewidth of 0.3 cm/sup -1/.

  9. High Biomass Specific Methyl Halide Production Rates of Selected Coastal Marsh Plants and its Relationship to Halide Content

    Manley, S. L.; Wang, N.; Cicerone, R. J.

    2002-12-01

    Salt tolerant coastal marsh plants (halophytes) have previously been shown to be globally significant producers of methyl chloride (MeCl) and methyl bromide (MeBr). While halophytes are known for their high salt content, there are few reports of their halide content. Our studies have attempted to quantify biomass specific methyl halide (MeX) production from these plants and relate it to tissue halide levels. MeCl, MeBr and MeI production rates and tissue chloride, bromide and iodide concentrations from selected coastal marsh plants were measured for nearly a year. Certain halophyte species (i.e. Batis and Frankenia) have very high summer biomass specific production rates for MeX (e.g. Frankenia: 1 ug MeCl /gfwt/hr; 80 ng MeBr/gfwt/hr; 8 ng MeI/gfwt/hr). These rates of MeCl and MeBr production are much higher than those from other coastal marsh plants or seaweeds. Plant halide levels remain high throughout the year, while MeX production peaks at a high level in mid summer falling to low winter rates. This implies a linkage to plant growth. Higher levels of chloride and bromide were seen in the fleshy marsh plants such as Batis (saltwort, approximately 20 percent dry wt chloride, 0.4 percent dry wt bromide) and Salicornia (pickleweed) than in the others such as Frankenia (alkali heath) approx 7 percent dry wt chloride, 0.1 percent dry wt bromide) or Spartina (cordgrass). No such trend was seen for iodide, which ranged from 4 - 10 ppm. Calculations show the daily halide losses from MeX production are far less than the variability in tissue halide content. MeX production removes a small fraction of the total tissue halide from these plants suggesting that MeX production is not a mechanism used by these species to control internal halide levels. Saltwort cell-free extracts incubated with bromide or iodide in the presence of S-adenosyl-L-methionine (SAM) produced the corresponding MeX. MeBr production was inhibited by caffeic acid the substrate of lignin-specific O-methyl transferase. MeX production may be a result of non-specific methylation of halides by SAM utilizing methyl transferases used in other biosynthetic pathways.

  10. Analysis of the exciton-LO-phonon coupling in single wurtzite GaN quantum dots

    Callsen, G.; Pahn, Gerald M. O.; Kalinowski, S.; Kindel, C.; Settke, J.; Brunnmeier, J.; Nenstiel, C.; Kure, T.; Nippert, F.; Schliwa, A.; Hoffmann, A.; Markurt, T.; Schulz, T.; Albrecht, M.; Kako, S.; Arita, M.; Arakawa, Y.

    2015-12-01

    In this study, we analyze the interaction between excitons and longitudinal-optical phonons (LO phonons) in single, wurtzite GaN quantum dots (QDs) by means of micro-photoluminescence (μ PL ) spectroscopy. We report on Stokes-shifted emission lines measured for hundreds of single QDs. A decrease of the Huang-Rhys factor (˜0.5 -0.01 ) is observed with increasing QD emission energy that can be modeled in an adiabatic approximation applying two-particle eight-band k .p wave functions. In order to obtain the QD dimensions and shape needed for these calculations, we conduct a scanning transmission electron microscope (STEM) analysis, not only focusing on the QD dimensions, but also on alloying effects. The QD height is identified as the most detrimental parameter for the exciton-LO-phonon interaction in strongly polar QD systems based on nitrides. Additionally, we extract the LO-phonon energy for a significant number of individual QDs from our μ PL data set scaling in-between the bulk values for the QD (GaN) and the matrix material (AlN). Such a large variation of the LO-phonon energy cannot be explained by the alloying effects attested by our STEM analysis. Hence, the exciton-LO-phonon interaction resides in a volume that encloses the QD in the growth direction and a fraction of the matrix material depending on the QD height. We approximate this exciton-LO-phonon interaction volume by a sphere with a constant diameter of 2.6 ± 0.2 nm.

  11. The ultraviolet photodissociation dynamics of the hydrogen halides

    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 (λ∼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: 3Π(1), 3Π(0+), 1Π(1) and 3Σ+(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 λ=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)

  12. Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

    Kasahara, Yuichi, E-mail: ykasahara@scphys.kyoto-u.ac.jp [Department of Physics, Kyoto University, Kyoto 606-8502 (Japan); Kuroki, Kazuhiko, E-mail: kuroki@phys.sci.osaka-u.ac.jp [Department of Physics, Osaka University, Toyonaka, Osaka 560-0043 (Japan); Yamanaka, Shoji, E-mail: syamana@hiroshima-u.ac.jp [Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8527 (Japan); Taguchi, Yasujiro, E-mail: y-taguchi@riken.jp [RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198 (Japan)

    2015-07-15

    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.

  13. Unconventional superconductivity in electron-doped layered metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I)

    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

  14. Chemistry of gaseous lower-valent actinide halides

    The objective of this program is to provide accurate thermochemical information for the uranium and thorium halide and oxyhalide systems, two of the key actinide systems, so that the basic factors underlying the chemical bonding and chemical reactivity in these systems can be elucidated in a systematic way. A complete characterization of the uranium and thorium systems will serve as a reliable base for modeling the behavior of the heavier actinides, which will be much more difficult to study because of sample size and safety restrictions. Our principal focus is on the the gaseous halides, mainly the lower-valent halides, because these molecular species largely will define the high-temperature chemistry in the nuclear applications of interest. 2 refs., 1 tab

  15. Metal-halide mixtures for latent heat energy storage

    Chen, K.; Manvi, R.

    1981-01-01

    Alkali metal and alkali halide mixtures are identified which may be suitable for thermal energy storage at temperatures above 600 C. The use of metal-halides is appropriate because of their tendency to form two immiscible melts with a density difference, which reduces scale formation and solidification on heat transfer surfaces. Also, the accumulation of phase change material along the melt interface is avoided by the self-dispersing characteristic of some metal-halides, in particular Sr-SrCl2, Ba-BaCl2, and Ba-BaBr2 mixtures. Further advantages lie in their high thermal conductivities, ability to cope with thermal shock, corrosion inhibition, and possibly higher energy densities.

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

    Nganou, Collins

    2013-07-01

    This paper studies the excitonic factor in the excited state energy transfer of phycobilisome (PBS) by using a polarized time-resolved pump-probe and by changing the ionic strength of the cofactors' medium in the PBS of Acaryochloris marina (A. marina). As a result, the interplay between the surrounding medium and the closely excited adjacent cofactors is shown to be a negligible factor of the excitonic decay kinetics at 618 nm of the phycocyanin (PC), while it appears as a driving factor of an increase in excitonic delocalization at 630 nm. The obtained anisotropy values are consistent with the contribution of ionic strength in the excitonic mechanism in PBS. These values were 0.38 in high ionic strength and 0.4 in low ionic strength at 618 nm, and 0.52 in high ionic strength and 0.4 in low ionic strength at 630-635 nm. The anisotropy value of 0.52 in high phosphate is similar at 630 nm and 635 nm, which is consistent with an excitonic delocalization band at 635 nm. The 635 nm band is suggested to show the true low energy level of PC in A. marina PBS. The anisotropy decay kinetic at 630 nm suggests that the excited state population of PC is not all equilibrated in 3 ps because of the existence of the 10 ps decay kinetic component. The presence of the slow kinetic decay component in high, and low ionic strength, is consistent with a 10 and 14 ps energy transfer pathway, while the 450 fs kinetic decay component is consistent with the presence of an additional excitation energy transfer pathway between adjacent α84 and β84. Furthermore, the 450 fs decay kinetic is suggested to be trapped in the trimer, while the 400 fs decay kinetic rules out an excitonic flow from low energy level PC to allophycoyanin. This excitonic flow may occur between β84 in adjacent trimers, towards the low energy state of the PBS rod.

  17. Halide-Dependent Electronic Structure of Organolead Perovskite Materials

    Buin, Andrei

    2015-06-23

    © 2015 American Chemical Society. Organometal halide perovskites have recently attracted tremendous attention both at the experimental and theoretical levels. These materials, in particular methylammonium triiodide, are still limited by poor chemical and structural stability under ambient conditions. Today this represents one of the major challenges for polycrystalline perovskite-based photovoltaic technology. In addition to this, the performance of perovskite-based devices is degraded by deep localized states, or traps. To achieve better-performing devices, it is necessary to understand the nature of these states and the mechanisms that lead to their formation. Here we show that the major sources of deep traps in the different halide systems have different origin and character. Halide vacancies are shallow donors in I-based perovskites, whereas they evolve into a major source of traps in Cl-based perovskites. Lead interstitials, which can form lead dimers, are the dominant source of defects in Br-based perovskites, in line with recent experimental data. As a result, the optimal growth conditions are also different for the distinct halide perovskites: growth should be halide-rich for Br and Cl, and halide-poor for I-based perovskites. We discuss stability in relation to the reaction enthalpies of mixtures of bulk precursors with respect to final perovskite product. Methylammonium lead triiodide is characterized by the lowest reaction enthalpy, explaining its low stability. At the opposite end, the highest stability was found for the methylammonium lead trichloride, also consistent with our experimental findings which show no observable structural variations over an extended period of time.

  18. Modified Becke-Johnson exchange potential: improved modeling of lead halides for solar cell applications

    Radi A. Jishi

    2016-01-01

    Full Text Available We report first-principles calculations, within density functional theory, on the lead halide compounds PbCl2, PbBr2, and CH3NH3PbBr3?xClx, taking into account spin-orbit coupling. We show that, when the modified Becke-Johnson exchange potential is used with a suitable choice of defining parameters, excellent agreement between calculations and experiment is obtained. The computational model is then used to study the effect of replacing the methylammonium cation in CH3NH3PbI3 and CH3NH3PbBr3 with either N2H5+or N2H3+, which have slightly smaller ionic radii than methylammonium. We predict that a considerable downshift in the values of the band gaps occurs with this replacement. The resulting compounds would extend optical absorption down to the near-infrared region, creating excellent light harvesters for solar cells.

  19. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.

    Kojima, Akihiro; Teshima, Kenjiro; Shirai, Yasuo; Miyasaka, Tsutomu

    2009-05-01

    Two organolead halide perovskite nanocrystals, CH(3)NH(3)PbBr(3) and CH(3)NH(3)PbI(3), were found to efficiently sensitize TiO(2) for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO(2) films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH(3)NH(3)PbI(3)-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH(3)NH(3)PbBr(3)-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%. PMID:19366264

  20. Effect of Electron-Hole Overlap and Exchange Interaction on Exciton Radiative Lifetimes of CdTe/CdSe Heteronanocrystals.

    Granados Del Águila, Andrés; Groeneveld, Esther; Maan, Jan C; de Mello Donegá, Celso; Christianen, Peter C M

    2016-04-26

    Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields. PMID:26982795

  1. Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films

    Christian Wehrenfennig

    2014-08-01

    Full Text Available The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3−xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL emission of vapor-deposited CH3NH3PbI3−xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion.

  2. Angular momentum dependence of transition rates in exciton model

    Based on statistical approach, simple analytical expressions have been derived for the spin part of the intermediate-state transition rates in the exciton model. The master-equation coefficients are related to those of exciton model without account of angular momentum conservation. The effect of spin conservation on the establishment of the thermodynamic equilibrium is discussed. (author). 13 refs., 6 figs

  3. Facile Preparation of Silver Halide Nanoparticles as Visible Light Photocatalysts

    Cui, Linfan; Jiao, Tifeng; Zhang, Qingrui; Zhou, Jingxin; Peng, Qiuming

    2015-01-01

    In this study, highly efficient silver halide (AgX)-based photocatalysts were successfully fabricated using a facile and template-free direct-precipitation method. AgX nanoparticles, which included silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI), were synthesized using different potassium halides and silver acetate as reactive sources. The size distribution of the AgX nanopar? ticles was determined by the reaction time and ratio of the reagents, which were monitored by U...

  4. Thallous and cesium halide materials for use in cryogenic applications

    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)

  5. Reactivity of halide and pseudohalide ligands in transition metal complexes

    The review generalizes experimental material on reactions of coordinated halide ligands, as well as cyanide, azide, thiocyanate and cyanate ligands in transition metal (Mo, W, Ru, etc.) complexes. It is shown that transformation of the intrasphere pseudohalide ligands is a very important method of directed synthesis of coordination compounds. The tendency of halide and pseudohalide ions to the formation of low-soluble salts or strong complexes can be widely used in preparation coordination chemistry for the ''forced'' introduction of solvent molecules into the complexes

  6. Optical diode based on exciton-polaritons

    Espinosa-Ortega, T.; Liew, T. C. H.; Shelykh, I. A.

    2013-11-01

    We propose theoretically an optical diode based on exciton-polaritons in semiconductor microcavities. A flow of polaritons in the bistable regime is used to send signals through an asymmetric fixed potential that favours the bridging of particles in one direction. Through dynamic modelling of the coherent polariton field, we demonstrate the characteristics of an ideal diode, namely, that the forward signal is fully transmitted while the transmission in the reverse direction tends to zero, without any additional external control. Moreover, the system proves to be robust to the presence of disorder, intrinsic to microcavities, and can function at gigahertz repetition rates.

  7. Bose-Einstein Condensation of Excitons and Biexcitons

    Moskalenko, S. A.; Snoke, D. W.

    2000-02-01

    Bose-Einstein condensation of excitons is a unique effect in which the electronic states of a solid can self-organize to acquire quantum phase coherence. The phenomenon is closely linked to Bose-Einstein condensation in other systems such as liquid helium and laser-cooled atomic gases. Covering theoretical aspects as well as recent experimental work, the book provides a comprehensive survey of the field. After introducing the relevant basic physics of excitons, the authors discuss exciton-phonon interactions as well as the behavior of biexcitons. They also cover exciton phase-transitions and give particular attention to nonlinear optical effects including the optical Stark effect and chaos in excitonic systems. The thermodynamics of equilibrium, quasiequilibrium, and nonequilibrium systems are examined in detail. Throughout, the authors interweave theoretical and experimental results. The book will be of great interest to graduate students and researchers in semiconductor and superconductor physics, quantum optics, and atomic physics.

  8. Dynamics of a single exciton in strongly correlated bilayers

    Rademaker, Louk; Wu, Kai; Zaanen, Jan

    2012-08-01

    We formulated an effective theory for a single interlayer exciton in a bilayer quantum antiferromagnet, in the limit when the holon and doublon are strongly bound onto one interlayer rung by the Coulomb force. Upon using a rung linear spin-wave approximation of the bilayer Heisenberg model, we calculated the spectral function of the exciton for a wide range of the interlayer Heisenberg coupling ? = J?/Jz. In the disordered phase at large ?, a coherent quasi-particle peak appears, representing free motion of the exciton in a spin singlet background. In the Nel phase, which applies to more realistic model parameters, a ladder spectrum arises due to Ising confinement of the exciton. The exciton spectrum is visible in measurements of the dielectric function, such as c-axis optical conductivity measurements.

  9. Calculating exciton downconversion rates in Coulombically coupled chromophores

    Chapman, Craig; Schatz, George

    2015-03-01

    Exciton downconversion is a second order energy transfer process that splits a high energy exciton in a donor chromophore into multiple lower energy excitons in acceptor chromophores. Downconversion has been seen in a variety of materials including rare-earth doped glassy matrices, organic crystals, and semiconductor nanocrystals, and has the potential to efficiently convert a single high energy photon into a broad distribution of lower energy excitons. A comprehensive mechanistic understanding of the energy conversion process will allow for the rational engineering of materials that can control the flow of energy in a guided fashion. To this end we formulate and implement a method for calculating multi-chromophore Frster-like exciton transfer rates using transition charges obtained from time-dependent density functional theory.

  10. Taming excitons in II–VI semiconductor nanowires and nanobelts

    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)

  11. Ultrastable PbSe nanocrystal quantum dots via in situ formation of atomically thin halide adlayers on PbSe(100).

    Woo, Ju Young; Ko, Jae-Hyeon; Song, Jung Hoon; Kim, Kyungnam; Choi, Hyekyoung; Kim, Yong-Hyun; Lee, Doh C; Jeong, Sohee

    2014-06-25

    The fast degradation of lead selenide (PbSe) nanocrystal quantum dots (NQDs) in ambient conditions impedes widespread deployment of the highly excitonic, thus versatile, colloidal NQDs. Here we report a simple in situ post-synthetic halide salt treatment that results in size-independent air stability of PbSe NQDs without significantly altering their optoelectronic characteristics. From TEM, NMR, and XPS results and DFT calculations, we propose that the unprecedented size-independent air stability of the PbSe NQDs can be attributed to the successful passivation of under-coordinated PbSe(100) facets with atomically thin PbX2 (X = Cl, Br, I) adlayers. Conductive films made of halide-treated ultrastable PbSe NQDs exhibit markedly improved air stability and behave as an n-type channel in a field-effect transistor. Our simple in situ wet-chemical passivation scheme will enable broader utilization of PbSe NQDs in ambient conditions in many optoelectronic applications. PMID:24919086

  12. The search for Bose–Einstein condensation of excitons in Cu2O: exciton-Auger recombination versus biexciton formation

    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)

  13. Exciton mechanisms and modeling of the ionoluminescence in silica

    Bachiller-Perea, D.; Jiménez-Rey, D.; Muñoz-Martín, A.; Agulló-López, F.

    2016-03-01

    A theoretical model is presented in order to discuss detailed kinetic data describing the evolution of the two main ionoluminescence bands at 650 nm (1.9 eV) and 460 nm (2.7 eV) in silica as a function of the irradiation fluence at room temperature. The model is based on the generation of self-trapped excitons (STEs), their hopping migration through the silica network and their recombination at non-bridging oxygen hole and Type II oxygen-deficient centers to produce the red and blue emission bands, respectively. For heavy ions, which have a high electronic stopping power, the two emission yields experience a fast initial growth with fluence up to a maximum value and then decrease at a fairly comparable rate. The fluence for such a maximum strongly increases on decreasing ion mass and stopping power and, finally, for H and He it is not observed for any of the investigated energy and fluence range. This kinetic behavior is explained in terms of the strong structural distortions (compaction) induced by the heavy-ion irradiations. In particular, it is proposed that these strong structural distortions cause a significant decrease in the STE migration length and, consequently, in the recombination rates at the two active recombination centers. The model offers a good quantitative accordance with detailed infrared spectroscopy reporting on the changes in the frequency ω 4 of a first-order vibrational mode in the SiO2 network as a function of irradiation fluence.

  14. PbSe Nanocrystal Excitonic Solar Cells

    Choi, Joshua J.

    2009-11-11

    We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that Is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date. © 2009 American Chemical Society.

  15. Two-photon absorption in an indirect-gap semiconductor quantum well systems II. Excitonic transitions

    A theory of phonon-assisted two-phonon exciton transitions in an indirect-band-gap semiconductor layered Quantum Wells (QW) and Quantum Well Wires (QWW) is developed. The expressions for the two-photon absorption coefficients in one-dimensional (1D), α(1)ex, 2D, α(2)ex and 3D (bulk), α(3)ex have been calculated. The spectra dependence of these expressions, at the vicinity of the band edge, are found to obey a law (2(h/2π)ω±(h/2π)Ω-EG+Ebex)β where (h/2π)ω((h/2π)Ω) is the photon (phonon) energy, EG is the effective indirect gap and Ebex is the exciton binding energy. The values of β varies from -1/2 up to 3/2 depending on the dimension of the system and the type of the coupling matrix element involved in each transition process. Before the edge, the final exciton states are of s-symmetry or p-symmetry according to the photon polarizations w.r.t. the confinement directions in the QW's and to the selection rules allowed by the momentum matrix elements. A numerical estimation for the case Si0.5Ge0.5 show that α(1)ex for both photon polarizations (parallel and perpendicular to the confinement directions of the QW's) is enhanced over the values of α(2)ex and α(3)ex (allowed and forbidden transitions). Furthermore α(2)ex is also enhanced over α(3)ex values for bulk materials. This behaviour of αex's is interpreted as due to (i) the additional confinement of the carriers which occur going from 3D → 2D → 1D systems (ii) the photon polarization configurations w.r.t. the confinement directions (iii) the coupling matrix elements. (author). 18 refs, 3 figs

  16. Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization.

    Fujita, Takatoshi; Atahan-Evrenk, Sule; Sawaya, Nicolas P D; Aspuru-Guzik, Alán

    2016-04-01

    Charge-transfer states in organic semiconductors play crucial roles in processes such as singlet fission and exciton dissociation at donor/acceptor interfaces. Recently, a time-resolved spectroscopy study of dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene (DNTT) thin films provided evidence for the formation of mixed Frenkel and charge-transfer excitons after the photoexcitation. Here, we investigate optical properties and excitation dynamics of the DNTT thin films by combining ab initio calculations and a stochastic Schrödinger equation. Our theory predicts that the low-energy Frenkel exciton band consists of 8-47% CT character. The quantum dynamics simulations show coherent dynamics of Frenkel and CT states in 50 fs after the optical excitation. We demonstrate the role of charge delocalization and localization in the mixing of CT states with Frenkel excitons as well as the role of their decoherence. PMID:27011327

  17. Coherent dynamics of mixed Frenkel and Charge Transfer Excitons in Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

    Fujita, Takatoshi; Sawaya, Nicolas P D; Aspuru-Guzik, Alan

    2016-01-01

    Charge transfer states in organic semiconductors play crucial roles in processes such as singlet fission and exciton dissociation at donor/acceptor interfaces. Recently, a time-resolved spectroscopy study of dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene (DNTT) thin films provided evidence for the formation of mixed Frenkel and charge-transfer excitons after the photoexcitation. Here we investigate optical properties and excitation dynamics of the DNTT thin films by combining ab initio calculations and a stochastic Schrodinger equation. Our theory predicts that the low-energy Frenkel exciton band consists of 8 to 47% CT character. The quantum dynamics simulations show coherent dynamics of Frenkel and CT states in 50 fs after the optical excitation. We demonstrate the role of charge delocalization and localization in the mixing of CT states with Frenkel excitons as well as the role of their decoherence.

  18. Gastric Banding

    ... gastric banding before deciding to have the procedure. Advertisements for a device or procedure may not include ... feeds Follow FDA on Twitter Follow FDA on Facebook View FDA videos on YouTube View FDA photos ...

  19. Excitons in coupled type-II double quantum wells under electric and magnetic fields: InAs/AlSb/GaSb

    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

  20. Effects of inter-molecular charge-transfer excitons on the external quantum efficiency of zinc-porphyrin/C60 heterojunction photovoltaic cells

    We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C60/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons.

  1. Effects of inter-molecular charge-transfer excitons on the external quantum efficiency of zinc-porphyrin/C60 heterojunction photovoltaic cells

    Ryuzaki, Sou; Kai, Toshihiro; Toda, Yasunori; Adachi, Satoru; Onoe, Jun

    2011-04-01

    We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C60/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons.

  2. Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods.

    Wu, Kaifeng; Hill, Lawrence J; Chen, Jinquan; McBride, James R; Pavlopolous, Nicholas G; Richey, Nathaniel E; Pyun, Jeffrey; Lian, Tianquan

    2015-04-28

    A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications. PMID:25803834

  3. Theory of exciton transfer and diffusion in conjugated polymers

    Barford, William, E-mail: william.barford@chem.ox.ac.uk [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); Tozer, Oliver Robert [Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford OX1 3QZ (United Kingdom); University College, University of Oxford, Oxford OX1 4BH (United Kingdom)

    2014-10-28

    We describe a theory of Frster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations.

  4. Theory of exciton transfer and diffusion in conjugated polymers

    We describe a theory of Frster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ?? < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ?10 nm are in good agreement with experiment. The spectral properties of the migrating excitons are also investigated. The emission intensity ratio of the 0-0 and 0-1 vibronic peaks is related to the effective Huang-Rhys parameter of the emitting state, which in turn is related to the chromophore size. The intensity ratios calculated from the effective Huang-Rhys parameters are in agreement with experimental spectra, and the time-resolved trend for the intensity ratio to decrease with time was also reproduced as the excitation migrates to shorter, lower energy chromophores as a function of time. In addition, the energy of the exciton state shows a logarithmic decrease with time, in agreement with experimental observations

  5. Excitonic and Raman Properties of Zinc SELENIDE/ZINC(1-X) Cadmium(x) Selenium Strained-Layer Quantum Wells

    Shastri, Vasant Krishna

    This dissertation presents for the first time the detailed excitonic and Raman scattering investigations of strained-layer single quantum-well ZnSe/Zn_ {1-x}Cd_{ x}Se heterostructures. Recently, the existing group of molecular-beam epitaxially (MBE) grown II-VI materials has been enlarged by the successful growth of cubic (zinc-blende) CdSe and Zn_ {1-x}Cd_{ x}Se on GaAs substrate. The heteroepitaxy of Zn_{rm 1-x}Cd _{x}Se on (100) GaAs by MBE results in single-phase zinc-blende crystals over the entire composition range from CdSe to ZnSe. In wide-gap II-VI semiconductors the luminescence spectra are dominated by excitonic transitions involving the electron subbands and hole subbands. The photoluminescence under direct and indirect excitations are investigated in detail to study the carrier generation, transport and recombination mechanisms. The temperature dependence of photoluminescence and resonant Raman scattering are investigated to study the exciton -phonon interaction and luminescence quenching mechanisms. Very strong 2LO phonon Raman scattering has been observed with single Zn_{0.86}Cd _{0.14}Se quantum wells, where the scattered photon energy is in resonance with an exciton transition. The experimental confined exciton energies are compared with a finite-square potential-well model including band-nonparabolicity. The bandgap of Zn _{1-x}Cd_ {x}Se as a function of composition 'x' has been computed. Lattice mismatch as high as 1.011% exists in samples of ZnSe/Zn_{1-x}Cd_{x} Se quantum wells with x = 0.14. The strain effect shifts the band-edge energy in the quantum-well and removes the degeneracy present at the zone center. The strain -induced effects on band structures are calculated and observed in the photoluminescence spectra. The main emission peak is attributed to the free-exciton transition between the lowest electron subband and the ground heavy-hole band of the Zn_{0.86}Cd _{0.14}Se quantum-well. The photoluminescence emission spectra show clear shifts to lower energies as the well width is increased. The temperature dependence of peak position, their intensities, and the linewidth are studied. The temperature dependency of the linewidth is consistent with a broadening model in which free excitons are scattered by LO and TO phonons and ionized donor impurities. By temperature tuning the bandgap, the free-exciton transition is brought into resonance with the Raman scattered photon hbaromega_{S2LO }. This investigation reveals the potential for realization of narrow-band emitters as blue light-emitting diodes (LED's), lasers, and photodetectors.

  6. 40 CFR 721.4095 - Quaternary ammonium alkyltherpropyl trialkylamine halides.

    2010-07-01

    ... 40 Protection of Environment 30 2010-07-01 2010-07-01 false Quaternary ammonium alkyltherpropyl... (CONTINUED) TOXIC SUBSTANCES CONTROL ACT SIGNIFICANT NEW USES OF CHEMICAL SUBSTANCES Significant New Uses for Specific Chemical Substances § 721.4095 Quaternary ammonium alkyltherpropyl trialkylamine halides....

  7. Methyl halide emission estimates from domestic biomass burning in Africa

    Mead, M. I.; Khan, M. A. H.; White, I. R.; Nickless, G.; Shallcross, D. E.

    Inventories of methyl halide emissions from domestic burning of biomass in Africa, from 1950 to the present day and projected to 2030, have been constructed. By combining emission factors from Andreae and Merlet [2001. Emission of trace gases and aerosols from biomass burning. Global Biogeochemical Cycles 15, 955-966], the biomass burning estimates from Yevich and Logan [2003. An assessment of biofuel use and burning of agricultural waste in the developing world. Global Biogeochemical Cycles 17(4), 1095, doi:10.1029/2002GB001952] and the population data from the UN population division, the emission of methyl halides from domestic biomass usage in Africa has been estimated. Data from this study suggest that methyl halide emissions from domestic biomass burning have increased by a factor of 4-5 from 1950 to 2005 and based on the expected population growth could double over the next 25 years. This estimated change has a non-negligible impact on the atmospheric budgets of methyl halides.

  8. Method for calcining nuclear waste solutions containing zirconium and halides

    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

  9. Halide glass containing trivalent uranium ions and its fabrication process

    This halide glass, showing an optical attenuation -1 in the near infrared from 2.2 to 304 micrometers, is prepared with a glass containing uranium ions as U4+ and/or U5+ reduced by ionizing radiations in U3+. Application is made to the fabrication of optical fibers and lasers doped with trivalent uranium

  10. Semiempirical and DFT Investigations of the Dissociation of Alkyl Halides

    Waas, Jack R.

    2006-01-01

    Enthalpy changes corresponding to the gas phase heats of dissociation of 12 organic halides were calculated using two semiempirical methods, the Hartree-Fock method, and two DFT methods. These calculated values were compared to experimental values where possible. All five methods agreed generally with the expected empirically known trends in the…

  11. Students' Understanding of Alkyl Halide Reactions in Undergraduate Organic Chemistry

    Cruz-Ramirez de Arellano, Daniel

    2013-01-01

    Organic chemistry is an essential subject for many undergraduate students completing degrees in science, engineering, and pre-professional programs. However, students often struggle with the concepts and skills required to successfully solve organic chemistry exercises. Since alkyl halides are traditionally the first functional group that is…

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

    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.

  13. Optical study on relaxation of excitons and initiation of photopolymerization in diacetylene crystals at low temperatures

    Chihiro Itoh

    1900-01-01

    Full Text Available Luminescence and optical absorption induced by irradiating with a pulse of 266nm laser beam have been measured in 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (DA-TCDU. In addition to the recombination luminescence of the lowest triplet state, luminescence from the excited single state is identified. Irradiation with a pulse of 266 nm laser beam having the fluence above 4.4 mJcm-2 leads to formation of series of optical absorption bands at 3.05 eV, 2.52 eV and 2.18 eV, which are ascribed to the optical transition due to dimer diradicals, trimer diradicals and the tetramer diradicals, respectively. The laser-fluence dependence of the integral intensity of the recombination luminescence of the singlet and the triplet exciton and the absorption strength of the 3.05 eV band have been measured. The nature of the triplet exciton and its relation to the initiation of the photopolymerization are discussed.

  14. Theoretical study of excitonic complexes in semiconductors quantum wells; Estudo teorico de complexos excitonicos em pocos quanticos de semicondutores

    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)

  15. Synthesis and Exciton Dynamics of Triplet Sensitized Conjugated Polymers

    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.

  16. Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

    Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, Bernhard; Wassner, T. A.; Eickhoff, Martin; Feneberg, M.; Goldhahn, R.

    2011-01-01

    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg(x)Zn(1-x)O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced fea...

  17. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites.

    Lee, Michael M; Teuscher, Joël; Miyasaka, Tsutomu; Murakami, Takurou N; Snaith, Henry J

    2012-11-01

    The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite. PMID:23042296

  18. Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

    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.

  19. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    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. Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

    Choi, Joshua J.

    2010-05-12

    Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices. © 2010 American Chemical Society.

  1. Spin-excitons in heavy-fermion semimetals

    Riseborough, Peter S.; Magalhaes, S. G.

    2016-02-01

    Spin-excitons are sharp and dispersive magnetic fluctuations in paramagnetic semiconductors where the dispersion relation lies within the semiconducting gap. Spin-excitons are found in the vicinity of magnetic quantum critical points in semiconductors, much the same as antiparamagnons are precursor fluctuations for quantum critical points in metals. Here we show that this concept of spin-exciton excitations can be extended to heavy-fermion semimetals and provides a natural explanation of the magnetic modes found by inelastic neutron scattering experiments on paramagnetic CeFe2Al10.

  2. Charged two-exciton emission from a single semiconductor nanocrystal

    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.

  3. Evidence of Excitonic Optical Tamm States using Molecular Materials

    Núñez-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.

  4. Dark and Bright Excitonic States in Nitride Quantum Dots

    Bagga, Anjana; Chattopadhyay, P.K.; Ghosh, Subhasis

    2004-01-01

    Formation of excitonic states in quantum dots of nitride based III-V semiconductors GaN and AlN including coulomb and exchange interactions are investigated. Dark exciton formation is found to occur for both GaN quantum dots(QDs) with wurtzite structure having positive crystal field splitting and GaN and AlN QDs with zero crystal field splitting with a transition from dark to bright exciton at about 40{\\AA}. In wurtzite AlN QDs with negative crystal field splitting the splitting between the d...

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

    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.

  6. Theory for electric dipole superconductivity with an application for bilayer excitons

    Qing-Dong Jiang; Zhi-qiang Bao; Qing-Feng Sun; Xie, X. C.

    2015-01-01

    Exciton superfluid is a macroscopic quantum phenomenon in which large quantities of excitons undergo the Bose-Einstein condensation. Recently, exciton superfluid has been widely studied in various bilayer systems. However, experimental measurements only provide indirect evidence for the existence of exciton superfluid. In this article, by viewing the exciton in a bilayer system as an electric dipole, we provide a general theory for the electric dipole superconductivity, and derive the London-...

  7. Exciton-photon coupling in organic materials with large intersystem crossing rates and strong excited-state molecular relaxation

    Holmes, R. J.; Forrest, S. R.

    2005-06-01

    We examine the influence of singlet-triplet intersystem crossing (ISC) and excited-state molecular relaxation on strong exciton-photon coupling in optical microcavities filled with small-molecular-weight organic materials. The effect of ISC is considered by comparing coupling effects in the phosphorescent organic platinum(II) octaethylporphyrin to those in the fluorescent free-base porphyrin tetraphenylporphyrin (TPP). The influence of excited-state molecular relaxation is studied by examining coupling to the Soret band of TPP. Both ISC and excited-state molecular relaxation prevent the population of polariton states under nonresonant optical excitation. The interplay between strong coupling and relaxation processes offers a unique opportunity to directly probe fundamental ultrafast excitonic phenomena. The competition between coupling in microcavities and these processes allows for estimation of their relative transition rates.

  8. Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

    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.

  9. Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

    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

  10. Control of Exciton Photon Coupling in Nano-structures

    Liu, Xiaoze

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.

  11. Decoherence in weakly coupled excitonic complexes.

    Mančal, Tomáš; Balevičius, Vytautas; Valkunas, Leonas

    2011-04-28

    Equations of motion for weakly coupled excitonic complexes are derived. The description allows one to treat the system in the basis of electronic states localized on individual chromophores while at the same time accounting for experimentally observable delocalization effects in optical spectra. The equations are shown to be related to the well-known Förster type energy-transfer rate equations, but unlike Förster equations, they provide a description of the decoherence processes leading to suppression of the resonance coupling by bath fluctuations. Linear absorption and two-dimensional photon echo correlation spectra are calculated for simple model systems in the homogeneous limit, demonstrating a distinct delocalization effect and reduction of the resonance coupling due to interaction with the bath. PMID:21338152

  12. Decoherence in Weakly Coupled Excitonic Complexes

    Mancal, Tomas; Valkunas, Leonas

    2010-01-01

    Equations of motion for weakly coupled excitonic complexes are derived. The description allows to treat the system in the basis of electronic states localized on individual chromophores, while at the same time accounting for experimentally observable delocalization effects in optical spectra. The equations are show to be related to the well-known Foerster type energy transfer rate equations, but unlike Foerster equations, they provide a description of the decoherence processes leading to suppression of the resonance coupling by bath fluctuations. Linear absorption and two-dimensional photon echo correlation spectra are calculated for simple model systems in homogeneous limit demonstrating distinct delocalization effect and reduction of the resonance coupling due to interaction with the bath.

  13. The optimization topography of exciton transport

    Scholak, Torsten; Buchleitner, Andreas

    2011-01-01

    Stunningly large exciton transfer rates in the light harvesting complex of photosynthesis, together with recent experimental 2D spectroscopic data, have spurred a vivid debate on the possible quantum origin of such efficiency. Different theoretical approaches come up with different suggestions for the efficiency's physical cause, from noise-assisted over driving-induced to fully coherent transport mechanisms. Here we compare the efficiency of noise-induced transport with that mediated by constructive multi-path quantum interference. We show that molecular configurations of the light harvesting complex that optimize a constructive interference condition from input to output site yield systematically higher -- quantum -- efficiencies of the transport than classical transport mechanisms induced by ambient dephasing noise.

  14. Excitons in a surface quantum well

    Arulmozhi, M.; Anitha, A.

    2014-11-01

    Binding energies of excitons in a Surface Quantum Well (SQW) composed of vacuum/GaAs/AlxGa1-xAs as a function of wellwidth are calculated. The effect of non-parabolicity is considered by using an energy dependent effective mass. The effect of mass anisotropy and the effect of image charges which arise due to the large dielectric discontinuity at the vacuum/GaAs interface are also considered. The average distances of the electron and the hole from the vacuum/GaAs interface, with and without image charges and the integrated probability of finding an electron and a hole inside the well are also calculated. The results agree well with the available experimental data.

  15. Benchmarking calculations of excitonic couplings between bacteriochlorophylls

    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 ...

  16. Exciton dynamics in emergent Rydberg lattices

    Bettelli, S; Fernholz, T; Adams, C S; Lesanovsky, I; Ates, C

    2013-01-01

    The dynamics of excitons in a one-dimensional ensemble with partial spatial order are studied. During optical excitation, cold Rydberg atoms spontaneously organize into regular spatial arrangements due to their mutual interactions. This emergent lattice is used as the starting point to study resonant energy transfer triggered by driving a $nS$ to $n^\\prime P$ transition using a microwave field. The dynamics are probed by detecting the survival probability of atoms in the $nS$ Rydberg state. Experimental data qualitatively agree with our theoretical predictions including the mapping onto XXZ spin model in the strong-driving limit. Our results suggest that emergent Rydberg lattices provide an ideal platform to study coherent energy transfer in structured media without the need for externally imposed potentials.

  17. Exciton transfer under dichotomic noise: GME treatment

    The exciton energy transfer between molecules in a dimer is investigated using a model, in which the influence of the phonons is described by a dichotomic stochastic process with colored noise giving rise to modulations of the molecular excitation energies. The solution of equations of motion for the density matrix of the system (obtained algebraically on a computer) is used to determine (also algebraically) the form of the memory functions which enter the Generalized Master Equation. The behaviour of the roots and their weights in the memory function is discussed thoroughly. Connection of the so called coherence time to stochastic parameters is treated analytically. Different limiting cases are investigated. (author). 14 refs, 9 figs

  18. Optical properties of Rydberg excitons and polaritons

    Zielińska-Raczyńska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David

    2016-02-01

    We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu20 crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.

  19. Spin excitons in heavy fermion semiconductors

    The heavy fermion semiconductors such as Ce3Bi4Pt3, YbB12, and SmB6 may be modeled as indirect hybridization gap semiconductors. The magnitudes of the gaps are smaller than the gaps predicted by local density functional electronic structure calculations, suggesting strong renormalizations due to electronic correlations. The inelastic neutron scattering spectra Im[?(q, ?)] are examined within the context of this model. Anomalously sharp and temperature dependent peaks are seen at energies within the indirect gap in the inelastic neutron scattering experiments on SmB6 and YbB12. The in-gap features are identified as a spin exciton excitations, which are induced by residual anti-ferromagnetic interactions between the renormalized quasi-particles

  20. Spin excitons in heavy fermion semiconductors

    Riseborough, P.S. E-mail: prise@photon.poly.edu

    2001-05-01

    The heavy fermion semiconductors such as Ce{sub 3}Bi{sub 4}Pt{sub 3}, YbB{sub 12}, and SmB{sub 6} may be modeled as indirect hybridization gap semiconductors. The magnitudes of the gaps are smaller than the gaps predicted by local density functional electronic structure calculations, suggesting strong renormalizations due to electronic correlations. The inelastic neutron scattering spectra Im[{chi}(q, {omega})] are examined within the context of this model. Anomalously sharp and temperature dependent peaks are seen at energies within the indirect gap in the inelastic neutron scattering experiments on SmB{sub 6} and YbB{sub 12}. The in-gap features are identified as a spin exciton excitations, which are induced by residual anti-ferromagnetic interactions between the renormalized quasi-particles.

  1. Thermodynamic Origin of Photoinstability in the CH3NH3Pb(I1-xBrx)3 Hybrid Halide Perovskite Alloy.

    Brivio, Federico; Caetano, Clovis; Walsh, Aron

    2016-03-17

    The formation of solid-solutions of iodide, bromide, and chloride provides the means to control the structure, band gap, and stability of hybrid halide perovskite semiconductors for photovoltaic applications. We report a computational investigation of the CH3NH3PbI3/CH3NH3PbBr3 alloy from density functional theory with a thermodynamic analysis performed within the generalized quasi-chemical approximation. We construct the phase diagram and identify a large miscibility gap, with a critical temperature of 343 K. The observed photoinstability in some mixed-halide solar cells is explained by the thermodynamics of alloy formation, where an initially homogeneous solution is subject to spinodal decomposition with I and Br-rich phases, which is further complicated by a wide metastable region defined by the binodal line. PMID:26952337

  2. Local excitations in the conduction band of xenon crystal

    The basic characteristics of the luminescence band with Emax = 2 eV (band A) are studied comprehensively as a function of temperature, impurity concentration, lattice structure perfection and irradiation dose. The band A parameters are compared with similar parameters of free excitons, localized holes Xe2+* and impurity centers Xe2O*, whose bands were detected concurrently. The photoexcitation spectra and time-resolved spectra of the band A luminescence are also analyzed. Radiation of similar structure with Emax = 2,05 eV was also observed in binary crystals Ar + Xe at high (? 10%) xenon concentrations. The conclusion is drawn that the observed luminescence originates from the intrinsic excited molecular type states localized in the bulk of the crystal and having energies within the conduction band close to 10 eV

  3. Luminescence of impurity-bound excitons in Li6GdB3O9:Ce3+ single crystals

    The anomalous (τ 6GdB3O9 doped with trivalent cerium ions, has been revealed for the first time and investigated through the low-temperature time-resolved vacuum ultraviolet synchrotron spectroscopy. It was shown that the optical transitions at 6.2 eV are due to electron transfer from the ground 4f1 states of Ce3+ ion onto the autoionized states near the conduction band bottom of a crystal. These transitions lead to the formation of impurity-bound excitons in the form of correlated electron-hole pair, in which the hole component is localized at 4f-level of the cerium ion and an electron component is located at the conduction band bottom in the attractive potential of this hole. It is established that the direct radiative recombination of the cerium impurity-bound exciton leads to a fast broadband emission at 4.25 eV. The energy threshold for creation of the impurity-bound excitons was determined on the basis of the obtained spectroscopic data. We calculated the H(k) functions of distribution of the elementary relaxations over the reaction rate constants and explained on this basis the decay kinetics and quenching processes, not only for the anomalous emission at 4.25 eV, but for the ordinary 5d-4f luminescence at 3.0 eV in Ce3+ ions. The paper discusses the decay channels for the impurity-bound excitons and their influence on the decay kinetics and spectra of luminescence in Li6GdB3O9 crystals.

  4. Inhomogeneous boundary conditions for Wannier-Mott excitons

    The specular reflection spectra of CdS crystals were determined at T = 2 K in the region of the An=1 exciton resonance. The great variability in the spectra could not be explained by the simple model of an exciton-free dead layer at the boundary of a crystal. Generalized boundary conditions were formulated for large excitons, which included inhomogeneous additional boundary conditions for the exciton polarization and inhomogeneous boundary conditions for the tangential component of the magnetic induction vector B. An analysis of the energy balance equations at the surface of a crystal yielded further relationships among the parameters of the theory. The proposed system of boundary conditions described the case with a sharp internal boundary at the dead layer and allowed for the effects of the intrinsic and extrinsic mechanisms for formation of this layer

  5. Excitonic condensation in systems of strongly correlated electrons

    Kuneš, Jan

    2015-01-01

    Roč. 27, č. 33 (2015), s. 333201. ISSN 0953-8984 Institutional support: RVO:68378271 Keywords : electronic correlations * exciton * Bose-Einstein condensation Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.346, year: 2014

  6. Excitonic condensation in systems of strongly correlated electrons

    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.

  7. Large excitonic effects in group-IV sulfide monolayers

    Tuttle, Blair R.; Alhassan, Saeed M.; Pantelides, Sokrates T.

    2015-12-01

    Large exciton binding energies are a distinguishing feature of two-dimensional semiconductors because of reduced screening, potentially leading to unique optoelectronic applications. Here we use electronic structure methods to calculate the properties of a two-dimensional material class: group-IV monosulfides including SiS, GeS, and SnS. Bulk SiS is predicted to be a metastable layered material. Quasiparticle excitations are calculated with the G0W0 method and the Bethe-Salpeter equation is are used to include electron-hole interactions. For monolayers, strongly bound excitons are found below the quasiparticle absorption edge. The predicted excitonic binding energies are as high as 0.7 eV. Due to large excitonic effects, these group-IV sulfide monolayers have great potential for nanoscale optoelectronic applications.

  8. Excitons in organic molecular nanostructures: Physics and applications

    Shen, Zilan

    Excitonic processes dominate the electric and optical properties of organic materials. From delocalized charge-transfer (CT) excitons in very closely packed organic molecular crystals (OMCs) to localized Frenkel excitons in loosely packed amorphous organic solids, they determine material characteristics such as absorption, photo-conduction and luminescence. Recently, organic light emitting displays (OLEDs) and organic thin film transistors whose functionality partially depends on fundamental excitations have attracted substantial interest due to their unique properties unattainable with conventional semiconductors. To optimize the device performance such as shifting absorption peak wavelength, enhancing current injection, controlling channel conduction, maximizing electroluminescence (EL) efficiency, and obtaining saturated red, green and blue emission colors, it is essential to understand how excitonic processes is modified in organic nanostructures. In this work, we examine CT and Frenkel excitons in OMCs and amorphous organic materials. A quantum mechanical model is developed to study electrooptical properties of delocalized CT excitons in closely packed OMC nanostructures. Based on this model, we analyze the electroabsorption (EA) spectrum in bulk PTCDA and the absorption spectral shifts in PTCDA/NTCDA multilayers, and obtain consistent values of effective masses and exciton radii along difference crystalline axes. The same treatment is extended to fit the GaAs EA spectrum, suggesting a common physical origin for both CT and Wannier excitons. We also examine the Frenkel excitons in more decoupled amorphous organic solids used for OLEDs. Assuming an exponentially distributed trap states in the lowest unoccupied molecular orbital and the highest occupied molecular orbital gap, we are able to explain current-voltage characteristics and EL efficiencies observed under various temperature and charge injection conditions. We infer that the traps are due to molecular polarons, which also determine the energy distribution of excitons, and hence the EL emission spectrum. To utilize some unique properties of OMCs, we demonstrate a vertically stacked, three color OLED which allows for independent tuning of color, gray scale and intensity. The radiative recombination of Frenkel excitons is influenced by the heterogeneous multilayer structure via microcavity interference. Controlling the recombination environment by varying the layer thickness, and emissive layer positions, color saturation and EL efficiency can be optimized.

  9. Excitonic quantum interference in a quantum dot chain with rings

    We demonstrate excitonic quantum interference in a closely spaced quantum dot chain with nanorings. In the resonant dipole-dipole interaction model with direct diagonalization method, we have found a peculiar feature that the excitation of specified quantum dots in the chain is completely inhibited, depending on the orientational configuration of the transition dipole moments and specified initial preparation of the excitation. In practice, these excited states facilitating quantum interference can provide a conceptual basis for quantum interference devices of excitonic hopping

  10. One dimensional models of excitons in carbon nanotubes

    Cornean, Horia Decebal; Duclos, P.; Pedersen, Thomas Garm

    Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable.......Excitons in carbon nanotubes may be modeled by two oppositely charged particles living on the surface of a cylinder. We derive three one dimensional effective Hamiltonians which become exact as the radius of the cylinder vanishes. Two of them are solvable....

  11. Quantum Many-Body Simulation using Cavity Coupled Monolayer Excitons

    Jiang, Jian-Hua; Wang, HaiXiao; Zhan, Alan; Xu, Yadong; Chen, Huanyang; Majumdar, Arka

    2016-01-01

    We propose an architecture for quantum simulation of strongly interacting bosons using monolayer excitons in MoS$_2$ quantum dots. The strongly coupled monolayer exciton and cavity photon form polaritons between which the interaction reaches $1\\sim 10$~meV---more than one order of magnitude higher than the state-of-art value. We discuss Mott transition in a chain of coupled cavities driven by photon blockade and polariton blockade. The cavity system can mimic optical lattices and superlattice...

  12. Optical absorption of charged excitons in semiconducting carbon nanotubes

    Rønnow, Troels Frimodt; Pedersen, Thomas Garm; Cornean, Horia

    2012-01-01

    In this article we examine the absorption coefficient of charged excitons in carbon nanotubes. We investigate the temperature and damping dependence of the absorption spectra. We show that the trion peak in the spectrum is asymmetric for temperatures greater than approximately 1 K whereas the...... absorption peak arising from excitons is symmetric. We expect the positive and negative trion absorption line shapes to be identical, independently of the chiral index (n,m)....

  13. Frequency combs with weakly lasing exciton-polariton condensates

    Rayanov, K.; Altshuler, B. L.; Rubo, Y. G.; Flach, S.

    2015-01-01

    We predict the spontaneous modulated emission from a pair of exciton-polariton condensates due to coherent (Josephson) and dissipative coupling. We show that strong polariton-polariton inter- action generates complex dynamics in the weak-lasing domain way beyond Hopf bifurcations. As a result, the exciton-polariton condensates exhibit self-induced oscillations and emit an equidistant frequency comb light spectrum. A plethora of possible emission spectra with asymmetric peak dis- tributions ap...

  14. Observation of two-dimensional exciton-phonon quasibound states

    Pelekanos, N. T.; Haas, H.; Magnea, N.; Belitsky, V. I.; Cantarero, A.

    1997-10-01

    We demonstrate the existence of robust exciton-phonon quasibound states (EPQBS) in a two-dimensional semiconductor system, resulting from the binding of the e1h1 and e1h2 heavy-hole quantum-well excitons with an LO phonon. We show that increasing quantum confinement drastically weakens these two-dimensional EPQBS. A theoretical model including phonon confinement accounts qualitatively for our results.

  15. Exciton front propagation in photoexcited GaAs quantum wells

    Yang, Sen; Butov, L. V.; Simons, B. D.; Gossard, A. C.; Levitov, Leonid

    2010-01-01

    We report on the study of spatiotemporal self-organization of carriers in photoexcited GaAs quantum wells. Propagating interfaces between electron-rich and hole-rich regions are seen as expanding and collapsing exciton rings in exciton emission patterns. The interfaces preserve their integrity during expansion, remaining as sharp as in the steady state, which indicates that the dynamics is controlled by carrier transport. The front propagation velocity is measured and compared to theoretical ...

  16. The excitonic resonance in semiconductor-metal nano-hybrids

    Climente, Juan I.; Movilla, Jose L.; Goldoni, Guido; Planelles, Josep

    2011-01-01

    We use a configuration interaction approach within the envelope function approximation to study the nature of the excitonic resonance in nano-hybrids, composite nanoparticles (NPs) combining a semiconducting and a metallic segment in contact. With reference to recent experimental reports, we specifically study CdS-based nanorods with metallic NPs deposited at the tips (matchstick) or metallic coatings (core-shell). The excitonic states are computed taking into account both the renormalization...

  17. Femtosecond THz Studies of Intra-Excitonic Transitions

    Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.

    2007-10-02

    Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.

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

    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

  19. Correlations between entropy and volume of melting in halide salts

    Melting parameters and transport coefficients in the melt are collated for halides of monovalent, divalent and trivalent metals. A number of systems show a deficit of entropy of melting relative to the linear relationships between entropy change and relative volume change on melting that are found to be approximately obeyed by a majority of halides. These behaviours are discussed on the basis of structural and transport data. The deviating systems are classified into three main classes, namely (i) fast-ion conductors in the high-temperature crystal phase such as AgI, (ii) strongly structured network-like systems such as ZnCl2, and (iii) molecular systems melting into associated molecular liquids such as SbCl3. (author). 35 refs, 1 fig., 3 tabs

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

    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.