WorldWideScience
1

Exciton-relaxation dynamics in lead halides  

OpenAIRE

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

Iwanaga, Masanobu; Hayashi, Tetsusuke

2002-01-01

2

Lattice defect formation via exciton dissociation in ammonium halide crystals  

International Nuclear Information System (INIS)

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

3

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

Science.gov (United States)

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. PMID:25247715

Wu, Kewei; Bera, Ashok; Ma, Chun; Du, Yuanmin; Yang, Yang; Li, Liang; Wu, Tom

2014-11-01

4

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

Science.gov (United States)

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

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

2014-04-01

5

Optical properties of halide and oxide compounds including the excitonic effects  

Science.gov (United States)

We have studied the optical properties of alkali halide and alkaline-earth oxide compounds including the excitonic effects by using the newly developed bootstrap kernel approximation for the exchange-correlation kernel of the Time-Dependent Density Functional Theory (TD-DFT) implemented in Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method in the elk code. The bootstrap calculations are computationally less expensive and give results the same quality as the Bethe-Salpeter equation. We found improved results when compared to normal Density Functional Theory calculations, and observed results are comparable with the experiments. The lower energy peak of imaginary part of dielectric spectra shifts to lower energy regions as we move from MgO to BaO indicating the decrease in the band gap of these compounds from MgO to BaO. In all the studied compounds, the lower energy peak of the imaginary part of dielectric function is due to the transition from halogen p or oxide p states to metal derived s/d states.

Shwetha, G.; Kanchana, V.

2014-04-01

6

Excitonic energy transport in wide-band inorganic scintillators  

Science.gov (United States)

The known experimental data on the energy transport in alkali-halide scintillators and rare gas crystals with a similar electronic structure can be explained only by taking into account the vibrational levels of a two-site self-trapped exciton (excimer quasi-molecule). In a crystal, these vibrational levels turn to narrow excitonic subbands with a very large effective mass which provides a possibility for a two-site exciton to be localized in shallow potential wells produced by impurity centers and lattice defects. A very high rate of the excitonic energy transfer, observed for alkali-halide at a low temperature, is explained by a coherent directional motion of two-site excitons with a velocity close to the sound velocity in the crystal. These two mechanisms, in their combination, provide an efficient energy transfer from the host crystal to a weak impurity or radiation defects by self-trapped two-site excitons formed after thermal relaxation of photoproduced electronic excitations.

Ratner, M.; Ratner, A.; Hryn'ova, T.

2002-06-01

7

Band filling with free charge carriers in organometal halide perovskites  

Science.gov (United States)

The unique and promising properties of semiconducting organometal halide perovskites have brought these materials to the forefront of solar energy research. Here, we present new insights into the excited-state properties of CH3NH3PbI3 thin films through femtosecond transient absorption spectroscopy measurements. The photoinduced bleach recovery at 760?nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via recombination of free electrons and holes. Additionally, charge accumulation in the perovskite films leads to an increase in the intrinsic bandgap that follows the Burstein-Moss band filling model. Both the recombination mechanism and the band-edge shift are studied as a function of the photogenerated carrier density and serve to elucidate the behaviour of charge carriers in hybrid perovskites. These results offer insights into the intrinsic photophysics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications.

Manser, Joseph S.; Kamat, Prashant V.

2014-09-01

8

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

International Nuclear Information System (INIS)

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

9

Plasmonic band gap engineering of plasmon-exciton coupling.  

Science.gov (United States)

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

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

2014-10-01

10

THE LATTICE-RELAXATION ENERGY ASSOCIATED WITH SELF-TRAPPING OF A POSITIVE HOLE AND AN EXCITON IN ALKALI-HALIDES  

OpenAIRE

The authors have evaluated the upper and lower bounds to the lattice relaxation energy gained on the self-trapping of excitons, using experimental values of transition energies for free and self-trapped excitons and theoretical values for lattice relaxation energies and optical transition energies. The lattice relaxation energy upon self-trapping of an exciton in alkali halides proves to be appreciably larger than that of a self-trapped hole. They discuss the implications for a number of soli...

Itoh, N.; Tanimura, K.; Stoneham, A. M.; Harker, A. H.

1989-01-01

11

f-band condensates in exciton-polariton lattice systems  

Science.gov (United States)

We report the condensation of microcavity exciton-polaritons at ? points located on the boundary between the third and higher Brillouin zones in hexagonal lattices: triangular and honeycomb geometries. We collect experimental evidence that supports the finite momentum condensation: (1) the coherent Bragg peaks formed at nonzero ? points; (2) the nonlinear intensity increase in the exciton-polariton emission at quantum degeneracy threshold; (3) the spectral linewidth behavior: narrowing near threshold and broadening above threshold; and (4) the equivalent 4fy3-3x2y-like orbital symmetry in real space. The f-orbital state at ? points appears as a metastable momentum valley to trap exciton-polaritons, which is explained by single-particle band-structure calculations.

Kim, Na Young; Kusudo, Kenichiro; Löffler, Andreas; Höfling, Sven; Forchel, Alfred; Yamamoto, Yoshihisa

2014-02-01

12

Exciton band structure of monolayer MoS2  

Science.gov (United States)

We address the properties of excitons in monolayer MoS2 from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin-zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the (n-1 /2 ) -2 pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with nonanalytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer MoS2 is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.

Wu, Fengcheng; Qu, Fanyao; MacDonald, A. H.

2015-02-01

13

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2013-12-21

14

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

International Nuclear Information System (INIS)

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

15

Shape Dependence of Band-Edge Exciton Fine Structure in CdSe Nanocrystals  

International Nuclear Information System (INIS)

The band-edge exciton fine structure of wurtzite CdSe nanocrystals is investigated by a plane-wave pseudopotential method that includes spin-orbit coupling, screened electron-hole Coulomb interactions, and exchange interactions. Large-scale, systematic simulations have been carried out on quantum dots, nanorods, nanowires, and nanodisks. The size and shape dependence of the exciton fine structure is explored over the whole diameter-length configuration space and is explained by the interplay of quantum confinement, intrinsic crystal-field splitting, and electron-hole exchange interactions. Our results show that the band-edge exciton fine structure of CdSe nanocrystals is determined by the origin of their valence-band single-particle wave functions. Nanocrystals where the valence-band maximum originates from the bulk A band have a 'dark' ground-state exciton. Nanocrystals where the valence-band maximum is derived from the bulk B band have a 'quasi-bright' ground-state exciton. Thus, the diameter-length configuration map can be divided into two regions, corresponding to dark and quasi-bright ground-state excitons. We find that the dark/quasi-bright ground-state exciton crossover is not only diameter-dependent but also length-dependent, and it is characterized by a curve in the two-parameter space of diameter and length.

16

Bands and excitons of gallium, indium and thallium chalcogenides  

International Nuclear Information System (INIS)

The theory of the band and exciton energy structures of A3B6 and A23B36 were discussed. The results of the optical and photoemission spectra of GaS, GaSe, GaTe, InS, InSe, InTe, TlSe, TlS, Tl2S, Tl2Se, In4Se3, In4Te3, In6Se7, In6S7, In6Te7, Ga2S3, Ga2Se3, Ga2Te3, In2S3, In2Se3, and In2Te3 crystals were investigated in the range 0.1 to 100 eV. Many optical fundamental functions were calculated by the Kramers-Kronig relation method. The most probable models of the energy spectra and fundamental optical functions of A3B6 crystals in a wide region of intrinsic transformations were established as a result of comparison between the theoretical calculations of bands with optical and photoemission spectra. The experimental and theoretical data on the intrinsic energy spectra of A3B6 crystals widely used in quantum and semiconductor electronics are summarized for the first time

17

Near-band-edge exciton polarization change in ZnO nanowires.  

Science.gov (United States)

Using the atomistic pseudopotential method complemented by configuration interaction calculations, we have studied the electronic and optical properties of ZnO nanowires (NWs) in the presence of quantum confinement effects. Our results indicate that the near-band-edge exciton experiences a crossover from an in-plane polarized A-exciton (for D? 3 nm) to an out-of-plane polarized C-exciton (for D diameter of 3 nm. The observed behavior is analyzed by a stepwise inclusion of correlation effects, leading to a comprehensive description of the excitonic fine structure. PMID:25418832

Zeng, Zaiping; Petoni, Alexia; Garoufalis, Christos S; Baskoutas, Sotirios; Bester, Gabriel

2015-01-14

18

Steric engineering of metal-halide perovskites with tunable optical band gaps  

Science.gov (United States)

Owing to their high energy-conversion efficiency and inexpensive fabrication routes, solar cells based on metal-organic halide perovskites have rapidly gained prominence as a disruptive technology. An attractive feature of perovskite absorbers is the possibility of tailoring their properties by changing the elemental composition through the chemical precursors. In this context, rational in silico design represents a powerful tool for mapping the vast materials landscape and accelerating discovery. Here we show that the optical band gap of metal-halide perovskites, a key design parameter for solar cells, strongly correlates with a simple structural feature, the largest metal–halide–metal bond angle. Using this descriptor we suggest continuous tunability of the optical gap from the mid-infrared to the visible. Precise band gap engineering is achieved by controlling the bond angles through the steric size of the molecular cation. On the basis of these design principles we predict novel low-gap perovskites for optimum photovoltaic efficiency, and we demonstrate the concept of band gap modulation by synthesising and characterising novel mixed-cation perovskites.

Filip, Marina R.; Eperon, Giles E.; Snaith, Henry J.; Giustino, Feliciano

2014-12-01

19

Phase diagram of exciton condensate in doped two-band Hubbard model  

Science.gov (United States)

Using the dynamical mean-field approximation we investigate the formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multicritical point. Rapid but continuous transition between ferromagnetic and nonmagnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential.

Kuneš, Jan

2014-12-01

20

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

CERN Document Server

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...

Karlický, František; Otyepka, Michal; 10.1063/1.4736998

2012-01-01

21

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

Science.gov (United States)

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.

Egorov, Vladimir V.

2014-07-01

22

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

Directory of Open Access Journals (Sweden)

Full Text Available 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.

Vladimir V. Egorov

2014-07-01

23

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

Energy Technology Data Exchange (ETDEWEB)

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{sup *}-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{sup *}-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{sup *}-aggregates. A similar enhancement in the H{sup *}-effect caused by the strengthening of the exciton coupling in H{sup *}-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted.

Egorov, Vladimir V., E-mail: egorov@photonics.ru [Photochemistry Center, Russian Academy of Sciences, Moscow, 119421 (Russian Federation)

2014-07-15

24

Exciton spectra and energy band structure of Cu2ZnSiSe4  

International Nuclear Information System (INIS)

Highlights: • Reflection spectra of Cu2ZnSiSe4 were studied for E ? c and E || c light polarizations. • Four excitonic series are revealed in the reflection spectra at 10 K. • Model of exciton dispersion and the presence of a dead-layer. • Exciton Rydberg energies and free carriers effective masses were calculated. • Reflectivity for E ? c and E || c were analyzed in the region 3–6 eV at 300 K. -- Abstract: Exciton spectra are studied in Cu2ZnSiSe4 single crystals at 10 and 300 K by means of reflection spectroscopy. The exciton parameters, dielectric constant and free carriers effective masses are deduced from experimental spectra by calculations in the framework of a model taking into account the spatial dispersion and the presence of a dead-layer. The structure found in the reflectivity was analyzed and related to the theoretical electronic band structure of close related Cu2ZnSiS4 semiconductor

25

Layer-controlled band gap and anisotropic excitons in few-layer black phosphorus  

Science.gov (United States)

We report the quasiparticle band gap, excitons, and highly anisotropic optical responses of few-layer black phosphorous (phosphorene). It is shown that these materials exhibit unique many-electron effects; the electronic structures are dispersive essentially along one dimension, leading to particularly enhanced self-energy corrections and excitonic effects. Additionally, within a wide energy range, including infrared light and part of visible light, few-layer black phosphorous absorbs light polarized along the structures' armchair direction and is transparent to light polarized along the zigzag direction, making them potentially viable linear polarizers for applications. Finally, the number of phosphorene layers included in the stack controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy window across a wide range.

Tran, Vy; Soklaski, Ryan; Liang, Yufeng; Yang, Li

2014-06-01

26

The initial production of defects in alkali halides: F and H centre production by non-radiative decay of the self-trapped exciton  

International Nuclear Information System (INIS)

Radiation damage in KCl can be produced by the decay of a self-trapped exciton into an F centre and an H centre. Calculations are presented of the energies of the states involved for various stages in the evolution of the damage. These lead to important conclusions about the very rapid damage process, and support strongly Itoh and Saidoh's suggestion (J.Physique;34:C19:101(1973)) that damage proceeds through an excited hole state. The results also help in understanding the prompt decay of F and H pairs at low temperatures, the thermal annihilation of F and H centres, the effects of optical excitation of the self-trapped exciton, and some of the trends within the alkali halides. The calculations use a self-consistent semi-empirical molecular-orbital method, here the CNDO method as implemented in the MOSES code. A large cluster of ions is used (either 42 or 57 ions) plus long-range Madelung terms. The ion positions were obtained from separate lattice-relaxation calculations with the HADES code. The choice of CNDO parameters and the adequacy of the method were checked by a number of separate predictions. These include the energy ? luminescence, where the 2.33 eV predicted is very close to the 2.31 eV observed. (author)

27

Exciton spectra and energy band structure of CuAlS2 crystals  

International Nuclear Information System (INIS)

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

28

Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic–Inorganic Halide Perovskites  

Science.gov (United States)

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

2015-01-01

29

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

International Nuclear Information System (INIS)

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

30

Temperature dependent band gap behavior and excitons in metallic carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

Resonant Raman spectroscopy is used to investigate the temperature dependence of the optical transitions of metallic and semiconducting nanotubes. While the semiconducting nanotubes show an approximately linear temperature dependence as known for bulk semiconductors, the metallic nanotubes show a different temperature dependent behavior with a non-monotonic dependence of the transition energy on the temperature. This result can be attributed to dissociation of bound electron-hole pairs (excitons), leading to a quasi band-to-band transition. First optical transition E{sub 11}{sup M} of metallic nanotubes and sketch of a metallic (13,1) tube. (Copyright copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

May, Patrick; Telg, Hagen; Thomsen, Christian; Maultzsch, Janina [Institut fuer Festkoerperphysik, Technische Universitaet Berlin, Hardenbergstrasse 36, 10623 Berlin (Germany)

2010-12-15

31

Steric engineering of metal-halide perovskites with tunable optical band gaps  

OpenAIRE

Owing to their high energy-conversion efficiency and inexpensive fabrication routes, solar cells based on metal-organic halide perovskites have rapidly gained prominence as a disruptive technology. An attractive feature of perovskite absorbers is the possibility of tailoring their properties by changing the elemental composition through the chemical precursors. In this context, rational in silico design represents a powerful tool for mapping the vast materials landscape and ...

Filip, Marina R.; Eperon, Giles E.; Snaith, Henry J.; Giustino, Feliciano

2014-01-01

32

Fine structure of the band edge excitons and trions in CdSe/CdS core/shell nanocrystals  

OpenAIRE

We present a theoretical description of excitons and positively and negatively charged trions in "giant" CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a ...

Shabaev, A.; Rodina, A. V.; Efros, Al L.

2012-01-01

33

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2014-02-25

34

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

Directory of Open Access Journals (Sweden)

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.

Simon A. Bretschneider

2014-04-01

35

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

Science.gov (United States)

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.

Bretschneider, Simon A.; Weickert, Jonas; Dorman, James A.; Schmidt-Mende, Lukas

2014-04-01

36

Recombination dynamics of band edge excitons in quasi-two-dimensional CdSe nanoplatelets.  

Science.gov (United States)

We report a time-resolved study of the photoluminescence of CdSe colloidal nanoplatelets with two different thicknesses. By studying the exciton recombination dynamics we assess the exciton fine structure in these systems. The splitting between bright and dark excitons is enhanced compared to epitaxial quantum well structures as result of dielectric confinement. Despite of strong variations in the absolute magnitude, by comparison with literature data we find a relatively slightly varying bright-dark exciton lifetime ratio in very different CdSe-based colloidal nanostructures, regardless of growth technique and of core and shell properties such as materials, dimensions, etc. This finding points to a universal mechanism in the dark exciton recombination. PMID:24559161

Biadala, Louis; Liu, Feng; Tessier, Mickael D; Yakovlev, Dmitri R; Dubertret, Benoit; Bayer, Manfred

2014-03-12

37

Hydrostatic pressure induced splitting of recombination luminescence band in KTaO 3 : Manifestation of charge transfer vibronic exciton phase  

Science.gov (United States)

Hydrostatic pressure induced splitting of luminescence transitions under UV photoexcitation was observed in KTaO 3 crystal. Two additional satellite bands of recombination luminescence arise at lower and higher energies with approximately the same shift with respect to the initial central band position. After the appearance of satellites at elevated hydrostatic pressures (1.9-6.1 GPa), the luminescence band shape does not exhibit any significant change with further increasing of the applied pressure up to 12.7 GPa. This effect is explained by light-induced occupation of Charge Transfer Vibronic Exciton (CTVE) ferroelectric phase. CTVE-phase occupation increases with hydrostatic pressure up to its proximity to the free CTVE level. Saturated ferroelectric order parameter of this phase splits the recombination luminescence band with two satellites' appearance. Hydrostatic pressure induced satellites in recombination luminescence can be considered as the CTVE-phase manifestation for KTaO 3 crystal.

Vikhnin, V. S.; Kutsenko, A. B.; Kapphan, S. E.

2006-12-01

38

Tuning the light emission properties by band gap engineering in hybrid lead halide perovskite.  

Science.gov (United States)

We report about the relationship between the morphology and luminescence properties of methylammonium lead trihalide perovskite thin films. By tuning the average crystallite dimension in the film from tens of nanometers to a few micrometers, we are able to tune the optical band gap of the material along with its photoluminescence lifetime. We demonstrate that larger crystallites present smaller band gap and longer lifetime, which correlates to a smaller radiative bimolecular recombination coefficient. We also show that they present a higher optical gain, becoming preferred candidates for the realization of CW lasing devices. PMID:25469762

D'Innocenzo, Valerio; Srimath Kandada, Ajay Ram; De Bastiani, Michele; Gandini, Marina; Petrozza, Annamaria

2014-12-24

39

Effect of temperature variation on shift and broadening of exciton band in Cs3Bi2I9 layered crystals  

International Nuclear Information System (INIS)

The exciton reflection spectra of Cs3Bi2I9 layered crystals is investigated in the temperature region 4.2-300 K with light polarization E perpendicular to c. It is estimated that the energy gap Eg equals 2.857 eV (T = 4.2 K) and the exciton binding energy Ry is 279 MeV. A nontraditional temperature shift of Eg(T) for the layered substances is found for the first time. It is learned that this shift is described very well by the Varshni formula. A transition region in the temperature broadening of the half-width H(T) of the exciton band with the increase of temperature is registered in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. A surge of H(T) at the point of the ferroelastic phase transition (Tc = 220 K) is also observed

40

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

International Nuclear Information System (INIS)

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

41

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

Energy Technology Data Exchange (ETDEWEB)

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.

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

42

High Density Excitation Effect of the Indirect Band Luminescence in Thallous Halides*  

Science.gov (United States)

A broad band luminescence is observed at the indirect gap region in highly purified TlBr and TlCl crystals under high density excitation. From the analysis of the line shape, power dependence, lifetime and some characteristic features of the luminescence, it is discussed the possibility that the luminescence is associated with an LA-phonon assisted indirect recombination (X6+-R6-) of electron-hole liquid (EHL). Equilibrium density, binding energy and other parameters of EHL are estimated and it is presumed that the hole-phonon scattering is dominant in their recombination process.

Takiyama, Ken; Nakahara, Junichiro; Takenaka, Hisashi; Kobayashi, Koichi; Fujita, Toshiaki; Fujii, Atsuhiro

1996-01-01

43

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

Czech Academy of Sciences Publication Activity Database

Ro?. 89, ?. 11 (2014), "115134-1"-"115134-8". ISSN 1098-0121 R&D Projects: GA ?R GA13-25251S Institutional support: RVO:68378271 Keywords : excitonic condensation * spin-state transition * dynamical mean - field theory Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.664, year: 2013

Kuneš, Jan; Augustinský, Pavel

2014-01-01

44

TWO-PHONON PROGRESSIONS ASSOCIATED WITH VIBRONIC EXCITONS IN LAYERED 3d-METAL COMPOUNDS  

OpenAIRE

We have observed two-phonon structures in some intraconfigurationa1 crystal field transitions of V-, Mn- and Ni-halides. We also report for the first time the crystal field spectrum of CrBr2(3d4), where, despite its unfilled eg semishell, exciton-phonon (ep) interaction vanishes to first order yielding a two-phonon sequence in 3A1(G) + 3A2(F) band.

Benedek, G.; Pollini, I.; Bauhofer, W.

1981-01-01

45

Determination of energy-band offsets between GaN and AlN using excitonic luminescence transition in AlGaN alloys  

Science.gov (United States)

We report the determination of the energy-band offsets between GaN and AlN using the linewidth (full width at half maximum) of an extremely sharp excitonic luminescence transition in AlxGa1-xN alloy with x=0.18 at 10 K. Our sample was grown on C-plane sapphire substrate by metal-organic chemical-vapor deposition at 1050 °C. The observed value of the excitonic linewidth of 17 meV is the smallest ever reported in literature. On subtracting a typical value of the excitonic linewidth in high-quality GaN, namely, 4.0 meV, we obtain a value of 13.0 meV, which we attribute to compositional disorder. This value is considerably smaller than that calculated using a delocalized exciton model [S. M. Lee and K. K. Bajaj, J. Appl. Phys. 73, 1788 (1993)]. The excitons are known to be strongly localized by defects and/or the potential fluctuations in this alloy system. We have simulated this localization assuming that the hole, being much more massive than the electron, is completely immobile, i.e., the hole mass is treated as infinite. Assuming that the excitonic line broadening is caused entirely by the potential fluctuations experienced by the conduction electron, the value of the conduction-band offset between GaN and AlN is determined to be about 57% of the total-band-gap discontinuity. Using our model we have calculated the variation of the excitonic linewidth as a function of Al composition in our samples with higher Al content larger than 18% and have compared it with the experimental data. We also compare our value of the conduction-band offset with those recently proposed by several other groups using different techniques.

Westmeyer, A. N.; Mahajan, S.; Bajaj, K. K.; Lin, J. Y.; Jiang, H. X.; Koleske, D. D.; Senger, R. T.

2006-01-01

46

Magneto-photoluminescence of GaN\\/AlGaN quantum wells valence band reordering and excitonic binding energies  

CERN Document Server

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for the observed values for the g-factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wavefunction from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al (phys. stat. sol. (b) 216, 371 (1999)) that uses a variational approach in the envelope function formalism that includes the effect of the electric field in the wells.

Shields, P A; Grandjean, N; Massies, J

2001-01-01

47

Charge transfer excitons and possible exitonic pairing in the extended three band Hubbard model  

CERN Document Server

Exact diagonalisations of the extended Hubbard model are performed. In the insulating regime it is shown that the nearest neighbour copper-oxygen repulsion, V, leads to Frenkel excitons in the charge transfer gap at values of V of the order of copper-oxygen hybridisation, t. In the metallic regime it is shown that the static charge-transfer and density-density correlation functions diverge as a function of V, indicating a charge-tansfer instability and phase separation. This is accompanied by a softening of the q \\rightarrow 0 mode of the dynamic correlation functions which is associated with the excitonic excitations responsible for the superconducting correlations observed in the proximity of the phase separation boundary of ref. \\cite{cv95b}.

Vermeulen, C; Gagliano, E R; Vermeulen, C; Barford, W; Gagliano, E R

1995-01-01

48

BRIEF COMMUNICATIONS: Mercury halide vapor molecular laser pumped by a wide-band optical radiation and emitting three-color visible radiation  

Science.gov (United States)

A description is given of the first mercury halide photodissociation laser pumped by wide-band thermal radiation from a plasma and emitting three-color visible radiation. Simultaneous lasing was observed for ionic-covalent electronic B-X transition in molecules of mercury monochloride ?lapprox558 and 559 nm, monobromide (?lapprox502 and 504 nm), and monoiodide (?lapprox443 and 444 nm). The laser radiation energy was ~0.3J emitted in the form of ~2.5?sec pulses and the lasing efficiency was ~0.07%.

Bazhulin, S. P.; Basov, N. G.; Bugrimov, S. N.; Zuev, V. S.; Kamrukov, A. S.; Kashnikov, G. N.; Kozlov, N. P.; Ovchinnikov, P. A.; Opekan, A. G.; Protasov, Yu S.

1986-07-01

49

On the interpretation of luminescence of lead halide crystals.  

Czech Academy of Sciences Publication Activity Database

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

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

2002-01-01

50

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2014-10-15

51

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

Energy Technology Data Exchange (ETDEWEB)

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

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

52

Excitonic properties related to valence band levels split by spin-orbit interaction in layered oxychalcogenide LaCuOCh(Ch=S,Se)  

International Nuclear Information System (INIS)

The energy splitting of valence band in oxychalcogenides LaCuOCh (Ch=S, Se) was studied through degenerative four-wave mixing (DFWM) signals excited by femtosecond laser pulses and high-resolution absorption spectra at 4 K. The DFWM signals for LaCuOS exhibited a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states were split by 9 meV, while the corresponding splitting in LaCuOSe was 125 meV. The spin-orbit interaction of Ch ion accompanied by the hybridization of Cu 3d orbital causes the splitting of the exciton levels

53

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

CERN Document Server

Fractionalized phases are studied in a low energy theory of exciton bose condensate in a multi-band insulator. It is shown that U(1) gauge theory with either fractionalized boson or fermion can emerge out of a single model depending on the coupling constants. Both the statistics and spin of the fractionalized particles are dynamically determined, satisfying the spin-statistics theorem in the continuum limit. We present two mutually consistent descriptions for the fractionalization. In the first approach, it is shown that fractionalized degree of freedom emerges from reduced phase space constrained by strong interaction and that the U(1) gauge field arises as a collective excitation of the low energy modes. In the second approach, complimentary descriptions are provided for the fractionalization based on world line picture of the original excitons. The emergent gauge structure is identified from the fluctuating web of exciton world lines which, in turn, realizes the string net condensation in a space-time pict...

Lee, S S; Lee, Sung-Sik; Lee, Patrick A.

2005-01-01

54

Band edge emission enhancement by quadrupole surface plasmon-exciton coupling using direct-contact Ag/ZnO nanospheres.  

Science.gov (United States)

Periodic Ag nanoball (NB) arrays on ZnO hollow nanosphere (HNS) supporting structures were fabricated in a large area by a laser irradiation method. The optimized laser power and spherical supporting structure of ZnO with a certain size and separation were employed to aggregate a sputtering-deposited Ag nano-film into an ordered, large-area, and two dimensional Ag NB array. A significant band edge (BE) emission enhancement of ZnO HNSs was achieved on this Ag NB/ZnO HNS hybrid structure and the mechanism was revealed by further experimental and theoretical analyses. With successfully fabricating the direct-contact structure of a Ag NB on the top of each ZnO HNS, the highly localized quadrupole mode surface plasmon resonance (SPR), realized on the metal NBs in the ultraviolet region, can effectively improve the BE emission of ZnO through strong coupling with the excitons of ZnO. Compared with the dipole mode SPR, the quadrupole mode SPR is insensitive to the metal nanoparticle's size and has a resonance frequency in the BE region of the wide band gap materials, hence, it can be potentially applied in related optoelectronic devices. PMID:23196786

Zang, Yashu; He, Xu; Li, Jing; Yin, Jun; Li, Kongyi; Yue, Chuang; Wu, Zhiming; Wu, Suntao; Kang, Junyong

2013-01-21

55

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

Energy Technology Data Exchange (ETDEWEB)

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.

Bhuyan, Sumi; Pal, Bipul; Bansal, Bhavtosh, E-mail: bhavtosh@iiserkol.ac.in [Indian Institute of Science Education and Research Kolkata, Mohanpur Campus, Nadia 741252, West Bengal (India); Das, Sanat K.; Dhar, Sunanda [Department of Electronic Science, University of Calcutta, 92 A.P.C. Road, Kolkata 700009 (India)

2014-07-14

56

Glycosyl Halides  

Science.gov (United States)

This chapter describes the preparations and chemical glycosylation reactions of glycosyl halides as glycosyl donors including glycosyl bromides, chlorides, iodides, and fluorides. For a survey on the general current methodological advances, glycosyl halide donors are classified into four groups based on the type of anomeric functional group and their activating methods. Among them, glycosyl fluorides, which are frequently and widely used in current glycosylation reactions, are particularly emphasized and discussed in this chapter.

Toshima, Kazunobu

57

Temperature-dependent study of the band-edge excitonic transitions of Cu{sub 2}ZnSiS{sub 4} single crystals by polarization-dependent piezoreflectance  

Energy Technology Data Exchange (ETDEWEB)

The temperature dependence of the band-edge excitonic transitions of Cu{sub 2}ZnSiS{sub 4} single crystals were characterized by using polarization-dependent piezoreflectance (PzR) in the temperature range of 10-300 K. The PzR measurements were carried out on the as-grown basal plane with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The PzR spectra revealed polarization-dependent E{sub perpendicular}{sup ex} and E{sub ||}{sup ex} features for E-perpendicular c and E||c polarization, respectively. Both E{sub perpendicular}{sup ex} and E{sub ||}{sup ex} features are associated with the interband excitonic transitions at {Gamma} point and can be explained by crystal-field splitting of valence band. From a detailed lineshape fit to the PzR spectra, the temperature dependence of the transition energies and broadening parameters of the band-edge excitons were determined accurately. The temperature dependence of near band-edge excitonic transition energies were analyzed using Varshni and Bose-Einstein expressions. The temperature dependence of the broadening parameter of excitonic features also has been studied in terms of a Bose-Einstein equation that contains the electron (exciton)-longitudinal optical phonon-coupling constant. The parameters that describe the temperature variation of the excitonic transition energies and broadening parameters were evaluated and discussed.

Levcenco, S.; Dumcenco, D. [Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan (China); Huang, Y.S., E-mail: ysh@mail.ntust.edu.t [Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan (China); Arushanov, E.; Tezlevan, V. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD 2028 (Moldova, Republic of); Tiong, K.K. [Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan (China); Du, C.H. [Department of Physics, Tamkang University, Tamsui 251, Taiwan (China)

2010-09-10

58

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

International Nuclear Information System (INIS)

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

59

Excitons in hexagonal nanonetwork materials  

CERN Document Server

Optical excitations in hexagonal nanonetwork materials, for example, Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. A permanent electric dipole moment, whose direction is from the B site to the N site, is considered along the BN bond. When the exciton hopping integral is restricted to the nearest neighbors, the flat band of the exciton appears at the lowest energy. The symmetry of this exciton band is optically forbidden, indicating that the excitons relaxed to this band will show quite long lifetime which will cause luminescence properties.

Harigaya, K

2003-01-01

60

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

DEFF Research Database (Denmark)

It is shown that the integrated luminescence intensity of localized excitons in solid solutions ZnSe(1 - c)Tec has a component slowly decaying with time. After the excitation above the mobility threshold, the long-time intensity decreases exponentially, with a fractional exponent changing from a value corresponding to the critical index of anomalous diffusion to the index of normal diffusion as the temperature increases from 5 to 80 K. This change allows estimation of the energy scale for the fluctuation tail of the conduction band. (C) 2000 MAIK "Nauka / Interperiodica".

Klochikhin, O.; Ogloblin, S. G.

2000-01-01

61

Mollwo-Ivey relations for optical absorption bands of the atomic and F' centres in alkali halides  

Energy Technology Data Exchange (ETDEWEB)

Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl and due to shallow electron traps or bound polarons according to Jacobs (Phys. Stat. Sol. B 129 (1985) 755) and Korovkin and Lebedkina (Fiz. Tverd. Tela (Russian) 35 (1993) 642), and (b) with maxima at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, RbCl:I and RbBr:I, due to on-centre STE localised at iodine-dimer according to Hirota et al. (J. Phys. Soc. Japan 63 (1994) 2774, Phys. Rev. B 52 (1995) 7779) and Edamatsu and Hirai (Mater. Sci. Forum 239-241 (1997) 525), are caused by the same defect. We propose that the defect is an atomic alkali impurity centre [M{sup +}]{sub c}{sup 0}e{sup -}, i.e. an electron e{sup -} trapped by a smaller size substitutional alkali cation impurity [M{sup +}]{sub c}{sup 0}. The Mollwo-Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E{sub 0} for NaCl, KCl, KBr, RbCl and NaBr, KCl:I and/or the low-energy edge values E{sub 0} for NaI, RbCl:I and RbBr:I versus anion-cation distance, d, are obtained for the first time. These data suggest that two types of the [M{sup +}]{sub c}{sup 0}e{sup -} centres are predominant: (i) [Na{sup +}]{sub c}{sup 0}e{sup -} in KX and RbX host crystals with the relation E{sub 0}{approx}6.15/d{sup 2.74} and (ii) [Li{sup +}]{sub c}{sup 0}e{sup -} in NaX host crystals with E{sub 0}{approx}29.4/d{sup 4.72}. The Mollwo-Ivey relation E{sub 0}{approx}18.36/d{sup 2.70} is fulfilled as well for the F' band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F' centre optical binding energy values for E{sub 0}.

Ziraps, V. E-mail: ziraps.v@cfi.lu.lv

2001-10-01

62

Mollwo-Ivey relations for optical absorption bands of the atomic and F' centres in alkali halides  

International Nuclear Information System (INIS)

Evidence indicates that two classes of the transient IR-absorption bands: (a) with maxima at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl and due to shallow electron traps or bound polarons according to Jacobs (Phys. Stat. Sol. B 129 (1985) 755) and Korovkin and Lebedkina (Fiz. Tverd. Tela (Russian) 35 (1993) 642), and (b) with maxima at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, RbCl:I and RbBr:I, due to on-centre STE localised at iodine-dimer according to Hirota et al. (J. Phys. Soc. Japan 63 (1994) 2774, Phys. Rev. B 52 (1995) 7779) and Edamatsu and Hirai (Mater. Sci. Forum 239-241 (1997) 525), are caused by the same defect. We propose that the defect is an atomic alkali impurity centre [M+]c0e-, i.e. an electron e- trapped by a smaller size substitutional alkali cation impurity [M+]c0. The Mollwo-Ivey plots for the transient IR-absorption bands of the zero-phonon line energy E0 for NaCl, KCl, KBr, RbCl and NaBr, KCl:I and/or the low-energy edge values E0 for NaI, RbCl:I and RbBr:I versus anion-cation distance, d, are obtained for the first time. These data suggest that two types of the [M+]c0e- centres are predominant: (i) [Na+]c0e- in KX and RbX host crystals with the relation E0?6.15/d2.74 and (ii) [Li+]c0e-]c0e- in NaX host crystals with E0?29.4/d4.72. The Mollwo-Ivey relation E0?18.36/d2.70 is fulfilled as well for the F' band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F' centre optical binding energy values for E0

63

Intrinsic excitonic photoluminescence and band-gap engineering of wide-gap p-type oxychalcogenide epitaxial films of LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) semiconductor alloys  

Science.gov (United States)

The optical spectroscopic properties of layered oxychalcogenide semiconductors LnCuOCh (Ln=La, Pr, and Nd; Ch=S or Se) on epitaxial films were thoroughly investigated near the fundamental energy band edges. Free exciton emissions were observed for all the films between 300 and ˜30 K. In addition, a sharp emission line, which was attributed to bound excitons, appeared below ˜80 K. The free exciton energy showed a nonmonotonic relationship with lattice constant and was dependent on lanthanide and chalcogen ion substitutions. These results imply that the exciton was confined to the (Cu2Ch2)2- layer. Anionic and cationic substitutions tune the emission energy at 300 K from 3.21 to 2.89 eV and provide a way to engineer the electronic structure in light-emitting devices.

Hiramatsu, Hidenori; Ueda, Kazushige; Takafuji, Kouhei; Ohta, Hiromichi; Hirano, Masahiro; Kamiya, Toshio; Hosono, Hideo

2003-11-01

64

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

Science.gov (United States)

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.

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

2012-05-01

65

Electronic states and the resonant optical non-linearity of exciton in a narrow band InSb quantum dot  

Energy Technology Data Exchange (ETDEWEB)

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGa{sub x}Sb{sub 1-x} quantum dot are computed as functions of dot radius and the Ga content. Optical properties are obtained using the compact density matrix approach. The dependence of non-linear optical processes on the dot sizes is investigated for different Ga concentrations. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of the exciton are calculated for different concentrations of gallium content. It is found that gallium concentration has great influence on the optical properties of InSb/InGa{sub x}Sb{sub 1-x} dots.

Narayanan, M. [Department of Physics, Yadava College Govindarajan Campus, Thiruppalai, Madurai-625 014 (India); John Peter, A., E-mail: a.john.peter@gmail.com [Center for Environmental Studies/Green Energy Center, Department of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of); Yoo, Chang Kyoo [Center for Environmental Studies/Green Energy Center, Department of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of)

2012-02-01

66

Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 1. Optical spectroscopy of excitonic interactions involving the soret band  

OpenAIRE

The photophysical properties of excited singlet states of zinc tetra-(p-octylphenyl)-porphyrin in 5-25-nm-thick films spin-coated onto quartz slides have been investigated by optical spectroscopy. Analysis of the polarized absorption spectra using a dipole-dipole exciton model with two mutually perpendicular transition dipole moments per molecule shows that the films are built from linear aggregates, i.e., stacks with a slipped-deck-of-cards configuration. The molecular planes of the porphyri...

Donker, H.; Koehorst, R. B. M.; Schaafsma, T. J.

2005-01-01

67

A variational Monte Carlo study of exciton condensation  

Science.gov (United States)

Exciton condensation in a two-band Hubbard model on a square lattice is studied with variational Monte Carlo method. We show that the phase transition from an excitonic insulator to a band insulator is induced by increasing the interband Coulomb interaction. To examine the character of the exciton condensation, the exciton pair amplitudes both in k-space and in real space are calculated. Using these quantities, we discuss the BCS-BEC crossover within the excitonic insulator phase.

Watanabe, Hiroshi; Seki, Kazuhiro; Yunoki, Seiji

2015-03-01

68

Fine structure and temperature dependence of shallow core excitons in insulators and semiconductors.  

Science.gov (United States)

The excitonic spectra associated with the shallow core levels K(+)3p, Ga3d, and In4d in potassium halide and III-V semiconductor single crystals were studied by high resolution reflectivity (DeltaE F centers, whereas weak coupling describes the core excitons in semiconductors. The contributions of initial and final states are discussed. Core surface excitons in semiconductors show a significantly smaller temperature coefficient than the volume excitons. PMID:20234725

Skibowski, M; Sprussei, G; Saile, V

1980-12-01

69

Optical absorption of triplet molecular excitons in alkali cyanides  

International Nuclear Information System (INIS)

The optical absorption spectra of alkali cyanides in the UV region present a set of weak absorption bands which are identified as triplet a'3?+ molecular excitons. The nature of the molecular exciton transitions in the ionic-molecular crystals is discussed and the existence of an admisture between molecular exciton and charge transfer exciton states is suggested. (Author)

70

Novel excitonic states in quantum Hall systems: Bound states of spin waves and a valence band hole  

OpenAIRE

If the Zeeman energy is small, the lowest energy excitations of a two dimensional electron gas at filling factor nu=1 are spin waves (spin flip excitations). At nu slightly larger (smaller) than unity, reversed spin electrons (spin holes) can form bound states with K spin waves that are known as skyrmions, S_K^- (antiskyrmions, S_K^+). It is suggested in this work that a valence hole can also bind K spin waves to form an excitonic complex X_K^+, analogous to the S_K^+. One s...

Quinn, John J.; Wojs, Arkadiusz

2002-01-01

71

The Silver Halides  

Science.gov (United States)

Illustrates the type of fractional bonding for solid silver halides. Treats the silver halides as electron excess compounds, and develops a model of a localized bonding unit that may be iterated in three dimensions to describe the bulk phase. (MLH)

Sahyun, M. R. V.

1977-01-01

72

Novel excitonic states in quantum Hall systems Bound states of spin waves and a valence band hole  

CERN Document Server

If the Zeeman energy is small, the lowest energy excitations of a two dimensional electron gas at filling factor nu=1 are spin waves (spin flip excitations). At nu slightly larger (smaller) than unity, reversed spin electrons (spin holes) can form bound states with K spin waves that are known as skyrmions, S_K^- (antiskyrmions, S_K^+). It is suggested in this work that a valence hole can also bind K spin waves to form an excitonic complex X_K^+, analogous to the S_K^+. One spin hole of the S_K^+ is simply replaced by the valence hole. At nu=1, the (S_K^-)-(X_K^+) attraction can potentially lead to different behavior.

Quinn, J J; Quinn, John J.; Wojs, Arkadiusz

2002-01-01

73

Exciton dynamics in 121 angstrom GaAs and GaAsP QWs having different valence band structures arising from built-in biaxial stress  

International Nuclear Information System (INIS)

Exciton dynamics in various 121 angstrom single quantum wells (QWs): a AlGaAs/GaAs QW and two AlGaAs/GaAsP QWs, under different built-in biaxial tension, has been investigated using time resolved photoluminescence (PL) spectroscopy at 5 K. Heavy-hole (hh) and light-hole (lh) exciton formation times from free electron-hole pair, hh (lh) exciton to lh (hh) exciton inter-subband relaxation times, exciton localization times to interface islands, and localized exciton annihilation decay times in the strained and nonstrained QWs have been determined by fitting the PL time profiles at the lowest emission energy with an analytical solution for the localized exciton population profile obtained by solving six level rate equations

74

Excitons in Alkali Halides: Analysis of Type I STE Spectrafootnote[1] Supported in part by the State of Vermont and the National Science Foundation under EPSCoR/grant RII-8610679.  

Science.gov (United States)

We have analyzed the newly observedfootnote[2] K. Edamatsu and M Hirai, Mat. Sci. Forum (in press). transient IR absorption spectra of ``Type I'' self-trapped excitons in NaBr and NaI for the exciton's spatial extent and the exciton Rydberg. A Vinti sum-rule analysis shows that the electron orbits extend well into the bulk crystal with radii 2.6 to 4 times the interatomic spacing. This corresponds to an electron distribution covering 100 or more lattice sites. The transient absorption line shape can be fit extremely well with a phonon-broadened hydrogenic spectrum. Gaussian broadening yields significantly better fits than Lorentzians. Rydberg values are 0.27 eV for NaBr and 0.19 eV for NaI. The large radii, small Rydberg and approximate Wannier-exciton-like properties of the absorption strongly suggest the originalfootnote[3]M. N. Kabler, Phys. Rev. 136, A1296 (1964); R. B. Murray and F. J. Keller, Phys. Rev. 137, A942 (1965). ``on-center'' model for STEs applies to these materials.

Malghani, M. S.; Smith, D. Y.

1997-03-01

75

Magneto-photoluminescence of GaN/AlGaN quantum wells: valence band reordering and excitonic binding energies  

OpenAIRE

A re-ordered valence band in GaN/AlGaN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g-factor in magneto-luminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlGaN barriers thus giving different barrier heights for the different quantum well hole states. From k.p calculations in the quasi-cubic approximation, a change in the valence-band ordering will account for...

Shields, P. A.; Nicholas, R. J.; Grandjean, N.; Massies, J.

2001-01-01

76

Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots.  

Science.gov (United States)

The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark-bright energy level splitting of 2.4-5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark-bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, in the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level in small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark ±2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range. PMID:21961786

Moreels, Iwan; Rainò, Gabriele; Gomes, Raquel; Hens, Zeger; Stöferle, Thilo; Mahrt, Rainer F

2011-10-25

77

Halogens and Halides (GCMP)  

Science.gov (United States)

Halogens and Halides: this is a resource in the collection "General Chemistry Multimedia Problems". In this problem we will study the oxidation-reduction reactions between the halogens and the halide ions. The halogens and halides will be dissolved in water and hexane. General Chemistry Multimedia Problems ask students questions about experiments they see presented using videos and images. The questions asked apply concepts from different parts of an introductory course, encouraging students to decompartmentalize the material.

78

Localized Excitons and Breaking of Chemical Bonds at III-V (110) Surfaces  

OpenAIRE

Electron-hole excitations in the surface bands of GaAs(110) are analyzed using constrained density-functional theory calculations. The results show that Frenkel-type autolocalized excitons are formed. The excitons induce a local surface unrelaxation which results in a strong exciton-exciton attraction and makes complexes of two or three electron-hole pairs more favorable than separate excitons. In such microscopic exciton "droplets" the electron density is mainly c...

Pankratov, Oleg; Scheffler, Matthias

1995-01-01

79

Reply to ``Comment on `Band structures and optical spectra of InN polymorphs: Influence of quasiparticle and excitonic effects'?''  

Science.gov (United States)

In their Comment, Bagayoko [Phys. Rev. B 76, 037101 (2007)] criticized the three-step electronic-structure calculations of Furthmüller [Phys. Rev. B 72, 205106 (2005)] to calculate the band structures and hence the fundamental energy gaps of crystalline InN polymorphs of 0.82eV (wurtzite InN) or 0.55eV (zinc-blende InN) [J. Furthmüller , Phys. Rev. B 72, 205106 (2005); F. Bechstedt and J. Furthmüller, J. Cryst. Growth 246, 315 (2002)]. In contrast, in their self-consistent density-functional calculations they compute the fundamental gaps at the theoretical lattice constants to 0.88 or 0.65eV in one step, already on the density-functional theory (DFT) local-density approximation (LDA) level of approximation [D. Bagayoko and L. Franklin, J. Appl. Phys. 97, 123708 (2005); D. Bagayoko , J. Appl. Phys. 96, 4297 (2004)].

Furthmüller, J.; Hahn, P. H.; Fuchs, F.; Bechstedt, F.

2007-07-01

80

Two-photon transitions to exciton polaritons  

International Nuclear Information System (INIS)

A semiclassical theory for the creation of excitonic polariton states by two-photon absorption, via an intermediate exciton state, is given. A band model has been introduced which gives the dominant contribution to this process. A numerical calculation is found to be in good agreement with a recent observation in CuCl. (author)

81

Localized molecular excitons in polyaniline  

Science.gov (United States)

An analysis of the UV absorption spectra of polyemeraldine and nigrosine reveals that the absorption band which appears at about 2.2 eV in both compounds is due to the creation of a localized molecular exciton with the electron on a quinoid moiety and the hole on the neighboring two benzoid moieties. The quinoid moiety is rotated approximately 90° relative to its ground-state conformation. Higher-energy absorption bands may be associated with molecular excitons localized on individual phenyl moieties.

Duke, C. B.; Conwell, E. M.; Paton, A.

1986-10-01

82

Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures  

OpenAIRE

Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot quantum well nanocrystals are determined by use of a multiband theory that includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian, and nonparabolicity of the conduction band. The multiband theory correctly describes the recently observed dark-exciton ground state and the lowest, optically active, bright-exciton states. Charge separation in pair states is identified. Previous sing...

Jaskolski, W.; Bryant, Garnett W.

1997-01-01

83

Radiative Lifetime of Excitons in Carbon Nanotubes  

CERN Document Server

We calculate the radiative lifetime and energy bandstructure of excitons in semiconducting carbon nanotubes, within a tight-binding approach. In the limit of rapid interband thermalization, the radiative decay rate is maximized at intermediate temperatures, decreasing at low temperature because the lowest-energy excitons are optically forbidden. The intrinsic phonons cannot scatter excitons between optically active and forbidden bands, so sample-dependent extrinsic effects that break the symmetries can play a central role. We calculate the diameter-dependent energy splittings between singlet and triplet excitons of different symmetries, and the resulting dependence of radiative lifetime on temperature and tube diameter.

Perebeinos, V; Avouris, P; Perebeinos, Vasili; Avouris, Phaedon

2005-01-01

84

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes  

OpenAIRE

Exciton effects are studied in single-wall boron-nitride (BN) nanotubes. Linear absorption spectra are calculated with changing the chiral index of the zigzag nanotubes. We consider the extended Hubbard model with atomic energies at the boron and nitrogen sites. Exciton effects are calculated using the configuration interaction technique. The Coulomb interaction dependence of the band gap, the lowest exciton energy, and the binding energy of the exciton are discussed. The op...

Harigaya, Kikuo

2007-01-01

85

A new luminescence due to an exciton-exciton collision process in lead iodide induced by two-photon absorption  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence properties of highly excited lead iodide have been investigated through two-photon absorption for band-to-band transitions at low temperature and a new luminescence band is observed. The intensity of the new luminescence band increases as 4th power of the excitation intensity and the peak energy is about 23 meV lower than the lowest exciton energy, whose value is almost equal to the energy difference between the n=1 and 2 excitonic states of lead iodide. These results indicate that the new luminescence band observed originates from an exciton-exciton collision process, in which one of the two excitons scatters into the n=2 excitonic state and the other radiatively recombines.

Tanaka, K. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Hosoya, T. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Fukaya, R. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Takeda, J. [Department of Physics, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan)]. E-mail: jun@ynu.ac.jp

2007-01-15

86

Photofragmentation of metal halides  

International Nuclear Information System (INIS)

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

87

Excitons in narrow-gap carbon nanotubes  

OpenAIRE

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-m...

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

2010-01-01

88

Spatially indirect excitons in coupled quantum wells  

Energy Technology Data Exchange (ETDEWEB)

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.

Lai, Chih-Wei Eddy

2004-03-01

89

Crown ether uranyl halide complexes  

International Nuclear Information System (INIS)

Uranium oxide is recovered from an aqueous solution of uranyl halides by extracting uranyl halide from the aqueous solution with an organic liquid, forming a crown ether uranyl halide complex in the organic liquid, and then contacting the uranyl halide crown ether complex with water, carboxylate ion, and light under suitable conditions. Also disclosed are certain novel crown ether uranyl halide complexes and their preparation. Also disclosed is the use of 18-crown-6 to selectively recover uranyl halide from a solution thereof containing other metal salts

90

Bivalent rare earth elements spectra in ammonium halide crystals  

International Nuclear Information System (INIS)

It is shown by optical methods that absorption band in ammonium halides with bivalent rare earth mixtures are conditioned transition from the main 4fn-a conditions in mixture 4fn-15d-a level. The methods to indignations theories are calculated split-pouring 5d-a level rare earth ions in crystalline field of ammonium halides. Electronic-background interaction and non-radioactive transitions are experimentally explored. (author)

91

Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates  

CERN Document Server

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 ...

Kühn, Oliver

2011-01-01

92

Binary technetium halides  

Science.gov (United States)

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

Johnstone, Erik Vaughan

93

Some uranium halide complexes  

International Nuclear Information System (INIS)

Uranium halide complex salts [MeEtim]X (where[MeEtim] 1-methyl-3-ethyl-imidazolium; X-Cl, Br, I) have been used for synthesis of room temperature ionic liquids AlX3 [Me Etim]X. The crystal structure of [MeEtim]I revealed significant H bonding between cation and anion. This interaction, detected here for the first time, is important for the structure of these ionic liquids which are efficient solvents for studying electrochemical and spectroscopic properties of novel uranium halide complex salts. The cyclic voltammogram of [UO2X4]2- salts showed irreversible and reversible reduction processes. X-ray structural analysis of this salt showed tetragonal symmetry and no evidence of H bonds between anions and cations. The X-ray structure of a pseudo-halide compound [U(NCS)]4- showed significant cation-anion bonding. (U.K.)

94

Exciton dispersion in molecular solids  

Science.gov (United States)

The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron–hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier–Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe–Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron–hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.

Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo

2015-03-01

95

Mott excitons in ferrodielectrics  

International Nuclear Information System (INIS)

Mott excitons in ferrodielectrics are investigated. It is shown that the two-particle equations for these excitons are different. These differences lead to the splitting of excitonic states. The generation of these excitons by electron beams is also discussed. (author)

96

Intermediate Exciton-Phonon Coupling in Tetracene  

Science.gov (United States)

Polarized absorption spectra of tetracene (naphthacene) single crystals have been investigated between 300 and 13 K, with particular emphasis on the b-polarized spectrum. The low energy tails of the a- and b-polarized first absorption bands are expressed by the Urbach rule with the steepness coefficients ?a'{=}1.37± 0.12, ?b'{=}1.58± 0.13, with which possibility of self-trapping of excitons is discussed along with the dimensionality of tetracene crystals. The exciton-phonon coupling constants are found to be ga{=}1.09± 0.1 and gb{=}0.95± 0.08, consistent with luminescence experiments which show that excitons in tetracene are self-trapped very shallowly. The line halfwidth, the oscillator strength of the first and the second absorption bands, refractive indices, and surface exciton structure are briefly discussed.

Mizuno, Kenichi; Matsui, Atsuo; Sloan, G. J.

1984-08-01

97

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

Science.gov (United States)

Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 1016 cm-3 to 6 × 1017 cm-3. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ˜3.47 eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1-2 × 1017 cm-3 is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.

Khromov, S.; Hemmingsson, C.; Monemar, B.; Hultman, L.; Pozina, G.

2014-12-01

98

Polarizable Optical Bistability of Frenkel Excitons  

Directory of Open Access Journals (Sweden)

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.

O. Derevyanchuk

2003-06-01

99

Exciton spin dynamics in spherical CdS quantum dots  

OpenAIRE

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spat...

Horodyska, P.; Nemec, P.; Sprinzl, D.; Maly, P.; Gladilin, V. N.; Devreese, J. T.

2010-01-01

100

Quantitative interpretation of the excitonic splittings in aluminum nitride  

OpenAIRE

Abstract We address the interpretation of the splitting between the ground state excitonic transition which indicates the energy of the lowest direct band gap in AlN bulk films and epilayers, and a 36–38 meV higher energy companion. We demonstrate that this splitting is consistent with the initial interpretation in terms of 1s–2s excitonic splitting by using a calculation of the exciton binding energy which includes mass anisotropy and anisotropy of the...

Gil, B.; Guizal, B.; Felbacq, D.; Bouchitte?, G.

2011-01-01

101

Excitonic Contribution on Light Emitting Properties of Nano silicon  

International Nuclear Information System (INIS)

We developed a phenomenological model by integrating the effects of excitonic energy states, localized surface states and quantum confinement to obtain an analytical expression for the room temperature photoluminescence (PL) intensity. The binding energy and oscillator strength of strongly confined excitons has been calculated to examine its contribution on optical band gap and electronic properties of silicon quantum dots. The band gap decreases as much as 0.23 eV with excitonic effects for the smallest dot. The effects of exciton states explain almost accurately the experimental PL data. Our model provides the mechanism for controlling the PL intensity by controlling model parameters. The results for the size dependence of the optical band gap and oscillator strength are presented the role excitonic effects on optical and electronic properties are discussed. (author)

102

Ultraviolet optical absorption of alkali cyanides and alkali halide cyanides  

International Nuclear Information System (INIS)

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

103

Indirect optical absorption and origin of the emission from ?-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions  

International Nuclear Information System (INIS)

We investigated the optical absorption of the fundamental band edge and the origin of the emission from ?-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained ?-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of ?-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7-0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of ?-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energtal indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

104

Exciton absorption of tetraalkylammonium and tetraalkylphosphonium iodides  

Energy Technology Data Exchange (ETDEWEB)

In this work the authors studied the intrinsic electronic absorption of tetraalkylammonium TAAI) ab tetraalkylphosphonium (TAPI), which are the closet and most comprehensively investigated analogs of the alkali metal halide and ammonium halide crystals. Results are given obtained in the investigation of the absorption spectra of higher molecular weight TAAI R/sub 4/NI and also TAPI R/sub 4/PI. The studies were carried out with crystalline films of R/sub 4/NI and R/sub 4/PI samples, deposited on a quartz plate by crystallization from an alcoholic solution or by spraying in vacuo. The spectra were measured by means of a Specord UV Vis spectrophotometer, using a vacuum cryostat in the range of 80-293 K. From the experimental facts obtained, and also from the results of the experiments on the examination of recombinational luminescence of TAAI and TAPI samples, it was concluded that the observed luminescence is intrinsic in nature, due to one- and two-halide autolocalization excitons. Graphs and tables are included.

Mamedov, A.P.; Dzhafarova, R.A.; Panova, L.Y.

1986-10-01

105

Exciton absorption of tetraalkylammonium and tetraalkylphosphonium iodides  

International Nuclear Information System (INIS)

In this work the authors studied the intrinsic electronic absorption of tetraalkylammonium TAAI) ab tetraalkylphosphonium (TAPI), which are the closet and most comprehensively investigated analogs of the alkali metal halide and ammonium halide crystals. Results are given obtained in the investigation of the absorption spectra of higher molecular weight TAAI R4NI and also TAPI R4PI. The studies were carried out with crystalline films of R4NI and R4PI samples, deposited on a quartz plate by crystallization from an alcoholic solution or by spraying in vacuo. The spectra were measured by means of a Specord UV Vis spectrophotometer, using a vacuum cryostat in the range of 80-293 K. From the experimental facts obtained, and also from the results of the experiments on the examination of recombinational luminescence of TAAI and TAPI samples, it was concluded that the observed luminescence is intrinsic in nature, due to one- and two-halide autolocalization excitons. Graphs and tables are included

106

Surface photovoltage in exciton absorption range in CdS  

Science.gov (United States)

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.

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

1977-01-01

107

Low-temperature dynamics of weakly localized Frenkel excitons in disordered linar chains  

OpenAIRE

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 non-e...

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

2003-01-01

108

Influence of Effects of Self-Polarization and Exciton-Phonon Interactions on the Exciton Energy in Lead Iodide Nanofilms  

Science.gov (United States)

In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.

Kramar, V. M.; Pugantseva, O. V.

2014-08-01

109

Exciton localization in films of ZnO nanoparticles  

Science.gov (United States)

Temperature dependent photoluminescence (PL) spectroscopy has been employed to study the exciton localization in ZnO nanostructures of various sizes. Low temperature (LT) near-band-edge UV emission from ZnO nanostructures of size less than 100 nm show signature of free exciton (FE), bound exciton and longitudinal-optical phonon replica of FE lines. Free and bound excitonic emission deviates from the Varshni equation at LT and exhibits S-shaped temperature dependence due to the exciton localization effect. Deviations of the measured data from the Varshni equation directly provide the exciton localization energies, ELoc. It was found that ELoc increases and dELocalization due to thermal activation occurs at higher temperature for smaller nanoparticles.

Ghosh, Manoranjan; Ghosh, Karabi; Patra, G. D.; Tyagi, M.; Sen, S.; Gadkari, S. C.

2012-06-01

110

Semiconductor quantum well excitons in strong, narrowband terahertz fields  

International Nuclear Information System (INIS)

Optical transitions between exciton states in semiconductors—intraexcitonic transitions—usually fall into the terahertz (THz) range and can be resonantly excited with narrowband, intense THz radiation as provided by a free-electron laser. We investigate this situation for two different quantum well structures by probing the near-infrared excitonic absorption spectrum near the band edge. We observe the dynamical Stark—or Autler–Townes—splitting of the 1s exciton ground state and follow its evolution for various THz photon energies and field strengths. The behavior is considerably more complex as compared to the atomic systems. At the highest field strengths, where the Rabi energy is of the same order of magnitude as the exciton level separation, the system cannot be described within the standard framework of a two-level system in rotating wave approximation. When the ponderomotive energy approaches the exciton binding energy, signatures of exciton field ionization are observed. (paper)

111

Exciton Mapping at Subwavelength Scales in Two-Dimensional Materials  

Science.gov (United States)

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.

Tizei, Luiz H. G.; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu

2015-03-01

112

Ultraviolet absorption spectra of mercuric halides.  

Science.gov (United States)

The gas phase transitions of the mercuric halides were observed in the UV region by operating at temperatures above 400 K and at vapor pressures on the order of 0.5 mm. Spectral features exhibited by the chloride, bromide, and iodide of mercury correlate energetically with bands previously designated as intermolecular charge transfer transitions. The solution spectra of mercuric iodide and deep color of the crystals (if not due to some solid state interactions) indicate that this molecule may also have longer wavelength transitions.

Templet, P.; Mcdonald, J. R.; Mcglynn, S. P.; Kendrow, C. H.; Roebber, J. L.; Weiss, K.

1972-01-01

113

Halide laser glasses  

International Nuclear Information System (INIS)

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

114

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

Scientific Electronic Library Online (English)

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

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

2006-04-01

115

Superfluid phase transition in two-dimensional excitonic systems  

International Nuclear Information System (INIS)

We study the superfluid phase transition in the two-dimensional (2D) excitonic system. Employing the extended Falicov–Kimball model (EFKM) and considering the local quantum correlations in the system composed of conduction band electrons and valence band holes we demonstrate the existence of the excitonic insulator (EI) state in the system. We show that at very low temperatures, the particle phase stiffness in the pure-2D excitonic system, governed by the non-local cross correlations, is responsible for the vortex–antivortex binding phase-field state, known as the Berezinskii–Kosterlitz–Thouless (BKT) superfluid state. We demonstrate that the existence of excitonic insulator phase is a necessary prerequisite, leading to quasi-long-range order in the 2D excitonic system.

116

Pulsed electron beam-induced formation of F-centers and autolocalized excitons in CsBr  

International Nuclear Information System (INIS)

Methods of optical spectroscopy with time resolution are used to study primary color centres formed in nonactivated cesium bromide crystals by accelerated electron pulses (E=0.25 MeV, j=2-300 A cm-2, tp=10-8 c). By the radiation pulse cease, with T=(80-160)K, instable absorption with Em maxima in spectrum, photon energies being 1.23, 2.4 and 3.1 eV, caused by two-halogen autolocalized excitons (THAE) in 3?4+-state are detected in CsBr. Low-energy absorption peak with Em being equal to 1.23 eV, is related to optical transitions in THAE electron part, and of Em=2.4 and 3.1 eV absorption band - to THAE hole centres. Efficiency of THAE induction by electron pulses is decreased with the temperature growth above 80 K according to dependence antibatic to the temperature dependence of formed per a pulse F- and H-centres number. Thermal activation energy of the processes responsible for formation of primary F-H-pairs and the following separation of the pair components in space is 0.06±0.01 and 0.09±0.01 eV, respectively. The processes model where thermoactivated diffusion motion of relaxing exciton halide nucleus is the defining mechanism common for formation and spatial separation of Frenkel defects in CsBr in high temperature range (>160 K) is discussed

117

Molecular excitons in a copper azadipyrrin complex.  

Science.gov (United States)

Exciton coupling is investigated in a copper azadipyrrin complex, Cu(L-aza)2. Exciton coupling in Cu(L-aza)2 assuming a single ?-?* state on the L-aza ligand fails to account for the electronic structure of Cu(L-aza)2, which displays two almost equal intensity transitions at 15?600 cm(-1) and 17?690 cm(-1). TD-UB3LYP/6-31G(d) calculations suggest multiple ?-?* transitions for the L-aza ligands and simple vector addition of the transition dipoles predicts two nearly orthogonal co-planar excitonic transitions that correctly reproduce the absorption band profile. Empirical modelling of absolute resonance Raman intensities using wavepacket dynamics confirms Cu(L-aza)2 has two equal intensity orthogonal exciton transitions. The phenyl substituents at the ?- and ?-positions of the pyrrole rings play a central role in determining the orientation of the transition dipoles. Consequently the ?-?* transitions for the L-aza ligands are oriented towards the substituent groups and are not in the plane of the pyrrole rings. Mode displacements in the Franck-Condon (FC) region obtained from the wavepacket model suggest that pyrrole ring and phenyl modes control the exciton FC dynamics. Our results suggest that Cu(L-aza)2 is an ideal model for theoretical, computational and experimental investigations of molecular excitons in molecular systems. PMID:25025874

McLean, T M; Telfer, S G; Elliott, A B S; Gordon, K C; Lein, M; Waterland, M R

2014-12-21

118

Synthetic Alkali Halides  

Science.gov (United States)

This complex experimental investigation uses alkali halides (NaCl, KCl, and mixtures of both) to simulate the melting of alkali feldspars which melt at too high of temperatures to work with in lab. Three hypotheses are tested: It is possible to crystallize alkali-chloride salts from a magma with any composition between NaCl and KCl. Because K+ and Na+ do not have the same ionic size, the atomic spacing in alkali chlorides will vary systematically with composition. Alkali chlorides are equally stable at high (just below liquidus) and low (subsolidus)temperatures. This project takes more than one class period, depending on how many students are in the class, because there will be lines at the scales, oven, and XRD. It is advisable to introduce the lab in class and have students complete various parts on their own time. There are three main parts. Part one: Synthesize all alkali halide compositions at high temperature (hopefully above the solvus. Part two: Put grown crystals back in an oven at lower temperature to see if they will unmix. Part three: Write a report evaluating and interpreting all results, relevant graphs, and the above three hypothesis.

Dexter Perkins

119

Observation of surface excitons in rare gas solids  

International Nuclear Information System (INIS)

Evidence is obtained for the excitation of surface excitons in solid Ar, Kr and Xe in optical transmission and reflection experiments using synchrotron radiation. They are located at photon energies ranging from 0.6 eV for Ar to 0.1 eV for Xe below the corresponding bulk excitons excited from the valence bands. Their halfwidths (20-50 MeV) is less than half the values found for the bulk excitons. Some are split by an amount considerably smaller than the spin orbit splitting of the valence bands. (orig.)

120

Exciton optical transitions in a hexagonal boron nitride single crystal  

International Nuclear Information System (INIS)

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.)

121

Ab initio many-body effects in TiSe?: A possible excitonic insulator scenario from GW band-shape renormalization  

OpenAIRE

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe?. 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 k?...

Cazzaniga, M.; Cercellier, H.; Holzmann, M.; Monney, Claude; Aebi, Philipp; Onida, Giovanni; Olevano, V.

2012-01-01

122

40 CFR 721.4095 - Quaternary ammonium alkyltherpropyl trialkylamine halides.  

Science.gov (United States)

...ammonium alkyltherpropyl trialkylamine halides. 721.4095 Section 721.4095 ...ammonium alkyltherpropyl trialkylamine halides. (a) Chemical substance and significant...ammonium alkyltherpropyl trialkylamine halides (PMNs...

2010-07-01

123

Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.  

Science.gov (United States)

Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique optical phenomena. The breakdown of the k-conversion rule and strong Coulomb interactions between carriers in NCs enhance Auger recombination rate and decrease the energy threshold for multiple exciton generation. We discuss this impact of the k-conservation rule on two-carrier radiative recombination and the three-carrier Auger recombination processes in indirect-gap semiconductor Si NCs. In NCs and SWCNTs, multiple exciton generation competes with Auger recombination, surface trapping of excitons, and cooling of hot electrons or excitons. In addition, we explore heterostructured NCs and impurity-doped NCs in the context of the optimization of charge carrier extraction from excitons in NCs. PMID:23421584

Kanemitsu, Yoshihiko

2013-06-18

124

Diamagnetic excitons and exciton magnetopolaritons in semiconductors  

International Nuclear Information System (INIS)

Interband magneto-absorption in semiconductors is reviewed in the light of the diamagnetic exciton (DE) concept. Beginning with a proof of the exciton nature of oscillating-magnetoabsorption (the DE discovery), development of the DE concept is discussed, including definition of observation conditions, quasi-cubic approximation for hexagonal crystals, quantum-well effects in artificial structures, and comprehension of an important role of the DE polariton. The successful use of the concept application to a broad range of substances is reviewed, namely quasi-Landau magnetic spectroscopy of the ‘Rydberg’ exciton states in cubic semiconductors such as InP and GaAs and in hexagonal ones such as CdSe, the proof of exciton participation in the formation of optical spectra in narrow-gap semiconductors such as InSb, InAs, and, especially, PbTe, observation of DE spectra in semiconductor solid solutions like InGaAs. The most fundamental findings of the DE spectroscopy for various quantum systems are brought together, including the ‘Coulomb-well’ effect, fine structure of discrete oscillatory states in the InGaAs/GaAs multiple quantum wells, the magneto-optical observation of above-barrier exciton. Prospects of the DE physics in ultrahigh magnetic field are discussed, including technological creation of controllable low-dimensional objects with extreme oscillator strengths, formation of magneto-quantum exciton polymer, and even modelling of the hydrogen behaviour in the atmosphere of a neutron star. (topical review)

125

Heat capacity of molten halides.  

Science.gov (United States)

The heat capacities of molten salts are very important for their practical use. Experimental investigation of this property is challenging because of the high temperatures involved and the corrosive nature of these materials. It is preferable to combine experimental investigations with empirical relationships, which allows for the evaluation of the heat capacity of molten salt mixtures. The isobaric molar heat capacities of all molten alkali and alkaline-earth halides were found to be constant for each group of salts. The value depends on the number of atoms in the salt, and the molar heat capacity per atom is constant for all molten halide salts with the exception of the lithium halides. The molar heat capacities of molten halides do not change when the anions are changed. PMID:25530462

Redkin, Alexander A; Zaikov, Yurii P; Korzun, Iraida V; Reznitskikh, Olga G; Yaroslavtseva, Tatiana V; Kumkov, Sergey I

2015-01-15

126

Optical response and excitons in gapped graphene  

DEFF Research Database (Denmark)

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

Pedersen, Thomas Garm; Jauho, Antti-Pekka

2009-01-01

127

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

Science.gov (United States)

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

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

2015-04-14

128

Temperature dependence of excitonic emission in cubic CdSe thin film  

International Nuclear Information System (INIS)

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16±1.5 meV was determined from the Arrhenius analysis

129

Temperature dependence of excitonic emission in cubic CdSe thin film  

Energy Technology Data Exchange (ETDEWEB)

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16{+-}1.5 meV was determined from the Arrhenius analysis.

Chia, C.H. [Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan (China)], E-mail: chchia@mail.nctu.edu.tw; Yuan, C.T.; Ku, J.T.; Yang, S.L. [Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan (China); Chou, W.C. [Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan (China)], E-mail: wuchingchou@mail.nctu.edu.tw; Juang, J.Y. [Department of Electrophysics, National Chiao Tung University, Hsin-Chu 30010, Taiwan (China); Hsieh, S.Y.; Chiu, K.C.; Hsu, J.S. [Department of Physics, Chung Yuan Christian University, Chung-Li 32023, Taiwan (China); Jeng, S.Y. [Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan (China)

2008-01-15

130

Coherently coupled exciton lasing  

OpenAIRE

Lowest reported threshold organic semiconductor VCSEL (4.9 muJ/cm[superscript 2]) is achieved when excitons coherently couple, upon sub-picosecond non-resonant optical excitation. Temperature dependence of lambda/2 n device, absent in ges lambda/n cavities, indicates excitons undergo condensate-like phase transition.

Bulovic, Vladimir; Tischler, Jonathan R.; Young, Elizabeth Renee; Nocera, Daniel G.

2009-01-01

131

Exciton laser rate equations  

Directory of Open Access Journals (Sweden)

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.

Garkavenko A. S.

2011-08-01

132

Methyl Halide Production by Fungi  

Science.gov (United States)

Methyl chloride (CH3Cl), methyl bromide (CH3Br) and methyl iodide (CH3I) are methyl halide gases that contribute significant amounts of halogen radicals to the atmosphere. In an effort to better understand the global budget of methyl halides and their impact on the atmosphere, we need to identify the natural sources in addition to the known anthropogenic sources of these compounds. We are investigating the role of fungi in the production of methyl halides in the soils and wetlands in southern New Hampshire, USA. Previous research has shown that wood decay fungi and ectomycorrhizal fungi, which are within a group of fungi called basidiomycetes, emit methyl halides. In our study, measurements of headspace gas extracted from flasks containing fungi grown in culture demonstrate that a variety of fungi, including basidiomycetes and non-basidiomycetes, emit methyl halides. Our research sites include four ecosystems: an agricultural field, a temperate forest, a fresh water wetland, and coastal salt marshes. We have collected and isolated fungi at each site by culturing tissue samples of fruiting bodies and plant material, by using wood baits, and from the direct culture of soil. We compared the rates of methyl halide emissions from the fungi in the four ecosystems. In addition, we measured emissions from previously assayed fungal isolates after reintroducing them to sterilized soils that were collected from their original environments. Fungal biomass was determined by substrate-induced respiration (SIR). The emission rate by the fungus was determined by a linear regression of the concentration of methyl halide in the sample headspace over time divided by the fungal biomass.

Dailey, G. D.; Varner, R. K.; Blanchard, R. O.; Sive, B. C.; Crill, P. M.

2005-12-01

133

Efficient creation of 1s ortho-excitons  

Science.gov (United States)

We report on the creation of a high density excitonic gas by two-photon IR pumping to high-energy excitonic states associated with the highest valence band and the second conduction band. The efficiency of this type of pumping in creating 1s ortho-excitons exceeds both types of excitation used so far, namely the one-photon band-to-band excitation to the lower conduction band and resonant two-photon absorption. We report results on time-resolved luminescence performed within first 50 ns after excitation with a sub-picosecond laser pulse of an energy 1.3-1.9 eV. In spite of the high excitation power density 100MW/cm2 an equilibrium temperature is reached within 300ps after laser pulse. The spectrum shows the two typical bands at 2.036eV and 2.023eV, attributed to the direct and phonon assisted recombination of the 1s ortho-exciton, respectively. In case of the direct transition a clearly single exponential decay in observed with a decay time of 3000ps. This is significantly longer than previously reported results (300ps). The decay of the phonon-assisted emission reveals an intersting, more complicated nature. We will discuss these results in terms possible phase transitions in the high density exciton gas.

Karpinska, K.; van Loosdrecht, P. H. M.; van der Marel, D.; Revcolevschi, A.

2002-03-01

134

Competitive hydrated positron reactions with halides  

International Nuclear Information System (INIS)

The hydrated positron, esub(aq)sup(+), reactions with halides and pseudohalides were studied by angular correlation technique. At halide concentrations below 0.02 M, the reaction of esub(aq)sup(+) with halides, x-, to form bound states [x-,e+] is diffusion controlled. On adding two halides to water containing a Ps inhibitor, the first positron transfer rate constants could be determined. For example, the reaction [Br-,e+] + I- ? Br- + [I-,e+] was studied. (Auth.)

135

On the exciton model for ion-beam damage: The example of TiO2  

OpenAIRE

The non-radiative exciton decay model recently developed to account for swift-ion-beam damage to LiNbO3 is, here, discussed within a general physical perspective, taking previous work on alkali halides as a reference. Some general rules for the validity of excitonic models have been put forward, allowing one to predict the irradiation behaviour of other materials. As a new example of application, some preliminary data on the generation and growth of uniform amorphous layers induced by irradia...

Rivera Mena, Antonio; Crespillo Almenara, Miguel; Olivares Roza, Jimena; Sanz, R.; Jensen, J.; Agullo Lopez, Fernando

2010-01-01

136

Radiochemical synthesis of pure anhydrous metal halides  

Science.gov (United States)

Method uses radiation chemistry as practical tool for inorganic preparations and in particular deposition of metals by irradiation of their aqueous metal salt solutions with high energy electrons. Higher valence metal halide is dissolved in organic liquid and exposed to high energy electrons. This causes metal halide to be reduced to a lower valence metal halide.

Philipp, W. H.; Marsik, S. J.; May, C. E.

1973-01-01

137

Preparation of cerium halide solvate complexes  

Science.gov (United States)

Crystals of a solvated cerium(III) halide solvate complex resulted from a process of forming a paste of a cerium(III) halide in an ionic liquid, adding a solvent to the paste, removing any undissolved solid, and then cooling the liquid phase. Diffusing a solvent vapor into the liquid phase also resulted in crystals of a solvated cerium(III) halide complex.

Vasudevan, Kalyan V; Smith, Nickolaus A; Gordon, John C; McKigney, Edward A; Muenchaussen, Ross E

2013-08-06

138

Probing Excitonic Dark States in Single-layer Tungsten Disulfide  

CERN Document Server

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...

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

2014-01-01

139

Excitonic effects in oxyhalide scintillating host compounds  

Energy Technology Data Exchange (ETDEWEB)

Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X?=?F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

Shwetha, G.; Kanchana, V., E-mail: kanchana@iith.ac.in [Department of Physics, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502 205, Telangana (India); Valsakumar, M. C. [School of Engineering Sciences and Technology (SEST), University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, Telangana (India)

2014-10-07

140

Quasienergy Spectroscopy of Excitons  

CERN Document Server

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 can be interpreted in terms of the quasienergy spectrum of the exciton, via a remarkably transparent expression for the susceptibility, and show that the effects of strongly avoided quasienergy crossings manifest themselves directly, both in the emission and absorption.

Johnsen, Kjell; Johnsen, Kristinn; Jauho, Antti-Pekka

1999-01-01

141

Multiple Exciton Generation in Colloidal Nanocrystals  

Directory of Open Access Journals (Sweden)

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

Charles Smith

2013-12-01

142

Phonon Scattering and Excitons in Carbon Nanotubes.  

Science.gov (United States)

Due to their low dimensionality, carbon nanotubes (CNTs) have striking properties, quite different from these of traditional semiconductors, with important implications for technology. The performance of electronic devices relies on carrier mobility, which is extraordinary high in CNTs at low fields. However, at high fields the mobility is dramatically reduced due to inelastic optical phonon scattering. Optical properties of CNTs, essential for electro-optical devices, are dominated by excitons with binding energies and oscillator strengths orders of magnitude larger than those in conventional semiconductors. We calculate the electron-phonon scattering and binding in CNTs, within a tight binding model [1]. We derive the mobility as a function of temperature, electric field, and nanotube chirality using a multi-band Boltzmann treatment. We find the drift velocity saturates at approximately half the graphene Fermi velocity. Polaronic binding give a band-gap renormalization of 70 meV, an order of magnitude larger than previously suggested. We calculate the properties of excitons in CNTs embedded in a dielectric, for a wide range of tube radii and dielectric environments, by solving the Bethe-Salpeter Equation in a tight binding basis. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength as a function the tube radius, the dielectric constant of the embedding material, and the chirality [2]. In addition we calculate optical absorption including the exciton-phonon interaction. We find a phonon sideband at 200 meV above the zero phonon line, due to the creation of exciton plus one optical phonon [3]. [1] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411021. [2] V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. 92, 257402 (2004). [3] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411618.

Perebeinos, Vasili

2005-03-01

143

Anharmonic properties of rubidium halides  

International Nuclear Information System (INIS)

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

144

Thermodynamic properties of Alkali Halides  

International Nuclear Information System (INIS)

The method of moments of [1], developed by the authors in [2] for strongly and harmonic crystals with f.c.c. structure is used here to investigate the main thermodynamic properties of the potassium halides. Their analytic expressions as functions of temperature are obtained and the comparison between the theoretical results and the experimental data is made. (author). 22 refs., 5 tabs

145

Exciton luminescence of boron nitride nanotubes and nano-arches  

Energy Technology Data Exchange (ETDEWEB)

We report photoluminescence (PL) and PL-excitation spectroscopy of BN nanotubes (nt-BN) mixed with some residual hexagonal crystalline (h-BN) starting material, and of pure h-BN microcrystalline powder. The nanotube phase exhibits a broad-band PL near 380 nm, in agreement with a published report of cathodoluminescence from a sample comprising >90% nanotubes. This emission is almost 3 eV lower in energy than unrelaxed exciton states found in recent all-electron theories of nt-BN and h-BN and about 1.4 eV lower than the lowest (perturbed dark?) exciton seen in absorption of nt-BN. This may suggest that excitons in nt-BN vibrationally relax to self-trapped states before emitting, a path found in many wide-gap solids, especially in quasi-1-dimensional forms. Exciton emission from bulk single-crystal h-BN has been shown to occur from vibrationally unrelaxed (free-exciton) states. We suggest a hypothesis in which known nano-arch reconstructions on the surface of h-BN may provide the low-dimensional environment to make exciton self-trapping on the surfaces of h-BN likely. This allows consistent interpretation of the surface-related 380 nm emission from h-BN powder within a half-nanotube self-trapped exciton hypo- thesis. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Berzina, B.; Trinkler, L.; Korsak, V.; Krutohvostov, R. [Institute of Solid State Physics, University of Latvia, Riga (Latvia); Carroll, D.L.; Ucer, K.B.; Williams, R.T. [Department of Physics and Center for Nanotechnology and Molecular Materials, Wake Forest University, Winston-Salem, NC 27109 (United States)

2006-11-15

146

Exciton-exciton annihilation in organic polariton microcavities  

OpenAIRE

Sublinear intensity dependence of photoluminescence from organic exciton-polariton microcavities under non-resonant excitation in two power regimes is shown. The sublinearity is attributed to exciton-exciton annihilation, which could compete with polariton-polariton scattering in these devices.

Bulovic, Vladimir; Tischler, Jonathan R.; Young, Elizabeth Renee; Bradley, Michael Scott; Nocera, Daniel G.; Akselrod, Gleb Markovitch

2009-01-01

147

Hot-carrier transport processes in stimulated desorption of alkali halides  

Science.gov (United States)

Electron- and photon-stimulated desorption of epitaxial thin films and bulk single crystals of alkali halides has been investigated. It has been found that the widely accepted model based on self-trapped exiton decay and thermally activated defect diffusion cannot account for a number of experimental observations. In particular, it cannot explain the stability of the very thin alkali-halide films against the electron beam, and it fails to interpret correctly the yield dependence of the halogen atoms emitted with nonthermal energies on the film thickness and sample temperature. It is shown that for satisfactory interpretation of those data one has to take into account early stages of crystal excitation, i.e., hot carrier formation and transport processes occurring prior to self-trapped exciton phase. Consequently, a comprehensive description of the observed desorption features is presented.

Kolodziej, J. J.; Szymonski, M.

1998-11-01

148

Engineering directed excitonic energy transfer  

OpenAIRE

We provide an intuitive platform for engineering exciton transfer dynamics. We show that careful consideration of the spectral density, which describes the system-bath interaction, leads to opportunities to engineer the transfer of an exciton. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. We carefully describe t...

Perdomo, Alejandro; Vogt, Leslie; Najmaie, Ali; Aspuru-guzik, Alan

2010-01-01

149

Development of Halide and Oxy-Halides for Isotopic Separations  

Energy Technology Data Exchange (ETDEWEB)

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.

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

2014-10-01

150

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

Energy Technology Data Exchange (ETDEWEB)

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.

Protin, L.

1994-10-05

151

Relaxation Between Bright Optical Wannier Excitons in Perovskite Solar Absorber CH$_3$NH$_3$PbI$_3$  

OpenAIRE

We study the light-absorbing states of the mixed-halide perovskite CH$_{3}$NH$_{3}$PbI$_2$Cl and tri-iodide perovskite CH$_{3}$NH$_{3}$PbI$_3$ with density functional and many-body calculations to explain the desirable photovolatic features of these materials. The short-lived electron-hole bound states produced in this photovoltaic material are of halide to lead electron transfer character, with a Wannier-type exciton. Bethe-Salpeter (GW+BSE) calculations of the absorption c...

Nanguneri, Ravindra; Parkhill, John

2014-01-01

152

Absorption lineshape of FA centers in alkali halides  

International Nuclear Information System (INIS)

The line shape of the absorption bands of FA centers in alkali halides have been studied for the first time. The new method used for this investigation is based on the determination of the overlap between the FA1 and FA2 bands from luminescence measurements. The experimental results have been compared with calculated values deduced from the theoretical FA bands of different shapes. For both FA(I) centers in KCl:Na+ and FA(II) centers in KCl:Li+ and RbCl:Li+ the absorption lineshape at low temperature is much closer to a sum of two Lorentzian curves than that of two Gaussian or Poissonian bands. This results shows an unexpected difference with the F centers, whose absorption lineshape is known to be Poissonian at the same temperatures

153

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

Science.gov (United States)

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

154

Photoluminescence-linewidth-derived exciton mass for InGaAsN alloys  

Energy Technology Data Exchange (ETDEWEB)

The authors report a measurement of the variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1 and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy. The samples were grown by metal-organic chemical vapor deposition and the photoluminescence measurements were performed a 4K. The authors find that the value of the excitonic linewidth increases as a function of pressure until about 100 kbars after which it tends to saturate. This change in the excitonic linewidth is used to derive the pressure variation of the reduced mass of the exciton using a theoretical formalism which is based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The variation of the excitonic reduced mass thus derived is compared with that recently determined using a first-principles band structure calculation based on local density approximation.

JONES,ERIC D.; ALLERMAN,ANDREW A.; KURTZ,STEVEN R.; MODINE,NORMAND A.; BAJAJ,K.K.; TOZER,S.T.; WEI,XING

2000-01-27

155

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

Science.gov (United States)

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

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

2014-10-01

156

Engineering directed excitonic energy transfer  

Energy Technology Data Exchange (ETDEWEB)

We provide an intuitive platform for engineering exciton transfer dynamics. We show that careful consideration of the spectral density, which describes the system-bath interaction, leads to opportunities to engineer exciton transfer. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. We carefully describe the validity of the model and use experimentally relevant material parameters to show counter-intuitive examples of directed exciton transfer in a linear chain of quantum dots.

Perdomo, Alejandro; Vogt, Leslie; Najmaie, Ali; Aspuru-Guzik, Alan

2010-01-01

157

Engineering directed excitonic energy transfer  

CERN Document Server

We provide an intuitive platform for engineering exciton transfer dynamics. We show that careful consideration of the spectral density, which describes the system-bath interaction, leads to opportunities to engineer the transfer of an exciton. Since excitons in nanostructures are proposed for use in quantum information processing and artificial photosynthetic designs, our approach paves the way for engineering a wide range of desired exciton dynamics. We carefully describe the validity of the model and use experimentally relevant material parameters to show counter-intuitive examples of a directed exciton transfer in a linear chain of quantum dots.

Perdomo, Alejandro; Najmaie, Ali; Aspuru-Guzik, Alán

2010-01-01

158

Enhancing multi-photon induced excitonic emission of ZnO single crystals by shaping fs laser pulses  

International Nuclear Information System (INIS)

We report on the control of multi-photon excited emission of the ZnO exciton band via spectral phase modulation of the femtosecond excitation pulses. It was observed that the optimum spectral phase that enhances the exciton emission results in a pulse-train temporal profile, whose separation corresponds to an energy of 82 meV, which is close to the energy of LO phonon sidebands in ZnO emission. Such a result suggests that exciton–LO phonon coupling can be explored to coherently enhance the exciton emission in ZnO single crystals with respect to the defect luminescence band. (letter)

159

Self-trapped exciton and core-valence luminescence in BaF2 nanoparticles  

International Nuclear Information System (INIS)

The influence of the BaF2 nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (h? ? Eg) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations

160

An optical criterion to obtain miscible mixed crystals in alkali halides  

Directory of Open Access Journals (Sweden)

Full Text Available This work gives a novel criterion to predict the formation of alkali halide solid solutions and discusses some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. These mixtures are miscible in any concentration of their components. The miscibility of these mixed crystals is quite related to the F center through the behavior observed in the spectral position of the optical absorption F band as a function of the lattice constant of the alkali halide where the F center was formed. By inspection of an energy graph of the F band energy versus lattice constant (Mollwo-Ivey law, a set of points is observed corresponding to several pure alkali halides (such as KCl, KBr, RbCl, which gives a notion of possible mixed materials that would correspond to adjacent points and a solid solution could be obtained, meaning a single phase crystal, which result in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows have a numerical criterion, based on the percentage respective of the F band energy, in order to predict possible solid solutions. We obtained experimental information using diffractograms of the mixed crystals, from which the lattice constant was obtained and compared with a theoretical calculus using a generalization of Vegard's law, finally it is discussed the case of a crystal growing, starting from five components, picking up five consecutive dots from the graph of Mollwo-Ivey's law.

R. Rodr\\u00EDguez-Mij\\u00E1ngos

2008-01-01

161

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

Science.gov (United States)

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%.

Schlosser, Herbert

1992-01-01

162

Exciton Ionization, Franz-Keldysh and Stark Effects in Carbon Nanotubes  

CERN Document Server

We calculate the optical properties of carbon nanotubes in an external static electric field directed along the tube axis. We predict strong Franz-Keldysh oscillations in the first and second band-to-band absorption peaks, quadratic Stark effect of the first two excitons, and the field dependence of the bound exciton ionization rate for a wide range of tube chiralities. We find that the phonon assisted mechanism dominates the dissociation rate in electro-optical devices due to the hot optical phonons. We predict a quadratic dependence of the Sommerfeld factor on the electric field and its increase up to 2000% at the critical field of the full exciton dissociation.

Perebeinos, V; Perebeinos, Vasili; Avouris, Phaedon

2007-01-01

163

Singlet exciton fission photovoltaics.  

Science.gov (United States)

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

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

164

Triplet exciton dissociation in singlet exciton fission photovoltaics  

Energy Technology Data Exchange (ETDEWEB)

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)

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

165

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

International Nuclear Information System (INIS)

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

166

Hygroscopicity Evaluation of Halide Scintillators  

Energy Technology Data Exchange (ETDEWEB)

A collaborative study of relative hygroscopicity of anhydrous halide scintillators grown at various laboratories is presented. We have developed a technique to evaluate moisture sensitivity of both raw materials and grown crystals, in which the moisture absorption rate is measured using a gravimetric analysis. Degradation of the scintillation performance was investigated by recording gamma-ray spectra and monitoring the photopeak position, count rate and energy resolution. The accompanying physical degradation of the samples exposed to ambient atmosphere was photographically recorded as well. The results were compared with ben

Zhuravleva, M [The University of Tennessee; Stand, L [The University of Tennessee; Wei, H [The University of Tennessee; Hobbs, C. L. [University of Tennessee, Knoxville (UTK); Boatner, Lynn A [ORNL; Ramey, Joanne Oxendine [ORNL; Burger, Arnold [Fisk University, Nashville; Rowe, E [Fisk University, Nashville; Bhattacharya, P. [Fisk University, Nashville; Tupitsyn, E [Fisk University, Nashville; Melcher, Charles L [University of Tennessee, Knoxville (UTK)

2014-01-01

167

On the low-temperature diffusion of localized Frenkel excitons in linear molecular aggregates  

CERN Document Server

We study theoretically diffusion of one-dimensional Frenkel excitons in J-aggregates at temperatures that are smaller or of the order of the J-band width. We consider an aggregate as an open linear chain with uncorrelated on-site (diagonal) disorder that localizes the exciton at chain segments of size smaller than the full chain length. The exciton diffusion over the localization segments is considered as incoherent hopping. The diffusion is probed by the exciton fluorescence quenching which is due to the presence of point traps in the aggregate. The rate equation for populations of the localized exciton states is used to describe the exciton diffusion and trapping. We show that there exist two regimes of the exciton diffusion at low temperatures. The first, slower one, involves only the states of the very tail of the density of states, while the second, much faster one, also involves the higher states that are close to the bottom of the exciton band. The activation energy for the first regime of diffusion is...

Malyshev, A V; Domínguez-Adame, F

2003-01-01

168

Dynamics of high-density excitons in PbI2 studied by two-photon absorption  

International Nuclear Information System (INIS)

Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI2 have been investigated through femtosecond transient grating and time-resolved luminescence measurements under two-photon absorption. We observed two luminescence bands at low temperature; one is due to an exciton-exciton collision process, while the other originates from localized excitonic states. By utilizing the transient grating spectroscopy, the lifetime and diffusion coefficient of the high-density excitons in PbI2 are estimated to be 72 ps and 4.6 cm2/s, respectively. The lifetime observed is almost equal to the values of the decay time of the luminescence due to the exciton-exciton collision process and the rise time of the luminescence due to the localized excitons. This strongly shows that the decay of the exciton-exciton collision process is dominated by the exciton trapping to the localized states arising from potential fluctuations due to imperfection of crystal

169

Dynamics of high-density excitons in PbI{sub 2} studied by two-photon absorption  

Energy Technology Data Exchange (ETDEWEB)

Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI{sub 2} have been investigated through femtosecond transient grating and time-resolved luminescence measurements under two-photon absorption. We observed two luminescence bands at low temperature; one is due to an exciton-exciton collision process, while the other originates from localized excitonic states. By utilizing the transient grating spectroscopy, the lifetime and diffusion coefficient of the high-density excitons in PbI{sub 2} are estimated to be 72 ps and 4.6 cm{sup 2}/s, respectively. The lifetime observed is almost equal to the values of the decay time of the luminescence due to the exciton-exciton collision process and the rise time of the luminescence due to the localized excitons. This strongly shows that the decay of the exciton-exciton collision process is dominated by the exciton trapping to the localized states arising from potential fluctuations due to imperfection of crystal.

Hosoya, Toshihiko [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Tanaka, Keita [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Fukaya, Ryo [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan); Takeda, Jun [Department of Physics, Graduate School of Engineering, Yokohama National University 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501 (Japan)]. E-mail: jun@ynu.ac.jp

2006-07-15

170

Tailoring the exciton fine structure of cadmium selenide nanocrystals with shape anisotropy and magnetic field.  

Science.gov (United States)

We use nominally spheroidal CdSe nanocrystals with a zinc blende crystal structure to study how shape perturbations lift the energy degeneracies of the band-edge exciton. Nanocrystals with a low degree of symmetry exhibit splitting of both upper and lower bright state degeneracies due to valence band mixing combined with the isotropic exchange interaction, allowing active control of the level splitting with a magnetic field. Asymmetry-induced splitting of the bright states is used to reveal the entire 8-state band-edge fine structure, enabling complete comparison with band-edge exciton models. PMID:25329623

Sinito, Chiara; Fernée, Mark J; Goupalov, Serguei V; Mulvaney, Paul; Tamarat, Philippe; Lounis, Brahim

2014-11-25

171

Shallow halogen vacancies in halide optoelectronic materials  

Science.gov (United States)

Halogen vacancies (VH ) are usually deep color centers (F centers) in halides and can act as major electron traps or recombination centers. The deep VH contributes to the typically poor carrier transport properties in halides. However, several halides have recently emerged as excellent optoelectronic materials, e.g., C H3N H3Pb I3 and TlBr. Both C H3N H3Pb I3 and TlBr have been found to have shallow VH , in contrast to commonly seen deep VH in halides. In this paper, several halide optoelectronic materials, i.e., C H3N H3Pb I3 , C H3N H3Sn I3 (photovoltaic materials), TlBr, and CsPbB r3 (gamma-ray detection materials) are studied to understand the material chemistry and structure that determine whether VH is a shallow or deep defect in a halide material. It is found that crystal structure and chemistry of n s2 ions both play important roles in creating shallow VH in halides such as C H3N H3Pb I3 , C H3N H3Sn I3 , and TlBr. The key to identifying halides with shallow VH is to find the right crystal structures and compounds that suppress cation orbital hybridization at VH , such as those with large cation-cation distances and low anion coordination numbers and those with crystal symmetry that prevents strong hybridization of cation dangling bond orbitals at VH . The results of this paper provide insight and guidance to identifying halides with shallow VH as good electronic and optoelectronic materials.

Shi, Hongliang; Du, Mao-Hua

2014-11-01

172

Equilibrium coverage of halides on metal electrodes  

Science.gov (United States)

The adsorption of halides on Cu(111) and Pt(111) has been studied using periodic density functional theory calculations. The equilibrium coverage of the halides as a function of the electrode potential was determined using a thermodynamic approach in which the electrochemical environment is not explicitly taken into account. For all considered systems, halide coverages between 1/3 and 3/8 should be stable over a wide potential range. Although some quantitative discrepancies with experiment are obtained, the qualitative trends derived from the calculations are consistent with experimental observations. The reasons for the remaining discrepancies with the experiment are discussed.

Gossenberger, Florian; Roman, Tanglaw; Groß, Axel

2015-01-01

173

An investigation of control mechanisms of the excitonic behavior in reactively sputtered ZnO on (0001) Al2O3  

Science.gov (United States)

Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton and neutral acceptor bound exciton (A0X) features. It is shown that the residual acceptors which bind excitons with an energy of 75meV reside about 312meV above the valence band, according to effective mass theory. An intra-band-gap absorption feature peaking at 2.5eV shows correlation with the characteristically narrow A-free exciton peak intensity, suggesting a compensation mechanism of the centers involving oxygen vacancy (VO) related donors. In order to enhance free exciton concentration relative to competing neutral bound exciton density, relevant annealing processes are performed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity.

Tüzemen, S.; Gür, Emre; Y?ld?r?m, T.; Xiong, G.; Williams, R. T.

2006-11-01

174

Exciton effects in boron-nitride (BN) nanotubes  

Science.gov (United States)

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

Harigaya, Kikuo

2012-12-01

175

Excitons with anisotropic effective mass  

CERN Document Server

We present a simple analytic scheme for calculating the binding energy of excitons in semiconductors that takes full account of the existing anisotropy in the effective mass, as a complement to the qualitative treatment in most textbooks. Results obtained for excitons in gallium nitride form the basis for a discussion of the accuracy of this approach.

Schindlmayr, A

1997-01-01

176

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

Science.gov (United States)

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.

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

2015-02-01

177

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

Science.gov (United States)

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.

Kaneko, Tatsuya; Ohta, Yukinori

2014-12-01

178

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

International Nuclear Information System (INIS)

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

179

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

International Nuclear Information System (INIS)

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

180

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

Energy Technology Data Exchange (ETDEWEB)

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.

Yang, Shize [International Center for Quantum Materials, School of Physics, Peking University and Collaborative Innovation Center of Quantum Matter, Beijing (China); Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Tian, Xuezeng; Wang, Lifen; Wei, Jiake; Qi, Kuo; Li, Xiaomin; Xu, Zhi, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn; Wang, Wenlong; Zhao, Jimin; Bai, Xuedong, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn [Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Wang, Enge, E-mail: xuzhi@iphy.ac.cn, E-mail: xdbai@iphy.ac.cn, E-mail: egwang@pku.edu.cn [International Center for Quantum Materials, School of Physics, Peking University and Collaborative Innovation Center of Quantum Matter, Beijing (China)

2014-08-18

181

Exciton related stimulated emission in ZnO polycrystalline thin film deposited by filtered cathodic vacuum arc technique  

International Nuclear Information System (INIS)

Random lasing was observed from ZnO polycrystalline thin film deposited by filtered cathodic vacuum arc technique. It is found that the random lasing action results from exciton-exciton scattering process when the excitation intensity is low. However, with increasing excitation intensity, the well-resolved discrete lasing modes evolve into broad stimulated emission band due to electron-hole plasma formation when the photogenerated exciton density exceeds the Mott density. The short spontaneous emission decay time measured at low temperature suggests that the radiative recombination rate is enhanced by the nanosized ZnO grains in the polycrystalline film through exciton-light coupling

182

Understanding excitons using spherical geometry  

International Nuclear Information System (INIS)

Using spherical geometry, we introduce a novel model to study excitons confined in a three-dimensional space, which offers unparalleled mathematical simplicity while retaining much of the key physics. This new model consists of an exciton trapped on the 3-sphere (i.e. the surface of a four-dimensional ball), and provides a unified treatment of Frenkel and Wannier–Mott excitons. Moreover, we show that one can determine, for particular values of the dielectric constant ?, the closed-form expression of the exact wave function. We use the exact wave function of the lowest bound state for ?=2 to introduce an intermediate regime which gives satisfactory agreement with the exact results for a wide range of ? values. -- Highlights: ? We introduce a novel model to study excitons. ? This provides a unified treatment of Frenkel and Wannier–Mott excitons. ? We determine the exact wave functions for particular values of the dielectric constant.

183

Exciton Effects in Optical Absorption of Boron-Nitride Nanotubes  

CERN Document Server

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

Harigaya, Kikuo

2007-01-01

184

Oxidation of hydrogen halides to elemental halogens  

Science.gov (United States)

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.

Rohrmann, Charles A. (Kennewick, WA); Fullam, Harold T. (Richland, WA)

1985-01-01

185

Cerium doped lanthanum halides: fast scintillators for medical imaging  

International Nuclear Information System (INIS)

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

186

Picosecond study of the exciton gas cooling  

International Nuclear Information System (INIS)

The exciton gas cooling in several materials, namely CdSe, CuI is investigated. The exciton gas temperature is measured directly from picosecond time resolved luminescence. It is shown that the cooling kinetics are well explained by emission of acoustical phonons by excitons. The lattice temperature is deduced, as well as the density of excitons created after the exciting laser pulse. (author)

187

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

Science.gov (United States)

Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependence and excitation power dependence of emission intensity. We have obtained the values of exciton diffusion length and absorption cross section for different chiralities, and diameter-dependent photoluminescence quantum yields have been observed. The simulations have also revealed the nature of a one-dimensional coalescence process, and an analytical expression for the power dependence of emission intensity is given.

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

2015-03-01

188

Efficient 1s ortho-exciton creation in Cu_2O  

Science.gov (United States)

A high density excitonic gas may undergo several types of phase transitions. We show that a high density 1s ortho-exciton gas in Cu_2O can be created by two-photon pumping to high-energy excitonic states associated with the highest valence band and the second conduction band. We observe a strong emission of 1s yellow exciton for the pump energy higher than 1.3eV. The efficiency of this type of pumping exceeds both types of excitation used so far namely the one-photon band-to-band excitation to the lower conduction band and two-photon absorption resonant on the 1s exciton. We report recent results on sub-picosecond time-resolved luminescence within the first 50 ns after two-photon excitation (1.46 eV). In spite of the high excitation power density 100MW/cm^2 an equilibrium temperature is reached within 300ps after the laser pulse. The studied spectrum consists of two bands at 2.036eV and 2.023eV attributed to the direct and phonon assisted recombination, respectively. In case of the direct transition a clearly single exponential decay in observed with a decay time of 3000ps. This is significantly longer than previously reported results (300ps). The decay of the phonon-assisted emission reveals an interesting, more complicated nature.

Karpinska, Katarzyna; van Loosdrecht, Paul; van der Marel, Dirk; Revcolevschi, Alexander

2002-03-01

189

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

OpenAIRE

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 additi...

Christian Wehrenfennig; Mingzhen Liu; Snaith, Henry J.; Johnston, Michael B.; Herz, Laura M.

2014-01-01

190

Observation of exciton localization in PbSrSe thin films grown by molecular beam epitaxy  

Science.gov (United States)

Detailed temperature and excitation-intensity dependence of radiative recombination characteristics in PbSrSe thin films grown by molecular beam epitaxy has investigated. For the first time, localized excitonic structure due to the alloy disorder has been observed at low temperature, and is gradually delocalized as wave-vector-nonconserving band-to-band transitions at high temperature in PbSrSe with low Sr composition. Furthermore, we are able to observe the evolution of the exciton localization, due to the strong alloy fluctuations and lattice distortions, into either free or trapped (strongly localized) excitons at low temperature by studying a PbSrSe thin film with the Sr composition as high as 0.276. The lattice deformation has been shown to play a key role in the composition-dependent excitonic peak broadening in PbSrSe at low temperature. This gives clear evidence for the observation of excitonic effects in lead salts due to their low carrier concentration and enhanced exciton binding energy. The results are discussed in the framework of theories taking into account the carrier migration and exciton trapping due to the alloy fluctuations and lattice vibrations.

Shen, W. Z.

2005-02-01

191

Superconductivity and Excitonic Charge Order  

Science.gov (United States)

It was suggested four decades ago that excitons as well as phonons could mediate superconductivity and that the temperature limits usually imposed by phonons could thus be avoided. In practice this form of excitonic pairing turned out to be elusive, because phonon softening typically causes a structural instability to occur before excitonic superconductivity has a chance to arise. Upon suppression of this CDW order however, superconductivity once again has an opportunity to materialise, as has recently been observed in for example pure TiSe2 under pressure. It is unclear what role is played by the excitons in such an environment of critical structural fluctuations, and whether they can have any effect on the pairing or indeed TC. Here we introduce a theoretical model to study the ways in which SC, CDW and excitonic order compete, coexist and even cooperate. Applying the model to TiSe2, we show that the hitherto elusive mechanism driving its CDW transition is a combination of excitonic and Jahn-Teller effects, and that under pressure it is likely to display an unusual type of superconductivity mediated by combinations of excitons and phonons.

van Wezel, Jasper; Nahai-Williamson, Paul; Saxena, Siddarth

2010-03-01

192

Origin of the variation of exciton binding energy in semiconductors.  

Science.gov (United States)

Excitonic effects are crucial to optical properties, and the exciton binding energy E(b) in technologically important semiconductors varies from merely a few meV to about 100 meV. This large variation, however, is not well understood. We investigate the relationship between the electronic band structures and exciton binding energies in semiconductors, employing first-principles calculations based on the density functional theory and the many-body perturbation theory using Green's functions. Our results clearly show that E(b) increases as the localization of valence electrons increases due to the reduced electronic screening. Furthermore, E(b) increases in ionic semiconductors such as ZnO because, contrary to the simple two-level coupling model, it has both conduction and valence band edge states strongly localized on anion sites, leading to an enhanced electron-hole interaction. These trends are quantized by electronic structures obtained from the density functional theory; thus, our approach can be applied to understand the excitonic effects in complex semiconducting materials. PMID:23383813

Dvorak, Marc; Wei, Su-Huai; Wu, Zhigang

2013-01-01

193

Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon  

International Nuclear Information System (INIS)

Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency

194

Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon  

Energy Technology Data Exchange (ETDEWEB)

Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

Thompson, Nicholas J.; Congreve, Daniel N.; Baldo, Marc A., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Energy Frontier Research Center for Excitonics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States); Goldberg, David; Menon, Vinod M., E-mail: vmenon@qc.cuny.edu, E-mail: baldo@mit.edu [Department of Physics, Queens College and Graduate Center, The City University of New York, Flushing, New York 11367 (United States)

2013-12-23

195

Excitonic effects of E11, E22, and E33 in armchair-edged graphene nanoribbons  

Energy Technology Data Exchange (ETDEWEB)

We explore excitonic effects of E11, E22, and E33, which are excitons formed between the three highest valence subbands and the three lowest conduction ones, in armchair-edged graphene nanoribbons by applying the extended tight-binding model including electron-electron interactions. Our results show that the excitation energies and the binding energies decrease inversely with the ribbon widths and can be classified into three categories based on their width indices. We found the relation between the band structures and the binding energies and explained some recent observations of strong excitonic effects in graphene.

Lu, Yan; Zhao, Shangqian; Lu, Wengang, E-mail: wglu@iphy.ac.cn; Liang, Wenjie [Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 (China); Liu, Hong [Physics Department, Nanjing Normal University, Nanjing 210046 (China)

2014-03-14

196

Strong plasmon-exciton coupling in a hybrid system of gold nanostars and J-aggregates  

Science.gov (United States)

Hybrid materials formed by plasmonic nanostructures and J-aggregates provide a unique combination of highly localized and enhanced electromagnetic field in metal constituent with large oscillator strength and extremely narrow exciton band of the organic component. The coherent coupling of localized plasmons of the multispiked gold nanoparticles (nanostars) and excitons of JC1 dye J-aggregates results in a Rabi splitting reaching 260 meV. Importantly, broad absorption features of nanostars extending over a visible and near-infrared spectral range allowed us to demonstrate double Rabi splitting resulting from the simultaneous coherent coupling between plasmons of the nanostars and excitons of J-aggregates of two different cyanine dyes.

Melnikau, Dzmitry; Savateeva, Diana; Susha, Andrey; Rogach, Andrey L.; Rakovich, Yury P.

2013-03-01

197

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

OpenAIRE

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6eV at the onsite interaction U=2t with the hopping integral t=1.1eV. The binding energy of the exciton is 0.50eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the bi...

Harigaya, Kikuo

2008-01-01

198

Picosecond pulse radiolysis of direct and indirect radiolytic effects in highly concentrated halide aqueous solutions.  

Science.gov (United States)

Recently we measured the amount of the single product, Br(3)(-), of steady-state radiolysis of highly concentrated Br(-) aqueous solutions, and we showed the effect of the direct ionization of Br(-) on the yield of Br(3)(-). Here, we report the first picosecond pulse-probe radiolysis measurements of ionization of highly concentrated Br(-) and Cl(-) aqueous solutions to describe the oxidation mechanism of the halide anions. The transient absorption spectra are reported from 350 to 750 nm on the picosecond range for halide solutions at different concentrations. In the highly concentrated halide solutions, we observed that, due to the presence of Na(+), the absorption band of the solvated electron is shifted to shorter wavelengths, but its decay, taking place during the spur reactions, is not affected within the first 4 ns. The kinetic measurements in the UV reveal the direct ionization of halide ions. The analysis of pulse-probe measurements show that after the electron pulse, the main reactions in solutions containing 1 M of Cl(-) and 2 M of Br(-) are the formation of ClOH(-•) and BrOH(-•), respectively. In contrast, in highly concentrated halide solutions, containing 5 M of Cl(-) and 6 M of Br(-), mainly Cl(2)(-•) and Br(2)(-•) are formed within the electron pulse without formation of ClOH(-•) and BrOH(-•). The results suggest that, not only Br(-) and Cl(-) are directly ionized into Br(•) and Cl(•) by the electron pulse, the halide atoms can also be rapidly generated through the reactions initiated by excitation and ionization of water, such as the prompt oxidation by the hole, H(2)O(+•), generated in the coordination sphere of the anion. PMID:21770462

Balcerzyk, Anna; Schmidhammer, Uli; El Omar, Abdel Karim; Jeunesse, Pierre; Larbre, Jean-Philippe; Mostafavi, Mehran

2011-08-25

199

Mechanically encoded single-photon sources: Stress-controlled excitonic fine structures of droplet epitaxial quantum dots  

Science.gov (United States)

We present numerical investigations based on the Luttinger-Kohn four-band k .p theory and, accordingly, establish a quantitatively valid model of the excitonic fine structures of droplet epitaxial GaAs/AlGaAs quantum dots under uniaxial stress control. In the formalisms, stressing a photoexcited quantum dot is equivalent creating a pseudomagnetic field that is directly coupled to the pseudospin of the exciton doublet and tunable to tailor the polarized fine structure of exciton. The latter feature is associated with the valence-band mixing of exciton, which is especially sensitive to external stress in inherently unstrained droplet epitaxial GaAs/AlGaAs quantum dots and allows us to mechanically design and prepare any desired exciton states of QD photon sources prior to the photon generation.

Cheng, Shun-Jen; Liao, Yu-Huai; Lin, Pei-Yi

2015-03-01

200

Thermal effects in exciton harvesting in biased one-dimensional systems  

OpenAIRE

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

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

2007-01-01

201

Strong plasmon-exciton coupling in a hybrid system of gold nanostars and J-aggregates  

OpenAIRE

Abstract Hybrid materials formed by plasmonic nanostructures and J-aggregates provide a unique combination of highly localized and enhanced electromagnetic field in metal constituent with large oscillator strength and extremely narrow exciton band of the organic component. The coherent coupling of localized plasmons of the multispiked gold nanoparticles (nanostars) and excitons of JC1 dye J-aggregates results in a Rabi splitting reaching 260 meV. Importantly, broad absorption features of nan...

Melnikau, D.; Savateeva, Diana; Susha, Andrey; Rogach, Andrey L.; Rakovich, Yury P.

2013-01-01

202

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

Directory of Open Access Journals (Sweden)

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.

V.I. Sugakov

2014-09-01

203

Halide Ion Enhancement of Nitrate Ion Photolysis  

Science.gov (United States)

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

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

2009-12-01

204

Exciton emission from hybrid organic and plasmonic polytype InP nanowire heterostructures  

Science.gov (United States)

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

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

2015-04-01

205

Transient excitons at metal surfaces  

Science.gov (United States)

Excitons, electron-hole pairs bound by the Coulomb potential, are the fundamental quasiparticles of coherent light-matter interaction relevant for processes such as photosynthesis and optoelectronics. The existence of excitons in semiconductors is well established. For metals, however, although implied by the quantum theory of the optical response, experimental manifestations of excitons are tenuous owing to screening of the Coulomb interaction taking place on timescales of a few femtoseconds. Here we present direct evidence for the dominant transient excitonic response at a Ag(111) surface, which precedes the full onset of screening of the Coulomb interaction, in the course of a three-photon photoemission process with ~15 fs laser pulses. During this transient regime, electron-hole pair Coulomb interactions introduce coherent quasiparticle correlations beyond the single-particle description of the optics of metals that dominate the multi-photon photoemission process on the timescale of screening at a Ag(111) surface.

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

2014-07-01

206

The Remarkable Reactivity of Aryl Halides with Nucleophiles  

Science.gov (United States)

Discusses the reactivity of aryl halides with nucleophilic or basic reagents, including nucleophilic attacks on carbon, hydrogen, halogen, and arynes. Suggestions are made concerning revisions of the sections on aryl halide chemistry courses and the corresponding chapters in textbooks. (CC)

Bunnett, Joseph F.

1974-01-01

207

Exciton dynamics in cuprous oxide  

Science.gov (United States)

This work addresses the mid-infrared properties of cuprous oxide and in particular induced absorption due to the presence of excitons. We probe the population of the non-radiative ground state of para-excitons via laser-induced changes of the transmission in the hydrogenic 1s-2p/1s-3p transition energy range in a stress-free sample.

Fishman, D. A.; Revcolevschi, A.; van Loosdrecht, P. H. M.

2006-08-01

208

Linear and nonlinear optical properties of free excitons in CdS  

International Nuclear Information System (INIS)

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

209

Growth and Studies of Halides doped Zinc Tris-Thiourea Sulphate(HZTS) Crystals  

Science.gov (United States)

Single crystals of Sodium chloride and Potassium iodide (Halides) doped Zinc tris-thiourea sulphate (ZTS) were grown from low temperature solution growth technique by slow evaporation method using water as solvent. The powder X-Ray diffraction pattern were recorded and indexed. The UV transmittance spectrum has been recorded. The optical band gap was estimated using Taucís plot. The TGA/DTA studies show the thermal properties of the crystals.

Suveetha, P.; Sathya, T.; Sudha, S.; Raj, M. B. Jessie

2012-10-01

210

Synthesis and vibrational absorption spectra of scandium and yttrium halide tetrahydrofuranates  

International Nuclear Information System (INIS)

Halides of MHal3x3THF type (M=Sc, Y; Hal=Cl, Br) solvated with tetrahydrofuran are prepared by HgCl2 effect on metallic scandium, yttrium or by HgBr2 effect on metallic scandium in tetrahydrofuran (THF). Infrared absorption spectra of the prepared compounds in the range of 200-3500 cm-1 are studied. Empiric attribution of the absorption bands, observed in the infrared spectra is carried out

211

Excitons in poly([ital para]-phenylenevinylene)  

Energy Technology Data Exchange (ETDEWEB)

We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90[plus minus]0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(2[sup [prime

Chandross, M. (Department of Physics, University of Arizona, Tucson, Arizona 85721 (United States)); Mazumdar, S. (Department of Physics and The Optical Sciences Center, University of Arizona, Tucson, Arizona 85721 (United States)); Jeglinski, S.; Wei, X.; Vardeny, Z.V. (Department of Physics, University of Utah, Salt Lake City, Utah 84112 (United States)); Kwock, E.W.; Miller, T.M. (AT T Bell Laboratories, Murray Hill, New Jersey 07974 (United States))

1994-11-15

212

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

International Nuclear Information System (INIS)

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

213

Recent advances in technetium halide chemistry.  

Science.gov (United States)

Transition metal binary halides are fundamental compounds, and the study of their structure, bonding, and other properties gives chemists a better understanding of physicochemical trends across the periodic table. One transition metal whose halide chemistry is underdeveloped is technetium, the lightest radioelement. For half a century, the halide chemistry of technetium has been defined by three compounds: TcF6, TcF5, and TcCl4. The absence of Tc binary bromides and iodides in the literature was surprising considering the existence of such compounds for all of the elements surrounding technetium. The common synthetic routes that scientists use to obtain binary halides of the neighboring elements, such as sealed tube reactions between elements and flowing gas reactions between a molecular complex and HX gas (X = Cl, Br, or I), had not been reported for technetium. In this Account, we discuss how we used these routes to revisit the halide chemistry of technetium. We report seven new phases: TcBr4, TcBr3, ?/?-TcCl3, ?/?-TcCl2, and TcI3. Technetium tetrachloride and tetrabromide are isostructural to PtX4 (X = Cl or Br) and consist of infinite chains of edge-sharing TcX6 octahedra. Trivalent technetium halides are isostructural to ruthenium and molybdenum (?-TcCl3, TcBr3, and TcI3) and to rhenium (?-TcCl3). Technetium tribromide and triiodide exhibit the TiI3 structure-type and consist of infinite chains of face-sharing TcX6 (X = Br or I) octahedra. Concerning the trichlorides, ?-TcCl3 crystallizes with the AlCl3 structure-type and consists of infinite layers of edge-sharing TcCl6 octahedra, while ?-TcCl3 consists of infinite layers of Tc3Cl9 units. Both phases of technetium dichloride exhibit new structure-types that consist of infinite chains of [Tc2Cl8] units. For the technetium binary halides, we studied the metal-metal interaction by theoretical methods and magnetic measurements. The change of the electronic configuration of the metal atom from d(3) (Tc(IV)) to d(5) (Tc(II)) is accompanied by the formation of metal-metal bonds in the coordination polyhedra. There is no metal-metal interaction in TcX4, a Tc?Tc double bond is present in ?/?-TcCl3, and a Tc?Tc triple bond is present in ?/?-TcCl2. We investigated the thermal behavior of these binary halides in sealed tubes under vacuum at elevated temperature. Technetium tetrachloride decomposes stepwise to ?-TcCl3 and ?-TcCl2 at 450 °C, while ?-TcCl3 converts to ?-TcCl3 at 280 °C. The technetium dichlorides disproportionate to Tc metal and TcCl4 above ?600 °C. At 450 °C in a sealed Pyrex tube, TcBr3 decomposes to Na{[Tc6Br12]2Br}, while TcI3 decomposes to Tc metal. We have used technetium tribromide in the preparation of new divalent complexes; we expect that the other halides will also serve as starting materials for the synthesis of new compounds (e.g., complexes with a Tc3(9+) core, divalent iodide complexes, binary carbides, nitrides, and phosphides, etc.). Technetium halides may also find applications in the nuclear fuel cycle; their thermal properties could be utilized in separation processes using halide volatility. In summary, we hope that these new insights on technetium binary halides will contribute to a better understanding of the chemistry of this fascinating element. PMID:24393028

Poineau, Frederic; Johnstone, Erik V; Czerwinski, Kenneth R; Sattelberger, Alfred P

2014-02-18

214

Dialkylzinc-mediated cross-coupling reactions of perfluoroalkyl and perfluoroaryl halides with aryl halides.  

Science.gov (United States)

A highly chemoselective perfluoroalkylation reaction of aromatic halides is reported. Thermally stable perfluoroalkylzinc reagents, generated by a rapid halogen-zinc exchange reaction between diorganozinc and perfluoroalkyl halide species, couple with a wide range of aryl halides in the presence of a copper catalyst, in moderate to high yields. Good stability of the perfluoroalkylzinc species was indicated by DFT calculation and the reagents were storable for at least three months under argon without loss of activity. This method is applicable to gram-scale synthesis, and its functional group tolerance compares favorably with reported protocols. PMID:25630706

Kato, Hisano; Hirano, Keiichi; Kurauchi, Daisuke; Toriumi, Naoyuki; Uchiyama, Masanobu

2015-03-01

215

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

International Nuclear Information System (INIS)

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

216

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

Science.gov (United States)

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

Filip, Marina R.; Giustino, Feliciano

2014-12-01

217

Excitonic splitting, delocalization, and vibronic quenching in the benzonitrile dimer.  

Science.gov (United States)

The excitonic S1/S2 state splitting and the localization/delocalization of the S1 and S2 electronic states are investigated in the benzonitrile dimer (BN)2 and its (13)C and d5 isotopomers by mass-resolved two-color resonant two-photon ionization spectroscopy in a supersonic jet, complemented by calculations. The doubly hydrogen-bonded (BN-h5)2 and (BN-d5)2 dimers are C2h symmetric with equivalent BN moieties. Only the S0 ? S2 electronic origin is observed, while the S0 ? S1 excitonic component is electric-dipole forbidden. A single (12)C/(13)C or 5-fold h5/d5 isotopic substitution reduce the dimer symmetry to Cs, so that the heteroisotopic dimers (BN)2-(h5 – h5(13)C), (BN)2-(h5 – d5), and (BN)2-(h5 – h5(13)C) exhibit both S0 ? S1 and S0 ? S2 origins. Isotope-dependent contributions ?iso to the excitonic splittings arise from the changes of the BN monomer zero-point vibrational energies; these range from ?iso((12)C/(13)C) = 3.3 cm(–1) to ?iso(h5/d5) = 155.6 cm(–)1. The analysis of the experimental S1/S2 splittings of six different isotopomeric dimers yields the S1/S2 exciton splitting ?exc = 2.1 ± 0.1 cm(–1). Since ?iso(h5/d5) ? ?exc and ?iso((12)C/(13)C) > ?exc, complete and near-complete exciton localization occurs upon (12)C/(13)C and h5/d5 substitutions, respectively, as diagnosed by the relative S0 ? S1 and S0 ? S2 origin band intensities. The S1/S2 electronic energy gap of (BN)2 calculated by the spin-component scaled approximate second-order coupled-cluster (SCS-CC2) method is ?el(calc) = 10 cm(–1). This electronic splitting is reduced by the vibronic quenching factor ?. The vibronically quenched exciton splitting ?el(calc)·? = ?vibron(calc) = 2.13 cm(–1) is in excellent agreement with the observed splitting ?exc = 2.1 cm(–1). The excitonic splittings can be converted to semiclassical exciton hopping times; the shortest hopping time is 8 ps for the homodimer (BN-h5)2, the longest is 600 ps for the (BN)2(h5 – d5) heterodimer. PMID:25353324

Balmer, Franziska A; Ottiger, Philipp; Leutwyler, Samuel

2014-11-26

218

Luminescence evidence for bulk and surface excitons in free xenon clusters  

CERN Document Server

Cathodoluminescence spectra of free xenon clusters produced by condensation of xenon-argon gas mixtures in supersonic jets expanding into vacuum were studied. By varying initial experimental parameters, including xenon concentration, we could obtain clusters with a xenon core (300-3500 atoms) covered by an argon outer shell as well as shell-free xenon clusters (about 1500 atoms). The cluster size and temperature (about 40 K for both cases) were measured electronographically. Luminescence bands evidencing the existence of bulk and surface excitons were detected for shell-free xenon clusters. The emission from bulk excitons in small clusters is supposed to be due to processes of their multiple elastic reflections from the xenon-vacuum interface. A presence of an argon shell causes extinction of the excitonic bands. In addition, some new bands were found which have no analogs for bulk xenon cryosamples.

Danylchenko, O G; Kovalenko, S I; Libin, M Yu; Samovarov, V N; Vakula, V L

2007-01-01

219

Optical generation of excitonic valley coherence in monolayer WSe2  

Science.gov (United States)

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

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

2013-09-01

220

Adiabatic preparation of a cold exciton condensate  

Science.gov (United States)

We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far blue-detuned IX mode, with its subsequent adiabatic transformation to an indirect exciton condensate by lowering IX energy via applied electric field. The following allows for generation of a spatially localized cold exciton gas, on the contrary to currently used methods, where IX cloud appears due to diffusion of carriers from spatially separated electron- and hole-rich areas.

Shahnazaryan, V.; Kyriienko, O.; Shelykh, I. A.

2015-02-01

221

Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals  

Science.gov (United States)

Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing. PMID:23974520

Grivas, Christos; Li, Chunyong; Andreakou, Peristera; Wang, Pengfei; Ding, Ming; Brambilla, Gilberto; Manna, Liberato; Lagoudakis, Pavlos

2013-01-01

222

Long excitonic spin relaxation in InAs/AlAs quantum dots  

International Nuclear Information System (INIS)

We investigated the excitonic spin relaxation in self-assembled InAs quantum dots, embedded in an AlAs matrix, by means of time-resolved, circularly polarized photoluminescence. The atomic-like electronic structure of self-assembled quantum dots suppresses not only elastic but also inelastic processes of spin relaxation mechanisms, thus increasing the exciton spin relaxation time ?s. The experimental determination of ?s is typically inhibited by the short exciton lifetime of a few nanoseconds. However, the novel InAs/AlAs quantum dot system, characterized by the lowest electron state at the direct ? or indirect X conduction band minima, exhibits exciton lifetimes of several milliseconds. At a temperature of 1.8 K the exciton spin relaxation time ?s is in the range of 100?s. It strongly depends on the optical excitation density, temperature and quantum dot size. Differences between the direct and indirect bandgap transitions have been observed in the temporal evolution of the non-equilibrium exciton spin-orientation induced by an external magnetic field or excitation with circularly polarized light. Spectral and temperature dependencies of photoluminescence kinetics are explained in terms of an electron redistribution between long-lived indirect and short-lived direct states of the quantum dot conduction band.

223

Vibrational spectroscopic characterization of the sulphate-halide mineral sulphohalite - Implications for evaporites  

Science.gov (United States)

The mineral sulphohalite - Na6(SO4)2FCl is a rare sodium halogen sulphate and occurs associated with evaporitic deposits. Sulphohalite formation is important in saline evaporites and in pipe scales. Sulphohalite is an anhydrous sulphate-halide with an apparent variable anion ratio of formula Na6(SO4)2FCl. Such a formula with oxyanions lends itself to vibrational spectroscopy. The Raman band at 1003 cm-1 is assigned to the (SO4)2- ?1 symmetric stretching mode. Shoulders to this band are found at 997 and 1010 cm-1. The low intensity Raman bands at 1128, 1120 and even 1132 cm-1 are attributed to the (SO4)2- ?3 antisymmetric stretching vibrations. Two symmetric sulphate stretching modes are observed indicating at least at the molecular level the non-equivalence of the sulphate ions in the sulphohalite structure. The Raman bands at 635 and 624 cm-1 are assigned to the ?4 SO42- bending modes. The ?2 (SO4)2- bending modes are observed at 460 and 494 cm-1. The observation of multiple bands supports the concept of a reduction in symmetry of the sulphate anion from Td to C3v or even C2v. No evidence of bands attributable to the halide ions was found.

Frost, Ray L.; Scholz, Ricardo; López, Andrés; Theiss, Frederick L.

2014-12-01

224

Excitonic emission of CuInS2 crystals using confocal microscopy system  

International Nuclear Information System (INIS)

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

225

Exciton in wurtzite GaN/Al x Ga1- x N coupled quantum dots  

International Nuclear Information System (INIS)

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al x Ga1- x N vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al x Ga1- x N coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al x Ga1- x N strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs

226

Tightly bound excitons in monolayer WSe2  

OpenAIRE

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally de...

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-01-01

227

Exciton-exciton interaction in quantum wells. Optical properties and energy and spin relaxation  

OpenAIRE

The gas of interacting excitons in quantum wells is studied. We obtain the Hamiltonian of this gas by the projection of the electron-hole plasma Hamiltonian to exciton states and an expansion in a small density. Matrix elements of the exciton Hamiltonian are rather sensitive to the geometry of the heterostructure. The mean field approximation of the exciton Hamiltonian gives the blue shift and spin splitting of the exciton luminescence lines. We also write down the Boltzmann...

De-leon, S. Ben-tabou; Laikhtman, B.

2000-01-01

228

Exciton-photon correlations in bosonic condensates of exciton-polaritons  

OpenAIRE

Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the "hidden variable" which characterizes the rate of exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.

Kavokin, A. V.; Sheremet, A. S.; Shelykh, I. A.; Lagoudakis, P. G.; Rubo, Y. G.

2014-01-01

229

The Additive Coloration of Alkali Halides  

Science.gov (United States)

Describes the construction and use of an inexpensive, vacuum furnace designed to produce F-centers in alkali halide crystals by additive coloration. The method described avoids corrosion or contamination during the coloration process. Examination of the resultant crystals is discussed and several experiments using additively colored crystals are…

Jirgal, G. H.; and others

1969-01-01

230

Molecular compressibility of some halides in alcohols  

Science.gov (United States)

After measuring ultrasonic velocity and density, the molecular compressibility values from Wada's formula were calculated, for alkali metal halide solutions in methyl, ethyl, butyl, and glycol alcohol. The temperature and concentration dependence were studied, finding deviations due to the hydrogen bonds of the solvent.

Serban, C.; Auslaender, D.

1974-01-01

231

Molten state and supercooling of lead halides.  

Czech Academy of Sciences Publication Activity Database

Ro?. 264, - (2004), s. 492-498. ISSN 0022-0248 R&D Projects: GA ?R GA203/00/1423; GA ?R GA202/03/0428 Institutional research plan: CEZ:AV0Z1010914 Keywords : melt supercooling * thermal history of the melt * halides Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.707, year: 2004

Nitsch, Karel; Cihlá?, Antonín; Rodová, Miroslava

2004-01-01

232

Formation of structured nanophases in halide crystals.  

Czech Academy of Sciences Publication Activity Database

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

Kulveit, Jan; Demo, Pavel; Polák, Karel; Sveshnikov, Alexey; Kožíšek, Zden?k

2013-01-01

233

Dissociation of alkali halides under electron bombardment  

International Nuclear Information System (INIS)

The low-energy electron bombardment of alkali halides results in their dissociation and emission of halogen. The alkali metal excess on the irradiated surface is found by the electron Auger-spectroscopy, neutron-activated analysis, electron microscopy, conductivity and secondary electron emission coefficient measurements

234

Breakdown processes in metal halide lamps  

Science.gov (United States)

Metal halide lamps typically have cold fills of tens to a few hundred Torr of a rare gas and the vapour from the dosing of a metal halide solid and mercury. Breakdown and starting of the lamp occurs following application of multi-kV pulses across electrodes separated by a few centimetres. Restarting of warm lamps is often problematic as the available voltage is insufficient to break down the higher pressure (>many atm) of metal halide vapour. In this paper, fundamental processes during breakdown in cold and warm, idealized metal halide lamps in mixtures of Ar and Hg are investigated using a two-dimensional fluid model for plasma transport. We find that the capacitances of the walls of the discharge tube and adjacent ground planes are important in determining the breakdown voltage and avalanche characteristics. The prompt capacitance represented by, for example, external trigger wires provides a larger E/N to sustain ionization early in the avalanche. This effect is lost as the walls charge and shield the plasma from the ground planes. More rapid breakdown occurs in slightly warm lamps having small vapour pressures of Hg due to the resulting Penning mixture. Warmer lamps, having larger mole fractions of Hg, have less efficient breakdown as the increase in momentum transfer of the electrons is not offset by the additional ionization sources of the Penning mixture.

Lay, Brian; Moss, Richard S.; Rauf, Shahid; Kushner, Mark J.

2003-02-01

235

Criterion for exciton lasing in pure crystals  

Energy Technology Data Exchange (ETDEWEB)

A scheme for lasing action involving excitons in a pure crystal is described. The lasing mechanism depends on the nonboson quality of excitons. We find this property to be exhibited at high exciton density. A criterion for lasing under these conditions is obtained.

Liu, K.C.; Liboff, R.L.

1983-10-01

236

Interwell excitons in GaAs superlattices  

DEFF Research Database (Denmark)

The formation of spatially indirect excitons in superlattices with narrow minibands is investigated experimentally. The interwell exciton is similar to the first Wannier-Stark localized exciton of an electrically biased superlattice. However, in the present case the localization is mediated by the Coulomb interaction of the electron and the hole without external fields. (C) 1997 Academic Press Limited.

Birkedal, Dan; Sayed, Karim El

1997-01-01

237

Excitons in a mirror: Formation of “optical bilayers” using MoS2 monolayers on gold substrates  

International Nuclear Information System (INIS)

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

238

Structure parameters and external electric field effects on exciton binding energies of CdTe/ZnTe quantum dots  

International Nuclear Information System (INIS)

We study the effects of the structure parameters of self-assembled CdTe/ZnTe quantum dots (QDs) under an electric field on the exciton binding energies due to Coulomb interaction between electrons and holes with a finite-element method based on the linear elasticity theory of solids and the eight-band k ·p Hamiltonian. The exciton binding energy is shown to decrease with increasing base width of the QD, regardless of its height. We point out that the monotonic decrease in the exciton binding energy is due to the confinement of the electron and the hole wavefunctions inside the QD. The exciton binding energy is also found to decrease as the wetting layer thickness increases, which can be attributed to the dipole-like wavefunction of the hole. The fact that the electron and the hole energies decrease parabolically and the exciton binding energy decreases with increasing electric field due to the Stark effect is demonstrated.

239

Exciton coupling in molecular crystals  

Science.gov (United States)

The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.

Ake, R. L.

1976-01-01

240

Dependence of nonproportionality in scintillators on diffusion of excitons and charge carriers  

Science.gov (United States)

The dipole-dipole and free-carrier Auger quenching processes that are generally regarded to be at the root of nonproportionality depend respectively on the 4th or 6th power of the electron track radius if modeled as cylindrical. In an initial time interval ? when nonlinear quenching and diffusion compete to reduce the density of excited states, the track radius expands as (Deff?)1/2 where Deff is the effective diffusion coefficient for the mixture of excitons and charge carriers. The range of Deff across semiconductor and scintillator radiation detectors is large, illustrated by 8 decades between mobilities of self-trapped holes in CsI:Tl and holes in high purity Ge. We present the functional form of nonlinear quenching predicted by diffusive track dilution and show that the simple model provides a surprisingly good fit of empirical nonproportionality across a wide range of semiconductor and oxide radiation detectors. We also show how diffusion drives nonlinear branching between excitons and free carriers in the track when electron and hole mobilities are unequal, and that this nonlinear branching coupled with linear trapping on defects can produce the "halide hump" seen in electron yield data for activated halide scintillators. Picosecond time-resolved spectroscopy in alkali halides, as well as quantitative comparison of recently measured 2nd order quenching rate constants K2 and results of K-dip spectroscopy, provide experimental benchmarks for consideration of carrier thermalization and the initial track or cluster radius r0 from which (nearly thermalized) diffusion is assumed to commence. The ratio of initial rate of 2nd order quenching to that of dilution by diffusion in a cylindrical track is proportional to K2/Deff and does not depend on r0 in lowest order; however, the absolute rates of both processes decrease with increasing r0.

Williams, R. T.; Li, Qi; Grim, Joel Q.; Ucer, K. B.

2011-09-01

241

Exciton Dissociation Dynamics in Model Donor-Acceptor Polymer Heterojunctions: I. Energetics and Spectra  

CERN Document Server

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band off-set in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band off-set at the heterojunction, the exciton will remain the lowest energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the off-set is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that i...

Bittner, E R; Karabunarliev, S; Bittner, Eric R.; Ramon, John Glenn Santos; Karabunarliev, Stoyan

2005-01-01

242

Theoretical study of excitonic complexes in semiconductors quantum wells  

International Nuclear Information System (INIS)

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

243

Direct observation of substrate induced exciton in carbon nanotube  

Science.gov (United States)

We have successfully measured the electroluminescence spectra of a single-walled carbon nanotube (CNT) grown to serpentine shape on quartz substrate. We observe two emission peaks: One locates at 0.85 eV and is identified as the usual E11 exciton peak, and the other locates at slightly higher energy of 0.94 eV with similar symmetrical line shape and comparable intensity. However, the extra peak is substantially wider and it broadens with increasing current at unusually faster speed. We show that the extra peak is not from interband transitions, and ascribe it to a type of exciton induced by the formation of substrate-CNT superlattice. The periodic surface potential of the substrate modulates the CNT band structure, causes degeneracy lifting and band flattering at the Brillouin zone, and generates the higher energy exciton. For confirmation, a similar device is fabricated using amorphous SiO2 substrate to avoid the formation of the superlattice. Indeed, the extra emission peak disappears.

Ye, Lin-Hui; Yu, Dang-Min; Wang, Sheng; Zhang, Zhiyong; Peng, Lian-Mao

2013-11-01

244

Emission characteristics of ZnO nanorods on nanosilicon-on-insulator: competition between exciton-phonon coupling and surface resonance effect  

International Nuclear Information System (INIS)

We investigated the optical properties of ZnO nanorods on nanosilicon-on-insulator using variable temperature photoluminescence (PL) spectroscopy, and explored the contribution of exciton-phonon coupling and surface resonance effect on the emission characteristics of the nanorods. The low-temperature (<100 K) PL spectra revealed different strengths of exciton-phonon interaction for nanorods of different surface structures. The exciton-phonon coupling strength was stronger for nanorods of rougher surfaces with enhanced contribution of longitudinal optical phonon replicas of free exciton. Despite exhibiting different coupling strengths of exciton-phonon interactions, the room-temperature PL showed an unchanged energy position at 3.28 eV for nanorods of different surface structures. The unchanged energy position of band-edge emission was caused by the competitive effect of the surface defects induced exciton-phonon interaction and the surface resonance effect in faceted nanorods.

245

Excitons in bulk liquid 4He  

International Nuclear Information System (INIS)

We present the first measurements of excitons in bulk liquid He, performed at several densities. Inelastic x-ray scattering was used to take these measurements of 4He. The exciton measurements are very similar to those for solid 4He, with the exciton excitations occurring at nearly the same energy transfers for the same pressure. This similarity suggests that excitons in solid helium rely mostly on short range order. The measured peaks for the liquid are asymmetric in nature and fit well to two Gaussians, suggesting that at least two exciton transitions are present in the signal peak

246

The electronic structure of F-centers in allkali halide crystals  

International Nuclear Information System (INIS)

A method is presented for estimating the number of bound states of different orbital angular momentum quantum number for an F-centre residing in an alkali halide crystal through the introduction of a spherically symmetric potential well of finite depth. The method is illustrated through its application to the F-centre in sodium chloride, and the results for F-centres in various alkali halide lattices are tabulated. The results provide a good account of the experimental features of the F-centre including the K-band which we suggest is due to transitions from the 2p to excited levels. Values for the effective range of penetration of the defect electron with the neighbouring cation in different crystals are determined by requiring that the well depth be independent of orbital angular momentum quantum number. The technique is applied to estimate the Madelung constant and Madelung potential for the host crystal. (Author)

247

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

CERN Document Server

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...

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

2011-01-01

248

Subdiffusive exciton transport in quantum dot solids.  

Science.gov (United States)

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications. PMID:24807586

Akselrod, Gleb M; Prins, Ferry; Poulikakos, Lisa V; Lee, Elizabeth M Y; Weidman, Mark C; Mork, A Jolene; Willard, Adam P; Bulovi?, Vladimir; Tisdale, William A

2014-06-11

249

A Variational Approach to Nonlocal Exciton-Phonon Coupling  

CERN Document Server

In this paper we apply variational energy band theory to a form of the Holstein Hamiltonian in which the influence of lattice vibrations (optical phonons) on both local site energies (local coupling) and transfers of electronic excitations between neighboring sites (nonlocal coupling) is taken into account. A flexible spanning set of orthonormal eigenfunctions of the joint exciton-phonon crystal momentum is used to arrive at a variational estimate (bound) of the ground state energy for every value of the joint crystal momentum, yielding a variational estimate of the lowest polaron energy band across the entire Brillouin zone, as well as the complete set of polaron Bloch functions associated with this band. The variation is implemented numerically, avoiding restrictive assumptions that have limited the scope of previous assaults on the same and similar problems. Polaron energy bands and the structure of the associated Bloch states are studied at general points in the three-dimensional parameter space of the mo...

Zhao, Y; Lindenberg, K; Zhao, Yang; Brown, David W.; Lindenberg, Katja

1996-01-01

250

Exciton emission and defect formation in yttrium trifluoride  

Energy Technology Data Exchange (ETDEWEB)

Two intrinsic emission bands at 220 and 280 nm have been detected in nominally pure YF{sub 3} powders at 10 K. Excitation spectra for both emissions have a sharp peak at 12 eV near the edge of interband transition. Observed emissions are assigned to the radiative decay of self-trapped excitons in YF{sub 3}. The strong thermal quenching of intrinsic luminescence was observed at temperature above 120 K. It was supposed that non-radiative decay of self-trapped exciton at high temperatures lead to defect formation in YF{sub 3}. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Pankratov, V. [Material Science Department, Institute for Electronic Materials, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt (Germany); Institute of Solid State Physics, University of Latvia, Kengaraga iela 8, LV1063 Riga (Latvia); Kirm, M. [Institute of Physics, University of Tartu, Riia 142, 51014 Tartu (Estonia); Seggern, H. von [Material Science Department, Institute for Electronic Materials, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt (Germany)

2005-01-01

251

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

DEFF Research Database (Denmark)

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.

Orani, D.; Polimeni, A.

1997-01-01

252

All conjugated copolymer excitonic multiferroics.  

Science.gov (United States)

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

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

2013-02-01

253

Angular momentum dependent exciton model  

International Nuclear Information System (INIS)

To consider the angular momentum and parity conservation the angular momentum (J) and parity (?) should be addressed in the master equation of the exciton model. Therefore the internal transition rates and the emission rates must be J? dependent. The angular momentum factor of the internal transition rates is given and the angular momentum conservation effect is discussed

254

Exciton dynamics in solid neon  

International Nuclear Information System (INIS)

The results of an experimental study of the photoelectric yield of pure and doped solid Ne in the extreme ultraviolet (h?/2? = 8-30 eV) by use of synchrotron radiation from the DESY-synchrotron are reported. Results for the range of impurity excitations, exciton states and interband transitions were obtained for Xe in Ne, Kr in Ne, and Ar in Ne. The photoemission data were utilized to extract new information concerning (a) energetics, (b) non-radiative relaxation phenomena, and (c) exciton dynamics in solid Ne. Concerning (a), a definite value for the electron affinity V0 in solid Ne was established experimentally. Concerning (b), the authors were able to show that the n = 2(1/2) exciton state decays in an Auger type process into an ionized impurity in the 3/2 state and a free electron on a time scale tau(Auger) approximately< 10sup(-13)sec, which is comparable to or even shorter than the n = 2 ? n = 1 nonradiative relaxation process. Concerning (c), the analysis of the energy-dependent photoemission line shape at different film thicknesses yielded information concerning the diffusion length of excitons and the escape length of electrons in solid Ne. (orig.)

255

Magnetic exciton dispersion in praseodymium  

DEFF Research Database (Denmark)

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 rare-earth metals.

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

1971-01-01

256

Bound exciton luminescence in phosphorus doped Cd1-xMnxTe crystals  

International Nuclear Information System (INIS)

Measurements of photoluminescence as a function of temperature and of magnetic field in p-type phosphorus doped Cd1-xMnxTe is reported. From the conduction band-acceptor level transition, the ionization energy of P-acceptors is obtained to be 54±1 MeV. The photoluminescence spectrum in the band edge region exhibits three maxima connected with the recombination of excitons bound to neutral acceptors (Ao,X), excitons bound to neutral donors (Do,X) and free excitons (X) at energies E(Ao,X) +1.606, E(Do,X) = 1.610, and EX = 1.614 eV, respectively. At T = 1.4 K a strong increase in PL intensity of (Ao,X) line 8-fold as a function of magnetic field is found and shown to originate from the magnetic field-induced lowering of the acceptor binding energy and increase in the hole effective volume. (author)

257

Photoluminescence and decay kinetics of indirect free excitons in diamonds under the near-resonant laser excitation  

Science.gov (United States)

Indirect edge emission spectra were observed in types-IIa, -Ib and -Ia diamond crystals, excited with a pulsed laser just above the indirect band gap. In the type-IIa the lifetime of indirect free excitons was also measured, whose temperature dependence was successfully analyzed using a rate-equation model including a bound exciton formation. Weak luminescence observed in other diamonds was qualitatively explained by a strong nonradiative process related to deep impurities.

Takiyama, K.; Abd-Elrahman, M. I.; Fujita, T.; Oda, T.

1996-09-01

258

Measurement of high exciton binding energy in the monolayer transition-metal dichalcogenides WS2 and WSe2  

OpenAIRE

Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorptio...

Hanbicki, A. T.; Currie, M.; Kioseoglou, G.; Friedman, A. L.; Jonker, B. T.

2014-01-01

259

75 FR 5544 - Energy Conservation Program: Energy Conservation Standards for Metal Halide Lamp Fixtures: Public...  

Science.gov (United States)

...Energy Conservation Standards for Metal Halide Lamp Fixtures: Public Meeting and Availability...conservation standards for certain metal halide lamp fixtures. This document announces...energy conservation standards for metal halide lamp fixtures and provide docket...

2010-02-03

260

76 FR 18127 - Energy Conservation Standards for Metal Halide Lamp Fixtures: Public Meeting and Availability of...  

Science.gov (United States)

...Energy Conservation Standards for Metal Halide Lamp Fixtures: Public Meeting and Availability...energy conservation standards for metal halide lamp fixtures (MHLFs); the analytical...appliance_standards/commercial/metal_halide_lamp_fixtures.html. DATES:...

2011-04-01

261

EVALUATION OF METHODS FOR THE DETERMINATION OF TOTAL ORGANIC HALIDE IN WATER AND WASTE  

Science.gov (United States)

Various methods for the determination of total organic halides (TOX) in groundwater and in waste oil samples have been evaluated. Of three inorganic halide species generation approaches and three inorganic halide determinative techniques evaluated for groundwater analyses, one co...

262

10 CFR 431.322 - Definitions concerning metal halide lamp ballasts and fixtures.  

Science.gov (United States)

...false Definitions concerning metal halide lamp ballasts and fixtures. 431...COMMERCIAL AND INDUSTRIAL EQUIPMENT Metal Halide Lamp Ballasts and Fixtures § 431.322 Definitions concerning metal halide lamp ballasts and fixtures....

2010-01-01

263

40 CFR 721.10181 - Halide salt of an alkylamine (generic).  

Science.gov (United States)

...2010-07-01 2010-07-01 false Halide salt of an alkylamine (generic). ...Chemical Substances § 721.10181 Halide salt of an alkylamine (generic). ...chemical substance identified generically as halide salt of an alkylamine (PMN...

2010-07-01

264

Lanthanide-halide based humidity indicators  

Science.gov (United States)

The present invention discloses a lanthanide-halide based humidity indicator and method of producing such indicator. The color of the present invention indicates the humidity of an atmosphere to which it is exposed. For example, impregnating an adsorbent support such as silica gel with an aqueous solution of the europium-containing reagent solution described herein, and dehydrating the support to dryness forms a substance with a yellow color. When this substance is exposed to a humid atmosphere the water vapor from the air is adsorbed into the coating on the pore surface of the silica gel. As the water content of the coating increases, the visual color of the coated silica gel changes from yellow to white. The color change is due to the water combining with the lanthanide-halide complex on the pores of the gel.

Beitz, James V. (Hinsdale, IL); Williams, Clayton W. (Chicago, IL)

2008-01-01

265

Computational Screening of Mixed Metal Halide Ammines  

DEFF Research Database (Denmark)

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

Jensen, Peter Bjerre; Lysgaard, Steen

266

The Optical Stark effect of the exciton due to dynamical coupling between quantized states of the electron and hole in quantum wells  

International Nuclear Information System (INIS)

Optical Stark effect of the exciton in quantum wells is treated by taking into account not only dynamical coupling by pump field between quantized states of the electron in the conduction band but also the coupling between quantized states of the hole in the valence band. The changes in the calculated one-photon absorption probability by exciton depend essentially on pump field detuning. (author). 19 refs, 3 figs

267

Dielectrically enhanced excitons in semiconductor-insulator quantum wires: Theory and experiment  

Science.gov (United States)

We present both theoretical and experimental investigations of optical properties of excitons in semiconductor-insulator quantum wires. Spectra of linear and nonlinear absorption, photoluminescence and its polarization, photoluminescence excitation, time-resolved photoluminescence of GaAs, CdSe, and InP quantum wires 4-6 nm in diameter, crystallized in dielectric matrix, demonstrate the prominent excitonic behavior. In these structures an essential difference of dielectric constants of constituent materials leads to a considerable enhancement of excitons, the binding energies ranging from 120 meV to 260 meV and exciton transitions being well distinguished in nanowires with sufficient dispersion of diameter even at room temperature. A theoretical approach to calculations of the exciton parameters in a semiconductor-insulator cylindrical quantum wire of finite diameter is developed. This approach accounts for a band-gap renormalization due to the spatial confinement and self-image effect, as well as for a dielectric enhancement of the electron-hole interaction. The calculated exciton transition energies and absorption spectra are consistent with the experimental results.

Muljarov, E. A.; Zhukov, E. A.; Dneprovskii, V. S.; Masumoto, Yasuaki

2000-09-01

268

Dynamical patterns of phase transformations from self-trapping of quantum excitons  

Science.gov (United States)

Phase transitions induced by short optical pulses is a new mainstream in studies of cooperative electronic states. Its special realization in systems with neutral-ionic transformations stands out in a way that the optical pumping goes to excitons rather than to electronic bands. We present a semi-phenomenological modeling of spacio-temporal effects applicable to any system where the optical excitons are coupled to a symmetry breaking order parameter. In our scenario, after a short initial pulse of photons, a quasi-condensate of excitons appears as a macroscopic quantum state which then evolves interacting with other degrees of freedom prone to instability. This coupling leads to self-trapping of excitons; that locally enhances their density which can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains which evolve through dynamical phase transitions and may persist even after the initiating excitons have recombined. We recover dynamic interplays of fields such as the excitons' wave function, electronic charge transfer and polarization, lattice dimerization.

Yi, Tianyou; Kirova, Natasha; Brazovskii, Serguei

2015-03-01

269

Process and composition for drying of gaseous hydrogen halides  

Science.gov (United States)

A process for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom, comprising: contacting the water impurity-containing gaseous hydrogen halide with a scavenger including a support having associated therewith one or more members of the group consisting of: (a) an active scavenging moiety selected from one or more members of the group consisting of: (i) metal halide compounds dispersed in the support, of the formula MX.sub.y ; and (ii) metal halide pendant functional groups of the formula -MX.sub.y-1 covalently bonded to the support, wherein M is a y-valent metal, and y is an integer whose value is from 1 to 3; (b) corresponding partially or fully alkylated compounds and/or pendant functional groups, of the metal halide compounds and/or pendant functional groups of (a); wherein the alkylated compounds and/or pendant functional groups, when present, are reactive with the gaseous hydrogen halide to form the corresponding halide compounds and/or pendant functional groups of (a); and M being selected such that the heat of formation, .DELTA.H.sub.f of its hydrated halide, MX.sub.y.(H.sub.2 O).sub.n, is governed by the relationship: .DELTA.H.sub.f .gtoreq.n.times.10.1 kilocalories/mole of such hydrated halide compound wherein n is the number of water molecules bound to the metal halide in the metal halide hydrate. Also disclosed is an appertaining scavenger composition and a contacting apparatus wherein the scavenger is deployed in a bed for contacting with the water impurity-containing gaseous hydrogen halide.

Tom, Glenn M. (New Milford, CT); Brown, Duncan W. (Wilton, CT)

1989-08-01

270

Computational screening of mixed metal halide ammines  

DEFF Research Database (Denmark)

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

Jensen, Peter Bjerre; Lysgaard, Steen

271

Pair spectra and exciton emission of sodium implanted ZnSe:Li  

International Nuclear Information System (INIS)

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.)

272

Exciton Seebeck effect in molecular systems  

Science.gov (United States)

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.

Yan, Yun-An; Cai, Shaohong

2014-08-01

273

Exciton localization in disordered poly(3-hexylthiophene).  

Science.gov (United States)

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the relative values of the disorder-induced localization length and the distances between the pi-conjugation breaks, these breaks sometimes serve to pin the low-lying localized excitons. The exciton confinement also causes a local spectrum of low-lying exciton states. Coulomb-induced intra- or interchain interactions between spectroscopic segments in close spatial proximity can delocalize an exciton across these segments, in principle causing phase coherent transition dipole moments. PMID:20687660

Barford, William; Lidzey, David G; Makhov, Dmitry V; Meijer, Anthony J H

2010-07-28

274

Magneto-optical properties and exciton dynamics in diluted magnetic semiconductor nanostructures  

International Nuclear Information System (INIS)

Nanostructures of diluted magnetic semiconductors were fabricated to study novel magneto-optical properties that are derived from quantum confined band electrons interacting with magnetic ions. Quantum dots (QDs) of Cd0.97Mn0.03Se were grown by the self-organization on a ZnSe substrate layer. QDs of Zn0.69Cd0.23Mn0.08Se and quantum wires (QWRs) of Cd0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se were fabricated by the electron beam lithography. A single quantum well (QW) of ZnTe/Zn0.97Mn0.03Te and double QWs of Cd0.95Mn0.05Te-CdTe were grown by molecular beam epitaxy. Magneto-optical properties and the formation and relaxation dynamics of excitons were investigated by ultrafast time-resolved photoluminescence (PL) spectroscopy. Excitons in these nanostructures were affected by the low-dimensional confinement effects and the interaction with magnetic ion spins. The exciton luminescence of the Cd0.97Mn0.03Se QDs shows the confined exciton energy due to the dot size of 4-6 nm and also shows marked increase of the exciton lifetime with increasing the magnetic field. The QDs of Zn0.69Cd0.23Mn0.08Se fabricated by the electron beam lithography display narrow exciton PL spectrum due to the uniform shape of the dots. The exciton luminescence from the QWRs of Cd0.92Mn0.92Mn0.08Se and Zn0.69Cd0.23Mn0.08Se shows the influence of the one-dimensional confinement effect for the exciton energy and the luminescence is linearly polarized parallel to the wire direction. The transient PL from the ZnTe/Zn0.97Mn0.03Te QWs displays, by the magnetic field, the level crossing of the exciton spin states of the nonmagnetic and magnetic layers and the spatial spin separation for the excitons. Cd0.95Mn0.05Te-CdTe double QWs show the injection of the spin polarized excitons from the magnetic well to the nonmagnetic QW

275

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

Science.gov (United States)

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

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

2013-11-01

276

Structure of luminescence spectra of UO2+2 impurity ions in halide salt solid solutions  

International Nuclear Information System (INIS)

Luminescence spectra of impurity polyatomic ions of UO22+ in water and aqueous solutions of alkali-halide salts are studied in the 1.62-77 K temperature range. The emission spectra of these solutions at 77 K are characterized by a pronounced vibrational structure specified by the interaction of an electron transition with an impurity ion intramolecular vibration #betta#1. At 1.62-4.2 K individual vibrational bands are splitted to phononless lines and structural phonon wings (FW). The PW structure is defined by an excitation of crystalline vibrations of a solution solid matrix. For the H2O-UO22+ solution the PW maximum frequences are shown to be comparable with the main critical points of the vibration spectrum of an orientation-disorded ice crystal. In case of aqueous solutions of alkali-halide salts with UO22+ impurity ions the PW luminescence spectra are compared with the density functions of undistorted vibrations of alkali-halide salt crystals and ice

277

Exciton dynamics in WSe2 bilayers  

OpenAIRE

We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\\tau_{D}$ ~ 3 ps and the indirect optical transition with $\\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignm...

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

2014-01-01

278

Exciton Binding Energy of Monolayer WS2.  

Science.gov (United States)

The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to 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 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023

Zhu, Bairen; Chen, Xi; Cui, Xiaodong

2015-01-01

279

The nature of singlet excitons in oligoacene molecular crystals  

Science.gov (United States)

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 -32 cm-1, far smaller than the measured value of 631 cm-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 601 cm-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.

Yamagata, H.; Norton, J.; Hontz, E.; Olivier, Y.; Beljonne, D.; Brédas, J. L.; Silbey, R. J.; Spano, F. C.

2011-05-01

280

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

International Nuclear Information System (INIS)

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

281

Three-particle anyon excitons  

International Nuclear Information System (INIS)

We propose a model of an anyon exciton (AE) consisting of a hole and several anyons, and apply is to the spectroscopy of an incompressible quantum liquid (IQL). Fractionalization of the electron charge makes properties of such entities quite different from those of usual magnetoexcitons. The model describes a number of properties established by few-particle simulations, including an abrupt change in emission spectra vs electron-hole asymmetry of the system

282

Study of the exciton condensed phase in 2D systems  

International Nuclear Information System (INIS)

A theory of exciton condensed phase formation in 2D systems is developed with regard for the temperature of a system and different lifetimes of free excitons and excitons in the condensed state. It is shown that the condensed phase in 2D systems consists of exciton islands. The mean radius of such an island increases with both lifetimes, the exciton creation rate, the temperature of a system, and the surface energy of condensed excitons

283

Excitonic coupling of chlorophylls in the plant light-harvesting complex LHC-II.  

OpenAIRE

Manifestation and extent of excitonic interactions in the red Chl-absorption region (Q(y) band) of trimeric LHC-II were investigated using two complementary nonlinear laser-spectroscopic techniques. Nonlinear absorption of 120-fs pulses indicates an increased absorption cross section in the red wing of the Q(y) band as compared to monomeric Chl a in organic solution. Additionally, the dependence of a nonlinear polarization response on the pump-field intensity was investigated. This approach r...

Schubert, Axel; Beenken, Wichard J. D.; Stiel, Holger; Voigt, Bernd; Leupold, Dieter; Lokstein, Heiko

2002-01-01

284

High-field magneto-excitons in unstrained GaAs/AlGaAs quantum dots  

OpenAIRE

The magnetic field dependence of the excitonic states in unstrained GaAs/AlGaAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb int...

Sidor, Y.; Partoens, B.; Peeters, F. M.; Schildermans, N.; Hayne, M.; Moshchalkov, V. V.; Rastelli, A.; Schmidt, O. G.

2006-01-01

285

Excitons in the wurtzite AlGaN/GaN quantum-well heterostructures  

OpenAIRE

We have theoretically studied exciton states and photoluminescence spectra of strained wurtzite AlGaN/GaN quantum-well heterostructures. The electron and hole energy spectra are obtained by numerically solving the Schr\\"odinger equation, both for a single-band Hamiltonian and for a non-symmetrical 6-band Hamiltonian. The deformation potential and spin-orbit interaction are taken into account. For increasing built-in field, generated by the piezoelectric polarization and by t...

Pokatilov, E. P.; Nika, D. L.; Fomin, V. M.; Devreese, J. T.; Photonics; Nanostructures, Semiconductor; Eindhoven, Tu; Netherlands, The

2008-01-01

286

Evidence for an excitonic insulator phase in 1T-TiSe$_{2}$  

OpenAIRE

We present a new high-resolution angle-resolved photoemission study of 1\\textit{T}-TiSe$_{2}$ in both, its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalisations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A theoretical calculation of the spectral function for an excitonic insulato...

Cercellier, H.; Monney, C.; Clerc, F.; Battaglia, C.; Despont, L.; Garnier, M. G.; Beck, H.; Patthey, L.; Berger, H.; Aebi, P.

2007-01-01

287

Exciton dynamics in WSe2 bilayers  

Science.gov (United States)

We investigate exciton dynamics in WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with ?D ? 3 ps and the indirect optical transition with ?I ? 25 ps. For temperatures between 4 to 150 K ?I remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time ?D. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation experiments.

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

2014-11-01

288

High Pressure Optical Studies of the Thallous Halides and of Charge-Transfer Complexes  

Science.gov (United States)

High pressure was used to study the insulator -to-metal transition in sulfur and the thallous halides and to study the intermolecular interactions in charge -transfer complexes. The approach to the band overlap insulator -to-metal transition was studied in three thallous halides and sulfur by optical absorption measurements of the band gap as a function of pressure. The band gap of sulfur continuously decreases with pressure up to the insulator -to-metal transition which occurs between 450 and 485 kbars. The results on the thallous halides indicate that the indirect gap decreases more rapidly than the direct gap; the closing of the indirect gap is responsible for the observed insulator -to-metal transitions. High pressure electronic and vibrational spectroscopic measurements on the solid-state complexes of HMB-TCNE were used to study the intermolecular interactions of charge -transfer complexes. The vibrational frequency shifts indicate that the degree of charge transfer increases with pressure which is independently confirmed by an increase in the molar absorptivity of the electronic charge-transfer peak. Induction and dispersion forces contribute towards a red shift of the charge-transfer peak; however, charge-transfer resonance contributes toward a blue shift and this effect is dominant for the HMB-TCNE complexes. High pressure electronic spectra were used to study the effect of intermolecular interactions on the electronic states of TCNQ and its complexes. The red shifts with pressure of the electronic spectra of TCNQ and (TCNQ)(' -) in polymer media and of crystalline TCNQ can be understood in terms of Van der Waals interactions. None of the calculations which considered intradimer distance obtained the proper behavior for either the charge-transfer of the locally excited states of the complexes. The qualitative behavior of both states can be interpreted as the effect of increased mixing of the locally excited and charge transfer states.

Jurgensen, Charles Willard

289

Nonlinear Photoluminescence in Atomically Thin Layered WSe2 Arising from Diffusion-Assisted Exciton-Exciton Annihilation  

OpenAIRE

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequ...

Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari

2014-01-01

290

Metal halide reduction with molten sodium/potassium alloy  

International Nuclear Information System (INIS)

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

291

Resonance Raman spectra of metal halide vapor complexes  

International Nuclear Information System (INIS)

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

292

Dielectric screenning, exciton binding energy and the insulator-metal transition in molecular hydrogen  

International Nuclear Information System (INIS)

Using an appropriate model for dielectric screening in molecular hydrogen we calculate the static dielectric constant ? (o) in the proximity of the insulator-metal transition. The dielectric constant ? (o) is constructed in terms of the energy gap, the exciton binding energy and the plasmon frequency. Considering the energy gap from the band-structure calculation of Friedli and Ashcroft and making an estimate of the lower exciton binding energy in the instability region, ? (o) is shown to diverge at a critical density given by r s*= 1,57 a.u. (Author)

293

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

International Nuclear Information System (INIS)

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

294

Importance of the dielectric contrast for the polarization of excitonic transitions in single GaN nanowires  

Science.gov (United States)

We investigate the polarization of excitonic transitions of single dispersed GaN nanowires with a diameter of about 50 nm. We observe donor-bound exciton transitions with a linewidth narrower than 250 ?eV at 10 K, whereas the luminescence from free excitons exhibits a width of up to 5 meV. This broadening is larger than that observed for free excitons in the as-grown nanowire ensemble and is the result of inhomogeneous strain introduced by the nanowire dispersion. This strain lowers the symmetry of the lattice structure and allows A excitons to emit light polarized parallel to the nanowire axis. The polarization anisotropy of A excitons, however, is found to largely vary from one nanowire to another. In addition, the various bound-exciton lines in a given nanowire do not show the same polarization anisotropies. These findings can be explained by the dielectric contrast between the nanowire and its environment, but only when taking into account the strong variations of the dielectric function of GaN at the near band-edge.

Corfdir, Pierre; Feix, Felix; Zettler, Johannes K.; Fernández-Garrido, Sergio; Brandt, Oliver

2015-03-01

295

Monte Carlo wavefunction approach to the exciton dynamics of molecular aggregates with exciton phonon coupling  

Science.gov (United States)

We develop the second-order Monte Carlo wavefunction (MCWF) approach to the exciton dynamics of molecular aggregate systems composed of dipole-coupled two-state monomers. The explicit form of Lindblad operator, which is indispensable for applying the MCWF approach, for population relaxation among exciton states is derived based on the quantum master equation involving weak exciton-phonon coupling. The exciton migration behaviors obtained by the MCWF approach are turned out to coincide with those by the conventional master equation approach, indicating the high potential of the MCWF approach to the dissipative exciton dynamics of extended molecular aggregates or supermolecules.

Ohta, S.; Nakano, M.; Kishi, R.; Takahashi, H.; Furukawa, S.

2006-02-01

296

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

International Nuclear Information System (INIS)

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

297

Structural Characterization of Methanol Substituted Lanthanum Halides  

OpenAIRE

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 throug...

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

2010-01-01

298

Ground state properties of heavy alkali halides  

OpenAIRE

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

Doll, Klaus; Stoll, Hermann

1998-01-01

299

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

CERN Document Server

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.

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

2013-01-01

300

Giant Enhancement of the Optical Second-Harmonic Emission of WSe_{2} Monolayers by Laser Excitation at Exciton Resonances.  

Science.gov (United States)

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

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

2015-03-01

301

Giant Enhancement of the Optical Second-Harmonic Emission of WSe2 Monolayers by Laser Excitation at Exciton Resonances  

Science.gov (United States)

We show that the light-matter interaction in monolayer WSe2 is strongly enhanced when the incoming electromagnetic wave is in resonance with the energy of the exciton states of strongly Coulomb bound electron-hole pairs below the electronic band gap. We perform second harmonic generation (SHG) spectroscopy as a function of laser energy and polarization at T =4 K . At the exciton resonance energies we record an enhancement by up to 3 orders of magnitude of the SHG efficiency, due to the unusual combination of electric dipole and magnetic dipole transitions. The energy and parity of the exciton states showing the strong resonance effects are identified in 1- and 2-photon photoluminescence excitation experiments, corroborated by first principles calculations. Targeting the identified exciton states in resonant 2-photon excitation allows us to maximize k -valley coherence and polarization.

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

2015-03-01

302

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

International Nuclear Information System (INIS)

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 conioned. 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 ?T=1. We stress the analogies and also important differences between the correlations functions in the two exciton superfluid systems. - Highlights: ? Establish the relations between photoluminescence and transport measurements. ? Stress the violations of the fluctuation–dissipation theorem in non-equilibrium systems. ? Explore the input–output relations between excitons and photons. ? Discuss differences between non-equilibrium systems and dissipative ones.

303

A new fundamental hydrogen defect in alkali halides  

International Nuclear Information System (INIS)

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

304

Exciton absorption spectrum of thin Ag sub 2 ZnI sub 4  

CERN Document Server

In Ag sub 2 ZnI sub 4 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 E sub g = 3.7 eV forbidden gap width. A strong exciton band at E = 3.625 eV (80 K) GAMMA 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 GAMMA(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

Yunakova, O N; Kovalenko, E N

2002-01-01

305

Self-localization of excitons in a periodically modulated molecular medium  

International Nuclear Information System (INIS)

Electromagnetic field propagation is analyzed in a one-dimensional Bragg grating consisting of periodically arranged linear molecules making up a resonant medium. Dye J-aggregates and conjugated polymers are considered as examples of the medium. Both adiabatic and nonadiabatic dynamics of the acoustic waves generated by electromagnetic field in the system are examined. The effects of exciton-phonon and exciton-phonon-photon interactions on the band structure and formation of self-localized excitations are examined on various time scales. A new mechanism for controlling bandgap parameters in a bistable regime is described. Some effects of electromagnetic-field nonuniformity on generation of phonons in molecules and exciton self-localization are investigated

306

Zeeman splitting of excitons in CdTe/CdMnTe quantum wells  

Science.gov (United States)

We present a calculation of the Zeeman splitting of the light- and heavy-hole excitons that takes into account the uniaxial strain due to lattice mismatch as well as the interface-enhanced paramagnetism. In the CdTe quantum well, the strain raises the light-hole band edge with respect to the heavy-hole one. The shallower well allows a larger penetration of the Cd 1- xMn xTe barrier region by the light hole. This adds to the effect of the lighter effective mass, leading to an "anomalous" enhancement of the Zeeman splitting of the light-hole exciton. We find that the ratio of the light- to heavy-hole exciton splitting (between the ? - and ? + components) increases with the quantum well width eventually exceeding unity, in agreement with experimental data.

Bhattacharjee, A. K.; Fishman, G.

1998-02-01

307

Fine structure of spectra of a bound exciton in tetragonal zinc diphosphide  

International Nuclear Information System (INIS)

Investigation into the low-temperature luminescence spectra recorded in different crystal geometry relative to the direction of incident radiation wave vector, has demonstrated the existence of saddle-shaped valent zone ceiling near k=0 in zinc tetragonal diphosphide monocrystals. Binding energies of free (2.2085 eV) and bound (A(2.1943eV)B(2.1765eV), C(2.1447eV)) excitons as well as the phonon energy value are determined by investigations into absorption spectrum and radiative recombination. Phonon-free lines of bound A0 and C0 excitons are splitted into 2.2 and 0.3 MeV respectively. The band exciton singlet-triplet state splittings in a magnetic field and their field dependences are obtained. The spectroscopic splitting factor g=1.9 is determined

308

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

CERN Document Server

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

Harigaya, Kikuo

2008-01-01

309

Theory of biexcitons and biexciton-exciton cascade in graphene quantum dots  

Science.gov (United States)

We present a microscopic theory of biexcitons in colloidal graphene quantum dots, and we discuss the possibility of a biexciton-exciton cascade generation. Assuming a pz orbital on each carbon atom, the single-particle properties are described in the tight-binding model. The screened direct, exchange, and scattering matrix elements of the Coulomb matrix are calculated using Slater pz orbitals. The many-body ground and excited states are constructed as a linear combination of a finite number of electron-hole pair excitations from the Hartree-Fock ground state by exact diagonalization techniques. The exciton and biexciton states are constructed exploiting the degeneracy of the valence- and conduction-band edges. The two degenerate exciton (X ) states and a corresponding biexciton (X X ) state are identified for generation of the X X -X cascade in threefold-symmetric quantum dots. Finally, the Auger coupling of the X X state with the excited X states is predicted.

Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

2015-03-01

310

The Role of Ligands in Determining the Exciton Relaxation Dynamics in Semiconductor Quantum Dots  

Science.gov (United States)

This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay.

Peterson, Mark D.; Cass, Laura C.; Harris, Rachel D.; Edme, Kedy; Sung, Kimberly; Weiss, Emily A.

2014-04-01

311

Exciton-polaritons in Bragg gratings  

Energy Technology Data Exchange (ETDEWEB)

We study the strong coupling between photons and bulk excitons in a one-dimensional Bragg grating. The dispersion of the resulting Bragg-polariton states resembles the dispersion of quantum-well microcavity polaritons. We report on a parametric scattering process at two 'magic frequencies' occurring due to the strong excitonic nonlinearity.

Creatore, C [Department of Physics ' A. Volta' , Universita di Pavia, via Bassi 6, I-27100, Pavia (Italy); Mouchliadis, L; Langbein, W [School of Physics and Astronomy, Cardiff University, The Parade, CF24 3AA, Cardiff (United Kingdom); Biancalana, F [Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1/Bau 24, 91058 Erlangen (Germany); Osborne, S, E-mail: creatore@fisicavolta.unipv.i [Tyndall National Institute, Lee Maltings, Prospect Row, Cork (Ireland)

2010-02-01

312

Exciton-polaritons in Bragg gratings  

International Nuclear Information System (INIS)

We study the strong coupling between photons and bulk excitons in a one-dimensional Bragg grating. The dispersion of the resulting Bragg-polariton states resembles the dispersion of quantum-well microcavity polaritons. We report on a parametric scattering process at two 'magic frequencies' occurring due to the strong excitonic nonlinearity.

313

Constraint on ?E and exciton number  

International Nuclear Information System (INIS)

It is shown that the constraint on ?E introduced by Alhassid, Levine, Karp and Steadman in their information-theoretical analysis of heavy ion collision data reflects the constraint on the exciton number nx. Arguments are based on Williams' state density formula for exciton model. Ericson's formula is also considered. (author)

314

Exciton Spin Dynamics in Semiconductor Quantum Wells  

OpenAIRE

In this paper we will review Exciton Spin Dynamics in Semiconductor Quantum Wells. The spin properties of excitons in nanostructures are determined by their fine structure. We will mainly focus in this review on GaAs and InGaAs quantum wells which are model systems.

Amand, Thierry; Marie, Xavier

2007-01-01

315

Tightly Bound Excitons in Monolayer WSe2  

Science.gov (United States)

Exciton binding energy and excited states in monolayers of tungsten diselenide (WSe2) are investigated using the combined linear absorption and two-photon photoluminescence excitation spectroscopy. The exciton binding energy is determined to be 0.37 eV, which is about an order of magnitude larger than that in III-V semiconductor quantum wells and renders the exciton excited states observable even at room temperature. The exciton excitation spectrum with both experimentally determined one- and two-photon active states is distinct from the simple two-dimensional (2D) hydrogenic model. This result reveals significantly reduced and nonlocal dielectric screening of Coulomb interactions in 2D semiconductors. The observed large exciton binding energy will also have a significant impact on next-generation photonics and optoelectronics applications based on 2D atomic crystals.

He, Keliang; Kumar, Nardeep; Zhao, Liang; Wang, Zefang; Mak, Kin Fai; Zhao, Hui; Shan, Jie

2014-07-01

316

Excitonic complexes in quantum Hall systems  

CERN Document Server

The formation and various possible decay processes of neutral and charged excitonic complexes in electronic integral and fractional quantum Hall systems are discussed. The excitonic complexes are bound states of a small number of the relevant negatively and positively charged quasiparticles (e.g., conduction electrons and valence holes, reversed-spin electrons and spin holes, Laughlin quasielectrons and quasiholes, composite fermions) that occur in an electron system under specific conditions (e.g., electron density, well width, electric and magnetic fields, or hydrostatic pressure). The examples of such bound states are interband neutral and charged excitons, fractionally charged "anyon excitons", spin waves, skyrmions, or "skyrmion excitons". Their possible decay processes include radiative recombination, experimentally observed in photoluminescence or far infrared emission, or spin transitions, important in the context of nuclear spin relaxation.

Wójs, A; Wojs, Arkadiusz; Quinn, John J

2003-01-01

317

Aqueous microsolvation of mercury halide species.  

Science.gov (United States)

The effects of aqueous solvation on the thermochemistry of reactions between mercury and small halogen molecules has been investigated by the microsolvation approach using ab initio and density functional theory (DFT) calculations. The structures, vibrational frequencies, and binding energies of 1, 2, and 3 water molecules with mercury-halide (HgBr2, HgBrCl, HgCl2, HgBr, and HgCl) and related mercury and halogen species (Br2, BrCl, Cl2, Cl, Hg, and Br) have been computed with second order Møller-Plesset perturbation theory (MP2) and the B3LYP density functional method. Accurate incremental water binding energies have been obtained at the complete basis set (CBS) limit using sequences of correlation consistent basis sets, including higher order correlation effects estimated from coupled cluster calculations. The resulting energetics were used to calculate the influence of water molecules on the thermochemistry of a number of reactions between mercury and small halogen-containing molecules. In general, the presence of water favors the formation of oxidized mercury halide species. PMID:17595065

Shepler, Benjamin C; Wright, Ashby D; Balabanov, Nikolai B; Peterson, Kirk A

2007-11-01

318

Defects induced melting in alkali halides  

International Nuclear Information System (INIS)

In the present paper we study the pressure dependence of melting of NaCl and CsCl crystals. A formulation has been presented for the pressure dependence of melting temperature on the basis of the vacancy model using the expression for the pressure dependence of the volume of Schottky defects from the Roy-Roy equation of state. Values of pressure derivatives of melting temperature have been calculated at elevated pressures to determine the rate of change of melting temperature with increase in pressures using the data of vacancy formation energy and effective volume of Schottky defects. The vacancy model revised in the present study takes into account the variation of bulk modulus with pressure, whereas in the Ksiazek and Gorecki model, it was treated constant. Results for pressure derivative of melting temperature are calculated for the solids under study. The melting curves have also been obtained and found to compare well with results based on molecular dynamics simulation and experimental data reported in recent literature. -- Research Highlights: ? We have studied the pressure dependence of melting in alkali halides on the basis of the vacancy model. ? We have taken into account the pressure dependence of volume of Schottky defects. ? The pressure derivatives of melting temperature are calculated for the alkali halide solids. ? The results have been found to compare well with the corresponding experimental data. ? This work will be useful in the analysisk will be useful in the analysis of thermal equation of state for solids.

319

Status of trivalent copper and charge-transfer excitons in high-TC cuprates.  

Science.gov (United States)

A chemical bonding approach based on tight-binding cluster and band calculations, taking into account on-site Coulomb repulsion (Hubbard U parameter) to differentiate doubly and singly occupied states, was applied to high- T C superconducting cuprates and related compounds. This work provides rational insight and explanations for issues such as (i) the actual oxidation number Cu (I+) for formally trivalent copper in oxides such as La 2Li 1/2Cu 1/2O 4, (ii) the dominant oxygen character of the doping holes in (CuO 2) ( n- ) planes, (iii) the Mott-Hubbard character of the insulator-to-metal transition triggered by hole doping, leading to an oxygen-to-copper charge transfer of avalanche type, (iv) the occurrence of an excitonic phase with anisotropic Frenkel-type excitons, (v) the role of Coulomb interactions between excitons and between doping holes and their exciton surroundings, and (vi) the on-time pairing of doping holes by means of an "excitonic glue". PMID:18821815

Pouchard, Michel; Doumerc, Jean-Pierre; Villesuzanne, Antoine

2008-10-01

320

Photoreflectance study of exciton energies and linewidths for homoepitaxial and heteroepitaxial GaN layers  

Science.gov (United States)

Photoreflectance has been applied to study the exciton energies and linewidths for hetero- and homoepitaxial GaN layers (i.e., GaN layers grown on sapphire and truly bulk GaN crystal obtained by ammonothermal method). In order to modulate the built-in electric field inside the samples and eliminate photoluminescence signal from photoreflectance spectra, the surface band bending was modulated by the laser light with the photon energy smaller than the energy gap of GaN, i.e., a 532 nm laser line. The Varshni-type and Bose-Einstein-type parameters that describe the temperature dependence of the exciton transition energies have been evaluated and compared for the two GaN epilayers. It has been concluded that the residual strain in the heteroepitaxial layer influences the exciton transition energy but does not influence the Varshni and Bose-Einstein parameters. It has been found that this strain significantly influences the exciton linewidth. The broadening parameter, which is associated with the temperature-independent mechanisms (?0 parameter corresponding to the exciton linewidth at 0 K), has been found to be ˜1 meV for the homoepitaxial layer. For the heteroepitaxial layer this parameter is twice higher (this work) or few times higher (previous papers).

Kudrawiec, R.; Rudzi?ski, M.; Serafinczuk, J.; Zaj?c, M.; Misiewicz, J.

2009-05-01

321

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

Energy Technology Data Exchange (ETDEWEB)

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 PbX{sub 2} (X = Cl, Br, and I). Lead halides are characterized by maximum phonon energies of less than {approx}200 cm{sup -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 {approx}808 nm, all investigated samples exhibited broad Nd{sup 3+} emission bands centered at {approx}2.6 and {approx}5.2 {mu}m with decay times in the millisecond range. The dominant contributions of these emission bands were assigned to the transitions {sup 4}I{sub 13/2} {yields} {sup 4}I{sub 9/2} (2.6 {mu}m) and {sup 4}I{sub 11/2} {yields} {sup 4}I{sub 9/2} (5.2 {mu}m), respectively. The temperature dependence of the 5.2 {mu}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:PbCl{sub 2}, Nd:PbBr{sub 2}, and Nd:PbI{sub 2}, respectively.

Brown, E. [Department of Physics, Hampton University, Hampton, VA 23668 (United States)], E-mail: eiei.nyein@hamptonu.edu; Hoemmerich, U. [Department of Physics, Hampton University, Hampton, VA 23668 (United States); Bluiett, A.G. [Department of Chemistry and Physics, Elizabeth City State University, Elizabeth City, NC 27909 (United States); Trivedi, S.B. [Brimrose Corporation of America, 19 Loveton Circle, Baltimore, MD 21152 (United States); Zavada, J.M. [US Army Research Office, Durham, NC 27709 (United States)

2008-01-15

322

The Study of Exciton relax Process in a Polymeric molecule  

Science.gov (United States)

As the polymer chain is a quasi-one-dimensional system, the property of the polymeric molecule is that its energy spectrum and bond distortion depend on the excitation. Our calculation is based on the quantum theroy. We think that the bond structure is easily distorted in this quasi-one-dimensional system, bacause there are the electron-lattice interaction, electron-electron interaction and the interaction of the electrons with the electric field. So, in our calculation, the Hamiltonian of exciton included three parts. The electron's energy spectrum and wave function and determined by the eigenequation of H. We get the total energy E in the Hartree-Fock approximation. At the same time, we can get the force of every lattice. Then, the lattice will distort and we can get the new displacement order parameter of distorted lattice. At last, the electron's energy spectrum, wave function and the functions of lattice distortion can be obtained by self-consistently solving the combined eigenequation and force equations. We found that the energy levels of the highest in valence band and the lowest in conducting band appear as periodic vibration during about 100 femtosecends. Lastly, the two electronic bound states created near the center of the energy gap, and caused self-trapping exciton.

Zhang, Xi-Juan; Shao, Zheng-Ming

2002-03-01

323

Luminescence of excitons in single-crystal garnets  

Science.gov (United States)

Comparative studies of the luminescence of Y3Al5O12:Ce and Lu3Al5O12:Ce single-crystal films and their volume analogues—Y3Al5O12 and Y3Al5O12:Ce single crystals, excited by synchrotron radiation with energy E=120-150 eV, have been performed. The films were grown from melt-solution by liquid-phase epitaxy and the crystals were grown from melt. The single-crystal films and single crystals studied are characterized by different degrees of structural order, in particular, different concentrations of substitutional defects of the Y{Al/3+} and LU{Al/3+} types. It was ascertained that the bands at 260 and 250 nm in the intrinsic luminescence spectra of Y3Al5O12:Ce and Lu3Al5O12:Ce single-crystal films and single crystals are due to the emission of self-trapped excitons. The luminescence band with ?max=300 nm and ?=0.36 ?s, which is present in the luminescence spectrum of single crystals and absent in the spectra of single-crystal films, is due to the recombination of electrons with holes localized at Y{Al/3+} centers. It is shown that an efficient energy transfer by excitons to activator ions occurs in Y3Al5O12 and Lu3Al5O2 single-crystal films doped with Ce3+ ions.

Zorenko, Yu. V.; Konstankevych, I. V.; Mikhailin, V. V.; Kolobanov, V. N.; Spasskii, D. A.

2004-05-01

324

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals  

Energy Technology Data Exchange (ETDEWEB)

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.

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

325

First-principles study on the electronic and optical properties of cubic ABX{sub 3} halide perovskites  

Energy Technology Data Exchange (ETDEWEB)

The electronic properties of ABX{sub 3} type compounds in the cubic phase are systematically studied using the first-principles calculations. The chemical trend of their properties as A or B or X varies is fully investigated. The optical properties of the ABX{sub 3} compounds are also investigated. Our calculations show that taking into account the spin–orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH{sub 3}NH{sub 3}SnBr{sub 3} is a promising material for solar cells absorber with a perfect band gap and good optical absorption.

Lang, Li; Yang, Ji-Hui; Liu, Heng-Rui; Xiang, H.J., E-mail: hxiang@fudan.edu.cn; Gong, X.G., E-mail: xggong@fudan.edu.cn

2014-01-10

326

Iridium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides  

Directory of Open Access Journals (Sweden)

Full Text Available Iridium-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were investigated by using [IrCl(cod]2 as catalyst, and dppe was found to be the most suitable phosphine ligand for the reaction. No oxidative insertion of the iridium into the carbon–halide bond was observed, and the reactions proceeded to provide the halogenated cycloadducts in good yield (75–94%. These results are the first examples of cycloadditions of alkynyl halides using an iridium catalyst.

Andrew Tigchelaar

2012-10-01

327

Optical and Spectral Studies on ? Alanine Metal Halide Hybrid Crystals  

Science.gov (United States)

We have synthesized and grown ? alanine metal halide hybrid crystals viz. ? alanine cadmium chloride (BACC), an amino acid transition metal halide complex crystal and ? alanine potassium chloride (BAPC), an amino acid alkali metal halide complex crystal by slow evaporation method. The grown crystals were found to be transparent and have well defined morphology. The optical characteristics of the grown crystals were carried out with the help of UV-Vis Spectroscopy. The optical transmittances of the spectrums show that BAPC is more transparent than BACC. The Photoluminescence of the materials were determined by the Photoluminescent Spectroscopy

Sweetlin, M. Daniel; Selvarajan, P.; Perumal, S.; Ramalingom, S.

2011-10-01

328

Potential surfaces and delocalization of excitons in dimers  

Science.gov (United States)

In the present work we will demonstrate that the nuclear dynamics have a strong influence on the delocalization of an exciton in a dimer, even if they do not effect the excitonic interaction. It will be shown that the internal nuclear conformation of the molecules forming the dimer depends critically on the delocalization of the exciton state in the dimer and vice versa. The resulting closed loop enforces a localization of the lower excitonic state, but, contrary to the commonly accepted view, a delocalization of the upper one. Qualitatively different time-evolution of the delocalization length for the lower and upper excitonic state will be shown. Besides, it will turn out that the nuclear motions inhibit a complete delocalization of the excitonic state in any case. To accomplish nuclear and exciton dynamics, the nonadiabatic coupling between the two excitonic states will be deduced. This causes a relaxation from the upper to the lower excitonic state, which limits the maximum reachable exciton delocalization.

Beenken, W. J. D.; Dahlbom, M.; Kjellberg, P.; Pullerits, T.

2002-09-01

329

Unraveling exciton dynamics in amorphous silicon dioxide: Interpretation of the optical features from 8 to 11 eV  

Science.gov (United States)

We introduce a model comprehensively describing the optical features of amorphous silicon dioxide (a-SiO2) in the spectral range from ~8 up to ~11 eV. Our model is grounded on the critical analysis of the temperature dependence of Kramers-Kronig-derived absorption spectra in the range from ~8 up to ~17.5 eV, together with the features of the Urbach absorption tail and of self-trapped exciton emission. In a paper we recently published [Phys. Rev. Lett. PRLTAO0031-900710.1103/PhysRevLett.105.116401105, 116401 (2010)] we showed the 10.4-eV resonance in the absorption spectra to feature a close Lorentzian line shape, thus implying a delocalized nature for excitons in a-SiO2. Here we provide estimations of the main parameters ruling exciton dynamics in SiO2, such as the energy of the mean lattice vibrational mode coupled to excitons (??0=0.083 eV), the half width of the excitonic energy band (B~2 eV), the root-mean-square amplitude of site-to-site energy fluctuations of exciton energy (D~0.7 eV), and the exciton-phonon coupling constant (g~2.1). The quantum yield of excitonic emission (?~10-3) at T=10 K in a-SiO2 is determined as well. Our model suggests that a-SiO2 features an indirect gap near ~9 eV and a direct one near ~11 eV, and allows a coherent description of the properties of the intrinsic Urbach absorption tail. The latter results are satisfactorily explained as arising from the momentary self-trapping of the 10.4-eV exciton. As far as near-edge absorption properties are concerned, our model places SiO2 in the wider context of wide-band-gap solids, such as LiF or NaF, where excitons are weakly scattered, but strongly coupled to phonons. On the whole, the present study shows that exciton dynamics accounts for all optical properties of a-SiO2 from ~8 up to ~11 eV.

Vella, Eleonora; Messina, Fabrizio; Cannas, Marco; Boscaino, Roberto

2011-05-01

330

Exciton and intracenter radiative recombination in ZnMnTe and CdMnTe quantum wells with optically active manganese ions  

Science.gov (United States)

The emission spectra of Zn1-x Mn x Te/Zn0.6Mg0.4Te and Cd1-x Mn x Te/Cd0.5Mg0.5Te quantum-well structures with different manganese concentrations and quantum-well widths are studied at excitation power densities ranging from 105 to 107 W cm-2. Under strong optical pumping, intracenter luminescence of Mn2+ ions degrades as a result of the interaction of excited managanese ions with high-density excitons. This process is accompanied by a strong broadening of the emission band of quantum-well excitons due to the exciton-exciton interaction and saturation of the exciton ground state. Under pumping at a power density of 105 W cm-2, stimulated emission of quantum-well excitons arises in CdTe/Cd0.5Mg0.5Te. The luminescence kinetics of the quantum-well and barrier excitons is investigated with a high temporal resolution. The effect of the quantum-well width and the managanese concentration on the kinetics and band shape of the Mn2+ intracenter luminescence characterized by the contribution of the manganese interface ions is determined.

Agekyan, V. F.; Akai, I.; Vasil'Ev, N. N.; Karasawa, T.; Karczewski, G.; Serov, A. Yu.; Filosofov, N. G.

2007-06-01

331

Excitonic properties of ZnO nanocrystalline films prepared by oxidation of zinc-implanted silica  

International Nuclear Information System (INIS)

Zinc oxide nanocrystalline films with (002) preferred orientation and intense ultravoilet (UV) emission were prepared by oxidation of zinc-implanted silica at 700 deg. C for 2 h in oxygen ambient. A TEM micrograph showed that ZnO nanocrystalline films with a thickness of about 90 nm were formed on the surface of the Zn-implanted silica substrate. The quality and excitonic properties of the ZnO nanocrystalline films were studied through absorption spectra at room temperature and photoluminescence (PL) spectra in the temperature range from 79 to 300 K. At room temperature, a strong free excitonic emission peak at 377 nm with a very weak deep-level emission can be observed. The intensity ratio of the UV near-band-edge emission to the deep-level emission can reach up to 40. The temperature-dependent PL indicated that the UV near-band-edge emission in the temperature range 79-187 K can be attributed to a free exciton (FE), a bound exciton and the one longitudinal-optical phonon replica of FE lines. The presence of a strong emission of FE lines at 79 K suggested that high-purity ZnO nanocrystalline films have been obtained

332

Preformed excitons, orbital selectivity, and charge density wave order in 1T-TiSe2  

Science.gov (United States)

Traditional routes to charge density wave (CDW) in transition-metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities, have recently been brought into question. While this calls for exploration of alternative views, a paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between, CDW and superconductive orders in transition-metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic-structure calculations, to an exci-tonic CDW order in 1T-TiSe2. We show that orbital-selective dynamical localization arising from preformed excitonic liquid correlations is somewhat reminiscent of states proposed for d and f band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition-metal dichalcogenides on the border of CDW. This brings the transition-metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in d and f band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

Koley, S.; Laad, M. S.; Vidhyadhiraja, N. S.; Taraphder, A.

2014-09-01

333

Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: fine structure and spin relaxation  

OpenAIRE

Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the ...

Tong, H.; Wu, M. W.

2011-01-01

334

Hydrostatic pressure and growth-direction magnetic field effects on the exciton states in coupled GaAs-(Ga, Al)As quantum wells  

International Nuclear Information System (INIS)

The effects of hydrostatic pressure and growth-direction applied magnetic fields on the exciton dispersion and in-plane effective mass in coupled GaAs-(Ga, Al)As quantum wells are investigated. Calculations for spatially direct and indirect excitons were performed within the variational procedure in the effective-mass and nondegenerate parabolic band approximations and by taking into account the coupling between the exciton centre-of-mass momentum and its internal structure. The pressure coefficient is also obtained as a function of both the hydrostatic pressure and growth-direction applied magnetic field

335

Excitons in V-Shaped and T-Shaped Semiconductor Quantum Well Wires  

OpenAIRE

We give a general framework for describing electronic states in isolated quantum wires. It provides a description of both the conduction band and the valence band, taking full account of the complexity of the ?8 valence band. This is applied to T-shaped wires and V-shaped wires. To make a useful comparison with experimental results we calculate the exciton Rydberg. We show that in the case of the T-shaped wires hole confinement is due to the interaction with the confined electron and not due...

Brinkmann, Dirk; Lo?ffler, Axel; Fishman, Guy

1997-01-01

336

Mass spectroscopic and matrix isolation studies on the ternary metal halides CsMCl3 (M = Pb, Cd, Co)  

International Nuclear Information System (INIS)

Mass spectrometric and matrix isolation infrared studies have been carried out on the vapor species present in the high temperature ternary halide systems Cs/Pb/Cl, Cs/Cd/cl, and Cs/Co/Cl, and for all three systems, both binary and ternary halide molecules are identified. In particular, the ternary species present in the Cs/Pb/Cl system is identified as molecular CsPbCl3, and on the basis of IR band intensities nd isotopic data, a C3v structure is proposed for this species. The high temperature vapors above the Cs/Cd/Cl and Cs/Co/Cl systems also contain ternary CsMCl3 species, but for these molecules, the vibrational data indicate planar C2v geometries involving bidentate coordination at the alkali metal

337

Optical nutation in the exciton range of spectrum  

Energy Technology Data Exchange (ETDEWEB)

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.

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

338

Formation of structured nanophases in halide crystals  

Science.gov (United States)

When halide crystals KCl and NaCl are slightly doped by PbCl_2, (in orders of 10^{-4} mol/mol) the structurally stable nanophases ("quantum dots") are formed via nucleation within the bulks of their matrices. Using lattice modeling we have found in KCl-Pb system natural nucleation pathway from single impurity-vacancy complex to Suzuki phase, not demonstrated in previous analyses; further transition to PbCl_2 is difficult due to high stability of this phase. In the case of NaCl-Pb, no stable "end point" of aggregation was observed and our calculations suggest nucleation may readily proceed to large PbCl_2 clusters when initially formed platelike cluster reaches a certain critical thickness. These results coincide with our experimental data.

Kulveit, J.; Demo, P.; Polák, K.; Sveshnikov, A. M.; Kožíšek, Z.

2011-04-01

339

Quantum-dot excitons in nanostructured environments  

DEFF Research Database (Denmark)

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

Hvam, JØrn Märcher; Stobbe, SØren

2011-01-01

340

Excitons in boron nitride nanotubes: Dimensionality effects  

OpenAIRE

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast con...

Wirtz, Ludger; Marini, Andrea; Rubio Secades, A?ngel

2006-01-01

341

EXCITONS AS A NEW QUANTUM SYSTEM  

OpenAIRE

In a first part, a brief review of some excitonic properties is presented, with the accent put on their relevance with respect to the possibility of Bose-Einstein condensation. In a second part, recent experimental evidence is described, which supports the idea that excitonic particles may form a highly quantum fluid. In Cu20, the analysis of the exciton decay spectrum shows a gradual evolution of the gas from a classical regime at low densities up to a strongly degenerate one at high densiti...

Mysyrowicz, A.

1980-01-01

342

Studies of the mechanism of excitonic self-trapping in quasi-one-dimensional materials  

Science.gov (United States)

The localization of electronic excitations via electron-lattice interactions is an important fundamental process in molecular-based electronic materials. In our previous work, we directly time-resolved the electronic and vibrational dynamics of the exciton self-trapping process in mixed-valence metal-halide linear chain (MX) complexes [Pt(en)_2][Pt(en)_2X_2], (X = Cl, Br, I) using femtosecond coherent phonon techniques. In this work, we present studies of the temperature dependence of the dynamics to further address the self-trapping mechanism. We find an increase in the excited state wave packet dephasing time at low temperature, indicating that the vibrational coherence properties of the exciton are not limited simply by anharmonicity of the potential energy surface associated with the localizing structural distortion. In addition, we find a new low-frequency modulation at low temperature, providing evidence for an acoustic phonon contribution to the self-trapping dynamics. This work was supported by the NSF.

Morrissey, F. X.; Dexheimer, S. L.

2004-03-01

343

Nonlinear photoluminescence in atomically thin layered WSe2 arising from diffusion-assisted exciton-exciton annihilation  

Science.gov (United States)

We studied multiexciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density and long-distance exciton diffusion, reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation (EEA) model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient EEA process determined the unusual multiexciton dynamics in atomically thin layered transition metal dichalcogenides.

Mouri, Shinichiro; Miyauchi, Yuhei; Toh, Minglin; Zhao, Weijie; Eda, Goki; Matsuda, Kazunari

2014-10-01

344

Full-potential study of the electronic structure of silver halides  

Energy Technology Data Exchange (ETDEWEB)

We present first-principles structural and electronic structure calculations of the silver halides AgF, AgCl, AgBr and AgI in their rocksalt NaCl-structure. The full-potential linearized augmented plane-wave (FP-LAPW) method is used, and for the exchange and correlation, the local density approximation (LDA) and generalized gradient approximations (GGA) are employed. The effect of spin-orbit coupling has also been incorporated. The results obtained using generalized gradient approximations are in better agreement with experimental data than those obtained using local density approximations. The inclusion of spin-orbit coupling leads to slight changes in the results. The location of the valence band maximum, the conduction band minimum as well as the peaks in the density of states (DOS) are correctly predicted for all the crystals. (Abstract Copyright [2002], Wiley Periodicals, Inc.)

Okoye, C.M.I. [Department of Physics and Astronomy, University of Nigeria, Nsukka (Nigeria)

2002-11-08

345

Novel Silver Cobaltacarborane Complexes with a Linearly Bridging Halide  

International Nuclear Information System (INIS)

The structural versatility of halides mainly originates from their coordinating abilities of adopting a bridging bond between two or more metal atoms, as well as a terminal bond. Moreover, a halide bridging bond angle is so flexible that thermodynamic stability can be endowed with proper geometry, which conceptually varies from acute to right, obtuse, and linear. In spite of innumerable reports on molecular metal halides, examples of the linearly bridging fashion are very scarce. The reason for the rarity of the linear M. X. M arrangement can be easily explained by the VSEPR (Valence Shell Electron Pair Repulsion) concept. The linear M. X. M formation has only been achieved by adopting a macrocyclic chelate ligand, which is structurally demanding, so that the VSEPR repulsions among lone-pair electrons on the halide atom could be overcome

346

Novel Silver Cobaltacarborane Complexes with a Linearly Bridging Halide  

Energy Technology Data Exchange (ETDEWEB)

The structural versatility of halides mainly originates from their coordinating abilities of adopting a bridging bond between two or more metal atoms, as well as a terminal bond. Moreover, a halide bridging bond angle is so flexible that thermodynamic stability can be endowed with proper geometry, which conceptually varies from acute to right, obtuse, and linear. In spite of innumerable reports on molecular metal halides, examples of the linearly bridging fashion are very scarce. The reason for the rarity of the linear M. X. M arrangement can be easily explained by the VSEPR (Valence Shell Electron Pair Repulsion) concept. The linear M. X. M formation has only been achieved by adopting a macrocyclic chelate ligand, which is structurally demanding, so that the VSEPR repulsions among lone-pair electrons on the halide atom could be overcome.

Park, Hyun Seo; Bae, Hye Jin; Do, Youngkyu [KAIST, Daejeon (Korea, Republic of); Park, Youngwhan [LG Chem/Research Park, Daejeon (Korea, Republic of); Go, Min Jeong; Lee, Junseong [Chonnam National Univ., Gwangju (Korea, Republic of)

2013-10-15

347

Palladium-catalyzed carboxylative coupling of allylstannanes and allyl halides  

Energy Technology Data Exchange (ETDEWEB)

A three-component carboxylative coupling between allyl halides, allylstannanes, and CO{sub 2} to produce allyl esters is catalyzed by Pd and Pt phosphine complexes. Tentative mechanistic pathways for the catalytic reaction are proposed.

Franks, R.J.; Nicholas, K.M.

2000-04-17

348

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

Science.gov (United States)

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.

Mance, J. G.; Felver, J. J.; Dexheimer, S. L.

2015-02-01

349

Tuning the near-gap electronic structure of tin-halide and lead-halide perovskites via changes in atomic layering  

Science.gov (United States)

Using density functional theory (DFT)-based calculations, we explore the extent to which achievable modes of atomic layering can tune the near-gap electronic structure of tin- and lead-halide perovskites with applications in dye-sensitized solar cells. We show that regardless of how atomic layering is achieved—whether by the growth of layered inorganic phases such as the Ruddlesden-Popper series, hybrid perovskites connected by organic linker molecules, or layered perovskite heterostructures—their band gaps can similarly be widened by several tenths of an eV or more. Because these classes of compounds are known to have band gaps spanning much of the visible region of the solar spectrum, the ability to controllably tune their near-gap electronic structure could further optimize their performance in solar energy conversion applications. Throughout this work, trends in band gap are explained based on the effects of atomic layering and quantum confinement on the character and energy of band-edge crystal orbitals.

Grote, Christopher; Ehrlich, Bradley; Berger, Robert F.

2014-11-01

350

Recent advances in new holographic silver halide materials  

OpenAIRE

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

Neipp Lo?pez, Cristian; Pascual Villalobos, Inmaculada; Bele?ndez Va?zquez, Augusto

2000-01-01

351

Size effect on the electronic and optical band gap of CdSe QD  

International Nuclear Information System (INIS)

Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed

352

Size effect on the electronic and optical band gap of CdSe QD  

Science.gov (United States)

Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed.

Sisodia, Namita

2014-04-01

353

Size effect on the electronic and optical band gap of CdSe QD  

Energy Technology Data Exchange (ETDEWEB)

Present paper deals with a critical and comprehensive analysis of the dependence of photo emission (PE) electronic band gap and optical absorption (OA) excitonic band gap on the size of CdSe QD, via connecting it with excitonic absorbance wavelength. Excitonic absorbance wavelength is determined through an empirical fit of established experimental evidences. Effective excitonic charge and Bohr radius is determined as a function of size. Increase in size of the CdSe QD results in greater Bohr radius and smaller effective excitonic charge. Excitonic binding energy as a degree of size of QD is also calculated which further relates with the difference in PE electronic and OA optical band gaps. It is also shown that with increase in size of CdSe QD, the excitonic binding energy decreases which consequently increases differences in two band gaps. Our results are very well comparable with the established results. Explanation for the origin of the unusual optical properties of CdSe QD has been also discussed.

Sisodia, Namita, E-mail: namitasisodiya@gmail.com [Department of Physics, Holkar Science College, Indore-45200 (India)

2014-04-24

354

Recombination of radiation defects in alkali halide crystals at low temperature  

International Nuclear Information System (INIS)

The complex investigation of the ionic-electronic processes of the charge transport and thermally stimulated luminescence of colour centres (I, ?, F, F', Vsub(K) and H) in X-irradiated KBr, NaCl, NaI and KCl crystals at 4.2 K has been carried out. The experiment has been performed on X-irradiated KBr at 4.2 K by a complex of thermally stimulated methods. A number of substages of temperature annealing of different defects was studied. The effect of electron tunneling ''recharge'' in the excited defect pair (F-H) during photostimulation in the F-band of KBr is observed which results in the charged Frenkel defect pair formation (?-I). The conclusion is drawn that the tunneling ''recharge'' of the non-excited F-H pair is unlikely. It was observed that the Frenkel defect (I, ?, ''H-Vsub(K)'', F, H') accumulation efficiencies increases by 10% or more if the X-irradiation at 4.6 K is carried out under an applied DC field of 3 kV/cm. Two competitive mechanism of the primary generation of the anion Frenkel defects are discussed. The formation of the self-trapped excitons and their luminescence as a result of the H and F centers direct recombination could be noneffective. It is found that anion exciton dissociation into the electron- hole pair is a process competitive to the exciton decay into the Frenkel defect pair

355

Genetic control of methyl halide production in Arabidopsis.  

Science.gov (United States)

Methyl chloride (CH(3)Cl) and methyl bromide (CH(3)Br) are the primary carriers of natural chlorine and bromine, respectively, to the stratosphere, where they catalyze the destruction of ozone, whereas methyl iodide (CH(3)I) influences aerosol formation and ozone loss in the boundary layer. CH(3)Br is also an agricultural pesticide whose use is regulated by international agreement. Despite the economic and environmental importance of these methyl halides, their natural sources and biological production mechanisms are poorly understood. Besides CH(3)Br fumigation, important sources include oceans, biomass burning, tropical plants, salt marshes, and certain crops and fungi. Here, 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. The encoded protein belongs to a group of methyltransferases capable of catalyzing the S-adenosyl-L-methionine (SAM)-dependent methylation of chloride (Cl(-)), bromide (Br(-)), and iodide (I(-)) to produce methyl halides. In mutant plants with the HOL gene disrupted, methyl halide production is largely eliminated. A phylogenetic analysis with 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. PMID:14561407

Rhew, Robert C; Østergaard, Lars; Saltzman, Eric S; Yanofsky, Martin F

2003-10-14

356

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

Science.gov (United States)

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 electron-phonon 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.

Morrissey, F. X.; Mance, J. G.; Van Pelt, A. D.; Dexheimer, S. L.

2013-04-01

357

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

Science.gov (United States)

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

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

2014-12-01

358

Giant Rydberg Excitons in Cuprous Oxide  

CERN Document Server

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...

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

2014-01-01

359

Topologically protected excitons in porphyrin thin films.  

Science.gov (United States)

The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials. PMID:25242533

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

2014-11-01

360

Excitons in boron nitride nanotubes: dimensionality effects  

CERN Document Server

We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed ``optical gap'' constancy for different tubes and bulk hBN [R. Arenal et al., to appear in Phys. Rev. Lett. (2005)].

Wirtz, L; Rubio, A; Wirtz, Ludger; Marini, Andrea; Rubio, Angel

2005-01-01

361

Excitonic Coherent States: Symmetries and Thermalization  

Science.gov (United States)

In this paper we considered the theoretical treatment of a physical system of excitons and its behaviour under temperature by means of a new coherent state construction of bounded states in a quantum field theoretical context.

Cirilo-Lombardo, Diego Julio

2015-04-01

362

10 CFR 431.324 - Uniform test method for the measurement of energy efficiency of metal halide ballasts.  

Science.gov (United States)

...measurement of energy efficiency of metal halide ballasts. 431.324 Section 431...COMMERCIAL AND INDUSTRIAL EQUIPMENT Metal Halide Lamp Ballasts and Fixtures Test Procedures...measurement of energy efficiency of metal halide ballasts. (a) Scope. This...

2010-01-01

363

Direct observation of C60 exciton  

International Nuclear Information System (INIS)

Photoexcitation of pure C60 thin films by picosecond absorption spectroscopy reveals a transient species with absorption maxima at 540 nm and 660 nm which is assigned to the exciton. Considering the dynamic behaviour of this species and luminescence properties of solid C60, it is concluded that a singlet exciton is observed both in its free (?1/21/2=250 ns) forms. (orig.)

364

Förster Coupling in Nanoparticle Excitonic Circuits  

OpenAIRE

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Förster matrix element, based on a three-dimensional real space grid and the self-consistent solution of the mesoscopic exciton in a macroscopic dielectric environment. This method enables the study of the role of the nanoparticle shape, spa...

Rebentrost, Patrick; Stopa, Michael; Aspuru-guzik, Ala?n

2010-01-01

365

Exciton in closed and opened quantum dot  

OpenAIRE

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...

Tkach, M. V.; Seti, Ju O.

2007-01-01

366

Exciton localization in disordered poly(3-hexylthiophene).  

OpenAIRE

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the ...

Barford, W.; Lidzey, Dg; Makhov, Dv; Meijer, Aj

2010-01-01

367

Enhancement Of Free Exciton Peak Intensity In Reactively Sputtered ZnO Thin Films On (0001) Al2O3  

Science.gov (United States)

Wide bandgap materials such as GaN with its direct bandgap structure have been developed rapidly for applications in short wavelength light emission. ZnO, II-VI oxide semiconductor, is also promising for various technological applications, especially for optoelectronic light emitting devices in the visible and ultraviolet (UV) range of the electromagnetic spectrum. Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton (FX) and neutral acceptor bound exciton (A°X) features. It is shown that the residual acceptors which bind excitons with an energy of 75 meV reside about 312 meV above the valence band, according to effective mass theory. An intra-bandgap absorption feature peaking at 2.5 eV shows correlation with the characteristically narrow A-free exciton peak intensity. Relevant annealing processes are presented as a function of time and temperature dependently for both Zn- and O- rich thin films. Enhancement of the free exciton peak intensity is observed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity.

Tüzemen, S.; Gür, Emre; Yildirim, T.; Xiong, G.; Williams, R. T.

2007-04-01

368

Enhancement Of Free Exciton Peak Intensity In Reactively Sputtered ZnO Thin Films On (0001) Al2O3  

International Nuclear Information System (INIS)

Wide bandgap materials such as GaN with its direct bandgap structure have been developed rapidly for applications in short wavelength light emission. ZnO, II-VI oxide semiconductor, is also promising for various technological applications, especially for optoelectronic light emitting devices in the visible and ultraviolet (UV) range of the electromagnetic spectrum. Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton (FX) and neutral acceptor bound exciton (A deg. X) features. It is shown that the residual acceptors which bind excitons with an energy of 75 meV reside about 312 meV above the valence band, according to effective mass theory. An intra-bandgap absorption feature peaking at 2.5 eV shows correlation with the characteristically narrow A-free exciton peak intensity. Relevant annealing processes are presented as a function of time and temperature dependently for both Zn- and O- rich thin films. Enhancement of the free exciton peak intensity is observed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity

369

Comparison of the spectroscopic behaviour of single crystals of lanthanide halides (X = Cl, Br)  

International Nuclear Information System (INIS)

Halides belong to large band gap matrices that have the perspective of wide applications when doped by optical active ions. This paper presents the results of spectroscopic studies of single crystals of halides (Cl, Br) of Ce, Pr and Nd. Their spectroscopic behaviour: electron-phonon coupling, ion pair interactions and the effect of covalency, is compared. Absorption, emission and emission excitation spectra of single crystals of LnCl3·yH2O (Ln = Nd, Pr, Ce; y=6, 7) were recorded at room temperatures and low temperatures down to 4.2 K. The intensities of the electronic lines and the Judd-Ofelt parameters were calculated (Nd, Pr) and compared to those of LnBr3·yH2O presented earlier by us. The relationship between the hypersensitivity and covalency was discussed. With increasing soft character of the halides (Br- > Cl-), the covalent character of Ln-ligand bond increases and the hypersensitive bands become more intense. The Judd-Ofelt intensity analysis resulted in a set of ?? parameters evaluated with quite low standard deviations. The temperature dependences of the intensities have been found and the vibronic coupling in the f-f transitions were analysed. At the low temperature (4.2 K), strong vibronic components occur in the electronic lines of the Nd(III) and Pr(III) ions, mainly with the Ln-X vibrations. The modes, which are in resonance with the splitting of the ground state multiplesplitting of the ground state multiplet, mediate in the cooperative transitions. Vibrational studies of the compounds under test were performed at the ambient temperature using IR and Raman spectroscopy. The assignment of the bands was done on the basis of the factor group analysis. The spectral features below 300 cm-1 point at the differences between the spectra of the bromides and chlorides of Nd and Pr. Although the spectral features within the FIR region are complex, the bands of the praseodymium monocrystals originated by halogen bridges are clearly visible

370

The Franz-Keldysh effect revisited: Electroabsorption including interband coupling and excitonic effects  

Science.gov (United States)

We study the linear optical absorption of bulk semiconductors in the presence of a uniform and constant (dc) electric field with an approach suitable for including excitonic effects while working with many-band models. The absorption coefficient is calculated from the time evolution of the interband polarization excited by an optical pulse. We apply the formalism to a numerical calculation for GaAs using a 14-band k · p model, which allows us to properly include interband coupling, and the exchange self-energy to account for the excitonic effects due to the electron-hole interaction. The Coulomb interaction enhances the features of the absorption coefficient captured by the k · p model; the enhancement depends on the strength of the dc field and the polarization of the optical field. With respect to the polarization dependence, we find that the anisotropy described by the independent particle approximation can be modified significantly by the Coulomb interaction.

Duque-Gomez, Federico; Sipe, J. E.

2015-01-01

371

Temperature dependence of the exciton gap in monocrystalline CuGaSe2  

International Nuclear Information System (INIS)

Near-band-edge photoluminescence properties of as-grown undoped CuGaSe2 single crystals have been investigated in the range between 10 K and room temperature. The temperature dependence of the exciton gap energy Eg was studied by means of a three-parameter thermodynamic model, the Einstein model, Varshni's model and the Paessler model. The values of the band gap energy at T = 0 K, the effective phonon energy and the Einstein temperature, the cut-off phonon energy of the phonon spectrum as well as a dimensionless constant related to the electron-phonon coupling were estimated. Comparing all applied models points out that a minimum set of four parameters is required for a complete analytical description of the experimentally determined temperature dependence of the exciton gap in CuGaSe2

372

Electron-hole liquid and exciton and biexciton gas in elastically deformed silicon crystals  

International Nuclear Information System (INIS)

Exciton condensation into an electron-hole liquid (EHL) in silicon deformed elastically along the 100, 110 and 111 crystallographic directions is investigated. The binding energy PHI and equilibrium EHL, density, N0 for various deformation directions are determined by analyzing the shape of the recombination emission spectrum. It is shown that on decrease of the multiplicity of degeneracy of the valency band and of the number of valleys in the conductivity band, PHI and N0 decrease. It is found that in Si crystals deformed along 100 the partial pressures of the exciton and biexciton gases along the gas-liquid equilibrium line are commensurable. The biexciton recombination emission spectrum is measured. The probability for biexciton annihilation is calculated and the shape of the biexciton emission spectrum is analyzed and the biexciton binding energy has been estimated with value 0.55+-0.15 mev

373

Evidence for an excitonic insulator phase in 1T-TiSe2.  

Science.gov (United States)

We present a new high-resolution angle-resolved photoemission study of 1T-TiSe2 in both its room-temperature, normal phase and its low-temperature, charge-density wave phase. At low temperature the photoemission spectra are strongly modified, with large band renormalizations at high-symmetry points of the Brillouin zone and a very large transfer of spectral weight to backfolded bands. A calculation of the theoretical spectral function for an excitonic insulator phase reproduces the experimental features with very good agreement. This gives strong evidence in favor of the excitonic insulator scenario as a driving force for the charge-density wave transition in 1T-TiSe2. PMID:17930692

Cercellier, H; Monney, C; Clerc, F; Battaglia, C; Despont, L; Garnier, M G; Beck, H; Aebi, P; Patthey, L; Berger, H; Forró, L

2007-10-01

374

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

International Nuclear Information System (INIS)

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

375

Photoluminescence of the incompressible Laughlin liquid Excitons, charged excitons, and fractionally charged excitons  

CERN Document Server

The photoluminescence (PL) of a two-dimensional electron gas (2DEG) in a high magnetic field is studied as a function of the filling factor and the separation d between the electron layer and the valence hole. Depending on the magnitude of d relative to the magnetic length lambda, two distinct regimes in the response of the 2DEG to the valence hole occur, with different elementary emission processes contributing to the PL spectrum. At dlambda ("weak coupling" regime), the hole decouples or binds one or two Laughlin quasielectrons to form fractionally charged excitons (FCX's). The binding energies as well as the emission energies and intensities of all X- and FCX states are calculated.

Wójs, A

2001-01-01

376

Excitonic Exchange Splitting and Radiative Lifetime in PbSe Quantum Dots  

International Nuclear Information System (INIS)

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 ?x is the energy difference between the lowest-energy optically dark exciton state and the first optically bright exciton state. We find that ?x 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 ?R ? 100 ns, considerably longer than the ?10 ns radiative lifetime of CdSe dots, in quantitative agreement with experimenttive agreement with experiment

377

T-shaped GaAs quantum-wire lasers and the exciton Mott transition  

International Nuclear Information System (INIS)

T-shaped GaAs quantum-wire (T-wire) lasers fabricated by the cleaved-edge overgrowth method with molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal are measured to study the laser device physics and fundamental many-body physics in clean one-dimensional (1D) systems. A current-injection T-wire laser that has 20 periods of T-wires in the active region and a 0.5 mm long cavity with high-reflection coatings shows a low threshold current of 0.27 mA at 30 K. The origin of the laser gain above the lasing threshold is studied with the high-quality T-wire lasers by means of optical pumping. The lasing energy is about 5 meV below the photoluminescence (PL) peak of free excitons, and is on the electron-hole (e-h) plasma PL band at a high e-h carrier density. The observed energy shift excludes the laser gain due to free excitons, and it suggests a contribution from the e-h plasma instead. A systematic micro-PL study reveals that the PL evolves with the e-h density from a sharp exciton peak, via a biexciton peak, to an e-h-plasma PL band. The data demonstrate an important role of biexcitons in the exciton Mott transition. Comparison with microscopic theories points out some problems in the picture of the exciton Mott transition

378

Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers  

Science.gov (United States)

Solar cells based on a light absorbing layer of the organometal halide perovskite CH3NH3PbI3 have recently surpassed 15% conversion efficiency, though how these materials work remains largely unknown. We analyze the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: Band gaps are much larger than what is predicted by the local-density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which partially accounts for the long diffusion lengths observed. The surface ionization potential (work function) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO2 and Au electrical contacts.

Brivio, Federico; Butler, Keith T.; Walsh, Aron; van Schilfgaarde, Mark

2014-04-01

379

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

Energy Technology Data Exchange (ETDEWEB)

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.

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

380

Nanomachining of inorganic transparent materials using an x-ray exciton method  

Science.gov (United States)

We have investigated microfabrication of inorganic transparent materials using laser plasma soft X-rays with the diffraction limit of 10 nm. As a soft X-ray source, we used Ta laser plasma soft X-rays, whch has a light emission band at around 10 nm. A SiO2 film was confirmed to have absorption band in the soft X-ray region. Micromachining a quartz plate was demonstrated by irradiation with focused soft X-rays and pulsed 266 nm Nd:YAG laser light. The quartz plates are ablated smoothly at 85 nm/shots. It is found that more than 40% of 266 nm laser light is absorbed by the quartz plates just after soft X-ray irradiation at room temperature. Thus, it is shown that a transient state such as an X-ray generated exciton (X-ray exciton), which have absorption band in UV region, are generated by soft X-ray irradiation. The micromachining technique (X-ray exciton method) can utilize the high space resolution of soft X-ray and the high energy density of conventional UV/visible laser light. Further, we found that a variety of materials such as SiO2, LiNO3, Si, CaF2, LiF and Al2O3 are ablated smoothly by irradiation of the focused laser plasma soft X-rays without 266 nm laser light.

Makimura, Tetsuya; Kenmotsu, Yoichi; Miyamoto, Hisao; Mori, Michiaki; Kondo, Kiminori; Murakami, Kouichi

2004-10-01

381

Theoretical Study on Exciton Dynamics in Dendritic Systems: Exciton Recurrence and Migration  

Directory of Open Access Journals (Sweden)

Full Text Available The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME approach based on the ab initio molecular orbital configuration interaction (MOCI method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the features of excion recurrence and migration motions in relation to their structural dependences. It is found that the nanostar dendrimer exhibits faster exciton migration from the periphery to the core than Cayley-tree dendrimer, which alternatively exhibits exciton recurrence motion among dendron parts in case of small relaxation parameters. Such strong structural dependence of exciton dynamics demonstrates the advantage of dendritic molecular systems for future applications in nano-optical and light-harvesting devices.

Kyohei Yoneda

2009-09-01

382

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

Directory of Open Access Journals (Sweden)

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.

Mihai E. Vaida

2011-09-01

383

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI2-PbI2 crystal system  

Science.gov (United States)

It has been shown using atomic-force microscopy that the PbI2 impurity is embedded in the CdI2 crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the 3 P 2 state of a free Pb2+ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25-45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI2 crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI2 in the CdI2 crystal lattice.

Bolesta, I. M.; Vistovskii, V. V.; Gloskovskaya, N. V.; Panasyuk, M. R.; Yaritskaya, L. I.

2011-04-01

384

Structure of luminescence spectra of UO/sup 2 +//sub 2/ impurity ions in halide salt solid solutions  

Energy Technology Data Exchange (ETDEWEB)

Luminescence spectra of impurity polyatomic ions of UO/sub 2//sup 2 +/ in water and aqueous solutions of alkali-halide salts are studied in the 1.62-77 K temperature range. The emission spectra of these solutions at 77 K are characterized by a pronounced vibrational structure specified by the interaction of an electron transition with an impurity ion intramolecular vibration ..nu../sub 1/. At 1.62-4.2 K individual vibrational bands are split to phononless lines and structural phonon wings (PW). The PW structure is defined by an excitation of crystalline vibrations of a solution solid matrix. For the H/sub 2/O-UO/sub 2//sup 2 +/ solution the PW maximum frequences are shown to be comparable with the main critical points of the vibration spectrum of an orientation-disorded ice crystal. In case of aqueous solutions of alkali-halide salts with UO/sub 2//sup 2 +/ impurity ions the PW luminescence spectra are compared with the density functions of undistorted vibrations of alkali-halide salt crystals and ice.

Kushnirenko, I.Ya.; Sheremet, V.I.

1982-10-01

385

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

International Nuclear Information System (INIS)

Knowledge of the infrared transition moments of hydrogen halides, namely HF, HCl, HBr, and HI, is essential for atmospheric, astrophysical, and laser applications. Recently, a new polynomial empirical dipole moment function (DMF) for HCl has been constructed using an efficient approach that involves a direct fit of experimental ro-vibrational intensities [Li et al. J Quant Spectrosc Radiat Transfer 2011;112:1543–50]. 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

386

Structured alkali halides for medical applications  

International Nuclear Information System (INIS)

Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron-hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper we present a new solution of the problem of combining a high MTF with a high sensitivity by structuring the image plates in form of thin needles acting as light guides. This suppresses the lateral spread of light which is detrimental to resolution. As doped CsBr, e.g. CsBr:Ga [Physica Medica XV (1999) 301], can pose a good storage phosphor evaporated layers are of interest in computed radiography. Needle structured CsI:Tl is used as scintillator in direct radiography [IEEE Trans. Nucl. Sci. 45 (3) (1998)]. CsBr layers have been produced by evaporation in vacuum and in inert gas atmosphere varying pressure and temperature. The resulting structures are of fibrous or columnar nature being in good agreement with the zone model of Thornton [Ann. Rev. Mater. Sci. 7 (1977) 239]. A zone model for CsBr has been developed. Measurements on doped alkali halide image plates having needle structure show good MTF at high sensitivity making a significant progress in image plate technology

387

Control of fine-structure splitting and excitonic binding energies in selected individual InAs/GaAs quantum dots  

CERN Document Server

A systematic study of the impact of annealing on the electronic properties of single InAs/GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within eight-band k.p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.

Seguin, R; Germann, T D; Rodt, S; Winkelnkemper, M; Pötschke, K; Strittmatter, P A; Pohl, U W; Hammerschmidt, T; Kratzer, P; Bimberg, D

2006-01-01

388

Exciton formation, relaxation, and decay in PCDTBT.  

Science.gov (United States)

The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband ?-?* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale (PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton. PMID:21087001

Banerji, Natalie; Cowan, Sarah; Leclerc, Mario; Vauthey, Eric; Heeger, Alan J

2010-12-15

389

Radiative lifetime of barbell excitons in semiparabolic double quantum wells under intense laser fields  

International Nuclear Information System (INIS)

The binding energy and optical properties of barbell excitons in GaAs–Ga1?xAlxAs semiparabolic double quantum wells under intense laser fields are investigated. Calculations are performed within the effective mass and envelope-function approximations, including the conduction band nonparabolicity. The dependence of the binding energy, oscillator strength and exciton absorption spectrum on the laser field in symmetric and asymmetric quantum wells is studied by using a finite difference method. It is shown that the exciton radiative lifetime can be tuned to a large extent by a proper choice of the structure design (double well size, middle barrier position and its thickness) as well as by varying the laser field intensity. -- Highlights: • Laser and heterostructure asymmetry are useful tools to tune the excitonic binding energy. • Radiative lifetime has an oscillatory behavior as the laser intensity grows. • Interwell coupling can be changed via the central barrier width and laser intensity. • Peak positions of the absorption coefficient are blue shifted by the laser field

390

Radiative lifetime of barbell excitons in semiparabolic double quantum wells under intense laser fields  

Energy Technology Data Exchange (ETDEWEB)

The binding energy and optical properties of barbell excitons in GaAs–Ga{sub 1?x}Al{sub x}As semiparabolic double quantum wells under intense laser fields are investigated. Calculations are performed within the effective mass and envelope-function approximations, including the conduction band nonparabolicity. The dependence of the binding energy, oscillator strength and exciton absorption spectrum on the laser field in symmetric and asymmetric quantum wells is studied by using a finite difference method. It is shown that the exciton radiative lifetime can be tuned to a large extent by a proper choice of the structure design (double well size, middle barrier position and its thickness) as well as by varying the laser field intensity. -- Highlights: • Laser and heterostructure asymmetry are useful tools to tune the excitonic binding energy. • Radiative lifetime has an oscillatory behavior as the laser intensity grows. • Interwell coupling can be changed via the central barrier width and laser intensity. • Peak positions of the absorption coefficient are blue shifted by the laser field.

Niculescu, E.C., E-mail: niculescu@physics.pub.ro

2013-11-15

391

Double strong exciton-plasmon coupling in gold nanoshells infiltrated with fluorophores  

Energy Technology Data Exchange (ETDEWEB)

We report on the broadband resonant energy transfer processes observed in dye doped gold nanoshells, consisting of spherical particles with a dielectric core (SiO{sub 2}) covered by a thin gold shell. The silica core has been doped with rhodamine B molecules in order to harness a coherent plasmon-exciton coupling between chromophores and plasmonic shell. This plasmon-exciton interplay depends on the relative spectral position of their bands. Here, we present a simultaneous double strong coupling plasmon-exciton and exciton-plasmon. Indeed, experimental observations reveal of a transmittance enhancement as function of the gain in a wide range of optical wavelengths (about 100?nm), while scattering cross sections remains almost unmodified. These results are accompanied by an overall reduction of chromophore fluorescence lifetimes that are a clear evidence of nonradiative energy transfer processes. The increasing of transmission in the range of 630–750?nm is associated with a striking enhancement of the extinction cross-section in the 510–630?nm spectral region. In this range, the system assumes super-absorbing features. This double behavior, as well as the broadband response of the presented system, represents a promising step to enable a wide range of electromagnetic properties and fascinating applications of plasmonic nanoshells as building blocks for advanced optical materials.

De Luca, A., E-mail: antonio.deluca@fis.unical.it; Dhama, R.; Rashed, A. R. [CNR-IPCF UOS Cosenza, Licryl Laboratory, Department of Physics, University of Calabria, Via P. Bucci, 87036 Rende (Italy); Coutant, C.; Ravaine, S.; Barois, P. [CRPP, Centre de Recherche Paul Pascal, CNRS and University of Bordeaux, 115 Avenue Schweitzer, 33600 Pessac (France); Infusino, M. [Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio 44106-7079 (United States); CNR-IPCF UOS Cosenza, Licryl Laboratory, Department of Physics, University of Calabria, Via P. Bucci, 87036 Rende (Italy); Strangi, G., E-mail: gxs284@case.edu [Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio 44106-7079 (United States)

2014-03-10

392

Self-organization of plasmonic and excitonic nanoparticles into resonant chiral supraparticle assemblies.  

Science.gov (United States)

Chiral nanostructures exhibit strong coupling to the spin angular momentum of incident photons. The integration of metal nanostructures with semiconductor nanoparticles (NPs) to form hybrid plasmon-exciton nanoscale assemblies can potentially lead to plasmon-induced optical activity and unusual chiroptical properties of plasmon-exciton states. Here we investigate such effects in supraparticles (SPs) spontaneously formed from gold nanorods (NRs) and chiral CdTe NPs. The geometry of this new type of self-limited nanoscale superstructures depends on the molar ratio between NRs and NPs. NR dimers surrounded by CdTe NPs were obtained for the ratio NR/NP = 1:15, whereas increasing the NP content to a ratio of NR/NP = 1:180 leads to single NRs in a shell of NPs. The SPs based on NR dimers exhibit strong optical rotatory activity associated in large part with their twisted scissor-like geometry. The preference for a specific nanoscale enantiomer is attributed to the chiral interactions between CdTe NP in the shell. The SPs based on single NRs also yield surprising chiroptical activity at the frequency of the longitudinal mode of NRs. Numerical simulations reveal that the origin of this chiroptical band is the cross talk between the longitudinal and the transverse plasmon modes, which makes both of them coupled with the NP excitonic state. The chiral SP NR-NP assemblies combine the optical properties of excitons and plasmons that are essential for chiral sensing, chiroptical memory, and chiral catalysis. PMID:25400100

Hu, Tao; Isaacoff, Benjamin P; Bahng, Joong Hwan; Hao, Changlong; Zhou, Yunlong; Zhu, Jian; Li, Xinyu; Wang, Zhenlong; Liu, Shaoqin; Xu, Chuanlai; Biteen, Julie S; Kotov, Nicholas A

2014-12-10

393

Excitonic emission of CuInS{sub 2} crystals using confocal microscopy system  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence (PL) spectra in the band-edge region on bulk single-crystals of CuInS{sub 2} grown by the traveling heater method have been investigated using a confocal microscopy system. The observed PL spectra are separated into two Lorentzian peaks which are assigned to be A and B free excitons, by the analysis of the excitation intensity dependence of the emissions. Consequently, we present the behaviour of B free exciton within a wide range of temperatures. The time-resolved emissions of A free exciton have also been examined. The decay of the emissions is analyzed using a double exponential curve. Fast and slow components are attributed to nonradiative relaxation and radiative recombination, respectively. The decay-time constant of the slow component corresponds to the radiative lifetime of A free exciton and is obtained over the wide temperature region until 300 K. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Horikawa, Yusuke; Matsuo, Shingo; Wakita, Kazuki [Department of Electrical, Electronics and Computer Engineering, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016 (Japan); Shim, YongGu [Department of Physics and Electronics, Osaka Prefecture University, 1-1 Gakuencho, Naka-ku, Sakai, Osaka 599-8531 (Japan)

2013-08-15

394

Luminescence study on exciton relaxation in single crystals of symmetric diacetylenes  

Energy Technology Data Exchange (ETDEWEB)

We have measured photoluminescence of the single crystals of symmetric diacetylenes with alkyl end groups. We found that the luminescence bands peaking around 2.6 eV are induced irrespective of the type of the end group. The 2.6-eV luminescence are characterized by large Stokes shifts and long decay time constants of 110 ms at 6 K. These characteristics are analogous to the 2.85-eV luminescence in 5,7-dodecadiyne-1,12-diole bis[phenyl carbamate] (DA-4Ph), in which it has been attributed to the recombination of the triplet self-trapped excitons. The results suggest that the excitons relax into the self-trapped exciton dominantly localized on the triple bonds. The 4.2-eV luminescence, which was tentatively assigned to the recombination luminescence of singlet self-trapped excitons in DA-4Ph, was not observed in the crystals used in the present study. It is inferred that the 4.2-eV luminescence is arising from the relaxed excited state localized at a phenyl ring.

Yamamoto, Daisaku [Department of Materials Science and Chemistry, Wakayama University 930 Sakaedani, Wakayama 640-8510 (Japan); Shirono, Takashi [Department of Materials Science and Chemistry, Wakayama University 930 Sakaedani, Wakayama 640-8510 (Japan); Izumi, Yuuichiro [Department of Materials Science and Chemistry, Wakayama University 930 Sakaedani, Wakayama 640-8510 (Japan); Kajimoto, Naoshi [Department of Materials Science and Chemistry, Wakayama University 930 Sakaedani, Wakayama 640-8510 (Japan); Itoh, Chihiro [Department of Materials Science and Chemistry, Wakayama University 930 Sakaedani, Wakayama 640-8510 (Japan)]. E-mail: citoh@sys.wakayama-u.ac.jp

2007-01-15

395

Process for oxidation of hydrogen halides to elemental halogens  

Science.gov (United States)

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.

Lyke, Stephen E. (Middleton, WI)

1992-01-01

396

Vitrification of IFR and MSBR halide salt reprocessing wastes  

Energy Technology Data Exchange (ETDEWEB)

Both of the genuinely sustainable (breeder) nuclear fuel cycles (IFR - Integral Fast Reactor - and MSBR - Molten Salt Breeder Reactor -) studied by the USA's national laboratories would generate high level reprocessing waste (HLRW) streams consisting of a relatively small amount ( about 4 mole %) of fission product halide (chloride or fluoride) salts in a matrix comprised primarily (about 95 mole %) of non radioactive alkali metal halide salts. Because leach resistant glasses cannot accommodate much of any of the halides, most of the treatment scenarios previously envisioned for such HLRW have assumed a monolithic waste form comprised of a synthetic analog of an insoluble crystalline halide mineral. In practice, this translates to making a 'substituted' sodalite ('Ceramic Waste Form') of the IFR's chloride salt-based wastes and fluoroapatite of the MSBR's fluoride salt-based wastes. This paper discusses my experimental studies of an alternative waste management scenario for both fuel cycles that would separate/recycle the waste's halide and immobilize everything else in iron phosphate (Fe-P) glass. It will describe both how the work was done and what its results indicate about how a treatment process for both of those wastes should be implemented (fluoride and chloride behave differently). In either case, this scenario's primary advantages include much higher waste loadings, much lower overall cost, and the generation of a product (glass) that is more consistent with current waste management practices. (author)

Siemer, D.D. [Idaho National Laboratory, 12N 3167E, Idaho Falls, ID 83402 (United States)

2013-07-01

397

Improved value for the silicon free exciton binding energy  

Directory of Open Access Journals (Sweden)

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.

Martin A. Green

2013-11-01

398

Exciton-induced D2 dissociation in xenon matrix  

International Nuclear Information System (INIS)

The results on VUV and UV cathodoluminescence spectra of solid compounds Xe-D2 studied in relation to D2 concentration and irradiation dose at T 4.2 K are presented. It is shown that during irradiation there occur a number of radiation-induced reactions in solid Xe-D2 mixtures (D2 dissociation, fragment diffusion, formation of (Xe2D)* excimer complexes, etc.), which result from the energy transport of band excitations of the Xe matrix. For solid compounds Xe-D2, a mechanism of D2 dissociation is suggested which involves an intermediate complex (XeD2)* formed due to localization of matrix excitons close to the impurity center.

399

Observation of Two-Exciton States in Perylene Bisimide Aggregates  

Directory of Open Access Journals (Sweden)

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.

Lochbrunner S.

2013-03-01

400

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects  

International Nuclear Information System (INIS)

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically. The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included. Numerical calculations are performed using a variational procedure within the single band effective mass approximation. Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed. The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton. It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased, and that the exciton binding energy, the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field. The obtained results are useful for the design of some opto-photoelectronic devices. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

401

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

CERN Document Server

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

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

2003-01-01

402

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

International Nuclear Information System (INIS)

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

403

Charged excitonic complexes in GaAs/Al0.35Ga0.65As p-i-n double quantum wells  

DEFF Research Database (Denmark)

Photoluminescence (PL) and PL excitation measurements (PLE) have been performed in GaAs/AlxGa1-xAs double quantum well (QW) structures under different applied electric fields. An emission due to charged excitons (trions) has been identified in the PL spectra similar to 3 meV below the heavy-hole exciton emission. These trions are localized by random potential fluctuations, at the interfaces or in the QW, as shown by the saturation of their emission intensity with respect to that of the heavy-hole excitons. Trions are positively charged, namely, they are made by two holes and one electron, as shown by (i) an analysis of the PL polarization for resonant excitation of the heavy- and the light-exciton ground state, and (ii) the analysis of the Zeeman effect for the trion PL band in the Faraday geometry, i.e., for a magnetic field normal to the QW's.

Timofeev, V. B.; Larionov, A. V.

1999-01-01

404

High-field magneto-excitons in unstrained GaAs/AlGaAs quantum dots  

CERN Document Server

The magnetic field dependence of the excitonic states in unstrained GaAs/AlGaAs quantum dots is investigated theoretically and experimentally. The diamagnetic shift for the ground and the excited states are studied in magnetic fields of varying orientation. In the theoretical study, calculations are performed within the single band effective mass approximation, including band nonparabolicity, the full experimental three-dimensional dot shape and the electron-hole Coulomb interaction. These calculations are compared with the experimental results for both the ground and the excited states in fields up to 50 Tesla. Good agreement is found between theory and experiment.

Sidor, Y; Peeters, F M; Schildermans, N; Hayne, M; Moshchalkov, V V; Rastelli, A; Schmidt, O G

2006-01-01

405

Solvated Positron Chemistry. Competitive Positron Reactions with Halide Ions in Water  

DEFF Research Database (Denmark)

It is shown by means of the angular correlation technique that the binding of positrons to halides is strongly influenced by solvation effects. For aqueous solutions we find increasing values for the binding energies between the halide and the positron with increasing mass of the halide. This is contrary to the calculations of Cade and Farazdel for the vacuum case

Christensen, Palle; Pedersen, Niels JØrgen

1979-01-01

406

Influence of excitonic oscillator strengths on the optical properties of GaN and ZnO  

Science.gov (United States)

We report on an extensive study of the excitonic properties of GaN and ZnO bulk samples with an accurate determination of excitonic parameters by linear and nonlinear spectroscopies. The in-depth comparative study is carried out between these two competitive wide band gap semiconductors for a better understanding of damping processes. In GaN, it is shown that due to microscopic disorder, such as lattice strain fluctuations, inhomogeneous broadening prevails over homogeneous broadening at low temperature. The opposite situation occurs in ZnO, where the homogeneous broadening dominates due to resonant Rayleigh scattering of exciton polaritons and their interaction with acoustic phonons. This comparative study also allows us to highlight the influence of oscillator strengths on spectrally resolved four-wave mixing and time-integrated four-wave mixing.

Mallet, E.; Réveret, F.; Disseix, P.; Shubina, T. V.; Leymarie, J.

2014-07-01

407

Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN Effect of Excitation-induced Dephasing  

CERN Document Server

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \\textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

Jho, Y D; Fischer, A J; Song, J J; Kenrow, J; Sayed, K E; Stanton, C J

2003-01-01

408

Exceptionally slow rise in differential reflectivity spectra of excitons in GaN: effect of excitation-induced dephasing  

International Nuclear Information System (INIS)

Femtosecond differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found only for positive time delay in both DRS and FWM experiments. The rise time at negative time delay for the DRS was much slower than the FWM signal or differential transmission spectroscopy at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

409

Resonance effects of excitons and electrons. Basics and applications  

Energy Technology Data Exchange (ETDEWEB)

Represents the first book on non-traditional resonance effects of excitons in semiconductors. Explains resonance phenomena of excitons and electrons in solids. Presents the Knight shift at the Bose-Einstein condensation of excitons. This book presents the various types of resonance effects on excitons, biexcitons and the local electronic centers (LEC) in solids, such as paramagnetic and paraelectric resonances on excitons, exciton acoustic resonance at intra- and interband transitions, radio-optical double resonance on excitons, hole-nuclear double resonance on localized biexcitons, ENDOR and acoustic ENDOR on LEC. The criteria for the generation of coherent photons, phonons and magnons by excitons are explained. The interactions of excitons and biexcitons with paramagnetic centers and nuclear spins, the indirect interaction between the PC through a field of excitons as well as the quasienergy spectrum of excitons and spin systems are discussed. It is proved that the interaction of paramagnetic centers with excitons increases the spin relaxation rate of paramagnetic centers in comparison with the case of their interaction with free carriers. The giant magneto-optical effects in semi-magnetic semiconductors are theoretically interpreted. In recent years, a new perspective has been added to these systems and their interactions: they can be used for storing and processing information in the form of quantum bits (qubits), the building blocks of quantum computers. The basics of this emerging technology are explained and examples of demonstration-type quantum computers based on localized spins in solids are discussed.

Geru, Ion [Moldovan Academy of Sciences, Chisinau (Moldova, Republic of). Inst. of Chemistry; Suter, Dieter [Technische Univ. Dortmund (Germany). Fakultaet Physik

2013-08-01

410

Interaction of Excitonic Magnetic Polaron Pairs in Diluted Magnetic Semiconductor  

Science.gov (United States)

Excitonic magnetic polarons in diluted magnetic semiconductor formed by the interaction between exciton carriers and magnetic ions within the range of exciton wave function are theoretically investigated. The Heitler-London model is applied to the polaron molecule. The binding energy of the exiton pair in triplet state is calculated and it is found that an attraction between triplet magnetic polaron pairs can occur.

Nakamura, Kiko

2006-05-01

411

Model of mixed Frenkel and charge-transfer excitons in donor-acceptor molecular crystals: investigation of vibronic spectra  

International Nuclear Information System (INIS)

The mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) in a molecular stack of regularly arranged donor (D) and acceptor (A) molecules is considered a model case and its vibronic line shapes have been calculated for several parameter sets. The two types of excitons (FE and CTE) are coupled linearly and quadratically with one vibrational mode of the D molecule (or of the A molecule). Using the methods of canonical transformation and of Green's functions (at T=0), as well as the vibronic approach which is applicable in the case of a narrow exciton band, the linear optical susceptibility is calculated for the three spectral regions: (a) excitonic, (b) one-phonon vibronics, and (c) two-phonon vibronics. As the study is directed to centrosymmetrical stacks, the cases of mixing of gerade excitons and of ungerade excitons have been treated separately in the calculation of the linear absorption coefficients. Because until now experimental observations of FE-CTE mixing in DA charge-transfer systems are absent, the numerical calculations have been performed for hypothetical sets of parameters which include: the parameters of CTEs in DA stacks (like anthracene-PMDA) and the parameters of FE-CTE mixing in a one-component stack (like that of PTCDA). The simulations establish the main features of the excitonic and vibronic spectra in the case of FE-CTE mixing, namely (i) the mutual influence on the positions and on the absorption intensities of all terms of the viorption intensities of all terms of the vibronic progressions stemming from FE and CTE levels; (ii) in the case of vibration of an A molecule (if the FE is assumed to be an excited electronic state of the D molecule), only one vibronic progression is manifest and the vibronic levels closer to the FE will be most enhanced; (iii) the redistribution of the absorption intensities depends on the sign of the mixing constant and may be so strong that the terms of the two vibronic progressions could have comparable absorption; (iv) spectral lines of different shape correspond to the bound and unbound exciton-phonon states; and (v) in the case of mixing of gerade excitons its possible impact on the absorption of the ungerade CTE-combination connected with the noncentral part k?0 of the Brillouin zone was established. The simulation of the FE-CTE mixing can be useful in the assignment of the linear absorption spectra and in the description of the FE-CTE-vibration coupling

412

Ultrafast spectroscopy of self trapped excitons in quasi-one-dimensional materials  

Science.gov (United States)

This dissertation studies the transition of electronic excitations from a delocalized state to a localized state. Such processes have a dramatic impact on optical and electronic properties and reflect fundamental interactions in the physics of condensed matter systems. An important example is the self-trapping of excitons: an initially extended free exciton interacts with a deformable lattice, resulting in the formation of a localized self-trapped exciton (STE). Experiments were performed in a series of quasi-one-dimensional materials called mixed-valence metal-halide linear chain (MX) complexes, in which variation of the chemical structure allows systematic control of the strength of the electron-phonon interaction that drives the dynamics. Results are compared in regimes ranging from the strong coupling limit (where electronic excitations are localized to nearly one unit cell) to the weak coupling limit (where electronic localizations are spread out over many unit cells). The MX materials studied here were [Pt(en)2][Pt(en)2 (Cl)2]·Y or PtCl, and [Pt(en)2I2][Pt(CN) 4] or PtICN. The following experiments were carried out: 1 - Transient optical absorption experiments were carried out on PtCl, a material with a very strong electron phonon coupling. These studies revealed the presence of a low frequency 68 cm-1 modulation that carries the system to the self trapped state. This frequency is used to calculate the spatial extent of the self trapped exciton, which is estimated to be localized to nearly 1 unit cell. 2 - Transient optical absorption experiments were carried out on PtICN, a material with a strong electron phonon coupling. These studies revealed the presence of a low frequency 15 cm-1 modulation that carries the system to the self trapped state. Using this frequency, the STE is calculated to be confined to nearly 3 unit cells, reflecting the unusually strong coupling in this novel material. 3 - The vibrational properties of the equilibrated STE were determined using 3-pulse pump-pump-probe techniques. Resonant impulsive Raman excitation of the equilibrated STE in both PtCl and PtICN revealed a downward shift in frequency as the lattice distorts to form the more localized STE.

Mance, Jason

413

Magneto-absorption spectra of hydrogen-like yellow exciton series in cuprous oxide: excitons in strong magnetic ?fields  

OpenAIRE

We study the absorption spectra of the yellow excitons in Cu$_2$O in high magnetic fields using polarization-resolved optical absorption measurements with a high frequency resolution. We show that the symmetry of the yellow exciton results in unusual selection rules for the optical absorption of polarized light and that the mixing of ortho- and para- excitons in magnetic field is important. Our calculation of the energies of the yellow exciton series in an arbitrary magnetic...

Artyukhin, Sergey L.; Fishman, Dmitry; Faugeras, Cle?ment; Potemski, Marek; Revcolevschi, Alexandre; Mostovoy, Maxim; Loosdrecht, Paul H. M.

2012-01-01

414

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

Directory of Open Access Journals (Sweden)

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.

A. P\\u00E9rez-Rodr\\u00EDguez

2006-01-01

415

Correlated exciton relaxation in Poly(3-hexylthiophene).  

Science.gov (United States)

Two-color 3 pulse photon echo peak shift (2C-3PEPS) measurements on poly(3-hexylthiophene) (3PHT) demonstrate that spectral regions in the photoluminescence remain correlated with the excitation, despite large differences in energy (>0.5 eV). The observations are explained in terms of exciton-phonon coupling that is dominated by only two motions: one high frequency bond stretch and a low frequency torsional motion. Numerical simulations of the 2C-3PEPS are shown to be consistent with the experimental observations. The results demonstrate that initial intramolecular exciton relaxation in P3HT is not primarily a stochastic process, but is driven by strong, selective exciton-phonon coupling to torsional motions. PMID:18352641

Wells, Nathan P; Blank, David A

2008-02-29

416

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

Energy Technology Data Exchange (ETDEWEB)

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.

Darma, Yudi; Rusydi, Andrivo, E-mail: phyandri@nus.edu.sg [Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 (Singapore); Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); NUSNNI-NanoCore, National University of Singapore, 2 Science Drive 3, Singapore 117542 (Singapore); Department of Physics, Institut Teknologi Bandung, Ganesa 10, Bandung 40132 (Indonesia); Seng Herng, Tun [Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 (Singapore); Department of Material Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 (Singapore); Marlina, Resti; Fauziah, Resti [Department of Physics, Institut Teknologi Bandung, Ganesa 10, Bandung 40132 (Indonesia); Ding, Jun [Department of Material Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 (Singapore)

2014-02-24

417

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

International Nuclear Information System (INIS)

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

418

An optical criterion to obtain miscible mixed crystals in alkali halides  

Scientific Electronic Library Online (English)

Full Text Available Este trabajo ofrece un nuevo criterio para predecir la formación de soluciones sólidas en halogenuros alcalinos cristalinos y discute algunos resultados obtenidos en el desarrollo de mezclas dieléctricas cristalinas miscibles de halogenuros alcalinos ternarias y cuaternarias. Estas mezclas son misci [...] bles en cualquier concentración de sus componentes. Tener el resultado de estas mezclas cristalinas está relacionado al centro F a través del comportamiento observado en la banda F de absorción en función de la constante de red de los halogenuros alcalinos donde el defecto fue formado (centro F). Dando un vistazo a la gráfica de Energía de banda F versus constante de red (ley de Mollwo-Ivey), se observa un conjunto de puntos, que dan la pauta (tal como KCl, KBr, RbCl), de posibles mezclas de materiales correspondientes a puntos adyacentes y una solución sólida podría formarse, significando un cristal de una sola fase, que dan por resultado cristales ternarios y cuaternarios. Así, la banda F de absorción nos permite tener un criterio numérico, basado en el porcentaje de cambio de la energía de la banda F que permite obtener soluciones sólidas. Encontramos información experimental, usando difractogramas de las mezclas cristalinas, se obtienen las constantes de red y se comparan con la obtenida teóricamente a través de la generalización de la Regla de Vegard, finalmente se discute la posibilidad de crecer cristales partiendo de cinco componentes, tomando cinco puntos consecutivos en la grafica de la Ley de Mollwo-Ivey. Abstract in english This work gives a novel criterion to predict the formation of alkali halide solid solutions and discusses some results obtained in the development of ternary and quaternary miscible crystalline dielectric mixtures of alkali halides. These mixtures are miscible in any concentration of their component [...] s. The miscibility of these mixed crystals is quite related to the F center through the behavior observed in the spectral position of the optical absorption F band as a function of the lattice constant of the alkali halide where the F center was formed. By inspection of an energy graph of the F band energy versus lattice constant (Mollwo-Ivey law), a set of points is observed corresponding to several pure alkali halides (such as KCl, KBr, RbCl), which gives a notion of possible mixed materials that would correspond to adjacent points and a solid solution could be obtained, meaning a single phase crystal, which result in ternary and quaternary mixed crystals. Thus, the optical absorption F band allows have a numerical criterion, based on the percentage respective of the F band energy, in order to predict possible solid solutions. We obtained experimental information using diffractograms of the mixed crystals, from which the lattice constant was obtained and compared with a theoretical calculus using a generalization of Vegard's law, finally it is discussed the case of a crystal growing, starting from five components, picking up five consecutive dots from the graph of Mollwo-Ivey's law.

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

2008-11-01

419

Spectral and luminescence properties of the neodymium ion in low-temperature halide glasses  

International Nuclear Information System (INIS)

The binary and ternary systems M'Hal3-ZnHal2 and M'Hal3-ZnHal2-M double-prime Hal, where M' is gallium or aluminum, Hal is chlorine or bromine, and M double-prime is an alkaline metal, when they contain zinc halide in excess of 25-30 mol%, form glasses that flow at room temperatures. Behavior of the rare-earth ion in these matrices has barely been studied, although they are transparent in a broad range, which allows observations of most of the absorption and luminescence bands. Low probability of the energy transfer on the Ln-Cl(Br) bond via vibronic coupling, high solubility, and low liquidus temperatures make these systems promising as media for pumped-liquid lasers as well as for model systems in studies of rare-earth behavior in melts and glasses. 10 refs., 4 tabs

420

Electrical field profile and doping in planar lead halide perovskite solar cells  

Science.gov (United States)

Hybrid lead halide perovskites (PVKs) have emerged as novel materials for photovoltaics and have rapidly reached very large solar to electricity power conversion efficiencies. As occurring with other kind of solar technologies establishing the working energy-band diagram constitutes a primary goal for device physics analysis. Here, the macroscopic electrical field distribution is experimentally determined using capacitance-voltage and Kelvin probe techniques. Planar structures comprising CH3NH3PbI3-xClx PVK exhibit p-doping character and form a p-n heterojunction with n-doped TiO2 compact layers. Depletion width at equilibrium within the PVK bulk has an extent about 300 nm (approximately half of the layer thickness), leaving as a consequence a significant neutral zone towards the anode contact. Charge collection properties are then accessible relying on the relative weight that diffusion and drift have as carrier transport driven forces.

Guerrero, Antonio; Juarez-Perez, Emilio J.; Bisquert, Juan; Mora-Sero, Ivan; Garcia-Belmonte, Germà

2014-09-01

421

High-density selective excitation effect on excitons in semimagnetic semiconductor CdMnTe  

International Nuclear Information System (INIS)

Many body effects among electron-hole pairs and exchange interaction of spins between electron/hole and Mn2+ ions have been investigated in a semimagnetic semiconductor of CdMnTe under the selective excitation of high-density localized excitons with use of a picosecond pulse laser. A new nonlinear luminescence band (X band) has been found on the high-energy side of conventional exciton magnetic polaron band and its peak energy shows a pronounced red shift with the increase of excitation power and/or with the application of magnetic field. Time-resolved luminescence without magnetic field has revealed the characteristic behaviour of the X band with a longer rise time for the higher energy side. However, this tendency disappears with the application of magnetic field. As the origin of the X band, we propose the high-density spin-polarized electron-hole pairs interacting with microscopic Mn2+-ion spins aligned through the exchange interaction. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

422

High-density selective excitation effect on excitons in semimagnetic semiconductor CdMnTe  

Energy Technology Data Exchange (ETDEWEB)

Many body effects among electron-hole pairs and exchange interaction of spins between electron/hole and Mn{sup 2+} ions have been investigated in a semimagnetic semiconductor of CdMnTe under the selective excitation of high-density localized excitons with use of a picosecond pulse laser. A new nonlinear luminescence band (X band) has been found on the high-energy side of conventional exciton magnetic polaron band and its peak energy shows a pronounced red shift with the increase of excitation power and/or with the application of magnetic field. Time-resolved luminescence without magnetic field has revealed the characteristic behaviour of the X band with a longer rise time for the higher energy side. However, this tendency disappears with the application of magnetic field. As the origin of the X band, we propose the high-density spin-polarized electron-hole pairs interacting with microscopic Mn{sup 2+}-ion spins aligned through the exchange interaction. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Katayama, K.; Miyajima, K.; Ashida, M.; Itoh, T. [Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531 (Japan)

2006-11-15

423

Efficient Use Of Silver Halide In X-Ray Film  

Science.gov (United States)

Silver halide is a unique imaging receptor because the absorption of only a few light photons can produce up to 109 silver atoms. Recently, the cost of silver has increased dramatically although there is no shortage of the metal. These price increases have focused attention on making more efficient use of this precious metal in silver halide films and on its recovery after use. Photographic manufacturers have developed technology necessary to make X-ray films using less silver without affecting photographic performance. Other non-silver imaging systems have been developed for use when high photographic speed is not required.

Wayrynen, R. E.

1981-07-01

424

Exciton diffusion and interfacial charge separation in meso-tetraphenylporphyrin/TiO2 bilayers: effect of ethyl substituents.  

Science.gov (United States)

The photoinduced charge separation efficiency in porphyrin/TiO2 bilayers has been determined using the time-resolved microwave conductivity (TRMC) technique. Porphyrins investigated are unsubstituted meso-tetraphenylporphyrin (TPP) and meso-tetra(4-ethylphenyl)porphyrin (TEPP). TEPP/TiO2 bilayers exhibit a charge separation efficiency per incident photon at the Soret band maximum of 6.2%, which is considerably higher than the efficiency of 1.2% found for TPP/TiO2 bilayers. Exciton diffusion lengths of 7 A for TPP and 75 A for TEPP are obtained from fitting a model for the charge separation efficiency to the experimental data. Optical measurements on the porphyrin derivatives on quartz yield a 20 times higher fluorescence quantum yield and a 7 times higher fluorescence rate constant for TEPP layers as compared to TPP layers. The exciton lifetime of 800 ps found for TEPP layers is considerably longer than the lifetime of 260 ps in TPP layers. The exciton diffusion coefficients, determined from the exciton diffusion length and the exciton lifetime, are found to be 2.10(-9) m(2)/s for TPP and 7.10(-8) m(2)/s for TEPP. The difference is discussed in terms of the presence of face-to-face dimers or larger aggregates in TPP layers. PMID:16853606

Huijser, Annemarie; Savenije, Tom J; Kroeze, Jessica E; Siebbeles, Laurens D A

2005-11-01

425

Exciton cascade model for fast neutron reactions  

International Nuclear Information System (INIS)

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

426

Device design for optimal exciton harvesting  

Science.gov (United States)

Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.

Ingram, G. L.; Lu, Z. H.

2014-10-01

427

Detecting an exciton crystal by statistical means  

International Nuclear Information System (INIS)

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

428

Intrinsic excitonic luminescence in crystalline terthiophene  

Science.gov (United States)

The intrinsic excitonic luminescence of crystalline terthiophene samples is reported and discussed in the context of their peculiar crystal structure, which is slightly different from that of all other unsubstituted oligothiophenes (OT’s). Starting from the pinwheel aggregate model, it is demonstrated that free-exciton radiative recombination is in principle allowed. Nevertheless, it can not be experimentally observed. Its oscillator strength is indeed so low that even the occurrence of cooperative effects, which enhance the corresponding transition in other OT crystals, cannot be effective in this case.

Meinardi, F.; Blumstengel, S.; Cerminara, M.; Macchi, G.; Tubino, R.

2005-07-01

429

Excitonic and photonic processes in materials  

CERN Document Server

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.

Williams, Richard

2015-01-01

430

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

Directory of Open Access Journals (Sweden)

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.

Christian Wehrenfennig

2014-08-01

431

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

Science.gov (United States)

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.

Wehrenfennig, Christian; Liu, Mingzhen; Snaith, Henry J.; Johnston, Michael B.; Herz, Laura M.

2014-08-01

432

Reclassifying exciton-phonon coupling in molecular aggregates: Evidence of strong nonadiabatic coupling in oligothiophene crystals  

Science.gov (United States)

Exciton-phonon (EP) coupling in molecular aggregates is reexamined in cases where extended intermolecular interactions result in low-energy excitons with high effective masses. The analysis is based on a single intramolecular vibrational mode with frequency ?0 and Huang-Rhys factor ?2. When the curvature Jc at the exciton band bottom is much smaller than the free-exciton Davydov splitting W, the strength of the EP coupling is determined by comparing the nuclear relaxation energy ?2?0 with the curvature. In this way, weak (?2?0?4?Jc), intermediate I (?2?0?4?Jc), and strong I (?2?0?4?Jc) coupling regimes are introduced. The conventional intermediate (?2?0?W) and strong (?2?0?W) EP coupling regimes originally defined by Simpson and Peterson [J. Chem. Phys. 26, 588 (1957)] are based solely on the Davydov splitting and are referred to here as intermediate II and strong II regimes, respectively. Within the intermediate I and strong I regimes the near degeneracy of the low-energy excitons allows efficient nonadiabatic coupling, resulting in a spectral splitting between the b- and ac-polarized first replicas in the vibronic progression characterizing optical absorption. Such spectral signatures are clearly observed in OT4 thin films and crystals, where splittings for the lowest energy mode with ?0=161cm-1 are as large as 30cm-1 with a small variation due to sample disorder. Numerical calculations using a multiphonon BO basis set and a Hamiltonian including linear EP coupling yield excellent agreement with experiment.

Spano, F. C.; Silvestri, L.; Spearman, P.; Raimondo, L.; Tavazzi, S.

2007-11-01

433

Nonadiabatic theory of excitons in wurtzite AlGaN/GaN quantum-well heterostructures  

Energy Technology Data Exchange (ETDEWEB)

We have theoretically studied exciton states and photoluminescence spectra of the strained wurtzite Al{sub x}Ga{sub 1-x}N/GaN quantum-well heterostructures. The electron energy spectrum is found through numerical solving of the one-band Schroedinger equation, while the hole and exciton energy spectra are determined from the Schroedinger equation with the non-symmetrical 6-band Hamiltonian, including deformational and spin-orbit interaction. The oscillator forces are calculated and the exciton states are determined, which are active in the process of the optical absorption. Three quantitative conclusions are made in a good agreement with experiments for different values of x as concerns: (i) the red shift of the zero-phonon photoluminescence (PL) peaks with increase of the quantum-well width, (ii) the relative intensities of the zero-phonon and one-phonon PL peaks, found within the non-adiabatic approach, and (iii) the values of the PL decay time as a function of the quantum-well width. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Pokatilov, Evghenii P.; Nika, Denis L. [PMS, Department of Theoretical Physics, State University of Moldova, 2009 Chisinau (Moldova); Fomin, Vladimir M. [PMS, Department of Theoretical Physics, State University of Moldova, 2009 Chisinau (Moldova); Theoretische Fysica van de Stoffen (TFVS), Departement Fysica, Universiteit Antwerpen, 2020 Antwerpen (Belgium); Photonics and Semiconductor Nanostructures (PSN), TU Eindhoven, 5600 MB Eindhoven (Netherlands); Devreese, Jozef T. [Theoretische Fysica van de Stoffen (TFVS), Departement Fysica, Universiteit Antwerpen, 2020 Antwerpen (Belgium); Photonics and Semiconductor Nanostructures (PSN), TU Eindhoven, 5600 MB Eindhoven (Netherlands)

2009-01-15

434

Photoluminescence properties and exciton dynamics in monolayer WSe2  

Science.gov (United States)

In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe2 to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260 K, providing further evidence for two-dimensional nature of monolayer material.

Yan, Tengfei; Qiao, Xiaofen; Liu, Xiaona; Tan, Pingheng; Zhang, Xinhui

2014-09-01

435

Magneto-optical properties of charged excitons in quantum dots  

CERN Document Server

We present both experimental and theoretical results on the influence of a magnetic field on excitons in semiconductor quantum dots. We find a pronounced difference between the strong and weak confinement regimes. For weak confinement, the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons. In this limit, we argue that the optical properties of excitons with two or more extra electrons are fundamentally different to those of the neutral exciton and trion.

Govorov, A O; Haft, D; Kalameitsev, A V; Chaplik, A; Warburton, R J; Karrai, K; Schoenfeld, W; García, J M; Petroff, P M

2002-01-01

436

Role of delocalized exciton states of light-harvesting pigments in excitation energy transfer in natural photosynthesis  

Science.gov (United States)

Photosynthesis is an extremely efficient converter of light into chemical energy, with an observed quantum yield for primary photochemistry approximately 90%. To achieve this the photosynthetic apparatus must be highly optimized, and some of the design principles that may be involved have been suggested. The role of delocalized exciton states of light-harvesting pigments in the energy transfer process has been considered by mathematical simulation of the light-harvesting process in model systems. Namely, it has been shown that aggregation of antenna pigments (allowing to consider each aggregate as a supermolecule) is biologically expedient, as an efficient strategy for light harvesting in photosynthesis. The question of whether this design principle is realized in a natural antenna has been examined for the 3D chlorosomal superantenna of green bacteria with the hole-burning spectroscopy. Spectral hole burning studies of intact cells of green bacteria Chlorobium phaeovibriodes. Chloroflexus aurantiacus and Chlorobium limicola have proven that the Qy- absorption system of antenna bacteriochlorophylls e or c (BChl e or BChl c) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 transition band of the lowest exciton state of BChl e and BChl c aggregates has been directly detected as the lowest energy inhomogeneously broadened band of the 1.8 K near-infrared excitation spectrum. These lowest energy bands have different spectral position of their maximums: approximately 739 nm in C.phaeovibriodes (BChl e band), approximately 752 nm in C.aurantiacus (BChl c band) and approximately 774 nm in C.limicola (BChl c band) cells. However, these bands display a number of fundamentally similar spectral features: (1) The magnitude of inhomogeneous broadening of these bands is 90 - 100 cm-1; (2) The width of each band is 2 - 3 times less than that of the monomeric BChl c (or BChl e) in vitro at 5 K; (3) Each band, being the lowest energy exciton band, manifests itself as the longest wavelength band in the circular dichroism spectrum; (4) At the wavelength of the maximum of each band for all the three species, the amplitude of the preburnt excitation spectrum makes up 20% of the maximum amplitude of the spectrum; (5) The weak exciton-phonon coupling of optical transitions corresponding to these bands is also a common feature. So, the hole spectra measured for C.aurantiacus, C.limicola and C.phaeovibrioides cells were shown to be consistent with the BChl c (and BChl e) organization in the chlorosomes as strongly exciton- coupled BChl c (or BChl e) aggregates, i.e. the delocalized excitons are in fact involved in the energy transfer process within these antennae. Thus, aggregation of the pigment molecules in natural light-harvesting antennae should be considered as one of the optimizing factors ensuring high efficiency of excitation energy transfer from antenna to reaction center.2362

Fetisova, Zoya; Mauring, Koit; Taisova, Alexandra

1995-02-01

437

Halide impact on emission of mononuclear copper(I) complexes with pyrazolylpyrimidine and triphenylphosphine.  

Science.gov (United States)

A series of mononuclear heteroleptic copper(I) halide complexes, [CuL(PPh3)X] (X = Cl, Br, I), based on 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine (L) and triphenylphosphine, have been synthesized by reaction between CuX (X = Cl, Br, I), L and PPh3 in a molar ratio of 1/1/1 in MeCN solutions. The copper atom, showing the distorted tetrahedral environment, is bound by the N,N-chelating ligand L, triphenylphosphine and a halide ion. The complexes [CuL(PPh3)Cl] and [CuL(PPh3)Br] are isostructural. In CH2Cl2 solutions, L and the complexes [CuL(PPh3)X] (X = Cl, Br, I) display a luminescence band with ?(max) = 377 nm and a lifetime of 1.9 ns (ligand-based luminescence (LL*)). However, the complex [CuL(PPh3)I] has an additional weak luminescence band with ?(max) = 681 nm and a lifetime of 96 ns of (3)MLCT origin. In the solid state, L shows the splitting of the luminescence band to ?(max) = 365 and 384 nm and a slight increase of the lifetime to 2.66 ns. Solid samples of the complexes [CuL(PPh3)X] demonstrate (3)MLCT luminescence bands at 620 nm (X = Cl), 605 nm (X = Br) and 559 nm (X = I) with lifetimes in the range 3.6-11.2 ?s, whereas the LL* band (377 nm) is absent. Quantum yields and rate constants of radiative and nonradiative processes were determined in CH2Cl2 solutions and in the solid state for all complexes. The luminescence quantum yield and lifetimes for the solid samples increase in the order [CuL(PPh3)Cl] < [CuL(PPh3)Br] < [CuL(PPh3)I]. This is due to the increase of radiative decay and simultaneous suppression of nonradiative decay. The complex [CuL(PPh3)I] shows a high quantum yield of 29.4% and an excited state lifetime of 11.2 ?s. PMID:24346321

Vinogradova, Katerina A; Plyusnin, Victor F; Kupryakov, Arkady S; Rakhmanova, Marianna I; Pervukhina, Natalia V; Naumov, Dmitrii Yu; Sheludyakova, Lilia A; Nikolaenkova, Elena B; Krivopalov, Viktor P; Bushuev, Mark B

2014-02-21

438

Theoretical Study on Exciton Dynamics in Dendritic Systems: Exciton Recurrence and Migration  

OpenAIRE

The optical functionalities such as exciton recurrence and migration for dendritic systems, e.g., dendrimers, are investigated using the quantum master equation (QME) approach based on the ab initio molecular orbital configuration interaction (MOCI) method, which can treat both the coherent and incoherent exciton dynamics at the first principle level. Two types of phenylacetylene dendrimers, Cayley-tree dendrimer and nanostar dendrimer with anthracene core, are examined to elucidate the featu...

Kyohei Yoneda; Masayoshi Nakano; Takuya Minami; Ryohei Kishi

2009-01-01

439

Charge transport and phase transition in exciton rings  

CERN Document Server

The macroscopic exciton rings observed in the photoluminescence (PL) patterns of excitons in coupled quantum wells (CQWs) are explained by a series of experiments and a theory based on the idea of carrier imbalance, transport and recombination. The rings are found to be a source of cold excitons with temperature close to that of the lattice. We explored states of excitons in the ring over a range of temperatures down to 380 mK. These studies reveal a sharp, albeit continuous, second order phase transition to a low-temperature ordered exciton state, characterized by ring fragmentation into a periodic array of aggregates. An instability at the onset of degeneracy in the cold exciton system, due to stimulated exciton formation, is proposed as the transition mechanism.

Butov, L V; Mintsev, A V; Simons, B D; Gossard, A C; Chemla, D S

2004-01-01

440

Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials  

Science.gov (United States)

The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

Quijada, Manuel A.; Henry, Ross

2007-01-01

441

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

Energy Technology Data Exchange (ETDEWEB)

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

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

442

Triplet superconductivity through quadrupolar exciton exchange in PrOs 4Sb 12  

Science.gov (United States)

Superconducting pair formation in heavy fermion (HF) compounds is usually attributed to the exchange of over-damped bosonic spin fluctuation modes in the itinerant quasiparticle system. A different mechanism based on the exchange of propagating CEF excitations, i.e., magnetic excitons has previously been identified for UPd 2Al 3 [Sato, et al., Nature 410 (2001) 340; McHale, et al., Phys. Rev. B 70 (2004) 014513]. We propose a similar model for the tetrahedral (T h) HF superconductor PrOs 4Sb 12. There low energy singlet-triplet CEF excitations exist which disperse into quadrupolar exciton bands as observed in inelastic neutron scattering (INS). Virtual exchange of these collective modes due to aspherical Coulomb scattering leads to Cooper pair formation. Calculations of the effective pairing interactions in non-retarded weak coupling approximation for the irreducible T h channels are presented. The triplet T u symmetry is favored for the gap function.

Thalmeier, P.

2006-05-01

443

Thermal effects in exciton harvesting in biased one-dimensional systems  

International Nuclear Information System (INIS)

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Recently, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (magnitude of the energy gradient towards the trap) are possible [S.M. Vlaming, V.A. Malyshev, J. Knoester, J. Chem. Phys. 127 (2007) 154719]. This paper generalizes the results for both homogeneous and disordered chains to nonzero temperatures. We show that thermal effects are appreciable only for low bias strengths, particularly so in disordered systems, and lead to faster harvesting

444

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

International Nuclear Information System (INIS)

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

445

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

International Nuclear Information System (INIS)

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

446

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

International Nuclear Information System (INIS)

Using coherent two-dimensional (2D) electronic spectroscopy in fully noncollinear geometry, we observe the excitonic coupling of ?,??-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)

447

Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport  

Science.gov (United States)

Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics,