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1

Excitonic Bands in the Photoconductivity Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units  

Science.gov (United States)

The photoconductivity (PC) spectra of the compounds CH3NH3PbI3, [CH3NH3][CH3C6H4CH2NH3]2Pb2I7, [CH3C6H4CH2NH3]2PbI4, [H2NC(I)=NH2]3PbI5 as well as the spectra of similar compounds based on C10H21SC(NH2)2, H3N(CH2)6NH3, C14H9CH2SC(NH2)2, SnI4, PbClxI4-x and PbBrxI4-x are described. The position, intensity and shape of the PC bands depend on the dimensionality (or size) of the inorganic network as well as on the nature of metal halide and the organic groups. Excitonic features are discussed.

Papavassiliou, G. C.; Mousdis, G. A.; Koutselas, I. B.; Papaioannou, G. J.

2001-08-01

2

Excitons and point defect creation in alkali halides  

International Nuclear Information System (INIS)

in the report, on an example of face- and body-centered alkali halide crystals (kcl and csbr) the following problems are biefly considered: 1. particularities of electronic excitations conditioned by the coexistence of free excitons and self-trapped excitons. 2. decay of excitons with the creation of neutral defects. 3. decay of excitons with the creation of the frenkel defects. 4. recombination of defects with the creation of luminescent self-trapped excitons. 5. creation of defects due to the recombination of electrons with self-trapped holes. 6. creation of cation defects

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

2014-10-01

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

Electromagnetically Induced Exciton Mobility in a Photonic Band Gap  

Science.gov (United States)

It is suggested that an exciton in the engineered vacuum of a photonic-band-gap quantum-well heterostructure exhibits electromagnetically induced anomalous quantum dynamics. The exciton is dressed by coherent emission and reabsorption of virtual photons near the photonic band edge and captured in momentum space, lowering its energy by 1 10 meV and lowering its effective mass by 4 5 orders of magnitude. The photonic band gap simultaneously enables strong coupling to confined optical modes and long exciton lifetime.

John, Sajeev; Yang, Shengjun

2007-07-01

6

Exciton dressing and capture by a photonic band edge  

Science.gov (United States)

We demonstrate electromagnetically induced anomalous quantum dynamics of an exciton in a photonic band gap (PBG)-quantum well (QW) heterostructure. Within the engineered electromagnetic vacuum of the PBG material, the exciton can propagate through the QW by the emission and reabsorption of virtual photons in addition to the conventional electronic hopping mechanism. When the exciton wave vector and recombination energy nearly coincide with a photonic band edge, the exciton kinetic energy is lowered by 1-10meV through coherent radiative hopping. This capture of the exciton by the photonic band edge is accompanied by strong electromagnetic dressing in which exciton’s renormalized effective mass is 4-5 orders of magnitude smaller than in the absence of the PBG environment. This dressed exciton exhibits a long radiative lifetime characteristic of a photon-atom bound state and is robust to phonon-assisted recombinative decay. By inheriting properties of the PBG electromagnetic vacuum, the bound electron-hole pair becomes a stable, ultramobile quantum excitation.

Yang, Shengjun; John, Sajeev

2007-06-01

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

Some peculiarities of optical spectra of CdS excitons near slip bands of dislocations  

International Nuclear Information System (INIS)

Studied were spectra of exciton reflection and luminescence of cadmium sulfide monocrystals with introduced slip bands of dislocations. A displacement of line position of free and bound excitons on wave length scale for 1-10 A depending on dislocation density in the band and its width has been observed on spectrograms of the above crystals near slip bands. The displacement from opposite sides of the band occurs in different directions. Observed is the difference in behaviour of lines of free excitons for dislocations with base and prismatic planes of slip: for base dislocations of A and B excitons are shifted in one direction, for prismatic ones - in the opposite direction. Correlation of exciton line displacement and pictures of mechanical stresses in polarization microscope permit to consider that the effect revealed is due to elastic deformations of lattice near slip band

9

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

Science.gov (United States)

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.

Ahmad, Shahab; Baumberg, Jeremy J.; Vijaya Prakash, G.

2013-12-01

10

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

11

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

12

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

13

Relaxation processes of the triplet states of self-trapped excitons in alkali-halide crystals  

International Nuclear Information System (INIS)

The spin lattice relaxation processes of the triplet state of the self trapped exciton have been studied in Alkali Chlorides and Bromides by measuring the transient response of the X ray excited luminescence under resonant pulse microwave. The symmetry properties and phonon coupling are discussed. Application of the model to magnetic circular polarization data also shows good agreement. (Author)

14

Band-bending in organic semiconductors: the role of alkali-halide interlayers.  

Science.gov (United States)

Band-bending in organic semiconductors, occurring at metal/alkali-halide cathodes in organic-electronic devices, is experimentally revealed and electrostatically modeled. Metal-to-organic charge transfer through the insulator, rather than doping of the organic by alkali-metal ions, is identified as the origin of the observed band-bending, which is in contrast to the localized interface dipole occurring without the insulating buffer layer. PMID:24338797

Wang, Haibo; Amsalem, Patrick; Heimel, Georg; Salzmann, Ingo; Koch, Norbert; Oehzelt, Martin

2014-02-12

15

Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3  

Science.gov (United States)

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed.

Ho, Ching-Hwa; Chen, Hsin-Hung

2014-08-01

16

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

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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

17

Multivalence-band calculation of the excitonic dielectric function for hexagonal GaN  

International Nuclear Information System (INIS)

The complex dielectric function of hexagonal gallium nitride (?-GaN) is obtained from a numerical solution of the excitonic Schrödinger equation taking into account the full 6 × 6 valence-band structure. The valence-band parametrization includes anisotropy, nonparabolicity, and the coupling of angular-momentum eigenstates. Spectra of excitonic eigenfunctions are obtained from a time-dependent initial-value problem, which is solved via an exponential split-operator method. In particular, we calculate the dielectric function and the reflectivity of a-plane GaN with polarization vectors parallel and perpendicular to the c-axis of the crystal. The simulated reflection spectra are in excellent agreement with recent experimental data and allow the unambiguous identification of the experimentally observed excitonic resonances. The binding energies of the FXA, FXB, and FXC excitons found in our calculation differ by up to 27%, depending on the chosen parameter set. An important consequence of this observation is that the experimentally observed splittings of the excitons cannot be used for the parametrization of the valence band near the ?-point, but need to be corrected by the differences of the binding energies. This is of general relevance for all spectroscopic measurements in semiconductors with a wide bandgap. (paper)

18

Multivalence-band calculation of the excitonic dielectric function for hexagonal GaN.  

Science.gov (United States)

The complex dielectric function of hexagonal gallium nitride (?-GaN) is obtained from a numerical solution of the excitonic Schrödinger equation taking into account the full 6 × 6 valence-band structure. The valence-band parametrization includes anisotropy, nonparabolicity, and the coupling of angular-momentum eigenstates. Spectra of excitonic eigenfunctions are obtained from a time-dependent initial-value problem, which is solved via an exponential split-operator method. In particular, we calculate the dielectric function and the reflectivity of a-plane GaN with polarization vectors parallel and perpendicular to the c-axis of the crystal. The simulated reflection spectra are in excellent agreement with recent experimental data and allow the unambiguous identification of the experimentally observed excitonic resonances. The binding energies of the FXA, FXB, and FXC excitons found in our calculation differ by up to 27%, depending on the chosen parameter set. An important consequence of this observation is that the experimentally observed splittings of the excitons cannot be used for the parametrization of the valence band near the ?-point, but need to be corrected by the differences of the binding energies. This is of general relevance for all spectroscopic measurements in semiconductors with a wide bandgap. PMID:23567664

Lenk, Steve; Schwarz, Felix; Goldhahn, Rüdiger; Runge, Erich

2013-05-01

19

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy  

International Nuclear Information System (INIS)

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.

20

Investigation of the excitonic luminescence band of CdTe solar cells by photoluminescence and photoluminescence excitation spectroscopy  

Energy Technology Data Exchange (ETDEWEB)

The excitonic luminescence band of polycrystalline cadmium telluride layers has been investigated by Photoluminescence (PL) and Photoluminescence excitation spectroscopy (PLE). CdTe was deposited by means of close space sublimation and the samples were activated by different chlorine containing compounds, i.e. cadmium chloride, hydrochloric acid, and sodium chloride as well as by simple air activation or received no post deposition treatment. In the PL spectra, four different peaks within the excitonic luminescence band were resolved. These include the free-exciton peak and two transitions of excitons bound to defects. Furthermore, free excitons and band to band transitions were detected by means of PLE. The PL and PLE spectra are discussed with respect to the post deposition treatments.

Kraft, C., E-mail: christian.kraft@uni-jena.de [Inst. fuer Festkoerperphysik, Universitaet Jena, Max-Wien-Platz 1, 07743 Jena (Germany); Haedrich, M.; Metzner, H.; Reisloehner, U. [Inst. fuer Festkoerperphysik, Universitaet Jena, Max-Wien-Platz 1, 07743 Jena (Germany); Schley, P. [Inst. fuer Physik, Techn. Universitaet Ilmenau, PF 100565, 98684 Ilmenau (Germany); Goldhahn, R. [Inst. fuer Physik, Techn. Universitaet Ilmenau, PF 100565, 98684 Ilmenau (Germany); Inst. fuer Exp. Physik, Universitaet Magdeburg, PF 4120, 39016 Magdeburg (Germany)

2011-08-31

 
 
 
 
21

Band Structure Parameters and Fermi Resonances of Exciton-Polaritons in CsI and CsBr under Hydrostatic Pressure  

Energy Technology Data Exchange (ETDEWEB)

Most alkali halides crystallize in the fcc sodium chloride structure. In contrast, with the exception of CsF, the Cs-halides form the simple cubic cesium chloride (CsCl) structure at ambient conditions and they have a substantially different electronic structure than other alkali halides; in particular, they have several nearly degenerate electronic levels near the Brillouin zone center. Highly resolved Three-Photon Spectroscopy (TPS) measurements allow direct observation of the near band edge structure and, in the case of CsI, probe more states than one-photon techniques. A number of interesting phenomena, among them level repulsion (Fermi resonance), occur as these levels are tuned through one another by application of hydrostatic pressure. To the best of our knowledge, this has been observed for CsBr for the first time. Doubling the photon energy range compared to a previous publication [see Yoo et al. PRL 84, 3875 (2000)] allows direct observation of the n=1, 2 and 3 exciton-polariton members of the {Lambda}{sub 8}{sup -}-{Lambda}{sub 6}{sup +} transition in CsI and lets us establish unambiguous values for the bandgap (6.139 eV), binding energy (0.265 eV) and their pressure dependence up to 7 kbar. Similarly to CsI, the CsBr linewidth of the lowest {Lambda}{sub 4}{sup -} polariton (A) decreases upon compression.

Lipp, M J; Yoo, C H; Strachan, D; Daniels, W B

2005-11-29

22

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

Science.gov (United States)

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

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

2014-01-01

23

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

Science.gov (United States)

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

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

2014-05-13

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

Influence of Coulomb-induced band couplings on linear excitonic absorption spectra of semiconducting carbon nanotubes  

Science.gov (United States)

Starting from the extended Su-Schrieffer-Heeger model, multiband semiconductor Bloch equations are formulated in momentum space and applied to the analysis of the linear optical response of semiconducting carbon nanotubes (SCNTs). This formalism includes the coupling of electron-hole pair excitations between different valence and conduction bands, originating from the electron-hole Coulomb attraction. The influence of these couplings, which are referred to as nondiagonal interband Coulomb interaction (NDI-CI), on the linear excitonic absorption spectra is investigated and discussed for light fields polarized parallel to the tube direction. The results show that the intervalley NDI-CI leads to a significant increase of the band gap and a decrease of the exciton binding energy that results in a blueshift of the lowest-frequency excitonic absorption peak. The strength of these effects depends on the symmetry of the SCNT. Furthermore, for zigzag SCNTs with higher symmetry other nonintervalley NDI-CI terms also affect the spectral positions of excitonic absorption peaks.

Liu, Hong; Schumacher, Stefan; Meier, Torsten

2014-04-01

26

The additional contribution caused by Coulomb interaction to the exciton dispersion in multiple quantum wells and superlattices for direct band gap cubic semiconductors  

International Nuclear Information System (INIS)

The exciton dispersion in multiple quantum wells and superlattices is studied for direct band gap cubic semiconductors in the three-band-model. The Coulomb interaction between the light and heavy exciton states in different wells is calculated. This leads to a direction dependence and split of the exciton dispersion. (author). 21 refs

27

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

28

Energy band structure and Frenkel excitons in PbGa2S4  

Science.gov (United States)

Optical reflection spectra are measured and calculated in PbGa2S4 crystals in the region of resonances related to excitons with large oscillator strength and binding energy (Frenkel excitons). The splitting of the upper valence band in the center of the Brillouin zone due to crystal field (?cf) and spin orbit (?so) interaction are determined. Optical reflection spectra are measured and calculated according to Kramers-Kronig relations in the region of 3-6 eV in ??? and ??? polarizations, and the optical constants n, k, ?1 and ?2 are determined. The observed electronic transitions in PbGa2S4 crystals are discussed in the frame of theoretical energy band structure calculation for thiogallate crystals.

Stamov, I. G.; Syrbu, N. N.; Ursaki, V. V.; Parvan, V. I.; Zalamai, V. V.

2012-11-01

29

Investigation of relations between absorption band positions and crystalline environment in Pb2+-doped alkali halides.  

Science.gov (United States)

The relationships between sp energy levels (A, B and C bands) as well as the charge transfer band (D band) of Pb(2+)-doped alkali halides and the crystalline environment were thoroughly investigated by means of dielectric theory of chemical bonds for complex crystals. It is found that the coordination number of the central ion, the bond volume polarizability, and the fractional covalence of the chemical bond between the central ion and the nearest anion are the major factors influencing the positions of A, B, C and D bands of Pb(2+). Our model has successfully built links between the E(A), E(B), E(C) and E(D) of Pb(2+) and the environmental factor h. Results indicated that the energies of sp levels and the charge transfer band of Pb(2+) all decrease with increase of h. The h has a linear relationship with the sp energy levels, and an exponential relationship with the charge transfer band. The model calculation results are in good agreement with experimental data. The current model can serve as a prediction tool and can be applied to assign and reassign the A, B, C and D band positions of Pb(2+). The model predicts the positions of B, C and D bands of NaF:Pb(2+) at 7.516, 8.688 and 12.796 eV, respectively; the D band of CsCl:Pb(2+), CsBr:Pb(2+) and CsI:Pb(2+) at 6.633, 6.389 and 5.275 eV, respectively. We reassign the C band of Pb(2+) in NaBr as 5.276 eV but not the reported 5.636 eV, which is more reasonable to be ascribed to the charge transfer band. PMID:20379510

Sun, Qiang; Qu, Baohan; Shi, Jinsheng

2010-04-28

30

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.

31

Tunable band gaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons.  

Science.gov (United States)

Doping of semiconductors is essential in modern electronic and photonic devices. While doping is well understood in bulk semiconductors, the advent of carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications raises some key questions about the role and impact of doping at low dimensionality. Here we show that for semiconducting carbon nanotubes, band gaps and exciton binding energies can be dramatically reduced upon experimentally relevant doping, and can be tuned gradually over a broad range of energies in contrast with higher dimensional systems. The latter feature is made possible by a novel mechanism involving strong dynamical screening effects mediated by acoustic plasmons. PMID:20482140

Spataru, Catalin D; Léonard, François

2010-04-30

32

Excitonic spectra and band structure of CdGa2Se4 birefractive crystals  

Science.gov (United States)

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

Syrbu, N. N.; Stamov, I. G.; Parvan, V. I.; Zalamai, V. V.; Tiginyanu, I. M.

2013-11-01

33

Excitonic spectra and band structure of CdGa2Se4 birefractive crystals  

International Nuclear Information System (INIS)

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

34

Evidence for ultrafast exciton localization in the Q sub y band of bacteriochlorophyll a-protein from Prosthecochloris aestuarii  

Energy Technology Data Exchange (ETDEWEB)

Time-dependent linear dichroism was observed for several wavelengths in the Q{sub y} band of the BChl a-protein antenna complex from Prosthecochloris aestuarii, and photobleaching spectra were obtained for evidence of exciton localization. The results indicate that excitation transport in this antenna proceeds largely through incoherent hopping between localized states at times longer than 2 ps.

Lyle, P.A.; Struve, W.S. (Iowa State Univ., Ames (USA))

1990-09-20

35

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

36

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

37

Investigation of the pressure dependence of band gaps for silver halides within a first-principles method  

Science.gov (United States)

The pressure dependence of the direct and indirect band gaps in rocksalt silver halides has been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange-correlation potential. It is found that indirect band gaps (L?X and L??) exhibit different responses to application of pressure. Similar trends are found for the indirect band gaps of AgCl and AgBr while the trend in AgI (L??) band gap is different. In all the compounds, the effect of pressure on the direct band gaps (???, X?X and L?L) show qualitatively similar results. The fundamental indirect band gap (L??) pressure coefficients are -4.19 meV (GPa) -1 and -3.81 meV (GPa) -1 for AgCl and AgBr while for AgI (L?X) it is -61.50 meV (GPa) -1. The band gap pressure coefficient as well as the volume deformation potential for the various band gaps of the compounds have also been investigated.

Okoye, C. M. I.

2004-01-01

38

Thermal redistribution of localized excitons and its effect on the luminescence band in InGaN ternary alloys  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Temperature-dependent photoluminescence measurements have been carried out in zinc-blende InGaN epilayers grown on GaAs substrates by metalorganic vapor-phase epitaxy. An anomalous temperature dependence of the peak position of the luminescence band was observed. Considering thermal activation and the transfer of excitons localized at different potential minima, we employed a model to explain the observed behavior. A good agreement between the theory and the experiment is achieved. At high te...

Xu, Sj; Xie, Mh; Che, Cm; Li, Q.; Cheng, Wc; Tong, Sy; Yang, H.

2001-01-01

39

Fine structure and energy spectrum of exciton in direct band gap cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

The influence of the cubic structure on the energy spectrum of direct exciton is investigated, using the new method suggested by Nguyen Van Hieu and co-workers. Explicit expressions of the exciton energy levels 1S, 2S and 2P are derived. A comparison with the experiments and the other theory is done for ZnSe. (author). 10 refs, 1 fig., 2 tabs

40

Exciton-plasma transition in semiconductors with account of band structure peculiarities  

International Nuclear Information System (INIS)

Multivalley effect of multilength, electron-hole mass ratio and anisotropy of their energy spectrum on critical density of exciton plasma transition is considered. For a homogeneous electron-hole system at T<< Esub(x) established are tends of change in critical density of exciton-plasma transition which criterium is the change in the spectrum character of one-particle excitations with provision for multilength, difference in the effective masses and anisotropy of energy spectra of electrons and holes

 
 
 
 
41

Temperature dependent study of the band edge excitons of ReS2 and ReSe2  

International Nuclear Information System (INIS)

ReS2 and ReSe2 are diamagnetic indirect semiconductors and belong to the family of transition-metal dichalcogenides crystallizing in the distorted octahedral layer structure of triclinic symmetry. We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps Edg in the temperature range between 25 and 525 K using piezoreflectance (PzR). The polarization dependence of the PzR spectra provides conclusive evidence that the features are associated with the interband excitonic transitions from different origins. From a detailed lineshape fit to the PzR spectra, we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated and discussed. (orig.)

42

Temperature dependent study of the band edge excitons of ReS{sub 2} and ReSe{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

ReS{sub 2} and ReSe{sub 2} are diamagnetic indirect semiconductors and belong to the family of transition-metal dichalcogenides crystallizing in the distorted octahedral layer structure of triclinic symmetry. We have measured the temperature dependence of the spectral features in the vicinity of the direct gaps E{sup d}{sub g} in the temperature range between 25 and 525 K using piezoreflectance (PzR). The polarization dependence of the PzR spectra provides conclusive evidence that the features are associated with the interband excitonic transitions from different origins. From a detailed lineshape fit to the PzR spectra, we have been able to determine accurately the temperature dependence of the energies and broadening parameters of the band-edge excitons. The parameters that describe the temperature variation of the transition energies and broadening function have been evaluated and discussed. (orig.) 21 refs.

Huang, Y.S.; Ho, C.H.; Liao, P.C. [National Taiwan Inst. of Technol., Taipei (Taiwan, Province of China). Dept. of Electron. Eng.; Tiong, K.K. [Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan, P. R. of (China)

1997-11-14

43

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

44

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

45

Verification of ?7 symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A exciton state  

International Nuclear Information System (INIS)

Circularly-polarized and angular-resolved magneto-photoluminescence spectroscopy was carried out to study the free A exciton 1S state in wurtzite ZnO at 5 K. The ? 7 symmetry of the top valence band symmetry is confirmed according to the unique selection rules of Zeeman splitting lines. The out-of-plane component B? of the magnetic field, which is parallel to ZnO's c-axis, leads to linear Zeeman splitting of both the dipole-allowed ? 5 exciton state and the weakly allowed ? 1 /? 2 exciton states. The in-plane field B?, which is perpendicular to the c-axis, increases the oscillator strength of the weak ? 1 /? 2 states by forming a mixed exciton state. For the ? 7 symmetry, the lower energy Zeeman splitting peak of the weak ? 1 /? 2 can only be ?+ polarization. (paper)

46

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

47

Excitonic ionizations of the electron centres in caesium iodide crystal and exoemission of electrons  

Energy Technology Data Exchange (ETDEWEB)

In the wide-band-gap alkali halide crystals recombination of defects may result in formation of the excitons, which can ionize an electron F-centre; thus the phenomenon of exoelectron emission takes place. According to this excitonic model, the energy spectrum and mean energy of CsI exoelectrons were attained. The results of theoretical evaluation are compared with experimental and the reasons suggested for the explanation of the observed difference are discussed. The conclusion based on the peculiarities of exoemission from CsI and CsBr crystals, was done.

Galiy, P.V. [Faculty of Electronics, Lviv National University 50 Dragomanov Str., Lviv 79005 (Ukraine); Mel' nyk, O.Ya. [Faculty of Electronics, Lviv National University 50 Dragomanov Str., Lviv 79005 (Ukraine)]. E-mail: moyafis@yahoo.com; Tsvetkova, O.V. [Faculty of Electronics, Lviv National University 50 Dragomanov Str., Lviv 79005 (Ukraine)

2005-04-15

48

Excitonic ionizations of the electron centres in caesium iodide crystal and exoemission of electrons  

International Nuclear Information System (INIS)

In the wide-band-gap alkali halide crystals recombination of defects may result in formation of the excitons, which can ionize an electron F-centre; thus the phenomenon of exoelectron emission takes place. According to this excitonic model, the energy spectrum and mean energy of CsI exoelectrons were attained. The results of theoretical evaluation are compared with experimental and the reasons suggested for the explanation of the observed difference are discussed. The conclusion based on the peculiarities of exoemission from CsI and CsBr crystals, was done

49

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

50

Ultrafast exciton dynamics after Soret- or Q-band excitation of a directly ?,?'-linked bisporphyrin.  

Science.gov (United States)

We report on a comprehensive transient absorption study with ?,?'-linked bis[tetraphenylporphyrinato-zinc(II)] and its corresponding monomer, covering the ultrafast dynamics from femtoseconds up to several microseconds. By exciting these porphyrins either to their first (S(1)) or second (S(2)) electronically excited states and by probing the subsequent dynamics, a multitude of reaction pathways have been identified. In the spectral region associated with the ground-state recovery of the bisporphyrin, transient absorption changes occur within the first few picoseconds, which are ascribable to excitonic interaction both in the S(2) (fs time-domain) and in the S(1) (ps time-domain) state. This is substantiated by complementary experiments with the monomeric porphyrin, in which the S(2) state exhibits a longer lifetime. In contrast to the picosecond dynamics the bisporphyrin and the monomer behave similarly on the nanosecond time-scale, that is nearly all excited molecules eventually reach a long-lived triplet excited state. PMID:22549252

Kullmann, Martin; Hipke, Arthur; Nuernberger, Patrick; Bruhn, Torsten; Götz, Daniel C G; Sekita, Michael; Guldi, Dirk Michael; Bringmann, Gerhard; Brixner, Tobias

2012-06-14

51

Interlayer-exciton mediated three-hole-Auger-decay in the ?*-band of highly oriented pyrolytic graphite  

Science.gov (United States)

Resonant photoemission spectroscopy (resPES) is used to probe the occupied ?- and unoccupied ?*-bands of carbon thin films with particular focus on the Auger decay. Highly Oriented Pyrolytic Graphite (HOPG) is studied at the C1s edge. We find strong resonant features at 285.5 eV and 292 eV in the resPES diagram. The normal two-hole Auger decay proceeds under constant kinetic energy (45°) only in the ?*-region. In the ?*-region, however, it proceeds under 67.5° in terms of a Ebind(??) diagram. We attribute this to a multiple Auger decay with a net three hole final state. For this novel decay process we propose a model. We discuss the long lifetime of the first resonant excitation and conclude that it arises from the strong excitonic character of the first resonant state. We use HOPG as a reference system and suggest that this novel process is a tool to identify interlayer-substrate interaction of the carbon layers involved.

Richter, Matthias; Friedrich, Daniel; Schmeißer, Dieter

2014-02-01

52

Exciton-exciton interaction in CdS  

International Nuclear Information System (INIS)

The exciton-exciton interaction in the indium-doped and undoped CdS platelet monocrystals (thickness 20 to 50 ?) has been studied in a temperature range of 8deg K to 110deg K under excitation levels g approximately 1024 to 1028 cm e-h pairs cm-3 sec-1. Luminescence studies show that at g >= 1026 e-h pairs cm-3 sec-1, the exciton-exciton interaction is the dominant channel of radiative annihilation of excitons (P-band). (M.G.B.)

53

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

International Nuclear Information System (INIS)

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

54

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".

Vadim, Lyssenko; Hvam, JØrn Märcher

2000-01-01

55

Polaron theory for description of electron excitations auto-localization peculiarity in alkali-halide crystals  

International Nuclear Information System (INIS)

In continual model for autolocalized excitons in low symmetry conditions the potential barrier height for excitons autolocalization of KI, CsI and KBr has been determined. Analytical expression for exciton full energy functional minimized by asymmetric oscillator wave function related to elastic uniaxial stress case of alkali halide crystal is received. In the work values of polaron effective mass for alkali halide crystal obtained with help continual integration are presented

56

Temperature-dependent band gap, interband transitions, and exciton formation in transparent p-type delafossite CuCr1-xMgxO2 films  

Science.gov (United States)

Temperature-dependent interband transitions and exciton excitations of sol-gel derived CuCr1-xMgxO2 (2%?x?8%) films have been investigated by transmittance spectra (8-300 K) and photoluminescence (PL) spectra (77-300 K). An abnormal dependence of the optical band gap with the temperature has been found for the films with x =0.02 and 0.04. At the low-temperature region, the gap energy shows a redshift trend with decreasing temperature. It is due to the strong Cr 3d-O 2p-Cu 3d interaction in the upper part of the valence band, which can be weaken by heavily Mg doping. The spin-orbit interactions of Cr3+ ions in an octahedral environment make the 3d states more disperse, which can contribute to the relatively high conductivity. A well-defined low-energy absorption has been assigned to the spin-allowed 3d ? 3d transition. Moreover, a strong exciton excitation around 1.8 eV has been observed due to the naturally low-dimensional structure of the delafossite, which can be modulated by temperature and hole concentration.

Li, Xurui; Han, Meijie; Zhang, Xiaolong; Shan, Chao; Hu, Zhigao; Zhu, Ziqiang; Chu, Junhao

2014-07-01

57

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

58

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

International Nuclear Information System (INIS)

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGaxSb1-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/InGaxSb1-x dots.

59

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

60

Band offsets and excitonic binding energies in In'xGa1-xP/GaAs heterostructures by photoluminescence spectroscopy  

International Nuclear Information System (INIS)

In this paper we report on the optical properties of disordered InxGa1-x P/GaAs (x = 0.54, 0.48, and 0.43) heterostructures grown by atomic layer molecular beam epitaxy (ALMBE). The samples, containing seven quantum wells (QWs), were investigated by means of photoluminescence (PL) and photoluminescence excitation (PLE) along with a mathematical model developed into the framework of a fractional dimensional space. A type I band offset has been deduced for all the investigated samples. For the sample with x = 0.54 and 0.48 (barrier lattice matched to GaAs) a band offset ratio about Ec /Ev = 20/80 has been estimated, whereas for the sample with x = 0.43 a value of ?Ev /?Ev = 30/70 was found. This means values of ?Ec = 0.155 eV and ?Ev = 0.360 for the sample with x = 0.43, and ?Ec = 0.085 eV and ?Ev = 0.340 for the sample with x = 0.54. These results point out the different degree of confinement for electrons and holes. We have also observed an increasing of the exciton binding energy (Eb) when the QW thickness (Lw) is reduced. The highest obtained values for Eb were 13.6 meV (x = 0.43), 11.3 meV (x = 0.48), and 10.7 meV (x = 0.54) for Lw ranging between 2 and 3 nm. The minimum values obtained for the exciton Bohr radius and the fractional dimension were 5.5 nm and 2.5, respectively, corresponding to a QW with Lw = 2.2 nm for the sample with x = 0.43. The most important consequences of all these results will be extensivelyll these results will be extensively discussed. (Full text)

 
 
 
 
61

Mn2+ and band exciton luminescence in ZnMnTe/ZnMgTe quantum well structures  

International Nuclear Information System (INIS)

The effect of excitation level on the luminescence of ZnMnTe/ZnMgTe quantum-well (QW) structures has been studied. We find that result depend strongly on the QW width and manganese concentration. The increasing optical pumping implies a degradation of the Mn2+ intracenter luminescence (IL) due to the interaction of the strongly excited Mn2+ system with the high density of excitons. The spectral and kinetic properties of the IL reveal the contribution of the Mn2+ ions located on the interface and inside the QW as well as the Mn2+ IL decay dependence on the QW width. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

62

Exciton characteristics and exciton luminescence of silicon quantum dot structures  

International Nuclear Information System (INIS)

The exciton binding energy, the energies of the basic radiative exciton transition, and the zerophonon radiative lifetime of excitons in silicon quantum dots embedded in the SiOx matrix are calculated in effective mass approximation with quadratic dispersion relation. In addition, the spectra of steady-state photoluminescence and of time-resolved photoluminescence of excitons in the silicon quantum dots are calculated, and the kinetics of the photoluminescence relaxation is considered. The theory is compared with the experiment. It is shown that, for nanostructures involving silicon quantum dots with diameters smaller than 4 nm, the governing factor in the broadening of the spectral photoluminescence bands is the effect of mesoscopic quantum fluctuations. In this case, either an even one dangling bond at the interface, or one intrinsic point defect, or one foreign atom located inside the small-sized nanocrystallite or in its close surroundings produces a pronounced effect on the energy of the exciton transition

63

Seven-fold exciton splitting of the 810-nm band in bacteriochlorophyll a-proteins from green photosynthetic bacteria  

Energy Technology Data Exchange (ETDEWEB)

Reseachers report comparative absorbance and fourth derivative absorbance spectra of two different bacteriochlorophyll a-proteins at 5 K in each of two different cryogenic solvent mixtures. In previous studies at 5 K each protein was observed in only one of these mixtures (not the same one). For the protein from Prosthecochloris aestuarii strain 2K, whose structure is known, the solvent effect is relatively small; for the protein from Chlorobium limicola f. sp. thiosulfatophilum strain 6230 (Tassajara), the effect is much more pronounced. From these results together with earlier results at 300 K, we conclude there may be slight conformational differences of the Prosthecochloris protein between the crystalline form used for x-ray diffraction studies and that in a cryogenic solvent. By comparing spectral features of the two proteins in the same solvent, we are able for the first time to assign all seven of the expected exciton levels in each protein. These occur at 793, 801, 806, 810, 814, 819, and 825 nm in the Prosthecochloris protein, and at 793, 802, 806, 810, 816, 820, and 823 nm in the Chlorobium protein.

Whitten, W.B. (Oak Ridge National Lab., TN); Olson, J.M.; Pearlstein, R.M.

1980-01-01

64

Seven-fold exciton splitting of the 810-nm band in bacteriochlorophyll A-proteins from green photosynthetic bacteria.  

Science.gov (United States)

We report comparative absorbance and fourth derivative absorbance spectra of two different bacteriochlorophyll a-proteins at 5 K in each of two different cryogenic solvent mixtures. In previous studies at 5 K each protein was observed in only one of these mixtures (not the same one). For the protein from Prosthecochloris aestuarii strain 2K, whose structure is known, the solvent effect is relatively small; for the protein from Chlorobium limicola f. sp. thiosulfatophilum strain 6230 (Tassajara), the effect is much more pronounced. From these results together with earlier results at 300 K, we conclude there may be slight conformational differences of the Prosthecochloris protein between the crystalline form used for X-ray diffraction studies and that in a cryogenic solvent. By comparing spectral features of the two proteins in the same solvent, we are able for the first time to assign all seven of the expected exciton levels in each protein. These occur at 793, 801, 806, 810, 814, 819, and 825 nm in the Prosthecochloris protein, and at 793, 802, 806, 810, 816, 820, and 823 nm in the Chlorobium protein. PMID:7388015

Whitten, W B; Olson, J M; Pearlstein, R M

1980-06-10

65

Luminescence excitation by VUV photons in alkali and silver halides  

International Nuclear Information System (INIS)

Excitation spectra of luminescence quantum yield in alkali and silver halides are measured at low temperatures using synchrotron radiation for the energy range up to 30eV. The yields at the valence exciton excitation are low (0.1 at most) in alkali halides (KCl, KBr and NaBr), but high (0.4) in AgCl. Enhancement of the yield indicative of multiple exciton production is observed at higher energies, and the yields in NaBr and AgCl exceed unity in the energy range studied. Discussion is given for the competition between radiative and non-radiative decay channels during relaxation of excited states. (author)

66

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

67

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

68

Bright and dark excitons in semiconductor carbon nanotubes  

International Nuclear Information System (INIS)

citonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large (?0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low luminescence efficiency in nanotubes.

69

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

70

Auger-free luminescence due to interatomic p-d transition and self-trapped exciton luminescence in Rb{sub 2}ZnCl{sub 4} crystals  

Energy Technology Data Exchange (ETDEWEB)

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb{sub 2}ZnCl{sub 4}. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p-d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton. - Highlights: Black-Right-Pointing-Pointer Studied Auger-Free (AF) and self-trapped exciton (STE) luminescence in Rb{sub 2}ZnCl{sub 4}. Black-Right-Pointing-Pointer Found AF luminescence due to interatomic p-d transitions in halide crystals for the first time. Black-Right-Pointing-Pointer Clarified the STE luminescence in this crystal for the first time.

Ohnishi, Akimasa, E-mail: ohnishi@sci.kj.yamagata-u.ac.jp [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan); Saito, Miki; Kitaura, Mamoru [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan); Itoh, Minoru [Department of Electrical and Electronic Engineering, Shinshu University, Nagano 380-8553 (Japan); Sasaki, Minoru [Department of Physics, Yamagata University, Yamagata 990-8560 (Japan)

2012-10-15

71

Absorption and magneto-optical properties of the 3A/sub 2g/? 1T/sub 2g//sup a/ transition in CsNiF3. II. Pure-exciton and hot-magnon bands in a one-dimensional ferromagnet  

International Nuclear Information System (INIS)

Absorption spectra associated with the lowest-energy region of the spin-forbidden transition 3A/sub 2g/? 1T/sub 2g//sup a/ in CsNiF3 are investigated in the one-dimensional ferromagnetic phase in magnetic fields up to 4.5 T. Pure-exciton bands are found to have a linear blue shift with a field applied in the easy plane, whereas the hot-magnon band shifts slightly to lower energies. When the field is increased the intensity of the exciton bands increases and the absorption due to the hot-magnon band decreases. An anomalous behavior of the band intensities and the half-widths is observed in fields below 1 T. The structure of the exciton spectra and their field dependence are discussed by using an energy-level scheme obtained by a single-ion approximation. The properties of the hot-magnon sideband are explained by the exciton creation accompanied by the annihilation of a thermally excited magnon at the Brillouin-zone edge. The anomalies of the exciton and the hot-magnon bands observed in low fields may be understood by the reduction of spin fluctuations in a magnetic field applied in the easy plane of magnetization

72

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

73

Excitons in narrow-gap carbon nanotubes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

74

Bright and dark excitons in semiconductor carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

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

Tretiak, Sergei [Los Alamos National Laboratory

2008-01-01

75

Core excitons and vibronic coupling in diamond and graphite  

International Nuclear Information System (INIS)

The C 1s exciton in diamond is studied by soft x-ray emission excited with high resolution monochromatic photons. A strong phonon sideband of 5 eV wide is observed in the exciton recombination spectrum. With the phonon contribution, the exciton binding energy is estimated to be 1.5 eV. The excited atoms is suggested to undergo Jahn-Teller relaxation similar to that of the nitrogen impurities in diamond. A similar exciton is observed at the ?*-band threshold in graphite. These results provide new insights to the understanding of the core exciton and the donor problem in diamond and other systems

76

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

77

Biexciton excitation profile for a semiconductor with intermediately bound excitons  

Energy Technology Data Exchange (ETDEWEB)

A method is presented for calculating the biexciton excitation profile for a semiconductor at low temperatures. The model consists of two levels (the valence and conduction bands) for cubium, with periodic boundary condition. The exciton which is discussed is intermediate between the Frenkel and Wannier type excitons. Using a formalism which is similar to Gorkov's theory of superconductivity, it is shown that excitons may interact to form charge transfer bound states.

Gumbs, G. (National Research Council of Canada, Ottawa, Ontario. Div. of Chemistry)

78

Combined characterization of composite tabular silver halide microcrystals by cryo-EFTEM/EELS and cryo-STEM/EDX techniques.  

Science.gov (United States)

The combination of cryo-energy filtering transmission electron microscopy (EFTEM)/electron spectroscopic diffraction (ESD)/electron energy-loss spectroscopy (EELS) and cryo-energy-dispersive X-ray (EDX) analysis in the scanning transmission (STEM) and scanning (SEM) modes was applied for the characterization of composite tabular Ag(Br,I) microcrystals. A low-loss fine structure in EEL spectra between 4 and 26 eV was attributed to excitons and plasmons possibly superimposed with interband transitions and many-electron effects. The contrast tuning under the energy-filtering in the low-loss region was used to image the crystal morphology, defect structure (random dislocations and ¿111¿ stacking faults) and bend and edge contours as well as electron excitations in the microcrystals. Sharp extra reflections at commensurate positions in between the main Bragg reflections and diffuse honeycomb contours in ESD patterns of the microcrystals taken near the [111] zone were assigned to the number of defects in the shell region parallel to the grain edges and polyhedral clusters of interstitial silver cations, respectively. The imaginary part of the energy-loss function, Im (-1/epsilon), and the real and imaginary parts, epsilon1 and epsilon2, of the dielectric permittivity were determined by means of a Kramers-Kronig analysis. An assignment of exciton peaks based on calculations of electronic band structure of silver bromide is proposed. Inner-shell excitation bands of silver halide were detected in line with EDX-analyses. The energy-loss near-edge structure (ELNES) of the AgM4,5-edge governed by spin-orbital splitting between the 3d3/2- and 3d5/2-states has been evaluated. Combined silver and halide distributions were obtained by a three-window method (EFTEM) and by EDX/STEM including area mapping and line profiling of iodide. PMID:9728882

Oleshko, V P; Gijbels, R H; Van Daele, A J; Jacob, W A; Xu, Y E; Wang, S E; Park, I Y; Kang, T S

1998-07-15

79

Exciton circular dichroism in channelrhodopsin.  

Science.gov (United States)

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

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

2014-10-16

80

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.)

 
 
 
 
81

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)

82

Exciton dynamics in disordered poly(p-phenylenevinylene). 2. Exciton diffusion.  

Science.gov (United States)

We present a first principles theory of exciton diffusion in conformationally disordered conjugated polymers. Central to our theory is that exciton transfer occurs from vibrationally relaxed states (VRSs) to local exciton ground states (LEGSs). LEGSs are determined by the diagonal and off-diagonal disorder induced by static density and torsional fluctuations, and VRSs are further localized by exciton-phonon coupling. The theory is implemented using the Frenkel-Holstein model to calculate the wave functions and energies of the LEGSs and VRSs. The coupling of VRSs and LEGSs via long-range dipole-dipole interactions leads to the familiar line-dipole approximation for the exciton transfer integral. The exciton transfer rates are derived from the Fermi Golden rule. The theory is applied to an ensemble of conformationally disordered poly(p-phenylenevinylene) chains using a kinetic Monte Carlo algorithm. The following are shown: (i) Torsional disorder and trans-cis defects reduce the exciton diffusion length. (ii) Radiative recombination occurs from VRSs in the tail of their density of states. (iii) Torsional disorder increases the band gap, the line width of the density of states, and the Stokes shift. As a consequence, it causes a blue shift in the vertical absorption, but a red shift in the emission. (iv) The energy of the radiated photon decreases as -log t, with a gradient that increases with torsional disorder. The predicted exciton diffusion lengths of ~8-11 nm are in good agreement with experimental values. PMID:23030335

Barford, William; Bittner, Eric R; Ward, Alec

2012-10-25

83

Exciton energies of wurtzite CdS nanoparticles  

Energy Technology Data Exchange (ETDEWEB)

We have determined exciton energies for wurtzite CdS nanoparticles, both theoretically and experimentally. The empirical pseudopotential method has been used to calculate the bulk band structure. The discretization of reciprocal space was considered to get the energy gap and the corresponding exciton energy as a function of the nanoparticles size. The CdS nanoparticles were prepared by colloidal methods and the exciton energies were determined from optical absorption measurements. A good agreement between the calculated and the experimental exciton energies is obtained when an average over the experimental size distribution of the nanoparticles is included in the calculation.

Bautista-Hernandez, A.; Loaiza-Gonzalez, G.; Meza-Montes, L.; Pal, U. [Instituto de Fisica, Universidad Autonoma de Puebla, Apdo. Postal J-48, Puebla, Pue. 72570 (Mexico)

2003-09-30

84

Bandwidth of excitons in LH2 bacterial antenna chromoproteins  

Science.gov (United States)

The bandwidth of the exciton manifold in LH2 peripheral antenna complexes from the photosynthetic bacterium Rhodobacter sphaeroides has been determined using polarized fluorescence excitation spectroscopy and model simulations. It is shown that the fluorescence anisotropy spectra reveal a hidden structure of the exciton band that is correlated with its boundaries. The estimated exciton coupling energy and exciton bandwidth in the native complex is ˜360 and ˜1620 cm -1, respectively. The corresponding numbers in a mutant with the B800 bacteriochlorophyll molecules absent are somewhat larger (˜420 and ˜1890 cm -1), probably due to tightening of the protein structure.

Timpmann, Kõu; Trinkunas, Gediminas; Olsen, John D.; Neil Hunter, C.; Freiberg, Arvi

2004-11-01

85

Relaxation of excitons in semimagnetic asymmetric double quantum wells  

International Nuclear Information System (INIS)

The steady-state circular-polarized photoluminescence in semimagnetic asymmetric double quantum wells based on Cd(Mn,Mg)Te is studied thoroughly in relation to the polarization of intrawell nonresonance photoexcitation in magnetic fields Bup to 9 T. In low fields B, in which the exciton in the magnetic well is higher in energy than the exciton in the nonmagnetic well, the complete interwell relaxation of excitons is observed. In fields higher than Bc = 3-6 T, at which the exciton level in the magnetic well crosses the field-independent exciton level in the nonmagnetic well, the magnetic-field-induced red shift of the exciton in the magnetic well is accompanied by the establishment of a nonequilibrium distribution of excitons. This suggests that spin relaxation plays an important part in the interwell separation of excitons in the spin-dependent potential of the heterostructure. The efficiency of spin relaxation is controlled by mixing of valence band states in the nonmagnetic well and by splitting of heavy and light holes ?hh-lh. Different modes of interwell tunneling are observed in different field regions separated by the field Bc* > Bc corresponding to the crossing of the localized excitons in the nonmagnetic well and free excitons in the magnetic well. Possible mechanisms of interwell tunnel relaxation are discussed.

86

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

87

Calculation of exciton wave functions in electric fields  

Energy Technology Data Exchange (ETDEWEB)

We describe a numerical method to compute exciton wave functions under the influence of an applied electric field. We present results for {alpha}-Gallium Nitride fully taking into account the complex 6 x 6 valence-band structure.

Lenk, S; Runge, E, E-mail: steve.lenk@tu-ilmenau.d [Institut fuer Physik and Institut fuer Mikro- und Nanotechnologien, Technische Universitaet Ilmenau, 98693 Ilmenau (Germany)

2010-02-01

88

Calculation of exciton wave functions in electric fields  

International Nuclear Information System (INIS)

We describe a numerical method to compute exciton wave functions under the influence of an applied electric field. We present results for ?-Gallium Nitride fully taking into account the complex 6 x 6 valence-band structure.

89

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

2014-11-12

90

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

91

Strong room-temperature ultraviolet to red excitons from inorganic organic-layered perovskites, (MX4 (M=Pb, Sn, Hg; X=I-, Br-)  

Science.gov (United States)

Many varieties of layered inorganic-organic (IO) perovskite of type (MX4 (where R: organic moiety, M: divalent metal, and X: halogen) were successfully fabricated and characterized. X-ray diffraction data suggest that these inorganic and organic structures are alternatively stacked up along c-axis, where inorganic mono layers are of extended corner-shared MX6 octahedra and organic spacers are the bi-layers of organic entities. These layered perovskites show unusual room-temperature exciton absorption and photoluminescence due to the quantum and dielectric confinement-induced enhancement in the exciton binding energies. A wide spectral range of optical exciton tunability (350 to 600 nm) was observed experimentally from systematic compositional variation in (i) divalent metal ions (M=Pb, Sn, Hg), (ii) halides (X=I and Br-), and (iii) organic moieties (R). Specific photoluminescence features are due to the structure of the extended MX42- network and the eventual electronic band structure. The compositionally dependent photoluminescence of these IO hybrids could be useful in various photonic and optoelectronic devices.

Ahmad, Shahab; Prakash, G. Vijaya

2014-01-01

92

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.)

93

Exciton Dynamics in soluble Poly(p-phenylene-vinylene): Towards an Ultrafast Excitonic Switch  

International Nuclear Information System (INIS)

We have applied a variety of ps transient and cw optical techniques to elucidate the dynamics, absorption, and emission properties of excitons in soluble derivatives of poly(p-phenylene vinylene) neat films and dilute solutions. We found that the photogenerated singlet excitons in both films and solutions are characterized by strong stimulated emission and two photoinduced absorption bands. We demonstrate that these bands can be used to form an ultrafast optical switch in the near IR spectral range with variable switching times. copyright 1997 The American Physical Society

94

Excitonic dynamical Franz-Keldysh effect.  

DEFF Research Database (Denmark)

The dynamical Franz-Keldysh effect is exposed by exploring near-band-gap absorption in the presence of intense THz electric fields. It bridges the gap between the de Franz-Keldysh effect and multiphoton absorption and competes with the THz ac Stark effect in shifting the energy of the excitonic resonance. A theoretical model which includes the strong THz field nonperturbatively via a nonequilibrium Green functions technique is able to describe the dynamical Franz-Keldysh effect in the presence of excitonic absorption.

Johnsen, Kristinn; Jauho, Antti-Pekka

1998-01-01

95

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

96

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

97

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)

98

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

99

Optical properties of halide and oxide scintillators  

Science.gov (United States)

Knowledge of optical properties of scintillators is important both for the optimization of scintillator systems and the development of new materials particularly ceramic scintillators. Recent theoretical developments, especially new density functionals that enable accurate prediction of band gaps, have made it possible to perform quantitative calculations of the optical properties of scintillator materials. We used these techniques to obtain optical properties of a large number of high light output halide scintillators. These calculations showed that many halide scintillators have remarkably little optical anisotropy and may be good candidates for development as ceramic scintillators. These include materials such as CaI2:Eu2+ that have very high light output and other favorable properties but are difficult to develop due to crystal growth issues. We review some of our recent results and present new results for CeCl3.

Singh, David J.

2011-09-01

100

Optical properties of halide and oxide scintillators  

Energy Technology Data Exchange (ETDEWEB)

Knowledge of optical properties of scintillators is important both for the optimization of scintillator systems and the development of new materials particularly ceramic scintillators. Recent theoretical developments, especially new density functionals that enable accurate prediction of band gaps, have made it possible to perform quantitative calculations of the optical properties of scintillator materials. We used these techniques to obtain optical properties of a large number of high light output halide scintillators. These calculations showed that many halide scintillators have remarkably little optical anisotropy and may be good candidates for development as ceramic scintillators. These include materials such as CaI{sub 2}:Eu{sup 2+} that have very high light output and other favorable properties but are difficult to develop due to crystal growth issues. We review some of our recent results and present new results for CeCl{sub 3}.

Singh, David J [ORNL

2011-01-01

 
 
 
 
101

Excitons on a glome  

CERN Document Server

We introduce a new model to study excitons confined in a three-dimensional space, which provides a unified treatment of Frenkel and Wannier-Mott excitons. The model consists of an exciton trapped on the 3-sphere (i.e. the surface of a four-dimensional ball), sometimes known as a glome. We show that one can determine, for particular values of the dielectric constant $\\epsilon$, the closed-form expression of the exact wave function. We use the exact wave function of the lowest bound state for $\\epsilon=2$ to introduce an intermediate regime which gives satisfactory agreement with exact diagonalization results for a wide range of $\\epsilon$ values.

Loos, Pierre-François

2011-01-01

102

Multiple exciton generation in colloidal silicon nanocrystals.  

Science.gov (United States)

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity. PMID:17645368

Beard, Matthew C; Knutsen, Kelly P; Yu, Pingrong; Luther, Joseph M; Song, Qing; Metzger, Wyatt K; Ellingson, Randy J; Nozik, Arthur J

2007-08-01

103

Effect of subband coupling on exciton binding energies and oscillator strengths in GaAs-Ga1-xAlxAs quantum wells  

International Nuclear Information System (INIS)

The authors calculate exciton binding energies and oscillator strengths in GaAs-GaAlAs quantum wells, starting from the subband structure which contains valence band mixing. The effect of Coulomb coupling between excitons belonging to different subbands is calculated and is shown to be important: the strongest interaction is with the exciton continuum. Calculated binding energies agree within 1meV with photoluminescence excitation experiments. It is shown that parity forbidden excitons have zero oscillator strength even when valence band mixing is included. Therefore, some transitions reported in the literature cannot be ground-state excitons, and are reinterpreted here as excitons in excited states

104

Excitonic effects in oxyhalide scintillating host compounds  

Science.gov (United States)

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.; Valsakumar, M. C.

2014-10-01

105

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

106

Phonon-assisted two-photon exciton transitions in semiconductors  

International Nuclear Information System (INIS)

The theory of phonon-assisted two-photon transitions to excitonic states in semiconductors has been theoretically investigated. The effects of both the nonparabolicity of the band and the degeneracy of the valence band have been taken into account. Expressions for the absorption coefficient through different band models are calculated. The numerical applications to CdI2 and GaP show that the 4-band model gives the dominant contribution which leads to a final s-exciton state. An exciton peak appears at an energy which is close to that recently observed in CdI2. The non-parabolic effect enhances the absorption coefficient by a two-order of magnitude. (author). 6 refs, 1 fig., 1 tab

107

The oscillator strength of extended exciton states and possibility for very fast scintillators  

Science.gov (United States)

Streak camera measurements of luminescence from ZnO single crystals as a function of temperature show that the spontaneous radiative lifetime of the "free exciton" band at 380 nm is 440 ps at room temperature, decreasing to 290 ps at 85 K. The donor-bound exciton lifetime is 50 ps from 85 K down to 16 K, the lowest temperature we measured. This is significantly faster than is consistent with the dipole matrix element of any single atom transition scaled to the same wavelength. We discuss the oscillator strength of extended exciton states based on the theory of defect-bound excitons, and examine its limit for free excitons. In that regard, we ask what physically limits the coherent dipole size and how a material might be engineered for "giant" exciton oscillator strength. The phenomenon applies to single excitons, not just coherently produced multiple excitations.

Wilkinson, John; Ucer, K. B.; Williams, R. T.

2005-01-01

108

The oscillator strength of extended exciton states and possibility for very fast scintillators  

International Nuclear Information System (INIS)

Streak camera measurements of luminescence from ZnO single crystals as a function of temperature show that the spontaneous radiative lifetime of the 'free exciton' band at 380 nm is 440 ps at room temperature, decreasing to 290 ps at 85 K. The donor-bound exciton lifetime is 50 ps from 85 K down to 16 K, the lowest temperature we measured. This is significantly faster than is consistent with the dipole matrix element of any single atom transition scaled to the same wavelength. We discuss the oscillator strength of extended exciton states based on the theory of defect-bound excitons, and examine its limit for free excitons. In that regard, we ask what physically limits the coherent dipole size and how a material might be engineered for 'giant' exciton oscillator strength. The phenomenon applies to single excitons, not just coherently produced multiple excitations

109

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

Scientific Electronic Library Online (English)

Full Text Available SciELO Mexico | Language: English Abstract in spanish 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.

110

Dynamical process of exciton-exciton scattering in CuI thin films  

International Nuclear Information System (INIS)

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

111

Excitons in motion in II-VI semiconductors  

Energy Technology Data Exchange (ETDEWEB)

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

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

2010-06-15

112

Multiscale photosynthetic exciton transfer  

CERN Document Server

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

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

2012-01-01

113

Exciton emissions in alkali cyanides  

International Nuclear Information System (INIS)

The emissions of Alkali Cyanides X irradiated at low temperature were measured. In addition to the molecular (Frenkel Type) exciton emissions, another emitting centre was found and tentatively assigned to a charge transfer self trapped exciton. The nature of the molecular exciton emitting state is discussed. (Author)

114

Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy  

Science.gov (United States)

The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ? 570?meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs. PMID:25318849

Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

2014-01-01

115

On the interpretation of luminescence of lead halide crystals  

Energy Technology Data Exchange (ETDEWEB)

Emission and excitation spectra, and the luminescence decay kinetics have been studied for PbCl{sub 2} crystals at the temperatures 0.45-100 K. It has been found that even at the lowest temperatures, where the non-radiative decay of exciton states in PbCl{sub 2} does not take place, the decay times of both the ultraviolet (UV) and the blue (B) exciton emission are by a few orders of magnitude shorter than the decay times of the emission arising from the triplet relaxed excited state of Pb{sup 2+} centres in lead-doped ionic crystals. A conclusion has been drawn that the luminescence of lead halides cannot be connected with single Pb{sup 2+} ions. It is proposed that the radiative decay of the self-trapped excitons of the type of {l_brace}(Pb{sub 2}){sup 3+} + hole{r_brace} is responsible for the UV and the B emission of PbCl{sub 2} crystals, while the lowest-energy (BG) emission arises from the tunnelling recombinations between the (Pb{sub 2}){sup 3+}-type self-trapped electrons and V{sub K}-type self-trapped holes. (orig.)

Babin, V.; Krasnikov, A.; Stolovits, A.; Zazubovich, S. [Tartu Univ. (Estonia). Inst. of Physics; Nikl, M. [ASCR, Prague (Czech Republic). Inst. of Physics

2002-02-01

116

Determination of the Exciton Binding Energy in CdSe Quantum Dots  

Energy Technology Data Exchange (ETDEWEB)

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

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

2009-10-27

117

Polarisation dependence of the free discrete exciton luminescence in ZnO microwires  

Energy Technology Data Exchange (ETDEWEB)

We report on the polarisation dependence of the free discrete exciton luminescence recorded from unstrained single-crystal ZnO microwires at temperatures between 10 K and room temperature. The threefold split topmost valence band in ZnO gives rise to the formation of three excitons (labeled A, B, C for increasing energy) with different energies and different selection rules for the coupling to light polarised parallel (pc) or perpendicular (sc) to the ZnO optical axis, specified by the symmetry of the valence bands. The sign of the spin-orbit and crystal field interaction and therefore the symmetry order of the three valence bands for ZnO is still a subject under debate. For our samples, at temperatures larger than 180 K, the emission from the A- and B-excitons was found to dominate the photoluminescence spectra for polarisation sc, while the C-exciton is more pronounced in the spectra for polarisation pc. This finding is concordant with predictions made for a valence band order {gamma}{sub 7}-{gamma}{sub 9}-{gamma}{sub 7}. At very low temperatures, no luminescence of the C-exciton for any polarisation could be detected, but the luminescence from the A- and B-excitons is suppressed for the polarisation pc. As the intensity of the C-exciton luminescence increases with increasing temperature, we attribute this finding to be caused in the temperature dependence of the occupation function of the exciton states.

Schmidt-Grund, Ruediger; Hilmer, Helena; Czekalla, Christian; Cao, Bingqjang; Sturm, Chris; Grundmann, Marius [Universitaet Leipzig (Germany). Fakultaet fuer Physik und Geowissenschaften

2008-07-01

118

Triplet exciton dynamics  

International Nuclear Information System (INIS)

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

119

Exciton-polariton condensates  

Science.gov (United States)

Recently a new type of system exhibiting spontaneous coherence has emerged--the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The polaritons have a lifetime that is typically comparable to or shorter than thermalization times, giving them an inherently non-equilibrium nature. Nevertheless, they exhibit many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions as to what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus on several physical phenomena exhibited by exciton-polariton condensates. In particular, we examine topics such as the difference between a polariton BEC, a polariton laser and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, and Berezinskii-Kosterlitz-Thouless and Bardeen-Cooper-Schrieffer physics. We also discuss the physics and applications of engineered polariton structures.

Byrnes, Tim; Kim, Na Young; Yamamoto, Yoshihisa

2014-11-01

120

Impurity and free excitons in zinc diarsenide  

Energy Technology Data Exchange (ETDEWEB)

The authors detect a resolved series of the free exciton in single crystals of ZnAs/sub 2/ with Eexclamationexclamationc and very close to it in energy a forbidden series with E exclamation c, which practically negates the effect of the crystal field on the structure of the upper valence band of ZnAs/sub 2/. In the reflectance and transmission spectra of ZnAs/sub 2/, structures are observed due to impurity excitons with extremely high oscillator strengths. The variability of the structures from sample to sample, as well as the different values of their displacement and the character of the decrease in intensity with rise in temperature, indicates their different nature.

Sobolev, V.V.; Kozlov, A.I.; Marenkin, S.F.; Sokolovskii, K.A.

1986-01-01

 
 
 
 
121

Towards Bose-Einstein condensation of excitons in an asymmetric multi-quantum state magnetic lattice  

CERN Document Server

An asymmetric multi-quantum state magnetic lattice is proposed to host excitons formed in a quantum degenerate gas of ultracold fermionic atoms to simulate Bose-Einstein condensation (BEC) of excitons. A Quasi-two dimensional degenerate gas of excitons can be collected in the in-plane asymmetric magnetic bands created at the surface of the proposed magnetic lattice, where the ultracold fermions simulate separately direct and indirect confined electronhole pairs (spin up fermions-spin down fermions) rising to the statistically degenerate Bose gas and eventually through controlled tunnelling to BEC of excitons. The confinement of the coupled magnetic quantum well (CMQWs) system may significantly improve the condition for long lived exciton BEC. The exciton BEC, formed in CMQWs can be regarded as a suitable host for the multi-qubits (multipartite) systems to be used in quantum information processors.

Abdelrahman, A; Alameh, K; Hannaford, P; Ham, Byoung S; Lee, Yong-Tak

2009-01-01

122

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

123

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

124

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

125

Excitonic dielectric function of hexagonal GaN  

Energy Technology Data Exchange (ETDEWEB)

We calculate the dielectric function of hexagonal GaN including the A-, B-, and C-excitons using a multi-valence band formalism. The importance of excitons for the interpretation of reflectance spectroscopy of GaN was emphasized by several experimental groups, but only recently theoretical calculations were presented. We derive the dielectric function from a numerical solution of an initial value problem via an exponential split-operator method, taking into account the full 6 x 6 valence band structures of several parametrizations. We present the complex dielectric function as well as the deduced reflectivity spectra of the excitons in GaN. These results show qualitative and quantitative agreement with recent experimental studies.

Lenk, Steve; Runge, Erich [Institut fuer Physik und Institut fuer Mikro- und Nanotechnologien, Technische Universitaet Ilmenau (Germany)

2010-07-01

126

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

127

Jahn-Teller effect on exciton states in hexagonal boron nitride single crystal  

Science.gov (United States)

Optical properties near the band edge of hexagonal boron nitride were studied at 8 K. The photoluminescence spectrum shows two series of bands, namely, sharp (S) and diffuse (D), which are also distinguished by their fast (0.6 ns) for S and slow (5 ns) for D radiative decay time. Each series is composed of four bands with large Stokes shifts that are attributed to self-trapped excitons by the strong exciton-phonon interaction. The respective four luminescence bands of the two series originate from the four free-exciton levels in which the doubly degenerated dark and bright exciton levels theoretically predicted are resolved with the Jahn-Teller distortion in the excited states.

Watanabe, Kenji; Taniguchi, Takashi

2009-05-01

128

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

129

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

130

Direct and indirect two-photon excitonic processes in solids  

International Nuclear Information System (INIS)

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

131

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

132

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

133

High Density Exciton Dynamics of CuBr Nanocrystals Embedded in PMMA  

Science.gov (United States)

When CuBr nanocrystals (NCs) embedded in poly methyl methacrylate are strongly excited, there appears a new luminescence band at the low energy side of exciton luminescence peak. The luminescence band is associated with biexcitons in the CuBr NCs. As the excitation intensity increases, the biexciton luminescence band shows obvious broadening, while the exciton band does not. Interaction among more than three excitons may cause this broadening effect. Optical gain has also been found by pump-probe measurement around the biexciton luminescence band at 77K. The decay time of the optical gain 15ps coincides with that of the biexciton luminescence band, which means that the lifetime of the biexciton is 15ps.

Oda, M.; Shen, M. Y.; Goto, T.

2001-08-01

134

Helium irradiation of alkali halides  

International Nuclear Information System (INIS)

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

135

Structure and optical properties of high light output halide scintillators  

Science.gov (United States)

Structural and optical properties of several high light output halide scintillators and closely related materials are presented based on first-principles calculations. The optical properties are based on the Engel-Vosko generalized gradient approximation and the recently developed density functional of Tran and Blaha. The materials investigated are BaBr2 , BaIBr, BaCl2 , BaF2 , BaI2 , BiI3 , CaI2 , Cs2LiYCl6 , CsBa2Br5 , CsBa2I5 , K2LaBr5 , K2LaCl5 , K2LaI5 , LaBr3 , LaCl3 , SrBr2 , and YI3 . For comparison results are presented for the oxide CdWO4 . We find that the Tran Blaha functional gives greatly improved band gaps and optical properties in this class of materials. Furthermore, we find that unlike CdWO4 , most of these halides are highly isotropic from an optical point of view even though in many cases the crystal structures and other properties are not. This general result is rationalized in terms of halide chemistry. Implications for the development of ceramic halide scintillators are discussed.

Singh, David J.

2010-10-01

136

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

Science.gov (United States)

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

Pandiyarajan, T; Udayabhaskar, R; Karthikeyan, B

2013-02-15

137

Exciton Mediated Superconductivity in PrOs4Sb12  

CERN Document Server

The most important character of the exotic superconductor PrOs4Sb12 is the existence of low-lying excitations (excitons) with a finite energy gap and it appears as the magnetic field-induced order above 4.5 T. We focus on the a_u conduction band, which hybridizes with a Pr 4f^2 state strongly, coupled to the excitons. It results in an attractive interaction between the conduction electrons. The symmetry of the superconducting order parameter is determined by dispersion relation of the exciton. For the bcc system PrOs4Sb12, a d-wave state [kx ky + omega ky kz + omega^2 kz kx, omega=exp(pm i 2 pi/3)] is stabilized with broken time reversal symmetry.

Matsumoto, M; Matsumoto, Masashige; Koga, Mikito

2004-01-01

138

Excitonic electroreflectance spectra of hexagonal GaN  

Energy Technology Data Exchange (ETDEWEB)

We calculate the bandstructure near the {gamma}-point of A-, B-, and C-excitons in hexagonal GaN in the presence of an external electric field. The parametrization of Chuang and Chang is used. The importance of excitons for the interpretation of electroreflectance spectroscopy was emphasized by several experimental groups, but only recently theoretical calculations were presented. We derive the imaginary part of the dielectric function from a numerical solution of the excitonic Schroedinger equation in a finite field, taking into account the full 6 x 6 valence band structure. Via Kramers-Kronig-Relation the real part of the dielectric function is evaluated. The derivative of the dielectric function yields the ER spectra. In particular, we compare the theoretical field-dependent oscillator strengths with experimental data.

Lenk, Steve; Runge, Erich [Institut fuer Physik, Technische Universitaet Ilmenau (Germany)

2008-07-01

139

Exciton interaction with impurity in barium fluoride crystals  

International Nuclear Information System (INIS)

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

140

Selective excitation of exciton transitions in PTCDA crystals and films  

Science.gov (United States)

Photoluminescence excitation studies on 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) single crystals and polycrystalline PTCDA films are compared to the calculated excitonic dispersion deduced from an exciton model including the coupling between Frenkel and charge transfer (CT) excitons along the stacking direction. For excitation energies below the 0-0 Frenkel exciton absorption band at 5 K these measurements enable the selective excitation of several CT states. The CT2 state involving stacked PTCDA molecules reveals two excitation resonances originating from different vibronic sublevels. Moreover, the fundamental transition of the CT1 exciton state delocalized over both basis molecules in the crystal unit cell has been identified from the corresponding excitation resonance. From the excitation energy dependence the fundamental transition energies of the CT2 and CT1 excitons have been deduced to occur at 1.95 and 1.98 eV, respectively. When the excitation energy exceeds ˜2.08eV , we observe a strong emission channel which is related to the indirect minimum of the lowest dispersion branch dominated by Frenkel excitons. Photoluminescence excitation spectroscopy measurements on polycrystalline PTCDA films reveal a strong CT2 signal intensity which is attributed to an increased density of defect-related CT2 states that are preferentially formed by slightly deformed or compressed stacked PTCDA molecules in the vicinity of defects or at grain boundaries. Temperature-dependent PL measurements in polycrystalline PTCDA films between 10 and 300 K at an excitation of 1.88 eV further allow a detailed investigation of the CT2 transition and its vibronic subband.

Gangilenka, V. R.; Titova, L. V.; Smith, L. M.; Wagner, H. P.; Desilva, L. A. A.; Gisslén, L.; Scholz, R.

2010-04-01

 
 
 
 
141

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

Science.gov (United States)

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

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

2013-10-21

142

Investigation of change regularity of energy states of Mn2+ in halides  

International Nuclear Information System (INIS)

Data on 4E, 4A1 (4G) and 4T1 (4G) energy states of Mn2+ ion in some halides have been obtained and analyzed. With use of the dielectric theory of the chemical bond for complex crystals, several chemical bond parameters were calculated. The change regularity of the energy states of Mn2+ in halides has been studied. The results show that the covalence, the coordination number and the radius of the central ion are the main factors influencing the energy states of Mn2+ ion in halides. The relationships between these factors and the energy state 4T1 (4G), the energy difference ?E (?E=4E, 4A1 (4G)?4T1 (4G)) of Mn2+ ion in halides were established: E=2.0898+0.8618 exp (?F/0.2431); ?E=0.3201+0.9713?F. These relationships allow us to predict the position of energy state 4T1 (4G) and the energy difference ?E of Mn2+ in halides. This work can be significant for further understanding the luminescent properties of Mn2+ and can be used to develop new Mn2+-doped phosphors. - Highlights: ? Relationship between F and energy state 4T1(4G) of Mn2+ in halides was set up. ? Relationship between F and energy difference ?E of Mn2+rence ?E of Mn2+ in halides was set up. ? Site occupation of Mn2+-doped halides with two or more cations can be made clear. ? Energy state 4T1(4G) and emission band of Mn2+ in halides can be predicted.

143

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)

144

Strongly bound excitons in gapless two-dimensional structures  

Science.gov (United States)

Common wisdom asserts that bound excitons cannot form in high-dimensional (d>1) metallic structures because of their overwhelming screening and the unavoidable resonance with nearby continuous bands. Strikingly we illustrate that this prevalent assumption is not quite true. A key ingredient has been overlooked: Destructive coherent effects are capable of thwarting the formation of resonance. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two-dimensional gapless structure that exhibits metallic screening. The binding energies calculated by first-principles simulations are surprisingly large. The low-energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.

Liang, Yufeng; Soklaski, Ryan; Huang, Shouting; Graham, Matthew W.; Havener, Robin; Park, Jiwoong; Yang, Li

2014-09-01

145

Localization length and intraband scattering of excitons in linear aggregates  

Science.gov (United States)

A theoretical model to describe the intraband scattering of excitons in linear aggregates of finite size which exhibit strong intermolecular interactions is presented. From the calculation of the aggregate eigenstates, the localization length of excitons is evaluated for various configurations featuring physical situations like trapping, edge effects, inclusion of diagonal and/or orientational disorders. The intraband scattering is studied by considering the exciton-phonon stochastic coupling induced by the thermal bath. This coupling creates local dynamical fluctuations in the site energies which are characterized by their amplitude ( ?) and their correlation time ( ?c). Expressions of scattering rates are provided and used in a Pauli master equation to calculate the time dependence of the eigenstates populations after initial excitation of the quasi exciton-band. It is shown that the time evolution of the lowest state population as well as the Stokes shift strongly depend on ?c. Comparison of the theoretical results to time-resolved experiments performed on triaryl pyrylium salts allows us to interpret the observed Stokes shift and to derive an average value of the exciton-phonon correlation time.

Lemaistre, J. P.

1999-07-01

146

BEC of dark excitons in stress-induced potentials  

International Nuclear Information System (INIS)

Full text: While it has long been known that the ground state of spatially indirect excitons is optically inactive, its relevance to Bose-Einstein condensation (BEC) has been overlooked. In this contribution, we review our experiments on trapped excitons, and the role of dark states. In order to confine the particles in the quantum wells, we apply stress-induced band-deformation to create macroscopic and harmonic traps. At moderate stress (or weak confining potential), the exciton luminescence (i.e., the ensemble of bright excitons) has a distribution dictated by the temperature and the shape of the trap. However, while keeping the particle number constant, by increasing the stress, a distinct and well-localized dark region develops at the centre of the trapping potential. We will consider evidence that this dark region is an ensemble of dark excitons, and we will discuss how this new phase transition depends on various experimental parameters, such as, the strength of the potential, the temperature and the particle density. (author)

147

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

148

Large K-exciton dynamics in GaN epilayers: the non-thermal and thermal regime  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a detailed investigation concerning the exciton dynamics in GaN epilayers grown on c-plane sapphire substrates, focussing on the exciton formation and the transition from the nonthermal to the thermal regime. The time-resolved kinetics of LO-phonon replicas is used to address the energy relaxation in the excitonic band. From ps time-resolved spectra we bring evidence for a long lasting non-thermal excitonic distribution which accounts for the rst 50 ps. Such a beh...

Vinattieri, A.; Bogani, F.; Cavigli, L.; Manzi, D.; Gurioli, M.; Martin, D.; Feltin, E.; Carlin, J. -f; Butte?, R.; Grandjean, N.

2012-01-01

149

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

150

Hyperspherical theory of anisotropic exciton  

CERN Document Server

A new approach to the theory of anisotropic exciton based on Fock transformation, i.e., on a stereographic projection of the momentum to the unit 4-dimensional (4D) sphere, is developed. Hyperspherical functions are used as a basis of the perturbation theory. The binding energies, wave functions and oscillator strengths of elongated as well as flattened excitons are obtained numerically. It is shown that with an increase of the anisotropy degree the oscillator strengths are markedly redistributed between optically active and formerly inactive states, making the latter optically active. An approximate analytical solution of the anisotropic exciton problem taking into account the angular momentum conserving terms is obtained. This solution gives the binding energies of moderately anisotropic exciton with a good accuracy and provides a useful qualitative description of the energy level evolution.

Muljarov, E A; Tikhodeev, S G; Bulatov, A E; Birman, Joseph L; 10.1063/1.1286772

2012-01-01

151

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

152

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

153

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

154

THz-manipulation of excitonic polarization in (GaIn)As/GaAs quantum wells  

Energy Technology Data Exchange (ETDEWEB)

We investigate the interaction of strong single-cycle THz-pulses with a coherent excitonic polarization. The initial bleaching is superimposed by a splitting of the 1s exciton resonance and eventually is accompanied by the emergence of side-bands on both the high and low energy sides of the former resonance. A quantum-mechanical many-body analysis associates these observations with the transition from excitationinduced bleaching and the regime of excitonic Rabi flopping to multi-THz-photon ionization and hence the population of states with high orbital quantum numbers. (copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Ewers, B.; Koester, N.S.; Woscholski, R.; Koch, M.; Chatterjee, S.; Klettke, A.C.; Kira, M.; Koch, S.W. [Faculty of Physics and Materials Science Center, Philipps-Universitaet Marburg (Germany); Khitrova, G.; Gibbs, H.M. [College of Optical Sciences, The University of Arizona, Tucson, AZ (United States)

2013-09-15

155

Optical study of possibility of exciton autolocalization in LiH crystals  

International Nuclear Information System (INIS)

Spectra of luminescence of free excitons and wide bands of luminescence in the region of 2.9-3.4 eV, which have been before attributed to radiation of autolocalized excitons, are investigated for LiH crystals at 10 K. It is established that luminescence of 2.9-3.4 eV is related to decomposition products of LiOH thin films on LiH surface. Autolocalization of excitons in LiH in experiments isn't developed, and it is highly improbable theoretically

156

Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy.  

Science.gov (United States)

The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ? 570?meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs. PMID:25318849

Klots, A R; Newaz, A K M; Wang, Bin; Prasai, D; Krzyzanowska, H; Lin, Junhao; Caudel, D; Ghimire, N J; Yan, J; Ivanov, B L; Velizhanin, K A; Burger, A; Mandrus, D G; Tolk, N H; Pantelides, S T; Bolotin, K I

2014-01-01

157

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

158

Analysis of confinement potential fluctuation and band-gap renormalization effects on excitonic transition in GaAs/AlGaAs multiquantum wells grown on (1 0 0) and (3 1 1)A GaAs surfaces  

International Nuclear Information System (INIS)

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/AlxGa1-xAs quantum wells grown on [1 0 0] and [3 1 1]A GaAs substrates is evaluated. The results clearly demonstrate the coexistence of the band-tail states filling related to potential fluctuations and the band-gap renormalization caused by an increase in the density of photogenerated carriers during the photoluminescence (PL) experiments. Both phenomena have strong influence on temperature dependence of the PL-peak energy (EPL(T)). As the photon density increases, the EPL can shift to either higher or lower energies, depending on the sample temperature. The temperature at which the displacement changes from a blueshift to a redshift is governed by the magnitude of the potential fluctuations and by the variation of BGR with excitation density. A simple band-tail model with a Gaussian-like distribution of the density of state was used to describe the competition between the band-tail filling and the BGR effects on EPL(T).

159

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

160

Excitonic Correlations in the Intermetallic $Fe_{2}$VAl  

CERN Document Server

The intermetallic compound Fe2VAl looks non-metallic in transport and strongly metallic in thermodynamic and photoemission data. It has in its band structure a highly differentiated set of valence and conduction bands leading to a semimetallic system with a very low density of carriers. The pseudogap itself is due to interaction of Al states with the d orbitals of Fe and V, but the resulting carriers have little Al character. The effects of generalized gradient corrections to the local density band structure as well spin-orbit coupling are shown to be significant, reducing the carrier density by a factor of three. Doping of this nonmagnetic compound by 0.5 electrons per cell in a virtual crystal fashion results in a moment of 0.5 bohr magnetons and destroys the pseudogap. We assess the tendencies toward formation of an excitonic condensate and toward an excitonic Wigner crystal, and find both to be unlikely. We propose a model is which the observed properties result from excitonic correlations arising from tw...

Weht, R; Weht, Ruben

1998-01-01

 
 
 
 
161

Excitons in silicon solar cells  

Energy Technology Data Exchange (ETDEWEB)

The effects of excitons, mobile electron-hole pairs in which the charge carriers remain loosely bound together, have until now been neglected in solar cell theory. This has seemed appropriate since their concentration in silicon at room temperature has been thought to be extremely small and the excitons do not carry charge. However, recent work elsewhere has proposed that a significant number of electrons and holes remain in the exciton form in silicon at room temperature, a finding which, if supportable by empirical evidence, contribute to solar cell currents by diffusing to the high-electric-field region of the cell where the electrons and holes may be separated from each other by the field. A theoretical study is presented of excitons as a third type of charge carrier in silicon solar cells. Initial results indicate a decrease in the undesirable `dark current`, leading to higher cell voltage, as well as an increase in short-circuit current when excitons are incorporated in the theory. However, the numerical results depend on assumptions which require experimental justification. (author). 3 figs., 16 refs.

Chan, D.S.P.; Green, M.A.; Corkish, R. [New South Wales Univ., Kensington, NSW (Australia). Centre for Photovoltaic Devices and Systems

1994-12-31

162

Halide transport in Xenopus oocytes.  

Science.gov (United States)

1. Radioisotopes and intracellular microelectrodes were used to characterize the permeability of Xenopus oocytes to chloride and other halides. 2. Uptake of 36Cl had a half-time for equilibration of approximately 3 h, with an initial rate of Cl- entry corresponding to a permeability coefficient of 3.9 x 10(-7) cm/s, and an equilibrium uptake of 36Cl of 33 mM. 3. Replacement of bathing Na+ by K+ depolarized the oocytes from -46 to -7 mV and stimulated influx approximately 3-fold. 4. Influx was linearly dependent on bathing [Cl-] and was temperature dependent with an activation energy of 46 kJ/mol. Influx of 125I of 36Cl was not affected by the presence of equal concentrations of other halides or thiocyanate. These results are consistent with a channel-mediated entry mechanism. 5. Diphenylamine-2-carboxylate (DPAC) and 9-anthracene carboxylate (9-AC), blockers of Cl- channels in other cells, inhibited Cl- entry with dissociation constants (Kds) of approximately 5 x 10(-4) and approximately 10(-3) M, respectively. Inhibitors of Cl(-)-HCO3- exchange or Na(+)-K(+)-2Cl- co-transport did not affect Cl- influx. 6. Attempts to lower or raise intracellular Ca2+ with BAPTA or A23187, respectively, were also without effect on Cl- influx. 7. The halide selectivity sequence determined with isotopes was I- (3.2) greater than Br- (1.3) greater than Cl- (1.0). However, DPAC inhibited almost all of the 36Cl influx but only a small fraction of 125I influx. 8. Replacement of bathing Cl- by I- or Br-resulted in hyperpolarizations, from which the same selectivity sequence was determined. 9. Replacement of bathing Cl- by gluconate caused a marked depolarization, which was inhibited by DPAC and, less potently, by 9-AC. PMID:1822540

Katayama, Y; Widdicombe, J H

1991-11-01

163

New layered manganese oxide halides.  

Science.gov (United States)

The first layered manganese(III) oxide chlorides, Sr2MnO3Cl and Sr4Mn3O8-yCl2, have been synthesised; Sr2MnO3Cl adopts a K2NiF4 type structure with sheets of MnO5 square based pyramids linked through oxygen and separated by SrCl layers; it is the end member of a new family of Ruddlesden-Popper type manganese oxide halides which includes the three-layer member Sr4Mn3O8-yCl2 also reported herein. PMID:12120392

Knee, Christopher S; Weller, Mark T

2002-02-01

164

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

165

Excitonic recombination near the mobility edge in CdSe/ZnSe nanostructures  

International Nuclear Information System (INIS)

The low temperature photoluminescence and Raman scattering studies of CdSe/ZnSe nanostructures with CdSe inclusions of 1.5 and 3.0 monolayer nominal thickness is reported. The energy position of the photoluminescence emission band is determined by interdiffusion of Cd and Zn in the region of the insert while the band shape is caused by strong interaction of the localized excitons with optical phonons in Zn1-xCdxSe alloy. The important role of multiphonon processes, including the acoustical phonons of the Brillouin band edge, in the excitonic relaxation is demonstrated. The results obtained are interpreted in the framework of the model of the efficient excitonic mobility edge

166

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)

167

Photoluminescence study of polycrystalline CsSnI3 thin films: Determination of exciton binding energy  

International Nuclear Information System (INIS)

We report on the determination of exciton binding energy in perovskite semiconductor CsSnI3 through a series of steady state and time-resolved photoluminescence measurements in a temperature range of 10–300 K. A large binding energy of 18 meV was deduced for this compound having a direct band gap of 1.32 eV at room temperature. We argue that the observed large binding energy is attributable to the exciton motion in the natural two-dimensional layers of SnI4 tetragons in this material. - Highlights: ? A unique method is presented to determine the exciton binding energy in a polycrystalline CsSnI3 compound. ? A large exciton binding energy of 18 meV was deduced for the natural two-dimensional excitons in CsSnI3. ? CsSnI3 is important semiconductor since it has a direct band gap of 1.32 eV at 300 K. ? Exciton lifetime increases monotonically with temperature up to near room temperature.

168

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.

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

2013-08-01

169

Charge carriers and excitons transport in an organic solar cell-theory and simulation  

Science.gov (United States)

An organic solar cell model is developed that consists of both excitonic and classical bipolar aspects of solar cells. In order to achieve this goal, the photon recycling term is imported into the equations to connect the Shockley-Queisser theory and the classical diode theory. This model for excitonic and classical bipolar solar cells can describe the combined transport and interaction of electrons, holes and excitons. For high mobilities this model reproduces the Shockley Queisser efficiency limit. We show how varying the respective mobilities of the different species changes the operation mode of the solar cell path between excitonic and bipolar. Then, the effect of conduction band offset on transport will be described in this paper. Finally, validity of reciprocity theorem between quantum efficiency and electroluminescence in this model will be discussed.

Shahini, Ali.; Abbasian, Karim.

2012-08-01

170

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.)

171

A variational estimate of the binding energy of charge-transfer excitons in the cuprate superconductors  

CERN Document Server

We present a variational estimate for the binding energy of a Frenkel exciton in the insulating cuprate superconductors. Starting from the three band Hubbard model we perform a canonical transformation to O(t^2), where t is the bare nearest neighbour copper-oxygen hopping integral. An effective Hamiltonian is then derived to describe the hopping of the exciton through the copper oxide plane. The critical parameter in the model is the nearest neighbour copper-oxygen coulomb repulsion, V. It is found that a critical value of V is needed to observe bound Frenkel excitons, and that these excitons have the same symmetry as the parent copper orbital, d_{x^2-y^2}. We determine the critical value of V using a variational approach, and attempt to fit the parameters of the model to known experimental results.

Vermeulen, C; Vermeulen, C; Barford, W

1995-01-01

172

Exciton size and mobility in (6,5) single-walled carbon nanotubes  

Science.gov (United States)

Using femtosecond pump probe spectroscopy with sub-20 fs resolution, we probe fundamental properties of the E11 exciton in (6,5) single walled carbon nanotubes, prepared by density gradient ultracentrifugation. From the initial photobleaching signal, measured faster than any relaxation process, we obtain the one-dimensional size of the excitonic wavefunction along the nanotube. Exciton decay is found pump-intensity dependent only at elevated pump intensities. Numerical modelling of decay kinetics yields an exciton diffusion coefficient of about 0.1 cm^2/s. Anisotropy measurements in highly purified samples show that there is virtually no depolarisation of the E11 bleach over 40 ps. A photoinduced absorption (PA) band, blueshifted against the E11 bleach, shows only weak anisotropy.

Lüer, Larry; Hoseinkhani, Sajjad; Polli, Dario; Crochet, Jared; Hertel, Tobias; Lanzani, Guglielmo

2008-04-01

173

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.”

174

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.

175

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2014-05-12

176

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

177

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)

178

Intramolecular radiationless transitions dominate exciton relaxation dynamics  

Science.gov (United States)

Reports of long-lived exciton coherences have lead researchers to expect that model dimer systems inevitably generate exciton superposition states observable by two-dimensional electronic spectroscopy. Here we report a careful photophysical characterization of a model dimer system, a diacetylene-linked perylenediimide dimer to examine that issue. The absorption spectrum of the dimer shows molecular exciton splitting, indicating that excitation is delocalized. The assignment of exciton states was supported by other photophysical measurements as well as theoretical calculations. Ultrafast two-dimensional electronic spectroscopy was employed to identify and characterize excitonic and vibrational features, as they evolve over time. Population transfer between the two exciton states is found to happen in <50 fs, thus preventing the sustainment of exciton coherences. We show that such fast radiationless relaxation cannot be explained by coupling to a solvent spectral density and is therefore missed by standard approaches such as Redfield theory and the hierarchical equations of motion.

Jumper, Chanelle C.; Anna, Jessica M.; Stradomska, Anna; Schins, Juleon; Myahkostupov, Mykhaylo; Prusakova, Valentina; Oblinsky, Daniel G.; Castellano, Felix N.; Knoester, Jasper; Scholes, Gregory D.

2014-04-01

179

Effects of higher exciton manifolds and exciton-exciton annihilation on optical bistable response of an ultrathin glassy film comprised of oriented linear Frenkel chains  

International Nuclear Information System (INIS)

We theoretically analyze the optical response from an ultrathin film built up of oriented molecular aggregates, the operating states of which are represented by Frenkel exciton states. A four-level model, involving transitions between the ground, one-exciton and two-exciton states, exciton-exciton annihilation from the two-exciton state as well as relaxation from the annihilation level back to the one-exciton and ground states, is used for describing the film optical response. It is proved that the exciton-exciton annihilation may act not as a destructive but, on the contrary, as a constructive factor tending towards the occurrence of bistability. In particular, the effect of inhomogeneous broadening of the exciton optical transition, preventing the bistable behavior, may be suppressed considerably due to a fast exciton-exciton annihilation

180

Fullerenes doped with metal halides  

International Nuclear Information System (INIS)

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

 
 
 
 
181

On excitons and other gap states in boron carbide  

International Nuclear Information System (INIS)

The excitons in boron carbide recently independently proved to be present by luminescence measurements (by Schmechel and co-workers) and by x-ray Raman scattering investigations in connection with ab initio calculations (by Feng and co-workers) are discussed, taking the actual structure of boron carbide within the homogeneity range (B4.3C-B?11C) into account. The excitonic levels at 1.560 and 1.5695 eV obtained from the luminescence spectrum are attributed to the central B atom in the C-B-C and the C-B-B chains respectively. Considering the transition energies related to the different gap states obtained by optical and electrical measurements for energies below the interband transition of 2.09 eV, an actualized energy band scheme of boron carbide is presented

182

Exciton scattering on symmetric branching centers in conjugated molecules.  

Science.gov (United States)

The capability of the exciton scattering approach, an efficient methodology for excited states in branched conjugated molecules, is extended to include symmetric triple and quadruple joints that connect linear segments on the basis of the phenylacetylene backbone. The obtained scattering matrices that characterize these vertices are used in application of our approach to several test structures, where we find excellent agreement with the transition energies computed by the reference quantum chemistry. We introduce topological charges, associated with the scattering matrices, which help to formulate useful relations between the number of excitations in the exciton band and the number of repeat units. The obtained features of the scattering phases are analyzed in terms of the observed excited state electronic structure. PMID:21194223

Li, Hao; Wu, Chao; Malinin, Sergey V; Tretiak, Sergei; Chernyak, Vladimir Y

2011-05-12

183

Excitonic Effects on Optical Absorption Spectra of Doped Graphene  

CERN Document Server

We have performed first-principles calculations to study optical absorption spectra of doped graphene with many-electron effects included. Both self-energy corrections and electron-hole interactions are reduced due to the enhanced screening in doped graphene. However, self-energy corrections and excitonic effects nearly cancel each other, making the prominent optical absorption peak fixed around 4.5 eV under different doping conditions. On the other hand, an unexpected increase of the optical absorbance is observed within the infrared and visible-light frequency regime (1 ~ 3 eV). Our analysis shows that a combining effect from the band filling and electron-hole interactions results in such an enhanced excitonic effect on the optical absorption. These unique variations of the optical absorption of doped graphene are of importance to understand relevant experiments and design optoelectronic applications.

Yang, Li

2011-01-01

184

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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...

Wehrenfennig, C.; Liu, M.; Snaith, Hj; Johnston, Mb; Herz, Lm

2014-01-01

185

Reactions of diiminopyridine ligands with chalcogen halides.  

Science.gov (United States)

The reactions of the chalcogen halides (Ch = S, Se, Te) with a series of diiminopyridine (DIMPY) ligands were explored. It was determined through these studies that varying both the substitution on the ?-carbon and the chalcogen halide reagent afforded different products. If methyl groups were present on the ?-carbon, reactivity was observed through the eneamine tautomer to yield N,N',C-bound neutral chalcogen complexes. In the cases where H and C(6)H(5) groups were in the same position, N,N',N?-chelated chalcogen cations or dications were produced. Many of the reactions resulted in complex mixtures postulated to occur by the release of halogen decomposing the product or, for reactions with the CH(3) substituted ligand, uncontrollable reactivity with the eneamine tautomer. This is the first report of reactions of sulfur and selenium halides with the ubiquitous diiminopyridine ligands and only the second example for a tellurium halide. PMID:22339169

Martin, Caleb D; Ragogna, Paul J

2012-03-01

186

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)

187

Exciton dynamics in disordered poly(p-phenylenevinylene). 2. Exciton diffusion.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We present a first principles theory of exciton diffusion in conformationally disordered conjugated polymers. Central to our theory is that exciton transfer occurs from vibrationally relaxed states (VRSs) to local exciton ground states (LEGSs). LEGSs are determined by the diagonal and off-diagonal disorder induced by static density and torsional fluctuations, and VRSs are further localized by exciton-phonon coupling. The theory is implemented using the Frenkel-Holstein model to calculate the ...

Barford, W.; Bittner, Er; Ward, A.

2012-01-01

188

Exciton and biexciton energies in bilayer systems  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report calculations of the energies of excitons and biexcitons in ideal two-dimensional bilayer systems within the effective-mass approximation with isotropic electron and hole masses. The exciton energies are obtained by a simple numerical integration technique, while the biexciton energies are obtained from diffusion quantum Monte Carlo calculations. The exciton binding energy decays as the inverse of the separation of the layers, while the binding energy of the biexciton with respect to...

Tan, M. Y. J.; Drummond, Neil; Needs, R. J.

2005-01-01

189

Analysis of Trapped Quantum Degenerate Dipolar Excitons  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The dynamics of quantum degenerate two-dimensional dipolar excitons confined in electrostatic traps is analyzed and compared to recent experiments. The model results stress the importance of artificial trapping for achieving and sustaining a quantum degenerate exciton fluid in such systems and suggest that a long-lived, spatially uniform, and highly degenerate exciton system was experimentally produced in those electrostatic traps. © 2006 American Institute of Physics.

Rapaport, R.; Chen, G.; Simon, S.

2006-01-01

190

Topologically protected excitons in porphyrin thin films  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

Yuen-zhou, Joel; Saikin, Semion S.; Yao, Norman Y.; Aspuru-guzik, Ala?n

2014-01-01

191

Luminescence Studies of the Excitonic System in Germanium: Phase Properties and Transport.  

Science.gov (United States)

This thesis presents the results of three studies of the excitonic system in Ge. These studies employ near -infrared spectroscopy and imaging techniques to examine the phase and transport properties of the low temperature exciton-plasma system. The first study, presented in Chapter II, examines the properties of the electron-hole liquid (EHL) phase as a function of -uniaxial stress. The application of stress along a axis greatly reduces the degeneracy of the conduction and valence bands, thereby reducing the stability of the EHL. This thesis reports the first observation of the EHL in the high-stress limit (sigma > 70 kgf/mm^2). The observation of the EHL in this high-stress regime serves as an important confirmation of theoretical calculations of EHL properties. The success of this experiment can be attributed to the use of the nonuniform Hertzian stress geometry. The second study, presented in Chapter III, uses time-resolved spectroscopy to study the phase properties of the exciton gas, EHL, and electron-hole plasma (EHP) in unstressed Ge. A detailed spectroscopic analysis of spectra taken near and above the liquid-gas critical temperature is presented. By employing ana analysis using three components (excitons, excitonic complexes, and EHP), evidence is found for a second exciton-plasma condensation at temperatures above the EHL critical temperature. A phase diagram for the excitonic system is proposed that includes the second condensed phase. Uniaxial stress is used to study the electron -hole droplet (EHD) cloud in Chapter IV. By applying stress along a axis, the two conduction band valleys perpendicular to the stress axis can be emptied of electrons in the EHL phase. This alteration of the electron Fermi surface causes remarkable changes to occur in the anisotropic EHD cloud structure. The changes in the cloud structure illustrate the important role that the anisotropic Fermi surface plays in giving the cloud its shape.

Simon, Andrew Herbert

192

Exciton trapping at heterojunctions in polymer blends.  

Science.gov (United States)

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency. PMID:16035815

Morteani, Arne C; Friend, Richard H; Silva, Carlos

2005-06-22

193

Exciton condensation in strongly correlated electron bilayers  

Science.gov (United States)

We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems, an exciton is the bound state of a double occupied and empty site. In the strong coupling limit, the exciton acts as a hard-core boson. Its physics is captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean-field theory, we constructed the ground-state phase diagram. Three homogeneous phases can be distinguished: the antiferromagnet, the exciton checkerboard crystal, and the exciton superfluid. For most model parameters, however, we predict macroscopic phase separation between these phases. The exciton superfluid exists only for large exciton hopping energy. Additionally, we studied the collective modes and susceptibilities of the three phases. In the superfluid phase, we find the striking feature that the bandwidth of the spin-triplet excitations, potentially detectable by resonant inelastic x-ray scattering (RIXS), is proportional to the superfluid density. The superfluid phase mode is visible in the charge susceptibility, measurable by RIXS or electron energy loss spectroscopy (EELS).

Rademaker, Louk; van den Brink, Jeroen; Zaanen, Jan; Hilgenkamp, Hans

2013-12-01

194

Excitons in the rare gas solids  

International Nuclear Information System (INIS)

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

195

Extraordinary exciton conductance induced by strong coupling  

CERN Document Server

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

Feist, Johannes

2014-01-01

196

Exciton induced photodesorption in rare gas solids  

Science.gov (United States)

This paper reviews our progress on the desorption induced by electronic transitions (DIET) in rare gas solids by selective excitation of valence excitons. Observation of metastable atoms desorbed by excitonic excitation gives us direct information on the exciton-induced desorption processes in rare gas solids. The validity of three desorption mechanisms, cavity ejection, excimer dissociation, and internal sputtering, is demonstrated by systematic measurements of kinetic energies and angular distributions of desorbed particles. The absolute yield of total and partial desorption was measured, which can lead us to the quantitative understanding of exciton-induced desorption processes.

Hirayama, Takato; Arakawa, Ichiro

2006-08-01

197

Exciton liquid in coupled quantum wells.  

Science.gov (United States)

Excitons in semiconductors may form correlated phases at low temperatures. We report the observation of an exciton liquid in gallium arsenide/aluminum gallium arsenide-coupled quantum wells. Above a critical density and below a critical temperature, the photogenerated electrons and holes separate into two phases: an electron-hole plasma and an exciton liquid, with a clear sharp boundary between them. The two phases are characterized by distinct photoluminescence spectra and by different electrical conductance. The liquid phase is formed by the repulsive interaction between the dipolar excitons and exhibits a short-range order, which is manifested in the photoluminescence line shape. PMID:24385625

Stern, Michael; Umansky, Vladimir; Bar-Joseph, Israel

2014-01-01

198

Ballistic spin transport in exciton gases  

Science.gov (United States)

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

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

2013-11-01

199

Emission related to exciton-polariton coupling in porous SiC  

Energy Technology Data Exchange (ETDEWEB)

The paper presents the results of SiC nanocrystal (NC) characterization using the photoluminescence (PL), its temperature dependence and X-ray diffraction (XRD) techniques. It is revealed that original n-type 6H-SiC wafers and porous 6H-SiC layers consisted inclusions of 2H-SiC, 4H-SiC, and 8H-SiC polytypes. The photoluminescence study has shown that in porous SiC layers the emission intensity of high energy PL bands enlarges. Temperature dependences of high energy PL bands testify that these PL bands related to excitons bounded at nitrogen donors in different SiC polytypes. The PL intensity enhancement of donor-bound exciton PL bands in the big size (50-250nm) SiC nanocrystals has been attributed to the realization of exciton weak confinement and exciton-polariton coupling in SiC nanocrystals. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Torchynska, T.V. [ESFM-Instituto Politecnico Nacional, Mexico D.F. 07738 (Mexico); Diaz Cano, A.; Yescas Hernandez, J.A. [UPIITA- Instituto Politecnico Nacional, Mexico D.F.07738 (Mexico); Vorobiev, Yu.V. [CINVESTAV-IPN, Unidad Queretaro, Queretaro 76230, QRO (Mexico); Shcherbyna, L.V. [V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences, 03028 Kyiv (Ukraine)

2011-06-15

200

Wannier excitons in the ferromagnetic semiconductors EuO and EuS  

International Nuclear Information System (INIS)

Fine structures in the thermoreflectance (TR) spectra of EuO and EuS in the ultraviolet region are ascribed for the first time to Wannier excitons in magnetic semiconductors. The characteristic temperature-induced splittings of the spectra of EuO and EuS at the ferromagnetic spin-ordering temperatures are successfully analyzed in terms of the exciton and the spin-polarization effect of the relevant one-electron bands, an s-like conduction and p-like valence bands, at the GAMMA point. The magnetic-spin-exchange energies, J/subc/ and J/subv/, between the localized Eu-4f electrons and the band electrons, the spin-orbit interaction energy lambda for the valence band, and the electron-hole spin-exchange energy ? for the exciton are estimated. It is demonstrated that the TR measurements open a new aspect of the optical study of magnetic-semiconductor excitons which become an ideal probe for directly detecting the interaction between the localized-magnetic-spin system and the nonlocalized-Bloch-electron system in the magnetic semiconductors

 
 
 
 
201

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

202

Resonant light scattering mediated by excitonic polaritons in semiconductors  

Science.gov (United States)

The field of polariton-mediated light scattering is obviously a very rich one. It first gives a direct visualisation of those fundamental hcoupled-mode excitations, the excitonic polaritons. It helps to obtain evidence for the predictions made by Hopfield and others of some extraordinary properties of these objects. Let us mention, for instance, the dispersive, multiple Brillouin peaks, the AS/S ratio in RBS larger than unity, the disappearence of sharp RRS peaks into luminescence bands at resonance, the huge cross sections of RIRS and its doubleresonance behavior on the n = 2 exciton-polariton peak. In this latter case, we encounter once more the situation where a third-order process with two strong interactions overcomes a two-step process involving weak interactions. But polariton-mediated light scattering is much more than just another theoretical description of the light-matter interaction. It provides a huge amount of information on exciton parameters [masses, L-T splittings (i.e., polarizability), fine structure parameters (exchange energy)], and semiconductor parameters (refractive indices, sound velocities, phonon energies, K-linear energy terms, impurity energy levels and concentration). Exciton-phonon (acoustic as well as optical phonons) and exciton-impurity interactions can be studied in great detail with this type of resonance scattering. Due to this wealth, and also due to the relative ease with which such experiments can be performed, polaritonmediated light scattering still has a bright future. There is a number of unresolved and challenging problems. In the first place, of course, the ABC problem due to its generality and fundamental character. The extremely complicated, somewhat uncontrolled, experimental situation at the crystal interface has been discussed in various portions of the text. We want to stress again that only complete sets of measurements, i.e., transmission, reflectivity, RBS (efficiency, linewidth, hlineshapes, high-resolution kinematics) can provide useful information to shed some light onto this delicate problem. Other remaining tasks concern realistic calculations of efficiencies and multiple scattering properties (in particular, thermalization of RRS events). Interesting experiments would be the observation of RBS for surface exciton polaritons, which would, as in the case of bulk polaritons, bring welcome information on these otherwise rather indirectly evidenced excitations.

Weisbuch, C.; Ulbrich, R. G.

203

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

Science.gov (United States)

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

Jiang, Jian-Hua; John, Sajeev

2014-07-01

204

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

205

Excitons in the wurtzite AlGaN/GaN quantum-well heterostructures  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

206

Experimental study of the electronic structure of uranium halides and oxyhalides by X-ray photoelectron spectroscopy  

International Nuclear Information System (INIS)

The trends observed in high resolution XPS spectra of uranium (+3 to +6) (oxy)halides (F, Cl, Br) for U4f levels, their shake-up satellites (relative intensities and energy separation), and valence levels (U5f and U-X bonding band energy separation and intensities) enlighten the nature of the bonding in these series of compounds. (orig.)

207

Colour centre phenomena in alkali halide crystals X-ray-irradiated under high electric fields  

International Nuclear Information System (INIS)

Some colour centre phenomena, such as the F-band absorption, thermoluminescence (TL) etc., in alkali halide crystals (NaCl and KCl particularly) subjected to high AC or DC electric fields and later irradiated with X-rays, have been studied. Similar investigations have also been carried out when these crystals are X-ray irradiated under high AC or DC fields. Interesting results are obtained in the work. An attempt is made to understand the results. (author)

208

Li 1s near-edge spectra in six lithium halides  

International Nuclear Information System (INIS)

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

209

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

210

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

Energy Technology Data Exchange (ETDEWEB)

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.

Munoz-Matutano, Guillermo; MartInez-Pastor, Juan [Instituto de Ciencias de los Materiales, Universitat de Valencia, PO Box 22085, 46071 Valencia (Spain); Alen, Benito [Instituto de Microelectronica de Madrid (CNM-CSIC), Isaac Newton 8, 28760 Tres Cantos, Madrid (Spain); Seravalli, Lucca; Frigeri, Paola; Franchi, Secondo [Istituto dei Materiali per l' Elettronica e il Magnetismo (CNR), Parco delle Scienze 37/a, I-43100 Parma (Italy)], E-mail: Guillermo.munoz@uv.es

2008-04-09

211

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

212

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)

213

On nature of luminescence off- and on-configuration of excitons in crystals CsI  

International Nuclear Information System (INIS)

In the paper low-temperature (at 80 K) effect of stress from uniaxial compression on migration process of electronic excitations in CsI crystals and mutual transformation of two exciton radiation bands of off- and on-configuration at 4.25 eV and 3.67 eV respectively. In the paper results of study of low-temperature (80 K) elastic deformation on the radiation spectra of CsI crystal under X radiation, corresponding to selective creation both excitons, electron-hole pairs are cited

214

Elementary processes of photolysis in lead halide crystals  

Energy Technology Data Exchange (ETDEWEB)

Emission spectra from PbCl{sub 2} crystals doped with various amount of Tl{sup +} ions have been measured at 4.8 K by varying the excitation photon energy. Two additional emission bands appear in the green region (G{sub 1} at 2.48 and G{sub 2} at 2.39 eV) with excitation in the low energy tail region of the excitonic absorption band. The G{sub 1}- and G{sub 2}-emissions show behavior similar to the intrinsic ultraviolet (UV at 3.78 eV) and blue-green (BG at 2.62 eV) emission, respectively, in their excitation spectra, temperature dependence and thermal quenching effect. By quenching the crystal from 620 K to room temperature, the intensities of the UV- and G{sub 1}-emissions are diminished drastically while those of the BG- and G{sub 2}-emissions are maintained. The G{sub 1}- and G{sub 2}-emissions are related to the [Tl{sup 0}(Cl{sup -}){sub 3}+hole] center and the [(PbTl){sup 2+}+hole] center corresponding to the [Pb{sup +}(Cl{sup -}){sub 3}+hole] center for the UV-emission and [(Pb{sub 2}){sup 3+}-STEL +hole] center for the BG-emission in the host crystal, respectively. A well-defined excitation band for the G{sub 1}-emission is established through spectral decomposition of the observed emission spectra. This means that the Tl{sup +}-related excitons maintain a rather strong localized nature leading to luminescence specific to the doped Tl{sup +} ion. The role of the anion vacancy will be discussed in relation to the lattice relaxation and the photolysis processes.

Nakagawa, H. E-mail: nakagawa@wbase.fuee.fukui-u.ac.jp; Terakami, M.; Yasuda, K

2001-10-01

215

Excitonic properties of graphene-based materials.  

Science.gov (United States)

First-principle density functional theory (DFT) calculations with quasiparticle corrections and many body effects are performed to study the electronic and optical properties of graphene-based materials. This review summarizes the excitonic properties including optical transition spectra and the distribution of exciton wavefunctions, thus providing the theoretical knowledge and predictions for promising optical applications of graphene materials. PMID:21960114

Wang, Min; Li, Chang Ming

2012-02-21

216

Radiative recombination of excitons in amorphous semiconductors  

International Nuclear Information System (INIS)

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

217

Synthesis of methyl halides from biomass using engineered microbes.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Methyl halides are used as agricultural fumigants and are precursor molecules that can be catalytically converted to chemicals and fuels. Plants and microorganisms naturally produce methyl halides, but these organisms produce very low yields or are not amenable to industrial production. A single methyl halide transferase (MHT) enzyme transfers the methyl group from the ubiquitous metabolite S-adenoyl methionine (SAM) to a halide ion. Using a synthetic metagenomic approach, we chemically synth...

Bayer, Ts; Widmaier, Dm; Temme, K.; Mirsky, Ea; Santi, Dv; Voigt, Ca

2009-01-01

218

Ternary halides: novel NLO compounds for LWIR  

Science.gov (United States)

We carried out studies to identify, synthesize, purify and grow crystals of a novel class of halides for nonlinear optical applications. Tl3PbBr5, Tl4PbI6, Tl4HgI6 and Tl3PbI5, were synthesized by reacting binary halides and crystals were grown. Optical quality was evaluated by fabricating cm size crystals. The homogeneity of bulk crystal was evaluated by studying transparency, etchpit and X-ray rocking curve and 2?-? scans. These halides have transparency from visible to far-IR wavelength region. The material of the compounds of Tl3PbBr5 composition showed self-poling during the growth.

Singh, N. B.; Suhre, D. R.; Green, K.; Fernelius, N.; Hopkins, F. K.

2005-08-01

219

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

220

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

 
 
 
 
221

Enhanced Quantum Efficiency From Hybrid Cesium Halide/Copper Photocathode  

Energy Technology Data Exchange (ETDEWEB)

The quantum efficiency of Cu is found to increase dramatically when coated by a CsI film and then irradiated by a UV laser. Over three orders of magnitude quantum efficiency enhancement at 266 nm is observed in CsI/Cu(100), indicating potential application in future photocathode devices. Upon laser irradiation, a large work function reduction to a value less than 2 eV is also observed, significantly greater than for similarly treated CsBr/Cu(100). The initial QE enhancement, prior to laser irradiation, is attributed to interface interaction, surface cleanliness and the intrinsic properties of the Cs halide film. Further QE enhancement following activation is attributed to formation of inter-band states and Cs metal accumulation at the interface induced by laser irradiation.

Kong, Lingmei; Joly, Alan G.; Droubay, Timothy C.; Gong, Yu; Hess, Wayne P.

2014-04-28

222

Enhanced quantum efficiency from hybrid cesium halide/copper photocathodes  

Energy Technology Data Exchange (ETDEWEB)

The quantum efficiency (QE) of Cu is found to increase dramatically when coated by a CsI film and then irradiated by a UV laser. Over three orders of magnitude quantum efficiency enhancement at 266?nm is observed in CsI/Cu(100), indicating potential application in future photocathode devices. Upon laser irradiation, a large work function reduction to a value less than 2?eV is also observed, significantly greater than for similarly treated CsBr/Cu(100). The initial QE enhancement, prior to laser irradiation, is attributed to interface interaction and the intrinsic properties of the Cs halide film. Further QE enhancement following activation is attributed to formation of inter-band states and Cs metal accumulation at the interface induced by laser irradiation.

Kong, Lingmei; Joly, Alan G.; Droubay, Timothy C.; Gong, Yu; Hess, Wayne P., E-mail: wayne.hess@pnnl.gov [Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 (United States)

2014-04-28

223

Vibrational spectroscopic studies of metal(II) halide benzimidazole  

Science.gov (United States)

Infrared spectra (4000-200 cm -1) are reported for metal halide(II) benzimidazole complexes of the following stoichiometries: M(benz)X 2 [M=Cd, Cu; X=Cl, Br; benz=benzimidazole], Co(benz) 2, and Co(benz) 2X 2 [X=Cl, Br, I]. Vibrational assignments are given for all the observed bands. The analysis of the vibrational spectra indicates that there are some structure-spectra correlations. For a given series of isomorphous complexes the sum of the difference between the values of the vibrational modes of uncoordinated benzimidazole and coordinated to metal ion benzimidazole was found to increase in the order of the second ionization potentials of metals.

Yurdakul, ?.; Kurt, M.

2003-05-01

224

Lattice theory of ultrafast excitonic and charge-transfer dynamics in DNA.  

Science.gov (United States)

We propose a lattice fermion model suitable for studying the ultrafast photoexcitation dynamics of ordered chains of deoxyribonucleic acid (DNA) polymers. The model includes both parallel (intrachain) and perpendicular (cross-chain) terms as well as diagonal cross-chain terms coupling neighboring bases. The general form of our Hamiltonian is borrowed from lattice fermion models of quantum chromodynamics. The band structure for this model can be determined analytically, and we use this as a basis for computing the singly excited states of the poly(dA)poly(dT) DNA duplex using configuration interaction singles. Parameters for the model are taken from various literature sources and our own ab initio calculations. Results indicate that the excited states consist of a low energy band of dark charge-separated states followed by separate bands of delocalized excitonic states which have weak mixing between the thymidine and adenosine sides of the DNA chain. We then propose a lattice exciton model based upon the transition dipole-dipole couplings between bases and compare the analytical results for the survival probability of an initially localized exciton to exact numerical results. The results herein underscore the competing role of excitonic and charge-transfer dynamics in these systems. PMID:16965121

Bittner, Eric R

2006-09-01

225

About the F center in cesium halides  

International Nuclear Information System (INIS)

The energy levels for the F center in cesium halides are determined. The point ion method and the ion size correction are applied. Vacancy-centered trial wave functions of three types are used. The structure of NaCl and the CsCl are considered in the calculations. It is concluded that some investigations probably used the NaCl structure in their calculations for cesium halides. For wave functions of the Gourary-Adrian type it is found that the Bartram et al. method can not be applied for values of the empirical parameter ? ? 0.53. Others papers are discussed. (author)

226

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

227

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

228

Piezoelectric and spontaneous polarization effects on exciton binding energies in Wurtzite GaN/AlGaN quantum wells  

International Nuclear Information System (INIS)

Piezoelectric and spontaneous polarization effects on the exciton binding energies of the GaN/AlxGa1-xN quantum well (QW) structure are investigated as functions of the well width and the Al composition in the barrier by using a self-consistent (SC) calculation. In the case of a very narrow QW, no significant difference between the SC model and the flat-band (FB) model without polarizations is observed in the exciton binding energy. For the QW with a large well width, on the other hand, the exciton binding energy in the SC model is remarkably reduced compared to that in the FB model. This results from a smaller overlap due to the spatial separation between the conduction and the valence wave functions as the well width gets larger. The exciton binding energy is also found to be nearly independent of the Al composition.

229

Electronic structure and core excitons in AlSb as studied by soft-x-ray spectroscopy  

Science.gov (United States)

The Al K? x-ray-emission and the Al K x-ray-absorption spectra in single crystalline AlSb are reported. A core exciton is observed 0.2 eV below the onset of the Al 1s-to-conduction-band transition. Its intensity is markedly lower than that of the Al 2p exciton in the same compound. This points towards strong dipole selection rules for the formation of core excitons. The corresponding core-exciton recombination radiation could not be identified in the Al K? spectrum. Structure in the emission and absorption spectra are discussed in terms of the one-electron densities of states. From all available experimental data, an empirical density of valence states is constructed for AlSb.

Sénémaud, C.; Gheorghiu, A.; Ley, L.

1991-05-01

230

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

231

Ultrafast pump-probe spectroscopy of linear molecular aggregates: Effects of exciton coherence and thermal dephasing  

Science.gov (United States)

We model the pump-probe spectrum of linear molecular aggregates measured by using ultrafast laser pulses. We analyze in particular the effects of coherences between exciton states created by the pump pulse and show that these give rise to a small induced-absorption peak that is red-shifted relative to the main one-exciton bleaching peak. The new coherent peak survives common values of the static disorder. With increasing pump-probe delay time, its amplitude oscillates and decays with a rate that is inversely proportional to the width of the absorption band. We also consider coupling to vibrations and show that with increasing temperature the frequency separation between the main bleaching and induced-absorption peaks increases and provides a measure for the exciton coherence size imposed by scattering on the vibrations.

Heijs, Dirk-Jan; Dijkstra, Arend G.; Knoester, Jasper

2007-11-01

232

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

Science.gov (United States)

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

Miloszewski, Jacek M.; Tomi?, Stanko; Binks, David

2014-06-01

233

Semiconducting monolayer materials as a tunable platform for excitonic solar cells.  

Science.gov (United States)

The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict power conversion efficiency limits in the 10-20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, ultrathin solar cells in which unexplored exciton and carrier transport regimes are at play. PMID:23062107

Bernardi, Marco; Palummo, Maurizia; Grossman, Jeffrey C

2012-11-27

234

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 1003cm(-1) is assigned to the (SO4)(2-) ?1 symmetric stretching mode. Shoulders to this band are found at 997 and 1010cm(-1). The low intensity Raman bands at 1128, 1120 and even 1132cm(-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 624cm(-1) are assigned to the ?4 SO4(2-) bending modes. The ?2 (SO4)(2-) bending modes are observed at 460 and 494cm(-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. PMID:25000566

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

2014-12-10

235

Ultrafast exciton dynamics in Type II ZnTe-ZnSe colloidal quantum dots.  

Science.gov (United States)

Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel. PMID:22964845

Cadirci, M; Stubbs, S K; Fairclough, S M; Tyrrell, E J; Watt, A A R; Smith, J M; Binks, D J

2012-10-21

236

Excitonic effects in ZnO nanowires and hollow nanotubes  

Science.gov (United States)

Energy levels and wave functions of ground and excited states of an exciton are calculated by the method of imaginary time. Energy levels as functions of radius of single and double wall nanotube are studied. Asymptotic behavior of energy levels at large and small values of the radius using perturbation theory and adiabatic approximation is considered. Spatially indirect exciton in semiconductor nanowire is also investigated. Experimental result from high quality reproducible ZnO nanowires grown by low temperature chemical engineering is presented. State of the art high brightness white light emitting diodes (HB-LEDs) are demonstrated from the grown ZnO nano-wires. The color temperature and color rendering index (CRI) of the HB-LEDs values was found to be (3250 K, 82), and (14000 K, 93), for the best LEDs, which means that the quality of light is superior to one obtained from GaN LEDs available on the market today. The role of V Zn and V ° on the emission responsible for the white light band as well as the peak position of this important wide band is thoroughly investigated in a systematic way.

Willander, M.; Lozovik, Y. E.; Zhao, Q. X.; Nur, O.; Hu, Q.-H.; Klason, P.

2007-02-01

237

Electrooptical functions and ellipsometric parameters of excitons in cylindrical quantum dots  

Science.gov (United States)

We show how to compute the optical functions (the complex electrosusceptibility tensor, dielectric function, electroreflection spectra and ellipsometric parameters) for semiconductor quantum dots (QD) exposed to a uniform electric field in the growth direction, including the excitonic effects. The method uses the microscopic calculation of the QD excitonic wave functions and energy levels, and the macroscopic real density matrix approach (RDMA) to compute the electromagnetic fields and susceptibilities. The electron-hole screened Coulomb potential is adapted and the valence band structure is taken into account in the cylindrical approximation. In the microscopic calculations we solve the 6-dimensional two-particles Schrödinger equation by transforming it into an infinite set of coupled second order 2-dimensional differential equations with the appropriate boundary conditions. These differential equations are solved numerically giving the eigenfunctions and the energy eigenvalues. Then we used the RDMA and computed the frequency- and electric field strength dependent complex excitonic susceptibility tensor. The above approach enables us to determine the relative oscillator strength connected with excitonic resonances and to find the averaged susceptibilities for light- and heavy-holes excitons. Having the frequency dependent complex susceptibility tensor, we calculate the electrooptical functions for a QD. Numerical calculations have been performed for a InGaAs QD with a constant electric field applied in the growth direction. The optical Stokes parameters and ellipsometric parameters ? and ? as functions of the frequency and the angle of incidence are also determined. A good agreement with experiment is obtained.

Schillak, Piotr; Czajkowski, Gerard

2010-02-01

238

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

239

Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field  

CERN Document Server

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculate...

Soubusta, J; Hlidek, P; Zvara, M; Smrcka, L; Malzer, S; Geisselbrecht, W; Döhler, G H

1999-01-01

240

Unraveling halide hydration: A high dilution approach  

Science.gov (United States)

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

Migliorati, Valentina; Sessa, Francesco; Aquilanti, Giuliana; D'Angelo, Paola

2014-07-01

 
 
 
 
241

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

242

Effect of halide binding on intramolecular exciplex of double-linked zinc porphyrin-fullerene dyad  

Science.gov (United States)

The effect of chloride or bromide ion coordination to a doubly-linked zinc porphyrin-fullerene dyad was studied by quantitative analysis of the absorption band in the red and near infrared spectral range. The absorption band in question corresponds to the direct excitation to the intramolecular exciplex and it can be analyzed using the semi-quantum charge transfer theory. Compared to the nonligated compounds the energy of the exciplex decreased by ?0.3 eV and the oscillator strength of the transition increased by ?15-20%. This is explained by the effect of the electric field created by the negative charge of the halide.

Al-Subi, Ali H.; Niemi, Marja; Ranta, Jenni; Tkachenko, Nikolai V.; Lemmetyinen, Helge

2012-04-01

243

First-principles study on the electronic and optical properties of cubic ABX3 halide perovskites  

Science.gov (United States)

The electronic properties of ABX3 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 ABX3 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 CH3NH3SnBr3 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.; Gong, X. G.

2014-01-01

244

Dependence of Resonance Energy Transfer on Exciton Dimensionality  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of locali...

Junis Rindermann, Jan; Pozina, Galia; Monemar, Bo; Hultman, Lars; Amano, Hiroshi; Lagoudakis, Pavlos G.

2011-01-01

245

Evidence of delocalized excitons in amorphous solids.  

Science.gov (United States)

We studied the temperature dependence of the absorption coefficient of amorphous SiO2 in the range from 8 to 17.5 eV obtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4 eV to feature a close Lorentzian shape redshifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to amorphous SiO2. Excitons turn out to be coupled to an average phonon mode of 83 meV energy. PMID:20867590

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

2010-09-10

246

Spectrum of exciton in a quantum wire  

International Nuclear Information System (INIS)

In the present work we obtain the wave function and the corresponding energy of exciton confined within a quantum wire. What we do is to obtain the approximate analytical solution of the corresponding Schroedinger equation for the quantum wire in the presence of Coulomb and confining terms. We then calculate the energy and the binding energy of the exciton. By using the obtained energy of exciton, we calculate the corresponding wave length. The comparison of the obtained wave length with the emitted wave length from the semiconductor under study shows a good agreement with experimental results. (author)

247

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

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

Lalov, I. J.; Zhelyazkov, I.

2011-01-01

248

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

249

FTIR Studies of Internal Water Molecules of Bacteriorhodopsin: Structural Analysis of Halide-bound D85S and D212N Mutants in the Schiff Base Region  

Science.gov (United States)

Bacteriorhodopsin (BR), a membrane protein found in Halobacterium salinarum, functions as a light-driven proton pump. The Schiff base region has a quadropolar structure with positive charges located at the protonated Schiff base and Arg82, and counterbalancing negative charges located at Asp85 and Asp212 (Figure 1A). It is known that BR lacks a proton-pumping activity if Asp85 or Asp212 is neutralized by mutation. On the other hand, binding of C1- brings different effects for pumping functions in mutants at D85 and D212 position. While C1--bound D85T and D85S pump C1-, photovoltage measurements suggested that C1--bound D212N pumps protons at low pH. In this study, we measured low-temperature FTIR spectra of D85S and D212N containing various halides to compare the halide binding site of both proteins. In the case of D85S, the N-D stretching vibrations of the Schiff base were halide-dependent. This result suggests that the halide is a hydrogen-bond acceptor of the Schiff base, being consistent with the X-ray crystal structure. On the other hand, no halide dependence was observed for vibrational bands of the retinal skeleton and the Schiff base in the D212N mutant. This result suggests that the halide does not form a hydrogen bond with the Schiff base directly, unlike the mutation at D85 position. Halide-dependent water bands in the Schiff base region also differ between D85S and D212N. From these results, halide binding site of both proteins and role of two negative charges in BR will be discussed.

Shibata, Mikihiro; Kandori, Hideki

2007-12-01

250

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

251

Photoluminescence quantum and surface states of excitons in ZnSe and CdS nanoclusters  

Energy Technology Data Exchange (ETDEWEB)

The optical spectra of quantum dots (QDs) of CdS and ZnSe grown in borosilicate glass by the sol-gel method are obtained and analyzed. It is found that at concentrations of the two semiconductors x<0.06% the emission spectra are due to annihilation of free (internal) excitons in quantum states. The mean size of the quantum dots for a given concentration of ZnSe and CdS is calculated and found to be in good agreement with the X-ray data, and the exciton binding energy is calculated with allowance for the dielectric mismatch between the semiconductor and matrix. It is proposed that this mismatch may be the cause giving rise to the exciton percolation level that is observed in QD arrays for both systems at x>0.06%. The emission from the surface level of CdS QDs in the region approx2.7 eV, formed by the outer atoms with dangling bonds, is observed for the first time, as is the emission band from surface localized states. The relation between the position of the maximum of this band and the energy of the 1S state of the free exciton is established. It is shown that the properties of surface localized states are largely similar to the analogous properties of localized states of 3D (amorphous semiconductors, substitutional solid solutions of substitution) and 2D (quantum wells and superlattices) structures.

Bondar, N.V., E-mail: jbond@iop.kiev.u [Institute of Physics of NAS of Ukraine, pr. Nauki 46, Kiev 03028 (Ukraine)

2010-01-15

252

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

253

Characteristics of excitonic emission in diamond  

Science.gov (United States)

The Editor's Choice [1] is a theoretical study of the excitonic emission properties in diamond, in particular with regard to possible Bose-Einstein condensation (BEC). The cover picture shows a simple phase diagram for electron-hole pairs in dependence on their density and temperature. It is predicted that exciton BEC does occur for highly excited exciton densities, e.g. on the order of 1018-1019 cm-3, at low temperatures.The first author, Hideyo Okushi, started to study homoepitaxial CVD diamond films for electronic devices in 1995, now at AIST. Since 2001, he has been the leader of the CREST project Ultraviolet nanodevices using high-density exciton in diamond.This issue contains papers presented at SBDD X. This year saw the 10th anniversary of the series of International Workshops on Surface and Bulk Defects in CVD Diamond Films, which are traditionally held in Diepenbeek-Hasselt, Belgium.

Okushi, Hideyo; Watanabe, Hideyuki; Kanno, Shokichi

2005-09-01

254

Exciton in type-II quantum dot  

Energy Technology Data Exchange (ETDEWEB)

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

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

2009-05-01

255

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

256

Electrochemistry of plutonium in molten halides  

International Nuclear Information System (INIS)

The electrochemistry of plutonium in molten halides is of technological importance as a method of purification of plutonium. Previous authors have reported that plutonium can be purified by electrorefining impure plutonium in various molten haldies. Work to eluciate the mechanism of the plutonium reduction in molten halides has been limited to a chronopotentiometric study in LiCl-KCl. Potentiometric studies have been carried out to determine the standard reduction potential for the plutonium (III) couple in various molten alkali metal halides. Initial cyclic voltammetric experiments were performed in molten KCL at 1100 K. A silver/silver chloride (10 mole %) in equimolar NaCl-KCl was used as a reference electrode. Working and counter electrodes were tungsten. The cell components and melt were contained in a quartz crucible. Background cyclic voltammograms of the KCl melt at the tungsten electrode showed no evidence of electroactive impurities in the melt. Plutonium was added to the melt as PuCl/sub 3/, which was prepared by chlorination of the oxide. At low concentrations of PuCl/sub 3/ in the melt (0.01-0.03 molar), no reduction wave due to the reduction of Pu(III) was observed in the voltammograms up to the potassium reduction limit of the melt. However on scan reversal after scanning into the potassium reduction limit a new oxidation wave was observed

257

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

258

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

CERN Document Server

We study the linear optical absorption of bulk semiconductors in the presence of a homogeneous 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 $\\textbf{k} \\cdot \\textbf{p}$ 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 $\\textbf{k} \\cdot \\textbf{p}$ model. We consider the dependence of the enhancement on the strength of the dc field and the polarization of the optical field.

Duque-Gomez, Federico

2014-01-01

259

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

Science.gov (United States)

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

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

1986-01-01

260

Solvation at nanoscale: Alkali-halides in water clusters  

Energy Technology Data Exchange (ETDEWEB)

The solvation of alkali-halides in water clusters at nanoscale is studied by photoelectron spectroscopy using synchrotron radiation. The Na 2p, K 3p, Cl 2p, Br 3d, and I 4d core level binding energies have been measured for salt-containing water clusters. The results have been compared to those of alkali halide clusters and the dilute aqueous salt solutions. It is found that the alkali halides dissolve in small water clusters as ions.

Partanen, Leena [Department of Physics, Tampere University of Technology, Box 692, 33101 Tampere (Finland); Mikkelae, Mikko-Heikki; Huttula, Marko [Department of Physics, University of Oulu, Box 3000, 90014 University of Oulu (Finland); Tchaplyguine, Maxim [MAX-lab, Lund University, Box 118, 22100 Lund (Sweden); Zhang Chaofan; Andersson, Tomas; Bjoerneholm, Olle [Department of Physics and Astronomy, University of Uppsala, Box 516, 75120 Uppsala (Sweden)

2013-01-28

 
 
 
 
261

Photoexcitation, photoionisation and photofragmentation of molecular rubidium halides  

International Nuclear Information System (INIS)

Photoionisation and photofragmentation of molecular rubidium halides following photoexcitation in the vacuum ultraviolet region are investigated. For this purpose total and partial ion yield spectra are measured from the vapours of RbF, RbCl, RbBr and RbI. Two different decay channels - spectator and participator type - are studied in different rubidium halide molecules. It is demonstrated that the opening of the spectator channel takes place at different excitation energies, depending on the halide atom in the molecule

262

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

CERN Document Server

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

Frazer, Laszlo; Ketterson, J B

2013-01-01

263

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

Science.gov (United States)

...Definitions concerning metal halide lamp ballasts and fixtures. 431.322 Section...INDUSTRIAL EQUIPMENT Metal Halide Lamp Ballasts and Fixtures § 431.322 Definitions concerning metal halide lamp ballasts and fixtures. Ballast...

2010-01-01

264

Exciton magnetic polarons in Cd1-xMnxTe quantum wells  

International Nuclear Information System (INIS)

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

265

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

266

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

Energy Technology Data Exchange (ETDEWEB)

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.

Tani, Toshiro [Strategic Research Initiative for Future Nano-Science and Technology, Tokyo University at Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan) and Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan)]. E-mail: ttani@cc.tuat.ac.jp; Oda, Masaru [Strategic Research Initiative for Future Nano-Science and Technology, Tokyo University at Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan); Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan); Hayashi, Toshihide [Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan); Ohno, Hiroyuki [Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan); Hirata, Kazuyuki [Department of Applied Physics, Tokyo University of Agriculture and Technology, Naka-cho 2-24-16, Kogane-i, Tokyo 184-8588 (Japan)

2007-01-15

267

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

268

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

269

Application of VUV harmonics light source for investigation of energy transfer mechanisms in the wide band gap solids  

International Nuclear Information System (INIS)

Complete test of publication follows. The experimental aims were the study of interaction mechanism of excitons generated in the crystals with a wide forbidden gap. For this investigation, we studied the dependence of luminescence from excitation density. As the source of VUV was used high harmonic source based on 1 kHz Ti:sapphire 800 nm 40 fs laser. For our experiments we used harmonics from 15 to 27 generated in Ar (23-42 eV). 1. Interaction of molecular excitons in CdWO4. CdWO4 is the good modeling system with one type of excitations - excitons. It has only one luminescence band, corresponding to Self-Trapped Exciton luminescence. Observation of these experiments is the strong quenching of luminescence at high density of excitation. The mechanism of this quenching is interaction between excitons. The basic interaction mode of electronic excitations is the dipole-dipole interaction. A. Vasiliev has made mathematical model, witch describes this interaction. This model gives the equation to describe the decay kinetics. We can find the parameters of exciton-exciton interaction via fitting measured decay curve data by this equation with known parameters of the crystal and excitation beam. Result: radius of dipole-dipole interaction of excitons in CdWO4 estimated from the fitting: Rd-d = 4.3 nm. 2. Interaction of core and valence band excitations in BaF2. BaF2 has two types of localized electronic excitations: Self Trapped Excitons and Auger Free Luminescence. In contrast to the CdWO4 in BaF2 it also presents interaction between excitons and core holes. Experimental evidence: strong quenching of luminescence at the expense of interaction between core holes and excitons. This interaction is much stronger and faster then exciton-exciton interaction.

270

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

271

Improved value for the silicon free exciton binding energy  

International Nuclear Information System (INIS)

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

272

Dynamics of indirect exciton transport by moving acoustic fields  

Science.gov (United States)

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

Violante, A.; Cohen, K.; Lazi?, S.; Hey, R.; Rapaport, R.; Santos, P. V.

2014-03-01

273

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

274

Non-conservation of excitons in finite molecular chain  

Energy Technology Data Exchange (ETDEWEB)

We have analyzed a linear molecular chain with exciton excitations when the number of excitons is not conserved. The dispersion law depends on two independent variables and it is surfaced in a 3D plot. The same conclusion is valid for the concentrations of excitons and exciton pairs. As it was expected, physical characteristics of the finite chain depend on spatial coordinates. All results are compared to the corresponding results of an infinite chain.

Tosic, Bratislav, E-mail: btosic@yahoo.co [Vojvodina Academy of Science and Arts, 21000 Novi Sad, Dunavska 37 (Serbia); Sajfert, Vjekoslav, E-mail: sajfertv@nadlanu.co [University of Novi Sad, Technical Faculty ' M. Pupin' , 23000 Zrenjanin, Djure Djakovica bb (Serbia); Maskovic, Ljiljana, E-mail: maskovicm@yahoo.co.u [Academy of Criminalistic and Police Studies, 11000 Belgrade, Zemun (Serbia); Bednar, Nikola, E-mail: bednar.nikola@gmail.co [University of Novi Sad, Faculty of Technical Sciences, 21000 Novi Sad, Trg Dositeja Obradovica 6 (Serbia)

2010-11-15

275

Non-conservation of excitons in finite molecular chain  

International Nuclear Information System (INIS)

We have analyzed a linear molecular chain with exciton excitations when the number of excitons is not conserved. The dispersion law depends on two independent variables and it is surfaced in a 3D plot. The same conclusion is valid for the concentrations of excitons and exciton pairs. As it was expected, physical characteristics of the finite chain depend on spatial coordinates. All results are compared to the corresponding results of an infinite chain.

276

Dynamics of Long-Living Excitons in Tunable Potential Landscapes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

A novel method to experimentally study the dynamics of long-living excitons in coupled quantum well semiconductor heterostructures is presented. Lithographically defined top gate electrodes imprint in-plane artificial potential landscapes for excitons via the quantum confined Stark effect. Excitons are shuttled laterally in a time-dependent potential landscape defined by an interdigitated gate structure. Long-range drift exceeding a distance of 150 um at an exciton drift vel...

Gartner, Andreas; Schuh, Dieter; Kotthaus, Jorg P.

2005-01-01

277

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

278

Excitonic recombinations in h-BN: From bulk to exfoliated layers  

Science.gov (United States)

Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices is now of intense research focus, and it becomes particularly important to evaluate the role played by crystalline defects on their properties. In this paper, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First, the link between the presence of structural defects and the recombination intensity of trapped excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replicas of exciton recombinations. Second, the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations, indicating extremely compact excitons in h-BN.

Pierret, A.; Loayza, J.; Berini, B.; Betz, A.; Plaçais, B.; Ducastelle, F.; Barjon, J.; Loiseau, A.

2014-01-01

279

Exciton states in GaAs ?-doped systems under magnetic fields and hydrostatic pressure  

Energy Technology Data Exchange (ETDEWEB)

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

Mora-Ramos, M.E. [Facultad de Ciencias, Universidad Autónoma del Estado de Morelos, Ave. Universidad 1001, CP 62209 Cuernavaca, Morelos (Mexico); Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia); Duque, C.A., E-mail: cduque@fisica.udea.edu.co [Instituto de Física, Universidad de Antioquia, AA 1226 Medellín (Colombia)

2013-04-15

280

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

Science.gov (United States)

We study the properties of one-dimensional exciton systems in which the commonly made Heitler-London approximation (HLA) is relaxed. The nonresonant interaction terms which then exist, mix the multi-exciton bands of the HLA. Our approach is based on the exact diagonalization of the Hamiltonian, which is possible using the Jordan-Wigner and Bogoliubov transformations. Exact expressions for transition dipoles between multi-particle states are given. Results of our exact theory for the ground state and one-particle energies, the superradiant enhancement, the pump-probe spectrum, and the linear absorption to multi-particle states are compared quantitatively to the HLA, to the Bose approximation (where the excitons are treated as bosons), and to perturbation theory. In this comparative study, we use parameter values that are relevant to much studied quasi-one-dimensional J aggregates, such as PIC and TDBC. We find that for these systems the strongest effects of the HLA occur in the oscillator strengths of the various optical transitions. In particular, the exciton delocalization length derived from the experimentally observed superradiant enhancement is overestimated by roughly 10% due to the HLA. Also, the transition between the ground state and three-particle states, which is strictly forbidden in the HLA, does obtain a finite oscillator strength due to the non-resonant interactions.

Bakalis, Lisette D.; Knoester, Jasper

1997-05-01

 
 
 
 
281

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

282

Enhanced rate of radiative decay in CdSe quantum dots upon adsorption of an exciton-delocalizing ligand.  

Science.gov (United States)

This paper describes the enhancement of the quantum yield of photoluminescence (PL) of CdSe quantum dots (QDs) upon the adsorption of an exciton-delocalizing ligand, phenyldithiocarbamate. Increasing the apparent excitonic radius by only 10% increases the value of the radiative rate constant by a factor of 1.8 and the PL quantum yield by a factor of 2.4. Ligand exchange therefore simultaneously perturbs the confinement energy of charge carriers and enhances the probability of band-edge transitions. PMID:25167466

Jin, Shengye; Harris, Rachel D; Lau, Bryan; Aruda, Kenneth O; Amin, Victor A; Weiss, Emily A

2014-09-10

283

Exciton spectra in MoSe2  

International Nuclear Information System (INIS)

Wavelength-modulated reflectivity spectra have been performed on 2H MoSe2 at temperatures from 3.1 to 300 K. The A exciton series for which the n = 3 state was observed for the first time is shown to be anomalous with a Rydberg of 109 meV and a reduced effective mass of 0.62 m0. Several additional structures are observed and interpreted as well; in particular, the first two excited states of the B exciton, seen here for the first time, allow the determination of the parameters of that exciton series. No shift in any of the spectral structures was observed in magnetic fields of up to 7.0 T. (author)

284

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

285

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

286

Radiation damage in the alkali halide crystals  

International Nuclear Information System (INIS)

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

287

Electric-field-dependent photoconductivity in CdS nanowires and nanobelts: exciton ionization, Franz-Keldysh, and Stark effects.  

Science.gov (United States)

We report on the electric-field-dependent photoconductivity (PC) near the band-edge region of individual CdS nanowires and nanobelts. The quasi-periodic oscillations above the band edge in nanowires and nanobelts have been attributed to a Franz-Keldesh effect. The exciton peaks in PC spectra of the nanowires and thinner nanobelts show pronounced red-shifting due to the Stark effect as the electric field increases, while the exciton ionization is mainly facilitated by strong electron-longitudinal optical (LO) phonon coupling. However, the band-edge transition of thick nanobelts blue-shifts due to the field-enhanced exciton ionization, suggesting partial exciton ionization as the electron-LO phonon coupling is suppressed in the thicker belts. Large Stark shifts, up to 48 meV in the nanowire and 12 meV in the thinner nanobelts, have been achieved with a moderate electric field on the order of kV/cm, indicating a strong size and dimensionality implication due to confinement and surface depletion. PMID:22642694

Li, Dehui; Zhang, Jun; Zhang, Qing; Xiong, Qihua

2012-06-13

288

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

289

Switching exciton pulses through conical intersections  

CERN Document Server

Exciton pulses transport excitation and entanglement adiabatically through Rydberg aggregates, assemblies of highly excited light atoms, which are set into directed motion by resonant dipole-dipole interaction. Here, we demonstrate the coherent splitting of such pulses as well as the spatial segregation of electronic excitation and atomic motion. Both mechanisms exploit local non-adiabatic effects at a conical intersection, turning them from a decoherence source into an asset. The intersection provides a sensitive knob controlling the propagation direction and coherence properties of exciton pulses.

Leonhardt, K; Rost, J -M

2013-01-01

290

Arsenic-bound excitons in diamond  

Science.gov (United States)

A set of new excitonic recombinations is observed in arsenic-implanted diamond. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, an ionization energy of 0.41 eV is deduced for arsenic in diamond, which shows that arsenic is a shallower donor than phosphorus (0.6 eV), in agreement with theory.

Barjon, J.; Jomard, F.; Morata, S.

2014-01-01

291

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

292

The coacervation of aqueous solutions of tetraalkylammonium halides  

International Nuclear Information System (INIS)

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

293

Metal-rich gadolinium hydride halides  

International Nuclear Information System (INIS)

Preparation, structures, and thermal behaviour of gadolinium hydride halides GdXH/sub n/ (X = Cl, Br, I; 0.7 2, and GdX3 in sealed Ta ampoules to 900 0C and 950 0C. The arrangement of the heavy atoms corresponds to the 3 slab (X-Gd-Gd-X) structures of ZrCl and ZrBr. Besides these, a new stacking variant containing only 2 slabs in the identity period (2s-GdBrH/sub 0.69/ and 2s-GdIH/sub 0.80/; nomenclature of niobium dichalcogenides) is found. (author)

294

Energy levels and predissociation in mercuric halides  

International Nuclear Information System (INIS)

Electron energy-loss spectra were obtained over an extended energy-loss range (approximately 5 to 15 eV) for incident electron energies of 200 eV and scattering angles of 20 in the mercuric halides HgBr2 and HgCl2. Under these conditions of impact energy and scattering angle, the energy-loss spectra correspond closely to optical absorption spectra, though the energy-loss spectra extend to much higher energies than previous photoabsorption spectra, and reveal many more excited states of these molecules than had previously been observed. This preliminary report gives raw spectra and tentative identifications of the principal features of these spectra

295

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)

296

Charge transfer excitons and the metal-insulator transition in the high temperature superconductors  

CERN Document Server

The effects of the nearest neighbour Coulomb repulsion, V, are considered in the one dimensional copper-oxide chain using the modified Lanczos method. Above a critical value of V we find that charge transfer excitons are the lowest lying energy exitations in the insulating phase. Close to the metal-insulator transition and V=0 there is good `particle-hole' symmetry, showing that the mapping from a two band model to a one band model is appropriate. The effect of the nearest neighbour repulsion is to create states in the gap and to pin the Fermi level upon hole doping.

Vermeulen, C; Gagliano, E R

1994-01-01

297

Exciton dynamics in single-walled nanotubes:?Transient photoinduced dichroism and polarized emission  

Science.gov (United States)

Ultrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes ( S -NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, t , follow a power law, (t)-? with ? in the range of 0.7 to 1. Whereas the PA bands in S -NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition S -NTs in D2O solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum. The PL emission shows a degree of polarization that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S -NTs are not free carriers, rather they are excitons that are confined along the nanotubes. We found that the transient relaxation kinetics of the excitons depend on the NT form. The fastest exciton dynamics (with sub-picosecond lifetime) characterizes the annealed film, whereas the slowest dynamics (with lifetime of tens of ps) characterizes the isolated NTs in D2O solution. From the polarization memory decay we could estimate the diffusion constant, D , and the diffusion length, LD , of the excitons along the nanotube. For the annealed films at room temperature we found D?100cm2s-1 and LD?100nm . From the average PL polarization degree, which remains constant across the PL spectrum, and the transient polarization memory decay, we estimate the PL lifetime in NT solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by nonradiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate. The weak radiative transition strength is consistent with recent excited state calculations that include electron-hole interaction, which predict that excitons in NTs are basically dark.

Sheng, C.-X.; Vardeny, Z. V.; Dalton, A. B.; Baughman, R. H.

2005-03-01

298

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

299

Radiative recombination of excitons in the transition-metal dichalcogenides MoS2:Cl2 and WS2:Br2  

International Nuclear Information System (INIS)

The comparative study of the excitonic radiative recombination in 2H-MoS2:Cl2 and 2H-WS2:Br2 layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region-a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS2:Br2 in the range T=2-7K is related to the presence of a long lifetime sublevel, placed 0.3meV above the lowest excitonic state

300

Radiative recombination of excitons in the transition-metal dichalcogenides MoS{sub 2}:Cl{sub 2} and WS{sub 2}:Br{sub 2}  

Energy Technology Data Exchange (ETDEWEB)

The comparative study of the excitonic radiative recombination in 2H-MoS{sub 2}:Cl{sub 2} and 2H-WS{sub 2}:Br{sub 2} layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region-a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS{sub 2}:Br{sub 2} in the range T=2-7K is related to the presence of a long lifetime sublevel, placed 0.3meV above the lowest excitonic state.

Charron, L. [Department of Physics, University of Ottawa, Ont., K1N 6N5 (Canada); Dumchenko, D. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of); Fortin, E. [Department of Physics, University of Ottawa, Ont., K1N 6N5 (Canada); Gherman, C. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of); Kulyuk, L. [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD-2028 (Moldova, Republic of)]. E-mail: kulyuk@phys.asm.md.ca

2005-04-15

 
 
 
 
301

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 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

302

Electron concentration dependence of exciton localization and freeze-out at local potential fluctuations in InN films  

International Nuclear Information System (INIS)

InN films with electron concentration ranging from n?1017 to 1020 cm-3 grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 1018 cm-3 show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy ? loc is found to follow the n 5/12 power relation, which testifies to the observed strong localization effects in InN with high electron concentrations. (orig.)

303

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

304

Peculiarities of the determination of shallow impurity concentrations in semiconductors from the analysis of exciton luminescence spectra  

CERN Document Server

An analysis was made of the applicability limits of the method for the determination of the content of shallow acceptors and donors in semiconductors from the ratio of the low-temperature (T = 1.8-4.2 K) luminescence intensities of exciton bands, in particular, induces by radiative annihilation of excitons bound to acceptors (donors) and free excitons. It is shown that correct data about the concentrations of shallow acceptors and donors as well as data on changes in their content as a result of various treatments may be obtained if the occupancy of the defects in question by holes and electrons does not depend on the excitation intensity or external treatments. A way to check the fulfillment of criteria for the method application is suggested. An example is given is given of the method application for determination of thermally stimulated changes in the concentration of shallow acceptors and donors in gallium arsenide

Glinchuk, K D

2002-01-01

305

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

Science.gov (United States)

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

Malinowski, F. K.; Smole?ski, T.; Goryca, M.; Koperski, M.; Wojnar, P.; Kossacki, P.

2014-08-01

306

Nonboson treatment of excitonic nonlinearity in optically excited media  

International Nuclear Information System (INIS)

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

307

Time evolution of two ground-state excitons  

Science.gov (United States)

This work essentially deals with the time evolution of two ground-state excitons when these excitons have the same center-of-mass momentum. Since all coupled states have a higher energy, scatterings toward same energy states are formally impossible. As a result, previous works on the time evolution of two-exciton states should not apply. This led us to carefully reconsider the requirement of “energy conservation in the large time limit” through the Fermi golden rule, when dealing with composite excitons and to enlighten the key role of coherence time in the time evolution of excitons which is experimentally observed.

Combescot, M.; Betbeder-Matibet, O.

2010-06-01

308

Multistability of cavity exciton-polaritons affected by the thermally generated exciton reservoir  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Until now, the generation of an excitonic reservoir in a cavity polariton system under quasi-resonant pumping has always been neglected. We show that in microcavities having a small Rabi splitting (typically GaAs cavities with a single quantum well), this reservoir can be efficiently populated by polariton-phonon scattering. We consider the influence of the exciton reservoir on the energy shifts of the resonantly pumped polariton modes. We show that the presence of this rese...

Vishnevsky, D. V.; Solnyshkov, D. D.; Gippius, N. A.; Malpuech, G.

2011-01-01

309

Excitonic and vibronic spectra of Frenkel excitons in a two-dimensional simple latice  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional (2D) lattice with one molecule per unit cell have been studied and their manifestation in the linear absorption is simulated. We use the Green function formalism, the vibronic approach (see Lalov and Zhelyazkov [Phys. Rev. B \\textbf{75}, 245435 (2007)]), and the nearest-neighbor approximation to find expressions of the linear absorption lineshape in closed form (in terms of the elliptic integrals) ...

Lalov, Ivan J.; Zhelyazkov, Ivan

2012-01-01

310

Strong charge-transfer excitonic effects and the Bose-Einstein exciton condensate in graphane  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound exciton...

Cudazzo, Pierluigi; Attaccalite, Claudio; Tokatly, I. V.; Rubio Secades, A?ngel

2010-01-01

311

Effects of surface passivation on the exciton dynamics of CdSe nanocrystals as observed by ultrafast fluorescence upconversion spectroscopy  

International Nuclear Information System (INIS)

The exciton dynamics of CdSe nanocrystals are intimately linked to the surface morphology. Photo-oxidation of the selenium surfaces of the nanocrystal leads to an increase in radiative decay efficiency from both the band edge and deep trap emission states. The addition of the primary amine hexadecylamine curtails nonradiative excitonic decay attributed to the dangling surface selenium orbitals by passivation of those trap sites by the methylene protons on the amine, leading to enhanced band edge emission and the absence of deep trap emission. Furthermore, CdSe/ZnSe core/shell nanocrystals are not immune from contributions from surface states because of the alignment of the band structures of the core and shell materials

312

Equation-of-motion coupled-cluster study on exciton states of polyethylene with periodic boundary condition.  

Science.gov (United States)

Equation-of-motion coupled cluster with singles and doubles (EOM-CCSD) method has been applied to exciton states of polyethylene using ab initio crystal Hartree-Fock method with one-dimensional periodic boundary condition. Full transformation of two-electron integrals from atomic-orbital basis to crystal-orbital basis has been performed for EOM-CCSD calculations. In order to make transformed integrals to have correct properties of translational symmetry, a lattice summation scheme has been proposed. The EOM-CCSD excitation energies have been obtained for the lowest singlet and triplet exciton states of polyethylene. The excitation energies converge with system size much faster than oligomer calculations using n-alkanes. Quasiparticle energy-level calculations by second-order many-body perturbation theory and by solving the inverse Dyson equation have also been performed to obtain exciton binding energies. Basis set dependencies on excitation energy, quasiparticle band gap, and exciton binding energy have been investigated. At the 6-31+G level, the excitation energy of the lowest singlet-exciton state and its binding energy are calculated to be 8.1 and 3.2 eV, respectively. The calculated excitation energy is well comparable with the corresponding experimental value, 7.6 eV. PMID:15974710

Katagiri, Hideki

2005-06-01

313

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

314

Exciton transport and dissociation at organic interfaces  

Science.gov (United States)

This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (>50nm) to values actually measured in CPs (structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers.

Beljonne, David

2011-03-01

315

Exciton Transfer Integrals Between Polymer Chains  

CERN Document Server

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

Barford, W

2007-01-01

316

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

317

Exciton dephasing in ZnSe quantum wires  

DEFF Research Database (Denmark)

The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering within the wire due to the electron trapping in surface states and exciton localization. The exciton-exciton scattering efficiency, determined by the density dependence of the exciton dephasing, is found to increase with decreasing win width. This is assigned to the reduced phase space in a quasi-one-dimensional system, enhancing the repulsive interaction between excitons due to Pauli blocking.

Wagner, Hans Peter; Langbein, Wolfgang Werner

1998-01-01

318

Heavy n-doping: Wannier-Mott and Mahan excitons in ZnO  

Energy Technology Data Exchange (ETDEWEB)

Transparent conductive oxides such as ZnO are highly interesting within the modern field of optoelectronics since they have large fundamental band gaps while intentional as well as unintentional n-doping renders them conductive. However, the free electrons in the material form a degenerate electron gas which occupies the lowest conduction-band states and whose effect on the optical properties is unknown. In addition to the Pauli blocking of the lowest optical transitions, the degenerate electron gas significantly influences the screening of the electron-hole interaction. We generalize the solution of the Bethe-Salpeter equation for the polarization function to investigate both of these aspects as well as their interplay with the excitonic effects for n-ZnO. We introduce k-dependent occupation numbers to account for the Pauli blocking. The additional screening due to the free electrons is taken into account by means of a Thomas-Fermi approach. Our approach essentially captures the involved physics, hence, we observe a Mahan exciton at the absorption edge - in perfect agreement with a measured result. We show that due to the strong decrease of the binding energy and the oscillator strength with an increasing free-electron concentration in the material an excitonic Mott transition is barely observable.

Schleife, Andre; Roedl, Claudia; Hannewald, Karsten; Bechstedt, Friedhelm [Institut fuer Festkoerpertheorie und -optik and European Theoretical Spectroscopy Facility, Friedrich-Schiller-Universitaet Jena, 07743 Jena (Germany)

2011-07-01

319

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

320

Multiple excitons and the electron phonon bottleneck in semiconductor quantum dots: An ab initio perspective  

Science.gov (United States)

The article presents the current perspective on the nature of photoexcited states in semiconductor quantum dots (QDs). The focus is on multiple excitons and photo-induced electron-phonon dynamics in PbSe and CdSe QDs, and the advocated view is rooted in the results of ab initio studies in both energy and time domains. As a new type of material, semiconductor QDs represent the borderline between chemistry and physics, exhibiting both molecular and bulk-like properties. Similar to atoms and molecules, the electronic spectra of QD show discrete bands. Just as bulk semiconductors, QDs comprise multiple copies of the elementary unit cell, and are characterized by valence and conduction bands. The electron-phonon coupling in QDs is weaker than in molecules, but stronger than in bulk semiconductors. Unlike either material, the QD properties can be tuned continuously by changing QD size and shape. The molecular and bulk points of view often lead to contradicting conclusions. For example, the molecular view suggests that the excitations in QDs should exhibit strong electron-correlation (excitonic) effects, and that the electron-phonon relaxation should be slow due to the discrete nature of the optical bands and the mismatch of the electronic energy gaps with vibrational frequencies. In contrast, a finite-size limit of bulk properties indicates that the kinetic energy of quantum confinement should be significantly greater than excitonic effects and that the electron-phonon relaxation inside the quasi-continuous bands should be efficient. Such qualitative differences have generated heated discussions in the literature. The great potential of QDs for a variety of applications, including photovoltaics, spintronics, lasers, light-emitting diodes, and field-effect transistors makes it crutual to settle the debates. By synthesizing different viewpoints and presenting a unified atomistic picture of the excited state processes, our ab initio analysis clarifies the controversies regarding the phonon bottleneck and the generation of multiple excitons in semiconductor QDs. Both the electron-hole and charge-phonon interactions are strong and, therefore, optical excitations can directly generate multiple excitons, while the electron-phonon relaxation exhibits no bottlenecks, except at low excitation energies and in very small QDs.

Prezhdo, Oleg V.

2008-07-01

 
 
 
 
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

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

323

Selective excitation of excitonic transitions in PTCDA crystals and thin films  

Science.gov (United States)

We study various exciton transitions in PTCDA crystals and in thin films at low temperatures (˜5 K) by photoluminescence excitation spectroscopy (PLE) using DCM and RG6 dyes. The investigated PTCDA crystals are grown by sublimation and thin films are deposited on Si substrate by organic molecular beam deposition (OMBD) at high vacuum. The PLE excitation energy ranging from 1.878 to 2.172 eV enables the selective excitation of Frenkel excitons and of charge transfer transitions between PTCDA molecules in the same unit cell (CT1) as well as between stacked molecules along the growth direction (CT2). The observed excitation energy dependence of the emission bands supports the assignment of the different recombination channels obtained from time resolved PL measurements [1]. [1] A. Yu. Kobitski, R. Scholz, I Vragovic, H. P. Wagner, and D. R. T. Zahn, Phys. Rev. B 66, 153204 (2002).

Gangilenka, V. R.; Desilva, A.; Titova, Lyubov V.; Smith, L. M.; Wagner, H. P.; Scholz, R.

2008-03-01

324

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)

325

Bandgap renormalization and excitonic binding in T-shaped quantum wires  

CERN Document Server

We calculate the electronic structure for a modulation doped and gated T-shaped quantum wire using density functional theory. We calculate the bandgap renormalization as a function of the density of conduction band electrons, induced by the donor layer and/or the gate, for the translationally invariant wire, incorporating all growth and geometric properties of the structure completely. We show that most of the bandgap renormalization arises from exchange-correlation effects, but that a small shift also results from the difference of wave function evolution between electrons and holes. We calculate the binding energy of excitons in the wire, which breaks translational invariance, using a simpler, cylindrical model of the wire. For a single hole and a one dimensional electron gas of density n_e, screening of the exciton binding energy is shown to approximately compensate for bandgap renormalization, suggesting that the recombination energy remains approximately constant with n_e, in agreement with experiment. W...

Stopa, M

1999-01-01

326

Excitonic Insulator State in Ta2NiSe5 Probed by Photoemission Spectroscopy  

Science.gov (United States)

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Wakisaka, Y.; Sudayama, T.; Takubo, K.; Mizokawa, T.; Arita, M.; Namatame, H.; Taniguchi, M.; Katayama, N.; Nohara, M.; Takagi, H.

2009-07-01

327

Metal halide-group III halide gas complexes with emphasis on aluminum chloride  

International Nuclear Information System (INIS)

The thermodynamics of the presently known gas complexation reactions between metal halides and group III halides are treated in a self-consistent manner. By focusing on aluminum chloride as a complexing agent, certain systematic trends are revealed. The partial pressures of the gaseous complexes display shallow minima near 8000K whenever the complex molecules involve more than one molecule of AlCl3. Increasing the aluminum chloride pressure from 1 atm. to 103 atm. decreases somewhat the differences in the partial pressures among the various gaseous complexes which span two to three orders of magnitude. The methods developed for characterizing the complexes, and their structures as well as some applications of gas complexation are discussed

328

The effects of crystallographic orientation and strain on the properties of excitonic emission from wurtzite InGaN/GaN quantum wells  

International Nuclear Information System (INIS)

We have examined in detail crystal orientation effects on the properties of excitonic emission from wurtzite InxGa1-xN/GaN quantum wells (QWs) with piezoelectric polarization using exciton binding and transition energy calculations based on a single-band effective-mass theory. We show numerical results for the bandgaps, effective heavy-hole masses, piezoelectric polarizations and fields, exciton wavefunctions, exciton binding and transition energies and radiative lifetimes of excitonic emission as a function of the QW crystallographic growth planes. Band-edge and effective-mass parameters for a continuum of GaN crystallographic orientations, on which InGaN/GaN QWs are grown, were obtained from In-composition- and strain-dependent k·p calculation for wurtzite InxGa1-xN, using the 6x6 k·p Hamiltonian in appropriate {hkil} representations. We have performed calculations for a continuum of technologically relevant QW growth planes {hh-bar0l} oriented at various angles ? relative to the (0001) c-plane. The excitonic ground- and first-excited-state energies and wavefunctions were calculated using an effective potential method. A strong reduction of average in-plane heavy-hole effective mass and normal to the plane piezoelectric polarization and field is observed as ? varies from ? = 0 deg. (i.e. the c-axis direction) to ? = 49.5 deg., where the piezoelectric polarization and electric field reverse their orientation with resp reverse their orientation with respect to the plane of the QW. The decrease of the electric field in the InGaN/GaN QW growth direction leads to an increased exciton transition energy and oscillator strength, which results in the increase of the exciton binding energy and decrease of the excitonic radiative lifetime. For angles ?>49.5 deg. only small variations on the order of ?10% in the exciton binding and transition energies and excitonic radiative lifetime are observed for narrow In0.12Ga0.88N/GaN QWs that have widths less than ? 3.5 nm. The average in-plane heavy-hole effective mass reaches its minimum for ? = 90 deg., i.e. m-plane {11-bar00} growth. These results indicate that InGaN/GaN QW structures grown on non-(0001)-oriented planes in a wide variety of angles 49.5 deg. ???90 deg. can be used for optimized operation of optoelectronic devices

329

Vibrational Spectroscopy of Sodium Halide and Hydrogen Halide Aqueous Solutions: Application to Atmospheric Aerosol Chemistry  

Science.gov (United States)

Heterogeneous reactions on the surfaces of atmospheric aerosols play an important role in atmospheric chemistry. These reactions are capable of converting alkyl and hydrogen halides (common constituents of marine boundary aerosols) into active halogen compounds. Fundamental questions still remain concerning surface species and reaction mechanisms pertaining to marine boundary aerosols. The first step in beginning to understand these heterogeneous reactions is to determine how ions in solution affect the structure of water at the interface. Vibrational sum frequency generation spectroscopy is used to examine the air-liquid interface of sodium halide and hydrogen halide (i.e. strong acid) solutions. In addition, comparison of the bulk water structure to that of the interface is accomplished using Raman spectroscopy. The hydrogen-bonding environment at the surface of NaCl is found to be similar to that of the air-water interface. In contrast, the interfacial water structure of NaBr, HCl, and HBr solutions is significantly altered from that of neat water. In the bulk, NaCl, NaBr, HCl, and HBr solutions disturb the hydrogen-bonding network of neat water. A comparison between the corresponding salts and acids show that the salts produce greater disorder (i.e. less coupling of the water symmetric stretching modes) in the bulk water structure.

Levering, L. M.; Liu, D.; Allen, H. C.

2003-12-01

330

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

331

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

Energy Technology Data Exchange (ETDEWEB)

An optical study of the band-edge property of ReX{sub 2} (X=S, Se) has been carried out using the techniques of transmittance, photoreflectance (PR), and piezoreflectance (PzR). The polarized transmittance measurements indicate that the absorption edge of E parallel b polarization presents a significant red-shift behavior with respect to that of the E perpendicular b polarization. Band-edge excitons E{sub 1}{sup ex} and E{sub 2}{sup ex} for ReX{sub 2} are characterized using polarized PzR measurements. The polarization dependence of E{sub 1}{sup ex} and E{sub 2}{sup ex} provides conclusive evidence that the band-edge excitons are interband excitonic transitions originated from different origins. In addition higher energy excitonic series in ReX{sub 2} layered compounds is also studied using low-temperature PR measurements at 25 K. Prominent and enlarged excitonic features positioned at higher energy side with respect to the E{sub 1}{sup ex} and E{sub 2}{sup ex} are observed in the PR spectra. The observed excitonic sequence has been shown to be corresponding to the Rydberg series starting with principal quantum number n=2. From the experimental results together with the theoretical density-of-states calculations, the near band-edge property of rhenium disulfide (ReS{sub 2}) and diselenide (ReSe{sub 2}) are characterized and discussed.

Ho, C.H.; Huang, C.E

2004-11-30

332

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

International Nuclear Information System (INIS)

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

333

Electronic structure, Compton profiles and cohesive properties of Cs-halides  

Energy Technology Data Exchange (ETDEWEB)

We have reported energy bands, density of states, valence electron charge densities and Compton profiles of CsCl, CsBr and CsI using linear combination of atomic orbitals with Hartree-Fock and density functional theories. We have also computed these properties, except the momentum densities, using full potential linearized augmented plane wave method. The general features of the energy bands and the density of states in these halides are found to be almost similar. To interpret the theoretical data on Compton line shapes, we have also measured the Compton profiles using our 20 Ci {sup 137}Cs spectrometer. It is seen that the Hartree-Fock calculations give relatively a better agreement with the experimental momentum densities. On the basis of equal-valence-electron-density profiles, a comparison of relative nature of bonding is made which is in agreement with the valence charge densities and atomic charges by means of Mulliken analysis. Using our experimental and theoretical Compton profiles, we have also computed the cohesive energy of the halides.

Sharma, Vinit; Tiwari, Shailja [Department of Physics, University College of Science, M.L. Sukhadia University, Udaipur - 313001 (Rajasthan) (India); Ahuja, B.L., E-mail: blahuja@yahoo.co [Department of Physics, University College of Science, M.L. Sukhadia University, Udaipur - 313001 (Rajasthan) (India)

2010-06-15

334

Electric field controlled exciton transport in a conjugated polymer chain  

International Nuclear Information System (INIS)

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

335

Diffusion Monte Carlo study on excitonic complexes in type-II core-multishell nanowires  

Energy Technology Data Exchange (ETDEWEB)

Binding energies of excitons and excitonic complexes in BeTe/ZnSe type-II core-multishell cylindrical nanowires are calculated by quantum Monte Carlo method. Binding energies are enhanced for negatively charged excitons, and reduced for positively charged excitons and biexcitons. These results are ascribed to excitonic Coulomb potential comes from different spatial distribution of electrons and holes.

Tsuchiya, Takuma, E-mail: t.t@physics.or [Division of Applied Physics, Graduate School of Engineering Hokkaido University (Hokudai) Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628 (Japan)

2010-02-01

336

Formation of structured nanophases in halide crystals  

Science.gov (United States)

When halide crystals KCl and NaCl are slightly doped by PbCl2, (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 PbCl2 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 PbCl2 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

337

A T-shaped selenenyl halide.  

Science.gov (United States)

The title selenenyl halide complex, 3-iodo-2-phenyl-3H-3-selenaindazole, C(12)H(9)IN(2)Se, has an almost planar conformation and a nearly ideal T-shape for the Se(INC) moiety [Se-I 2.8122 (12), Se-C 1.881 (7) and Se-N2 2.051 (6) A; C-Se-N 79.6 (3), C-Se-I 96.8 (2) and N-Se-I 176.17 (17) degrees ]. This arrangement, together with the two selenium lone pairs, leads to a distorted trigonal-bipyrimidal geometry about the Se atom. Intermolecular interactions are largely limited to stacking forces. PMID:15263218

Majeed, Z; McWhinnie, W R; Lowe, P R

2000-03-15

338

Taming excitons in II-VI semiconductor nanowires and nanobelts  

Science.gov (United States)

Excitons are one of the most important fundamental quasi-particles, and are involved in a variety of processes forming the basis of a wide range of opto-electronic and photonic devices based on II-VI semiconductor nanowires and nanobelts, such as light-emitting diodes, photovoltaic cells, photodetectors and nanolasers. A clear understanding of their properties and unveiling the potential engineering for excitons is of particular importance for the design and optimization of nanoscale opto-electronic and photonic devices. Herein, we present a comprehensive review on discussing the fundamental behaviours of the excitons in one-dimensional (1D) II-VI semiconductor nanomaterials (nanowires and nanobelts). We will start with a focus on the unique properties (origin, generation, etc) and dynamics of excitons and exciton complexes in the II-VI semiconductor nanowires and nanobelts. Then we move to the recent progress on the excitonic response in 1D nanomaterials and focus on the tailoring and engineering of excitonic properties through rational controlling of the physical parameters and conditions, intrinsically and extrinsically. These include (1) exciton-exciton interaction, which is important for 1D nanomaterial nanolasing; (2) exciton-phonon interaction, which has interesting applications for laser cooling; and (3) exciton-plasmon interaction, which is the cornerstone towards the realization of plasmonic lasers. The potential of electric field, morphology and size control for excitonic properties is also discussed. Unveiling and controlling excitonic properties in II-VI semiconductor nanowires and nanobelts would promote the development of 1D nanoscience and nanotechnology.

Xu, Xinlong; Zhang, Qing; Zhang, Jun; Zhou, Yixuan; Xiong, Qihua

2014-10-01

339

Excitons, polarons, and laser action in poly(p-phenylene vinylene) films  

Science.gov (United States)

We have used a multitude of linear and nonlinear cw optical spectroscopies to study the optical properties of water precursor poly(p-phenylene vinylene) (PPV) thin films. These spectroscopies include absorption, photoluminescence, photoinduced absorption and their respective optically detected magnetic resonance, and electroabsorption spectroscopy. We have studied singlet and triplet excitons, polarons, and laser action in PPV films. We found that the lowest-lying absorption band is excitonic in origin. It consists of two absorption components due to a bimodal distribution of the polymer chain conjugation lengths. Electroabsorption spectroscopy unambiguously shows the positions of the lowest-lying odd parity exciton 1Bu at 2.59 eV and two of the higher-lying even-parity excitons, namely, mAg at 3.4 eV and kAg at 3.7 eV. From these exciton energies we obtained a lower bound for the exciton binding energy in PPV, Eb(min)=E(mAg)-E(1Bu)=0.8 eV. The quantum efficiency spectrum for triplet exciton photogeneration consists of two contributions; the intersystem crossing and, at higher energies, singlet fission. From the onset of the singlet fission process at ESF=2ET, we could estimate the energy of the lowest-lying triplet exciton, 1 3Bu, at 1.55 eV, with a singlet-triplet splitting as large as 0.9 eV. From photoinduced absorption spectroscopy we measured the triplet-triplet transition, T?T*, to be 1.45 eV. The quantum efficiency spectrum for polaron photogeneration also consists of two contributions: one extrinsic and the other intrinsic. The latter shows a monotonically increasing function of energy with an energy onset at 3.3 eV. The intrinsic photogeneration process is analyzed with a model of free-electron-hole pair photogeneration, which separate more effectively at higher energy. The carrier generation quantum yield at 3.65 eV is estimated to be 0.5%. The quantum efficiency for photoluminescence, on the other hand, shows one single step-function process, with an onset at 2.4 eV. The photoluminescence spectrum is well structured, showing five phonon side bands with 190 meV separation. We have also studied laser action in PPV thin films and microcavities such as microrings and microdisks. The effective gain spectrum is calculated and the estimated threshold excitation intensity for laser action for the 0-1 transition is found to be in good agreement with the data, with an estimated exciton density of 2×1018 cm-3. Lasing from microring devices shows several narrow waveguide modes, with intermode spacing of 0.45 nm that corresponds to an effective mode refractive index, neff=1.45. The spectral width of the laser modes is resolution limited and gives a lower estimate of the cavity quality factor, Q. For microrings we found Q>5000, which is limited by self-absorption in the polymer film.

Österbacka, R.; Wohlgenannt, M.; Shkunov, M.; Chinn, D.; Vardeny, Z. V.

2003-05-01

340

Excitons in Ultrathin PbI2 Crystals  

Science.gov (United States)

Due to their weak inter-layer van der Waals bonding, layered materials offer the unique possibility to produce natural quantum wells in the form of single and few atomic layer samples. A technique known as micromechanical cleavage, involving repeated cleaving, is used to isolate samples of all thicknesses [1]. Here, we present a combined experimental and theoretical study of band-edge excitons in the layered compound PbI2 and, in particular, on their behavior as a function of sample thickness. Results of photoluminescence and reflection experiments are reported on samples with thicknesses ranging from a few micrometers down to a few monolayers, as determined by atomic force microscopy measurements. The data display striking and well reproducible changes in the transition from three to two-dimensions, which will be compared with results of first-principles calculations of the electronic band structure based on density functional and many-body perturbation theory. Computational resources were provided by the DOE NERSC facility. [1] A. K. Geim et. al. Nature Materials 6, 183 (2007)

Toulouse, Alexis; Isaacoff, Benjamin; Shi, Guangsha; Kioupakis, Emmanouil; Matuchová, Marie; Merlin, Roberto

2013-03-01

 
 
 
 
341

Ultrafast spectroscopy of excitons in semiconducting carbon nanotubes  

Science.gov (United States)

Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t)-?, with ? in the range of 0.7 to 1. The PL emission of S-NTs in D2O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.

Sheng, Chuanxiang; Vardeny, Zeev V.; Dalton, Alan B.; Baughman, Ray H.

2005-04-01

342

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2012-12-15

343

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

International Nuclear Information System (INIS)

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

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

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

346

Photoluminescence dynamics due to biexcitons and exciton-exciton scattering in the layered-type semiconductor PbI2  

Science.gov (United States)

The dynamics of photoluminescence due to biexcitons and exciton-exciton scattering (M and P emissions, respectively) has been investigated in the layered-type semiconductor PbI2 by using the optical Kerr gate method. We simultaneously observed P and M emissions under high-density excitation. The M emission emerges instantaneously, whereas the P emission shows a delayed onset whose latency increases as the excitation photon energy increases. The latency to onset indicates that the P emission takes place after the relaxation of excitons with excess energy toward the bottleneck region via exciton-longitudinal optical (LO) phonon scattering processes. Based on the time-dependent peak energy shift of the P emission and a line-shape analysis of the M emission, we evaluated the effective temperatures of both photogenerated excitonic and biexcitonic systems as well as the self-energy due to the collisions among biexcitons. We conclude that these systems are separately formed in space owing to potential fluctuations between PbI2 layers, and independently reach thermal equilibrium after ˜30 ps with different cooling processes. The exciton-exciton and exciton-LO phonon scattering processes play an important role in cooling the excitonic system, whereas the biexciton-biexciton and biexciton-exciton collisions are dominant in cooling the biexcitonic system.

Ando, M.; Yazaki, M.; Katayama, I.; Ichida, H.; Wakaiki, S.; Kanematsu, Y.; Takeda, J.

2012-10-01

347

Crystal structure and density data. 2. Titanium halides and chalcogenides  

Energy Technology Data Exchange (ETDEWEB)

The following data are tabulated for titanium binary and complex halides and chalcogenides: compound, structural type, crystal system and space group, unit cell dimensions, number of formula units in the unit cell and density.

Franzen, H.F. (Iowa State Univ., of Science and Technology, Ames (USA). Dept. of Chemistry; Ames Lab., IA (USA))

1983-01-01

348

Dipole-dipole van der Waals interaction in alkali halides  

International Nuclear Information System (INIS)

Values of van der Waals dipole-dipole constants and interaction energetics of alkali halides are reported using the recent data. The values obtained are somewhat larger than those of earlier workers. (orig.)

349

Catalytic effect of halide additives ball milled with magnesium hydride  

Energy Technology Data Exchange (ETDEWEB)

The influence of various halide additives milled with magnesium hydride (MgH{sub 2}) on its decomposition temperature was studied. The optimum amount of halide additive and milling conditions were evaluated. The MgH{sub 2} decomposition temperature and energy of activation reduction were measured by temperature programmed desorption (TPD) and differential scanning calorimetry (DSC). The difference in catalytic efficiency between chlorides and fluorides of the various metals studied is presented. The effects of oxidation state, valence and position in the periodic table for selected halides on MgH{sub 2} decomposition temperature were also studied. The best catalysts, from the halides studied, for magnesium hydride decomposition were ZrF{sub 4}, TaF{sub 5}, NbF{sub 5}, VCl{sub 3} and TiCl{sub 3}. (author)

Malka, I.E.; Bystrzycki, J. [Department of Advanced Materials and Technologies, Military University of Technology, Kaliskiego 2, 00-908 Warsaw (Poland); Czujko, T. [Department of Advanced Materials and Technologies, Military University of Technology, Kaliskiego 2, 00-908 Warsaw (Poland); CanmetENERGY, Hydrogen Fuel Cells and Transportation Energy, Natural Resources (Canada)

2010-02-15

350

Excitonic exchange splitting in bulk semiconductors  

International Nuclear Information System (INIS)

We present an approach to calculate the excitonic fine-structure splittings due to electron-hole short-range exchange interactions using the local-density approximation pseudopotential method, and apply it to bulk semiconductors CdSe, InP, GaAs, and InAs. Comparing with previous theoretical results, the current calculated splittings agree well with experiments. Furthermore, we provide an approximate relationship between the short-range exchange splitting and the exciton Bohr radius, which can be used to estimate the exchange splitting for other materials. The current calculation indicates that a commonly used formula for exchange splitting in quantum dot is not valid. Finally, we find a very large pressure dependence of the exchange splitting: a factor of 4.5 increase as the lattice constant changes by 3.5%. This increase is mainly due to the decrease of the Bohr radius via the change of electron effective mass. copyright 1999 The American Physical Society

351

Frenkel-exciton-polaritons in organic microcavities  

Scientific Electronic Library Online (English)

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

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

352

Directional exciton transport in supramolecular nanostructured assemblies.  

Science.gov (United States)

Supramolecular nanostructured host-guest compounds provide an intriguing strategy for improved materials in optoelectronic devices. Here, conjugated organic guest molecules are embedded in channel-forming organic or inorganic hosts, like perhydrotriphenylene (PHTP) and zeolites, leading to weakly coupled, highly ordered, and brightly emissive materials. In-depth material characterization allows for a full understanding of the exciton transport mechanism in the weak coupling regime through steady-state time-resolved fluorescence studies combined with quantum-chemical based Monte-Carlo simulations without adjustable parameters. Despite weak chromophore coupling and dilution by the host, the exciton diffusion length exceeds 50 nm similar to molecular crystals, and might be tuned to the ?m range by the proper choice of host and guest materials. PMID:22941317

Gierschner, Johannes

2012-10-14

353

Microcavity controlled coupling of excitonic qubits  

CERN Document Server

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

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

2012-01-01

354

Can Natural Sunlight Induce Coherent Exciton Dynamics?  

CERN Document Server

Excitation of a model photosynthetic molecular aggregate by incoherent sunlight is systematically examined. For a closed system, the excited state coherence induced by the sunlight oscillates with an average amplitude that is inversely proportional to the excitonic gap, and reaches a stationary amplitude that depends on the temperature and coherence time of the radiation field. For an open system, the light-induced dynamical coherence relaxes to a static coherence determined by the non-canonical thermal distribution resulting from the entanglement with the phonon bath. The decay of the excited state population to the common ground state establishes a non-equilibrium steady-state flux driven by the sunlight, and it defines a time window to observe the transition from dynamical to static coherence. For the parameters relevant to photosynthetic systems, the exciton dynamics initiated by the sunlight exhibits a non-negligible amount of dynamical coherence (quantum beats) on the sub-picosecond timescale; however, ...

Olšina, Jan; Wang, Chen; Cao, Jianshu

2014-01-01

355

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)

356

Surface influence on the exciton characteristics of semiconductors  

CERN Document Server

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

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

2002-01-01

357

Localized excitons mediate defect emission in ZnO powders  

Science.gov (United States)

A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.

Foreman, J. V.; Simmons, J. G.; Baughman, W. E.; Liu, J.; Everitt, H. O.

2013-04-01

358

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

DEFF Research Database (Denmark)

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

Masri, Zarifi; Ruseckas, Arvydas

2013-01-01

359

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

DEFF Research Database (Denmark)

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

Nielsen, Martin Meedom

2013-01-01

360

Ultrafast dynamics of excitons in tetracene single crystals  

Energy Technology Data Exchange (ETDEWEB)

Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

Birech, Zephania; Schwoerer, Heinrich, E-mail: heso@sun.ac.za [Laser Research Institute, Stellenbosch University, Stellenbosch 7600 (South Africa); Schwoerer, Markus [Department of Physics, University of Bayreuth, Bayreuth (Germany); Schmeiler, Teresa; Pflaum, Jens [Experimental Physics VI, University of Würzburg and Bavarian Center for Applied Energy Research, Würzburg (Germany)

2014-03-21

 
 
 
 
361

Energy relaxation of quantum--well excitons during transport  

CERN Document Server

We investigate the energy relaxation of excitons during the real--space transport in a ZnSe quantum well by using microphotoluminescence with spatial resolution enhanced by a solid immersion lens. The spatial evolution of the LO--phonon sideband, originating from the LO--phonon assisted recombination of hot excitons, is measured directly. By calculating the LO--phonon assisted recombination probability, we obtain the non--thermal exciton energy distribution and observe directly the energy relaxation of hot excitons during their transport. We find the excitons remain hot during their transport on a length scale of several micrometers. Thus the excitonic transport on this scale cannot be described by classical diffusion.

Zhao, H; Kalt, H; Zhao, Hui; Moehl, Sebastian; Kalt, Heinz

2002-01-01

362

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

Energy Technology Data Exchange (ETDEWEB)

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

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

1998-12-10

363

Charge and exciton transport through molecular wires  

CERN Document Server

As functional elements in opto-electronic devices approach the singlemolecule limit, conducting organic molecular wires are the appropriateinterconnects that enable transport of charges and charge-like particles such as excitons within the device. Reproducible syntheses and athorough understanding of the underlying principles are therefore indispensable for applications like even smaller transistors, molecularmachines and light-harvesting materials. Bringing together experiment and theory to enable applications in real-life devices, this handbookand ready reference provides ess

Siebbeles, Laurens D A

2011-01-01

364

Excitons in small hydrogenated Si clusters  

Science.gov (United States)

The excitonic states of four small hydrogenated Si clusters (SiH4,Si2H6,Si5H12, and Si10H16) are studied using the diffusion quantum Monte Carlo approach. The importance of using accurate guiding wave functions is stressed and we show that the quantum chemical singles-only configuration interaction method and the time-dependent density functional theory within the adiabatic local-density approximation can provide suitable zeroth-order approximations in these systems.

Porter, A. R.; Towler, M. D.; Needs, R. J.

2001-07-01

365

Microcavity controlled coupling of excitonic qubits  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupli...

Albert, F.; Sivalertporn, K.; Kasprzak, J.; Strauß, M.; Schneider, C.; Ho?fling, S.; Kamp, M.; Forchel, A.; Reitzenstein, S.; Muljarov, E. A.; Langbein, W.

2013-01-01

366

Excitonic Dynamical Franz-Keldysh Effect  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The Dynamical Franz-Keldysh Effect is exposed by exploring near-bandgap absorption in the presence of intense THz electric fields. It bridges the gap between the DC Franz- Keldysh effect and multi-photon absorption and competes with the THz AC Stark Effect in shifting the energy of the excitonic resonance. A theoretical model which includes the strong THz field non-perturbatively via a non-equilibrium Green Functions technique is able to describe the Dynamical Franz-Keldysh ...

Nordstrøm, K. B.; Johnsen, Kristinn; Allen, S. J.; Jauho, Antti-pekka; Birnir, B.; Kono, J.; Noda, T.; Akiyama, H.; Sakaki, H.

1998-01-01

367

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

Science.gov (United States)

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

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

2014-04-28

368

Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Monolayer transition metal dichalcogenides feature Coulomb-bound electron-hole pairs (excitons) with exceptionally large binding energy and coupled spin and valley degrees of freedom. These unique attributes have been leveraged for electrical and optical control of excitons for atomically-thin optoelectronics and valleytronics. The development of such technologies relies on understanding and quantifying the fundamental properties of the exciton. A key parameter is the intrin...

Moody, Galan; Dass, Chandriker Kavir; Hao, Kai; Chen, Chang-hsiao; Li, Lain-jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Bergauser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

2014-01-01

369

Excitons in anisotropic two-dimensional semiconducting crystals  

Science.gov (United States)

The excitonic behavior of anisotropic two-dimensional crystals is investigated using numerical methods. We employ a screened potential arising due to the system polarizability to solve the central-potential problem using the Numerov approach. The dependence of the exciton energies on the interaction strength and mass anisotropy is demonstrated. We use our results to obtain the exciton binding energy in phosphorene as a function of the substrate dielectric constant.

Rodin, A. S.; Carvalho, A.; Castro Neto, A. H.

2014-08-01

370

Study of exciton transfer in dense quantum dot nanocomposites  

Science.gov (United States)

Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies.Nanocomposites of colloidal quantum dots (QDs) integrated into conjugated polymers (CPs) are key to hybrid optoelectronics, where engineering the excitonic interactions at the nanoscale is crucial. For such excitonic operation, it was believed that exciton diffusion is essential to realize nonradiative energy transfer from CPs to QDs. In this study, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites of dense QDs, where exciton transfer can be as efficient as 80% without requiring the assistance of exciton diffusion. This is enabled by uniform dispersion of QDs at high density (up to ~70 wt%) in the nanocomposite while avoiding phase segregation. Theoretical modeling supports the experimental observation of weakly temperature dependent nonradiative energy transfer dynamics. This new finding provides the ability to design hybrid light-emitting diodes that show an order of magnitude enhanced external quantum efficiencies. Electronic supplementary information (ESI) available: Information about the functionalized polymer structure, TEM of the QDs, XPS analysis of the nanocomposites, derivation of the energy transfer models and temperature dependent steady state PLs. See DOI: 10.1039/c4nr03456b

Guzelturk, Burak; Hernandez-Martinez, Pedro Ludwig; Sharma, Vijay Kumar; Coskun, Yasemin; Ibrahimova, Vusala; Tuncel, Donus; Govorov, Alexander O.; Sun, Xiao Wei; Xiong, Qihua; Demir, Hilmi Volkan

2014-09-01

371

Hall effect for indirect excitons in an inhomogeneous magnetic field  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We study the effect of non-homogeneous out-of-plane magnetic field on the behaviour of 2D spatially indirect excitons. Due to the difference of magnetic field acting on electrons and holes the total Lorentz force affecting the center of mass motion of an indirect exciton appears. Consequently, an indirect exciton acquires an effective charge proportional to the gradient of the magnetic field. The appearance of the Lorentz force causes the Hall effect for neutral bosons which...

Arnardottir, K. B.; Kyriienko, O.; Shelykh, I. A.

2012-01-01

372

Halide and proton binding kinetics of yellow fluorescent protein variants.  

Science.gov (United States)

A T203Y substitution in green fluorescent protein causes a red shift in emission to yield a class of mutants known as yellow fluorescent protein (YFP). Many of these YFP mutants bind halides with affinities in the millimolar range, which often results in the chromophore pK values being shifted into the physiological range. While such sensitivities may be exploited for halide and pH sensors, it is desirable to reduce such environmental sensitivities in other studies, such as in Förster resonance energy transfer probes to measure conformational changes within fusion proteins. Venus and Citrine are two such variants that have been developed with much reduced halide sensitivities. Here we compare the kinetics of halide binding, and the coupled protonation reaction, for several YFP variants and detect slow kinetics (dissociation rate constants in the range of 0.1-1 s(-1)), indicative of binding to an internal site, in all cases. The effective halide affinity for Venus and Citrine is much reduced compared with that of the original YFP 10C construct, primarily through a reduced association rate constant. Nuclear magnetic resonance studies of YFP 10C confirm halide binding occurs on a slow time scale (<4 s(-1)) and that perturbations in the chemical shift occur throughout the sequence and structure. PMID:23514090

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

2013-04-01

373

Relaxation of nonthermal hh and lh excitons in ZnSe quantum wells  

DEFF Research Database (Denmark)

The strong exciton-LO phonon coupling in ZnSe QWs gives a direct access to the relaxation dynamics of nonthermal, free heavy-hole and light-hole excitons. Narrow hot-exciton distributions can be generated by LO-phonon assisted exciton formation. The thermalization of these excitons is monitored by cw or time-resolved phonon-sideband spectroscopy. A thermal exciton distribution evolves on a 100 ps timescale at low exciton densities when only scattering with acoustic phonons is possible. Exciton-exciton scattering accelerates the relaxation dynamics at high densities. Filling of the free heavy-hole exciton states and thermal population of the light-hole exciton states is observed. Measurements of the polarization degree after excitation with a sigma(+)-polarized picosecond pulse show that light-hole excitons with zero in-plane momentum change sign of their spin with each emission of an LO phonon.

Langbein, Wolfgang Werner; Hvam, JØrn Märcher

1998-01-01

374

Pattern formation of indirect excitons in coupled quantum wells  

International Nuclear Information System (INIS)

Using a nonlinear Schroedinger equation including short-range two-body attraction and three-body repulsion, we investigate the spatial distribution of indirect excitons in semiconductor coupled quantum wells. The results obtained can explain the experimental phenomenon that the annular exciton cloud first contracts then expands when the number of confined excitons is increased in the impurity potential well, as observed by Lai et al (2004 Science 303 503). In particular, the model reconciles the patterns of exciton rings reported by Butov et al (2002 Nature 418 751). At higher densities, the model predicts much richer patterns, which could be tested by future experiments

375

Exciton molecule in semiconductors by two-photon absorption  

International Nuclear Information System (INIS)

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect

376

Exciton management in organic photovoltaic multidonor energy cascades.  

Science.gov (United States)

Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures. PMID:24702468

Griffith, Olga L; Forrest, Stephen R

2014-05-14

377

Disorder-enhanced exciton delocalization in an extended dendrimer.  

Science.gov (United States)

The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory. PMID:25215792

Pouthier, Vincent

2014-08-01

378

Density of states model and photoluminescence spectra of localized excitons in ZnHgTe and CdHgTe alloys  

Science.gov (United States)

A tractable model of the tail-state distribution in alloys is obtained by minimizing the entropy of composition fluctuation in an assumed spherical well. This model is used to analyse the photoluminescence band due to recombination of localized excitons in Zn-rich ZnHgTe and Cd-rich CdHgTe.

Ouadjaout, D.; Marfaing, Y.; Lusson, A.; Heurtel, A.

1990-04-01

379

Excitonic instability and electric-field-induced phase transition towards a two dimensional exciton condensate  

CERN Document Server

We present an InAs-GaSb-based system in which the electric-field tunability of its 2D energy gap implies a transition towards a thermodynamically stable excitonic condensed phase. Detailed calculations show a 3 meV BCS-like gap appearing in a second-order phase transition with electric field. We find this transition to be very sharp, solely due to exchange interaction, and so, the exciton binding energy is greatly renormalized even at small condensate densities. This density gradually increases with external field, thus enabling the direct probe of the Bose-Einstein to BCS crossover.

Naveh, Y

1996-01-01

380

Exciton-exciton correlations revealed by two-quantum, two-dimensional fourier transform optical spectroscopy.  

Science.gov (United States)

The Coulomb correlations between photoexcited charged particles in materials such as photosynthetic complexes, conjugated polymer systems, J-aggregates, and bulk or nanostructured semiconductors produce a hierarchy of collective electronic excitations, for example, excitons, and biexcitons, which may be harnessed for applications in quantum optics, light-harvesting, or quantum information technologies. These excitations represent correlations among successively greater numbers of electrons and holes, and their associated multiple-quantum coherences could reveal detailed information about complex many-body interactions and dynamics. However, unlike single-quantum coherences involving excitons, multiple-quantum coherences do not radiate; consequently, they have largely eluded direct observation and characterization. In this Account, we present a novel optical technique, two-quantum, two-dimensional Fourier transform optical spectroscopy (2Q 2D FTOPT), which allows direct observation of the dynamics of multiple exciton states that reflect the correlations of their constituent electrons and holes. The approach is based on closely analogous methods in NMR, in which multiple phase-coherent fields are used to drive successive transitions such that multiple-quantum coherences can be accessed and probed. In 2Q 2D FTOPT, a spatiotemporal femtosecond pulse-shaping technique has been used to overcome the challenge of control over multiple, noncollinear, phase-coherent optical fields in experimental geometries used to isolate selected signal contributions through wavevector matching. We present results from a prototype GaAs quantum well system, which reveal distinct coherences of biexcitons that are formed from two identical excitons or from two excitons that have holes in different spin sublevels ("heavy-hole" and "light-hole" excitons). The biexciton binding energies and dephasing dynamics are determined, and changes in the dephasing rates as a function of the excitation density are observed, revealing still higher order correlations due to exciton-biexciton interactions. Two-quantum coherences due to four-particle correlations that do not involve bound biexciton states but that influence the exciton properties are also observed and characterized. The 2Q 2D FTOPT technique allows many-body interactions that cannot be treated with a mean-field approximation to be studied in detail; the pulse-shaping approach simplifies greatly what would have otherwise been daunting measurements. This spectroscopic tool might soon offer insight into specific applications, for example, in detailing the interactions that affect how electronic energy moves within the strata of organic photovoltaic cells. PMID:19691277

Stone, Katherine W; Turner, Daniel B; Gundogdu, Kenan; Cundiff, Steven T; Nelson, Keith A

2009-09-15

 
 
 
 
381

Excitonic effects in time-dependent density-functional theory: An analytically solvable model  

CERN Document Server

We investigate the description of excitonic effects within time-dependent density-functional theory (TDDFT). The exchange-correlation kernel f_xc introduced in TDDFT allows a clear separation of quasiparticle and excitonic effects. Using a diagrammatic representation for f_xc, we express its excitonic part f_xc^Ex in terms of the effective vertex function Lambda. The latter fulfills an integral equation which thereby establishes the exact correspondence between TDDFT and the standard many-body approach based on Bethe-Salpeter equation (BSE).The diagrammatic structure of the kernel in the equation for Lambda suggests the possibility of strong cancellation effects. Should the cancellation take place, already the first-order approximation to f_xc^Ex is sufficient. A potential advantage of TDDFT over the many-body BSE method is thus dependent on the efficiency of the above-quoted cancellation. We explicitly verify this for an analytically solvable two-dimensional two-band model. The calculations confirm that the ...

Stübner, R; Pankratov, O

2004-01-01

382

Exciton trapping in vibrationally excited organic molecules near a ZnO surface  

CERN Document Server

We present a systematic study of the exciton population dynamics at the interface of the spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2',7'-ditertbutyl-9.9'-spirobifluorene (SP6) and the non-polar (10-10) surface of ZnO, using time-resolved excited state optical transmission spectroscopy. The photoexcited dye first undergoes intramolecular vibrational relaxation in the S1 state on a 2 to 9 ps timescale. Subsequently, the excited state transmission reveals transitions from two distinct vibrational levels of S1, with a lifetime of the vibrationally excited state that is comparable to the one of the vibrational ground state (vGS). The electronic population relaxes by (i) decay to the electronic ground state (ii) transfer to a long-lived dark state that remains populated for longer than 5 microseconds, and (iii) diffusion-limited charge transfer to the ZnO conduction band. Remarkably, the lifetime of the vibratioanlly trapped excition (exciton-vibron) and vGS exciton are not equally affected by a change of s...

Foglia, Laura; Wolf, Martin; Stähler, Julia

2014-01-01

383

Mott transition of excitons in ZnSe studied by phase resolved reflection  

Energy Technology Data Exchange (ETDEWEB)

The Mott transition of excitons in a semiconductor with increasing carrier density is in principle well understood as a consequence of screening of the Coulomb interaction between carriers. While the position of the exciton stays widely unchanged the exciton peak disappears due to band gap shrinkage. However, a more quantitative check of applied theoretical models for the screening and of the role of quantum kinetic effects in this process is still open.We demonstrate that the phase-resolved reflection in shallow-confined ZnSSe heterostructures opens the possibility of a detailed study of the Mott transition due to the appearance of pronounced interferences effects of propagating polariton modes. Our theoretical approach for the investigation of the influence of excited carriers on the dielectric susceptibility is based (i) on a quasi-particle approximation for the carrier energies and damping, and (ii) on the semiconductor Bloch equations including dynamical screening and a quantum kinetic treatment of scattering. These manybody effects lead to drastic changes of both amplitude and phase of the reflected light: Jumps of the phase are steepened, show abrupt changes from +{pi} to -{pi} and are smoothed out, and interference peaks in the amplitude decrease and disappear completely with increasing excitation. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Manzke, Guenter; Henneberger, Klaus; Seemann, Matthias; Stolz, Heinrich [University Rostock (Germany). Institute of Physics

2009-02-15

384

Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles  

International Nuclear Information System (INIS)

We report systematic study of structural, phonon and optical properties of Cr-doped ZnO nanoparticles. These particles are synthesized through simple sol–gel technique. Structural studies carried out by X-ray diffraction method, confirm that the prepared particles are in hexagonal wurtzite structure and lattice parameters change considerably while increasing the doping. Raman and Fourier transform infrared spectral studies show that the intensity of the phonon modes decreased and also blue shift due to ion doping, respectively. Apart from this, transmission electron microscopic studies show reduction in particle size where the particle diameters reduced from 36 to 11 nm. Optical absorption spectral measurements show a blue shift in the band-gap and increment in excitonic oscillator strength. Photoluminescence studies show doping altered the near-band edge emission but there is no change in the other emission bands which is due to oxygen vacancy, surface defects and surface dangling bonds.

385

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

Science.gov (United States)

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

2010-01-01

386

Strong coupling of exciton and photon modes in photonic crystal infiltrated with organic-inorganic layered perovskite  

Science.gov (United States)

Large vacuum Rabi-splitting, the evidence of strong coupling of photon and exciton modes, was observed at room temperature in an ordered array of silica microspheres infiltrated with organic-inorganic layered perovskite. By natural sedimentation of a colloidal suspension of monodispersed silica microspheres with a diameter of 256 nm, three-dimensional ordered array of silica microspheres (silica opal) were prepared. Into an air space of the silica opal, organic-inorganic perovskite, bis-(phenethylammonium) tetraiodoplumbate (PAPI), which exhibits intense exciton absorption at 2.40 eV, was infiltrated. The silica opal infiltrated with PAPI (23% of air space was filled) exhibited a stop band at 2.13 eV, when observed at a normal direction to the sample surface. By changing the observation angle, the coupling of the stop band at around 2.1-2.4 eV and the exciton band at 2.40 eV due to PAPI was attained. From angle-tuning measurements of reflection spectra, a vacuum Rabi-splitting of 240 meV was evaluated.

Sumioka, Kazuhiro; Nagahama, Hiroyuki; Tsutsui, Tetsuo

2001-03-01

387

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

388

Color silver halide hologram production and mastering  

Science.gov (United States)

Color reflection holograms recorded with the Denisyuk geometry have been demonstrated by the recently formed HOLOS Corporation in New Hampshire. The Slavich red-green-blue (RGB) sensitized ultra-high resolution silver halide emulsion was used for the hologram recording. The employed laser wavelengths were 647 nm, 532 nm, and 476 nm, generated by an argon ion, a frequency doubled Nd:YAG, and a krypton ion laser, respectively. A beam combination mechanism with dichroic filters enabled a simultaneous RGB exposure, which made the color balance and overall exposure energy easy to control as well as simplifying the recording procedure. HOLOS has been producing limited edition color holograms in various sizes from 4' X 5' to 12' X 16'. A 30 foot long optical table and high power lasers will enable HOLOS to record color holograms up to the size of one meter square in the near future. Various approaches have been investigated in generating color hologram masters which have sufficiently high diffraction efficiency to contact copy the color images onto photopolymer materials. A specially designed test object including the 1931 CIE chromaticity diagram, a rainbow ribbon cable, pure yellow dots, and a cloisonne elephant was used for color recording experiments. In addition, the Macbeth Color Checker chart was used. Both colorimetric evaluation and scattering noise measurements were performed using the PR-650 Photo Research SpectraScan SpectraCalorimeter.

Bjelkhagen, Hans I.; Huang, Qiang

1997-04-01

389

Combination bands in vibronic spectra of molecular crystals  

Energy Technology Data Exchange (ETDEWEB)

The molecular crystals' vibronic spectra with combination bands have been studied using the complete vibronic approach. Those vibronic spectra consist of one electronic excitation (Frenkel exciton) plus vibrational quanta of two different intramolecular vibrational modes. The linear absorption spectra of one-dimensional models of some aromatic crystals have been calculated applying the methods of canonical transformations and Green functions (at T = 0) in the following cases: (i) vibronic spectra of naphthalene and benzene which consist of Frenkel exciton plus one phonon of non-totally symmetrical vibration plus one/two phonons of totally symmetrical vibrations; (ii) vibronic spectra of an anthracene-like model and other two models with combination bands with two different totally symmetrical vibrations (one of them is weakly coupled with Frenkel exciton). In this paper, we show that the linear absorption demonstrates considerable changes in the one-phonon and two-phonon vibronic spectra caused by the indirect coupling between vibrational modes (appearing because of their coupling with a Frenkel exciton). Our simulations of absorption spectra prove the opportunity to observe some peculiarities like the manifestation of various molecular configurations and edge anomalies in the shape of the absorption bands.

Lalov, I.J. [Faculty of Physics, Sofia University, BG-1164 Sofia (Bulgaria); Zhelyazkov, I. [Faculty of Physics, Sofia University, BG-1164 Sofia (Bulgaria)], E-mail: izh@phys.uni-sofia.bg

2008-09-03

390

Combination bands in vibronic spectra of molecular crystals  

International Nuclear Information System (INIS)

The molecular crystals' vibronic spectra with combination bands have been studied using the complete vibronic approach. Those vibronic spectra consist of one electronic excitation (Frenkel exciton) plus vibrational quanta of two different intramolecular vibrational modes. The linear absorption spectra of one-dimensional models of some aromatic crystals have been calculated applying the methods of canonical transformations and Green functions (at T = 0) in the following cases: (i) vibronic spectra of naphthalene and benzene which consist of Frenkel exciton plus one phonon of non-totally symmetrical vibration plus one/two phonons of totally symmetrical vibrations; (ii) vibronic spectra of an anthracene-like model and other two models with combination bands with two different totally symmetrical vibrations (one of them is weakly coupled with Frenkel exciton). In this paper, we show that the linear absorption demonstrates considerable changes in the one-phonon and two-phonon vibronic spectra caused by the indirect coupling between vibrational modes (appearing because of their coupling with a Frenkel exciton). Our simulations of absorption spectra prove the opportunity to observe some peculiarities like the manifestation of various molecular configurations and edge anomalies in the shape of the absorption bands

391

Combination bands in vibronic spectra of molecular crystals  

Science.gov (United States)

The molecular crystals' vibronic spectra with combination bands have been studied using the complete vibronic approach. Those vibronic spectra consist of one electronic excitation (Frenkel exciton) plus vibrational quanta of two different intramolecular vibrational modes. The linear absorption spectra of one-dimensional models of some aromatic crystals have been calculated applying the methods of canonical transformations and Green functions (at T = 0) in the following cases: (i) vibronic spectra of naphthalene and benzene which consist of Frenkel exciton plus one phonon of non-totally symmetrical vibration plus one/two phonons of totally symmetrical vibrations; (ii) vibronic spectra of an anthracene-like model and other two models with combination bands with two different totally symmetrical vibrations (one of them is weakly coupled with Frenkel exciton). In this paper, we show that the linear absorption demonstrates considerable changes in the one-phonon and two-phonon vibronic spectra caused by the indirect coupling between vibrational modes (appearing because of their coupling with a Frenkel exciton). Our simulations of absorption spectra prove the opportunity to observe some peculiarities like the manifestation of various molecular configurations and edge anomalies in the shape of the absorption bands.

Lalov, I. J.; Zhelyazkov, I.

2008-09-01

392

Localized excitons in II-VI semiconductor alloys: Density-of-states model and photoluminescence line-shape analysis  

Science.gov (United States)

A simple but tractable model of the density of exciton states associated with potential fluctuations in semiconductor alloys is proposed. It consists of minimizing the fluctuation entropy related to the composition fluctuation ?x averaged over a volume V, for a given localization energy ?. A critical volume Vc(?x), defined as the smallest volume in which the fluctuation ?x can occur, is introduced. This model leads to a density-of-states tail of the form exp[-(?/?0)3/2]. The characteristic energy ?0 depends on the manner an exciton can be localized: as a whole or through electron and/or hole confinement. It is shown that the most probable event is determined by two physical parameters of the system: the electron-hole mass ratio and the ratio between the coefficients of variation with composition of the conduction- and valence-band edges. The density of states is used to model the exciton photoluminescence line shape of three representative alloys Zn0.97Hg0.03Te, CdS0.36Se0.64, and Cd0.92Hg0.08Te in which exciton localization occurs, respectively, via the electron, via the hole, or by electron-hole confinement. In each case a good agreement with the experimental results is obtained.

Ouadjaout, Djamel; Marfaing, Yves

1990-06-01

393

Single exciton spectroscopy of semimagnetic quantum dots  

CERN Document Server

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

Fernández-Rossier, J

2006-01-01

394

Self-trapped excitons in caesium iodide  

International Nuclear Information System (INIS)

The magnetic circular and linear polarisation of the self-trapped exciton (STE) luminescence of pure CsI and CsBr are studied experimentally for 1.3 < T < 50 K and 0 < B < 6T. By comparing the 4.2 K data with numerical calculations based on the model proposed in the previous paper, the orbital and spin symmetries and the best set of parameters characterising the static and kinetic properties of the lowest triplet STE states have been determined. The transient behaviour of the luminescence observed under microwave pulse (ODEPR) is also studied and good agreement with the calculation is obtained. (author)

395

Dynamical excitonic effects in metals and semiconductors  

CERN Document Server

The dynamics of an electron--hole pair induced by the time--dependent screened Coulomb interaction is discussed. In contrast to the case where the static electron--hole interaction is considered we demonstrate the occurrence of important dynamical excitonic effects in the solution of the Bethe--Salpeter equation.This is illustrated in the calculated absorption spectra of noble metals (copper and silver) and silicon. Dynamical corrections strongly affect the spectra, partially canceling dynamical self--energy effects and leading to good agreement with experiment.

Marini, A; Marini, Andrea; Sole, Rodolfo Del

2003-01-01

396

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

397

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

398

Design and construction of a copper-halide laser  

International Nuclear Information System (INIS)

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

399

Optically induced rotation of a quantum dot exciton spin  

CERN Document Server

We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two non-degenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time-difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.

Poem, E; Kodriano, Y; Benny, Y; Khatsevich, S; Avron, J E; Gershoni, D

2011-01-01

400

Band gap tuning of ZnO nanoparticles via Mg doping by femtosecond laser ablation in liquid environment  

International Nuclear Information System (INIS)

Highlights: ? Femtosecond laser ablation synthesis of Mg doped ZnO nanoparticles. ? Electronic properties of ZnO are modified by Mg. ? Band gap and exciton energy shifts to the blue. ? The exciton energy shift is saturated at Mg content of about 20%. ? Phase separation at Mg content is at more than 25%. ? Mechanism of exciton pinning – recombination via new surface states. - Abstract: We use multiphoton IR femtosecond laser ablation to induce non-thermal non-equilibrium conditions of the nanoparticle growth in liquids. Modifications of the electronic properties of ZnO NP were achieved by Mg ion doping of targets prepared from mixtures of Zn and Mg acetylacetonates. The nanoparticle sizes were 3–20 nm depending on the ablation conditions. X-ray fluorescence indicates that stoichiometric ablation and incorporation of Mg in nanocrystalline ZnO occurs. HRTEM observations show that nanoparticles retain their wurtzite structure, while at high Mg concentrations we detect the MgO rich domains. Exciton emissions exhibit relatively narrow bands with progressive and controlled blue shifts up to 184 meV. The exciton energy correlates to band edge absorption indicating strong modification of the NP band gaps. Stabilisation of the exciton blue shift is observed at high Mg concentration. It is accompanied by the formation of structure defects and ZnO/MgO phase separation within the nanoparticles.

 
 
 
 
401

Self-luminescence of several fluorite-structure halides doped with curium or berkelium  

International Nuclear Information System (INIS)

The self-luminescence emission spectra of several fluorite-structure halides doped with 0.1 cation % 244Cm or 249Bk were measured in the range 14 X 105 to 50 X 105m-1 (200 - 700 nm) between 295 and 600 K. The hosts included CaF2, SrF2, BaF2, and SrCl2 (for Cm) and BaF2 and SrCl2 (for Bk). The room-temperature spectra of the Cm-doped samples all have relatively sharp peaks at approximately 16.5 X 105m-1 and broad, asymmetric bands with maxima near 35 X 105m-1 (fluorides) or 28 X 105m-1(SrCl2). The sharp peaks are attributed to the Cm dopant and are stable to at least 600 K, while the broad bands appear to be associated with host anion centers and disappear on sample heating. The room-temperature spectrum of Bk-doped BaF2 also displays an anion-related band at 35 X 105m-1, while that of Bk-doped SrCl2 has both an anion band at 28 X 105m-1 and a peak at 19 X 105m-1 (believed to be an additional host effect). The luminescence intensity of all samples decreased with room-temperature storage, but could be partially restored by annealing. (Auth.)

402

Probing excitonic dark states in single-layer tungsten disulphide.  

Science.gov (United States)

Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, 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. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (?0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing. PMID:25162523

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

2014-09-11

403

Probing excitonic dark states in single-layer tungsten disulphide  

Science.gov (United States)

Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, 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. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing.

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

2014-09-01

404

Structure and properties of lanthanide halides  

Science.gov (United States)

Lanthanum and cerium bromides and chlorides form isomorphous alloy systems with the UCl 3 type structure. These scintillating alloys exhibit high luminosity and proportional response, making them the first scintillators comparable to room temperature semiconductors for gamma spectroscopy; Ce(III) activated lanthanum bromide has recently enabled scintillating gamma ray spectrometers with < 3% FWHM energy resolutions at 662 keV. However brittle fracture of these materials impedes development of large volume crystals. Low fracture stress and perfect cleavage along prismatic planes cause material cracking during and after crystal growth. These and other properties pose challenges for material production and post processing; therefore, understanding mechanical behavior is key to fabricating large single crystals, and engineering of robust detectors and systems. Recent progress on basic structure and properties of the lanthanide halides is reported here, including thermomechanical and thermogravimetric analyses, hygroscopicity, yield strength, and fracture toughness. Observations including reversible hydrate formation under atmospheric pressure, loss of stoichiometry at high temperature, anisotropic thermal expansion, reactivity towards common crucible materials, and crack initiation and propagation under applied loads are reported. The fundamental physical and chemical properties of this system introduce challenges for material processing, scale-up, and detector fabrication. Analysis of the symmetry and crystal structure of this system suggests possible mechanisms for deformation and crack initiation under stress. The low c/a ratio and low symmetry relative to traditional scintillators indicate limited and highly anisotropic plasticity cause redistribution of residual process stress to cleavage planes, initiating fracture. This proposed failure mechanism and its implications for scale up to large diameter crystal growth are also discussed.

Doty, F. P.; McGregor, Douglas; Harrison, Mark; Findley, Kip; Polichar, Raulf

2007-09-01

405

Photophysics of quasi-one-dimensional excitons in pi-conjugated polymers and semiconducting single-walled carbon nanotubes  

Science.gov (United States)

In this work we studied the ultrafast dynamics of photoexcitations in pi-conjugated organic semiconductors and semiconducting single-walled carbon nanotubes (S-NTs), using a low-intensity high-repetition rate laser system in the spectral range from 0.13 to 1.05 eV, and high-intensity low-repletion rate laser system in the spectral range from 1.2 to 2.5 eV, in the time domain from 100 fs to 1 ns. We also measured cw photomodulation (PM) spectroscopy of pi-conjugated polymers and photoluminescence (PL) spectra of both polymers and isolated nanotubes. In polymers, we found that excitons are the primary photoexcitations in single polymer chains. However, polarons and polaron pairs may also be photogenerated at early time in films. We consider this process to be extrinsic in nature, namely, dependent on materials properties, temperatures, excitation photon energies, as well as the quality of films. Both annealed and unannealed thin NT films and D2O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands were observed, which also showed photoinduced dichroism and decay together after taking into account the PB spectral shift. The PL emission shows polarization degree. We therefore conclude that the primary photoexcitations in S-NT are excitons that are confined along the NTs. Prom the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime in isolated NTs in solution to be of the order of 500 ps, coupling with the minute PL emission quantum efficiency, which indicates weak radiative transition strength. In S-NTs and pi-polymers, the emission spectra relative to the absorption bands are very similar, as well as transient photoinduced absorption bands (PA) with a low-energy PA1 and a higher-energy PA2 in all cases. Theoretical calculations of excited state absorptions within a correlated pi-electron Hamiltonian show the same excitonic energy spectrum, illustrating the universal features of quasi-one-dimensional excitons in carbon-based pi-conjugated systems. In both cases PA1 gives the lower limit of the binding energy of the lowest optical exciton. The binding energy of lowest exciton belonging to the widest S-NTs with diameters ? 1 nm in films is 0.3-0.4 eV, as well as ˜ 0.9 eV in PPV polymers.

Sheng, Chuanxiang

406

Magneto-optical studies of the type I type II crossover and band offset in ZnTe/Zn1-xMnxTe superlattices in magnetic fields up to 45 T  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report measurements of magneto-reflectivity in ZnTe/Zn1-xMnxTe superlattices in magnetic fields up to 45 Tesla. From an analysis of the Zeeman splitting, we investigate the change of band alignment with field and the band offset ratio. A crossing of the 1 s exciton transitions from the ZnTe buffer layer and the 1 s heavy hole sigma(+) exciton of the superlattice is observed, providing unambiguous evidence of a band alignment change from type I to type II. The excitonic energy levels for bo...

Cheng, Hh; Nicholas, Rj; Lawless, Mj; Ashenford, DE; Lunn, B.

1996-01-01

407

Signature of excitonic effect in BaFe2As2 revealed by angle-resolved photoemission spectroscopy  

Science.gov (United States)

We report on an electronic structural study of BaFe2As2 which has a spin-density-wave (SDW) transition and is a parent material of FeAs-based superconductors. Angle-resolved photoemission spectroscopy on BaFe2As2 shows that, across the SDW transition, the spectral weight at 0-100 meV below EF is reconstructed to form flat bands and Fermi surfaces in the SDW state. The coexistence of the Fermi surfaces and the flat band supports the orbital-dependent excitonic coupling between electron and hole pockets rather than the simple Fermi surface nesting as the origin of the SDW state.

Wakisaka, Y.; Sudayama, T.; Takubo, K.; Morinaga, R.; Sato, T. J.; Arita, M.; Namatame, H.; Taniguchi, M.; Mizokawa, T.

2010-12-01

408

Signature of excitonic effect in BaFe2As2 revealed by angle-resolved photoemission spectroscopy  

International Nuclear Information System (INIS)

We report on an electronic structural study of BaFe2As2 which has a spin-density-wave (SDW) transition and is a parent material of FeAs-based superconductors. Angle-resolved photoemission spectroscopy on BaFe2As2 shows that, across the SDW transition, the spectral weight at 0-100 meV below EF is reconstructed to form flat bands and Fermi surfaces in the SDW state. The coexistence of the Fermi surfaces and the flat band supports the orbital-dependent excitonic coupling between electron and hole pockets rather than the simple Fermi surface nesting as the origin of the SDW state.

409

Near band gap photoluminescence properties of hexagonal boron nitride  

CERN Document Server

Near band-gap luminescence (hnu > 5 eV) of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 eV and 5.3 eV. The high-energy emission at 5.5 eV is composed of fixed sub-bands assigned to bound excitons at 5.47 eV, 5.56 eV and 5.61 eV. The non-structured low-energy emission at 5.3 eV undergoes a large blue shift (up to 120 meV) with a linear slope 5.7 eV, the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the 5.3 eV band to quasi donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photo-induced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.

Museur, Luc

2008-01-01

410

Chemistry of gaseous lower valent actinide halides. Final technical report, August 1979-August 1985  

International Nuclear Information System (INIS)

The objective of this program was to provide accurate thermochemical information for key actinide-halide and oxyhalide systems, starting with uranium halides, so that the basic factors underlying the chemical bonding and chemical reactivity in these systems can be elucidated in a systematic way. Our principal focus was on the gaseous halides, mainly the lower valent halides, because these molecular species largely will define the high temperature chemistry in the nuclear applications of interest. 6 refs., 2 tabs

411

Methyl halide emissions from greenhouse-grown mangroves  

Science.gov (United States)

Two mangrove species, Avicennia germinans and Rhizophora mangle, were greenhouse grown for nearly 1.5 years from saplings. A single individual