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1

Exciton-relaxation dynamics in lead halides  

CERN Document Server

We survey recent comprehensive studies of exciton relaxation in the crystals of lead halides. The luminescence and electron-spin-resonance studies have revealed that excitons in lead bromide spontaneously dissociate and both electrons and holes get self-trapped individually. Similar relaxation has been also clarified in lead chloride. The electron-hole separation is ascribed to repulsive correlation via acoustic phonons. Besides, on the basis of the temperature profiles of self-trapped states, we discuss the origin of luminescence components which are mainly induced under one-photon excitation into the exciton band in lead fluoride, lead chloride, and lead bromide.

Iwanaga, M; Iwanaga, Masanobu; Hayashi, Tetsusuke

2003-01-01

2

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

3

Band structure and excitons in CsCdBr3  

International Nuclear Information System (INIS)

CsCdBr3 has a very unique electronic band structure. The conduction band is split up into two parts: a lower isolated and very narrow band (?E?4000 cm-1) and higher lying ones. Just below the lowest conduction band we observe an absorption band of excitons localized at lattice defects. Both of them, the low-lying conduction band as well as the localized excitons, play an important role in the fast and very efficient energy transfer within the crystal. This can be shown by analyzing the decay dynamics of the fluorescence of the dopant RE-ions and of the localized and lattice excitons (excitons of the perfect lattice). Also, excitation into either the excitons or the RE-ion clearly shows an energy transfer back and forth between both of them. We deem this mechanism to be the most important one for the observed upconversion processes in CsCdBr3. (orig.)

2000-04-12

4

Self-trapped excitons in pure and Na- and Tl-doped caesium halides and the recombination luminescence  

International Nuclear Information System (INIS)

[en] On the basis of a method used earlier to predict the off-centre relaxed triplet self-trapped excitons in alkali halides, similar systems in caesium halides are studied. This work confirms the expected off-centre relaxation along the [100] cubic axes with increasing magnitude in the order CsI, CsBr and CsCl. The calculated emission energies are in reasonable agreement with observed values. The well-known 2.95 eV emission band of CsI:Na has been studied as a tunnelling recombination between a close pair consisting of a VK centre and a Na atom. For a number of close-pair geometries the emission energies are close to 3 eV. The strong emission bands of CsI:Tl at 2.25 eV and 2.55 eV have been interpreted as arising from tunnelling recombination of close pairs each consisting of a Tl0 and a VK centre. The calculated emission energies and polarizations are discussed in conjunction with the experimental data. (author)

1999-07-19

5

F-center and self-trapped exciton formation in strongly excited alkali halide crystals  

International Nuclear Information System (INIS)

Method of luminescent and absorption spectroscopy with time resolution was used to study the effect of density of electron pulse excitation (tp=10-8 s, P=(105-108) WXcm-2) on efficiency of ?? two-halide autolocalized exciton (TALE) and F-centers (?F) formation in CsI, CsBr, KBr, KI alkali halide crystals. It was established that for all studied systems the elevation of P power of electron beam (EB) from 105 up to 5X107 WXcm-2 resulted to sufficient decrease of production efficiency and yield of TALE luminescence. In the case when F-centers of colour are induced predominantly by pulsed irradiation in crystals, F-center yield is independent of P. If F-centers and TALE are produced in comparable amounts (CsBr crystals, T=80 K), ?? decrease with P growth is accompanied by ?F growth.

1988-01-01

6

Basic mechanisms of color centres production by excitons in activated alkali halides  

International Nuclear Information System (INIS)

[en] The paper deals with some peculiarities of colour centers formation which are caused by introduction of the activator in alkali halide crystals. The crystals of KBr and KI activated with Tl+, In+, Sn++ in concentrations 1017-1018 cm-3 and irradiated with ultraviolet light are studied. Excitation spectra of photostimulated activator luminescence and thermoluminescence were measured. The kinetics of the photostimulated activator luminescence is studied. The conclusion is made that the activator does not affect the primary reaction of exciton decay with F-H pair generation, but only the secondary reactions of colour center production

1981-01-01

7

1D exciton band and exciton-phonon interaction in a single polymer chain  

CERN Multimedia

The excitonic luminescence of an isolated polydiacetylene polymer chain in its monomer matrix is studied by micro-photoluminescence. These chains behave as perfect 1D excitonic systems with the expected 1/sqrt{E} density of states between 5 and 50 K. The temperature dependence of the homogeneous width is quantitatively explained by interaction with longitudinal acoustic phonons of the crystal in the range of temperature explored.

Dubin, F

2002-01-01

8

Band structure and excitons in CsCdBr{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

CsCdBr{sub 3} has a very unique electronic band structure. The conduction band is split up into two parts: a lower isolated and very narrow band ({delta}E{proportional_to}4000 cm{sup -1}) and higher lying ones. Just below the lowest conduction band we observe an absorption band of excitons localized at lattice defects. Both of them, the low-lying conduction band as well as the localized excitons, play an important role in the fast and very efficient energy transfer within the crystal. This can be shown by analyzing the decay dynamics of the fluorescence of the dopant RE-ions and of the localized and lattice excitons (excitons of the perfect lattice). Also, excitation into either the excitons or the RE-ion clearly shows an energy transfer back and forth between both of them. We deem this mechanism to be the most important one for the observed upconversion processes in CsCdBr{sub 3}. (orig.)

Wenzel, M.; Altwein, M.; Demirbilek, R.; Leu, B.; Heber, J.; Kuebler, J. [Technische Univ. Darmstadt (Germany). Inst. fuer Festkoerperphysik; Bleeker, B.; Meijerink, A. [University of Utrecht, Utrecht (Netherlands)

2000-04-12

9

Physics of halide scintillators  

International Nuclear Information System (INIS)

[en] The paper reviews the results obtained within the last 5-6 years in the field of the physics of some halide crystals which have led to a better understanding of their characteristics. The luminescence and scintillation characteristics of caesium iodides, widely used in the existing detectors, as well as the properties of some other selected materials are discussed in view of their possible applications. For all the systems concerned, the emission important for their practical applications arises from the radiative decay of excitons of various types. The following features are considered in more detail: (1) the excited states structure, the mechanism of excitation of the localized exciton emission in the impurity-induced absorption bands, the processes of defects creation, and the scintillation mechanism in CsI:Tl and CsI:Na scintillators; (2) the luminescence of on-centre excitons in CsI and RbI crystals; (3) the luminescence of free cation excitons in the CsPbX3-type aggregates thermally created in CsX:Pb single crystals (X=Cl, Br); (4) the model of the excited states responsible for the cation exciton luminescence of lead halides

2001-01-01

10

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

International Nuclear Information System (INIS)

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

1987-01-01

11

Exciton-phonon interaction in one dimension: Half-filled-band sector  

International Nuclear Information System (INIS)

[en] We propose and investigate a continuum-field theory which describes the long-distance behavior of the Frenkel excitons interacting with phonons in one dimension in the half-filled band sector. For both spinless electrons and spin-1/2 electrons, the system exhibits a superconducting instability above a critical coupling

1988-10-01

12

Excitonic characteristics in direct wide-band-gap CuScO2 epitaxial thin films  

Science.gov (United States)

Thin films of a delafossite compound CuScO2 were grown on spinel MgAl2O4 (111) substrates, yielding in highly crystalline and (0001)-oriented epitaxial structures. Absorption spectra at 20 K revealed a sharp exciton resonance at 3.97 eV, which persisted up to 300 K. Its direct transition band gap at 20 K and exciton binding energies were determined to be about 4.35 and 380 meV, both of which are considerably larger than those of ZnO. In view of its capability of naturally layered structure and p-type doping, this compound will be interesting for exciton physics as well as implementation of heterostructured devices.

Hiraga, H.; Makino, T.; Fukumura, T.; Ohtomo, A.; Kawasaki, M.

2009-11-01

13

Observation of excitonic and band-to-band behavior in ordered InGaP{sub 2} alloys  

Energy Technology Data Exchange (ETDEWEB)

Photoluminescence measurements on ordered InGaP{sub 2} were studied as a function of temperature, laser power density, and magnetic field. The temperature varied between 1.4 and 300 K, the laser power densities ranged from 10 nW/cm{sup 2} to 20 W/cm{sup 2}, and the maximum magnetic field was 13.6 T. The data show both excitonic and band-to-band behavior, depending upon the incident laser power density. A consistent interpretation of all data leads to a type-II valence-band offset between the ordered domains.

Jones, E.D.; Follstaedt, D.M.; Lee, H.; Nelson, J.S.; Schneider, R.P. Jr. [Sandia National Labs., Albuquerque, NM (United States); Alonso, R.G.; Horner, G.S.; Machol, J.; Mascarenhas, A. [National Renewable Energy Lab., Golden, CO (United States)

1994-09-01

14

Polaritonic stop-band transparency via exciton-biexciton coupling in CuCl.  

UK PubMed Central (United Kingdom)

Radiation is almost completely reflected within the exciton-polariton stop band of a semiconductor, as in the typical case of CuCl. We predict, however, that a coherently driven exciton-biexciton transition allows for the propagation of a probe light beam within the stop band. The phenomenon is reminiscent of electromagnetically induced transparency effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystalline structure via a frequency and wave-vector selective polaritonic mechanism. A well-developed transparency, favored by the narrow linewidth of the biexciton, is established within the stop band where a probe pulse may propagate with significant delays. The transparency window can be controlled via the pump beam detuning and intensity.

Chesi S; Artoni M; La Rocca GC; Bassani F; Mysyrowicz A

2003-08-01

15

Polaritonic stop-band transparency via exciton-biexciton coupling in CuCl.  

Science.gov (United States)

Radiation is almost completely reflected within the exciton-polariton stop band of a semiconductor, as in the typical case of CuCl. We predict, however, that a coherently driven exciton-biexciton transition allows for the propagation of a probe light beam within the stop band. The phenomenon is reminiscent of electromagnetically induced transparency effects occurring in three-level atomic systems, except that it here involves delocalized electronic excitations in a crystalline structure via a frequency and wave-vector selective polaritonic mechanism. A well-developed transparency, favored by the narrow linewidth of the biexciton, is established within the stop band where a probe pulse may propagate with significant delays. The transparency window can be controlled via the pump beam detuning and intensity. PMID:12906631

Chesi, S; Artoni, M; La Rocca, G C; Bassani, F; Mysyrowicz, A

2003-07-29

16

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)

[en] 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

1992-01-01

17

Coherent infrared multidimensional spectra of the OH stretching band in liquid water simulated by direct nonlinear exciton propagation.  

Science.gov (United States)

The two-dimensional vibrational response of the disordered strongly fluctuating OH exciton band in liquid water is investigated using a new simulation protocol. The direct nonlinear exciton propagation generalizes the nonlinear exciton equations to include nonadiabatic time dependent Hamiltonian and transition dipole fluctuations. The excitonic picture is retained and the large cancellation between Liouville pathways is built-in from the outset. The sensitivity of the photon echo and double-quantum-coherence techniques to frequency fluctuations, molecular reorientation, intermolecular coupling, and the two-exciton coherence is investigated. The photon echo is particularly sensitive to the frequency fluctuations and molecular reorientation, whereas the double-quantum coherence provides a unique probe for intermolecular couplings and two-exciton coherence. PMID:19449930

Falvo, Cyril; Palmieri, Benoit; Mukamel, Shaul

2009-05-14

18

Coherent infrared multidimensional spectra of the OH stretching band in liquid water simulated by direct nonlinear exciton propagation.  

UK PubMed Central (United Kingdom)

The two-dimensional vibrational response of the disordered strongly fluctuating OH exciton band in liquid water is investigated using a new simulation protocol. The direct nonlinear exciton propagation generalizes the nonlinear exciton equations to include nonadiabatic time dependent Hamiltonian and transition dipole fluctuations. The excitonic picture is retained and the large cancellation between Liouville pathways is built-in from the outset. The sensitivity of the photon echo and double-quantum-coherence techniques to frequency fluctuations, molecular reorientation, intermolecular coupling, and the two-exciton coherence is investigated. The photon echo is particularly sensitive to the frequency fluctuations and molecular reorientation, whereas the double-quantum coherence provides a unique probe for intermolecular couplings and two-exciton coherence.

Falvo C; Palmieri B; Mukamel S

2009-05-01

19

Exciton spectra and energy band structure of CuAlS{sub 2} crystals  

Energy Technology Data Exchange (ETDEWEB)

Three exciton series are investigated in the reflectivity spectra of CuAlS{sub 2} crystals at the temperature of 10 K. The n{sup A}=1 ({omega}{sub T}=3.543 eV, {omega}{sub L}= 3.546 eV) and n{sup A}=2 (3.565 eV) lines of the {Gamma}{sub 4} (A-series) excitons are observed in the E-parallel c polarization. In the E-perpendicular c polarization, the {Gamma}{sub 5} excitons (n{sup B}=1 at 3.668 eV and n{sup B}=2 at 3.686 eV) of the B-series, and n{sup C}=1 at 3.813 eV of the C-series are observed. The parameters of the excitons and the more exact values of the {Gamma}{sub 7} - {Gamma}{sub 6}, {Gamma}{sub 6} - {Gamma}{sub 6}, and {Gamma}{sub 7} - {Gamma}{sub 6} energy gaps are determined. The crystal field and spin-orbit splitting of the valence band is calculated. The electron (m{sub c1}{sup *}) and hole (m{sub V1}*, m{sub V2}*, and m{sub V3}*) effective masses have been estimated.

Syrbu, N.N. [Technical University of Moldova, 168 Stefan cel Mare avenue, 2004 Chisinau (Moldova, Republic of); Korzun, B.V.; Fadzeyeva, A.A.; Mianzelen, R.R. [Institute of Physics of Solids and Semiconductors, P. Brovki 17, Minsk 220072 (Belarus); Ursaki, V.V., E-mail: ursaki@yahoo.co [Institute of Applied Physics, Academy of Sciences of Moldova, 5 Academy street, 2028 Chisinau (Moldova, Republic of); Galbic, I. [Technical University of Moldova, 168 Stefan cel Mare avenue, 2004 Chisinau (Moldova, Republic of)

2010-08-15

20

Exciton spectra and energy band structure of CuGaSe{sub 2} single crystals  

Energy Technology Data Exchange (ETDEWEB)

Photoreflectivity, wavelength modulation spectroscopy and photoluminescence measurements of CuGaSe{sub 2} are used to determine the exciton and band parameters as well as the energy band structure of CuGaSe{sub 2} at photon energies higher than the fundamental band gap. The spectral dependences of the real {epsilon}{sub 1} and imaginary {epsilon}{sub 2} components of the complex dielectric function {epsilon}(E) = {epsilon}{sub 1} (E) + i{epsilon}{sub 2}(E) are calculated using the Kramers-Kronig relations. As a result, the energy band structure of CuGaSe{sub 2} at photon energies higher than the fundamental band gap is derived from the analysis of the structures observed both in the {epsilon} ({omega}) and in the wavelength modulated reflectivity spectra.

Levchenko, S; Tezlevan, V E; Arushanov, E [Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD 2028 (Moldova, Republic of); Syrbu, N N [Laboratory of Optoelectronics, Technical University of Moldova, 168, Bul. Stefan cel Mare, Chisinau, MD-2012 (Moldova, Republic of); Merino, J M; Leon, M [Universidad Autonoma de Madrid, Departamento Fisica Aplicada, C-XII, 28049 Madrid (Spain)], E-mail: maximo.leon@uam.es

2008-03-07

 
 
 
 
21

Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity  

CERN Document Server

Motivated from the experimental observation of metal induced gap states (MIGS) at insulator/metal interfaces by Kiguchi {\\it et al.} [Phys. Rev. Lett. {\\bf 90}, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density functional method. We have found that, on top of the usual evanescent state, MIGS generally have a long tail on halogen sites with a $p_z$-like character, whose penetration depth ($\\lambda$) is as large as half the lattice constant of bulk alkali halides. This implies that $\\lambda$, while little dependent on the carrier density in the jellium, is dominated by the lattice constant (hence by energy gap) of the alkali halide, where $\\lambda_{\\rm LiF} < \\lambda_{\\rm LiCl} < \\lambda_{\\rm LiI}$. We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity.

Arita, R; Kuroki, K; Aoki, H; Arita, Ryotaro; Tanida, Yoshiaki; Kuroki, Kazuhiko; Aoki, Hideo

2003-01-01

22

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

23

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

International Nuclear Information System (INIS)

[en] 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

1987-01-01

24

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

Science.gov (United States)

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

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

2010-08-31

25

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

UK PubMed Central (United Kingdom)

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

West BA; Womick JM; McNeil LE; Tan KJ; Moran AM

2011-05-01

26

Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We describe the internal structure of acceptor-bound excitons in wurtzite semiconductors. Our approach consists in first constructing, in the context of angular momentum algebra, the wave functions of the two-hole system that fulfill Paulis exclusions principle. Second, we construct the acceptor-bou...

Gil, Bernard; Bigenwald, Pierre; Paskov, Plamen; Monemar, Bo

27

Beyond band alignment: hole localization driven formation of three spatially separated long-lived exciton states in CdSe/CdS nanorods.  

UK PubMed Central (United Kingdom)

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron-hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials.

Wu K; Rodríguez-Córdoba WE; Liu Z; Zhu H; Lian T

2013-08-01

28

Beyond band alignment: hole localization driven formation of three spatially separated long-lived exciton states in CdSe/CdS nanorods.  

Science.gov (United States)

Colloidal one-dimensional semiconductor nanoheterostructures have emerged as an important family of functional materials for solar energy conversion, although the nature of the long-lived exciton state and their formation and dissociation dynamics remain poorly understood. In this paper we study these dynamics in CdSe/CdS dot-in-rod (DIR) NRs, a representative of 1D heterostructures, and DIR-electron-acceptor complexes by transient absorption spectroscopy. Because of a quasi-type II band alignment of CdSe and CdS, it is often assumed that there exists one long-lived exciton state with holes localized in the CdSe seed and electrons delocalized among CdSe and CdS. We show that excitation into the CdS rod forms three distinct types of long-lived excitons that are spatially localized in the CdS rod, in and near the CdSe seed and in the CdS shell surrounding the seed. The branching ratio of forming these exciton states is controlled by the competition between the band offset driven hole localization to the CdSe seed and hole trapping to the CdS surface. Because of dielectric contrast induced strong electron-hole interaction in 1D materials, the competing hole localization pathways lead to spatially separated long-lived excitons. Their distinct spatial locations affect their dissociation rates in the presence of electron acceptors, which has important implications for the application of 1D heterostructures as light-harvesting materials. PMID:23829512

Wu, Kaifeng; Rodríguez-Córdoba, William E; Liu, Zheng; Zhu, Haiming; Lian, Tianquan

2013-07-15

29

Band-Edge Exciton Fine Structure of Single CdSe-ZnS Nanocrystals in External Magnetic Fields  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We report a spectroscopic study of the two lowest-energy exciton levels of individual CdSe-ZnS nanocrystals under applied magnetic fields. Field-induced coupling between the bright and the dark excitonic states is directly observed in the low-temperature photoluminescence spectrum and decay and allo...

Biadala, Louis; Louyer, Yann; Tamarat, Philippe; Lounis, Brahim

30

Zeeman splitting and dynamics of an isoelectronic bound exciton near the band edge of ZnO  

Science.gov (United States)

Comprehensive time-resolved photoluminescence and magneto-optical measurements are performed on a bound exciton (BX) line peaking at 3.3621 eV (labeled as I*). Though the energy position of I* lies within the same energy range as that for donor bound exciton (DX) transitions, its behavior in an applied magnetic field is found to be distinctly different from that observed for DXs bound to either ionized or neutral donors. An exciton bound to an isoelectronic center with a hole-attractive local potential is shown to provide a satisfactory model that can account for all experimental results of the I* transition.

Chen, S. L.; Chen, W. M.; Buyanova, I. A.

2012-12-01

31

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

International Nuclear Information System (INIS)

[en] 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

1991-11-15

32

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

Energy Technology Data Exchange (ETDEWEB)

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.

Li, Y.; Krieger, J.B. (Department of Physics, Brooklyn College, City University of New York, Brooklyn, New York 11210 (United States)); Norman, M.R. (Material Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)); Iafrate, G.J. (U.S. Army Research Office, Research Triangle Park, North Carolina 27709-2211 (United States))

1991-11-15

33

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

34

Ion-electron and secondary electron emissions from defective alkali halide crystals  

International Nuclear Information System (INIS)

The influence of color centers (CC), produced by low energy electron irradiation, on ion- and secondary-electron emission (IEE and SEE), thermoluminescence (TL) and F1-absorption of alkali halide crystals (AHC) has been investigated. It has been shown that IEE and SEE- ? and ? coefficients, the light sum S of TL and F1-centers concentration NF1 versus electron irradiation dose change nonmonotonically due to the generation of CC in near-surface crystal layers. It has been found that at kinetic IEE the interaction of the excited electrons from the valence band with the CC electrons will be selective by means of exciton generation.

1992-01-01

35

Violet laser emission in copper halides  

Science.gov (United States)

With two-photon excitation into the longitudinal exciton or into the excitonic continuum copper halides show laser emission at wavelengths of 413.3 nm (CuI), 422.5 nm (CuBr), and 391.9 nm (CuCl). The linewidth of the emission is very narrow in CuI and CuBr (below 0.004 nm) and much broader in CuCl (0.4 nm). The laser transition occurs between the bottleneck of the exciton polariton and the longitudinal optical phonon.

Reimann, K.; Ruebenacke, St.

1994-10-01

36

Exciton Multiplication from First Principles.  

UK PubMed Central (United Kingdom)

Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron-phonon energy relaxation. Multiple excitons are generated through impact ionization within picoseconds. The basis of exciton multiplication in quantum dots is the collective result of photoexcitation, dephasing, and nonadiabatic evolution. Each process is characterized by a distinct time-scale, and the overall multiple exciton generation dynamics is complete by about 10 ps. Without relying on semiempirical parameters, we computed quantum mechanical probabilities of multiple excitons for small model systems. Because exciton correlations and coherences are microscopic, quantum properties, results for small model systems can be extrapolated to larger, realistic quantum dots.

Jaeger HM; Hyeon-Deuk K; Prezhdo OV

2013-03-01

37

Exciton Multiplication from First Principles.  

Science.gov (United States)

Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron-phonon energy relaxation. Multiple excitons are generated through impact ionization within picoseconds. The basis of exciton multiplication in quantum dots is the collective result of photoexcitation, dephasing, and nonadiabatic evolution. Each process is characterized by a distinct time-scale, and the overall multiple exciton generation dynamics is complete by about 10 ps. Without relying on semiempirical parameters, we computed quantum mechanical probabilities of multiple excitons for small model systems. Because exciton correlations and coherences are microscopic, quantum properties, results for small model systems can be extrapolated to larger, realistic quantum dots. PMID:23459543

Jaeger, Heather M; Hyeon-Deuk, Kim; Prezhdo, Oleg V

2013-03-01

38

Dynamic quasi-energy-band modulation and exciton effects in biased superlattices driven by a two-color far-infrared field: Disappearance of dynamic localization  

Science.gov (United States)

A theoretical study of the optical and electronic properties of semiconductor superlattices in ac-dc fields, termed the dynamic Wannier-Stark ladder (DWSL), is done. The biased superlattices are driven by two far-infrared fields with different frequencies and relative phase of ?. Here, the frequency of the first laser is equal to the Bloch frequency ?B of the system under study, while that of the second laser is equal to 2?B. Quasienergies of the DWSL are calculated based on the Floquet theorem, and the associated linear photoabsorption spectra are evaluated. For ?=0, a gourd-shaped quasi-energy structure characteristic of both dynamic localization (DL) and delocalization (DDL), similar to the usual DWSL driven by a single laser, appears. By changing the ratio of the two laser strengths, however, the width of the quasi-energy band and the locations of both DL and DDL vary noticeably. As for ??0, on the other hand, band collapse and the associated DL do not necessarily follow. In fact, DL vanishes and the quasi-energy degeneracy is lifted in a certain range of ?. Just DDL remains over the entire range of the laser strength, eventually resulting in a plateaulike band structure in the linear absorption spectra. The basic physics underlying this phenomenon, which can be readily interpreted in terms of a closed analytical expression, is that all quasi-energies for given crystal momenta are out of phase with each other as a function of laser strength without converging to a single point of energy. This is a feature of this DWSL which sharply distinguishes it from a conventional DWSL generated using a single laser to drive it. Furthermore, an exciton effect is incorporated with the above noninteracting problem, so that exciton dressed states are formed. It is found that this effect gives rise to more involved quasi-energy structures and a more pronounced release of the energy degeneracy of DL, leading again to the formation of a band structure in the absorption spectra. These are exclusively due to unevenly spaced exciton energy levels.

Yashima, Kenta; Hino, Ken-Ichi; Toshima, Nobuyuki

2003-12-01

39

Photophysics of Molecular Crystals: I. Optical and Electrical Properties of Phosphorescence Multiplets in Mixed Crystals: 2,4,5-TRIMETHYLBENZALDEHYDE Guest in a Durene Host. I. Exciton Band to Band Dynamics: 1,4 - and Naphthalene.  

Science.gov (United States)

The photophysical behavior of molecular crystals were investigated from the different viewpoints of a chemically mixed crystal system in the approximation of an oriented gas and of pure crystals forming band states characterizing the whole crystal. For the mixed crystal 2,4,5-trimethylbenzaldehyde in durene we have obtained the electric field effect on each of the doublet origins in the T(,1)(--->)S(,0) transition. The change in dipole moments based on a Lorentz local field approximation was found to be 0.91 (+OR-) 0.02 D and 0.90 (+OR-) 0.02 D for the 3794. 3 (ANGSTROM) and 3797.7 (ANGSTROM) sites respectively. Directions for these vectors were determined and indicate molecular orientation in basic agreement with that given from previous absorption Stark splitting analysis. Selective laser emission excitation shows that the sites in the S(,1)()S(,0) transitions are directly linked but reversed in relative energy ordering while investigation of the T(,2)()S(,0). This result is compared to similar selective emission excitation of Stark components and discussed in terms of the orbital configuration of the excited states. Different lifetimes of 10.4 and 8.0 msec. found for the two sites corroborate this discussion. Concentration studies are reported that indicate no guest to guest energy transfer or interaction occurs in these crystals up to 0.4 mole percent impurity. The dynamic behavior of molecular crystal exciton bands were investigated by observing the band to band transitions for the pure crystals 1,4-dibromonaphthalene (1,4 DBN) and naphthalene using pulsed laser and fast detection electronics. The exciton band of 1,4 DBN at 21(DEGREES)K is found to achieve full thermal equilibrium essentially as soon as the excitation process is finished (nsec.). This result is compared to microseconds thermalization time reported in isotopically doped 1,4 DBN (D. D. Smith and A. H. Zewail, J. Chem. Phys., 71, 3533 (1979)). Differences in the results are discussed in terms of the experimental procedures. Investigation of the band to band transitions in naphthalene at 1,5(DEGREES)K shows that the k states do not reach complete equilibrium until 1 to 2 nanoseconds after initial excitation. This is the first observation of time resolved k to k' state scatter in other than a 1 D molecular crystal band.

Leong, Baldwin

40

Coherent motion and anomalous transport properties of exciton and hole polarons with intrinsic vibrational structure  

CERN Document Server

It is shown that intrinsic vibrational degrees of freedom, inherent in two-atom exciton and hole polarons, drastically affect their transport properties in wide-band dielectrics (rare-gas solids and alkali halides). A fast excitonic energy transport and a comparatively high hole mobility, experimentally observed and attributed to two-site polarons tightly bound with the lattice, cannot be explained by the conventional theory of small-radius polarons that predicts their negligibly weak diffusion, exponentially small in the ratio of the binding energy to temperature. The theory, developed below with allowance for the intrinsic vibrational structure of two-site polarons describes qualitatively a large relevant set of experimental data which seem anomalous from the viewpoint of the conventional theory.

Ratner, A M B

2003-01-01

 
 
 
 
41

Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design.  

Science.gov (United States)

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed. PMID:22309185

Rolczynski, Brian S; Szarko, Jodi M; Son, Hae Jung; Liang, Yongye; Yu, Luping; Chen, Lin X

2012-02-22

42

Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design.  

UK PubMed Central (United Kingdom)

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.

Rolczynski BS; Szarko JM; Son HJ; Liang Y; Yu L; Chen LX

2012-03-01

43

Exciton characteristics and exciton luminescence of silicon quantum dot structures  

International Nuclear Information System (INIS)

[en] 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

2006-01-01

44

Exciton spectra of AgAsS2 crystals  

International Nuclear Information System (INIS)

[en] Reflectivity and wavelength modulated reflectivity spectra of AgAsS2 crystals are investigated in the region of exciton resonances. The exciton and energy band structure parameters were determined.

2011-01-01

45

The J- and H-bands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation  

CERN Document Server

The validity of the CES approximation is investigated by comparison with direct diagonalisation of a model vibronic Hamiltonian of $N$ identical monomers interacting electronically. Even for quite short aggregates (N\\gtrsim 6) the CES approximation is shown to give results in agreement with direct diagonalisation, for all coupling strengths, except that of intermediate positive coupling (the H-band region). However, previously excellent agreement of CES calculations and measured spectra in the H-band region was obtained [A. Eisfeld, J. S. Briggs, Chem. Phys. 324, 376]. This is shown to arise from use of the measured monomer spectrum which includes implicitly dissipative effects not present in the model calculation.

Roden, Jan; Briggs, John S; 10.1016/j.chemphys.2008.07.001

2010-01-01

46

The J- and H-bands of dye aggregate spectra: Analysis of the coherent exciton scattering (CES) approximation  

International Nuclear Information System (INIS)

The validity of the CES approximation is investigated by comparison with direct diagonalisation of a model vibronic Hamiltonian of N identical monomers interacting electronically. Even for quite short aggregates (N?6) the CES approximation is shown to give results in agreement with direct diagonalisation, for all coupling strengths, except that of intermediate positive coupling (the H-band region). However, previously excellent agreement of CES calculations and measured spectra in the H-band region was obtained [A. Eisfeld, J.S. Briggs, Chem. Phys. 324 (2006) 376]. This is shown to arise from use of the measured monomer spectrum which includes implicitly dissipative effects not present in the model calculation

2008-09-03

47

Theory of Doped Excitonic Insulators  

CERN Document Server

The theory of doped excitonic insulators is reinvestigated in light of recent experiments on hexaborides. For the appropriate valley-degenerate X_3,X_3' band structure, ``intra-valley'' condensation is energetically favored. Ferromagnetism occurs upon doping due to the quenching of kinetic energy at the otherwise direct first-order excitonic insulator-metal transition. The phase diagram includes states of spatially inhomogeneous density and magnetization at low temperatures.

Balents, L; Balents, Leon; Varma, Chandra M.

1999-01-01

48

Photoemission of bands above the Fermi level: The excitonic insulator phase transition in 1T-TiSe2  

Digital Repository Infrastructure Vision for European Research (DRIVER)

High-resolution angle-resolved photoemission spectroscopy (ARPES) was used to investigate the transition metal dichalcogenide (TMC) 1T-TiSe2 above and below the phase transition. We find that this system fulfills special conditions such as narrow band width and flat dispersion for ...

Pillo, Th.; Hayoz, J.; Berger, Helmuth; Lévy, F.; Schlapbach, Louis; Aebi, Philipp

49

Defect production in alkali halide crystals by monochromatic x and uv radiation  

International Nuclear Information System (INIS)

The spectral dependence of the F-center production efficiency of several alkali halides was observed in diverse parts of the electromagnetic spectrum. Using low flux monochromatic sources, this efficiency was measured in the hard x-ray region at the K-edge of bromine in KBr and in the near-ultraviolet part of the spectrum corresponding to the exciton region of the alkali halide. A significant increase in the production efficiency was observed at the K-edge, indicating a new F-center production mechanism. In the ultraviolet, maxima in the F-center production efficiency were observed in the long wavelength tail of the first exciton peak and even in the region of the alpha band, providing the first direct measurements of the F-center production efficiency in this region. These studies were made possible by the development of an extremely sensitive new method of detecting F-centers in ultralow concentrations, which utilizes the luminescent properties of these centers. This laser-induced luminescence method enables detection of concentrations of F-centers that are five orders of magnitude smaller than can be detected by optical absorption measurements. Thus, a new regime of F-center production, the early part of the so-called stage I region, could be studied; it was observed to be linear for about four orders of magnitude

1985-01-01

50

Weak Exciton Scattering in Molecular Nanotubes Revealed by Double-Quantum Two-Dimensional Electronic Spectroscopy  

Science.gov (United States)

The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry.

Abramavicius, Darius; Nemeth, Alexandra; Milota, Franz; Sperling, Jaroslaw; Mukamel, Shaul; Kauffmann, Harald F.

2013-01-01

51

Two-photon transitions to exciton polaritons  

International Nuclear Information System (INIS)

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

1979-01-01

52

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

Energy Technology Data Exchange (ETDEWEB)

Absorption spectra associated with the lowest-energy region of the spin-forbidden transition /sup 3/A/sub 2g/..-->.. /sup 1/T/sub 2g//sup a/ in CsNiF/sub 3/ 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.

Tsuboi, T.; Laiho, R.

1985-08-01

53

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

54

Mechanoluminescence of ?-irradiated alkali halide crystals  

International Nuclear Information System (INIS)

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

1984-01-01

55

Quantum Dynamics and Spectroscopy of Excitons in Molecular Aggregates  

CERN Document Server

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

Kühn, Oliver

2011-01-01

56

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

57

Bright and dark excitons in semiconductor carbon nanotubes  

International Nuclear Information System (INIS)

[en] 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 (?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.

2008-01-01

58

Theory of core excitons  

International Nuclear Information System (INIS)

[en] The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material

1980-08-29

59

Total organic halide measurements  

Energy Technology Data Exchange (ETDEWEB)

Currently, the most popular method of analyzing total dissolved halide in water involves carbon adsorption, oxidative combustion, and measurement of the hydrogen halide by microcoulometry. Total dissolved organic halide can be used as an indicator of water quality, providing a direct measure of disinfection by-product formation and, in some circumstances, a surrogate measure of individual organic pollutants. An important application of organic halide measurements is in unit process design, control, and monitoring. Each potential application, however, must be carefully assessed to determine the usefulness of organic halide measurements.

Dressman, R.C.; Stevens, A.A.

1988-11-01

60

Ultraviolet laser ablation of halides and oxides  

International Nuclear Information System (INIS)

We compare and contrast recent measurements of the behavior of ions and excited ions desorbed from samples of alkali halides and oxide ferroelectrics by an excimer laser at 308 nm wavelength. At the intensities used in these experiments, the density of local electronic excitation is low in the halides and high in the ferroelectrics, corresponding to two- and one-photon band-to-band transitions, respectively. The observed desorption yields and changes in the sample surfaces are discussed in terms of the density of electronic excitation, the relative strengths of electron-lattice coupling, and the role of thermal relaxation processes in the two materials. (orig.)

1992-01-01

 
 
 
 
61

Exciton Regeneration at Polymeric Semiconductor Heterojunctions  

CERN Multimedia

Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation, and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2-3.1nm) which may collapse into an exciplex and then endothermically (E=100-200meV) back-transfer towards the exciton.

Morteani, A C; Herz, L M; Friend, R H; Silva, C; Morteani, Arne C.; Sreearunothai, Paiboon; Herz, Laura M.; Friend, Richard H.; Silva, Carlos

2004-01-01

62

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; C. Zenkova; V. Kramar; B. Nitsovich; O. Poterejko

2003-01-01

63

Are excitons really bosons?  

Science.gov (United States)

Non-bosonic commutation relations of exciton operators discovered by Keldysh and Kozlov (1968 Zh. Eksp. Teor. Fiz. 54 978; 1968 Sov. Phys.-JETP 27 521 (Engl. Transl.)) led to a long-standing question about the possible difference between excitons and bosons in spite of the integer spin of excitons. In this paper the problem is analysed and it is shown that, in spite of the difference between separate excitons and bosons, the exciton gas is a Bose gas. Bosons comprising this gas are mixtures of separate excitons, and the non-bosonic nature of excitons leads only to a renormalization of the interaction between them. Features of the exciton luminescence coming from the interaction and non-bosonic nature of excitons are considered. PMID:21483066

Laikhtman, B

2007-06-11

64

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

CERN Document Server

Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot quantum well nanocrystals are determined by use of a multiband theory that includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian, and nonparabolicity of the conduction band. The multiband theory correctly describes the recently observed dark-exciton ground state and the lowest, optically active, bright-exciton states. Charge separation in pair states is identified. Previous single-band theories could not describe these states or account for charge separation.

Jaskolski, W; Bryant, Garnett W.

1998-01-01

65

Enhanced exciton separation through negative energy band bending at grain boundaries of Cu2ZnSnSe4 thin-films  

Science.gov (United States)

Local surface potential of Cu2ZnSnSe4 thin-films was investigated by Kelvin probe force microscopy. The surface potential profile across grain boundaries (GBs) shows a rise of 200-600 meV at GBs in a Cu-poor and Zn-poor film with 3.8% efficiency, which means positively charged GBs. In contrast, the GBs in a Cu-poor and Zn-rich film with 2% efficiency exhibit lowering of surface potential by 40 meV. The results indicate that GBs of Cu2ZnSnSe4 films play a role for exciton separation and governing defects for high efficiency could be not only CuZn but also VCu as explained theoretical predictions.

Jeong, A. R.; Jo, W.; Jung, S.; Gwak, J.; Yun, J. H.

2011-08-01

66

Theory and it ab initio calculation of radiative lifetime of excitons in semiconducting carbon nanotubes  

CERN Document Server

We present theoretical analysis and first-principles calculation of the radiative lifetime of excitons in semiconducting carbon nanotubes. An intrinsic lifetime of the order of 10 ps is computed for the lowest optically active bright excitons. The intrinsic lifetime is however a rapid increasing function of the exciton momentum. Moreover, the electronic structure of the nanotubes dictates the existence of dark excitons nearby in energy to each bright exciton. Both effects strongly influence measured lifetime. Assuming a thermal occupation of bright and dark exciton bands, we find an effective lifetime of the order of 10 ns at room temperature, in good accord with recent experiments.

Spataru, C D; Capaz, R B; Louie, S G; Spataru, Catalin D.; Ismail-Beigi, Sohrab; Capaz, Rodrigo B.; Louie, Steven G.

2005-01-01

67

Creation of color centers in alkali halides by pulsed irradiation  

International Nuclear Information System (INIS)

Regularities of creation and destruction of primary color centers in some alkali-halide crystals (AHC) under pulsed electron irradiation (E = 0.25 MeV, tp = 10 ns, fluence P 7.6 x 1011 el x cm-2) were investigated by the methods of optical spectroscopy with 7 ns time resolution. Values of ? yield and characteristic destruction time ? of short-lived F,H - pairs of color centers and autolocalized excitons (ALE) in 80-400 K range were determined. Correlation between processes of formation of stable and short-lived F-centers in the group of crystals of chlorides and bromides of K and Rb was established. Model of two-halide ALE decay to defects, common for AHC, was suggested. Regularities in AHC series, revealing the relationship between characteristics of defect formation (?F, ?eh, ?F, ?eh, Ep, ET) and crystal parameters (ratio of length of two-halide ALE nucleus to interanion distance) were established.

1990-01-01

68

Exciton Dynamics in Hexagonal InP Nanowires  

Science.gov (United States)

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

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

2013-03-01

69

Energy levels of exciton in a gapped graphene sheet  

Science.gov (United States)

A theory is presented for exciton formation in a graphene sheet using the center-of-mass approximation. The energy levels and wavefunctions of exciton are calculated analytically which show that the exciton can form if the band gap of graphene is not zero. We show that the energy gap of graphene plays the role of the mass which if not zero, leads to formation of the excitons. It is shown that the main quantum number of the exciton ground state changes with the graphene dielectric constant. Also, all of the states are found to be four-fold degenerate. The binding energy of exciton can reach as high as 1/4 of the energy gap of graphene which is notable among the conventional quasi-2D systems. This result can play an important rule in the photonics of graphene.

Fallah, Farhang; Esmaeilzadeh, Mahdi

2013-08-01

70

Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons  

CERN Multimedia

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies excitonic and electron-hole plasma contributions. For low temperatures and low densities the excitonic emission is extremely sensitive to even minute optically active exciton populations making it possible to extract a phase diagram for incoherent excitonic populations.

Chatterjee, S; Mosor, S; Khitrova, G; Gibbs, H M; Hoyer, W; Kira, M; Koch, S W; Prineas, J P; Stolz, H

2003-01-01

71

Optical properties and structural phase transitions of lead-halide based inorganic-organic 3D and 2D perovskite semiconductors under high pressure  

Science.gov (United States)

Optical absorption, photoluminescence and Raman scattering of lead-halide based inorganic-organic perovskite semiconductors were measured under quasi-hydrostatic pressure at room temperature. For the 3D perovskite semiconductor, (CH3NH3)PbBr3, the free exciton photoluminescence band exhibits red-shifts with pressure, and jumps to a higher energy by 0.07 eV at 0.8 GPa, which is associated with a phase transition from a cubic to an orthorhombic structure confirmed by Raman scattering. Above the phase transition pressure, the exciton band shows blue-shifts with further increasing pressure, and eventually disappears above 4.7 GPa. The results are compared with those for the 2D perovskite semiconductor, (C4H9NH3)2PbI4. First principles pseudopotential calculations were performed to investigate changes in octahedral distortion and electronic band structures with pressure. The calculations have explained the origins of the intriguing changes in the electronic states with pressure in view of bonding characters between atomic orbitals in octahedra.

Matsuishi, K.; Ishihara, T.; Onari, S.; Chang, Y. H.; Park, C. H.

2004-11-01

72

Exciton optical transitions in a hexagonal boron nitride single crystal  

Energy Technology Data Exchange (ETDEWEB)

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

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

2011-06-15

73

Exciton optical transitions in a hexagonal boron nitride single crystal  

International Nuclear Information System (INIS)

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

2011-01-01

74

Excitonic dynamical Franz-Keldysh effect.  

Digital Repository Infrastructure Vision for European Research (DRIVER)

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 reso...

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

75

Multiple Exciton Generation and Recombination in Carbon Nanotubes and Nanocrystals.  

UK PubMed Central (United Kingdom)

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

Kanemitsu Y

2013-02-01

76

Multiple Exciton Generation and Recombination in Carbon Nanotubes and Nanocrystals.  

Science.gov (United States)

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

Kanemitsu, Yoshihiko

2013-02-19

77

Depolarizers for lithium halide batteries  

Energy Technology Data Exchange (ETDEWEB)

The present invention relates to a depolarizer for use in a lithium halide battery which comprises a halogen and a charge transfer complex consisting of the halogen and an organic component of (A) quaternary amine halide selected from the group consisting of (I) polyvinylalkylpyridinium halide or polyvinylalkylquinolinium halide and (II) poly (N,n'-dialkyl 1,2 bis (4-pyridinium halide)) ethylene or poly (N-alkyl 1,2 bis (4pyridinium halide) ethylene) and (B) phenyl methane dyes wherein said halogen is present in an amount of from about 3 to 50 parts by weight for each part of organic component.

Harney, M.J.

1981-06-30

78

Exciton absorption in GaN  

International Nuclear Information System (INIS)

The absorption coefficient of the wurtzite-type GaN is described in terms of three almost totally decoupled excitons A, B, C associated with split valence bands of ?9, ?7 and ?7 symmetries using a spherically symmetric approximation for the standard effective-mass Hamiltonian. The excitonic equations are solved within the subspace of optically active states of s-symmetry. The effects of crystal field and spin-orbit coupling are discussed. The results show that the spherical approximation is in perfect agreement with experimental findings. (author)

1998-01-01

79

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.

NordstrØm, K.B.; Johnsen, Kristinn

1998-01-01

80

Optical response and excitons in gapped graphene  

DEFF Research Database (Denmark)

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

Pedersen, Thomas Garm; Jauho, Antti-Pekka

2009-01-01

 
 
 
 
81

Formation of excitons nearby the defects in alkali halide crystals  

International Nuclear Information System (INIS)

Through the conducted studies by methods of pulsed spectroscopy it is shown that presence of defects, including electroneutral ones relative to the crystal lattice, essentially influences topography of distribution of electron excitations created by radiation. The lattice deformation in the area of defects is the most probable cause of the defects impact on the topography of electron excitations distribution. It is shown that there exists an oscillating potential relief, the presence whereof leads to the carriers capture and their localization in this area

1996-01-01

82

Excitons at Interfaces  

Science.gov (United States)

Solar photovoltaics based on molecular and nano materials commonly involve excitons. This results from strong Coulomb attraction between an electron and a hole due to the low dielectric constants of molecules or quantum confinement of nano materials. In this lecture, I will address the question of how excitons dissociate at donor/acceptor interfaces. The first example deals with charge separation in organic photovoltaics. Due to the low dielectric constant of organic materials, an electron-hole pair across an organic donor/acceptor interface is bound by the Coulomb potential. This gives rise to a set of H-atom like states called charge-transfer excitons, as observed experimentally. The lowest energy charge transfer exciton state has a binding energy much higher than kT at room temperature. This leads to the conclusion that hot charge transfer exciton states must be involved in charge separation in organic photovoltaics. The second example deals with hot exciton dissociation due to electron transfer from photo-excited semiconductor nanocrystals (PbSe) to an electron acceptor (TiO2), an issue of particular interest to hot carrier solar cells with theoretical solar conversion efficiency surpassing the Shockley-Queisser limit. We show that, with appropriate chemical treatment of the nanocrystal surface, ultrafast transfer of a hot electron can be competitive with hot exciton relaxation due to phonon scattering. The last example will show recent development on hot carrier scattering and multiple exciton generation (MEG) in semiconductor nanorystals.

Zhu, Xiaoyang

2011-03-01

83

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.

2011-01-01

84

Optically programmable excitonic traps.  

UK PubMed Central (United Kingdom)

With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits.

Alloing M; Lemaître A; Galopin E; Dubin F

2013-01-01

85

Optically programmable excitonic traps  

Science.gov (United States)

With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits.

Alloing, Mathieu; Lemaitre, Aristide; Galopin, Elisabeth; Dubin, Francois

2013-01-01

86

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.

2008-01-01

87

Exciton annihilation on dendrimeric trees  

International Nuclear Information System (INIS)

Exciton-exciton annihilation on Cayley tree like dendrimer molecules are investigated via Monte Carlo simulations. Annihilation reaction of the type A+A->0 is considered to calculate the exciton density decay for multiexciton diffusion on dendrimers. Exciton density decays as a power law with a continuously varying exponent in a linear potential. For the case of realistic nonlinear potential of phenylacetylene dendrimers (Phys. Rev. Lett. 77 (1998) 4656) the excitons accumulate around the free energy minimum and annihilate each other quickly.

2005-01-01

88

Multiple exciton generation in colloidal silicon nanocrystals.  

UK PubMed Central (United Kingdom)

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.

Beard MC; Knutsen KP; Yu P; Luther JM; Song Q; Metzger WK; Ellingson RJ; Nozik AJ

2007-08-01

89

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds  

CERN Document Server

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

Ohara, J; Ohara, Jun; Yamamoto, Shoji

2006-01-01

90

Theory of ferromagnetism in doped excitonic condensates.  

UK PubMed Central (United Kingdom)

Nesting in a semimetal can lead to an excitonic-insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. We present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.

Bascones E; Burkov AA; MacDonald AH

2002-08-01

91

Theory of Ferromagnetism in Doped Excitonic Condensates  

CERN Multimedia

Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.

Bascones, E; MacDonald, A H

2002-01-01

92

One-dimensional continuum and exciton states in quantum wires  

International Nuclear Information System (INIS)

High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth with molecular-beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Characterization by micro-photoluminescence (PL) and PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp spectral width, and a small Stokes shift of one-dimensional (1D) excitons. The PLE spectrum for 1D states shows a large peak of ground-state excitons and a small absorption band ascribed to 1D continuum states with an onset at 11 meV above the exciton peak.

2003-01-20

93

One-dimensional continuum and exciton states in quantum wires  

CERN Multimedia

High-quality T-shaped quantum wires are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Characterization by micro-photoluminescence (PL) and PL excitation (PLE) spectroscopy at 5 K reveals high uniformity, a sharp spectral width, and a small Stokes shift of one-dimensional (1-D) excitons. The PLE spectrum for 1-D states shows a large peak of ground-state excitons and a small absorption band ascribed to 1-D continuum states with an onset at 11 meV above the exciton peak.

Akiyama, H; Yoshita, M; Pinczuk, A; West, K W; Akiyama, Hidefumi; Pfeiffer, Loren N.; Yoshita, Masahiro; Pinczuk, Aron; West, Ken W.

2002-01-01

94

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

1987-01-01

95

The mechanism of the electron sputtering process of alkali halides  

International Nuclear Information System (INIS)

The energy distributions of the particles leaving a polycrystalline alkali halide target under bombardment with electrons are measured. NaBr, KBr, RbBr, CsBr, NaI, KI, RbI, and CsI at target temperatures between 40 and 4000C are investigated. Among the possible ways to cause sputtering the diffusion of excitons is very improbable because no particles with energies higher than 1 eV have been found. Compared to it the experimental results are in agreement with the focussed collision replacement sequence. This mechanism is described.

1977-01-01

96

Quasienergy Spectroscopy of Excitons  

CERN Multimedia

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

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

1999-01-01

97

Huge excitonic effects in layered hexagonal boron nitride.  

UK PubMed Central (United Kingdom)

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

Arnaud B; Lebègue S; Rabiller P; Alouani M

2006-01-01

98

Huge excitonic effects in layered hexagonal boron nitride.  

Science.gov (United States)

The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor. PMID:16486604

Arnaud, B; Lebègue, S; Rabiller, P; Alouani, M

2006-01-18

99

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

Scientific Electronic Library Online (English)

Full Text Available 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 mét (more) odo 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 effect, 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.

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

2006-04-01

100

Multiscale photosynthetic exciton transfer  

CERN Multimedia

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

 
 
 
 
101

Scintillation mechanisms in Ce3+ doped halide scintillators  

Science.gov (United States)

Last couple of years witnessed the development of various new Ce3+ doped halide scintillators (LaCl3, LaBr3, LuI3, Cs2LiYCl6, Cs2LiYBr6) that possess excellent gamma ray or thermal neutron detection properties. The scintillation pulse in LaBr3:Ce3+ is 20 times faster than in the most commonly used scintillator NaI:Tl. This, combined with a more then two times better energy resolution and higher gamma ray stopping power, makes it ideally suited for many different applications. In this work the scintillation properties and mechanisms of Ce3+ doped inorganic halide (Cl, Br, I) compounds are reviewed; especially the role of Vk centers and self trapped excitons (STEs) in the energy transfer from the ionization track to Ce3+ is treated. Aspects of the lifetime and thermal stability of the self trapped exciton, the migration speed of Vk centers and STEs, spectral overlap between STE emission and Ce3+ absorption, and the influence of the type of anion (Cl, Br, I) are addressed.

Dorenbos, Pieter

2005-01-01

102

Identification of bound exciton complexes in ZnO  

Science.gov (United States)

An identification of shallow bound exciton centers in ZnO is presented based on magneto-optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I4, I6, I8 and I9 exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the 7 symmetry of the upper valence band. The presence of two-electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I4, I6, I8 and I9 lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry.

Strassburg, M.; Rodina, A.; Dworzak, M.; Haboeck, U.; Krestnikov, I. L.; Hoffmann, A.; Gelhausen, O.; Phillips, M. R.; Alves, H. R.; Zeuner, A.; Hofmann, D. M.; Meyer, B. K.

2004-02-01

103

Two-photon excitation of excitons in CuCl in total reflection geometry.  

Science.gov (United States)

We have observed emission spectra of a CuCl film on a TiO2 prism surface(110) associated with the two-photon excitation of the exciton system in total reflection geometry. The I1 bound exciton emission, which resonantly appeared at the two-photon excitation of the Z3-longitudinal excitons and Z1,2-exciton band, was observed. The dependence of the emission intensities on the polarization of the excitation light was explained from the field intensity and the penetration depth of the evanescent light in CuCl accompanying the totally reflected light at the TiO2/CuCl interface. PMID:12641758

Hasuo, M; Shimamoto, A; Fujimoto, T

2003-03-01

104

Two-photon excitation of excitons in CuCl in total reflection geometry.  

UK PubMed Central (United Kingdom)

We have observed emission spectra of a CuCl film on a TiO2 prism surface(110) associated with the two-photon excitation of the exciton system in total reflection geometry. The I1 bound exciton emission, which resonantly appeared at the two-photon excitation of the Z3-longitudinal excitons and Z1,2-exciton band, was observed. The dependence of the emission intensities on the polarization of the excitation light was explained from the field intensity and the penetration depth of the evanescent light in CuCl accompanying the totally reflected light at the TiO2/CuCl interface.

Hasuo M; Shimamoto A; Fujimoto T

2003-03-01

105

Engineering directed excitonic energy transfer  

CERN Document Server

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

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

2010-01-01

106

Sulfides, sulfohalides and halides  

International Nuclear Information System (INIS)

[en] Experimental material on hydrothermal synthesis of simple and complex chalcogenides Asub(x)Bsub(y) (A=Cd, In, Cu, Zn, Hg, Pb, As, Sb, Bi, Ag; B=S, Se, Te), chalcohalogenides Asub(x)Bsub(y)Csub(z) (A=Sb, Bi, Hg, Pb, Cu, Au; B=S, Se, Te; C=Cl, Br, I), halides ACsub(x) (A=Sb, Bi, Cu; C=Br, I), K(TR)Fsub(x) (TR - rare earths) has been generalized. Physicochemical conditions of synthesis in multicomponent systems are investigated, morphogenetic peculiarities and their characteristic properties are described

1984-01-01

107

Halide laser glasses  

International Nuclear Information System (INIS)

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

1982-03-01

108

Halide laser glasses  

Energy Technology Data Exchange (ETDEWEB)

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

Weber, M.J.

1982-01-14

109

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.).

1981-01-01

110

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

111

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

112

Nonlinear optical spectroscopy of biexcitons and excitonic polaritons in CuCl (CuBr)  

Energy Technology Data Exchange (ETDEWEB)

Copper halides (especially CuCl) have played an important role in the development of nonlinear optical spectroscopy. The binding energies of excitons and of biexcitons are so large that the optical response of these materials and its dynamics could be fully described in the frame of these quasi-particle states and of their mutual interactions. We shall go through the different steps of the development of this research and present the contribution of the Strasbourg Group to this activity. (orig.).

Grun, J.B. [Strasbourg-1 Univ., 67 (France). Inst. de Physique et Chimie des Materiaux; Hoenerlage, B. [Strasbourg-1 Univ., 67 (France). Inst. de Physique et Chimie des Materiaux; Levy, R. [Strasbourg-1 Univ., 67 (France). Inst. de Physique et Chimie des Materiaux

1995-06-01

113

Multiple Exciton Generation in Colloidal Silicon Nanocrystals  

Energy Technology Data Exchange (ETDEWEB)

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_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. 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.

Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.

2007-01-01

114

Synthetic Alkali Halides  

Science.gov (United States)

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

Perkins, Dexter

115

Singlet Exciton Fission Photovoltaics.  

UK PubMed Central (United Kingdom)

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.

Lee J; Jadhav P; Reusswig PD; Yost SR; Thompson NJ; Congreve DN; Hontz E; Van Voorhis T; Baldo MA

2013-04-01

116

Excitonic effects in the luminescence of quantum wells  

International Nuclear Information System (INIS)

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

2005-11-15

117

Experimental evidence of exciton-induced production of close pairs F, In++ in KBr-In  

International Nuclear Information System (INIS)

[en] Optical creation of anion excitons under irradiation in the fundamental exciton absorption band at room temperature leads to energy accumulation in the form of close F-In2+ defect pairs in KBr-In crystals. Further destruction of these pairs by illumination in the F-absorption band (F-illumination) results in the production of the close ?a+In+ defect pairs where ?a+ is an anion vacancy. Results may be considered as one of experimental evidences of the close F-In2+ defect pairs' production at the optical anion exciton creation

1990-01-01

118

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

CERN Document Server

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

Perebeinos, V; Perebeinos, Vasili; Avouris, Phaedon

2007-01-01

119

Exciton ionization, Franz-Keldysh, and Stark effects in carbon nanotubes.  

UK PubMed Central (United Kingdom)

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

Perebeinos V; Avouris P

2007-03-01

120

Quantum statistical properties of exciton and bielectron Bose gases  

International Nuclear Information System (INIS)

[en] A single approach to the study of the collective properties of excitons and bielectrons is developed. The relationship of the spectral properties of these quasiparticles with the rotational-vibrational spectra of diatomic molecules is parametrized. The band nature of high-temperature superconductivity is correlated with the collective properties of a bielectron Bose gas. 39 refs., 1 fig., 1 tab

1993-01-01

 
 
 
 
121

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

1987-01-01

122

Excitons in intact cells of photosynthetic bacteria.  

UK PubMed Central (United Kingdom)

Live cells and regular crystals seem fundamentally incompatible. Still, effects characteristic to ideal crystals, such as coherent sharing of excitation, have been recently used in many studies to explain the behavior of several photosynthetic complexes, especially the inner workings of the light-harvesting apparatus of the oldest known photosynthetic organisms, the purple bacteria. To this date, there has been no concrete evidence that the same effects are instrumental in real living cells, leaving a possibility that this is an artifact of unnatural study conditions, not a real effect relevant to the biological operation of bacteria. Hereby, we demonstrate survival of collective coherent excitations (excitons) in intact cells of photosynthetic purple bacteria. This is done by using excitation anisotropy spectroscopy for tracking the temperature-dependent evolution of exciton bands in light-harvesting systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic organisms.

Freiberg A; Pajusalu M; Rätsep M

2013-09-01

123

Exciton luminescence spectra of CuCl films and their change by photo-irradiation  

Energy Technology Data Exchange (ETDEWEB)

We have observed luminescence spectra of vapor-deposited CuCl films on rutile TiO{sub 2} under photo-irradiation with picosecond light pulses at 4.6K. Decrease in the Z{sub 3}-exciton and I{sub 1} luminescence intensities and an appearance of new exciton luminescence are found to be induced by the photo-irradiation. From the excitation photon energy dependence of the luminescence spectra, it is found that the Z{sub 3}-exciton luminescence is resolved into two components: one is attributed to the optical phonon Raman lines and the other to the free Z{sub 3}-exciton luminescence. The intensity of the Raman lines resonantly increases at the Z{sub 3}-exciton band resulting in the optical phonon structures in the excitation spectrum. The excitation spectra of the new exciton and I{sub 1} luminescence are similar to the absorption spectrum of CuCl, which suggest that the resonant Raman process observed in the Z{sub 3}-exciton luminescence hardly contributes to the generation of the excitons associated with the new exciton and I{sub 1} luminescence.

Hasuo, M. [Department of Engineering Physics and Mechanics, Graduate School of Engineering, Kyoto University, Kyoto 606-8501 (Japan)]. E-mail: hasuo@kues.kyoto-u.ac.jp; Shimamoto, A. [Department of Engineering Physics and Mechanics, Graduate School of Engineering, Kyoto University, Kyoto 606-8501 (Japan); Fujimoto, T. [Department of Engineering Physics and Mechanics, Graduate School of Engineering, Kyoto University, Kyoto 606-8501 (Japan)

2005-04-15

124

Exciton luminescence spectra of CuCl films and their change by photo-irradiation  

International Nuclear Information System (INIS)

[en] We have observed luminescence spectra of vapor-deposited CuCl films on rutile TiO2 under photo-irradiation with picosecond light pulses at 4.6K. Decrease in the Z3-exciton and I1 luminescence intensities and an appearance of new exciton luminescence are found to be induced by the photo-irradiation. From the excitation photon energy dependence of the luminescence spectra, it is found that the Z3-exciton luminescence is resolved into two components: one is attributed to the optical phonon Raman lines and the other to the free Z3-exciton luminescence. The intensity of the Raman lines resonantly increases at the Z3-exciton band resulting in the optical phonon structures in the excitation spectrum. The excitation spectra of the new exciton and I1 luminescence are similar to the absorption spectrum of CuCl, which suggest that the resonant Raman process observed in the Z3-exciton luminescence hardly contributes to the generation of the excitons associated with the new exciton and I1 luminescence

2005-01-01

125

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2012-05-15

126

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

Energy Technology Data Exchange (ETDEWEB)

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

Protin, L.

1994-10-05

127

Evidence of exciton recombination at very high temperature in InGaN  

Energy Technology Data Exchange (ETDEWEB)

Time-resolved electroluminescence measurements are carried out on the blue light emitting diodes with InGaN active layer at temperatures from 30 to 530 K. The decay mechanisms of the ultraviolet optical pulses corresponding to the band-to-band recombinations are investigated. The exciton-related recombination is found mainly responsible for this band-edge radiative recombination from 250 to 425 K. A thermal equilibrium model, in which the exciton dissociation process is taken into account, is used to fit the experimental results. The fitted exciton binding energy is about 48.3 meV. This high exciton binding energy is attributed to the indium-related localization effect in InGaN. Moreover, it is also found that the nonradiative lifetimes in these samples are quite long. This is ascribed to the suppression of the nonradiative recombination centers because of the incorporation of indium in GaN material. {copyright} 2001 American Institute of Physics.

Wang, Xiaojun; Choa, Fow-Sen; Liu, Feng; Worchersky, T. L.

2001-06-01

128

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

UK PubMed Central (United Kingdom)

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.

Pandiyarajan T; Udayabhaskar R; Karthikeyan B

2013-02-01

129

A Review of Luminescent Anionic Nano System: d10 Metallocyanide Excimers and Exciplexes in Alkali Halide Hosts  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Dicyanoaurate, dicyanoargentate, and dicyanocuprate ions in solution and doped in different alkali halide hosts exhibit interesting photophysical and photochemical behavior, such as multiple emission bands, exciplex tuning, optical memory, and thermochromism. This is attributed to the formation of d...

Xiaobo Li; Howard H. Patterson

130

Excitons in solid C60  

Energy Technology Data Exchange (ETDEWEB)

Exciton levels in undoped, solid C60 are calculated using a model Hamiltonian. We find excitation energies of 1.58 and 1.30 eV for the lowest singlet and triplet exciton, respectively, in comparison with the measured energies of 1.83 and 1.55 eV. Singlet and triplet states have similar energy diagrams, wherein exciton states having T{sub 2g}, T{sub 1g},G{sub g}, and H{sub g} symmetries are separated by up to several tenths of an electron volt. As a function of crystal momentum, exciton energies exhibit dispersion from 20 to 40 meV. Theoretical pressure derivatives of exciton energies are presented.

Shirley, Eric L.; Benedict, Lorin X.; Louie, Steven G.

1995-10-01

131

Actinide halide complexes  

Energy Technology Data Exchange (ETDEWEB)

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

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

1992-01-01

132

Diamond C 1s core-level excitons: Surface sensitivity  

Science.gov (United States)

The effect of surface termination on C 1s core-level bulk excitons found near the surface in single-crystal diamond is reported. By simultaneously recording bulk (total electron yield) and surface (partial electron yield) near-edge x-ray absorption fine-structure (NEXAFS) spectra with a range of single-crystal surface terminations, variable bulk exciton energy blueshifts and linewidth broadening were observed, most notably in the reconstructed single-crystal diamond. Investigation of polycrystalline diamond and ultrananocrystalline diamond films, using the same technique, allowed for a comparison between the reconstructed single-crystal diamond surface and nanodiamond bulk excitons. These findings suggest that surface-related effects can be misinterpreted as quantum confinement in some nanodiamond NEXAFS studies. Band bending is suggested as a possible contributor to these results.

Stacey, Alastair; Cowie, Bruce C. C.; Orwa, Julius; Prawer, Steven; Hoffman, Alon

2010-09-01

133

Exciton effects in boron-nitride (BN) nanotubes  

Science.gov (United States)

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

Harigaya, Kikuo

2012-12-01

134

Mechanoluminescence produced during cleavage of ?-irradiated alkali halide crystals  

International Nuclear Information System (INIS)

When g-irradiated alkali halide crystals are cleaved, then in their mechanoluminescence (ML) intensity versus time curve, two peak intensities Im1 and Im2 are observed at time tm1 and tm2 respectively. The ML intensity both beyond tm1 and tm2 decrease exponentially. It is shown that ML provides a new technique for determining the pinning time of dislocations and lifetime of the electrons in dislocation band. (author)

2001-01-01

135

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

UK PubMed Central (United Kingdom)

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.

Al-Otaify A; Kershaw SV; Gupta S; Rogach AL; Allan G; Delerue C; Binks DJ

2013-09-01

136

Exciton energy transfer between nanoparticles and nanowires  

Science.gov (United States)

In this paper, we study the processes of exciton transfer in coupled nanoparticles and nanowires, and obtain convenient equations and numerical results for energy-transfer rates. For large distances, the energy-transfer rates are proportional to 1/d5 , where d is the nanoparticle-nanowire distance. Our models incorporate both semiconductor and metal material systems. In this paper, we show that excitonic energy generated optically in nanoparticles can be efficiently extracted and channeled to nanowires/nanotubes by utilizing the Förster energy-transfer mechanism. In particular, we propose a complex composed of an indirect-band semiconductor nanowire (silicon) and direct-band nanoparticles (CdTe). The efficiency of light harvesting in the CdTe-Si complex becomes strongly enhanced due to a strongly increased absorption cross section and a reduced absorption anisotropy. The system has potential for optoelectronic applications. Hybrid metal-semiconductor complexes have more efficient energy transfer and may be used for sensors. This paper describes available experimental data and suggests further experimental realizations.

Hernández-Martínez, Pedro L.; Govorov, Alexander O.

2008-07-01

137

Exciton-phonon interaction and exciton energy in semiconductor nanofilms  

International Nuclear Information System (INIS)

The Bethe variational method has been used to estimate the binding energy of the exciton ground state in a flat semiconductor nanofilm. The Green's function method has been applied to study the dependence of the exciton energy on the film thickness taking the exciton-phonon interaction into account at a temperature of 0 K. Calculations were executed in the framework of the rectangular finite depth quantum well model and making use of Al0.3Ga0.7As/GaAS/Al0.3Ga0.7As and dS/HgS/CdS nanofilms as examples.

2009-01-01

138

Topological insulator phase in halide perovskite structures  

Science.gov (United States)

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

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

2012-09-01

139

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

140

Exciton Dynamics in Semiconductor Nanocrystals.  

UK PubMed Central (United Kingdom)

This review article provides an overview of recent advances in the study and understanding of dynamics of excitons in semiconductor nanocrystals (NCs) or quantum dots (QDs). Emphasis is placed on the relationship between exciton dynamics and optical properties, both linear and nonlinear. We also focus on the unique aspects of exciton dynamics in semiconductor NCs as compared to those in bulk crystals. Various experimental techniques for probing exciton dynamics, particularly time-resolved laser methods, are reviewed. Relevant models and computational studies are also briefly presented. By comparing different materials systems, a unifying picture is proposed to account for the major dynamic features of excitons in semiconductor QDs. While the specific dynamic processes involved are material-dependent, key processes can be identified for all the materials that include electronic dephasing, intraband relaxation, trapping, and interband recombination of free and trapped charge carriers (electron and hole). Exciton dynamics play a critical role in the fundamental properties and functionalities of nanomaterials of interest for a variety of applications including optical detectors, solar energy conversion, lasers, and sensors. A better understanding of exciton dynamics in nanomaterials is thus important both fundamentally and technologically.

Wheeler DA; Zhang JZ

2013-04-01

 
 
 
 
141

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

142

Exciton-polariton mediated superconductivity.  

UK PubMed Central (United Kingdom)

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

Laussy FP; Kavokin AV; Shelykh IA

2010-03-01

143

Exciton-polariton mediated superconductivity.  

Science.gov (United States)

We revisit the exciton mechanism of superconductivity in the framework of microcavity physics, replacing virtual excitons as a binding agent of Cooper pairs by excitations of an exciton-polariton Bose-Einstein condensate. We consider a model microcavity where a quantum well with a two-dimensional electron gas is sandwiched between two undoped quantum wells, where a polariton condensate is formed. We show that the critical temperature for superconductivity dramatically increases with the condensate population, opening a new route towards high-temperature superconductivity. PMID:20366440

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

2010-03-09

144

Origin of the variation of exciton binding energy in semiconductors.  

UK PubMed Central (United Kingdom)

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

Dvorak M; Wei SH; Wu Z

2013-01-01

145

Origin of the variation of exciton binding energy in semiconductors.  

Science.gov (United States)

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

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

2013-01-02

146

Excitons into one-axis crystals of zinc phosphide (Zn3P2)  

Directory of Open Access Journals (Sweden)

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

D.M. Stepanchikov; G.P. Chuiko

2009-01-01

147

Effects of environmental and exciton screening in single-walled carbon nanotubes  

Energy Technology Data Exchange (ETDEWEB)

The ground-state exciton binding energy for single-walled carbon nanotubes (SWCNTs) in vacuum calculated ignoring the screening of Coulomb interaction appears to be much greater than the corresponding band gap. The most essential contributions to the screening of electron-hole (e-h) interaction potential in semiconducting SWCNTs, which return the ground-state exciton binding energy into the energy gap, are considered. Our estimates on the screening effects and exciton binding energies are in satisfactory agreement with the corresponding experimental data for concrete nanotubes.

Adamyan, Vadym M; Smyrnov, Oleksii A; Tishchenko, Sergey V [Department of Theoretical Physics, Odessa I I Mechnikov National University, 2 Dvoryanskaya St., Odessa 65026 (Ukraine)], E-mail: smyrnov@onu.edu.ua

2008-10-15

148

Exciton-exciton annihilation in mixed-phase polyfluorene films  

Energy Technology Data Exchange (ETDEWEB)

Singlet-singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of {beta}-phase chains using time-resolved fluorescence. On a timescale of >15 ps after excitation, the results are fitted well by a time-independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time-dependent annihilation rate is observed during the first 15 ps in the glassy phase and in the {beta}-phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low-energy sites. The annihilation rate in the mixed-phase films increases with increasing fraction of {beta}-phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersed{beta}-phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line-dipole approximation for an exciton wavefunction extending over 2.5 nm. The results indicate that glassy and {beta}-phase chromophores are intimately mixed rather than clustered or phase-separated. (Abstract Copyright [2010], Wiley Periodicals, Inc.)

Shaw, Paul E.; Ruseckas, Arvydas; Samuel, Ifor D.W. [Organic Semiconductor Centre, SUPA School of Physics and Astronomy University of St. Andrews North Haugh, St. Andrews, KY16 9SS (United Kingdom); Peet, Jeffrey; Bazan, Guillermo C. [Center for Polymers and Organic Solids Department of Chemistry and Biochemistry University of California Santa Barbara, CA 93106-9510 (United States)

2010-01-08

149

Exchange narrowing and exciton delocalization in disordered J aggregates: Simulated peak shapes in the two dimensional spectra  

Science.gov (United States)

Delocalized excitons in elementary linear J aggregates of two-level molecules absorb a photon into the low-energy edge of an exciton band. Absorption of a second photon is blue-shifted as the lowest energy state is occupied. This setup of states leads to a double-peak feature in a set of two dimensional photon echo spectra for excitonic bands. The delocalization properties of excitons, thus, strongly affect the peak lineshapes and their relative amplitudes. Simulations of various two dimensional spectra of a linear J aggregate are presented and possible schemes to quantitatively characterize the peak profiles are suggested. This allows to relate observable peak lineshapes to the exciton delocalization.

Han, Jiao; Zhang, Houyu; Abramavicius, Darius

2013-07-01

150

Exchange narrowing and exciton delocalization in disordered J aggregates: Simulated peak shapes in the two dimensional spectra.  

UK PubMed Central (United Kingdom)

Delocalized excitons in elementary linear J aggregates of two-level molecules absorb a photon into the low-energy edge of an exciton band. Absorption of a second photon is blue-shifted as the lowest energy state is occupied. This setup of states leads to a double-peak feature in a set of two dimensional photon echo spectra for excitonic bands. The delocalization properties of excitons, thus, strongly affect the peak lineshapes and their relative amplitudes. Simulations of various two dimensional spectra of a linear J aggregate are presented and possible schemes to quantitatively characterize the peak profiles are suggested. This allows to relate observable peak lineshapes to the exciton delocalization.

Han J; Zhang H; Abramavicius D

2013-07-01

151

High duty factor rare gas halide laser  

Energy Technology Data Exchange (ETDEWEB)

This patent describes a high duty factor rare gas halide laser is described comprising: (a) a rare gas halide gas mixture composed of at least one rare gas and one halide compound; (b) a hollow discharge tube of dielectric material containing the laser gas mixture; (c) means for exciting the laser gas mixture by high frequency electrodeless discharge; and (d) means for producing rapid recombination throughout the discharge tube of the halide compound after disassociation, comprising inner wall surfaces of the hollow discharge tube that are chemically inert to atomic and molecular species derived from the halide compound.

Christensen, C.P. Jr.; Moutoulas, C.

1989-01-03

152

Linear, diatomic crystal: single-electron states and large-radius excitons  

International Nuclear Information System (INIS)

The large-radius exciton spectrum in a linear crystal with two atoms in the unit cell was obtained using the single-electron eigenfunctions and the band structure, which were found by the zero-range potentials method. The ground-state exciton binding energies for the linear crystal in vacuum appeared to be larger than the corresponding energy gap for any set of the crystal parameters

2009-01-01

153

Linear diatomic crystal: single-electron states and large-radius excitons  

Science.gov (United States)

The large-radius exciton spectrum in a linear crystal with two atoms in the unit cell is obtained using the single-electron eigenfunctions and band structure found by the zero-range potentials method. The ground-state exciton binding energies for the linear crystal in vacuum turn out to be larger than the corresponding energy gaps for any set of the crystal parameters.

Adamyan, V. M.; Smyrnov, O. A.

2009-05-01

154

Hyperspherical theory of anisotropic exciton  

CERN Multimedia

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

155

Excitonic ferromagnetism in the hexaborides  

CERN Document Server

A ferromagnet with a small spontaneous moment but with a high Curie temperature can be obtained by doping an excitonic insulator made from a spin triplet exciton condensate. Such a condensate can occur in a semimetal with a small overlap or a semiconductor with a small bandgap. We propose that it is responsible for the unexpected ferromagnetism in the doped hexaboride material Ca_{1-x}La_xB_6.

Zhitomirsky, M E; Anisimov, V I

1999-01-01

156

Anharmonic properties of rubidium halides  

International Nuclear Information System (INIS)

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

1988-01-01

157

Metallic halides of the lanthanides  

International Nuclear Information System (INIS)

A short survey of the now known metallic halides of the lanthanides is given. These have the formula types MX2, M'MX3 and M2X5. Metallic behaviour arises from the 4fn-15d1 valence state of the lanthanide element (M). 16 refs.

1991-01-01

158

Investigation of the exciton electroluminescence of 6H-SiC based p-n structures grown by sublimation epitaxy  

International Nuclear Information System (INIS)

Electroluminescence spectra of 6H-SiC obtained by sublimation epitaxy were investigated. It is shown that exciton band intensity increased very quickly with increasing forward current (J) and became to predominate in the diode radiation spectrum at high values of J and elevated temperatures. Investigation of the temperature bias of the band maximum position shows that this radiation is connected with recombination of a free exciton

1997-01-01

159

Excitonic splittings in jet-cooled molecular dimers.  

UK PubMed Central (United Kingdom)

In more than 60 years of research on molecular excitons, there has been extensive theoretical work but few experimental investigations have rigorously tested the predictions of exciton coupling theories. In centrosymmetric doubly H-bonded molecular dimers with identical chromophores, the S0-->S1 electronic transition dipole moments of the monomers combine in a parallel and antiparallel fashion, giving the S0-S1 and S0-->S2 transitions of the dimer. One of these is strictly symmetry-forbidden and the other fully allowed. Minimal perturbations such as 12C/13C or H/D isotopic substitution lift the symmetry restrictions sufficiently to render both transitions allowed. The excitonic (Davydov) splitting can then be measured as the energy difference between the respective vibrationless O0(0) bands. We have measured the mass-specific vibronic spectra of the centrosymmetric H-bonded dimers (2-pyridone)2 and (2-aminopyridine)2 that are supersonically cooled to a few K and isolated in molecular beams, using two-color resonant two-photon ionization spectroscopy. Comparison of the all-12C- and 13C- isotopomer spectra yield excitonic splittings of delta(exp) = 43.5 and 10.5 cm(-1), respectively. The corresponding splittings calculated by high-level ab initio methods (RI-CC2/aug-cc-pVTZ) are 20 to 50 times larger. These purely electronic ab initio exciton splittings need to be reduced ('quenched') by vibronic coupling to the optically active vibrational modes. Only after quenching are the experimentally observed exciton splittings correctly reproduced.

Ottiger P; Leutwyler S

2011-01-01

160

Optical Absorption in Band-Edge Region of ( CH3NH3)3Bi2I9 Single Crystals  

Science.gov (United States)

Optical absorption spectra were measured around the band edge in ( CH3NH3)3Bi2I9 single crystals. In the very thin crystals, an intrinsic exciton band appears clearly at 2.51 eV at low temperatures, and the absorption peak remains even at room temperature reflecting a large binding energy of more than 300 meV. The exciton band does not show a discontinuous shift near the temperature where the methylammonium ions induce a structural phase transition. This fact indicates that the exciton states are independent of the methylammonium ions. The origin of the exciton band is examined on the basis of the annealing effect on the absorption bands in vacuum-evaporated films. The lower energy tail of the exciton band obeys the Urbach rule between 78 and 350 K. The detailed analysis by the Urbach rule reveals that the exciton-phonon interaction in the present system is relatively strong.

Kawai, Taketoshi; Ishii, Atsuko; Kitamura, Takako; Shimanuki, Shigetaka; Iwata, Makoto; Ishibashi, Yoshihiro

1996-05-01

 
 
 
 
161

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  

Energy Technology Data Exchange (ETDEWEB)

The competition between confinement potential fluctuations and band-gap renormalization (BGR) in GaAs/Al{sub x}Ga{sub 1-x}As 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 (E{sub PL}(T)). As the photon density increases, the E{sub PL} 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 E{sub PL}(T).

Lourenco, S.A., E-mail: sidneylourenco@utfpr.edu.br [Engenharia de Materiais, Universidade Tecnologica Federal do Parana, Londrina, PR 86036-370 (Brazil); Teodoro, M.D. [Departamento de Fisica, Universidade Estadual de Londrina, Londrina, PR 86051-970 (Brazil); Gonzalez-Borrero, P.P. [Departamento de Fisica, Universidade Estadual do Centro-Oeste, Guarapuava, PR 85040-080 (Brazil); Dias, I.F.L.; Duarte, J.L. [Departamento de Fisica, Universidade Estadual de Londrina, Londrina, PR 86051-970 (Brazil); Marega, E. [Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, SP 13560-970 (Brazil); Salamo, G.J. [Arkansas Institute for Nanoscale Materials Science and Engineering, University of Arkansas, Fayetteville, AR 72701 (United States)

2012-06-15

162

Organic halide electroreduction on silver  

Energy Technology Data Exchange (ETDEWEB)

Silver, whose extraordinary electrolytically properties for organic halide reduction have been recently evidenced, has been used as cathode material for systematic preparative electrolyses in membrane-divided cells. To better elucidate the substrate role on the remarkable positive shift of reduction potentials, and on the cage effect i. e. the promotion of intermolecular reaction on adsorbed intermediates, three halide substrate patterns are here compared in terms of both voltammetric characterization and preparative electroreduction products: aliphatic halides (adamantanes), aromatic halides (phenols) and anomeric glycosyl halides. The preparative electroreductions result mainly in dimerization in the case of glycosyl halides, in H {yields} Br substitution in the case of bromophenols, in dimerization + substitution in the case of haloadamantanes. The product analysis, both at the end of the reaction and at intermediate times, allows discussing the reaction pathways in terms of intermediate stability and of active surface accessibility. The possibility of complete dehalogenation on a wider substrate variety with remarkably lower energy consumption and almost quantitative current yields makes the process potentially very interesting for environmental purposes. [Italian] L'argento, di cui sono state recentemente evidenziate straordinarie proprieta' elettrocatalitiche per la riduzione degli alogenuri organici, e' stato utilizzato come materiale catodico per sistematiche elettrolisi preparative in celle a membrana. Per mettere in risalto il ruolo del substrato organico sul notevole anticipo del potenziale di riduzione e sull'effetto gabbia, ovvero la promozione di reazioni intermolecolari su intermedi adsorbiti, vengono qui confrontate, in termini sia di caratterizzazione voltammetrica sia di prodotti di elettroriduzioni preparative, tre tipologie di alogenuri: alifatici (adamantani), aromatici (fenoli) e glicosidici. Le elettroriduzioni preparative danno luogo principalmente a dimerizzazione nel caso di glicosilalogenuri, a H {yields} Br sostituzione nel caso di bromofenoli, a dimerizzazione + sostituzione nel caso degli alogenoadamantani. L'analisi dei prodotti, sia al termine della reazione, sia a tempi intermedi e di accessibilita' della superficie attiva. La possibilita' di completa dealogenazione di una piu' vasta categoria di substrati con notevole riduzione del consumo energetico e rendimento di corrente pressoche' unitario conferisce al processo un potenziale notevole interesse ambientale.

Fiori, G.; Mussini, P.; Rondinini, S.; Vertova, A. [Milan Univ., Milan (Italy). Dept. of Physical Chemistry and Electrochemistry

2001-04-01

163

Identification of bound exciton complexes in ZnO  

Energy Technology Data Exchange (ETDEWEB)

An identification of shallow bound exciton centers in ZnO is presented based on magneto-optical measurements and diffusion experiments. The thermalization behavior of the Zeeman split components confirms that the I{sub 4}, I{sub 6}, I{sub 8} and I{sub 9} exciton lines stem from donor bound exciton complexes. The results are supported by theoretical analysis of shallow bound exciton complexes revealing the {gamma}{sub 7} symmetry of the upper valence band. The presence of two-electron satellites related to the respective transitions is further evidence for the donor bound complexes and enabled the determination of donor binding energies. Hydrogen, aluminum, gallium and indium were identified to origin the I{sub 4}, I{sub 6}, I{sub 8} and I{sub 9} lines by doping, diffusion and annealing experiments combined with photoluminescence and secondary ion mass spectrometry. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Strassburg, M.; Rodina, A.; Dworzak, M.; Haboeck, U.; Krestnikov, I.L.; Hoffmann, A. [Institute of Solid State Physics, Technical University of Berlin, Hardenbergstrasse 36, 10623 Berlin (Germany); Gelhausen, O. [Institute of Solid State Physics, Technical University of Berlin, Hardenbergstrasse 36, 10623 Berlin (Germany); Microstructural Analysis Unit, University of Technology, Sydney, Broadway 2007 (Australia); Phillips, M.R. [Microstructural Analysis Unit, University of Technology, Sydney, Broadway 2007 (Australia); Alves, H.R.; Zeuner, A.; Hofmann, D.M.; Meyer, B.K. [I. Physics Institute, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen (Germany)

2004-03-01

164

Quantum condensation in electron-hole systems: excitonic BEC-BCS crossover and biexciton crystallization  

Energy Technology Data Exchange (ETDEWEB)

Quantum condensation of electron-hole (e-h) systems in photoexcited semiconductors is reviewed from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type condensed state, the exciton Mott transition, and the biexciton crystallization. First, we discuss the crossover between the exciton BEC and the e-h BCS states at low temperature using the self-consistent t-matrix and local approximations, applied to the high-dimensional two-band Hubbard model with both repulsive and attractive on-site interactions. We also study the metal-insulator transition (called the 'exciton Mott transition') at zero and finite temperatures, investigated with the dynamical mean-field theory. Away from half-filling we find excitonic/biexcitonic insulating phases and the first-order transition between metallic and insulating states. Second, in a one-dimensional e-h system, we employ the exciton bosonization and renormalization-group techniques to clarify quantum orders at zero temperature. The most probable ground state exhibits the biexciton crystallization, which reflects the Tomonaga-Luttinger liquid properties, the e-h backward scattering, and the long-range Coulomb interaction. The one-dimensional e-h system is insulating even at the high-density limit, hence the exciton Mott transition never occurs at zero temperature in one dimension.

Ogawa, Tetsuo; Tomio, Yuh; Asano, Kenichi [Department of Physics, Osaka University and CREST, JST, Toyonaka, Osaka 560-0043 (Japan)

2007-07-25

165

Quantum condensation in electron-hole systems: excitonic BEC-BCS crossover and biexciton crystallization.  

UK PubMed Central (United Kingdom)

Quantum condensation of electron-hole (e-h) systems in photoexcited semiconductors is reviewed from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type condensed state, the exciton Mott transition, and the biexciton crystallization. First, we discuss the crossover between the exciton BEC and the e-h BCS states at low temperature using the self-consistent t-matrix and local approximations, applied to the high-dimensional two-band Hubbard model with both repulsive and attractive on-site interactions. We also study the metal-insulator transition (called the 'exciton Mott transition') at zero and finite temperatures, investigated with the dynamical mean-field theory. Away from half-filling we find excitonic/biexcitonic insulating phases and the first-order transition between metallic and insulating states. Second, in a one-dimensional e-h system, we employ the exciton bosonization and renormalization-group techniques to clarify quantum orders at zero temperature. The most probable ground state exhibits the biexciton crystallization, which reflects the Tomonaga-Luttinger liquid properties, the e-h backward scattering, and the long-range Coulomb interaction. The one-dimensional e-h system is insulating even at the high-density limit, hence the exciton Mott transition never occurs at zero temperature in one dimension.

Ogawa T; Tomio Y; Asano K

2007-07-01

166

Nonlinear optical spectroscopy of biexcitons and excitonic polaritons in CuCl (CuBr)  

International Nuclear Information System (INIS)

[en] Copper halides (especially CuCl) have played an important role in the development of nonlinear optical spectroscopy. The binding energies of excitons and of biexcitons are so large that the optical response of these materials and its dynamics could be fully described in the frame of these quasi-particle states and of their mutual interactions. We shall go through the different steps of the development of this research and present the contribution of the Strasbourg Group to this activity. (orig.)

1995-01-01

167

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

2013-01-01

168

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

UK PubMed Central (United Kingdom)

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 C; Li C; Andreakou P; Wang P; Ding M; Brambilla G; Manna L; Lagoudakis P

2013-01-01

169

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.

2012-01-01

170

Scaling of excitons in carbon nanotubes  

CERN Multimedia

Light emission from carbon nanotubes is expected to be dominated by excitonic recombination. Here we calculate the properties of excitons in nanotubes embedded in a dielectric, for a wide range of tube radii and dielectric environments. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength.

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

2004-01-01

171

Excitonic polaritons in Fibonacci quasicrystals.  

Science.gov (United States)

The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission. PMID:18825174

Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M

2008-09-29

172

Time-domain chirally-sensitive three-pulse coherent probes of vibrational excitons in proteins  

CERN Multimedia

The third order optical response of bosonic excitons is calculated using the Green's function solution of the Nonlinear Exciton Equations (NEE) which establish a quasiparticle-scattering mechanism for optical nonlinearities. Both time ordered and non ordered forms of the response function which represent time and frequency domain techniques, respectively, are derived. New components of the response tensor are predicted for isotropic ensembles of periodic chiral structures to first order in the optical wavevector. The nonlocal nonlinear response function is calculated in momentum space, where the finite exciton-exciton interaction length greatly reduces the computational effort. Applications are made to coupled anharmonic vibrations in the amide I infrared band of peptides. Chirally-sensitive and non sensitive signals for alpha helices and antiparallel beta sheets are compared.

Abramavicius, D; Abramavicius, Darius; Mukamel, Shaul

2005-01-01

173

Peculiarities of exciton and EPR spectra of 2H-PbI2 layered crystals with high concentration of Mn impurity  

International Nuclear Information System (INIS)

Recent results on the study of exciton and EPR spectra of PbI2 layered crystals of the 2H polytype doped with a high concentration of Mn impurity are discussed. The anomalous temperature shift of the exciton band n = 1 is found and explained by means of anharmonic vibrations of the layered lattice at T 40 K

2006-01-01

174

Excitonic pairing between nodal fermions  

CERN Multimedia

We study excitonic pairing in nodal fermion systems characterized by a vanishing quasiparticle density of states at the point-like Fermi surface and a concomitant lack of screening for long-range interactions. By solving the gap equation for the excitonic order parameter, we obtain a critical value of the interaction strength for a variety of power-law interactions and densities of states. We compute the free energy and analyze possible phase transitions, thus shedding further light on the unusual pairing properties of this peculiar class of strongly correlated systems.

Khveshchenko, D V

2006-01-01

175

Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals  

Directory of Open Access Journals (Sweden)

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

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

2013-01-01

176

High-field franz-keldysh effect and exciton ionization in semiconductor quantum wires  

UK PubMed Central (United Kingdom)

We investigate the Franz-Keldysh effect and exciton ionization in semiconductor quantum wires. Absorption spectra are calculated near the band gap by solution of the low-density semiconductor Bloch equations in real space. The Sommerfeld factor and field-induced tunnel ionization of the exciton significantly affect the continuum portion of the absorption spectrum and remove the well known divergence problem associated with the 1D density of states at all field strengths. For reasonable electric field strengths substantial and tunable absorption oscillations appear above the band gap. Moreover, for very large fields, transparency can be achieved in the continuum for certain spectral positions.

Hughes S; Citrin DS

2000-05-01

177

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

International Nuclear Information System (INIS)

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

1991-01-01

178

Observation of surface excitons in rare gas solids  

International Nuclear Information System (INIS)

[en] Evidence is obtained for the excitation of surface excitons in solid Ar, Kr and Xe in optical transmission and reflection experiments using synchrotron radiation. They are located at photon energies ranging from 0.6 eV for Ar to 0.1 eV for Xe below the corresponding bulk excitons excited from the valence bands. Their halfwidths (20-50 MeV) is less than half the values found for the bulk excitons. Some are split by an amount considerably smaller than the spin orbit splitting of the valence bands. (orig.)[de] Die Anregung von Oberflaechenexzitonen in festem Ar, Kr und Xe wird in optischen Transmissions- und Reflexionsexperimenten mit Synchrotronstrahlung nachgewiesen. Diese Exzitonen liegen bei Photonenenergien von 0,6 eV in Ar bis zu 0,1 eV in Xe unter den entsprechenden Volumenexzitonen, die aus Valenzbaendern angeregt wurden. Ihre Linienbreiten (20-50 MeV) betragen weniger als die Haelfte der Werte fuer Volumenexzitonen. Einige werden bei Werten aufgespalten, die erheblich unterhalb der Spin-Orbit-Aufspaltung der Valenzbaender liegen. (orig./AK)

1976-01-01

179

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

CERN Multimedia

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

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

2005-01-01

180

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra.  

Science.gov (United States)

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases. PMID:15974774

Bittner, Eric R; Ramon, John Glenn Santos; Karabunarliev, Stoyan

2005-06-01

 
 
 
 
181

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra.  

UK PubMed Central (United Kingdom)

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

Bittner ER; Ramon JG; Karabunarliev S

2005-06-01

182

All conjugated copolymer excitonic multiferroics.  

UK PubMed Central (United Kingdom)

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.

Lohrman J; Liu Y; Duan S; Zhao X; Wuttig M; Ren S

2013-02-01

183

The excitonic optical Stark effect in GaN  

Energy Technology Data Exchange (ETDEWEB)

The dynamic Stark effect of excitons in GaN at 10 K with excitation well below the excitonic resonances was studied using nondegenerate femtosecond pump-probe spectroscopy with co- and cross-linear polarization configurations. In contrast to two-dimensional GaAs/AlGaAs quantum wells, which have Bloch eigenstates similar to those of GaN and a large spin-orbit coupling, we observed that the Stark effect in GaN is strongly dependent on the pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of the A and B valence bands induced by a linearly polarized pump. (orig.)

Choi, C.K.; Lam, J.B.; Gainer, G.H.; Shee, S.K.; Krasinski, J.S.; Song, J.J. [Oklahoma State Univ., Stillwater (United States). Center for Laser and Photonics Research; Chang, Y.C. [Illinois Univ., Urbana, IL (United States). Dept. of Physics and Materials Research Lab.

2002-03-16

184

Synthesis of aryl halides via organoborane chemistry  

Energy Technology Data Exchange (ETDEWEB)

A method for the rapid synthesis of a variety of substituted aryl halides by the reaction of organoboranes with halide ions in the presence of chloramine-T is described in detail. The products were purified by column chromatography on silica gel using a mixture of petroleum ether-ethyl acetate as eluent.

Kabalka, G.W.; Sastry, K.A.R.; Sastry, U.; Somayaji, V.

1982-01-01

185

Electronic properties of metal-induced gap states formed at alkali-halide/metal interfaces  

CERN Document Server

The spatial distribution and site- distribution of metal induced gap states (MIGS) are studied by thickness dependent near edge x-ray absorption fine structure (NEXAFS) and comparing the cation and anion edge NEXAFS. The thickness dependent NEXAFS shows that the decay length of MIGS depends on rather an alkali halide than a metal, and it is larger for alkali halides with smaller band gap energy. By comparing the Cl edge and K edge NEXAFS for KCl/Cu(001), MIGS are found to be states localizing at anion sites.

Kiguchi, M; Ikeda, S; Saiki, K; Kiguchi, Manabu; Yoshikawa, Genki; Ikeda, Susumu; Saiki, Koichiro

2005-01-01

186

Photophysics of excitons in quasi-one-dimensional organic semiconductors: Single-walled carbon nanotubes and ? -conjugated polymers  

Science.gov (United States)

The nature of the primary photoexcitations in semiconducting single-walled carbon nanotubes (S-SWCNTs) is of strong current interest. We have studied the emission spectra of S-SWCNTs and two different ? -conjugated polymers in solutions and films, and have also performed ultrafast pump-probe spectroscopy on these systems with unprecedented spectral range from 0.1to2.6eV . The emission spectra relative to the absorption bands are very similar in S-SWCNTs and polymers, with redshifted photoluminescence in films showing exciton migration. We also found that the transient excited state spectra of both polymers and SWCNTs contain two prominent photoinduced absorption (PA) bands ( PA1 and PA2 ) that are due to photogenerated excitons; in the polymers these PA bands are correlated with a stimulated emission band, which is absent in the S-SWCNTs. In order to understand the similarities in the PA spectra we have performed theoretical calculations of excited state absorptions in ? -conjugated polymers as well as S-SWCNTs within the same correlated electron Hamiltonian. We find strong similarities in the excitonic energy spectra of these two classes of quasi-one-dimensional materials, although there exist also subtle differences such as the occurrence of dark excitons below the optical excitons in the S-SWCNTs. In the polymers PA1 is an excited state absorption from the optical exciton to a two-photon exciton that occurs below the continuum band threshold. In the S-SWCNTs PA1 occurs from both the optical exciton and the dark exciton, to final states which are close in energy and again below the continuum band threshold. PA1 therefore gives the lower limit of the binding energy of the lowest optical exciton in both ? -conjugated polymers and S-SWCNTs. The binding energy of lowest exciton that belongs to the widest S-SWCNTs with diameters ?1nm in films is 0.3-0.4eV , as determined by both experimental and theoretical methods.

Zhao, H.; Mazumdar, S.; Sheng, C.-X.; Tong, M.; Vardeny, Z. V.

2006-02-01

187

Quaternary system of cesium halides  

International Nuclear Information System (INIS)

[en] The state diagram of the quaternary system consisting of fluorides, chlorides, bromides, and iodides of cesium has been studied by visual-polythermal, partially X-ray phase and thermographical analyses. The crystallization volume of the quaternary system involves the crystallization volume of cesium fluoride and the crystallization volume of the ternary solid solutions of the rest cesium halides. A quaternary nonvariant point corresponding to melting point 360 deg C appears on the crystallization surface which separates the cesium fluoride volume from the volume of the ternary solid solutions

1977-01-01

188

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

Directory of Open Access Journals (Sweden)

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

C. Spitz; S. Daehne

2006-01-01

189

Tuning the excitonic and plasmonic properties of copper chalcogenide nanocrystals.  

UK PubMed Central (United Kingdom)

The optical properties of stoichiometric copper chalcogenide nanocrystals (NCs) are characterized by strong interband transitions in the blue part of the spectral range and a weaker absorption onset up to ~1000 nm, with negligible absorption in the near-infrared (NIR). Oxygen exposure leads to a gradual transformation of stoichiometric copper chalcogenide NCs (namely, Cu(2-x)S and Cu(2-x)Se, x = 0) into their nonstoichiometric counterparts (Cu(2-x)S and Cu(2-x)Se, x > 0), entailing the appearance and evolution of an intense localized surface plasmon (LSP) band in the NIR. We also show that well-defined copper telluride NCs (Cu(2-x)Te, x > 0) display a NIR LSP, in analogy to nonstoichiometric copper sulfide and selenide NCs. The LSP band in copper chalcogenide NCs can be tuned by actively controlling their degree of copper deficiency via oxidation and reduction experiments. We show that this controlled LSP tuning affects the excitonic transitions in the NCs, resulting in photoluminescence (PL) quenching upon oxidation and PL recovery upon subsequent reduction. Time-resolved PL spectroscopy reveals a decrease in exciton lifetime correlated to the PL quenching upon LSP evolution. Finally, we report on the dynamics of LSPs in nonstoichiometric copper chalcogenide NCs. Through pump-probe experiments, we determined the time constants for carrier-phonon scattering involved in LSP cooling. Our results demonstrate that copper chalcogenide NCs offer the unique property of holding excitons and highly tunable LSPs on demand, and hence they are envisaged as a unique platform for the evaluation of exciton/LSP interactions.

Kriegel I; Jiang C; Rodríguez-Fernández J; Schaller RD; Talapin DV; da Como E; Feldmann J

2012-01-01

190

Coexistence of bound and localized exciton magnetic polarons in Cd(0.8)Mn(0.2)Te.  

Science.gov (United States)

Fundamental magneto-optical properties of single crystals of the semimagnetic semiconductor Cd(0.8)Mn(0.2)Te are carefully measured with the use of band-to-band excitation. Two new phenomena are presented: the peculiar magnetic field dependence of the magnetic polaron luminescence and the existence of two components for the polaron luminescence. Theoretical consideration with regard to the Zeeman effect ascribes the two components of luminescence to the exciton magnetic polaron spectrum and the neutral acceptor-bound exciton magnetic polaron spectrum. The calculated result for the luminescence peak positions is also consistent with the experimental result of the circularly polarized luminescence under the presence of an applied magnetic field. In other words, the ?(+) luminescence is due to the exciton magnetic polaron, whereas the ?(-) luminescence is due to the neutral acceptor-bound exciton magnetic polaron. PMID:21690991

Okada, Takanori; Itoh, Tadashi

2007-04-05

191

Exciton ionization, Franz-Keldysh, and Stark effects in carbon nanotubes.  

Science.gov (United States)

We calculate the optical properties of carbon nanotubes in an external static electric field directed along the tube axis. We predict strong Franz-Keldysh oscillations in the first and second band-to-band absorption peaks, quadratic Stark effect of the first two excitons, and the field dependence of the bound exciton ionization rate for a wide range of tube chiralities. We find that the phonon-assisted mechanism dominates the dissociation rate in electro-optical devices due to the hot optical phonons. We predict a quadratic dependence of the Sommerfeld factor on the electric field and its increase up to 2000% at the critical field of the full exciton dissociation. PMID:17261074

Perebeinos, Vasili; Avouris, Phaedon

2007-01-30

192

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

DEFF Research Database (Denmark)

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

Orani, D.; Polimeni, A.

1997-01-01

193

Temperature-Dependent Maximum Density of 1D Excitons in Carbon Nanotubes  

Science.gov (United States)

Previous studies have shown that an upper limit exists on the density of 1D excitons in single-walled carbon nanotubes (SWNTs) due to very efficient exciton-exciton annihilation (EEA). A recent theoretical study based on a dark-bright two-band exciton model predicts that there is a temperature at which the achievable exciton density will be maximized, surpassing the room-temperature upper limit. Therefore, we performed temperature-dependent (300 K to 11 K) photoluminescence (PL) on HiPco SWNTs embedded in an i-carrageenan matrix under high resonant excitation. To achieve high densities, we used pump fluences up to ˜ 10^14 photons/cm^2, utilizing intense fs pulses from a wavelength-tunable optical parametric amplifier. We found that for each temperature the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Below that critical temperature, the PL intensity decreases with decreasing temperature. Within the framework of diffusion-limited EEA, we successfully estimated the upper limit of the density of 1D excitons in SWNTs as a function of temperature and chirality

Searles, Thomas; Walsh, Ian; Nosaka, Takayuki; Rice, William; Kono, Junichiro

2011-03-01

194

Transient photoluminescence enhancement as a probe of the structure of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$  

CERN Document Server

We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF$_2$:Yb$^{2+}$. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.

Reid, Michael F; Wells, Jon-Paul R; Berden, Giel; Meijerink, Andries; Salkeld, Alex; Duan, Chang-Kui

2011-01-01

195

ZnSe/ZnTe(shell/shell) radial quantum-wire heterostructures: the excitonic properties  

International Nuclear Information System (INIS)

The ground-state characteristics of spatially indirect excitons trapped in radially heteronanostructured type-II band alignment ZnSe/ZnTe nanotubes as functions of the magnetic field for nanotubes with a radial size both smaller and larger than the effective Bohr radius are theoretically investigated. In the former case, dominated by the net kinetic energy of the electron and hole, the magnetic field modifies the exciton spectrum through the well-known Zeeman splitting, intra-orbital-state Aharonov-Bohm oscillations and inter-orbital-state crossovers occurring in very strong magnetic field strengths. However, in the latter case, dominated by the electron-hole Coulomb attraction, the magnetic field adjusts the exciton lines only by means of the Zeeman splitting and inter-orbital-state transitions happening in typical magnetic fields. As a result, the angular momentum transitions occurr at lower magnetic fields when the radial size of the nanotube is increased. Most importantly, another consequence is the substantially unusual exciton oscillator strength in such heteronanostructures. It is shown that when the exciton is optically active, due to the full cylindrical symmetry of the problem, the exciton oscillator strength shows undamped oscillations. This effect is associated with the periodic redistribution of the exciton density as the magnetic field is varied. Also, the magnitude of the magnetically induced excitonic persistent current is decreased with increasing radial size of the nanotube. This study may provide a platform to investigate new photonic quantum interference as well as polarization-sensitive photodetector and photovoltaic devices based on the Aharonov-Bohm effect. (fast track communication)

2010-07-14

196

Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes  

Digital Repository Infrastructure Vision for European Research (DRIVER)

We studied the exciton energy transfer in pairs of semiconducting nanotubes using high-resolution optical microscopy and spectroscopy on the nanoscale. Photoluminescence from large band gap nanotubes within bundles is observed with spatially varying intensities due to distance-dependent internanotub...

Qian, Huihong; Georgi, Carsten; Anderson, Neil; Green, Alexander A.; Hersam, Mark C.; Novotny, Lukas; Hartschuh, Achim

197

Fractionally charged magneto-excitons  

CERN Document Server

The photoluminescence (PL) spectrum of a two-dimensional electron gas (2DEG) in the fractional quantum Hall regime is studied. The response of the 2DEG to an optically injected valence hole depends on the separation d between the electron and hole layers. At d smaller than the magnetic length lambda, the PL spectrum shows recombination of neutral (X) and charged (X-) excitons. At d>lambda, the hole binds one or two Laughlin quasielectrons (QE) of the 2DEG to form fractionally charged excitons (FCX), hQE or hQE2. Different FCX states have different optical properties, and their stability depends critically on the presence of QE's in the 2DEG. This explains discontinuities observed in the PL spectrum at such (Laughlin) filling factors as nu=1/3 or 2/3.

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

2001-01-01

198

Surface phonons and exciton-polariton coupling in SiC nanocrystals  

International Nuclear Information System (INIS)

The paper presents the results of SiC nanocrystal characterization using scanning electron microscopy (SEM), Raman scattering and photoluminescence spectroscopy techniques, as well as X-ray diffraction (XRD). The Raman scattering investigation of porous SiC with different NC sizes have shown new features specific for nano-crystallite materials: appearing of surface phonon modes and the enlargement of diffusing background scattering. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton related PL bands. The intensity enhancement for exciton-related PL bands is attributed to exciton recombination rate increasing due to the realization of exciton confinement and exciton-polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC). XRD study has confirmed that the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes.

2010-01-01

199

Exciton and biexciton signatures in femotosecond transient absorption of {pi}-conjugated oligomers  

Energy Technology Data Exchange (ETDEWEB)

The authors report femotosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene (PPV) prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photo-chemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long-wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, the authors see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. They attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.

Klimov, V.; McBranch, D. [Los Alamos National Lab., NM (United States); Barashkov, N.; Ferraris, J. [Univ. of Texas, Dallas, TX (United States)

1997-10-01

200

Halide glasses and their optical properties  

Energy Technology Data Exchange (ETDEWEB)

The different families of halide glasses are considered by reference to the glass-forming ability of divalent, trivalent and quadrivalent halides. The passive optical behavior, such as that in the IR transmission range, the low-loss potentially and scattering problems, is discussed as well as the active function of halide glasses considered as new matrices for rare-earth or transition-metal spectroscopy. Some other properties such as electrical or magnetic ones and those associated with purification, devitrification, chemical durability will be discussed. (orig.).

Lucas, J.; Adam, J.L. (Rennes-1 Univ., 35 (France). Lab. de Chimie Minerale)

1989-12-01

 
 
 
 
201

Spontaneous coherence in a cold exciton gas.  

UK PubMed Central (United Kingdom)

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose-Einstein condensation. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons--bound pairs of electrons and holes--form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin.

High AA; Leonard JR; Hammack AT; Fogler MM; Butov LV; Kavokin AV; Campman KL; Gossard AC

2012-03-01

202

Dynamic theory of vibronic spectra of charge transfer excitons  

Science.gov (United States)

The vibronic spectra of charge transfer excitons (CTE) in a molecular chain are treated in the framework of a dynamic approach (which neglects the thermal excitation of the excitons and of the intramolecular vibration). The model includes two mechanisms of coupling between CTEs and vibrational quanta, generalizes in this way the simple CTE Hamiltonian as well as the vibronic Hamiltonian of Frenkel excitons and allows to study the vibronics of CTEs. The linear optical susceptibility is calculated in the vibronic regime (one CTE plus one vibrational quantum). A double splitting of the vibronics of CTEs is established: (1) a splitting connected with the location of the intramolecular vibration on donors or on acceptors and (2) a splitting connected with the symmetry of the vibronic states (in the degenerate case, realized in the absence of an external electric field). The investigation of the general structure of the vibronic spectra of CTEs, which includes two-particle bands and bound CTE-phonon states, allows to model the optical absorption spectra in two cases: (1) without proper transfer of a vibrational quantum between neighboring molecules and (2) with transfer of a vibrational quantum between donor and acceptor molecules.

Lalov, I. J.; Supritz, C.; Reineker, P.

2005-03-01

203

Mechanism and Selectivity in Nickel-Catalyzed Cross- Electrophile Coupling of Aryl Halides with Alkyl Halides.  

UK PubMed Central (United Kingdom)

The direct cross-coupling of two different electro- philes, such as an aryl halide with an alkyl halide, offers many ad- vantages over conventional cross-coupling methods that require a carbon nucleophile. Despite its promise as a versatile synthetic strategy, a limited understanding of the mechanism and origin of cross selectivity has hindered progress towards reaction develop- ment and design. Herein, we shed light on the mechanism for the nickel-catalyzed cross-electrophile coupling of aryl halides with alkyl halides and demonstrate that the selectivity arises from an unusual catalytic cycle that combines both polar and radical steps to form the new C-C bond.

Biswas S; Weix DJ

2013-08-01

204

Spin-dependent exciton-exciton interaction potential in two- and three-dimensional structure semiconductors under excitation.  

Science.gov (United States)

Analytical expressions of the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D structure materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass rati...

Nguyen Ba An Hoang Ngoc Cam Nguyen Trung Dan

1990-01-01

205

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

International Nuclear Information System (INIS)

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

2003-05-21

206

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

Energy Technology Data Exchange (ETDEWEB)

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

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

2003-05-21

207

Spin-dependent exciton-exciton interaction potential in two- and three-dimensional structure semiconductors under excitation  

International Nuclear Information System (INIS)

Analytical expressions of the exciton-exciton interaction potentials have been approximately derived in both 2D and 3D structure materials exhibiting explicit dependences on exciton momentum difference, momentum transfer, electron-hole effective mass ratio and two-exciton state spin symmetry. Numerical calculations show that the character of the exciton-exciton interaction is determined by all of the above-mentioned dependences. (author). 32 refs, 7 figs.

1990-01-01

208

Reactions of diiminopyridine ligands with chalcogen halides.  

UK PubMed Central (United Kingdom)

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.

Martin CD; Ragogna PJ

2012-03-01

209

Oxidation of hydrogen halides to elemental halogens  

Energy Technology Data Exchange (ETDEWEB)

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

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

1985-01-01

210

Interaction of boron halides with carbon preparations  

International Nuclear Information System (INIS)

Interaction of boron trichloride and tribromide with hydroxyfunctional groups of diamond and technical carbon surface under the pressure of halide vapours in the range 0.33-3.52 kPa and in the temperature range 32-200 deg C has been investigated. It is shown that interaction under the conditions takes place irreversibly. The rate of boron halide adsorption is described by kinetic equation for homogeneous surface. Activation energies of chemisorption and rate constants are found.

1987-01-01

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

2008-04-09

212

Optical spectra and exciton Mott transition in correlated electron-hole systems  

International Nuclear Information System (INIS)

Optical properties in the insulating states of three-dimensional electron-hole systems are studied using a two-band Hubbard model with both repulsion U and attraction U'. For two types of insulators, the Mott-Hubbard type and the biexciton type at half-filling, in particular, interband absorption spectra are calculated analytically within the two-site dynamical mean-field theory and ladder approximation. We show that an excitonic peak structure appears due to an 'exciton' in the sea of correlated fermions when U' is larger than a certain critical value.

2008-01-01

213

Electronic and surface properties of PbS nanoparticles exhibiting efficient multiple exciton generation.  

UK PubMed Central (United Kingdom)

Ultrafast transient absorption measurements have been used to study multiple exciton generation in solutions of PbS nanoparticles vigorously stirred to avoid the effects of photocharging. The threshold and slope efficiency of multiple exciton generation are found to be 2.5 ± 0.2 ×E(g) and 0.34 ± 0.08, respectively. Photoemission measurements as a function of nanoparticle size and ageing show that the position of the valence band maximum is pinned by surface effects, and that a thick layer of surface oxide is rapidly formed at the nanoparticle surfaces on exposure to air.

Hardman SJ; Graham DM; Stubbs SK; Spencer BF; Seddon EA; Fung HT; Gardonio S; Sirotti F; Silly MG; Akhtar J; O'Brien P; Binks DJ; Flavell WR

2011-12-01

214

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)

1987-01-01

215

Optical spectra and excitonic BEC-BCS crossover in coherent electron-hole pair condensation  

Energy Technology Data Exchange (ETDEWEB)

We investigate coherent pair condensation in three-dimensional electron-hole (e-h) systems using the two-band Hubbard model with both repulsion and attraction, especially from the viewpoint of optical properties. Applying the self-consistent t-matrix and local approximations, the crossover behavior between the e-h BCS-like state and the excitonic Bose-Einstein condensation (BEC) at finite temperatures is described as a function of the e-h attraction strength. The changes of absorption and luminescence spectra are also discussed along the excitonic BEC-BCS crossover.

Tomio, Yuh [CREST, JST and Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)], E-mail: tomio@acty.phys.sci.osaka-u.ac.jp; Ogawa, Tetsuo [CREST, JST and Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan)

2008-05-15

216

Optically pumped divalent metal halide lasers  

Energy Technology Data Exchange (ETDEWEB)

A laser system, emitting energy in or near the visible region of the spectrum, is described comprising: (a) enclosure means; (b) a molecular divalent metal halide and an inert buffer gas, present within the enclosure means; (c) a heat source, for vaporization of the molecular divalent metal halide, capable of maintaining the enclosure means at a temperature within the range from about 50/sup 0/ to about 550/sup 0/C; (d) a source of dissociative energy, located in proximity to the enclosure means and in communication therewith, for effecting dissociation of the vaporized molecular divalent metal halide into diatomic metal halide fragments thereof present in a ground electronic energy state; (e) optical pumping means, located in proximity to the enclosure means and in communication therewith; whereby the diatomic metal halide fragments are raised from the ground electron energy state to one or more vibrational energy levels of a low-lying electronic excited state; and (f) an optical cavity, disposed about the enclosure means, aligned to obtain lasing from optical transition in which the diatomic metal halide fragments return to their ground electronic energy state.

Eden, J.G.; Greene, D.P.; Kileen, K.P.

1988-04-05

217

Propagation of exciton pulses in semiconductors  

CERN Document Server

We examine the propagation of excitons in Cu$_2$O, which form localized pulses under certain experimental conditions. The formation of these waves is attributed to the effect of dispersion, non-linearity and the coupling of the excitons to phonons, which acts as a dissipative mechanism.

Jackson, A D

2002-01-01

218

Constraint on ?E and exciton number  

International Nuclear Information System (INIS)

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

1986-01-01

219

Even-parity excitons in condensed xenon  

Energy Technology Data Exchange (ETDEWEB)

Excitation spectra of two-photon excited solid xenon have been obtained in the energy range of 8.4 to 11.1 eV. The measurements were performed at temperatures from 160 to 100 K. The even-parity {Gamma}(3/2) {ital n}=2, 3, and 4 Wannier excitons are observed in solid xenon. The {ital n}=2 and 3 excitons are shifted to higher energies relative to the energy of the corresponding odd-parity excitons by approximately 80 and 50 meV, respectively. In addition, the excitation spectrum revealed a previously unobserved series of excitons between 10.7 and 11 eV. These excitons form a Rydberg series and are possibly assignable as excitons at the {ital X} point of the Brillouin zone. The absorption linewidths (full width at half maximum) of the 2{ital p}, 3{ital p}, and 4{ital p} excitons are measured to be approximately 80, 52, and 22 meV, respectively. Several models are investigated in an attempt to explain the shift in energy between the {ital s} and {ital p} excitons in liquid and solid xenon. {copyright} {ital 1996 The American Physical Society.}

Pournasr, H.; Holder, J.P.; Keto, J.W. [Department of Physics, The University of Texas at Austin, Austin, Texas 78712 (United States)

1996-07-01

220

Electrochemically promoted carbon-halide bond cleavage in 4-nitrobenzyl halides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

This manuscript reports the study of the carbon-halide bond cleavage in 4-nitrobenzyl halides, taking special attention to the iodide and fluoride derivatives. The electrochemical reduction mechanism has been disclosed for both compounds by terms of cyclic voltammetry and controlled potential ele...

Cabán Huertas, Zahilia

 
 
 
 
221

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

Digital Repository Infrastructure Vision for European Research (DRIVER)

An application of the N,N’-(5-nitro-1,3-phenylene)bisglutaramide (NPG) hydroxylation assay for spectrophotometric determination of the rate of oxidising species generation in Fenton-like systems in the presence of halide and pseudo-halide antioxidants was evaluated. Using ion chromatography it was d...

Maleši?, J.; Kolar, J.; Strli?, M.; Polanc, S.

222

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

CERN Multimedia

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

223

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

UK PubMed Central (United Kingdom)

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 usually 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 KJ; Ma F; Zheng K; Zidek K; Mousa A; Abdellah MA; Messing ME; Wallenberg LR; Yartsev A; Pullerits T

2013-07-01

224

Influence of size on the excitonic absorbance of CdSe nano crystal  

Science.gov (United States)

The size quantization effect is noticed as a shift of the transition energies toward higher values with decreasing particle size. Based on the SEM images report a curve is drawn and fitted with MATLAB programming to obtain a relation between excitonic absorbance and diameter of the nano crystal. Nonlinear fit of the curve is used to provide an empirical formula for the estimation of the excitonic absorbance as a dependent parameter of diameter of nano crystal. Some adjustable parameter is required to calculate the excitonic absorbance wavelength of CdSe nano crystal. Theoretical findings are graphically represented and compared with available experimental results. The theoretical model predicts a decrease in absorbance wavelength or blue shift in absorbance spectrum which consequently increases the band gap of CdSe nano crystal with decreasing of crystalline sizes. Results are comparable with the established experimental findings.

Sisodia, Namita; Bhadkare, Bhavana; Bhavsar, Priyanka; Sisodia, Amitesh

2013-06-01

225

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

CERN Multimedia

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

Harigaya, Kikuo

2008-01-01

226

Exciton trapping at Au and Ag atoms in rare gas matrices  

Energy Technology Data Exchange (ETDEWEB)

The fluorescence of Au and Ag atoms in Ar and Kr matrices is investigated as a function of the excitation wavelength in the UV and VUV range. The occurrence of a fluorescence of the dopants during excitation into the exciton bands of the matrices demonstrates the existence of energy transfer from the solid host to the embedded metal atoms. The emission spectra after exciton and inner-shell excitation, respectively, as well as the temperature dependence of the emission yield show striking similarities. From this it is concluded that the exciton trapping leads to an excitation of the same inner-shell states that are also populated by direct inner-shell excitation of the metal atoms. (orig.).

Schrimpf, A.; Herkert, B.; Manceron, L.; Schriever, U.; Stoeckmann, H.J. (Fachbereich Physik, Philipps-Univ. Marburg und Wissenschaftliches Zentrum fuer Materialwissenschaften (Germany, F.R.))

1991-06-01

227

Spontaneous exciton condensation in 1T-TiSe2 : BCS-like approach  

Science.gov (United States)

Recently we found strong evidence in favor of a BCS-like condensation of excitons in 1T-TiSe2 [Cercellier , Phys. Rev. Lett. 99, 146403 (2007)]. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the exciton condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. The scope of this paper is to present the detailed calculations in the framework of this model. They represent an extension of the original excitonic insulator phase model of Jérome [Phys. Rev. 158, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and comparison with experiment is presented. Finally, the disagreement with density-functional theory is discussed.

Monney, C.; Cercellier, H.; Clerc, F.; Battaglia, C.; Schwier, E. F.; Didiot, C.; Garnier, M. G.; Beck, H.; Aebi, P.; Berger, H.; Forró, L.; Patthey, L.

2009-01-01

228

Electronic structure, lattice energies and Born exponents for alkali halides from first principles  

Directory of Open Access Journals (Sweden)

Full Text Available First principles calculations based on DFT have been performed on crystals of halides (X = F, Cl, Br and I) of alkali metals (M = Li, Na, K, Rb and Cs). The calculated lattice energies (U0) are in good agreement with the experimental lattice enthalpies. A new exact formalism is proposed to determine the Born exponent (n) for ionic solids. The values of the Born exponent calculated through this ab-initio technique is in good agreement with previous empirically derived results. Band Structure calculations reveal that these compounds are wide-gap insulators that explains their optical transparency. Projected density of states (PDOS) calculations reveal that alkali halides with small cations and large anions, have small band gaps due to charge transfer from X ? M. This explains the onset of covalency in ionic solids, which is popularly known as the Fajans Rule.

C. R. Gopikrishnan; Deepthi Jose; Ayan Datta

2012-01-01

229

Use of core binding energies in the assignment of the ultraviolet photoelectron spectra of the manganese pentacarbonyl halides  

Energy Technology Data Exchange (ETDEWEB)

By using core binding energy data to correct for the effects of potential (atomic charges) and relaxation energy, one can largely resolve the ambiguities in the assignment of the first four bands in the ultraviolet photoelectron spectra of the manganese pentacarbonyl halides.

Jolly, W.L.

1983-01-06

230

Two-dimensional donor-bound excitons in the extreme magnetic quantum limit  

Science.gov (United States)

Two-dimensional (2D) impurity-bound exciton complexes are studied in the extreme quantum limit of a magnetic field B. For a 2D magnetoexciton bound to the ionized donor (D+, X) the localized spectra consist of (i) the discrete low-lying bound states below the free magnetoexciton (MX) band, (ii) the resonant states with finite widths within the MX band, and (iii) the excited discrete states above the MX band. Stable states of a magnetoexciton bound to the neutral donor (D0, X) and the magnetobiexciton bound to the ionized donor (D+, BX) with electrons in the spin-singlet state are found. Optical transitions associated with 2D impurity-bound exciton complexes in a strong magnetic field B are discussed. The support of Professor S.G. Tikhodeev is gratefully acknowledged. I would also like to thank Dr. S. Huant for drawing my attention to ref. [2] and for providing me with a copy of ref. [22].

Dzyubenko, Alexander B.

1993-02-01

231

Effect of intrinsic luminescence of alkali halide amplification by low temperature deformation  

International Nuclear Information System (INIS)

By using luminescence spectroscopy, we have established the influence of low-temperature uniaxial deformation on the configuration of a self-trapped exciton (STE) in alkali halide crystals (AHC) at the instant of radiative relaxation. In face-centered crystals, there occurs the redistribution of the luminescence intensity from the asymmetric configuration of STE to the symmetric one (III ? II ? I- types), whereas in volume-centered crystals, on the contrary, the evidence counts in favour of the asymmetric configuration of STE (I? II–types). The external deformation in the direction leads to an effective sliding of anions in the direction that coincides with the direction of the STE squeeze which classically makes for the creation, of the symmetric STE configuration in the direction, acting perpendicularly to the STE’ length results in their stretching, which in turn brings into the creation of the asymmetric STE configuration with a higher degree of the polarization.

2012-12-17

232

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

Science.gov (United States)

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

Ye, Jinwu; Sun, Fadi; Yu, Yi-Xiang; Liu, Wuming

2013-02-01

233

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

CERN Multimedia

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

234

Excitonic spectrum of the ZnO/ZnMgO quantum wells  

Energy Technology Data Exchange (ETDEWEB)

Excitonic spectrum of the wurtzite ZnO/Zn{sub 1-x}Mg{sub x}O quantum wells with a width on the order of or larger than the Bohr radius of the exciton has been studied; the quantum wells have been grown by the method of molecular beam epitaxy (with plasma-assisted activation of oxygen) on substrates of sapphire (0001). Low-temperature (25 K) spectra of photoluminescence excitation (PLE) have been experimentally measured, making it possible to resolve the peaks of exciton absorption in the quantum well. The spectrum of excitons in the quantum well is theoretically determined as a result of numerical solution of the Schroedinger equation by the variational method. The value of elastic stresses in the structure (used in calculations) has been determined from theoretical simulation of measured spectra of optical reflection. A comparison of experimental data with the results of calculations makes it possible to relate the observed features in the PLE spectra to excitons, including the lower level of dimensional quantization for electrons and two first levels of holes for the A and B valence bands of the wurtzite crystal. The values of the electron and hole masses in ZnO are refined, and the value of the built-in electric field introduced by spontaneous and piezoelectric polarizations is estimated.

Bobrov, M. A., E-mail: largaseal@gmail.com; Toropov, A. A.; Ivanov, S. V. [Russian Academy of Sciences, Ioffe Physical Technical Institute (Russian Federation); El-Shaer, A.; Bakin, A.; Waag, A. [TU Braunschweig, Institute of Semiconductor Technology (Germany)

2011-06-15

235

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 ...

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

236

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.

1982-01-01

237

Synthesis and vibrational absorption spectra of scandium and yttrium halide tetrahydrofuranates  

Energy Technology Data Exchange (ETDEWEB)

Halides of MHal/sub 3/x3THF type (M=Sc, Y; Hal=Cl, Br) solvated with tetrahydrofuran are prepared by HgCl/sub 2/ effect on metallic scandium, yttrium or by HgBr/sub 2/ effect on metallic scandium in tetrahydrofuran (THF). Infrared absorption spectra of the prepared compounds in the range of 200-3500 cm/sup -1/ are studied. Empiric attribution of the absorption bands, observed in the infrared spectra is carried out.

Kravchenko, O.V.; Kravchenko, S.E.; Makhaev, V.D.; Polyakova, V.B.; Slobodenchuk, G.V.; Semenenko, K.N. (AN SSSR, Moscow. Inst. Novykh Khimicheskikh Problem)

1982-01-01

238

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.

2003-07-07

239

Instantaneous Rayleigh scattering from excitons localized in monolayer islands  

DEFF Research Database (Denmark)

We show that the initial dynamics of Rayleigh scattering from excitons in quantum wells can be either instantaneous or delayed, depending on the exciton ensemble studied. For excitation of the entire exciton resonance, a finite rise time given by the inverse inhomogeneous broadening: of the exciton resonance is observed. Instead, when exciting only a subsystem of the exciton resonance, in our case excitons localized in quantum well regions of a specific monolayer thickness, the rise has an instantaneous component. This is due to the spatial nonuniformity of the initially excited exciton polarization, which emits radiation also into nonspecular directions.

Langbein, Wolfgang; Leosson, Kristjan

2000-01-01

240

Temperature and composition dependent excitonic luminescence and exciton-phonon coupling in CdSeS nanocrystals.  

UK PubMed Central (United Kingdom)

The yellow- and red-emitting CdSeS nanocrystals (NCs) synthesized through one-step organometallic synthesis method are uniformly assembled in polymethyl methacrylate (PMMA). A higher-energy emission band originates from band-edge excitonic state appeared at low temperature. With the Se dopant concentration increasing, the luminescent spectra of CdSeS NCs have a red-shifted emission peak and a shorter luminescent lifetime, which is attributed to the existence of trapping state caused by surface defect and Se dopant. CdSeS NC shows a shorter luminescence lifetime and higher energy emission peak in PMMA matrix than that in toluene, indicating that the former is more favorable to transfer energy through exciton-phonon coupling. The upconversion luminescence (UCL) is observed using 800?nm femtosecond laser excitation. The pump power dependence demonstrated UCL spectra of yellow-emitting CdSeS NCs has a slope of 2.2, while that of red-emitting CdSeS NCs has a slope of 1.4. The results demonstrate that the two-photon absorption plays a dominating role when Se concentration of CdSeS NCs is lower, while phonon-assisted UCL by one-photon excitation gradually takes place with the amount of Se dopants increasing.

Wu W; Yu D; Ye HA; Gao Y; Chang Q

2012-01-01

 
 
 
 
241

Chemistry of gaseous lower-valent actinide halides  

Energy Technology Data Exchange (ETDEWEB)

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

Hildenbrand, D.L.

1983-05-24

242

Competing ground states of metal-halide ladders  

CERN Document Server

Based on a symmetry argument, we investigate the ground-state properties of newly synthesized metal-halide ladder compounds (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_(ClO_4_)_4_ 2H_2_O (X=Cl, Br, I). Employing a fully dressed two-band Peierls-Hubbard model, we systematically reveal possible charge- or spin-ordered states. Numerical phase diagrams demonstrate a variety of competing Peierls and Mott insulators with particular emphasis on the transition between two types of mixed-valent state of Pt^II^ and Pt^IV^ driven by varying interchain hopping integrals and Coulomb interactions.

Funase, K; Funase, Kei-ichi; Yamamoto, Shoji

2006-01-01

243

Excitonic instability in layered degenerate semimetals  

Energy Technology Data Exchange (ETDEWEB)

We study excitonic pairing in quasi-two-dimensional degenerate semimetals such as graphite, where the electron density of states nearly vanishes at the Fermi level and, therefore, the Coulomb interactions remain unscreened. By focusing on the Dirac-like low-energy electronic excitations and numerically solving a non-linear gap equation for the excitonic order parameter, we obtain a critical value of the Coulomb coupling and establish the Kosterlitz-Thouless-like nature of the putative semimetal-to-excitonic insulator transition which can also be identified as a formal analog of chiral symmetry breaking in relativistic fermion theories.

Khveshchenko, D.V. E-mail: khvesh@physics.unc.edu; Leal, H

2004-05-31

244

Dynamical mean-field theory for the exciton Mott transition in electron-hole systems  

Energy Technology Data Exchange (ETDEWEB)

In electron-hole (e-h) systems in photoexcited insulators, the exciton Mott transition may take place, which is a typical photoinduced phase transition. To understand how the exciton Mott transition depends on the e-h carrier density and the Coulomb correlation, we study an e-h two-band Hubbard model by means of the dynamical mean-field theory assuming that electron-hole pairs do not condense. The phase diagram on the plane of interactions at zero temperature is obtained. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott-Hubbard insulator and the biexciton-like insulator. Away from half-filling we find another insulating phase, the exciton-like insulator, which is followed by the formation of incoherent (not condensed) e-h pairs, while the Mott-Hubbard insulator phase disappears. The exciton Mott transition is found to be the first-order transition. Linear optical susceptibilities are also discussed.

Ogawa, Tetsuo; Tomio, Yuh; Asano, Kenichi [Department of Physics, Osaka University, and CREST, JST, Toyonaka, Osaka 560-0043 (Japan)

2005-01-01

245

Femtosecond spectroscopy study of the exciton relaxation dynamics in silicon quantum dots  

Energy Technology Data Exchange (ETDEWEB)

This contribution is targeted to the development of surface-modified silicon quantum dots (Siqdots) with tailored luminescence properties. The surface modification of Siqdots with sizes between 1 and 5 nm has been successfully achieved via two different synthesis routes, first, by controlled oxidation followed from silanization and second, by thermal hydrosilylation with chromophores. The luminescence properties of ethanolic Siqdots dispersions were characterized using stationary and time-resolved luminescence spectroscopy techniques, whereas the ultrashort exciton relaxation dynamics were examined using femtosecond transient absorption spectroscopy. Silanized Siqdots were observed to exhibit two species of photoluminescence (PL): the blue emission at 380 nm is assigned to localized surface states, whereas radiative recombination of quantum confined excitons gives rise to a broad PL band around 800 nm. Whereas the latter is ascribed to Siqdots with sizes larger than 3 nm, for Siqdots smaller than 1.5 nm exciton relaxation dynamics is understood to occur predominantly by trapping due to lower-lying surface states which may radiatively decay. Siqdots terminated with suited chromophores were observed to exhibit only one PL band in the visible that is ascribed to exciton states involving resonant couplings to the conjugated electron system of the chromophores.

Kryschi, Carola; Kuntermann, Volker; Cimpean, Carla [Institut fuer Physikalische Chemie I, FAU, Erlangen (Germany); Haarer, Dietrich [BIMF, Universitaet Bayreuth (Germany)

2008-07-01

246

Multiple Exciton Generation Solar Cells  

Energy Technology Data Exchange (ETDEWEB)

Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.

Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.

2012-01-01

247

Polarizabilities of the alkali halide dimers. II  

International Nuclear Information System (INIS)

We have measured the average electric dipole polarizabilities of twenty alkali halide dimers, using a molecular beam electric deflection method. This is a continuation of the earlier measurements of five alkali halide dimers [R. Kremens et al., J. Chem. Phys. 81, 1676 (1984)]. We compare our measurements with a simple combination of bond (via a ''spring'' model) and effective ionic polarizabilities, using literature values for the required molecular constants. Agreement between our measured and calculated values is reasonable, in most cases, considering the relative crudeness of the model.

1991-01-01

248

Exciton Cascade Model for Fast Neutron Reactions.  

Science.gov (United States)

A more sophisticated version of the exciton cascade model, treating equilibrium and pre-equilibrium particle emissions in a unique way has been developed and applied to the description of neutron induced reactions, using realistic input data. The master e...

Z. Schram G. Kluge K. Sailer

1987-01-01

249

Exciton in closed and opened quantum dot  

Directory of Open Access Journals (Sweden)

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

M.V.Tkach; Ju.O.Seti

2007-01-01

250

Interband electronic Raman scattering of polarized light in cubic semiconductors with degenerate valence bands  

International Nuclear Information System (INIS)

[en] The cross-sections are calculated for the Raman scattering processes with polarized light in cubic semiconductors where the valence band top is fourfold degenerate ?8. The exciton effect is taken into account. (author). 17 refs

1987-01-01

251

Trapping of Cold Excitons with Laser Light  

CERN Multimedia

Optical trapping and manipulation of neutral particles has led to a variety of experiments from stretching DNA-molecules to trapping and cooling of neutral atoms. An exciting recent outgrowth of the technique is an experimental implementation of atom Bose-Einstein condensation. In this paper, we propose and demonstrate laser induced trapping for a new system--a gas of excitons in quantum well structures. We report on the trapping of a highly degenerate Bose gas of excitons in laser induced traps.

Hammack, A T; Butov, L V; Smallwood, L E; Ivanov, A L; Gossard, A C

2006-01-01

252

Exciton spectra of the nanostructured zinc oxide  

International Nuclear Information System (INIS)

In the present work we studied the technological aspects of synthesis, structure and spectral properties of the zinc oxide nanostructures. The main attention was given to the study of the exciton states especially in the vicinity of liquid helium temperature. The exciton luminescence and absorption spectra are known to be quite informative for the study of the peculiarities of structure and energy transfer in the nanosystems.

2008-01-01

253

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

254

Method of producing halide-free metal and hydroxides  

Energy Technology Data Exchange (ETDEWEB)

A method for producing a halide-free oxide or hydroxide of a subject element from the corresponding subject element halide, is described comprising: in a first reaction step, providing a liquid medium comprising a water-free alcohol and contacting the alcoholic medium with the subject element halide and further contacting the alcohol and the subject element halide with a replacement species that reacts with halides, physically separating the medium containing the alkoxide from the precipitate salt, and in a second reaction step, hydrolyzing the subject element alkoxide with 18 mega ohm purity water to produce the corresponding subject element oxide or subject element hydroxide.

Melas, A.A.

1988-05-03

255

Orientation of luminescent excitons in layered nanomaterials.  

UK PubMed Central (United Kingdom)

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling-MoS? and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS? mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

Schuller JA; Karaveli S; Schiros T; He K; Yang S; Kymissis I; Shan J; Zia R

2013-04-01

256

Exciton Stark ladder in semiconductor superlattices  

Science.gov (United States)

We present a method for calculating the 1s exciton states of type-I and type-II superlattices in an applied static electric field. Our approach is based upon choosing localized exciton wave functions as the basis in which to expand the exciton eigenstates. We calculate the excitonic optical-absorption spectra of a number of very different type-I GaAs/Ga1-xAlxAs superlattices, as a function of electric field, and find our results for the 65-Å superlattice of Agulló-Rueda, Mendez, and Hong [Phys. Rev. B 40, 1357 (1989)] to be in good agreement with the experimental photocurrent spectra over the full range of field strengths. In addition, we explain many of the low-field effects generally seen, such as the ladder absorption asymmetry, oscillations in the absorption of individual levels, and the nonlinear dependence of the exciton-Stark-ladder energy levels on the field strength. Finally, we discuss the electric-field range for which a modified single-particle picture provides an adequate picture of the qualitative features of the exciton Stark ladder.

Dignam, M. M.; Sipe, J. E.

1991-02-01

257

Excitons in semiconducting superlattices, quantum wells, and ternary alloys  

Energy Technology Data Exchange (ETDEWEB)

Semiconducting layered structures can now be fabricated with precisely defined layer thicknesses down to one monolayer. An example is the superlattice'' (SL) structure, in which to semiconductors with different band gaps are interleaved. The electronic and optical properties of the SL are quite different from those of the constitutents and offer interesting new possibilities both in device design and in basic physics. This proposal aims to improve our understanding of optically excited states in SL's, particularly in the so-called Type 2 indirect'' SL's in which in electron and hole created by optical excitation are separated both in real and in momentum space. We study these structures by time-resolved tunable laser spectroscopy, with and without external perturbations such as magnetic field, electric field, and uniaxial stress. In SLs with only a few atomic layers per period the familiar effective mass model'' of semiconductor states breaks down. We have made precise optical experiments on well-characterized material to test current first principles'' calculations of the band structure. Our work under this grant has shown that the material we are using is of sufficiently high quality to test the theoretical predictions. Comparison of theory and experiment provides a new and sensitive probe of the interface quality on a fine scale. Statistical analysis of the temperature dependence of the exciton decay dynamics provides complementary information. From a careful study of the exciton spectra of the recently discovered mixed type 1- type 2 CdTe/CdZnTe SLs we have obtained the band offset at the CdTe/CdZnTe interface to unprecedented accuracy.

Sturge, M.D. (Dartmouth Coll., Hanover, NH (United States). Dept. of Physics); Nahory, R.E.; Tamargo, M.C. (Bellcore, Red Bank, NJ (United States))

1992-06-01

258

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

Science.gov (United States)

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

Nganou, Collins

2013-07-01

259

Tetraethylene glycol adducts of alkaline earth halides  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Reaction of commercially available tetraethylene glycol with different alkaline earth metal halides affords tetraethylene glycol complexes with different coordination numbers at the metal ion and structural features; depending on the ionic radius of the metal ion, one or two ligand molecules will be...

Gschwind, Fabienne; Fromm, Katharina M.

260

Reactivity of halide and pseudohalide ligands  

International Nuclear Information System (INIS)

Reactivity of halide and pseudohalide (cyanide, azide, thiocyanate, cyanate) ligands tending to form bridge bonds in transition metal (Re, Mo, W) complexes is considered. Complexes where transition metal salts are ligands of other, complex-forming ion, are described. Transformation of innerspheric pseudohalide ligands is an important way of directed synthesis of these metal coordination compounds.

1987-01-01

 
 
 
 
261

Holographic interconnectors in silver halide sensitized gelatin  

Digital Repository Infrastructure Vision for European Research (DRIVER)

When making interconnection holographic systems, it is important to obtain high diffraction efficiency and low noise levels. Silver halide sensitized gelatins have been shown to be an excellent material to use in transmission holographic systems because these two objetives can be met. Using a copyin...

Fuentes Rosillo, Rosa; Pascual Villalobos, Inmaculada; Beléndez Vázquez, Augusto; Fimia Gil, Antonio; Egozcue Rubí, Juan José

262

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.

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

263

Ionic alkali halide XUV laser feasibility study  

Energy Technology Data Exchange (ETDEWEB)

The objective of this work is to assess the feasibility of a select set of ionic alkali halide XUV laser concepts by obtaining the relevant kinetic and spectroscopic parameters required for a proof-of-principle and conceptual design. The proposed lasers operate in the 80--200 nm spectral region and do not require input from outside radiation sources for their operation. Frequency up-conversion and frequency mixing techniques and therefore not considered in the work to be described. An experimental and theoretical study of a new type of laser operating in the extreme ultraviolet wavelength region has been conducted. The lasing species are singly ionized alkali halide molecules such as Rb{sup 2+}F{sub {minus}}, Rb{sup 2+}Br{sup {minus}} and Cs{sup 2+}F{sup {minus}}. These species are similar in electronic structure to the rare gas halide excimers, such as XeF and Krf, except that the ionic molecules emit at wavelengths of 80--200 nm, much shorter than the conventional rare-gas halide excimer laser. The radiative lifetime of these molecules are typically near 1 ns, which is about an order of magnitude shorter than that for rare-gas halide systems. The values of the cross section for stimulated emission are on the order of 1 {times} 10{sup {minus}16}cm{sup 2}. Because of the fundamental similarity to existing UV lasers, these systems show promise as a high power, efficient XUV lasers. 55 refs., 50 figs., 5 tabs.

Yang, T.T.; Gylys, V.T.; Bower, R.D.; Harris, D.G.; Blauer, J.A.; Turner, C.E.; Hindy, R.N.

1989-11-10

264

Optical spectra and energy band structure of AgAsS2 crystals  

Science.gov (United States)

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS2 crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E?c and E?c polarizations in AgAsS2 crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.

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

2012-11-01

265

Charge transfer excitons: dynamic theory of vibronic spectra  

International Nuclear Information System (INIS)

The vibronic spectra of charge transfer excitons (CTE) in a molecular one-component or alternatingly ordered two-component chain are treated in the framework of a dynamic approach (neglecting thermal excitations of the intramolecular vibrations). The model introduces two mechanisms of coupling between CTEs and vibrational quanta: (1) shift of the equilibrium positions of the nuclei in the ionized donor or acceptor; (2) change of the vibrational frequency in the ionized molecule. This model allows to generalize the simple CTE Hamiltonian and the vibronic Hamiltonian of Frenkel excitons. The linear optical susceptibility is calculated in the vibronic region (one CTE and one vibrational quantum). The double splitting of vibronics of CTEs was analyzed: (1) the splitting connected with the location of the intramolecular vibration on the donors or on the acceptors; (2) the splitting connected with the symmetry of the vibronic spectra (in the degenerate case). The general structure of the vibronic spectra of CTEs is established. It contains structureless absorption lines, which correspond to two-particle bands (the phonon is excited on a neutral molecule neighboring the donor or the acceptor) and Lorentz-type lines of one-particle states, which correspond to the bound propagation of the CTE and the phonon.

2004-12-10

266

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

Directory of Open Access Journals (Sweden)

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

Babkina A.N.; Shirshnev P.S.; Tsekhomsky V.A.; Nikonorov N.V.

2013-01-01

267

Ultrafast exciton relaxation in the B850 antenna complex of Rhodobacter sphaeroides.  

UK PubMed Central (United Kingdom)

Spectral changes were measured with femtosecond resolution following low-intensity, broad-band excitation of the peripheral antenna complex of the purple photosynthetic bacterium Rhodobacter sphaeroides. Absorption anisotropy decays also were measured. We identified a 35-fs relaxation of the absorption and emission spectra of the excited state, as well as a 20-fs anisotropy decay. We interpret these results as interlevel relaxation and dephasing, respectively, of extensively delocalized exciton states of the circular bacteriochlorophyll aggregate.

Nagarajan V; Alden RG; Williams JC; Parson WW

1996-11-01

268

Ultrafast exciton relaxation in the B850 antenna complex of Rhodobacter sphaeroides.  

Science.gov (United States)

Spectral changes were measured with femtosecond resolution following low-intensity, broad-band excitation of the peripheral antenna complex of the purple photosynthetic bacterium Rhodobacter sphaeroides. Absorption anisotropy decays also were measured. We identified a 35-fs relaxation of the absorption and emission spectra of the excited state, as well as a 20-fs anisotropy decay. We interpret these results as interlevel relaxation and dephasing, respectively, of extensively delocalized exciton states of the circular bacteriochlorophyll aggregate. PMID:8943011

Nagarajan, V; Alden, R G; Williams, J C; Parson, W W

1996-11-26

269

Ultrafast exciton relaxation in the B850 antenna complex of Rhodobacter?sphaeroides  

Science.gov (United States)

Spectral changes were measured with femtosecond resolution following low-intensity, broad-band excitation of the peripheral antenna complex of the purple photosynthetic bacterium Rhodobacter sphaeroides. Absorption anisotropy decays also were measured. We identified a 35-fs relaxation of the absorption and emission spectra of the excited state, as well as a 20-fs anisotropy decay. We interpret these results as interlevel relaxation and dephasing, respectively, of extensively delocalized exciton states of the circular bacteriochlorophyll aggregate.

Nagarajan, V.; Alden, R. G.; Williams, J. C.; Parson, W. W.

1996-01-01

270

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)

1997-01-01

271

Anatomy of an exciton: vibrational distortion and exciton coherence in H- and J-aggregates.  

UK PubMed Central (United Kingdom)

In organic materials, coupling of electronic excitations to vibrational degrees of freedom results in polaronic excited states. Through numerical calculations, we demonstrate that the vibrational distortion field accompanying such a polaron scales as the product of the excitonic interaction field and the exciton coherence function. This scaling relation is derived analytically in the regime where excitonic interactions are weak, yet it is shown to remain valid for interaction strengths ranging up to physically relevant values. Moreover, it is not affected by the magnitude of exciton-vibrational coupling or the presence of disorder in the molecular transition energies, despite the dramatic changes observed in the excited state. An application to helical MOPV4 aggregates is presented, followed by a quantitative study of the vibrational distortion field when excitonic interactions are strong. Our findings allow for a straightforward interpretation of widely varying polaron profiles, thereby facilitating the characterization of organic excited states.

Tempelaar R; Stradomska A; Knoester J; Spano FC

2013-01-01

272

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

CERN Multimedia

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\\sim 4$ meV to zero, at a field of $\\sim $35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.

Aichmayr, G; Viña, L; Dickerson, J; Camino, F; Méndez, E E

1999-01-01

273

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

Directory of Open Access Journals (Sweden)

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

Masayoshi Nakano; Ryohei Kishi; Takuya Minami; Kyohei Yoneda

2009-01-01

274

Comment on "Inverse exciton series in the optical decay of an excitonic molecule"  

CERN Document Server

Tokunaga et al. [Phys. Rev. B Vol. 59, R7837 (1999)] claim the first successful observation of the inverse exciton M series in the emission spectrum of excitonic molecules. We assert that actually such a series was observed as early as 1989 in the beta-ZnP_2 crystal. We show that the objections of Tokunaga et al. against the biexciton nature of the inverse exciton series in beta-ZnP_2 are ungrounded. In particular, their estimations for the ratio of the intensities of M2 and M1 emission lines in this crystal give two order smaller value because they do not take into account the reabsorption of the M1 line photons. We report the observation of additional inverse exciton M' series in the emission spectrum of excitonic molecules in beta-ZnP_2.

Gorban, I S; Dmitruk, I M; Yeshchenko, O A

1999-01-01

275

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

International Nuclear Information System (INIS)

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

2007-07-25

276

Dependence of resonance energy transfer on exciton dimensionality.  

Science.gov (United States)

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 localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations. PMID:22182117

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

2011-11-30

277

Excitons and rare-earth ions in CsCdBr3  

International Nuclear Information System (INIS)

CsCdBr3 has a quasi-linear crystal structure. It consists of covalently bound [CdBr]64- chains separated by chains of Cs+ ions. The trivalent rare-earth (RE) ions substitute for divalent Cd ions forming predominantly pair centers of the type RE3+-(Cd vacancy)-RE3+. A minority of RE ions forms 'single-ion' centers with more distant charge compensation. The electronic structure around the band gap is determined by the [CdBr]64- octahedra. The lowest excitonic states of the lattice are charge-transfer states of these octahedra. At low temperatures they form self-trapped excitons which become mobile around 80 K. In addition we find defect-localized excitons at the RE pairs and single ions with slightly modified spectra. There is a strong energy transfer between the RE ions and the defect-localized excitons in both directions with transition times below 10-8 s. For the cooperative fluorescence transition 1D2x1G4?3H4x3H4 in Pr3+: CsCdBr3 a frequency-modulated vibronic sideband spectrum was found with up to four repetitions of the frequency of the localized optical phonon mode at the ion pair.

2004-10-20

278

Excitons and rare-earth ions in CsCdBr{sub 3}  

Energy Technology Data Exchange (ETDEWEB)

CsCdBr{sub 3} has a quasi-linear crystal structure. It consists of covalently bound [CdBr]{sub 6}{sup 4-} chains separated by chains of Cs{sup +} ions. The trivalent rare-earth (RE) ions substitute for divalent Cd ions forming predominantly pair centers of the type RE{sup 3+}-(Cd vacancy)-RE{sup 3+}. A minority of RE ions forms 'single-ion' centers with more distant charge compensation. The electronic structure around the band gap is determined by the [CdBr]{sub 6}{sup 4-} octahedra. The lowest excitonic states of the lattice are charge-transfer states of these octahedra. At low temperatures they form self-trapped excitons which become mobile around 80 K. In addition we find defect-localized excitons at the RE pairs and single ions with slightly modified spectra. There is a strong energy transfer between the RE ions and the defect-localized excitons in both directions with transition times below 10{sup -8} s. For the cooperative fluorescence transition {sup 1}D{sub 2}x{sup 1}G{sub 4}{yields}{sup 3}H{sub 4}x{sup 3}H{sub 4} in Pr{sup 3+}: CsCdBr{sub 3} a frequency-modulated vibronic sideband spectrum was found with up to four repetitions of the frequency of the localized optical phonon mode at the ion pair.

Heber, J.; Demirbilek, R.; Nikitin, S.I

2004-10-20

279

Exciton and negative trion dissociation by an external electric field in vertically coupled quantum dots  

CERN Multimedia

We study the Stark effect for an exciton confined in a pair of vertically coupled quantum dots. A single-band approximation for the hole and a parabolic lateral confinement potential are adopted which allows for the separation of the lateral center-of-mass motion and consequently for an exact numerical solution of the Schr\\"odinger equation. We show that for intermediate tunnel coupling the external electric field leads to the dissociation of the exciton via an avoided crossing of bright and dark exciton energy levels which results in an atypical form of the Stark shift. The electric-field-induced dissociation of the negative trion is studied using the approximation of frozen lateral degrees of freedom. It is shown that in a symmetric system of coupled dots the trion is more stable against dissociation than the exciton. For an asymmetric system of coupled dots the trion dissociation is accompanied by a positive curvature of the recombination energy line as a function of the electric field.

Szafran, B; Peeters, F M; Bednarek, S; Adamowski, J; Partoens, B

2005-01-01

280

Synergetics in multiple exciton generation effect in quantum dots  

Science.gov (United States)

We present detailed analysis of the non-Poissonian population of excitons produced by multiple exciton generation (MEG) effect in quantum dots on the base of statistic theory of MEG and synergetic approach for chemical reactions. From the analysis we can conclude that a non-Poissonian distribution of exciton population is evidence of nonlinear and nonequilibrium character of the process of multiple generation of excitons in quantum dots at a single photon absorption.

Turaeva, N. N.; Oksengendler, B. L.; Uralov, I.

2011-06-01

 
 
 
 
281

Probing Bose-Einstein Condensation of Excitons with Electromagnetic Radiation  

CERN Multimedia

We examine the absorption spectrum of electromagnetic radiation from excitons, where an exciton in the $1s$ state absorbs a photon and makes a transition to the $2p$ state. We demonstrate that the absorption spectrum depends strongly on the quantum degeneracy of the exciton gas, and that it will generally manifest many-body effects. Based on our results we propose that absorption of infrared radiation could resolve recent contradictory experimental results on excitons in Cu$_2$O.

Johnsen, Kjell

2001-01-01

282

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

283

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.

2010-01-01

284

Bound spin-excitons in two-dimensional electron gas  

International Nuclear Information System (INIS)

A theory of the spin exciton capture by a magnetic impurity in a 2D electron gas is developed. We consider a resonance model for the electron scattering by a transition metal impurity and calculate the binding potential for spin excitons. This potential is spin selective and is capable of binding a spin exciton with zero angular momentum. In order to trap an exciton with a nonzero angular momentum m, the potential must exceed a certain threshold value, depending on m.

2001-01-01

285

Vibronic spectra of charge transfer excitons and of mixed charge transfer and Frenkel excitons  

Science.gov (United States)

The linear absorption spectra in the excitonic and vibronic regions in the case of mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been theoretically studied for the exciton parameters of the crystals of MePTCDI and PTCDA. Two coupling parameters for the exciton phonon coupling are introduced: the FE phonon coupling and the CTE phonon coupling. The main features of the vibronics in the absorption spectra are the following: (a) the existence of a doublet structure in the vibronic spectra of CTEs; (b) the vibronic levels of the FE at intermediate values of both coupling parameters are located in the continuum of the many-particle exciton phonon states which makes its absorption line wide and flat; (c) in the case of strong coupling (coupling constants larger than 1) a doublet of bound states appears above this continuum; (d) in the case of vanishing CTE phonon coupling the vibronics of the charge transfer excitons practically disappear in the absorption spectra.

Lalov, I. J.; Supritz, C.; Reineker, P.

2007-01-01

286

Photoluminescence from excitons at the mini-Brillouin-zone boundary in a GaAs/AlAs superlattice  

Energy Technology Data Exchange (ETDEWEB)

We have investigated the photoluminescence (PL) properties related to the mini-Brillouin-zone boundary ({pi} point) in a (GaAs){sub 24}/(AlAs){sub 2} superlattice. In order to detect the PL from the {pi}-point exciton that has a very fast relaxation process in momentum space, we measured time-resolved PL (TRPL) spectra with a streak-camera system. Photoreflectance spectroscopy was adopted to characterize the optical transition energies at the mini-Brillouin-zone center ({gamma} point) and the {pi} point, which results in the clear observation of the heavy-hole-exciton transition in the first quantization (n=1) miniband at the {pi} point, H11({pi}), in addition to the {gamma}-point exciton, H11({gamma}) usually observable. We have found that PL band relevant to the H11({pi}) transition appears in the TRPL spectra by tuning the excitation energy to the H11({gamma}) transition. Furthermore, the H11({pi})-PL band has a delay time relative to the H11({gamma})-PL band, which reflects the pileup of excitons and carriers from the {gamma} point to the {pi} point in the miniband dispersion. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Hasegawa, T.; Nakayama, M. [Department of Applied Physics, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka (Japan)

2006-11-15

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

2012-06-01

288

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

UK PubMed Central (United Kingdom)

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.

Li D; Zhang J; Zhang Q; Xiong Q

2012-06-01

289

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

290

Lanthanide-halide based humidity indicators  

Energy Technology Data Exchange (ETDEWEB)

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

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

2008-01-01

291

Excitonic Bloch Oscillations in a Terahertz Electric Field  

Science.gov (United States)

We present the results of a calculation of the time-dependent interband and intraband polarization due to excitons in a semiconductor superlattice in combined DC and terahertz electric fields. The excitons are considered to be photo-excited using an ultra-short laser pulse which results in a motion which is best described as excitonic Bloch

Dignam, Marc M.

1998-03-01

292

Forster coupling in nanoparticle excitonic circuits.  

UK PubMed Central (United Kingdom)

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Forster matrix element, based on a three-dimensional real space grid and the self-consistent solution of the mesoscopic exciton in a macroscopic dielectric environment. This method enables the study of the role of the nanoparticle shape, spatially varying dielectric environments, and externally applied electric fields. Depending on the orientation of the transition dipole, the Forster coupling is shown to be either increased or decreased as a function of the nanoparticle shape and of the properties of the dielectric environment. In the presence of an electric field, we investigate the relation between excitonic binding and confinement effects. We also study a type II core-shell quantum dot where electron and hole are spatially separated due to a particular configuration of the bandstructure.

Rebentrost P; Stopa M; Aspuru-Guzik A

2010-08-01

293

Proposal of structures possessing high exciton concentration  

International Nuclear Information System (INIS)

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5x10-2. The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values.

2008-01-01

294

Frenkel-exciton-polaritons in organic microcavities  

Scientific Electronic Library Online (English)

Full Text Available 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 exciton-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 (more) 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.

Zoubi, Hashem; La Rocca, G. C.

2006-06-01

295

Second Thresholds in BEC-BCS-Laser Crossover of Exciton-Polariton Systems  

Science.gov (United States)

The mechanism of second thresholds observed in several experiments is theoretically revealed by studying the BEC-BCS-laser crossover in exciton-polariton systems. We find that there are two different types of second thresholds: one is a crossover within quasiequilibrium phases and the other is into nonequilibrium (lasing). In both cases, the light-induced band renormalization causes gaps in the conduction and valence bands, which indicates the existence of bound electron-hole pairs in contrast to earlier expectations. We also show that these two types can be distinguished by the gain spectra.

Yamaguchi, Makoto; Kamide, Kenji; Nii, Ryota; Ogawa, Tetsuo; Yamamoto, Yoshihisa

2013-07-01

296

Exciton-exciton interaction engineering in coupled GaN quantum dots  

CERN Multimedia

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

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

2002-01-01

297

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.

2005-01-01

298

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

CERN Multimedia

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

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

2003-01-01

299

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

300

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

 
 
 
 
301

Complexes in polyvalent metal - Alkali halide melts  

International Nuclear Information System (INIS)

[en] Experimental evidence is available in the literature on the local coordination of divalent and trivalent metal ions by halogens in some 140 liquid mixtures of their halides with alkali halides. After brief reference to classification criteria for main types of local coordination, we focus on statistical mechanical models that we are developing for Al-alkali halide mixtures. Specifically, we discuss theoretically the equilibrium between (AlF6)3- and (AlF4)- complexes in mixtures of AlF3 and NaF as a function of composition in the NaF-rich region, the effect of the alkali counterion on this equilibrium, the possible role of (AlF5)2- as an intermediate species in molten cryolite, and the origin of the different complexing behaviours of Al-alkali fluorides and chlorides. We also present a theoretical scenario for processes of structure breaking and electron localization in molten cryolite under addition of sodium metal. (author). 26 refs, 2 tabs

1991-01-01

302

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

Science.gov (United States)

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.

Lalov, I. J.; Warns, C.; Reineker, P.

2008-08-01

303

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.

2008-01-01

304

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

Energy Technology Data Exchange (ETDEWEB)

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{ne}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.

Lalov, I J [Faculty of Physics, Sofia University, Sofia 1164 (Bulgaria); Warns, C; Reineker, P [Institute of Theoretical Physics, Ulm University, Albert-Einstein-Allee 11, 89069 Ulm (Germany)], E-mail: peter.reineker@uni-ulm.de

2008-08-15

305

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

CERN Document Server

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

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

2003-01-01

306

Solvation at nanoscale: alkali-halides in water clusters.  

UK PubMed Central (United Kingdom)

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 L; Mikkelä MH; Huttula M; Tchaplyguine M; Zhang C; Andersson T; Björneholm O

2013-01-01

307

Process and composition for drying of gaseous hydrogen halides  

Energy Technology Data Exchange (ETDEWEB)

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

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

1989-08-01

308

Exciton cascade model for fast neutron reactions  

International Nuclear Information System (INIS)

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

1987-01-01

309

Probing molecular chirality via excitonic nonlinear response  

International Nuclear Information System (INIS)

[en] The complete set of fifth-rank tensor components which describe the chirality-induced response of excitons to three femtosecond laser pulses is calculated to first order in the optical wavevector, by including the electric dipole and quadrupole and the magnetic dipole contributions to the induced current. The photon echo signal for the ? - ?* and n - ?* electronic transitions of ? helical polypeptides is calculated using the Green's function solution of the nonlinear exciton equations (NEE). Two-dimensional correlation plots of selected tensor components show considerably enhanced crosspeaks with high sensitivity to structural variations

2006-12-28

310

Heat pumping with optically driven excitons  

CERN Multimedia

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

Gauger, Erik M

2010-01-01

311

Excitonic Dynamical Franz-Keldysh Effect  

CERN Multimedia

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 Effect in the presence of excitonic absorption.

Nordstrom, K B; Allen, S J; Jauho, A P; Birnir, B; Kono, J; Noda, T; Akiyama, H; Sakaki, H

1998-01-01

312

Probing molecular chirality via excitonic nonlinear response  

Energy Technology Data Exchange (ETDEWEB)

The complete set of fifth-rank tensor components which describe the chirality-induced response of excitons to three femtosecond laser pulses is calculated to first order in the optical wavevector, by including the electric dipole and quadrupole and the magnetic dipole contributions to the induced current. The photon echo signal for the {pi} - {pi}* and n - {pi}* electronic transitions of {alpha} helical polypeptides is calculated using the Green's function solution of the nonlinear exciton equations (NEE). Two-dimensional correlation plots of selected tensor components show considerably enhanced crosspeaks with high sensitivity to structural variations.

Abramavicius, Darius [Institute of Physics, Savanoriu Avenue 231, 02300 Vilnius (Lithuania); Zhuang, Wei [Department of Chemistry, University of California Irvine, Irvine, CA 92697-2025 (United States); Mukamel, Shaul [Department of Chemistry, University of California Irvine, Irvine, CA 92697-2025 (United States)

2006-12-28

313

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

Science.gov (United States)

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

314

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.

2008-02-28

315

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

Science.gov (United States)

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

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

2005-11-01

316

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

UK PubMed Central (United Kingdom)

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

Huijser A; Savenije TJ; Kroeze JE; Siebbeles LD

2005-11-01

317

Engineering the spin-flip limited exciton dephasing in colloidal CdSe/CdS quantum dots.  

Science.gov (United States)

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ?110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied. PMID:22564176

Accanto, Nicolò; Masia, Francesco; Moreels, Iwan; Hens, Zeger; Langbein, Wolfgang; Borri, Paola

2012-05-15

318

Engineering the spin-flip limited exciton dephasing in colloidal CdSe/CdS quantum dots.  

UK PubMed Central (United Kingdom)

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ?110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.

Accanto N; Masia F; Moreels I; Hens Z; Langbein W; Borri P

2012-06-01

319

Excitonic optical bistability in n-type doped semiconductors  

International Nuclear Information System (INIS)

[en] A resonant monochromatic pump laser generates coherent excitons in an n-type doped semiconductor. Both exciton-exciton and exciton-donor interactions come into play. The former interaction can give rise to the appearance of optical bistability which is heavily influenced by the latter one. When optical bistability occurs at a fixed laser frequency both its holding intensity and hysteresis loop size are shown to decrease with increasing donor concentration. Two possibilities are suggested for experimentally determining one of the two parameters of the system - the exciton-donor coupling constant and the donor concentration, if the other parameter is known beforehand. (author). 36 refs, 2 figs

1991-01-01

320

Nonboson treatment of excitonic nonlinearity in optically excited media  

International Nuclear Information System (INIS)

[en] 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

1990-01-01

 
 
 
 
321

Phase diagram for the exciton Mott transition in infinite-dimensional electron-hole systems  

Energy Technology Data Exchange (ETDEWEB)

To understand the essence of the exciton Mott transition in three-dimensional electron-hole systems, the metal-insulator transition is studied for a two-band Hubbard model in infinite dimensions with interactions of electron-electron (hole-hole) repulsion U and electron-hole attraction -U{sup '}. By using the dynamical mean-field theory, the phase diagram in the U-U{sup '} plane is obtained (which is exact in infinite dimensions) assuming that electron-hole pairs do not condense. When both electron and hole bands are half-filled, two types of insulating states appear: the Mott-Hubbard insulator for U>U{sup '} and the biexciton-like insulator for Uexciton- or biexciton-like insulator and a metallic state. This transition can be assigned to the exciton Mott transition, whereas the Mott-Hubbard transition is absent.

Tomio, Yuh [CREST, JST and Department of Physics, Osaka University, Toyonaka, Osaka 560-0043 (Japan)]. E-mail: tomio@acty.phys.sci.osaka-u.ac.jp; Ogawa, Tetsuo [CREST, JST and Department of Physics, Osaka University, Toyonaka, Osaka 560-0043 (Japan)

2005-04-15

322

Indirect excitons in coupled quantum dots and exciton polaritons in optical microcavities in magnetic field  

Energy Technology Data Exchange (ETDEWEB)

Quasi-two-dimensional (2D) exciton polaritons in single and coupled quantum wells or quantum dots embedded in an optical microcavity in magnetic field are considered. The dependencies of exciton properties on control parameters are obtained. Magnetic field influence on polariton resonance, splitting and dispersion is studied. Effects of spontaneous coherence and Kosterlitz-Thouless transition and Bose-Einstein condensation for such systems are discussed.

Kaputkina, Natalia E. [Moscow Institute for Steel and Alloys, Leninskii prosp. 4, 117936 Moscow (Russian Federation)], E-mail: kaputkina@mail.ru; Lozovik, Yurii E. [Institute of Spectroscopy, Academy of Sciences of Russia, 142092 Troitsk, Moscow Region (Russian Federation)

2008-04-01

323

Indirect excitons in coupled quantum dots and exciton polaritons in optical microcavities in magnetic field  

International Nuclear Information System (INIS)

Quasi-two-dimensional (2D) exciton polaritons in single and coupled quantum wells or quantum dots embedded in an optical microcavity in magnetic field are considered. The dependencies of exciton properties on control parameters are obtained. Magnetic field influence on polariton resonance, splitting and dispersion is studied. Effects of spontaneous coherence and Kosterlitz-Thouless transition and Bose-Einstein condensation for such systems are discussed

2008-04-01

324

Vibronic Spectra of Mixed Frenkel and Charge-Transfer Excitons  

Science.gov (United States)

The excitonic and vibronic spectra of a molecular chain and of crystal of MePTCDI are studied in the case when: (i) a Frenkel exciton (FE) and charge transfer excitons (CTEs) mix strongly; (ii) two mechanisms of coupling between these mixed excitons and intramolecular vibrations, notably linear and quadratic coupling, are acting. Using a convenient canonical transformation and the Green function method, we calculate the linear optical susceptibility. The spectra of linear absorption in the excitonic and vibronic regions have been calculated introducing the exciton parameters of MePTCDI. These spectra exhibit: (i) a relative separation of vibronics of FE and CTEs and transfer of intensity from a FE to CTEs excitonic and vibronic lines; (ii) a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; (iii) the appearance of a spectral doublet of vibronics of the CTEs-its splitting depends on the parameters of linear and quadratic exciton-phonon coupling in the neutral excited molecule and ions. Moreover, in the case of weak linear exciton-phonon coupling the vibronic line of FE, being wide and flat, lies in two-particle continuum while in the case of intermediate and strong linear coupling the linear absorption is dominated by the bound exciton-phonon states and their narrow Lorentzian maxima depend strongly on the quadratic coupling.

Lalov, I. J.; Zhelyazkov, I.

2007-04-01

325

Vibronic spectra of mixed Frenkel and charge transfer excitons  

Science.gov (United States)

The excitonic and vibronic spectra of a molecular chain and of a crystal of NN' -dimetyl 3,4,9,10-perylene tetracarboxylic diimide (MePTCDI) are studied in the case when: (i) a Frenkel exciton (FE) and charge transfer excitons (CTEs) mix strongly; and (ii) two mechanisms of coupling between these mixed excitons and intramolecular vibrations, notably linear and quadratic coupling, are acting. Using a convenient canonical transformation and the Green function method, we calculate the linear optical susceptibility: (a) in the exciton region taking into account the contribution of the transition dipoles of FE; and (b) in one-phonon vibronic regime. The spectra of linear absorption in the excitonic and vibronic regions have been calculated introducing the exciton parameters of MePTCDI. These spectra exhibit: (i) a relative separation of vibronics of FE and CTEs; (ii) a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; and (iii) the appearance of a spectral doublet of vibronics of the CTEs—its splitting depends on the parameters of linear and quadratic exciton-phonon coupling in the neutral excited molecule and ions. Moreover, in the case of weak linear exciton-phonon coupling the vibronic line of FE, being wide and flat, lies in many-particle continuum while in the case of intermediate and strong linear coupling the linear absorption is dominated by the bound exciton-phonon states and their narrow Lorentzian maxima depend strongly on the quadratic coupling.

Lalov, I. J.; Zhelyazkov, I.

2006-07-01

326

Exciton dynamics in the chlorosomal antenna of the green bacterium Chloroflexus aurantiacus: experimental and theoretical studies of femtosecond pump-probe spectra.  

UK PubMed Central (United Kingdom)

Femtosecond absorption difference spectra were measured for chlorosomes isolated from the green bacterium Chloroflexus aurantiacus at room temperature. Using the relative difference absorption of the oligomeric BChl c and monomeric BChl a bands, the size of a unit BChl c aggregate as well as the exciton coherence size were estimated for the chlorosomal BChl c antenna under study. A quantitative fit of the data was obtained within the framework of the exciton model proposed before [Fetisova et al. (1996) Biophys J 71: 995-1010]. The size of the antenna unit was found to be 24 exciton-coupled BChl c molecules. The anomalously high bleaching value of the oligomeric B740 band with respect to the monomeric B795 band provided the direct evidence for a high degree of exciton delocalization in the chlorosomal B740 BChl c antenna. The effective delocalization size of individual exciton wavefunctions (the thermally averaged inverse participation ratio) in the chlorosomal BChl c antenna is 9.5, whereas the steady-state wavepacket corresponds to the coherence size (the inverse participation ratio of the density matrix) of 7.4 at room temperature.

Yakovlev A; Novoderezhkin V; Taisova A; Fetisova Z

2002-01-01

327

Ultrafast dynamics of exciton fission in polycrystalline pentacene.  

UK PubMed Central (United Kingdom)

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ?80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.

Wilson MW; Rao A; Clark J; Kumar RS; Brida D; Cerullo G; Friend RH

2011-08-01

328

Ultrafast dynamics of exciton fission in polycrystalline pentacene.  

Science.gov (United States)

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ?80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937

Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H

2011-07-14

329

Förster Coupling in Nanoparticle Excitonic Circuits  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Exciton transport in semiconductor nanoparticles underlies recent experiments on electrically controlled nanostructures and proposals for new artificial light-harvesting systems. In this work, we develop a novel method for the numerical evaluation of the Förster matrix element, based on a three-dime...

Rebentrost, Patrick; Stopa, Michael; Aspuru-Guzik, Alán

330

Singlet exciton fission in a hexacene derivative.  

UK PubMed Central (United Kingdom)

Hexacene, an acene with six benzene rings, is notable for its exceptionally small triplet energy, around one third of the singlet energy. Herein, singlet fission, i.e., conversion of a singlet exciton into two triplets, is demonstrated in a thin film of hexacene derivative, employing both transient absorption spectroscopy and magnetic field effects on photocurrent.

Lee J; Bruzek MJ; Thompson NJ; Sfeir MY; Anthony JE; Baldo MA

2013-03-01

331

Ghost excitonic insulator transition in layered graphite.  

Science.gov (United States)

Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( ABAB...) configuration. PMID:11736528

Khveshchenko, D V

2001-11-20

332

Ghost excitonic insulator transition in layered graphite.  

UK PubMed Central (United Kingdom)

Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( ABAB...) configuration.

Khveshchenko DV

2001-12-01

333

Exciton Transfer Integrals Between Polymer Chains  

CERN Multimedia

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

334

Effective models for excitons in carbon nanotubes  

DEFF Research Database (Denmark)

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

Cornean, Horia; Duclos, Pierre

2007-01-01

335

Thermal effects in exciton harvesting in biased one-dimensional systems  

International Nuclear Information System (INIS)

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Recently, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (magnitude of the energy gradient towards the trap) are possible [S.M. Vlaming, V.A. Malyshev, J. Knoester, J. Chem. Phys. 127 (2007) 154719]. This paper generalizes the results for both homogeneous and disordered chains to nonzero temperatures. We show that thermal effects are appreciable only for low bias strengths, particularly so in disordered systems, and lead to faster harvesting

2008-01-01

336

Thermal effects in exciton harvesting in biased one-dimensional systems  

Energy Technology Data Exchange (ETDEWEB)

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Recently, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (magnitude of the energy gradient towards the trap) are possible [S.M. Vlaming, V.A. Malyshev, J. Knoester, J. Chem. Phys. 127 (2007) 154719]. This paper generalizes the results for both homogeneous and disordered chains to nonzero temperatures. We show that thermal effects are appreciable only for low bias strengths, particularly so in disordered systems, and lead to faster harvesting.

Vlaming, S.M.; Malyshev, V.A. [Centre for Theoretical Physics and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen (Netherlands); Knoester, J. [Centre for Theoretical Physics and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen (Netherlands)], E-mail: j.knoester@rug.nl

2008-05-15

337

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

338

Excitonic luminescence of ZnSe single crystals doped with Au  

Energy Technology Data Exchange (ETDEWEB)

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

Nedeoglo, N.D. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of)]. E-mail: nedeoglo@usm.md; Avdonin, A.N. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Ivanova, G.N. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Nedeoglo, D.D. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Kolibaba, G.V. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of); Sirkeli, V.P. [Department of Physics, State University of Moldova, A. Mateevich str. 60, MD 2009 Kishinau (Moldova, Republic of)

2005-04-15

339

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).

1983-01-01

340

Second-order optical nonlinearity in alkali halide crystals containing point defects  

Science.gov (United States)

Model calculation of second order susceptibilities for FA color centers in wide band gap materials are reported. The second order optical nonlinearity in crystals of alkali halide due to FA color centers evaluated theoretically. The density matrix formalism is employed and the equation of motion is solved by second order perturbation to evaluate the nonlinear optical susceptibilty for second harmonic generation as well as frequency mixing. It is found that the system shows large resonance-enhanced second order susceptibility (~10-16 mV-1) for color center concentration of ~1023 m-3. A scheme of phase matching in terms of anomalous dispersion of the centers and coherent length are discussed.

Majles Ara, Mohammad H.

2003-12-01

 
 
 
 
341

Recombination of radiation defects in alkali halide crystals at low temperature  

International Nuclear Information System (INIS)

[en] 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

1981-01-01

342

Pigment organization and exciton dynamics in the B808-866 antenna of the green bacterium Chloroflexus aurantiacus.  

UK PubMed Central (United Kingdom)

The model for the B808-866 antenna of the green photosynthetic bacterium Chloroflexus aurantiacus based on femtosecond pump-probe studies is proposed. The three-dimensional structure of the B808-866 antenna is assumed to be similar to the structure of the B800-850 antenna of purple bacteria, i.e. it has the form of two concentric rings of N strongly coupled BChl866 pigments and of N/2 weakly coupled BChl808 monomers. The Qy transition dipoles of BChl808 and BChl866 molecules form the angles 43 degrees +/- 3 degrees and 8 degrees +/- 4 degrees, respectively, with the plane of the corresponding rings. The lowest limit of the BChl866 aggregate size is N = 18. The anomalously high bleaching value of the BChl866 band with respect to the monomeric BChl808 band provides evidence for a high degree of exciton delocalization. To account for this phenomenon, the exciton model describing exciton dynamics in the spectrally disordered circular BChl866 aggregate is developed. According to this model, the effective exciton size in this antenna (Neff) is 6-8.

Novoderezhkin V; Taisova A; Fetisova Z

1998-06-01

343

Investigation of doped cuprous halides for photovoltaic and display applications  

Digital Repository Infrastructure Vision for European Research (DRIVER)

The thesis mainly focuses on the growth and optoelectronic characterisation of the doped cuprous halides (CuX) with high UV/blue emission properties and the light harvesting in the CuBr/Si based heterojunction photovoltaic (PV) cells. Since cuprous halides are short wavelength emitters with high exc...

Vijayaraghavan, Rajani K.

344

Silver-halide sensitized holograms and their applications  

Digital Repository Infrastructure Vision for European Research (DRIVER)

Silver-halide sensitized gelatin (SHSG) has the sensitometric and spectral response of silver halide emulsions and the holographic characteristics of dichromated gelatin, and therefore when properly processed it has little scattering and absorption. SHSG is one of the most promising techniques today...

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

345

Regioselective Heck reaction of aliphatic olefins and aryl halides.  

UK PubMed Central (United Kingdom)

A regioselective Heck reaction of aliphatic olefins and aryl bromides is realized at internal carbons of olefins. Methanol solvent promoted halide ionization from neutral arylpalladium halide complexes via hydrogen bonding, so as to create cationic aryl-Pd species for regioselective olefin insertion.

Qin L; Hirao H; Zhou JS

2013-09-01

346

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

1979-01-01

347

EXAFS measurement of molten cuprous halides  

International Nuclear Information System (INIS)

EXAFS measurement of molten CuCl and CuBr has been carried out near the melting temperatures in order to study the partial structure. EXAFS oscillation was clearly observed and the structural parameters for the nearest neighboring Cu-halogen pairs were obtained from standard curve fit analyses. The obtained Cu-halogen distance is, however, about 0.1A shorter than the distance obtained from neutron diffraction measurement with isotopic substitution. The result may suggest cluster formation such as a CuX trimer (X=Cl, Br) in molten cuprous halides. (author).

1993-01-01

348

Metal-rich gadolinium hydride halides  

International Nuclear Information System (INIS)

[en] 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)

1985-01-01

349

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.

1981-01-01

350

A new fundamental hydrogen defect in alkali halides  

International Nuclear Information System (INIS)

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

1978-01-01

351

Excitonic recombination radiation from CVD. CVD diamond no reikishi hakko  

Energy Technology Data Exchange (ETDEWEB)

An excitonic recombination radiation was discussed on undoped and boron doped diamonds formed under a microwave plasma CVD process, using a cathode luminescence (CL) method. A scanning electron microscope was used for the CL measurement. A free exciton has electrons and holes formed in pairs by coulomb force, and a free exciton bound by an ionizing acceptor or neutral acceptor is a bound exciton. In the case of undoped diamond, recombination radiation by a free exciton is predominant, with no bound excitons related to acceptors observed. Regions of the recombination radiation by the free excitons are observed as monochromatic CL images, which are suitable for evaluating crystallinity and purity. In the case of boron doped diamond, recombination radiation by a bound exciton is predominant. When a peak strength of the radiation spectrum is termed B for the case of a free exciton and D for the case of a bound exciton, the acceptor concentration may be evaluated quantitatively by B/D. 7 refs., 7 figs.

Kawarada, H. (Waseda University, Tokyo (Japan). School of Science and Engineering)

1991-08-25

352

Excitation energy transfer between closely spaced multichromophoric systems: Effects of band mixing and intraband relaxation  

CERN Multimedia

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of F\\"orster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that in addition to the well-known fact that the point-dipole approximation breaks down (enabling energy transfer between optically forbidden states), also the perturbative treatment of the electronic interactions between donor and acceptor system, which underlies the F\\"orster approach, in general loses its validity due to overlap of the exciton bands. We therefore propose a nonperturbative method, in which donor and acceptor bands are mixed and the energy transfer is described in terms of a phonon-assisted energy relaxation process between the two new (renormalized) bands. ...

Didraga, C; Knöster, J

2006-01-01

353

Size-dependent decoherence of excitonic states in semiconductor microcrystallites  

International Nuclear Information System (INIS)

[en] The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition aB0??. In general, the larger size of the microcrystallite corresponds to the shorter coherence time. If the initial state is a superposition of two different excitonic coherent states, the coherence time depends on both the overlap of two excitonic coherent states and the size of the microcrystallite. When the system with fixed size is initially in the even or odd coherent state, the larger average number of the excitons corresponds to the faster decoherence. When the average number of the excitons is given, the bigger size of the microcrystallite corresponds to the faster decoherence. The decoherence of the exciton states for the materials GaAs and CdS is numerically studied by our theoretical analysis

2003-01-01

354

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

355

Fluorescence Spectroscopy, Exciton Dynamics and Photochemistry of Single Allophycocyanin Trimers  

International Nuclear Information System (INIS)

[en] We report a study of the spectroscopy and exciton dynamics 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 exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible ixciton trap formation within the three quasi-independent pairs of strong interacting a84 and B84 chromophores in APC, as well a 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 exciton dynamics and photochemistry of light-harvesting complexes

1998-01-01

356

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

357

Fluorescence Spectroscopy, Exciton Dynamics and Photochemistry of Single Allophycocyanin Trimers  

Energy Technology Data Exchange (ETDEWEB)

We report a study of the spectroscopy and exciton dynamics 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 exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible ixciton trap formation within the three quasi-independent pairs of strong interacting a84 and B84 chromophores in APC, as well a 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 exciton dynamics and photochemistry of light-harvesting complexes.

Ying, Liming (ASSOC WESTERN UNIVERSITY); Xie, Xiaoliang (HARVARD UNIVERSITY)

1998-01-01

358

$Q$-deformed description of excitons and associated physical results  

CERN Document Server

We consider excitons in a quantum dot as q-deformed systems. Interaction of some excitonic systems with one cavity mode is considered. Dynamics of the system is obtained by diagonalizing total Hamiltonian and emission spectrum of quantum dot is derived. Physical consequences of q-deformed exciton on emission spectrum of quantum dot is given. It is shown that when the exciton system deviates from Bose statistics, emission spectra will become multi peak. With our investigation we try to find the origin of the q-deformation of exciton. The optical response of excitons, which affected by the nonlinear nature of q-deformed systems, up to the second order of approximation is calculated and absorption spectra of the system is given.

Harouni, M Bagheri; Naderi, M H; 10.1088/0953-4075/42/9/095501

2011-01-01

359

Q-deformed description of excitons and associated physical results  

Energy Technology Data Exchange (ETDEWEB)

We consider excitons in a quantum dot as q-deformed systems. The interaction of some excitonic systems with one cavity mode is considered. The dynamics of the system is obtained by diagonalizing the total Hamiltonian, and the emission spectrum of a quantum dot is derived. The physical consequences of a q-deformed exciton on the emission spectrum of a quantum dot are given. It is shown that when the exciton system deviates from Bose statistics, the emission spectra will become multi-peak. With our investigation we try to find the origin of the q-deformation of the exciton. The optical response of excitons, which is affected by the nonlinear nature of q-deformed systems, up to the second order of approximation is calculated and the absorption spectrum of the system is given.

Harouni, M Bagheri; Roknizadeh, R; Naderi, M H [Quantum Optics Group, Department of Physics, Faculty of Science, University of Isfahan, Hezar Jerib Str., 81746-73441, Isfahan (Iran, Islamic Republic of)], E-mail: m-baghreri@phys.ui.ac.ir, E-mail: rokni@sci.ui.ac.ir, E-mail: mhnaderi@phys.ui.ac.ir

2009-05-14

360

Nanotransformation and current fluctuations in exciton condensate junctions.  

UK PubMed Central (United Kingdom)

We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale.

Soller H; Dolcini F; Komnik A

2012-04-01

 
 
 
 
361

Nanotransformation and current fluctuations in exciton condensate junctions.  

Science.gov (United States)

We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale. PMID:22587267

Soller, H; Dolcini, F; Komnik, A

2012-04-10

362

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

363

Internal Transitions of Negatively Charged Magneto-Excitons and Many Body Effects in GaAs Quantum Wells  

CERN Multimedia

Spin-singlet and spin-triplet internal transitions of quasi-two-dimensional, negatively charged magneto-excitons (X-) and their evolution with excess electron density have been studied in GaAs/AlGaAs quantum-wells by optically detected resonance (ODR) spectroscopy. In the dilute electron limit, due to magnetic translational invariance, the ODR spectra are dominated by bound-to-continuum bands in contrast to the superficially similar negatively-charged-donor system D-, which exhibits strictly bound-to-bound transitions. With increasing excess electron density in the wells in the magnetic field region corresponding to Landau level filling factors nu 2. The blue-shifted transitions are explained in terms of a new type of collective excitation --- magnetoplasmons bound to a mobile valence band hole, which demonstrates the many-body nature of "exciton-like" magnetoluminescence for nu < 2.

Nickel, H A; Dzyubenko, A B; McCombe, B D; Petrou, A; Sivachenko, A Yu; Schaff, W J; Umansky, V

2001-01-01

364

One-dimensional excitonic states and lasing in highly uniform quantum wires formed by cleaved-edge overgrowth with growth-interrupt annealing  

International Nuclear Information System (INIS)

High quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth with molecular beam epitaxy on an interface improved by a growth-interrupt high temperature annealing. Microphotoluminescence and photoluminescence (PL) excitation spectroscopy at low temperatures reveals the formation of quantum wires with unprecedentedly high quality, and intrinsic structures of one-dimensional (1D) free excitons, exciton excited states, and 1D continuum states. At high pumping levels, the PL evolves from showing a sharp free exciton peak via exhibiting a biexciton peak to a Coulomb correlated electron-hole plasma PL band. Lasing has been achieved with a low lasing threshold, and its emission patterns are measured in imaging experiments. The lasing energy is in the plasma PL band and is about 5 meV below the free exciton level. The observed shift excludes the possibility of free excitons in the lasing, and suggests a contribution from the electron-hole plasma. Single T-wire samples such as a single-quantum-wire laser and a field-effect-transistor-type doped single quantum wire are fabricated and studied optically

2004-09-08

365

Charge and exciton transport through molecular wires  

CERN Multimedia

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

366

Ghost excitonic insulator transition in layered graphite  

CERN Multimedia

We discuss the effect of possible proximity of a graphite monolayer to the excitonic insulator phase whose origin is shown to be formally analogous to the non-perturbative phenomenon of dynamical chiral symmetry breaking in relativistic theories of 2+1-dimensional Dirac fermions. In the framework of this scenario, we propose a new explanation for the observed linear energy dependence of the quasiparticle damping in layered graphite and predict other potentially observable features associated with this behavior.

Khveshchenko, D V

2001-01-01

367

Long-range excitons in conjugated polymers with ring torsions  

CERN Document Server

Ring torsion effects on optical excitation properties in poly(para-phenylene) (PPP) and polyaniline (PAN) are investigated by the intermediate exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We find that ring torsions affect the long-range excitons in PAN more easily than in PPP, due to the larger torsion angle of PAN and the large number of bonds whose hopping integrals are modulated by torsions.

Harigaya, K

1997-01-01

368

Multiple exciton generation in single-walled carbon nanotubes.  

UK PubMed Central (United Kingdom)

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

Wang S; Khafizov M; Tu X; Zheng M; Krauss TD

2010-07-01

369

Exciton BCS or BEC state in a semiconductor bilayer system?  

CERN Multimedia

We calculate the off-diagonal long range order (ODLRO) terms of the exciton--exciton correlation function of a semiconductor bilayer system with Coulomb interaction and a transverse magnetic field. We show that the formation of a BEC state is very sensitive to the width of the interaction in momentum space. This dependence is analytically derived and represents the key physical ingredient for the formation (or not) of an exciton condensate state.

Ferreira, E H Martins; Nemes, M C

2008-01-01

370

Microcavity controlled coupling of excitonic qubits.  

UK PubMed Central (United Kingdom)

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 coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton-photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample's coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

Albert F; Sivalertporn K; Kasprzak J; Strauß M; Schneider C; Höfling S; Kamp M; Forchel A; Reitzenstein S; Muljarov EA; Langbein W

2013-01-01

371

Process and composition for drying of gaseous hydrogen halides  

Energy Technology Data Exchange (ETDEWEB)

This patent describes a scavenger, having utility for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom and produce an essentially completely water-free gaseous hydrogen halide effluent characterized by residual water concentration of below 0.1 part per million by volume (ppm), the scavenger comprising: a support; and associated with the support an active scavenging moiety selected from one or more members of the group consisting of: metal halide compounds dispersed in the support, of the formula MX{sub y}; metal halide pendant functional groups, of the formula {minus}MX{sub y{minus}1}, covalently bonded to the support. This patent describes an apparatus for drying a gaseous hydrogen halide of the formula HX, wherein X is selected from the group consisting of bromine, chlorine, fluorine, and iodine, to remove water impurity therefrom, comprising: a vessel containing a bed of a scavenger according to the above claim; means for introducing the water impurity-containing gaseous hydrogen halide to the vessel for passage through the bed therein; and means for discharging water impurity-depleted gaseous hydrogen halide from the vessel.

Tom, G.M.; Brown, D.W.

1989-08-01

372

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals  

Energy Technology Data Exchange (ETDEWEB)

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

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

2010-02-15

373

Phonon-induced dephasing of excitons in semiconductor quantum dots: multiple exciton generation, fission, and luminescence.  

UK PubMed Central (United Kingdom)

Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phonon-induced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub-10 fs. Generally, dephasing is faster for higher-energy and higher-order excitons and increased temperatures. MEF is slow because it is facilitated only by low-frequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials.

Madrid AB; Hyeon-Deuk K; Habenicht BF; Prezhdo OV

2009-09-01

374

Phonon-Induced Dephasing of Excitons in Semiconductor Quantum Dots: Multiple Exciton Generation, Fission, and Luminescence  

Science.gov (United States)

Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phonon-induced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub-10 fs. Generally, dephasing is faster for higher-energy and higher-order excitons and increased temperatures. MEF is slow because it is facilitated only by low-frequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials.

Madrid, Angeline; Kim, Hyeon-Deuk; Habenicht, Bradley; Prezhdo, Oleg

2010-03-01

375

Excitonic energy level structure and pigment-protein interactions in the recombinant water-soluble chlorophyll protein. II. Spectral hole-burning experiments.  

UK PubMed Central (United Kingdom)

Persistent spectral hole burning at 4.5 K has been used to investigate the excitonic energy level structure and the excited state dynamics of the recombinant class-IIa water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The hole-burned spectra are composed of four main features: (i) a narrow zero-phonon hole (ZPH) at the burn wavelength, (ii) a number of vibrational ZPHs, (iii) a broad low-energy hole at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, and (iv) a second satellite hole at ~658 and ~673 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The doublet of broad satellite holes is assigned to an excitonically coupled chlorophyll dimer. The lower-energy holes at ~665 and ~683 nm for chlorophyll b- and chlorophyll a-WSCP, respectively, represent the lower exciton states. Taking into account the parameters of electron-phonon coupling, the lower exciton state can be assigned as the fluorescence origin. The lower exciton state is populated by two processes: (i) exciton relaxation from the higher exciton state and (ii) vibrational relaxation within the lower exciton state. Assuming identical site energies for the two excitonically coupled chlorophyll molecules, the dipole-dipole interaction energy J is directly determined to be 85 and 100 cm(-1) for chlorophyll b- and chlorophyll a-WSCP, respectively, based on the positions of the satellite holes. The Gaussian low-energy absorption band identified by constant fluence hole burning at 4.5 K has a width of ~150 cm(-1) and peaks at 664.9 and 682.7 nm for chlorophyll b- and chlorophyll a-WSCP, respectively. The action spectrum is broader and blue-shifted compared to the fluorescent lower exciton state. This finding can be explained by a slow protein relaxation between energetically inequivalent conformational substates within the lowest exciton state in agreement with the results of Schmitt et al. (J. Phys. Chem. B2008, 112, 13951).

Pieper J; Rätsep M; Trostmann I; Schmitt FJ; Theiss C; Paulsen H; Eichler HJ; Freiberg A; Renger G

2011-04-01

376

Isolation and molecular structures of novel organotellurium(IV) halides, oxyhalide and mixed halide.  

UK PubMed Central (United Kingdom)

The formation of chiral (R)-[2-(4-ethyl-2-oxazolinyl)phenyl]tellurium(IV) trihalides (trichloride (5), tribromide (6) and triiodide (7)) and (R)-[2-(4-ethyl-2-oxazolinyl)phenyl]tellurium(II) bromide (8) is described. The molecular structure of the first chiral organotellurium(IV) triiodide (7) is discussed. The most interesting feature of the structure of 5 is the observation of the shortest intramolecular TeN distance for any organotellurium(IV) trichloride. Compounds 5 and 6 do not undergo electrophilic addition reaction with olefins. Attempted addition reaction of 6 with olefin in methanol leads to the isolation of the rare partially hydrolysed tellurium(IV) halide, methyl-2-(dibromooxotellurium)benzoate (9). Novel stable mixed halide 12 was isolated from the bromination of 11. DFT calculations indicate that both the isomers 12a and 12b have similar stability.

Rakesh P; Singh HB; Butcher RJ

2012-09-01

377

Excitonic bistabilities, instabilities and chaos in laser-pumped semiconductor  

International Nuclear Information System (INIS)

[en] The Hurwitz criteria are used for a stability analysis of the steady state excitonic optical bistability curves in a semiconductor pumped by an external laser resonant with the exciton level. Besides the middle branch of the bistability curves which is unstable in the sense of the linear stability theory, we have found other domains of instability in the upper and lower branches of the steady state curves. Numerical results show that a possible route to chaos in the photon-exciton system is period-doubling self-oscillation process. The influence of the presence of free carriers that coexist with the excitons is also discussed. (author). 16 refs, 6 figs

1992-01-01

378

Dark and Bright Excitonic States in Nitride Quantum Dots  

CERN Multimedia

Formation of excitonic states in quantum dots of nitride based III-V semiconductors GaN and AlN including coulomb and exchange interactions are investigated. Dark exciton formation is found to occur for both GaN quantum dots(QDs) with wurtzite structure having positive crystal field splitting and GaN and AlN QDs with zero crystal field splitting with a transition from dark to bright exciton at about 40{\\AA}. In wurtzite AlN QDs with negative crystal field splitting the splitting between the dark and bright excitonic states is very small and vanishes at about 15{\\AA}.

Bagga, A; Ghosh, S; Bagga, Anjana; Ghosh, Subhasis

2004-01-01

379

Lateral superlattices as voltage-controlled traps for excitons  

CERN Multimedia

We demonstrate the localisation of quantum well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localisation originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments, the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilisation of excitons in the presence of strong in-plane electric fields.

Zimmermann, S; Hansen, W; Kotthaus, J P; Bichler, M; Wegscheider, W

1997-01-01

380

Excitonic electroluminescence from ZnO-based heterojunction light emitting diodes  

International Nuclear Information System (INIS)

This work reports on the fabrication and characteristics of n-ZnO/p-GaN and n-ZnO/n-MgZnO/n-ZnO/p-GaN heterojunction light emitting diodes (LEDs). Both devices exhibited diode-like rectifying current-voltage characteristics. Room temperature electroluminescence (EL) spectra for both LEDs consisted of dominant emission at 375 nm and two weaker bands centred at 415 nm and 525 nm, which were attributed to ZnO excitonic transition and defect-related emissions from GaN and ZnO, respectively. Moreover, it was demonstrated that the single heterojunction required a higher injection current to obtain an excitonic EL than that for the n-ZnO/n-MgZnO/n-ZnO/p-GaN LEDs. This indicated that the insertion of the MgZnO layer confined the injection carriers and thus increased the intensity of excitonic emission in the ZnO active region.

2008-08-07

 
 
 
 
381

Excitonic recombination in superstoichiometric nanocrystalline TiO2 grown by cluster precursors at room temperature.  

UK PubMed Central (United Kingdom)

Unprecedented room temperature excitonic emissions are achieved from TiO(2) nanocrystals synthesized at 300 K by supersonic cluster beams. Transmission electron microscopy studies show the crystalline nature of the nanoparticles (NPs) with a diameter ranging from 5 to 30 nm. All the samples show mixed rutile and anatase phases as confirmed by Raman spectroscopy. XPS core level analyses evidence an O/Ti ratio of the as-grown nanoparticles of 2.30 ± 0.04. Two room temperature cathodoluminescence excitonic peaks observed at 3.16 and 3.25 eV are ascribed to the coexistence of rutile and anatase crystallographic phases respectively. Subsequent thermal treatments at 450 °C cause the complete quenching of the UV excitonic emissions and result in a more conventional broad visible band centered at 2.5 eV. HRTEM and XPS studies reveal that, after annealing, the NPs remain single crystals in nature with an O/Ti ratio of 2.20 ± 0.04. These results suggest a correlation between the emission properties and the oxygen concentration of our NPs. The achieved ability to tune the optical properties of TiO(2) nanoparticles is very promising for sensing and energy applications.

Detto F; Armani N; Lazzarini L; Toccoli T; Verucchi R; Aversa L; Nardi MV; Rossi B; Salviati G; Iannotta S

2012-04-01

382

Ultrafast exciton dynamics in InAs/ZnSe nanocrystal quantum dots.  

UK PubMed Central (United Kingdom)

Colloidal nanocrystal quantum dots with a band gap in the near infra-red have potential application as the emitters for telecommunications or in vivo imaging, or as the photo-absorbing species in next generation solar cells or photodetectors. However, electro- and photoluminescence yields and the efficiency with which photo-generated charges can be extracted from quantum dots depend on the total rate of recombination, which can be dominated by surface-mediated processes. In this study, we use ultrafast transient absorption spectroscopy to characterise the recombination dynamics of photo-generated charges in InAs/ZnSe nanocrystal quantum dots. We find that recombination is dominated by rapid, sub-nanosecond transfer of conduction band electrons to surface states. For the size of dots studied, we also find no evidence of significant multiple exciton generation for photon energies up to 3.2 times the band gap, in agreement with our theoretical modelling.

Cadirci M; Stubbs SK; Hardman SJ; Masala O; Allan G; Delerue C; Pickett N; Binks DJ

2012-11-01

383

Ultrafast exciton dynamics in InAs/ZnSe nanocrystal quantum dots.  

Science.gov (United States)

Colloidal nanocrystal quantum dots with a band gap in the near infra-red have potential application as the emitters for telecommunications or in vivo imaging, or as the photo-absorbing species in next generation solar cells or photodetectors. However, electro- and photoluminescence yields and the efficiency with which photo-generated charges can be extracted from quantum dots depend on the total rate of recombination, which can be dominated by surface-mediated processes. In this study, we use ultrafast transient absorption spectroscopy to characterise the recombination dynamics of photo-generated charges in InAs/ZnSe nanocrystal quantum dots. We find that recombination is dominated by rapid, sub-nanosecond transfer of conduction band electrons to surface states. For the size of dots studied, we also find no evidence of significant multiple exciton generation for photon energies up to 3.2 times the band gap, in agreement with our theoretical modelling. PMID:22968520

Cadirci, Musa; Stubbs, Stuart K; Hardman, Samantha J O; Masala, Ombretta; Allan, Guy; Delerue, Christophe; Pickett, Nigel; Binks, David J

2012-09-11

384

Structure of uranyl ion luminescence spectra in pressed alkali-halide matrices  

International Nuclear Information System (INIS)

[en] The luminescent properties of the polyatomic UO22+ ions present as impurity in pressed alkali-halide matrices of KCl and KBr are studied. At 77-4.2 K the spectra of absorption, photoexcitation and radiation of such systems exhibit a vibronic structure. In this case two series of vibronic bands due to the interaction of electron transitions sup(1)?sub(u)?sup(1)?sub(g)sup(+)(sup(1)?sub(u) ? sup(1)?sub(g)sup(+)) and sup(1)?sub(u)sup(+) ? sup(1)?sub(g)sup(+)) with intramolecular impurity vibration appear in the luminescence spectra. At 4.2 K the emission vibronic bands of the sup(1)?sub(u)sup(+) ? sup(1)?sub(g)sup(+) transition are splitted to phononless lines and structural phonon wings (PW). Part of PW structure maxima are shown to be associated with an excitation of crystal vibrations of the KCl and KBr matrices

1984-01-01

385

Palladium-catalyzed amination of aryl halides on solid support.  

UK PubMed Central (United Kingdom)

[reaction: see text] The first examples of the Pd(0)-catalyzed amination of aryl halides using Rink-resins as nitrogen source are described. Pd(2)dba(3)/BINAP/NaO-t-Bu was found to be the most efficient catalyst/base system, while a solvent mixture of dioxane and tert-butyl alcohol was shown to enhance the selectivity toward the desired monoarylation. Moderate to good yields and excellent purities of the amination products were found with electron-poor aryl halides, while electon-rich aryl halides failed to react under these conditions.

Weigand K; Pelka S

2002-12-01

386

Palladium-catalyzed amination of aryl halides on solid support.  

Science.gov (United States)

[reaction: see text] The first examples of the Pd(0)-catalyzed amination of aryl halides using Rink-resins as nitrogen source are described. Pd(2)dba(3)/BINAP/NaO-t-Bu was found to be the most efficient catalyst/base system, while a solvent mixture of dioxane and tert-butyl alcohol was shown to enhance the selectivity toward the desired monoarylation. Moderate to good yields and excellent purities of the amination products were found with electron-poor aryl halides, while electon-rich aryl halides failed to react under these conditions. PMID:12489962

Weigand, Klaus; Pelka, Sylvie

2002-12-26

387

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; William Tam

2012-01-01

388

Study of the anharmonicity properties of copper halides  

International Nuclear Information System (INIS)

The anharmonic properties of copper halides have been investigated by means of a three-body interaction potential extended to include the van der Waals effects. For this purpose, the third-order elastic constants and the pressure derivatives of second-order elastic constants have been computed after deriving their correct expressions from the present potential following the lines of Sharma and Verma. Our results have, generally, followed a systematic trend and given a satisfactory prediction of the available experimental data on the anharmonic properties of copper halides. This potential has as scope of its application to describe the dynamical and dielectric properties of these halides.

1987-01-01

389

Straightforward synthesis of thiazoline-incorporated chalconoids from phenacyl halides.  

UK PubMed Central (United Kingdom)

A series of thiazoline-incorporated chalconoids, designed based on natural product scaffold, were efficiently synthesized via the reaction of 3-methyl-4-arylthiazole-2(3H)-thiones and appropriate phenacyl halides, and subsequent desulfurization. The starting 3-methyl-4- arylthiazole-2(3H)-thiones were also prepared from phenacyl halides. The structural aspects and (Z)-geometry of compounds were confirmed by IR and (1)H NMR spectral data. This chemistry provides a new library of compounds basically originated from phenacyl halides as building blocks, with potential activity for biomedical screening.

Ayati A; Emami S

2013-02-01

390

Excitons in coupled InAs/InP self-assembled quantum wires  

CERN Document Server

Optical transitions in coupled InAs/InP self-assembled quantum wires are studied within the single-band effective mass approximation including effects due to strain. Both vertically and horizontally coupled quantum wires are investigated and the ground state, excited states and the photoluminescence peak energies are calculated. Where possible we compare with available photo-luminescence data from which it was possible to determine the height of the quantum wires. An anti-crossing of the energy of excited states is found for vertically coupled wires signaling a change of symmetry of the exciton wavefunction. This crossing is the signature of two different coupling regimes.

Sidor, Y; Peeters, F M; Ben, T; Ponce, A; Sales, D L; Molina, S I; Fuster, D; González, L; Gonzalez, Y

2007-01-01

391

Excitons or spaced-charged-perturbed fields in irradiated SiO2  

International Nuclear Information System (INIS)

Flat-band voltage shifts in MOS capacitors irradiated by a kilovolt electron beam have been measured. The relatively low dose conditions employed resulted in the data points being obtained under non-equilibrium conditions and therefore sensitive to the exact manner in which the bias voltage across the sample was cycled. All the results obtained can be explained by considering the transient growth and decay of hole space-charges at the oxide interfaces with the metal electrode and the silicon substrate. No evidence was found to support the role of excitons in generating hole space charge at the oxide/silicon interface as postulated by previous authors. (author).

1985-01-01

392

Nonresonant two-photon generation of excitonic matter in a CuCl pellet  

Energy Technology Data Exchange (ETDEWEB)

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.

Jang, J.I. [Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States)], E-mail: joon-jang@northwestern.edu; Ketterson, J.B. [Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (United States); Anderson, M.A.; Chang, R.P.H. [Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive Evanston, IL 60208 (United States)

2008-10-15

393

Application of the extended effective mass theory to the exciton spectra of CuCl  

International Nuclear Information System (INIS)

[en] An extended effective mass theory has been applied to the interpretation of exciton spectra of Cu Cl. Various parameters entering the theory have been calculated using the known crystal structure and band structure of this crystal. The screening parameters have been chosen so as to give agreement between theory and experiment for n=1 state. The proposed theory and experimental findings are found to converge to the same result. It is also found that the empirical series formulas suggested by previous workers are defective. They need to be replaced by series relationships suggested in the present article. (JA)

1977-01-01

394

Nonresonant two-photon generation of excitonic matter in a CuCl pellet  

International Nuclear Information System (INIS)

[en] A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor

2008-01-01

395

Excitonic splitting and coherent electronic energy transfer in the gas-phase benzoic acid dimer  

International Nuclear Information System (INIS)

The benzoic acid dimer, (BZA)2, is a paradigmatic symmetric hydrogen bonded dimer with two strong antiparallel hydrogen bonds. The excitonic S1/S2 state splitting and coherent electronic energy transfer within supersonically cooled (BZA)2 and its 13C?, d1?, d2?, and 13C/d1? isotopomers have been investigated by mass-resolved two-color resonant two-photon ionization spectroscopy. The (BZA)2-(h?h) and (BZA)2-(d?d) dimers are C2h symmetric, hence only the S2?S0 transition can be observed, the S1?S0 transition being strictly electric-dipole forbidden. A single 12C/13C or H/D isotopic substitution reduces the symmetry of the dimer to Cs, so that the isotopic heterodimers (BZA)2?13C, (BZA)2?(h?d), (BZA)2?(h13C?d), and (BZA)2?(h?d13C) show both S1?S0 and S2?S0 bands. The S1/S2 exciton splitting inferred is ?exc= 0.94 ± 0.1 cm?1. This is the smallest splitting observed so far for any H-bonded gas-phase dimer. Additional isotope-dependent contributions to the splittings, ?iso, arise from the change of the zero-point vibrational energy upon electronic excitation and range from ?iso= 3.3 cm?1 upon 12C/13C substitution to 14.8 cm?1 for carboxy H/D substitution. The degree of excitonic localization/delocalization can be sensitively measured via the relative intensities of the S1?S0 and S2?S0 origin bands; near-complete localization is observed even for a single 12C/13C substitution. The S1/ S2 energy gap of (BZA)2 is ?calcexc=11 cm?1 when calculated by the approximate second-order perturbation theory (CC2) method. Upon correction for vibronic quenching, this decreases to ?vibronexc=2.1 cm?1 [P. Ottiger et al., J. Chem. Phys. 136, 174308 (2012)], in good agreement with the observed ?exc= 0.94 cm?1. The observed excitonic splittings can be converted to exciton hopping times ?exc. For the (BZA)2?(h?h) homodimer ?exc= 18 ps, which is nearly 40 times shorter than the double proton transfer time of (BZA)2 in its excited state [Kalkman et al., ChemPhysChem 9, 1788 (2008)]. Thus, the electronic energy transfer is much faster than the proton-transfer in (BZA)2*.

2012-11-28

396

Ground state properties of heavy alkali halides  

CERN Document Server

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

Doll, K; Doll, Klaus; Stoll, Hermann

1998-01-01

397

Thermoluminescence of alkali halides and its implications  

International Nuclear Information System (INIS)

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

2012-03-01

398

Study of Ammonium Halides by Neutron Spectrometry  

International Nuclear Information System (INIS)

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

1963-01-01

399

The convergence of longitudinal excitons onto the ?5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons.  

Science.gov (United States)

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ? c) or perpendicular (K ? c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton ?5 in the K?c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ? c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton-photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers. PMID:23880965

Elgawadi, Amal; Gainer, Gordon; Krasinski, Jerzy

2013-07-24

400

The convergence of longitudinal excitons onto the ?5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons.  

UK PubMed Central (United Kingdom)

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ? c) or perpendicular (K ? c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton ?5 in the K?c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ? c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton-photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers.

Elgawadi A; Gainer G; Krasinski J

2013-08-01

 
 
 
 
401

The convergence of longitudinal excitons onto the ?5 transverse exciton in GaN and the thermal activation energy of longitudinal excitons  

Science.gov (United States)

The crystal orientation dependence of GaN excitons was investigated via the photoluminescence (PL) technique. The PL emissions at a temperature of 10 K were obtained from two experimental configurations where the emission K vector (the propagation vector) was either parallel (K ? c) or perpendicular (K ? c) to the crystal c-axis. Longitudinal, transverse and donor-bound excitons were observed in the two configurations. However, the longitudinal excitons converged onto the transverse free exciton ?5 in the K?c emission. This behavior was discussed in terms of electron screening due to the scattering of electrons moving perpendicular to charged dislocation lines. Additionally, the thermal activation energy of the longitudinal excitons was calculated from the temperature dependent PL measurements collected from the K ? c emission, and was found to be 5 to 6 times as high as the binding energy of the free excitons. This high energy was interpreted tentatively in view of the creation of polaritons in strong exciton–photon coupling regimes. These findings present fundamental concepts for applications such as vertical cavity surface-emitting lasers (VCSELs) and polariton lasers.

Elgawadi, Amal; Gainer, Gordon; Krasinski, Jerzy

2013-08-01

402

Effects of hydrostatic pressure and applied electric fields on the exciton states in GaAs-(Ga,Al)As quantum wells  

Energy Technology Data Exchange (ETDEWEB)

The effects of both hydrostatic pressure and electric fields applied perpendicular to the layers on the direct-exciton states in single GaAs-(Ga,Al)As quantum wells are studied. Theoretical calculations are performed within the variational procedure, in the framework of the effective-mass and non-degenerate parabolic-band approximations. Both heavy- and light-hole exciton energies and corresponding quantum-confined Bohr radii are obtained. The pressure coefficient is also theoretically evaluated and found in good agreement with available experimental measurements.

Raigoza, N. [Instituto de Fisica, Universidad de Antioquia, AA 1226 Medellin (Colombia); Duque, C.A. [Instituto de Fisica, Universidad de Antioquia, AA 1226 Medellin (Colombia)]. E-mail: cduque@fisica.udea.edu.co; Reyes-Gomez, E. [Department of Theoretical Physics, University of Havana, San Lazaro y L, Vedado 10400, Havana (Cuba); Oliveira, L.E. [Instituto de Fisica, Unicamp, CP 6165, Campinas, Sao Paulo 13083-970 (Brazil)

2005-10-01

403

Long-range exciton dissociation in organic solar cells  

Digital Repository Infrastructure Vision for European Research (DRIVER)

It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the ...

Caruso, Domenico; Troisi, Alessandro

404

Tunable exciton Aharonov-Bohm effect in a quantum ring  

Energy Technology Data Exchange (ETDEWEB)

We studied the optical Aharonov-Bohm effect for an exciton in a semiconductor quantum ring. A perpendicular electric field applied to a quantum ring with large height, is able to tune the exciton ground state energy such that it exhibits a weak observable Aharonov-Bohm oscillations. This Aharonov-Bohm effect is tunable in strength and period.

Li Bin; Magnus, W; Peeters, F M, E-mail: phymilky@gmail.co [Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen (Belgium)

2010-02-01

405

Angular momentum dependence of transition rates in exciton model  

International Nuclear Information System (INIS)

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

1994-01-01

406

Halide and proton binding kinetics of yellow fluorescent protein variants.  

UK PubMed Central (United Kingdom)

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.

Seward HE; Basran J; Denton R; Pfuhl M; Muskett FW; Bagshaw CR

2013-04-01

407

Near band gap photoluminescence properties of hexagonal boron nitride  

CERN Multimedia

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

408

Correlated exciton fluctuations in cylindrical molecular aggregates.  

UK PubMed Central (United Kingdom)

Femtosecond electronic relaxation dynamics of a cylindrical molecular aggregate are measured with transient grating (TG) and two-dimensional Fourier transform photon echo (PE) spectroscopies. The aggregates are double-walled cylindrical structures formed by self-assembly of amphiphilic cyanine dye molecules in water. The diameters of the inner and outer cylinders are approximately 6 and 10 nm. The linear absorption spectrum of the aggregate exhibits four spectrally resolved single exciton transitions corresponding to excited states localized on particular regions of the structure: (1) an excited state localized on the inner cylinder corresponds to the lowest energy transition at 16670 cm(-1); (2) a transition at 17150 cm(-1) represents a state localized on the outer cylinder, (3) whereas an overlapping peak found at 17330 cm(-1) is more closely associated with the inner cylinder; (4) an excited state delocalized between the inner and outer cylinder is assigned to a transition in the linear absorption spectrum at 17860 cm(-1). TG spectra show a series of resonances reflecting the electronic structure of both the single and double exciton manifolds. In addition, PE spectra reveal coherent modulation of both diagonal and cross-peak amplitudes persisting for 100 fs, where the coherence frequency matches the energy gap between transitions 1 and 4 in the linear absorption spectrum. PE line shapes suggest correlated energy level fluctuations for the exciton states associated with these two transitions, which is consistent with this fairly long-lasting coherence at room temperature in aqueous solution. The impact of these correlations on Forster energy transfer efficiency is discussed. The observations imply fairly long-range correlations between the molecular sites (>0.6 nm), which in turn reflects the length scale of the environmental motion inducing the fluctuations. We suggest that this environmental motion is most likely associated with water confined inside the cylinder and/or fluctuations of the dye's aliphatic functional groups.

Womick JM; Miller SA; Moran AM

2009-05-01

409

Correlated exciton fluctuations in cylindrical molecular aggregates.  

Science.gov (United States)

Femtosecond electronic relaxation dynamics of a cylindrical molecular aggregate are measured with transient grating (TG) and two-dimensional Fourier transform photon echo (PE) spectroscopies. The aggregates are double-walled cylindrical structures formed by self-assembly of amphiphilic cyanine dye molecules in water. The diameters of the inner and outer cylinders are approximately 6 and 10 nm. The linear absorption spectrum of the aggregate exhibits four spectrally resolved single exciton transitions corresponding to excited states localized on particular regions of the structure: (1) an excited state localized on the inner cylinder corresponds to the lowest energy transition at 16670 cm(-1); (2) a transition at 17150 cm(-1) represents a state localized on the outer cylinder, (3) whereas an overlapping peak found at 17330 cm(-1) is more closely associated with the inner cylinder; (4) an excited state delocalized between the inner and outer cylinder is assigned to a transition in the linear absorption spectrum at 17860 cm(-1). TG spectra show a series of resonances reflecting the electronic structure of both the single and double exciton manifolds. In addition, PE spectra reveal coherent modulation of both diagonal and cross-peak amplitudes persisting for 100 fs, where the coherence frequency matches the energy gap between transitions 1 and 4 in the linear absorption spectrum. PE line shapes suggest correlated energy level fluctuations for the exciton states associated with these two transitions, which is consistent with this fairly long-lasting coherence at room temperature in aqueous solution. The impact of these correlations on Forster energy transfer efficiency is discussed. The observations imply fairly long-range correlations between the molecular sites (>0.6 nm), which in turn reflects the length scale of the environmental motion inducing the fluctuations. We suggest that this environmental motion is most likely associated with water confined inside the cylinder and/or fluctuations of the dye's aliphatic functional groups. PMID:19382770

Womick, Jordan M; Miller, Stephen A; Moran, Andrew M

2009-05-14

410

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

411

Pattern Formation in Exciton System near Quantum Degeneracy  

CERN Multimedia

We discuss models of the modulational instability in a cold exciton system in coupled quantum wells. One mechanism involves exciton formation in a photoexcited electron-hole system in the presence of stimulated binding processes which build up near exciton degeneracy. It is shown that such processes may give rise to Turing instability leading to a spatially modulated state. The structure and symmetry of resulting patterns depend on dimensionality and symmetry. In the spatially uniform 2d electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. Wavelength selection mechanism is analyzed, revealing that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface. Another mechanism that could possibly drive the instability involves long-range attraction of the excitons. We illustrate how such an interaction can result from current induced by exciton recombination ('pl...

Levitov, L S; Butov, L V

2005-01-01

412

Activated singlet exciton fission in a semiconducting polymer.  

UK PubMed Central (United Kingdom)

Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.

Musser AJ; Al-Hashimi M; Maiuri M; Brida D; Heeney M; Cerullo G; Friend RH; Clark J

2013-08-01

413

Feasibility study of a nuclear exciton laser  

Science.gov (United States)

Nuclear excitons known from Mössbauer spectroscopy describe coherent excitations of a large number of nuclei—analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.

ten Brinke, Nicolai; Schützhold, Ralf; Habs, Dietrich

2013-05-01

414

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

International Nuclear Information System (INIS)

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

2011-04-13

415

Manifestation of protein conformations in the B850 absorption band of light-harvesting complex LH2  

Science.gov (United States)

As experimentally established the absorption bands corresponding to the B850 ring of LH2 complexes from Rhodoblastus acidophilus and Rhodobacter sphaeroides broaden and shift by changing the temperature. The peak shift with the temperature cannot be explained in terms of the conventional disordered exciton model, therefore the modified (dichotomous) exciton model is formulated. The modified exciton model, which assumes the presence of two conformational states for every bacteriochlorophyll molecule constituting the B850 ring, is postulated. Since the conformational states are characterized by their free energies, the population ratio of the conformational states is temperature dependent. As the result, the disorder becomes also temperature dependent and, thus, both the broadening and the shift of the B850 absorption bands are explained within the frame of the modified exciton model.

Meldaikis, Julius; Zerlauskiene, Oksana; Abramavicius, Darius; Valkunas, Leonas

2013-09-01

416

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

International Nuclear Information System (INIS)

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

1993-01-01

417

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

UK PubMed Central (United Kingdom)

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

Vaida ME; Tchitnga R; Bernhardt TM

2011-01-01

418

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

Directory of Open Access Journals (Sweden)

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

Mihai E. Vaida; Robert Tchitnga; Thorsten M. Bernhardt

2011-01-01

419

Interaction of osmium tetroxide with chalcogen halides and chalcogen halide complexes of osmium  

International Nuclear Information System (INIS)

Chalcogen halide coordination and simple osmium complexes of OsCl4(ECl4)2 composition, where E=S, Se, Te; OsS2O2Cl3; OsS4Cl3; OsS4Cl2; OsS2Br4; OsBr4(SeBr4)2 are prepared in nonaqueous chlorotropic media. Magnetic properties of the compounds, electron and oscillating IR spectra are investigated. The conclusion on complex composition is made based on recent data.

1988-01-01

420

GaN nanotubes grown by halide vapor phase epitaxy  

International Nuclear Information System (INIS)

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

2011-01-01

 
 
 
 
421

Multiple exciton generation effect in quantum dots  

International Nuclear Information System (INIS)

Full text: Recently among various investigations in the field of nanophysics a great intensification in studying of multiple exciton generation (MEG) effect in quantum dots at absorption of photons with the energy several times higher than the gap in electronic spectra (h?>>Eg) has been made [1-2]. There have been proposed four theoretical approaches to explain the MEG effect. However, the statistic approach basing on the Fermi theory of multiple pi-mesons generation (1950) is the most adequate related to others. In this work: 1. the results of statistic approach have been generalized; 2. the microscopic mechanism basing on theory of electronic 'shaking' has been proposed; 3. the problem of deviation of mean multiplicity of MEG effect from the Poisson law of fluctuations has been investigated that is a evidence of strong non-linear and non-equilibrium character of exciton generation process; 4. the role of interface electronic states of quantum dot and ligand has been considered by means of perturbation theory of closed states related to surface Tamm electronic states of quantum dot and legend; 5. the problem of size optimization of quantum dot to receive the maximum multiplicity has been studied; 6. the efficiency of MEG effect for the third generation of solar cells has been calculated on the basis of proposed theory. (authors)

2012-01-01

422

On the theory of energetic structure of the Wannier-Mott exciton in a dielectric medium  

International Nuclear Information System (INIS)

The theory of the fine structure of the Wannier-Mott exciton, due to the interaction of the electron and hole with the polarizable vibrations of the lattice, is developed. The ''nonhydrogen-like'' corrections to the exciton energy, and also the corrections to the effective mass of the exciton, depending on the internal state of the exciton, are calculated.

1973-01-01

423

On the theory of phonoriton in cubic semiconductors with a degenerate valence band  

International Nuclear Information System (INIS)

[en] The ''phonoriton'' is an elementary excitation constructed from an exciton polariton and phonon in semiconductors under intense excitation by an electromagnetic wave near the exciton resonance (L.V. Keldysh and A.L. Ivanov, 1982). In this paper we develop a theory of phonoriton in direct band gap cubic semiconductor with a degenerate valence band using the simple model of J.L. Birman and B.S. Wang (1990). In addition to experimental proofs of the existence of phonoriton we propose an experiment to measure its flight time. (author). 33 refs

1992-01-01

424

PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation.  

Science.gov (United States)

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts. PMID:16522105

Murphy, James E; Beard, Matthew C; Norman, Andrew G; Ahrenkiel, S Phillip; Johnson, Justin C; Yu, Pingrong; Mi?i?, Olga I; Ellingson, Randy J; Nozik, Arthur J

2006-03-15

425

PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation.  

UK PubMed Central (United Kingdom)

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.

Murphy JE; Beard MC; Norman AG; Ahrenkiel SP; Johnson JC; Yu P; Mi?i? OI; Ellingson RJ; Nozik AJ

2006-03-01

426

Birefringence and excitonic spectra of CdGa2S4 crystals  

International Nuclear Information System (INIS)

[en] The spectral dependence of ?n = no - ne of CdGa2S4 crystals in short- and long- wavelength part of isotropic wavelength ?0 = 485.7 nm (300 K) was determined. It was established what in the case when ? > ?0 ?n is positive and at ? 0 it is negative. The wavelength ?0 = 485.7 nm shifts in short - wavelength at decreasing of temperature. Ground and excited states of three excitonic series (A, B and C) with binding energies of 53 meV, 52 meV and 46 meV, respectively were found out at 10 K. The effective mass of electrons for k = 0 was determined form calculation of excitonic spectra (mc|| (E||c) = 0.21m0 and mcperpendicular (E perpendicular c) = 0.19m0). The holes masses are equal to 0.59m0, 0.71m0 and 0.71m0, respectively. The value of valence bands V1-V2 splitting by crystalline field is corresponding to 24 meV and bands V2-V3 splitting due to the spin-orbital interaction is equal to 130 meV. (authors)

2013-01-01

427

Spatial dispersion in CaF$_2$ caused by the vicinity of an excitonic bound state  

CERN Document Server

The microscopic mechanism beyond the optical anisotropy of an ionic crystal which occurs for short wavelengths is investigated. The electron-hole, two particle propagator and its analytical behaviour close to the band edge of the one particle continuum plays a major role for the mechanism of this optical anisotropy. Especially for an ionic crystal the two particle bound state, the exciton, is of special importance. In this way we argue that the so called ``intrinsic birefringence'' in CaF$_2$ is neither intrinsic to the material nor it is birefringence. Instead it is spatial dispersion caused by the vicinity of a dispersive optical absorption given by the excitonic bound state. We propose a model which connects the bound state dispersion with the band structure and a model potential for a screened coulomb interaction. Based on these considerations we predict a wavelength dependence of the dielectric function approaching close to the bound state level $\\epsilon \\sim (\\lambda - \\lambda_0)^{-1}$, where $\\lambda_...

Letz, M; Brinkmann, M; Moersen, E

2002-01-01

428

Exciton hopping and nonradiative decay in AlGaN epilayers  

Science.gov (United States)

Monte Carlo simulation of phonon-assisted localized exciton hopping has been employed to describe the photoluminescence linewidth variation with temperature and to reveal band potential profile of ternary AlGaN epilayers with different carrier lifetimes. The lifetimes of 30 and 190 ps were experimentally determined in the layers with AlN buffers grown by conventional metal-organic chemical vapor deposition (MOCVD) and by migration-enhanced MOCVD (MEMOCVD™), respectively. The potential profile in AlGaN is shown to consist of double-scaled fluctuations. Exciton hopping in Al0.26Ga0.74N occurs within the random potential fluctuations (on the scale ?~19 meV) in isolated low-potential regions with the average localization energy dispersed on the scale ?~19 meV. Such a pattern of band