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Sample records for halide exciton bands

  1. Exciton-relaxation dynamics in lead halides

    International Nuclear Information System (INIS)

    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

  2. Optical Constants and Excitons of Alkali Cuprous Halide Crystals

    Science.gov (United States)

    Edamatsu, Keiichi; Nanba, Takao; Ikezawa, Mikihiko

    1989-01-01

    The anisotropic reflection spectra of K2AgI3, Rb2AgI3 and Rb2AgBr3 have been measured in the range from 4 to 26 eV using synchrotron radiation. The optical constants have been obtained through the Kramers-Kronig analysis. The characters of the band edge excitons of K2AgI3 and Rb2AgI3 are very similar to those of the alkali iodides. However, the perturbation calculation has indicated that the spin-orbit interaction scarcely affect the outline of the valence band structure of Rb2AgI3 and that no spin-orbit doublet appears in the absorption spectra. The anisotropic absorption due to the core excitons of K+ or Rb+ have been found around 20 or 17 eV, respectively. The prominent structures due to plasmons of valence electrons have been found at 10-12 eV in the energy loss spectra.

  3. The self-trapping of anion excitons in alkali halides at elastic deformation

    International Nuclear Information System (INIS)

    The self-trapping of electronic excitations (EE) (excitons, holes and electrons) in alkali halides (AH), fluorides and oxides plays an important roles in luminescence and defect formation. Therein the specific features of self-trapping of EE in various materials are essentially different. In particular, the self-trapping of excitons in some AH (i.e. alkali iodides and bromides) occurs with overcoming of the potential barrier and in other AH (e.g. alkali fluorides and chlorides) such a barrier is absent. Here we develop the continuum theory of self-trapping of within the adiabatic approximation elastically stressed AH. In the continuum model of solids the functional of the total energy of are interacting exciton-phonon system in the deformed ionic crystal just as in the undeformed crystal depends on the dilation ?(r) described by the deformation potential of acoustic phonon, the electrostatic potential ?[r) due to the lattice polarization at optical lattice vibrations and the wave function of exciton chosen for hydro statically and uniaxially stressed 3D crystals. The functionals of the total energy of the interfacing exciton-phonon system E{?(r),?(r),?(r)} are minimized relative to ?, ? and ? for the cases of isotropic and anisotropic 3D crystals. As a result, we obtained the functionals depending on ? and determined their possible extremum. We have show that the linear deformations under the hydrostatic and uniaxial stress at 80 K lead to the decreasing of t 80 K lead to the decreasing of the self trapping barrier for exciton and to the increasing of the luminescence of self-trapped excitons (STE). While the nonlinear deformations under the such stress at 80 K lead to the increasing of the self-trapping barrier for excitons and to the decreasing at the STE luminescence in AH. At T=0 K the small hydrostatic and uniaxial pressures lead to the same effects. Further at hydrostatic and uniaxial compressions of AH the minimums of the adiabatic potentials of quasifree and STE are shifted to higher energies. The above predictions are consistent with the experimental data

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

    International Nuclear Information System (INIS)

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

  5. Valence and conduction bands engineering in halide perovskites for solar cell applica- tions

    OpenAIRE

    Meloni, Simone; Palermo, Giulia; Astani, Negar Ashari; Curchod, Basile F. E.; Graetzel, Michael; Roethlisberger, Ursula

    2014-01-01

    We performed ab initio simulations aimed at identifying the atomistic and electronic structure origin of the high valence and conduction band, and band gap tunability of halide perovskites. We found that the two key ingredients are the overlap between bivalent cation and halide atomic orbitals, and the electronic charge of the bottom of the conduction band (CBM) state on the Sn or Pb atoms. In particular, we found that lower gaps are associated to higher negative antibonding...

  6. Exciton Binding energies and effective masses in Organo-lead Tri-Halide Perovskites

    Science.gov (United States)

    Portugall, Oliver; Miyata, Atsuhiko; Mitioglu, Anatol; Plochocka, Paulina; Wang, Jacob Tse-Wei; Stranks, Samuel; Snaith, Henry; Nicholas, Robin; Lncmi Toulouse Team; Oxford University Team

    2015-03-01

    Solid-state perovskite-based solar cells have made a dramatic impact on emerging PV research with efficiencies of over 17% already achieved. However, to date the basic electronic properties of the perovskites such as the electron and hole effective masses and the exciton binding energy are not well known. We have measured both for methyl ammonium lead tri-iodide using magneto absorption in very high magnetic fields up to 150T showing that the exciton binding energy at low temperatures is only 16 meV, a value three times smaller than previously thought and sufficiently small to completely transform the way in which the devices must operate. Landau level spectroscopy shows that the reduced effective mass of 0.104 me is also smaller than previously thought. In addition by using a fast pulse 150T magnet we measure the band structure change due to the structural phase transition that occurs in this system at around 160K. We also observe Landau levels in the high temperature phase as used for device production, which has a very similar effective mass and the analysis suggests an exciton binding energy which is even smaller than in the low temperature phase.

  7. First-principles method of propagation of tightly bound excitons: exciton band structure of LiF and verification with inelastic x-ray scattering

    OpenAIRE

    Lee, Chi-Cheng; Chen, Xiaoqian M.; Gan, Yu; Yeh, Chen-Lin; Hsueh, H. C.; Abbamonte, Peter; Ku, Wei

    2012-01-01

    We propose a simple first-principles method to describe propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energy. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is comput...

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

    International Nuclear Information System (INIS)

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

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

    OpenAIRE

    D Innocenzo, V.; Grancini, G.; Alcocer, Mj; Kandada, Ar; Stranks, Sd; Lee, Mm; Lanzani, G.; Snaith, Hj; Petrozza, A.

    2014-01-01

    Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estima...

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

    Science.gov (United States)

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

    2014-12-01

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

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

    International Nuclear Information System (INIS)

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

  12. Temperature Behavior of Exciton Absorption Bands in PbI2 Layer Crystals

    Directory of Open Access Journals (Sweden)

    V. Kramar

    2002-06-01

    Full Text Available The influence of bending waves on the warm-up behavior of exciton absorption bands in layer crystals has been investigated. The effective mass of the current carriers in the layer semiconductor PbJ2 has been computed and used to obtain the values of the exciton-phonon interaction function by pseudopotential method energy spectra calculations. It was shown that the different signs of the warm-up dynamics of an exciton absorption peak shift and existence of inversion points is related with the concurrent influence of two exciton energy relaxation mechanisms – on both the bending waves and the lattice phonons.

  13. First-Principles Method of Propagation of Tightly Bound Excitons: Verifying the Exciton Band Structure of LiF with Inelastic x-Ray Scattering

    Science.gov (United States)

    Lee, Chi-Cheng; Chen, Xiaoqian M.; Gan, Yu; Yeh, Chen-Lin; Hsueh, H. C.; Abbamonte, Peter; Ku, Wei

    2013-10-01

    We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures.

  14. First-principles method of propagation of tightly bound excitons: verifying the exciton band structure of LiF with inelastic x-ray scattering.

    Science.gov (United States)

    Lee, Chi-Cheng; Chen, Xiaoqian M; Gan, Yu; Yeh, Chen-Lin; Hsueh, H C; Abbamonte, Peter; Ku, Wei

    2013-10-11

    We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures. PMID:24160627

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

    Science.gov (United States)

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

    2014-01-01

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

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

    OpenAIRE

    Kunes, Jan

    2014-01-01

    Using the dynamical mean-field approximation we investigate formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first-order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a mul...

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

    OpenAIRE

    Falvo, Cyril; Palmieri, Benoit; Mukamel, Shaul

    2009-01-01

    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-q...

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

    Science.gov (United States)

    Egorov, Vladimir V.

    2014-07-01

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

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

    International Nuclear Information System (INIS)

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-07-15

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

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

    Science.gov (United States)

    Karlický, František; Zbo?il, Radek; Otyepka, Michal

    2012-07-01

    Density functional theory calculations of the electronic structure of graphane and stoichiometrically halogenated graphene derivatives (fluorographene and other analogous graphene halides) show: (i) localized orbital basis sets can be successfully and effectively used for such two-dimensional materials; (ii) several functionals predict that the band gap of graphane is greater than that of fluorographene, whereas HSE06 gives the opposite trend; (iii) HSE06 functional predicts quite good values of band gaps with respect to benchmark theoretical and experimental data; (iv) the zero band gap of graphene is opened by hydrogenation and halogenation and strongly depends on the chemical composition of mixed graphene halides; (v) the stability of graphene halides decreases sharply with increasing size of the halogen atom - fluorographene is stable, whereas graphene iodide spontaneously decomposes. In terms of band gap and stability, the C2FBr and C2HBr derivatives seem to be promising materials, e.g., for (opto)electronics applications, because their band gaps are similar to those of conventional semiconductors, and they are expected to be stable under ambient conditions. The results indicate that other fluorinated compounds (CaHbFc and CaFbYc, Y = Cl, Br, I) are stable insulators.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2011-08-31

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

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

    International Nuclear Information System (INIS)

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

  4. Exciton spectra and energy band structure of Cu2ZnSiSe4

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

    2015-01-01

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

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

    Science.gov (United States)

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

    2014-03-01

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

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

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

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

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

    OpenAIRE

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

    2014-01-01

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

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

    Directory of Open Access Journals (Sweden)

    Simon A. Bretschneider

    2014-04-01

    Full Text Available The field of thin-film photovoltaics has been recently enriched by the introduction of lead halide perovskites as absorber materials, which allow low-cost synthesis of solar cells with efficiencies exceeding 16%. The exact impact of the perovskite crystal structure and composition on the optoelectronic properties of the material are not fully understood. Our progress report highlights the knowledge gained about lead halide perovskites with a focus on physical and optoelectronic properties. We discuss the crystal and band structure of perovskite materials currently implemented in solar cells and the impact of the crystal properties on ferroelectricity, ambipolarity, and the properties of excitons.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-02-25

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

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

    Science.gov (United States)

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

    2014-12-24

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

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

    Science.gov (United States)

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

    2014-03-12

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

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

    International Nuclear Information System (INIS)

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

  15. Shallow electron traps in alkali halide crystals: Mollwo-Ivey relations of the optical absorption bands

    Science.gov (United States)

    Ziraps, Valters

    2001-03-01

    Evidences are given that two classes of the transient IR- absorption bands: (a) with max. at 0.27-0.36 eV in NaCl, KCl, KBr, KI and RbCl (due to shallow electron traps according G. Jacobs or due to bound polarons according E.V. Korovkin and T.A. Lebedkina) and (b) with max. at 0.15-0.36 eV in NaI, NaBr, NaCl:I, KCl:I, RbCl:I and RbBr:I (due to on-center STE localized at iodine dimer according M. Hirai and collaborators) are caused by the same defect- atomic alkali impurity center [M+]c0e- (electron e- trapped by a substitutional smaller size alkali cation impurity [M+]c0). The Mollwo-Ivey plots (for the transient IR-absorption bands) of the zero-phonon line energy E0 (for NaCl, KCl, KBr, RbCl and NaBr, KCl:I) and/or the low-energy edge valued E0 (for NaI, RbCl:I, RbBr:I) versus anion-cation distance (d) evidence that two types of the [M+]c0e- centers are predominant: (a) [Na+]c0e- in the KX and RbX host crystals with the relation E0approximately equals 6.15/d2.74, (b) [Li+]c03- in the NaX host crystals - E0approximately equals 29.4/d4.72. The Mollwo-Ivey relation E0approximately equals 18.36/d(superscript 2.70 is fulfilled as well for the F' band in NaCl, KCl, KBr, KI, RbCl, RbI if we use the F' center optical binding energy values E0.

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

    International Nuclear Information System (INIS)

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

  17. Exciton Bose condensation: the ground state of an electron-hole gas. 2. Spin states, screening and band structure effects

    International Nuclear Information System (INIS)

    We first generalize the approach of the previous paper by including spin degrees of freedom. We classify the various spin states and we discuss the effect of interband exchange interactions. We then introduce screening, in the framework of a generalized RPA which incorporates Bose condensation of bound electron-hole pairs. We discuss in detail the low density limit: screening corrections do not change the sign of the compressibility, which remains positive, in contrast to previous estimates. We show that such RPA corrections reduce to an approximate form of the Van der Waals attraction between excitons. Viewing this RPA approach as an interpolation procedure at intermediate densities, we propose several interpolation schemes that should account for the Mott transition, and we give some preliminary very rough numerical estimates. Finally, we discuss the effect of band degeneracy on the ground state: different degeneracies in the two bands should lead to a normal plasma at high density while at low densities bound excitons 'Bose condense'', with a breakdown of their internal symmetry; we expect a first order transition with a liquid-gas phase separation

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

    Czech Academy of Sciences Publication Activity Database

    Kuneš, Jan; Augustinský, Pavel

    2014-01-01

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

  19. Observation of excited-state excitons and band-gap renormalization in hole-doped carbon nanotubes using photoluminescence excitation spectroscopy

    Science.gov (United States)

    Kimoto, Yoshio; Okano, Makoto; Kanemitsu, Yoshihiko

    2013-05-01

    The higher Rydberg states of the E11 exciton in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) were studied using one- and two-photon photoluminescence excitation spectroscopy. Increasing the hole-dopant concentration resulted in a redshift of the first excited state (2g) and a blueshift of the ground state (1u) of the E11 exciton. From the redshift of higher Rydberg states, we found that a reduction of the band-gap energy occurs in hole-doped SWCNTs. These findings show that the exciton and electronic band structures are modified by the presence of a small number of holes in the SWCNT one-dimensional structures.

  20. Muon radiolysis in alkali halides

    International Nuclear Information System (INIS)

    In order to shed new light on the initial loss of muon spin polarization, or so-called ''missing fraction'', which is commonly observed in non-metallic solids, we have studied muon-induced excitation in various alkali halides by measuring the luminescences associated with the radiative decay of the self-trapped excitons (STE). The result strongly suggests that the spin-exchange interaction between muonium and muon radiolysis products including STE's causes fast muon depolarization in those materials. (orig.)

  1. Zn(S, Se)-based quantum wells : band offsets, excitons and optical properties

    OpenAIRE

    Martino, G.; Girlanda, R.; Tomassini, N.; D Andrea, A.; Atanasov, R.

    1993-01-01

    The Wannier exciton properties in a single quantum well (SQW) of Zn(S,Se)/ZnS strained materials are reproduced by an accurate variational envelope function. A systematic study of binding energy and oscillator strength is performed as a function of well thicknesses and strain parameter values. Optical response and second harmonic generation effects (SHG) in asymmetric single quantum wells (ASQWs) is computed and briefly discussed.

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

    International Nuclear Information System (INIS)

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-10-15

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-11-15

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

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

    International Nuclear Information System (INIS)

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

  6. Excitons in Mott insulators

    OpenAIRE

    Wrobel, P.; Eder, R.

    2001-01-01

    Motivated by recent Raman and resonant inelastic X-ray scattering experiments performed for Mott insulators, which suggest formation of excitons in these systems, we present a theory of exciton formation in the upper Hubbard band. The analysis based on the spin polaron approach is performed in the framework of an effective t-J model for the subspace of states with one doubly occupied site. Our results confirm the existence of excitons and bear qualitative resemblance to expe...

  7. Proposal for observation of band-edge effect and entanglement generation of quantum dot excitons coupled to nanowire surface plasmons

    CERN Document Server

    Chen, G Y; Chuu, D S

    2008-01-01

    The radiative decay of quantum dot (QD) excitons into surface plasmons in a cylindrical nanowire is investigated theoretically. Maxwell's equations with appropriate boundary conditions are solved numerically to obtain the dispersion relations of surface plasmons. The radiative decay rate of QD excitons is found to be greatly enhanced at certain values of the exciton bandgap. Such an enhanced phenomenon is due to the zero-slope of dispersion relations and indicates that the radiative dynamics of the QD excitons there should be non-Markovian. Besides, due to the one dimensional propagating feature of nanowire surface plasmons, remote entangled states can be generated via super-radiance and may be useful in future quantum information processing.

  8. Exciton-exciton interaction in CdS

    International Nuclear Information System (INIS)

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

  9. Temperature dependence of energies and broadening parameters of the band-edge excitons of Mo{sub 1-x}W{sub x}S{sub 2} single crystals

    Energy Technology Data Exchange (ETDEWEB)

    Ho, C.H.; Wu, C.S.; Huang, Y.S. [Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan (China); Liao, P.C. [Department of Electronic Engineering, Kuang Wu Institute of Technology and Commerce, Peitou, Taipei 112, Taiwan (China); Tiong, K.K. [Department of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan (China)

    1998-10-19

    We have measured the temperature dependence of the spectral features in the vicinity of direct band-edge excitonic transitions of Mo{sub 1-x}W{sub x}S{sub 2} single crystals from 25 to 300 K using piezoreflectance (PzR). From a detailed lineshape fit of the PzR spectra, the energies and broadening parameters of the A and B excitons have been determined accurately. The origin of these excitonic transitions is discussed. The transition energies and their splittings vary smoothly with the tungsten composition x, indicating that the natures of the direct band edges are similar for the Mo{sub 1-x}W{sub x}S{sub 2} compounds. In addition, the parameters that describe the temperature variation of the energies and broadening function of the excitonic transitions are evaluated and discussed. (author)

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

    International Nuclear Information System (INIS)

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

  11. Manifestations of charged lattice defects in excitonic luminescence and control over properties of wide-band scintillation crystals

    International Nuclear Information System (INIS)

    A fast energy transfer from the host crystal to impurity centres and charged lattice defects (e.g., vacancies in an ionic crystal) is due to the two-site exciton motion on high vibrational levels. At low temperatures this motion occurs in a coherent directional way with the sound velocity whereas a small separation between vibrational levels provides an easy exciton trapping by shallow potential wells near impurity centres. Charged defects have a much larger trapping section than isovalent impurity centres and manifest themselves in luminescence at a concentration smaller by two orders of magnitude

  12. Exciton linewidth due to fracton-exciton interaction

    Science.gov (United States)

    Wang, Xiao-Bing; Jiang, Qing; Tian, De-Cheng

    1994-08-01

    The contribution of the fracton-exciton interaction to the exciton linewidth is calculated. It is found that the linewidth is significantly dependent on the fractal dimension and the absorption process of a fracton by an exciton at the bottom of the band is forbidden for some dimensions. In the high temperature limit, the linewidth increases linearly with temperature. Possible relevance of the calculation to experiments is also discussed.

  13. A variational Monte Carlo study of exciton condensation

    Science.gov (United States)

    Watanabe, Hiroshi; Seki, Kazuhiro; Yunoki, Seiji

    2015-03-01

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

  14. Exciton condensations in thin film topological insulator

    OpenAIRE

    Moon, Eun Gook; Xu, Cenke

    2010-01-01

    We study the many-body physics in thin film topological band insulator, where the inter-edge Coulomb interaction can lead to an exciton condensation transition. We investigate the universality class of the exciton condensation quantum critical point. With different chemical potentials and interactions, the exciton condensation can belong to z = 2 mean field, or 3d XY, or Yukawa-Higgs universality classes. The interplay between exciton condensate and the time-reversal symmetr...

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

    International Nuclear Information System (INIS)

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

  16. Exciton characteristics and exciton luminescence of silicon quantum dot structures

    International Nuclear Information System (INIS)

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

  17. Exciton dispersion from first principles

    OpenAIRE

    Matteo Gatti; Francesco Sottile

    2013-01-01

    We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar “exciton revi...

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

    OpenAIRE

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

    2012-01-01

    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.

  19. Photofragmentation of metal halides

    International Nuclear Information System (INIS)

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

  20. Photochemistry of Phenyl Halides

    OpenAIRE

    Karlsson, Daniel

    2008-01-01

    We have studied fundamental aspects of photo-induced dissociation kinetics and dynamics in several phenyl halides. By combining femtosecond pump-probe measurements with ab initio calculations we are able to account for several observations. In mixed phenyl halides, the dissociation kinetics is found to be dependent on the nature, the number, and the position of the substituents, and also on the excitation wavelength. A surprisingly large reduction in the dissociation time constant, compared t...

  1. Crown ether uranyl halide complexes

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

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

  3. Charge-transfer excitons in strongly coupled organic semiconductors

    OpenAIRE

    Glowe, Jean-Francois; Perrin, Mathieu; Beljonne, David; Hayes, Sophia C.; Gardebien, Fabrice; Silva, Carlos

    2009-01-01

    Time-resolved and temperature-dependent photoluminescence measurements on one-dimensional sexithiophene lattices reveal intrinsic branching of photoexcitations to two distinct species: self-trapped excitons and dark charge-transfer excitons (CTX; > 5% yield), with radii spanning 2-3 sites. The significant CTX yield results from the strong charge-transfer character of the Frenkel exciton band due to the large free exciton bandwidth (~400 meV) in these supramolecular nanostruc...

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

    International Nuclear Information System (INIS)

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

  5. Optically Controlled Excitonic Transistor

    OpenAIRE

    Andreakou, P.; Poltavtsev, S. V.; Leonard, J. R.; Calman, E. V.; Remeika, M.; Kuznetsova, Y. Y.; Butov, L. V.; Wilkes, J; Hanson, M.; Gossard, A. C.

    2013-01-01

    Optical control of exciton fluxes is realized for indirect excitons in a crossed-ramp excitonic device. The device demonstrates experimental proof of principle for all-optical excitonic transistors with a high ratio between the excitonic signal at the optical drain and the excitonic signal due to the optical gate. The device also demonstrates experimental proof of principle for all-optical excitonic routers.

  6. Some uranium halide complexes

    International Nuclear Information System (INIS)

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

  7. Excitons in narrow-gap carbon nanotubes

    OpenAIRE

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

    2010-01-01

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

  8. Optical second harmonic generation from Wannier excitons

    DEFF Research Database (Denmark)

    Pedersen, Thomas Garm; Cornean, Horia

    2007-01-01

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

  9. Spatially indirect excitons in coupled quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Lai, Chih-Wei Eddy

    2004-03-01

    Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

  10. Pulsed copper halide vapor lasers

    Science.gov (United States)

    Kazarian, M. A.; Petrash, G. G.; Trofimov, A. N.

    The theory and the performance characteristics of copper halide lasers are examined with reference to recent theoretical and experimental research in the field of metal vapor lasers. The discussion covers gas discharge tubes, comparative characteristics of copper halide lasers, determination of the metastable level decomposition rates in copper chloride and copper bromide lasers, and spectroscopic measurements of the discharge plasma in copper halide lasers. Attention is also given to the kinetics of copper atoms formation in copper halide lasers and calculation of the kinetic characteristics of copper halide lasers.

  11. Exciton spectra of anthracene-cadmium sulphide system

    International Nuclear Information System (INIS)

    The exciton behaviour at the boundary of CdS and anthracene are experimentally studied at low temperatures (1.7-70 K). It is shown that the presence of contact of two crystals leads to changes in reflection and luminescence spectra both the Vanje-Mott excitons and Frenkel excitons. Accumulation of the Frenkel excitons at the interface and exciton luminescence flare-up of anthracene in the 1.7-30 K temperature range, and for the Vanje-Mott excitons essential is development of the ''dead'' layer due to contact of the crystals. In the CdS luminescence spectrum ionized donor levels can be seen, which are exciton traps, and a new reflection band of anthracene in A-polarization, caused by its interaction with inorganic semiconductor. The role of the interface of two crystals, possible deformations due to different coefficients of linear expansion in the exciton behaviour is discussed

  12. Bright and dark excitons in semiconductor carbon nanotubes

    International Nuclear Information System (INIS)

    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 nanotubescitonic 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.

  13. Band gap bowing and exciton localization in strained cubic InxGa1-xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

    Science.gov (United States)

    Chichibu, S. F.; Sugiyama, M.; Kuroda, T.; Tackeuchi, A.; Kitamura, T.; Nakanishi, H.; Sota, T.; DenBaars, S. P.; Nakamura, S.; Ishida, Y.; Okumura, H.

    2001-11-01

    Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1-xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1-xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1-xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material's inhomogeneity is much enhanced compared to that of the hexagonal polytype.

  14. Effect of fractons on the exciton dynamics in dilute magnets

    Science.gov (United States)

    Wang, Xiao-Bing; Tian, De-Cheng; Jiang, Qing; Zhang, Zhe-Hua

    1995-02-01

    The effect of fractons on the exciton dynamics in dilute magnets is investigated theoretically. It is predicted that the total intensity of the hot exciton-fracton absorption band varies with temperature as Td¯+2d¯/D, where d¯ is the fracton dimension and D the fractal dimension. Also, the fracton-exciton interaction results in a quadratic temperature dependence of the exciton intersublattice relaxation rate in dilute antiferromagnets, substantially different from that (cubic temperature dependence) of magnon-exciton scattering. Implication of the results for experimental study of the magnon-fracton crossover in dilute magnets is demonstrated.

  15. Exciton dispersion in molecular solids

    Science.gov (United States)

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

    2015-03-01

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

  16. Excitonic ferromagnetism in the hexaborides

    Science.gov (United States)

    Zhitomirsky, Michael

    2000-03-01

    The unusual high-temperature weak ferromagnetism in doped hexaborides is explained as a spontaneous spin polarization in a novel ground state of a low-density electron-hole plasma -- a triplet excitonic insulator.footnote M. E. Zhitomirsky, T. M. Rice, and V. I. Anisimov, Nature 402, 251 (1999). The excitonic insulating state is a condensate of bound electron-hole pairs formed in a narrow-gap semiconductor or a weak semimetal. Homogeneous excitonic insulator polarizes under doping because of a coherent interaction of doped electrons with the condensate. The origin of small ferromagnetic moments is attributed to a time-reversal symmetry breaking transition into a nonunitary triplet state. Two mechanisms which can lead to the broken time-reversal symmetry and to ferromagnetism even in undoped excitonic insulator are also proposed. A possibility of gas-liquid type phase transition between a dilute excitonic gas and a dense electron-hole plasma for varying bandgap (band overlap) is investigated.footnote M. E. Zhitomirsky and T. M. Rice,

  17. Intermediate Exciton-Phonon Coupling in Tetracene

    Science.gov (United States)

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

    1984-08-01

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

  18. Exciton Regeneration at Polymeric Semiconductor Heterojunctions

    CERN Document Server

    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

    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.

  19. Polarizable Optical Bistability of Frenkel Excitons

    Directory of Open Access Journals (Sweden)

    O. Derevyanchuk

    2003-06-01

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

  20. Anisotropic exciton Stark shift in black phosphorus

    Science.gov (United States)

    Chaves, A.; Low, Tony; Avouris, P.; ?ak?r, D.; Peeters, F. M.

    2015-04-01

    We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction-dependent field-induced polarizability of the exciton.

  1. Studies of rare gas halide lasers

    OpenAIRE

    Hogan, Daniel Christopher.; Webb, Colin E.; Dr. C. E. Webb

    1983-01-01

    This thesis presents the results of a study of the mechanisms responsible for limiting the laser pulse duration obtainable in xenon chloride lasers which are excited by UV-preionized, self-sustained gas discharges. The xenon chloride laser system, the principal emission band of which is centred around 308 nm, belongs to the class of high pressure gas lasers known as 'rare-gas halides'(RGH). RGH lasers are now well known for their high peak power output at a number of wavelen...

  2. 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

  3. Elastic exciton-exciton scattering in photoexcited carbon nanotubes

    OpenAIRE

    Nguyen, D.T.; Voisin, C; Roussignol, Ph.; Roquelet, C.; Lauret, Jean-Sébastien; Cassabois, Guillaume

    2011-01-01

    We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behav...

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

    Scientific Electronic Library Online (English)

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

    2006-04-01

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

  5. Excitonic Contribution on Light Emitting Properties of Nano silicon

    International Nuclear Information System (INIS)

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

  6. Interaction of Dirac Fermion excitons and biexciton-exciton cascade in graphene quantum dots

    Science.gov (United States)

    Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

    2015-03-01

    We present a microscopic theory of interacting Dirac quasi-electrons and quasi-holes confined in graphene quantum dots. The single particle states of quantum dots are described using a tight binding model and screened direct, exchange, and scattering Coulomb matrix elements are computed using Slater pz orbitals. The many-body ground and excited states are expanded in a finite number of electron-hole pair excitations from the Hartree-Fock ground state and computed using exact diagonalization techniques. The resulting exciton and bi-exciton spectrum reflects the degeneracy of the top of the valence and bottom of the conduction band characteristic of graphene quantum dots with C3 symmetry. We study the interaction of multi-electron and hole complexes as a function of quantum dot size, shape and strength of Coulomb interactions. We identify two degenerate bright exciton (X) states and a corresponding biexciton (XX) state as XX-X cascade candidates, a source of entangled photon pairs. We next calculate the exciton to bi-exciton transitions detected in transient absorption experiments to extract the strength of exciton-exciton interactions and biexciton binding energies. We further explore the possibility of excitonic instability.

  7. Atomic lattice excitons: from condensates to crystals

    International Nuclear Information System (INIS)

    We discuss atomic lattice excitons (ALEs), bound particle-hole pairs formed by fermionic atoms in two bands of an optical lattice. Such a system provides a clean set-up, with tunable masses and interactions, to study fundamental properties of excitons including exciton condensation. We also find that for a large effective mass ratio between particles and holes, effective long-range interactions can mediate the formation of an exciton crystal, for which superfluidity is suppressed. Using a combination of mean-field treatments, bosonized theory based on a Born-Oppenheimer approximation, and one-dimensional (1D) numerical computation, we discuss the properties of ALEs under varying conditions, and discuss in particular their preparation and measurement

  8. "Dilute" excitons in a double layer system: single-exciton and mean-field approach

    OpenAIRE

    Jamell, Christopher R.; ZHANG, CHANG-HUA; Joglekar, Yogesh N.

    2009-01-01

    Double layer systems where one layer has electrons and the holes are in a parallel layer a distance d away are expected to undergo excitonic condensation at low temperature. This excitonic condensate is traditionally described by a many-body wavefunction that encodes the coherence between electron and hole bands. Here we compare the mean-field ground state in the limit of dilute electron (hole) density with the ground state of a single electron-hole pair in double-layer syst...

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

    Science.gov (United States)

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

    2015-03-01

    Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.

  10. Many body exciton due to Fano resonance in graphene

    Science.gov (United States)

    Yadav, Premlata; Ghosh, Subhasis

    2015-06-01

    The excitonic effect is thought to be generally unimportant in zero gap systems (at K point of the Brillouin zone) like monolayer graphene, but excitonic transition in graphene at the saddle point (M) of Brillouin zone has received increasing attentions. There are two important issues with excitons in graphene. Firstly: in contrast to excitonic transitions in semiconductors the line shape of excitonic peaks in graphene is asymmetric which is due to Fano resonance, a many body coupling between discrete excitonic state and continuous band states, Secondly due to many body effects the excitonic peak is sensitive to dielectric environment. Hence it is desirable to vary the dielectric environment of graphene without varying carrier concentration. To investigate this completely new method for obtaining graphene monolayer has been developed using chemical exfoliation technique. We show that there is shift in excitonic peak position with change in the dielectric environment of graphene and this has been achieved by varying Fermi velocity without varying the carrier concentration. The observed distinctive effect is decrease in exciton binding energy with increase in dielectric value of exfoliating solvents, resulting into a scaling relation between the dielectric environment and the exciton binding energy of graphene.

  11. Excitonic polarons in low-dimensional transition metal dichalcogenides

    Science.gov (United States)

    Thilagam, A.

    2015-05-01

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

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

    OpenAIRE

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

    2008-01-01

    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 bands within 5kBT of the Fermi energy. These prerequisites allow ARPES to observe energy dispersion of bands above EF without normalization procedures and a leading edge of the Fermi-Dirac distribution cutoff, which...

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

    Science.gov (United States)

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

    2015-04-14

    The last two years have seen the unprecedentedly rapid emergence of a new class of solar cells, based on hybrid organic-inorganic halide perovskites. The success of this class of materials is due to their outstanding photoelectrochemical properties coupled to their low cost, mainly solution-based, fabrication techniques. Solution processed materials are however often characterized by an inherent flexible structure, which is hardly mapped into a single local minimum energy structure. In this perspective, we report on the interplay between structural and electronic properties of hybrid lead iodide perovskites investigated using ab initio molecular dynamics (AIMD) simulations, which allow the dynamical simulation of disordered systems at finite temperature. We compare the prototypical MAPbI3 (MA = methylammonium) perovskite in its cubic and tetragonal structure with the trigonal phase of FAPbI3 (FA = formamidinium), investigating different starting arrangements of the organic cations. Despite the relatively short time scale amenable to AIMD, typically a few tens of ps, this analysis demonstrates the sizable structural flexibility of this class of materials, showing that the instantaneous structure could significantly differ from the time and thermal averaged structure. We also highlight the importance of the organic-inorganic interactions in determining the fluxional properties of this class of materials. A peculiar spatial localization of the valence and conduction band edges is also found, with a dynamics in the range of 0.1 ps, which is associated with the positional dynamics of the organic cations within the cubo-octahedral perovskite cage. This asymmetry in the spatial localization of the band edges is expected to ease exciton dissociation and assist the initial stages of charge separation, possibly constituting one of the key factors for the impressive photovoltaic performances of hybrid lead-iodide perovskites. PMID:25766785

  14. Superfluid phase transition in two-dimensional excitonic systems

    International Nuclear Information System (INIS)

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

  15. Photoluminescence due to exciton-exciton scattering in a GaAs/AlAs multiple quantum well

    International Nuclear Information System (INIS)

    We have investigated photoluminescence (PL) properties of a GaAs (20 nm)/AlAs (20 nm) multiple quantum well at 10 K under intense excitation conditions. It has been found that a PL band due to exciton-exciton scattering, the so-called P emission, is observed in addition to the biexciton PL under an excitation energy higher than the fundamental heavy-hole exciton by the energy of the longitudinal optical phonon. On the other hand, the P band could never be observed at an excitation energy much higher than the exciton energy, where a band-filling phenomenon appears in the PL spectrum. Furthermore, we confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band using a variable-stripe-length method

  16. Elastic exciton-exciton scattering in photoexcited carbon nanotubes.

    Science.gov (United States)

    Nguyen, D T; Voisin, C; Roussignol, Ph; Roquelet, C; Lauret, J S; Cassabois, G

    2011-09-16

    We report on original nonlinear spectral hole-burning experiments in single wall carbon nanotubes that bring evidence of pure dephasing induced by exciton-exciton scattering. We show that the collision-induced broadening in carbon nanotubes is controlled by exciton-exciton scattering as for Wannier excitons in inorganic semiconductors, while the population relaxation is driven by exciton-exciton annihilation as for Frenkel excitons in organic materials. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. PMID:22026798

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2011-06-15

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

  18. Defect Induced Photoluminescence from Dark Excitonic States in Individual Single-Walled Carbon Nanotubes

    OpenAIRE

    Harutyunyan, Hayk; Gokus, Tobias; Green, Alexander A.; Hersam, Mark C.; Allegrini, Maria; Hartschuh, Achim

    2008-01-01

    We show that new low-energy photoluminescence (PL) bands can be created in the spectra of semiconducting single-walled carbon nanotubes by intense pulsed excitation. The new bands are attributed to PL from different nominally dark excitons that are “brightened” because of a defect-induced mixing of states with different parity and/or spin. Time-resolved PL studies on single nanotubes reveal a significant reduction of the bright exciton lifetime upon brightening of the dark excitons. The l...

  19. Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.

    Science.gov (United States)

    Kanemitsu, Yoshihiko

    2013-06-18

    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

  20. Preparation of cerium halide solvate complexes

    Science.gov (United States)

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

    2013-08-06

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

  1. Exciton dynamics in organic molecular crystals and nanostructures

    Science.gov (United States)

    Bardeen, Chris

    2014-03-01

    The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.

  2. Isotopic mixed crystal exciton spectra in the far infrared: Naphthalene

    International Nuclear Information System (INIS)

    The far infrared vibrational exciton spectra of isotopic mixed crystals of naphthalene-h8 and d8 were studied. The two observed translational phonon modes were determined to fall into the amalgamated band limit while the lowest energy B3sub(u), Asub(u) and B1sub(u) vibrational exciton bands were found to be in the separated band limit. The lowest energy B3sub(u) butterfly mode with its large (15 cm-1) exciton splitting was found to agree well with CPA calculations of mixed crystal spectra. A peak at 185 cm-1 was also assigned as a peak in the vibrational exciton density-of-states of the B3sub(u) mode. (orig.)

  3. Exciton condensation in quantum wells. Excitonic hydrodynamics. Autosolitons

    OpenAIRE

    Sugakov, V. I.

    2013-01-01

    The hydrodynamics equations for indirect excitons in the double quantum wells are obtained taking into account 1) a possibility of an exciton condensed phase formation, 2) the presence of a pumping, 3) the finite value of the exciton lifetime, 4) the exciton scattering by defects.New types of solutions in the form of bright and dark autsolitons are obtained for the exciton density. The role of localized and free exciton states is analyzed in a formation of the emission spectra.

  4. Electronic structure and core exciton of hexagonal boron nitride

    Science.gov (United States)

    Robertson, John

    1984-02-01

    A new tight-binding band calculation is presented for the layer compound boron nitride which correctly describes the widths of both ? and ? bands. The binding energy and wave function of the B 1s-->2p core exciton are studied. The core exciton is found to be small and bound by the medium-range part of the core-hole potential. Its binding energy far exceeds the interlayer band dispersion at the conduction-band minimum. Its incomplete ?-like polarization is found to be due to a small admixture of B ? states from adjacent layers into the A-1 final state.

  5. Excitonic dynamical Franz-Keldysh effect.

    DEFF Research Database (Denmark)

    NordstrØm, K.B.; Johnsen, Kristinn

    1998-01-01

    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.

  6. Diamagnetic excitons and exciton magnetopolaritons in semiconductors

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

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

  8. Excitons in cuprous oxide

    OpenAIRE

    Fishman, Dmitry,

    2008-01-01

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

  9. Electrostatic Conveyer for Excitons

    OpenAIRE

    Winbow, A. G.; Leonard, J. R.; Remeika, M.; Kuznetsova, Y. Y.; High, A. A.; Hammack, A. T.; Butov, L. V.; Wilkes, J; Guenther, A. A.; Ivanov, A. L.; Hanson, M.; Gossard, A. C.

    2011-01-01

    We report on the study of indirect excitons in moving lattices - conveyers created by a set of AC voltages applied to the electrodes on the sample surface. The wavelength of this moving lattice is set by the electrode periodicity, the amplitude is controlled by the applied voltage, and the velocity is controlled by the AC frequency. We observed the dynamical localization-delocalization transition for excitons in the conveyers and measured its dependence on the exciton densit...

  10. Exciton laser rate equations

    Directory of Open Access Journals (Sweden)

    Garkavenko A. S.

    2011-08-01

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

  11. Secondary emission of indirect exciton in BiI3

    International Nuclear Information System (INIS)

    Resonant secondary emissions of BiI3 crystals in indirect exciton edge are investigated at liquid helium temperature by using a tunable dye laser. The emission spectra depend critically on the excitation energy, and consist of 1st-, 2nd- and 3rd-order resonant Raman lines and hot-luminescence bands of LO phonon assisted exciton recombinations. From an analysis of Raman lines, the associated phonon energies and the exciton effective mass are estimated. Preliminary discussion on the hot-luminescence is also given. (orig.)

  12. Huge excitonic effects in layered hexagonal boron nitride

    CERN Document Server

    Arnaud, B; Rabiller, P; Alouani, M

    2005-01-01

    The calculated quasiparticle band structure of bulk hexagonal boron nitride using the all-electron GW approximation shows that this compound is an indirect-band-gap semiconductor. The solution of the Bethe-Salpeter equation for the electron-hole two-particle Green function has been used to compute its optical spectra and the results are found in excellent agreement with available experimental data. A detailed analysis is made for the excitonic structures within the band gap and found that the 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 {\\it et al} using a Wannier model to interpret their experimental results and assuming that h-BN is a direct-band-gap semiconductor.

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

    CERN Document Server

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

    2014-01-01

    Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an important two-dimensional semiconductor with promising potentials for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDC has a sizable band gap. More interestingly, when thinned down to a monolayer, TMDC transforms from an indirect bandgap to a direct bandgap semiconductor, exhibiting a number of intriguing optical phenomena such as valley selective circular dichroism, doping dependent charged excitons, and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. The observed optical resonance was initially considered to be band-to-band transitions. In contrast, first-principle calculations predicted a much larger quasiparticle band gap size and an optical response that is dominated by excitonic effects. Here, we report experimental evidence of the exciton dominance mechanism by discovering a series of exc...

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-10-01

    The goal of this project was to synthesize a volatile form of Np for introduction into mass spectrometers at INL. Volatile solids of the 5f elements are typically those of the halides (e.g. UF6), however fluorine is highly corrosive to the sensitive internal components of the mass separator, and the other volatile halides exist as several different stable isotopes in nature. However, iodide is both mono-isotopic and volatile, and as such presents an avenue for creation of a form of Np suitable for introduction into the mass separator. To accomplish this goal, the technical work in the project sought to establish a novel synthetic route for the conversion NpO2+ (dissolved in nitric acid) to NpI3 and NpI4.

  15. Excitonic effects in oxyhalide scintillating host compounds

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-10-07

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

  16. Absorption lineshape of FA centers in alkali halides

    International Nuclear Information System (INIS)

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

  17. Spatial confinement, self-polarization and exciton-phonon interaction effect on the location of exciton line in lead iodide nanofilms

    Science.gov (United States)

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

    2014-08-01

    Theoretical investigation of the spatial confinement, self-polarization and exciton-phonon interaction influence on the exciton state in plane double nanoheterostructure (nanofilm)-lead iodide in polymeric matrix is performed within the effective mass approximation for the electron and dielectric continuum for the phonons in the framework of infinitely deep single quantum well. It is shown that spatial confinement is the dominating feature determining the energy of the bottom of exciton ground band and its binding energy. The relationship of two others depends on nanofilm thickness: in ultrathin films the influence of self-polarization effect is essentially bigger than the role of exciton-phonon interaction.

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

    OpenAIRE

    Thompson, Nicholas J.; Goldberg, David; Menon, Vinod M.; Baldo, Marc A.; Congreve, Daniel Norbert

    2013-01-01

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

  19. EXCITON SCATTERING ON SPIN WAVES IN QUASI-ONE-DIMENSIONAL ANTIFERROMAGNET

    OpenAIRE

    Eremenko, V.; Shapiro, V.; Kachur, I.

    1988-01-01

    Exciton absorption bandwidths have been measured in a quasi-one-dimensional antiferromagnet CsMnCl3.2 H2O and a three-dimensional MnF2 in magnetic fields oriented along crystal “easy” axes. In one-dimensional antiferromagnets exciton-magnon interaction is shown to result in a considerable broadening of exciton bands in the vicinity of the critical field of the spin-flop transition.

  20. Multiple Exciton Generation in Colloidal Nanocrystals

    Directory of Open Access Journals (Sweden)

    Charles Smith

    2013-12-01

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

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

    Directory of Open Access Journals (Sweden)

    R. Rodr\\u00EDguez-Mij\\u00E1ngos

    2008-01-01

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

  2. Huge Excitonic Effects in Layered Hexagonal Boron Nitride

    Science.gov (United States)

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

    2006-01-01

    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).1476-112210.1038/nmat1134] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

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

    OpenAIRE

    Nanguneri, Ravindra; Parkhill, John

    2014-01-01

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

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

    Energy Technology Data Exchange (ETDEWEB)

    Protin, L.

    1994-10-05

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

  5. Curved nanowire structures and exciton binding energies

    International Nuclear Information System (INIS)

    Growth of quantum-confined semiconductor structures is a complicated process that may lead to imperfect and complex shapes as well as geometrical nonuniformities when comparing a large number of intended identical structures. On the other hand, the possibility of tuning the shape and size of nanostructures allows for extra optimization degrees when considering electronic and optical properties in various applications. This calls for a better understanding of size and shape effects. In the present work, we express the one-band Schroedinger equation in curved coordinates convenient for determining eigenstates of curved quantum-wire and quantum-dash structures with large aspect ratios. Firstly, we use this formulation to solve the problem of single-electron and single-hole states in curved nanowires. Secondly, exciton states for the curved quantum-wire Hamiltonian problem are found by expanding exciton eigenstates on a product of single-particle eigenstates. A simple result is found for the Coulomb matrix elements of an arbitrarily curved structure as long as the radius-of-curvature is much larger than the cross-sectional dimensions. We use this general result to compute the groundstate exciton binding energy of a bent nanowire as a function of the bending radius-of-curvature. It is demonstrated that the groundstate exciton binding energy increases by 40 meV as the radius-of-curvature changes from 20 to 2 nm while keeping the total length (and volume) of the nanowire conal length (and volume) of the nanowire constant.

  6. Anharmonic relaxation times of molecular vibrational modes in alkali halide crystals

    International Nuclear Information System (INIS)

    Low-temperature infrared-saturation and hole-burning measurements have been performed on an internal vibrational mode of a spherical-top molecule embedded in a variety of alkali halide crystals. The vibrational relaxation times T1 and T2 satisfy the relation T2approx. =2T1 but T1 (< or =38 nsec) is orders of magnitude smaller than expected for multiphonon decay. A study of the alkali halide dependence of these relaxation times suggests that a specific combination of internal, local, and band modes is involved in the decay process

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

    Science.gov (United States)

    Ellis, Jason; Wen, Xiaodong; Martin, Richard

    2014-03-01

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

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

    OpenAIRE

    Kiguchi, Manabu; Yoshikawa, Genki; Ikeda, Susumu; Saiki, Koichiro

    2005-01-01

    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...

  9. Excitonic condensation in spatially separated one-dimensional systems

    Science.gov (United States)

    Abergel, D. S. L.

    2015-05-01

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

  10. Failure of classical Fresnel relationships in the exciton absorption region

    International Nuclear Information System (INIS)

    The optical reflection spectra of CdS single crystals, determined at 4.20K in the region of the A exciton band, were compared with the spectrum calculated from the Fresnel formula. In these calculations use was made of the absorption coefficients eta (?) and refractive indices n(?) determined experimentally in refs. 1 and 2 for thin flake-like single crystals. The experimental and calculated curves differed in a fundamental way in a frequency range equal to the half-width of the exciton absorption band. This discrepancy was explained by applying the spatial dispersion theory

  11. Fundamental collapse of the exciton-exciton effective scattering

    OpenAIRE

    Pilozzi, Laura; Combescot, Monique; Betbeder-Matibet, Odile; D'Andrea, Andrea

    2010-01-01

    The exciton-exciton effective scattering which rules the time evolution of two excitons is studied as a function of initial momentum difference, scattering angle and electron-to-hole mass ratio. We show that this effective scattering can collapse for energy-conserving configurations provided that the difference between the two initial exciton momenta is larger than a threshold value. Sizeable scatterings then exist in the forward direction only. We even find that, for an ele...

  12. Exciton-exciton interaction and biexciton formation in bilayer systems

    OpenAIRE

    Lee, R. M.; Drummond, Neil; Needs, R. J.

    2008-01-01

    We report quantum Monte Carlo calculations of biexciton binding energies in ideal two-dimensional bilayer systems with isotropic electron and hole masses. We have also calculated exciton-exciton interaction potentials, and pair distribution functions for electrons and holes in bound biexcitons. Comparing our data with results obtained in a recent study using a model exciton-exciton potential [C. Schindler and R. Zimmermann, Phys. Rev. B \\textbf{78}, 045313 (2008)], we find a...

  13. The effects of polarization fields on exciton binding energy in GaN quantum dots

    International Nuclear Information System (INIS)

    In this paper the exciton binding energy in wurtzite GaN quantum dot is investigated by considering the influences of polarization in details. We find that the strong built-in electric field gives rise to an obvious modification of conduction band profile of QDs and leads to remarkable electron-hole spatial separation. This effect has a significant influence on exciton states and optical properties of the QDs. The relationship between exciton states and QD size and QD confinement potential is studied. Our results represent the decrease of exciton binding energy by considering piezoelectric polarization.

  14. Freeze Enhanced Halate Halide Reactions

    Science.gov (United States)

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

    2014-12-01

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

  15. Halide substitution in magnesium borohydride

    OpenAIRE

    Baricco, Marcello; DAMIN, Alessandro Ali; Zavorotynska, Olena; Corno, Marta

    2012-01-01

    The synthesis of halide-substituted Mg(BH4)2 by ball-milling, and characterization with respect to thermodynamics and crystal structure, has been addressed. The ball-milled mixture of Mg(BH4)2 and MgX2 (X = Cl, Br) has been investigated by in situ/ex situ synchrotron powder X-ray di?raction (SR-PXD), di?erential scanning calorimetry (DSC), and infrared and Raman spectroscopy. High resolution SR-PXD patterns reveal that the unit cell volume of ?-Mg(BH4)2 in milled and annealed mixtu...

  16. Exciton-exciton annihilation in organic polariton microcavities

    OpenAIRE

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

    2009-01-01

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

  17. Low-temperature magnetic circular dichroism spectra of thin layers of copper and silver halides

    International Nuclear Information System (INIS)

    The electronic and magnetic circular dichroism spectra of thin layers of Cu1 and Ag1 halides have beem measured at low ( about10 K) temperatures. The med spectra show clear A-terms which, in the case of the silver halides and for one band of cuprous iodide, have unexpected negative signs. A qualitative theoretical interpretation of these signs has been attempted by considering halogen-p and metals/d atomic orbital mixing within the framework of a simple, tight-binding model. Reasonable agreement of theory with experiment has been achieved for the positive copper halide A-terms, but it has not proved possible to obtain by calculation the experimentally observed negative A-term sign of the silver salts and cuprous iodide

  18. Engineering directed excitonic energy transfer

    OpenAIRE

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

    2010-01-01

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

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

    International Nuclear Information System (INIS)

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

  20. Exciton-exciton annihilation in MoSe2 monolayers

    OpenAIRE

    Kumar, Nardeep; Cui, Qiannan; Ceballos, Frank; He, Dawei; Wang, Yongsheng; Zhao, Hui

    2013-01-01

    We investigate the excitonic dynamics in MoSe2 monolayer and bulk samples by femtosecond transient absorption microscopy. Excitons are resonantly injected by a 750-nm and 100-fs laser pulse, and are detected by a probe pulse tuned in the range of 790 - 820 nm. We observe a strong density-dependent initial decay of the exciton population in monolayers, which can be well described by the exciton-exciton annihilation. Such a feature is not observed in the bulk under comparable ...

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

    OpenAIRE

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

    2014-01-01

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

  2. Exciton formation stimulated by optically pumped electron - hole plasma in semiconductor nanostructures

    International Nuclear Information System (INIS)

    A mechanism to increase the exciton binding energy in wide band-gap semiconductors due to the presence of optically pumped electron-hole plasma is proposed. These exciton states with high binding energy (>150 MeV) can exist at room temperature when the dielectric permittivity of a semiconductor approaches zero in the infrared region. Calculations for CdS show that the density of electron-hole plasma should be above 1019 cm-3 to form such excitons. There are large numbers of closely spaced energy levels of exciton states with a high binding energy in the forbidden band gap of semiconductors. We believe these excitons are involved in lasing process observed in optically pumped semiconductor nanocrystals

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

    International Nuclear Information System (INIS)

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

  4. Multiscale photosynthetic exciton transfer

    CERN Document Server

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

    2012-01-01

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

  5. Structure of polyvalent metal halide melts

    International Nuclear Information System (INIS)

    A short review is given of recent progress in determining and understanding the structure of molten halide salts involving polyvalent metal ions. It covers the following three main topics: (i) melting mechanisms and types of liquid structure for pure polyvalent-metal chlorides; (ii) geometry and stability of local coordination for polyvalent metal ions in molten mixtures of their halides with alkali halides; and (iii) structure breaking and electron localization on addition of metal to the melt. (author). 28 refs, 3 figs, 1 tab

  6. Engineering directed excitonic energy transfer

    CERN Document Server

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

    2010-01-01

    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.

  7. Fullerenes doped with metal halides

    International Nuclear Information System (INIS)

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

  8. EXAFS studies of zinc halide glasses

    International Nuclear Information System (INIS)

    Structure of mixed halide glasses of ZnCl2-ZnBr2, ZnCl2-ZnI2 and ZnX2-KX' (X, X'=Cl, Br, I) systems was investigated by means of Zn K-edge, Br K-edge and I L3-edge EXAFS spectroscopy. The network structure of the ZnCl2-ZnBr2 and ZnCl2-ZnI2 glasses is composed of corner-sharing tetrahedral units, ZnClnX4-n (X=Br, I). The bond distances between zinc and halide ions are similar to those of the corresponding end members though they are slightly varied by the substitution of halide ion. The addition of K+ ions introduces non-bridging halide ions and decreases the bond distance. (author)

  9. Electron-Hole Asymmetry in Single-Walled Carbon Nanotubes Probed by Direct Observation of Transverse Quasi-Dark Excitons

    OpenAIRE

    Miyauchi, Yuhei; Ajiki, Hiroshi; Maruyama, Shigeo

    2009-01-01

    We studied the asymmetry between valence and conduction bands in single-walled carbon nanotubes (SWNTs) through the direct observation of spin-singlet transverse dark excitons using polarized photoluminescence excitation spectroscopy. The intrinsic electron-hole (e-h) asymmetry lifts the degeneracy of the transverse exciton wavefunctions at two equivalent K and K' valleys in momentum space, which gives finite oscillator strength to transverse dark exciton states. Chirality-d...

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

    International Nuclear Information System (INIS)

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

  11. Exciton-polariton condensates

    Science.gov (United States)

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

    2014-11-01

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

  12. 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.)

  13. First-principles study of ?-ray detector materials in perovskite halides

    Science.gov (United States)

    Im, Jino; Jin, Hosub; Stoumpos, Constantinos; Chung, Duck; Liu, Zhifu; Peters, John; Wessels, Bruce; Kanatzidis, Mercouri; Freeman, Arthur

    2013-03-01

    In an effort to search for good ?-ray detector materials, perovskite halide compounds containing heavy elements were investigated. Despite the three-dimensional network of the corner shared octahedra and the extended nature of the outermost shell, its strong ionic character leads to a large band gap, which is one of the essential criteria for ?-ray detector materials. Thus, considering high density and high atomic number, these pervoskite halides are possible candidate for ?-ray detector materials. We performed first-principles calculations to investigate electronic structures and thermodynamic properties of intrinsic defects in the selected perovskite halide, CsPbBr3. The screened-exchange local density approximation scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the calculated band gap of CsPbBr3 is found to be suitable for ?-ray detection. Furthermore, defect formation energy calculations allow us to predict thermodynamic and electronic properties of possible intrinsic defects, which affect detector efficiency and energy resolution. In an effort to search for good ?-ray detector materials, perovskite halide compounds containing heavy elements were investigated. Despite the three-dimensional network of the corner shared octahedra and the extended nature of the outermost shell, its strong ionic character leads to a large band gap, which is one of the essential criteria for ?-ray detector materials. Thus, considering high density and high atomic number, these pervoskite halides are possible candidate for ?-ray detector materials. We performed first-principles calculations to investigate electronic structures and thermodynamic properties of intrinsic defects in the selected perovskite halide, CsPbBr3. The screened-exchange local density approximation scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the calculated band gap of CsPbBr3 is found to be suitable for ?-ray detection. Furthermore, defect formation energy calculations allow us to predict thermodynamic and electronic properties of possible intrinsic defects, which affect detector efficiency and energy resolution. Supported by the office of Nonproliferation and Verification R &D under Contract No. DE-AC02-06CH11357

  14. Oxidation of hydrogen halides to elemental halogens

    Science.gov (United States)

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

    1985-01-01

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

  15. Thermochemical properties of zirconium halides. A review

    Energy Technology Data Exchange (ETDEWEB)

    Van der Vis, M.G.M.; Cordfunke, E.H.P.; Konings, R.J.M. [Netherlands Energy Research Foundation ECN, Petten (Netherlands)

    1997-10-22

    Physico-chemical experiments are reviewed and the thermodynamic properties of solid and gaseous zirconium halides of the general formula ZrX{sub n} (X=F, Cl, Br, I; n=1-4) are evaluated and, where necessary, estimated by analogy with other metal halide systems. Thermodynamic key values at 298.15K and heat capacity equations are tabulated up to 1500K for the solid phase and 3000K for the gas phase

  16. Singlet fission of hot excitons in ?-conjugated polymers.

    Science.gov (United States)

    Zhai, Yaxin; Sheng, Chuanxiang; Vardeny, Z Valy

    2015-06-28

    We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(?)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent ?-conjugated polymers. From the high energy step in the triplet EXPA(?) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E?2ET (=2.8?eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8?eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2ET, which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(?) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=Eg (?2.25?eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2Ag exciton, dominates the triplet photogeneration in PDA polymer as Eg>2ET. The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in ?-conjugated polymers is a much more general process than thought previously. PMID:25987576

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

    International Nuclear Information System (INIS)

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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2012-05-15

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

  19. Propagation dynamics of exciton spins in a high-density semiconductor quantum dot system

    International Nuclear Information System (INIS)

    We study propagation dynamics of exciton spins in a system composed of high-density self-assembled quantum dots (QDs) of CdSe and a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin-polarized excitons are generated in the DMS layer and are subsequently injected into the QDs. The degree of circular polarization, P, of excitonic photoluminescence at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulsed excitation, indicating the exciton-spin injection from the ZnMnSe to CdSe-QDs. The P value tends to decay gradually because of the exciton-spin relaxation in the QDs after the injection. The spin-polarized excitons in the QD ensemble migrate simultaneously from the QDs with higher exciton energies to those with lower exciton energies, where the exciton-spins relax inside each dot during the migration. Therefore, the observed P values in the QD emission band are affected by this feeding of the relaxed spins. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  20. FIELD DEPENDENCE OF THE BOUND STATE OF Cr3+ EXCITON ACCOMPANIED WITH Yb3+ SPIN FLIP IN YbCrO3

    OpenAIRE

    Kojima, N.; Tsujikawa, I.

    1988-01-01

    In YbCrO3, the Cr3+ exciton -Yb3+ spin flip combined excitation (R' band) appears in the neighborhood of the Cr3+ exciton lines. We found that a bound state appears on the lowest energy side of the R' band and has a strong dependence on external magnetic fields.

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

    OpenAIRE

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

    1998-01-01

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

  2. Thermodynamics of a cold dipolar exciton gas

    OpenAIRE

    Andreev, Sergey

    2013-01-01

    I address some general aspects of thermodynamics of a cold dipolar exciton gas. I argue that at the typical exciton densities the system exhibits universal scaling behaviour. This theoretical finding paves the way to a systematic study of exciton condensates.

  3. Quantum-chemical simulation of production and tunneling recombination of pairs of Frenkel defects in alkali halides

    International Nuclear Information System (INIS)

    Quantum-chemical simulation of the mechanism of non-irradiating annihilation of self-localized exciton resulting in production of Frenkel defects in alkali-halide crystals is performed. LiF crystal is considered, in particular. It is shown that at drawing the anion escaping along the axis together with the following anion up to smaller distance characteristic for H-centre the electron tunneling back in anion vacancy with production of F-, H-centre pair becomes energetically advisable. Calculations of effectiveness of non-irradiating annihilation reveal that release of almost the total energy of self-localized exciton on an anion is possible, and time of transition does not exceed 10-11 s. The non-irradiating channel dominates for tunnel recombination of F-, H-pairs of defects while the irradiating one - for F-Vsub(k)-pairs

  4. Exciton-Plasmon Interaction Effects in Individual Carbon Nanotubes

    Science.gov (United States)

    Bondarev, Igor; Meliksetyan, Areg

    2013-03-01

    We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions -- despite the mathematical statements of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence for no free-exciton BEC in carbon nanotubes. We have recently developed a theory for the electrostatically controlled coupling between excitons and low-energy inter-band plasmons in individual semiconducting carbon nanotubes. Here, we report on our studies towards the applications of this effect of both applied and fundamental interest. One practical application is the electromagnetic absorption/photoluminescence control for individual nanotubes. Another, fundamental one, comes from the fact that the coupling of the excitons to the same inter-band plasmon resonance results in their entanglement, a pre-requisite for strong quantum correlations/quantum phase transitions in many-particle systems. Our coupled exciton-plasmon excitation is a quasi-1D Bose system and could possibly be Bose-condensed in an individual carbon nanotube under appropriately created external conditions -- despite the mathematical statements of the BEC impossibility in ideal 1D and 2D quantum systems and previously reported evidence for no free-exciton BEC in carbon nanotubes. DOE-DE-SC0007117 (I.B.), ARO-W911NF-11-1-0189 (A.M.)

  5. Exciton states in cylindrical nanowires

    OpenAIRE

    Slachmuylders, A. F.; Partoens, B.; Magnus, W; Peeters, F.

    2005-01-01

    The exciton ground state and excited state energies are calculated for a model system of an infinitely long cylindrical wire. The effective Coulomb potential between the electron and the hole is studied as function of the wire radius. Within the adiabatic approximation, we obtain `exact' numerical results for the effective exciton potential and the lowest exciton energy levels which are fitted to simple analytical expressions. Furthermore, we investigated the influence of a ...

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

    Science.gov (United States)

    Yang, Shize; Tian, Xuezeng; Wang, Lifen; Wei, Jiake; Qi, Kuo; Li, Xiaomin; Xu, Zhi; Wang, Wenlong; Zhao, Jimin; Bai, Xuedong; Wang, Enge

    2014-08-01

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

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

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

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

    2015-02-01

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

  9. Excitation mechanisms and excited states of the I1 bound exciton in CdS

    International Nuclear Information System (INIS)

    For the neutral-acceptor-bound exciton emission (I1) of CdS three new excitation lines with light E perpendicular c are found at 1.2, 2.5, and 3.7 meV on the high energy side of the I1 line. The nearest one is interpreted as due to the simultaneous excitation of the bound exciton and a transverse-acoustic phonon. The other two are understood as the direct creation of the I1 bound exciton in excited states with one of the equal holes belonging to a higher configuration. With light E parallel c the excitation mechanisms via the free exciton states Asub(L) and Asub(F) and the bound exciton states Isub(1B) and Isub(1B') are discussed. An excitation intensity dependent energy shift of the long wavelength tail of the band edge is reported for the first time. (author)

  10. Frenkel-like Wannier-Mott excitons in few-layer Pb I2

    Science.gov (United States)

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

    2015-04-01

    Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

  11. Signature of Anomalous Exciton Localization in the Optical Response of Self-Assembled Organic Nanotubes

    Science.gov (United States)

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

    2015-04-01

    We show that the disorder scaling of the low-temperature optical absorption linewidth of tubular molecular assemblies sharply contrasts with that known for one-dimensional aggregates. The difference can be explained by an anomalous localization of excitons, which arises from the combination of long-range intermolecular interactions and the tube's higher-dimensional geometry. As a result, the exciton density of states near the band bottom drops to zero, leading to a strong suppression of exciton localization. Our results explain the strong linear dichroism and weak exciton-exciton scattering in tubular J aggregates observed in experiments and suggest that for nanoscale wirelike applications a tubular shape is to be preferred over a truly one-dimensional chain.

  12. Identification of electric dipole moments of excitonic complexes in nitride-based quantum dots

    Science.gov (United States)

    Hönig, Gerald; Rodt, Sven; Callsen, Gordon; Ostapenko, Irina A.; Kure, Thomas; Schliwa, Andrei; Kindel, Christian; Bimberg, Dieter; Hoffmann, Axel; Kako, Satoshi; Arakawa, Yasuhiko

    2013-07-01

    The built-in dipole moments of excitonic and multiexcitonic complexes of GaN/AlN quantum dots (charged and uncharged excitons and biexcitons) are investigated in detail, both experimentally and theoretically. The calculations are done with Hartree-Fock wave functions in the framework of an 8-band k·p approximation. Electric dipole moments of different excitonic complexes are found to vary significantly. Hence, their Coulomb coupling to charged defects in the vicinity of a QD produces emission energy variations, known as spectral diffusion, which can be used to identify the emitting excitonic complex. This method was exemplary applied to the presented cathodoluminescence and ?-photoluminescence data and opens a way to identify excitonic complexes by their response to external electric fields.

  13. 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

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2004-07-01

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

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

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

    Webster, R.; Bernasconi, L.; Harrison, N. M.

    2015-06-01

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

  17. Quantitative interpretation of the excitonic splittings in aluminum nitride

    Science.gov (United States)

    Gil, B.; Guizal, B.; Felbacq, D.; Bouchitté, G.

    2011-02-01

    We address the interpretation of the splitting between the ground state excitonic transition which indicates the energy of the lowest direct band gap in AlN bulk films and epilayers, and a 36-38 meV higher energy companion. We demonstrate that this splitting is consistent with the initial interpretation in terms of 1s-2s excitonic splitting by using a calculation of the exciton binding energy which includes mass anisotropy and anisotropy of the dielectric constant. Analytical expressions are proposed to compute the evolution of 1s and 2s excitonic energies using an anisotropy parameter. We show that the values of the dielectric constant that are required to fit the data are \\varepsilon bot ? 8.7 and \\varepsilon allel ? 10, values different from the couple of values \\varepsilon bot ? 7.33 and \\varepsilon allel ? 8.45 erroneously obtained after a fitting procedure using a spherical description of the long range Coulomb interaction and the classical textbook n-2 spectrum of the excitonic eigenstates. Starting from now, our values are the recommended ones.

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

    International Nuclear Information System (INIS)

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

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

    Directory of Open Access Journals (Sweden)

    D.M. Stepanchikov

    2009-01-01

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

  20. Hot trion and excited exciton states of single InGaAs/GaAs quantum dots

    International Nuclear Information System (INIS)

    For a detailed understanding of the electronic structure of self-organized quantum dots (QDs) high-resolution measurements that reveal the spectrum of excited states are essential. Here, polarized photoluminescence excitation (PLE) spectra of the positive trion and the exciton of a number of MBE-grown single InGaAs/GaAs QDs were recorded and compared to results of 8-band k.p calculations. The exciton and trion absorption spectra comprise two characteristic parts. ?E=60 meV above the ground state energy exists a quasi continuous band of absorption features. The corresponding transitions involve hybrid states of QDs and wetting layer. In our study we concentrate on the regime below ?E=60 meV where the PLE spectrum of the exciton consists of well separated sharp resonances, corresponding to absorptions into excited exciton states, and a broad resonance around 35 meV due to exciton-phonon coupling. Upon adding a positive charge carrier, the PLE spectrum has a more complex structure due to the singlet-triplet splitting of the hot trion states. The sharp resonances, of both trion and exciton show clear dependence on linearly or circularly polarized excitation and detection. Moreover the spectra of different dots, detected on the ground state energy of the same excitonic complex, reveal substantial similarities

  1. Understanding excitons using spherical geometry

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

    Filip, Marina R.; Giustino, Feliciano

    2014-12-01

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

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

    Science.gov (United States)

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

    2015-03-01

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

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

    International Nuclear Information System (INIS)

    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.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-12-23

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

  6. Collective $d-$wave Excitonic Modes in the Fe-Superconductors

    OpenAIRE

    Scalapino, D. J.; Devereaux, T. P.

    2009-01-01

    Calculations of the pairing interaction in multi-band models of the Fe superconductors show that it is attractive in both the $A_{1g}$ (s-wave) and $B_{1g}$ (d-wave) channels. This raises the possibility that these materials may have collective excitonic modes. Here, assuming an s-wave groundstate, we investigate the d-wave collective excitonic mode and its coupling to the Raman scattering.

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

    OpenAIRE

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

    2013-01-01

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

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

    Science.gov (United States)

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

    2015-03-01

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

  9. 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.

  10. Decay dynamics of free and trapped excitons in tetragonal mercuric iodide

    Science.gov (United States)

    Anderson, Richard J. M.

    1996-10-01

    We present the time-resolved photoluminescence spectra of tetragonal mercuric iodide from 4.2 to 100 K. Decay of photoluminescence is dominated by transport of the exciton-polariton to sites for recombination: the surface (band 1 at about 532 nm) and the band 2 trap site (about 560 nm). The decay rate of the exciton-polariton in the bulk of the material decreases precipitously (a factor of nearly 100) over this temperature range, which we ascribe to a manifestation of polariton (radiative) decay. Inelastic scattering of exciton-polaritons from thermally populated phonons reduces the cooling rate of the exciton-polariton to energies below the knee of the exciton-polariton dispersion curve, which slows transport and thus blocks the dominant pathway for radiative recombination and reduces the decay rate. Band 2, which is present to some extent in all mercuric iodide crystals, shows a rapid rise time and two decay channels. The decay rate of the radiative channel is less than the decay rate of band 1 at low temperatures, and changes at higher temperatures to be indistinguishable from that of band 1. At higher temperatures a nonradiative channel begins to dominate; its decay rate increases strongly with temperature, which leads to quenching of all luminescence even at temperatures well below 300 K.

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

    Science.gov (United States)

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

    2015-04-01

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

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

    International Nuclear Information System (INIS)

    The electronic properties of ABX3 type compounds in the cubic phase are systematically studied using the first-principles calculations. The chemical trend of their properties as A or B or X varies is fully investigated. The optical properties of the ABX3 compounds are also investigated. Our calculations show that taking into account the spin–orbit coupling effect is crucial for predicting the accurate band gap of these halide perovskites. We predict that CH3NH3SnBr3 is a promising material for solar cells absorber with a perfect band gap and good optical absorption.

  13. Exciton condensation in strongly correlated electron bilayers

    OpenAIRE

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

    2013-01-01

    We studied the possibility of exciton condensation in Mott insulating bilayers. In these strongly correlated systems an exciton is the bound state of a double occupied and empty site. In the strong coupling limit the exciton acts as a hard-core boson. Its physics are captured by the exciton t-J model, containing an effective XXZ model describing the exciton dynamics only. Using numerical simulations and analytical mean field theory we constructed the ground state phase diagr...

  14. Unraveling halide hydration: A high dilution approach

    Science.gov (United States)

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

    2014-07-01

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

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

    OpenAIRE

    Sugakov, V. I.

    2014-01-01

    The hydrodynamic equations for indirect excitons in the double quantum wells are studied taking into account 1) a possibility of an exciton condensed phase formation, 2) the presence of pumping, 3) finite value of the exciton lifetime, 4) exciton scattering by defects. The threshold pumping emergence of the periodical exciton density distribution is found. The role of localized and free exciton states is analyzed in the formation of emission spectra.

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

    Directory of Open Access Journals (Sweden)

    V.I. Sugakov

    2014-09-01

    Full Text Available The hydrodynamic equations for indirect excitons in the double quantum wells are studied taking into account 1 a possibility of an exciton condensed phase formation, 2 the presence of pumping, 3 finite value of the exciton lifetime, 4 exciton scattering by defects. The threshold pumping emergence of the periodical exciton density distribution is found. The role of localized and free exciton states is analyzed in the formation of emission spectra.

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

    OpenAIRE

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

    2013-01-01

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

  18. Exciton scattering by free carriers and excitons in semiconductor quantum well structures

    International Nuclear Information System (INIS)

    The excitons scattering due to free carriers and excitons in semiconducting quantum well structures as well as the effects of an applied electric field, are theoretically investigated. It is found that effects due to exciton - exciton interactions may gain importance over that due to free carrier - exciton interactions in the presence of the electric field. As the exciton linewidth and its scattering cross sections are strongly correlated, the present results could be of interest to quantum well designers working on optical devices. (author)

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

    International Nuclear Information System (INIS)

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

  20. Ordered "excitonic" phase with flat bands and a gap increasing with decreasing doping as consequence of electronic topological transition in 2D system on a square lattice Applications to high-$T_{c}$ cuprates

    CERN Document Server

    Kiselev, M N; Pfeuty, P

    2000-01-01

    We study the ordered "excitonic" phase which develops around electronic topological transition (ETT) in a 2D electron system on a square lattice. We consider the case of hoping between more than nearest neighbours for which the quantum critical point (QCP) associated with ETT (at \\delta=\\delta_c, n=1-\\delta is electron concentration) is quite unusual. We show that the amplitude of the order parameter and of the gap in the electron spectrum increase with increasing the distance from the QCP, \\delta_c - \\delta, unlike the ordinary case when they decrease when going away from the point which is a motor for instability. The electron spectrum is characterized by a gap on Fermi level which opens at "hot spot" and extends until the saddle point (SP) whatever is a doping concentration. On the other hand it has a characteristic flat shape in a proximity of SP. The existence of the gap around SP, its increasing with decreasing doping, the flat shape of the spectrum and the angle dependence of the gap have a striking si...

  1. Observation of Rapid Exciton-Exciton Annihilation in Monolayer Molybdenum Disulfide

    Science.gov (United States)

    Sun, Dezheng; Rao, Yi; Reider, Georg; Chen, Gugang; You, Yumeng; Brézin, Louis; Harutyunyan, Avetik; Heinz, Tony

    2015-03-01

    In this paper, we present ultraist pump-probe spectroscopy results for monolayer MoS2 crystals in which we explore exciton dynamics as a function of exciton density. After a femtosecond excitation pulse of near-resonant radiation to create A excitons, we have monitored the temporal evolution of the exciton density using a continuum probe pulse. We observe a decay rate as long as 100 ps for samples at room temperature and at relatively low exciton density. The decay rate increases strongly with increasing exciton density. We are able to fit the entire set of density-dependent exciton dynamics using a simple model in which the dominant decay channel is an exciton-exciton annihilation process. From these measurements, we infer an exciton-exciton annihilation rate of (4.3 +/- 1.1) × 102 cm2/s. We compare this rate with that observed in other nanostructured materials.

  2. Excitonic correlations in the intermetallic Fe2VAl

    Science.gov (United States)

    Weht, Ruben; Pickett, W. E.

    1998-09-01

    The intermetallic compound Fe2VAl looks nonmetallic in transport and strongly metallic in thermodynamic and photoemission data. It has in its band structure a highly differentiated set of valence and conduction bands leading to a semimetallic system with a very low density of carriers. The pseudogap itself is sensitive to the presence of Al states, but the resulting carriers have only minor Al character. The effects of generalized gradient corrections to the local density band structure are shown to be important, reducing the carrier density by a factor of 3. Spin-orbit coupling results in a redistribution of the holes among pockets at the Brillouin zone center. Doping of this nonmagnetic compound by 0.5 electrons per cell in a virtual crystal fashion results in a moment of 0.5?B and destroys the pseudogap. We assess the tendencies toward the formation of an excitonic condensate and toward an excitonic Wigner crystal and find both to be unlikely. We propose a model in which the observed properties result from excitonic correlations arising from two interpenetrating lattices of distinctive electrons (eg on V) and holes (t2g on Fe) of low density (one carrier of each sign per 350 formula units).

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

    CERN Document Server

    Weht, R; Weht, Ruben

    1998-01-01

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

  4. Models of coherent exciton condensation

    OpenAIRE

    EASTHAM, PAUL

    2004-01-01

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

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

    International Nuclear Information System (INIS)

    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

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

    Science.gov (United States)

    Ogawa, Tetsuo; Tomio, Yuh; Asano, Kenichi

    2007-07-25

    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. PMID:21483057

  7. Computational Screening of Mixed Metal Halide Ammines

    DEFF Research Database (Denmark)

    Jensen, Peter Bjerre; Lysgaard, Steen

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

  8. Lanthanide-halide based humidity indicators

    Science.gov (United States)

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

    2008-01-01

    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.

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

    OpenAIRE

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

    2015-01-01

    Pump-dependent photoluminescence imaging and 2nd order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature that enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these is attributed to the effect of environmental disorder in setting the exciton mean free-path and capture-limited Auger recombination at this lengthscale. A suppre...

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

    Science.gov (United States)

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

    2015-06-01

    We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interactions as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated. PMID:25998778

  11. Exciton-polariton behaviour in bulk and polycrystalline ZnO

    International Nuclear Information System (INIS)

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

  12. Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

    Science.gov (United States)

    Cobet, Munise; Cobet, Christoph; Wagner, Markus R.; Esser, Norbert; Thomsen, Christian; Hoffmann, Axel

    2010-01-01

    The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands ?7,?9,?7. The transverse-longitudinal splittings which separate the upper polariton branch from the lower branch, corresponding to the k-vector of the used light, are deduced precisely for each subband. Mainly for E ?c, additional absorption peaks are observed at the longitudinal B-exciton and closely above. One is considered to be a mixed-mode and the other is seen as a consequence of interference effects in an exciton free surface layer which is also visible in reflectance anisotropy spectroscopy.

  13. Anharmonic properties of potassium halide crystals

    OpenAIRE

    Raju, Krishna Murti

    2011-01-01

    An effort has been made to obtain the anharmonic properties of potassium halides starting from primary physical parameters viz. nearest neighbor distance and hardness parameters assuming long- and short- range potentials at elevated temperatures. The elastic energy density for a deformed crystal can be expanded as power series of strains for obtaining coefficients of quadratic, cubic and quartic terms which are known as the second, third and fourth order elastic constants respectively...

  14. Formation of structured nanophases in halide crystals.

    Czech Academy of Sciences Publication Activity Database

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

    Praha : Czechoslovak Association for Crystal Growth (CSACG), 2012 - (Kožíšek, Z.; Nitsch, K.). s. 46-46 ISBN 978-80-260-2357-9. [Joint Seminar – Development of materials science in research and education /22./. 03.09.2012-07.09.2012, Lednice] R&D Projects: GA ?R GAP108/12/0891 Institutional research plan: CEZ:AV0Z10100521 Keywords : halides * nucleation * nanophases Subject RIV: BM - Solid Matter Physics ; Magnetism

  15. Lanthanide doped strontium-barium cesium halide scintillators

    Energy Technology Data Exchange (ETDEWEB)

    Bizarri, Gregory; Bourret-Courchesne, Edith; Derenzo, Stephen E.; Borade, Ramesh B.; Gundiah, Gautam; Yan, Zewu; Hanrahan, Stephen M.; Chaudhry, Anurag; Canning, Andrew

    2015-06-09

    The present invention provides for a composition comprising an inorganic scintillator comprising an optionally lanthanide-doped strontium-barium, optionally cesium, halide, useful for detecting nuclear material.

  16. Computational screening of mixed metal halide ammines

    DEFF Research Database (Denmark)

    Jensen, Peter Bjerre; Lysgaard, Steen

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

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

    International Nuclear Information System (INIS)

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

  18. 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

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

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

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

    2013-11-01

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

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

    International Nuclear Information System (INIS)

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

  2. Spin Texture in a Cold Exciton Gas

    OpenAIRE

    High, A. A.; Hammack, A. T.; Leonard, J. R.; Yang, Sen; Butov, L. V.; Ostatnicky, T.; Kavokin, A.V.; Gossard, A. C.

    2011-01-01

    We report on the observation of a spin texture in a cold exciton gas in a GaAs/AlGaAs coupled quantum well structure. The spin texture is observed around the exciton rings. The observed phenomena include: a ring of linear polarization, a vortex of linear polarization with polarization perpendicular to the radial direction, an anisotropy in the exciton flux, a skew of the exciton fluxes in orthogonal circular polarizations and a corresponding four-leaf pattern of circular pol...

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

    International Nuclear Information System (INIS)

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

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

    OpenAIRE

    de-Leon, S. Ben-Tabou; Laikhtman, B.

    2000-01-01

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

  5. Exciton-photon correlations in bosonic condensates of exciton-polaritons

    OpenAIRE

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

    2014-01-01

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

  6. Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using 13C-Labeled, Highly Enriched (6,5) Single-Walled Carbon Nanotubes

    International Nuclear Information System (INIS)

    A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton-phonon coupling opens up additional absorption and emission channels, some of which may 'brighten' the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare 12C and 13C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright 1E11 singlet exciton, one absorptive sideband ?200 meV above, and one emissive sideband ?140 meV below, the bright singlet exciton. Both sidebands demonstrate a ?50 cm-1 isotope-induced shift, which is commensurate with exciton-phonon coupling involving phonons of A'1 symmetry (D band, ? ? 1330 cm-1). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ?25 meV (for the (6,5) SWCNT) above the bright ?-momentum singlet. This study represents the first use of 13C-labeled SWCNTs highly enriched in a single nanotube species ton a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton.

  7. Spontaneous exciton dissociation in carbon nanotubes

    OpenAIRE

    Kumamoto, Y.; Yoshida, M.; Ishii, A; Yokoyama, A; Shimada, T.; Kato, Y. K.

    2013-01-01

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. Correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating during their relaxation into the lowest exciton state. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and...

  8. Split donor centers and split excitons in a semiconductor heterostructure

    Science.gov (United States)

    Gribnikov, Z. S.; Haddad, G. I.

    2005-10-01

    The first subject considered in the article is a donor center embedded in a thin heterostructural barrier separating a semiconductor medium into two halves. As a result of the small thickness of this barrier, the wave function of an electron bound by the donor center shifts almost completely into both halves of the surrounding semiconductor medium. The ground and first excited electron states of such a donor center are separated from each other by a narrow energy gap determined by the symmetric-antisymmetric tunnel splitting. Such structures can be implemented in both GaAs/AlXGa1-XAs and Si/GeXSi1-X material systems. The second considered subject is an exciton formed in analogous heterostructures when the staggered band alignment takes place between the heterobarrier and semiconductor medium. As a result of such band alignment, the hole participating in the exciton creation is located in the formed quantum well and the electron, which is the hole's opponent, is separated into halves (on different sides of the quantum well) as before. Unlike the donor center, the exciton can be shifted and localized in arbitrary positions along the staggered ``barrier-well'' boundary by inhomogeneous electric fields of external controlling gates.

  9. Theoretical study of excitonic complexes in semiconductors quantum wells

    International Nuclear Information System (INIS)

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

  10. Exciton coupling in molecular crystals

    Science.gov (United States)

    Ake, R. L.

    1976-01-01

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

  11. 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

  12. Metal halide reduction with molten sodium/potassium alloy

    International Nuclear Information System (INIS)

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

  13. Photoluminescence properties of silicon nanocrystals interacting with gold nanoparticles via exciton-plasmon coupling

    Science.gov (United States)

    Potrick, Karsten; Huisken, Friedrich

    2015-03-01

    The photoluminescence (PL) properties of silicon nanocrystals (Si NCs) deposited on gold nanostructures have been studied regarding PL intensity and lifetime. In contrast to most investigations, which attempt to optimize the overlap between the plasmon resonance and the Si NCs' PL band, we chose much smaller gold nanoparticles to achieve an improved exciton-plasmon coupling. PL enhancements of up to 20 were observed near the plasmon resonance. In the maximum of the PL band at 710 nm, we determined a PL enhancement of ˜4 while the average PL lifetime was found to increase from 45 to 66 ? s . The experimental observations were successfully explained with a model invoking bidirectional energy transfer between excitonic and plasmonic states, being operative in addition to the normal exciton recombination rate enhancement.

  14. Resonant Fluorescence from Quantum Dot Molecular Excitonic Transitions

    Science.gov (United States)

    Kerfoot, Mark; Bracker, Allan; Gammon, Daniel; Scheibner, Michael

    2011-03-01

    Quantum dot molecules formed by two vertically stacked quantum dots are a rich testing ground for basic concepts regarding the measurement and control of quantum states. The well defined geometry is ideal for studying interaction mechanisms, such as the interaction of two dipoles each located in one of the quantum dots of the quantum dot molecule. A prerequisite for doing so is the ability to detect the interaction mediated changes in the properties of the individual, uncoupled quantum dots. Here we use resonant fluorescence to study exciton transitions in quantum dot molecules. We measure the photoluminescence of the same transition we optically excite with a narrow band laser. With this method, features on the scale of the homogeneous line width of the intradot exciton transition are well resolved. This enables us to study the fine-structure of different charge and spin configurations with high sensitivity.

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

    OpenAIRE

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

    2014-01-01

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

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

    International Nuclear Information System (INIS)

    We study the optical properties of the lowest-energy exciton states in highly photostable individual CdSe/ZnS nanocrystals at low temperatures. We observe two sharp zero-phonon lines which we attribute to the radiative recombination from the two lowest-energy levels of the band-edge exciton fine structure. By using resonant laser excitation on these two lines we measure a spectral broadening of 10 ?eV over integration times of 100 ms.

  17. Relaxation time in the exciton model

    International Nuclear Information System (INIS)

    The equation for the time-dependent average number of excitons is derived making use of the system of kinetic equations of the exciton model. The relaxation time is obtained as a function of the excitation energy and of the number of nucleons with and without account of the Pauli principle for the level densities of states with fixed number of excitons. The equation is as well written for the mean-square fluctuation of the exciton number. Relation is discussed between the exciton model equations and the stochastic differential equation for the number of collisions in a medium with random scatterers whose average density is time dependent

  18. Exciton binding energy in semiconductor quantum dots

    International Nuclear Information System (INIS)

    In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass ? = ?(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy Eex(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii aex, the exciton binding energy Eex(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.

  19. Motional enhancement of the exciton magnetic moment

    International Nuclear Information System (INIS)

    This paper overviews our recent results on the magneto-optics of excitons in wide quantum wells, when the well width is much larger than the exciton Bohr radius. We have demonstrated that the magnetic moment of the exciton increases by an order of magnitude due to its motion along the magnetic field. The effect was observed for a set of quantum well structures with different well widths and based on various semiconductor compounds. The reason for this effect is mixing of the exciton's center-of-mass motion and the internal motion of electron and hole in the exciton

  20. Radiation damage in the alkali halide crystals

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

    Yi, Tianyou; Kirova, Natasha; Brazovskii, Serguei

    2015-03-01

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

  2. 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

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

    Directory of Open Access Journals (Sweden)

    S. Daehne

    2006-11-01

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

  4. Berry Phase Effect on the Exciton Transport and on the Exciton Bose-Einstein Condensate

    International Nuclear Information System (INIS)

    With the exciton lifetime much extended in semiconductor quantum-well structures, the exciton transport and Bose-Einstein condensation have become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict a spin-dependent transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation

  5. Angular momentum dependent exciton model

    International Nuclear Information System (INIS)

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

  6. Stirring Potential for Indirect Excitons

    OpenAIRE

    Hasling, M. W.; Kuznetsova, Y. Y.; Andreakou, P.; Leonard, J. R.; Calman, E. V.; Dorow, C; Butov, L. V.; Hanson, M.; Gossard, A. C.

    2014-01-01

    We demonstrate experimental proof of principle for a stirring potential for indirect excitons. The azimuthal wavelength of this stirring potential is set by the electrode periodicity, the amplitude is controlled by the applied AC voltage, and the angular velocity is controlled by the AC frequency.

  7. Excitonic transitions in ZnO/MgZnO quantum well heterostructures

    Science.gov (United States)

    Coli, Giuliano; Bajaj, K. K.

    2001-03-01

    We present a calculation of the excitonic transition energies in ZnO/MgZnO quantum well heterostructures, including the effects of the exciton-optical phonon interaction. The results of our calculations clearly show that the use of the static screened Coulomb potential to describe the electron-hole interaction and of the polaron masses for the electron and the hole leads to a poor agreement with the available experimental data[1]. On the other hand, including the exciton-phonon interaction in the calculation of the exciton binding energies, leads to the values of the excitonic transition energies which agree very well with the recently published experimental data[1]. A critical discussion of the choice of the various physical parameters used in ZnO is also presented. This leads us to suggest a value for the heavy-hole band mass of 0.78m0 and a conduction to valence band offset ratio in the range 60/40-70/30. [1] T. Makino, C. H. Chia, N. T. Tuan, H.D. Sun, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura and H. Koinuma, Appl. Phys. Lett. 77, 975 (2000)

  8. 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.

  9. Exciton binding energy in wurtzite InGaN/GaN quantum wells

    International Nuclear Information System (INIS)

    The internal field and carrier density effects on the exciton binding energies in wurtzite (WZ) InGaN/GaN quantum-well (QW) structures are investigated using the multiband effective-mass theory, and are compared with those obtained from the at-band model and with those of GaN/AlGaN QW structures. The exciton binding energy is significantly reduced with increasing sheet carrier density, suggesting that excitons are nearly bleached at densities around 1012 cm-2 for both InGaN/GaN and GaN/AlGaN QW structures. With the inclusion of the internal field, the exciton binding energy is substantialy reduced compared to that of the at-band model in the investigated region of the wells. This can be explained by a decrease in the momentum matrix element and an increase in the inverse screening length due to the internal field. The exciton binding energy of the InGaN/GaN structure is smaller than that of the GaN/AlGaN structure because InGaN/GaN structures have a smaller momentum matrix element and a larger inverse screening length than GaN/AlGaN structures.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-01-10

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

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

    OpenAIRE

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

    2002-01-01

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

  12. Excitons in the wurtzite AlGaN/GaN quantum-well heterostructures

    OpenAIRE

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

    2008-01-01

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

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

    International Nuclear Information System (INIS)

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

  14. Exciton trapping in a periodically modulated magnetic field

    Scientific Electronic Library Online (English)

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

    2002-06-01

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

  15. Exciton line broadening in solid solutions with degenerated valent zone

    International Nuclear Information System (INIS)

    It is shown, that exciton line broadening in solid solutions with degenerated valent zone in the case, when the exciton translation mass is negative, is irregularly large and determined by exciton scattering in composition fluctuations with large momentum transfer

  16. Transport of Indirect Excitons in a Potential Energy Gradient

    OpenAIRE

    Leonard, J. R.; Remeika, M.; Chu, M. K.; Kuznetsova, Y. Y.; High, A. A.; Butov, L. V.; Wilkes, J; Hanson, M.; Gossard, A. C.

    2012-01-01

    We realized a potential energy gradient - a ramp - for indirect excitons using a shaped electrode at constant voltage. We studied transport of indirect excitons along the ramp and observed that the exciton transport distance increases with increasing density and temperature.

  17. Kinetics of exciton-exciton annihilation in molecular crystals

    International Nuclear Information System (INIS)

    The theoretical investigation of the time dependence of the concentration of incoherent excitons formed by pulsed pumpering is carried out. The basis of this investigation is a system of an infinite number of coupled equations for multiparticle distribution functions. Using the Kirkwood method of uncoupling, this system is reduced to a system of differential equations for one-and two-particle distribution functions. The algorithm for calculating these equations by means of an electronic computer is developed. A simple phenomenological differential equation for exciton density has been obtained. The solution of this phenomenological equation coincides with good accuracy with the solution obtained by numerical integration of the above-mentioned system of differential equations for one- and two-particle distribution functions. (orig.)

  18. Bright and dark singlet excitons via linear and two-photon spectroscopy in monolayer transition metal dichalcogenides

    CERN Document Server

    Berkelbach, Timothy C; Reichman, David R

    2015-01-01

    We discuss the linear and two-photon spectroscopic selection rules for spin-singlet excitons in monolayer transition metal dichalcogenides. Our microscopic formalism combines a fully $k$-dependent few-orbital band structure with a many-body Bethe-Salpeter equation treatment of the electron-hole interaction, using a model dielectric function. We show analytically and numerically that the single-particle, valley-dependent selection rules are preserved in the presence of excitonic effects. Furthermore, we definitively demonstrate that the bright (one-photon allowed) excitons have $s$-type azimuthal symmetry and that dark $p$-type excitons can be probed via two-photon spectroscopy. The screened Coulomb interaction in these materials substantially deviates from the $1/\\varepsilon_0 r$ form; this breaks the "accidental" angular momentum degeneracy in the exciton spectrum, such that the 2$p$ exciton has a lower energy than the 2$s$ exciton by at least 50 meV. We compare our calculated two-photon absorption spectra t...

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2008-04-09

    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.

  20. Optical signatures of quantum dot excitons in carbon nanotubes

    OpenAIRE

    Matthias S. Hofmann; Glückert, Jan T.; Högele, Alexander

    2012-01-01

    We report optical studies of quantum dot excitons in individual suspended carbon nanotubes at cryogenic temperatures. Narrow optical linewidths, strongly suppressed spectral wandering, and photoluminescence lifetimes in the range of nanoseconds emerge as key signatures of exciton localization. We infer exciton quantum dot formation with a characteristic length of a few exciton Bohr radii. Localization inhibits exciton diffusion and protects the exciton from dephasing by stru...

  1. Exciton-polariton emission from organic semiconductor optical waveguides

    Science.gov (United States)

    Ellenbogen, Tal; Crozier, Kenneth B.

    2011-10-01

    We photoexcite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Nonresonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations.

  2. Exciton-polariton emission from organic semiconductor optical waveguides

    OpenAIRE

    Ellenbogen, Tal; Crozier, Kenneth B.

    2011-01-01

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

  3. Exciton-related electroluminescence from monolayer MoS2

    OpenAIRE

    Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang

    2013-01-01

    Excitons in MoS2 dominate the absorption and emission properties of the two-dimensional system. Here, we study the microscopic origin of the electroluminescence from monolayer MoS2 fabricated on a heavily p-type doped silicon substrate. By comparing the photoluminescence and electroluminescence of a MoS2 diode, direct-exciton and bound-exciton related recombination processes can be identified. Auger recombination of the exciton-exciton annihilation of bound exciton emission ...

  4. The trion as an exciton interacting with a carrier

    OpenAIRE

    Combesccot, M.; Betbeder-Matibet, O.

    2003-01-01

    The X$^-$ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X$^-$ trion creation operator in terms of...

  5. EFFECT OF EXCITON DIFFUSION AND REABSORPTION ON DECAY CURVE OF EXCITON LUMINESCENCE

    OpenAIRE

    Nishimura, H

    1985-01-01

    Well-known wavelength-dependent decay time of exciton luminescence is interpreted in a model based on exciton diffusion and reabsorption of the short wavelength of the luminescence spectrum. The decay curves of the singlet-exciton luminescence in anthracene, depending on wavelength, experimental geometry and penetration depth of incident light, are simulated very well by the model.

  6. Exciton dynamics in WSe2 bilayers

    OpenAIRE

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

    2014-01-01

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

  7. Topologically protected excitons in porphyrin thin films

    OpenAIRE

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

    2014-01-01

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

  8. Exciton Binding Energy of Monolayer WS2

    OpenAIRE

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2014-01-01

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

  9. Exciton Binding Energy of Monolayer WS2

    Science.gov (United States)

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2015-03-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 +/- 0.01 eV around K valley in the Brillouin zone.

  10. Exciton Binding Energy of Monolayer WS2

    Science.gov (United States)

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2015-01-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01?eV around K valley in the Brillouin zone. PMID:25783023

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

    International Nuclear Information System (INIS)

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

  12. Tellurium halide IR fibers for remote spectroscopy

    Science.gov (United States)

    Zhang, Xhang H.; Ma, Hong Li; Blanchetiere, Chantal; Le Foulgoc, Karine; Lucas, Jacques; Heuze, Jean; Colardelle, P.; Froissard, P.; Picque, D.; Corrieu, G.

    1994-07-01

    The new family of IR transmitting glasses, the TeX glasses, based on the association of tellurium and halide (Cl, Br, or I) are characterized by a wide optical window extending from 2 to 18 micrometers and a strong stability towards devitrification. Optical fibers drawn from these glasses exhibit low losses in the 7 - 10 micrometers range (less than 1 dB/m for single index fibers, 1 - 2 dB/m for fibers having a core-clad structure). The TeX glass fibers have been used in a remote analysis set-up which is mainly composed of a FTIR spectrometer coupled with a HgCdTe detector. This prototype system permits qualitative and quantitative analysis in a wide wavelength region lying from 3 to 13 micrometers , covering the fundamental absorption of more organic species. The evolution of a lactic and an alcoholic fermentation has been monitored by means of this set-up.

  13. Exciton dynamics and annihilation in WS2 2D semiconductors

    Science.gov (United States)

    Yuan, Long; Huang, Libai

    2015-04-01

    We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr00383k

  14. Towards exciton condensation in quantum semiconductor structures

    International Nuclear Information System (INIS)

    Full text: The phenomena of exciton condensation, analogous to Bose-Einstein Condensation (BEC) of bosonic atoms is a rapidly advancing field with recent observations of macroscopically ordered states in a two-dimensional system, The basic requirements of an exciton system include long radiative lifetime, repulsive interactions and low potential disorder. In this paper we present measurements of the luminescence properties of cold excitons in a range of systems including coupled quantum wells, quantum ring-like topologies in the optical Aharonov-Bohm regime and selectively patterned 2D geometries and comment on their applicability to the observation of exciton condensation. Copyright (2005) Australian Institute of Physics

  15. Extraordinary exciton conductance induced by strong coupling.

    Science.gov (United States)

    Feist, Johannes; Garcia-Vidal, Francisco J

    2015-05-15

    We demonstrate that exciton conductance in organic materials can be enhanced by several orders of magnitude when the molecules are strongly coupled to an electromagnetic mode. Using a 1D model system, we show how the formation of a collective polaritonic mode allows excitons to bypass the disordered array of molecules and jump directly from one end of the structure to the other. This finding could have important implications in the fields of exciton transistors, heat transport, photosynthesis, and biological systems in which exciton transport plays a key role. PMID:26024185

  16. Extraordinary exciton conductance induced by strong coupling

    CERN Document Server

    Feist, Johannes

    2014-01-01

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

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

    OpenAIRE

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

    2014-01-01

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

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

    International Nuclear Information System (INIS)

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

  19. Recent advances in new holographic silver halide materials

    OpenAIRE

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

    2000-01-01

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

  20. Localized Excitons in Carbon Nanotubes.

    Science.gov (United States)

    Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-03-01

    It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.

  1. Cavity modes of exciton-polaritons in an organic crystal with distributed Bragg reflectors

    International Nuclear Information System (INIS)

    We study optical spectra of cavity modes of exciton-polaritons in an organic crystal. The cavity is composed of distributed Bragg reflectors and anthracene crystals occupy the gap of the cavity. Many transmittance peaks corresponding to the cavity modes appear in the photonic band gap region of the transmittance spectra. The spectral positions of the cavity modes converge to the exciton resonance in anthracene. Dispersion of the cavity modes is obtained from the spectral peaks. It looks like a dispersion of the exciton-polariton in anthracene; however it differs slightly from the dispersion curve of the polariton. This discrepancy between the dispersion of the modes and the polariton curve is ascribed to the effective cavity length varying with the refractive index of anthracene

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

    Science.gov (United States)

    Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

    2015-03-01

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

  3. The role of ligands in determining the exciton relaxation dynamics in semiconductor quantum dots.

    Science.gov (United States)

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

    2014-01-01

    This article reviews the mechanisms through which molecules adsorbed to the surfaces of semiconductor nanocrystals, quantum dots (QDs), influence the pathways for and dynamics of intra- and interband exciton relaxation in these nanostructures. In many cases, the surface chemistry of the QDs determines the competition between Auger relaxation and electronic-to-vibrational energy transfer in the intraband cooling of hot carriers, and between electron or hole-trapping processes and radiative recombination in relaxation of band-edge excitons. The latter competition determines the photoluminescence quantum yield of the nanocrystals, which is predictable through a set of mostly phenomenological models that link the surface coverage of ligands with specific chemical properties to the rate constants for nonradiative exciton decay. PMID:24364916

  4. Localized suppression of longitudinal-optical-phonon-exciton coupling in bent ZnO nanowires

    International Nuclear Information System (INIS)

    Using confocal micro-Raman and photoluminescence spectroscopy, we studied bending effects on optical properties of individual ZnO nanowires. Raman spectroscopy shows that local tensile strain can be introduced by bending the nanowire. The strain is expected to reduce the band gap on the bent part and modify the local phonon-exciton interaction. The corresponding micro-photoluminescence spectra indicate local suppression of the longitudinal-optical (LO) phonon-exciton interaction, which is determined by the intensity ratio of the second-order LO-phonon replica of the free exciton (FX-2LO) to the first-order process (FX-1LO). Our results may provide insight into the modulation of local electrical and optical properties by deforming the nanostructures.

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

    Science.gov (United States)

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

    2014-04-01

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

  6. Correlation effects in the optical spectra of porphyrin oligomer chains: Exciton confinement and length dependence

    Science.gov (United States)

    Hogan, Conor; Palummo, Maurizia; Gierschner, Johannes; Rubio, Angel

    2013-01-01

    Excited states of ethylene-linked free-base porphyrin oligomers and polymer are studied using many-body perturbation theory (MBPT) within the GW approximation and the Bethe-Salpeter equation. Trends in the electronic levels with oligomer length are analysed and the correct long-range behaviour in the band gap is obtained. High polarizabilities and strong redshifts in the optical absorption peaks are predicted in agreement with observations on other strongly conjugated oligoporphyrins. We explain these trends by means of spatial and spectral analyses of the exciton character. Although Wannier-Mott and charge-transfer excitons are identified in the optical spectra, the strongest polarizabilities are actually associated with small, tightly bound excitons (Frenkel-like), in contrast to expectations. Furthermore, the common procedure of extrapolating polymer properties from oligomer calculations is examined from a MBPT perspective.

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

    Science.gov (United States)

    Consani, Cristina; Koch, Federico; Panzer, Fabian; Unger, Thomas; Köhler, Anna; Brixner, Tobias

    2015-06-01

    Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution. By combining the results of the two techniques, we differentiate between an initial exciton relaxation, which is not characterized by significant exciton mobility, and energy transport between different chromophores in the aggregate.

  8. Ultrafast electron trapping at the surface of semiconductor nanocrystals: excitonic and biexcitonic processes.

    Science.gov (United States)

    Saari, Jonathan I; Dias, Eva A; Reifsnyder, Danielle; Krause, Michael M; Walsh, Brenna R; Murray, Christopher B; Kambhampati, Patanjali

    2013-04-25

    Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory. PMID:23186016

  9. Seasonal variations in halides in marine brown algae from Porbandar and Okha coasts (NW coast of India)

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, Ch.K.; Singbal, S.Y.S.

    1995-01-01

    percentage of enrichment factor) of halides shows that the tissue concentration of each halide was independent of other halides accumulation. The extent of bioaccumulation could be related to intrinsic property such as x space (spectroscopic electronegativity...

  10. Atomic force microscopy studies of alkali halide surfaces nanostructured by DIET

    Science.gov (United States)

    Goryl, M.; Such, B.; Krok, F.; Meisel, K.; Kolodziej, J. J.; Szymonski, M.

    2005-11-01

    We report on surface topography modification of single crystal alkali halides due to creation of the excitonic states by keV electron irradiation. The DIET—structured surfaces have been studied with nanometer scale resolution by means of a dynamic (non-contact) atomic force microscopy (DFM) in UHV. The force microscopy studies reveal that randomly spread rectangular pits of monolayer depth in the topmost layer of the crystal are formed during irradiation. Growth and coalescence of the pits lead to almost layer-by-layer desorption mode. It is demonstrated that varying surface topography affects the yield of both the halogen and the alkali atom desorption component, as well as velocity spectrum of desorbing halogen atoms (thermal versus non-thermal ratio). We propose a model in which periodic changes of the surface topography with the increasing electron fluence (from initially flat to rough at about half monolayer desorbed, back to flat after a complete monolayer removal) are modulating the surface recombination probability for the excited F-centers. By controlling the population of traps in the bulk these surface processes are causing modulation of the diffusion range of mobile defects migrating from the bulk of the material towards its surface and the bulk recombination probabilities of F- and H-centres.

  11. Recombination of radiation defects in alkali halide crystals at low temperature

    International Nuclear Information System (INIS)

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

  12. Magneto-optical and temperature investigations of exciton spectra in Cdsub(1-x)Mnsub(x)Se

    International Nuclear Information System (INIS)

    The results are presented of studying the temperature, DE Esub(ex)(T), and magnetic field, DE Esub(ex)(H), dependences of exciton reflection (ER) optical band shift and photoluminescence (RL) Cdsub(1-x)Mnsub(x)Se With H1=0.013, x2=0.1, x3=0.15. In Cdsub(0.85)Mnsub(0.15)Se peculiarities of the effect of the giant splitting of photoluminescence band related to exciton-impurity complex (EIC) are revealed. The shift DE Esub(ex)(H)=Esub(ex)(H)-Fsub(ex)(O) of the EIC exciton level is considerably less than the free exciton level shift DE Esub(ex)sup(ER)(H) > DE Esub(ex)sup(PL)(H). When measuring magnetic field dependences I(H) of the photoluminescence bands intensity it has been foUnd that with the field increasing from 0 to 30 kE the I(H) value sharply decreases (8 to 10 times) in the sample with x1=0.0013, while in the sample with x3=0.15 it increases 2-Q.5 times. As the authors statee the effects revealed are related to both the change in parameters of the exciton-phonop interaction and peculiarities of spin-spin interactions in mixed crystal

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

    Science.gov (United States)

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

    2015-01-01

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

  14. 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)

  15. Exciton Spin Dynamics in Semiconductor Quantum Wells

    OpenAIRE

    Amand, Thierry; Marie, Xavier

    2007-01-01

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

  16. Photoacoustic measurements of excitons in CdSe nanorods

    Energy Technology Data Exchange (ETDEWEB)

    Awad, H; Abdallah, T; Talaat, H [Physics Department, Faculty of Science, Ain Shams University, Abbassia, Cairo (Egypt); Mohammed, M B [NILES, Cairo University, Giza (Egypt); Easawi, K; Negm, S, E-mail: hassantalaat@hotmail.co [Department of Mathematical and Physical Engineering, Faculty of Engineering (Shoubra), Banha University, Cairo (Egypt)

    2010-03-01

    Photoacoustic (PA) spectra were obtained for CdSe nanorods (NRs) of different aspect ratios, prepared via the organometallic synthesis. The second derivative spectra were used to have an accurate determination of the different excitonic transitions. Using the lowest transition energy (band gap) 1S(e)-1S1/2(h) and applying the effective mass model, the NRs diameters were determined. The obtained diameters were then compared to direct measurements of scanning tunneling microscopy (STM) and XRD. It is observed that the band gap depends on the diameter of the rods due to quantum confinement effect, since diameters are of the order of the bulk CdSe Bohr radius. The second derivative of the PA spectra for CdSe NRs also shows clearly a second excitonic transition1P(e)-1P1/2(h) in contrast to UV-Vis absorption spectra carried out for colloidal samples. The thermal parameters for samples were also measured and compared to the bulk values.

  17. Relaxation time in the exciton model

    International Nuclear Information System (INIS)

    An equation for the time-dependent mean number of excitons is derived from the system of kinetic equations of the exciton model. The relaxation time is obtained as a function of the excitation energy and the number of nucleons, both with and without the effect of the Pauli exclusion principle in the densities of states with a fixed number of excitons. An equation for the mean square fluctuation of the number of excitons is also obtained. We discuss the connection between the equations of the exciton model and the stochastic differential equation for the number of collisions in a medium with a random distribution of scatterers having a time-dependent average density

  18. High Pressure Electrochemistry: Application to silver halides

    Science.gov (United States)

    Havens, K.; Kavner, A.

    2007-12-01

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

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

    Directory of Open Access Journals (Sweden)

    Mihai E. Vaida

    2011-09-01

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

  20. Gastric Banding

    Science.gov (United States)

    ... two FDA approved gastric banding devices on the market designed to treat obesity: Lap-Band Gastric Banding ... moving of the gastric band from its original position, requiring another surgery to reposition it erosion of ...

  1. Reaction, diffusion and dissociation of excitons on carbon nanotubes

    OpenAIRE

    Allam, J.; Sutton, R.; Sajjad, Mt; Litvinenko, K.; Wang, Z.; Siddique, S.; Yang, Q-h; Brown, T.; Loh, W.

    2010-01-01

    Photoexcitation of carbon nanotubes generates excitons which decay by exciton-exciton annihilation at sufficient density. We examine this decay under conditions of one, few and many excitons per nanotube. A classic ID reaction-diffusion behaviour is observed, with decay limited by diffusion for t>3ps and by reaction for t

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

    International Nuclear Information System (INIS)

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

  3. Process for oxidation of hydrogen halides to elemental halogens

    Science.gov (United States)

    Lyke, Stephen E. (Middleton, WI)

    1992-01-01

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

  4. Design and construction of a copper-halide laser

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

    Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm2/(eVs).

  6. Excitation Energy Dependence of the Exciton Inner Ring

    OpenAIRE

    Kuznetsova, Y. Y.; Leonard, J. R.; Butov, L. V.; Wilkes, J.; Muljarov, E. A.; Campman, K. L.; Gossard, A. C.

    2012-01-01

    We report on the excitation energy dependence of the inner ring in the exciton emission pattern. The contrast of the inner ring is found to decrease with lowering excitation energy. Excitation by light tuned to the direct exciton resonance is found to effectively suppress excitation-induced heating of indirect excitons and facilitate the realization of a cold and dense exciton gas. The excitation energy dependence of the inner ring is explained in terms of exciton transport ...

  7. Magnetic field tuning of exciton polaritons in a semiconductor microcavity

    OpenAIRE

    Pietka, B.; D. Zygmunt; M. Kr\\xf3l; M. R. Molas; Nicolet, A. A. L.; F. Morier-Genoud; Szczytko, J.; J. \\u0141usakowski; Zieba, P; I. Tralle; P. Stepnicki; Matuszewski, M.; Potemski, M.; Deveaud, B.

    2015-01-01

    We detail the influence of a magnetic field on exciton-polaritons inside a semiconductor microcavity. Magnetic field can be used as a tuning parameter for exciton and photon resonances. We discuss the change of the exciton energy, the oscillator strength, and redistribution of the polariton density along the dispersion curves due to the magnetically induced detuning. We have observed that field-induced shrinkage of the exciton wave function has a direct influence not only on the exciton oscil...

  8. Exciton/Charge-transfer Electronic Couplings in Organic Semiconductors

    OpenAIRE

    Difley, Seth; Van Voorhis, Troy

    2010-01-01

    Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for obtaining CT?exciton electronic couplings between CT states and excitons is presented. This method is applied to two organic heterodimers to obtain their CT?exciton coupling and adiabatic energy surfaces near their CT?exciton diabatic surface crossings. The results show ...

  9. Optical nutation in the exciton range of spectrum

    International Nuclear Information System (INIS)

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

  10. Optical nutation in the exciton range of spectrum

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-08-15

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

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

    Science.gov (United States)

    Kasahara, Yuichi; Kuroki, Kazuhiko; Yamanaka, Shoji; Taguchi, Yasujiro

    2015-07-01

    In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides MNX (M = Ti, Zr, Hf; X = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors.

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

    Science.gov (United States)

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

    2002-12-01

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

  13. Time-dependent density-functional theory of exciton-exciton correlations in the nonlinear optical response

    OpenAIRE

    Turkowski, Volodymyr; Michael N. Leuenberger

    2013-01-01

    We analyze possible nonlinear exciton-exciton correlation effects in the optical response of semiconductors by using a time-dependent density-functional theory (TDDFT) approach. For this purpose, we derive the nonlinear (third-order) TDDFT equation for the excitonic polarization. In this equation, the nonlinear time-dependent effects are described by the time-dependent (non-adiabatic) part of the effective exciton-exciton interaction, which depends on the exchange-correlatio...

  14. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system.

    Science.gov (United States)

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-05-01

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system. PMID:25641931

  15. Analysis of the exciton-exciton interaction in semiconductor quantum wells

    OpenAIRE

    Schindler, Christoph; Zimmermann, Roland

    2008-01-01

    The exciton-exciton interaction is investigated for quasi-two-dimensional quantum structures. A bosonization scheme is applied including the full spin structure. For generating the effective interaction potentials, the Hartree-Fock and Heitler-London approaches are improved by a full two-exciton calculation which includes the van der Waals effect. With these potentials the biexciton formation in bilayer systems is investigated. For coupled quantum wells the two-body scatteri...

  16. Dynamique collective des excitons et exciton-polaritons dans des hétérostructures nanométriques

    OpenAIRE

    Visnevski, Dmitri

    2013-01-01

    Dans ma thèse, je discute des phénomènes collectifs dynamiques impliquant des excitons et des exciton-polaritons dans des nanostructures de semiconducteurs. Dans le premier chapitre j'introduis brièvement des éléments de physique des semiconducteurs. Les quatre chapitres suivants sont dédiés à la présentation de résultats originaux. Le chapitre 2 décrit les phénomènes d'interaction cohérente entre phonons et condensats d'exciton. Le chapitre 3 décrit un laser à boite quantiqu...

  17. Excitons and biexcitons bound to a positive ion in a bismuth-doped inorganic-organic layered lead iodide semiconductor

    Science.gov (United States)

    Fujisawa, Jun-Ichi; Ishihara, Teruya

    2004-11-01

    We have studied optical properties of an inorganic-organic layered lead iodide semiconductor doped with bismuth ions (Bi3+) . A new absorption band was observed at 2.15eV that is by 0.21eV lower than two-dimensional 1s excitons (2.36eV) and is assigned to excitons bound to the bismuth ions. In the photoluminescence spectra with high excitation density (˜0.4MW/cm2) , a novel emission band was observed at the energy of 2.51eV that is by 0.15eV higher than the exciton resonance, and is attributed to biexcitons bound to the bismuth ions. On the basis of the experimental results, the binding energies of the bismuth-bound exciton and biexciton were estimated approximately as 210 and 10meV , respectively. It is found that the binding energy of the positive-ion (Bi3+) bound biexciton is by far smaller than that of the corresponding bound exciton. The much lower stability of the positive-ion bound biexciton is discussed.

  18. Quantum-dot excitons in nanostructured environments

    DEFF Research Database (Denmark)

    Hvam, JØrn Märcher Technical University of Denmark,

    2010-01-01

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

  19. Spontaneous Exciton Dissociation in Carbon Nanotubes

    Science.gov (United States)

    Kumamoto, Y.; Yoshida, M.; Ishii, A.; Yokoyama, A.; Shimada, T.; Kato, Y. K.

    2014-03-01

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies.

  20. Harvesting Excitons Through Plasmonic Strong Coupling

    CERN Document Server

    Gonzalez-Ballestero, Carlos; Moreno, Esteban; Garcia-Vidal, Francisco J

    2015-01-01

    Exciton harvesting is demonstrated in an ensemble of quantum emitters coupled to localized surface plasmons. When the interaction between emitters and the dipole mode of a metallic nanosphere reaches the strong coupling regime, the exciton conductance is greatly increased. The spatial map of the conductance matches the plasmon field intensity profile, which indicates that transport properties can be tuned by adequately tailoring the field of the plasmonic resonance. We exploit this effect in a plasmonic nanostructure composed of three metal nanospheres, showing efficient transport of excitons into a very deep sub- wavelength region.

  1. Spectrum of exciton in a quantum wire

    International Nuclear Information System (INIS)

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

  2. Spontaneous exciton dissociation in carbon nanotubes.

    Science.gov (United States)

    Kumamoto, Y; Yoshida, M; Ishii, A; Yokoyama, A; Shimada, T; Kato, Y K

    2014-03-21

    Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. The correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons why photoconductivity measurements in single-walled carbon nanotubes are straightforward despite the large exciton binding energies. PMID:24702413

  3. Strong localization induced anomalous temperature dependence exciton emission above 300 K from SnO2 quantum dots

    Science.gov (United States)

    Pan, S. S.; Li, F. D.; Liu, Q. W.; Xu, S. C.; Luo, Y. Y.; Li, G. H.

    2015-05-01

    SnO2 quantum dots (QDs) are potential materials for deep ultraviolet (DUV) light emitting devices. In this study, we report the temperature and excitation power-dependent exciton luminescence from SnO2 QDs. The exciton emission exhibits anomalous blue shift, accompanied with band width reduction with increasing temperature and excitation power above 300 K. The anomalous temperature dependences of the peak energy and band width are well interpreted by the strongly localized carrier thermal hopping process and Gaussian shape of band tails states, respectively. The localized wells and band tails at conduction minimum are considered to be induced by the surface oxygen defects and local potential fluctuation in SnO2 QDs.

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

    International Nuclear Information System (INIS)

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

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

    Science.gov (United States)

    Lai, Ying-Yu; Chen, Jee-Wei; Chang, Tsu-Chi; Chou, Yu-Hsun; Lu, Tien-Chang

    2015-03-01

    We report on the fabrication and characterization of a membrane-type ZnO microcavity (MC). The ZnO membrane was cut from a single crystalline ZnO substrate by using focused ion beam milling, and was then placed onto a SiO2 substrate by using glass microtweezers. Through changing the pumping regime, manipulation of P-band exciton lasing and whispering-gallery mode (WGM) photon lasing could be easily achieved. P-band exciton lasing was observed only when the pumping laser was focused at the center of the ZnO MC with a small pumping size because of the innate ring-shaped WGM distribution. Furthermore, the lasing threshold of the ZnO MC could be reduced to an order lower by using a larger pumping spot because of the more favorable spatial overlap between the optical gain and WGM.

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

    Science.gov (United States)

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

    2015-01-01

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

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

    CERN Document Server

    Duque-Gomez, Federico

    2014-01-01

    We study the linear optical absorption of bulk semiconductors in the presence of a homogeneous constant (dc) electric field with an approach suitable for including excitonic effects while working with many-band models. The absorption coefficient is calculated from the time evolution of the interband polarization excited by an optical pulse. We apply the formalism to a numerical calculation for GaAs using a 14-band $\\textbf{k} \\cdot \\textbf{p}$ model, which allows us to properly include interband coupling, and the exchange self-energy to account for the excitonic effects due to the electron-hole interaction. The Coulomb interaction enhances the features of the absorption coefficient captured by the $\\textbf{k} \\cdot \\textbf{p}$ model. We consider the dependence of the enhancement on the strength of the dc field and the polarization of the optical field.

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

    Science.gov (United States)

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

    1986-01-01

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

  9. Exciton spectra of the ZnSe1-xTex solid solutions

    International Nuclear Information System (INIS)

    Photoluminescence and reflection spectra in the region of the self-absorption edge of ZnSe1-xTex solid solution crystals in the whole composition range were investigated at T=2 K. Excition line in reflection spectra is observed only for samples with the content of TeX?2% and X >or approx. 40%. It is established that for compositions 12% a wide luminescence band testifying to the close spatial excitation localization is observed. Further increase of Te content (x>30%) is accompanied with luminescence band necking. Exciton localization in this case is performed with large-scale composition fluctuations. Energy migration begins to reveal in luminescence spectra of samples with x>60% along with the exciton localization

  10. Far IR spectra of Th(IV) halide complexes of some heterocyclic bases

    International Nuclear Information System (INIS)

    The synthesis and IR spectra of Th(IV) perchlorato, nitrato and thiocyanato complexes of some heterocyclic bases have been reported. Halogens are common ligands in coordination chemistry forming coordinate bonds with metals readily. Metal halogen (M-X) stretching bands show a strong absorption in the far-IR region. Very little information is available on Th-X stretching frequencies. In the present communication, adducts of Th(IV) halide with certain nitrogen heterocyclic bases such as pyridine, ?-picoline, 2-amino pyridine, 2:4-lutidine, 2:6-lutidine, quinoline, 2,2'-bipyridine and 1,10-phenanthroline were synthesised and characterised. Experimental details are given. Results are presented and discussed. (author)

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

    Scientific Electronic Library Online (English)

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

    2008-11-01

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

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

    OpenAIRE

    Aichmayr, G.; M. Jetter; Vina, L.; Dickerson, J.; Camino, F.; Mendez, E. E.

    1999-01-01

    We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\\sim 4$ meV to zero, at a field of $\\sim $35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separat...

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

    International Nuclear Information System (INIS)

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

  14. Layered structures of organic/inorganic hybrid halide perovskites

    OpenAIRE

    Huan, Tran Doan; Tuoc, Vu Ngoc; Minh, Nguyen Viet

    2015-01-01

    Organic/inorganic hybrid halide perovskites, formed by substituting the cations A of ABX$_3$ halide perovskites with certain organic cations, may be used for solar thermoelectric applications. In this work, we systematically study three lead-free hybrid perovskites, i.e., methylammonium tin iodide CH$_3$NH$_3$SnI$_3$, ammonium tin iodide NH$_4$SnI$_3$, and formamidnium tin iodide HC(NH$_2$)$_2$SnI$_3$, by first-principles calculations. We find that in addition to the commonl...

  15. 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)

  16. Exciton Energy Transfer in Pairs of Single-Walled Carbon Nanotubes

    OpenAIRE

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

    2008-01-01

    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 internanotube transfer. The range of efficient energy transfer is found to be limited to a few nanometers because of competing fast nonradiative relaxation responsible for low photoluminescence quantum yield.

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

    Directory of Open Access Journals (Sweden)

    A. P\\u00E9rez-Rodr\\u00EDguez

    2006-01-01

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

  18. Dynamics of excitons and trions in semiconducting carbon nanotubes

    Science.gov (United States)

    Okano, Makoto; Nishihara, Taishi; Yamada, Yasuhiro; Kanemitsu, Yoshihiko

    2013-03-01

    We report the optical properties and exciton dynamics of undoped and hole-doped single-walled carbon nanotubes (SWCNTs). In the one-dimensional structures of SWCNTs, an electron and a hole form an exciton with a huge exciton binding energy. Stable excitons govern the optical responses of SWCNTs even at room temperature. With hole doping of SWCNTs, new peaks due to positive trions (positively charged excitons) appear below the E11 exciton peaks in the absorption and photoluminescence spectra. Trions are also stable at room temperature. Using femtosecond pump-probe transient absorption spectroscopy, we revealed that the exciton decay dynamics depends on the number of holes in SWCNTs. The exciton lifetime of hole-doped SWCNTs is much shorter than that of undoped SWCNTs. Fast decay components with lifetimes of a few picoseconds are attributed to trion formation and exciton-hole scattering in holedoped SWCNTs.

  19. Excitonic nonlinearities in single-wall carbon nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Nguyen, D.T.; Voisin, C.; Roussignol, P. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Roquelet, C.; Lauret, J.S. [Laboratoire de Photonique Quantique et Moleculaire de l' Ecole Normale Superieure de Cachan (France); Cassabois, G. [Laboratoire Pierre Aigrain, Ecole Normale Superieure, UPMC, Universite Paris Diderot, CNRS UMR8551, Paris (France); Laboratoire Charles Coulomb, UMR5221, Universite Montpellier 2, Montpellier (France); CNRS, Laboratoire Charles Coulomb, UMR5221, Montpellier (France)

    2012-05-15

    Excitons are composite bosons that allow a fair description of the optical properties in solid state systems. The quantum confinement in nanostructures enhances the excitonic effects and impacts the exciton-exciton interactions, which tailor the performances of classical and quantum optoelectronic devices, such as lasers or single-photon emitters. The excitonic nonlinearities exhibit significant differences between organic and inorganic compounds. Tightly bound Frenkel excitons in molecular crystals are for instance affected by an efficient exciton-exciton annihilation (EEA). This Auger process also governs the population relaxation dynamics in carbon nanotubes that share many physical properties with organic materials. Here, we show that this similarity breaks down for the excitonic decoherence in carbon nanotubes. Original nonlinear spectral-hole burning experiments bring evidence of pure dephasing induced by exciton-exciton scattering (EES) in the k-space. This mechanism controls the exciton collision-induced broadening, as for Wannier excitons in inorganic semiconductors. We demonstrate that this singular behavior originates from the intrinsic one-dimensionality of excitons in carbon nanotubes, which display unique hybrid features of organic and inorganic systems. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

  20. Topologically protected excitons in porphyrin thin films.

    Science.gov (United States)

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

    2014-11-01

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

  1. Giant Rydberg Excitons in Cuprous Oxide

    CERN Document Server

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

    2014-01-01

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

  2. Cavity-Enhanced Transport of Excitons

    Science.gov (United States)

    Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido

    2015-05-01

    We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.

  3. Cavity-enhanced transport of excitons.

    Science.gov (United States)

    Schachenmayer, Johannes; Genes, Claudiu; Tignone, Edoardo; Pupillo, Guido

    2015-05-15

    We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions. PMID:26024186

  4. Exciton spectra of orthorhombic indium bromide

    International Nuclear Information System (INIS)

    Absorption and reflection spectra of InBr single crystal are measured at 2 K with polarized light. Dichroic exciton peaks are observed in the reflection spectrum around 2.33 eV. Binding energy of the direct exciton for E parallel c is estimated as (12 +- 1) meV. Indirect absorption edge is confirmed for E parallel c by using a wavelength modulation technique. The indirect exciton gap is determined to be 2.129 eV. Absorption components associated with five different phonon energies are resolved, one of which is presumably due to a two-phonon process. The binding energy of the indirect exciton is estimated to be (6.9 +- 0.4) meV. The indirect gap increases rapidly with increasing temperature. (author)

  5. Excitonic Coherent States: Symmetries and Thermalization

    Science.gov (United States)

    Cirilo-Lombardo, Diego Julio

    2015-04-01

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

  6. Can disorder enhance incoherent exciton diffusion?

    CERN Document Server

    Lee, Elizabeth M Y; Willard, Adam P

    2015-01-01

    Recent experiments aimed at probing the dynamics of excitons have revealed that semiconducting films composed of disordered molecular subunits, unlike expectations for their perfectly ordered counterparts, can exhibit a time-dependent diffusivity in which the effective early time diffusion constant is larger than that of the steady state. This observation has led to speculation about what role, if any, microscopic disorder may play in enhancing exciton transport properties. In this article, we present the results of a model study aimed at addressing this point. Specifically, we present a general model, based upon F\\"orster theory, for incoherent exciton diffusion in a material composed of independent molecular subunits with static energetic disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule transition rates which we demonstrate has two important consequences related to exciton transport. First, the distribution of local site-specific diffusivity is broadened in a manner that results i...

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

    Science.gov (United States)

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

    2014-08-01

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

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

    Directory of Open Access Journals (Sweden)

    Christian Wehrenfennig

    2014-08-01

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

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

    OpenAIRE

    Frazer, Laszlo; Schaller, Richard D.; Ketterson, J. B.

    2013-01-01

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

  10. Unexpectedly slow two particle decay of ultra-dense excitons in cuprous oxide

    Science.gov (United States)

    Laszlo Frazer, N.; Schaller, Richard D.; Ketterson, J. B.

    2013-09-01

    For an ultra-dense exciton gas in cuprous oxide (Cu2O), exciton-exciton interactions are the dominant cause of exciton decay. This study demonstrates that the accepted Auger recombination model overestimates the exciton decay rate following intense two photon excitation. Two exciton decay is relevant to the search for collective quantum behavior of excitons in bulk systems. These results suggest the existence of a new high density regime of exciton behavior.

  11. EXCITON LOCALIZATION IN STRUCTURALLY DISORDERED MATERIALS

    OpenAIRE

    BLUMEN, A; Lemaistre, J.-P.

    1985-01-01

    We study the localization of excitonic eigenstates in structurally disordered materials. For two-level systems the problem leads to the quantum-percolation model, of which the Anderson-localization is a special case. The general Hamiltonian admits both diagonal and off-diagonal randomness. Applications to the triplet excitons of naphthalene and anthracene mixed crystals involve mostly the diagonal disorder, on which we focus. As localization criterium we use the inverse participation ratios o...

  12. Förster Coupling in Nanoparticle Excitonic Circuits

    OpenAIRE

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

    2010-01-01

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

  13. 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

  14. Phase-Resolved Imaging of Exciton Polaritons

    OpenAIRE

    Nardin, Gae?l

    2011-01-01

    In this PhD thesis, new imaging techniques have been developed in order to explore the physics of semiconductor microcavities. In these structures, composite bosons called exciton polaritons are the result of strong coupling between the cavity mode and quantum well excitons. A spectroscopic imaging technique has been developed to image the eigenstates of polaritons confined in the traps of a patterned GaAs microcavity. Polariton probability densities ...

  15. Exciton in closed and opened quantum dot

    OpenAIRE

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

    2007-01-01

    The theory of exciton spectrum in spherically symmetric states for the three- shell closed spherical quantum dot is proposed. The evolution of the exciton spectrum while varying the outer well thickness from zero (stationary spectrum of single closed spherical quantum dot) to infinity (quasistationary spectrum of a single open spherical quantum dot) is investigated. The mechanism of damping (semiwidth) of quasistationary states due to the redistribution over the energy levels of probability o...

  16. Electric-field modulation of excitons in two-dimensions

    Science.gov (United States)

    Cocoletzi, Gregorio H.; Ulloa, Sergio E.

    1996-03-01

    In this paper we study the behavior of excitons confined to a quantum well in the z-direction, in addition to an applied field E_x(x) which exhibits strong x-dependence. Such modulation can now be achieved experimentally by interdigitated gates deposited near a quantum well. The field modulation produces a strong spatial separation of the electron and hole of the system and, very importantly, a folding of the center of mass dispersion relation to an effective Brillouin zone due to the periodicity of the field. This folding of the dispersion produces center-of-mass minibands and associated features in the photoluminescence, as the recombination processes can now occur with various values of center-of-mass momentum. We present the behavior of these additional and novel features with respect to structure parameters and applied electric fields, as the first few minibands of the system are calculated using a variational function which considers the band-folding effects and polarization of the exciton in this geometry. Supported by CONACYT Grant No. 481100--5--5264E ^** Supported by the US DOE

  17. Optimization of exciton currents in photosynthetic systems

    Science.gov (United States)

    Guan, Chengbo; Wu, Ning; Zhao, Yang

    2013-03-01

    In an approach analogous to that used to treat electronic currents in semiconductor quantum dots, we investigate the exciton current in a pigment network that is sandwiched between two exciton reservoirs, also known as the emitter and the acceptor. Employing the master equation for the reduced density matrix, the exciton current is obtained analytically for a two-site model, and numerically for an eight-site Fenna-Matthews-Olson (FMO) subunit model. It is found that, to maximize the exciton current with a specific network configuration, there exist optimal emitter temperatures and exciton transfer rates between the network and the reservoirs. The steady state current in the FMO model is consistent with the trapping time calculated by network optimization in the one-exciton picture. The current optimization with respect to various control parameters is discussed for the FMO model. At and below the biologically relevant transfer rate 1 ps-1, the FMO network is more efficient for excitation energy transfer than the two-site model. Beyond this scale, the FMO network shows robustness with respect to the interplay with the reservoirs.

  18. The phenomenon of exciton-fermion condensation

    International Nuclear Information System (INIS)

    Conditions are investigated under which a finite density of excitons can arise spontaneously, which then mediate BCS-type pairing of free electrons through virtual excitonic transitions involving higher state (excited) excitonic levels. This cooperative exciton-fermion condensation, which takes place primarily via a first-order phase transition that is close to second order, is characterized by large values of the ratio of the superconducting gap to the transition temperature, even when the coupling constant is small. In the limit of small free-electron densities and high temperatures, polaron effects give rise to a discontinuity in the exciton concentration, leading to a considerable relaxation of the conditions for subsequent appearance of the superconducting condensate. It is shown that the maximum transition temperatures for a given position of the Fermi level correspond to a line of critical points. Near this critical line, the nature of the exciton transition turns out to be irrelevant, which eliminates any direct or indirect influence of the isotopic shift on the transition temperature. The applicability of the model to layered superconducting oxides is discussed

  19. Exciton Bose condensation: the ground state of an electron-hole gas. I. Mean field description of a simplified model

    International Nuclear Information System (INIS)

    We consider an electron-hole gas in a simple model semiconductor, with direct gap and isotropic, non degenerate bands. We study the Bose condensed ground state of that system as a function of density, using a mean field variational ansatz. In a first-stage, we ignore screening as well as the spin structure of the carriers. We thus describe the smooth transition between Bose condensation of atomic excitons at low densities, and the ''excitonic insulator'' state and ultimately electron-hole plasma at high densities. As compared to previous treatments, our approach includes the effect of electron-hole pairing on the ground state, within a simple realistic ansatz

  20. Exciton-polariton emission from organic semiconductor optical waveguides

    CERN Document Server

    Ellenbogen, Tal

    2011-01-01

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

  1. Exciton dephasing in ZnSe quantum wires

    OpenAIRE

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

    2006-01-01

    The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering within the wire due to the electron trapping in surface states and exciton localization. The exciton-exciton scattering efficiency, determined by the density dependence of the exciton dephasing, is f...

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

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

    A reduction in the quantity of gelatinous solids which are formed in aqueous zirconium-fluoride nuclear reprocessing waste solutions by calcium nitrate added to suppress halide volatility during calcination of the solution while further suppressing chloride volatility is achieved by increasing the aluminum to fluoride mole ratio in the waste solution prior to adding the calcium nitrate

  4. Extraction of tellurium halide complexes with tetrasubstituted alkylamines

    International Nuclear Information System (INIS)

    Solvent extraction of tellurium (4) from halogen hydracid acid solutions by tetrabutyl-ethylenediamine, tetra exylethylenediamine, tetrhcaoyetylenediamine has been studied. The composition of the halide complexes of tellurium, extracted by the alkyldiamines, has been determined. A method of separation of tellurium from lead tin and iron and a procedure of extraction-photometric determination of tellurium in ores and cast iron have been developed

  5. Alternative route to metal halide free ionic liquids

    International Nuclear Information System (INIS)

    An alternative synthetic route to metal halide free ionic liquids using trialkyloxonium salt is proposed. Utility of this synthetic route has been demonstrated by preparing 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid through the reaction between 1-methylimidazole and triethyloxonium tetra-fluoroborate in anhydrous ether. (author)

  6. CO2 electrochemical reduction via adsorbed halide anions

    Science.gov (United States)

    Ogura, Kotaro; Salazar-Villalpando, Maria D.

    2011-01-01

    The electrochemical reduction of CO2 was studied utilizing halide ions as electrolytes, specifically, aqueous solutions of KCl, KBr, KI. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were a Pt wire and an Ag/AgCl electrode, respectively. The results of our work suggest a reaction mechanism for the electrochemical reduction of CO2 where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO2. Electron-transfer to CO2 may occur via the X- ad(Br-, Cl-, I-)-C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO2. The stronger the adsorption of the halide anion to the electrode, the more strongly CO2 is restrained, resulting in higher CO2 reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons; leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the over potential necessary for CO2 reduction, and thus increase the rate of electrochemical CO2 reduction.

  7. Different temperature dependence of excitonic and defect-related photoluminescence spectra in ZnS nanobelts and nanowires

    International Nuclear Information System (INIS)

    In this paper, both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultraviolet (UV) and visible emission regions. In UV emission regions, due to high-quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (Do, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10 K. The peak and relative intensity of the FX and (Do, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, weak donor-acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires are commonly observed, but the Y band emission is only observed at 10 K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50 K. The peak position and full width at half maximum of DAP and SA emission bands display different temperature dependences. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures. (paper)

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

    Science.gov (United States)

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

    2015-02-01

    Monolayer transition-metal dichalcogenides are direct gap semiconductors with great promise for optoelectronic devices. Although spatial correlation of electrons and holes plays a key role, there is little experimental information on such fundamental properties as exciton binding energies and band gaps. We report here an experimental determination of exciton excited states and binding energies for monolayer WS2 and WSe2. We observe peaks in the optical reflectivity/absorption spectra corresponding to the ground- and excited-state excitons (1s and 2s states). From these features, we determine lower bounds free of any model assumptions for the exciton binding energies as E2sA-E1sA of 0.83 eV and 0.79 eV for WS2 and WSe2, respectively, and for the corresponding band gaps Eg?E2sA of 2.90 and 2.53 eV at 4 K. Because the binding energies are large, the true band gap is substantially higher than the dominant spectral feature commonly observed with photoluminescence. This information is critical for emerging applications, and provides new insight into these novel monolayer semiconductors.

  9. Photoluminescence temperature behavior and Monte Carlo simulation of exciton hopping in InGaN multiple quantum wells

    International Nuclear Information System (INIS)

    Application of Monte Carlo simulation of exciton (carrier) hopping for the analysis of the photoluminescence (PL) temperature behavior in In0.2Ga0.8N/GaN multiple quantum wells is reported. The PL linewidth and peak position measured in the 10-300 K range exhibited a W-shaped and S-shaped temperature behavior, respectively. The W-shaped linewidth dependence was fitted with the results of Monte Carlo simulation, which involved phonon-assisted exciton hopping through energy states confined in the band potential fluctuation minima. The simulation yielded the values of the standard deviation for potential fluctuations within In-rich regions (31 meV), dispersion of the average exciton energy in different regions (29 meV), and the temperature dependence of the band gap, which was found to be in a fair agreement with the photoreflectance data. Our results, which infer in-plane motion of localized excitons within the wells, are consistent with the model of large In-rich regions (''segmented quantum wells'' or ''quantum discs'') with band potential fluctuations inside these regions. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

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

    International Nuclear Information System (INIS)

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

  11. Theory for Optical Absorption, Direct and Indirect Excitons in BaBiO 3

    Science.gov (United States)

    Hasanuzzaman, S.; Iwano, Kaoru; Nasu, Keiichiro

    1999-04-01

    The infrared and visible spectral shapes of optical absorption in a three dimensional charge density wave (CDW) state of BaBiO3 (BBO) are theoretically studied by using an extended Peierls-Hubbard model, so as to clarify the direct and indirect exciton effects in this material. For this purpose we introduce the adiabatic approximation for phonons and the Hartree-Fock approximation for inter-electron coulombic interactions. The electron-hole correlation on the Bi atoms and the classical fluctuation of the oxygen sub-lattice coordinates are also taken into account, so as to obtain exciton effects as well as thermal fluctuations of the lattice. The energy splitting due to CDW formation in the 6s band of Bi is found to be very large (2.0 eV), whereas, the minimum gap is much smaller (0.55 eV) than it. This large optical gap appears in the visible region as a dominant peak of the optical conductivity, and it is mainly due to the direct exciton. While the smaller gap appears in the infrared region, and it is due to the indirect exciton.This theory well agrees with the results of recent optical experiments on BaBiO3.

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

    Science.gov (United States)

    Spano, F C

    2015-05-14

    The properties of polaritons in J-aggregate microcavities are explored using a Hamiltonian which treats exciton-vibrational coupling and exciton-photon coupling on equal footing. When the cavity mode is resonant with the lowest-energy (0-0) transition in the J-aggregate, two polaritons are formed, the lowest-energy polariton (LP) and its higher-energy partner (P1), separated by the Rabi splitting. Strong coupling between the material and cavity modes leads to a decoupling of the exciton and vibrational degrees of freedom and an overall reduction of disorder within the LP. Such effects lead to an expanded material coherence length in the LP which leads to enhanced radiative decay rates. Additional spectral signatures include an amplification of the 0-0 peak coincident with a reduction in the 0-1 peak in the photoluminescence spectrum. It is also shown that the same cavity photon responsible for the LP/P1 splitting causes comparable splittings in the higher vibronic bands due to additional resonances between vibrationally excited states in the electronic ground state manifold and higher energy vibronic excitons. PMID:25978905

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

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

    We report on the broadband resonant energy transfer processes observed in dye doped gold nanoshells, consisting of spherical particles with a dielectric core (SiO2) covered by a thin gold shell. The silica core has been doped with rhodamine B molecules in order to harness a coherent plasmon-exciton coupling between chromophores and plasmonic shell. This plasmon-exciton interplay depends on the relative spectral position of their bands. Here, we present a simultaneous double strong coupling plasmon-exciton and exciton-plasmon. Indeed, experimental observations reveal of a transmittance enhancement as function of the gain in a wide range of optical wavelengths (about 100?nm), while scattering cross sections remains almost unmodified. These results are accompanied by an overall reduction of chromophore fluorescence lifetimes that are a clear evidence of nonradiative energy transfer processes. The increasing of transmission in the range of 630–750?nm is associated with a striking enhancement of the extinction cross-section in the 510–630?nm spectral region. In this range, the system assumes super-absorbing features. This double behavior, as well as the broadband response of the presented system, represents a promising step to enable a wide range of electromagnetic properties and fascinating applications of plasmonic nanoshells as building blocks for advanced optical materials

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

    Science.gov (United States)

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

    2014-12-10

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

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

    CERN Document Server

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

    1999-01-01

    We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of $\\sim 4$ meV to zero, at a field of $\\sim $35 kV/cm. This decrease, due to the field-induced deformation of the exciton wavefunction, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole 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.

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

    Science.gov (United States)

    Aichmayr, G.; Jetter, M.; Viña, L.; Dickerson, J.; Camino, F.; Mendez, E. E.

    1999-09-01

    We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of ~4 meV to zero, at a field of ~35 kV/cm. This decrease, due to the field-induced deformation of the exciton wave function, 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.

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

    DEFF Research Database (Denmark)

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

    1999-01-01

    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.

  19. Exciton formation, relaxation, and decay in PCDTBT.

    Science.gov (United States)

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

    2010-12-15

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

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

    International Nuclear Information System (INIS)

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

  1. Intrinsic Exciton Linewidth in Monolayer Transition Metal Dichalcogenides

    Science.gov (United States)

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

    2015-03-01

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

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

    Directory of Open Access Journals (Sweden)

    Lochbrunner S.

    2013-03-01

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

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

    International Nuclear Information System (INIS)

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

  4. Coherence Length of Excitons in a Semiconductor Quantum Well

    OpenAIRE

    Zhao, Hui; Moehl, Sebastian; Kalt, Heinz

    2002-01-01

    We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300 ~ 400 nm, about 25 ~ 30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured ...

  5. Exciton condensation in semiconductor quantum wells in nonuniform electric field

    OpenAIRE

    Chernyuk, A. A.; Kopp, V. S.; Sugakov, V. I.

    2007-01-01

    The structure appearance in exciton density distribution in semiconductor double quantum well with transverse electric field applied is studied, in the case when the metal electrode contains a round window. It is suggested that there is exciton condensed phase, free energy of which can be described by phenomenological Landau model. For the exciton density determination the traditional theory of phase transitions was used generated for the case of the finite exciton lifetime,...

  6. Electron teleportation between quantum dots using virtual dark exciton

    OpenAIRE

    Combescot, M.; Betbeder-Matibet, O.; Voliotis, V.

    2006-01-01

    We here propose a mechanism to teleport electrons between quantum dots through the transformation of a virtual bright exciton into a dark exciton. This mechanism relies on the interactions of two composite bosons: a pair of electrons with opposite spins, trapped in two dots and an electron-hole pair in a free exciton coupled to an unabsorbed pump pulse, which makes it ``bright'' but virtual. This bright exciton first turns ``dark'' by dropping its electron and stealing the t...

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

    OpenAIRE

    Elistratov, Andrei; Lozovik, Yurii

    2015-01-01

    We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of tr...

  8. Intracavity optical pumping of J-aggregate microcavity exciton polaritons

    OpenAIRE

    Bulovic, Vladimir; Bradley, Michael Scott

    2010-01-01

    We demonstrate intracavity optical pumping of J-aggregate microcavity exciton polaritons. The use of ultrathin layer-by-layer J-aggregate thin films as the strongly coupled exciton medium allows for inclusion of a thermally evaporated luminescent cavity spacer layer, through which the lower-branch exciton-polariton states are resonantly pumped. We present a measurement of the lower-branch exciton-polariton occupation in room-temperature microcavity devices containing J-aggregated molecular th...

  9. Berry Phase Effect on Exciton Transport and Bose Einstein Condensate

    OpenAIRE

    Yao, Wang; Niu, Qian

    2008-01-01

    With exciton lifetime much extended in semiconductor quantum-well structures, their transport and Bose-Einstein condensation become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict spin-dependent topological transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and in...

  10. Optically induced rotation of a quantum dot exciton spin

    OpenAIRE

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

    2011-01-01

    We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initializes its spin state as a coherent superposition of its two non-degenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the exciton's spin direct...

  11. Frenkel versus charge-transfer exciton dispersion in molecular crystals

    OpenAIRE

    Cudazzo, Pierluigi; Gatti, Matteo; Rubio Secades, Ángel; Sottile, Francesco

    2013-01-01

    By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excito...

  12. Strong exciton-plasmon coupling in semiconducting carbon nanotubes

    OpenAIRE

    Bondarev, I.V.; Woods, L M; Tatur, K.

    2009-01-01

    We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (~1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption line shape exhibits Rabi splitting ~0.1 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube. We also show that the quantum confined Stark effect may be ...

  13. Exciton-Phonon Dynamics with Long-Range Interaction

    OpenAIRE

    Laskin, Nick

    2011-01-01

    Exciton-phonon dynamics on a 1D lattice with long-range exciton-exciton interaction have been introduced and elaborated. Long-range interaction leads to a nonlocal integral term in the motion equation of the exciton subsystem if we go from discrete to continuous space. In some particular cases for power-law interaction, the integral term can be expressed through a fractional order spatial derivative. A system of two coupled equations has been obtained, one is a fractional di...

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-01-01

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

  15. Spontaneous coherence of indirect excitons in a trap

    OpenAIRE

    High, Alexander A.; Leonard, Jason R.; Remeika, Mikas; Butov, Leonid V.; Hanson, Micah; Gossard, Arthur C.

    2011-01-01

    We report on the emergence of spontaneous coherence in a gas of indirect excitons in an electrostatic trap. At low temperatures, the exciton coherence length becomes much larger than the thermal de Broglie wavelength and reaches the size of the exciton cloud in the trap.

  16. Thermodynamic model of the macroscopically ordered exciton state

    OpenAIRE

    Andreev, Sergey

    2012-01-01

    We explain the experimentally observed instability of cold exciton gases and formation of a macroscopically ordered exciton state (MOES) in terms of a thermodynamic model accounting for the phase fluctuations of the condensate. We show that the temperature dependence of the exciton energy exhibits fundamental scaling behavior with the signature of the second order phase transition.

  17. Bose-Einstein Condensation of Excitons: Reply to Tikhodeev's Criticism

    OpenAIRE

    Loutsenko, I.; Roubtsov, D.

    2000-01-01

    The extended version of our reply to Comment on ``Critical Velocities in Exciton Superfluidity'' by S. G. Tikhodeev (Phys. Rev. Lett., 84 (2000), 3502 or from http://prl.aps.org/) is presented here. The principal question is discussed: does the moving exciton-phonon packet contain the coherent `nucleus', or the exciton-phonon condensate?

  18. Massive creation of entangled exciton states in semiconductor quantum dots

    OpenAIRE

    Hohenester, Ulrich

    2002-01-01

    An intense laser pulse propagating in a medium of inhomogeneously broadened quantum dots massively creates entangled exciton states. After passage of the pulse all single-exciton states remain unpopulated (self-induced transparency) whereas biexciton coherence (exciton entanglement) is generated through two-photon transitions. We propose several experimental techniques for the observation of such unexpected behavior.

  19. Resonance effects of excitons and electrons. Basics and applications

    Energy Technology Data Exchange (ETDEWEB)

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

    2013-08-01

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

  20. Isolated flat bands and spin-1 conical bands in two-dimensional lattices

    Science.gov (United States)

    Green, Dmitry; Santos, Luiz; Chamon, Claudio

    2010-08-01

    Dispersionless bands, such as Landau levels, serve as a good starting point for obtaining interesting correlated states when interactions are added. With this motivation in mind, we study a variety of dispersionless (“flat”) band structures that arise in tight-binding Hamiltonians defined on hexagonal and kagome lattices with staggered fluxes. The flat bands and their neighboring dispersing bands have several notable features: (a) flat bands can be isolated from other bands by breaking time-reversal symmetry, allowing for an extensive degeneracy when these bands are partially filled; (b) an isolated flat band corresponds to a critical point between regimes where the band is electron-like or hole-like, with an anomalous Hall conductance that changes sign across the transition; (c) when the gap between a flat band and two neighboring bands closes, the system is described by a single spin-1 conical-like spectrum, extending to higher angular momentum the spin-1/2 Dirac-like spectra in topological insulators and graphene; (d) some configurations of parameters admit two isolated parallel flat bands, raising the possibility of exotic “heavy excitons”; and (e) we find that the Chern number of the flat bands, in all instances that we study here, is zero.

  1. Hydrostatic deformation potentials and the question of exciton binding energies and splittings in aluminium nitride

    Science.gov (United States)

    Gil, Bernard

    2010-05-01

    By comparing a series of optical experiments performed on bulk aluminum nitride crystals and heteroepitaxial films, we determine the hydrostatic excitonic deformation potentials of AlN. The agreement between the whole available experimental data and our analysis consolidates this determination. Using the previously determined values of the valence-band deformation potentials which account for the strain-induced variation in the crystal-field splitting: d3=-8.19eV and d4=4.10eV we obtain values of -6.04 and 2.15 eV for the hydrostatic excitonic deformation potentials a1 and a2 in the context of the quasicubic approximation. This constitutes the first series of values coherent with the whole set of experimental data. The experimental value of 1s-2s splitting disagrees with the theory of excitons in anisotropic semiconductors. This disagreement, we attribute it to our poor knowledge of the valence-band dispersion relations of AlN and to the difficulty we face for including in the calculation plausible values for the anisotropic hole effective mass, dielectric constant.

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

    Science.gov (United States)

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

    1992-11-01

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

  3. Exciton-exciton interaction in GaAs/AlGaAs quantum wells under intense optical excitation

    International Nuclear Information System (INIS)

    The results of investigation of exciton photoluminescence (PL) spectra of GaAs/AlGaAs single quantum wells of different width, depending on optical excitation, are reported. PL brought about by exciton-exciton collisions is observed in the structures. It is shown that the emission due to collision of two-dimensional excitons has much in common with similar process in bulk semiconductors

  4. Theory of highly excited semiconductor nanostructures including Auger coupling: exciton-bi-exciton mixing in CdSe nanocrystals

    OpenAIRE

    Korkusinski, Marek; Voznyy, Oleksandr; Hawrylak, Pawel

    2011-01-01

    We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intuitive picture of breaking neutral multi-exction complexes into positively and negatively charged multi-exciton complexes is given. The general approach is illustrated by analyzing...

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

    OpenAIRE

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

    2012-01-01

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

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

    International Nuclear Information System (INIS)

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

  7. Image excitons and plasmon-exciton strong coupling in two-dimensional perovskite semiconductors

    Science.gov (United States)

    Niu, Wendy; Ibbotson, Lindsey A.; Leipold, David; Runge, Erich; Prakash, G. Vijaya; Baumberg, Jeremy J.

    2015-04-01

    We present evidence for "image biexcitons" within organic-inorganic perovskite-coated silver gratings. These composite quasiparticles are formed by the interaction between an exciton and its image in the metal mirror below, with binding energy 100 meV at room temperature. By changing the polar and azimuthal angles of the incident light, we observe strong coupling between excitons and surface plasmon polaritons on the grating, with Rabi splittings of 150 and 125 meV for the exciton and biexciton, respectively. A detailed analysis of the field polarizations and dipole orientations shows how these Rabi couplings arise from the strongly compressed field volume.

  8. Excitonic effects in the Casimir force: A-exciton in CdS

    Science.gov (United States)

    Hernández de la Luz, A. D.; Alvarado-García, A. F.; Cocoletzi, Gregorio H.; Esquivel-Sirvent, R.

    2004-12-01

    Calculations of the Casimir force are performed between parallel nonlocal excitonic semiconductor slabs. Using a frequency and wave vector dependent dielectric response we describe the An=1 excitonic transition in CdS and calculate the electromagnetic waves reflectivity in the free space between the slabs. The results of the normal incidence reflectivity coefficients are then employed to determine the Casimir forces, which are investigated as functions of the layer thickness and the vacuum gap width between slabs. Comparisons with the local case show that the excitons are important as they produce a change of the Casimir force within current experimental precision.

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

    Energy Technology Data Exchange (ETDEWEB)

    Holland, Justin M.; Cecala, David M.

    2015-05-26

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

  10. Excitons confined by split-gate potentials

    Science.gov (United States)

    Cocoletzi, Gregorio H.; Ulloa, Sergio E.

    1994-03-01

    Quasi-one-dimensional excitons in a GaAs-AlxGa1-xAs quantum well are studied; they are produced by an applied twin-split-gate potential which confines the particles laterally and allows free motion in one dimension. A variational approach is used to calculate the binding energies Eex and oscillator strength fex of these excitonic transitions as functions of the applied voltage and width of the induced potential wells. In the limit of high electrostatic confinement the excitons are strongly polarized and the system resembles a type II structure where electron and hole are spatially separated. The resulting Eex and fex show a strong dependence on applied voltage and structure width. Strong oscillations are found, which should be observed experimentally, as a consequence of subtle competition between confinement and Coulomb attraction.

  11. Exciton annihilation and diffusion in semiconducting polymers

    Science.gov (United States)

    Shaw, P. E.; Lewis, A. J.; Ruseckas, A.; Samuel, I. D. W.

    2006-08-01

    We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate ?, which was measured to be ? = (2.8+/-0.5)×10 -8 cm 3s -1 in MEH-PPV and ? = (5.2+/-1)×10 -10 cm 3s -1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1+/-0.2)×10 -9 cm 3s -1 and the diffusion length to 4.7 nm.

  12. Exciton spectra in MoSe2

    International Nuclear Information System (INIS)

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

  13. Optical model calculation of exciton decay rates

    International Nuclear Information System (INIS)

    The decay rate of an exciton is calculated using the imaginary part of both the phenomenological optical potential of Becchetti and Greenlees and the microscopic potential of Jeukenne et al. The decay rate thus calculated is found to be about the same for proton- and neutron-induced reactions and weakly sensitive to mass number. Kalbach's two body matrix element, obtained from the analysis of precompound reactions using the Williams exciton model, also leads to a decay rate in agreement with the phenomenological potential estimate. However, the exciton model of Gadioli et al. is inconsistent with optical model estimates. Further, remarks are made on the concept of a mean free path in finite nuclear matter

  14. 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

  15. Topological excitonic superfluids in three dimensions

    Energy Technology Data Exchange (ETDEWEB)

    Hankiewicz, Ewelina M. [Wuerzburg University (Germany); Kim, Youngseok; Gilbert, Matthew [University of Illinois, Urbana (Germany)

    2013-07-01

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

  16. Theory of lifetime of exciton incoherently created below its resonance frequency by inelastic scattering

    OpenAIRE

    Bamba, Motoaki; Wakaiki, Shuji; Ichida, Hideki; Mizoguchi, Kohji; Kim, DaeGwi; Nakayama, Masaaki; Kanematsu, Yasuo

    2014-01-01

    When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process, because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is cre...

  17. Quantum Dot Lattice Embedded in An Organic Medium: Hybrid Exciton State and Optical Response

    OpenAIRE

    Huong, Nguyen Que; Birman, Joseph L.

    1998-01-01

    We propose a new model to implement organic exciton-semiconductor exciton hybridization by embedding a semiconductor quantum dot array into an organic medium. A Wannier-Mott transfer exciton is formed when the exciton in each semiconductor dot interacts via the multipole-multipole coupling with other excitons in the different dtos of the array. A new hybrid exciton appears in the system owing to strong dipole-dipole interaction of the Frenkel exciton of the organic molecules...

  18. Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials

    Science.gov (United States)

    Quijada, Manuel A.; Henry, Ross

    2007-01-01

    The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

  19. Exciton dynamics in InGaAsN/GaAs heterostructures

    Energy Technology Data Exchange (ETDEWEB)

    Vinattieri, A.; Alderighi, D.; Zamfirescu, M.; Colocci, M. [Dipartimento di Fisica, INFM and LENS, via Sansone 1, 50019 Sesto Fiorentino (Italy); Polimeni, A.; Capizzi, M. [INFM and Dipartimento di Fisica,Universita degli Studi di Roma ' ' La Sapienza' ' , Piazzale A. Moro 2, I-00185 Roma (Italy); Gollub, D.; Fischer, M.; Forchel, A. [Technische Physik, Universitaet Wuerzburg, Am Hubland, 97074 Wuerzburg (Germany)

    2003-02-01

    We present a detailed experimental study on the exciton dynamics in InGaAsN/GaAs quantum well structures after picosecond excitation. Time-resolved photoluminescence (PL) data are reported for samples grown in the very same conditions but differing for the nitrogen content. By varying the excitation density and the lattice temperature, we show that the PL from samples with nitrogen arises from localized excitons. Besides, the emission from localized states saturates even at low excitation power with a broadening of the PL band on the high energy side. No major quenching of the quantum yield is observed upon temperature increase. This indicates that the recombination dynamics is affected by non-radiative channels, which do not strongly depend on temperature. The similar dynamics observed in nitrogenated and not-nitrogenated samples suggest the dominance of non-radiative channels due to the highly-strained host matrix. (Abstract Copyright [2003], Wiley Periodicals, Inc.)

  20. Localized excitons in InxGa1-xN/GaN quantum well structure

    International Nuclear Information System (INIS)

    Photoluminescence (PL) photoreflectance (PR) have been employed to study the optical transitions of InxGa1-xN/GaN quantum well (QW) structures grown by metal-organic chemical vapor deposition (MOCVD). The main Pl peak at 2.895 eV is attributed to the excitons localized at trap centers, which originate from the In-rich region within the well. Several emission bands on both sides of the main peak are attributed to the interference fringe effects and the recombination of excitons localized at several levels in the In0.183Ga0.817 N well. The PL peak at 3.040 eV is ascribed to the lowest n=1 quantized transition which agrees well the calculated result

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

    Science.gov (United States)

    Shimazaki, Tomomi; Nakajima, Takahito

    2015-05-01

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

  2. 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)

  3. Dielectric and anharmonic behaviour of alkali halide crystals

    International Nuclear Information System (INIS)

    The dielectric and anharmonic behaviour of alkali halide crystals is studied by determining the volume derivatives of low frequency dielectric constant and polarizability. An interionic force model which takes into account the contributions arising from the short range overlap repulsion and the van der Waals interactions is used in calculations. The implicit and explicit temperature derivatives of low frequency and high frequency dielectric constants are obtained separately for all the alkali halide crystals with NaCl and CsCl structures. The anharmonic contribution to the dielectric constant is calculated using theoretical models due to Szigeti and Havinga. The recent analysis of pressure dependence of effective charge parameter and dielectric constant of ionic crystals performed by Varotsos is shown to be invalid. (author)

  4. Correlations between entropy and volume of melting in halide salts

    International Nuclear Information System (INIS)

    Melting parameters and transport coefficients in the melt are collated for halides of monovalent, divalent and trivalent metals. A number of systems show a deficit of entropy of melting relative to the linear relationships between entropy change and relative volume change on melting that are found to be approximately obeyed by a majority of halides. These behaviours are discussed on the basis of structural and transport data. The deviating systems are classified into three main classes, namely (i) fast-ion conductors in the high-temperature crystal phase such as AgI, (ii) strongly structured network-like systems such as ZnCl2, and (iii) molecular systems melting into associated molecular liquids such as SbCl3. (author). 35 refs, 1 fig., 3 tabs

  5. Optical radiation from electron sputtering of alkali halides

    International Nuclear Information System (INIS)

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

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

    International Nuclear Information System (INIS)

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

  7. Probing of molecular aggregates with exciton annihilation

    International Nuclear Information System (INIS)

    It is characteristic for the kinetics of singlet-singlet exciton annihilation in a finite-size molecular aggregate to approach with time the steady state corresponding to the very last single excitation. The elimination of corresponding signal from the kinetic trace exposes the dominance of the exponential decay. The characteristic time constant representing the slowest annihilation mode is related to the number of exciton hopping sites and the pairwise hopping time by solving the single excitation trapping problem. The relevance of proposed approach for the analysis of annihilation kinetics is demonstrated for the bacterial light-harvesting complex LH1, the reaction center complex D1-D2 and C60 structures

  8. Excitonic and photonic processes in materials

    CERN Document Server

    Williams, Richard

    2015-01-01

    This book is expected to present state-of-the-art understanding of a selection of excitonic and photonic processes in useful materials from semiconductors to insulators to metal/insulator nanocomposites, both inorganic and organic.  Among the featured applications are components of solar cells, detectors, light-emitting devices, scintillators, and materials with novel optical properties.  Excitonic properties are particularly important in organic photovoltaics and light emitting devices, as also in questions of the ultimate resolution and efficiency of new-generation scintillators for medical diagnostics,  border security, and nuclear nonproliferation.  Novel photonic and optoelectronic applications benefit from new material combinations and structures to be discussed.

  9. Spin cutoff parameter for the exciton model

    International Nuclear Information System (INIS)

    In order to estimate the angular momentum dependence of the state density for the exciton model, the spin cutoff parameter for a noninteracting harmonic oscillator shell model (HOSM) is computed. A simple but useful estimate is obtained. A simpler estimation based on the level density formula of Williams is also given by introducing the ''approximation of restricted population'' (ARP). The spin cutoff parameters for HOSM and ARP are functions of not only the exciton number nx but also of excitation energy Ex. The ARP reproduces the average behavior of HOSM. ARP provides an improved version of the formula for the spin cutoff parameter based on the equi-distance model. (author)

  10. Multiple Exciton Generation in Silicon QD arrays

    Science.gov (United States)

    Kryjevski, Andrei; Kilin, Dmitri

    2014-03-01

    We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication.

  11. Excitonic clusters in coupled quantum dots

    CERN Document Server

    Filinov, A V; Lozovik, Yu E

    2003-01-01

    We present a first-principles path integral Monte Carlo study of a finite number of strongly correlated electron-hole pairs in two symmetric vertically coupled quantum dots. In this system, the intra- and interdot correlations depend on the distance d between the dots, the density n (strength of confinement potential) and temperature T. For fixed d and T > 0, increasing n leads to four qualitatively different states: an exciton 'liquid', an exciton 'crystal', orientationally decoupled electron and hole 'crystals' and an electron (hole) liquid.

  12. Device design for optimal exciton harvesting

    Science.gov (United States)

    Ingram, G. L.; Lu, Z. H.

    2014-10-01

    Organic light emitting diodes (OLEDs) show potential as the next generation solid state lighting technology. A major barrier to widespread adoption at this point is the efficiency droop that occurs for OLEDs at practical brightness (~ 5000 cd/m2) levels necessary for general lighting. We highlight recent progress in highly efficient OLEDs at high brightness, where improvements are made by managing excitons in these devices through rational device design. General design principles for monochrome OLEDs are discussed based on recent device architectures that have been successfully implemented. We expect that an improved understanding of exciton dynamics in OLEDs in combination with innovative device design will drive future development.

  13. Detecting an exciton crystal by statistical means

    International Nuclear Information System (INIS)

    We investigate an ensemble of excitons in a coupled quantum well excited via an applied laser field. Using an effective disordered quantum Ising model, we perform a numerical simulation of the experimental procedure and calculate the probability distribution function P(M) to create M excitons as well as their correlation function. It shows clear evidence of the existence of two phases corresponding to a liquid and a crystal phase. We demonstrate that not only the correlation function but also the distribution P(M) is very well suited to monitor this transition.

  14. Excitonic Dynamical Franz-Keldysh Effect

    CERN Document Server

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

    1998-01-01

    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.

  15. Magneto-optical properties of charged excitons in quantum dots

    CERN Document Server

    Govorov, A O; Haft, D; Kalameitsev, A V; Chaplik, A; Warburton, R J; Karrai, K; Schoenfeld, W; García, J M; Petroff, P M

    2002-01-01

    We present both experimental and theoretical results on the influence of a magnetic field on excitons in semiconductor quantum dots. We find a pronounced difference between the strong and weak confinement regimes. For weak confinement, the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons. In this limit, we argue that the optical properties of excitons with two or more extra electrons are fundamentally different to those of the neutral exciton and trion.

  16. Ring emission and exciton-pair scattering in semiconductor microcavities

    International Nuclear Information System (INIS)

    Polariton microemitters are semiconductor devices that take advantage of the strong coupling of light and matter to produce efficient optical sources. Injecting an exciton gas into such a semiconductor microcavity can produce annular optical emission of a specific cone angle. We demonstrate that at higher exciton densities or temperatures, this ring continuously decreases in diameter due to the greater energy that can be carried off in exciton-pair scattering processes. These Coulomb-mediated exciton-exciton scatterings are shown to dominate the polariton relaxation and optical emission

  17. Inversion of exciton level splitting in quantum dots

    CERN Document Server

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

    2005-01-01

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

  18. Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

    Science.gov (United States)

    Andreakou, P.; Cronenberger, S.; Scalbert, D.; Nalitov, A.; Gippius, N. A.; Kavokin, A. V.; Nawrocki, M.; Leonard, J. R.; Butov, L. V.; Campman, K. L.; Gossard, A. C.; Vladimirova, M.

    2015-03-01

    Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude.

  19. Photoluminescence properties and exciton dynamics in monolayer WSe2

    International Nuclear Information System (INIS)

    In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe2 to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260?K, providing further evidence for two-dimensional nature of monolayer material.

  20. Scattering process between polaron and exciton in conjugated polymers

    OpenAIRE

    Sun, Zhen; Liu, Desheng; Stafström, Sven; An, Zhong

    2011-01-01

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

  1. Excitonic optical bistability in n-type doped semiconductors

    International Nuclear Information System (INIS)

    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

  2. Nonboson treatment of excitonic nonlinearity in optically excited media

    International Nuclear Information System (INIS)

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

  3. Electronic structure of copper halides CuI and CuCl: A comparative X-Ray photoelectron and absorption spectroscopy study

    Science.gov (United States)

    Generalov, A. V.; Vinogradov, A. S.

    2013-06-01

    The energy distributions of the occupied and unoccupied electronic states for copper halides CuCl and CuI have been investigated using X-ray photoemission and absorption spectroscopy with a highenergy resolution on the equipment of the Russian-German beamline for outlet and monochromatization of synchrotron radiation from the electron storage ring BESSY II. A quasi-molecular analysis of the obtained experimental spectra has revealed that there is a fundamental similarity of the energy structures of the valence band and the conduction band of copper halides Cu X ( X = Cl, I) due to the identical atomic structure of the studied compounds. The differences in the positions of individual energy subbands in the valence band and the conduction band of Cu X and in their intensities in the spectra are associated with different degrees of hybridization of the Cu 3 d, 4 s and X( n + 1) s, np valence states, as well as with different sizes of structural units (CuCl4 and CuI4 quasi-molecules) of the studied crystals.

  4. Collisional electron-capture processes at sodium halide surfaces

    International Nuclear Information System (INIS)

    A new class of inelastic collisional excitation processes accounts for the electron spectra of ion bombarded sodium halide surfaces. These spectra indicate that the excitation-deexcitation process consists of a sequence of lattice-ion collisions in which localized electron transfer occurs. Such collisional processes demonstrate how collisions can initiate electronic change in molecular compounds and that electron-transfer processes must play an essential role in chemistry activated by energetic collisions

  5. Electroreduction of praseodymium in solutions of alkali metal halides

    International Nuclear Information System (INIS)

    The process of praseodymium complexes electric reduction in solutions of alkali metal halides was studied by voltammetric and polarographic methods. The mechanism of praseodymium reduction is suggested. 3.0-3.2 M KCl solutions with pH 1.9-2.0 were chosen as optimal background electrolytes for praseodymium determination by voltammetric method. Praseodymium detection limit using the method mentioned equals 5x10-5 mol/l. Neodymium, terbium, gadolinium ions interfere in praseodymium determination

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

    OpenAIRE

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

    1991-01-01

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

  7. Organometal halide perovskites as useful materials in sensitized solar cells.

    Science.gov (United States)

    Singh, Surya Prakash; Nagarjuna, P

    2014-04-14

    Organometal halide perovskites offer dual potential properties as a light harvester and at the same time as a hole conductor in inorganic-organic hybrid heterojunction solar cells. The sequential deposition route provides a power conversion efficiency of 15% under standard AM1.5G test conditions. In this perspective, we will briefly summarize the development of perovskite-sensitized solar cells from their first report up to the present. PMID:24577264

  8. Separation parameters in the reaction between volatile halides and hydrogen

    International Nuclear Information System (INIS)

    Equations for calculating the separation coefficient and factor in reactions of volatile halides of group III-VI metals (M=B, Zr, Hf, V, Nb, Ta, Mo) with hydrogen from the equilibrium constants of the corresponding reactions and initial reactant concentrations were obtained. The concept of the limiting impurity concentration was introduced. The values specified were calculated for the reduction of volatile chlorides with hydrogen

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

    Directory of Open Access Journals (Sweden)

    Tomislav Cigula

    2010-09-01

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

  10. Molybdenum bis(?-cyclopentadienyl) dihydride complexes with transition metal halides

    International Nuclear Information System (INIS)

    Stable complexes of molybdenum bis (eta-cyclopentadienyl) dihydride with copper, silver, zinc, mercury, iron, cobalt halides are prepared. The analysis of IR spectra of the compounds permits to make a conclusion on the participation of hydrogen hydride atoms of Cp2MoH2 in the formation of the bond with transition metal atom. A possible similarity in the structure of the complexes prepared and the active centre of nitrogenase ferment is discussed

  11. Combination bands in vibronic spectra of molecular crystals

    International Nuclear Information System (INIS)

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

  12. Characterization of Catalytically Active Octahedral Metal Halide Cluster Complexes

    Directory of Open Access Journals (Sweden)

    Satoshi Kamiguchi

    2014-04-01

    Full Text Available Halide clusters have not been used as catalysts. Hexanuclear molecular halide clusters of niobium, tantalum, molybdenum, and tungsten possessing an octahedral metal framework are chosen as catalyst precursors. The prepared clusters have no metal–metal multiple bonds or coordinatively unsaturated sites and therefore required activation. In a hydrogen or helium stream, the clusters are treated at increasingly higher temperatures. Above 150–250 °C, catalytically active sites develop, and the cluster framework is retained up to 350–450 °C. One of the active sites is a Brønsted acid resulting from a hydroxo ligand that is produced by the elimination of hydrogen halide from the halogen and aqua ligands. The other active site is a coordinatively unsaturated metal, which can be isoelectronic with the platinum group metals by taking two or more electrons from the halogen ligands. In the case of the rhenium chloride cluster Re3Cl9, the cluster framework is stable at least up to 300 °C under inert atmosphere; however, it is reduced to metallic rhenium at 250–300 °C under hydrogen. The activated clusters are characterized by X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure analysis, thermogravimetry–differential thermal analysis, infrared spectrometry, acid titration with Hammett indicators, and elemental analyses.

  13. Photoexcitation, -ionization and fragmentation of molecular rubidium halides

    International Nuclear Information System (INIS)

    Full text: Photoionization and fragmentation of molecular rubidium halides following photoexcitation in the vacuum ultraviolet region are investigated. The experiments were performed at beamline 52 at MAX-I storage ring (Lund, Sweden) using Wiley- McLaren type time-of-flight mass spectrometer. Ion time-of-flight spectra, total and partial ion yield spectra are measured from the vapours of RbF, RbCl, RbBr and RbI, containing both the monomer and dimer molecules. From the partial ion yield spectra of the Rb+ covering the 8-18 eV photon energy range, a number of resonating excited states are identified. In the same spectral region similar resonances were observable in the case of Br+ and I+ partial ion yield spectra, but not in the case of F+ and Cl+ partial ion yield spectra. The fragmentation patterns of the excited and ionized rubidium halide molecules RbX and also the Rb2X2 dimers are studied in detail from ion time-of-flight mass spectra recorded at and around the resonances. Molecular dissociation following the resonant excitation is discussed and the different fragmentation of rubidium halide molecules is explained with two different electronic decay channels

  14. Colloidal nanocrystals with inorganic halide, pseudohalide, and halometallate ligands.

    Science.gov (United States)

    Zhang, Hao; Jang, Jaeyoung; Liu, Wenyong; Talapin, Dmitri V

    2014-07-22

    We investigate simple halides and pseudohalides as an important class of inorganic ligands for nanocrystals (NCs) in solution phase ligand exchange. These short, robust, and easy to model ligands bind to the NC surface and provide electrostatic stabilization of NC dispersions in N-methylformamide. The replacement of organic ligands on NCs with compact halide and pseudohalide ligands greatly facilitates electronic communication between NCs. For example, a high electron mobility of ? ? 12 cm(2) V(-1) s(-1) has been observed in thin films made of I(-)-capped CdSe NCs. We also studied charge transport properties of thin films based on the pseudohalide N3(-)-capped InAs NCs, suggesting the possibility of obtaining "all III-V" NC solids. In addition, we extend the surface chemistry of halometallates (e.g., CH3NH3PbI3), which can stabilize colloidal solutions of lead chalcogenide NCs. These halide, pseudohalide, and halometallate ligands enrich the current family of inorganic ligands and can open up more opportunities for applications of NCs in the fields of electronics, optoelectronics, and thermoelectrics. PMID:24988140

  15. Discrete hopping model of exciton transport in disordered media

    Science.gov (United States)

    Burlakov, V. M.; Kawata, K.; Assender, H. E.; Briggs, G. A. D.; Ruseckas, A.; Samuel, I. D. W.

    2005-08-01

    A model of dispersive exciton transport has been developed for a medium with exciton energy levels randomly distributed in both space and energy scale. For a boxcar density of states of excitons an analytical solution is given describing the exciton density as a function of time and the proximity to the exciton quenching interfaces. The model parameters, such as exciton lifetime, effective number of exciton energy levels within inhomogeneously broadened density of states, and hopping distance and hopping rate constants, could be determined using time-resolved photoluminescence data. The developed model is verified via comparison with experimental data on the time-dependent photoluminescence decay of the conjugated polymer MEH-PPV and on the spectrum of internal quantum efficiency of the heterojunction photovoltaic device based on the MEH-PPV/TiO2 nanostructure.

  16. Spatiotemporal dynamics of excitons in monolayer and bulk WS2

    Science.gov (United States)

    He, Jiaqi; He, Dawei; Wang, Yongsheng; Cui, Qiannan; Ceballos, Frank; Zhao, Hui

    2015-05-01

    Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 +/- 1 and 110 +/- 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 +/- 20 and 3.5 +/- 0.5 cm2 s-1 are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications.

  17. Spatiotemporal dynamics of excitons in monolayer and bulk WS2.

    Science.gov (United States)

    He, Jiaqi; He, Dawei; Wang, Yongsheng; Cui, Qiannan; Ceballos, Frank; Zhao, Hui

    2015-05-21

    Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 ± 1 and 110 ± 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 ± 20 and 3.5 ± 0.5 cm(2) s(-1) are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications. PMID:25947347

  18. Polaronic Effects of an Exciton in a Cylindrical Quantum Wire

    International Nuclear Information System (INIS)

    The effects of exciton-optical phonon interaction on the binding energy and the total and reduced effective masses of an exciton in a cylindrical quantum wire have been investigated. We adopt a perturbative-PLL [T.D. Lee, F. Low, and D. Pines, Phys. Rev. B90 (1953) 297] technique to construct an effective Hamiltonian and then use a variational solution to deal with the exciton-phonon system. The interactions of exciton with the longitudinal-optical phonon and the surface-optical phonon have been taken into consideration. The numerical calculations for GaAs show that the influences of phonon modes on the exciton in a quasi-one-dimensional quantum wire are considerable and should not be neglected. Moreover the numerical results for heavy- and light-hole exciton are obtained, which show that the polaronic effects on two types of excitons are very different but both depend heavily on the sizes of the wire.

  19. Exciton dephasing in ZnSe quantum wires

    DEFF Research Database (Denmark)

    Wagner, Hans Peter; Langbein, Wolfgang Werner

    1998-01-01

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

  20. Two Types of Self-Trapped Excitons in a Quasi-One-Dimensional Crystal Piperidinium Tribromoplumbate

    Science.gov (United States)

    Azuma, Junpei; Tanaka, Koichiro; Kan'no, Ken-ichi

    2002-03-01

    Photo-excited states of piperidinium tribromoplumbate (C5H10NH2PbBr3) single crystals were investigated by spectroscopic methods. The strong anisotropy was observed in polarized reflection spectra. A peak with large oscillator strength appears at 3.9 eV only for the light polarized parallel to the [PbBr3]n1- chain, which is assigned as the lowest exciton transition. These polarized reflection spectra indicate that electrons and holes are strongly confined within the chain, suggesting that the system can be considered as natural multiple quantum wires. Two luminescence bands appear around 3.0 eV (V emission) and 1.9 eV (R emission) under excitation into the intrinsic absorption region. The large Stokes-shifts of the luminescence bands indicate strong electron-phonon coupling in the excited states. Temperature dependence of luminescence intensities and decay lifetimes reveal that population transfer from the initial state of the V emission to that of the R emission is thermally activated via a potential barrier of about 35 meV. The excitation spectra and the decay kinetics of the luminescence bands strongly suggest that the initial states of luminescence bands are qualitatively different kinds of self-trapped excitons.

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

    OpenAIRE

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

    2011-01-01

    The Photoluminescent (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. ...

  2. Tailoring quantum dot assemblies to extend exciton coherence times and improve exciton transport

    Science.gov (United States)

    Seward, Kenton T.

    Electron energy transfer (EET) through nanostructured assemblies plays a crucial role in a wide range of emerging technologies such as quantum dot solar cells, quantum computing, molecular electronics, excitonic transistors, and light emitting diodes. These technologies are very dependent on excitonic lifetimes which are short on the order of a nanosecond. In order to efficiently use this short time scale, EET needs to be as fast as possible. This leads to an interest in the application of coherent exciton transfer. To examine the possibility of coherent transfer, we ask a simple question: How rapidly do coherent superpositions of excitonic states dephase between quantum dots?. We assume that the major source of decoherence at room temperature is from the internal phonon modes of silicon quantum dots. The question is then addressed using a combination of ab initio calculations and a master equation formulation for the evolution of the electronic density operator for a dimer of interacting two-level systems coupled to a shared bath of harmonic oscillators. A combination of density functional theory (DFT) and frozen phonon method (FPM) analysis was used to obtain exciton-phonon coupling in various sizes of silicon quantum dots. As expected, coherent EET is faster in comparison to incoherent EET in assemblies of identical nanostructures. In cases of non-identical assemblies of nanostructures, the low energy regions act as trap states for the exciton. In this case, a combination of coherent and incoherent transport leads to the fastest transport rate.

  3. Metal Halide Solid-State Surface Treatment for High Efficiency PbS and PbSe QD Solar Cells.

    Science.gov (United States)

    Crisp, Ryan W; Kroupa, Daniel M; Marshall, Ashley R; Miller, Elisa M; Zhang, Jianbing; Beard, Matthew C; Luther, Joseph M

    2015-01-01

    We developed a layer-by-layer method of preparing PbE (E = S or Se) quantum dot (QD) solar cells using metal halide (PbI2, PbCl2, CdI2, or CdCl2) salts dissolved in dimethylformamide to displace oleate surface ligands and form conductive QD solids. The resulting QD solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. We find that the PbI2 treatment is the most successful in removing alkyl surface ligands and also replaces most surface bound Cl(-) with I(-). The treatment protocol results in PbS QD films exhibiting a deeper work function and band positions than other ligand exchanges reported previously. The method developed here produces solar cells that perform well even at film thicknesses approaching a micron, indicating improved carrier transport in the QD films. We demonstrate QD solar cells based on PbI2 with power conversion efficiencies above 7%. PMID:25910183

  4. Transport of Triplet Excitons along Continuous 100 nm Polyfluorene Chains.

    Science.gov (United States)

    Li, Xiang; Bird, Matthew; Mauro, Gina; Asaoka, Sadayuki; Cook, Andrew R; Chen, Hung-Cheng; Miller, John R

    2015-06-18

    Triplet excitons created in poly-2,7-(9,9-dihexyl)fluorene (pF) chains with end trap groups in solution are efficiently transported to and captured by the end groups. The triplets explore the entire lengths of the chains, even for ?100 nm long chains, enabling determination of the completeness of end-capping. The results show that the chains are continuous: they may contain transient barriers or traps, such as those from fluctuations of dihedral angles, but they are free of major defects that stop motion of the triplets. Quantitative determinations are aided by the addition of a strong electron donor, TMPD, which removes absorption bands of the end-trapped triplets. For chains having at least one end trap, triplet capture is quantitative on the 1 ?s time scale imposed by the use of the donor. Fractions of chains having no end traps were 0.15 for pF samples with anthraquinone (AQ) end traps and 0.063 with naphthylimide (NI) end traps. These determinations agreed with measurements by NMR for short (<40 polymer repeat units (PRU)) chains, where NMR determinations are accurate. The results find no evidence for traps or barriers to the transport of triplets, and places limits on the possible presence of defects as impenetrable barriers to less than one per 300 PRU. The present results present a paradigm different from the current consensus, derived from observations of singlet excitons, that conjugated chains are divided into "segments," perhaps by some kind of defects. For the present pF chains, the segmentation either does not apply to triplet excitons or is transient so that the defects are healed or surmounted in times much shorter than 1 ?s. Triplets on chains without end trap groups transfer to chains with end traps on a slower time scale. Rate constants for these bimolecular triplet transfer reactions were found to increase with the length of the accepting chain, as did rate constants for triplet transfer to the chains from small molecules like biphenyl. A second set of polyfluorenes with 2-butyloctyl side chains was found to have a much lower completeness of end-capping. PMID:25521161

  5. LIMITS OF KINETIC SCHEMES FOR EXCITON REACTIONS

    OpenAIRE

    BLUMEN, A; Zumofen, G.; Klafter, J.

    1985-01-01

    In this communication we analyze exciton annihilation reactions under spatial constraints. Such limitations often lower the effective dimension, an aspect which can be well-modelled through fractals. Thus, qualitative deviations of the reaction pattern from the kinetic scheme occur and allow to highlight the limits of the kinetic approach.

  6. Effective models for excitons in carbon nanotubes

    DEFF Research Database (Denmark)

    Cornean, Horia; Duclos, Pierre

    2007-01-01

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

  7. On the nonstationary processes in exciton systems

    International Nuclear Information System (INIS)

    In consequence of the nonconservation of excitons, the excitation number operator as well as the exciton pair creation and annihilation operators are expressed as functions of time and of the corresponding stationary operators. The nonstationary pair correlation processes and the nonstationary excitation density fluctuation processes are analysed using the nonstationary operators. It is found that the nonstationary pair correlations are characterized by a specific kind of dissipation that is linearly proportional with time. In contradistinction to stationary processes, the corresponding nonstationary ones, lead to elementary excitations having a double energy: E (nonstationary) = 2E (stationary). It is shown that in contrast to exciton density fluctuations, pair correlations have a tendency of localization related to a lattice point. It is also shown that the exciton nonstationary pair correlations are in many respect similar to the density fluctuations that take place in the crystal due to a near slow neutron passage. Finally, a general method for the evaluation of the correlation function, applicable even in the case when the Green's function has a n-th order pole, is developed in the Appendix. (author)

  8. Effective models for excitons in carbon nanotubes

    OpenAIRE

    Cornean, Horia; Duclos, Pierre; Ricaud, Benjamin

    2006-01-01

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

  9. Excitons in T-shaped quantum wires

    CERN Document Server

    Szymanska, M H; Needs, R J

    2000-01-01

    We calculate energies, oscillator strengths for radiative recombination, and two-particle wave functions for the ground state exciton and around 100 excited states in a T-shaped quantum wire. We include the single-particle potential and the Coulomb interaction between the electron and hole on an equal footing, and perform exact diagonalisation of the two-particle problem within a finite basis set. We calculate spectra for all of the experimentally studied cases of T-shaped wires including symmetric and asymmetric GaAs/Al$_{x}$Ga$_{1-x}$As and In$_{y}$Ga$_{1-y}$As/Al$_{x}$Ga$_{1-x}$As structures. We study in detail the shape of the wave functions to gain insight into the nature of the various states for selected symmetric and asymmetric wires in which laser emission has been experimentally observed. We also calculate the binding energy of the ground state exciton and the confinement energy of the 1D quantum-wire-exciton state with respect to the 2D quantum-well exciton for a wide range of structures, varying t...

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

    International Nuclear Information System (INIS)

    We investigate the breakup of bound electron–hole pairs, known as Mott transition of excitons, in GaAs-GaAlAs quantum wells with increasing excitation, comparing two different theoretical approaches. Firstly, a thermodynamic approach is used to investigate the ionization equilibrium between electrons, holes and excitons, where the abrupt jump of the degree of ionization from 0 to 1 indicates the Mott density. It is extended to a self-consistent quasi-particle approximation (QPA) for the carrier properties, including dynamical screening of the Coulomb interaction between carriers. Secondly, a spectral approach based on the semiconductor Bloch equations within linear optical response is used, considering the quasi-particle (QP) properties of carriers and the dynamical screening between electron–hole pairs. While the first is effectively a one-particle approach, in the second the whole two-particle spectrum is analyzed. Within the thermodynamic approach, a simple criterion for the Mott transition can be given: namely, if the sum of chemical potentials of carriers, reflecting the effective shrinkage of the band edge, crosses the exciton energy with increasing excitation. We demonstrate that this simple picture cannot be maintained in the two-particle approach. Here, a compact quantity, which describes the behavior of the band edge, does not exist. In fact, the behavior of the single states in the spectrum is generated by the interplay of dynamical screening in the intplay of dynamical screening in the interband self-energy and the effective interaction of the electron–hole pairs. Moreover, the band edge cannot be clearly resolved, since it is merged with excited exciton states (e.g. 2s state), which show up only for densities far below the Mott density. Instead of a Mott density, only a density range can be given, where the Mott transition appears. We demonstrate that a small damping as a prerequisite for the validation of the extended QPA in the thermodynamic approach breaks down, analyzing (i) the dephasing processes with increasing excitation, (ii) the strong increase of the excitonic linewidth and (iii) comparing with the lifetime of carriers in the QP description. (paper)

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

    OpenAIRE

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

    2009-01-01

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

  12. 10 CFR 431.324 - Uniform test method for the measurement of energy efficiency of metal halide ballasts.

    Science.gov (United States)

    2010-01-01

    ...method for the measurement of energy efficiency of metal halide ballasts...ENERGY ENERGY CONSERVATION ENERGY EFFICIENCY PROGRAM FOR CERTAIN COMMERCIAL...method for the measurement of energy efficiency of metal halide ballasts....

  13. Exciton localization and dissociation dynamics in CdS and CdS-Pt quantum confined nanorods: effect of nonuniform rod diameters.

    Science.gov (United States)

    Wu, Kaifeng; Rodríguez-Córdoba, William; Lian, Tianquan

    2014-12-11

    One-dimensional colloidal multicomponent semiconductor nanorods, such as CdSe-CdS dot-in-rod, have been extensively studied as a promising class of new materials for solar energy conversion because of the possibilities of using the band alignment of component materials and the rod-diameter-dependent quantum confinement effect to control the location of electrons and holes and to incorporate catalysts through the growth of Pt tips. Here we used CdS nanorods as an example to study the effect of nonuniform diameters along the rod on the exciton localization and dissociation dynamics in CdS and (platinum tipped) CdS-Pt nanorods. We showed that, in CdS nanorods prepared by seeded growth, the presence of a bulb with a larger diameter around the CdS seed resulted in an additional absorption band lower in energy than the exciton in the CdS rod. As a result, excitons generated in the CdS rod could undergo ultrafast localization to the bulb region in addition to trapping on the CdS rod. We observed that the Pt tip led to fast exciton dissociation by electron transfer. However, excitons localized on the CdS bulb showed slower average ET rates than those localized in the rod region. Our findings suggested that the effect of rod morphology should be carefully considered in designing multicomponent nanorods for solar energy conversion applications. PMID:24945594

  14. Exciton quantum confinement effect in nanostructures formed by laser radiation on the surface of CdZnTe ternary compound

    International Nuclear Information System (INIS)

    Self-organizing structures of nanometer size are observed on the surface of CdZnTe crystal irradiated by strongly absorbed Nd:YAG laser radiation (LR) at intensities within 4-12 MW/cm2. The effect of exciton quantum confinement manifested by a shift to higher energies of the A0,X exciton band of the photo-luminescent spectrum is present in structures of 10-15 nm in diameter at the top of nano-hills. A graded band gap structure with optical window is formed at the top of nano-hills. (copyright 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

  15. Waveshifting fiber readout of lanthanum halide scintillators

    International Nuclear Information System (INIS)

    Newly developed high-light-yield inorganic scintillators coupled to waveshifting optical fibers provide the capability of efficient X-ray detection and millimeter scale position resolution suitable for high-energy cosmic ray instruments, hard X-ray/gamma ray astronomy telescopes and applications to national security. The CASTER design for NASA's proposed Black Hole Finder Probe mission, in particular, calls for a 6-8 m2 hard X-ray coded aperture imaging telescope operating in the 20-600 keV energy band, putting significant constraints on cost and readout complexity. The development of new inorganic scintillator materials (e.g., cerium-doped LaBr3 and LaCl3) provides improved energy resolution and timing performance that is well suited to the requirements for national security and astrophysics applications. LaBr3 or LaCl3 detector arrays coupled with waveshifting fiber optic readout represent a significant advance in the performance capabilities of scintillator-based gamma cameras and provide the potential for a feasible approach to affordable, large area, extremely sensitive detectors. We describe some of the applications and present laboratory test results demonstrating the expected scintillator performance

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

    Energy Technology Data Exchange (ETDEWEB)

    Elangovan, P. [Department of Physics, Maamallan Institute of Technology, Chennai 602105 (India); John Peter, A., E-mail: a.john.peter@gmail.com [Department of Physics, Government Arts College, Melur 625 106. Madurai (India); Kyoo Yoo, Chang [Center for Environmental Studies/Green Energy Center, Deptartment of Environmental Science and Engineering, College of Engineering, Kyung Hee University, Seocheon-dong 1, Giheung-gu, Yongin-Si, Gyeonggi-Do, 446-701 (Korea, Republic of)

    2013-11-15

    Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: • Binding energy of an exciton in a wurtzite ZnO/Zn{sub 1?x}Mg{sub x}O strained quantum well is investigated. • The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. • The oscillator strength and the exciton lifetime for radiative recombination are computed. • The internal field induced transition energy of strained ZnO/Zn{sub 1?x}Mg{sub x}O well is investigated. • The results show that the nonlinear optical properties strongly depend on Mg composition.

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

    International Nuclear Information System (INIS)

    Binding energy of an exciton in a wurtzite ZnO/Zn1?xMgxO strained quantum well is investigated theoretically in which the strong built-in electric field due to the spontaneous and piezoelectric polarizations is included. Numerical calculations are performed using variational procedure within the single band effective mass approximation by varying the Mg composition in the barrier. The exciton oscillator strength and the exciton lifetime for radiative recombination as functions of well width and Mg content have been computed. The internal field induced interband emission energy of strained ZnO/Zn1?xMgxO well is investigated with the various structural parameters. The total optical absorption coefficients and the changes of refractive index as a function of normalized photon energy in the presence of built-in internal field are analyzed. The result shows that the strong built-in electric field has influence on the oscillator strength and the recombination life time of the exciton. The optical absorption coefficients and the refractive index changes strongly depend on Mg composition. The occurred blue shift of the resonant peak due to the incorporation of Mg ions will give the information about the variation of two energy levels in the quantum well. -- Highlights: • Binding energy of an exciton in a wurtzite ZnO/Zn1?xMgxO strained quantum well is investigated. • The built-in internal fields due to the spontaneous and piezoelectric polarizations are included. • The oscillator strength and the exciton lifetime for radiative recombination are computed. • The internal field induced transition energy of strained ZnO/Zn1?xMgxO well is investigated. • The results show that the nonlinear optical properties strongly depend on Mg composition

  18. Electric field controlled exciton transport in a conjugated polymer chain

    International Nuclear Information System (INIS)

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

  19. Taming excitons in II-VI semiconductor nanowires and nanobelts

    Science.gov (United States)

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

    2014-10-01

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

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

    International Nuclear Information System (INIS)

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

  1. Structural and chemical aspects of complex formation in the metal halide-macrocyclic polyether systems

    International Nuclear Information System (INIS)

    Structural data for more than two hundred complexes of oxygen-containing crown ethers with the period 2-6 element halides, as well as uranium and thorium halides are reviewed. The main types of complexes are distinguished. The characteristic features of the formation of complexes and coordination fragments are discussed

  2. Crystal-liquid interface energy and surface stress of alkali halides

    International Nuclear Information System (INIS)

    A model to determine surface stress (f) and crystal-liquid interface energy (?) of (1 0 0) face of NaCl type alkali halides without free parameters is established. While the predicted ? values are thermodynamically reasonable, the model predictions on f are in agreement with other theoretical calculations for (1 0 0) face of NaCl type alkali halides

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

    OpenAIRE

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

    2014-01-01

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

  4. First Principles Calculations of Linear and Second-Order Optical Responses in Rhombohedrally Distorted Perovskite Ternary Halides, CsGeX3 (X = Cl, Br, and I)

    Science.gov (United States)

    Tang, Li-Chuan; Chang, Yia-Chung; Huang, Jung-Yau; Lee, Ming-Hsien; Chang, Chen-Shiung

    2009-11-01

    Systematic studies based on first-principles calculations of second-order optical susceptibilities as well as the dielectric function of CsGeX3 (X = Cl, Br, and I; CGX) are presented. The relationship between structural properties and optoelectronic responses are examined. The structural factors ??, and dGe, dX are proposed to describe the degree of distortion from an ideal perovskite structure. ?? and dGe increase when halide anions are changed from Cl to I; while halide anion displacement, dX, decreases. The structural distortion effect on these rhombohedral CGX crystals is analyzed by first-principles calculations. The dielectric function and the second harmonic generation (SHG) response coefficient also increase with increasing ?? and dGe. The direct band gaps (EG) of CsGeX3 all occur at the R-point, ?ER. The experimental band gaps of CGX crystals become smaller, i.e., ECGCG (=3.67 eV) >ECGBG (=2.32 eV) >ECGIG (=1.53 eV), as ?? and dGe increase, i.e., dCGCGegermanium. The calculated magnitudes of ?(2)ijk are close to some reported experimental values near the band gap.

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

    Directory of Open Access Journals (Sweden)

    David T. Moore

    2014-08-01

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

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

    International Nuclear Information System (INIS)

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

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

    Energy Technology Data Exchange (ETDEWEB)

    Moore, David T.; Sai, Hiroaki; Wee Tan, Kwan; Estroff, Lara A.; Wiesner, Ulrich, E-mail: ubw1@cornell.edu [Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 (United States)

    2014-08-01

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

  8. Excitons in ZnCdSe/ZnSe quantum dots with parabolic confinement potential

    International Nuclear Information System (INIS)

    Material distribution profile is found to have significant influence on emission properties of quantum structures. It was reported that core/shell ZnCdSe/ZnSe semiconductor nanocrystals individually exhibit continuous, non-blinking photoluminescence, which was explained by softening the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe(Xiaoyong Wang et al, Nature 459, 686-689, 2009). We present exact calculation of electronic states for spherical core*shell quantum dots with realistic boundary condition: inside the dot potential is taken equal to -ar2, outside the dot -zero. Calculations are carried out for ZnCdSe/ZnSe structure. We use single band effective mass approximation for finding single particle states of electrons and holes. Exciton states with corresponding energies are calculated by direct diagonalization of Hamilton matrix for 8 lowest excitonic states and their radiative decay probability in dependence of quantum dot radius are calculated. We found that for quantum dots with radius less than 8 nm there is no electron levels inside the dot. For this radius exciton binding energy is 56 meV.

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

    CERN Document Server

    Foglia, Laura; Wolf, Martin; Stähler, Julia

    2014-01-01

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

  10. Metal-halide lamp design: atomic and molecular data needed

    Energy Technology Data Exchange (ETDEWEB)

    Lapatovich, Walter P [OSRAM SYLVANIA, Central Research and Services Laboratory, 71 Cherry Hill Drive, Beverly, MA 01921 (United States)], E-mail: walter.lapatovich@sylvania.com

    2009-05-15

    Metal-halide lamps are a subset of high intensity discharge (HID) lamps so named because of their high radiance. These lamps are low temperature ({approx}0.5 eV), weakly ionized plasmas sustained in refractory but light transmissive envelopes by the passage of electric current through atomic and molecular vapors. For commercial applications, the conversion of electric power to light must occur with good efficiency and with sufficient spectral content throughout the visible (380-780 nm) to permit the light so generated to render colors comparable to natural sunlight. This is achieved by adding multiple metals to a basic mercury discharge. Because the vapor pressure of most metals is very much lower than mercury itself, metal-halide salts of the desired metals, having higher vapor pressures, are used to introduce the material into the basic discharge. The metal compounds are usually polyatomic iodides, which vaporize and subsequently dissociate as they diffuse into the bulk plasma. Metals with multiple visible transitions are necessary to achieve high photometric efficiency (efficacy) and good color. Compounds of Sc, Dy, Ho, Tm, Ce, Pr, Yb and Nd are commonly used. The electrons, atoms and radicals are in local thermodynamic equilibrium (LTE), but not with the radiation field. Strong thermal (10{sup 6} K m{sup -1}) and density gradients are sustained in the discharge. Atomic radiation produced in the high-temperature core transits through colder gas regions where it interacts with cold atoms and un-dissociated molecules before exiting the lamp. Power balance and spectral output of the lamp are directly affected by the strength of atomic transitions. Attempts to simulate the radiative output of functional metal-halide lamps have been successful only in very simple cases. More data (e.g. the atomic transition probabilities of Ce i) are necessary to improve lamp performance, to select appropriate radiators and in scaling the lamp geometry to various wattages for specific applications.

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

    Science.gov (United States)

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

    2015-04-01

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

  12. Photochemistry of (5,10,15,20-tetraphenylporphyrinato)iron(III) halide complexes, Fe(TPP)(X)

    Energy Technology Data Exchange (ETDEWEB)

    Hendrickson, D.N.; Kinnaird, M.G.; Suslick, K.S.

    1987-02-18

    The photochemistry of metalloporphyrins and related macrocycles is of intense current interest. Nonetheless, the photochemistry of ferric porphyrins and heme proteins remains largely unexplored and not well understood. The ability of Fe(III) porphyrins to act as catalysts for hydrocarbon oxidations with various oxidants suggested to us their possible use as photocatalysts. In this paper, they assign a near-ultraviolet absorption in Fe/sup III/(TPP)(X) (where X = F, Cl, Br, I, N/sub 3/; TPP = 5,10,15,20-tetraphenylporphyrinate(-2)) as a halide ligand-to-metal charge-transfer (LMCT) transition. Irradiation into this band leads to rapid photoreduction of the iron atom and dissociation of X/sup 0/. In the presence of O/sub 2/, photoinitiation of hydrocarbon oxidation occurs.

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

    Directory of Open Access Journals (Sweden)

    Howard H. Patterson

    2013-06-01

    Full Text Available 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 different sizes of nanoclusters in solution and in doped hosts. A series of spectroscopic methods (luminescence, UV-reflectance, IR, and Raman as well as theoretical calculations have confirmed the existence of excimers and exciplexes. This leads to the tunability of these nano systems over a wide wavelength interval. The population of these nanoclusters varies with temperature and external laser irradiation, which explains the thermochromism and optical memory. DFT calculations indicate an MLCT transition for each nanocluster and the emission energy decreases with increasing cluster size. This is in agreement with the relatively long life-time for the emission peaks and the multiple emission peaks dependence upon cluster concentration.

  14. Ultrafast dynamics of excitons in tetracene single crystals

    Energy Technology Data Exchange (ETDEWEB)

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

    2014-03-21

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

  15. Optical response of localized excitons in layered systems.

    Science.gov (United States)

    Cocoletzi, Gregorio H.; Coyotecatl, Honorato A.

    1998-03-01

    Theoretical studies are presented of exciton bound states effects on the reflectivity of semiconductor superlattices and thin films. Semiconductors of direct gap are considered and model with extrinsic potentials near surfaces to account for the exciton interactions. Using a multistep method and the N× N transfer matrix approach, with N being the number of propagating modes, we solve analytically the polariton exciton equations. Results are presented for the A_n=1 excitonic transition of CdS and interpreted in terms of exciton bound states and Fabry-Perot resonances of the transverse and longitudinal modes in the excitonic layers. It is found that for the superlattice, the reflectivity peaks of the bound states are notably enhanced as compared with the corresponding of the single isolated film.

  16. Excitonic ring formation in ultrapure bulk GaAs

    Science.gov (United States)

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

    2014-11-01

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

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

    DEFF Research Database (Denmark)

    Masri, Zarifi; Ruseckas, Arvydas

    2013-01-01

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

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

    DEFF Research Database (Denmark)

    Masri, Zarifi; Ruseckas, Arvydas

    2013-01-01

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

  19. On the problem of superfluidity in exciton systems

    International Nuclear Information System (INIS)

    The problem of superfluidity in the system of Frenkel excitons, corresponding to multi-level scheme of molecular excitations, is considered. It is shown that the elementary excitations arising in the process of disintegration of exciton drops can have the superfluid properties. As a mechanism leading to the formation of exciton drops the exciton-phonon interaction was taken. The three-level exciton scheme is analyzed in more detail and it was found that only one branch of elementary excitations is superfluid. In the same time, the second branch can exist only for strictly defined values of momenta. It is shown also that the excitations arising due to the disintegration of exciton drops have to be considered only as a fluctuation phenomenon in the system. Some hints and ideas for experimental testing of the results are given. (Auth.)

  20. Two-exciton states and spectroscopy of phenylacetylene dendrimers

    Science.gov (United States)

    Chernyak, Vladimir; Poliakov, Evgeni Y.; Tretiak, Sergei; Mukamel, Shaul

    1999-09-01

    The two-exciton wave functions of conjugated dendrimers with fractal geometries are calculated using the Frenkel-exciton model. Self-similarity and the high degree of symmetry make it possible to express the two-photon spectra of these chromophore aggregates in a compact form using irreducible representations of optical excitations, single-exciton states, and an effective two-exciton transition dipole moment. The explicit calculation of the complete manifold of two-exciton states which involves an expensive l3×l3 diagonalization, l being number of generations, is totally avoided. A real space analysis shows that the two-exciton states and resonances are dominated by periphery chromophores due to their exponentially large number.