ADVANCED FUNCTIONAL MATERIALS, Issue 17 2010
Chang-Lyoul Lee
Abstract The triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH-PPV) photovoltaic devices are investigated by quasi-steady-state photo-induced absorption (PIA) spectroscopy. According to the low-temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet-triplet (T1 - Tn) absorption of MEH-PPV in the blend system originates from the triplet-triplet energy transfer from PtOEP to MEH-PPV. The PtOEP blended MEH-PPV/C60 bilayer photovoltaic device shows a roughly 30%,40% enhancement in photocurrent and power-conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power-conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet-triplet energy transfer between PtOEP and MEH-PPV. [source]

Exciton,Exciton Annihilation in Mixed-Phase Polyfluorene Films

ADVANCED FUNCTIONAL MATERIALS, Issue 1 2010
Paul E. Shaw
Abstract Singlet,singlet annihilation is studied in polyfluorene (PFO) films containing different fractions of , -phase chains using time-resolved fluorescence. On a timescale of >15,ps after excitation, the results are fitted well by a time-independent annihilation rate, which indicates that annihilation is controlled by 3D exciton diffusion. A time-dependent annihilation rate is observed during the first 15,ps in the glassy phase and in the , -phase rich films, which can be explained by the slowdown of exciton diffusion after excitons reach low-energy sites. The annihilation rate in the mixed-phase films increases with increasing fraction of , -phase present, indicating enhanced exciton diffusion. The observed trend agrees well with a model of fully dispersed, -phase chromophores in the surrounding glassy phase with the exciton diffusion described using the line-dipole approximation for an exciton wavefunction extending over 2.5,nm. The results indicate that glassy and, -phase chromophores are intimately mixed rather than clustered or phase-separated. [source]

Illuminating Dark Plasmons of Silver Nanoantenna Rings to Enhance Exciton,Plasmon Interactions

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2009
Hong-Mei Gong
Abstract The chemical growth of silver nanorings that possess singly twinned crystals and a circular cross section via a reductive reaction solution is reported. The wire and ring diameters of the synthesized nanorings are in the ranges 80,200,nm and 4.5,18.0,,m, respectively. By lighting up the multipolar dark plasmons with slanted illumination, the silver nanoring exhibits unique focused scattering and large local-field enhancement. We also demonstrate strong exciton,plasmon interactions between a monolayer of CdSe/ZnS semiconductor quantum dots and a single silver antenna-like nanoring (nanoantenna) at the "hot spots" located at the cross points of the incident plane and nanoring; the position of these spots are tunable by adjusting the incidence angle of illumination. The tunable plasmonic behavior of the silver nanorings could find applications as optical nanoantennae or plasmonic nanocavities. [source]

Exciton,Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals

ADVANCED MATERIALS, Issue 48 2009
Michele Saba
Exciton,exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton,exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm,2 for continuous wave lasing. [source]

Exciton dephasing in quantum dots: Coupling to LO phonons via excited states

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2008
E. A. Muljarov
Abstract We have found a novel mechanism of spectral broadening and dephasing in quantum dots (QDs) due to the coupling to longitudinal-optical (LO) phonons. In theory, this mechanism comes into play only if the complete manifold of exciton levels (including those in the wetting-layer continuum) is taken into account. We demonstrate this nontrivial dephasing in different types of QDs, using the exactly solvable quadratic coupling model, here generalized to an arbitrary number of excitonic states. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Exciton,phonon interaction and Raman spectra of [(CH3)2NH2]5Cd2CuCl11 crystals

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2004
V. Kapustianik
Abstract Temperature evolution of the exciton,phonon interaction (EPI) in ((CH3)2NH2)5Cd2CuCl11 solid solution was studied on the basis of absorption spectroscopy data. The obtained values of effective phonon energies were compared with the data of Raman spectroscopy. It is shown that the (T) and E, parameters of Urbach's rule show the continuous anomalous change characteristic of the second-order phase transition at T1 = 176 K. The anomalous behaviour of the EPI and other spectral parameters at T0 = 310,315 K was related to the complex co-operative effect involving weakening of the hydrogen bonds and variation of the Jahn,Teller distortion of metal,halogen polyhedra with temperature. This process takes place only within the copper,chlorine sublattice and due to this would be hardly related to the usual phase transition. At the same time, the considered temperature change of the tetragonal distortion of the metal,halogen octahedra is followed by nonfulfillment of Urbach's rule in the temperature range TT0. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Optical properties of Ga1,xInxAs/GaAs(001) quantum well superlattices: Exciton and polariton dispersion curves

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2004
N. Tomassini
Abstract Wannier exciton wavefunctions and energies have been computed in superlattices of strained Ga1,xInxAs/GaAs(001) quantum wells (SLQWs) using Luttinger Hamiltonian and accurate variational envelope functions. Exciton dispersion curves of the SLQWs are then obtained by computing exciton energies for different K -points of the corresponding first Brillouin zone. Photon dispersion curves, due to the background dielectric constant modulation, and the polariton dispersion curves have been computed in the semiclassical self-consistent framework. The results are discussed for the case of exciton energies far from the photonic gaps. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Ultrafast Hole-Transfer Dynamics in Polymer/PCBM Bulk Heterojunctions

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2010
Artem A. Bakulin
Abstract Ultrafast dynamics of the hole-transfer process from methanofullerene to a polymer in a polymer/PCBM bulk heterojunction are directly resolved. Injection of holes into MDMO-PPV is markedly delayed with respect to [60]PCBM excitation. The fastest component of the delayed response is attributed to the PCBM,polymer hole-transfer process (30,±,10,fs), while the slower component (,150,fs) is provisionally assigned to energy transfer and/or relaxation inside PCBM nanoclusters. The charge generation through the hole transfer is therefore as fast and efficient as through the electron-transfer process. Exciton harvesting efficiency after PCBM excitation crucially depends on the concentration of the methanofullerene in the blend, which is related to changes in the blend morphology. Ultrafast charge generation is most efficient when the characteristic scale of phase separation in the blend does not exceed ,20,nm. At larger-scale phase separation, the exciton harvesting dramatically declines. The obtained results on the time scales of the ultrafast charge generation after PCBM excitation and their dependence on blend composition and morphology are instrumental for the future design of fullerene-derivative-based photovoltaic devices. [source]

Charge Transfer Excitons in Polymer/Fullerene Blends: The Role of Morphology and Polymer Chain Conformation

ADVANCED FUNCTIONAL MATERIALS, Issue 22 2009
Markus Hallermann
Abstract Here, it is shown how carrier recombination through charge transfer excitons between conjugated polymers and fullerene molecules is mainly controlled by the intrachain conformation of the polymer, and to a limited extent by the mesoscopic morphology of the blend. This experimental result is obtained by combining near-infrared photoluminescence spectroscopy and transmission electron microscopy, which are sensitive to charge transfer exciton emission and morphology, respectively. The photoluminescence intensity of the charge transfer exciton is correlated to the degree of intrachain order of the polymer, highlighting an important aspect for understanding and limiting carrier recombination in organic photovoltaics. [source]

Illuminating Dark Plasmons of Silver Nanoantenna Rings to Enhance Exciton,Plasmon Interactions

Design of Multilayered Nanostructures and Donor,Acceptor Interfaces in Solution-Processed Thin-Film Organic Solar Cells,

ADVANCED FUNCTIONAL MATERIALS, Issue 10 2008
Hiroaki Benten
Abstract Multilayered polymer thin-film solar cells have been fabricated by wet processes such as spin-coating and layer-by-layer deposition. Hole- and electron-transporting layers were prepared by spin-coating with poly(3,4-ethylenedioxythiophene) oxidized with poly(4-styrenesulfonate) (PEDOT:PSS) and fullerene (C60), respectively. The light-harvesting layer of poly-(p -phenylenevinylene) (PPV) was fabricated by layer-by-layer deposition of the PPV precursor cation and poly(sodium 4-styrenesulfonate) (PSS). The layer-by-layer technique enables us to control the layer thickness with nanometer precision and select the interfacial material at the donor,acceptor heterojunction. Optimizing the layered nanostructures, we obtained the best-performance device with a triple-layered structure of PEDOT:PSS|PPV|C60, where the thickness of the PPV layer was 11,nm, comparable to the diffusion length of the PPV singlet exciton. The external quantum efficiency spectrum was maximum (ca. 20%) around the absorption peak of PPV and the internal quantum efficiency was estimated to be as high as ca. 50% from a saturated photocurrent at a reverse bias of ,3,V. The power conversion efficiency of the triple-layer solar cell was 0.26% under AM1.5G simulated solar illumination with 100,mW,cm,2 in air. [source]

Electronic and Mechanical Coupling in Bent ZnO Nanowires

ADVANCED MATERIALS, Issue 48 2009
Xiaobing Han
A red shift of the exciton of ZnO nanowires is efficiently produced by bending strain, as demonstrated by a low-temperature (81,K) cathodoluminescence (CL) study of ZnO nanowires bent into L- or S-shapes. The figure shows a nanowire (Fig. a) with the positions of CL measurements marked. The corresponding CL spectra,revealing a peak shift and broadening in the region of the bend,are shown in Figure b. [source]

Exciton,Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals

High-Gain Broadband Solid-State Optical Amplifier using a Semiconducting Copolymer,

ADVANCED MATERIALS, Issue 1 2009
Dimali Amarasinghe
A dilute fluorene copolymer produces enhanced optical amplification. High gain with 1000 times optical amplification and a long lifetime is achieve in only 1mm of the material, and exciton,exciton annihilation is suppressed. [source]

Numerical studies of optical switching and optical bistability phenomena of nano- or meso-size spheres

JOURNAL OF MICROSCOPY, Issue 3 2003
T. Okamoto
Summary We propose the use of numerical calculations of the optical response of nonlinear Kerr-spheres, modifying the Mie theory. As a numerical example, we use a CuCl sphere coated with a Kerr-nonlinear sphere, taking into account the excitation of the Z3 -exciton. The results clearly show that optical bistable and/or optical switching devices can be realized on a nano- and meso-size scale when the real part of the dielectric constant of CuCl is negative. [source]

Competition between Host Aggregates and Isolated Guest Chromophores in Trapping Excitons in Polybenzazole Copolymers and Blends

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 24 2006
Shanfeng Wang
Abstract Summary: Host,guest systems have been prepared using the blends and copolymers consisting of the host molecule poly(p -phenylene benzobisoxazole) (PBO) with a higher bandgap of 2.93 eV and the guest molecule poly(2,5-thienylbenzobisoxazole) (PBOT) with a lower bandgap of 2.57 eV. These systems have been investigated using photoluminescence (PL) spectra and time-resolved PL decay dynamics. Both PBOT-PBO copolymers and PBOT/PBO blends with the PBOT compositions less than 20% demonstrate higher intensities and narrower bandwidths in solid-state emission compared to that of PBOT, as well as larger fractions of the shorter lifetime component in PL decay dynamics. A general scheme on intrachain and interchain exciton migration and trapping mechanism has been proposed to interpret the phenomena in both solutions and thin films. Particularly, a competition in trapping exciton between PBO aggregates and isolated PBOT chromophores has been revealed. General scheme of exciton migration and trapping paths for host (D: donor),guest (A: acceptor) systems involving the isolated chromophores (A and D) and aggregates (AA and DD) in both ground state and excited state marked without or with an asterisk. [source]

Light emission from different ZnO junctions and nanostructures

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2009
M. Willander
Abstract We will discuss our experimental results for optical spectra produced by hole-injection from different p-type organic and inorganic materials into n-type ZnO nanowires. The influence of different growth techniques and conditions on the nanowires and their emission spectral characteristics will then be analyzed and discussed. The latest findings on the mixture of the green emission band responsible for visible light emission from ZnO and the blue light emission from the organic polymer will be presented. Different high brightness light emitting diodes (HB-LEDs) from our grown ZnO nanowires are demonstrated. The p-type multi layer organic structures contain PEDOT:PSS as hole injectors combined with a hole transporting layer, and in some structures, a final top electron blocking/hole barrier stepping layer is placed. The purpose of this layer is to adjust the hole and electron emission from the corresponding junction side to optimize the LED performance. Structural scanning electron microscopy (SEM), electrical (I ,V characteristics), photoluminescence (PL) and electroluminescence (EL) characteristics of these devices are displayed. Theoretically, we study the superfluidity of a two-dimensional system of excitonic polaritons in an optical microcavity with an embedded quantum well. Using the effective low-energy action for thermodynamic phase fluctuations, we obtain an expression for the analogue of the superfluid density in the system in terms of the "current,current" correlation function. The Kosterlits,Thouless transition temperature to the superfluid state as a function of the controlling parameters is calculated. Two methods are considered for producing traps for a polariton system in an optical microcavity. The behaviour of a two-component Bose condensate of photons and excitons is analyzed theoretically for both types of the trap. The Bose condensate is described by the coupled system of equations of the Gross,Pitaevskii type. The approximate wave functions and the spatial profiles of coupled photon and exciton condensates are obtained. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Cathodoluminescence as a tool to determine the phosphorus concentration in diamond

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 9 2007
J. Barjon
Abstract In n-type diamond doped with phosphorus, exciton properties have been investigated by cathodoluminescence as a function of the phosphorus concentration. A series of homoepitaxial diamond layers were grown by microwave plasma-assisted chemical vapor deposition and doped with a liquid organic precursor of phosphorus (tertiarybutylphosphine). Their phosphorus concentration ranges from 5.2 × 1016 cm,3 to 3.3 × 1018 cm,3 as measured by secondary ion mass spectrometry. It is shown that the ratio between the luminescence intensities of the neutral phosphorus-bound exciton and the free exciton recombinations follows the donor concentration. A calibration graph is presented to determine the phosphorus contents in diamond with cathodoluminescence spectroscopy at 102 K. The influence of electrical compensation on the optical spectra is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Excitons and defects in homoepitaxial diamond films from cathodoluminescence of p,/p+ samples

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2004
M. Wade
Abstract We study the signals from excitons and defects from the cathodoluminescence spectra (CL) of the p,/pseudo substrate sample. The CL from the p, film are determined from the variation of the CL of the sample as the voltage of the exciting electron beam increases. At the lower voltages, it probes the p, film. The CL is dominated by the free exciton associated with the TO phonon (FETO, FWHM = 6 meV), but shows also small signals (ratio = 5 × 10,2) mainly from the A band (and other 2.6, 3.6 and 4.05 eV wide bands) and tiny signals from the 5 RL defect. At the higher voltages it probes also the Ib substrate and is dominated by the signals of the H3 defects in the Ib substrate. The p, film appears almost homogeneous, and of good quality from the characteristics of its exciton. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Unified description of collective modes in superconductors and semiconductors with an exciton condensed phase

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 9 2010
Z. G. Koinov
Abstract It is shown that the Bethe,Salpeter approach, the Bardeen, Cooper, and Schrieffer (BCS) based vertex method, and a generalized random-phase approximation (GRPA) to the many-electron problem in the presence of a condensed quantum phase yield the same theoretical excitation spectrum to collective modes. This spectrum reveals a secondary peak in optical absorption in semiconductors that can be understood as signaling the existence of an excitonic Bose,Einstein condensate (BEC). The analysis shows as well that there is an additional, non-trivial linearly-dispersive " moving" Cooper-pair solution for superconductors in both weak and strong coupling. [source]

Resonant Raman scattering in spherical quantum dots: II,VI versus III,V semiconductor nanocrystals

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2010
Mikhail I. Vasilevskiy
Abstract Resonant Raman scattering (RRS) in nearly-spherical nanocrystal (NC) quantum dots (QDs) is discussed with respect to the underlying (Fröhlich-type and optical deformation potential, ODP) mechanisms of the exciton,phonon interaction. Their relative contribution for different QD materials, both II,VI and III,V is compared. It is shown that the (usually overlooked) ODP interaction is entirely responsible for an additional peak in the RRS spectra, situated near the transverse-optical (TO) phonon frequency, which has been observed for InP, InAs and, recently, CdTe QDs. RRS spectra calculated using continuum models for confined phonons and excitons and taking into account both interaction mechanisms are in excellent agreement with these experimental data. [source]

Spin properties of trions in a dense 2DEG

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2010
V. Kochereshko
Abstract Reflectivity and photoluminescence spectra from CdTe/CdMgTe modulation-doped quantum well structures were studied. We have found that in reflectivity spectra the value and the sign of the Zeeman splitting of the trion lines depend on the electron concentration in the quantum well, whereas, the value and sign of the exciton line splitting are constant for all studied electron concentrations. On the other hand, in the photoluminescence spectra the sign and value of the Zeeman splitting are the same for trion and exciton. Such "renormalization" of the trion g -factor is explained in the model of combined exciton,electron processes. [source]

Fine structure of emission lines from charged CdSe/ZnSe/ZnMnSe quantum dots

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2010
E. A. Chekhovich
Abstract Photoluminescence spectroscopy has been employed to study CdSe/ZnSe/ZnMnSe quantum dots. For most of the dots studied here luminescence comes in three spectrally separated features: neutral exciton (X), biexciton (XX), and charged exciton (XC) states. Spectral properties of X and XX emission are well understood, however, in a marked contrast with previous studies, the observed fine structure of XC can not be explained within a commonly accepted model of a ground state trion luminescence. We find that at zero magnetic field luminescence from the charged state exhibits fine structure that varies gradually between different dots from a single unpolarized line to a quartet with the maximum splitting of 2,meV. Several models including magnetic polaron formation and double charging have been considered, but a plausible explanation can be given only if one considers the influence of a charge trapped in a nearby dot. [source]

Temperature dependent luminescence from quantum dot arrays: phonon-assisted line broadening versus carrier escape-induced narrowing

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2010
M. B. Smirnov
Abstract The paper presents a theoretical model describing the temperature dependence of the photoluminescence spectrum of self-ordered quantum dots arrays taking into account exciton,phonon interaction and thermal carriers transfer. This model is applied to the photoluminescence behaviour of InAs quantum dots grown on GaAs vicinal substrates. It allows distinguishing between effects caused by the different temperature-induced mechanisms and thus provides information about the physical and electronic structure of the quantum dot arrays. [source]

Temperature dependent high resolution resonant spectroscopy on a charged quantum dot

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2009
M. Kroner
Abstract We present temperature dependent high resolution resonant optical spectroscopy on a single, negatively charged InGaAs quantum dot. We performed laser transmission measurements yielding the natural linewidth of the excitonic ground state transition of a quantum dot in a temperature range from 4.2 K up to 25 K. Here, we describe the linewidth evolution and the temperature induced red shift of the resonance energy with simple models based on the exciton,phonon coupling in the quantum dot. The resonant spectroscopy measurements are complemented with results from non-resonant PL measurements on the very same quantum dot. Here we observe a simple linear behavior of the linewidth according to an effect of a fluctuating environment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Three-dimensional localization of excitons in the InAs/GaAs wetting layer , magnetospectroscopic study

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 4 2009
A. Babinski
Abstract Magnetospectroscopic studies of individual excitonic states confined in potential fluctuations in the InAs/GaAs wetting layer (WL) are presented. A neutral exciton and a trion emission have been identified. They split in magnetic field in two components of orthogonal circular polarizations. The respective Zeeman splitting changes linearly with magnetic field up to 10 T. A significant scatter of the effective excitonic g *-factor is observed, reflecting the distribution of sizes and compositions of potential fluctuations in the WL. The distribution affects also diamagnetic shift of the excitonic emission. The observed properties of the excitons are consistent with a picture of shallow quantum dots formed in the WL due to In composition fluctuations. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Excitons in motion: universal dependence of the magnetic moment on kinetic energy

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2008
V. P. Kochereshko
Abstract We have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence on the translational wavevector Kz. The behaviour of the Zee-man splittings corresponds to enhancement of the magnetic moments of the excitons. This enhancement is particularly marked when their translational kinetic energy becomes comparable with the exciton Rydberg and can be described by what appears to be a universal function of Kz. A model for the behaviour is outlined which involves motionally-induced mixing between the 1S hydrogenic exciton ground state and excited nP states. The observations imply that there are significant changes in the structure of the exciton as its translational kinetic energy increases. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

First and second optical transitions in single-walled carbon nanotubes: a resonant Raman study

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007
H. Telg
Abstract Resonant Raman spectroscopy was performed to study electron,phonon coupling in single-walled carbon nanotubes separated in solution. By varying the excitation energy from 1.26 eV to 1.93 eV we obtained radial breathing mode resonance profiles of the first and second optical transitions E11 and E22 of the (9,1) and (8,3) tubes. We observe up to 16 times stronger Raman intensities for the E11 transitions which can mostly be attributed to a two times broader linewidth of the E22 transition. Comparison of the matrix element ratio ,11/,22 to theoretical predictions on the electron,phonon coupling show a deviation of a factor 1.7 which might be associated with the change of the exciton,photon matrix element. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Strong coupling in artificial semimagnetic Cd(Mn,Mg)Te quantum dot molecule

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2006
S. V. Zaitsev
Abstract Exciton photoluminescence in a pair of strongly coupled artificial asymmetric quantum dots (QDs) has been studied in a magnetic field up to 8 T. The QD molecules have been fabricated by a selective interdiffusion technique applied to asymmetric semimagnetic CdTe/Cd(Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane, induced by the diffusion, gives rise to effective zero-dimensional exciton localization. In contrast to a typically positive exciton Lande g -factor, an exciton transition in the non-magnetic QD demonstrates a nearly zero g -factor, indicating a strong electron tunnel coupling between the QDs. The strong coupling results in the formation of an inter-QDs indirect exciton, which is a ground exciton state at high magnetic field, as found in the experiment and confirmed by our calculations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Computational methods for studies of multiexciton complexes,

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2006
T. Vänskä
Abstract Powerful computational methods are presented for studies of energy levels, photon-recombination rates, and phonon-relaxation rates of neutral and charged multiexciton complexes at correlated levels of theory. The electron,hole system is described by a two-band effective-mass Hamiltonian. The one-particle functions are expanded in a basis set consisting of anisotropic Gaussian functions. The many-body Hamiltonian constructed in the space of the antisymmetric products of one-particle functions is diagonalized using general coupled-cluster and configuration-interaction methods. The expansion coefficients of the coupled-cluster and configuration-interaction wave functions are obtained by solving the corresponding equations using direct iterative algorithms. We demonstrate the potential of the computational approaches by calculating total energies of multiexciton complexes at coupled-cluster and configuration-interaction levels. Computational methods for studies of radiative recombination and phonon-relaxation rates have also been developed and results are reported for radiative recombination rates and recombination energies of the exciton, biexciton, and of the positive and the negative trions confined in a InGaAs/GaAs quantum-dot sample. Phonon-relaxation rates have been calculated for a few low-lying ,g states of the exciton complex of the same quantum-dot sample. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]