[show abstract][hide abstract] ABSTRACT: A method to calculate the quantum states of exciton–phonon complexes in semiconductor nanocrystals is presented. The exciton–phonon complexes are built from a basis set made of products of phonon states and electron–hole pairs, which are coupled through the electron–phonon Fröhlich interaction, and the electron–hole Coulomb and exchange interactions. In CdSe nanocrystals, the conduction band electrons are described by the effective mass equation, while the holes are represented by the spherical 4 × 4 Baldereschi–Lipari Hamiltonian. It is shown that a flexible and complete electron–hole basis, not limited to the 1s–1S3/2 octet, is essential to obtain converged eigenvalues and the correct polaron shift to the exciton energy. A study of the spectral properties is presented; in particular, the spectral region which involves the lowest exciton–phonon complex eigenstates is analysed in details. Specifically, the non-adiabatic nature of the exciton–phonon dynamics in the nanocrystals examined is clearly shown by the vibron eigenstates that were obtained.
[show abstract][hide abstract] ABSTRACT: Polycrystalline bismuth sample was compressed in a diamond anvil cell and X-ray diffraction patterns from the body centered cubic phase of bismuth (bcc-Bi) recorded in the pressure range 12–222 GPa. The analysis of diffraction data indicates that the factor S=(S11−S12−S44/2) is positive, where Sij are the single-crystal elastic compliances. The data suggest S/S11≅0.5 in the entire pressure range if stress continuity across the grain boundaries is assumed, and S/S11≅0.9 if a condition halfway between the stress and strain continuities is assumed. These results are compared with the first principle calculations of the elastic moduli carried out recently. The upper bound of the uniaxial stress component (the difference between the axial and radial stress components) increases linearly from very low value at 12 to ≈3 GPa at 222 GPa.
Journal of Physics and Chemistry of Solids 01/2006; · 1.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: The electronic structure of bulk gamma -alumina is investigated using Density Functional Theory. Recent theoretical structures, including occupation of nonspinel positions, are used as starting point for energy minimization, allowing for relaxation of the cell shape and the ionic positions. A comparison of simulated diffraction patterns of the present and recent theoretical and experimental structural models is presented. The electronic structure is described in terms of band structure, density of states, charge density, electron localization function, and ionic charges. The valence band density of states of gamma -alumina is similar to alpha - and kappa -alumina, although a smaller bandgap is found. It is shown that gamma -alumina ionicity is similar to other alumina phases, with a high localization of the electrons at oxygen atoms. The smaller bandgap of gamma -alumina is attributed to the structural disorder.
Physical Review B 01/2005; 72. · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Quantum Stark effect in semiconductor nanocrystals is theoretically investigated, using the effective mass formalism within a 4×4 Baldereschi-Lipari Hamiltonian model for the hole states. General expressions are reported for the hole eigenfunctions at zero electric field. Electron and hole single-particle energies as functions of the electric field (EQD) are reported. Stark shift and binding energy of the excitonic levels are obtained by full diagonalization of the correlated electron-hole Hamiltonian in presence of the external field. Particularly, the structure of the lower excitonic states and their symmetry properties in CdSe nanocrystals are studied. It is found that the dependence of the exciton binding energy upon the applied field is strongly reduced for small quantum-dot radius. Optical selection rules for absorption and luminescence are obtained. The electric-field induced quenching of the optical spectra as a function of EQD is studied in terms of the exciton dipole matrix element. It is predicted that photoluminescence spectra present anomalous field dependence of the emission lines. These results agree in magnitude with experimental observation and with the main features of photoluminescence experiments in nanostructures.
[show abstract][hide abstract] ABSTRACT: The electronic structure of crystalline CdTe, CdO, α-TeO2, CdTeO3, and Cd3TeO6 is studied by means of first principles calculations. The band structure, total and partial density of states, and charge densities are presented. For α-TeO2 and CdTeO3, Density Functional Theory within the Local Density Approximation (LDA) correctly describes the insulating character of these compounds. In the first four compounds, LDA underestimates the optical bandgap by roughly 1 eV. Based on this trend, we predict an optical bandgap of 1.7 eV for Cd3TeO6. This material shows an isolated conduction band with a low effective mass, thus explaining its semiconducting character observed recently. In all these oxides, the top valence bands are formed mainly from the O 2p electrons. On the other hand, the binding energy of the Cd 4d band, relative to the valence band maximum, in the ternary compounds is smaller than in CdTe and CdO.
[show abstract][hide abstract] ABSTRACT: The adiabatic theory of resonance one-phonon Raman scattering in semiconductor nanocrystals is revised and extended with perturbative non-adiabatic corrections, given by the Albrecht's B term. This theory is confronted with the time-dependent perturbation approach, pointing at their differences and similarities. It is shown that both theories are equivalent in the limit of weak electron-phonon coupling and non-degenerate or uncoupled resonant states. Evaluations of the A and B terms for the confined LO phonon in CdSe and CdS nanocrystals are reported. These evaluations show that the B term can usually be neglected. Comment: 8 pages, twocolumn, 0 figure. Sent to PRB
[show abstract][hide abstract] ABSTRACT: During the obtaining of semiconductor materials by some growth techniques the film structure forms randomly according to how the species arrive to the subtract. If the film is a material with three or more elements they can organize in several compounds, which have only local order and even become amorphous. As a consequence the physico-chemical parameters manifest non-typical behaviors other than in pure materials. In the present work we develop a dynamical-probabilistic model, which describes quantitatively such composition mixture and was applied on the behavior of the absorption profiles of CdTeO films grown by radio frequency (rf) sputtering with different oxygen concentrations. The model can be applied to films obtained by other growth techniques.
Modern Physics Letters B 01/2001; 15:643-646. · 0.48 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have studied the multiphonon resonant Raman scattering from confined and interface polar optical phonons in spherical nanocrystallites. The intermediate virtual states in the scattering process are taken into account as Wannier-Mott confined excitons in a spherical dot. Fröhlich interaction between excitons and optical phonons has been considered and general selection rules for the exciton-phonon matrix elements and multiphonon scattering processes in the case of spherical quantum dots have been derived. It is shown that for a second-order process, two phonons are created with the same angular momentum (lp1=lp2) while, in a third-order process, the second emitted (or absorbed) phonon with angular momentum lp2 must fulfill the triangular property |lp1-lp3|<~lp2<~lp1+lp3. In the general case, the sum of phonon momentum projections on the z axis mp1+mp2+⋯=0. We have performed multiphonon Raman cross-section calculations of CdSe quantum dots of various sizes up to third order and present detailed comparison with available experimental data. The effect of size distribution is studied; we show that a broad dispersion of nanocrystal sizes has important consequences on the multiphonon Raman spectra. The experimental relative intensities between phonon overtones are correctly described in the framework of the present model. Also, an analysis of the applicability of the Huang-Rhys factor for quantum dot systems is presented and several contradictions found in the literature concerning this parameter have been explored.
Physical Review B 10/2000; 62(16). · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: A theoretical treatment for first-order resonant Raman scattering in self-assembled quantum dots (SAQD’s) of different materials is presented. The dots are modeled as cylindrical disks with elliptical cross section, to simulate shape and confinement anisotropies obtained from the SAQD growth conditions. Coulomb interaction between electron and hole is considered in an envelope function Hamiltonian approach and the eigenvalues and eigenfunctions are obtained by a matrix diagonalization technique. By including excitonic intermediate states in the Raman process, the scattering efficiency and cross section are calculated for long-range Fröhlich exciton-phonon interaction. The Fröhlich interaction in the SAQD is considered in an approach in which both the mechanical and electrostatic matching boundary conditions are fulfilled at the SAQD interfaces. Exciton and confined phonon selection rules are derived for Raman processes. Characteristic results for SAQD’s are presented, including InAs dots in GaAs, as well as CdSe dots in ZnSe substrates. We analyze how Raman spectroscopy would give information on carrier masses, confinement anisotropy effects, and SAQD geometry.
Physical Review B 12/1999; 60(24). · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: A theoretical model of resonant hyper-Raman scattering by an ensemble of spherical semiconductor quantum dots has been developed. The electronic intermediate states are described as Wannier-Mott excitons in the framework of the envelope function approximation. The optical polar vibrational modes of the nanocrystallites (vibrons) and their interaction with the electronic system are analized with the help of a continuum model satisfying both the mechanical and electrostatic matching conditions at the interface. An explicit expression for the hyper-Raman scattering efficiency is derived, which is valid for incident two-photon energy close to the exciton resonances. The dipole selection rules for optical transitions and Fr\"ohlich-like exciton-lattice interaction are derived: It is shown that only exciton states with total angular momentum $L=0,1$ and vibrational modes with angular momentum $l_p=1$ contribute to the hyper-Raman scattering process. The associated exciton energies, wavefunctions, and vibron frequencies have been obtained for spherical CdSe zincblende-type nanocrystals, and the corresponding hyper-Raman scattering spectrum and resonance profile are calculated. Their dependence on the dot radius and the influence of the size distribution on them are also discussed. Comment: 12 pages REVTeX (two columns), 2 tables, 8 figures
[show abstract][hide abstract] ABSTRACT: Resonant Raman scattering in semiconductor quantum dots with spherical shape is theoretically investigated. The Frohlich-like interaction between electronic states and optical vibrations has been considered. The Raman profiles are studied for the following intermediate electronic state models: (I) Uncorrelated electron-hole pairs in the strong size quantized regime, (II) Wannier-Mott excitons in an infinite potential well, and (III) Excitons in a finite confinement barrier. It is shown that the finite confinement barrier height and the electron-hole correlation determine the absolutes values of the scattering intensities and substantially modify the Raman line shape, even in the strong confinement regime. Comment: 12 pages REVTeX (preprint), 1 table, 3 figures. Subj-clas: Mesoscopic systems
Semiconductor Science and Technology 08/1998; · 1.92 Impact Factor
[show abstract][hide abstract] ABSTRACT: We examine the dependence on size distribution of the Raman line shape of optical vibron modes in CdSe dots. A detailed comparison with the theory of electron-hole correlated Raman scattering in spherical quantum dots, which includes LO-confined modes, is carried out. The obtained CdSe vibron frequencies are found to map well on the ab initio calculated LO phonon dispersion relations of wurtzite CdSe along the  direction of the Brillouin zone.