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Register a new userElectronic structure and optical properties of ZnS/CdS nanoheterostructures
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J. Perez-Conde
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The electronic and optical properties of spherical nanoheterostructures are studied within the semi-empirical $sp^{3}s^{*}$ tight-binding model including the spin-orbit interaction. We use a symmetry-based approach previously applied to CdSe and CdTe quantum dots. The complete one-particle spectrum is obtained by using group-theoretical methods. The excitonic eigenstates are then deduced in the configuration-interaction approach by fully taking into account the Coulomb direct and exchange interactions. Here we focus on ZnS/CdS, ZnS/CdS/ZnS and CdS/ZnS nanocrystals with particular emphasis on recently reported experimental data. The degree of carrier localization in the CdS well layer is analyzed as a function of its thickness. We compute the excitonic fine structure, i.e., the relative intensities of low-energy optical transitions. The calculated values of the absorption gap show a good agreement with the experimental ones. Enhanced resonant photoluminescence Stokes shifts are predicted.
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We have measured the specific heat of zincblende ZnS for several isotopic compositions and over a broad temperature range (3 to 1100 K). We have compared these results with calculations based on ab initio electronic band structures, performed using both LDA and GGA exchange- correlation functionals. We have compared the lattice dynamics obtained in this manner with experimental data and have calculated the one-phonon and two-phonon densities of states. We have also calculated mode Grueneisen parameters at a number of high symmetry points of the Brillouin zone. The electronic part of our calculations has been used to investigate the effect of the 3d core electrons of zinc on the spin-orbit splitting of the top valence bands. The effect of these core electrons on the band structure of the rock salt modification of ZnS is also discussed.
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Optical properties of ZnMnO layers grown at low temperature by Atomic Layer Deposition and Metalorganic Vapor Phase Epitaxy are discussed and compared to results obtained for ZnMnS samples. Present results suggest a double valence of Mn ions in ZnO lattice. Strong absorption, with onset at about 2.1 eV, is tentatively related to Mn 2+ to 3+ photoionization. Mechanism of emission deactivation in ZnMnO is discussed and is explained by the processes following the assumed Mn 2+ to 3+ recharging.
The complex optical properties of a single crystal of hexagonal FeCrAs ($T_N simeq 125$ K) have been determined above and below $T_N$ over a wide frequency range in the planes (along the $b$ axis), and along the perpendicular ($c$ axis) direction. At room temperature, the optical conductivity $sigma_1(omega)$ has an anisotropic metallic character. The electronic band structure reveals two bands crossing the Fermi level, allowing the optical properties to be described by two free-carrier (Drude) contributions consisting of a strong, broad component and a weak, narrow term that describes the increase in $sigma_1(omega)$ below $simeq 15$ meV. The dc-resistivity of FeCrAs is ``non-metallic, meaning that it rises in power-law fashion with decreasing temperature, without any signature of a transport gap. In the analysis of the optical conductivity, the scattering rates for both Drude contributions track the dc-resistivity quite well, leading us to conclude that the non-metallic resistivity of FeCrAs is primarily due to a scattering rate that increases with decreasing temperature, rather than the loss of free carriers. The power law $sigma_1(omega) propto omega^{-0.6}$ is observed in the near-infrared region and as $Trightarrow T_N$ spectral weight is transferred from low to high energy ($gtrsim 0.6$ eV); these effects may be explained by either the two-Drude model or Hunds coupling. We also find that a low-frequency in-plane phonon mode decreases in frequency for $T < T_N$, suggesting the possibility of spin-phonon coupling.
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