Papers by Keyword: Band Structure

Abstract: The crystal structure, electronic and optical properties of organic molecular crystal Trans-4-(trifluoromethyl) cinnamic acid (4-TFMCA, C₁₀H₇F₃O₂) were studied using the density functional theory (DFT). Since 4-TFMCA undergoes solid-state photodimerization under an external light source (UV), the optical properties were a subject of interest. In the calculations, experimental lattice parameters were obtained from previous studies and used as an initial geometry. Structural optimization was achieved using the vdW-DF2 functional with norm-conserving pseudopotentials (NCPP). To optimize the crystal structure, the Birch-Murnaghan equation of state (EOS) was used, and the total volume showed a decrease of 2%. The electronic band structure of the 4-TFMCA crystal was first calculated. The electronic and optical band gaps were predicted, and an artificial acceptor level, attributed to the unsaturated carbon and hydrogen atoms in the molecule, was observed. Additionally, optical properties such as the dielectric function, reflectivity, loss function, refractive index, and absorption coefficient were computed.

Abstract: We perform the density functional theory calculations (DFT) to study the effect of biaxial strain on the band structures of monolayer GaN. We apply compressive and tensile strains up to 10%. There is no change of bandgap for the applied tensile strains below 8%. The compressive strains have a constant bandgap which is slightly smaller than that of the zero strain. We find that the applied tensile strain above 8% affects its electronic structure and decreases its bandgap energy by about 0.05 eV while the compressive strain above 4% decreases its bandgap about 0.22 eV.

Abstract: Optical absorption and photoluminescence (PL) spectra were measured on defect-stannite-type semiconductor ZnGa₂Se₄ at temperatures T from 11 to 300 K. The square of the absorption coefficient spectra showed distinct two absorption edges, which were E_0A,B and E_0C,D transitions at Γ point in the Brillouin zone. The temperature dependence of the direct-gap energies, E_0A,B and E_0C,D, of ZnGa₂Se₄ were determined and fit using the analytical four-parameter expression developed for the explanation of the band-gap shrinkage effect in semiconductors. The PL emissions at near band-edge and at higher energy than band-edge were also observed at T ≤ 150 K.

Abstract: In this work we investigated the structural, electronic and elastic properties of TlN, TlP, TlAs and TlSb compounds in the zinc-blende phase, the lattice parameter, bulk modulus, band structure, and elastic constants have been calculated by employing the full potential linearized augmented plane wave method based on density functional theory of the exchange-correlation potentials including local density approximation, PBE generalized gradient, and Wu-Cohen generalized gradient are used. Furthermore, the modified Backe-Johnson (mBJ) potential has been utilized for the calculation of the energy gap. The present results are compared with other available theoretical values obtained.

Abstract: In the present study, we have computed the electronic band structure and electronic charge density of the alloy (Be, Cd)Se in the zinc-blende structure; using the local Empirical Pseudopotential Method (EPM), which takes into account the disorder effect into the Virtual Crystal Approximation (VCA) by introducing an effective potential disorder. The obtained results show a reasonable agreement with the available experimental data. Detailed plots of the valence charge distribution along the [111] direction and in the (110) plane are also presented and discussed.

Abstract: We measured the low temperature wavelength modulated absorption and low temperature photoluminescence spectra of relatively high purity (n~10¹⁴-10¹⁵ cm^-3) 6H SiC boule and epi layers at high wavelength resolution (0.1-0.7 Å) to adequately separate and identify phonon-assisted absorption and recombination processes due to free excitons. As a result we have identified newly resolved or weak features in both spectra which we associate with previously unidentified momentum conserving (MC) phonons. We obtain the energies of 21 of the 36 possible MC phonons in 6H SiC and a more accurate estimate of the exciton bandgap, E_gx = 3.0225±0.0003 eV. In several of the phonon-assisted absorption onsets we also observe fine structure and variations in the measured spin-orbit splitting, which relate to a fourfold splitting of the free exciton energy bands.

Abstract: The effect of strain on the band structure of the GeH monolayer has been investigated by first-principles calculations based on density functional theory. The results show that the change of the band gap under the zigzag strain, the armchair strain and the biaxial strain is nonlinear. The effect of the biaxial strain on the band gap is the most obvious. In addition, the changes of energy under the three kinds of strain are asymmetric and the biaxial strain make the energy change the most. This work has significant implication of strain to tune optical catalytic properties of GeH monolayer.

Abstract: We use thick, relatively high purity 4H SiC boule material to measure the wavelength modulated absorption spectrum with improved wavelength resolution and sensitivity with respect to previous work. We observe several small 0.6 ± 0.1 meV splittings, which we attribute to electron mass anisotropy and electron-hole exchange interaction. In addition, we identify several features in the absorption spectrum as signatures of nonparabolicity in the free exciton dispersion relations, the primary origin of which is likely the nonparabolic energy dispersion of the valence bands, as revealed by published band structure calculations based on density functional theory.

Abstract: Electronic structures and optical anisotropy of α- and β-phase copper phthalocyanine (CuPc) molecular crystals have been systemically investigated by first-principles calculations based on Density Functional Theory (DFT). Both crystals were shown to be small gap organic semiconductors with relatively flat and dispersionless bands. The α-CuPc was a direct band gap semiconductor, whereas the β-CuPc was an indirect band gap semiconductor. The analysis of Partial Density of States (PDOS) showed that the top of valance band was mainly contributed by N 2p and C 2p states; the bottom of the conduction band was mainly contributed by N 2p, C 2p and Cu 3d states. The interband optical properties, such as the complex dielectric function, absorption coefficient and complex refractive index, showed a high degree of anisotropy that can be traced to the unique structures of these molecular crystals. The calculated dielectric function for α-CuPc in the low energy region was consistent with the experiment results proposed in the literature. These calculations provided particular interpretations on electronic structure and optical properties of α- and β-CuPc organic semiconductors that were critical to optoelectronics, which would promote the applications of these materials in semiconductor optoelectronic devices.

Abstract: We briefly review the spin-polarized positron annihilation experiments on some ferromagnets (Fe, Co, Ni, Gd, Co₂MnSi, Co₂MnAl and NiMnSb) using positron beams generated with ⁶⁸Ge-⁶⁸Ga sources. The differential DBAR spectra between majority and minority spin electrons are well interpreted by the first principles band structure calculation. This further provides information about the half-metallicity of the Heusler alloys. The surfaces of Fe, Co and Ni are more negatively spin-polarized, that is, there are more majority than minority spin electrons. To explain the observed spin polarization quantitatively, detailed theoretical calculations and further experiments are required.