Ali Mokhtari

Density functional study of the group II phosphide semiconductor compounds under hydrostatic pressure

The full-potential all-electron linearized augmented plane wave plus local orbital (FP-LAPW+lo) method, as implemented in the suite of software WIEN2k, has been used to systematically investigate the structural and electronic properties of the group II phosphide semiconductor compounds M3P2 (M = Be, Mg and Ca). The exchange–correlation functional was approximated as a generalized gradient functional introduced by Perdew–Burke–Ernzerhof (GGA96) and Engel–Vosko (EV-GGA). Internal parameters were optimized by relaxing the atomic positions in the force directions using the Hellman–Feynman approach. The structural parameters, bulk modules, cohesive energy, band structures and density of states have been calculated and compared to the available experimental and theoretical results. These compounds are predicted to be semiconductors with the direct band gap of about 1.60, 2.55 and 2.62 eV for Be3P2, Mg3P2 and Ca3P2, respectively. The effects of hydrostatic pressure on the behavior of band parameters such as band gap, valence bandwidths and anti-symmetric gap (the energy gap between two parts of the valence bands) are investigated using both GGA96 and EV-GGA. The contribution of s, p and d orbitals of different atoms to the density of states is discussed in detail.

High pressure study of the zinc phosphide semiconductor compound in two different phases

Electronic and structural properties of the zinc phosphide semiconductor compound are calculated at hydrostatic pressure using the full-potential all-electron linearized augmented plane wave plus local orbital (FP-LAPW+lo) method in both cubic and tetragonal phases. The exchange-correlation potential is treated by the generalized gradient approximation within the scheme of Perdew, Burke and Ernzerhof, GGA96 (1996 Phys. Rev. Lett. 77 3865). Also, the Engel and Vosko GGA formalism, EV-GGA (Engel and Vosko 1993 Phys. Rev. B 47 13164), is used to improve the band-gap results. Internal parameters are optimized by relaxing the atomic positions in the force directions using the Hellman-Feynman approach. The lattice constants, internal parameters, bulk modulus, cohesive energy and band structures have been calculated and compared to the available experimental and theoretical results. The structural calculations predict that the stable phase is tetragonal. The effects of hydrostatic pressure on the behavior of band parameters such as band-gap, valence bandwidths and internal gaps (the energy gap between different parts of the valence bands) are studied using both GGA96 and EV-GGA.

Ab initio study of the stability and electronic properties of wurtzite and zinc-blende BeS nanowires

In this work we study the structural stability and electronic properties of the Beryllium sulfide nanowires (NWs) in zinc-blende (ZB) and wurtzite (WZ) phases (with triangle and hexagonal cross sections), using first principle calculations within the plane-wave pseudopotential method. A phenomenological model is used to explain the role of dangling bonds in the stability of the NWs. In contrast to the bulk phase, the ZB-NWs with diameters less than 133.3 A are found to be less favorable over the WZ-NWs, in which the surface dangling bonds (DBs) on the NW facets play an important role to stabilize the NWs. Furthermore, both ZB- and WZ-NWs are predicted to be semiconductor and the values of the band gaps are dependent on the surface DBs as well as the size and shape of the NWs. Finally, we obtain atom projected density of states (PDOSs) by calculating the localized density of states on the surface atoms, as well as on the core and edge atoms.

First-principles investigation of electronic and structural properties and bowing parameters in SrFClxBr1?x alloy

The first ab?initio calculations have been performed to study the structural and electronic properties of technologically important SrFClxBr1?x quaternary alloys (for x equal to 0.0, 0.25, 0.5, 0.75 and 1.0) using the full-potential linearized augmented-plane-wave method within density-functional theory. The Perdew et al generalized-gradient approximation (GGA96), which is based on exchange?correlation energy optimization, is utilized to optimize the internal parameters by relaxing the atomic positions in the force directions and to calculate the total energy. Both the Engel?Voskos generalized-gradient approximation (EV-GGA), which optimizes the exchange?correlation potential, and GGA96 are used for band structure calculations. The effect of composition on the equilibrium volume, cohesive energy, band gap and mean values of the bond length, shows nonlinear dependence, but on the bulk modulus it exhibits nearly linear concentration dependence (LCD). The results obtained show that the quaternary alloy of interest could be an appropriate material for using in an optical apparatus.

Developing a method for the evaluation of dislocation parameters from the Rietveld refinement procedure

In this investigation, the ability of Rietveld refinement method was used to simultaneously refine the structure and microstructure and evaluate the linear defects of cubic crystals. To do this, the basis of Stephans theory, about the anisotropic strain broadening, was developed and the values of dislocation density as well as the fraction of dislocation types were estimated. The reliability of this procedure was checked by selecting four different nanocrystaslline samples and evaluating the microstructure of these materials. Finally, the results were compared with those extracted from the whole powder pattern modeling method. Good agreement was achieved between the results of two methods.

Simulation BeSe Nanowires in Two Phasese Zinc-Blende and Wurtzite Using Density Functional Theory

In this work, we are reporting on the simulation of the beryllium selenide (BeSe) nanowires (NWs) by computational package Q-Espresso / PWSCF according to the ab-initio calculations. Structural and electronic properties, including cohesive energy and Density Of State (DOS) BeSe NWs in two phases on the zinc–blende (ZB) and wurtzite (WZ), using density functional theory based on pseudo-potential approximation and generalized gradient approximation (GGA) up to 20 angstrom in diameter has been calculated. Due to dangling bonds (DBs) in the side surface NWs, cohesive energy is obtained less than the amount of this energy in bulk state of this compound, but with increasing diameter of NWs, the amount of this energy will approach to the bulk state. Comparison of cohesive energy with beryllium selenide NWs in two phases, we find these NWs in WZ phase is more stable and have good compatibility for this result with other results in NWs of similar compounds. The value of energy gap in these NWs on various diameters is obtained less than the amount of the bulk state. It is observed that by increasing the diameter of NWs, the cohesive energy approaches to its value in bulk state.