Band structure of cubic and rhombohedral GeTe

Magnesium silicide (Mg2Si) has been identified a promising advanced thermoelectric material in temperature range from 300 to 700K. In order to understand thermoelectric properties of Co-doped magnesium silicide, the band structure and electronic density of states have been calculated using a first-principle pseudopotential method. It is shown that the band gap gradually decreases, at the same time degeneracy of the band and the density of states at the fermi level increase as the content of cobalt increases. It was properly predicted that the Seebeck coefficient and electrical conductivity increase, and thermal conductivity decreases as the content of cobalt increases.

GeTe Thermoelectrics

The effect of spin-orbital coupling on the lattice parameters, volume and band gap of rhombohedral (R3m) GeTe has been reported using density functional theory. The effect of applied pressure on rhombohedral GeTe has been studied. This structure has two different bond lengths for Ge-Te bond i.e. 2.86 Å and 3.24 Å. As the applied pressure increases, the both bond lengths decreases and the band gap become narrow. At a pressure of 6.47 GPa, the band gap becomes equal to zero and GeTe has semimetal phase. With pressure applied under a constraint, the larger bond length starts to decrease and the smaller bond increases. As a result, GeTe shows transition to semimetal phase at a relatively lower pressure of 2.87 GPa. The effect of pressure on band structures has been discussed in terms of atomic positions and bond lengths in the real space.

High pressure structural and electronic properties of Lithium Fluoride (LiF) have been studied by employing an ab-initio pseudopotential method and a linear response scheme within the density functional theory (DFT) in conjunction with quasi harmonic Debye model. The band structure and electronic density of states conforms that the LiF is stable and is having insulator behavior at ambient as well as at high pressure up to 1 Mbar. Conclusions based on Band structure, phonon dispersion and phonon density of states are outlined.

First‐principles calculations are carried out to investigate the structural and electronic properties of the α modification of zinc diphosphide (α‐ZnP2). We use the conjugate gradient (CG) minimization method, in which both the local density approximation (LDA) and the generalized gradient approximation (GGA) as implemented in the SIESTA code are employed. The ground state properties such as the lattice parameters, bulk modulus, pressure derivative of the bulk modulus at zero pressure, bond lengths and bond angles are determined and compared with the available experimental data. The energy band structure and electron density of states (DOS) of α‐ZnP2 are also presented. By analyzing the electronic band structure, a band gap of 1.91 eV is obtained. Based on the study of properties of α‐ZnP2 at high pressure, it is found that the structural and electronic properties of α‐ZnP2 at high pressure are quite stable. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Semiconductor nanowires are recently of special interest due to the strong electron confinement in their one-dimensional structure, which, in turn, implies easily tunable transport properties of charge carriers. This is especially important for use of nanowires in electrical sensing devices as well as field-effect transistors. High surface-to-volume ratio typical for nanowires leads to strong dependence of their electrical properties on surface states. The quantum confinement of the charge carriers in nanowires might be even more strengthened due to manufacturing shell nanowires via the chemical etching of a core in core-shell structures [1]. The higher confinement in the shell nanowires in comparison to typical nanowires implies also higher surface-to-volume ratio, thus even stronger impact of surface states on electronic band structure and electron population of the shell structures. Here, we report on a theoretical study of impact of surface states on the electron band structure and electron density in intrinsic cylindrical InAs nanowires and InAs shell nanowires (Fig., inset). For the calculation of the electron conductivity, we describe the density of the surface states as a function of energy by the U-type dependence with the minimum corresponding to the neutrality level at the surface. We assume that in accordance with the experimental data for surface of bulk InAs presented in [2] the neutrality level is located about 160 meV above the conduction band edge. We calculate the band structure of the conduction band by the self-consistent solution of Poisson and Schroedinger equations in cylindrical coordinate system where the Schroedinger equation is solved for the envelope functions within the effective mass approach.We show that for both nanowires and shell nanowires the conduction band bending and electron density in the nanowires are strongly affected by the charged surface states at both low and room temperature. Due to the location of the neutrality level above the conduction band edge the conduction band in both nanowires and shell nanowires is bent downwards at the surface giving rise to the formation of the electron accumulation channel at the surface (Fig., inset). The conduction band bending increases with increase of total density of surface states and for the density exceeding 1012 cm-2 the band profile remains almost without changing. Then, Fermi level at the surface in both types of structures is pinned in the vicinity of the neutrality level. The electron density in both nanowires and shell nanowires increases with increase in the total surface states density due to the donor-type states delivering electrons in the structures. The electron density reaches its maximum value for the surface states density above 1012 cm-2. The figure displays that for the total density of surface states above 1012 cm-2 intrinsic nanowires and shell nanowires show significant two-dimensional electron density which increases with the external nanowire radius. Donor-type states of the internal surface in shell nanowires introduce additional electrons into the structure volume, thus increasing their two-dimensional electron density in comparison with conventional nanowires, This does not hold only for shell structures with the shell thickness below 15 nm which are less populated then the nanowires due to the strong electron confinement. The study delivers criteria for development of a building block for tube-like electrical devices.Fig.: Two-dimensional electron density vs external radius InAs nanowires (symbols) shell nanowires (solid line) with the shell thickness of 10, 20 and 30 nm at 300 K and the total surface states density above 1012 cm-2; inset: sketch of InAs nanowire and shell nanowire and their band structure.[1] T. Rieger et al. Nano Lett. 12, 5559 (2012). [2] H. Hasegawa and T. Sawada, J. Vac Sci. Technol. 21, 457 (1982).

Structural, electronic and elastic properties of the new ternary alkali metal chalcogenides KLiX (X = S, Se and Te)

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Theoretical analysis of magnetic, optoelectronic, and thermoelectric properties of Cs2CuCrX6 (X = Cl and Br) double perovskites for spintronic and data storage devices

The optical properties of metal-free and metal phthalocyanine were calculated by using density functional theory with various metals including copper, zinc, cobalt, iron and manganese. The polymorphic form of these crystals was employed only for β from. The molecules were optimized with the symmetry of D4h. For the alignments of the molecule in the crystal structures of this polymorphic form which have not been reported in detail, the variation of total energy was examined as a function of the align angles. The align angle at minimum total energy was used for the band calculation. The density functional theory and plane-wave pseudopotential method were used to calculate the energy band structure and electron density of state. The calculated band structures of various metal phthalocyanines can be divided in two groups according to the peak wavelength of the maximum absorption. The first group with the peak wavelength at about 230 nm consists of β-CuPc, β-H2Pc, and β-MnPc while the wavelength of another group for β-CoPc, β-FePc, and β-ZnPc occurs at 350 nm. From the density of state calculation, it indicates that these two transitions originate from the different band and the ratio of the absorption between these states depending on the type of metal in phthalocyanine. The optical absorption was derived to examine the absorption spectra for various metal compositions while the variation in intrinsic electrical conductivity can be estimated from the shape of the band. The phonon and infrared spectra were also determined in order to investigate the vibration mode of molecule in the crystals

The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.

First-principles study on the structural, elastic and electronic properties of platinum carbide

Ground state properties of face centered cubic fluorite BaF2 is investigated using first principle method. The calculated lattice constants, bulk modulus are reported and compared with available experimental and theoretical data. The band structure and electronic density of states conforms that the BaF2 is ionic. The phonon dispersion and phonon density of states are also presented and analyzed.

Structural, electronic and magnetic properties of L10 ordered CoPt nanoparticles: An experimental and DFT study

This work aims to test the structural, electronic properties of RbGeCl3. The physical properties of the RbGeCl3 crystal are investigated using the generalized gradient approximation (GGA) using density functional theory (DFT). RbGeCl3 is in the cubic phase. The calculated band gap of RbGeCl3 is 1.03eV. The band structures are further evaluated by the total density of state (TDOS). The total density of states confirms the degree of electron localization. We studied electronic band structure and electronic density of states. The calculated electronic band structure shows that RbGeCl3 has a direct band gap.