Electronic Properties of Orthorhombic, Tetragonal and Monoclinic KDP by First-Principles Calculations

The electronic properties of KDP (KH2PO4), in orthorhombic, tetragonal and monoclinic phases, have been investigated by using the generalized gradient approximation (GGA) within density functional theory. The comparison of the calculated band gap of the tetragonal crystal and the experimental results shows that the band gap of this paper is very close to the experimental results. In addition, this work calculates the effective mass of KDP for the first time. The effective mass of electrons in the conduction bands of the orthorhombic phase is greater than that of the tetragonal phase, so the mobility of electrons in the tetragonal phase is higher than that of the orthorhombic phase. The calculated DOS shows that the lower conduction band is composed of the K-3p states, P-3p states and O-2p states. There is a strong hybridization between P and O orbitals.

Electronic Property and Negative Thermal Expansion Behavior of Si136-xGex (x = 8, 32, 40, 104) Clathrate Solid Solution from First Principles

We present the electronic and vibrational studies on Si136-xGex (x = 8, 32, 40, 104) alloys, using the local density approximation (LDA) scheme. We find that a “nearly-direct” band gap exists in the band structure of Si104Ge32 and Si96Ge40, when compared with the similarly reported results obtained using a different computational code. The calculated electronic density of state (EDOS) profiles for the valence band remain nearly identical and independent of the Ge concentration (x = 32, 40, 104) even though some variation is found in the lower conduction band (tail part) as composition x is tuned from 8 (or 40) to 104. The negative thermal expansion (NTE) phenomenon is explored using quasi-harmonic approximation (QHA), which takes the volume dependence of the vibrational mode frequencies into consideration, while neglecting the temperature effect on phonon anharmonicity. Determined macroscopic Grüneisen parameter trends show negative values in the low temperature regime (1 K < T < 115 K), indicating the NTE behavior found in Si128Ge8 is analogous to the experimental result for Si136. Meanwhile, calculations for the ratio of the vibrational entropy change to the volume change at several characteristic temperatures reconfirm the existence of NTE in Si128Ge8 and Si104Ge32.

Real-time observation of the intravalley spin-flip process in single-layer WS2

We use helicity-resolved transient absorption spectroscopy to track intravalley scattering dynamics in monolayer WS2. We find that spin-polarized carriers scatter from upper to lower conduction band by reversing their spin orientation on a sub-ps timescale.

Влияние размерных эффектов на электронную структуру гексагонального теллурида галлия

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.

Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality

The concept of electronic dimensionality,i.e., the connectivity of the atomic orbitals that comprise the lower conduction band and upper valence band, is introduced to better account for the device performance of the perovskite-based solar cells.

First-Principles Calculations on the Geometry and Electronic Structure of Rutile TiO2 (110) Surface

In this paper, both geometrical and electronic properties of rutile TiO2 (110) surfaces have been investigated using First-Principles Density-Functional calculations with CASTEP code, the model of stoichiometric surface is a (2x1) super-cell which has 12 atomic-layer slabs with the bottom 6 held fixed, the bridging-oxygen vacancy surface has been constructed by removing a neutral bridging oxygen atom from this surface. For the stoichiometric surface, the atom relaxations are: Ti6f (+0.2865Å), Ti5f (-0.1039Å), O3f (+0.2433Å) and Ob (+0.0075Å), we find no reconstruction and no surface states in the band gap, the density of states (DOS) is similar to the bulk except the lower conduction band intensity, in accord with recent experiments. Whereas, as a result of bridging-oxygen vacancy, the atom relaxations exchanged and reconstruction occur. The 2 excess electrons left behind removal of one bridge O atom are localized on the Ti-t2g conduction band orbitals, convert some of the Ti4+ ions into Ti3+ ions and result a compensatory shift in the Fermi level. The band gaps we calculated for stoichiometric surface is similar to the bulk, but its increase can be found for Ob vacancy surface.

Corrole-sensitized TiO2 solar cells

We are investigating the properties of corrole-sensitized TiO 2 solar cells. The TiO 2-adsorbed free base and Ga III corroles display cell efficiencies under AM 1.5 illumination that are about half that of a standard N 3-sensitized cell ( N 3 = cis-bis(4,4'-dicarboxy-2,2'-bipyridine)dithiocyanato ruthenium(II)), while that of the Sn IV-based cell is much lower. The properties of the corrole- TiO 2 solar cells, along with results obtained with electrodes of lower conduction band energies clearly reveal that the reducing power of the singlet excited states of the free base and Ga III corrole, but not of the Sn IV derivative, is sufficiently high for efficient injection into the TiO 2 conduction band.

A New Concept for Solving the Boltzmann Transport Equation in Ultra-fast Transient Situations

A concept based on relaxation of the hydrodynamic parameters is introduced to arrive at a computational model for the extreme non-equilibrium distribution function of carriers in multi-valley bandstructure. The relaxation times are taken to describe the evolution scale of the distribution function. The developed model is able to account for transport phenomena at the momentum relaxation scale. The model, together with the Monte Carlo simulation, is applied to obtain the electron distribution function in each valley of the lower conduction band in GaAs, and to study the evolution of the distribution function in GaAs subjected to rapid changes in field.