Ab initio driven density functional theory-based high throughput simulations have been conducted to search for stable two-dimensional (2D) structures based on transition metal halides. Binary MeX2 and MeXY (Me-transition element, X and Y-Cr, Br, I, where X not equal Y) 2D structures in two structural polymorphic modifications, which are 1T-phase and 1H-phase, have been studied. The main structural stability criteria, such as heat formation energy, elasticity constants, and phonon spectra and the following ab initio molecular dynamics simulations have been used to determine the stability of studied compounds. It has been shown that 35 MeX2 and 32 MeXY 2D structures comply with given stability criteria. Photocatalytic properties of these stable 2D MeX2 and 2D MeXY have been investigated. Based on the calculated band gap size E g, work function CYRILLIC CAPITAL LETTER EF and electron affinity chi, it has been found that among all stable compounds 13 MeX2 and 16 MeXY 2D structures are promising photocatalysts for water splitting. However, only 7 compounds have solar-to-hydrogen (STH) efficiency overcome the 10% threshold, which is a critical parameter for solar hydrogen generation to be an economically viable resource. Among MeX2 2D structures 1T-CdI2 and 1H-VBr2 possess a STH efficiency of 11.58% and 17.23%. In the case of 2D MeXY, STH efficiencies are 22.79% (1T-ZnClI), 15.20% (1T-CdClI), 22.13% (1T-ZnBrI), 12.11% (1T-CdBrI) and 19.76% (1H-VClBr). Moreover, as a result of this work, a comprehensive publicly available database, containing detailed calculation parameters and fundamental properties of the discovered 2D transition metal halides, has been created.

Methods for modeling the interaction of electromagnetic radiation with a carbon nanostructured composite containing various structural elements, such as carbon fibers, nanoparticles, air gaps, and transition shells are considered. It is shown that for effective and adequate modeling of the interaction of electromagnetic radiation with a carbon nanostructured composite, it is necessary to create software due to the presence of a large set of input parameters, as well as the complexity of calculations. Functional requirements for the software are formulated, reflecting the choice of models and the presence of a convenient interactive interface. The architecture of the software that implements models of interaction of electromagnetic radiation with a carbon nanostructured composite is described, and the software implementation (class structure, methods, user interface) is considered. The developed code is based on proven physical and mathematical models, which make it possible to obtain results close to experimental data. The software architecture allows to effectively manage the modeling process, and also includes the ability to modify models and take into account new structural elements. An interactive interface for users has been developed that allows to easily manage the model parameters and analyze the results of numerical experiments.

A new family of ternary nitride materials, Zn2(V, Nb, Ta)N3 monolayers, is predicted. A fabrication mechanism of the Zn2(V, Nb, Ta)N3 monolayers is proposed based on the chemical vapor deposition approach used for their bulk counterparts. The calculations show that these monolayers are thermodynamically and environmentally stable and that the Zn2VN3 monolayer is the most stable and the easiest to synthesize. The Zn2VN3 monolayer also has the highest strength and elasticity. The Zn2(V, Nb, Ta)N3 monolayers are semiconductors with nearly equal direct and indirect band gaps. Considering optoelectronic properties, the predicted monolayers are transparent to the visible light and provide shielding in the ultraviolet region. Thus, the predicted Zn2(V, Nb, Ta)N3 monolayers are promising for applications in LED devices and as blocking layers in tandem solar cells.

The results of a comparison of the exchange interaction mechanisms in low dimensional magnetic systems are presented. It has been shown that ZnO crystal may be used as a semiconductor non-magnetic matrix for the formation of quasi-one-dimensional and quasi-zero-dimensional magnetic systems by introducing impurity atoms of Cr, Mn, Fe, Co and Ni. Structural parameters, electronic and magnetic properties were calculated at the atomic level in the framework of quantum mechanical simulation. The exchange interaction integrals were calculated at the microscopic level using the Heisenberg model. The exchange interaction mechanisms were determined on the obtained dependences of the exchange interaction integral on the structural and electronic properties, as well as on the features of the low-dimensional magnetic systems partial density of electronic states. The results of studying the exchange interaction mechanisms in two-dimensional magnetic systems formed in materials of the MAX3 (M= Cr, Fe, A = Ge, Si, X= S, Se, Te) group are summarized. The established mechanisms made it possible to compare the conditions for the formation of a ferromagnetic order in systems with different dimensions of magnetic interaction. The ferromagnetic order in all the structures under study is formed due to the indirect superexchange interaction between orbitals of different symmetry. Strategies aimed at enhancing the superexchange interactions between orbitals of different symmetry or attenuating the contributions of the exchange interaction between orbitals of the same symmetry contribute to the formation of stable hightemperature ferromagnetism.

This paper reviews the theoretical and experimental works concerning one of the most important parameters of wurtzite gallium nitride – thermal conductivity. Since the heat in gallium nitride is transported almost exclusively by phonons, its thermal conductivity has a temperature behavior typical of most nonmetallic crystals: the thermal conductivity increases proportionally to the third power of temperature at lower temperatures, reaches its maximum at approximately 1/20 of the Debye temperature and decreases proportionally to temperature at higher temperatures. It is shown that the thermal conductivity of gallium nitride (depending on fabrication process, crystallographic direction, concentration of impurity and other defects, isotopical purity) varies significantly, emphasizing the importance of determining this parameter for the samples that closely resemble those being used in specific applications. For isotopically pure undoped wurtzite gallium nitride, the thermal conductivity at room temperature has been estimated as high as 5.4 W/(cm·K). The maximum room temperature value measured for bulkshaped samples of single crystal gallium nitride has been 2.79 W/(cm·K).

The article presents the results of a magnetism study in quasi-two-dimensional MAX(3) (M=Cr, A=Ge, Si and X=S, Se, Te) systems. We calculated the microscopic magnetic parameters using quantum mechanical methods and showed that MAX(3) can have a high spin polarization. The easy magnetization axis lies normal to the layer plane. The main magnetic order of the CrGeSe3, CrGeTe3, CrSiSe3, and CrSiTe3 atomic systems is ferromagnetism. CrGeS3 and CrSiS3 exhibit antiferromagnetism. The low energy stability of the magnetic order is confirmed by the calculated values of the exchange interaction integral (J). We showed that the magnetic order realizes only at low temperatures. A study of the dependences of J and the magnetic anisotropy energy on the structural (distance between magnetic ions, distortion of the octahedral complex) and electronic properties (population and hybridization of atomic and molecular orbitals) has been performed. The dependences indicate three possible mechanisms of the exchange interaction. We have given ways of influencing a specific mechanism for managing exchange interaction.

The thermal stability of devices based on GaN, AlN, and Al0.5Ga0.5N semiconductors is a critical property for efficient and reliable operation. The thermal conductivity of these materials has anisotropic nature. We proposed an approach for calculating the anisotropic thermal conductivity based on harmonic and anharmonic interatomic force constants of a lattice. The thermal-conductivity coefficient of GaN, AlN, and Al0.5Ga0.5N in the [100], [001], and [111] directions were calculated using ab initio methods by solving the linearized Boltzmann transport equation. It equals lambda[100] = 259.28, lambda([) (001]) = 335.96 and lambda([111]) = 309.56 W/(m center dot K) for GaN;lambda([100]) = 396.06, lambda([001)] = 461.65 and lambda([111]) = 435.05 W/(m center dot K) for AlN; and lambda([100]) = 186.74, lambda([001]) = 165.24 and lambda([111]) = 177.62 W/(m center dot K) for Al0.5Ga0.5N at 300 K. The dependence of the coefficient.(T) on temperature in the range from 250 to 750 K is presented. A comparative analysis of the GaN thermal conductivity investigations has been carried out for experimental studies and theoretical calculations.

The influence of the exchange-correlation functional on the crystal fundamental property calculation is shown. CrGeTe3, compound with transition metals, was used for the simulation of structural and electronic properties. The calculations were carried out using such functional classes as LDA and GGA. It has been shown that LDA exhibits 0.4 % and 5.2 % overestimations of the lattice constants for a and c, respectively. GGA (OR) overestimates a by 0.58 % and underestimates c by 4 %. The influence of the Hubbard correction on the band gap was also investigated. If Ueff is applied to the d-electrons, then the band gap will decrease. This is due to the hybridization of the p-electrons of the chalcogen and the d-electrons of the transition metal. Thus, GGA demonstrates better agreement with the experiment. The convergence of the calculation of the total energy with a change in the k-points and the cutoff energy were also investigated.

The search for fundamental physical laws which lead to stable high-temperature ferromagnetism is an urgent task. In addition to the already synthesized two-dimensional materials, there remains a wide list of possible structures, the stability of which is predicted theoretically. The article suggests the results of studying the electronic properties of MAX3 (M = Cr, Fe, A = Ge, Si, X = S, Se, Te) transition metals based compounds with nanostructured magnetism. The research was carried out using quantum mechanical simulation in specialized VASP software and calculations within the Heisenberg model. The ground magnetic states of twodimensional MAX3 and the corresponding energy band structures are determined. We found that among the systems under study, CrGeTe3 is a semiconductor nanosized ferromagnet. In addition, one is a semiconductor with a bandgap of 0.35 eV. Other materials are antiferromagnetic. The magnetic moment in MAX3 is localized on the transition metal atoms: in particular, the main one on the d-orbital of the transition metal atom (and only a small part on the p-orbital of the chalcogen). For CrGeTe3, the exchange interaction integral is calculated. The mechanisms of the formation of magnetic order was established. According to the obtained exchange interaction integrals, a strong ferromagnetic order is formed in the semiconductor plane. The distribution of the projection density of electronic states indicates hybridization between the d-orbital of the transition metal atom and the p-orbital of the chalcogen. The study revealed that the exchange interaction by the mechanism of superexchange is more probabilistic.

Magnetic anisotropy in low dimensional semiconductors can lead to long-range ferromagnetic order. This feature is useful for various applications such as spintronic and sensoric. The exchange interaction integrals have been observed through magnetic, structural and electronic properties of Cr2Ge2Te6 and Cr2Si2Te6 first-principles simulation in the VASP software package and Heisenberg model approach. The structural parameters for the bulk layered Cr2Ge2Te6 and Cr2Si2Te6 were determined and in good agreement with the experiment. Calculations have established that the number of layers does not affect the magnetic properties and entire magnetic moment is localized on Cr atoms. Exchange interaction integrals between the first, second and third neighbors have been calculated for monolayers. The dependence of the exchange interaction integral on the Hubbard coefficient has been established. The exchange integral with the first neighbors J1 was -2.34 (Cr2Ge2Te6) and -1.61 (Cr2Si2Te6) meV at the Hubbard coefficient of 3 eV. Thus, strong ferromagnetic order exists in plane of monolayers of Cr2Ge2Te6 , Cr2Si2Te6 and magnetic anisotropy leads to long-range ferromagnetism

The GaN high electron mobility transistor (HEMT) thermal characteristics were evaluated employing an integrated approach based on the combined use of ab initio (first-principles) and device simulations. The necessity of utilizing such a method arises when model parameters required for device simulation are unavailable or not suited to certain conditions. A fine example is thermal conductivity of crystalline materials, which is strongly dependent on the defect density, isotopic purity and temperature. Since the developed temperature due to self-heating is highly sensitive to the thermal conductivities of certain regions of the device structure, it is of great importance to use correct thermal conductivity models that are incorporated into the heat flow equation. A combination of ab initio calculations and solutions of the linearized phonon Boltzmann transport equation is a high-end tool to estimate the thermal properties of crystalline materials. In this paper, firstly, the values of the thermal conductivity and thermal capacity of AlN, GaN, Al 0.21 Ga 0.79 N and Al 0.5 Ga 0.5 N were calculated in the range of temperature from 20 K to 1000 K. Secondly, device simulation of a GaN HEMT was performed and the thermal characteristics were evaluated.

The article presents the results of quantum-mechanical computer simulation. The purpose of studying the electronic and magnetic properties of twenty crystalline structures based on perovskites of transition metals with the general formula ABO3 (where A - Ca, Ce, Y, Na; B - Ti, Ta, Nb, Mn, Fe ion; O - oxygen ion) is to assess the possibility of using this group of materials in modern electronic devices. Systematization of fundamental characteristics will allow further describing of the physical mechanisms that occur in structures. Calculations of the fundamental properties of crystals were performed using first-principle methods based on the density functional theory (Density Functional Theory - DFT). The VASP software package (Vienna Ab initio Simulation Package) was used as the simulation tool, which is designed to perform quantum-mechanical calculations. As a result of the simulation, the following characteristics of perovskites of transition metals were established: ABO3 unit cells have cubic syngony; a number of compounds have a magnetic moment (from 0.26 to 4.39 p®); an analysis of the band structures shows the presence of compounds with a semiconductor (band gap from 1.65 to 2.99 eV) and metallic type of conductivity. The direct-gap type of conductivity was established for only CeTiO3 compound. The results obtained quantitatively and qualitatively characterize the electronic and magnetic properties of crystalline structures based on ABO3 perovskites and can be used to develop methods for calculating the basic electrophysical parameters of promising electronic components.

The results of the device simulation of a three-dimensional magnetometer based on Hall sensors integrated in a standard CMOS technology are presented. The Hall voltage vs magnetic field, Hall voltage vs magnetic field deviation angle, and sensitivity vs temperature curves were measured. The first-principles simulation of chalcogenide spinel CuCr2Se4 used in creation of a magnetic field concentrator was performed.

In systems with breaking of inversion symmetry a perpendicular electric field arises that interacts with the conduction electrons. It may give rise to electron state splitting even without influence of external magnetic field due to the spin-orbital interaction (SOI). Such a removal of the spin degeneracy is called the Rashba effect. Nanostructure with the Rashba effect can be part of a spin transistor. Spin degeneracy can be realized in a channel from a material of this type without additive of magnetic ions. Lack of additive increases the charge carrier mobility and reliability of the device. Ab initio simulations of BiTeX (X=Cl, Br, I) monolayers have been carried out using VASP wherein implemented DFT method. The study of this structures is of interest because such sort of structures can be used their as spin-orbitronics materials. The crystal parameters of BiTeCl, BiTeBr, BiTeI have been determined by the ionic relaxation and static calculations. It is necessary to note that splitting of energy bands occurs in case of SOI included. The values of the Rashba coefficient aR (in the range from 6.25 to 10.00 eV·Å) have high magnitudes for spintronics materials. Band structure of monolayers structures have ideal Rashba electron gas, i.e. there no other energy states near to Fermi level except Rashba states.

The electronic and magnetic properties of ZnO containing Cr doped atoms and Zn and O vacancies in its crystal structure are theoretically investigated. Calculations are performed using Atomistix Tool Kit and Vienna Ab-initio Simulation Package software implementing the electron density functional theory method with the Hubbard correction. It is shown that the magnetic moment of a defect supercell strongly depends on the impurity concentration and presence of vacancies. The doping of an oxygen atom increases the probability of zinc-vacancy formation.

Double-layer heterostructures were studied. The energetic influence of the quasi-two-dimensional materials (MoS2 and WSe2) on the electrical properties of graphene was simulated. Electron density functional (DFT) implemented into VASP program was chosen to take into account van der Waals forces. Interlayer distances were determined for the systems studied by suitable electron density functional (DFT-D2). The distance is 3.50 angstrom for WSe2/G and 3.45 angstrom for MoS2/G respectively. Energy band structures were calculated; the influence of electric field on band structure being taken into account. A quantum-mechanical simulation was performed for determining dielectric permittivity, absorption coefficient, reflected index, Brewster angle and the critical angle.

Magnetic properties of ZnO with intrinsic point defects, such as Co impurity, were calculated in the framework of quantum-mechanical Heisenberg model. Direct exchange in the chain model of magnetic ion impurities was studied, the magnetic chain was being created in (001) and (100) planes. In both cases the ground states of systems have antiferromagnetic order. Exchange interaction integrals were obtained (-1.5 and 0.5 meV). We have also calculated microscopic magnetic parameters: Curie temperature (T-c_in =34.04 and T-c_out=10.7 K), stiffness constant (D-in=82.4, D-out=25.8 meV angstrom(2)), saturation magnetization (M-sat_in=2.88x105, M-sat_out=2.89x105A/m), exchange constant (A(in)=4.46x 10(-13) and A(out)=1.42x10(-12) J/m), and Bloch constant (B-in=6.53x10(-5) Bout=3.7x10(-4) K-3/2). For the energy of magnetic anisotropy of ZnO:Co structure it was found that the easy magnetization axis lies in (100) plane. The value of magnetic anisotropy for the whole cell is 0.06 meV. The anisotropy energy density is 5.54 x 10(4) J / m(3).

AbstractThe electronic and magnetic properties of ZnO containing Cr doped atoms and Zn and O vacancies in its crystal structure are theoretically investigated. Calculations are performed using Atomistix Tool Kit and Vienna Ab-initio Simulation Package software implementing the electron density functional theory method with the Hubbard correction. It is shown that the magnetic moment of a defect supercell strongly depends on the impurity concentration and presence of vacancies. The doping of an oxygen atom increases the probability of zinc-vacancy formation.

Heterostructures based on graphene were investigated by using density function theory (DFT). It is found that energy gap appear in ZnS-graphene heterostructure. Heterostructures graphene-ZnO and graphene-phosphorene have almost zero-band gap. Binding energy between heterostructure layers was determined.