Yu. F. Zhukovskii

Structural and electronic properties of single-walled AlN nanotubes of different chiralities and sizes

Four models of single-walled AlN nanotubes (NTs), which possess (i) two different chiralities (armchair or zigzag type) and (ii) two different uniform diameters for both types of NTs (1 or 6 nm) have been constructed, in order to analyse the dependence of their properties on both morphology and thickness. Periodic one-dimensional (1D) DFT calculations performed on these models have allowed us to analyse how the chirality and curvature of the NT change its properties as compared to both AlN bulk with either wurtzite or zinc-blende structures and their densely packed surfaces. We have found that the larger the diameter of the AlN NT, the smaller the width of its bandgap, the strengths of its bonds and the charge separations in them .T his confirms the recent experimental finding of the possibility to adjust electronic properties in ultimate nanoscale optoelectronic devices produced from AlN and other group III nitrides. (Some figures in this article are in colour only in the electronic version)

A first‐principles DFT study of UN bulk and (001) surface: Comparative LCAO and PW calculations

LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic‐covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.

Influence of F centres on structural and electronic properties of AlN single-walled nanotubes

We analyse the influence of uncharged N vacancies (neutral F centres), created either under conditions of AlN nanotube growth or by its soft irradiation, on the atomic and electronic structure. Periodic one-dimensional (1D) density functional theory (DFT) calculations on models of defective single-walled nanotubes (SW NTs) allow us to analyse how NT chirality and concentration of F centres change their properties compared to the corresponding defect-free nanotubes. We have simulated reconstruction around periodically repeated F centres on 1 nm AlN SW NTs with armchair- and zigzag-type chiralities. To achieve the limit of an isolated vacancy for both chiralities, we have considered different inter-defect distances repeated along the axes of these nanotubes. For dF‐F 20 ˚ A, the interaction between defects is found to be negligible, since energy dispersion does not exceed 0.02 eV. We also analyse the influence of F centres on the energy cost required to wrap up AlN graphitic nanosheets (NSs) of both chiralities into the corresponding 1 nm thick SW NTs. The electronic properties of defective NS and NTs of both chiralities have been compared with those for defective AlN bulk three-dimensional (3D) structures of wurtzite and zinc-blend. The presence of N vacancies in various aluminium nitride structures (including SW NTs) results in the appearance of defect energy levels in the band gap with the prevailing contribution from 3s and 3p atomic orbitals of the nearest Al atoms. We have found that the larger the concentration of F centres is, the smaller the maximal energy gap between defect levels is, i.e. an increase 7 Author to whom any correspondence should be addressed.

The electronic properties of an oxygen vacancy at ZrO2-terminated (001) surfaces of a cubic PbZrO3: computer simulations from the first principles

Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO(3) (001) surface (with ZrO(2)- and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO(2) termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO(2)-surface attracts approximately 0.3 e (0.7 e in the bulk PbZrO(3)), while the remaining electron density from the missing O(2-) ion is localized mostly on atoms nearest to a vacancy. The calculated defect formation energy is smaller than in the bulk which should lead to the vacancy segregation to the surface. Unlike Ti-based perovskites, the vacancy-induced (deep) energy level lies in PbZrO(3) in the middle of the band gap.

Hydrogen adsorption on the ZnO ( ) surface: ab initio hybrid density functional linear combination of atomic orbitals calculations

Hydrogen atoms unavoidably presented in ZnO samples or thin films during their synthesis considerably affect electrical conductivity. Results of first principles hybrid functional linear combination of atomic orbitals calculations are discussed for hydrogen atoms incorporated in bulk or adsorbed upon non-polar ZnO (1¯ 100) surfaces. The energy of H incorporation, atomic relaxation, electronic density redistribution and modification of the electronic structure are compared for both surface adsorption and bulk absorption. It is shown that hydrogen forms a strong bonding with the surface O ions (Eads = 2.7eV) whereas its incorporation into bulk is energetically quite unfavorable. Hydrogen adsorption reduces the surface energy. Surface hydrogen atoms are very shallow donors, thus contributing to the electronic conductivity and ZnO metallization.

Ab initio modeling of sulphur doped TiO2 nanotubular photocatalyst for water-splitting hydrogen generation

In order to construct an efficient visible-light-driven TiO2 photocatalyst for water splitting applications, one has to perform improvements of its electronic structure. In this theoretical study we consider single-walled anatase TiO2 nanotubes having following morphologies: (101) 3-layered wall with chirality indexes (n,0) and (n,n), (101) 6-layered wall with (n,0) and (0,n), (001) 6-layered wall with (n,0) and (0,n), and (001) 9-layered wall with (n,0) and (0,n). The latter configuration occurs to be the most energetically stable, due to possessing negative strain energy. In our study the most stable 9-layered anatase (001) (0,n) nanotube has been doped with sulphur. According to obtained results sulphur dopant creates the mid-gap states making the TiO2 nanotube to be a good candidate for efficient photocatalyst working under day light irradiation.

Quantum-chemical simulations of free and bound hole polarons in corundum crystal

Abstract The semi-empirical method of the so-called intermediate neglect of differential overlap (INDO) has been applied to the calculations of the hole small-radius polarons in corundum crystals. Results for optimized atomic and electronic structure using two different approaches (the molecular cluster and periodic, supercell model) are critically compared. It is shown that the main results are similar in both cases.

The Effect of Oxygen Vacancies on the Atomic and Electronic Structure of Cubic ABO3 Perovskite Bulk and the (001) Surface: Ab initio Calculations

We employed the hybrid DFT-LCAO and GGA-PW approaches as implemented in the CRYSTAL and VASP codes, respectively, for large supercell calculations of neutral O vacancies with trapped electrons (known as F centers) in the bulk and on the (001) surface of three cubic perovskite crystals (SrTiO 3 , PbTiO 3 , and PbZrO 3 ). The local lattice relaxation, charge redistribution, and positions of defect energy levels within the band gap are compared for three perovskites under study. We demonstrate how the difference in chemical composition of host materials leads to quite different defect properties.

Hybrid DFT calculations of the F centers in cubic ABO3 perovskites

We employed the hybrid DFT-LCAO approach as implemented in the CRYSTAL code for 135 atom supercell calculations of O vacancies with trapped electrons (known as the F centers) in three cubic perovskite crystals: SrTiO3, PbTiO3 and PbZrO3. The local lattice relaxation, charge redistribution and defect energy levels in the optical gap are compared. We demonstrate how difference in a chemical composition of host materials leads to quite different defect properties.

Simulation of Fundamental Properties of CNT- and GNR-Metal Interconnects for Development of New Nanosensor Systems

Cluster approach based on the multiple scattering theory formalism, realistic analytical and coherent potentials, as well as effective medium approximation (EMA-CPA), can be effectively used for nano-sized systems modeling. Major attention is paid now to applications of carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) with various morphology which possess unique physical properties in nanoelectronics, e.g., contacts of CNTs or (GNRs) with other conducting elements of a nanocircuit, which can be promising candidates for interconnects in high-speed electronics. The main problems solving for resistance C-Me junctions with metal particles appear due to the influence of chirality effects in the interconnects of single-wall (SW) and multi-wall (MW) CNTs, single-layer (SL) and multi-layer (ML) GNRs with the fitting metals (Me = Ni, Cu, Ag, Pd, Pt, Au) for the predefined carbon system geometry. Using the models of ‘liquid metal’ and ‘effective bonds’ developed in the framework of the presented approach and Landauer theory, we can predict resistivity properties for the considered interconnects. We have also developed the model of the inter-wall interaction inside MW CNTs, which demonstrates possible ‘radial current’ losses. CNT- and GNR- Metal interconnects in FET-type nanodevices provide nanosensoring possibilities for local physical (mechanical), chemical and biochemical influences of external medium. At the same time, due to high concentrations of dangling bonds CNT- and GNR- Metal interconnects as interfaces are also considered as electrically, magnetically and chemically sensitive elements for novel nanosensor devices.