Yuri F. Zhukovskii

Adhesion trends and growth mode of ultra-thin copper films on MgO

Ab initio simulations are performed for Cu atoms adsorbed on the perfect MgO(001) substrate, with an ordered metal coverage varied from 1 monolayer (ML), i.e. almost single atoms, up t o1M L. As trong dependence of the adhesion energy and the sub-monolayer film distance from the substrate on the surface coverage and adsorbate positions (Mg 2+ or O 2− )i s discussed. The nature of interfacial bonding at all coverages is physisorption .W hen increasing Cu atomic fraction, a decrease of the substrate-induced polarization of adatoms accompanied by an increase of both in-plane metallic bonding and the interfacial distance has been found. Combining results of ab initio calculations with thermodynamic theory (taking into account the lattice mismatch), we show that the metal cluster formation becomes the predominant growth mode even at low Cu coverages, in agreement with experiment. (Some figures in this article are in colour only in the electronic version)

Symmetry and models of single-walled TiO2 nanotubes with rectangular morphology

The formalism of line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for symmetry analysis of single-walled titania nanotubes (SW TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of anatase structure. Either six- or twelve-layer (101) slabs have been cut from TiO2 crystal in a stable anatase phase. After structural optimization, the latter keeps the centered rectangular symmetry of initial slab slightly compressed along a direction coincided with large sides of elemental rectangles. We have considered two sets of SW TiO2 NTs with optimized six- and twelve-layer structures, which possess chiralities (−n, n) and (n, n) of anatase nanotubes. To analyze the structural and electronic properties of titania slabs and nanotubes, we have performed their ab initio LCAO calculations, using the hybrid Hartree-Fock/Kohn-Sham exchange-correlation functional PBE0. The band gaps (Δɛgap) and strain energies (Estrain) of six-layer nanotubes have been computed and analyzed as functions of NT diameter (DNT). As to models of 12-layer SW TiO2 NTs of both chiralities, their optimization results in structural exfoliation, i.e., the multi-walled structure should be rather formed in nanotubes with such a number of atomic layers.

Energetic stability and photocatalytic activity of SrTiO 3 nanowires: ab initio simulations

First principles periodic calculations based on the density functional theory within the localized atomic orbital approach (DFT-LCAO) using the hybrid exchange–correlation potential PBE0 have been performed in order to simulate the structural and electronic properties of both stoichiometric and nonstoichiometric [001]-oriented four-faceted SrTiO3 (STO) nanowires (NW) of cubic structure. Their diameters have been varied from 0.3 up to 2.4 nm with a corresponding consequent change of NW cross-section from 2 � 2t o 5� 5 extension of the lattice constant in bulk. Energetic stability of STO NW (both stoichiometric and non-stoichiometric) has been found to increase with the decrease of their formation energies together with the increase of NW diameter. The electronic structure calculations have shown that the width of the band gap changes in STO NWs of different structural types as compared to that in bulk being consequently reduced with the growth of NW diameter although the character of such a decrease depends on the morphology of the nanowire. Analysis of these changes shows that stoichiometric and non-stoichiometric TiO2-terminated strontium titanate nanowires can be quite promising candidates for further applications in photocatalytic processes under solar irradiation whereas SrO-terminated NWs are rather not suitable for this purpose.

Ab initio modelling of silver adhesion on the corundum (0001) surface

The Ag/a-Al2O3(0001) interface was simulated using ab initio slab calculations. We have studied silver adhesion on both Al- and Oterminated corundum substrates. The latter case may be considered as silver adhesion on a defective Al-terminated corundum surface with external aluminium vacancies. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two favorable Ag adsorption positions and for two metal coverages (a 1/3 monolayer of the Ag(111) crystallographic plane and a full Ag(111) monolayer, 1 ML). The two different terminations (Al- and O-) give rise to qualitatively different results. In the former case, the small adhesion energies per Ag atom are accompanied by a minor interfacial charge transfer, indicating physisorption, which may be explained by a weak atomic polarization. In contrast, for O-terminated corundum, substantial adhesion energies per Ag atom combined with a noticeable charge transfer towards the substrate up to formation of Ag + ions give a clear indication for a strong interfacial ion bonding. We compare these results with the data obtained earlier for the perfect and defective Ag/MgO(001) interfaces and found some qualitative similarities in metal adhesion nature on both alumina and magnesia substrates. D 2002 Elsevier Science B.V. All rights reserved.

Resistance simulations for junctions of SW and MW carbon nanotubes with various metal substrates

This theoretical study focuses on junctions between the carbon nanotubes (CNTs) and contacting metallic elements of a nanocircuit. Numerical simulations on the conductance and resistance of these contacts have been performed using the multiple scattering theory and the effective media cluster approach. Two models for CNT-metal contacts have been considered in this paper: a) first principles “liquid metal” model and b) semi-empirical model of “effective bonds” based on Landauer notions on ballistic conductivity. Within the latter, which is a more adequate description of chirality effects, we have simulated both single-wall (SW) and multi-wall (MW) CNTs with different morphology. Results of calculations on resistance for different CNT-Me contacts look quantitatively realistic (from several to hundreds kOhm, depending on chirality, diameter and thickness of MW CNT). The inter-wall transparency coefficient for MW CNT has been also simulated, as an indicator of possible ‘radial current’ losses.

First-Principles Modeling of Oxygen Interaction with SrTiO3(001) Surface: Comparative Density-Functional LCAO and Plane-Wave Study

Large scale first-principles calculations based on density functional theory (DFT) employing two different methods (atomic orbitals and plane wave basis sets) were used to study the energetics, geometry, the electronic charge redistribution and migration for adsorbed atomic and molecular oxygen on defect-free SrTiO3(001) surfaces (both SrO- and TiO2-terminated), which serves as a prototype for many ABO3-type perovskites. Both methods predict substantial binding energies for atomic O adsorption at the bridge position between the oxygen surface ions and an adjacent metal ion. A strong chemisorption is caused by formation of a surface molecular peroxide ion. In contrast, the neutral molecular adsorption energy is much smaller, ∼0.25 eV. Dissociative molecular adsorption is energetically unfavorable, even at 0 K. Adsorbed O atoms migrate along the (001) direction with an activation energy of ∼1 eV which is much larger than that for surface oxygen vacancies (0.14 eV). Therefore, the surface O vacancies control encounter with the adsorbed O atoms and oxygen penetration to the surface which is the limiting step for many applications of ABO3-type perovskites, including resistive oxygen sensors, permeation ceramic membranes and fuel cell technology.

Simulation of electromagnetic properties in carbon nanotubes and graphene-based nanostructures

As carbon nanotubes (CNT) and graphene nanostructures (GNR) constitute the basis of high-speed nanoelectronics and nanosensors, we examine the fundamental properties of var- ious CNT-metal (Me), GNR-Me, and CNT-graphene interconnects. The cluster approach based on the multiple scattering theory as well as effective medium approximation were used to model the dispersion law, electronic density of states (DOS), and conductivity, etc. Multiple scattering problems were solved for nanostructures with radial (quantum dots) and axial (nanowires, nano- tubes) symmetry. Interconnect capacitances and impedances have been evaluated in the GHz and THz regimes. Parametrical numerical simulations of conductivity were carried out for zig-zag ðm;0Þ, armchair ðm;mÞ, and chiral ðm;nÞ CNTs, and the sensitivity of conductivity to the local electronic DOS in CNTs with local impurities (N and B atoms) was demonstrated. CNTs, CNT- Me, and GNR-Me based nanostructures are prospective nanosensor structures.

Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications

In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO2 nanotubes, undoped, doped, and codoped with N and S atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree–Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO2 nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O2/H2O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level.

Comparative analysis of the electronic structures of mono- and bi-atomic chains of IV, III-V and II-VI group elements calculated using the DFT LCAO and LACW methods

Using the first principle non-relativistic linear combination of atomic orbitals (LCAO) and relativistic linearized augmented cylindrical wave (LACW) methods, the band structure of the covalent and partially ionic ANB8−N single atom width chain is calculated. Both the LCAO and LACW methods show that the chains of C, Si, Ge, Sn, and Pb are metallic. However, there is a great difference between the relativistic and non-relativistic band structures. The π bands crossing the Fermi level are orbitally doubly degenerate in the non-relativistic model. The relativistic LACW calculations demonstrate that the spin and orbital motion of electrons are coupled, thereby splitting the π bands. The spin–orbit gaps are equal to 1.5 meV, 28 meV, 0.22 eV, 0.45 eV, and 4 eV for the C, Si, Ge, Sn, and Pb chains, respectively. The mass–velocity corrections result in a lowering of all the valence band levels. In the carbon and silicon chains, the corrections are possibly negligible (2–5 and 10–30 meV, respectively), while in the Ge, Sn, and Pb chains the low-energy shifts are equal to 0.6, 2.2, and 3.7 eV, respectively, due to these effects. The Darwin corrections are several times smaller in comparison to the mass–velocity contributions. The transition from the covalent chains to the partially ionic ones is accompanied by a drastic change in the band structure. The C chain with all bond lengths equal has a metal type electronic structure while the BN chain is an insulator with an energy gap equal to 6–8 eV. The differences between the covalent and partially ionic chains are explained by the presence of the antisymmetric components of the electron potential in the latter case. The transition from the BN chain to the AlP, GaAs, and InSb ones is accompanied by a gradual decrease in the gaps; for example, the AlP chain is a semiconductor. According to the LCAO calculations, the GaAs chain is a semiconductor, but it is a metal according to the relativistic LACW method. The InSb chain possesses a metal type band structure, but the spin–orbit interaction splits the π states, forming the two π+ and π− sub-bands, and noticeably complicates the band structure and density of states in the vicinity of the Fermi level. In the case of compounds from the same horizontal row in the periodic table, the transition from the AIIIBV chains to the AIIBVI ones is accompanied by a sharp increase in the band gap. The calculations indicate the metallic nature of the InSe chain, but the CdTe one is an insulator. Among the atomic ANB8−N chains, there are compounds with different electrical properties: from metals to semiconductors and insulators.

The eect of Zn vacancies and Ga dopants on the electronic structure of ZnO: Ab initio simulations

Zinc oxide modied by metal dopants can be used as a low-cost material for production of transparent conducting lms. Its optical and electronic properties vary with the type and the concentration of dopants. In this study we have performed rst-principle calculations on ZnO with Zn vacancies and that with Ga dopants in wurtzite type hexagonal morphology using density functional theory approach. Dependence of the electronic properties on the concentration of dopants has been studied using supercells of dierent sizes.