A. A. Safonov

Structure and electronic properties of zirconium and hafnium nitrides and oxynitrides

The atomic structure, stability and electronic properties of zirconium and hafnium nitrides and oxynitrides (MN, M3N4, and M2N2O; M=Zr, Hf) have been studied using first-principles density functional theory calculations. It is found that the orthorhombic Pnam structure of M3N4, which was observed experimentally for zirconium nitride, is more stable for this stoichiometry than the cubic spinel and rock-salt-type structures. The calculated band structures and electronic densities of states demonstrate that both the MN and M3N4 phases of zirconium and hafnium nitrides in the rock-salt-type structure are characterized by metallic properties, while the orthorhombic structure of the M3N4 phase exhibits an insulating behavior in agreement with experimental observations. The formation of nitrogen vacancies in the insulating M3N4 phase converts it into the metallic MN phase. Calculations of Zr2N2O and Hf2N2O in the cubic Bixbyite-type and hexagonal P3–ml crystal structures predict that these materials are insulato...

First-principles-based investigation of kinetic mechanism of SiC(0001) dry oxidation including defect generation and passivation

The key stages of the dry oxidation of the SiC(0001) surface are analyzed based on first-principles calculations. It is found that an abrupt SiC/SiO2 interface model results in a large activation barrier of oxygen penetration to the silicon carbide, and thus the penetration is probably the rate-limiting step for the entire dry-oxidation process. The subsequent reactions of SiC oxidation after oxygen penetration are investigated, and it is found that CO release is competing with carbon dimer formation. These dimers probably are responsible for near-interface traps in the silica layer generated during SiC oxidation. The possible passivation reactions of a carbon dimer defect by active species, such as O2, NO, and H2 are investigated. It is found that an oxygen molecule can break a Si–C bond via dissociation in the triplet state and finally can produce two CO molecules from the carbon dimer defect. The NO molecule can easily break a Si–C bond of a carbon dimer defect and form cyano groups –CN, which can fina...

Atomistic modeling of chemical vapor deposition: silicon nitride CVD from dichlorosilane and ammonia

Abstract The mechanism and kinetics of chemical vapor deposition of silicon nitride films from SiH2Cl2 and NH3 have been studied theoretically by ab initio (MP2/MC-31G(d,p) and MP2/6-31G(d)) methods combined with the transition state and RRKM theories. Reactions involving the starting reagents and no more than one of the initial reaction products are included in the analysis. It has been found that, in the gas phase at least at T SiN surface groups. The calculated SiN bond length 1.62 A is considerably shorter than typical lengths of crystalline SiN bonds (1.74–1.76 A), and the surface atoms of these diatomic groups are significantly displaced from their bulk crystalline positions.

Computational study of ZrSiO4 polymorphs

Using the density functional theory in a local density approximation and generalized gradient approximation (GGA) with a plane wave basis set we have revealed eight new polymorphs of ZrSiO4 within the energy range ∼1eV above the most stable zircon which have higher and lower density than experimentally known zircon and reidite. Two structures, which have both silicon and zirconium atoms sixfold coordinated, orthorhombic AlTaO4-like (alumotantite), and monoclinic PbWO4-like (raspite), have similar energies at a GGA level ∼0.35eV above reidite and density intermediate between zircon and reidite. Among two low-density structures, which can be potentially revealed experimentally in the nanocrystalline thin films, the orthorhombic CaSO4-like form has an energy similar to reidite but with much lower density.

First-principles investigation of the WC∕HfO2 interface properties

The thermodynamic and electronic properties of tungsten carbide surfaces and interfaces with monoclinic hafnia (WC∕m-HfO2) are investigated through first-principles calculations. We show that oxidation of the WC surface and of the WC∕m-HfO2 interface is energetically favorable. An oxygen monolayer on the W-terminated WC(0001) surface gives rise to a larger vacuum work function than that for the C-terminated WC(0001) surface, while the opposite result is obtained for the WC(0001) effective work function on hafnia: a carbon intermediate layer results in larger work function than an oxygen intermediate layer. This result is explained by the atomic structure of the intermediate layers neighboring the interface which differ if the interface is O or C rich.

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.

Fluorescence Spectra and Structure of the Difluoro(dibenzoylmethanato)boron Monomers and Dimers Absorbed on Silica Gel

It has been shown that difluoro(dibenzoylmethanato)boron ((dbm)BF2) can be absorbed on silica gel in the form of fluorescent monomers and dimers with the emission properties that change in the presence of vapors of volatile organic compounds, such as ethanol, acetone, toluene, and meta-xylene. Fluorescence quenching was observed for the (dbm)BF2 monomers and dimers in the case of ethanol and acetone, whereas the formation of fluorescent exciplexes with monomers and enhancement of the dimer fluorescence were observed in the case of toluene and meta-xylene. Results of the quantum-chemical calculations of the structure of the (dbm)BF2 monomer complex with the matrix and toluene and (dbm)BF2 dimers with matrix are presented.

Multiscale computer design of photonic crystal based materials for optical chemosensors

A multiscale method is proposed for modeling elements of optical chemosensors based on photonic crystals. The method is based on the first-principles quantum-chemical and electrodynamic calculations. A technique is proposed that takes into account electromagnetic emission sources in the framework of the finite-difference time-domain method. An end-to-end simulation of fluorescence in a photonic crystal is performed: the absorption and emission spectra of a dye on a substrate are calculated by quantum chemistry, and it is shown how the dye emission spectra are modified in a three-dimensional photonic crystal.

Atomistic simulations of materials for optical chemical sensors: DFT-D calculations of molecular interactions between gas-phase analyte molecules and simple substrate models

The structures of complexes of some small molecules (formaldehyde, acetaldehyde, ammonia, methylamine, methanol, ethanol, acetone, benzene, acetonitrile, ethyl acetate, chloroform, and tetrahydrofuran, considered as possible analytes) with ethylbenzene and silanol (C6H5C2H5 and SiH3OH, considered as models of polystyrene and silica gel substrates) and with acridine (C13H9N, considered as a model of an indicator dye molecule of the acridine series) and the corresponding interaction energies have been calculated using the DFT-D approximation. The PBE exchange-correlation potential was used in the calculations. The structures of complexes between the analyte and the substrate were determined by optimizing their ground-state geometry using the SVP split-valence double-zeta plus polarization basis set. The complex formation energies were refined by single-point calculations at the calculated equilibrium geometries using the sufficiently large triple-zeta TZVPP basis set. The calculated interaction energies are used to assess the possibility of using dyes of the acridine series adsorbed on a polystyrene or silica substrate for detecting the small molecules listed above.

Segregation trends of the metal alloys Mo-Re and Mo-Pt on HfO2 : A first-principles study

Using first-principles calculations, we compared the segregation trends at the surface of metal alloys with those at an interface with HfO2. The choice of this oxide was motivated by its significance as a potential replacement for SiO2 in advanced transistors. We considered Mo–Re and Mo–Pt alloys as typical examples of disordered and ordered alloys, respectively. The segregation to the surface/interface was analyzed in terms of metal and oxygen adsorption energies. It is shown that chemical bonding at the metal/oxide interface strongly influences segregation both in Mo–Re and Mo–Pt alloys. In particular, bonding with oxygen atoms at the oxide/Mo–Re alloy interface depletes the Re content of the interfacial layer. In the case of Mo–Pt on HfO2 an oxygen-rich interface promotes the formation of one monolayer (but not two monolayers) of Mo separating PtMox from HfO2, while a stoichiometric interface favors an abrupt PtMox∕HfO2 interface. This study also shows that the presence of Mo in the alloy stabilizes Pt...