Anharmonic effects on Be(0 0 0 1): A molecular dynamics study

Using molecular dynamics simulations and a modified analytic embedded atom method (MAEAM), the anharmonic effects of Be(0 0 0 1) surface have been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer separation, mean square vibrational displacement, phonon frequencies and phonon line width, and layer structure factor are calculated. The obtained results for temperature dependence of interlayer separation and mean square displacement show that the anharmonic effects are small in the temperature range from 0 K to 1100 K. The calculated layer order parameters indicate that Be(0 0 0 1) surface loses its long-range translational order, but do not premelt up to 50 K below the bulk melting point. The surface disordering may result from strongly contracted c/a ratio of Be.     

Microstructure and optical absorption of FeS2 films formed by sulfurizing precursive iron of various crystallite scales

Precursive iron films with various crystallite scales were prepared by sputtering on substrates heated at different temperatures. The iron films were annealed in sulfur vapor at 673 K to form pyrite films. The structural and optical characters were determined. High substrate temperatures produce large crystallites in the precursive iron films. The pyrite films are composed of a surface layer with coarse columnar grains and a bottom layer with fine equiaxed grains. Sufficient formation and growth of iron grains result in improved crystallinity of the pyrite films. The optical absorption characteristics depend mainly on the crystallinity of the pyrite films or the crystallizing status of the precursive iron films. The optical absorption coefficient of the pyrite films decreases with the increase of iron grain diameter from 12 to 39 nm. The band gap of the pyrite films increases with the increase of iron grain diameter from 12 to 53 nm.     

Molecular dynamics study on vanadium pentoxide

We have studied bulk structure and three low-index surfaces of V₂O₅ using molecular dynamics (MD) simulation. The calculated infra-red (IR) absorption bands of V₂O₅ bulk structure are consistent with the experimental result. The (0 0 1) surface was calculated to be the most stable, small energy difference between the (0 0 1) surface and bulk corresponds their similarity. Atoms with small coordination relax much more than bulk like atoms, they undergo vertical as well as lateral relaxations in order to compensate the missing bonds at the top layer. The driving force which determines the direction of relaxation seems to be the improvement of local environments of the top layer atoms. The vanadyl oxygens exposed to the (0 0 1) and (0 1 0) as well as (1 0 0) surfaces seem to act as active sites in the oxidation process of hydrocarbons.     

Energetics of Ge addimers on the Si(1 0 0) and Ge(1 0 0) surfaces: a comparative study

The adsorption energetics of Ge dimers on the (1 0 0) surfaces of Ge and Si has been investigated using the first-principles molecular dynamics method. Four high-symmetry configurations have been considered and fully relaxed. The most stable configuration for Ge dimers on Si(1 0 0) is found to be in the trough between two surface dimer rows, oriented parallel to the substrate Si dimers. These results are consistent with recent experimental studies of the system using the scanning tunneling microscopy (STM), and help to clarify some existing controversies on the interpretation of the STM images. In contrast, for Ge dimers on Ge(1 0 0), the most stable configuration is on top of the substrate dimer row.     

Crystal data and indirect optical transitions in Tl2InGaSe4 crystals

The room temperature crystal data and the optical properties of the Bridgman method grown Tl₂InGaSe₄ crystals are reported and discussed. The X-ray diffraction technique has revealed that Tl₂InGaSe₄ is a single phase crystal of monoclinic structure. The unit cell lattice parameters, which were recalculated from the X-ray data, are found to be a = 0.77244 nm, b = 0.64945 nm, c = 0.92205 nm and β = 95.03°. The temperature dependence of the optical band gap of Tl₂InGaSe₄ single crystal in the temperature region of 290–500 K has also been investigated. The absorption coefficient was calculated from the transmittance and reflectance data in the incident photon energy range of 1.60–2.10 eV. The absorption edge is observed to shift toward lower energy values as temperature increases. The fundamental absorption edge corresponds to indirect allowed transition energy gap of 1.86 eV that exhibited a temperature coefficient γ = −3.53 × 10⁻⁴ eV/K.     

The oxidation kinetics of magnesium at low temperatures and low oxygen partial pressures

The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10^− 8 Pa and 1.3 × 10^− 5 Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO ( 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer.     

Dissociation pathways of oxygen on copper (1 1 0) surface: a first principles study

We have carried out first principles calculations to examine the adsorption and dissociation of oxygen on the Cu (1 1 0) surface. At low coverage, our results indicate two absorption species: a peroxo- and a superoxo-like species which have direct implications to the dissociation process. In agreement with experimental studies, the most favourable absorption site for O₂ is found to be the 4-fold hollow site. The dissociation barrier at this site is only 150 meV, which is consistent with experimental findings. Static calculations also reveal other possible dissociation pathways with significantly different energy barriers. First principles MD simulations are used to probe the dynamics of the dissociation process. We find only selected pathways are likely to be utilised because of interaction with the surface.     

Theoretical investigation of atomic structure and electronic properties of Ca/Si(110)-(2 × 1) reconstruction

We have presented first-principles total-energy calculations for the adsorption of Ca metals onto a Si(110) surface. The density functional method was employed within its local density approximation to study the atomic and electronic properties of the Ca/Si(110) structure. We considered the (1 × 1) and (2 × 1) structural models for Ca coverages of 0.5 monolayer (ML) and 0.25 ML, respectively. Our total-energy calculations indicate that the (1 × 1) phase is not expected to occur. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The Ca/Si(110)-(2 × 1) produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. One filled and two empty surface states were observed in the gap; these empty surface states originate from the uppermost Si dangling bond states and the Ca 4 s states. It is found that the Ca-Si bonds have an ionic nature and complete charge being transferred from Ca to the surface Si atoms. Finally, the key structural parameters of the equilibrium geometry are detailed and compared with the available results for metal-adsorbed Si(110) surface, Ca/Si(001), and Ca/Si(111) structures.     

Understanding spectroscopic phonon-assisted defect features in CVD grown 3C-SiC/Si(1 0 0) by modeling and simulation

New phonon-assisted defect features are observed using photoluminescence (PL) and Raman scattering spectroscopy on 3C-SiC/Si(1 0 0) films grown by chemical vapor deposition (CVD) technique. The ultraviolet excitation room-temperature (RT) PL-Raman spectra show a luminescence band near 2.3 eV due to RT recombination over the 3C-SiC indirect band gap. In addition to the strong Raman lines characteristic of Si substrate and 3C-SiC we also observed weaker impurity modes near 620, 743 and 833 cm⁻¹. These frequencies are compared with the results of Green's function simulations of impurity modes with plausible defect structures to best support the observed Raman features as well as modes of some prototypical defect center.     

Adsorption of Ni on Si(1 0 0) surface

The chemisorption of one monolayer Ni atoms on ideal Si(1 0 0) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of adsorption systems of Ni atoms on different sites are calculated. It is found that Ni atoms can adsorb at fourfold site above the surface and bridge site below the surface. The adsorption of Ni atoms can readily diffuse and penetrate into the subsurface. A Ni, Si mixed layer might exist at the Ni-Si(1 0 0) interface. The layer projected density states are calculated and compared with that of the clean surface. The charge transfers are also investigated.     

Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3

Surface reactivity and near-surface electronic properties of SrO-terminated SrTiO₃ and iron doped SrTiO₃ were studied with first principle methods. We have investigated the density of states (DOS) of bulk SrTiO₃ and compared it to DOS of iron-doped SrTiO₃ with different oxidation states of iron corresponding to varying oxygen vacancy content within the bulk material. The obtained bulk DOS was compared to near-surface DOS, i.e. surface states, for both SrO-terminated surface of SrTiO₃ and iron-doped SrTiO₃. Electron density plots and electron density distribution through the entire slab models were investigated in order to understand the origin of surface electrons that can participate in oxygen reduction reaction. Furthermore, we have compared oxygen reduction reactions at elevated temperatures for SrO surfaces with and without oxygen vacancies. Our calculations demonstrate that the conduction band, which is formed mainly by the d-states of Ti, and Fe-induced states within the band gap of SrTiO₃, are accessible only on TiO₂ terminated SrTiO₃ surface while the SrO-terminated surface introduces a tunneling barrier for the electrons populating the conductance band. First principle molecular dynamics demonstrated that at elevated temperatures the surface oxygen vacancies are essential for the oxygen reduction reaction.     

Effect of growth parameters on the heteroepitaxy of 3C-SiC on 6H-SiC substrate by chemical vapor deposition

Epitaxial 3C-SiC(1 1 1) films were grown on 6H-SiC(0 0 0 1) Si face on axis substrates by chemical vapor deposition under H₂, SiH₄ and C₃H₈ in a cold wall vertical reactor. Two temperatures were studied (1450 and 1700 °C) with various C/Si ratio and deposition time. It was found that under conditions giving high lateral growth (low C/Si and/or high temperature), homoepitaxial growth occurred even at temperatures as low as 1450 °C. For other conditions, the 3C-SiC polytype was detected and always together with the formation of double positioning boundaries whose density was found to depend on the growth conditions but not on the initial surface reconstruction. Single domain enlargement was observed when growth was performed at 1700 °C over a nucleation layer grown at 1450 °C.     

Preparation of a novel Bi2MoO6 flake-like nanophotocatalyst by molten salt method and evaluation for photocatalytic decomposition of rhodamine B

The flake-like nanoscale γ-bismuth molybdate (Bi₂MoO₆) powders as a novel photocatalyst was prepared using Bi(NO₃)₃·5H₂O and Na₂MoO₄·2H₂O as raw materials by a simple low-temperature molten salt method at 350 °C for 1, 4 and 8 h, respectively. The as-prepared samples were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV–vis absorption spectra (UV–vis). The photocatalytic activity of Bi₂MoO₆ crystals was evaluated using the photocatalytic oxidation of rhodamine B (RhB) at room temperature under ultraviolet irradiation. It was found that the band gap adsorption edge of Bi₂MoO₆ is estimated to be 477 nm corresponding to the band gap energy to be 2.60 eV. In addition, the Bi₂MoO₆ powders exhibit good photocatalytic properties to photodegrade RhB at room temperature. Finally, the mechanism of photocatalytic property of Bi₂MoO₆ had been also discussed.     

Relativistic effects, infrared intensities, diffusion and lithiation in InSb

We consider In₄Sb₄H₁₈ cluster with zinc-blend structure of InSb. The cohesive energy and band gap of InSb were calculated by Unrestricted Hartree–Fock (UHF) and Density Functional Theory (DFT) method and it was shown that underestimated values of band gap can be decreased by considering the relativistic effects and in some cases InSb crystal even to be a semimetal. By calculating time-dependent DFT (TDDFT), the first three singlet and triplet excited energies were calculated and it was shown the 0.23 eV (band gap of InSb at 77 K) is placed between them. The infrared intensities were calculated and it is shown that the 367 cm⁻¹ (294 cm⁻¹ experimentally) mode can be attributed to the LO(Γ) in InSb. Finally it was shown that the Cadmium (Cd) diffuses substantially as Cd_In and changes the first three singlet and triplet excited energies, but Lithium (Li) is placed in empty spaces of InSb crystal in the lithiation of InSb.     

Characterization of activated reactive evaporated MoO3 thin films for gas sensor applications

Thin films of molybdenum trioxide (MoO₃) were prepared by activated reactive evaporation technique on Pyrex glass substrates. The influence of oxygen partial pressure, substrate temperature and glow power on the structure, surface morphology and optical properties of MoO₃ thin films was studied. The MoO₃ films deposited in an oxygen partial pressure of 1×10⁻³ Torr, glow power of 10 W and substrate temperature of 573 K exhibited predominantly a (0 k 0) orientation corresponding to the orthorhombic layered structure of -MoO₃. The evaluated optical band gap was 3.24 eV. The sensing property of these MoO₃ films for gases like NH₃ and CO was also studied to see the applicability for environmental monitoring. We have observed that the MoO₃ thin films of -phase are capable of detecting NH₃ and CO gases at concentrations lower than 10 ppm in dry air.     

Photoelectron spectroscopy of ion-irradiated B-doped CVD diamond surfaces

Chemical vapour deposition (CVD) diamond films were irradiated by 1 keV argon ions at room temperature with doses ranging from 3.6 × 10¹² to 1.1 × 10¹⁶ Ar⁺ cm². The influence of sputtering on the valence band density of states of a boron-doped CVD diamond film was investigated by ultraviolet photoelectron spectroscopy and the changes in the plasmon features were observed by X-ray photoelectron spectroscopy of the carbon Is core level and its loss region. A gradual change from typical diamond features to amorphous carbon was observed after prolonged bombardment times. Above a critical dose D_crit of 5.8 × 10¹⁴ Ar⁺ cm² the damaged surface layer is characterized by a splitting of the C Is bulk peak into two components: a bulk-like diamond peak at 285.3 eV binding energy and a defect peak with 1 eV lower binding energy, which is attributed to the production of an amorphous sp²-rich carbon matrix. Moreover additional occupied states in the range of 0–4 eV binding energy, completely different to those observed on reconstructed diamond surfaces, were observed in the valence band spectra of the ion-irradiated diamond surface. These filled states can also be attributed to the amorphous carbon matrix which is formed at high doses. At very low doses (< 3 × 10¹⁴ ions cm²) only a band bending of the C Is diamond core level peak, along with the formation of some occupied states in the band structure at around 3.8 eV binding energy was observed. A comparison with annealed hydrogen-free CVD diamond surfaces shows some similarities concerning these filled states. The obtained spectra are compared with other crystalline and amorphous forms of carbon and the results are discussed in terms of an irradiation-induced change in the atomic structure of the surface. A comparison of ion bombarded and annealed diamond samples clearly shows that no graphitization takes place in the latter case.     

Shallow defect states in hydrogenated amorphous silicon oxide

The theoretical cluster-Bethe-lattice method is used in this study to investigate the shallow defect states in hydrogenated amorphous silicon oxide. The electronic density of states (DOS) for the SiO₂ Bethe lattice of various Si–O–Si angles, non-bridging oxygen Si–O^√, peroxyl radical Si–O–O^√, threefold coordinated O₃ and Si–H bonds are calculated. The variation of the Si–O–Si bond angle causes the bandgap fluctuation and induces tail states near the conduction band minimum. The Si–O^√ and Si–O–O^√ bonds introduce shallow defect states in the energy gap near the top of the valence band. The Si–H bond induces a defect state, in the energy gap near the conduction band minimum, in a-SiO_x with high oxygen concentration, but not low oxygen concentration. The O₃ bond itself does not induce defect state in the energy gap. The O₃⁺D⁻ complex, formed by the O₃ and threefold coordinated silicon, induces shallow state in the energy gap near the conduction band minimum. This defect state can explain the energy shift of photoluminescence of a-SiO_x:H under annealing.     

First-principles study of the (1 1 0) polar surface of cubic PbTiO3

Under GGA, the cleavage energy, surface energy, surface grand potential, surface relaxation, and surface electronic structure have been calculated for five different terminations of PbTiO₃ (1 1 0) surface by using PAW method implemented in VASP. Taking into account the results of two neutral PbTiO₃ (1 0 0) surfaces, the favorable PbTiO₃ (1 1 0) and (1 0 0) surfaces are the TiO₂-terminated (1 0 0) surface, the PbO-terminated (1 0 0) surface, and the O-terminated (1 1 0) surface successively in view of surface energy minimization. The surface grand potential calculations show that two neutral PbO- and TiO₂-terminated (1 0 0) surfaces are favored in the moderate Pb and O chemical potentials, two mutual complementary TiO- and Pb-(1 1 0) terminations are stable in Pb-poor environment and in O- and Pb-rich conditions, respectively. A non-negligible rumpling of O-terminated (1 1 0) surface is found in the third O₂ layer and large lateral displacements between Ti and O atoms on the PbTiO layer lead to the initial O-Ti-O alignment broken. Different from the Fermi levels of the three nonstoichiometric TiO-, Pb- and O-terminations which are located in the band gap, the Fermi level of the PbTiO- termination is located at the bottom of the conduction band and that of the O₂-termination is located at the top of the valence band due to increment and decrement of the occupation states for polarity compensation.     

Computer modeling of grain boundaries in Ni3Al

Computer simulation of symmetric tilt grain boundaries (GB) Σ=5 [1 0 0] (0 1 2) and Σ=5 [1 0 0] (0 1 3) in alloy Ni₃Al was carried out. The energy of GB was calculated out by a method of construction of γ-surface with using Morse's empirical central-force potentials. Investigations shown that GB have several steady states: one is stable and other – metastable. These states differ by energy and atomic structure GB. It is shown, that GB in model CSL are unstable, the stabilization is achieved by additional displacement on some vector along plane of defect. Directions and value of potential barriers of GB slippage are calculated.     

Analysis of VUV and optical spectra of Cs2NaYF6 crystals doped with Tm

This study presents a detailed interpretation and analysis of the reported interconfigurational spectra of Tm³⁺ in Cs₂NaYF₆ [V.N. Makhov, N.M. Khaidukov, D. Lo, J.C. Krupa, M. Kirm, E. Negodin, Opt. Mater. 27 (2005) 1131]. Since only spin-forbidden d–f emission bands are observed for Tm³⁺ in this host, it is unsuitable for use as a scintillator because the emission lifetime is too long. The d–f emission spectrum is well-explained by calculation and most of the intensity is located in one band: 4f¹¹5d¹ (high-spin) → 4f^{12 3}H₆. Measurements from the bands in the d–f excitation spectrum do not provide the accurate separation energy of the high and low-spin states. Strict O_h point group selection rules are operative for the optical spectra. The electronic states of the 4f¹¹5d¹ configuration (briefly, d-electron states, hereafter) are calculated to span from 58,318 cm⁻¹ to 86,900 cm⁻¹. At least five structured bands are observed in the excitation spectrum and their intensities are fairly well simulated by calculation. The 4f¹² → 4f¹³ (np₆)⁻¹ charge transfer band is assigned between 140 and 120 nm (83,000–71,000 cm⁻¹) and excitation into this band leads to f–f emission, bypassing the d-electron states.