Ab initio study of structural and magnetic properties of cubic Fe4N(0 0 1) surface

First-principles calculations are employed to study the structural and magnetic properties of fully-relaxed cubic Fe₄N(0 0 1) surfaces with both Fe₂- and Fe₂N-termination. The results of surface stability calculations show that the (0 0 1) surface of Fe₄N is most possibly existing with Fe₂N-termination. Slab structures have more localized features in the density of states especially for the Fe₂N-terminated surface due to structure relaxation. The average magnetic moments of Fe atoms increase with increasing thickness of slabs. The calculated interlayer distances indicate that the decreases of d₁₂ and d₂₃ result in stronger hybridization and shorter bond distances between Fe2 atom in the second layer and other atoms in surface or the third layers, which lead to variation of magnetic moments with different slab thicknesses.     

Surface vibrational thermodynamics from ab initio calculations for fcc(1 0 0)

We present vibrational dynamics and thermodynamics for the (1 0 0) surfaces of Cu, Ag, Pd, Pt and Au using a real space approach. The force field for these systems is described by density functional theory. The changes in the vibrational dynamics and thermodynamics from those in bulk are confined mostly to the first-layer. A substantial enhancement of the low-frequency end of the acoustic branch was found and is related to a loosening of the bond at the surface. The thermodynamics of the first-layer also show significant differences (higher heat capacity, lower free energy and higher mean vibrational square amplitudes) from what obtains in bulk. Comparing these results with those calculated using embedded-atom method potentials, we discovered that for Ag(1 0 0) and Cu(1 0 0), the two methods yield very similar results while for Pd(1 0 0), Pt(1 0 0) and Au(1 0 0) there are substantial differences.     

Structural stabilities and diffusion of small Fe clusters on Fe (1 1 0) surface: A molecular dynamics study

Using Embedded-atom-method (EAM) potential of iron, structural stabilities of small Fe clusters on a Fe (1 1 0) surface have been investigated by molecular dynamics studies. It is presented that a tetramer and heptamer clusters are more stable than other sizes. These two clusters have high transition energies. They can be a critical nucleus at low and high temperature, respectively. A dimer diffuses more easily with lower energy barrier than single adatom. The trimer's rotation and dimer shearing mechanisms have been investigated in this paper.     

Nonlinear magnetoelastic effects in ultrathin epitaxial FCC Co(0 0 1) films:: an ab initio study

Those linear and nonlinear magnetoelastic coupling coefficients which determine the magnetostrictive stress and the strain-induced out-of-plane magnetic anisotropy in epitaxially grown FCC Co(0 0 1) films are calculated by the ab initio density functional electron theory. The nonlinear couplings have a strong effect on the change Δσ₁^m of the in-plane magnetostrictive stress resulting from a change of the magnetization direction from [0 1 0] to [1 0 0], but a negligibly small effect on the out-of-plane anisotropy e_MCA. The calculations confirm the experimental result that the measured out-of-plane anisotropy cannot be totally attributed to volume magnetoelastic effects. Estimates are given for the nonlinear magnetoelastic coupling coefficients m₁^γ,2 and m₂^γ,2.     

Ge adsorption on SiC(0 0 0 1): An ab initio study

In this work we have performed an ab initio total energy investigation of the Ge adsorption process on the Si-terminated SiC(0 0 0 1)- and (3 × 3) surfaces. We find that Ge adatoms lying on the topmost sites of the and (3 × 3) surfaces represent the energetically more stable configurations at the initial stage of the Ge adsorption on the SiC(0 0 0 1) surface. The Si → Ge substitutional adsorption processes have been examined as a function of the Si and Ge chemical potentials. Increasing the Ge coverage, we verify that the formation of Ge wetting layer on the surface, and Ge nanocluster on the (3 × 3) surface are the energetically more stable configurations, in accordance with recent experimental findings.     

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom-restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH₃OH dissociation favouring a basic cleavage (adsorption of OH and CH₃ fragments) rather than an acidic one (adsorption of H and OCH₃ fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.     

Adsorption of molecular SiO2 on a clean Si(1 0 0) surface

We have investigated the adsorption of molecular (gaseous) SiO₂ on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. The SiO₂ molecule is found to be chemisorbed on various sites on the Si surface and the most energetically favourable structure is on top of the dimers. The minimum energy pathways for the various adsorption channels indicate that the reaction is barrierless in all cases. The corresponding vibrational spectrum is also calculated and the adsorbed molecules are, as expected, found to have red-shifted vibrational frequencies. The energetically favourable adsorption sites and adsorption energies are comparable to the results found for SiO.     

Anharmonic effects on B2-FeAl(1 1 0) surface: A molecular dynamics study

Using molecular dynamics simulations and the analytic embedded-atom method (AEAM), the surface anharmonicity of B2-FeAl(1 1 0) has been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer spacing, mean square vibrational amplitudes, surface phonon frequencies and line-widths, and layer structure factor have been calculated. The obtained results indicate that the anharmonic effects are small in the temperature range from 0 K to 900 K. The temperature dependences of the interlayer spacing indicates that the rippling effect of the B2-FeAl(1 1 0) surface is exhibited by the contraction of Fe surface atoms and the expansion of Al atoms, which persists at high temperatures. The temperature dependence of the layer structure factors shows that the B2-FeAl(1 1 0) surface does not disorder until the temperature of 1300 K.     

Ellipsometric detection of GaAs(0 0 1) surface hydrogenation in H2 atmosphere

Optical properties of MBE-grown GaAs(0 0 1) surfaces have been studied by spectroscopic ellipsometry under dynamic conditions of ramp heating and cooling after desorption of passivating As-cap-layer with low pressure H₂ atmosphere (14 Torr) applied to the surface. The temperature dependence of GaAs pseudo-dielectric function with atomically smooth (0 0 1) surface carrying the fixed Ga-rich (4 × 2) reconstruction was obtained for the temperature range of 160–600 °C. It is shown ellipsometrically that GaAs(0 0 1) heating in the molecular hydrogen atmosphere results in the formation of hydrogenated layer on the surface.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

AFM study of a SiC film grown on Si(1 0 0) surface using a C2H4 beam

A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C₂H₄) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).     

Adsorption of Xe atoms on the TiO2(1 1 0) surface: A density functional study

The interaction of xenon atoms with the TiO₂(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO₂(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO₂(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.     

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F_s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.     

Adsorption and NiO(1 0 0) formation by atomic and molecular oxygen on Ni(1 1 0)

The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0).     

Adsorption of Ag on Si(1 0 0) surface

The chemisorption of one monolayer Ag atoms on an ideal Si(1 0 0) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The adsorption energies (E_ad) of different sites are calculated. It is found that the adsorbed Ag atoms are more favorable on C site (fourfold site) than on any other sites on Si(1 0 0) surface, the polar covalent bond is formed between Ag atom and surface Si atom, a Ag and Si mixed layer does not exist and does form an abrupt interface at the Ag–Si(1 0 0) interface. This is in agreement with the experiment results. The layer-projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated. Comparing with the Au/Si(1 0 0) system, the interaction is weaker between Ag and Si than between Au and Si.     

Atomic and electronic properties of furan on the Si(0 0 1)-(2 × 2) surface

The atomic and electronic properties of the adsorption of furan (C₄H₄O) molecule on the Si(1 0 0)-(2 × 2) surface have been studied using ab initio calculations based on pseudopotential and density functional theory. We have considered two possible chemisorption mechanisms: (i) [4 + 2] and (ii) [2 + 2] cycloaddition reactions. We have found that the [4 + 2] interaction mechanism was energetically more favorable than the [2 + 2] mechanism, by about 0.2 eV/molecule. The average angle between the CC double bond and Si(1 0 0) surface normal was found to be 22°, which is somewhat smaller than the experimental value of 28°, but somewhat bigger than other theoretical value of 19°. The electronic band structure, chemical bonds, and theoretical scanning tunneling microscopy images have also been calculated. We have determined a total of six surface states (one unoccupied and five occupied) in the fundamental band gap. Our results are seen to be in good agreement with the recent near edge X-ray absorption fine structure and high resolution photoemission spectroscopy data.     

Bistability in a H-terminated Si(1 0 0)2 × 1 surface obtained by ab initio transport calculations

We have analyzed electron tunneling due to the electric field from a hydrogen-terminated Si(1 0 0)2 × 1 ultrathin film on a metal substrate by density functional transport calculations. We have obtained a hysteresis loop in the tunneling current, which comes from the existence of two electronic structures. Furthermore, we have clarified that, as a condition of bistable electron transport, a double-barrier potential structure is not necessarily required for zero field, because it can be induced by the electric field.