Reconstruction and de-reconstruction of the Ir(1 0 0) surface and ultrathin Fe/Ir(1 0 0) films

The structure, energetics and magnetic properties of the quasihexagonal reconstruction of the Ir(1 0 0) surface and nanostructures formed by Fe atoms on this surface have been investigated using first-principles density functional theory with generalized gradient corrections. We find the reconstructed (1 × 5) surface to be 0.10 eV/(1 × 1) area lower in energy than the unreconstructed surface and we demonstrate that first-principles calculations can achieve quantitative agreement with experiment even for such long-period and deep-going reconstructions. For Fe coverage of 0.4 monolayers (ML) we have studied the stripe-like structure with biatomic Fe rows placed in the troughs of the (1 × 5)-reconstructed surface. Results of nonmagnetic calculations agree well with the structure inferred from STM data. Higher Fe coverages lead to a de-reconstruction of the Ir substrate. At 0.8 ML coverage a surface compound with composition Fe₄Ir is formed, which shows an appreciable buckling. In this case, a ferromagnetic calculation leads to good agreement with the low-temperature LEED data. We predict that the (1 × 5) periodicity of the mixed interface layer will persist also in thicker films with a pure Fe surface. Films with 1-4 ML Fe are predicted to be tetragonally distorted and ferromagnetic, with an axial ratio corresponding well to an elastic distortion of the Fe lattice.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

First principles study of H2S adsorption and dissociation on Fe(1 1 0)

We report first principles density functional theory (DFT) results of H₂S and HS adsorption and dissociation on the Fe(1 1 0) surface. We investigate the site preference of H₂S, HS, and S on Fe(1 1 0). H₂S is found to weakly adsorb on either the short bridge (SB) or long bridge (LB) site of Fe(1 1 0), with a binding energy of no more than 0.50 eV. The diffusion barrier from the LB site to the SB site is found to be small (∼0.10 eV). By contrast to H₂S, HS is predicted to be strongly chemisorbed on Fe(1 1 0), with the S atom in the LB site and the HS bond oriented perpendicular to the surface. Isolated S atoms also are predicted to bind strongly to the LB sites of Fe(1 1 0), where the SB is found to be a transition state for S surface hopping between neighboring LB sites. The minimum energy paths for H₂S and HS dehydrogenation involve rotating an H atom towards a nearby surface Fe atom, with the S-H bonds breaking on the top of one Fe atom. The barrier to break the first S-H bond in H₂S is low at 0.10 eV, and breaking the second S-H bond is barrierless, suggesting deposition of S on Fe(1 1 0) via H₂S is kinetically and thermodynamically facile.     

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.     

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces through first-principles calculations

First-principles calculations were performed to study the properties of O adsorption on Ni₃Al (0 0 1), (0 1 1), and (1 1 1) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption sites are identified. The atomic and electronic structures and adsorption energies are predicted. The adsorption sites for O on the Ni₃Al (0 0 1) surface are at the 2Ni–2Al fourfold hollow site, whereas O prefers to adsorb at the Ni–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollow site on (1 1 1) surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized when O adsorbs on (0 0 1) surface, while the affinity of O for Al on (0 1 1) surface is weaker than (0 0 1) surface, and (1 1 1) surface is the weakest. The stronger O and Al affinity indicates more stable Al₂O₃ when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in different orientations improves in the order of (1 1 1), (0 1 1), and (0 0 1) surface, suggesting that the oxidation in different crystallographic orientations may be related to the affinity of O for Al in the surface.     

Self-organized Fe nanostructures on W(1 1 0)

The magnetic domain structure of Fe wires, ribbons and islands with different shapes that have been prepared under ultra high vacuum conditions on W(1 1 0) are studied with spin polarized low energy electron microscopy. The dimensions of the nanostructures, which are of the order of tens and hundreds of nm, can be controlled by the temperature at which they are produced, by the average Fe coverage and by the substrate morphology. The domain structure of the nanostructures depends on their shape and is determined by the competition between magnetoelastic, shape and magnetocrystalline anisotropies.     

FeO(1 1 1) formation by exposure of Fe(1 1 0) to atomic and molecular oxygen

The growth, structure, and morphology of ultrathin iron oxide layers formed on a Fe(1 1 0) single crystal surface are investigated by Auger electron spectroscopy, low energy electron diffraction, and grazing ion scattering. For Fe oxidation by atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude because surface sticking and dissociation are not limiting the growth process. A well-ordered FeO(1 1 1) film with low defect density is only obtained with atomic oxygen. Compared to the bulk, the FeO lattice is laterally compressed by about 5-6% resulting in an in-plane oxygen (Fe) nearest-neighbor distance of 2.87 Å. Independent of the preparation method, long-range structural order is poor if the oxide film thickness is increased to 3-5 layers. This is attributed to the relatively large lattice mismatch between FeO(1 1 1) and Fe(1 1 0).     

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.     

Theoretical study of Ni adsorption on the GaN(0 0 0 1) surface

First-principles pseudo-potential calculations within density-functional theory framework are performed in order to study the structural and electronic properties of nickel adsorption and diffusion on a GaN(0 0 0 1)-2×2 surface. The adsorption energies and potential energy surfaces are investigated for a Ni adatom on the Ga-terminated (0 0 0 1) surface of GaN. This surface is also used to study the effect of the nickel surface coverage. The results show that the most stable positions of a Ni adatom on GaN(0 0 0 1) are at the H₃ sites and T₄ sites, for low and high Ni coverage respectively. In addition, confirming previous experimental results, we have found that the growth of Ni monolayers on the GaN(0 0 0 1) surface is possible.     

Diffusion properties of Cu(0 0 1)-c(2 × 2)-Pd surface alloys

Structural and diffusion properties of a Cu(0 0 1)-c(2 × 2)-Pd surface and sub-surface ordered alloys are studied by using interaction potentials obtained from the embedded-atom method. The calculated diffusion energies are in agreement with observed kinetics of the surface alloy formation and confirm stability of the underlayer alloy. Activation energy of planar diffusion of palladium at the initial stage of the alloy formation as well as the activation energy of the overlayer-underlayer diffusion of the Pd atoms are in good agreement with those obtained by the scanning tunneling microscopy and low energy electron diffraction measurements, respectively.     

Theoretical study of oxygen adsorption at the Fe(1 1 0) and (1 0 0) surfaces

Electronic, magnetic and structural properties of atomic oxygen adsorbed in on-surface and subsurface sites at the two most densely packed iron surfaces are investigated using density functional theory combined with a thermodynamics formalism. Oxygen coverages varying from a quarter to two monolayers (MLs) are considered. At a 1/4 ML coverage, the most stable on-surface adsorption sites are the twofold long bridge sites on the (1 1 0), and the fourfold-hollow sites on the (1 0 0) surface. The presence of on-surface oxygen atoms enhances the magnetic moments of the atoms of the two topmost Fe layers. Detailed results on the surface magnetic properties, due to O incorporation, are presented as well. Subsurface adsorption is found unfavored. The most stable subsurface O, in tetrahedral positions at the (1 0 0) and octahedral ones at the (1 1 0) surface, are characterized by substantially lower binding than that in the on-surface sites. Subsurface oxygen increases the interplanar distance between the uppermost Fe layers. The preadsorbed oxygen overlayer enhances binding of subsurface O atoms, particularly for tetrahedral sites beneath the (1 1 0) surface.     

Structural stability and electronic structure of iron adsorption on the GaN(0 0 0 1) surface

In this work, we have investigated by means of first-principles spin-polarized calculations, the electronic and magnetic properties of iron (Fe) adsorption and diffusion on the GaN(0 0 0 1) surface using density functional theory (DFT) within a plane-wave pseudopotential scheme. In the surface adsorption study, results show that the most stable positions of a Fe adatom on GaN(0 0 0 1) surface are the H3 sites and T4 sites, for low and high Fe coverage respectively. We found that the Fe-H3 2 × 2 surface reconstruction exhibits a half-metallic behavior with a spin band gap and stable ferromagnetism ordering, which is a desirable property for high-efficiency magnetoelectronic devices. In addition, confirming previous experimental results, we found that the iron monolayers present a ferromagnetic order and a large thermal stability. This is interesting from a theoretical point of view and for its technological applications.     

Structure and reactivity of the hydrated hematite (0 0 0 1) surface

The structure of the hydroxylated hematite (0 0 0 1) surface was investigated using crystal truncation rod diffraction and density functional theory. The combined experimental and theoretical results suggest that the surface is dominated by two hydroxyl moieties—hydroxyls that are singly coordinated and doubly coordinated with Fe. The results are consistent with the formation of distinct domains of these surface species; one corresponding to the hydroxylation of the surface Fe-cation predicted to be most stable under UHV conditions, and the second a complete removal of this surface Fe species leaving the hydroxylated oxygen layer. Furthermore, our results indicate that the hydroxylated hematite surface structures are significantly more stable than their dehydroxylated counterparts at high water partial pressures, and this transition in stability occurs at water pressures orders of magnitude below the same transition for α-alumina. These results explain the observed differences in reactivity of hematite and alumina (0 0 0 1) surfaces with respect to water and binding of aqueous metal cations.     

Adsorption of N@C60 on Si(1 0 0)

The interactions between endohedrally doped N@C₆₀ molecules and the Si(1 0 0) surface have been explored via ab initio total energy calculations. Configurations which have the cage located upon the dimer row bonded to two dimers (r2) and within the dimer trench bonded to four dimers (t4) have been investigated, as these have previously been found to be the most stable for the C₆₀ molecule. We have investigated the differences between the adsorption of the C₆₀ and N@C₆₀ molecules upon the Si(1 0 0) surface and found that there are only minimal differences. Two interesting cases are the r2g and t4d configurations, as they both exhibit differences that are not present in the other configurations. These subtle differences have been explored in-depth. It is shown that the effects on the endohedral nitrogen atom, due to its placement within the fullerene cage, are small. Bader analysis has been used to explore differences between the C₆₀ and N@C₆₀ molecules.     

Adsorption of O2 on the CaO and SrO (1 0 0) surfaces: A first-principles study

First-principles calculations based on density functional theory (DFT) have been performed to investigate the adsorption of O₂ on the CaO and SrO (1 0 0) surfaces. The present results indicate that the bridge-top site for both the CaO and SrO (1 0 0) surfaces is the most favorable site for O₂ adsorption, with predicted adsorption energies of 1.437 eV and 1.236 eV, respectively. Detailed analysis of density of states, Mullikan population and vibration frequency are performed. The calculated results imply the possible formation of a peroxo (O₂²⁻) when O₂ is adsorbed with the bridge-top mode on both CaO and SrO (1 0 0) surfaces.     

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(1 1 1)-(7 × 7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(1 1 1) surfaces for which the Fermi level lies near the middle of the band gap.     

Atomistic simulation of the self-diffusion in Fe (1 1 1) surface

The formation energies, the intra- and inter-layer self-diffusion activation energies of a single vacancy in Fe (1 1 1) surface have been investigated with the modified analytical embedded-atom method (MAEAM). The results show that the effect of the surface is down to the sixth layer for the formation and intra-layer migration of the vacancy. It is easier for a vacancy to form and to migrate in intra-layer in the first (especially), the second and the third layer. For inter-layer migration, a single vacancy in each of the first six layers is favorable to migrate to the upper layers. On the contrary, a single vacancy in the seventh, the eighth and the ninth layers is favorable to migrate to the lower layers.     

Dissociative adsorption and adsorbate-induced reconstruction on Fe{2 1 1}

The products of CO, NO, O₂ and N₂ dissociation on Fe{2 1 1} have been studied by means of first-principles density functional theory. Preferred adsorption sites for adatoms C, N and O are identified, and trends in charge transfer and surface magnetism described. An experimentally observed (2 × 1) reconstruction induced by O is confirmed to be energetically stable, and a similar reconstruction induced by N is tentatively predicted. It is argued that these reconstructions may be important not only in the context of the catalytic reactivity of the Fe{2 1 1} surface, but also for the initial stages of surface nitridation and oxidation.     

First-principles study of oxidized Nb(1 0 0) surface structures

The atomic positions of the oxygen-induced c(2 × 2)-O, (3 × 1)-O and (4 × 1)-O surface structures on Nb(1 0 0) are determined by first-principles electronic structure calculations within the density functional theory comparing experimentally observed scanning tunneling microscopy (STM) images. STM images of these surfaces are calculated on the basis of the theory of Tersoff and Hamann. The theoretical and experimental STM images of the oxygen-chemisorbed c(2 × 2)-O structural model agree well. However, only the oxide-covered (3 × 1)-O and (4 × 1)-O structural models with two layers of NbO and contraction of the unit length along longitudinal 〈1 0 0〉 direction by 10% result in the theoretical STM images that agree with the experimental ones.     

Structural properties of oxygen on InN(0 0 0 1) surface

First-principles calculations are performed to study the various structures of oxygen (O) adsorbed on InN(0 0 0 1) surfaces. It is found that the formation energy of O on InN(0 0 0 1) decreases with decreasing oxygen coverage. Of all the adsorbate induced surface structures examined, the structure of InN(0 0 0 1)-(2 × 2) as caused by O adsorption at the H₃ sites with 0.25 monolayers coverage is most energetically favorable. Meanwhile, nitrogen (N) vacancy can form spontaneously. Oxygen atoms may also substitute N atoms, or accumulate at the voids inside InN film or simply stay on the surface during growth. The oxygen impurity then acts as a potential source for the n-type conductivity of InN as well as the large energy band gap measured.