Usage of pattern recognition scheme in kinetic Monte Carlo simulations: Application to cluster diffusion on Cu(1 1 1)

We have invoked a simple pattern recognition scheme in kinetic Monte Carlo simulations of post-deposition evolution of two dimensional islands on fcc(1 1 1) surfaces. On application of the technique to the diffusion of small Cu clusters (8-100 atoms) on Cu(1 1 1) we find that, at room temperature, clusters with certain magic numbers show stick-slip type of motion with striking patterns rather than the random paths followed by the others. At higher temperatures all clusters display random motion. The calculated diffusion coefficients show dependence on size and temperature with an effective barrier ranging between 0.62 eV and 0.84 eV. Small asymmetries in diffusion barriers lead to a large difference in the frequencies of adatom diffusion along the two types of micro-facetted steps on Cu(1 1 1) leading to consequences in their shape evolution. The pattern recognition scheme revealed 49 basic periphery single atom diffusion processes whose activation energy barriers were calculated using the nudged elastic band technique and interatomic potentials from the embedded atom method.     

Self-diffusion dynamic behavior of atomic clusters on Re(0 0 0 1) surface

Using molecular dynamics simulations and a modified analytic embedded atom potential, the self-diffusion dynamics of rhenium atomic clusters up to seven atoms on Re(0 0 0 1) surface have been studied in the temperature ranges from 600 K to 1900 K. The simulation time varies from 20 ns to 200 ns according to the cluster sizes and the temperature. The heptamer and trimer are more stable comparing to other neighboring non-compact clusters. The diffusion coefficients of clusters are derived from the mean square displacement of cluster's mass-center, and diffusion prefactors D₀ and activation energies E_a are derived from the Arrhenius relation. It is found that the Arrhenius relation of the adatom can be divided into two parts at different temperature range. The activation energy of clusters increases with the increasing of the atom number in clusters. The prefactor of the heptamer is 2-3 orders of magnitude higher than a usual prefactor because of a large number of nonequivalent diffusion processes. The trimer and heptamer are the nuclei at different temperature range according to the nucleation theory.     

A theoretical study of hydrogen adsorption and diffusion on a W(1 1 0) surface

We have used density functional theory to investigate hydrogen adsorption and diffusion on a W(1 1 0) surface. Hydrogen adsorption structures were examined from low coverage to one monolayer, and a threefold hollow site was found to be the most stable site at all coverages. In contrast to previous assertions, the work function decrease is not due to electron transfer from the hydrogen atoms to the W surface, but due to electron depletion at the vacuum region above the hydrogen atoms. Hydrogen atoms can diffuse via short-bridge sites and long-bridge sites at a coverage of θ = 1.0. Although the calculated activation energy for hydrogen diffusion via a short-bridge site is as small as 0.05 eV, field emission microscope experiments have shown that the activation energy for hydrogen diffusion is about 0.20 eV, which agrees fairly well with our calculated value of the activation energy via a long-bridge site. This discrepancy can be related to hydrogen delocalization on the W(1 1 0) surface, which has been suggested by electron energy loss spectroscopy experiments.     

Diffusion of single adatom Cu on Cu (0 0 1) and (1 1 0) surfaces

With static relaxation, the surface diffusion activation energies of a single Cu adatom migrated by both atomic exchange and hopping mechanisms and the forces acted on the diffusing adatom from other atoms of Cu (0 0 1) or (1 1 0) surface are calculated by using the MAEAM. When adatom migrated on Cu (0 0 1) or (1 1 0) surface, the increment curves of the system energy by hopping mechanism are symmetrical and the saddle points are in the midpoints of the migration path, but the ones by the exchange mechanism are dissymmetrical and the saddle points are always close to the initial hole positions of the adatom and away from the initial equilibrium positions of the exchanged atom. From minimization of both the diffusion activation energy and the force acted on the diffusing adatom from other atoms, we found that, on Cu (0 0 1) surface the favorable diffusion mechanism is hopping mechanism, however, on Cu (1 1 0) surface, hopping via long bridge is easier than the exchange mechanism but the hopping via short bridge is more difficult than the exchange mechanism.     

Dynamics of copper atoms on Si(1 1 1)-7 × 7 surfaces

This study investigated the dynamics of copper atoms adsorbed on Si(1 1 1)-7 × 7 surfaces between 300 K and 623 K using a variable-temperature scanning tunneling microscope (STM). The diffusion behavior of copper clusters containing up to ∼6 atoms into a particular half unit cell of the 7 × 7 reconstructed Si(1 1 1) surface was considered. The movements and the formation of copper clusters were tracked in detail. The activation energies and pre-exponential factors for various diffusion paths were estimated. Finally, the Cu-etching-Si process and the quasi-5 × 5 incommensurated phase of Cu/Si islands were discussed.     

Diffusions of small clusters on Pt(1 1 1) and Cu(1 1 1) surfaces

Diffusions of small cluster Pt₆ on Pt(1 1 1) surface and Cu₆ on Cu(1 1 1) are studied by molecular dynamics simulation, respectively. The atomic interaction is modeled by the semiempirical potential. The results show that the diffusion processes in the two systems are far different. For example, on Pt(1 1 1) surface, the hopping of single atom and the shearing of two atoms of hexamer only occur on the adatom(s) adsorbed at B-step, while on Cu(1 1 1) surface they can appear on the adatom(s) either at A-step or B-step. To the concerted translation of the parallelogram hexamer, the anisotropy in the diffusion path is observed in the two systems, the mechanisms and then the preferential paths, however, are completely different. The reasons for these diffusion characteristics and differences are discussed.     

First-principles study on field evaporation of surface atoms from W(0 1 1) and Mo(0 1 1) surfaces

The simulations of field-evaporation processes for surface atoms on W(0 1 1) and Mo(0 1 1) surfaces are implemented using first-principles calculations based on the real-space finite-difference method. The threshold values of the external electric field for evaporation of the surface atoms, which are ∼6 V/Å for tungsten and ∼5 V/Å for molybdenum, are in agreement with the experimental results. While the threshold value of the electric field and the local-field enhancement around the evaporating atoms agree with those expected from the conclusion of the previous study using structureless jellium, the induced charge around the surface atom has a significant difference from that obtained by the jellium model.     

Double rotation mechanism in small Cu clusters concerted diffusion over Cu{1 1 1} surfaces

In this work, we study the role of the double rotation mechanism in the concerted diffusion of two-dimensional small Cu clusters (up to 10 atoms) over Cu{1 1 1} surfaces. Our results show that the necessary energy to diffuse the cluster on any direction over the surface (overall activation energy) increases proportionally to the cluster size. However, the minimum energy necessary to just move the cluster center of mass presents a nonmonotonic increase. The reason for this behavior relies on the double rotation mechanism, which is observed in some clusters with diamond shape configuration. Consequently, clusters as big as hexamers can be expected to be surprisingly mobile with activation energies around 0.15 eV.     

Interaction of Ag and O on W(1 1 0) studied by scanning tunneling microscopy

The structural changes of Ag films on W(1 1 0) upon coadsorption of oxygen have been studied by scanning tunneling microscopy. The exposure of one monolayer Ag to oxygen leads to a phase separation into an Ag bilayer and patches of O-covered W(1 1 0). The effective Ag island thickness increases linearly with oxygen exposure. For Ag submonolayer-islands the onset of the bilayer formation is delayed, the induction period increases with the available free W area. We conclude that the steps of the transport process are (1) dissociation of oxygen on W and on the Ag islands, (2) site exchange of atomic oxygen with Ag atoms predominantly at the island edges - while on W(1 1 0) the oxygen is immobile, (3) diffusion of the displaced Ag atoms to the island edges where they are incorporated into the monolayer and (4) initiation of Ag bilayer formation, once the W(1 1 0) is saturated with O. This indicates an unexpected activity of the Ag monolayer on W(1 1 0) towards oxygen dissociation. In case of a reversed deposition sequence, where submonolayer quantities of Ag are adsorbed on an oxygen-precovered W(1 1 0) surface, growth of Ag clusters is observed. The distribution of cluster size and cluster height depends critically on the spatial order within the predeposited oxygen overlayer - it is obvious that the oxygen overlayer on the W surface acts as a structured template for preferential Ag nucleation.     

Density functional theory study of hydrogenation mechanism in Fe-doped Mg(0 0 0 1) surface

Using density functional theory (DFT) in combination with nudged elastic band (NEB) method, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and Fe-doped Mg(0 0 0 1) surfaces are studied. Firstly, the dissociation pathway of H₂ and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0 0 0 1) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0 0 0 1) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe. Then, the diffusion processes of atomic hydrogen on pure and Fe-doped Mg(0 0 0 1) are presented. The obtained diffusion barrier to the first subsurface is 0.45 eV and 0.98 eV, respectively. Finally, Chou method was used to investigate the hydrogen sorption kinetic mechanism of pure MgH₂ and Mg mixed with 5 at.% Fe atoms composites. The obtained activation energies are 0.87 ± 0.02 and 0.31 ± 0.01 eV for H₂ dissociation on the pure surface and H atom diffusion in Fe-doped Mg surfaces, respectively. It suggests that the rate-controlling step is dissociation of H₂ on the pure Mg surface while it is diffusion of H atom in the Fe-doped Mg surface. And both of fitting data are matching well with our calculation results.     

Structures and stability of Ag clusters on Ag(1 1 1) and Ni(1 1 1) surfaces

The structures of the lowest total energy for small Ag_N clusters with N = 2-20, which are grown on Ag(1 1 1) and Ni(1 1 1) surfaces, have been determined using a combination of the embedded-atom method and the basin-hopping algorithm. It is found that the particularly stable Ag clusters with N<18 have similar geometries on both surfaces when comparing clusters of the same size. On the other hand, the geometries of the less stable Ag clusters in the same size range differ for the two surfaces. From N?18, the sizes of the particularly stable structures are different for the two different substrates. Due to the large size mismatch of the two types of atoms it is energetically unfavorable for Ag to form a pseudomorphic monolayer structures on Ni(1 1 1) and there is considerable strain produced at the interface. The effect of this strain and the increased adatom-substrate interactions lead to irregular and elongated structures of the adsorbed Ag clusters.     

Electron-induced H atom desorption patterns created with a scanning tunneling microscope: Implications for controlled atomic-scale patterning on H-Si(1 0 0)

We have used scanning tunneling microscopy (STM) to explore the details of single and multiple H atom desorption from the H-Si(1 0 0)-2 × 1 surface induced by the inelastic scattering of electrons from an STM tip. The desorption of pairs of H atoms from individual Si dimers is rarely observed. Two-H atom desorption most often involves pairs of dimers, in the same or adjacent rows. This suggests that recombinative H₂ desorption via an interdimer reaction pathway, like that observed recently under nanosecond laser heating, may also be operative for electron-induced excitation using STM. Repeatable fabrication of desired size-selected dangling bond (DB) clusters is also achieved. The single atomic precision of the fabrication is a result of the intrinsically unfavorable paired H atom desorption from a single dimer, but does not result from the spatial localization of excitation energy of the Si-H bond under the STM tip as suggested in previous studies.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

The mechanism of amine formation on Si(1 0 0) activated with chlorine atoms

The dissociation of NH₃ on a Si(1 0 0) surface activated with Cl atoms was investigated using X-ray photoelectron spectroscopy. Gas phase UV-Cl₂ (0.1-10 Torr Cl₂ for 10-600 s under 1000 W Xe lamp illumination) completely replaced the H-termination on aqueous-cleaned Si(1 0 0) with 0.82 ± 0.06 ML of Cl at 298 K. A single spin-orbit split Cl 2p doublet indicated that the Cl atoms were bound to Si dimer atoms, forming silicon monochloride (Cl-Si-Si-Cl). Exposing the Cl-terminated surface at 348 K to NH₃ (1-1000 Torr for 5-60 min) replaced one Cl atom with one N atom up to a coverage of 0.33 ± 0.02 ML. Cl atoms lowered the activation energy barrier for reaction to form a primary amine (Si-NH₂). Oxygen was coadsorbed due to competition by H₂O contamination. The presence of Cl on the surface even after high NH₃ exposures is attributed to site blocking and electrostatic interactions among neighboring Cl-Si-Si-NH₂ moieties. The results demonstrate a low temperature reaction pathway for depositing N-bearing molecules on Si surfaces.     

Simulation of STM images of p(3 × 2)/Na/Ge(0 0 1) surface

Adsorption of Na on the Ge(0 0 1) surface is known to be a cause of surface reconstruction. It is expected to find one Na atom per unit cell of the reconstructed surface, however, the precise atomic configuration of this system is still a matter of controversy. Consequently, the aim of our present theoretical study is to examine the atomic structure of stable p(3 × 2)/Na/Ge(0 0 1) surfaces with and without the possible change of the number of Ge atoms in the surface layer (so-called mass transport). Structural and electronic properties of the considered system are investigated using the local-orbital density functional method. Our considerations are completed by a simulation of STM images of the structures following from molecular dynamics calculations.     

The gold and oxygen (3 × 1) structures on W(1 1 2)

The adsorption of two very different adsorbates, gold and oxygen, induce the formation of a (3 × 1) surface structure on both W(1 1 2) and Mo(1 1 2). In spite of similar adsorbate unit cells, the surface electronic structure, derived from photoemission, exhibits pronounced differences for the two adsorbates. Indeed, both experiment and simulations indicate substantial changes in electronic structures of (1 × 1) and (3 × 1) gold overlayers supported by highly anisotropic (1 1 2) plane. We speculate that (3 × 1) is a favored periodicity in the atomic rearrangement of the (1 1 2) surfaces of molybdenum and tungsten due in part as a result of the initial state band structure of these surfaces.     

Atomic and electronic structure of methanol on Ge(1 0 0)

We have performed density-functional theory (DFT) calculations to investigate the adsorption structures of methanol on a Ge(1 0 0) surface. Among many possible adsorption configurations, the most favorable configurations at room temperature were found to be those in which the OH-dissociated methanol molecule forms O-Ge bonds, with the methoxy group either parallel or perpendicular to the Ge surface. The spatial arrangement of methoxy group relative to the Ge(1 0 0) surface is not critical. The dissociated H is bonded to an adjacent up-Ge atom, passivating the dangling bond. The possibility of H diffusion to other Ge atoms is also investigated. The corresponding simulated images explain well the adsorption features observed experimentally. The reaction pathways explain the feasibility of OH-dissociative structures at room temperature. The two OH-dissociative configurations where methoxy groups are either parallel or perpendicular to Ge surfaces are similar in thermodynamic and kinetic aspects.     

Adsorption of gold on hydrogen terminated Si(0 0 1): Formation of chain structure

Possible formation of stable Au atomic wire on the hydrogen terminated Si(0 0 1): 3×1 surface is investigated under the density functional formalism. The hydrogen terminated Si(0 0 1): 3×1 surface is patterned in two different ways by removing selective hydrogen atoms from the surface. The adsorption of Au on such surfaces is studied at different sub-monolayer coverages. At 4/9 monolayer (ML) coverage, zigzag continuous Au chains are found to be stable on the patterned hydrogen terminated Si(0 0 1): 3×1 surface. The reason for the stability of the wire structures at 4/9 ML coverage is explained. It is to be noted that beyond 4/9 ML coverage, the additional Au atoms may introduce clusters on the surface. The continuous atomic gold chains on the substrate may be useful for the fabrication of atomic scale devices.     

A density functional study of atomic oxygen and water molecule adsorption on Ni(1 1 1) and chromium-substituted Ni(1 1 1) surfaces

Density functional theory (DFT) for generalized gradient approximation calculations has been used to study the adsorption of atomic oxygen and water molecules on Ni(1 1 1) and different kind of Ni-Cr(1 1 1) surfaces. The fcc hollow site is energetically the most favorable for atomic oxygen adsorption and on top site is favorable for water adsorption. The Ni-Cr surface has the highest absorption energy for oxygen at 6.86 eV, followed by the hcp site, whereas the absorption energy is 5.56 eV for the Ni surface. The Ni-O bond distance is 1.85 Å for the Ni surface. On the other hand, the result concerning the Ni-Cr surface implies that the bond distances are 1.93-1.95 Å and 1.75 Å for Ni-O and Cr-O, respectively. The surface adsorption energy for water on top site for two Cr atom substituted Ni-Cr surface is 0.85 eV. Oxygen atoms prefer to bond with Cr rather than Ni atoms. Atomic charge analysis demonstrates that charge transfer increases due to the addition of Cr. Moreover, a local density of states (LDOS) study examines the hybridization occurring between the metal d orbital and the oxygen p orbital; the bonding is mainly ionic, and water bonds weakly in both cases.     

First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.