Comment on the “cross-channel diffusion mechanism on the Pt(110) surface” BY S.H. Garofalini and T. Halicioglu

It is known that the Pt(110) surface exhibits in its stable state a 1 × 2 reconstruction. It seems therefore not appropriate to compare theoretical surface diffusion coefficients calculated for the regular, non-reconstructed Pt(110) surface with the experimental data obtained by field ion microscopy. In particular, the process of cross-channel diffusion by atom exchange which is observed on the regular (110) surface by molecular dynamics simulations may have no relationship to the experimental findings of cross-channel adatom diffusion on reconstructed Pt(110) and Ir(110) surfaces. An alternate mechanism for easy cross-channel adatom diffusion on reconstructed (110)?1 × 2 surfaces of Pt and Ir is proposed.     

Formation of carbon composite structures on the Ge(110) surfaces

We present our first-principles calculation of the adsorption and diffusion of a carbon adatom on the H-terminated and clean Ge(110) surfaces, which are essential processes in the nucleation and growth of a monolayer graphene on Ge(110) by chemical vapor deposition. On the H-terminated surface, the C adatom spontaneously substitutes H atom(s) to form a monohydride structure (CH) or a dihydride structure (CH₂) and makes direct bonds with the substrate Ge atoms. The resulting diffusion barriers of the C adatom are 2.67 and 6.45 eV parallel to and perpendicular to the zigzag Ge chains of the surface, respectively. On the clean surface, the C adatom embeds into the zigzag Ge chain with nearly no barrier, kicking out a Ge atom out of the chain at the same time. The kicked-out Ge atom, instead of the C adatom, becomes a diffusion species with the barrier less than 0.63 eV. The formation of the C composite structures makes the C adatom difficult to diffuse both on the H-terminated and clean Ge(110) surfaces, which suggests that the nucleation and growth of the graphene islands from C seeds is much suppressed. We propose a growth mechanism of graphene monolayer going round the diffusion of the C adatoms on the Ge(110) surfaces.     

Surface atom displacement processes

Progress in field ion microscope studies of adatom displacements on metal surfaces is reviewed. It is concluded that of the displacement processes that contribute to surface diffusion only displacements between low-coordination (terrace) sites are well characterised. Procedures and preliminary results of FIM studies of adatom displacement over steps are described. Activation energies measured for passage of Ta, W, Re, Ir and Pt adatoms across (110) W steps are found to equal activation energies for diffusion over (110) W, despite the highly reflecting character of the step for all the adsorbates except Pt. Displacements of adatoms interacting with other adatoms are discussed. Results presented show that interaction of transition metal adatoms forming close-packed dimers on (110) W is rather weak, with a minimum interaction energy [?U(r) < 4kJ/mol] for Re₂ corresponding to a very weak attraction for Re adatoms 0.27 nm apart.     

Diffusion of adatoms and vacancies on otherwise perfect surfaces: A molecular dynamics study

Using computer simulation by the technique of molecular dynamics, we have investigated the influence of the terrace structure on the type and the dynamical aspects of atomic mechanisms for surface diffusion in fcc structure crystals. On the (100) terraces, vacancies are much more mobile than adatoms, while the opposite is true for (111) terraces. On the latter, vacancies migrate through the creation in their vicinity of paired, adatom-vacancy, defects. On the (100) face, the adatom jump length incrases with increasing temperature and reaches a value equal to several times the nearest neighbour distance. Adatoms are also fully delocalized on the (111) face and spend much more time in flight over the surface than by vibrations in the equilibrium sites. Large dynamical correlations are present in the vacancy movement on the (100) face and have been identified as new mechanisms of the defect migration by multiple jumps. On the (110) terrace, despite its anisotropic structure, two-dimensional diffusion takes place by an original atomic exchange mechanism. This mechanism has been identified to be the same as the one proposed by Halicioglu to explain two-dimensional diffusion on (110) Pt terraces, and recently corroborated by the FIM experiments of Wrigley and Ehrlich.     

Mechanism for the self-diffusion of Au and Ir adatoms on Pt(110) surface

A molecular dynamics computer technique was used to simulate the diffusion of a Au and and Ir adatom on the Pt(110) surface. Details of the exchange mechanism associated with cross-channel diffusion were observed. For the Au adatom at the low temperature, the expected channel diffusion occurred, whereas at the higher temperature a temporary exchange of the Pt wall atom by the Au adatom was seen. In the Ir case at the low temperature no exchange was observed, although there was a partial displacement of the wall atom by the adatom. At the higher temperature, the exchange mechanism was observed. The results are consistent with experimental observations and also indicate the importance of the stability of the channel wall atoms (via their thermal motion) on the occurrence of the exchange mechanism.     

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.     

A very low energy electron reflection study of hydrogen adsorption on W(100) and W(110) surfaces

Experimental results of the elastic backscattering of electrons with energies between 0 and 20 eV from the surface systems H/W(100) and H/W(110) are reported and interpreted on the basis of the Darwin model. On W(100) the adatom distance from the reconstructed substrate is found to be different from that on the relaxed surface. Large vertical displacements of the substrate atoms during reconstruction can be excluded. On W(110) the results indicate two different binding states which are occupied sequentially and which differ significantly in their distances from the surface. Work function change data are also reported for both systems.     

Adatom ascending at step edges and faceting on fcc metal (110) surfaces

Using first-principles total-energy calculations, we show that an adatom can easily climb up at monatomic-layer-high steps on several representative fcc metal (110) surfaces via a place exchange mechanism. Inclusion of such novel adatom ascending processes in kinetic Monte Carlo simulations of Al(110) homoepitaxy as a prototypical model system can lead to the existence of an intriguing faceting instability, whose dynamical evolution and kinetic nature are explored in comparison with experimental observations.     

金属铝在半导体表面的吸附

本文采用集团模型,用自洽的EHT方法计算金属Al在Si(111)和GaAs(110)面上吸附的稳定的几何构型和电子态,结果表明,Al在Si(111)面的三度开位上的离子吸附比顶位的共价吸附更稳定,态密度与实验符合也更好,Al吸附在GaAs(110)面上,将取代表面Ga原子,形成AlAs,此时GaAs(110)面将恢复到不弛豫的理想位置。 关键词：     

Stacking-faults-free zinc blende GaAs/AlGaAs axial heterostructure nanowires during vapor-liquid-solid growth

Pure zinc blende structure GaAs/AlGaAs axial heterostructure nanowires (NWs) are grown by metal organic chemical vapor deposition on GaAs(111) B substrates using Au-catalyzed vapor-liquid-solid mechanism.Al adatom enhances the influence of diameters on NWs growth rate.NWs are grown mainly through the contributions from the direct impingement of the precursors onto the alloy droplets and not so much from adatom diffusion.The results indicate that the droplet acts as a catalyst rather than an adatom collector.     

分子动力学模拟Gd原子在Cu(110)表面的扩散过程

为了分析Gd吸附原子在Cu(110)表面的扩散机理，用分子动力学对该扩散过程进行模拟.模拟 结果表明在［1 1 0］方向Gd原子通过跳跃机理扩散，而且多步跳跃频率很高.而在［0 0 1］方向则通过交换机理扩散.吸附原子在［1 1 0］方向的扩散能力要比［0 0 1］ 方向强.通过对扩散频率的拟合，发现两种扩散机理都符合Arrhenius公式，从而确定了跳跃 机理的扩散势垒为0.097eV，交换机理的扩散势垒为0.33eV.另外还用能量弛豫的方法确定了 跳跃机理的扩散势垒. 关键词： 分子动力学 表面扩散 跳跃机理 交换机理 扩散势垒     

异质原子在Cu（001）表面扩散的分子动力学模拟

采用分子动力学方法模拟单个增原子Ag,Pd和Cu在Cu（001）表面上的扩散过程.通过对自扩散和异质扩散过程中扩散机制的观察,统计三种不同的增原子在不同温度下的扩散频率,拟合给出扩散势垒和扩散频率的指前因子,并与扩散势垒的静力学计算结果进行比较.结果表明：在800 K以下时,三种增原子均以简单跳跃机制为主扩散,与衬底不互溶的Ag增原子的跳跃频率最大,与衬底互溶的Pd增原子的跳跃频率最小.同质增原子与异质增原子的扩散频率和温度的关系均较好地符合Arrhenius公式,由Arrhenius公式拟合给出的三种不同增原子的扩散势垒与表面结构和增原子表面结合能有关.Pd和Cu增原子从跳跃机制为主向交换机制为主的转换温度分别在825和937 K左右. 关键词： 表面扩散 分子动力学模拟     

Nanoscale self-organization using standing surface acoustic waves

The diffusion of an adatom on a substrate submitted to a standing surface acoustic wave is theoretically studied. By performing large scale molecular dynamic simulations, we show that the wave dynamically structures the substrate by encouraging the presence of the adatom in the vicinity of the maximum displacements of the substrate. Using an analytical model, we explain this feature introducing an effective potential induced by the wave. Applied in an atomic deposition experiment, this dynamic structuring process should govern the nucleation sites distribution opening the route to accurately control the self-organization process at the nanoscale.     

A first-principles study of oxygen adsorption and interaction with Al adatoms on Al(110)

We use first-principles density-functional theory to identify several stable binding sites for adsorbed O₂ and O on Al(110). Our calculations indicate that it is energetically favorable for O₂ to dissociate to two atoms on Al(110). When O₂ dissociates, it is energetically favorable for the resulting O atoms to exist as dimers. We identify several possible configurations for O dimers on this surface, and quantify atomic interactions between an Al adatom and these dimers. Our work provides insight into the initial stages of oxidation of Al(110), as well as the role of oxygen impurities in Al thin-film epitaxy.     

Surface diffusion of tungsten adatoms induced by low-energy bombardment with He atoms

Using low-temperature field ion microscope techniques, we studied at the atomic level the elementary events of radiation-induced surface diffusion of tungsten adatoms on the ideally pristine surface. The experiments on surface-damage formation and adatom displacements have been performed in situ with a source of neutral helium atoms with an energy of 5?keV. It was demonstrated that the low-energy He atomic bombardment at grazing incidence was able to induce a substantial surface mobility of adatoms. Most of the radiation-induced adatom displacements were oriented along the direction of irradiation. The long impact-induced jumps of adatoms, spanning more than a nearest-neighbor distance, were revealed. Performed molecular dynamics simulations reproduce the general experimental trends and conclusions.     

表面羟基辅助甲醛分子在金红石型TiO2(110)表面迁移动力学研究

As the photo-dissociation product of methanol on the TiO₂(110) surface,the diffusion and desorption processes of formaldehyde (HCHO) were investigated by using scanning tunneling microscope (STM) and density functional theory (DFT).The molecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation.The diffusion was found to be mediated by hydrogen adatoms nearby,which were produced from photodissociation of methanol.Diffusion of HCHO was significantly decreased when there was only one H adatom near the HCHO molecule.Furthermore,single HCHO molecule adsorbed on the bare TiO₂(110) surface was quite stable,little photo-desorption was observed during laser irradiation.The mechanism of hydroxyl groups assisted diffusion of formaldehyde was also investigated using theoretical calculations.     

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.     

Fabrication and modification of the supported Al nanoparticles on the Al(001) and (110) surfaces using the reversible vertical single-atom manipulation with the Cu and Pt trimer-apex tips: a first-principles study

First-principles simulations show that at the step edge of the stepped Al(001) and (110) surfaces and at the edge of the small supported nanoparticles like the dimer, trimer, and tetramer, single Al atoms can be extracted and repositioned using the Cu trimer-apex tip and the Pt tip of a Al apex, while a more weakly adsorbed single Al adatom on the plane terrace of the flat surface or of the nanocluster cannot be vertically picked up by these two tips. This result suggests in principle a non-electrically assisted method of fabricating and modifying metal nanoparticles at the atomic scale using the vertical single-atom manipulation. As an illustration, a pyramidal nanocluster of five Al atoms is assembled on the Al(001) surface in an atom-by-atom way with the Cu trimer-apex tip, moreover, it can be modified to be two-dimensional in shape.     

Potential energy of an isolated gold atom on the (100) surface of alkali halide crystals

The energy of an isolated gold atom on the (100) surface of several alkali halide crystals (NaCl, NaF, KCl, KBr) is calculated as a function of distance above the crystal surface. The calculations include dispersion, electrostatic and repulsive potentials and are performed by direct summation over near neighbours and integration over the rest of the crystal. Detailed potential maps of the (100) surface as seen by a gold adatom are obtained, preferred adsorption sites are identified, and surface quantities such as adsorption energies, minimum energy barriers for diffusion and vibrational frequencies are computed. The effects of relaxations of the (100) surface ions due to both the presence of the surface and the presence of the adatom are examined. Both are found to be significant. Some comparison with experimental results is made.     

Calculation of geometries and chemisorption energies of adatoms on simple metals

We report results of calculations with a formalism that in principle applies quite generally for chemisorption on a real metallic substrate. Including the substrate structure within perturbation theory on a self-consistent jellium-plus-adatom calculation, we have computed the dependence of the binding energy of an adatom on the surface geometry. Specifically, in the case of hydrogen on Al, our model calculation predicts that the stable positions are bridge configurations on the (100) and (110) surfaces and atop positions on the (111) surface, and that they have almost the same heats of chemisorption (1.8–2.0 eV). For geometrical reasons the bridge configuration seems to be a reasonable result while the atop result for the (111) surface is more uncertain. Thus, chemisorption of H on Al should require predissociation of the H₂ gas. In addition, the predicted values for hydrogen desorption imply that measurements on H on Al surfaces should be performed at low temperatures to avoid desorption. Results for H on a jellium of Na density indicates that hydrogen should be absorbed in rather than adsorbed on Na metal.