理论预测晶体表面吸附二维原子岛的形状

提出了凝结势概念用以建立一种预测晶体表面吸附二维原子岛几何结构的理论方法.基于半经验相互作用势（SEAM势和OJ势）的计算表明,同相外延生长的二维原子岛在Cu和Ag的（111）面呈现正六边形结构, 而在Pt（111）面呈现截角三角形状;Cu和Ag的（100）面二维岛则形成正方形.这些理论预测均与实验观测结果一致.由于凝结势的计算不受原子数量的限制,该模型可普遍应用于预测各种表面二维原子岛形状. 关键词： 表面吸附 二维原子岛 量子点     

Long-range electronic interactions at a high temperature: bromine adatom islands on Cu(111)

Long-range electronic interactions between Br adatom islands, which are formed at approximately 600 K, on Cu(111) are mediated by substrate surface-state electrons at that elevated temperature. Using scanning tunneling microscopy at 4 K, we have quantified nearest neighbor island separations and found favored spacings to be half-multiples of the Fermi wavelength of Cu(111). The strong interaction potential and decay length of the interisland interactions are discussed in terms of the interaction of Br with the substrate surface state.     

Cu(111)三维表面岛对表面原子扩散影响的分子动力学研究

采用嵌入原子方法的原子间相互作用势，利用分子动力学方法计算了同质外延生长中不同层数的三维Cu(111)表面岛上表面原子扩散激活能，分析了三维表面岛的层数对表面原子交换扩散和跳跃扩散势垒的影响. 研究结果表明，二维Enrilich-Schwoebel(ES)势垒小于三维ES势垒，且三维ES势垒不随表面岛层数的增加而显著变化. 对于侧向表面为(100)的表面岛，表面原子沿〈011〉方向上的扩散行为，随表面岛层数增加而逐渐变化；在表面岛层数达到3层时，扩散路径上的势垒变化趋于稳定，表面原子扩散以下坡扩散为主. 对于侧面取向为(111)的表面岛，当表面岛层数大于3层后，开始呈现上坡扩散的可能. 关键词： 表面原子 扩散 分子动力学模拟     

Reversible shape transition of pb islands on Cu(111)

Using low-energy electron microscopy, we have observed a reversible transition in the shape of Pb adatom and vacancy islands on Cu(111). With increasing temperature, circular islands become elongated in one direction. In previous work we have shown that surface stress domain patterns are observed in this system with a characteristic feature size which decreases with increasing temperature. We show that the island shape transition occurs when the ratio of the island size to this characteristic feature size reaches a particular value. The observed critical ratio matches the value expected from stress domains.     

Quantum size effects in adatom island decay

The decay of hexagonal Ag adatom islands on top of larger Ag adatom islands on a Ag(111) surface is followed by a fast-scanning tunneling microscope. Islands do not always show the expected increase in decay rate with decreasing island size. Rather, distinct quantum size effects are observed where the decay rate decreases significantly for islands with diameters of 6, 9.3, 12.6, and 15.6 nm. We show that electron confinement of the surface state electrons is responsible for this enhancement of the detachment barrier for adatoms from the island edge.     

Vacancy-mediated and exchange diffusion in a Pb/Cu(111) surface alloy: concurrent diffusion on two length scales

Pb diffuses in a Pb/Cu(111) surface alloy predominantly by exchange with surface vacancies and, much less frequently, by exchange with thermal Cu adatoms. Because the infrequent adatom exchanges transport Pb atoms much farther, both processes affect observations of Pb transport in the Pb/Cu(111) surface alloy.     

Reversible Vertical Manipulations of Single Pt Adatom on Pt（111） Surface with a Triple-Apex Tip

With a triple-apex tip, we investigate theoretically the vertical manipulation of single Pt adatom on the Pt（111） surface. The adatom adsorbed on the f cc site of the flat Pt（111） surface can be transferred vertically to the tip by adjusting the tip height properly. Moreover, based on the strong vertical trapping ability and the relatively weak lateral trapping ability of the tip, we propose a simple method to realize a reversible vertical manipulation of the Pt adatom from the highly coordinated sites, the kink and the step sites, of the stepped Pt（111） surface. All the vertical manipulations are completed using only the atomic force between the tip and the adatom, without the electric field.     

Catalytic active sites on sputtered metal interfaces

By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied “π” as well as “σ” states at the Fermi level. Simple group theoretical arguments give that the “π” states, which represent the highest occupied molecular orbitais (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π¹, assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these “π” states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived “σ” states are introduced close to the Fermi level.     

Atom transfer and single-adatom contacts

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump to contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the adatom and the surface atoms.     

动力学晶格蒙特卡洛方法模拟Cu薄膜生长

利用动力学晶格蒙特卡洛方法模拟了Cu薄膜在Cu(100)面上的三维生长过程。模型中考虑了四个动力学过程:原子沉积、增原子迁移、双原子迁移和台阶边缘原子迁移,各动力学过程发生的概率由多体势函数确定。讨论了基底温度、沉积速率及原子覆盖率对Cu原子迁移、成核和表面岛生长等微观生长机制的影响;获得了Cu薄膜的表面形貌图并计算了表面粗糙度。模拟结果表明,随基底温度升高或沉积速率下降,岛的平均尺寸增大,数目减少,形状更加规则。低温时,Cu薄膜表现为分形的离散生长,高温时,Cu原子迁移能力增强形成密集的岛。Cu薄膜表面粗糙度随着基底温度的升高而迅速减小;当基底温度低于某一临界温度时,表面粗糙度随原子覆盖率或沉积速率的增大而增大;当基底温度超过临界温度时,表面粗糙度随原子覆盖率或沉积速率的变化很小,基本趋于稳定。     

Surface-state localization at adatoms

Low-temperature scanning tunneling spectroscopy of magnetic and nonmagnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.     

Energy barriers for interlayer diffusion in Pt/Pt(111) and Rh/Rh(111) homoepitaxy: Small islands

We performed detailed molecular statics calculations of energy barriers for various adatom movements in the vicinity of small island on (111) surfaces of Pt and Rh using Rosato-Guillope-Legrand interatomic potential. We found that for both systems the exchange processes are energetically favorable in comparison with direct jumps of an adatom. We observed that Ehrlich-Schwoebel barriers for exchange decrease significantly on small islands in comparison to single step on the surface. A comparison of both materials is presented. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. Financial support for this work was provided by the COST project P3.80.     

Size-dependent detachment-limited decay kinetics of two-dimensional TiN islands on TiN(111)

In situ high-temperature (T(a)=1050-1250 K) scanning tunneling microscopy was used to determine the coarsening and decay kinetics of two-dimensional TiN adatom and vacancy islands on atomically smooth TiN(111) terraces. We report the first observation of an abrupt decrease in decay rates, irrespective of T(a), of adatom islands with areas less than a critical value of 1600 A(2). However, no decay rate transition was observed for vacancy islands. We attribute the size-dependent island decay behavior, which is consistent with detachment-limited kinetics, to anisotropic attachment and detachment barriers.     

Bonding configurations and collective patterns of Ge atoms adsorbed on Si(111)-(7 x 7)

We report scanning tunneling microscopy observations of Ge deposited on the Si(111)-(7 x 7) surface for a sequence of submonolayer coverages. We demonstrate that Ge atoms replace so-called Si adatoms. Initially, the replacements are random, but distinct patterns emerge and evolve with increasing coverage, until small islands begin to form. Corner adatom sites in the faulted half unit cells are preferred. First-principles density functional calculations find that adatom substitution competes energetically with a high-coordination bridge site, but atoms occupying the latter sites are highly mobile. Thus, the observed structures are indeed more thermodynamically stable.     

Island formation and manipulation of prochiral azobenzene derivatives on Au(111)

Based on previous work with very similar azobenzene derivatives, this study of para-TBA (2,2',5,5'-tetra-tert-butylazobenzene) molecules aims to identify single intact molecules and investigate their adsorption behavior on a Au(111) surface. The molecules are found to be mobile on the surface at the deposition temperature, leading to highly ordered and enantiomerically pure molecular islands. Voltage pulses between the surface and the tip of a scanning tunneling microscope are used to change the chirality of the adsorbate molecules. On the Cu(111) surface instead, single molecules are found on the terraces, which points to a stronger molecule-substrate interaction.     

First-principles calculation of the single impurity surface Kondo resonance

We perform first-principles calculations of the surface and bulk wave functions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co. By analyzing the calculated hybridization energies, we find the bulk states dominate the contribution to the Kondo temperature, in agreement with recent experiments. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances (<6 A). Our results indicate the need for a new formulation of the problem at larger distances.     

Nuclear magnetic resonance study of epitaxial Co layers on single-crystal substrates

Nuclear magnetic resonance (NMR) is a powerful tool to study the local structure of ultrathin magnetic films and multilayers. This is demonstrated with an NMR study of ultrathin molecular beam epitaxy-grown Co layers on single-crystal Cu(111), Cu(110) and Pd(111) substrates. Co on Cu(111) results in a mixture of fcc and hep phases and the Cu(111)/Co interface is near to perfect. Co on Cu(110) grows mainly fcc in long stripe-shaped islands with rough Co/Cu interfaces. Finally, Co on Pd(111) gives a (111)-oriented fcc structure. The top surface is flat, but the Pd(111)/Co surface is intermixed.     

The effect of embedded Pb on Cu diffusion on Pb/Cu(1 1 1) surface alloys

We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect.     

Excitation of frustrated translation and nonadiabatic adatom hopping induced by inelastic tunneling

The dynamics of lateral manipulation for cobalt/Cu(111) has been investigated combining the model of vibrational heating and first-principles density functional calculations. The frustrated translational mode responsible for lateral excitation is identified as a vibrational resonance involving a concerted motion between the adatom and surface phonons. The calculated frequency shows good agreement with the onset energy for adatom hopping induced by inelastic tunneling. Simulation of the power law, compared with experiment, suggests that the atom hopping overcomes a nonadiabatic barrier due to the nonequilibrium local heating of the translational mode.     

Monolayered adatom aggregation induced by metallofullerene molecules on Cu(100)

The adsorption and self-assembly of Gd@C₈₂ molecules on Cu(100) surface have been investigated using scanning tunneling microscopy (STM). The metallofullerene molecules in the assemblies showed two characteristic apparent heights in the STM images. STM manipulation and spectroscopy was performed and revealed the formation of Cu adatom islands underneath the Gd@C₈₂ molecules. The monolayered Cu aggregates were resulted from the adatom–molecule complexation, which is supported by density functional theory (DFT) calculations that show charge transfer and electrostatic interactions between Gd@C₈₂ and adatoms. In addition, sub-molecularly resolved STM images demonstrated the structural and orientational ordering of Gd@C₈₂ assemblies upon thermal annealing. DFT calculations demonstrated that Gd atom located at the lower part of the carbon cage is a favored adsorption configuration for Gd@C₈₂ molecules adsorbed on Cu(100).