Angle-resolved photoemission studies of a surface state for Ag overlayers on Cu(111)

We have performed an angle-resolved photoemission study of a surface state on Cu(111) covered by various thicknesses of Ag. The growth mode of Ag on Cu(111) was determined to be layer-by-layer despite the large mismatch between the two lattices. The Cu(111) surface state was observed to evolve monotonically for increasing Ag overlayer thickness to eventually become the Ag(111) surface state. The measured rate of shift of the surface state binding energy can be explained qualitatively in terms of the degree of localization of the surface-state wave functions.     

Adsorption-desorption properties and surface structural chemistry of chlorine on Cu(111) and Ag(111)

The adsorption, desorption, and structural properties of chlorine adlayers on Cu(111) and Ag(111) have been studied by LEED, Auger, Δ?, and thermal desorption measurements. Ancillary experiments were also carried out on cuprous chloride for purposes of comparison with the Cu(111)-Cl data. Chlorine adsorption is rapid on both metals and follows precursor kinetics, the absolute initial sticking probabilities being ~1.0 (Cu) and ~0.5 (Ag). Δ? results suggest that significant depolarisation of the chemisorption bond occurs at high coverages, the maximum values being + 1.2 eV (Cu) and + 1.8 eV (Ag). On Cu(111), adsorption leads to the formation of a sequence of well-ordered phases; in order of increasing coverage, these are as follows: (√3 × √3)R30°, (12√3 × 12√3)R30°, (4√7 × 4√7)R19.2°, and (6√3 × 6√3)R30°. On Ag(111) (√3 × √3)R30°, and (10 × 10) structures are observed. All six structures are susceptible to a straightforward interpretation in terms of coincidence lattices resulting from the progressive uniform compression of a hexagonal layer of Cl atoms. This interpretation is consistent with all the experimental results, and gives values for the nearest-neighbour ClCl spacing on both Cu(111) and Ag(111) which are in good agreement with other work on other surfaces. Chlorine desorbs exclusively as atoms from both metals with first-order desorption kinetics, and apparent desorption energies of 236 (Cu) and 209 (Ag) kJ mol^?1. These values, which depend on an assumed pre-exponential factor of 10¹³ s^?1, are shown to be inconsistent with the thermochemical constraints on the system necessitated by the complete absence of Cl₂ desorption. Lower limits for the pre-exponential factors are then deduced, and the values are found to be consistent with the differences between the CuCl and AgCl systems.     

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.     

The molecular dynamic study of anharmonic effects at Cu(111) and Ag(111) surfaces in the presence of Cu- and Ag-trimer island

The molecular dynamics (MD) technique based on semi-empirical potentials, is used to carry out the diffusion of Cu- and Ag-trimer on Cu- and Ag(111) surface at 300, 500 and 700 K temperatures. The constant energy MD simulation elaborates the anharmonic effects at the surface such as fissures, dislocations and vacancy creation, in the presence of island. The fissures and dislocations formed are in the range of 1.5–4 Å and 1–7 Å, respectively, from the island's position. The Cu and Ag islands both diffuse easily on Cu(111) surface, manipulate that the trend of diffusion is faster on Cu surface as compared to Ag surface. The process of breaking and opening of the island has also been observed. Moreover, a surface atom popped-up at 700 K by creating a vacancy near the Cu island on Ag surface. The rate of diffusion increases with the increase in temperature, both for homo- and hetero-cases.     

Wetting and structural transition induced by segregation at grain boundaries: a monte carlo study.

Wetting of the Sigma = 5 (310) <001> symmetrical tilt grain boundary (GB) close to the solubility limit in the Cu(Ag) solid solution has been observed by means of Monte Carlo simulations at T = 600 K. More precisely, a finite thickness film almost pure in Ag, separating the two initial Cu(Ag) grains, can be obtained from a critical intergranular germ induced by the strong segregation of Ag in the GB. As this film is actually a single crystal, this implies a complete rearrangement of the GB core structure. Thus the initial GB is replaced by two Cu(Ag)/Ag(Cu) interfaces. Evidence is presented for the increase of the film thickness when approaching the solubility limit, as expected in wetting phenomena.     

Density functional theory calculation of platinum surface segregation energy in Pt3Ni (111) surface doped with a third transition metal

We have used density functional theory method to calculate the Pt surface segregation energy in the Pt₃Ni (111) surface doped with a third transition metal M and thus investigated the influence of component M on the extent of Pt segregation to the outermost layer of these Pt₃Ni/M (111) surface. As a third component in the Pt₃Ni/M (111) surface, V, Fe, Co, Mo, Tc, Ru, W, Re, Os, and Ir were predicted to lead to even more negative Pt surface segregation energies than that in the based Pt₃Ni (111) surface; Ti, Cr, Mn, Cu, Zr, Nb, Rh, Hf, and Ta would still retain the preference of Pt segregation to the surface but with less extent than the replaced Ni, while Pd, Ag, and Au would completely suppress the Pt segregation to the Pt₃Ni/M (111) surfaces. Furthermore, we examined the relation between the Pt surface segregation energy in the Pt₃Ni/M (111) surfaces and the material properties (lattice parameter, heat of solution, and Pt surface segregation energy) of binary alloys Pt₃M. It was found that the surface energy effect, strain effect, and heat of solution effect induced by the doped element M would collectively affect the Pt surface segregation energy in the Pt₃Ni/M (111) surfaces.     

Manifestation of work function difference in high order Gundlach oscillation

Gundlach oscillation (or the standing-wave state) is a general phenomenon manifesting in the tunneling spectrum acquired from a metal surface using scanning tunneling spectroscopy. Previous studies relate the energy shift between peaks of the lowest-order Gundlach oscillation observed on the thin film and the metal substrate to the difference in their work functions. By observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111), and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely.     

Electronic structure and ordering of multilayers of Co and Ag on Cu(111) investigated by photoelectron spectroscopy

The growth and the electronic structure of multilayers of Co and Ag on Cu(111) at room temperature have been studied with photoelectron spectroscopy and low-energy electron diffraction (LEED). The coverage range spans from Co and Ag layers between one monolayer (ML) to stacking of several monolayers. Surface states and ordered structures have been identified at room temperature. A Ag-related surface state with a binding energy of 0.30 eV is identified in normal emission in the ultraviolet photoelectron spectra when silver constitutes the top layer. Core-level binding energy shifts of Ag 3d_5/2 reflect the changing surroundings of Ag. Hexagonal diffraction patterns are observed for sandwiches of consecutive layers of Co and Ag up to 5 layers. Since no interlayer diffusion is observed in the layer-by-layer formation of the films, multilayers of consecutive silver and cobalt on Cu(111) offer preparation of sandwiched magnetic–non-magnetic structures.     

Ag-Cu双金属团簇中Ag原子偏析行为的 分子动力学研究

采用了恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程。分析低温退火结构可得团簇中Ag原子的偏析行为：在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致。通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现：当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同。     

银和铜膜中异常晶粒生长和织构变化的实验研究

用透射电子显微镜(TEM)和x射线衍射(XRD)方法对经300℃，2h退火的Ag和Cu自由膜和Si基体上的Ag和Cu附着膜的异常晶粒生长和织构变化进行了实验研究.XRD分析表明：Ag和Cu沉积膜均有(111)和(100)择优取向.但经退火处理后，Ag和Cu自由膜的(111)织构稍有加强.相反，Si基体上的Ag和Cu附着膜的(100)和(110)织构明显加强，同时用TEM在Cu附着膜中观察到了两个(110)和四个(211)取向的异常大晶粒.根据表面能和应变能的各向异性对实验结果进行了分析.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.     

Scanning tunneling microscopy study of a Ag monolayer on Cu(111)

The structure of submonolayer Ag deposited on Cu(111) has been examined with scanning tunneling microscopy. The long diffusion length of Ag leads to the formation of large (111)-like islands nucleating from the “downhill” side of Cu step edges. The growth fronts of the Ag islands are mostly straight and lie along the 〈1&#x0304;10〉 directions. The Ag overlayer is apparently slightly compressed relative to bulk Ag, and forms a mixture of superstructures rotated slightly relative to the substrate.     

A combined XPS/AES study of Cu segregation to the high and low index surfaces of a Cu-Ni alloy

Quantitative information was obtained regarding the equilibrium segregation of Cu to the high index surfaces (cut 5° off the (100) plane in the [001] and [011] directions) and low index surfaces ((100), (111) and (110)) of a Cu: Ni, 5: 95% alloy crystal. Data regarding segregation to the (111) surface of a Cu: Ni, 50: 50% alloy crystal was also obtained. Equilibrium surfaces were obtained by careful annealing in the temperature range 850–920 K to avoid Cu loss by evaporation. The surface composition profile was calculated using data from the combination of the X-ray Photo-electron Spectroscopy and Auger Electron Spectroscopy techniques. Using these techniques, a wide range of electron kinetic energies and associated electron escape depths can be probed, yielding information about the topmost layer concentration and about the concentration profile into the bulk. Extensive Cu segregation was observed for the 5% and 50% Cu alloys. Topmost layer compositions of 85–100% Cu were found for both the high and low index surfaces of the 5% Cu alloy. In the next two or three atomic layers the composition was found to drop swiftly to near bulk Cu levels. For atomic layers deeper than this, some experimental evidence suggested a rise in Cu composition over three or four layers to 8–17% Cu before bulk levels were finally regained. For the 50% Cu alloy sample, a topmost layer composition was found of 95–100% Cu. Bulk levels of Cu were regained in about four atomic layers. These results are discussed in relation to other theoretical and experimental studies of segregation in these Cu-Ni alloys. The significance of the measured composition in relation to an equilibrium surface is also considered.     

用改进嵌入原子法计算Cu晶体的表面能

用改进嵌入原子法(MEAM)计算了Cu晶体12个晶面的表面能.结果表明，密排面(111)的表面能最小.其他晶面的表面能随其晶面与(111)晶面夹角的增加而增加，据此可以粗略地估计各晶面表面能的相对大小.给出的几何结构因子的确定方法及结果可以直接用于计算其他面心立方晶体的表面能及其他特性.在Cu,Ag等面心立方薄膜中出现(111)择优取向或织构的机理是表面能的最小化. 关键词： 改进嵌入原子法 铜 表面能 计算     

The anomalous RHEED patterns observed from (111)Ag-(111)Cu bilayers

RHEED patterns obtained from thin expitaxial bilayers consisting of Ag grown on smooth (111)Cu surfaces have been shown by Gradmann and Krause to contain anomalous reflections. Previously, these reflections were interpreted as due to a regular arrangement of interfacial dislocations or to multiple diffraction effects. In the present work a thin film technique was used to form smooth, easily detachable (111)Ag/Cu bilayers from which RHEED patterns containing these extra reflections were obtained. These films were grown, annealed, and examined in situ by UHV-RHEED techniques. They were then removed from the vacuum system, detached from their substrates and examined by TEM and TED. Thus, contrary to previous work, the present observations consisted of a series of experiments using RHEED, TED, and TEM carried out on the same specimen. No evidence was obtained to indicate that the anomalous reflections were due to interfacial dislocations. Rather, all the observed phenomena could be explained entirely by multiple diffraction effects.     

Mechanism of Cu transport along clean Si surfaces

Cu diffusion along clean Si(111), (110) and (100) surfaces are investigated by Auger electron spectroscopy and low energy electron diffraction. The effective diffusion coefficients of copper are measured in the temperature range from 500 to 650°C. It is shown that the Cu transport along silicon surface occurs by the diffusion of Cu atoms through Si bulk and the segregation of Cu atoms to the surface during the diffusion process. It is found that the segregation coefficients of Cu to silicon surface during the diffusion process depend on surface orientation.     

Adsorbed layers on Au(111) — The inelastic transitions

Measurements have been made of the inelastic energy changes between layers of inert gases adsorbed on Au(111) and He atoms. The monolayers appear to show dispersionless effects while, as the layers build up in thickness, and screen the stronger forces between the He and Au(111), the vibrational modes soften. A seven layer film clearly shows dispersion. The holding potential for Au surfaces is predicted to be larger than for Ag and Cu both of which are quite similar. Comparisons between the lowest energy transition in a monolayer on Au(111) and other faces of Ag and Cu are made.     

Exploring magnetic properties of ultrathin epitaxial magnetic structures using magneto-optical techniques

We describe magneto-optic Kerr effect studies of ultrathin Fe and Ni films on single crystal surfaces of Ag and Cu. Monolayer Fe films on Ag(100) exhibit the theoretically predicted spin-orbit anisotropy, but also yield some interesting discrepancies between behavior predicted by Kerr effect and by spin-polarized photoemission experiments. Layer-dependent studies of the magnetic moment of Ni on Ag(111) and Ag(100) suggest sp-d hybridization effects quench the first layer magnetic moment on Ag(111) but not on Ag(100). Temperature dependent studies of thin film magnetization obtained from Kerr effect measurements yield thickness dependent Curie temperatures, and critical exponents for several thin film systems.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

Temperature dependence of photo-hole decay in 4d derived Quantum Well States in monolayer Ag(111) films on Pd(111), Ni(111), Mo(110) and Cu(100)

We present experimental data on the temperature dependence of photo-hole decay obtained by Angle Resolved Photoemission (ARPES) measurements from 4d derived Quantum Well States (QWS) on Ag(111) monolayer films deposited on Pd(111), Ni(111), Mo(110) and Cu(100). We have found a significant increase of the Ag 4d electron–phonon (e-ph) coupling strength with respect to the bulk values. The increase is attributed to different mechanisms that are associated with the interaction of the Ag film with under laying substrate. It is proposed that the main channels that contribute to the increased e-ph coupling originate from the inter-band transitions that involve bulk states of the substrates.