Ag(111)和Au(111)上铋的初始生长行为

半金属铋(Bi)的表面合金具有的Rashba效应,和其具体结构性质有重要关联.本文结合扫描隧道显微镜(STM)和密度泛函理论(DFT),系统地研究了Bi原子在Ag(111)和Au(111)上的不同初始生长行为.在室温Ag(111)上,连续的Ag2Bi合金薄膜会优先在Ag台阶边缘形成;在570 K Ag(111)上,随着...     

Structure of V thin films on Al(100) using XPD,LEED, and LEIS

We have used X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and low energy ion scattering (LEIS) to determine the atomic structure of V thin films grown on the Al(100) single crystal surface. For V film thicknesses ranging from 0.75 ML–7 ML, the LEED patterns show no significant changes from the p(1 × 1) symmetry of a clean Al(100) surface, other than becoming more diffuse for higher V coverage. XPD and LEIS spectra indicate that during the initial deposition (1–3 ML) V atoms diffuse into the Al lattice, and tend to accumulate at the surface during subsequent deposition. Strain associated with the lattice mismatch for V and Al is likely relieved by the formation of a surface alloy during the first few ML's of V deposition. For 7 ML V coverage, XPD and LEED showed that an ordered V structure is stabilized on the Al(100) surface. Due to the close resemblance of the V and Al XPD polar scans, we conclude that V and Al are occupying similar lattice sites in tetragonally distorted bcc lattice, although the specific lattice location of the V and Al atoms is not clear.     

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.     

Photoemission study of Gd metal overlayers on a GaAs(110) surface

We have studied the chemical reactions of Gd metal on an in situ cleaved GaAs(110) surface by photoemission spectroscopy of Ga 3d and As 3d core-levels as well as the Gd 4f level on- and off-resonance valence band using synchrotron radiation. We find that the Fermi-level pinning is completed before 0.13 ML coverage, and the deposited Gd atoms start to react with the GaAs substrate at a very low coverage (critical coverage < 0.067 ML). As more Gd atoms are deposited, they form stable compounds with As atoms which are then trapped in the relatively narrow interfacial layer of thickness less than about 3.3 ML, while Ga atoms diffuse out towards the surface and eventually become metallic. The thickness of the GdGa intermixed layers is estimated to be about 6.7 ML, which is somewhat greater than that for a interface.     

Electrical conductance at initial stage in epitaxial growth of Pb, Ag, Au and In on modified Si(1 1 1) surface

The electronic properties of thin metallic films of Pb, Ag, Au and In atoms deposited at 105 K on well defined metallic surface, i.e. Si(1 1 1)-(6 × 6)Au surface with 10 ML of annealed Pb, were investigated using four-point probe method in UHV condition. The structure of the substrate and deposited metals were monitored by the RHEED system. The electrical conductance, measured during the deposition of In and Pb atoms, shows the local minimum for the coverage equals about 0.3 ML whereas for Au and Ag atoms the conductance decreases during the first monolayer growth. For Au atoms the local maximum in the conductance was observed for the coverage about 0.55 ML, which can be connected with localized states. To describe theoretically the conductance behavior the tight-binding Hamiltonian and equation of motion for the Green’s function were used and good qualitative agreement was obtained.     

Influence of the metal substrate on the adsorption properties of thin oxide layers: Au atoms on a thin alumina film on NiAl(110)

Thin oxide films grown on metal substrates are widely used in surface science to model bulk oxides, assuming their chemical and electronic properties to be similar. In some cases, however, this might not be justified as the present scanning tunneling microscopy studies demonstrate for Au atoms on a thin alumina film on NiAl(110). Au atoms were evaporated onto the oxide film at a sample temperature of approximately 10 K. At low coverage, this leads to the formation of one-dimensional clusters with unusually large Au-Au distances of 5.6-6.0 A. A direct interaction between the Au atoms can be excluded, and a substrate-mediated mechanism is supposed instead. This assumption is strengthened by the finding that the Au chains exhibit a preferential orientation: They are almost aligned with the [001] direction of the NiAl(110) substrate, clearly indicating that the metal substrate participates in the binding of the Au atoms.     

Formation of and phase transitions in electrodeposited tellurium atomic layers on Au(1 1 1)

Detailed studies of the structures formed by the electrodeposition of atomic layers of Te on Au(1 1 1) surfaces from aqueous solutions were performed using in situ scanning tunneling microscopy (STM), as well as by UHV-EC techniques such as low energy electron diffraction and Auger electron spectroscopy. There are two features in the voltammetry that may be considered underpotential deposition (UPD). However, from the voltammetry, it is clear that the deposition process is kinetically slow, and from this study it appears that several atomic layer structures are actually formed at overpotentials. Prior to deposition, a surface excess of a tellurium oxide species coats the surface. This layer is then converted to a Au(1 1 1)(√3×√3)R30°–Te structure with an array of domain walls, at 1/3 ML. The initial structure appears to have a symmetric array of walls, resulting in a (13×13) periodicity, which then converts to a less symmetric structure where the domain walls form rhombi, with a larger periodicity. During the second UPD feature, the coverage increases, forming a (√7×√13) unit cell at 0.36 ML and then a (3×3) at 0.44 ML. Commensurate with the formation of these higher coverage structures, a roughening transition takes place, where the surface becomes pitted, resulting in about 40% of the surface being covered with single atom deep pits. This process appears to be related to the pits formed in the surfaces of self-assembled monolayers (SAM) of thiols on Au surfaces, and layers of Se and S on Au surfaces. Several theories have been suggested to account for these pits. The model that appears to best explain the pits is based on shrinking of the size of the underlying Au atoms, reconstructing the underlying Au. There also appears to be a high coverage structure, near 0.9 ML, that forms at potentials near where the (3×3) forms, but only by holding the potential for an extended period of time. Subsequent dissolution of this high coverage structure produces domains of disordered Te atoms, which gradually decrease in coverage until the (3×3) is again formed at 0.44 ML.     

The initial growth of Au on GaAs(001)-c(4 × 4)

The metal growth when depositing a monolayer (ML) of Au at 200° C on MBE-grown surfaces of GaAs(001)-c(4 × 4) was studied by AES and RHEED. The surface interaction can be characterized to proceed in two stages depending on the surface coverage of Au. At a coverage of less than 0.3 ML the gold atoms are mainly dispersed on the surface with a small in depth diffusion. Above 0.4 ML there is a rapid intermixing and a tendency of arsenic accumulation to the surface.     

Adsorption of Ag and Au atoms on wurtzite ZnO (0001) surface

We perform first-principles calculations to investigate various surface structures in the absorption of Ag and Au atoms on wurtzite ZnO (0001) surface. The results show that both Ag and Au atoms prefer to be absorbed on the H₃ sites (the center of Zn–O ring) of the surface, and the most favorable monolayer (ML) coverage is 1. The calculated electron structure shows that the Ag- and Au-adsorbed ZnO (0001) surfaces exhibit metallic characteristics even the ML coverage of the adatoms is very low. Finally, the work functions of Ag- and Au-adsorbed ZnO (0001) surfaces are calculated and discussed for the first time in the present work.     

LEED-AES-TDS characterization of Sb overlayers on GaAs(110)

The GaAs(110)-Sb system is studied with Auger Electron Spectroscopy, Low Energy Electron Diffraction, Soft X-Ray Photoemission Spectroscopy and Thermal Desorption. Sb evaporated at room temperature forms a continuous film and a sharp interface with the substrate. For a coverage of one monolayer, LEED indicates a well ordered (1 × 1) structure which produces diffracted intensities very different from those measured from the clean substrate. Thermal desorption experiments show the particularly strong bonding between the first Sb monolayer and the substrate, confirming the large chemical stability observed during photoemission experiments. Two types of structures were considered for GaAs(110)-Sb(1 ML). Sb chains extending along the (110) direction, parallel and anti-parallel to the top layer Ga-As chains, and Sb dimers placed above the surface with one Sb bond to the surface Ga. The preliminary LEED analysis favors the later model.     

氯原子在Cu(111)表面的吸附结构和电子态

密度泛函理论(DFT)总能计算研究了不同覆盖度下氯原子在Cu(111)表面的吸附结构和表面电子态。计算结果表明,清洁Cu(111)表面自由能 为15.72 ,表面功函数φ为4.753eV。在1/4ML和1/3ML覆盖度下,每个氯原子在Cu(111)表面fcc谷位的吸附能分别等于3.278eV/atom和3.284eV/atom。在1/2ML覆盖度下,两个紧邻氯原子分别吸附于fcc和hcp谷位,氯原子的平均吸附能为2.631eV/atom。在1/3ML覆盖度下,fcc和hcp两个位置每个氯原子吸附能的差值约为2meV/atom,与正入射X光驻波实验结合蒙特卡罗方法得到结果(<10meV/atom)基本一致。在1/4ML、1/3ML和1/2ML覆盖度下,吸附后Cu(111)表面的功函数依次为5.263eV、5.275eV和5.851eV。吸附原子和衬底价轨道杂化形成的局域表面电子态位于费米能级以下约1.2eV、3.6eV和4.5eV等处。吸附能和电子结构的计算结果表明,氯原子间的直接作用和表面铜原子紧邻氯原子数目是决定表面结构的两个重要因素。     

Coverage-dependent molecular tilt of carbon monoxide chemisorbed on Pt{110}: A combined LEED and DFT structural analysis

The adsorption of carbon monoxide on the Pt{110} surface at coverages of 0.5 ML and 1.0 ML was investigated using quantitative low-energy electron diffraction (LEED IV) and density-functional theory (DFT). At 0.5 ML CO lifts the reconstruction of the clean surface but does not form an ordered overlayer. At the saturation coverage, 1.0 ML, a well-ordered p(2 × 1) superstructure with glide line symmetry is formed. It was confirmed that the CO molecules adsorb on top of the Pt atoms in the top-most substrate layer with the molecular axes tilted by ± 22° with respect to the surface normal in alternating directions away from the close packed rows of Pt atoms. This is accompanied by significant lateral shifts of 0.55 Å away from the atop sites in the same direction as the tilt. The top-most substrate layer relaxes inwards by ? 4% with respect to the bulk-terminated atom positions, while the consecutive layers only show minor relaxations. Despite the lack of long-range order in the 0.5 ML CO layer it was possible to determine key structural parameters by LEED IV using only the intensities of the integer-order spots. At this coverage CO also adsorbs on atop sites with the molecular axis closer to the surface normal (< 10°). The average substrate relaxations in each layer are similar for both coverages and consistent with DFT calculations performed for a variety of ordered structures with coverages of 1.0 ML and 0.5 ML.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

Submonolayer structures of Ce adsorbed on Mo(110) surface

The surface structure of Ce adsorbed on a Mo(110) surface was studied by reflection high energy electron diffraction. Five kinds of surface structures were found with different deposition coverages in the submonolayer region. These structures have a common feature that Ce atoms possess a periodicity with double spacing at the atomic rows in the Mo[ 10] direction. Observed at 1 ML of Ce coverage was a surface structure related to a deformed fcc Ce(111) plane.     

Alloy formation at the Ni–Al interface for nickel films deposited on Al(110) surfaces

Alloy formation at the Ni–Al interface for thin nickel films deposited on Al(110) surfaces has been studied using high-energy ion scattering/channeling (HEIS) and X-ray photoelectron spectroscopy (XPS). For nickel atoms deposited at room temperature on Al(110), a large amount of nickel–aluminum intermixing occurs at the interface. For the first two monolayers (ML) of deposited nickel, an NiAl-like compound is formed. The intermixing continues with a different rate, forming an Ni₃Al-like compound for nickel coverages from 2 to 8 ML, at which point a nickel metal film begins to grow on the surface. Nickel atoms deposited at 250°C on the Al(110) surface exhibit no surface compound formation, but diffuse up to 400 Å into the aluminum substrate. Interatomic potentials based on the embedded-atom method (EAM) are used in a Monte Carlo approach to simulate the evolution of the Ni–Al(110) interface as a function of the nickel coverage. The calculated ion-scattering yields and X-ray photoelectron intensities from nickel and aluminum atoms in these simulated interfaces are in good quantitative agreement with the experimental results. The simulations show a high-density Ni–Al alloy forming at the Al(110) surface which apparently inhibits outward diffusion of aluminum, leading to the more nickel-rich alloy and finally nickel film growth. The ion-scattering simulations show an unusually large amount of backscattering occurring below the Ni–Al(110) interface, apparently associated with defocusing of the incident ion beam.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

STM studies on the growth of monolayers: Co on Cu(110) with one half monolayer of preadsorbed oxygen

The growth of the first cobalt monolayer (ML) on the Cu(110)-(2×1)O surface was studied by scanning tunneling microscopy. Extensive exchange of Cu and Co atoms takes place in the first stages of the deposition. The displaced Cu atoms form new Cu---O---Co mixed islands, with the same structure as those of the terrace surface. At 0.25 ML Co, a new structure nucleates, which contains three Cu atoms, four Co atoms and two O atoms per 2×2 cell. The structure consists of rows in the [ 10] direction with an internal periodicity of two lattice units. The rows are separated from one another by two lattice units along the [001] direction, and are found both in-phase and out-of-phase relative to one another. The result is a mixed p(2×2) and c(2×4) surface. The fraction of the surface covered by the new structure increases with Co coverage, and completely covers the surface at 1 ML Co.     

MOLECULAR DYNAMICS SIMULATION OF ENERGETIC ATOM DEPOSITIONS OF Au/Au(100) FILM

The energetic atom deposition of thin Au/Au(100) film has been studied by molecular dynamics simulation using the Au-Au interatomic interaction potential with embedded atom method. By investigating the variation of coverage curves and Bragg diffraction intensities during the film growth, the transition of Stranski-Kranstanov growth mode to Frank-van der Merwe growth mode was observed with the increase of the incident energy of deposition atoms. The role of energetic atoms in the film growth is discussed by analyzing the transport properties of deposited atoms and the evolution of incident energy and substrate temperatures.     

Enhanced activity for supported Au clusters: Methanol oxidation on Au/TiO2(110)

Gold clusters supported on TiO₂(110) exhibit unusual activity for the oxidation of methanol to formaldehyde. Temperature programmed desorption studies of methanol on Au clusters show that both Au and titania sites are necessary for methanol reaction. Isotopic labeling experiments with CD₃OH demonstrate that reaction occurs via OH bond scission to form a methoxy intermediate. When the TiO₂ surface is oxidized with ¹⁸O₂ before or after Au deposition, methanol reaction produces H₂¹⁸O below 300 K, indicating that oxygen from titania promotes OH bond scission and is incorporated into desorbing products. XPS experiments provide additional evidence that during methanol reaction on the Au/TiO₂ surface, methanol adsorption occurs on TiO₂, given that the titania support becomes slightly oxidized after exposure to methanol in the presence of Au clusters. While the role of TiO₂ is to dissociate the OH bond and form the reactive methoxy intermediate, the role of the Au sites is to remove hydrogen from the surface as H₂, thus preventing the recombination of methoxy and hydrogen to methanol. The decrease in formaldehyde yield with increasing Au coverage above 0.25 ML suggests that reaction occurs at Au–titania interfacial sites; scanning tunneling microscopy images of various Au coverages confirm that the number of interfacial sites at the perimeter of the Au clusters decreases as the Au coverage is increased between 0.25 and 5 ML.     

Bi induced step-flow growth in the homoepitaxial growth of Au(1 1 1)

Homoepitaxial growth of Au on Bi-covered Au(1 1 1) was studied at room temperature using reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). From observations of RHEED it is found that the Au(1 1 1) (23 × 1) reconstruction structure changes to a (1 × 1) by about 0.16-0.5 ML deposition of Bi and to a (2√3 × 2√3)R30° by about 1.0 ML deposition of Bi, respectively. The surface morphology evolution by Bi deposition leads to a change of Au homoepitaxial growth behavior from layer-by-layer to step flow. This indicates that the surface diffusion distance of Au atoms on the Bi-precovered (1 × 1) and (2√3 × 2√3)R30° surfaces is longer than that on the Au(1 1 1) (23 × 1) clean surfaces. A strong surface segregation of Bi was found at top of surface. It is concluded that Bi atoms acted as an effective surfactant in the Au homoepitaxial growth by promoting Au intralayer mass transport.