Ag-Co clusters deposition on Ag(100): an atomic scale study

The slowing down of Co₁₀Ag₁₉₁ and Co₂₈₅Ag₃₀₁ nanoclusters on a Ag (100) surface is studied at the atomic scale by means of classical Molecular Dynamics simulations. The slowing down energy, 0.25 to 1.5 eV/atom, is characteristic of low energy cluster beam deposition and aerosol focused beam techniques. The two clusters differentiate by their size, stoechiometry and structure. While Co forms one or several groups just beneath the cluster surface in Co₁₀Ag₁₉₁, Co₂₈₅Ag₃₀₁ displays a core-shell structure where Ag forms one complete monolayer around the Co core. As a consequence of the impact, the smallest cluster undergoes deep reorganization and becomes fully epitaxial with the substrate. The larger one only undergoes partial accommodation and partially retains the memory of its initial morphology. For both, after impact, the Co forms one group covered by Ag. The substrate damage is significant and depends on the slowing down energy. It results in a Ag step surrounding the cluster which may be more than one atomic layers high and isolated add-atoms or small monolayer islands apart from the step. The latter originate from the cluster and the former from the substrate. Further details in the consequences of the impact are given, concerning the cluster penetration, its deformation and lattice distortions, with emphasis on the cluster size and stoechiometry.Received: 11 June 2004, Published online: 31 August 2004PACS: 35.40.-c - 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 07.05.Tp Computer modeling and simulation     

Magnetism of Mg atomic chains on the NaCl(100) surface

Based on the first-principles calculations within the density-functional theory, we have investigated the magnetism of 1D Mg nanowires, both unsupported and supported by the NaCl(100) surface. It is shown that Mg can exhibit magnetism in both the geometry of linear and zigzag chains. The freestanding Mg linear chain shows magnetization when the bonds are compressed. For an Mg zigzag chain, however, ferromagnetism is predicted at the equilibrium bond length. It is found that the magnetism of Mg chains can be maintained when deposited on the NaCl(100) surface. We have also analyzed the results by using the Stoner theory and the calculated electronic band structures.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Ordering and segregation at (001) surfaces of Cu3Au

The interplay of surface ordering and segregation at the (001) surface of Cu₃Au is studied. The calculations are carried out using the tetrahedron approximation of the cluster variation method. The almost continuous disordering observed by low-energy electron diffraction and the Au enrichment observed by low-energy ion scattering spectroscopy can be consistently and accurately reproduced by our model.     

Formation of an ensemble of nanoclusters under rapid deposition of atoms on a surface

The results of an experimental study of the formation of nanometer-size Au clusters on NaCl(100) and HOPG(0001) surfaces under pulsed laser deposition are presented. No clusters of small sizes (d ≤ 1 nm) have been found in the cluster size distribution. The distribution itself at d < 5 nm has the form of a percolation distribution. It has been established that the perimeter of clusters with sizes d < 5 nm has a fractal structure. The fractal dimension of clusters is different for NaCl(100) and HOPG(0001) surfaces with different symmetries; it decreases with increasing cluster size from D _f ≈ 1.2–1.4 at d ≈ 1.5 nm to D _f ≈ 1 at d ≈ 5 nm. A physical mechanism of nanocluster formation is suggested. Under pulsed laser deposition, the attainable densities of adatoms are close to the percolation threshold in the region of thermodynamically unstable states and many-particle correlation regions are formed in a spatially inhomogeneous adsorbate. Clusters are formed on the surface from many-particle correlation regions in several diffusion jumps. The suggested mechanism allows the fractal dimension of the clusters forming on surfaces with different symmetries, its dependence on cluster size, and the cluster size distribution functions to be calculated.     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综(NVT)分子动力学,模拟了在10 K条件下,Au959团簇负载于MgO(100)表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大.     

Systematic study of Au6 to Au12 gold clusters on MgO(100) F centers using density-functional theory

We present an optimized genetic algorithm used in conjunction with density-functional theory in the search for stable gold clusters and O2 adsorption ensembles in F centers at MgO(100). For Au8 the method recovers known structures and identifies several more stable ones. When O2 adsorption is investigated, the genetic algorithm is used to imitate structural fluxionality, increasing the O2 bond strength by up to 1 eV. Extending the method to Au(6,10,12), strong O2 adsorption configurations are found for all sizes. However, the effect of fluxionality appears to wear off with increasing cluster size.     

Oxygen induced surface segregation of Cu on the Au0.7Cu0.3(100) surface

The oxygen induced surface segregation of Cu on the Au_0.7Cu_0.3(100) surface was investigated by means of LEED and AES techniques. The dissociative adsorption of O₂ did not take place on this clean surface for a long time exposure at least up to 10⁴ L, and so the oxygen was forcibly introduced onto the surface through a pre-deposition of few a layers of Cu and its successive oxidation. The oxygen coverage was controlled by a heat treatment, which leads the system to a thermal equilibrium state. For the clean surface, the segregation of Au was clearly observed and the surface concentration of Au was estimated to be about 86%, greater than the bulk concentration of 70%. At low coverages below 0.16 ML, no remarkable oxygen induced segregation of Cu was observed. But, above 0.2 ML, the surface concentration of Cu was proportional to the oxygen coverage. The (2 × 4) LEED pattern was observed in a wide range of oxygen coverage. The maximum intensity of the (2 × 4) was observed at about 0.45 ML.     

Atom deposition on surface clusters: thin film growth on Pd(111)

We have studied the vapor phase deposition of single metal atoms on two-dimensional clusters at the Pd(111) surface. The system considered here are heptamers and dodecamers for the substrate temperature of 100 K. Using molecular dynamics simulations with the corrected effective medium potentials for Pd, Cu and Rh, we have found that an adatom either diffuses on the cluster or incorporates itself into the cluster depending on the size of clusters and the kind of metals in the gas/cluster system. We also discovered that incorporation of a deposited gas atom into a surface cluster at 100 K was not a consequence of the reduction of the barrier heights at the step edges and that the type of post-impact dynamics for hexagonal heptamers depends on the impact point.     

A helium diffraction study of the reconstructed Au(100) surface

The reconstructed Au(100) surface has been investigated using He-diffraction techniques. The topmost layer is found to have a dominant (1 × 5) periodicity, whereby the corrugation appears one-dimensional in the direction of the longer unit cell. The complex LEED patterns observed for this surface must be due to secondary structural features such as minor atomic displacements in the topmost or lower layers. Several reconstruction models were considered and surface electron density calculations were performed to enable a comparison with the corrugation functions deduced from the He-diffraction data.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Image-potential-induced surface state at Si(100)

We have identified a surface state on Si(100) (2×1) at a binding energy of 0.69±0.05 eV with respect to the vacuum level. Band-structure calculations within the GW method reveal that almost 80% of the probability density of the resonance is located in front of the surface. We therefore assign the surface resonance to an image-potential state. It has a lifetime of about 10 fs and contributes significantly to two-photon photoemission from Si(100). PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

EELS of methane physisorbed on NaCl(100)

EELS spectra of CH₄, CD₄ and CH₂D₂ physisorbed on NaCl(100) at 40 K are presented. For the clean NaCl surface, increasing the incident beam energy to the 30 to 50 eV range is sufficient to overcome charging effects. However, when adsorbate is present charging prevents extended signal averaging. All methane modes appear to contribute to the EELS spectra. Loss peaks corresponding to adsorbate vibrations are very broad (400 to 600 cm⁻¹, FWHH), the low resolution being probably due to a combination of effects including surface disorder, scattering of electrons by gas phase molecules and charging effects.     

In-situ observation of an ordered sulfate adlayer on Au(100) electrodes

An ordered adlayer of sulfate or bisulfate has been found on the unreconstructed Au(100) surface in 0.1 M H₂SO₄ by in-situ STM. The ordered adlayer is formed concomitantly with lifting of the (hex)-reconstruction. The new structure consists of sulfate/bisulfate rows that run parallel to the edges of the islands that were created during the (hex)→(1×1) transition. The distance between the adions within the rows is 4.0±0.3 Å, and that between rows 10.4±0.7 Å, corresponding to a superstructure with a surface coverage of about 0.2.