无格点基底表面的杂质分布对分枝状凝聚体的影响

根据含杂质熔融玻璃表面金原子凝聚的实验规律，在原子团簇具有随机的线扩散步长和刚性转动角的特征条件下，建立了含杂质无格点基底表面上改进的杂质限制团簇-团簇(IRCCA)凝聚模型.对团簇的扩散、刚性转动以及凝聚全过程进行了计算机模拟，系统地研究了杂质区域分布情况对分枝状凝聚体诸多特性的影响.结果表明规则分布的杂质对凝聚体生长的影响比随机分布的杂质大，导致杂质规则分布的基底表面上的分枝状凝聚体的数密度更大，分枝状凝聚体的回旋半径，凝聚体平均大小及分形维数更小. 关键词： 薄膜生长 Monte Carlo模拟 分形 杂质     

Role of Au(+) in supporting and activating Au(7) on TiO(2)(110)

The adhesion properties and catalytic activity of rutile TiO(2)(110)-supported Au(7) nanoclusters in different oxidation states are investigated by means of density functional theory. The calculations cover both surface science conditions of reduced TiO(2) and real catalyst conditions of oxidized (alkaline) TiO(2) supports. Large adhesion energies of Au(7) are found only when modeling real catalysts where the cluster becomes cationic with Au(+) ions in Au-O or Au-OH bonds. The full catalytic cycle for oxidation of CO by O(2) over Au(7) on alkaline TiO(2)(110) is calculated and found to involve only small activation barriers. In the presence of the CO reductant, the Au(+) sites are capable of cycling between bonding of atomic and molecular oxygen. We confirm our findings by comparison of calculated and experimental infrared stretch frequency data for adsorbed CO.     

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.     

Computer simulations of two-phase flow with surface tension in the percolation cluster

Percolation clusters with varying occupation probability were constructed. Viscous fingering (VF) in the percolation cluster, based on the assumption that throat radii are Rayleigh distributed, is investigated by means of a successive over-relaxation technique. The fractal dimension and the sweep efficiency of VF in the percolation cluster when surface tension is considered are larger than when surface tension is neglected. The fractal dimension of VF will increase as the percolation probability increases or the viscous ratio decreases. VF's fractal dimension of porous media in the limit viscous ratio → ∞ is found to be identical with the DLA. The topology and the geometry of the porous medium have a strong effect on the displacement processes and the structure of the VF.     

Study of nanoclusters growth at initial stages of ultrathin film deposition by kinetic modeling

Comparative analysis of Au, Cu, Pt, Ni and Fe nanoclusters growth on amorphous carbon substrate by proposed kinetic model based on rate equations is present. Partial sticking coefficients introduced into the model let to discriminate elementary processes such as adatom adsorption and diffusion on bare substrate and on top of islands, nucleation and mobility of islands and its coalescence, 2-d and 3-d island growth modes. The quantitative fittings of experimental time dependencies of surface coverage, clusters density, cluster size are performed by solving model equations. From the best fittings the values of phenomenological coefficients defining elementary processes are found for different materials. Comparative analysis of those coefficients let to discover mechanisms of nanoclusters formation and growth of different materials. It is shown that clusterization for Cu and Au is more favorable than for Pt and Ni. Diffusivity for Pt and Ni on amorphous carbon (a-C) substrate is significantly less than for Au and Cu. In opposite, diffusivity on the top of islands for Ni and Pt is significantly higher than for Au and Cu. The mobility of islands for Au and Cu is much higher than for Ni and Pt. The fitting of experimental curves of Fe deposition on a-C at different temperatures showed that temperature mainly influences sticking process but not diffusion.     

Synthesis of LECBD grown cluster assembled SeO2 thin films

Cluster assembled selenium oxide (SeO₂) thin films, as a function of oxygen flow pressure (OFP) have been synthesized by a low energy cluster beam deposition (LECBD) technique. The OFP dependent surface morphology leading to well separated nanoclusters (size ranging from 50 to 200 nm) and fractal features are confirmed from transmission electron microscopic (TEM) measurements. A diffusion limited aggregation (DLA) mediated fractal growth with dimension as 1.71 ± 0.01 has been observed for high OFP (60 mbar). Structural analysis by glancing angle X-ray diffraction (GXRD) and selected area diffraction (SAD) studies identify the presence of tetragonal phase SeO₂ in the deposit. Micro-Raman studies indicate the shifts in bending and stretching vibrational phonon modes in cluster assembled SeO₂ as compared to their bulk counter part due to the phonon confinement effect.     

Study of the tunnel conductance and electronic screening in nanoclusters of d metals on graphite surface

The evolution of the electronic structure of pulsed-laser-deposited nanoclusters of noble and transition metals (Au, Cu, Ni, Co, Cr) on a graphite surface with a decrease in their size from 10 to 1 nm has been investigated by scanning tunnel spectroscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The data obtained suggest that the properties of nanoclusters differ from metallic when the lateral cluster size reaches ～2–3 nm and the height reaches ～1 nm and that the shape of clusters (transition from three-dimensional to two-dimensional ones) significantly affects their transition to the nonmetallic state.     

On the size dependence of the heats of melting of metal nanoclusters

The size dependence of the heats of melting of transition metal nanoclusters (Ni, Au, Cu and Ag) is investigated using two alternative methods of computer simulation (isothermal molecular dynamics and Monte Carlo). Au and Cu nanoclusters are subjected to more detailed study over a wide range of their sizes. The heats of melting are shown to fall along with nanocluster size and to tend in some approximation to a linear dependence on inverse particle radii.     

On the melting of gold nanoclusters formed by pulsed laser deposition on different substrates

The experimental results are presented for the backscattering of 500-eV electrons on Au nanoclusters formed on the surface of highly oriented pyrolytic graphite HOPG(0001) and amorphous SiO₂. It has been found that the measured intensity of the elastically backscattered electrons nonmonotonically depends on the size of nanoclusters. It has been shown that the observed features can be explained by an increase in the rms deviation of the atoms of the Au nanocluster with a decrease in its size. The difference in the dependence of the rms deviation of atoms on the size of the nanoclusters formed on the surfaces of HOPG(0001) and amorphous SiO₂ is qualitatively explained by an increase in the roughness of the nanocluster surface accompanying their formation under the strongly nonequilibrium conditions of pulsed laser deposition.     

Copper nanoclusters in amorphous hydrogenated carbon

A study of the geometric characteristics of copper nanoclusters incorporated in an amorphous hydrogenated carbon matrix is reported. It makes use of small-angle x-ray scattering and transmission electron microscopy (100 keV). The fractal dimension and nanocluster diameter have been determined from the x-ray scattering indicatrix for different copper concentrations. TEM images of copper nanoclusters have been analyzed, and a cluster distribution function in size constructed. The shape of the distribution function is discussed in terms of the theory of nucleation of a new phase. Fiz. Tverd. Tela (St. Petersburg) 40, 568–572 (March 1998)     

Deposition and fabrication of alkanethiolate gold nanocluster films on TiO2(1 1 0) and the effects of plasma etching

Deposition and fabrication of films of Au nanoclusters protected by alkanethiolate ligands are attempted on a TiO₂(1 1 0) surface and the structures of films are observed by a scanning tunneling microscope (STM). Effects of oxygen and hydrogen-plasma etching in addition to UV irradiation on the structure and chemical composition of the films are also investigated by using STM and X-ray photoelectron spectroscopy. Alkanethiolate Au nanoclusters are produced using a modified Brust synthesis method and their LB films are dip-coated on TiO₂(1 1 0). Alkanethiolate Au nanoclusters are weakly bound to the substrate and can be manipulated with an STM tip. Net-like structures of alkanethiolate Au nanoclusters are formed by a strong blast of air. Oxygen-plasma etching removes alkanethiolate ligands and simultaneously oxidizes Au clusters. At room temperature, prolonged oxygen-plasma etching causes agglomeration of Au nanoclusters. UV irradiation removes ligands partly, which makes Au nanoclusters less mobile. The net-like structure of alkanethiolate Au clusters produced by a blast of air is retained after oxygen and hydrogen-plasma etching.     

Deposition of metal clusters on single-layer graphene/Ru(0001): Factors that govern cluster growth

Fabrication of nanoclusters on a substrate is of great interest in studies of model catalysts. The key factors that govern the growth and distribution of metal on graphene have been studied by scanning tunneling microscopy (STM) based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moiré pattern formed on Ru(0001). Our experimental findings show that Pt and Rh form finely dispersed small clusters located at fcc sites on graphene while Pd and Co form large clusters at similar coverages. These results, coupled with previous findings that Ir forms the best finely dispersed clusters, suggest that both metal–carbon (M–C) bond strength and metal cohesive energies play significant roles in the cluster formation process and that the M–C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Furthermore, experimental results show Au behaves differently and forms a single-layer film on graphene, indicating other factors such as the effect of substrate metals and lattice match should also be considered. In addition, the effect of annealing Rh on graphene has been studied and its high thermal stability is rationalized in terms of a strong interaction between Rh and graphene as well as sintering via Ostwald ripening.     

Charging of metal atoms on ultrathin MgO/Mo(100) films

The chemical activity of supported metal nanoclusters is enhanced by electronic charging induced by the interaction with surface defects. We use density functional theory plane wave calculations to show that charging of metal atoms with high electron affinity like Au is possible also in the absence of defects by atom deposition on ultrathin MgO films (1 to 3 layers) grown on Mo(100). The Au 6s level falls below the Fermi level of Mo, leading to electron transfer from Mo to Au by direct tunneling through the insulating MgO film. The effect is not observed for Pd, whose 5s empty level falls just above the Fermi level of Mo, or for thicker MgO films.     

FRACTAL PATTERN GROWTH OF METAL ATOM CLUSTERS IN ION IMPLANTED POLYMERS

The fractal and multi-fractal patterns of metal atoms are observed in the surface layer and cross section of a metal ion implanted polymer using TEM and SEM for the first time. The surface structure in the metal ion implanted polyethylene terephthalane (PET) is the random fractal. Certain average quantities of the random geometric patterns contain self-similarity. Some growth origins appeared in the fractal pattern which has a dimension of 1.67. The network structure of the fractal patterns is formed in cross section, having a fractal dimension of 1.87. So it can be seen that the fractal pattern is three-dimensional space fractal. We also find the collision cascade fractal in the cross section of implanted nylon, which is similar to the collision cascade pattern in transverse view calculated by the TRIM computer program. Finally, the mechanism for the formation and growth of the fractal patterns during ion implantation is discussed.     

Pulse regime in formation of fractal fibers

The pulse regime of vaporization of a bulk metal located in a buffer gas is analyzed as a method of generation of metal atoms under the action of a plasma torch or a laser beam. Subsequently these atoms are transformed into solid nanoclusters, fractal aggregates and then into fractal fibers if the growth process proceeds in an external electric field. We are guided by metals in which transitions between s and d-electrons of their atoms are possible, since these metals are used as catalysts and filters in interaction with gas flows. The resistance of metal fractal structures to a gas flow is evaluated that allows one to find optimal parameters of a fractal structure for gas flow propagation through it. The thermal regime of interaction between a plasma pulse or a laser beam and a metal surface is analyzed. It is shown that the basic energy from an external source is consumed on a bulk metal heating, and the efficiency of atom evaporation from the metal surface, that is the ratio of energy fluxes for vaporization and heating, is 10^–3–10^–4 for transient metals under consideration. A typical energy flux (~10⁶ W/cm²), a typical surface temperature (~3000 K), and a typical pulse duration (~1 μs) provide a sufficient amount of evaporated atoms to generate fractal fibers such that each molecule of a gas flow collides with the skeleton of fractal fibers many times.     

Fractal aggregates evolution of methyl red in liquid crystal

The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.     

Fractal Aggregation Under Rotation

By means of the Monte Carlo simulation, a fractal growth model is introduced to describe diffusion-limited aggregation (DLA) under rotation. Patterns which are different from the classical DLA model are observed and the fractal dimension of such clusters is calculated. It is found that the pattern of the clusters and their fractal dimension depend strongly on the rotation velocity of the diffusing particle. Our results indicate the transition from fractal to non-fractal behavior of growing cluster with increasing rotation velocity, i.e. for small enough angular velocity ω; thefractal dimension decreases with increasing ω;, but then, with increasing rotation velocity, the fractal dimension increases and the cluster becomes compact and tends to non-fractal.     

Observation of fractal nanoclusters on the pulsed laser deposition of gold

The results of studying the fractal structure of Au clusters arising in pulsed laser deposition under conditions of strong deviations from thermodynamic equilibrium are presented. A mechanism is suggested in which the formation of fractal nanoclusters is considered as a result of the decay of a strongly correlated initial state in a non-equilibrium system of adatoms. The geometrical shape of the fractal structures observed experimentally can be described within the given mechanism, and their dimensionality can be estimated.     

密封界面微孔结构的三维分形重构技术与泄漏特性

基于分形粗糙表面的三维数值重构技术,对界面微孔结构的分形表征进行详细研究,并应用分形多孔介质输运理论构建界面泄漏机理模型,建立微观形貌与宏观泄漏率之间的理论关系式.用有限元分析法对单粗糙峰的接触变形进行模拟,获得最大孔径的变形位移,实现微观接触力学与微观泄漏模型的有效耦合.应用所提出的预测模型对金属垫片泄漏率进行计算,结果与已有实验结果相吻合.由预测模型可知,粗糙表面分形维数、特征尺度系数以及变形位移量对微孔几何结构影响显著,是影响界面泄漏率的关键因素.     

On fractal nature of the structure of nanoporous carbon obtained from carbides

The curves describing small-angle x-ray scattering at npor-C nanoporous carbon samples obtained from polycrystalline α-SiC, TiC, and Mo₂C and a 6H-SiC single crystal have been analyzed. An algorithm is developed for taking into account the corrections to experimental curves for the intensity of the primary beam transmitted through the sample and the height of the inlet slit in these measurements. Two systems of nanoclusters observed in the npor-C structure differ in the type of stacking of structural elements: small-scale mass fractals of a dimension 1<D ₂<3 and a size L ₂=50–90 Å, which depend on the type of the initial carbide, and large-scale nanoclusters having a size L ₁>550 Å. In most samples, large-scale nanoclusters can be regarded as objects with a fractal surface and a dimension 2<D ₁<13, which also depends on the type of the initial carbide. Large-scale nanoclusters in npor-C obtained from Mo₂C prove to be mass fractals with a dimension D ₁>2. Peculiarities of the structure formation of nanoporous carbon obtained from various carbides are discussed.