Crystal growth study using combination of molecular dynamics and Monte Carlo methods

Molecular dynamics method although provides details of energies of the system as a function of time, is not suited to simulate the processes involving activation processes. Therefore, we attempted to combine the molecular dynamics and Monte Carlo methods. Using molecular dynamics, the energies of the system were calculated which were subsequently combined with Monte Carlo method using random numbers, epitaxial growth of (111) plane of copper, silver, and gold. While surface adsorption and surface diffusion for copper, silver, and gold were simulated by use of molecular dynamics method, the relation between the growth rate of thin films and the packing density of atoms were obtained using Monte Carlo simulation. Thus, by combining the results of the molecular dynamics method and the Monte Carlo method the growth process of thin films at elevated temperatures were obtained, which is too tedious to be calculated by molecular dynamics alone.     

Computer simulation of point defects in fcc metals using EAM potentials

The activation energies for the motion of interstitials and vacancies, vacancy clusters have been calculated using embedded atom potentials. The most exciting results obtained are that the stable interstitial configuration is a [1 0 0] split type and the saddle point was a body-centered interstitial. This is quite different from widely accepted computational results. The activation energies for the motion of vacancies, di-vacancies and tri-vacancies have been calculated. The atom above the center of a 60° tri-vacancy relaxes to the center of the tetrahedron in copper but does not relax to the center of the tetrahedron in silver. In the case of gold the energy relaxed to the center is very small and the atoms move around in the tetrahedron. The details depend upon the potentials.     

Thermodynamic properties of silver nanoclusters

Icosahedral models of magic number silver clusters have been constructed using molecular dynamics simulations with the Doyama-Kogure potential. We have calculated the energy, heat capacity, entropy, Gibbs energy change, and excess surface energy as functions of cluster size and temperature. The vibrational contributions to the heat capacity and entropy are weak functions of cluster size (N), and the statistical degeneracy is lifted starting at 250–300 K. The surface energy density of the silver clusters is size-independent down to N = 13. The saturated vapor pressure over the clusters has been estimated. The calculated thermodynamic properties of fcc silver agree well with standard thermodynamic data.     

Au-Ag系统的扩散系数及其扩散激活能的研究

在银基底上蒸镀80nm的金薄膜,然后用不同温度和时间间隔对这些样品进行热处理。再用PHI 550型俄歇电子能谱仪测出上述样品金膜中银的浓度剖面分布。根据银的浓度剖面分布及Hall公式算出Au-Ag系统的互扩散系数。由Leclaire公式计算出该系统的晶界扩散系数。从Arrhenius方程得出互扩散及晶界扩散激活能。并对实验和计算结果进行了讨论。     

Substrate-diffusion-controlled film growth: Silver and copper on lead (111)

Silver and copper were vapour deposited onto atomically clean lead (111) surfaces under ultrahigh vacuum conditions. Low energy electron diffraction, reflection high energy electron diffraction and Auger electron spectroscopy measurements in the same chamber showed that, for a 0.1 nm min^-1 impingement of silver or copper, the deposit completely re-evaporated. At 10 nm min^-1 both silver and copper condensed onto a lead (111) surface at room temperature. The silver deposit was a continuous-layer single crystal which had an atomically flat upper surface covered by an equilibrium monolayer of substrate atoms epitaxed to it. The results for the copper experiments were similar to those for silver, except for the presence of small amounts of misoriented copper (111) and polycrystalline copper.It is proposed that substrate diffusion during the condensation of silver or copper onto lead (111) is crucial in determining the mode of film growth. It is thought that two-dimensional nucleation results from a mixed mobile layer of adsorbate and substrate atoms.     

Local investigation of electron-induced processes in water-metal systems

Abstract

Electrons tunnelling inelastically in a scanning tunnelling microscope have been used to induce the diffusion of water monomers adsorbed on the (111) faces of Ag, Cu, and Au via electrons coupling to vibrational modes of the molecules. Electrons of higher energies transform amorphous water clusters into crystalline clusters. At energies above 1.6 eV, the water molecules within the clusters are dissociated only after crystallization of the amorphous ice clusters. When the energy is sufficient to populate an anti-bonding orbital at 3 V the surface is oxidized.     

Zinc porphyrin-driven assembly of gold nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.     

Initial epitaxial growth of (111) Au/(111) Cu and (111) Cu/(111) Au

The room temperature modes of growth of Au/(111) Cu and Cu/(111) Au are described. For the former growth mode initial deposits (2.4 Å) of gold on copper form smooth flat islands delineated by coincidence lattice misfit dislocations. For 6.0 Å of gold deposit, both thick and thin gold areas were observed with almost complete substrate coverage. For a 10 Å deposit, surface coverage was complete. Strain measurements and dislocation densities obtained on the (111) Au/(111) Cu films suggest the presence of two separate misfit dislocation networks at the interface. The coincidence lattice networks were large enough for transmission electron microscopy observation but contributed little to total overlayer strain. The (van der Merwe) natural lattice misfit dislocations were too closely spaced for direct observation but their presence was inferred because of the strain measurements. The initial epitaxy of Cu/(111) Au was similar to the Stranski-Krastanov model: the initial monolayer of copper (also delineated by coincidence misfit dislocations) grew smoothly on the gold; additional copper formed essentially stress-free “nuclei” on top of the initial copper layer.     

噻二唑型缓蚀剂缓蚀性能的研究

利用电化学测试和表面分析技术,研究了2,5-二巯基-1,3,4噻二唑(DMTD)在硫-乙醇溶液中对金属银、铜的缓蚀性能,结合量子化学计算和分子动力学模拟对DMTD在金属表面的吸附行为和缓蚀作用机理进行了分析讨论。结果表明,DMTD在50mg/L的硫-乙醇溶液中,对金属银、铜均起到较好的缓蚀作用。极化曲线结果表明,当缓蚀剂DMTD浓度达到50mg/L时,缓蚀效率可以达到92.3%。表面分析技术表明,缓蚀剂的加入在金属表面形成吸附膜,明显抑制了腐蚀速率。量化计算和分子动力学模拟得到了缓蚀剂分子的活性位点和缓蚀剂在金属表面的吸附形态。     

Surface structures on thin gold and platinum crystals

The surface structures on gold and platinum crystal films 40 Å thick formed by vacuum evaporation onto 111 silver substrates have been investigated by transmission electron microscopy using dark field images that are forbidden by structure factor. The gold crystals show contrast features which are attributed to atom steps. The platinum crystals show a pronounced hillock structure which is believed to be due to the 4% misfit between the platinum and silver surfaces.     

The effect of surface relaxation and atomic vibration on the equilibrium shape of gold and copper crystallites

Summary The free energy of surfaces along the <011> pole in gold and copper is determined to assess the effect of surface relaxation and atomic vibration on the equilibrium crystal shape of gold and copper. The Wulff construction is performed on the -plots to determine the equilibrium shape of gold and copper crystallites at different temperatures. It is shown that surface relaxation and atomic vibration do not have any discernible effect on the equilibrium shape of EAM gold or copper crystallites. The equilibrium shape of EAM gold crystallites is formed entirely from {111} and {100} facets, while that of EAM copper shows small {110} facets in addition to the {111} and {100} facets.     

Orientation dependence of fracture toughness measured by indentation methods and its relation to surface energy in single crystal silicon

Fracture toughness of silicon crystals has been investigated using indentation methods, and their surface energies have been calculated by molecular dynamics (MD). In order to determine the most preferential fracture plane at room temperature among the crystallographic planes containing the 〈001〉, 〈110〉 and 〈111〉 directions, a conical indenter was forced into (001), (110) and (111) silicon wafers at room temperature. Dominant {110}, {111} and {110} cracks were introduced from the indents on (001), (011) and (111) wafers, respectively. Fracture occurs most easily along {110}, {111} and {110} planes among the crystallographic planes containing the 〈001〉, 〈011〉 and 〈111〉 directions, respectively. A series of surface energies of those planes were calculated by MD to confirm the orientation dependence of fracture toughness. The surface energy of the {110} plane is the minimum of 1.50 Jm⁻² among planes containing the 〈001〉 and 〈111〉 directions, respectively, and that of the {111} plane is the minimum of 1.19 Jm⁻² among the planes containing the 〈011〉 direction. Fracture toughness of those planes was also derived from the calculated surface energies. It was shown that the K _IC value of the {110} crack plane was the minimum among those for the planes containing the 〈001〉 and 〈111〉 directions, respectively, and that K _IC value of the {111} crack plane was the minimum among those for the planes containing the 〈011〉 direction. These results are in good agreement with that obtained conical indentation.     

Auger analysis of thick silver films and of silver layers grown on copper

A sensitive new measure of the line broadening of an Auger doublet, called the R factor, was applied to the growth of (1) thick silver films and (2) thin silver films on a thick copper substrate. It is shown that periodic fluctuations in R with thickness give evidence for monocrystalline layer growth while the absence of such fluctuations is characteristic of polycrystalline growth. By combining measurements of the Auger amplitude and the R factor as functions of the silver film thickness for silver films deposited onto copper, it was shown that a Stranski-Krastanov growth mode is characteristic of the epitaxial growth of Ag(111) on Cu(111) at 210°C. The initial layer in this case is two (111) atom spacings thick on which high flat islands form. Continued growth of these islands is shown to take place by two-atom-thick flat islands.     

The Electronic Structures and Photographic Properties of Nanometer-sized Silver Clusters

The electronic structures and spectroscopic properties of nanometer-sized silver clusters Ag_n (n = 1 to 15) have been examined in the framework of self-consistent-field local density theory SCC-DV-X_α. The results show that there is a quantum size effect in Agn clusters, which can be observed as a red shift in the absorption threshold and semiconductor-conductor transition with increasing n. Based on the molecular orbital energies we studied the microscopical mechanisms of the transition point from lower efficiency step to higher efficiency step in latent image growth. The results also show that there are positive charges on the surface silver atoms of some large Ag" clusters, which is favourable for the latent image particles to adsorb developer anion in development.     

Absolute amplitudes in the de Haas-van Alphen effect

Absolute amplitudes of the neck oscillations in the de Haas-van Alphen effect of silver, gold, and copper have been measured for various orientations around 111 infields up to 100 kG by means of a new kind of torque magnetometer. These amplitudes are found to agree with the predictions of the Lifshitz-Kosevich theory within an uncertainty of order 20% for silver, 5% for gold, and 10% for copper. This confirms theoretical expectation that many-body modifications to the Lifshitz-Kosevich theory should not be appreciable in our experimental conditions.     

The treasure of gold and silver artifacts from the Royal Tombs of Sipán, Peru — a study on the Moche metalworking techniques

In 1987–1990, a spectacular treasure of gold and silver ornamental and ceremonial artifacts was recovered scientifically from the unlooted Royal Tombs of Sipán, Peru (dated to approximately AD 50–300). These objects give evidence of the outstanding craftsmanship of the Moche metalsmiths and reflect the various elaborate metalworking techniques available at that time. The present paper summarizes the results of a study on an array of artifacts stemming mainly from the tomb of the “Lord of Sipán.” Most of the objects were found to be made of thin sheet metal (1–<0.1 mm thick), which was further worked by cutting, embossing, punching, and chasing. Three-dimensional structures were created from pieces of the sheet metal by mechanical or metallurgical joining (soldering or welding). The Moche metalsmiths were masters in making objects that looked like pure gold or silver. In the case of copper objects, the surfaces were often found to be gilded electrochemically by the deposition of very thin gold films. In the case of objects made of alloys of copper with gold and some silver (tumbaga) or of copper with silver, the surface gilding or silvering was achieved by the depletion of copper, mostly by selectively oxidizing the surface copper and etching away the copper oxides that are formed.     

X-ray photoelectron spectroscopy of silver nanoparticles in phosphate glass

Phosphate glasses doped with silver nanoparticles have been characterized by X-ray photoelectron spectroscopy. A study on the relation of Ag binding energies with the optical properties of the nanocomposites has revealed binding energies for 3d doublets higher than those reported for bulk silver with a tendency towards the bulk value with increasing wavelength of surface plasmon resonance peak position. The data is interpreted in terms of quantum confinement effects in small silver clusters and an increase in particle size with plasmon absorption redshift.     

Mobility of small clusters of self-interstitial atoms in dilute Fe–Cr alloy studied by means of atomistic calculations

Atomistic simulations have been used to characterize the interaction and mobility of small clusters of self-interstitial atoms (SIAs) in dilute Fe–Cr alloys. The variety of migration mechanisms for Di- and Tri-SIA clusters in the bcc Fe matrix were studied using the nudged elastic band method. The corresponding binding and migration energies for the SIA clusters interacting with isolated Cr atoms and Cr–Cr close pairs were calculated using the two-band model interatomic potential. The obtained results are discussed in the light of available experimental data for dilute Fe–Cr alloys and are compared with results obtained using ab initio calculations.     

High-temperature thermal expansion of six metallic elements measured by dilatation method and X-ray diffraction

The thermal expansion of silver, gold, copper, nickel, molybdenum and tungsten has been measured by the dilatation method. The thermal expansion values of silver, gold, copper and nickel have also been evaluated from lattice parameter measurements by X-ray diffraction. These six metals were found to exhibit a nearly uniform expansion over the temperature ranges covered. The thermal expansion values obtained by the dilatation method are in good agreement with those determined by X-ray diffraction. The present results appear also to agree well with those reported previously in the literature.     

Structural and magnetic resonance properties of permalloy films epitaxially grown on f.c.c. metals

Single-crystal permalloy films have been prepared by epitaxy on different f.c.c. metals epitaxied on mica. Various methods of preparation of the f.c.c. metals have been tested: epitaxy of gold on mica at 400°C; epitaxy of silver on mica at 300°C; epitaxy of copper on mica at 500°C; and recrystallization of a thin silver film on mica.