Computer simulation of surface diffusion of copper, silver and gold

The binding energies to copper, silver and gold (111) surfaces of self-atom clusters have been calculated. The activation energies of motion of these ad-atom clusters, vacancies and divacancies on copper, silver and gold (111) surface, and of the conversion of ad-atom clusters on (111) and (100) have been calculated by use ofn-body embedded atom potentials and molecular dynamics.     

Monte Carlo simulation of nucleation and growth of thin films

We study thin film growth using a lattice-gas, solid-on-solid model employing the Monte Carlo technique. The model is applied to chemical vapour deposition (CVD) by including the rate of arrival of the precursor molecules and their dissociation. We include several types of migration energies including the edge migration energy which allows the diffusive movement of the monomer along the interface of the growing film, as well as a migration energy which allows for motion transverse to the interface. Several well-known features of thin film growth are mimicked by this model, including some features of thin copper films growth by CVD. Other features reproduced are—compact clusters, fractal-like clusters, Frank-van der Merwe layer-by-layer growth and Volmer-Weber island growth. This method is applicable to film growth both by CVD and by physical vapour deposition (PVD).     

Simulation of structure evolution in Cu films

The Monte Carlo method was used to simulate grain growth in thin Cu films. The model, based on energetic principles, was compared with the evolution of measured film structures. Surface, interface, grain boundary, and elastic strain energies were applied to determine the preferred microstructure in terms of different annealing conditions and film thicknesses. Four microstructural cases, relating to different film thicknesses, were developed in this paper. Twinning in the Cu films is simulated by arbitrary re-assignment of randomly selected crystallite lattice orientations. The observed evolution in crystallographic texture for each film thickness can be obtained from the Monte Carlo simulations.     

Study of initial stages of thin film growth by means of atomistic computer simulation and image analysis

Nucleation and initial stages of three-dimensional film growth were studied by a computer experiment. The experiment consisted of two parts - preparation of simulated thin-film structures and computational analysis of their images. The simulated structures were generated by a model combining the molecular dynamics and Monte Carlo approaches. The analysis of the obtained structures was based on the ’Quadrat Counts’ method, which proved to be one of the most promising algorithms of image processing. The results obtained by image analysis of simulated thin films were discussed and the most suitable algorithms were suggested for the analysis of experimentally prepared films.     

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。     

Observations on the electromigration in various thin films of group I–IV

The transport of ions under the influence of a direct current has been found to be directed towards the cathode in thin films of silver, copper and gold, and towards the anode in aluminum, magnesium, indium, tin and lead films. The direction of electromigration in silver, copper and gold films is opposite to the one usually found in bulk samples of these metals. In silver films with large as well as with small grains the direction of electrotransport is the same. The experiments show that electromigration in single-crystalline silver films is small. This suggests that grain boundary electrotransport predominates in silver films over surface and volume electrotransport, which is also true for aluminum films.     

Film growth of metals and semiconductors with ionized beams and surface imperfections

The nucleation and growth processes of thin films of metals and semiconductors on a solid surface are discussed in terms of the controlled treatment of substrate surfaces by bombardment with photons, electrons and ions, In particular, epitaxial film growth with ionized beams is discussed in conjunction with surface damage.For metal deposits such as gold and silver on alkali halide crystals, nuclei are trapped by imperfections on the surface and grow into islands. Vapor deposits of semiconductors are produced with partially ionized beams and show good epitaxy, with lowering of the epitaxial temperature. These results are reviewed and discussed as a problem involved in the growth processes of thin films.     

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.     

Nanostructured copper and copper oxide thin films fabricated by hydrothermal treatment of copper hydroxide nitrate

Metallic copper and copper oxide thin films were fabricated on surface of glass slide substrates. Copper oxide thin films were prepared by a hydrothermal method using an α-phase layered hydroxide, copper hydroxide nitrate as a precursor. Morphology, thickness and crystallite size of the obtained copper oxide thin films changed by changing the time of hydrothermal treatment. Accordingly, the copper oxide thin films showed various water contact angles and optical band gaps. As, the optical band gap of the nanostructured copper oxide thin films increased with an increase in hydrothermal time from 1.85 to 2.95 eV. Moreover, the water contact angles changed from 16.4 to 38.8° by changing the hydrothermal time. By a reductive hydrothermal-treatment route, the copper oxide thin film was reduced to metallic copper thin film without any particle growth.     

氧化物异质外延薄膜生长的计算机模拟

利用Monte Carlo方法分别模拟了在SrTiO3基底上沉积MgO薄膜和在MgO基底上沉积SrTiO3薄膜.模拟中,选取与实验中薄膜生长相近的参数条件,引入了新的参数扩散势垒,得到了在晶格正失配(张应力)和负失配(压应力)下薄膜生长的形貌图以及薄膜粗糙度的变化曲线图,分析了张应力和压应力对薄膜生长形貌的影响.模拟结果与文献报道的外延薄膜生长模式的实验观察结果一致.     

Vibrational spectra and surface-enhanced vibrational spectra of 1-nitropyrene

The present report on the vibrational spectra of 1-nitropyrene (1NP) describes the infrared and Raman spectra; their interpretation is aided by local density functional theory (DFT) calculations at the B3LYP/6-311G(d, p) level of theory and by the surface-enhanced vibrational spectra (SEVS) with the final objective of trace organic analytical applications. The surface-enhanced Raman scattering (SERS) on silver island films and mixed silver/gold island films was investigated with several laser lines in the visible region. Surface-enhanced infrared absorption (SEIRA) was attempted on silver and gold island films. The interface of the organic 1-NP with smooth metal surfaces of silver and copper was also probed using reflection-absorption infrared (RAIRS) spectra that, in conjunction with the transmission spectra, allow one to extract the molecular orientation in vacuum evaporated thin solid films. Chemical adsorption of 1-NP on silver and further photochemical decomposition of the 1-NP-metal adsorbates was detected with all visible laser lines. Resonance Raman scattering (RRS) using UV-laser excitation at 325 nm was also recorded.     

Sample metallization for performance improvement in desorption/ionization of kilodalton molecules: quantitative evaluation, imaging secondary ion MS, and laser ablation

The metallization procedure, proposed recently for signal improvement in organic secondary ion mass spectrometry (SIMS) (Delcorte, A.; Médard, N.; Bertrand, P. Anal.Chem. 2002, 74, 4955)., has been thoroughly tested for a set of kilodalton molecules bearing various functional groups: Irganox 1010, polystyrene, polyalanine, and copper phthalocyanine. In addition to gold, we evaluate the effect of silver evaporation as a sample treatment prior to static SIMS analysis. Ion yields, damage cross sections, and emission efficiencies are compared for Ag- and Au-metallized molecular films, pristine coatings on silicon, and submonolayers of the same molecules adsorbed on silver and gold. The results are sample-dependent but as an example, the yield enhancement calculated for metallized Irganox films with respect to untreated coatings is larger than 2 orders of magnitude for the quasimolecular ion and a factor of 1-10 for characteristic fragments. Insights into the emission processes of quasimolecular ions from metallized surfaces are deduced from kinetic energy distribution measurements. The advantage of the method for imaging SIMS applications is illustrated by the study of a nonuniform coating of polystyrene oligomers on a 100-microm polypropylene film. The evaporated metal eliminates sample charging and allows us to obtain enhanced quality images of characteristic fragment ions as well as reasonably contrasted chemical mappings for cationized PS oligomers and large PP chain segments. Finally, we report on the benefit of using metal evaporation as a sample preparation procedure for laser ablation mass spectrometry. Our results show that the fingerprint spectra of Au-covered polystyrene, polypropylene, and Irganox films can be readily obtained under 337-nm irradiation, a wavelength for which the absorption of polyolefins is low. This is probably because the gold clusters embedded in the sample surface absorb and transfer the photon energy to the surrounding organic medium.     

Electron emission from condensed phase material induced by fast protons

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport codes commonly rely on elastic and inelastic electron scattering cross sections determined using theoretical methods supplemented with gas-phase data; experimental condensed phase data are often unavailable or infeasible. The largest uncertainties in the theoretical methods exist for low-energy electrons, which are important for simulating electron track ends. To test the reliability of these codes to deal with low-energy electron transport, yields of low-energy secondary electrons ejected from thin foils have been measured following passage of fast protons. Fast ions, where interaction cross sections are well known, provide the initial spectrum of low-energy electrons that subsequently undergo elastic and inelastic scattering in the material before exiting the foil surface and being detected. These data, measured as a function of the energy and angle of the emerging electrons, can provide tests of the physics of electron transport. Initial measurements from amorphous solid water frozen to a copper substrate indicated substantial disagreement with MC simulation, although questions remained because of target charging. More recent studies, using different freezing techniques, do not exhibit charging, but confirm the disagreement seen earlier between theory and experiment. One now has additional data on the absolute differential electron yields from copper, aluminum and gold, as well as for thin films of frozen hydrocarbons. Representative data are presented.     

Mechanisms,models and methods of vapor deposition

The condensation and assembly of atomic fluxes incident upon the surface of a thin film during its growth by vapor deposition is complex. Mediating the growth process by varying the flux, adjusting the film temperature, irradiating the growth surface with energetic (assisting) particles or making selective use of surfactants is essential to achieve the level of atomic scale perfection needed for high performance films. A multiscale modeling method for analyzing the growth of vapor deposited thin films and nanoparticles has begun to emerge and is reviewed. Ab-initio methods such as density functional theory are used to provide key insights about the basic mechanisms of atomic assembly. Recent work has explored the transition paths and kinetics of atomic hopping on defective surfaces and is investigating the role of surfactants to control surface atom mobility. New forms of interatomic potentials based upon the embedded atom method, Tersoff and bond order potentials can now be combined with molecular dynamics to investigate many aspects of vapor phase synthesis processes. For example, the energy distribution of atoms emitted from sputtering targets, the effects of hot atom impacts upon the mechanisms of surface diffusion, and the role of assisting ions in controlling surface roughness can all be investigated by this approach. They also enable the many activation barriers present during atomic assembly to be efficiently evaluated and used as inputs in multipath kinetic Monte Carlo models or continuum models of film growth. This hierarchy of modeling techniques now allows many of the atomic assembly mechanisms to be incorporated in film growth simulations of increasing fidelity. We identify new opportunities, to extend this modeling approach to the growth of increasingly complicated material systems. Using the growth of metal multilayers that exhibit giant magnetoresistance as a case study, we show that the approach can also lead to the identification of novel growth processes that utilize adatom energy control, very low energy ion assistance, or highly mobile, low solubility chemical species (surfactants) to control surface diffusion controlled film growth. Such approaches appear capable of enabling the creation of multilayered materials with exceptionally smooth, unmixed interfaces, with significantly superior magnetoresistance.     

Molecular dynamics modeling of microstructure evolution during growth of amorphous carbon films

Summary Classical molecular dynamics simulations, using Brenner's bond-order interatomic potential model, is used to study the bonding microstructure formation during quench from liquid and during growth on a diamond surface. For a 64-atom quench simulation we find 56 sp³- and 8 sp²-bonded carbon atoms, in qualitative agreement with tight-binding simulations. The growth of amorphous carbon films was simulated by depositing carbon and hydrogen atoms onto a diamond surface at energies up to 100 eV The simulated films are amorphous with a maximal density near the deposition energies (20–40 eV) used to grow films on magnetic disks. Lower deposition energies yield open graphitic structures, while much higher deposition energies cause the surface to ablate, leading to a poorly defined interface. The hardness calculated from the densest simulated films is about twice that found experimentally.     

Reversible scaling: Optimized free-energy determination using atomistic simulation techniques

We present a new simulation technique that allows accurate and very efficient determination of free energies as a function of temperature using a single constant temperature molecular dynamics or Monte Carlo simulation. The method is based on the dynamical reversible scaling of the potential energy function of the system of interest and is implemented using the adiabatic switching method. Application to the calculation of the free energy of crystalline silicon using a semi-empirical interatomic potential demonstrates that the reversible-scaling method provides an accurate and very efficient tool for the calculation of free energies over a wide temperature interval. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microstructure formation in electrochemically deposited Copper thin films

The influence of the electrochemical potential and deposition mode on the preferred orientation, morphology and microstructure of crystallites in electrochemically deposited (ECD) copper thin films was investigated using X‐ray diffraction (XRD), scanning electron microscopy (SEM) and the diffraction of back‐scattered electrons (EBSD). As a working electrode for the ECD process, thin gold layers were employed that were deposited on floating‐glass substrates in a vacuum evaporation process. With increasing negative electrochemical potential in the ECD process, the deposition mode changed from the charge transfer controlled one to the diffusion controlled one. At the highest electrochemical potentials, the copper deposition and the hydrogen release were running concurrently. The change of the deposition mode was accompanied by a change of the surface roughness of the thin films and by a change of the direction and degree of the preferred orientation of crystallites. The surface roughness of the deposited copper thin films increased with increasing electrochemical potential. Compact plate‐like crystallites with the preferred orientation {111} grew in the transport controlled deposition mode. Development of the {111} texture was supported by the {111} preferred orientation of the gold layers and by surface energy of copper, which is the lowest in the (111) plane. The diffusion controlled deposition mode was characterized by the growth of globular {110}‐oriented crystallites. The {110} texture resulted from the minimization of the anisotropic strain energy of copper via reduction of the structure defects in this deposition modus. For highest electrochemical potentials, the copper deposition run simultaneously with the development of hydrogen that resulted in growth of needle‐like crystallites with the {100} texture.     

薄膜生长的三维蒙特卡罗模型

构造三维蒙特卡罗模型,研究了六边形基底薄膜生长的过程.在模型中针对每个原子考虑了原子沉积、原子扩散及原子脱附三个动力学过程,并认为这三个过程是相互独立的,即在同一计算步长中三个过程依据各自的概率发生.经过生长过程可视化的结果表明,薄膜原子之间的相互作用能、基底温度和沉积速率对薄膜的生长方式有显著的影响.这一结论得到了实验的验证.