It is well established that nucleation of metal clusters on oxide and halide surfaces is typically dominated by defect sites. Rate equation models of defect nucleation have been developed and applied to these systems. By comparing the models with nucleation density experiments, energies for defect trapping, adsorption, surface diffusion and pair binding have been deduced in favourable cases, notably for Pd deposited on Ar-cleaved MgO(001). However, the defects responsible remain largely unknown. More recently, several types of ab initio calculation have been presented of these energies for Pd and related metals on MgO(001) containing several types of surface defect; these calculated values are surveyed, and some are widely divergent. New rate equation nucleation density predictions are presented using the calculated values. Some calculations, for some defect types, are much closer to experiment than others; the singly charged F(s)(+) centre and the neutral divacancy emerge as candidate defects. In these two cases, the Pd/MgO(001) nucleation density predictions agree well with experiment, and the corresponding surface defects deserve to be taken seriously. Energy and entropy values are discussed in the light of differences in calculated charge redistribution between the metal atoms, clusters and (charged) surface defects, and (assumed or calculated) cluster geometries.
Calculations of the nucleation and growth of thin films are presented. These atomistic calculations depend on adsorption (${E}_{a}$), diffusion (${E}_{d}$), and lateral binding (${E}_{b}$) energies. A simplified pair binding model of small two-dimensional clusters is used to make the calculations explicit for layer-plus-island (or Stranski-Krastanov) growth systems. Within such a model it is found that at least a crude (Einstein) representation of surface vibrations is needed to make reasonable predictions at low supersaturation. The calculations are applied to extract parameter values from nucleation and growth experiments on Ag/W(110), Ag/Mo(100), Ag/Si(111), and Ag/Si(100), and for rare gases onto various (plated) substrates. Comments are made about the parameters obtained for these systems, and about the role of surface crystallography and defects.
A stage for an electron microscope has been constructed which is stable and versatile. It is a two-axis tilting stage which will operate at temperatures between liquid helium temperature and 50°C.
Abstract Electron microscopy and diffraction have been used to study twins in annealed single crystals of α-N2. It is shown that although in principle two different types of twins may occur, only one type (a type II twin) is seen in practice. Calculations of the energies of such twin configurations and stacking faults, using lattice relaxation methods, also indicate that the energy of a type II twin which is observed in α-N2 is less than the energy of a type I twin. It is also less than the energy of various stacking faults which are not observed.
Abstract Recent developments in the theory of nucleation of vapour deposits on crystalline substrates are reviewed. To facilitate comparison, the theories are formulated in a common dimensionless notation, and an examination of the major underlying assumptions reveals the basic similarity of many of them. The capture and dissociation rates are expressed in terms of the cluster geometry and the pertinent energy parameters, and from these the ‘chemical’ rate equations are set up. The following types of approximate solutions are discussed: (a) long-time asymptotic solutions, from which the conditions for saturation in the cluster concentration may be deduced, (b) a generalization of the type of approximate solution used by Logan (1969), and (c) numerical solutions employing a minimum number of simplifying assumptions. Based on a simple model, agreement between the latter two seems reasonably good. For any given set of fixed parameters (energies, geometrical constants, and arrival rate from the vapour) several temperature ranges may be distinguished. The main division is between ‘initially complete condensation’ at low temperatures and ‘initially incomplete condensation’ at high temperatures. Within each of the latter cases there are further transitions (a) between different values of the ‘critical size’ i*, and (b) from negligible growth to rapid growth of the supercritical clusters. The influence of all of these factors on the final cluster concentration is described. The distributions of the clusters in size and spacing are discussed briefly and qualitatively, as are the types of effects that can be induced by defects or other ‘special sites’ on the substrate. Comparisons are made with some recent experimental studies. In many of these, defects in the substrate seem to play a dominant role, and no detailed comparison with theory seems possible. One notable exception is the nucleation of rare gas crystals on graphite substrates (Venables and Ball 1970), and here, for at least two of the three gases studied, excellent quantitative agreement is obtained.
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