Nucleation of AuAg binary alloys from the vapour phase on KBr single crystals

Abstract

The nucleation, growth and composition of particulate AuAg alloy deposits on a KBr(100) crystal cleavage surface was investigated by coevaporation of the metals in ultrahigh vacuum. Series of specimens were prepared by increasing the deposition time at constant fluxes of the vapour beams. Analysis of the nucleation rates by transmission electron microscopy revealed that they are mainly determined by the nucleation of gold. Correspondingly, the preferred condensation of gold was found in the early stages of particle growth, as measured by quantitative X-ray fluorescence analysis. During further deposition the composition approached that of the vapour beam, while the substrate coverage increased. The results were interpreted by means of an extended kinetic nucleation model in which it is assumed that the nucleation and growth of alloy particles is mainly governed by the differences in the adatom adsorption energies E a, in the activation energies E d for surface diffusion and in the pre-exponential constants of the components. Evaluation of the experimental data resulted in E a, Au − E a, Ag = 0.03 eV and E d, Au − E d, Ag = -0.02 eV, in close agreement with data for AuAg on NaCl reported in earlier work. As small variations of these values were found to result in large deviations in the condensation behaviour, particularly in the composition in the nucleation phase, the error limits were only about ±0.02 eV. The ratio of the pre-exponential constants was found to be about unity.