THE NUCLEATION AND GROWTH OF PD-AU ALLOY PARTICLES ON NACL(100)

The early stages of thin film formation of Pd-Au alloys by simultaneous deposition of the components onto NaCl(100) cleavage surfaces had been investigated both experimentally and by a theoretical model in terms of rate equations, combined with an adatom diffusion model. Whereas the condensation coefficient of gold was initially very low, that of palladium was near unity even at an early stage. As a straightforward analytical solution of the rate equation approach is not feasible for such a complex case, we simulated the initial stages of condensation numerically by calculating the composition of the particles from atomic capture rates, taking into account the development in time of the adatom and particle number densities, and of the growth rates. Comparison of the results was also made with Monte Carlo calculations, which will be presented in more detail elsewhere. A close fit of experimentally measured compositions and particle growth rates was obtained by an appropriately chosen difference of the adatom energies for adsorption and for surface diffusion of the components, which is (E(a)-E(d))Pd-(E(a)-E(d))Au = 0.12 +/- 0.02 eV. However, the experimentally measured particle number dentities varied much less with the deposition fluxes than was theoretically expected. We suspect that this is caused by dimer and cluster mobilities and/or defect decoration, which are not included in our simplified model. It is argued that this may affect the absolute values of the energy parameters, but is only of minor importance for the difference given above.