Direct observation of small cluster mobility and ripening

Abstract

New direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single-crystalline thin graphite substrates have been studied by in situ transmission electron microscopy (TEM) under controlled environmental conditions (residual gas pressure of 10 -9 torr) in the temperature range from 25° to 450°C. It was possible to monitor all stages of the experiments ( i.e. sputter cleaning of the substrate surface, metal deposition and annealing) by TEM observation of the same specimen area. Various techniques ( e.g. pseudo-stereographic presentation of micrographs in different annealing stages, the observation of the annealing behavior at cast shadow edges, and measurements with an electronic image analyzing system) were employed to aid the visual perception and the analysis of changes in deposit structure recorded during annealing. Slow Ostwald ripening was found to occur over the entire temperature range but the overriding surface transport mechanism was short distance cluster mobility. This was concluded from in situ observations of individual particles during annealing and from measurements of cluster size distributions, cluster number densities, area coverages and mean cluster diameters.