Structural rearrangements in small gold particles

Abstract

A knowledge of the structure of small metal particles is fundamental to understanding their role in heterogeneous catalysis. It is therefore of much topical interest that structural rearrangements have been found to take place as a result of intense electron irradiation within the electron microscope. Initial observations were of gold particles on supports of carbon and silica-covered silicon, but further studies confirmed that qualitatively similar behaviour also took place for particles of Pt, Ru and Rh. Some of the factors which influence the motion have been noted, including the particle size, the nature of the substrate and, obviously, the current density of the electron beam. Considerable debate has been generated by these observations. It has been suggested, for example, that the structural fluctuations should be dampened by an electrically conducting substrate which would prevent the particles from becoming charged. It was also proposed that the “quasimelting” of gold particles was triggered by Auger decay, in effect heating the particles electronically rather than thermally. The energetics of these fluctuations have also been considered.In this paper, we report our initial efforts to quantify some of the experimental factors involved in this quasimelting process. Three sets of samples were prepared: one was a normal holey carbon support film, the second was a holey silicon support film, and the third was a carbon film upon which graphitised carbon had been deposited.