Abstract. Coalescence dynamics of size-selected gold (Au) clusters (each with nominal 923 atoms), on amorphous Si 3 N 4 substrate at room temperature, has been studied via time-resolved transmission electron microscopy (TEM). We found that the clusters approached each other in two stages. In the first stage, the drift velocity was independent of the particle separation and could be attributed to beam-induced random motion. In the second stage, the clusters were found to jump into contact with a much higher final averaged speed. This is independent of beam dose rates and is attributed to the van der Waal attraction. IntroductionCoalescence of supported metal nanoparticles/clusters is a common phenomenon in the field of catalysis, whose process is known to relate to many factors, such as size, structure, orientation, temperature, support and environment [1,2]. It is critical to quantify these factors for understanding the physical mechanism of the coalescence if one wants to rationally design a catalyst system. Previous experimental studies of nanoparticle coalescence are mainly on particles of different sizes [1,3], thus it is difficult to differentiate the effect of size as to other factors such as temperature, structure, etc. Here we utilised the unique capability of an atomically size-controlled cluster source in our group, together with a fast acquisition feature of a microscope camera, to study the coalescence phenomena between gold (Au) clusters. Specifically, we reported the effects of the cluster size and electron dose on the coalescence of Au clusters.

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