MELTING, FREEZING, AND COALESCENCE OF GOLD NANOCLUSTERS

We present a detailed molecular-dynamics study of the melting, freezin g, and coalescence of gold nanoclusters within the framework of the em bedded-atom method. Concerning melting, we find the process first to a ffect the surface (''premelting''), then to proceed inwards. The curve for the melting temperature vs cluster size is found to agree reasona bly well with predictions of phenomenological models based on macrosco pic concepts, in spite of the fact that the clusters exhibit polymorph ism and structural transitions. Upon quenching, we observe a large hys terisis of the transition temperature, consistent with recent experime nts on lead. In contrast, we find macroscopic sintering theories to be totally unable to describe the coalescing behavior of two small clust ers. We attribute this failure to the fact that the nanocrystals are f aceted, while the sintering theories are formulated for macroscopicall y smooth crystallites. The time for coalescence from our calculations is predicted to be much longer than expected from the macroscopic theo ry. This has important consequences for the morphology of cluster-asse mbled materials.