Formation of ordered assemblies from deposited gold clusters

Abstract

A novel technique for producing intense, parallel beams of mass-selected nanocrystals from a magnetron cluster source has been used to study the low-energy impact of size-selected gold nanocrystals, 2.0-8 nm diameter, on 2 nm thick carbon films. The measurements of the sizes using transmission electron microscopy show that it is possible to deposit, intact, nanocrystals with a very narrow size range ( D(FWHM)/D = 0.06) as long as the impact energy is below 40 eV. The subsequent surface motion of the nanocrystals after impact (with the substrate) results in cluster-cluster collisions, which for large clusters (>4 nm) produces aggregations but for small clusters (<3.5 nm) results in complete fusion and reformation into larger aggregated clusters with approximate spherical symmetry. However, when the energy is reduced to 10 eV, clusters of a size of around 5 nm form some localized ordered arrays rather than random aggregates. This implies that the clusters forming these arrays can undergo local spatial rearrangement during their formation in order to reduce the total energy. The clear absence of sintering other than that expected from random impact collisions requires either a repulsive cluster-cluster interaction or surface passivation of the particles. These issues are discussed in this article.

Island formation and dynamics of gold clusters on amorphous carbon films

“…Moreover, we are able to reproduce experimental results 4,5,6 for the diffusion and agglomeration of size selected clusters published previously.…”

“…While the formation of small islands on short time scales has been observed previously for larger sizeselected clusters for Co on microgrid substrates, 4 for Au on amorphous carbon, 5 as well as for Ag on graphite, 6 the new features here are dark areas typically covering 25×70 nm 2 underlying the larger clusters. As described in this paper in detail we were able to identify these areas as islands consisting of Au films of roughly 4 to 8 monolayers thickness with immersed larger Au clusters.…”

“…Figure 12 shows a series of snapshots 71 from a simulation run of 200 initially randomly placed effective Au 147 clusters. Panel (a) shows that after 3000 MC steps per cluster small islands and chains of clusters have formed very similar to those observed on short time scales for monodisperse 4.8 nm diameter Au clusters on an a-C substrate, 5 Ag 5000 clusters on graphite, 6 and 8.5 nm diameter Co clusters on microgrid substrates. 4 It is important to note that the time scales for the diffusion and fusion processes depend on the temperature, the substrate properties, and the cluster sizes so that these metastable structures can be observed only in a system specific time window.…”

“…1). The fusion process of adsorbed clusters has been observed experimentally for Co clusters 4 of diameter 8.5 nm as well as for clusters 5 of Au 5000 and and has been modeled for free clusters with MD methods using embedded atom 60 and glue potentials 61 for Au 225 to Au 3805 . The objective here is to obtain an effective inter-cluster potential in the framework of the better suited many body Gupta potentials Eq.…”

“…Larger clusters fuse on much longer time scales or at higher temperatures. 4,5,6,60,61 From the MC simulations a diffusion coefficient cannot be determined quantitatively because of the lack of a time constant in the MC procedure. Panels (b), (c) and (d) of Fig.…”