Formation of ordered assemblies from deposited gold clusters

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 (DeltaD(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.