Temperature Dependence of Cluster-cluster Coalescence in Monodispersed Co Cluster Assemblies

Using a plasma-gas-condensation-type cluster beam deposition system, we deposited monodispersed Co clusters with mean diameter d = 8.5 nm on quartz and microgrid substrates. The cluster-cluster coalescence process of the Co cluster assemblies was investigated by in situ electrical conductivity measurements and ex situ transmission electron microscopy (TEM). The change of magnetic properties induced by the inter-cluster coalescence was also discussed. The electrical conductivity measurement indicated that, at temperatures T < 100°C, the Co clusters in the assemblies maintain their original size as deposited at room temperature, while the inter-cluster coalescence takes place at T > 100°C. The TEM observation showed that the size distribution and the interface morphology of the clusters do not change markedly at substrate temperatures Ts ≤ 200°C, while cluster-cluster coalescence starts at Ts > 200°C. Above Ts = 300°C, the interfacial area of coalesced clusters is crystalline, having its own orientation different from those of two connected cluster cores.