Geometrical and electrical percolation in nanometre-sized Co-cluster assemblies

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Abstract
We deposited monodispersed Co-clusters in the size range of 6-13 nm on substrates using the plasma-gas-condensation cluster deposition system. The assembling process of the clusters from discontinuous to continuous networks was investigated by transmission electron microscopy (TEM) and in situ electrical conductivity measurement, and discussed in terms of the two-dimensional (2D) percolation concept. The electrical conductivity measurement indicates that the percolation process of Co clusters does not agree with a simple scaling-law: the critical conductivity exponent increases with increasing mean cluster diameter, d, although it is predicted to be independent of d in the ordinary 2D percolation theory. This anomaly is interpreted by the soft-percolation model, implying that there is distribution of electrical contacts between the clusters. The critical coverage of clusters (0.63) is much higher than the predicted one (0.45) irrespective of d, due mainly to the partial overlapping of deposited clusters, and partly to an attractive interaction between the clusters. Such cluster-overlapping also increases the critical thickness of electrical percolation with increasing d.