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

We deposited monodispersed Go-clusters in the size range of 6-13 nm on subs trates using the plasma-gas-condensation cluster deposition system. The ass embling process of the clusters from discontinuous to continuous networks w as investigated by transmission electron microscopy (TEM) and in situ elect rical conductivity measurement, and discussed in terms of the two-dimension al (2D) percolation concept. The electrical conductivity measurement indica tes that the percolation process of Co clusters does not agree with a simpl e 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 s oft-percolation model, implying that there is distribution of electrical co ntacts between the clusters. The critical coverage of clusters (0.63) is mu ch higher than the predicted one (0.45) irrespective of d, due mainly to th e partial overlapping of deposited clusters, and partly to an attractive in teraction between the clusters. Such cluster-overlapping also increases the critical thickness of electrical percolation with increasing d.