Ac and dc electrical conductivity of polynuclear metal cluster compounds

Ac and dc electrical conductivity data are presented for two polynuclear metal cluster compounds Au₅₅(PPh)₃)₁₂Cl₆ and Pd₅₆₁Phen₃₆O₂₀₀. The temperature dependence of the low field dc conductivity increases with electrical field, the rate of increase becoming stronger with decreasing temperature. These results are compared with data for granular metal systems, and a close correspondence is observed. The conductivity is found to increase with frequency in analogy to what is observed in amorphous systems. A scaling law for low frequencies that has been recently proposed is shown to describe the data quite well.     

Frequency and field dependent conductivity in Pd-based metal cluster compounds

Ligand stabilized metal cluster compounds and colloids consist of nanometer-size metal cores surrounded by ligand shells. We study the dielectric properties of randomly packed aggregates of these compounds as a function of temperature T, electric field E and frequency ω. Cores were formed by Pd-atoms and stabilized by a selection of ligands. The core size varied between 2.4 and 3.6 nm (monodisperse) and 18 nm (5–10% dispersion). The dielectric properties of all compounds show scaling: the same functional dependence of the normalized conductivity on normalized frequency irrespective of temperature. The results imply that at room temperature at all frequencies below 1 THz exclusively hopping between the particles is seen. More details of the hopping processes are obtained by varying E. The observed dependences are consistent with stochastic models that use random packing of clusters inside the sample.