Electron localization in granular materials

We have investigated the structural and electrical properties of granular Pd/C as well as granular Au/C thin films. As revealed by transmission electron microscopy the films consist of small metallic clusters of either Pd or Au embedded within amorphous carbon. Films with low metal volume fraction, x consist of structurally isolated clusters and show exponential resistance behavior. Above the so-called percolation threshold x(p), metallic clusters form a percolating network rendering metallic conductivity. For metallic films with x much greater than x(p) we observe two dimensional (2d) electronic transport behavior at low temperatures via small additional contributions to the Boltzmann-resistance as arising from weak electron localization (WEL) and enhanced electron-electron interaction (EEI). When approaching the percolation threshold these additional resistance contributions grow in absolute magnitude and the 2d transport behavior changes to 3d-behavior close to x(p). For both cases resistance as well as magnetoresistance data can be well explained using theories for WEL and EEI in 2d and 3d respectively.