Thickness induced metal to insulator charge transport and unusual hydrogen response in granular palladium nanofilms

Abstract

This work reports a systematic study of the evolution of charge transport properties in granular ultra-thin films of palladium of thicknesses varying between 6 nm and 2 nm. While the films with thickness >4 nm exhibit metallic behaviour, that at 3 nm thickness undergoes a metal–insulator transition at 19.5 K. In contrast, the 2 nm thick film remained insulating at all temperatures, with transport following Mott's variable range hopping. At room temperature, while the thicker films exhibit resistance decrease upon H₂ exposure, the insulating film showed an anomalous initial resistance increase before switching to a subsequent decrease. The nanostructure dependent transport and the ensuing H₂ response is modeled on a percolation model, which also explores the relevance of film thickness as a macroscopic control parameter to engineer the desired system response in granular metal films.