Electrical conduction in nanogranular cluster-assembled metallic films

A film fabricated by the assembling of nanoparticles that retain, at least partially, their individuality is expected to show substantially different structural and functional properties compared to the case where atoms or molecules are used as building blocks. Although films assembled with metallic clusters or nanoparticles have been predicted to have unusual functional properties, it has been tacitly assumed that cluster-assembled metallic films have the same conduction behavior observed for polycrystalline metallic thin films grown from atoms. Unexpectedly, in the last decade, several studies showed that nanogranular metallic films show a non-linear electric behavior, substantially different from their polycrystalline counterparts. Here we review and discuss the electrical transport properties of cluster-assembled films. Our aim is to provide a background and a common language for the systematic investigation and exploitation of nanogranular metallic thin films where the extremely high density of defects and grain boundaries causes the departure from ohmic behavior. We will focus on the non-linear electrical conduction and resistive switching relevant for neuromorphic applications.