Characterization of the reorganization at the microscale of nanostructured films during resistive switching by means of in operando optical dark field microscopy

Nanostructured films assembled by metallic nanoparticles deposited by Supersonic Cluster Beam Deposition have been investigated for their resistive switching (RS) activities in view of unconventional and neuromorphic computing applications. The physical phenomena underlying RS in nanostructured metallic films have been attributed to structural and morphological rearrangements of grain boundaries, at the nano- and meso-scale, triggered by joule heating and electromigration. However, the direct in operando observation in ambient condition of the structural and morphological reorganization at different scales of the nanostructured films under voltage stimuli has never been reported. By means of dark field optical microscopy we obtained the first real-time evidence of the irreversible morphological reorganization over the entire nanostructured network, as also the reversible local ones, occurring in a gold cluster-assembled film under electrical stimulation, responsible for the emerging collective RS behavior. We have identified distinct film reorganization phenomena: an irreversible roughening of the entire film network is observed during the electrical forming process, which triggers the onset of RS. After this initial modification, reversible structural rearrangements occur at both the macro- and micro-scale. These results support the proposed model, which relies the RS mechanism in cluster-assembled films on the modulation of the complex cluster-assembled film structural connectivity.