In Situ TEM Investigation of Thermally Induced Modifications of Cluster-Assembled Gold Films Undergoing Resistive Switching: Implications for Nanostructured Neuromorphic Devices

Cluster-assembled nanostructured gold films are renowned for exhibiting neuromorphic properties, making them pivotal components in receptrons. Receptrons, which extend the perceptron framework, are specifically crafted for the classification of Boolean functions due to their distinctive properties. However, the current understanding of the phenomena underpinning this behavior is based on overall electrical measurements and speculative modeling. In situ biasing and heating transmission electron microscopy (TEM) imaging were performed to directly investigate the nanoscale origin of the films' neuromorphic properties. The films, resulting from the self-assembling of preformed gold nanoclusters, were deposited just above the percolation threshold to keep them electrically conductive and sufficiently thin for TEM observation and displayed a highly branched structure at the nano- and microscale. They were studied upon in situ biasing by imaging the whole area in between the biasing electrodes. The main biasing effect consisted of very confined, pronounced, and fast retraction of the branches, with concomitant formation of a few thick nanosized gold particles, and spatially limited rearrangement of the film structure. Similarly, upon overall in situ heating, the films retracted their branched structure without apparent mass loss and again with the formation of separated gold polycrystalline nanosized islands over the whole heating substrate. These general results indicate the likely occurrence of extremely intense and very local hot spots during in situ biasing, whose temperature and extension increased with the applied voltage. They also provide direct evidence of the local arrangements occurring in the film nanostructure and morphology due to electrical biasing and help understand the origin of the neuromorphic behavior of the cluster-assembled gold films.