In Situ Heating and Biasing TEM Investigation of the Microstructural Evolution Occurring in Gold Nanogranular Films Showing Neuromorphic Behavior

Nanogranular gold films prepared by supersonic cluster beam deposition, with thickness above the electrical percolation threshold, are well-known to show an overall neuromorphic behavior. Indeed, they exhibit electrical features like those of biological neural networks, such as spiking and adaptation, and this makes them promising candidates for neuromorphic computing applications [1]. The current understanding for the phenomena underpinning such behavior is based on electrical measurements and speculative modelling. Thus, <i>in situ</i> heating and biasing transmission electron microscopy (TEM) imaging was performed, both in high resolution mode and at low magnification with high temporal resolution, to directly investigate what gives rise to the films’ neuromorphic properties. Nanogranular gold films were deposited just above the percolation threshold, in order to ensure their electrical conductivity while keeping them sufficiently thin for TEM imaging. Upon overall <i>in situ</i> heating, the films de-percolated, i.e., they retracted their branched structure with no apparent mass loss. As a consequence, quite thick, well separated gold polycrystalline islands were formed over the whole heating substrate, a behavior resembling what previously reported for atom assembled films [2]. However, when subjected to sole <i>in situ</i> biasing in a two-electrode configuration, the thin, percolating films show electrical behavior quite similar to the thicker ones. Aiming at understanding what happened to the films morphology globally and locally, the whole area comprised between the biasing electrodes was imaged with high temporal resolution by a CMOS direct detection camera (Gatan K3) working with a limited dose rate, while the films were electrically stimulated first over a voltage ramp and then at constant voltage. Again, the main effect of the biasing is branches retraction, but in this case so spatially confined that the films undergo a very local de-percolation, thus with spatially limited rearrangement of their microstructure and concomitant formation of few, small and thick gold islands in the close, little areas involved. These results overall indicate a likely occurrence of extremely intense and local hot spots. Their temperature and extension are shown to increase with the applied voltage and can finally bring to local melting-dictated breaks of the amorphous silicon nitride constituting the deposition substrate of the films. Besides, further aspects still need to be investigated, such as, for instance, the possible determination of the temperature in the very local hot spots, and the chance of a not trivial concomitance of local heating by Joule effect and electromigration of gold atoms, which in previous studies have been indicated as possible physical mechanisms giving origin to the neuromorphic behavior of metal films. In any case, the <i>in situ</i> TEM imaging demonstrates its capability in providing new insights on the underlying physical phenomena giving rise to the unique electrical behavior of metal nanogranular films, with several implications for further developing of advanced materials with neuromorphic properties.<br/><br/>[1] M. Mirigliano, <i>et al</i>. Binary classifier based on a reconfigurable dense network of metallic nanojunctions. <i>Neuromorph. Comput. Eng.</i> <b>2021</b>, <i>1</i>, 024007A. DOI: 10.1088/2634-4386/ac29c9<br/><br/>[2] F. Niekiel, <i>et al</i>. The process of solid-state dewetting of Au thin films studied by in situ<br/>scanning transmission electron microscopy. <i>Acta Mater.</i> <b>2015</b>, <i>90</i>, 118. DOI: 10.1016/j.actamat.2015.01.072