MICROSTRUCTURE AND ELECTRICAL TRANSPORT PROPERTIES OF METALLIC CLUSTER-ASSEMBLED FILMS

Nanostructured cluster-assembled metallic films have unique properties that can deviate remarkably from their atom-assembled counterparts, making them auspicious materials for a wide range of applications in different fields, including but not limited to chemoresistive gas sensing, electrochemical devices (lithium-ion batteries, fuel cells), memristive electronic devices. In all these cases, the understanding of the electrical transport properties of the films and the link with their micro- and nanostructure is of fundamental importance. In this context, I present a comparative study of the electrical conduction properties in cluster-assembled nanostructured Sn, Pt, Ni and W films produced by Supersonic Cluster Beam Deposition (SCBD), investigated in-situ during film growth, spanning from insulating phase to percolation and three-dimensional growth beyond percolation. Ex-situ morphological characterization provided information about the evolution of the nanoparticle morphology, microscale growth and oxidation after exposure to air. The investigated materials include very low and very high melting point metals, as well as less prone and more prone to oxidation ones. My research highlights nanoparticle size and coalescence's critical role in the electrical conduction behavior at the percolation for the investigated cluster-assembled films. Specifically, I have observed that for Sn films, the deposition rate interferes with coalescence dynamics at the early stages of the growth. Lower deposition rates favor the coalescence of primeval size particles, resulting in enlarged islands and denser films. This, in turn, shifts the percolation threshold to a higher thickness with an abrupt onset of conductance. Conversely, higher deposition rates promote an advanced formation of percolation paths, with the growth of smaller islands and more porous structures, leading to an earlier percolation threshold with a more gradual transition from insulating to conducting phase. This behavior contrasts with Pt, Ni and W films, where coalescence plays a much less significant role at percolation, and particles with a smaller size are efficiently formed well-connected paths, achieving percolation at reduced thicknesses. Following the percolation phase, electrical conduction stabilizes, and film resistivity sets at values 2-3 orders of magnitude larger than bulk one for both materials, allegedly due to the nanogranular nature of cluster-assembled films. In this three-dimensional growth beyond percolation, resistivity shows an increasing trend with thickness, a feature ascribable to the decrease of the density of interconnections between particles in the topmost layer during film growth, as expected in ballistic growth. Understanding the impact of nanoparticle size and coalescence and its implications on the electrical transport of nanostructured metallic films is essential for tailoring the properties and optimizing their performance in different applications. Additionally, below the percolation threshold, cluster-assembled metallic films exhibit nonlinear current-voltage (I-V) characteristics, indicating that quantum tunneling is the primary charge transport mechanism across nanoscale gaps. This consistent behavior across all studied materials highlights a common conduction mechanism in this regime. However, Fowler-Nordheim tunnelling currents were only observed in Sn films, highlighting the microstructural differences resulting from coalescence in comparison with the high melting point studied materials. Preliminary results demonstrate the effectiveness of combining two metals (Sn and Pt) for chemoresistive gas sensing applications. Additionally, according to the scientific literature, distinct mechanisms of non-linear electrical conduction and conductance switching, relevant to neuromorphic applications, were observed in Sn films near the percolation threshold, Ni and Pt films beyond the percolation threshold, and WOₓ films.