Adhesion, mobility and aggregation of nanoclusters at surfaces: Ni and Ag on Si, HOPG and graphene

Abstract: An experimental investigation of Ag and Ni nanoparticles (NPs) deposited on Silicon with its native oxide, on highly oriented pyrolytic graphite and on graphene flakes is reported. The NPs were physically synthesized with a magnetron based gas aggregation source and the produced beam was mass-filtered and deposited in vacuum on the substrates. The study was concentrated on the morphology for the different cases, shedding some light on the interaction of pre-formed NPs with surfaces, a crucial aspect both of technological and scientific relevance. The nature of adhesion can be strongly influenced by the intrinsic properties of the surface (like for instance the energetics of interaction between the NP surface atoms and the first layers of the substrate) and/or the extrinsic properties, like the presence of defects, step edges, impurities and other irregularities. After adhesion, the NPs mobility and their mutual interaction are very relevant. In this work, the study was concentrated on NP/surface morphology, by using atomic force microscopy, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Article highlights: Morphology of physically synthesized metal Nano-Particles (NPs) on Si, HOPG and Graphene was investigated. The NPs were pure Ag and Ni. Coalescence, diffusion and self-aggregation and preferential adhesion were observed, with possible applications in sensor technology.Possible explanations are: NP softness, NP/surface bonding interaction and presence of contaminant species molecules between NP.