Nanostructured nickel films by supersonic cluster beam deposition: Morphology, oxidation, and clues of hollow structures by Kirkendall effect at room temperature

Abstract

Supersonic Cluster Beam Deposition was exploited to study morphology, nanostructure and room temperature spontaneous oxidation of Nickel films obtained by cluster assembling in high-vacuum conditions. Size analysis shows the dominant presence of particles with dimensions below 10 nm, while the lognormal profile of the size distribution confirms that gas-phase homogeneous nucleation and aggregation prior to deposition is at the base of the mechanism governing clusters formation. The relationship between surface roughness and film thickness is found to follow a power law, suggesting a growth process ascribable to ballistic deposition model. Although ultrafine dimensions, clusters oxidation is not complete as residual metallic Nickel, beside NiO and Ni(OH)2, is detected by Electron Spectroscopy and observed by Transmission Electron Microscopy and Electron Diffraction, within core-shell structures. Cubic symmetry is identified in both metallic and NiO phases, while hydroxide phase appears as amorphous. The presence of hollow particles is also unveiled, which may represent a first clue of room temperature Kirkendall effect in ultrafine Nickel particles produced by Supersonic Cluster Beam Deposition. The coexistence of core-shell and hollow structures of the same size suggests that particle dimension per-se is not enough to define unambiguously the outcomes of room temperature oxidation process.