Spectroscopic characterization of Ni films on sub-10-nm silica layers: Thermal metamorphosis and chemical bonding

The thermal evolution in the chemical and physical characteristics of the Ni film of thickness 1-50 nm deposited on silica of thickness less than 10 nm was studied. The chemical composition of silica affected the thermal behavior of the Ni overlayer substantially. Nickel deposited on native oxide may diffuse downward into native oxide during annealing and was oxidized. It mainly produced Ni₃O₂ and silicides below 150 °C. Increasing the temperature to 300 °C caused further oxidation of Ni to yield NiO. The sub-10-nm silicon dioxide layer, on the other hand, can inhibit the diffusion of Ni atoms downward when the Ni-deposited sample was annealed. Instead, these atoms aggregated into small particles on the surface at elevated temperatures, causing the substrate to be exposed. The size of the particles produced can be controlled, as it increased almost linearly with the thickness of the Ni film deposited in the low thickness regime. The thinner Ni films yielded smaller, round nanoparticles with better dispersity. The particles formed were strongly adhered to the silicon dioxide surface. The bulk of the particles formed was mainly metallic. Exposing to the air of the Ni particles formed on silicon dioxide mainly produces Ni₂O₃ on the particles.