Formation of (111)-oriented nickel nanocrystals by thermal annealing of nickel thin films.

We have produced Ni nanocrystals with face centered cubic structure by thermally annealing Ni films deposited on SiO2-covered Si(001) substrates in a flow of mixed hydrogen and argon gas. Ni films thicker than 5 nm self-assemble into highly (111)-oriented Ni nanocrystals on a flat and continuous SiO2 interlayer during the thermal annealing, while Ni films of 5 nm thickness aggregate to the irregularly shaped nanoparticles. The lateral width of the nanocrystals ranges from tens of nanometers to hundreds of nanometers, and the crystal height is under 100 nm. The nanocrystals have wide (111) top facets of hexagonal shape and narrow (100) sidewalls of truncated pyramidal shape, as a result of each crystal minimizing its total surface energy. Our results demonstrate that the formation of nanocrystals during thermal annealing is strongly affected by the morphology of the SiO2 interlayer, the Ni film thickness, the annealing temperature, and the partial pressure of hydrogen gas.