Self-Assembled Cobalt Nanodots along Al$_{2}$O$_{3}$ (0001) step edges*

Ultrathin films of Co with thicknesses of 0.80 monolayers (ML) and 0.20 ML were grown on Al$_{2}$O$_{3}$ (0001) via molecular beam epitaxy at room temperature. \textit{In situ} non-contact atomic force microscopy images obtained after annealing the 0.80 ML film in-situ at 400 $^{\circ}$C revealed the formation of nano-dots distributed randomly on the sapphire surface with an average diameter of approximately 18 nm and a height of 0.4 nm. The annealing of the 0.20 ML film showed the formation of nanodots with diameters of 16 nm for the film annealed at 500 $^{\circ}$C and 28 nm for an annealing temperature of 500 $^{\circ}$C. In general, the nanodots increase their sizes with the annealing temperature and they move towards the top of the sapphire step edges, indicating a certain degree of self organization.