Time-resolved scanning tunneling microscopy (STM) is used to investigate a massive sulfur-induced transformation of a homogeneous array of $\sim 2\mathrm{nm}$ Co nanoparticles into a new cobalt sulfide phase. The underlying atomistic mass-transport process is revealed and, surprisingly, found to be mediated exclusively by the formation and detachment of monosized ${\mathrm{Co}}_3{\mathrm{S}}_4$ complexes at the perimeter of the Co nanoparticles. The process is followed by fast diffusion, agglomeration of the complexes, and subsequent crystallization into a cobalt sulfide phase.

• Received 13 November 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.116104