Emergence of New Collective Properties of Cobalt Nanocrystals Ordered in fcc Supracrystals: I, Structural Investigation

Abstract

As is now well-established, metallic nanocrystals with very low size distribution self-organize into long-range ordered three dimensional (3D) superlattices. Here, we report the annealing induced atomic phase transition of Co nanocrystals, arranged in long-range ordered fcc supracrystal 3D assemblies, from a poorly crystallized fcc structure to hcp monocrystals. We stress that this transition takes place at the atomic scale and that the fcc suprastructure of the superlattice remains unchanged. In addition, neither coalescence between nanocrystals nor oxidation of the Co material is observed. In fact, improvement in the long-range fcc order of the supracrystals is found. To our knowledge, this is the first report of the fabrication of long-range fcc supracrystals made with hcp Co nanocrystal building blocks.