Au nanocrystal superlattices: nanocrystallinity, vicinal surfaces, and growth processes

Vicinal Au supracrystal surfaces were prepared from Au-single single domain nanocrystals (NCs), whereas by replacing Au-single with their polycrystalline counterparts common low-energy supracrystal surfaces were produced. By analogy to atomic crystalline surfaces, we propose a mechanism to explain the formation of such unexpected supracrystal vicinal surfaces, composed of only Au-single NCs which are non-compact (bct structure) with a coherent alignment of the atomic planes oriented along the [111] superlattice axis and a slight tilt configuration (8.1 degrees) of NCs. In the presence of Co(epsilon) NCs, these Au-single supracrystals lose both the slightly tilted configuration of NCs and their orientational order leading to a superlattice transition from bct to fcc. In contrast, supracrystals of Au-poly NCs are insensitive to the presence of Co(epsilon) NCs. These intriguing structural changes obtained with Au-single compared to Au-poly supracrystals in the absence and presence of Co(epsilon) NCs could explain the formation of vicinal surfaces. Note that the solvent used to disperse the nanocrystals plays a key role in the formation of supracrystal vicinal surfaces. Here, a new analogy between supracrystals and atomic crystals is presented.