Controlled Self-Assembly of Colloidal Cobalt Nanocrystals

There is growing interest in organizing materials on the nanometer scale for fundamental investigations [ 1–3 ] and novel technological applications [ 5 , 6 ]. One of the most promising routes to create highly ordered structures on the nanometer scale is by the self-assembly of nanoscale building blocks comprised of colloidal nanocrystals [ 7 , 8 ]. However, the mechanisms influencing the self-assembly of such nanocrystals are not understood [ 9 ] and there is a critical need to develop general approaches for predicting and controlling this behavior. Here, using surfactant-stabilized Co nanocrystals [ 10 ] with narrow size distributions and controlled shapes as building blocks, we describe our ability to finely tune their self-assembled arrays. We can selectively achieve square packing, hexagonal close packing, linear chains, arrays spatially segregated as a function of particle size and lyotropic liquid-crystal-like arrays with orientation order. This richness in self-assembly is obtained in a single system as a function of size and shape, and in which one of a set of competing forces (steric, van der Waals, depletion, entropy or magnetostatic) dominates to determine the resulting organization.