  Rigorous understanding of the self-assembly of colloidal nanocrystals is
crucial to the development of tailored nanostructured materials. Despite
extensive studies, a mechanistic understanding of self-assembly under
non-equilibrium driven by an external field remains an ongoing challenge. We
demonstrate self-assembly by optical tweezers imposing an external attractive
field for cubic-phase sodium yttrium fluoride nanocrystals. We show that
surface roughness of the nanocrystals is a decisive factor for contact leading
to assembly between the nanocrystals, manifested by the roughness-dependent
hydrodynamic resistivity. This provides direct evidence that dynamics are
equally important to energetics in understanding self-assembly. These results
have implications in a wide variety of different fields, such as in
understanding the factors that mediate oriented attachment-based crystal growth
or in interpreting the structure of binding sites on viruses.
