Dynamics of Dielectrophoretic-Force-Directed Assembly of NaYF4 Colloidal Nanocrystals into Tunable Multilayered...

The dynamics of dielectrophoretic-force-directed assembly of polarizable colloidal upconverting β-NaYF4 nanocrystals into tunable multilayers on charge micropatterns written by atomic force microscopy is investigated. Multilayered nanocrystal assembly by this nanoxerography process occurs in two phases. During the first phase typically lasting a few minutes, the nanocrystal assemblies grow up to a maximum thickness under the influence of strong dielectrophoretic forces exerted by the charge patterns. Subsequently, the nanocrystals start to diffuse back into the solvent, leaving a single layer attached to the charge patterns. A theoretical model based on the Fokker-Planck equation is formulated to describe this dynamics involving an interplay of diffusive and dielectrophoretic forces. Being in good agreement with the experimental results, this approach may be reliably extended to simulate the directed assembly of other types of polarizable colloids from liquid phase by nanoxerography.