Coerced mechanical coarsening of nanoparticle assemblies

Coarsening is a ubiquitous phenomenon(1-3) that underpins countless processes in nature, including epitaxial growth(1,3,4), the phase separation of alloys, polymers and binary fluids(2), the growth of bubbles in foams(5), and pattern formation in biomembranes(6). Here we show, in the first real-time experimental study of the evolution of an adsorbed colloidal nanoparticle array, that tapping-mode atomic force microscopy (TM-AFM) can drive the coarsening of Au nanoparticle assemblies on silicon surfaces. Although the growth exponent has a strong dependence on the initial sample morphology, our observations are largely consistent with modified Ostwald ripening processes(7-9). To date, ripening processes have been exclusively considered to be thermally activated, but we show that nanoparticle assemblies can be mechanically coerced towards equilibrium, representing a new approach to directed coarsening. This strategy enables precise control over the evolution of micro- and nanostructures.