Crystallization of Opals from Polydisperse Nanoparticles

We report the reversible formation of crystals of nanoparticles (opals) from solutions of polydisperse gold nanocrystals. The structures are identified by transmission electron microscopy, and are characterized by hexagonal domains of large particles at the center, surrounded radially by successively smaller particles. Simulated annealing Monte Carlo calculations are used to demonstrate that these configurations correspond to minimization of the mesoscopic van der Waals energy of polydisperse particles, and the driving force for ordering is the size dependence of dispersional attractions.