Computational Study of the Self-Organization of Bidisperse Nanoparticles

We study computationally the self-organization of bidisperse mixtures of thiol-stabilized gold particles in two dimensions through random sequential adsorption (RSA) coupled with the Metropolis algorithm for determining surface diffusion. It was previously shown [Doty et al. Phys. Rev. E 2002, 65, 061503] that ordered lattices of bidisperse particles cannot form with hard sphere interactions. Here we include the effects of interparticle forces. Osmotic and steric interactions provide a repulsive force at close distances, while at longer ranges the van der Waals interaction leads to attraction. Two size ratios (sigma) of 0.375 and 0.577, determined experimentally to form LS (the two-dimensional NaCl analogue) and LS2 (the two-dimensional AlB2 analogue) lattices, were studied. The calculated jamming limits for RSA fall well below the minimum surface coverage necessary for stable ordering as determined by melting simulations. Uniform compression of the particles' positions, as a model of the convection and lateral capillary forces that would be experienced during solvent evaporation, allowed this critical surface coverage to be achieved, and LS lattice formation was observed for sigma = 0.375. No LS2 lattice formation was observed for sigma = 0.577 with compression. The melting coverage of the LS2 lattice far exceeds the coverage observed experimentally and so is not observed.