(80a) Surfactant-Coated Nanoparticle on a Stepped Surface

Traditionally, ultrafine metal or semiconductor particles are prepared as sols or electrically stabilized aqueous colloids, which tend to form fractal aggregates in solutions or when precipitated. However, to fully realize the technological potential of nanometer-sized particles, they need to be fabricated into predesigned structures or multidimensional ordered arrays. Recently it was discovered that by capping nanoparticles with surfactant molecules during synthesis, the resultant surfactant-coated nanoparticles can remain well dispersed in solutions and tend to self-assemble into ordered arrays when deposited on solid substrates and on the free surface of water in a Langmuir trough. Despite great promises, the current fabrication methods of surfactant-coated nanoparticles are still highly empirical in nature and produce partially satisfactory results. From a fundamental perspective and based on the analysis of key experimental results, it can be hypothesized that nanoparticle chain arrays could be used as nanobuilding blocks and controllably produced by using templated substrates with one-dimensional structures. To assess the feasibility of the hypothesis, we have simulated gold thiol nanoparticles on a stepped fcc(14,13,13) surface and on a corresponding flat surface with molecular dynamics (MD) method and atomic-scale models for nanocrystals, surfactants, solvent, and solid surfaces. Compared to the flat surface, surface steps provide extra energy barrier against diffusion across the steps and thus promote anisotropic nanoparticle diffusion preferably along the step edges. This implies that surface steps could be considered to be used as ?canals? to transport nanoparticles on surface, and as ?templates? to promote the formation of one-dimensional nanoparticle chain arrays. Stepped surfaces freshly cut from bulk crystals have steps that are of single atom height and relatively small compared to the size of typical nanoparticles. For better effectiveness in promoting anisotropic surface mobility and forming nanoparticle chain arrays, reconstructed or specially prepared surfaces having larger steps would be desired.