| The mobility of three-dimensional (3-D), nanoscale metal particles on substrates impacts many technologically important processes, including catalysis and self-assembly. A particularly plaguing impact: the sintering of heterogeneous catalysts, where catalytically active metal nanoparticles migrate and coalesce into larger, less active particles. Crucial to mitigating sintering is determining whether this mechanism, or the alternate mechanism, Ostwald ripening (where particles are immobile) dominates. In contrast to earlier approaches utilizing size distributions, we study the role of particle mobility in determining the sintering mechanism.; We have developed a 3-D Monte Carlo model capable of simulating particle diffusion, shaping, and vapor phase transport. Simulated data yield scaling relations between diffusion coefficient and size and Arrhenius dependencies for two diffusion mechanisms, evaporation-condensation and periphery diffusion. Simulation results indicate evaporation condensation is driven by surface diffusion of adatoms to critical evaporation locations, while particle shaping drives the periphery diffusion mechanism. Evaporation condensation simulations yield concentrations of free adatoms in equilibrium with particles. Results fit the Gibbs-Thomson relation; however, the surface free energy exceeds the expected value, revealing a decreased stability of particle atoms due to the small particle size.; We measure diffusion of 3-D Pd nanoparticles on TiO2(110) with atom-tracking scanning tunneling microscopy (STM). We observe simply activated diffusion of small particles, with hops essentially confined to the [001] direction; scaling analyses and the 3-D Monte Carlo model allow interpretation of the diffusion mechanism. The instability of the small particles impedes faceting; incremental growth leads to more stable particles, the onset of faceting, and a potential shift to the periphery diffusion mechanism, causing larger particles to become stationary on the experimental time scale.; We also utilize atom-tracking STM to quantify tip effects on diffusion measurements. While the primary tip effect, electric field, yields small changes in the overall diffusion barrier, these small changes vary linearly with field. These results, coupled with first principles calculations, demonstrate correspondence between negative field and a walking diffusion mechanism, positive field and a piecewise diffusion mechanism, for Ge-Si ad-dimers on Si(001). We confirm positive field facilitates Ge intermixing, as predicted by theoretical calculations, revealing a method to control structuring at interfaces. |
