In search of the smoothest nanoparticle surface: diffusion and mobility on Ag clusters

Surface diffusion is the key atomic process in nanoparticle growth. Regular shapes and low-defect surfaces can only be obtained if the deposited atoms are able to move over the entire surface of the nanoparticle-something that may be hindered by the presence of edges separating adjacent facets. Edge crossing is the rate-limiting step for adatom diffusion on nanoparticle surfaces and, consequently, edges of different sharpness are expected to affect diffusion processes differently. Here, we investigate this problem in the case of a silver adatom diffusing on top of nanoparticles with different geometric shapes: tetrahedron, octahedron, Mackay icosahedron, and chiral icosahedron. All structures have close-packed (111) facets-on which diffusion is very fast-separated by edges of different types. Using molecular dynamics simulations, we identify the most relevant edge-crossing processes and estimate their activation barriers. Our results clearly show that the geometrical shape of the nanoparticle strongly influences the inter-facet diffusion of atoms, affecting the energy barriers associated with edge-crossing processes. Jump and exchange diffusion barriers depend on the edge sharpness in opposite ways, so that-interestingly-the smoothest surfaces for adatom diffusion are both the sharpest (the tetrahedron) and the most rounded (the chiral icosahedron). Our results for Ag clusters are expected to hold for other fcc metals as well.