Dynamics of Polar Surfaces on Ceria Nanoparticles Observed In Situ with Single-Atom Resolution

摘要

Atomic hopping processes on ceria nanoparticle surfaces are observed by in situ phase contrast high-resolution electron microscopy with an aberration-corrected imaging lens. It is shown that single-atom resolution is possible, and single-atom dynamics for cerium are observable. Discrete changes in contrast and discrete positional changes of contrast maxima can be safely interpreted as visual fingerprints of atomic displacements. Both single-atom movements and spontaneous sequential relocations of entire atomic rows are observed. Exclusive occurence of the effect on {100} type facets indicates polar dipole field mediated atomic rearrangements, while {111} facets are found to be stable. Molecular modelling confirms that the relocations follow genuine pathways involving partially occupied oxygen-terminated surfaces, by means of temperature induced fluctuations. A series of images tracks the detailed atomic motions over a time of 120 s and quantifies the ratio of reversible atom hopping versus atom ablation.