Triggered fast relaxation of metastable Pb crystallites

A small fraction of Ru-supported Pb crystallites, prepared by quenching from just below the melting point (∼600 K), is found in a nearly spherical metastable configuration that relaxes after an STM tip interaction. The metastable structure could not be reproduced by exposure to hydrogen, water, carbon monoxide or oxygen. However, AES measurements demonstrate the segregation of oxygen from the Pb/Ru interface to the surface of the continuous Pb film upon post-deposition annealing, suggesting surface oxygen as a possible origin of the metastability. The rapid relaxation of the metastable crystallites has been measured using STM, providing access to the early stages of relaxation where crystalline layers are removed on the time scale of seconds, in comparison with the time scales of minutes to hours for quenched structures. The shape evolution (facet radius vs. time) for different crystallites is consistent with predictions of shape-preserving relaxation from an initial Pokrovsky–Talapov shape for T<400 K. Evaluation of the overall kinetics of single layer relaxation shows that the concentration gradients of the diffusing species across the top facet of the crystallite are on the order of 1% of the equilibrium concentration.