ADSORBATE-PROMOTED MASS-FLOW ON THE GOLD(111) SURFACE OBSERVED BY SCANNING TUNNELING MICROSCOPY

Using time-lapse sequences of scanning tunneling microscopy images, we have studied the room temperature mass flow from nanoscale nonequilibrium gold structures formed on the gold (111) surface. Isolated clusters of adatoms (islands) and vacancies (pits) formed in air decay in size according to the power law scaling r is-proportional-to (t0 - t)n, where n = 1/2, r is the cluster radius, t is the observation time, and t0 is the time at which the cluster vanishes. This behavior is consistent with a model used to describe nucleation and growth in which the mass flow is driven by the curvature of the step edges, and is limited by the generation of adatoms (vacancies) at the edges of the islands (pits). In contrast, we have found no significant mass flow at room temperature for similar structures formed on clean samples under high, and ultrahigh vacuum (UHV) conditions. However, mass flow can be induced on these surfaces by admitting certain gases into the vacuum which adsorb on the surface. The adsorbate-induced mass flow is consistent with a model in which the adsorbates promote the creation of adatoms and vacancies at step edges. Using time-lapse image sequences, we compare the mass flow observed under ambient, UHV, and adsorbate-contaminated conditions, and discuss the role of the adsorbates in promoting the mass flow process at room temperature.