Spatial and temporal dynamics of individual step merging events on Ni(977) measured by scanning tunneling microscopy

Step dynamics induced by the chemisorption of oxygen on Ni(977) at low coverages and elevated temperatures have been studied using scanning tunneling microscopy (STM). Previous experiments using both local probe and scattering techniques have assessed the surface reconstruction behavior and structural transformations for this vicinal system, At temperatures above 390 K, the surface is capable of undergoing step doubling or merging if exposed to small amounts of oxygen. These double steps exist up to 565 K, where step-adsorbed oxygen dissolves into the nickel lattice destabilizing this morphology. Real-time STM measurements have been made on the behavior of individual merging events as a function of local step density, and hence local oxygen concentration at step edges, at 465 K. Results indicate that there is an optimal oxygen coverage, corresponding to complete titration of the single step density, that enables fast step merging to occur. An areal sweep rate of similar to 60 Angstrom (2) s(-1) was found for step doubling under these conditions. For oxygen coverages greater than the single step density in which four adjacent single steps are embedded in an otherwise doubled local environment, step merge motion was punctuated in time. We attribute this observation to local energetics, in which specific structural fluctuations, including adsorbate step decoration and local step and kink configurations, enable the doubling transition. Moreover, under these same conditions, strong spatial and temporal correlations were observed for the coalescence of adjacent pairs of steps. These time-lapse STM studies advance our understanding of the atomic-level mechanisms which contribute to the initial stages of oxidation and facetting for metallic surfaces.