Crystal surfaces normally aren't atomically flat; instead they are a mosaic of flat terraces joined by atomic steps. These steps can move and even merge into doubled steps under the right conditions. But how do these mergers, involving many atoms, occur in practice. Some new high-tech "movies" from the University of Chicago Materials Center give a startling answer. Studying the microscopic details of nanoscale processes involving metal surfaces is useful in a number of modern technologies. Specifically, investigating the concerted rearrangement of atoms over the scale of nanometers (billionths of a meter) can reveal information needed to design structures on this tiny scale.
Materials Center researchers Thomas Pearl and Steven Sibener have used time-lapse scanning tunneling microscopy (STM) to observe the oxygen-induced reconstruction behavior of Ni(977), a stepped metallic surface. Previous studies using helium atom diffraction resolved details of the macroscopic process for the reversible step-doubling and -singling of this stepped surface (Figure 1). Sequential STM imaging recorded at high temperature has now revealed atomic-level details for the merging of steps in the presence of small amounts of adsorbed oxygen, less than 2% of a single atomic layer. At temperatures above 120 ºC, the surface is capable of undergoing oxygen-mediated step doubling or merging. These double steps exist up to 290 ºC where step-adsorbed oxygen dissolves into the nickel lattice reverting the surface to single steps once again. Pearl and Sibener were looking for two things. First, how does a merging event get started. Second, once started, how does the merged step expand? These results give a real-space view of the atomic-level surface structural changes that accompany the initial stages of metallic oxidation of interfaces containing extended surface defects.
Point contact between neighboring steps decorated with oxygen atoms facilitates rapid step coalescence by means of "zippering." As shown in Figure 2, a step bulge forms to make contact with a neighboring step. This process was found to occur exclusively in the "downstairs" direction of this stepped surface and is triggered by the presence of oxygen adsorbed to mobile step edges. Measurements for two steps highlighted the degree to which oxygen tips the surface energy balance in favor of this microfaceted structure. An optimal oxygen concentration of step edge saturation was found to enable the step merging to proceed most rapidly.
Real-time STM measurements also revealed the extent to which the behavior of individual merging events (Figure 3) as well as conversion of many steps is influenced by the local step density, and hence local oxygen concentration at step edges. Excess oxygen was found to hinder the coalescence of neighboring steps through the possible growth of overlayer structures on the terraces. Local step density influences the coalescence behavior by defining the number of available step edge sites. Regions of the surface with trace amounts of non-single steps exhibited slower rates of doubling compared to regions that started with single steps exclusively.
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 surprisingly 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 faceting for metallic surfaces.
by Seth B. Darling, created 03/02