The dynamics of the room temperature faceting of Ag(110) upon exposure to oxygen has been studied using scanning tunneling microscopy (STM). Vicinal surfaces are exposed to oxygen while STM images are continuously collected. The surfaces are observed to break up into regions of high step density (‘‘step bunches’’) and (110) facets. This faceting is driven thermodynamically by the formation of an n×1 oxygen overlayer on the (110) facet, in which rows of oxygen atoms form along the next-nearest neighbor [001] direction. The dynamics of this breakup are found to depend sensitively on the step-edge orientation of the original clean vicinal surface. The reversibility, and thus probable thermodynamic origin, of the faceting is demonstrated by removal of the oxygen through chemical reaction with CO.

Step fluctuations on Ag(110) surfaces have been investigated with STMT atomic events that underlie these thermal fluctuations are quantified using a Langevin analysis. From the t1- scaling of the step-position correlation function, we deduce random attachment and detachment processes along the step edge, with a characteristic interval of 350 ms between successive detachments. Upon oxygen adsorption on vicinal Ag(110), a dramatic change in step configuration occurs. Steps which are originally equi-spaced due to repulsive step-step interactions are compressed into bunches. This process is driven by the formation of large (110) facets, on which oxygen atoms arrange into chains along [001]. The faceting dynamics are sensitive to the orientation of the step edge: proceeding by nucleation for close-packed steps and by spinodal decomposition for steps at an acute angle to the oxygen chains. A closer inspection of these oxygen chains reveals that each incorporates an additional row of silver atoms. When oxygen dosing pressures are kept below 10-6 Torr, silver atoms detaching from the step edge provide a sufficient supply necessary for the formation of the added row. With higher oxygen pressures, the silver atoms required for the oxygen chains are extracted directly from the terraces, resulting in the formation of large, long-lived etch pits.

The distribution of terrace widths on vicinal Ag(110) surfaces is measured with scanning tunneling microscopy as a function of mean terrace width [l] and interpreted in terms of a step-step interaction potential. As previously observed on vicinal Cu(100) surfaces, but in contrast to reports involving semiconductor surfaces, the distribution does not scale simply with (1). For vicinal Ag(110) surfaces with [l] = 22 angstrom, the distribution resembles that expected for noninteracting steps. For [l] = 30 angstrom, the distribution narrows significantly, suggesting repulsive interactions. For [l] = 40 angstrom, the distribution has a form expected for attractive interactions. The absence of simple scaling of this distribution reveals that the decay of step-step interactions is more complicated than the 1-2 behavior expected for elastic or dipolar repulsions. A nonmonotonic potential is found to reproduce the observed terrace-width distributions, as demonstrated with Monte Carlo simulations of a terrace-step-kink model. According to this trial potential, step interactions are dominated by a repulsive l-2 term at short l and by an RKKY-like oscillatory term at longer 1. The form of the latter term is reminiscent of indirect interactions between adsorbates.

Step configurations on Ag(110) near pinning sites have been measured under ultrahigh vacuum conditions with scanning tunneling microscopy (STM). At isolated pinning sites the surface structure is generally characterized by a high density of monatomic steps (step bunch) on the “lower” side of the pinning site and a very large (> 1000 Å) terrace on the “upper” side of the pinning site. The step bunch partially envelopes the pinning site, resulting in a curved interface between the step bunch and the upper terrace. We propose that the shape of the step bunches near these pinning sites reflects the local thermodynamic equilibrium between the surface tension of the curved step front and the pressure due to step-step interactions. These interactions are evaluated for steps with the same net orientation as the pinned steps via STM measurements of terrace width distributions. The distributions indicate that step-step interactions are predominantly entropie with a step stiffness of ~ kT/(5 Å). The Gibbs-Thomson relation, together with these measurements of step-step interactions, are used to predict the curvature of the step bunch-upper terrace interface at the pinning site. The predicted radius of curvature of 3000 Å for 50 Å mean step separation agrees with experimental observations.