A Dynamic View of Step Configurations on Ag(110) and Their Role in the Formation of Oxygen Overlayers
- PDF / 3,847,472 Bytes
- 7 Pages / 414.72 x 648 pts Page_size
- 106 Downloads / 159 Views
J.S. OZCOMERT',1,2 W.W. PAIT,1 N.C. BARTELT 12, and J.E. REUTf-ROBEY2 , Department of Physics' and Department of Chemistry2 , University of Maryland, College Park, MD 20742.
ABSTRACT Step fluctuations on Ag(1 10) surfaces have been investigated with STM. The atomic events that underlie these thermal fluctuations are quantified using a Langevin analysis. From the t12 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 vicinalAg(1 10), 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. INTRODUCTION In this paper, we give some highlights of work over the last two years using Scanning Tunneling Microscopy (STM) to study several aspects of Ag(110) surface structure and reactivity. Equilibrium fluctuations of the surface at room temperature has been studied by monitoring the position of individual steps as a function of time. These observations can be analyzed to obtain the fundamental mass transport mechanisms and the rate of motion at step edges. The faceting of vicinal Ag(1 10) surfaces upon exposure to oxygen has also been observed with STM. This striking surface rearrangement is found to be thermodynamic in origin, as it is reversible upon exposure to CO. In addition, the dynamics of the faceting is sensitive to the step edge orientation. Finally, we report some results of a study investigating the microscopic nature of the oxygen overlayer and Ag(1 10) steps as a source of atoms for surface overlayer formation. The kinetic balance between the step-edge supply rate of Ag atoms and the rate of oxygen chemisorption determines the kinetic mechanism for the oxygen overlayer formation. Each of these subjects is treated as an overview; more details can be found elsewhere, as indicated in the text.
115 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
EXPERIMENTAL Experimental studies were carried out in an ultrahigh-vacuum chamber that has been described in detail previously [1,2]. All STM measurements were performed at room temperature with a negative sample bias
Data Loading...
