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Abstract
As part of an effort to characterize the morphological instability of bcc(111) surfaces, the surface restructuring of W(111) induced by various overlayers has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). Three-sided pyramidal facets having {211} faces (as confirmed by atomic-resolution STM-images) can be induced by ultrathin Pd films (>1 monolayer) on W(111), upon annealing to >700K. We also observe the {011} faceting of Pd/W(111) in coexistence with {211} faceting upon prolonged annealing. While {211} facets are found to grow in size from ∼30Å to 150Å with increasing annealing temperatures, the dimensions of {011} facets appear to be limited to ∼25Å. The coexistence of two types of pyramidal facets is consistent with energetic considerations based on first-principles calculations; the remarkable difference in their typical sizes is interpreted as an evidence for kinetic factors in the faceting processes.
In contrast, a sulfur-covered W(111) surface heated to >800K reconstructs with (4 x 4) periodicity. Upon further annealing, the terrace-step configurations restructure and form triangular terraces, which coalesce and enlarge at T > 1000K. A size-mismatch mechanism based on charge transfer is proposed to explain the formation of (4 x 4) structures. An atomic vacancy found in each (4 x 4) unit cell is believed to play a significant role in the strain relief mechanism. We have also explored a terrace growth mechanism for S(4 x 4)/W(111) based on structural variety of the atomic steps, coupled with domain mismatch. This leads to general principles of surface domain growth, which provide insights into the relationship between the surface reconstruction and the morphological changes.
A brief report of oxygen-induced restructuring on W(111) is documented, and we report coadsorption of pairs of overlayer materials on W(111). When significant amounts of carbon impurity is found on Pd/W(111), coexistence of two spatially separated phases (i.e., planar regions with a (2 x 2) structure and faceted regions with {211} pyramids) is observed upon annealing. For a W(111) surface coadsorbed with Pd and S, the as-deposited morphology depends remarkably on the order of deposition, whereas the post-annealing morphology is fairly similar in all cases.
As byproducts of this thesis work, two methods have been developed for analyzing different aspects of STM data (i.e., the size distribution of pyramidal facets and the volume measurement of three-dimensional clusters). The first method, being designed for statistical counting of objects in two dimensional arrays, is expected to have such applications as real time statistical analysis and straightforward model-testing in various discretized imaging techniques.
