C.-H. Nien

FACETING INDUCED BY ULTRATHIN METAL FILMS ON W(111) AND Mo(111): STRUCTURE, REACTIVITY, AND ELECTRONIC PROPERTIES

We have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger spectroscopy, LEED, thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films ≥ 1 monolayer thick of certain metals, i.e. they form faceted structures. For example, using a UHV STM to study Pd/W(111), we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage θc. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-co...

Atomic structures on faceted W(111) surfaces induced by ultrathin films of Pd

Abstract The faceting of Pd W (111) surfaces has been studied using a Scanning tunneling microscope (STM). Three-sided pyramidal facets having {211} faces with dimensions ranging from ∼3 to 15 nm can be induced by ultrathin Pd films (≥ 1 monolayer), upon annealing to 700 K or higher. From atomic-resolution STM-images of these surfaces, we obtain direct confirmation of the {211} structure on individual facets of the 3-sided pyramids. In addition, the atomic structure of the facet edges indicates that edge energy may play a role in faceting. When the as-deposited coverage of Pd is greater than the critical value (∼ 1 monolayer) for inducing faceting, the extra Pd atoms diffuse to form 3-dimensional clusters, some with discernible crystalline structures, upon annealing.

Faceting of W(111) induced by Pd and Pt adlayers — a photoemission study

High resolution photoelectron spectroscopy using synchrotron radiation is used to probe indirectly the substrate electronic response of faceting of W(111) induced by adsorbed Pd and Pt monolayers. The W4f 7/2 core level is measured in this study. For clean W(111), there are three components observed in W 4f 7/2 photoemission due to surface, subsurface and bulk atoms, respectively ; the core level shift between surface and bulk atoms is 430 meV. Upon adsorption of Pd or Pt at 300 K, only a bulk-like W 4f 7/2 peak is observed. The interface peak associated with W atoms at the Pd-W or Pt-W interface of the W(111) face coincides with the W bulk peak. Upon annealing of the metal-covered W(111) above ∼750 K, the entire surface undergoes a transition from planar to faceted ; the W 4f 7/2 core level associated with Pd- and Pt-covered W(211) facets broadens and its centroid shifts to the lower binding energy side of the W bulk peak. The centroid shift and peak broadening are attributed to W atoms at the Pd-W or Pt-W(211) interface of the facets. The relation between the interfacial energy and the binding energy difference of the interface W 4f 7/2 peak and the surface W 4f 7/2 peak is discussed.

SURFACE RESTRUCTURING OF W(111) INDUCED BY SULFUR OVERLAYERS

The restructuring of S/W(111) surfaces has been studied using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Dosing W(111) with a saturation coverage of H2S followed by annealing to >800 K causes the substrate to reconstruct, forming a structure with (4× 4) periodicity. In addition, the terrace-step configurations restructure and form triangular domains with a preferential orientation. These domains coalesce and enlarge, and also form multiple steps when the surface is heated to T>1000 K. The low reactivity of sulfided W(111) to high exposures of oxygen demonstrates that the surface is passivated by sulfur. Adsorption of S onto a faceted Pd/W(111) surface causes the facets to disappear, restoring the surfaces planar form upon annealing. The resulting features are dominated by a (2× 2) structure. A size-mismatch mechanism, based on charge transfer between S/W and coadsorbed Pd/S on W, has been proposed to explain the formation of (4× 4) and (2× 2) structures, as well as the transition between these structures.