Ultrathin Metal Films on TUNGSTEN(111) and MOLYBDENUM(111): Surface Reconstruction and Morphology
Abstract
The (111) surfaces of body centered cubic (bcc) metals such as tungsten and molybdenum are atomically rough and have high surface energies, thus are energetically unstable and may undergo substantial reconstruction to lower energy, stable surfaces under certain conditions. Previous studies have indicated that in the presence of ultrathin films of Pt, Pd and Au with coverages >1.0 ML, the W(111) surface undergoes extensive surface reconstruction upon annealing, forming pyramidal facets with {211} orientations due to the enhancement of anisotropy of surface free energy caused by the adsorption of the overlayers. A systematic study of the morphology of W(111) and Mo(111) has been conducted in the presence of ultrathin films of the first row transition metals, noble metals, platinum group metals and a rare earth metal by using low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and Temperature Programmed Desorption (TPD). The studies on W(111) based systems show that, in addition to ultrathin films of Pt, Pd and Au, the adsorption of Rh, Ir, and S as well, also effectively induce surface instability and cause faceting of the substrate, forming pyramidal facets with {211 } orientations. It has been found however, that not all ultrathin films of metals are active in terms of inducing faceting reconstruction of W(111). A correlation between overlayer properties and the faceting behavior of the substrate has been found: the elements with Pauling electronegativity greater than 2.0 seem to be capable of inducing faceting on W(111), others do not. The study on the faceting of metal surfaces is extended to Mo(111), a surface similar to W(111) in terms of atomic size, surface lattice constant and electronic structure. In addition to all the overlayer elements examined on W(111) except Ir, two more elements, i.e., Mn and Fe, are included in the investigation. The faceting behavior of Mo(111) is found to be strikingly similar to W(111): Rh, Pd, Pt and Au, and sulfur as well, are found also to be the active overlayer elements that effectively induce faceting on Mo(111). The correlation between faceting of Mo(111) and the Pauling electronegativity of the overlayer elements is the same as found for W(111). For W(111) and Mo(111) surfaces, the faceting behavior seems to be dominated by the electronic structure of overlayer elements, which in turn influences the anisotropy of surface free energy, leading to surface faceting.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT.......113G
- Keywords:
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- TUNGSTEN;
- MOLYBDENUM;
- Chemistry: Physical; Physics: Condensed Matter