Electronic Structure of Copper Chemisorbed on Low-Index Tungsten Surfaces
The growth and electronic structure of thin copper overlayers chemisorbed on the (110), (100), and (111) facets of a thermally-annealed tungsten field emitter have been examined by exploiting the sensitivity of field emission and photofield emission in p-polarization to he metal-vacuum interface. For the first time, aggregates of Cu atoms have been observed in overlayers on the (110) and (100) facets following adsorption at 300 K. Aggregates on the (110) facet form on the bare substrate, while those on the (100) facet form only after an initial monolayer of Cu has been adsorbed. Aggregates were not observed at 300 K on the (111) facet even after two monolayers of Cu had been adsorbed. Aggregates on the (110) facet are found to be island-like structures that probably consist of a single layer of Cu atoms and grow primarily by atoms binding to the edge. Aggregates on the (100) facet appear to also have a flat profile, but the details of the structure are not known. Cu overlayers strongly influence the electronic structure of the W substrates and introduce additional features in the surface density of states. In the absence of aggregation, additional features appear at the (100) facet as the coverage increases in the first monolayer, whereas no additional features are detected during adsorption of the first two monolayers on the (111) facet. Additional features also emerge in the presence of Cu aggregates. On the (110) facet, striking differences are observed between the surface density of states near the centre of an aggregate and that near the edge. Features that are prominent near the edge might reflect the electronic structure of singly -adsorbed Cu atoms. On the (100) facet, aggregation destroys those features in the surface density of states associated with the first monolayer of Cu. Additional features detected at the centre of a (100) aggregate may be the surface states of a close-packed Cu layer. Some of the Cu-induced features are detected in photofield emission in p-polarization at the final state energy. These results, which are the first observations of final state effects in photofield emission using p-polarized light, demonstrate that electronic states of the absorbed overlayer can act as the final states in surface photoexcitations.
- Pub Date:
- Physics: Condensed Matter