ELECTRON FIELD EMISSION TUNNELING SPECTROSCOPY OF PLATINUM GROUP AND TUNGSTEN METALS IN THE PRESENCE OF MOLECULAR GAS AND NOBLE METAL ADSORBATES.

摘要

Using field-electron energy-distribution, (FEED), measurements, three adsorbate/substrate systems have been investigated. Molecular gases, (hydrogen, carbon monoxide, oxygen, nitrogen, nitric oxide, ammonia and hydrazine), have been adsorbed on several crystal faces of iridium. Gold and copper have been adsorbed on iridium, rhodium and platinum. Finally gold and copper have been adsorbed on crystal faces of tungsten.; In results similar but not identical to those of other experimentalists, we find that on W{lcub}100{rcub} the surface state present in the clean FEED spectrum disappears immediately as adsorption of gold and copper begins. However, at both one and two monolayer coverages, the surface state and, in fact, the entire clean energy spectrum returns. We believe these results can be explained in terms of theoretical models and the symmetry of the surface atomic layer. The symmetry is broken by the initial adsorbate coverage but is reestablished at integral monolayer coverages by perfect pseudomorphic ordering of the overlayers.; For copper the results depend on substrate temperature. After four monolayers of copper on a 530K substrate an extremely large resonance feature appears at 0.8 eV. This is probably caused by reconstruction of copper into its normal face-centered-cubic structure and may then be a manifestation of the surface state which is seen on Cu{lcub}111{rcub}. Copper deposited on colder substrates behaves like gold except for minor enhancement at 0.8 eV.; Copper on the W{lcub}111{rcub} plane produces results related but not identical to those on W{lcub}100{rcub}. At high temperatures and coverages a very large peak is again seen although it shows a large energy shift with coverage. When copper is deposited at 78K the clean spectrum remains for a significant fraction of a monolayer. This is interpreted as due to nucleated 1 x 1 island growth of the adsorbed layer such that the surface symmetry is not destroyed.; Interpretation of the results for copper and gold on the platinum group metals is inhibited by the lack of experimental or theoretical work with which to compare the present results. A large binding energy anisotropy between the Ir{lcub}111{rcub} plane and the emitter surface around it is found for both gold and copper. Either metal rapidly diffuses off the flat {lcub}111{rcub} plane at 600K. However, after a monolayer has accumulated on the surrounding areas, invasion of the {lcub}111{rcub} plane begins. A number of work-function and emission micrograph phenomena are found to be associated with this effect.; The most common result obtained in the gas adsorption experiments is that the interesting d-band energy structure found on the clean iridium surface is removed, leaving a free-electron-like FEED spectrum. In several instances this result is preceded by interesting enhancement or suppression of certain regions of the spectra. Unfortunately without a quantitative theory of field emission in the presence of adsorbates we are usually unable to interpret such results.; Complex phenomena are observed for nitric oxide. On Ir{lcub}111{rcub} adsorption is associative and thermal desorption occurs between 400K and 530K. On Ir{lcub}100{rcub} NO is probably molecular from 78K to 400K but dissociates before desorption at around 600K. The work-functions and energy spectra appear to be dominated by nitrogen since similar results are obtained with nitrogen, ammonia and hydrazine. It is not clear what happens to the oxygen which other experiments have shown not to desorb until 800 to 1270K. Experiments with nitric oxide on a macroscopic Ir{lcub}100{rcub} crystal should be performed.