Field emission microscopy of gold on single-crystal planes of tungsten

Abstract

Adsorption of gold on (110), (100), (211) and (310) planes of tungsten has been examined by field emission microscopy and energy analysis. Thermally equilibrated submonolayer amounts of gold produce an increase in the work function φ hkl at (100), (211) and (310) which is ascribed to the formation of gold-tungsten dipoles with the gold negatively charged. The observed decrease in φ 110 is thought to result from formation of dipoles of the opposite sign, the gold adatoms losing electronic charge to the metal owing to the high work function at (110). Electron spectroscopy of gold near (100) indicates that the bonding level forms a broad resonance with no discernible features within the accessible range of electron energies. This agrees with the findings of Young and Gomer for gold adsorbed on (111). A decrease in φ hkl attends completion of the first layer at (100), (211) and (310) and, as the second layer forms, φ hkl lies in the range 5.1–5.2 eV on each plane. The increase in φ to its final value, which has been reported previously for φ and is termed the transition, occurs in a similar manner on (100) and (310), reaching a final value in the range 5.5–5.6 eV. In contrast with this, φ 211 increases sharply at a coverage which depends upon the temperature at which the layer is heated. The sharpness of the transition is believed to reflect the ease with which the layer adopts its fully transformed structure on (211), and the final value of 6.1 eV is tentatively ascribed to the effect of a surface band structure formed by the gold layer. By monitoring changes in the probe-hole current, surface diffusion on (110), (100) and (211) has been examined. On (110) diffusion proceeds with an activation energy of about 0.1 eV which, as expected, is lower than that for invasion of the plane over the surrounding step. Similar measurements confirm a low activation energy for diffusion on (100) and (211). A fall in probe-hole current marks the transition, and rate measurements give for this process an activation energy in the region of 1 eV which is much higher than that for surface diffusion and which supports the view that the transition involves structural changes in the adsorbed layer.