Adsorption of Nitrogen on Single-Crystal Faces of Tungsten

Work functions of the (110), (100), and (111) regions of a thermally annealed tungsten field emitter were measured as a function of nitrogen coverage. The experiments were carried out with the emitter at 79° and 295°K, using the probe—hole technique with a Faraday cage. It is shown that faces deviating even less than 10° from a flat plane often show considerable differences in behavior on nitrogen adsorption. This is concluded from changes in work function as well as in the pre-exponential term of the Fowler—Nordheim equation. A simple model is outlined which is in qualitative agreement with the experimental results. Quantitatively the situation is more complicated, as several quantities are not precisely known and some basic assumptions have to be made. However, in general, there exists quite a reasonable correlation between surface structure on the one hand and changes in work function and pre-exponential term on the other.