Adsorption of CO on Tungsten: Field Emission from Single Planes

Abstract

The work functions and Fowler–Nordheim pre-exponentials on the (110), (100), (211), (111), and (210) regions of a tungsten field emitter have been determined by means of a probe-hole method as function of thermal treatment of full CO layers; as function of coverage in incremental dosing at 100°K; and as functions of readsorption on beta layers. The results shows that virgin, alpha, and beta states are formed on all regions investigated, but that their properties vary with substrate orientation. The results on the close-packed planes (110) and (100) indicate complex behavior of beta layers on heating, and suggest surface reconstruction at T > 700°K. Temperatures for the onset of surface diffusion of CO were also determined, and indicate that diffusion occurs most readily on the zones (211)–(110) (at 275°–300°K). This is interpreted to mean that diffusion over (110)-like regions is rapid, but that trapping at the edges of (110) diffusion steps occurs. Adsorption on (110) is not seen below 800°K, and on its vicinals below 500°K, indicating that diffusion over these regions occurs without trapping below these temperatures. This is considered further evidence for surface reconstruction on (110) at high temperature. Diffusion into the vicinals of (111) occurs at 575°K and into (111) at 700°K. Diffusion into (100) was noted at 750°K.