The adsorption of methane on tungsten (100) and (111)

Abstract

The work function of methane adsorbed on a tungsten field emitter at temperatures ranging from 300 to 1000 K was examined with a probe hole microscope. Significant differences in behaviour were found on the (111) and (100) planes. On the former, φ 111 increased with exposure up to a value of ~2000 L (1 L = 1 × 10 −6 Torr sec) beyond which little change was observed. Subsequent heating of the tip resulted in a slow decrease towards the clean value at 2000 K. Emission patterns and the Fowler-Nordheim pre-exponential term B 111it also indicated a clean (111) plane at this temperature. On the (100) plane φ 100 decreased by ~0.3 eV at low exposure, but for adsorption temperatures below 500 K and exposures greater than 2000 L this was followed by an increase beyond the clean value by ≅0.2 eV showing saturation characteristics similar to that on the (111). On slight heating to 500 K, φ 100 again decreased below the clean value while on further heating Δφ 100 approached zero at 2000 K. However, B 100 value and emission patterns indicated that the (100) plane was not clean at this flash temperature. The presence of an electropositive species indicated by Δφ 100, b 100it and emission current values was confirmed by emission patterns showing a bright (100) pole under certain conditions of exposure and temperature. It is suggested that work function increase on the (111) plane are caused by electronegative species CH 3 and H while on the (100) the most abundant species is an electropositive methylene group CH 2 which is responsible for the observed decreases. For adsorption at 700 K and higher where the CH 4 sticking probability is higher and where carbon is the only surface species, the work function on the (111) always increased. Subsequent flashing of the tip resulted in little change but between 2000 and 2500 K, depending on the initial carbon coverage, all signs point to a clean surface. This may be due to the ease of carbon penetration and inward diffusion on the (111) resulting in a depletion of surface carbon. On the (100) plane however, φ 100 always decreased and flashing to 2500 K resulted in a surface carbon layer. The work function decrease on the (100) is in accord with the accepted bonding theories of interstitial carbides but by the same token the increase on the (111) is anomalous.