SURFACE-ADSORPTION AND BULK DIFFUSION IN METALLIC-FILMS SENSED BY RESISTIVITY CHANGE

Adsorption of H-2 and CO on Nb, Co, Ni and Pd films was monitored by c hanges of the film resistance during the exposure to the gases. Films with thicknesses varying from 6 to 400 nm were deposited onto mica or sapphire substrates. The increase in resistance was measured near room temperature during exposure to 1000 Langmuir of either Hz or CO. When the gas dosing was discontinued and the chamber was evacuated, the fi lm resistance decreased until it reached its initial value. The change in resistance of very thin, rough Co and relatively thick, smooth Co, Nb and Ni films upon exposure to either H-2 or CO are comparable in m agnitude. There is a relationship between this effect and surface roug hness. The change in resistace is related to weakly adsorbed states of these molecules close to the surface. On the other hand, the change i n resistance of thin Pd films is related to the diffusion of atomic hy drogen into the bulk of the films.