Abstract
Contact potential difference (cpd) measurements were carried out on {112̄} surfaces of high conductivity CdS cleaved in ultrahigh vacuum. A modified Kelvin method was employed. A change in cpd upon illumination with white light (surface photovoltage) was observed, indicating the presence of intrinsic surface states in these surfaces. Long exposure (about 24 hr) to water vapor (2 × 10 −8 torr) caused an increase of about 1.0 V in cpd and a reduction in surface photovoltage. This increase in cpd was attributed primarily to a decrease in the electron affinity of the surfaces brought about by the molecular dipole moment of physically adsorbed water molecules. The “clean” surfaces exhibited no appreciable affinitity for spectroscopically pure oxygen, except under white light illumination apparently due to the increase of the density free electrons at the surface.
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