Photo-electron emission microscopy of work function changes

Abstract

Photo-electron emission microscopy (PEEM) under ultra-high vacuum (UHV) conditions is one of the various surface imaging techniques least developed but nevertheless most promising. Excited by ultra-violet radiation, the low energy photo-electrons are only surface sensitive via the effect of the work function. Work function changes due to different materials, orientations, adsorption or reaction layers, and step densities can be visualized microscopically with a lateral resolution of some 10 nm. PEEM and Auger electron spectroscopy (AES) have been combined in a true UHV apparatus to complement surface imaging by the surface-sensitive elemental analysis. First investigations were focused on the orientation dependence of the photoemission in polycrystalline Ni and on contrast changes by work function changes due to the adsorption of oxygen on differently oriented Ni grains. By recrystallization large Ni(111) and Ni(110) grains were obtained, the orientations of which were determined by LEED. Relative measurements of the photoemission current by a spot photometer, and the Auger signals of O and Ni depending on oxygen exposure clearly show the different stages of chemisorption and oxidation. Measured differences due to different orientations and the sensitivity to low oxygen coverages are much higher for PEEM than for AES. Present instrumental limitations and future improvements are discussed.