FIM studies of clean and graphitized rhodium using lithium and oxygen as imaging species

Selected areas of a clean or Li-doped Rh field emitter tip were studied by low-temperature O+2-FIM and Li+-FDM in order to reveal the promoting influence of Li on the early stages of the oxidation process. The experiments were conducted by graphitizing a Rh tip specimen in a first step and reacting off subsequently the surface carbon to create two single “windows” with a clean Rh surface in the region of (1 0 0) and (1 2 3). Both windows were imaged by Li+-FDM. At high field strengths Li+ formation was restricted to the border between the clean surface areas of the windows and the graphite-like overlayer, i.e. ring-like ion images appeared on an otherwise dark background. Since the Li source was originally present on the shank of the Rh tip it must be concluded that Li atoms penetrate the graphite-like layer in order to form an intercalation structure ensuring high mobility to sustain steady supply for image formation in Li+-FDM. Field strengths and temperatures could also be adjusted so that the initially clean Rh windows were precovered by Li without desorption occurring. This enabled us to study the influence of Li on the rate of surface oxidation while imaging in O+2-FIM at 78 K. We found an enhancement of this rate (as compared to the clean surface) and explain this observation by an increase of the sticking probability of O2 on the Li-covered surface. In another experiment, we found that O+2-FIM of Rh surfaces covered by COad yield higher brightness than those covered by Oad.