Atomically resolved surface photovoltage imaging and tunneling barrier imaging were performed with a scanning tunneling microscope to elucidate the nature of the initial adsorption state of potassium on room temperature Si(111)7×7 surface, which appears as a missing center Si-adatom. No evidence was found to support the fact that this adsorption mode is charged. K islands start to grow (and accumulate at 7×7 domain boundaries) already at coverages as low as 4×1012 cm−2. These islands may be responsible for the previously reported work function lowering and enhanced chemical reactivity upon K deposition.
Abstract.Highly oriented pyrolytic graphite (HOPG) as well as polycrystalline graphite (pcgraphite) were bombarded with 3.5keV H + ions by means of a Penning ion source. The implanted graphite was characterized by in situ electron spectroscopy techniques such as UPS, XPS and EELS. Our UPS valence band measurements of the hydrogen saturated graphite reveal it to be an insulating phase, and XPS measurements show a shift of the Cls core level to higher binding energy with respect to pristine graphite. This behavior is explained by a Fermi energy shift upon hydrogen bombardment of graphite. In addition, a close resemblance in the electronic structure of hydrogen bombarded graphite and amorphous hydrogenated carbon films (a-C :H) is shown which suggests the modification of pristine graphite to an amorphous network [1] of mostly tetrahedrally bonded carbon atoms by hydrogen implantation. 79.20-m, 79.60-i, 73.60-n Graphite is an interesting material in the field of plasma fusion research because of its outstanding physical properties as a first wall material for limiters in fusion reactors (Tokamak) [2][3][4][5]. In particular, one requires a low-Zmaterial with a low vapor pressure. Graphite meets these specifications.
PACS:During a tokamak discharge, the first wall interacts with the plasma and is bombarded by hydrogen species which leads to a considerable modification of the surface. Due to the immobility of the implanted hydrogen ions at room temperature, an upper limit of the hydrogen content nn/nc --0.45 is fou/ad in the near-surface region
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