| Summary: | 碩士 === 國立清華大學 === 物理學系 === 94 === Transition metal carbides and nitrides are the potential and cheap substitutes for the noble metals in heterogeneous catalysis. In addition, the carbide overlayers grown on the surfaces of transition metal have the advantages of carbides and metal. The carbide overlayer of (4x4) structure on Mo(110) surface has been confirmed to possess the noble metal-like catalytic properties and been studied in many investigations of surface reaction. However, the (4x4) structure in real space is unknown so far. In this study, molybdenum carbide overlayers were fabricated by carburizing the Mo(110) surface with ethylene (C2H4). Under various carburizing conditions, two carbide structures were produced: the (4x4) and “rectangle” structures. The structures of carbide overlayers were investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM).
The carburization was carried out on clean Mo(110) surface which could be obtained by standard cleaning recipe of metal surfaces. Carburizing conditions include two main factors: the exposure of C2H4 and the post-annealing. Three recipes of C2H4 exposure were carried out in this study: exposing on 800 K and RT surface and exposure of ionized C2H4 on RT surface. Each exposure recipe was followed by annealing at various temperatures, including 600, 800, 900, and 1100 K. The contamination of Mo(110) surface in UHV was also investigated and compared with the oxidized Mo(110) surface.
The (4x4) structure was formed at higher exposures, and from the STM images the carbon coverage on this surface was likely to be 1/16 ML. At lower exposures, the “rectangle” structure was formed which had two domains and a possible carbon coverage of 1/30 ML. The “rectangle” structure coexisted with non-identified area which probably was the (1x1) structure of Mo(110) surface or structure of much lower carbon coverage. The inward diffusion of carbon atoms during annealing was confirmed from the results of this study.
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