Date of Award
Doctor of Philosophy
Patricia A. Thiel
The structures formed on the Ni(100) surface during oxygen adsorption, leading to oxidation, are studied with Video-LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). The temperature- and exposure-dependence in the development of LEED patterns observed during oxidation on Ni(100), at oxidation temperatures of 80 to 400 K, are investigated extensively. Integrated diffraction spot intensities and fractional spot profiles are measured quantitatively and continuously, allowing unambiguous correlation of various surface processes. A LEED pattern with 7th order spots around the substrate integral spots, (7 x 7), is observed. Possible models to explain this observation are discussed. AES is used to measure the oxidation onset during adsorption and the final relative thickness of the oxide. It also helps to characterize the structural differences between NiO(111), (7 x 7) and NiO(100). We find a strong temperature-dependence in the development of LEED patterns associated with NiO. The formation of NiO(111) is favored by adsorption temperatures below 300 K, whereas a (7 x 7)-like structure is favored by adsorption temperatures of 300 to 400 K. Room temperature is a "crossover" point between these two forms of the oxide. The final depth of the oxide is independent of adsorption temperature and, therefore, of epitaxial orientation, between 80 and 400 K. Also, the exposure at which oxidation begins as measured by AES, correlates well with the onset of the NiO(111) pattern or the (7 x 7)-like pattern, whichever is favored at a given adsorption temperature. NiO(100) is formed when the oxygen-covered surface, showing either a NiO(111) or (7 x 7) pattern, is heated above 550-600 K. A possible mechanism to explain the formation sequence of different oxide structures is proposed throughout this dissertation. In addition, the Debye-Waller factor and, therefore, Debye temperatures of surface and bulk NiO are estimated experimentally. The linearity of the Debye-Waller effect is found to hold at temperatures as low as 1/2 the surface Debye temperature. The pressure effect, postulated by others, on the epitaxy of oxide growth and the depth of oxide layer is also checked in this work. As oxygen pressure varies from 10[superscript]-7 to 10[superscript]-9 Torr, only the intensity of the grown oxide superstructure is affected, not the nature of the superstructure itself nor the exposure-dependent progression of superstructures.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Wang, Wen-Di, "Study of O/Ni(100) with LEED and AES: from chemisorption to oxidation " (1990). Retrospective Theses and Dissertations. 9908.