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Author

Yan Chen

Date of Award

2-1995

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Supervisor

Professor P.T. Dawson

Abstract

This thesis reports a comprehensive study of transition metal nitride surfaces and the effects of electron beam stimulation on the adsorption processes. Since the nitrides of transition metals in the fourth to sixth group of the periodic table have important industrial applications as coating materials and heterogeneous catalysts, the understanding of the surface nitriding processes has tremendous technological importance.

AES studies have been carried out on the N-Zr system over a wide energy range which has led to the identification of new Auger cross transition features induced by nitriding and to an understanding of the intensity variation and energy shift of Auger features during nitriding. Consequently, a quantitative surface analysis method has been established for this system.

The effect of electron beam stimulation on the processes of ammonia adsorption and nitride film growth has been investigated on polycrystalline Ti, Cr, Zr, W and three single crystal tungsten surfaces. This study demonstrates methods for performing experiments properly when surface science tools utilising electron probes are used, and provides a potential application for the development of new techniques for preparing transition metal nitride films.

Titanium and tungsten represent extremes in the stability of their nitrides. Extensive studies of the interaction of ammonia with these metals are reported in this thesis.

The adsorption of N₂ and NH₃ gases with and without electron beam stimulation has been studied on titanium surfaces. This has led to an understanding of many surface processes including the adsorption of gas molecules, the diffusion of nitrogen atoms into the bulk and the initial stage of nitride film growth. It has been found that only with electron stimulation does the adsorption and diffusion behaviour of N₂ gas mimic that of ground state NH₃ gas.

AES and LEED studies on the interaction of NH₃ with tungsten single crystal surfaces have been performed including the stimulation effect of electrons in ammonia adsorption. Nitrogen desorption and diffusion have been observed during annealing and surface structural changes have been monitored. A remarkable surface reconstruction involving nitrogen atom transfer from the outermost layer to the underlayer has been revealed on the W(100) surface. These results lead to new insights into the complex surface chemistry of the NH₃-W(100) system.

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