Ultra-thin oxide films

Thesis

Oxide ultra-thin film surfaces have properties and structures that are significantly different from the terminations of the corresponding bulk crystals. For example, surface structures of epitaxial ultra-thin oxide films are highly influenced by the crystallinity and electronegativity of the metal substrates they grown on. Some enhanced properties of the novel reconstructions are related to catalysis, sensing and microelectronics, which has resulted in an increasing interest in this field.

Ultra-thin TiO_x films were grown on Au(111) substrates in this work. Two well-ordered structures within monolayer coverage – honeycomb (HC) and pinwheel – were generated and investigated. Special attention has been paid to the uniform (2 × 2) Ti₂O₃ HC phase including its regular structure and imperfections such as domain boundaries (DBs) and point defects. Linear DBs with long-range repeating units have been observed; density functional theory (DFT) modelling has been used to simulate their atomic structures and calculate their formation energies. Rotational DBs/defects show up less frequently, however a six-fold symmetrical 'snowflake' DB loop stands out. Two types of point defects have been discovered and assigned to Ti vacancies and oxygen vacancies/hydroxyl groups. Their diffusion manners and pairing habits have been discussed within an experimental context.

The results of growing NbO_x ultra-thin films on Au(111) are also presented in this thesis. An identical looking (2 × 2) HC structure to the Ti₂O₃ ultra-thin film has been formed; a stoichiometry of Nb2O3 is suggested. Another interesting reconstruction is a hollow triangle structure. Various sizes have been found, and sides of these equilateral triangles all show a double-line feature aligned along the 〈 1 ^ˉ₁ 0〉 directions of the Au(111) lattice. Chemical composition characterisations of NbO_x thin films are still required as is DFT modelling.

Experimental techniques used in this thesis include scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Ultra-thin oxide films were created by physical vapour deposition (PVD) in ultra-high vacuum (UHV) systems.

Authors: Hu, X

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Au(111) substrate; epitaxial growth; thin film; titanium oxide; 2D material; scanning tunnelling microscope; domain boundary; niobium oxide; surface science; surface reconstruction; nanomaterial
• Subjects: Materials