Epitaxial oxide thin films

Growing transition metal oxides in the form of an ultrathin film can create new stoichiometries and structures that do not exist naturally, which may open new perspectives on the studies of oxide properties. This thesis focuses on the structural characterization of Nb, Ti, and V oxide thin films on the Au(111) substrates using scanning tunneling microscopy (STM). The work starts with ring patterns created by M-Au substitutions (M = Nb, Ti, V, Mn). According to the rings' radii and centers, we can conduct a three-dimensional (3D) mapping of the substitutional sites. The ring's morphology is related to the curvature anisotropy of the Au isoenergy surface in k-space. Nb oxide has been studied most intensively in this project. Four structures of oxide monolayers have been identified which are (i) NbOx cluster, (ii) NbOy triangle, (iii) pinwheel, and (iv) the (2 × 2) Nb2O3 honeycomb (HC) structure. The pinwheel and honeycomb structures can form free films on the surface or embedded monolayers on the second layer. Nb-induced rings act as a precursor structure for an embedded monolayer. Fourfold coordinated Nb atoms in Nb2O3 HC appear much brighter in STM images. The actual oxidation state of a fourfold coordinated Nb atom is between +4 and +5 due to the electron transfer to Au. According to the contrast difference and coordination number, mapping the Nb oxidation states with atomic resolution at room temperature can be realized. Besides, fourfold coordinated Nb atoms are involved in point defects, edges, and domain boundaries. Discussions on HC edges and defects are organized in two chapters. Comparative analysis between Nb2O3 and Ti2O3 constitutes pivotal sections. Triangles and pinwheels are common structures in 2D materials. Nb and Ti oxides can form independently-standing triangles, and Nb/Ti/V oxides can generate both free and embedded pinwheel monolayers. A triangle building block model is proposed to elucidate the pinwheel comprising various sizes of triangle elements. Besides, Nb/Ti oxide triangles/pinwheels can be incorporated into HC monolayers by forming unique boundaries.