Monolayer oxide films
Keywords: Honeycomb lattice
Transition metal oxide
Density functional theory
Scanning tunneling microscopy

This thesis focuses on the monolayer films and nanostructures of Ti oxide and Nb oxide supported on the gold substrate. Particular emphasis have been placed on learning about the point and extended defects that occur in the films.
The thesis begins with a significant improvement in the resolving power of the STM that can be achieved through automated distortion correction and multi-frame averaging. The broad utility of this approach is demonstrated with three examples: the Si(111)-(7 × 7) reconstruction, the Ti2O3 (2 × 2) film on Au(111) and the (4 × 4) reconstructed SrTiO3(111) surface.
Ti2O3 monolayer films with a honeycomb lattice often contain local defects in which the hexagonal elements are replaced by four, five, seven, and eight-membered rings. The structure and energetics of Stone-Wales and divacancy defects in the Ti2O3 monolayers are investigated using scanning tunneling microscopy (STM) and density functional theory (DFT). The substrate significantly influences the energetics, and hence favors vacancy-type defects, in compressively strained 2D materials. A variety of non-stoichiometric defects are observed and they can be generated by applying a voltage pulse using the STM tip.
An epitaxial (2 × 2) honeycomb Nb2O3 monolayer is grown on an Au(111) surface. The films form a well-ordered honeycomb lattice and adopt a (2 × 2) periodicity with respect to the Au(111) substrate. The Nb atoms are located in Au(111) three-fold hollow sites and the O atoms are located in on-top positions. The existence of a strong interfacial interaction is characterized by a large electron transfer to the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High resolution STM images are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the surface.
A variety of monolayer niobium oxide (NbOx) nanostructures are grown on Au(111) surfaces. The NbOx nanostructures can be classified as atomic clusters consisting of only a few atoms, or larger triangular islands of various sizes. The atomic clusters have rectangular, pentagonal and hexagonal shapes, and they can merge to form larger clusters. The atomic clusters have stoichiometries ranging from NbO to NbO1.5. The triangular islands have a close-packed structure and locally adopt the (1 × 1) periodicity of the Au(111) substrate. Triangular islands of different sizes have similar atomic structures and stoichiometries close to NbO. The nanostructures can be converted into (2 × 2) Nb2O3 honeycomb monolayer films when annealed for prolonged periods in 10-6 Pa of oxygen.
The structures and properties of defects in monolayer films of Ti2O3 and Nb2O3 on Au (111) are compared. For both systems, a rich variety of defects including island edges and domain boundaries are observed. Characteristic domain boundaries consist of 4, 5, 7 and 8-membered rings. For the Nb2O3 monolayer films, a larger variety of structures within domain boundaries have been observed. Furthermore, the edge structures for both systems have been studied. While the Ti2O3 forms disordered edges containing nonhexagonal rings, the Nb2O3 forms armchair and zigzag edges containing only hexagonal rings. It is surprising that although both Ti2O3 and Nb2O3 form the same structure, the defects in these systems are distinctly different.