Oxygen defective metal oxides for room temperature electronics

Due to their wide range of important properties, there is increasing interest in developing defective functional metal oxides. Among the various ionic defect species, oxygen defects are of notable significance. These are primarily in the form of oxygen vacancies, although in certain unique structures with relatively high activation energy, oxygen interstitials can also be predominant. Their significance stems largely from their higher mobility than cation defects, offering extensive possibilities for strategic manipulation to modify functional oxides' physical and chemical properties. This feature encompasses potential alterations in electronic and phase structures, thermal transport, and oxygen diffusivity. A deep understanding of the mechanisms underlying the tuning of oxygen defect chemistry is crucial to optimise the figures of merit for each specific application. Moreover, expanding the application of oxygen-deficient oxide devices necessitates developing these functional oxides at room temperature.

In the studied materials, the limitation of reduced temperature for oxygen vacancy formation is partially attributed to the low mobility of ionic charge carriers and their mechanical and Coulombic interactions with host crystal structures. Reducing the device dimensions to the nanoscale, such as creating ultrathin films with thicknesses of just a few nanometers or even two-dimensional (2D) materials, can effectively decrease the transport distance for oxygen vacancies while maintaining relatively high performance. This approach provides a strategy to optimise the application of functional oxides at room temperature. However, transitioning to low-dimensional thin films introduces new constraints that impact their physical and chemical performance. For instance, in-plane strain or other structural defects arising from the mismatch between the substrate and the thin film, as well as at the surfaces and interfaces of these low-dimensional thin films, can be critically important factors influencing the properties of the thin films.

This thesis aims to fundamentally understand oxygen vacancies in low-dimensional thin film materials fabricated using Pulsed Laser Deposition (PLD) with highly coherent epitaxial characteristics. PLD is adopted as a synthetic method for the superior control of the material's structural and chemical properties, including the possibility of tuning the oxygen defect content in the asdeposited state.

Several key targets are outlined for the thesis:

(1) Understand the functionalities of oxygen vacancies in correlated oxides, particularly their conductivity as a function of oxygen vacancy content. The coupling between strain and oxygen vacancies is a new freedom to control the film's properties.

(2) Explore high-temperature oxygen ionic conductors with mixed electronic and ionic properties in the nanoscale for room-temperature application. We mainly explored ceria based (CeGdO₂) properties at room temperature.

(3) Topotactic transitions in cobaltite (La,Sr)CoO_3-δ perovskites. This class of materials yields exemplary transition of room temperature properties. However, the transition mainly occurs at high temperatures and in large chemical gradients. We mainly studies how to realize the room temperature topotactic transition.

(4) Enrich the methodology for generating oxygen vacancies at room temperature in the designed double-layer heterostructure devices. This configuration aids in understanding the structural and chemical mechanisms leading to the kinetics of oxygen vacancy transport at the nanoscale. It also explores how oxygen vacancies can be formed at room temperature, particularly at the heterostructure interfaces.