title: Dislocation-mediated Electrical Conductivity in Metal Oxides Titania and Yttria-Stabilized Zirconia
Creators: name: Muhammad, Qaisar Khushi
Publisher: name: UNSPECIFIED
Year: 2022

description: Using dislocations as one-dimensional dopants is a novel concept that utilizes their elastic strain field, charged core, and associated compensating space charge to engineer the functional properties of metal oxides. In contrast to comparatively mature research fields, such as the mechanical deformation of metals, little is known about the plastic deformation of metal oxides due to their brittle nature. Consequently, the tailoring and impact of mechanically introduced dislocations on functional properties, especially electrical conductivity, remain under investigated. In this work, these issues are addressed by investigating the influence of mechanically generated dislocations on the electrical properties of the two important metal oxides. Model material systems are chosen to be rutile (TiO2) and Yttria Stabilized Zirconia (YSZ) due to their many technological applications such as solar cells, water splitting, memory devices, Li-ion batteries, and solid electrolytes in fuel cells (SOFCs). A comprehensive framework is developed by utilizing dislocations to tune the electronic and ionic conductivity of metal oxides. It is illustrated that understanding the mechanics of the subjected material system helps introduce significantly large deformation in metal oxides, which are considered brittle otherwise. Several dislocation configurations can be systematically achieved by changing the deformation conditions. The resulting electrical response of induced dislocation networks is accessed via electrochemical impedance spectroscopy, including bulk and microcontact modes, supplemented by scanning probe microscopy. These measurements indicated that the electrical conductivity could both be increased and decreased by merely controlling the mesoscopic dislocation structure. Dislocation configuration is identified as a tuning parameter over which it is shown that unprecedented control allows us to engineer the electrical conductivity above what can be achieved by point defect doping. Induced dislocation networks profoundly impact the electronic conductivity of rutile and can induce behavior akin to the donor and/or acceptor doping in the pristine material. Arranged dislocation regions showed several orders of magnitude higher electrical conductivity compared to the pristine regions. The physical interpretation of the data results in a quantitative description of the impact of dislocations as highly conductive pathways in rutile. This route is further expanded to study the influence of mechanically generated dislocations on the ionic conductivity of YSZ. Highly aligned dislocation-rich and -deficient regions are generated; an in-depth electrical characterization of these regions exhibited highly conducting effects of dislocation-induced strain inside the bulk material. The underlying mechanism for the observed enhancement in the ionic conductivity is discussed in detail. So far, such effects were only illustrated via DFT calculations and in strained thin films. However, in this work, the potential of mechanically induced dislocations is presented as a design element to tune the bulk ionic transport. The underlying mechanism responsible for the observed enhancement in ionic conductivity is discussed in detail. Furthermore, it is emphasized that dislocations possess the potential to tune the electronic and ionic conductivity of metal oxides. These effects are explained by deconvoluting the dislocation character, core charge properties, possibly existing space charge, and their mesoscopic arrangement. The combined concepts of dislocation mechanics and solid-state ionics indicate that dislocation-mediated, highly stable electrical conductivity can be used to modify the electronic and ionic charge transport locally and globally. Therefore, these results allow an additional degree of freedom for tuning various functional oxides' electronic/ionic properties apart from chemical doping strategies.