| Summary: | The ability to study and control oxygen ion transport in metal oxide systems at reduced temperatures (<400°C) is key to the design of new generations of energy conversion/storage, memory and functional device applications, but has remained a significant challenge in the field of Solid-State Ionics. In this thesis, electric field and optical-based tools were developed capable of manipulating and quantifying the ionic transport properties of model oxygen ion conducting metal oxides systems at these temperatures.
First, we show that ionic mobilities near room temperature in a model mixed conducting thin film of Pr0.1Ce0.9O2-δ are sensitive to frozen in defect concentrations. This was achieved by exposing a thin film to different thermal histories and quantifying the quenched in defect concentrations by measuring the film’s optical absorption, related to the oxidation state of the Pr ion. A dynamic current-voltage technique was applied to isolate the oxygen ion mobility in the quenched-in state. A 13-fold increase in ionic mobility with increases in oxygen nonstoichiometry from 0.032 ± 0.001 to 0.042± 0.001 was observed at 60°C. We discuss how nonobvious entropic effects can lead to these ionic mobility – defect concentration trends, contrary to expectations and how being able to control and quantify these trends can ultimately aid in elucidating the origins of variations seen in nano-ionic devices.
Next, we demonstrate how applied electric fields can be used to reversibly redistribute oxygen ions between two mixed conducting metal oxide thin films (Pr0.1Ce0.9O2-x/Ce0.15La1.85CuO4+y). Field induced changes in resistance in each layer are correlated with respective changes in defect concentrations and defect chemical properties. We demonstrate for the first time the importance of defect chemical models in interpreting the origin of the resistance changes induced by ion exchange under field in nano-ionics devices. In turn, information on the electronic transport properties of the respective layers and their defect formation energetics are obtained from these models. These findings highlight the unique opportunities such bilayer studies offer in investigating the defect chemistry of metal oxide films near ambient temperatures. Dynamic current voltage measurements applied to these bilayer device structures were successful in isolating the oxygen ion mobility within the dominant layer, that was, in turn, correlated with the variations in defect concentration of the layer. The results, when compared to the results of thermal annealing studies, showed good agreements in trends, thus confirming the viability of using such experimental tools for controlling and extracting ionic transport properties in a bilayer device. Characteristic evolutions of the dynamic current voltage measurement curves at lower sweep rates were also identified in these bilayer systems and were assigned to the solid ion exchange process. This broadens the capabilities of such a technique in being able to measure the rate controlling ion transfer kinetics occurring under field between two metal oxides films.
Finally, we show, for the first time, how above band gap illumination can be used to modulate ion transport across interfaces in metal oxides. This was demonstrated through selective changes in grain boundary ionic transport in a model oxygen solid electrolyte Gd0.03Ce0.97O2-δ and by ruling out the impact of optical heating and gas atmosphere. The observed changes are assigned to the modulation of the local grain boundary space charge potentials and ionic charge carrier depletion zones. Models to describe the observed response are developed and are supported by a combination of impedance spectroscopy (IS) and intensity-modulated photocurrent spectroscopy (IMPS).
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