Phase Transitions in Strongly Correlated Oxides Modulated Through Electrochemical Gating

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

Non-technical Description: The project investigates an unusual property seen in certain class of metal oxides known as metal-insulator phase transition, wherein the electrical conductivity of an insulating oxide can be dramatically increased in ultrafast timescale through application of electric field in the presence of a liquid electrolyte. Oxides that show this property are of utmost importance in number of next-generation electronic, optical, and energy harvesting devices, such as rechargeable metal-air batteries, solar cells, field effect transistors, and devices employed in communications, detection, and stealth technologies. Despite their importance, the underlying mechanism of insulator-to-metal phase transition is not well understood, and is a bottleneck for improving the devices performances and further expanding the scope of these oxide-based devices. This project studies the fundamental aspects of interaction of ions in the electrolyte with structural defects in nanostructure of metal oxide and their roles in inducing metal-insulator transition through state-of-the-art electrical and optical spectroscopic techniques. Research efforts encompass nanomaterial materials fabrication, advanced spectroscopic characterization and device fabrication. The research team comprises of students from all levels from graduate to K-12, who will be trained in this multidisciplinary study that interfaces with material science, electrochemistry, advance spectroscopy, and solid-state physics through course work, energy workshops, summer camps, and interactive hands-on demo modules. The impacts of current work are brought to the awareness of the broader public through participation in science fairs, international conferences, and leadership workshops. Technical Description: Electrochemical gating is a powerful technique for modulating number of unusual properties of strongly-correlated oxides, such as high-temperature superconductivity, colossal magnetoresistance, and metal-insulator phase transitions that have applications in diverse optoelectronic devices. However, the fundamental mechanism of this gating process is not well understood, especially the role of vacancy defects in inducing phase transitions. This research project studies metal-insulator transitions in strongly correlated oxides of vanadium, nickel and manganese. The study is motivated by the recent observation by PI and coworkers of a new type of phase transition in nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition induced by electrochemical gating. The overall goal of this project is to elucidate the origin of various types of phase transitions during gating by probing the nature of interactions between vacancy defects and redox species. The research employs a threefold experimental approach using advanced spectroscopic techniques to: (1) determine the nature of electronic phase transitions during electrochemical gating via in-operando electrical and electrochemical measurements, (2) monitor vacancy-ion interactions by in-situ photoluminescence, and (3) monitor the changes in the oxidation state of metal ions and identify the chemical nature of adsorbed species by ex-situ X-ray photoelectron spectroscopy. Collective results from these measurements will serve as the basis for developing defect-property-function correlation. The main outcome of this work would be a better understanding of the interrelationships between the electronic structures, defect equilibria, chemical nature of redox species, and device properties, which can have a transformative impact on the performance and functionality oxide-based devices in diverse fields such as electrocatalysis, optoelectronics, batteries, fuel cells and sensors.