Phase Transitions in Strongly Correlated Oxides Modulated Through Electrochemical Gating

非技术描述：该项目研究在某些类型的金属氧化物中发现的一种不寻常的性质，称为金属-绝缘体相变，其中绝缘氧化物的电导率可以在超快时间尺度上通过在液体电解液存在的情况下施加电场来显著增加。表现出这一特性的氧化物在许多下一代电子、光学和能量收集设备中具有极其重要的作用，例如可充电的金属-空气电池、太阳能电池、场效应晶体管以及用于通信、探测和隐身技术的设备。尽管它们很重要，但绝缘体到金属相变的潜在机制还不是很清楚，这是提高器件性能和进一步扩大这些氧化物基器件范围的瓶颈。本项目通过最先进的电学和光学光谱技术，研究电解液中离子与金属氧化物纳米结构中的结构缺陷相互作用的基本方面，以及它们在诱导金属-绝缘体转变中的作用。研究工作包括纳米材料制造、先进的光谱表征和器件制造。研究团队由从研究生到K-12的所有级别的学生组成，他们将在这项多学科研究中接受培训，通过课程作业、能源研讨会、夏令营和互动动手演示模块与材料科学、电化学、高级光谱学和固态物理相结合。通过参加科学博览会、国际会议和领导力研讨会，使更广泛的公众认识到当前工作的影响。技术描述：电化学门控是一种强大的技术，用于调制强关联氧化物的许多特殊性质，如高温超导、巨磁电阻和金属-绝缘体相变，这些特性在各种光电子器件中都有应用。然而，这种门控过程的基本机制还不是很清楚，特别是空位缺陷在诱导相变中的作用。这项研究项目研究了强相关的钒、镍和锰氧化物中的金属-绝缘体转变。这项研究是由PI和他的同事最近观察到的非化学计量比氧化镍中的一种新型相变引起的，该相变涉及由电化学门控引起的半导体到绝缘体到金属的转变。这个项目的总体目标是通过探索空位缺陷和氧化还原物种之间相互作用的性质来阐明在浇注过程中各种类型的相变的起源。这项研究采用了三种实验方法，使用先进的光谱技术：(1)通过操作内电学和电化学测量来确定电化学门控过程中电子相变的性质，(2)通过原位光致发光来监测空位-离子相互作用，以及(3)通过非原位X射线光电子能谱来监测金属离子的氧化态的变化和识别被吸附物种的化学性质。这些测量的综合结果将作为开发缺陷-属性-功能关联的基础。这项工作的主要成果将是更好地理解电子结构、缺陷平衡、氧化还原物种的化学性质和器件属性之间的相互关系，这些关系可以对不同领域的氧化物器件的性能和功能产生革命性的影响，如电催化、光电子学、电池、燃料电池和传感器。

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.