Transition-metal oxides show a wide range of fascinating properties, derived from the complex interplay between spin, charge, orbital and lattice degrees of freedom [1]. A major challenge in modern condensed matter physics is to understand, manipulate and utilize these properties. One way to control the properties of oxides is to change the number of charge carriers, which has been widely exploited and lead to the discovery of e.g. high-temperature superconductivity in cuprates and the large magnetoresistance in manganites. Charge doping in complex oxides is often realized by cation substitution or variation of the oxygen content. Through this, the level of charge doping is set at the stage of the growth and cannot be easily changed afterwards. Since for potential device applications fast and reversible switching between different states by an external stimulus is required, electrostatic and electrochemical doping is of high interest. Achieving very large induced charge densities is possible by electrolyte gating with so-called ionic liquids. An ionic liquid is a molten salt at room temperature, often with negligible vapor pressure and high thermal, chemical and electrochemical stability, properties usually associated with solids. Furthermore, the possibility to mix them with a suitable polymer acting as a matrix opens new perspectives for applications since so-called ionic liquid gels have a persistent structure that is mechanically robust and can be easier handled than a liquid [2-4].
In this project, we propose to investigate changes in physical properties when an ionic liquid gel is brought in contact with a transition-metal oxide thin film or multilayer. Recently a controversial discussion emerged about the electrochemical versus electrostatic nature of doping effects in transition-metal oxides induced by ionic liquid gating [5-7]. The aim of this project is to gain further insight into this subject and explore the effects of ionic liquid gating in other oxides.
The first part of the project is dedicated to the preparation of the ionic liquid gel. Following the procedure described in ref. [4], the ionic liquid gels will be prepared as freestanding layers by spin coating on a supporting plate and afterwards transferred to the oxide films. The first goal is to improve the polymer quality by testing different solvent concentrations and optimizing the spin coating procedures. In addition, we like to investigate systematically the variation of the concentration of ionic liquid in the gel. In the second part of the project, the optimized gels will be applied to two different perovskite oxide heterostructures based on rare-earth nickelates and vandates, grown in our institute by either pulsed- laser deposition or ozone-assisted molecular beam epitaxy. By measuring electrical transport, atomic force microscopy and x-ray diffraction, the effects of ionic liquid gel gating on the electronic and structural properties of the heterostructures will be investigated.
[1] E. Dagotto, Science 309, 257 (2005).
[2] K. H. Lee et al., Advance Materials 24, 4457 (2012).
[3] P. C. Marr & A. C. Marr, Green Chemistry, 18, 105 (2016).
[4] T. P. Lodge, Science, 321, 50 (2008).
[5] M. Nakano et al., Nature 487, 459 (2013).
[6] J. Jeong et al., Science 339, 1402 (2013).
[7] J. Walter et al., ACS Nano, 10, 7799 (2016).