Study of flexoelectric effect in oxide materials

Abstract

Flexoelectricity describes a material's electrical polarization in response to a stain gradient. Recent studies have expanded the intrinsic flexoelectric effects to a new modulation strategy in a wide range of functional materials, making the enhanced flexoelectricity a great potential for application as flexible electronics. The nature of flexoelectricity eliminates the restrictions, like the Curie temperature in many piezoelectric materials, and allows the existence and coupling with other physical properties in all materials from insulator to semiconductor as well as conductor. The coupling between flexoelectricity and photovoltaics (PV), as one of the most promising phenomena from the application point of view, has triggered many investigations and discussions in the flexoelectric community. However, there still lacks a clear physical picture to describe the flexo-photovoltaic effect, where the interactions between photon, charge, and polarization make the mechanism more complicated. In this work, the photo-flexoelectric (photoflexoelectric) effect in perovskite structured SrTiO3 (STO) single crystal was demonstrated and the coupling mechanism between its photovoltaic and flexoelectric effect was revealed. Driven by the flexoelectric field, incident light-induced electrons can tunnel through the Schottky junction at the Au/STO interface, giving rise to enhanced flexoelectricity, i.e., photoflexoelectric effect. Thermal annealing in vacuum induces oxygen vacancies in STO and results in stronger light absorption and enlarged photoflexoelectric effect. These results help us to understand the mechanism of flexoelectricity and the photoflexoelectric effect and may provide hints of more correlation effects between flexoelectricity and photon-charge interaction. The increased flexoelectric coefficient may also have application prospects like energy harvesters and sensors. The doping effect has also been studied in TiO2 crystals. It is found that by introducing hydrogen dopants in TiO2, the effective flexoelectric coefficient can be enhanced by more than two orders of magnitude compared to the pristine sample. A hydrogen charging technique has been achieved, and hydrogen doping in TiO2 has been obtained. The mechanism of influence on the flexoelectricity by hydrogen doping in TiO2 single crystal is attributed to the potentially formed polarization by crystal symmetry breaking due to the existence of H ions in the crystal lattice. These results broaden the horizon of study on the flexoelectricity effect in dielectric materials. The results of this thesis demonstrate some new approaches and techniques to enhance both the intrinsic and extrinsic flexoelectricity of STO and TiO2 crystals. These methods could be extended to different materials and find potential applications in flexible electronic devices. The physics and mechanism behind also deserve further studies.