Exploiting Functional Defects in Epitaxial Oxide Thin Films for Giant Piezoelectricity and Synaptic Electronics*

Transition metal oxide thin films exhibit many interesting properties, including ferroelectricity, piezoelectricity and electrochemical activities for energy applications. Defects, such as oxygen vacancies, have long been known to be detrimental to certain functional properties of oxide thin films. However, by properly designing and controlled synthesis, the defects can be beneficial and even greatly improve the functional properties.

In this talk, I will discuss our recent work on the giant piezoelectricity and memristive behaviour induced by antisite defects in NaNbO₃ (NNO) epitaxial thin films. Through precise control of Na deficiency, Nb atoms occupy vacant Na positions and form the Nb_Na antisite defects in NNO films. These antisite defects self-assembled to form vertical nanopillar structures. Such nanopillar regions with local structural and polar heterogeneity give rise to giant piezoelectric coefficients while maintain a high Curie temperature (Science, 369, 292, 2020). The antisite defects also induce resistive switching behaviour and tunable resistance states by electric field. The NNO film based devices exhibit synaptic plasticity under applied electrical pulses, promising for the application in neuromorphic computing.