Domain Wall-Defect Interactions in Ferroelectric Films

Ferroelectric materials are in widespread commercial use as capacitors, precision positioning devices, fuel injectors, non-volatile memory elements, and as medical ultrasound transducers, among other applications. The measured properties of ferroelectric materials are a function of both the intrinsic response of the crystal to applied fields and the extrinsic response due to motion of domain walls under applied electric fields and stresses. The role of domain wall motion in optimising the extrinsic properties of bulk piezoelectrics has been extensively investigated but in thin films there have been few coherent studies that have attempted to understand how domain walls interact with defects.

For piezoelectric thin film based micro electromechanical systems (i.e. nano-motors and actuators) to gain widespread usage, their properties need to be optimised and their reliability improved. One aspect of reliability is the ability to maintain the same piezoelectric response over the lifetime of the device. This is currently limited by progressive pinning of domain walls during cycling by defects such as grain boundaries, point and planar defects within the film. This proposal aims to establish a fundamental understanding of the role of defects in pinning domain walls by applying a combination of advanced transmission electron microscopy, piezo-force microscopy and classic FEstack measurements to the study of a bespoke series of ferroelectric films in which specific types of defects have been engineered. The UK effort concentrates exclusively on TEM and links directly to an already funded programme between Pennsylvania State University (Trolier-McKinstry, film deposition and FEStack) and Oak Ridge National Laboratory (Kalinin, piezo-force microscopy).

Piezoelectric based micro electromechanical systems (MEMS) have the potential to gain widespread usage in the fabrication of nano-motors, nano-sensors, nano-pumps and nano-actuators. They offer the possibility of larger displacements and greater voltage outputs than many competitor systems. However, they are currently limited in exploitation by their ability to perform at optimum displacement/voltage over the lifetime of the device. This problem relates to a progressive pinning of domain walls under cyclic field which ultimately suppresses the extrinsic contribution to the piezoelectric response. The project is fundamental in nature and is designed to understand the complex interactions of domain walls with pinning centres in thin films Currently, there is no coherent overview of how domain walls interact on an atomic scale with grain boundaries, dislocations and point defects. For the first time, a combination of transmission electron microscopy, piezo-force microscopy and conventional FEstack measurement will be applied to a bespoke set of films in which various types of defect have been engineered. The programme is in collaboration with the Materials Research Institute at Pennslylvania State University (Trolier-McKinstry) and the Centre for Nanophasic Material Sciences (Kalinin) at Oak Ridge National Laboratories. The goal is to combine the expertise of these two world renowned institutes with the exceptional knowledge of electroceramics, crystal chemistry, and electron microscopy at Sheffield (Reaney, Rainforth). The high profile of the subject matter and the reputation of the assembled team ensures, in the short term, potent academic impact with the possibility, in the long term, of improving the performance of ferroelectric thin films in MEMS based applications.