Scanning probe microscopy studies of nanoscale ferroelectrics

Abstract

Scanning Probe Microscopy (SPM) has emerged as an indispensable tool to probe and characterise materials, correlate local responses spatially, and manipulate and write features directly. Concurrently, nanoscale ferroelectrics have seen a surge in interest in the past decade for photovoltaic, nanoelectronics and micromechanical systems. Here, SPM techniques are used to probe electrical and material properties in a variety of nanoscale ferroelectrics. Initially, Band-Excitation Piezoresponse Force Microscopy (BE-PFM) is used to confirm the ferroelectric behaviour of synthesised nanoparticles of a multiferroic material, BiFeO3 (BFO). PFM and other SPM techniques are also employed to measure novel properties arising at topological defects in BFO thin films. It is shown that by applying a large bias to the SPM tip, centre-type and closure domain structures can be formed in BFO. Further experiments show that by writing ring domains, and therefore curved domain walls, control over electrical properties can be obtained. Such results are discussed with respect to existing thermodynamic frameworks to explain carrier accumulation at charged interfaces to account for observed conductivity. Later, voltage-based spectroscopy is used to decipher the origins of a field-induced phase transition in mixed-phase BFO, and subsequent experiments, along with phase-field models, highlight the ability to control the transition electrically by a probe motion. This allows creation of frustrated structures, and importantly paves the way for electric field-induced control of magnetic moments. Finally, AC spectroscopic techniques are used to probe the electromechanical response of domains in Pb(Zr,Ti)O3 and BFO thin films at the single domain wall level. The results highlight the method by which motion of a strained interface contributes to the local electromechanical response, and further allows the fundamental size limits of macroscopic theories to be tested. Overall, these studies suggest that SPM and its extension to various spectroscopies can provide very detailed nanoscale insight into electrical and material properties in ferroelectrics, test long-standing macroscopic theories and their applicability in nanoscale regimes, and help evolve emerging paradigms that consider domains and topological defects as individual elements in functional nanoscale devices.