Novel Scanning Probe Investigations of Domains and Domain Walls in Lithium Niobate

Over the last 25 years, scanning probe microscopy techniques have revolutionised the way in which we understand ferroelectric domains and domain walls. This trend shows no sign of stopping. Here, for example, we have used two key imaging modes to investigate new characteristics of charged (head-to-head) conducting domain walls in single crystal LiNbO₃: Kelvin Probe Force Microscopy (KPFM) and Charge Gradient Microscopy (CGM).<br/><br/>In detail, newly configured “high-voltage” in-situ KPFM (imaging relatively large voltages while driving current) has allowed us to explicitly determine the resistivity of conducting lithium niobate domain walls for the first time (without the confounding influence of contact resistance): conducting domain walls were written in x-cut lithium niobate films by partial poling between coplanar thin film electrodes. In-situ KPFM mapping of the interelectrode region was then performed, while driving current along the conducting walls. Knowing the current density and the potential gradient developed allowed room temperature "four-probe" wall conductivity to be explicitly determined.<br/><br/>Equally, the combination of KPFM and CGM has revealed interesting surface potential profiles, associated with the microstructure around the charged walls, and forced us to reconsider the fundamental imaging mechanism responsible for CGM contrast [1]: KPFM scans revealed a triangular potential profile perpendicular to a head-to-head wall, which was an almost perfect match to the integrated CGM signal obtained in the same region. This strongly suggests that CGM does not actually measure charge gradients, but rather potential gradients.<br/> <br/>[1] Maguire, J.R., Waseem, H., McQuaid, R.G., Kumar, A., Gregg, J.M. and Cochard, C., 2022. Imaging Ferroelectrics: Reinterpreting Charge Gradient Microscopy as Potential Gradient Microscopy. <i>Advanced Electronic Materials</i>, p.2101384.