Towards Spatially Resolved Measurements of Domain Wall Thermal Transport Using Scanning Thermal Microscopy

It is now a well-established idea that ferroelectric domain walls can be considered as extended structural defects that affect thermal transport through phonon scattering¹. Additionally, the fact that some types of domain walls exhibit dramatically enhanced electrical conductivity suggests enhanced thermal transport within the wall due to charge transport might be plausible.<br/>Measurement approaches used to date to investigate domain wall thermal properties, such as time-domain thermoreflectance and steady state heat flow setups, typically measure large sample areas and domain wall contributions to the overall sample response must be back extracted. Direct, local measurements of domain wall thermal transport properties have not yet been reported and would require thermal imaging with sufficiently nanoscale spatial resolution.<br/>Scanning Thermal Microscopy is a promising technique for mapping thermal properties of a sample with nanoscale spatial resolution. Here, we describe an approach for locally mapping variations in thermal properties associated with microstructural heterogeneity. A thin gold bar is deposited on the sample surface and is periodically Joule heated, allowing lock-in measurements of temperature to be mapped locally using the scanning probe as a temperature sensor². To validate that spatial heterogeneity in thermal properties can be imaged, a multilayer ceramic capacitor was used due to the expected variation in thermal response between the metal electrode and ceramic layers. We find that temperature contrast between the metal and ceramic layers can indeed be resolved and that the lock-in temperature measurements help mitigate against topographical crosstalk issues seen in static temperature measurements. Exploratory measurements are also currently underway to image thermal signal contrast associated with electrically conducting domain walls in ion-sliced LiNbO₃ thin films.<br/><br/>[1] Wang, J. J., Wang, Y., Ihlefeld, J. F., Hopkins, P. E. & Chen, L. Q. Tunable thermal conductivity via domain structure engineering in ferroelectric thin films: A phase-field simulation. <i>Acta Mater.</i> <b>111</b>, 220–231 (2016).<br/>[2] Menges, F. <i>et al.</i> Temperature mapping of operating nanoscale devices by scanning probe thermometry. <i>Nat. Commun.</i> <b>7</b>, 1–6 (2016).