Near-field scanning thermoreflectance imaging (NeSTRI) as a nano-optical technique for contactlessly mapping the thermal conductivity of 2D materials at the nanoscale

To date, virtually all techniques used to image the thermal properties of 2D materials and thin films at the nanoscale have required to position the sample in contact with probes that act as undesirable thermal sinks and dramatically affect the measurements. Thermoreflectivity, an optical technique in which thermal transport properties are measured by contactlessly probing the heat-induced changes in reflectivity at the air-sample interface, has been utilized to image and map the thermal conductivity of solids at the macroscopic and microscopic level, but, so far, has been diffraction-limited in its applicability at the nanoscale. In this paper, we show how our group has tackled such an issue by coupling thermoreflectivity mapping with near-field scanning optical microscopy (NSOM) in a pump-probe nano-optical technique [Nanoscale 9 (2017) 4097]. We show that our technique is successful in investigating the local impact on the thermal conductivity of edges and wrinkles of non-ideal domains of 2D materials. Further on, we investigate the thermal properties of a graphene thin film decorated with copper particles and demonstrate that contactless near-field scanning thermoreflectance imaging can map the electron-phonon coupling in graphene-based nanocomposites.