The understanding of heat transport processes at the nanoscale largely relies on our ability to measure temperature with high-spatial and temporal discrimination. Several thermometry techniques have been introduced in the recent past to tackle this problem, but they are typically inadequate for high-resolution screening or are applicable to a restricted set of samples. We are exploiting the unique properties of the NV center to develop new forms of scanning thermal microscopy. An example is presented in the figure below where we use a nanocrystal-hosted NV as an atomic-size sensor to monitor the temperature of a hot AFM tip. Upon contact with a room-temperature substrate heat flow into the sample causes a temperature drop in the tip and, consequently, in the diamond nanocrystal attached to it. Leveraging on the temperature dependence of the NV resonance frequency, we manage to record this temperature drop and, from it, we succeed in reconstructing a map of the substrate thermal conductivity with resolution limited by the tip size (~10 nm).
Relevant research areas that can benefit from the present technique are the investigation of phonon dynamics in confined structures or the study of radiative heat transport through nano-gaps. More in general, we foresee applications to a range of problems where local differences in the thermal conductivity can be exploited to indirectly gain information on the local sample structure or electronic dynamics. Examples are the investigation of heterogeneous phase transitions or catalytic processes. Ongoing complementary activities are oriented towards the use of AFM-hosted NVs for nanoscale thermometry (where the temperature change is induced by heat flow from the substrate onto the tip). This form of nanoscale sensing may find use in the characterization of the ‘hot spots’ formed at the junctions of semiconductor heterostructures, or in the investigation of exothermal reactions.