Spin-assisted nanoscale sensing

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).

(a) Experimental setup. An electrical current circulates along the arms of a thermal AFM cantilever (phosphorous-doped Si) and heats up the end section above the tip (intrinsic Si). A high-NA objective excites and collects the fluorescence emitted by a diamond-nanocrystal-hosted NV attached to the AFM tip. A wire on the sample surface serves as the mw source. We determine the tip temperature by monitoring the NV spin resonance frequency. (b) NV-assisted thermal conductivity image of an 18 nm thick gold structure on sapphire. The tip temperature drops when in contact with the E-shaped gold pattern, indicating higher thermal conductivity. (c) Measured tip temperature near the edge of the gold structure at two different heater temperatures, 465 K (full black circles, left vertical axis) and 540 K (empty red circles, right vertical axis). The topographic curve displaying the 18-nm-thick gold film edge as measured with the AFM (blue squares) is also present for reference. Higher spatial resolution (approaching the tip size) is possible at lower heater temperatures. (d) AFM topographic image of the same structure. From Laraoui et al., Nature Commun. (2015).

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.