Thermal Transport Mechanisms at Nanoscale Point Contacts

We have experimentally investigated the heat transfer mechanisms at a 90±10 nm diameter point contact between a sample and a probe tip of a scanning thermal microscope (SThM). For large heated regions on the sample, air conduction is the dominant tip-sample heat transfer mechanism. For micro/nano devices with a submicron localized heated region, the air conduction contribution decreases, whereas conduction through the solid-solid contact and a liquid meniscus bridging the tip-sample junction become important, resulting in the sub-100 nm spatial resolution found in the SThM images. Using a one dimensional heat transfer model, we extracted from experimental data a liquid film thermal conductance of 6.7±1.5 nW/K. Solid-solid conduction increased linearly as contact force increased, with a contact conductance of $0.76±0.38W/m2-K-Pa,$ and saturated for contact forces larger than 38±11 nN. This is most likely due to the elastic-plastic contact between the sample and an asperity at the tip end.