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Nanoscale heat transport is of increasing importance as it often defines performance of modern processors and thermoelectric nanomaterials, and affects functioning of chemical sensors and biosensors. Scanning thermal microscopy (SThM) is the leading tool for nanoscale mapping of thermal properties, but it is often negatively affected by unstable tip-surface thermal contacts. While operating SThM in-liquid environment may allow unimpeded thermal contact and open new application areas, it has so far been regarded as impossible due to increased heat dissipation into the liquid, and the perceived reduced spatial thermal resolution. Nevertheless, in this paper we show that such liquid immersion SThM (iSThM) is fully feasible and, while its thermal sensitivity and spatial resolution is somewhat below that of in-air SThM, it has sufficient thermal contrast to detect thermal conductivity variations in few tens of nm thick graphite nanoflake and metal-polymer nanostructured interconnects. Our results confirm that thermal sensing in iSThM can provide nanoscale resolution on the order of 30 nm, that, coupled with the absence of tip snap-in due to the elimination of capillary forces, opens the possibility for nanoscale thermal mapping in liquids, including thermal phenomena in energy storage devices, catalysts and biosystems.

Van der Waals (vdW) atomically thin materials and their heterostructures offer a versatile platform for the management of nanoscale heat transport and the design of novel thermoelectrics. These require the measurement of highly anisotropic heat transport in vdW‐based nanolayers, a major challenge for nanostructured materials and devices. In the present study, a novel effective method of cross‐sectional scanning thermal microscopy was used to map and quantify the anisotropic heat transport in nanoscale thick layers of vdW materials and the material‐substrate interfaces. This technique measures the heat conducted into a vdW crystal via the nanoscale apex of a heat‐sensitive probe. The crystal is nano‐polished via Ar ion beams generating an oblique nearly atomically flat surface. By measuring the thermal conductance variation as a function of increasing layer thickness, the transition between the cross‐plane and in‐plane heat transport (defined by heat conductivity anisotropy) is acquired. By using an analytical model validated by finite element simulations, anisotropic thermal transport in a gamma indium selenide crystal nano‐thin flake on a Si substrate was studied, obtaining results corresponding to anomalously low anisotropic thermal conductivities of kxy = 2.16 Wm−1 K−1 in‐plane and kz = 0.89 Wm−1 K−1 cross‐plane confirming its potential for thermoelectric applications. [ABSTRACT FROM AUTHOR]

Scanning thermal microscopy (SThM), which enables measurement of thermal transport and temperature distribution in devices and materials with nanoscale resolution is rapidly becoming a key approach in resolving heat dissipation problems in modern processors and assisting development of new thermoelectric materials. In SThM, the self-heating thermal sensor contacts the sample allowing studying of the temperature distribution and heat transport in nanoscaled materials and devices. The main factors that limit the resolution and sensitivities of SThM measurements are the low efficiency of thermal coupling and the lateral dimensions of the probed area of the surface studied. The thermal conductivity of the sample plays a key role in the sensitivity of SThM measurements. During the SThM measurements of the areas with higher thermal conductivity the heat flux via SThM probe is increased compared to the areas with lower thermal conductivity. For optimal SThM measurements of interfaces between low and high thermal conductivity materials, well defined nanoscale probes with high thermal conductivity at the probe apex are required to achieve a higher quality of the probe-sample thermal contact while preserving the lateral resolution of the system. In this paper, we consider a SThM approach that can help address these complex problems by using high thermal conductivity nanowires (NW) attached to a tip apex. We propose analytical models of such NW-SThM probes and analyse the influence of the contact resistance between the SThM probe and the sample studied. The latter becomes particularly important when both tip and sample surface have high thermal conductivities. These models were complemented by finite element analysis simulations and experimental tests using prototype probe where a multiwall carbon nanotube (MWCNT) is exploited as an excellent example of a high thermal conductivity NW. These results elucidate critical relationships between the performance of the SThM probe on one hand and thermal conductivity, geometry of the probe and its components on the other. As such, they provide a pathway for optimizing current SThM for nanothermal studies of high thermal conductivity materials. Comparison between experimental and modeling results allows us to provide direct estimates of the contact thermal resistances for various interfaces such as MWCNT-Al (5×10 −9 ±1×10 −9 K m 2 W −1 ), Si 3 N 4 –Al (6×10 −8 ±2.5×10 −8 K m 2 W −1 ) and Si 3 N 4 −graphene (~10 −8 K m 2 W −1 ). It was also demonstrated that the contact between the MWCNT probe and Al is relatively perfect, with a minimal contact resistance. In contrast, the thermal resistance between a standard Si 3 N 4 SThM probe and Al is an order of magnitude higher than reported in the literature, suggesting that the contact between these materials may have a multi-asperity nature that can significantly degrade the contact resistance. [ABSTRACT FROM AUTHOR]