Direct Measurements of Anisotropic Thermal Transport in gamma-InSe Nanolayers via Cross-Sectional Scanning Thermal Microscopy

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 k(xy) = 2.16 Wm(-1) K-1 in-plane and k(z) = 0.89 Wm(-1) K-1 cross-plane confirming its potential for thermoelectric applications.