Extreme in-plane thermal conductivity anisotropy in Rhenium-based dichalcogenides

Anisotropies in thermal conductivity are important for thermal management in a variety of applications, but also provide insight on the physics of nanoscale heat transfer. As materials are discovered with more extreme transport properties, it is interesting to ask what the limits are for how dissimilar the thermal conductivity can be along different directions in a crystal. Here we report on the thermal properties of rhenium-based transition metal dichalcogenides (TMDs), specifically rhenium disulfide (ReS2) and rhenium diselenide (ReSe2), highlighting their extraordinary thermal conductivity anisotropy. Along the basal crystal plane of ReS2, a maximum of 169 +/- 11 W mK(-1) is detected along the b-axis and a minimum of 53 +/- 4 W mK(-1) perpendicular to it. For ReSe2, the maximum and minimum values of 116 +/- 3 W mK(-1) and 27 +/- 1 W mK(-1) are found to lie 60 degrees and 150 degrees away from the b-axis, along the polarization direction of some of the principal Raman modes. These measurements demonstrate a remarkable anisotropy of 3.2 x and 4.3 x in the conductivity within the crystal basal planes, respectively. The through-plane thermal conductivities, recorded at 0.66 +/- 0.01 W mK(-1) for ReS2 and 2.31 +/- 0.01 W mK(-1) for ReSe2, highlight the impact of their layered structures, contributing to notably high in-plane to through-plane thermal conductivity ratios of 256 x for ReS2 and 50 x for ReSe2. This research demonstrates the unique thermal properties that these comparatively underexplored TMDs have, shedding light on the need for further exploration into the intricate thermal behavior of such materials, while underscoring their potential significance for future applications in the fields of semiconductor devices and nanotechnology.