Anisotropic thermal conductivity in 2D tellurium

Two-dimensional tellurium (2D-Te) has been recently synthesized and shown potential in electronics, optoelectronics, and thermoelectric applications, with the merits of high mobility, environmental stability, high thermoelectric power-factor, and simplicity of mass production. These 2D-Te films have unique atomic structures: the Te atoms form trigonal helical chains and are then stacked into hexagonal lattice by van der Waals force, which brings up distinctive transport behaviors. Here we report anisotropic thermal conductivity of suspended 2D-Te films measured by micro-Raman thermometry and the time-domain thermal reflectance (TDTR) method. The in-plane along-chain and cross-chain thermal conductivities are found to be around 2.5 and 1.7 W m−1 K−1, respectively, for thicker films (>100 nm), and reduced to 1.6 and 0.64 W m−1 K−1 for the thinner films (<20 nm). The measured anisotropy is >1.3 for all the films studied. The cross-plane (also across-chain) thermal conductivity is found to be around 0.8 to 1.2 W m−1 K−1 for thicker films, slightly lower than that along the in-plane across-chain direction due to the stronger suppression by the thin film boundary. Theoretical modeling reveals that the anisotropy mainly originates from anisotropic phonon dispersion. The long mean-free-path phonons in Te are also shown to be strongly suppressed by boundary scattering. The large reduction of anisotropic thermal conductivity from the bulk makes it the best single-element thermoelectric material and enables potential thermoelectric generation or cooling devices at room temperature. Our results also provide critical information for thermal management of 2D-Te electronic devices.