Unusually low thermal conductivity of atomically thin 2D tellurium.

Tellurium is a high-performance thermoelectric material due to its superior electronic transport and low lattice thermal conductivity ($\kappa_L$). Here, we report the ultralow $\kappa_L$ in the monolayer tellurium, i.e., tellurene, which has been successfully synthesized in recent experiments. We find tellurene has a compellingly low room temperature $\kappa_L$ of 2.16 and 4.08 W m$^{-1}$ K$^{-1}$ along the armchair and zigzag directions, respectively, which is lower than any reported values for other 2D materials. We attribute this unusually low $\kappa_L$ to the soft acoustic modes, extremely low-energy optical modes and the strong scattering among optical-acoustic phonons, which place tellurene as a potential novel thermoelectric material. Finally, we disclose that $\kappa_L$ is proportional to the largest acoustic phonon frequency ($\omega_{D}^{a}$) and the lowest optical phonon frequency at $\Gamma$ point ($\omega_{\Gamma}^{o}$) in 2D materials, which reflect both harmonic and anharmonic thermal properties respectively.