Ultra-low thermal conductivity in disordered, layered tungsten diselenide

Ultra low thermal conductivity of tungsten diselenide (WSe$_{2})$ thin films is achieved by controlling order and disorder of two dimensional WSe$_{2}$ sheets. We prepared highly textured nanocrystalline WSe$_{2}$ films by modulated elemental reactant (MER) method. Synchrotron X-ray diffraction shows the WSe$_{2}$ sheets are well aligned with the Si (100) substrate and the films have completely random crystalline orientation in the a-b plane. The cross-plane thermal conductivity of thin films of WSe$_{2}$ is as small as 0.05 W/m-K at room temperature, 30 times smaller than the c-axis thermal conductivity of single-crystal WSe$_{2}$ and a factor of 6 smaller than the predicted minimum thermal conductivity for this material. Molecular dynamics simulation on model structures suggests that the ultra-low thermal conductivity in layered, disordered crystals is a general phenomenon and not restricted to WSe$_{2}$. Ion irradiation of the samples disrupted the layered structure and the crystallinity of the WSe$_{2}$ sheets and lead to an increase with a factor of 3 in thermal conductivity. We attribute the ultra-low thermal conductivity to the localization of lattice vibrations induced by the random stacking of two-dimensional crystalline WSe$_{2}$ sheets.