Drastically Reduced Thermal Conductivity in Chlorinated Diamane due to the Emergence of Extremely Low-Frequency Optical Phonons

We have carried out a computational study on the lattice dynamics and phonon transport properties of chlorine-functionalized diamane (ClD), and compared them with those of hydrogenated and fluorinated diamane (shortened as HD and FD, respectively), which shed some light on understanding how the functional group would modify the lattice dynamics and phonon scattering of two-dimensional materials from a microscopic view. Our study indicates that the heavy functional groups of chlorine could make the thermal conductivity drop by as much as ∼95% as compared to HD. Such a giant reduction originates from the sensitive suppression of the phonon group velocities and lifetimes of the out-of-plane acoustic (ZA) modes. More specifically, the heavy mass of the functional group induces the softening of lattice vibration, resulting in the emergence of extremely low-frequency optical phonons, which eventually suppresses the phonon group velocities and intensifies the scattering of phonons, especially the ZA phonons. These findings suggest that the phonon transport behavior in 2D materials is highly susceptible to basal plane functionalization. Thus, one can tune the thermal conductivity of 2D materials to a large extent by engineering the functional group. This strategy is promising for optimizing the figure merit of 2D thermoelectric materials.