Phononic thermal properties of two-dimensional materials

With many novel two-dimensional (2-D) materials beyond graphene emerging for various applications ranging from electronics, photonics, to thermal management and energy storage, phononic and thermal properties of 2-D materials are of fundamental interest, for exploring both fundamental physics and practical applications. In this talk, we first summarize and compare the phonon properties, such as phonon dispersion and relaxation time, of pristine 2-D materials with the single layer graphene to understand the role of crystal structure on their thermal conductivity. We then compare the phonon properties, between an idealized 2-D crystal, realistic 2-D crystals, and 3-D crystals, and present the physical picture on how the thermal conductivity of 2-D materials changes with sample sizes. The geometric effects, such as layer numbers and nanoribbon width, and other physical effects like defects, mechanical strains, and substrates, on the thermal properties of 2-D materials are discussed. Intercalation could affect both the group velocities and phonon relaxation times of layered crystals, and thus tune the thermal conductivity along both the through-plane and basal-plane directions. We also briefly discuss the challenges in theoretical and experimental studies of thermal transport in 2-D materials. The rich and special phonon physics in 2-D materials make them promising candidates for exploring novel phenomena like topological phonon effect and applications like phononic quantum devices, as discussed in the outlook.