Softened sp2-sp3 bonding network leads to strong anharmonicity and weak hydrodynamics in graphene+

Graphene+, a novel carbon monolayer with sp2-sp3 hybridization, is recently reported to exhibit graphene-like Dirac properties and unprecedented out-of-plane half-auxetic behavior [Yu et al, Cell Reports Physical Science, 3 100790 (2022)]. Herein, from comprehensively state-of-the-art first-principles studies, we report the exceptional lattice thermal transport properties of graphene+ driven by the unique sp2-sp3 crystal configuration. At room temperature, the thermal conductivity of graphene+ is calculated to be ~170 W/mK, which is much lower than that of graphene (~3170 W/mK) Despite the buckling structure, weak phonon scattering phase space is trapped in graphene+. Thus, the reduction in thermal conductivity magnitude stems from soft bonding due to the unique sp2-sp3 crystal configuration. Soft bonding suppresses the vibrations of acoustic phonons, which leads to strong anharmonicity and weak phonon hydrodynamics. Further, lower group velocity, relaxation time and smaller phonon mean free path emerge in graphene+, and the significantly decreased thermal conductivity is achieved. Our study provides fundamental physical insights into the thermal transport properties of graphene+, and it serves as an ideal model to study atomic bonding versus thermal transport properties due to weak scattering phase space.