  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.
