Phonon stability and phonon transport of graphene-like borophene.

The last decades have seen tremendous progress in quantitative understanding of phonon transport, which is critical for the thermal management of various functional devices and the proper optimization of thermoelectric materials. In this work, using first-principles based calculation combined with non-equilibrium Green's function and phonon Boltzmann transport equation, we provide a systematic study on the phonon stability and phonon transport of monolayer boron sheet with honeycomb, graphene-like structure (graphene-like borophene) in both ballistic and diffusive regimes. For free-standing graphene-like borophene, phonon instabilities occur near the centre of Brillouin zone, implying elastic instability. Investigation of the electronic structures shows that the phonon instability is due to the deficiency of electrons. Our first-principles results show that with net charge doping and in-plane tensile strain, the graphene-like borophene is becoming thermodynamic stable in ideal plat nature, because the bonding characteristic is modified. At room temperature, the ballistic thermal conductance of graphene-like borophene 7.14 nWK-1 nm-2) is higher than that of graphene (4.1 nWK-1 nm-2), due to high phonon transmission. However, its diffusive thermal conductivity is two orders of magnitude lower than graphene, because the phonon relaxation time is dramatically reduced comparing with its carbon counterpart. Although the phonon group velocity and phonon anharmonicity are comparable with that of graphene, the suppressed phonon space results in dramatically strong phonon-phonon scattering. These thermal transport characteristics in both ballistic and diffusive regimes are of fundamental and technological relevance and provide guidance for applications of boron based nanomaterials in which their thermal conduction is major concern.