Anomalous thermal conductivity of monolayer boron nitride

In this paper, we use nonequilibrium molecular dynamics modeling to investigate the thermal properties of monolayer hexagonal boron nitride nanoribbons under uniaxial strain along their longitudinal axis. Our simulations predict that hexagonal boron nitride shows an anomalous thermal response to the applied uni- axial strain. Contrary to three dimensional materials, under uniaxial stretching the thermal conductivity of boron nitride nanoribbons rst increases rather than decreasing until it reaches its peak value and then starts decreasing. Under compressive strain the thermal conductivity of monolayer boron nitride ribbons monolithically reduces rather than increasing. We use phonon spectrum and dispersion curves to investi- gate the mechanism responsible for the unexpected behavior. Our molecular dynamics modeling and density functional theory (DFT) results show that application of longitudinal tensile strain leads to the reduction of the group velocities of longitudinal and transverse acoustic modes. Such a phonon softening mechanism acts to reduce the thermal conductivity of the nanoribbons. On the other hand, a signicant increase in the group velocity (stiening) of the exural acoustic modes is observed which counteracts the phonon softening eects of the longitudinal and transverse modes. The total thermal conductivity of the ribbons is a results of competition between these two mechanisms. At low tensile strain, the stiening mechanism overcomes the softening mechanism which leads to an increase in the thermal conductivity. At higher tensile strain, the softening mechanism supersedes the stiening and the thermal conductivity slightly reduces. Our simulations show that the decrease of the thermal conductivity under compressive strain is attributed to the formation of buckling defects which reduces the phonon mean free path.