First-principles study of thermal properties of borophene.

Very recently, a new single-element two-dimensional (2D) material borophene was successfully grown on a silver surface under pristine ultrahigh vacuum conditions which attracts tremendous interest. In this paper, the lattice thermal conductivity, phonon lifetimes, thermal expansion and temperature dependent elastic moduli of borophene are systematically studied by using first-principles. Our simulations show that borophene possesses unique thermal properties. Strong phonon-phonon scattering is found in borophene, which results in its unexpectedly low lattice thermal conductivity. Thermal expansion coefficients along both the armchair and zigzag directions of borophene show impressive negative values. More strikingly, the elastic moduli are sizably strengthened as temperature increases, and the negative in-plane Poisson's ratios are found along both the armchair and zigzag directions at around 120 K. The mechanisms of these unique thermal properties are also discussed in this paper.