Anisotropic phonon thermal transport in nitrophosphorene monolayer

Abstract

Recent studies have shown that although black phosphorene ($\ensuremath{\alpha}$-P) has promising electrical, thermal, and mechanical properties, its poor stability in the air makes its applications challenging. Very recently, monolayer puckered-phase nitrophosphorene ($\ensuremath{\alpha}$-NP) has emerged as a new two-dimensional material with improved stability compared to $\ensuremath{\alpha}$-P, while it preserves the appealing electronic and mechanical properties of $\ensuremath{\alpha}$-P. However, the phonon transport properties of $\ensuremath{\alpha}$-NP remain poorly understood. In this study, we systematically compare phonon transport and thermal conductivity of $\ensuremath{\alpha}$-NP and $\ensuremath{\alpha}$-P by means of an iterative solution for the Boltzmann transport equation and first-principles calculations. We pay particular attention to careful enforcement of the Born-Huang sum rules on the interatomic force constants so that the flexural phonon branch of $\ensuremath{\alpha}$-NP becomes perfectly quadratic close to the center of the Brillouin zone. We find that $\ensuremath{\alpha}$-NP has a less anisotropic thermal conductivity compared to $\ensuremath{\alpha}$-P, with the room-temperature values of 37.65 and $18.66\phantom{\rule{4pt}{0ex}}{\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in the armchair and zigzag directions, respectively. Interestingly, the anisotropy in the thermal conductivity of $\ensuremath{\alpha}$-NP is found to be reversed compared to that of $\ensuremath{\alpha}$-P, which has a higher thermal conductivity in the zigzag direction than the armchair direction. We found that as opposed to the usual understanding, considering only the directional dependence of the phonon group velocity fails to explain this reversed anisotropic thermal conductivity, and other phonon behaviors, such as avoided crossing of the acoustic and optical phonon branches, play a key role in determining the phonon relaxation time and thermal conductivity in $\ensuremath{\alpha}$-NP. Our results may provide theoretical guidance for future fundamental studies on the anisotropic phonon thermal transport in low-dimensional materials, and they may also enable efficient heat management of nanoelectronic devices based on $\ensuremath{\alpha}$-NP.