Tensile Strain Induced Anomalous Enhancement in the Lattice Thermal Transport of Monolayer ZnO: A First Principles Study

Abstract

Density functional theory based calculations have been performed for solving the phonon Boltzmann transport equation to investigate the thermal transport properties of monolayer (ML) ZnO under in-plane isotropic biaxial tensile strain. The in-plane lattice thermal conductivity ($\kappa_{\text{L}}$) of ML-ZnO increases dramatically in response to the biaxial tensile strain ranging from 0% to 10%, conflicting with the general belief. The strain-induced stiffening of the ZA phonon mode and the resulting concomitant increase in group velocity and decrease in phonon population is found to play a significant role behind the unusual enhancement of $\kappa_{\text{L}}$. The mode resolved analysis shows the tensile strain driven competitive behavior between different phonon properties, mainly the group velocity and phonon lifetimes, being responsible for the observed unusual enhancement in $\kappa_{\text{L}}$. Additionally, the phonon scattering calculations show the importance of inclusion of 4-phonon scattering in the thermal transport calculations suggesting the significance of higher-order anharmonicity in ML-ZnO. A strikingly high 4-phonon scattering strength in ML-ZnO primarily results from the strong anharmonicity, quadratic ZA mode dispersion, large frequency gap in phonon dispersion, and reflection symmetry induced selection rule. The incorporation of 4-phonon scattering significantly alters the transport characteristics of all the phonon modes, in general and ZA phonons, in particular. At large strains, a linear dispersion of the ZA mode and closure of the frequency gap is observed, which results in a significant reduction of 4-phonon scattering strength in ML-ZnO.