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

Density functional theory-based calculations have been performed to solve 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 (κL) of ML-ZnO is found to increase dramatically in response to biaxial tensile strain. This result contradicts the general belief that tensile strain leads to the deterioration of thermal transport properties. The strain-induced quadratic to linear transition of the out-of-plane acoustic or ZA mode dispersion and the resulting concomitant increase in group velocity and decrease in phonon population are found to play a significant role in the unusual enhancement of κL. The mode-resolved analysis further reveals that the tensile-strain-driven competition between different phonon properties, primarily group velocity and phonon lifetime, is responsible for the observed anomalous increase in κL. Additionally, the phonon scattering calculations elucidate the crucial role of 4-phonon scattering in the thermal transport, highlighting the importance of higher-order anharmonicity in ML-ZnO. A strikingly high 4-phonon scattering strength is found in ML-ZnO, which primarily results from the strong anharmonicity, quadratic ZA mode dispersion, large frequency gap in phonon dispersion, and reflection symmetry-induced selection rule. The inclusion of 4-phonon scattering significantly alters the transport characteristics of all of the phonon modes, in general, and ZA phonons, in particular. This work, therefore, highlights a valuable approach to enhance the thermal transport properties of ML-ZnO while providing critical insight into the underlying 3-phonon and 4-phonon scattering mechanisms.