Significant enhancement of lattice thermal conductivity of monolayer AlN under bi-axial strain: a first principles study

Abstract

Using rigorous ab initio calculations within the framework of phonon Boltzmann transport theory, we have carried out a detailed investigation to probe the effects of uniform bi-axial strain and finite size on the lattice thermal conductivity (κ) of monolayer AlN. We show that implementation of bi-axial tensile strain can shoot up the value of κ of monolayer AlN by a large amount unlike in the case of analogous 2D materials. The value of κ for monolayer AlN is calculated to be 306.5 W m⁻¹ K⁻¹ at room temperature (300 K). The value of κ can be raised by one order of magnitude to up to 1500.9 W m⁻¹ K⁻¹ at 300 K by applying a bi-axial strain of about 5%. A similar trend persists when the finite size effect is incorporated in the calculation. As the sample size is varied from 10 nm to 10 000 nm along with increasing tensile strain, a huge variation of κ (from 20.7 W m⁻¹ K⁻¹ to 558.9 W m⁻¹ K⁻¹) is observed. Our study reveals that the major part of the lattice thermal conductivity of monolayer AlN comes from the contribution of the flexural acoustic (ZA) phonon modes. The anomalous trend of drastic increment in the value of κ with tensile strain is found to be a direct effect of interaction between nitrogen lone-pair electrons and bonding electrons in the ionic lattice which results in the reduction of phonon anharmonicity with increasing tensile strain. Our study provides a detailed analysis of the strain modulated and size-tuned thermal transport properties of monolayer AlN revealing that it is an impactful 2D material to be used in thermal management devices.