Thickness and strain engineering of structural and electronic properties for 2D square-octagon AlN

Two-dimensional (2D) semiconductors exhibit great potential to minimize the size and drastically reduce the energy consumption of optoelectronic devices due to promising features induced by quantum confinement. It has achieved many successes in infrared and visible light optoelectronic devices. The study on ultra-wide band gap 2D semiconductors excepth-BN are still limited, however, the requirement is more and more urgent. Inspired by the progresses of III-nitride semiconductors in recent several decades, 2D AlN is highly expected to be a new member of ultra-wide band gap 2D semiconductors. In this work, we employed the first-principles calculations to investigate the structural and electronic properties of 2D AlN. We revealed that few-layer AlN acquires a square-octagon (so-AlN) configuration in the vertical direction when the number of atomic layersnis smaller than 16. With increasing the thickness from 2 ML to 8 ML, the band gap decreased due to the weakening of quantum confinement effect. We demonstrated the intrinsic indirect band gap can be tuned to be direct by applying different direction strains forso-AlN. Our results open new avenues for their application in nano-optoelectronics.