First principle study on optical properties of defects in two-dimensional materials

Two-dimensional (2D) materials with direct bandgaps have drawn a lot of attention due to their great potential for use in future optoelectronics. However, since 2D materials are quite susceptible to defects, we need to identify possible defect structures created and to understand the effects of such defects on their optoelectronic properties. It is expected that these defects will play crucial roles in optical properties, as spatially localized electron transitions occur through the mid-gap states induced by them, leading to the single-photon emission. In this study, we carry out ab initio calculations based on density functional theory to investigate the structural and electronic properties of various defects in hexagonal boron nitride and WSe₂, as prototypical 2D systems. We explore the stability of various defects by evaluating their formation energies as a function of different parameters including chemical potentials, charge states, and temperature, which can be adjusted by an external electric field or an electron beam under experimental conditions. We further solve the Bethe–Salpeter equation with the GW approximation to study their optical absorption spectra and compare them with the results of their defect-less pristine counterparts to extract the defect effects.