Effect of uniaxial strain on the excitonic properties of monolayer C3N : A symmetry-based analysis

In recent years, the application of mechanical stress has become a widespread experimental method to tune the electronic and optical properties of two-dimensional (2D) materials. In this work, we investigate the impact of uniaxial tensile strain along zigzag and armchair directions on the excitonic properties of graphene-like ${\mathrm{C}}_{3}\mathrm{N}$, a single-layer indirect-gap material with relevant mechanical and optical properties. To do that, we develop a tight-binding Bethe-Salpeter equation framework based on a Wannier-function description of the frontier bands of the system, and use it to compute both dark and bright excitons of ${\mathrm{C}}_{3}\mathrm{N}$ for different applied strain configurations. Then, we use this model approach to classify excitons of pristine and strained ${\mathrm{C}}_{3}\mathrm{N}$ according to the crystal symmetry and to explain the appearance of bright excitons with intense optical anisotropy in strained ${\mathrm{C}}_{3}\mathrm{N}$, even at small strains. Finally, the effect of strain on the exciton dispersion at small center-of-mass momenta is discussed, with special focus on the implications for 2D linear-nonanalytic dispersions.