The impact of point defects and temperature on the excitonic properties of monolayer germanium selenide*

We utilize first-principles density functional theory and many-body perturbation theory to study the optoelectronic properties of monolayer germanium selenide (GeSe), emphasizing the role of point defects and electron-phonon interactions; two phenomena that will be present in and can dominate the properties of real materials. First, we study the impact of a single Se vacancy on the optoelectronic properties of the monolayer, demonstrating the presence of localized excitons trapped by the defect. Additionally, by approximating the role of electron-phonon interactions, we study the role of vibrations on exciton photophysics. We determine that the optical absorption spectrum is red-shifted by ~0.1 eV at room-temperature phonons with both acoustic and optical phonons coupling to the excitonic state. Overall, we determine that the excitonic properties of GeSe are significantly affected by the presence of defects and phonons.