Localized Excitons in Defective Monolayer Germanium Selenide

TL;DR

This study uses advanced computational methods to reveal how selenium vacancies in monolayer GeSe create localized states and alter optical properties, emphasizing the significance of defects in 2D material design.

Contribution

It provides a detailed theoretical analysis of defect-induced electronic and optical changes in monolayer GeSe, highlighting the impact of selenium vacancies.

Findings

Vacancies create mid-gap trap states that localize charge carriers.

Optical absorption peaks shift below the pristine material’s gap.

Defects significantly reduce exciton size and induce out-of-plane dipoles.

Abstract

Germanium Selenide (GeSe) is a van der Waals-bonded layered material with promising optoelectronic properties, which has been experimentally synthesized for 2D semiconductor applications. In the monolayer, due to reduced dimensionality and, thus, screening environment, perturbations such as the presence of defects have a significant impact on its properties. We apply density functional theory and many-body perturbation theory to understand the electronic and optical properties of GeSe containing a single selenium vacancy in the $-2$ charge state. We predict that the vacancy results in mid-gap "trap states" that strongly localize the electron and hole density and lead to sharp, low-energy optical absorption peaks below the predicted pristine optical gap. Analysis of the exciton wavefunction reveals that the 2D Wannier-Mott exciton of the pristine monolayer is highly localized around the defect, reducing its Bohr radius by a factor of four and producing a dipole moment along the out-of-plane axis due to the defect-induced symmetry breaking. Overall, these results suggest that the vacancy is a strong perturbation to the system, demonstrating the importance of considering defects in the context of material design.