Localized excitons in defective monolayer germanium selenide

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.

Modeling Excited States of Point Defects in Materials from Many-Body Perturbation Theory

“…93 For low-dimensional materials, incorporation of excitonic effects is critical because the reduced screening means that electron−hole interactions are strong even for the pristine material and that the defect poses a strong perturbation to the system. For defective 2D materials, exciton binding energies of ∼0.5−1.5 eV have been predicted from GW/BSE calculations for doped graphene, 87 defective monolayer chalcogenides, 90,104,105 and doped boron nitride nanostructures. 106 Such strong excitonic effects result in trapping of the excited state at the defect site.…”

“…106 Such strong excitonic effects result in trapping of the excited state at the defect site. 104,107 Additionally, the symmetry breaking associated with defects can result in a modification of valley selectivity in 2D transition metal dichalcogenides. 68,90 Substrate screening can significantly influence the excitonic properties of defective 2D materials; 105,108 thus, it is necessary to incorporate these screening effects for interpretation of defectrelated energetics.…”

“…As an example of the strong excitonic effects in 2D materials, Figure 6 shows the GPP-G 0 W 0 @PBE/BSE-predicted imaginary component of the dielectric function (eq 6) of monolayer GeSe with the addition of a single charged Se vacancy. 104 The vacancy introduced midgap states into the bandstructure (not shown) that resulted in low-energy transitions well below the 1.5 eV optical gap of pristine GeSe. Analysis of the exciton wave function (inset of Figure 6) showed that the lowest energy excited state is a defect-to-defect transition that is localized around the defect center with a binding energy of ∼0.3 eV.…”

“…Cohen et al predicted that the exciton can be described as a hydrogenic system with a Bohr radius of 5 Å, while the pristine Wannier−Mott exciton was predicted to extend over a Bohr radius of 2 nm. 104 MBPT calculations have also resulted in reinterpretation of optoelectronic device properties. DFT + G 0 W 0 @PBE calcu-lations by Flores et al predicted that oxygen impurities improve the performance of CdTe/CdSe solar cells due to antisiteoxygen complex formation.…”

“…For low-dimensional materials, incorporation of excitonic effects is critical because the reduced screening means that electron–hole interactions are strong even for the pristine material and that the defect poses a strong perturbation to the system. For defective 2D materials, exciton binding energies of ∼0.5–1.5 eV have been predicted from GW/BSE calculations for doped graphene, defective monolayer chalcogenides, ,, and doped boron nitride nanostructures . Such strong excitonic effects result in trapping of the excited state at the defect site. , Additionally, the symmetry breaking associated with defects can result in a modification of valley selectivity in 2D transition metal dichalcogenides. , Substrate screening can significantly influence the excitonic properties of defective 2D materials; , thus, it is necessary to incorporate these screening effects for interpretation of defect-related energetics.…”