Engineering Optical Excitations of 2D materials with Defects and Molecules

Two-dimensional (2D) materials are the subject of significant ongoing research for exploring exciton physics and device applications. One of the most promising classes of 2D materials, monolayer transition metal dichalcogenides (TMDs), features strong excitonic emission and the locking of the valley and spin degrees of freedom, leading to the selective excitation of states in different valleys by left- and right-hand circularly polarized light. These unique properties make such materials desirable for optical manipulation and enable their application in valleytronics. Here, we explore the modulation of the excitons and valley-selective circular dichroism in monolayer TMDs by proximity effects from the adsorption of chiral molecules and induction of point defects, using ab initio GW and Bethe-Salpeter equation calculations of quasiparticle energy levels and the optical spectra. We also study the charge and energy transfer at the molecule/TMD (perfect or defective with point defects) heterointerfaces. Our results suggest a pathway for manipulating the valley degree of freedom in 2D materials for valleytronics via defect engineering and molecular functionalization.