van der Waals Engineering for Discrete Control of Homogeneous and Inhomogeneous Exciton Broadening in Monolayer 2D Semiconductors.

Controlling exciton broadening is essential for maintaining stable and coherent optical signals. In monolayer transition metal dichalcogenides (TMDs), neutral excitons can exhibit radiative decay-dominated homogeneous broadening at low temperatures, yet the observed emission spectra contain significant disorder-induced inhomogeneous broadening. However, only a few approaches, such as hexagonal boron nitride (hBN) encapsulation and its thickness control, have partially suppressed these contributions. Here, discrete control of both homogeneous and inhomogeneous exciton broadening in monolayer MoSe2 is demonstrated by constructing van der Waals heterostructures that incorporate hBN, graphene, and additional TMD layers. Interfacing MoSe2 with graphene suppresses inhomogeneous broadening and quenches charged exciton emission through rapid nonradiative transfer, resulting in a single and homogeneously broadened neutral exciton emission. Adding TMD or graphene layers on graphene/MoSe2 reduces the homogeneous exciton linewidth by weakening Coulomb interactions. A 40% reduction in total exciton linewidth and more than a 3-fold enhancement in the homogeneous broadening contribution are achieved compared to solely hBN-encapsulated monolayer MoSe2 by adjusting the number of graphene and top TMD layers. This approach offers a practical pathway for achieving coherent, lifetime-limited exciton emission in monolayer TMDs, positioning them as promising light sources for quantum optoelectronic applications.