The influence of disorder in the external environment of 2D semiconductors on their electronic and optical properties

Disorder in solids typically stems from local fluctuations of material structure itself like composition, strain, and size. Here, we highlight a new source of disorder in atomically thin, two-dimensional (2D) materials: The variation in the effective strength of Coulomb interactions in the 2D material resulting from fluctuations in the dielectric screening of the adjoining environment. We experimentally monitor the influence of dielectric disorder for monolayer WS₂ and WSe₂ on SiO₂, PDMS and h-BN by probing correlations between ground and excited state exciton resonances, which exhibit different sensitivities to the external dielectric environment. Our observations are described in a theoretical framework that considers variation in external dielectric screening and intrinsic phonon scattering channels as contributing to the ground and excited-state exciton linewidths. Even moderate fluctuations in the external dielectric permittivity are shown to induce inhomogeneous variations of the bandgap and exciton binding energies on the order of 100's of meV, constituting the major source of disorder in the studied samples. We identify elimination of dielectric disorder as key to achieving high material quality through encapsulation of 2D semiconductors in other van der Waals materials.