Nano-spectroscopy of excitons in atomically-thin transition metal dichalcogenides

Excitons play a dominant role in optoelectronic properties of atomically thin van der Waals(vdW) semiconductors, such as transition metal dichalcogenides (TMDs). These excitons are amenable to on-demand engineering with diverse controls, including dielectric screening, interlayer hybridization, and moiré potential. However, external stimuli frequently yield heterogeneous excitonic responses at the nano- and meso-scales, which are beyond the spatial resolution of conventional diffraction-limited optics. We use a scattering-type scanning near-field optical microscope (s-SNOM) to acquire exciton spectra in atomically thin transition metal dichalcogenide microcrystals with previously unattainable 20 nm resolution. Our nano-optical data reveal material- and stacking-dependent exciton spectra of MoSe₂, WSe₂ monolayers, and their heterostructures. Furthermore, the complex dielectric function of these vdW semiconductors can be accessed at the nanoscale. In addition, s-SNOM hyperspectral images uncover how the dielectric screening modifies excitons at length scales as short as few nanometers. This work paves the way towards understanding and manipulation of excitons in atomically thin layers at the nanoscale.