Near-field Imaging of Transition Metal Dichalcogenides*

Monolayer transition metal dichalcogenide (TMD) (e.g., MoSe₂, WSe₂, MoS₂, WS₂) semiconductors have received massive attention after the discovery of strongly bound excitons at the +K and -K valleys. When TMDs are stacked in a heterostructure, the lattice mismatch and/or twist angle causes a periodic modulation of the bands resulting in trapped moiré excitons at the potential minima. To visualize these trapped moiré excitons requires an imaging technique with spatial resolution on the order of the moiré wavelength.  Previous measurements of excitons in these systems have primarily relied on far-field optical spectroscopy techniques which are diffraction-limited to several hundred nanometers.  Here, we present a study of the exciton spectra of TMD heterostructures using a cryogenic scattering-type scanning near-field optical microscope (s-SNOM). Using a tunable visible source, we map the exciton resonances in the TMD materials with sub 100 nm spatial resolution.