Cryogenic Nano-Imaging of Excitons in a Monolayer Semiconductor

Optical measurements of 2D semiconductors have primarily relied on far-field spectroscopy techniques, which are diffraction limited to several hundred nanometers. Precisely imaging nanoscale spatial disorder requires an order of magnitude increase in resolution capabilities. Here, we present a spatially resolved study of the exciton spectra of monolayer MoSe2 in the visible range using cryogenic scattering-type scanning near-field optical microscopy (s-SNOM) operating down to 11 K. Mapping the exciton resonance across an hBN-encapsulated MoSe2 monolayer, we achieve sub-50 nm spatial resolution and an energy resolution below 1 meV. We further investigate the material's near-field spectra and dielectric function, demonstrating the ability of cryogenic visible s-SNOM to reveal nanoscale disorder. Comparison to room-temperature measurements illustrates enhanced capabilities of the cryogenic s-SNOM to reveal fine-scale material heterogeneity. These results establish cryogenic visible s-SNOM as an effective nanoscale excitonic probe, offering valuable insights into 2D material heterogeneity and nanoscale sensing.