Cryogenic Near-field Imaging in Spatial Disorder in Transition Metal Dichalcogenides

Excitons dominate the optoelectronic properties of two-dimensional transition metal dichalcogenides (TMDs) across near-infrared and visible frequencies due to their large binding energy and prominent oscillator strength. Previous measurements of excitons in these systems have primarily relied on far-field optical spectroscopy techniques which are diffraction limited to several hundred nanometers in the visible spectrum. To precisely image nanoscale spatial details requires an order of magnitude increase in resolution capabilities. Here, we present a study of the exciton spectra of TMDs using a cryogenic scattering-type scanning near-field optical microscope (s-SNOM) at 10 K. We map the spatial variation in exciton resonance energy across an hBN encapsulated MoSe₂ monolayer region with 30 nm resolution. Due to the exciton resonance narrowing at low temperatures, we can detect energy variations of less than one meV with nanometer resolution. We then construct high resolution images characterizing the spatial variation of the real and imaginary parts of the dielectric function of our device at 10 K. Comparison to measurements at room temperature illustrate the importance of cryogenic capabilities to observe this disorder. These results demonstrate cryogenic visible s-SNOM to be an effective nanoscale excitonic probe with relevance towards better understanding 2D material heterogeneity.