Combined Nano-Optical/Scanning Tunneling Microscopy of Localized Excitons in 1L-WSe₂

The transition metal dichalcogenides (TMDs) are among the most studied low dimensional material classes largely due to their tightly bound exciton states which strongly interact with light. Extensive experimental and theoretical work has shown that monolayer TMDs host a rich variety of exciton complexes, from dark excitons, biexcitons, to moiré excitons. Because of this strong light-matter coupling, optical probes are often the tool of choice, however for a full understanding combined electronic, optical, and structural probes are needed to disentangle the many degrees of freedom. Combining these probes is however experimentally challenging because of the large scale mismatch of light, ~500 nm, and high resolution electronic/structural techniques such as scanning tunneling microscopy (STM), ~1 nm.<br/><br/>In this presentation, I will show results from a newly constructed scanning tunneling microscope integrated with a nano-optical probe that allows for co-localized near-field light delivery, at high excitation intensities, with tunneling current measurements. I will show the application of this tool to strain-localized excitons in nanobubbles of monolayer WSe₂. We observe strong modification of the density of states in scanning tunneling spectroscopy as a function of pump fluence. Further, by hole tunneling we are able to map the STM-luminescence, revealing a localized exciton edge state with a resolution &lt;10 nm. Our work represents a novel demonstration of the effect of optical pumping on the single particle electronic structure in TMDs at the exciton’s native length scale.