Nanoscale Mapping of Strain-engineered Excitons in Monolayer WSe$_2$

Abstract

Excitons in monolayer transition metal dichalcogenides (TMDs) are highly sensitive to external stimuli, allowing control of resonance energies and lifetimes through electric and magnetic fields, heterostructures, moire potentials and mechanical strain. Observations of single-photon emission (SPE) near sample edges, wrinkles, and strained nanobubbles have motivated strain-engineering aimed at realizing localized emitters in 2D materials. The origin of these emitters remains unclear and prior studies have relied on far-field spectroscopy with strain profiles inferred separately through topography, limiting direct correlation between local strain and excitonic response. We directly probe strain-modified excitonic spectra in monolayer WSe$_2$ using cryogenic scattering-type scanning near-field optical microscopy (s-SNOM). We compare two strain-engineering techniques, nano-indentation and patterned substrates, used to produce SPEs. At 9 K, we spatially resolve exciton resonance shifts with sub-diffraction limit resolution, simultaneously measuring near-field optical response and topography. We observe that strain transferred from nano-indentation into a deformable polymer substrate differs from that produced by draping TMDs over patterned dielectric pillars. Adhesion and tearing during plastic deformation drive a compressive exciton blueshift, while draping produces dominant tensile strain. Comparing the strain model results to the experimentally observed energy shifts shows that the nano-indentation strain is over an order of magnitude smaller than previously inferred, due to puncturing of the fragile monolayer, whereas pillar-strained devices remain intact under more modest strain. These differences suggest SPE arises from complex, strain-mediated interactions rather than simple confinement potentials, establishing s-SNOM as a powerful probe of quantum emission in strained 2D materials.