Self-Assembled Oxide Nanoneedle-Induced Strain for Tunable Excitonic Emission in Monolayer MoS₂

Strain engineering has emerged as a powerful approach for tailoring the optical response of two-dimensional semiconductors. In this work, we show that monolayer MoS₂ exhibits tunable excitonic emission, when nanoneedle-like oxide protrusions form across the monolayer surface during a photo-oxidation-assisted exfoliation process. These protrusions locally deform the lattice, generating spatially varying strain fields within the MoS₂ layer. Photoluminescence and Raman measurements reveal a clear correlation between strained regions, exciton peak shifts, and enhanced emission, with the photoluminescence intensity increasing by approximately 3-fold, accompanied by a red shift of the in-plane (E_2g ¹) vibrational mode, indicative of strain. Furthermore, selected monolayers and unexfoliated regions exhibit additional emergent near-infrared photoluminescence features absent in pristine MoS₂. Unlike conventional strain-engineering approaches requiring external mechanical deformation, specialized substrate prepatterning, or complex heterostructures, this simple one-step process generates intrinsic nanoneedle-induced strain, providing an effective route for tailoring excitonic recombination and extending the optical response of monolayer TMDCs.