Engineering nanopore in monolayer WS ₂ for single-molecule imaging on the atomic-scale flat 2D surfaces

Two-dimensional (2D) materials often face limitations in molecular adsorption due to the absence of dangling bonds, affecting their performance as surface-enhanced Raman scattering (SERS) substrates. We address this by engineering defects, specifically nanopores, in monolayer WS₂ through controlled plasma treatments. These nanopores create dangling bonds to improve molecular adsorption for molecule enrichment and dynamically modulate the WS₂ bandgap to optimize band alignment with rhodamine 6G (R6G) molecules. This leads to a shift in photoluminescence peaks from red to blue with increasing defect density. Furthermore, the Raman signal is amplified by localized electromagnetic fields around the nanopores and carefully chosen resonance laser wavelengths. This combination allows blue-shifted WS₂ to achieve single-molecule SERS detection, with a sensitivity limit reaching 1 × 10^-20 M for R6G. Using SERS mapping, we visualize adsorption sites, confirming the nanopores' role as active sites. Our study introduces a pioneering method for developing ultrasensitive 2D SERS substrates and advances the understanding of SERS mechanisms on 2D materials.