Plasmonic nanogap grid arrays for tunable SERS enhancement and strain-induced shifts in 2D materials

Gap plasmons in metallic nanogaps confine electromagnetic fields to sub-wavelength volumes, offering a significant advantage for surface-enhanced Raman scattering (SERS). The performance of a nanogap SERS substrate relies heavily on its geometry, which can be customized using e-beam lithography with high fidelity and resolution. In this work, we fabricated nanogap grid arrays with various gap widths between 7 and 60 nm and periods of 150, 200, and 300 nm using e-beam lithography and explored their geometrical effects on SERS enhancement and strain-induced shifts in 2D materials. The measured transmission and reflection spectra show good agreement with simulations, implying electric field enhancement due to gap plasmon excitation. In SERS experiments on rhodamine 6G with varying gap widths, we demonstrated that the field enhancement and the resulting SERS signal increase with decreasing gap width. Meanwhile, a comparison of the SERS signals with different periods revealed a trade-off between proximity effects and nanogap density. Furthermore, we transferred a few layers of molybdenum disulfide (MoS₂) onto nanogap grid arrays, and then observed a redshift in the peaks of the SERS on these samples, revealing that larger gap widths induce greater stretching in the 2D material. Our findings provide insights into optimizing nanogap SERS substrates and leveraging grid structures to induce strain effects in 2D materials, highlighting potential applications in biomolecular sensing, chemical detection, and optoelectronics.