Geometry-Dependent Field Localization in Plasmonic Nanoporous Silicon for Raman Enhancement

Raman spectroscopy is a powerful technique for molecular identification and quantitative analysis, but inherently weak scattering limits its use at low analyte concentrations. Here, we developed a lithography-free route to fabricate three-dimensional metal-decorated silicon nanopores for Raman enhancement. Nanopores were first produced by metal-assisted chemical etching and then modified by physical vapor deposition. This deposition introduced metal nanoparticles onto the pore sidewalls. Owing to geometric shadowing, the sidewall nanoparticles exhibited a depth-dependent size distribution. Meanwhile, residual catalyst nanoparticles remained near the pore bottoms. This configuration provides a structural basis for vertically distributed plasmonic coupling within the nanopores. By adjusting catalyst size and oxidant concentration, the pore width, pore depth, and nanoparticle spacing were regulated. Representative electromagnetic analysis further indicates that appropriate pore geometry promotes field localization through coupling between sidewall and bottom metallic nanoparticles. Under 532 nm excitation, the optimized substrates enabled experimental detection of melamine and methylene blue down to 10 -7 M and 10 -10 M, respectively. The corresponding enhancement factors, calculated at these measured concentrations, reached 4.56 × 10 6 and 7.32 × 10 8 . These results show the potential of the proposed metal decorated nanopores for Raman based detection of chemically distinct analytes.