Sculptured silicon nanopillars bridging face to face nanogaps with metal-insulator-metal coating for surface enhanced Raman spectroscopy

A multilayer structure of metal-insulator-metal (MIM) thin film on a sculptured silicon nanopillar (Si NP) is designed and optimized for surface-enhanced Raman spectroscopy (SERS) applications. A facile fabrication method of large-area periodic nanopillars was developed by combining reactive ion etching and monolayer polystyrene (PS) beads on a Si wafer to fabricate a regular array of Si NPs. The MIM-coated Si NP showed better Raman signal enhancement than a single metal layer-coated Si NP due to multidimensional coupled plasmonic enhancement at metal-dielectric interface. Further, the MIM-coated Si NPs showed better SERS activity than the MIM-coated unprocessed PS beads due to increased surface area for hotspot generation and coupling of plasmonic modes induced by the formation of closely located branches in sculpture pillar morphology. The shape- and size-dependent SERS activity of fabricated NPs and the dielectric gap layer variation for tuning plasmonic properties were investigated using rhodamine 6G as a probe molecule. The investigation of plasmonic enhancement using finite difference time domain (FDTD) simulation provided insight into the distribution of hotspots and the amount of local electric field enhancement. Furthermore, the developed pillar exhibited distinct and enhanced peak of R6G up to 10−15 M concentrations.