Improving analytical utility of Surface Enhanced Raman Spectroscopy through unique lithographic substrate development

Surface enhanced Raman spectroscopy (SERS) has the potential to be a useful analytical technique due to large signal enhancements. Unfortunately, SERS has several drawbacks, including a lack of reproducibility, which inhibits it from being a practical option. These large signals often arise from “hot spots” of extremely high enhancement on nanofeatured metallic substrates, the most common being comprised of aggregated silver colloid. It is difficult to reproducibly create these hot spots due to the randomness of the colloid substrates. However, through controlled substrate fabrication, many problems associated with SERS analysis can be overcome. Electron beam lithography (EBL) combined with reactive-ion etching (RIE) was used to fabricate a wide variety of aggregate-like structures that allow for methodically surveying the system to determine if areas of high enhancement are present. Any well performing areas were then recreated consistently to produce areas of similar enhancement.

While the aforementioned “combinatorial” approach has its advantages, simple structures are often easier to fabricate and theoretically model. As such, a single structure consisting of a metal disc on a silicon pillar was created. A variety of tests were performed on these structures to determine the overall utility of the simple pillar system. The system was found to possess extremely high enhancement, making it an ideal system to both theoretically model and test experimentally. The system also has strong enough overall signal to allow for potential analytical implications.

Studies were also conducted to determine the feasibility of using a strong enhancing silicon nanopillar system to make analytical measurements without a metal surface present. A special fabrication process using EBL and RIE was used to created tall, high aspect ratio pillars of known diameters. These nanopillars were then observed to exhibit special optical properties not seen in bulk silicon. Aside from modest Raman enhancement, these structures also demonstrated the ability to enhance the signal of specific analytes similar to SERS. Surface enhanced fluorescence (SEF) was also observed for different analytes, allowing for a variety of potential analytical areas.