Understanding the Role of Nanoscale Metallic Patterns in Surface Enhanced Raman Scattering (SERS)
Mentor:Hyuck Choo, Assistant Professor of Electrical Engineering, California Institute of Technology
SERS, which occurs when analytes are adsorbed onto roughened metal substrates in the nanoscale, is a highly useful technique that increases the weak Raman effects by several orders of magnitude. It has been reported that this method allows the detection of the presence of single molecules. This study focused on identifying nanoscale structures on SERS substrates that are responsible for the surface-originated Raman enhancement, which would enable us to engineer substrates with far superior, more spatially uniform Raman enhancement. Our SERS substrates were prepared by roughening the surfaces of silicon chips, followed by deposition of 150-nm gold layer and incubation in a BPE solution for a BPE-monolayer formation. We performed a Raman scan to identify the locations of SERS hotspots based on the Raman intensity at 1200 cm-1, and then imaged each identified hotspot using scanning electron microscopy (SEM) and atomic force microscopy (AFM). By superimposing the AFM and optical Raman measurements at the hotspots, we observed the shapes and organizations of the nanostructures that occurred most frequently in hotspots: more frequently seen traits are more likely to be responsible for the Raman enhancement. The superimposed AFM images revealed that these hotspots were all located on large nanoscale flat regions and surrounded by smaller nanocone-shaped structures. This indicates that the organization of a large structure surrounded by a group of smaller structures may serve as an optical antenna or focusing point for the SERS effect. Energy in forms of surface-plasmon-polariton may be focused by surrounding structures and directed towards the flat region. We also observed that the locations of the intense gold autofluorescence overlap with those of the hotspots from the Raman maps, which is consistent with SERS-electromagnetic-enhancement theory. Further work will include simulation and fabrication of the identified nanostructures and experimental verification of hotspot formation.