Highly Efficient Surface Enhanced Raman Scattering (SERS) Nanowire/Ag Composites

Introduction: Optically based sensing provides advantages over electronic sensing because optical spectra can uniquely fingerprint a chemical compound, significantly reducing false alarms and simplifying the detection process. In addition, light can easily be directed over long distances, enabling remote sensing. One of the most promising optical sensing techniques is surface enhanced Raman scattering (SERS). In Raman scattering (RS) of light from a chemical of interest, the vibrational modes in the chemical redshift the frequency of the scattered light, producing a spectrum characteristic of that molecule. Ordinary Raman scattering cross-sections are very small, resulting in low sensitivity (1E-8 of the intensity of the exciting laser); this is not a problem for most solids and liquids, because of the large numbers of molecules or atoms exposed to the laser light, but in the case of trace amounts of molecules in gases or liquids, detection through ordinary Raman scattering is virtually impossible. However, SERS enhances the Raman signal by many orders of magnitude by the use of a substrate of metal nanoparticles.1 The SERS enhancement of molecules adsorbed on the roughened metal surface is caused by local electromagnetic fields that are created by the laser excitation of surface plasmons at the metal surface. Even larger SERS effects can be produced by local “hot spots” in the electric fields, produced by interactions of localized plasmons on adjacent or neighboring nanoparticles.2 Although the SERS effect has been recognized for a long time, a full understanding of the phenomenon has not yet been achieved. This lack of understanding limits its application potential, as it is difficult to produce highly sensitive, inexpensive, and repeatable SERS substrates. To address these issues, we have developed a new SERS substrate material consisting of dielectric/Ag metal shell nanowires that exhibit high SERS sensitivity due to their plasmonic coupling. These nanowires are sensitive at low concentrations, quite repeatable, and inexpensive to produce.