Nanofabrication of densely packed metal-polymer arrays for surface-enhanced Raman spectrometry.

A key element to improve the analytical capabilities of surface-enhanced Raman spectroscopy (SERS) resides in the performance characteristics of the SERS-active substrate. Variables such as shape, size, and homogeneous distribution of the metal nanoparticles throughout the substrate surface are important in the design of more analytically sensitive and reliable substrates. Electron-beam lithography (EBL) has emerged as a powerful tool for the systematic fabrication of substrates with periodic nanoscale features. EBL also allows the rational design of nanoscale features that are optimized to the frequency of the Raman laser source. In this work, the efficiency of EBL fabricated substrates are studied by measuring the relative SERS signals of Rhodamine 6G and 1,10-phenanthro-line adsorbed on a series of cubic, elliptical, and hexagonal nanopatterned pillars of ma-N 2403 directly coated by physical vapor deposition with 25 nm films of Ag or Au. The raw analyte SERS signals, and signals normalized to metal nanoparticle surface area or numbers of loci, are used to study the effects of nanoparticle morphology on the performance of a rapidly created, diverse collection of substrates. For the excitation wavelength used, the nanoparticle size, geometry, and orientation of the particle primary axis relative to the excitation polarization vector, and particularly the density of nanoparticles, are shown to strongly influence substrate performance. A correlation between the inverse of the magnitude of the laser backscatter passed by the spectrometer and SERS activities of the various substrate patterns is also noted and provides a simple means to evaluate possible efficient coupling of the excitation radiation to localized surface plasmons for Raman enhancement.