Large area vertical Ag columnar thin films as highly sensitive SERS substrates

Abstract

Noble metal nanostructured thin films are of great interest as competitive surface enhanced Raman scattering (SERS) substrates due to their remarkable plasmonic properties in the visible wavelength. In this work, large-area vertical Ag columnar thin films of different thickness have been prepared on the glass substrates by the simple, cost-effective glancing angle deposition technique. The Raman spectra of R6G from these substrates are measured and the experimental SERS enhancement factor is found to have the maximal value of 2.5E+8 at the optimized thickness of 680 nm. Finite difference time domain simulations have been utilized to study the near-field plasmonic properties upon these films and the simulated structure is a geometric copy obtained from the SEM image topology rather than simplified regular nanostructures. The areal electric field enhancement is sensitive to the gap size and areal column density. The wavelength and polarization dependence of localized electric enhancement in subwavelength gaps, "hot spots", are studied and the electric enhancement at different film depths is also analyzed. The simulation SERS enhancement factors are calculated and show good agreement with the experimental ones. When the thickness increases, the areal electric field enhancement decreases while the number of adsorded molecules increases, so there exists an optimized thickness to maximize the SERS performance. These results help to further our understanding of the plasmonic properties of noble metal nanostructured thin films.