Study of the C 60 /Ag Interface of a Large Area Nanoarchitectured Ag Substrate Using Surface-Enhanced Raman Scattering Akram A. Khosroabadi, †,§ Dallas L. Matz, ‡,§ Palash Gangopadhyay,* ,† Jeanne E. Pemberton, ‡ and Robert A. Norwood* ,† † College of Optical Sciences, The University of Arizona, Tucson, Arizona 85721, United States ‡ Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States ABSTRACT: Plasmonic Ag nanopillars have been fabricated and used as a surface- enhanced Raman scattering (SERS) substrate. The effective surface area of the sample is determined using underpotential deposition (UPD) of thallium and agrees well with a geometrical calculation using ImageJ analysis of SEM images. In order to find the SERS enhancement factor (SEF), a similar sample is coated with Pt, which shows no plasmon response at the excitation wavelength of 532 nm. SEF values on the order of 10 5 are obtained for Ag nanopillar substrates. Several monolayers of C 60 were deposited on these Ag nanopillars, and the Raman spectral results indicate charge delocalization at the interface between C 60 and Ag. FDTD simulation of the electric field confirms the experimental results; on the basis of these simulations, the electric field modulates with increasing diameter of the pillars, while the pitch (center-to-center distance) is fixed at 200 nm. ■ INTRODUCTION Since its discovery, SERS has drawn substantial attention due to its potential to overcome the low sensitivity that plagues traditional Raman spectroscopy. 1 SERS not only improves the surface sensitivity but also facilitates the study of various interfacial processes by enhancing the Raman scattering from analytes on metal/semiconductor surfaces. 2,3 With potential applications in fields ranging from plasmonics to diagnostics, 4 the SERS effect is predominantly an electromagnetic effect arising from an increase in the local optical field due to multiplicative amplification of the excitation laser and the reradiated Raman scattered light. 5 This optical enhancement is commonly associated with the excitation of surface plasmon oscillations in most SERS systems. 5 Nanostructured metal/ metal oxide surfaces often lead to surface plasmon resonance formation and a coupling between the localized surface plasmon polaritons (SPP) and electromagnetic radiation incident on the substrate surface resulting in intense absorption in the near-IR and visible-near UV region and enhancing the Raman scattering signal intensity by many orders of magnitude. 6-8 For surface nanofeatures smaller than the incident optical wavelength, the surface plasmon resonance normal modes of oscillation are resonant with both the excitation and scattered photons. 9 The frequency of the surface plasmon resonance depends on the dielectric constant of the metal/metal oxide and the dimensions of the nanofeatures which are responsible for the SERS effect. The SERS intensity decreases significantly with nanostructures that are either significantly larger than ∼100 nm or smaller than ∼10 nm. 10 However, SERS enhancement depends greatly on the geo- metrical configuration of the nanostructures and their interstructure interactions. The fact that the nanostructure plasmon resonance allows direct coupling of light to the resonant electron plasmon oscillation has spurred tremendous efforts in the design and fabrication of highly enhancing substrates based on nanostructured films and metallic nano- particles in both engineered and random arrays. 11 The most established substrates are those that are sprayed with Ag or Au colloids, resulting in intense SERS signals at the narrow junctions between the particles. Junctions between aggregated nanoparticles are believed to be SERS “hot spots” where large field enhancements allow for single molecule detection in some cases. Although spraying Au or Ag colloids on a substrate provides extremely high enhancement factors at local hot spots, it has thus far been difficult to achieve reliable, stable, and uniform SERS signals spanning a wide dynamic range on large area substrates using this method. 12,13 Furthermore, such substrates suffer from limited stability and reproducibility and, in general, are not amenable to large-scale production of SERS- based sensors. 14 More reliable and uniform surface enhance- ments are expected from substrates containing anisotropic nanostructured plasmonic materials. Anisotropic metallic nanostructures allow: (1) tunable plasmon absorption bands that can be achieved by adjusting the nanostructure aspect ratio and separation to be in resonance with common laser radiation sources used for Raman excitation in order to optimize the electromagnetic enhancement mechanism; 15 (2) symmetry breaking leading to more complex plasmon propagation, potentially giving more intense electromagnetic field generation from the structure and in gaps formed between these Received: May 30, 2014 Revised: July 3, 2014 Article pubs.acs.org/JPCC © XXXX American Chemical Society A dx.doi.org/10.1021/jp505364d | J. Phys. Chem. C XXXX, XXX, XXX-XXX