title: Gold nanoparticle dimer plasmonics: finite element method calculations of the electromagnetic enhancement to surface-enhanced Raman spectroscopy.
abstract: Finite element method calculations were carried out to determine extinction spectra and the electromagnetic (EM) contributions to surface-enhanced Raman spectroscopy (SERS) for 90-nm Au nanoparticle dimers modeled after experimental nanotags. The calculations revealed that the EM properties depend significantly on the junction region, specifically the distance between the nanoparticles for spacings of less than 1 nm. For extinction spectra, spacings below 1 nm lead to maxima that are strongly red-shifted from the 600-nm plasmon maximum associated with an isolated nanoparticle. This result agrees qualitatively well with experimental transmission electron microscopy images and localized surface plasmon resonance spectra that are also presented. The calculations further revealed that spacings below 0.5 nm, and especially a slight fusing of the nanoparticles to give tiny crevices, leads to EM enhancements of 10(10) or greater. Assuming a uniform coating of SERS molecules around both nanoparticles, we determined that regardless of the separation, the highest EM fields always dominate the SERS signal. In addition, we determined that for small separations less than 3% of the molecules always contribute to greater than 90% of the signal.
authors.affiliation: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA.
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: McMahon
authors.forename: Anne-Isabelle
authors.initials: AI
authors.lastname: Henry
authors.forename: Kristin L
authors.initials: KL
authors.lastname: Wustholz
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Natan
authors.forename: R Griffith
authors.initials: RG
authors.lastname: Freeman
authors.forename: Richard P
authors.initials: RP
authors.lastname: Van Duyne
authors.forename: George C
authors.initials: GC
authors.lastname: Schatz
mesh_terms: D019281:Dimerization; D055590:Electromagnetic Phenomena; D020342:Finite Element Analysis; D006046:Gold / Q000737:chemistry*; D008422:Materials Testing; D053768:Metal Nanoparticles / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D036103:Nanotechnology; D010316:Particle Size; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013059:Spectrum Analysis, Raman / Q000379:methods*; D020349:Surface Plasmon Resonance; D013499:Surface Properties
chemical_list: D006046:Gold