BANIK ET AL. VOL. 6 ’ NO. 11 ’ 10343–10354 ’ 2012 www.acsnano.org 10343 October 23, 2012 C 2012 American Chemical Society Surface-Enhanced Raman Trajectories on a Nano-Dumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse Mayukh Banik, † Patrick Z. El-Khoury, †,§ Amit Nag, † Alejandro Rodriguez-Perez, † Nekane Guarrottxena, ‡ Guillermo C. Bazan, ‡ and Vartkess A. Apkarian †, * † Department of Chemistry, University of California, Irvine, California 92697, United States and ‡ Department of Chemistry, University of California, Santa Barbara, California 93106, United States. § Present address: Pacific Northwest National Laboratory. R aman spectroscopy of single molecules is accessible at junctions of metallic nanostructures, where local fields are dramatically enhanced by the plasmonic response. 1 Since Raman scattering is a tensor quantity and local fields of nanojunctions are inhomogeneous on molecular length scales, given knowledge of the local field, spectra should be sufficient to track the location and orientation of molecules in 3D space. Alter- natively, given knowledge of the molecular polarizability tensor, spectra should be sufficient to determine the local vector field. We implement this proposition through mea- surements on the prototypical junction for- med between two metallic nanospheres. 25 A chemically engineered nano-dumbbell consisting of dibenzyldithio-linked silver spheres (∼30 nm diameter) serves as our experimental platform, with all evidence indicating that we are tracking single molecules. 6,7 Two illustrative spectral se- quences will be presented: An uneventful sequence, which highlights optical activity of the junction and the spectral sensitivity to orientation that can be attained. A more eventful trajectory is recorded during the fusion of the nanospheres. The sequence identifies line spectra due to surface- enhanced dipolequadrupole Raman (SEQRS), surface-enhanced dipolemagnetic dipole Raman (SEMRS), 8 and band spectra when the junction gap reaches the conductiv- ity limit of plasmons. 913 In good agreement * Address correspondence to aapkaria@uci.edu. Received for review September 16, 2012 and accepted October 23, 2012. Published online 10.1021/nn304277n ABSTRACT By taking advantage of the tensor nature of surface- enhanced Raman scattering (SERS), we track trajectories of the linker molecule and a CO molecule chemisorbed at the hot spot of a nano-dumbbell consisting of dibenzyldithio-linked silver nano- spheres. The linear Stark shift of CO serves as an absolute gauge of the local field, while the polyatomic spectra characterize the vector components of the local field. We identify surface-enhanced Raman optical activity due to a transient asperity in the nanojunction in an otherwise uneventful SERS trajectory. During fusion of the spheres, we observe sequential evolution of the enhanced spectra from dipole-coupled Raman to quadrupole- and magnetic dipole-coupled Raman, followed by a transition from line spectra to band spectra, and the full reversal of the sequence. From the spectrum of CO, the sequence can be understood to track the evolution of the junction plasmon resonance from dipolar to quadrupolar to charge transfer as a function of intersphere separation, which evolves at a speed of ∼1 Å/min. The crossover to the conduction limit is marked by the transition of line spectra to Stark-broadened and shifted band spectra. As the junction closes on CO, the local field reaches 1 V/Å, limited to a current of 1 electron per vibrational cycle passing through the molecule, with associated Raman enhancement factor via the charge transfer plasmon resonance of 10 12 . The local field identifies that a sharp protrusion is responsible for room- temperature chemisorption of CO on silver. The asymmetric phototunneling junction, AgCOAg, driven by the frequency-tunable charge transfer plasmon of the dumbbell antenna, combines the design elements of an ideal rectifying photocollector. KEYWORDS: SERS . SEROA . plasmon . charge transfer plasmon . break junction . dumbbell . nanosphere . dibenzyldithiol . biphenyl . carbon monoxide . rectenna . antenna ARTICLE