title: Surface-enhanced Raman trajectories on a nano-dumbbell: transition from field to charge transfer plasmons as the spheres fuse.
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 nanospheres. 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.
authors.affiliation: Department of Chemistry, University of California, Irvine, California 92697, United States.
authors.forename: Mayukh
authors.initials: M
authors.lastname: Banik
authors.forename: Patrick Z
authors.initials: PZ
authors.lastname: El-Khoury
authors.forename: Amit
authors.initials: A
authors.lastname: Nag
authors.forename: Alejandro
authors.initials: A
authors.lastname: Rodriguez-Perez
authors.forename: Nekane
authors.initials: N
authors.lastname: Guarrottxena
authors.forename: Guillermo C
authors.initials: GC
authors.lastname: Bazan
authors.forename: Vartkess A
authors.initials: VA
authors.lastname: Apkarian
mesh_terms: D003198:Computer Simulation; D004579:Electron Transport; D008027:Light; D008422:Materials Testing; D008956:Models, Chemical; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D010316:Particle Size; D012542:Scattering, Radiation; D055672:Static Electricity; D020349:Surface Plasmon Resonance / Q000379:methods*