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.

Quantum effects and nonlocality in strongly coupled plasmonic nanowire dimers

“…The nanoparticles thus appear conductively connected prior to direct contact, and the transition between the non-touching and conductive contact regimes is continuous. In particular, the charge transfer plasmon associated with interparticle charge transfer [51][52][53][54][55][56][57] progressively emerges in the optical response of the system, as has been fully confirmed in recent experiments [28,29]. These quantum effects can be reproduced with the Quantum Corrected Model (QCM) [33] that treats the junction between the nanoparticles as an effective medium mimicking quantum effects within the classical local Maxwell theory [28,29].…”

Detection of electron tunneling across plasmonic nanoparticle–film junctions using nitrile vibrations

“…Acting as a molecular voltmeter 30-32 , the vibrational frequency shifts can be utilized as a direct indicator of the local electric fields. The VSE occurs when the local electric field perturbs the electronic environment of a chemical bond and results in a change in its vibrational energy, which can be directly measured with infrared or Raman spectroscopy.…”

Substrate-mediated charge transfer plasmons in simple and complex nanoparticle clusters

“…For example, the higher order plasmon modes have been identied, and the enhanced Raman spectroscopic signature of molecules in the interparticle junction as two adjacent nanoparticles approach each other to the point of fusion has been investigated. [29][30][31] Far fewer studies have addressed the effects of charge transfer on more complex plasmonic clusters that support Fano resonances. 31,32 Here we report an experimental study of the onset of charge transfer plasmons in metallic nanoparticle clusters of simple (dimer) and more complex (nonamer) geometries, mediated by the introduction of a conductive, metallic substrate.…”

“…[29][30][31] Far fewer studies have addressed the effects of charge transfer on more complex plasmonic clusters that support Fano resonances. 31,32 Here we report an experimental study of the onset of charge transfer plasmons in metallic nanoparticle clusters of simple (dimer) and more complex (nonamer) geometries, mediated by the introduction of a conductive, metallic substrate. With this approach, the geometry of the cluster can be held constant, while charge transfer is induced by the optical frequency conductivity of the substrate.…”