Drastic chemical interface plasmon damping is induced by the ultrathin (∼2 nm) titanium (Ti) adhesion layer; alternatively, molecular adhesion is implemented for lithographic fabrication of plasmonic nanostructures without significant distortion of the plasmonic characteristics. As determined from the homogeneous linewidth of the resonance scattering spectrum of individual gold nanorods, an ultrathin Ti layer reduces the plasmon dephasing time significantly, and it reduces the plasmon scattering amplitude drastically. The increased damping rate and decreased plasmon amplitude are due to the dissipative dielectric function of Ti and the chemical interface plasmon damping where the conduction electrons are transferred across the metal-metal interface. In addition, a pronounced red shift due to the Ti adhesion layer, more than predicted using electromagnetic simulation, suggests the prevalence of interfacial reactions. By extending the experiment to conductively coupled ring-rod nanostructures, it is shown that a sharp Fano-like resonance feature is smeared out due to the Ti layer. Alternatively, vapor deposition of (3-mercaptopropyl)trimethoxysilane on gently cleaned and activated lithographic patterns functionalizes the glass surface sufficiently to link the gold nanostructures to the surface by sulfur-gold chemical bonds without observable plasmon damping effects.

Using methylene blue (MB) as a model system, we demonstrate surface plasmon-enhanced resonant excitation that leads to N-demethylation reaction under visible light irradiation (λ = 633 nm) at low photon flux. The chemical changes are monitored by detecting the vibrational signatures of the reactant and product species in situ using surface-enhanced Raman scattering (SERS) spectroscopy. Drastic temporal evolution of SERS spectra has been observed upon continuous irradiation. While the SERS spectra acquired immediately after irradiation are the same as the Raman spectrum of MB solid powder, the spectra recorded a few seconds later are remarkably similar to that of thionine solid powder, indicating N-demethylation of MB. No demethylation reaction has been observed under resonant excitation (λ = 633 nm) of MB adsorbed on nonplasmonic surfaces. Similarly, excitations of plasmon resonances at 532 and 808 nm wavelengths that do not overlap with the MB electronic transition do not lead to transformation of MB to thionine. The reaction mechanism is discussed in terms of resonant excitation of MB and hot electron transfer to adsorbed species. Considering that both MB and thionine have large SERS signal due to the combination of resonance Raman and electromagnetic enhancement effects that provide high detection sensitivity, we suggest that this demethylation reaction serves as a convenient model system for future mechanistic studies.

Quadrupole plasmon and (octupolar) Fano resonances are induced in lithographically fabricated theta-shaped ring-rod gold nanostructures. The optical response is characterized by measuring the light scattered by individual nanostructures. When the nanorod is brought within 3 nm of the ring wall, a weak quadrupolar resonance is observed due to capacitive coupling, and when a necklike conductive bridge links the nanorod to the nanoring the optical response changes dramatically bringing the quadrupolar resonance into prominence and creating an octupolar Fano resonance. The Fano resonance is observed due to the destructive interference of the octupolar resonance with the overlapping and broadened dipolar resonance. The quadrupolar and Fano resonances are further enhanced by capacitive coupling (near-field interaction) that is favored by the theta-shaped arrangement. The interpretation of the data is supported by FDTD simulation.