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Reversible tuning of localized surface plasmon resonances is achieved with strong resonance intensity in the near-IR region using hydrogen uptake into dimeric nanostructures of Pd–Ag layered nanodots. The resonance feature in the present dimer is characterized by hybridized plasmon modes between two adjacent Pd–Ag layered nanodots with similar plasmonic features. In contrast, the resonance of a conventional Pd–Ag heterodimer is considered to be a perturbed plasmon mode between Pd and Ag nanodots with different plasmonic features. The strong symmetric interactions in the layered dimers lead to a large peak shift with hydrogen uptake into the Pd layers, while the weak asymmetric interactions in the heterodimers result in a decrease in the band intensity. The flexible tailoring of the resonance intensity, wavelength, and tunability of the layered nanostructures provides the possibility for novel functionalized plasmonic materials or devices to be realized.

Polarized Raman scattering measurement was carried out using a hybridized system of Ag nanodimer structures and organic dye molecules. Tuning of the localized surface plasmon resonance energy leads to modulation of the hybridized polariton energy. The anticrossing behavior of the polariton energy implies a strong coupling regime with maximum Rabi splitting energy of 0.39 eV. The observation proves the effective Raman enhancement via the excitation of the upper and the lower branches of the hybridized states at the gap of the metal dimer. Maximum Raman enhancement was obtained at an optimized resonant energy between the hybrid states and Raman excitation.

The in situ observation of geometrical and electronic structural dynamics of a single molecule junction is critically important in order to further progress in molecular electronics. Observations of single molecular junctions are difficult, however, because of sensitivity limits. Here, we report surface-enhanced Raman scattering (SERS) of a single 4,4'-bipyridine molecule under conditions of in situ current flow in a nanogap, by using nano-fabricated, mechanically controllable break junction (MCBJ) electrodes. When adsorbed at room temperature on metal nanoelectrodes in solution to form a single molecule junction, statistical analysis showed that nontotally symmetric b(1) and b(2) modes of 4,4'-bipyridine were strongly enhanced relative to observations of the same modes in solid or aqueous solutions. Significant changes in SERS intensity, energy (wavenumber), and selectivity of Raman vibrational bands that are coincident with current fluctuations provide information on distinct states of electronic and geometrical structure of the single molecule junction, even under large thermal fluctuations occurring at room temperature. We observed the dynamics of 4,4'-bipyridine motion between vertical and tilting configurations in the Au nanogap via b(1) and b(2) mode switching. A slight increase in the tilting angle of the molecule was also observed by noting the increase in the energies of Raman modes and the decrease in conductance of the molecular junction.

Nanostructure-enhanced optical trapping of polymer beads was investigated by means of fluorescence microspectroscopy. It was found that trapping behavior was quite sensitive to the particle size as well as excitation light intensity. We present a 2D closely packed assembly of polystyrene nanospheres on a gold nanostructure that is triggered by gap-mode localized surface plasmon (LSP) excitation. We discuss the trapping mechanism from the viewpoints of not only the radiation force but also of the thermal force (thermophoresis and thermal convection) induced by near-infrared laser irradiation. Thermophoresis worked as a repulsive force whose direction was opposed to that of the radiation force. On the other hand, thermal convection acted in favor of trapping: It supplied nanospheres toward the LSP excitation area. By suppressing the repulsive force, the assembled trapped nanospheres took the form of hexagonal shapes on a gold nanostructure. By optimizing irradiation parameters, we achieved 2D manipulation o...

This Perspective describes studies aimed at effective excitation of molecules by localized surface plasmon polaritons. Recently developed bottom-up and top-down techniques allow the controlled fabrication of well-defined metal structures exhibiting desirable localization of plasmon energy. Under certain conditions, molecules display unique florescence and Raman scattering behavior in such localized fields, suggesting selective resonant excitation of specific electronic/vibrational modes. Finally, several examples of improvements in the efficiencies of photochemical and photoelectrochemcal systems are briefly discussed to find a way to overcome challenges for enhancement of photoenergy conversion in future.

One of the recent hot topics in the fields of plasmonics and related nanophotonics is optical trapping of nano/microparticles based on surface plasmon. Experimental demonstration of such trapping by gap-mode plasmon has hitherto been limited so far to a few reports in which submicrometer polymer beads were trapped with intense irradiation at MW/cm2, satisfying an energetic condition of U > kT. (U is the potential energy of the trap and kT is an averaged thermal background energy.) We demonstrate plasmon-based optical trapping of a luminescent quantum dot (Q dot, diameter ≥10 nm) with a very weak irradiation (0.5−10 kW/cm2). The most important discovery is that the Q dot trapping was clearly observed through luminescence detection even under an energetic condition of U < kT, on the basis of which we propose a novel concept that is peculiar to plasmon-based trapping at a nanogap.