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.
The use of localized surface plasmons (LSPs) for highly sensitive biosensors has already been investigated, and they are currently being applied for the optical manipulation of small nanoparticles. The objective of this work was the optical trapping of λ-DNA on a metallic nanostructure with femtosecond-pulsed (fs) laser irradiation. Continuous-wave laser irradiation, which is generally used for plasmon excitation, not only increased the electromagnetic field intensity but also generated heat around the nanostructure, causing the DNA to become permanently fixed on the plasmonic substrate. Using fs laser irradiation, on the other hand, the reversible trapping and release of the DNA was achieved by switching the fs laser irradiation on and off. This trap-and-release behavior was clearly observed using a fluorescence microscope. This technique can also be used to manipulate other biomolecules such as nucleic acids, proteins, and polysaccharides and will prove to be a useful tool in the fabrication of biosensors.
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 of nanospheres on a substrate. Our method has advantages over the conventional optical tweezers technique because of its weak light intensity, and could be a promising method of creating and manipulating a 2D colloidal crystal on a plasmonic substrate. ■ INTRODUCTIONLocalized surface plasmons (LSP) have been investigated because they demonstrate highly sensitive spectroscopies 1−5 and the enhancement of photochemical reactions. 6−10 These applications are enabled by the enhancement effect of an incident resonant electromagnetic field (EMF) at the surfaces of noble metallic nanostructures. 11−13 In particular, the application of LSPs at the nanogaps between adjacent noble metallic nanostructures (gap-mode LSP) has recently attracted much attention for achieving the effective optical trapping (OT) of nanoparticles at the nanogaps; this is known as LSPbased OT (LSP-OT). Since Grigorenko et al. first experimentally demonstrated the LSP-OT of polystyrene microspheres in 2008, 14 various researchers have been exploring the phenomenon to reveal the features and mechanisms of the LSP-OT of polymer beads, metallic nanoparticles, and bacteria. 15−24 We also demonstrated the LSP-OT of semiconductor nanocrystals (quantum dots) and polystyrene nanospheres and by means of confocal fluorescence microspectroscopy. 22−24 Such LSP-OT has a great advantage with respect to incident light intensity: the laser intensity can be much decreased (to the order of kW/cm 2 ) and still achieve stable trapping, as compared to conventional optical tweezers (∼MW/cm 2 ). 25−29 Thus, LSP-OT could enable a new technique for manipulating not only nanoparticles but also smaller molecules such as polymer chains, 24 proteins, and DNA.According to recent research, however, the process of LSP-OT should not be so simple. Simultaneously with LSP excitation, other favorable or unfavorable physical processes take place competing with the enhanced radiation force (RF), which is the driving force for OT. We currently consider that photothermal effects (local t...
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.
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.
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