Toward Plasmon-Induced Photoexcitation of Molecules - The Journal

PERSPECTIVE pubs.acs.org/JPCL

Toward Plasmon-Induced Photoexcitation of Molecules Hideki Nabika, Mai Takase, Fumika Nagasawa, and Kei Murakoshi* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan

ABSTRACT 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.

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A pioneering proposal to adopt a localized plasmon for the enhancement of photochemical reactions was first made by Nitzan and Brus in 19812,3 based on their theoretical calculations of highly localized electromagnetic fields on rough metal surfaces. They were inspired by early studies on the phenomenon of surface-enhanced Raman scattering (SERS), in which localized electromagnetic fields interact with molecules adsorbed on the metal surface and give rise to very intense Raman scattering. Soon thereafter, the first experimental evidence confirming the enhancement of a photochemical reaction by surface plasmons was reported by Chen and Osgood.4 They observed anisotropic Cd metal particle growth via photodissociation of dimethyl cadmium under polarized light illumination with an energy corresponding to the plasmon resonance of Cd (Figure 1). Randomly distributed Cd particles showed elliptical growth patterns that were well aligned with the polarization direction of the light. Such an effect was not observed with other metals such as In, Al, and Ag, whose resonance energies were different from that of the illuminating light, indicating that the photochemical reaction was accelerated by a localized field above the nanoparticle. Since then, numerous studies have been carried out on the control of photochemical reactions by localized plasmons, mainly in systems comprising chemically synthesized metal/semiconductor nanoparticles, as described in review articles.5-8 One interesting example was reported by Augustynski et al,9 who found that increments in the photocurrent caused a reduction of carbon dioxide in solution at a roughened Ag electrode. The quantum yield of the photocurrent exhibited a sharp maximum at the peak wavelength of Ag surface plasmons. These results suggest that plasmon-mediated photoinduced electron transfer at a metal electrode may be possible, if an appropriate electron acceptor/donor exists at the interface. Metal deposition and dissolution was also involved in the

ecent attempts to improve the efficiency of photochemical and photoelectrochemical processes have focused on developing novel methods of drastically changing the interaction between photons and molecules. Optimized perturbation of photons to excite molecules can be achieved when the polarization direction of the electromagnetic field is tuned to match that of the electronic states of molecular orbitals.1 However, the intrinsic cross section of molecules for interaction with photons also limits the probability of the electronic excitation. Thus, both energy localization and precise control of the polarization directions of photons are necessary to excite target molecules effectively, although this is difficult to achieve, especially under ambient room-temperature conditions with relatively large thermal fluctuations. One possible means of solving this problem is to use localized surface plasmon polaritons, hereafter referred to simply as localized plasmons, in metal nanostructures. Collective excitation of free electrons in metals under illumination produces highly localized electromagnetic fields on the metal surface. Localized plasmons allow the localization, energy, phase, and wave vector of the field to be controlled by changing the structure of the metal on a nanometer scale. It is expected that an electromagnetic field with well-controlled anisotropic characteristics will be effective in exciting target molecules.

The utilization of localized plasmons is a promising means of improving the efficiency of photochemical reactions, especially in the visible-near-infrared wavelength region.

r 2010 American Chemical Society

Received Date: July 5, 2010 Accepted Date: July 19, 2010 Published on Web Date: August 02, 2010

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DOI: 10.1021/jz100914r |J. Phys. Chem. Lett. 2010, 1, 2470–2487


Figure 1. TEM photographs of cadmium particles (a) before photochemical deposition, showing randomly distributed spheres, and (b-d) after photochemical deposition, showing elliptical similar growth to similar sizes, well aligned with the electric field of the light (Reprinted with permission from ref 4. Copyright 1983 by the American Physical Society.)

Figure 2. By extending the polyol reaction for a given time period, various polyhedral shapes capped with {100} and {111} faces can be obtained in high yield. SEM images of cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra, respectively (scale bar: 100 nm). (Reprinted with permission from ref 21. Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 2006.)


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Figure 3. (upper) SEM images of the obtained quadrangular pyramidal and 3D hierarchical structures of Au. (left) Low-magnification image of the Au quadrangular pyramidal structure and (right) high-magnification image of a 3D hierarchical structure. (Reproduced with permission from ref 22. Copyright 2009 by APEX/JJAP.) (lower) SEM images of Au nanoframes obtained through chemical etching of Auframed OT-Ag nanocubes. A typical large area image (a) and images with high resolution of individual Au nanoframes (b-d). (Reference 23. Reproduced with permission of The Royal Society of Chemistry, http://dx.doi.org/10.1039/b901469a. Copyright 2009.)

reaction enhancement for structural control.9-11 These studies have shown that the utilization of localized plasmons is a promising means of improving the efficiency of photochemical reactions, especially in the visible-near-infrared wavelength region. On the other hand, it has not yet been clarified that the intrinsic probability of excitation of a molecule is improved by localized plasmon excitation. Although localization of the electromagnetic field may indeed increase the apparent reaction probability, whether the overall efficiency of a plasmon excitation reaction is improved in comparison with the space-averaged efficiency of a normal photoreaction using far-field photon irradiation is something that needs to be considered. Collateral effects associated with plasmon excitation, such as the generation of heat and mechanical oscilla-


tion, may hinder a clear understanding of the phenomena involved.5,6 Uncertainties in the key processes involved in the enhancement sometimes make it difficult to design optimized systems. The aim of this Perspective is to summarize the present status of localized plasmon mode control and its application to the excitation of molecules and materials. The interaction between a molecule and localized plasmon is discussed in terms of various plasmon-enhanced phenomena, such as fluorescence enhancement and SERS. Finally, we will focus on the possibilities of using effective excitation of molecules to promote photoenergy conversion processes via excitations by localized plasmon. Structural control of plasmonic nanomaterials is an important issue to construct the systems for plasmon-enhanced

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Figure 4. (left) Scanning electron micrographs of representative nanoparticle arrays (all 35 nm in height); (A) cylindrical nanoparticles with a diameter of 200 nm in the hexagonal arrangement, period 260 nm; (B) cylindrical nanoparticles with a diameter of 200 nm in the square arrangement, period 250 nm; (C) trigonal prism nanoparticles with a perpendicular bisector of 170 nm in the hexagonal arrangement, period 230 nm; (D) cylindrical nanoparticles with a diameter of 200 nm in square arrangement, period 350 nm (from ref 24). (right) SEM images of representative bowtie antennas. Each bowtie is composed of two opposing, 75 nm long triangles, with gaps of 20 (A) and 285 nm (B). The Au films are 20 nm thick with a 5 nm Cr adhesion layer and are deposited onto an ITO-coated fused silica coverslip. The “halo” of lighter contrast around the particles is likely due to charging induced during SEM imaging and is ignored when determining dimensions (from ref 25).

Figure 5. (left) Surface charge density of interacting cylindrical, spherically capped gold nanorods placed 1.5 nm apart and with dimensions of 78 nm 24 nm, calculated using the electrostatic approximation, where blue represents one charge (for example, -ve) and the red the opposite charge (þve) of the dipole. The plasmon resonance for a single rod of these dimensions calculated using this method is 707 nm. The wavelength given is the resonance wavelength for the coupled plasmon mode. The general trends in the resonance wavelengths are identical to those calculated using DDA (from ref 30). (right) (a) Schematic illustration of a sphere-plane system for gap-mode plasmon excitation and plasmon hybridization diagrams for a (b) Au sphere/Au plane system and a (c) Au sphere/Pt plane system (from ref 32).

metals to semiconductors19 and oxides.20 In recent studies on structural control, the emphasis has gradually shifted toward achieving much more precisely control not only of size and anisotropy but also crystallinity, shape, and uniformity. Nanostructures with sharp vertexes, such as triangular plates and polyhedra, show multipolar plasmon modes.21 These vertexes are predicted to produce much stronger localization of the plasmon field than is possible with a spherical shape (Figure 2). In addition

photochemical reactions. Following the development of a method to produce metal nanorods by a chemical synthesis technique using a “shape-inducing” reagent,12 researches in this area became focused on tuning the plasmon resonance energy by controlling the structural anisotropy.13 Methods for producing dot,14 triangular,15 cubic,16 wire,17 and rod18 shapes via chemical and/ or photochemical synthesis have also developed. The choice of materials for nanostructures has also been extended from


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Figure 6. (a) FE-SEM images of gold octahedron dimers with varying tip-to-tip separations (s, 25-125 nm). Individual dimers were prepared on ITO/ quartz substrates using a four-probe nanomanipulator installed in the FE-SEM chamber. (b, c) Microscattering spectra acquired with the scattering light polarization parallel to the incident light wave. In these spectra, the incident light wave vectors kin were chosen to be parallel (b) and perpendicular (c) to the dimer axis; (d) plasmon energies of gold octahedron dimers as a function of dimer tip-to-tip separation. The upper curve corresponds to the in-phase-coupled (antibonding, bright) plasmonic dimers, while the lower curve corresponds to the antiphase-coupled (bonding, dark) plasmonic dimers. The orientations of corresponding dipole moments (m) are normal to the air-substrate interface (modified from ref 33).

to the fine control of the unit structure of nanoparticles, a method for constructing three-dimensional structures has been also developed. Interesting examples for increasing the surface to volume ratio includes hierarchical structures of Au composed of micrometer-scale quadrangular prisms with a nanometer-scale fine pillar array22 and a jungle gym structure with a Au nanoframe less than 30 nm in width23 (Figure 3). These systems show unique optical properties leading to effective three-dimensional localization of the electromagnetic field In addition to bottom-up chemical techniques, top-down technique using electron beam lithography (EBL) are also valuable, especially for controlling the statistical uniformity of the shape and size of multiple nanostructures24-27 (Figure 4). Recent advances in precision control have led to Au nanostructures with sizes corresponding to about 4-12 layers of Au atoms and a separation between nanodot structures of less than single nm.27 The usefulness of EBL in fabricating such structures was confirmed by the observation of dipole coupling between EBL-prepared nanoparticles. In addition, bowtie structures with controlled distance


prepared by EBL also shows very effective resonance works as an optical antenna.25 Control of the plasmon coupling allows the construction of a localized field with a well-defined spatial distribution and energy. The coupling has been regarded as similar to the hybridization of the wave function of molecules with the creation of bonding and antibonding states.28,29 Differently coupled nanostructures offer a wide variety of plasmon responses. For example, it was found that Au nanorod dimers arranged side-to-side, end-to-end, and at right angles in different orientations exhibited both red- and blue-shifted surface plasmon resonances, consistent with the plasmon hybridization model (Figure 5).30 This concept also led to the construction of a resonant field at the surface of non-plasmonresonant metals, such as Pt, by positioning a plasmon-active metal tip or dot at a controlled distance from the surface (Figure 5).31,32 Recent studies have also shown that for a faceto-face arrangement of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying separation (25-125 nm), carefully positioned using a

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Figure 7. (upper) Self-assembled nanoshell clusters as nanoplasmonic components. (A) Nanoshell clusters can be tailored to support tunable electric, magnetic, and Fano-like resonances. Electric dipole resonances generally exist for all plasmonic nanostructures and are shown here for individual nanoshells and coupled dimers. Packed trimer clusters exhibit magnetic dipole resonances and can be described as a closed loop of nanoinductors and nanocapacitors. Fano-like resonances are supported by heptamers. These simulations use nanoshells with [r1, r2] = [62.5, 85] nm (where r1 and r2 are the inner and outer radii of the gold shell, respectively), and the clusters have 2 nm wide gaps filled with a dielectric spacer with a dielectric constant of e = 2.5. E is the electric field. (lower) Experimental and theoretical s-polarized scattering spectra for the trimer with the cross polarizer. Both spectra now exhibit a clearly visible magnetic dipole peak, matching in peak position and line width. The inset shows the calculated magnitude of the trimer magnetic dipole moment in the z direction, confirming the nature of the spectral peak near 1400 nm. (Modified with permission from AAAS, ref 34. Copyright 2010.)

manipulator combined with a scanning electron microscope, antiphase plasmon coupling modes (“dark” plasmons) could be observed (Figure 6).33 In addition to the electric resonance, magnetic34 and Fano-like resonances35,36 are also expected to complete broad-ranging manipulation of the electromagnetic field at the nanoscale (Figure 7). Several core-shell and composite structures using different metals and oxides have been theoretically proposed to induce Fano-like resonance (Figure 8).36 In this size regime, chemical synthesis techniques are required, especially for the preparation of cores with thin shells composed of a different material.37 The combination of top-down and bottom-up techniques is likely to prove invaluable for the construction of nanostructures with complete control of the electric, magnetic, and Fanolike resonances.

The combination of top-down and bottom-up techniques is likely to prove invaluable for the construction of nanostructures with complete control of optical properties.


Figure 8. Fano resonance of an individual dolmen structure (d) (scale bar: 100 nm) with the following parameters: L1, 160 nm; w1, 110 nm; L2, 135 nm; w2, 100 nm; S, 55 nm; G, 20 nm; and height H, 60 nm. (a, b) Experimentally and numerically obtained extinction spectra, respectively, with polarization as defined by the arrows in panel d. (c) The evolution of the experimentally measured extinction as the polarization direction is changed in regular 10° steps. (e, f) Calculated surface charge distributions of the dipolar mode and the Fano extinction dip, respectively (after ref 36).

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Figure 9. (upper) (a) Sketch of the experimental arrangement. Inset: SEM image of a gold particle attached to the end of a pointed optical fiber. (b) Field distribution (factor of 2 between successive contour lines) of an emitting dipole (λ = 650 nm) located 2 nm underneath of the surface of a glass substrate and faced by a gold particle separated by a distance of z = 60 nm from the glass surface. (lower) (a) Fluorescence rate as a function of particle-surface distance z for a vertically oriented molecule (solid curve: theory; dots: experiment). The horizontal dashed line indicates the background level. (b) Fluorescence rate image of a single molecule acquired for z = 2 nm. The dip in the center indicates fluorescence quenching. (c) Corresponding theoretical image. (Reprinted with permission from ref 39. Copyright 2006 by the American Physical Society.)

The effects that electromagnetic fields have on molecules can be determined by measuring the scattered or emitted photons from these molecules. It is well-known that the structures described above enhance the fluorescence and Raman scattering from molecules located at points where the electromagnetic field is localized. If the mechanisms involved can be fully understood, these enhanced responses can be used as a measure of the level of optimization of photoreactions by localized plasmons. The fluorescence enhancement phenomenon is generally well-understood from both a theoretical and experimental viewpoint.38-45 It is recognized that a localized field affects


both the excitation and radiation processes, leading to an increase in the transition rate.38,39 The existence of metal nanostructures, however, leads to quenching via energy transfer from molecules to the metal. Thus, the fluorescence intensity largely depends on the separation between the molecules and the metal nanostructure. Experimental measurements of the fluorescence intensity from nile blue molecules as a function of distance from a single gold single particle 80 nm in diameter shows that the enhancement becomes apparent at distance of less than 20-30 nm and reaches a maximum at a distance of ∼5 nm (Figure 9).39 The fluorescence begins to be quenched at distances shorter than this. The distance

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Figure 10. (left) (a, b) AFM and (c, d) emission images taken for Ag nanodot arrays with different sizes. (right-upper) Emission spectra shown for (blue) smaller Ag dot arrays (shown in (a)) and (red) larger Ag dot arrays (shown in (b)). Metal-sizedependent emission was observed. Larger particles showed red-shifted emission compared with that from smaller size particles. (right-lower) Typical intensity changes of a single emissive site acquired on the successive TIRFM images. Time course of the emission shows on-off behavior, indicating that a single Ag cluster site on a Ag nanoparticle contributes to the emission (modified from ref 44).

dependence of the fluorescence enhancement is reproduced by theoretical calculations of the florescence rate based on two-level transitions in the local field. From previous reports, the optimal distance varies from ∼5 to 20 nm,39,43 indicating that it depends on the dielectric properties and microscopic structures of the system. Not only the fluorescence intensity but also the spectral shape was found to depend on the properties of localized plasmons.41,42,44 One example is the observation of selective emission coupled with localized plasmons from a photoactivated Ag cluster on preformed triangular Ag nanoparticles (Figure 10).44 Comparison of the extinction and emission spectra revealed that a strong interaction at the emission wavelength between the Ag clusters and the localized plasmons was responsible for the observed wavelength-selective enhancement. Experiments have shown that emission at a desired wavelength can be produced by tuning the localized plasmon energy of the Ag nanoparticles. Single-molecule fluorescence behavior suggests that a single emissive site whose energy of the emitting levels is matched to that of the plasmon can selectively resonate, even in the presence of many weakly or nonresonant clusters in close proximity. Spectral shaping by plasmon tuning was also observed for the emissive sites, such as organic dyes,39,41,42 metal complexes,40 and quantized semiconductor nanodots.45 These results imply that the excited states and lifetimes of molecules in localized plasmons can be tuned if the geometric arrangement of the molecule and metal nanostructures


can be controlled based on the energy of the target excited states and the localized plasmons. SERS is also a promising tool for detecting the interaction between molecules and localized plasmons. Over the past three decades, numerous studies on SERS have shown that it can be applied as a versatile probe to detect molecules with ultrahigh sensitivity.46,47 There is still, however, a major unresolved issue with SERS. Although the electromagnetic enhancement by localized plasmons in the SERS process has been quantitatively evaluated and shows good agreement with numerical results obtained by finite difference time domain calculations of the field,48,49 this explanation cannot account for the fact that there are SERSactive and SERS-inactive molecules. To explain the molecular specificity of the enhancement, the contribution of charge-transfer resonance involving electron transfer between the molecules and metal has been considered from the early stages of study on SERS.50 Due to uncertainties in the mechanism of the huge enhancement, there is still controversy concerning the concept of single-molecule SERS.51 Despite a very large number of studies on the subject, there has been no clear experimental verification of single-molecule SERS. There are several approaches that may be taken to solve this problem. In situ measurement of SERS using a system with a controlled surface coverage of bianalyte is one method to determine the number of molecules at SERS-active sites.52-55 In systems where the surface coverage of adsorbed molecules is less than one

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Figure 11. SERS spectra at the periodic Ag dimer array measured in the aqueous solution mixtures of 4,40 -bipyridine and 2,20 bipyridine in a 1 μM solution mixture. The exposure time is for 1 s at 30 μW excitation. Possible model of the SERS-active site was also shown (from ref 54).

monolayer, SERS spectra which can be assigned to just one of the two analytes are frequently observed. It should be the case that one of the analyte molecules exists at the most intense SERS-active sites. If it is confirmed that the SERS signal comes from a single active site, the observation of one of the two analytes can be attributed to a SERS signal from a single molecule at the SERS-active sites. The results for a system with a controlled surface coverage of bianalyte also suggest that the SERS-active sites are comparable in size to a single molecule, indicating the importance of the charge-transfer process bewteen the localized electronic states of a single molecule and those at the surface of the metal (Figure 11).53

The results for a system with a controlled surface coverage suggest that the SERS-active sites are comparable in size to a single molecule.

Figure 12. Simultaneously measured Stokes and anti-Stokes Raman spectra of solid 4,40 -bipyridine (a) and those observed at the periodic Ag dimer array in the 1 mM aqueous solution of 4,40 bipyridine at different sites (b, c, d). Excitation power: 200 μW; exposure time: (b, c) 2 s and (d) 100 s. The relative intensities of the bands were significantly dependent upon the observed site. Several vibrational modes showed a quite large relative intensity ratio to normal Raman up to ∼75. This extraordinary value cannot be explained by the effect of the vibrational pumping. (Reproduced with permission from ref 62. Copyright CJASS, 2007.)

Recent strategies for proving that single-molecule SERS is possible involve controlling the number of molecules at the SERS-active sites. Tip-enhanced Raman spectroscopy31 and the simultaneous measurement of single-molecule conductivity and Raman spectra in molecular junctions56 can be used to identify the molecule responsible for the enhanced Raman emission. Measurement of the conductivity provides information on the orientation and binding states of molecules. The dependence of the SERS spectra on the tip distance and/or the conductivity has been successfully determined. Although the origin of the correlation between the conductivity and the SERS spectral shape has


not yet been clarified, the observed spectra should contain characteristics of single-molecule SERS spectrum. If the characteristics of single-molecule SERS were fully clarified, SERS could be used as a powerful probe to obtain vibrational information on individual molecules excited in localized plasmon fields. One of the very unique aspects of

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Figure 13. Polarization response of a nanoparticle trimer. (A) SEM image of a trimer. A red arrow indicates the position of the molecule that leads to the best agreement between experiment and calculation. (B) Normalized Raman intensity at 555 (black squares) and 583 nm (red circles) as a function of the angle of rotation of the incident polarization. The intensities at both wavelengths show approximately the same profile, but the maximal intensity is observed at 75°, which does not match any pair of nanoparticles in the trimer. The green line is the result of a calculation assuming that the molecule is situated at the junction with a gap of 1 nm marked with red arrow in the SEM image and the corresponding geometrical parameters from the SEM image, R1 = 44 nm, R2 = 35 nm, and R3 = 28 nm, as the input. (C) Depolarization ratio (F) measured at 555 (black squares) and 583 nm (red circles). Depolarization profiles are wavelength-dependent in this case and are aligned differently than the intensity profiles. The black and red lines show the result of calculations at the two wavelengths, assuming that the molecule is situated at the junction marked with the red arrow in SEM image. (D) SEM image of a second trimer. A red arrow indicates the position of the molecule that leads to the best agreement between experiment and calculation. (E) Normalized RS intensity at 555 (black squares) and 583 nm (red circles) as a function of the angle of rotation of the incident polarization. As in (B), the intensity profile does not peak along the direction of the axis connecting particles 2 and 3. (F) Depolarization ratio (F) measured at 555 (black squares) and 583 nm (red circles). As in (C), depolarization profiles are wavelength-dependent and are aligned differently than the intensity profiles. The black and red lines show the result of calculations at the two wavelengths, assuming that the molecule is situated at the junction marked with red arrow in the SEM image. (G) Wavelength dependence of the parameter r0 (representing the ratio between g tensor elements), calculated from measurements on the trimer in (A) (blue) and (D) (orange). (H) Wavelength dependence of the parameter (representing the phase difference between Δ0 tensor elements), calculated from measurements on the trimer in (A) (blue) and (D) (orange). (Reprinted with permission from ref 71. Copyright 2008, National Academy of Sciences, U.S.A.)

SERS spectra is a relatively strong anti-Stokes scattering intensity. Deviation of the anti-Stokes/Stokes ratio of a SERS spectrum from that of a normal Raman spectrum at a comparable temperature has been considered as a means of observation and control of the vibrational population of molecules.57 Several enthusiastic attempts have been made to explain the observed characteristics of the “strange” anti-Stokes/Stokes ratio observed in SERS spectra based on vibrational pumping, localized heating via anisotropic thermal dissipation, and coupled plas-


mon resonance of the molecular state with electrons in the metal.57-61 Although all of these factors can alter the antiStokes/Stokes ratio from that of a normal Raman spectrum, the vibrational mode dependence of the anti-Stokes/Stokes ratio in SERS has not yet been fully explained (Figure 12).62 If the asymmetric anti-Stokes/Stokes ratio is due to a change in the vibrational population of the molecules, an effective singlemolecule SERS process can open the way to controlling the thermodynamic characteristics of individual molecules.

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Figure 14. Nanometric trapping and quenching of Brownian motion near the nanostructured substrate. (a, b) The y position of a 200 nm bead as a function of the y position of the beam focus moving at a distance of a = 14 (a) and 0.7 μm (b) from the substrate. Green and blue arrows indicate the y direction of motion for the laser trap (with a speed of 4 μm s-1). Green circles correspond to positive motion and blue hexagons to negative (the graph is shown for two cycles of motion). (c, d) Bead position as a function of time (time step, 5 ms) at a fixed position of a = 0.7 μm of the beam focus above glass (green circles, c) and a nanodot pair (red circles, d). The electron micrograph of the sample is scaled to demonstrate the amplitude of the Brownian motion with respect to the size of double-dot nanomolecules. (Reprinted with permission from Macmillan Publishers Ltd: ref 79, copyright 2008).

The observed Raman scattering spectrum is the consequence of the interaction of a localized plasmon with the molecule, which can be expressed in terms of an induced dipole moment. Thus, polarized SERS measurements give information on both the incident and Raman scattering polarizations.63-71 Several interesting polarized SERS studies have been carried out. Haran et al. reported that an asymmetric metal nanoparticle trimer generates elliptically polarized scattered light (Figure 13). This result suggests that the polarization of light scattered from a single molecule can be manipulated on the nanometer scale.71 Although details of the vibrational structure of molecules are at present not taken into account in the analysis of polarized SERS spectra, contributions from totally and nontotally symmetric modes of molecule can be considered separately to estimate the

effect of the charge-transfer resonance in SERS. It is important to clarify the vibrational band assignment, especially for identifying the charge-transfer band assigned to the nontotally symmetric mode which may play an important role in localized plasmon excitation of the molecule. Before moving on to the topic of photoexcitation, an additional perturbation that was applied to molecules located in a localized plasmon field will be discussed. Theoretical calculations predict that the intensity gradient due to the highly localized electromagnetic field generates an optical force on the molecule. For a single Ag dimer separated by a gap of 1 nm and subjected to incident illumination with an intensity of 1 mW/μm2, the net optical force in the gap is 62 pN.72 The value indicates that a single molecule can be trapped at the gap at room temperature. The optical force between metal nanoparticles under plasmon excitation was also investigated theoretically73,74 and experimentally.75,76 An experiment on the plasmon-based radiation force exerted on a single polystyrene bead was also reported.77-79 Recent developments in the structural control using top-down techniques have allowed the fabrication of plasmon-active metal nanostructures for the investigation of optical trapping of a metal nanoparticle.80 In the case of the single polystyrene bead, the gap width was in the range of 20-200 nm, and an illumination condition of 1.5 mW/μm2 was used. The trapping light intensity depends on the metal nanostructure. Another report described the optical trapping of polystyrene beads

Theoretical calculations predict that the intensity gradient due to the highly localized electromagnetic field generates an optical force to trap the molecule even at room temperature.


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Figure 15. (left-upper) Surface morphology of the AR-NSL gold substrate used in the present study. (a) Transmission electron micrograph. Scale: the bar = 200 nm. (b) AFM image in 3D. (right) Optical trapping behavior of Q dots in the presence of polyethylene glycol (PEG). The scale of the luminescence intensity (counts in the CCD camera) and the PEG concentration are given. (a) Modulation (temporal profile) of the photoluminescence intensity in line with the plasmonic excitation by repeatedly switching the 808 nm irradiation (I = 1.0 kW/cm2) on and off. Data points were collected every 1.0 s in 3.8 ms intervals. (b) Photoluminescence spectra. Spectra before irradiation at 808 nm are plotted in black, while those measured during irradiation at 808 nm are plotted in colors. (left-lower) Theoretical calculation using a Maxwell equation based on the discrete dipole approximation (DDA). (a) Top view of the gold nanodimer used in this calculation and enhancement factor of the electric field intensity as a function of x and y at z = 0. The color scale is on a logarithmic scale. (b) Side view of gold nanodimer used in this calculation and enhancement factor of the electric field intensity as function of z and y at x = 0. The color scale is on a logarithmic scale. (c) Spatial distribution of the trapping potential (U) at the nanovalley (marked with a white dotted-line box in (b) when I = 1.0 kW/cm2 and the radius of the Q dot particle is 15 nm. The color scale gives the potential energy normalized with kT on a logarithmic scale (from ref 81).

(d = 200 nm) under much less intense irradiation conditions ( 10 nm) under very weak irradiation (5100 μW/μm2) using a substrate Au dimer array81 (Figure 15). These results imply that the optical force can be used to trap molecules under localized plasmon excitation even at room temperature if the system is optimized. Finally, the possibility of plasmon-enhanced photochemical reactions and photoenergy conversion is discussed in reference to several important experimental results. As mentioned previously, plasmon enhancement of photochemical reactions is expected to improve the intrinsic efficiency of the reactions. One interesting possibility is to induce mutiphoton absorption by localized plasmons. The two-photon


sensitivity of photopolymers has been exploited for nearfield imaging of nanoparticle plasmon modes under laser excitation.82,83 Two-photon ring-opening photochromic reaction of diarylethene was also shown to be driven by irradiation from a weak, near-infrared continuous-wave laser light source.84 Furthermore, recent attempts to use arrays of gold nanoblocks specially tailored for maximum near-field localization and intensity enhancement led to detectable two-photon absorption even under irradiation by incoherent light sources. Two-photon polymerization of the photoresist surrounding the nanoparticles was found in the high-plasmonic field regions after irradiation by an incoherent light source85,86 (Figure 16). This result demonstrated that two-photon absorption induced by localized plasmons triggered a photochemical reaction in the absence of a laser source. This can be

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Figure 16. (a) SEM image of a pair of gold nanoblocks measuring 100 100 40 nm3 and separated by a 5.6 nm wide nanogap before irradiation by an attenuated femtosecond laser beam. (b) SEM image of other nanoblock pairs after 0.01 s of exposure to the laser beam polarized linearly along the long axis of the pair. (c) SEM image of another pair after 100 s of exposure to the laser beam polarized in the perpendicular direction. (d, e) Theoretically calculated near-field patterns at selected planes for the excitation conditions of the samples shown in (b) and (c), respectively. In (d), the field pattern is shown on the x-y plane bisecting the nanoblocks at half of their height (i.e., 20 nm above the substrate), and in (e), the field is calculated on the plane coincident with the line c-c shown in (d). The field intensity is normalized to that of the incident wave and therefore represents the intensity enhancement factor (from ref 85).

Figure 17. (a) SEM image of the Au nanorods. The figure on the right side shows a SEM image taken from directly above the substrate. The figures on the center and left side show a SEM image of the same substrate tilted by 75°. The lengths of scale bars are 500 (black line) and 100 nm (white line), respectively. (b) Overview of the photoelectrochemical measurement system using a TiO2 semiconductor electrode with Au nanorods as the working electrode. (from ref 92).

considered the first experimental verification of twophoton absorption without a coherent light source via localized plasmons.

Application of localized plasmon excitation to photoenergy conversion is also expected to improve the conversation efficiency and extend the optical response to the longer wavelength region. Successful improvements in optical absorption were demonstrated for a system involving dye sensitization and organic photovoltaic cells.87-90 In addition to the improvements in the optical absorption of molecules, plasmon-active metal nanoparticles themselves have been known to show sensitization for photocurrent generation.91 Quite recently, it was found that an elaborate array of Au nanorods formed on the surface of TiO2 single-crystal electrodes via a top-down nanostructuring process achieved plasmonic photoelectric conversion at visible to near-infrared wavelengths92 (Figures 16- 18).

Plasmon enhancement of photochemical reactions is expected to improve the intrinsic efficiency of the reactions.


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Figure 18. (a) Extinction spectrum of the Au NRs in water. Black: under irradiation of nonpolarized light. Red and blue: minor-axis direction (T-mode) and major-axis direction (L-mode) under irradiation of linearly polarized light, respectively. (b) Red: current-potential curve under the condition that light with a wavelength of 500-1300 nm was used to irradiate the TiO2 electrode with the Au NRs. Blue: current-potential curve of the TiO2 electrode without the Au NRs. Black: dark current of the TiO2 electrode with the Au NRs. The lower figure depicts the I-t curve under the conditions that light with a wavelength of 500-1300 nm was used to irradiate the TiO2 electrode with the Au NRs and 0.3 V was applied. (c) Photoelectric conversion efficiency of the action spectrum measured at each wavelength with monochromatic light (wavelength interval: 25 nm) irradiated with a monochromator. Black: with nonpolarized light. Red: with polarized light in the minor-axis direction. Blue: with polarized light in the major-axis direction. The inset figure shows the dependence of the photocurrent on irradiation intensity at wavelengths of 450, 650, and 1000 nm. (d) Internal quantum efficiency (IQE) obtained by standardizing the photoelectric conversion efficiency (from ref 92).

The photocurrent induced by monochromatic light was proportional to the first order of the light intensity. Thus, a photocurrent could be produced in response to quite a wide range of the solar spectrum, from visible to infrared light, merely by using simple Au nanorods, without the need for different dye sensitizers for different wavelengths or a semiconductor with different band gaps used as a tandem solar cell. The system had the quite unique characteristic that the internal quantum efficiency was dependent upon the irradiation light wavelength and exhibited a maximum at the peak wavelength of the plasmon mode of the structure. This suggests that the electromagnetic field enhancement gives rise to a nonlinear improvement of the intrinsic efficiency of photoinduced electron transfer from the Au nanostructure to TiO2. The higher localization resulted in an improvement of the quantum efficiency of the photoenergy conversion process. Effective excitation of molecules and materials can be achieved by localized plasmons. Metal nanostructures can


act as optical antennas to collect photons and then localize the electromagnetic field to excite target molecules and/or materials in the vicinity of the surface. Advanced structural controls of metal nanostructures offer a wide variety of choices on the energy of localized plasmons, the shape of the enclosed nanospace, and materials to confine target molecules. Recent theoretical studies have highlighted the contribution of electron tunneling between the gaps of metal dimers, indicating the importance of molecular-scale structural control, especially for localization on a subnanometer scale.93 Although the interaction between localized plasmons and molecules on the surface of metal nanostructure has not yet been clarified, several experimental results have established that the perturbation from plasmon excitation can be confined within a single molecule under certain conditions. This situation resulted in the improvement of the efficiencies of mutiphoton absorption and photoenergy conversion under visible to near-infrared light illumination. The exotic behavior of molecules in the plasmonic

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fields should be an indication of the strong interaction between the excited states of molecules and plasmons.94 Novel states in which molecules and plasmons are hybridized should be characterized both experimentally and theoretically to develop concepts and techniques for plasmon-induced excitation of molecules and materials, which can be applied to photochemistry and photoenergy conversion.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. Phone: þ81-11706-2704. Fax: þ81-11-706-4810. E-mail: [email protected].

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Biographies

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Hideki Nabika is Assistant Professor of Physical Chemistry in the Department of Chemistry, Faculty of Science, at Hokkaido University. He obtained a B.Eng. from the Department of Applied Chemistry at Kobe University in 1999 and a M.Eng. from Kobe University in 2001. After being a visiting student at Kiel University (Germany), he received his Ph. D in Science from Kobe University in 2004. He joined the Department of Chemistry of Hokkaido University as Assistant Professor in 2003. His current research interests are the synthesis, characterization, functionalization, and manipulation of nanomaterials. Mai Takase is a Ph D. student at Hokkaido University. She obtained a B.Sc. in Science from the Department of Chemistry at Hokkaido University in 2006 and a M.Sc. from Hokkaido University in 2009. Her current research interests are to control single-molecule photoelectrochemical reactions. Fumika Nagasawa is a graduate student at Hokkaido University. She obtained a B.Sc. in Science from the Department of Chemistry at Hokkaido University in 2009. She has developed single-molecule spectroscopy to observe an interaction between metal surfaces and molecules. Kei Murakoshi is Professor of Physical Chemistry at Hokkaido University. He obtained his B.Sc. in Science from the Department of Chemistry at Hokkaido University in 1986, graduated with a M.Sci. from Hokkaido University in 1989, and completed his Ph.D. at the Department of Chemistry in 1992. After postdoctoral positions at Centre National de la Recherche Scientifique (CNRS) in Muedon, France, he joined the Department of Engineering of Osaka University as Assistant Professor in 1993 and was promoted to Associate Professor in the Department of Science and Engineering there in 2003. He was the appointed Full Professor at Hokkaido University. His current research interests are in the uses of single-molecule spectroelectrochemistry to segregate, manipulate, and photoexcite a very small number of molecules in ultrasmall spaces.

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ACKNOWLEDGMENT The authors are very grateful to Professors Hiroaki Misawa, Kosei Ueno, and Katsuyoshi Ikeda at Hokkaido University for fruitful discussions. We also thank Dr. Manabu Kiguchi at Tokyo Institute of Technology. This work was supported by funding from the Ministry of Education, Culture, Sports, Science, and Technology of Japan: KAKENHI Grant-in-Aid for Scientific Research on the Priority Area “Strong Photon-Molecule Coupling Fields” (No. 470 (No. 19049003).

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