Article pubs.acs.org/JPCC
Photoelectrochemical Analysis of Allowed and Forbidden Multipole Plasmon Modes of Polydisperse Ag Nanorods Emiko Kazuma and Tetsu Tatsuma* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan S Supporting Information *
ABSTRACT: Resonant wavelengths of multipole (i.e., higherorder) plasmon resonance of polydisperse Ag nanorods (NRs) and their dependence on incident angle are analyzed by a photoelectrochemical means coupled with spectroscopy. It also allows us to observe nanostructures of multipole-resonant Ag NRs. Multiple dip formation in extinction spectra is caused by selective oxidation of multipole-resonant NRs on the basis of multipole plasmon-induced charge separation on a TiO2 substrate. The dip wavelengths are the dipole resonant wavelengths of the NRs that are dipole- or multipole-resonant at the irradiation wavelength. Even order plasmon modes (mode index number m = 2, 4) of the NRs are forbidden at perpendicular incidence (θ = 0°), and in the present system, the third-order plasmon mode (m = 3) is also forbidden at a specific incident angle, θ = 37.5°.
M
etal nanoparticles (NPs) exhibit light absorption and scattering and focus optical near field in the close vicinity of the NPs due to localized surface plasmon resonance (LSPR),1 which is collective oscillation of conduction electrons coupled with incident electric field. LSPR is therefore applied to chemical sensors and biosensors,2,3 surface-enhanced Raman scattering,4 fluorescence enhancement,5 photovoltaic cells6,7 and photocatalysts,7,8 image displays and data storage,9 and nanoimaging10 and nanofocused photochemical reactions.11 Most of the investigations and the applications of LSPR have focused on dipole (fundamental, first-order; mode index number m = 1) resonance mode, while multipole (higherorder; m = 2, 3, 4, ...) modes have increasingly attracted attention.12−23 The multipole plasmon modes are often observed for relatively large NPs. In particular, nanorods (NRs) exhibit discrete multipole plasmon modes of electron oscillation along their long axes (longitudinal mode). As Figure 1 shows, the resonant wavelength decreases as the order m increases. On the other hand, the resonant wavelength of each order of LSPR increases as the aspect ratio (and dimension) of NRs increases (Figure 1).12,17−19 The longitudinal plasmon mode of a NR also causes strong nanofocusing of optical near field at their tips,10 and the field would be used for nanoimaging and nanofabrication. Although the field enhances as the aspect ratio increases, the resonant wavelength also increases. A longer resonant wavelength corresponds to lower photon energy, which restricts applications related to charge separation6−8 and photochemical reactions.11 Actually, the charge separation has been reported at ≤1300 nm for Ag24 and ≤1325 nm for Au NPs.25 Such an issue could be addressed by taking advantage of the multipole plasmon, which is excited by photons of higher energy; for instance, charge separation induced by NRs with high aspect ratio or nanowires could be possible via multipole plasmon © 2012 American Chemical Society
Figure 1. Schematic illustration for multipole plasmon modes (m = 1, 2, and 3) of Ag NRs along the long axes. Blue arrows and red waves indicate the directions and amplitudes of collective electron oscillations. Colors of NRs qualitatively represent the resonant wavelengths (blue < green < yellow < pink < gray). Note that all longitudinal plasmon modes (m = 1, 2, 3, ...) of Ag NRs with the same width have an identical dispersion relation and that the NR length is not proportional to the resonant wavelength because the dispersion relation is not linear.13,16
resonance, even if it is not possible via dipole resonance. It may also be possible to excite the NRs and nanowires with avoiding infrared absorption of a medium such as water. Thus, it could be advantageous to chemical and biosensing, nanofabrication, and data storage in a wide wavelength range. Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: January 6, 2012 Revised: March 29, 2012 Published: April 23, 2012 2435
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
The Journal of Physical Chemistry C
Article
Multipole plasmon modes are typically studied by using NRs and nanowires because of their discrete multipole resonances. However, it is not necessarily easy to synthesize monodisperse NRs and nanowires.17,20,26 Nanolithographic approaches are powerful but costly and time-consuming.12,27 Alternatively, single particle observation may be applied to polydisperse NPs based on scanning near-field optical microscopy (SNOM),15,16 dark-field spectroscopy,21 and electron energy loss spectroscopy (EELS).22,23 However, all these methods require sophisticated equipment, and it is difficult to analyze incident angle dependence of the multipole plasmon. It is known that the intensity of the multipole plasmon resonance strongly depends on the incident angle θ, and even order modes (m = 2, 4, ...) are optically forbidden at perpendicular incidence (θ = 0°) because the symmetrically distributed electrical polarizations are canceled out in integration along the long axis as indicated in Figure 1 (m = 2).13,17−19 When the symmetry is lost at oblique incidence (θ > 0°), the even-order modes are allowed. Likewise, the third-order plasmon resonance should also be forbidden at a specific angle of incidence (θ > 0°) at which the polarizations are canceled out,13 but it has barely been studied. In this study, we analyzed the optical properties of Ag NRs and nanowires that exhibit multipole plasmon resonance at specific wavelengths, even in the presence of NRs and nanowires of different aspect ratios, by conventional spectroscopy. We irradiate an ensemble of Ag NRs on TiO2 at a specific wavelength. As a result, NRs resonant at the wavelength are oxidized due to the plasmon-induced charge separation, and an extinction dip forms at the excitation wavelength (Figure 2).7,9,24 In addition, extra dips form at longer wavelengths by oxidation of longer NRs due to multipole plasmon-induced charge separation. The present method allows us to observe the morphology of Ag NRs oxidized selectively by the multipole plasmon-induced charge separation at a specific wavelength, simply by using conventional atomic force microscopy (AFM). It is also suitable for studying incident angle dependence of the multipole plasmon behavior, and we evidenced that the thirdorder plasmon resonance is forbidden at a specific incident angle (θ ∼ 37.5°). We also proved for the first time that multipole plasmon induces charge separation and extinction dip formation as well as dipole plasmon. Extinction dip formation at longer wavelengths by multipole plasmon excitation would be applied to data storage, image display, and chemical and biosensing in the near-infrared region.
Figure 2. (a) Typical AFM image of biaxially oriented Ag NRs on the TiO2(100) surface. Inset is the extinction spectrum. (b) Schematic illustration for oxidation of a Ag NR and redeposition of small Ag nanoparticles on the basis of plasmon-induced charge separation. (c− e) Schematic illustration for the concept of multiple dip formation. (c) Spectral changes (black line → colored line) caused by plasmoninduced oxidation and decreases in length of NRs due to excitation of the dipole ((1) m = 1) and multipole ((2) m = 2, (3) m = 3) plasmon mode at 900 nm (yellow band). The peaks of the excited modes are assumed to be blueshifted by 100 nm.24 All spectra are obtained by multiplying the spectrum calculated for a single Ag NR model on rutile TiO2 by the population of equivalent NRs found on the real sample. (d) Resultant difference extinction spectra obtained from (c) for cases (1)−(3) (colored curves) and (e) the sum of those difference spectra (black curve).
Evaluation of Plasmon-Induced Oxidation of NRs. The prepared sample of polydisperse Ag NRs on TiO2 was irradiated with polarized visible or infrared light in N2 gas at about 50% relative humidity (RH) at room temperature. The light source was a Xe lamp (Luminar Ace LA-251Xe, Hayashi Watch Works for excitation wavelength λexc = 600 nm) or a halogen lamp (HA-150UX, Myutron for λexc = 700−1200 nm) equipped with a long-pass filter (>460 nm, SCF-50S-48Y, Sigma Koki) to cut off UV light completely, a bandpass filter (fwhm = 10 nm for λexc = 600 nm and fwhm = 40 nm for λexc = 700, 800, 900, and 1000 nm, CVI Melles Griot; fwhm = 55 and 63 nm for λexc = 1100 and 1200 nm, respectively, Optical Coatings Japan), and a linear polarizer (SPF-30C-32, Sigma Koki for λexc = 600 nm; colorPol VISIR, Codixx for λexc = 700− 1200 nm). Extinction spectra of the ensemble of the polydisperse Ag NRs were collected by a Jasco V-670 UV/ vis/NIR spectrophotometer thorough the polarizer mounted between the sample and the light source. Morphological changes of single Ag NRs were observed by AFM (Nanonavi Station/E-sweep, SII Nanotechnology) in a tapping mode (driving frequency = 110−150 kHz, scan rate = 0.4−0.7 Hz) by using a silicon cantilever (SI-DF20, SII Nanotechnology) with a
■
EXPERIMENTAL METHODS Preparation of Ag NRs. A rutile TiO2(100) single-crystal substrate (10 × 10 × 0.5 mm, Shinkosha) was washed with acetone and ultrapure water, etched in 20% aqueous HF for 10 min, rinsed with water, and dried, followed by annealing at 900 °C for 1 h under atmospheric conditions to obtain an atomically flat surface (step height = 0.46 nm). Biaxially oriented Ag NRs with various aspect ratios and dimensions were deposited on the TiO2 surface on the basis of epitaxial growth28 by photoelectrochemical reduction of Ag+ ions under UV light (310 nm, 1 mW cm−2) for 8 min in 3 mM aqueous AgNO3 mixed with an equal volume of ethanol (99.5 vol %). The mixture contained acetaldehyde (550 ppm). The UV light source was a Hg−Xe lamp (Luminar Ace LA-300UV, Hayashi Watch Works) equipped with a bandpass filter (full width at half-maximum (fwhm) = 10 nm). After the growth of the NRs, the sample was rinsed thoroughly with ultrapure water to remove the residual salts and organic molecules and dried. 2436
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
The Journal of Physical Chemistry C
Article
normal spring constant of 15 N m−1 and tip radius of curvature of 10 nm. All AFM measurements were performed under atmospheric pressure in a closed chamber filled with dry N2 gas, which was flowed for 1 h before each measurement. AFM images were corrected by a Morphology Filter (SII Nanotechnology) to eliminate the influence of the tip shape. Parameters used for the correction were determined by evaluating the probe shape before and after measurements. Simulation of Spectra and Electric Field Distribution. Absorption, scattering, and extinction spectra of a semicylindrical Ag NR with quarter spherical caps on rutile TiO2 and spatial distribution of polarization around the NR were calculated on the basis of a finite-difference time-domain (FDTD) method via FDTD Solutions (Lumerical Solutions). The simulation domain consisted of 10 nm cubic cells, and the central region was further meshed with a 3D grid of 1 nm spacing. The dielectric functions of Ag and TiO2 were extracted from the data of Palik.29
irradiation wavelength and a peak at a shorter wavelength (case 1 in Figure 2d). The aspect ratio of the reacted NRs shows an almost linear relationship with excitation wavelength (= dip wavelength), supporting that the aspect ratio-selective oxidation of NRs is responsible for the dip formation.24 So far, the plasmon-induced charge separation has been observed only for dipole excitation. However, charge separation by multipole plasmon excitation of Ag NRs might also be possible. In multipole plasmon resonance, as is clear from its principle (Figure 1), the resonant wavelength should become shorter as the order of oscillation m increases. Thus, if oxidative dissolution of NRs is induced by multipole plasmon excitation (m ≥ 2) at a specific wavelength λ, we should be able to observe not only a dip at λ but also extra dips at longer wavelengths. Figure 2c−e schematically explains the dip formation mechanisms. The Ag NR ensemble on TiO2 involves NRs with various aspect ratios. Irradiation at wavelength λ excites several different types of NRs: (1) NRs of which the first-order resonance wavelength (λm=1) equals λ (i.e., λm=1 = λ) (Figure 2c and d, (1)), (2) NRs of which second-order resonance wavelength (λm=2) equals λ (i.e., λm=2 = λ) (Figure 2c and d, (2)), and (3) NRs of λm=3 = λ (Figure 2c and d, (3)). Likewise, NRs of λm≥4 = λ are also excited. If all those excited NRs (λm≥1 = λ) are oxidized and shortened, their extinction peaks due to dipole and multipole resonance are blueshifted and suppressed as Figure 2c shows. These changes are reflected in a difference spectrum by formation of multiple dips as shown in Figure 2e, which is the sum of the difference spectra for the different types of NRs (Figure 2d). Since dipole peaks are much more prominent than multipole peaks for all types of NRs, the observable dips should be ascribed to dipole resonance, as Figure 2c−e shows. However, the second dip (dip 2 in Figure 2e) is formed by the second-order resonance (m = 2) and the third dip (dip 3) by the third-order resonance (m = 3), so that those extra dips involve information of the multipole plasmon resonance. For instance, formation of dip 2 by irradiation at λ indicates that the NR ensemble contains Ag NRs that exhibit second-order resonance at λ. If the ensemble is irradiated under conditions that prohibit the second-order resonance, dip 2 would not be formed. In this study, we examine the formation of those extra dips and study oxidation of Ag NRs by multipole (i.e., higher-order) plasmon resonance. For this purpose, we prepared samples of NRs with higher aspect ratios exhibiting stronger extinction in the infrared region (Figure 2a), by longer photodeposition (8 min) than the previous one.24 The obtained Ag NR ensemble was irradiated with polarized light (//TiO2[001]) at 800 nm (10 mW cm−2) with incident angle θ (θ = 0−45°). Figure 3a shows polarized difference extinction spectra (//TiO2[001]). Besides the dip at the excitation wavelength (λ = 800 nm) (dip 1), extra dips (dips 2, 3, and 4) were observed at longer wavelengths (∼1370, ∼1900, and ∼2400 nm, respectively). Intensities of dips 2 and 3 are plotted in Figure 3b as functions of the incident angle. Dip 2 is almost suppressed at the vertical incidence (θ = 0°), and dip 3 is damped at 37.5°. Dip 4 appeared only at 45° among the incident angles examined. These dependencies on the incident angle are described below. All these dips observed for polarized spectra parallel to the excitation polarization were negligible in perpendicularly polarized spectra, indicating that the dips are associated with the longitudinal mode. Multiple Dip Formation by Multipole Resonance Excitation. The formation of extra dips has never been
■
RESULTS AND DISCUSSION Extra Dip Formation and Its Incident Angle Dependence. In this study, we used ensembles of biaxially oriented Ag NRs (//TiO2[001] and //TiO2[010]) with various aspect ratios (1 to >26) deposited on a TiO2(100) substrate (Figure 2a).28 Since the resonant wavelength of the longitudinal mode of NRs is approximately proportional to aspect ratio,24 our sample has a broad extinction (= absorption + scattering) band in the visible-infrared region (Figure 2a, inset). In comparison with the aspect ratio, the distribution of width (40 ± 6.7 nm) and height/width ratio (0.47 ± 0.05) is much smaller, so that their effect on the extinction wavelength is limited ( 0°) and at ∼1300 nm for the firstorder mode (Figure 4b). The second-order resonance is absent at θ = 0°, and its intensity increases with increasing incident angle. A NR of which the aspect ratio is 5.3 has extinction peaks at ∼800 nm for the third-order mode and at ∼1900 nm for the first-order mode (Figure 4c), and a NR of which the aspect ratio is 7.2 has peaks at ∼800 nm for the fourth-order and at ∼2400 nm for the first-order mode. All these calculated results are in line with the experimental results that the λdip2, λdip3, and λdip4 are ∼1370, ∼1900, and ∼2400 nm for 800 nm excitation. Therefore, the dips 2, 3, and 4 correspond to dipole resonance (m = 1) of NRs which have second- (m = 2), third- (m = 3), and fourth-order (m = 4) resonances, respectively, at 800 nm. In other words, those extra dips reflect the multipole plasmon resonances at 800 nm. To investigate the formation of the four dips in further detail, morphological changes of NRs were analyzed. After the 800 nm light irradiation, the Ag NR ensemble on TiO2 was carefully observed by AFM (232 μm2), to find the oxidized Ag NRs, which were accompanied by satellite Ag NPs (Figure 5a) as a result of recombination of electrons and Ag+ ions released from the Ag NR (Figure 2b).24a Then, aspect ratios of the oxidized NRs were measured by AFM and plotted in Figure 5b. It is obvious from the figure that the aspect ratio values are discretely distributed at 1.56 ± 0.13, 3.47 ± 0.18, 5.58 ± 0.30, and 7.66 ± 0.25 (average ± standard deviation, the number of reacted NRs n = 8−15). Due to spectral simulation, aspect ratios of NRs that exhibit the first-, second-, third-, and fourthorder resonance at 800 nm are 1.5, 3.5, 5.3, and 7.2 (dotted lines in Figure 5b). These theoretically expected values are roughly in agreement with the experimental data. Since we observed the NRs after the reaction, the values should be slightly lower than those before excitation, but the shifts are likely to be smaller than the dispersion of the experimentally obtained values. Actually, when we observe the same location on the sample before and after the irradiation (Figure 5c), the decrements in the aspect ratio were 4.3 ± 1.6% (aspect ratio = 6.6−7.4, n = 7). We therefore assign dips 1, 2, 3, and 4 to blueshifted and damped first-order resonance, resulting from oxidative dissolution of NR tips by excitation of first-, second-, third-, and fourth-order plasmon modes, respectively (Figure 2c, d). Here the maximum depth of a dip is extinction peak height × population of the NR with the corresponding aspect ratio. The experimentally obtained difference spectra are qualitatively in agreement with the theoretically expected spectrum shown in Figure 2e. Incidentally, NRs which are oxidized by excitation of the third plasmon mode at ∼800 nm have the second-order mode at ∼1020 nm as well as the first-order mode at ∼1900 nm
Figure 3. (a) Difference extinction spectra divided by the initial extinction (Ext0) for the polarization angle //TiO2[001] after irradiation with light at 800 nm (10 mW cm−2 for 60 min, polarization //TiO2[001]) at different incident angles (θ = 0°, 15°, 30°, 37.5°, and 45°). (b) Diagram for the depths of dip 2 and dip 3 relative to the depth of dip 1 as functions of the incident angle. (c) Difference extinction spectra (//TiO2[001]) after irradiation with 700, 800, 900, 1000, and 1100 nm light (10 mW cm−2 for 1 h, 10 mW cm−2 for 1 h, 10 mW cm−2 for 2 h, 10 mW cm−2 for 12 h, 20 mW cm−2 for 16 h, respectively, //TiO2[001], θ = 30°). (d) Dependencies of the experimentally observed (symbols) and calculated (dotted lines) dip wavelengths on the excitation wavelength.
observed for any systems with Ag NPs. The formation of these multiple dips can be explained by the oxidative dissolution of NPs which have multiple resonance modes at different wavelengths. If NPs resonant at λdip1 and λdip2 are dissolved by irradiation at one of the wavelengths, two dips are formed at both wavelengths. In the case of NRs, multiple resonance is possible for longitudinal/transverse modes or for multipole resonances. Regarding the former, a NR exhibits the transverse mode with oscillation along the short axis, in addition to the longitudinal mode with oscillation along the long axis. If NPs with transverse mode resonance at λdip1 and longitudinal mode resonance at λdip2 reacted by excitation at λdip1, dip 2 at λdip2 should be observed preferentially in the polarized spectrum perpendicular to the excitation polarization. In our experimental results, however, dips were observed only for the parallel polarization as described above. In addition, the transverse mode of Ag NRs on TiO2 cannot be excited at 800 nm but at ≤700 nm.24,28 Formation of the multiple dips is 2438
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
The Journal of Physical Chemistry C
Article
Figure 5. (a) Typical tapping-mode AFM images of Ag NRs after irradiation with 800, 900, and 1000 nm light polarized along the long axis of the NRs. All scale bars are 50 nm. (b) Aspect ratio of oxidized NRs (symbols, measured by AFM) as a function of the excitation wavelength. Calculated results in Figure 4d are also plotted (dot lines). (c) Morphological changes of NRs caused by irradiation with 900 nm light (polarization // long axis). Red arrows indicate the direction of incident electric field.
Figure 4. (a−c) Absorption spectra calculated by a FDTD method for a semicylindrical Ag NR with quarter spherical caps (width = 40 nm, height = 20 nm) on rutile TiO2 at different incident angles (θ = 0°, 15°, 30°, 37.5°, and 45°, the model is shown in Figure 3a, inset). Aspect ratio AR of the NR is (a) 1.5, (b) 3.5, and (c) 5.3. Spatial distributions of polarization around the peak wavelength at m = 1 (θ = 0°), 2 (θ = 30°), and 3 (θ = 0°) are shown in (c) (red, positive; blue, negative). White dot lines indicate the interface between TiO2 and Ag or air. (d) Calculated resonant peak wavelengths as a function of aspect ratio of the Ag NR.
Multipole Resonance Excitation at Different Wavelengths. The main dip at the excitation wavelength and extra dips at longer wavelengths were observed not only for excitation at 800 nm but also at other wavelengths (600− 1200 nm, θ = 30°, Figure 3c). The extra dips redshift linearly with increasing excitation wavelength as shown in Figure 3d (symbols). We simulated spectra of 40 nm wide NRs on TiO2 by the FDTD method, and the results showed linear correlations between the peak wavelength and aspect ratio of NRs, for respective order of resonance (Figure 4d). On this basis, we calculated the extra dip wavelengths (λdip2, λdip3, and λdip4), which are wavelengths of the first-order resonance of the NRs oxidized by excitation of the second-, third-, and fourthorder resonances, respectively, and plotted them against the excitation wavelength in Figure 3d (dotted lines). The plots agree well with the experimental ones, supporting our conclusion further. Again, experimentally observed aspect
according to the simulation (Figure 4c). However, because all the spectra shown in Figure 3a were measured under the perpendicular incidence (θ = 0°), even modes were not detected. Even for θ > 0°, the dip was not observed at ∼1020 nm (Supporting Information, Figure S1) because the secondorder peak was much lower than the first-order peak as evidenced by the simulation (Figure 4c). This also applies to, for instance, the second- and third-order resonances of the fourth-order excitation. 2439
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
The Journal of Physical Chemistry C
Article
Notes
ratio of the reacted NRs falls into discrete values, and the values are roughly in agreement with the theoretical expectation, as Figure 5b indicates. On the basis of all the present results, we conclude that the plasmon-induced charge separation is possible by excitation of multipole plasmon (m ≥ 2) as well as dipole plasmon (m = 1). The dipole-based charge separation of Ag NRs on TiO2 has been possible only at ≤1300 nm for an aspect ratio ≤3.7.24 By taking advantage of the charge separation by the multipole plasmon, however, NRs of higher aspect ratios can also be oxidized, and dips can be formed at longer wavelengths. Damped Multipole Resonance at Specific Incident Angles. The incident angle dependence of the dip depth, which is mentioned above, should be related to the dependence of the multipole resonance intensity on the incident angle. In particular, it is well documented that even order resonance modes are optically forbidden at θ = 0° because the electrical polarizations are canceled out in integration along the long axis (Supporting Information, Figure S2).13,17−19 This explains that dip 2, which is formed by excitation of the second-order resonance, is suppressed at the vertical incidence (Figure 3a, b). Likewise, odd order modes except for m = 1 could also be forbidden or significantly suppressed at a specific incident angle. In the present case, the simulation revealed that the third-order mode at 800 nm of a Ag NR of which aspect ratio is 5.3 is almost suppressed at θ = 37.5° (Figure 4c and Supporting Information, Figure S2). In the experiment, dip 3 was significantly damped at θ ∼ 37.5° by irradiation at 800 nm as described above (Figure 3a, b), which means that the thirdorder plasmon mode of few NRs was excited and oxidized, as predicted theoretically. In addition, dip 4 at ∼2400 nm appears at θ ≥ 45°. Therefore, various different dip patterns can be obtained by taking advantage of this incident angle dependence; dips 1 and 2 (θ ∼ 37.5°), dips 1 and 3 (θ ∼ 0°), dips 1−3 (0° < θ < 37.5°), and dips 1−4 (θ ≥ 45°).
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was conducted as a part of “R&D on Innovative PV Power Generation Technology” which The University of Tokyo contracted with New Energy and Industrial Technology Development Organization (NEDO). E.K. thanks a JSPS Research Fellowship for Young Scientists.
■
(1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Mayer, K. M.; Hafner, J. H. Chem. Rev. 2011, 111, 3828−3857. (3) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Nat. Mater. 2008, 7, 442−453. (4) Stiles, P. L.; Dieringer, J. A.; Shah, N. C.; Van Duyne, R. P. Annu. Rev. Anal. Chem. 2008, 1, 601−626. (5) Lakowicz, J. R.; Ray, K.; Chowdhury, M.; Szmacinski, H.; Fu, Y.; Zhang, J.; Nowaczyk, K. Analyst 2008, 133, 1308−1346. (6) Atwater, H. A.; Polman, A. Nat. Mater. 2010, 9, 205−213. (7) Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632−7637. (8) Kowalska, E.; Abe, R.; Ohtani, B. Chem. Commun. 2009, 241− 243. (9) Ohko, Y; Tatusma, T.; Fujii, T.; Naoi., K.; Niwa, C.; Kubota, Y.; Fujishima, A. Nat. Mater. 2003, 2, 29−31. (10) Kawata, S.; Inouye, Y.; Verma, P. Nat. Photonics 2009, 3, 388− 394. (11) Ueno, K.; Juodkazis, S.; Shibuya, T.; Yokota, Y.; Mizeikis, V.; Sasaki, K.; Misawa, H. J. Am. Chem. Soc. 2008, 130, 6928−6929. (12) Krenn, J. R.; Schider, G.; Rechberger, W.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R.; Weeber, J. C. Appl. Phys. Lett. 2000, 77, 3379−3381. (13) Schider, G.; Krenn, J. R.; Hohenau, A.; Ditlbacher, H.; Leitner, A.; Aussenegg, F. R.; Schaich, W. L.; Puscasu, I.; Monacelli, B.; Boreman, G. Phys. Rev. B 2003, 68, 155427. (14) Kumbhar, A. S.; Kinnan, M. K.; Chumanov, G. J. Am. Chem. Soc. 2005, 127, 12444−12445. (15) Hohenau, A.; Krenn, J. R.; Schider, G.; Ditlbacher, H.; Leitner, A.; Aussenegg, F. R.; Schaich, W. L. Europhys. Lett. 2005, 69, 538−543. (16) Imura, K.; Nagahara, T.; Okamoto, H. J. Chem. Phys. 2005, 122, 154701. (17) Payne, E. K.; Shuford, K. L.; Park, S.; Schatz, G. C.; Mirkin, C. A. J. Phys. Chem. B 2006, 110, 2150−2154. (18) Khlebtsov, N. B.; Khlebtsov, N. G. J. Phys. Chem. C 2007, 111, 11516−11527. (19) Encina, E. R.; Coronado, E. A. J. Phys. Chem. C 2008, 112, 9586−9594. (20) Bok, H.; Shuford, K. L.; Kim, S.; Kim, S. K.; Park, S. Nano Lett. 2008, 8, 2265−2270. (21) Wei, H.; Reyes-Coronado, A.; Nordlander, P.; Aizpurua, J.; Xu, H. ACS Nano 2010, 4, 2649−2654. (22) Guiton, B. S.; Iberi, V.; Li, S.; Leonard, D. N.; Parish, C. M.; Kotula, P. G.; Varela, M.; Schatz, G. C.; Pennycook, S. J.; Camden, J. P. Nano Lett. 2011, 11, 3482−3488. (23) Rossouw, D.; Couillard, M.; Vickery, J.; Kumacheva, E.; Botton, G. A. Nano Lett. 2011, 11, 1499−1504. (24) (a) Kazuma, E.; Sakai, N.; Tatsuma, T. Chem. Commun. 2011, 47, 5777−5779. (b) Kazuma, E.; Tatsuma, T. Chem. Commun. 2012, 48, 1733−1735. (25) Nishijima, Y.; Ueno, K.; Yokota, Y.; Murakoshi, K.; Misawa, H. J. Phys. Chem. Lett. 2010, 1, 2031−2036. (26) Lyvers, D. P.; Moon, J.; Kildishev, A. V.; Shalaev, V. M.; Wei, A. ACS Nano 2008, 2, 2569−2576. (27) Tabor, C.; Van Haute, D.; El-Sayed, M. A. ACS Nano 2009, 3, 3670−3678. (28) Kazuma, E.; Matsubara, K.; Kelly, K. L.; Sakai, N.; Tatsuma, T. J. Phys. Chem. C 2009, 113, 4758−4762.
■
CONCLUSIONS We revealed that multipole plasmon resonance of polydisperse Ag NRs can be analyzed by a photoelectrochemical means coupled with spectroscopy. We can also observe by AFM Ag NRs that exhibit multipole resonance at a given wavelength. It is also possible to know incident angle dependence of the multipole plasmon resonance. Plasmon-induced charge separation based on multipole resonance allowed these analyses as well as dip formation at long wavelengths at which the charge separation based on dipole plasmon cannot be induced. The oxidation and dip formation based on multipole plasmon would be applied to nanofabrication, data storage, and chemical sensing.
■
ASSOCIATED CONTENT
S Supporting Information *
Difference extinction spectra measured under polarized light with various incident angles. Dependencies of spatial distributions of polarization and electric field on incident angle. This material is available free of charge via the Internet at http://pubs.acs.org.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 2440
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
The Journal of Physical Chemistry C
Article
(29) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: New York, 1985. (30) Tian, Y.; Tatsuma, T. Chem. Commun. 2004, 1810−1811. (31) Takahashi, Y.; Tatsuma, T. Nanoscale 2010, 2, 1494−1499. (32) Kawahara, K.; Suzuki, K.; Ohko, Y.; Tatsuma, T. Phys. Chem. Chem. Phys. 2005, 7, 3851−3855. (33) Tatsuma, T.; Takada, K.; Miyazaki, T. Adv. Mater. 2007, 19, 1249−1251.
2441
dx.doi.org/10.1021/jp300175y | J. Phys. Chem. C 2013, 117, 2435−2441
