Crystal Face Dependent Chemical Effects in Surface-Enhanced

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Crystal Face Dependent Chemical Effects in Surface-Enhanced Raman Scattering at Atomically Defined Gold Facets Katsuyoshi Ikeda,*,†,‡ Shuto Suzuki,† and Kohei Uosaki*,†,‡,§ †

Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), Sapporo 060-0810, Japan § International Center for Materials Nanoarchitechtonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan ‡

ABSTRACT: Among electromagnetic and chemical (CM) contributions to surface-enhanced Raman scattering (SERS), the former is becoming controllable according to the recent progress in nanofabrication of plasmonic metal structures. However, it is still difficult to control the latter effect. Here, the degree of each contribution to SERS signals is examined on well-defined single crystalline facets of gold by using optical field localization within sphere-plane type plasmonic cavities. Crystal face dependent SERS studies of aminobenzenthiol adsorbates clearly show the distinction between CM enhancements on different surfaces, suggesting that the CM-activity of “SERS-hotspots” is closely related to interfacial dipoles formed at metal
molecular junctions. KEYWORDS: Surface-enhanced Raman scattering, charge transfer enhancement, electromagnetic enhancement, single-crystalline surface

M

etal
organic interactions have been the focus of intense multidisciplinary research such as catalytic chemistry, materials science, and molecular electronics.1
5 Chemical and physical interactions of molecules with a metal surface are strongly affected by its geometric and electronic surface structures. Various catalytic and electrocatalytic reactions are indeed known to be crystal-face dependent.6,7 For the study of such metal
organic interactions, vibrational spectroscopy is a powerful experimental approach to obtaining molecular level information. Among various spectroscopic methods, surface-enhanced Raman scattering (SERS) is recognized as a promising technique because of its high sensitivity to molecular adsorbetes on a metal surface.8,9 It is believed that the enhancement of Raman scattering intensity is attributed to electromagnetic (EM) and chemical (CM) effects.10
13 The former is caused by efficiency enhancement of photon
molecule coupling via plasmonic localization of optical fields, which occurs on metallic nanostructures. The latter is ascribed to charge transfer resonances between metal and molecular electronic states. Since the plasmonic field localization requires matching the wave vectors of the incident light and the plasmon modes, SERS experiments have been usually carried out at electrochemically roughened metal substrates having surface features much larger than 20 Å in mean diameter.9 In such a system, however, there are many uncontrollable variables such as molecular geometries and orientations because various adsorption sites are exposed on the surface. Uncontrollable metal
molecule interfacial structures cause the difficulty in quantitative and reproducible spectroscopic investigations on metal
organic interactions. Actually, SERS spectroscopy has suffered from this limitation from its discovery in 1970s.14 Despite extensive experimental and theoretical efforts, the problem of disentangling the EM r 2011 American Chemical Society

and CM contributions to SERS signals is still under hot discussion.15 Especially, microscopic origin of CM-active sites is not understood well. According to the recent progress in nanotechnology, artificial metallic nanostructures with a well-defined shape on a nanometric or submicrometric scale can be constructed by various top-down fabrication methods. Thus EM contributions are now controllable in SERS experiments.16
20 However, it is still difficult to manage CM contributions in such a nanostructure having uncontrolled surface features on an atomic scale. For further advancement of SERS spectroscopy, therefore, it is indispensable to fabricate plasmonic nanostructures with well-defined surfaces. Recently, we have experimentally showed that SERS observations become possible even at an atomically controlled smooth surface of a single crystalline metal substrate when sphere-plane type plasmonic cavities are introduced on it.21
23 Although this cavity itself has been utilized by several researchers,15,20,24,25 the application to defined single crystalline faces provides us a simple solution to the longstanding problem of surface roughness in SERS. Therefore, our technique without uncertain variables should be useful for studying detailed information of metal
organic interfaces, which is hardly obtainable from conventional SERS spectra. Indeed, we have already reported that adsorption geometry of self-assembled isocyanide monolayers depends on crystal orientation of a Pt surface.22,23 A similar SERS observation on a single crystalline Pt surface has also been conducted by Tian et al. using a shell-isolated Au-NP enhancement method.26 Received: January 22, 2011 Revised: March 14, 2011 Published: March 21, 2011 1716

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Figure 1. (a) An energy diagram for plasmon hybridization between plasmons of the sphere and the planar surface. (b) Calculated extinction spectra of a gold-sphere above a gold-plane with various distances. (c) A schematic illustration of Au-NPs/SAMs/Au(hkl) system for gap-mode plasmon excitation. A STM image of MBT-SAM/Au(111) and a SEM image of Au-NPs adsorbed on the SAM are also presented.

Such a crystal face dependent SERS study should reveal how the molecule
metal structure influences the SERS activities. In this Letter, we investigate contributions of EM and CM effects in SERS signals by observing well-defined metal
molecular interfaces. SERS spectra of well-organized self-assembled monolayers (SAMs) are measured at atomically smooth single crystalline gold substrates with different crystal orientations

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under ambient and electrochemical conditions. The crystal face dependent SERS studies show that CM contributions are sensitive to interfacial electronic and geometric structures. The details of the sphere-plane type plasmonic cavity have been studied by Metiu et al.27,28 Briefly, when a metal nanosphere is nearly in touch with a metal plane, highly localized electromagnetic fields can be excited even at atomically smooth surface. This is because dipole-active bonding plasmons, so-called gapmode plasmons, are formed by hybridization between the plasmon of the sphere and that of the planar surface as shown in Figure 1a.29 Hereinafter, we called this SERS technique as sphere-plane cavity induced surface-enhanced Raman scattering spectroscopy (sphere-plane SERS). The degree of the hybridization is related to the sphere diameter and the sphere-plane distance.19,27,28 Figure 1b shows extinction spectra of a gold sphere with a diameter of 20 nm above a gold planar substrate under a given space, which are theoretically calculated by Wind’s method.30,31 One can see that the plasmon resonance peak is red shifted with decreasing the distance. Since the plasmonic localization of optical fields within the sphere-plane gap contributes to the magnitude of the EM effect, it is essentially important to control the distance on a subnanometric scale. Experimentally, a large number of such cavities can be easily formed on a SAMcovered metal substrate as follows (see Figure 1c).21
23,32 First, a single crystal gold bead having well-defined (111) and (100) facets was fabricated by the Clavilier method.33 Next, the gold bead was immersed in an ethanoic solution containing 1 mM thiol molecules for two hours, so that the well-organized SAM was formed on the substrate. Here, 4-aminobenzenethiol (ABT) and 4-methylbenzenethiol (MBT), purchased from SigmaAldrich, Inc., were utilized to form the molecular layer. Finally, citrate-reduced gold nanoparticles (Au-NPs) with the diameter of ca. 20 nm were deposited on the SAM-covered gold bead by immersing in the colloidal solution. Note that the sphere-plane distance in this system is precisely determined by the thickness of the organic monolayer. Moreover, the surface density of the cavities, which determines the overall enhancement factor (EF), can be estimated from SEM observations. Figure 3c right bottom panel shows a SEM image of Au-NPs on the MBT-covered Au(111). One can confirm that the physisorbed Au-NPs are welldispersed on the SAM-covered surface; typical coverage of the Au-NPs is around 30%, not depending on the crystal orientation. Hence, the EM-induced enhancement is expected to be highly reproducible in this system. In addition, the well-defined interfacial structure of SAM-covered single crystal facets, shown in the molecularly resolved STM image (Figure 3c left bottom panel), should contribute to giving reproducible CM enhancements √ in SERS. According to Borguet et al., the MBT-SAM has a (2 3 √ 3) structure with the ideal molecular density of 7.6  10
10 mol 3 cm
2 on Au(111).34 The surface concentration of 6.3  10
10 mol 3 cm
2 in our case, determined by the reductive desorption method,35 is in good agreement with the STM observation. Raman scattering signals were monitored with a home-built inverted microscope Raman system with an objective lens (40, 0.6 N.A.).21
23 According to the calculated plasmon resonance spectra shown in Figure 1b, a He
Ne laser (radiation wavelength of 632.8 nm and intensity of 0.02 mW) was chosen as an excitation light source for SERS measurements. The backscattered Raman signals from each facet were monitored by a CCDpolychromator system (PIXIS 400B, Princeton Instruments) after Rayleigh scattering light was filtered by an edge filter 1717

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Figure 2. Conventional and sphere-plane SERS spectra of (a) ABT and (b) MBT measured on three different substrates of roughened Au or defined Au(111).

(LP02
633-RU, Semrock). In electrochemical SERS observations, the substrate potential of the single-crystal gold bead was controlled in an electrochemical cell filled with Ar-bubbled 0.1 M NaClO4 solution. To compare sphere-plane SERS spectra with conventional SERS spectra, an electrochemically roughened polycrystalline Au substrate was also utilized in the experiments. We first examine the reproducibility of conventional SERS and sphere-plane SERS spectra. Figure 2a shows conventional and sphere-plane SERS spectra of ABT measured on three different substrates of roughened Au or defined Au(111). Assignments of the observed Raman bands are summarized in Table 1. The conventional SERS method is obviously not reproducible with respect to both of the overall intensity and spectral features. Especially, the ratios of the nontotally symmetric b2 modes to the totally symmetric a1 modes were largely different one another. Conversely, the sphere-plane SERS are highly reproducible; for example, the intensity of 7a and the ratio of 9b(b2) to 7a(a1) were reproduced within (10 and (15% for different samples, respectively. A similar experiment for MBT is presented in Figure 2b. Again, one can see the similar tendency of the spectral reproducibility for each measurement method; the sphere-plane SERS is much better than the conventional one. The 7a-intensity of the Au(111)/MBT was reproduced within (5%. The difference of the reproducibility between ABT and MBT is presumably due to the difference of the degree of the molecular ordering; the

Table 1. Raman Band Frequencies of ABT Au(111)

Au(100)

SERS

assignmentsa

389

393

395

νCS þ γCCC, 6a(a1)

631

556 644

556 643

γCCC, 16b(b1) γCCC, 12(a1)

703

706

708

πCH þ πCS þ πCC, 4b(b1)

815

820

815

νCH þ νCS þ νCC, 1(a1)

934

936

937

πCH, 5b(b1)

1004

1007

1008

γCC þ γCCC, 18a(a1)

1077

1079

1080

νCS, 7a(a1)

1138

1142

1144

δCH, 9b(b2)

1180 1384

1180 1395

1176 1397

δCH, 9a(a1) δCH þ νCC, 3(b2)

1430 1582

1432

1442

νCC þ δCH, 19b(b2)

1483

1483

νCC þ δCH, 19a(a1)

1582

1582

νCC, 8a(a1)

Approximate description of the modes (ν, stretch; δ and γ, bend; π, wagging). For ring vibrations, the corresponding vibrational modes of benzene and the symmetry species under C2v symmetry are indicated.

a

surface density of MBT on Au(111) is 1.7 times higher than that of ABT. Interestingly, there is no b2 mode found in the spectra, which will be discussed later. These results indicate that the 1718

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Figure 3. Raman spectra of (a) ABT or (b) MBT on Au(111) and Au(100) without and with Au-NPs. The overall signal intensity for each spectrum is normalized by the surface molecular density.

sphere-plane SERS method is a rather simple but powerful technique to extract quantitative information from SERS spectra. Next, we elucidate crystal face dependence of the EFs in sphere-plane SERS. For precise estimation of the EF, it is indispensable to determine the surface density of adsorbates accurately. For the accurate determination, the reductive desorption method35 is applicable to a well-defined surface while not to a roughened surface. In the case of the sphere-plane SERS, therefore, the signal intensity is easily normalized by the molecular density among different facets. Then, each EF can be estimated by comparison with normal Raman intensity of the solid thiols. Figure 3 shows the normalized Raman spectra of Au(111)/ABT and Au(100)/ABT, and Au(111)/MBT and Au(100)/MBT with and without adsorption of Au-NPs. In the absence of Au-NPs, no Raman signal was detected both at Au(111) and Au(100) faces because surface plasmons could not be excited at such a smooth surface. Conversely, when AuNPs were deposited on the molecular layers, the signal intensities were substantially enhanced. The EFs for Au(100)/ABT, Au(100)/MBT, and Au(111)/MBT were estimated to be the order of 105 with respect to the 7a(a1)-intensity. On the other hand, the EF for the Au(111)/ABT was 4-fold larger than the others. Furthermore, the ratio of b2 to a1 was significantly different in the Au(111)/ABT spectrum; for example, the ratio of 9b(b2) to 7a(a1) is 0.43 for Au(111)/ABT and 0.06 for Au(100)/ABT. Note that the EM contributions to the EF should be comparable between the different facets. This is because the plasmonic property of the sphere-plane cavity, which is characterized by plasma frequency of the bulk metal, is independent of crystal orientations.27,28 Consequently, it is naturally concluded that the crystal face dependence of the EFs must be ascribed to the difference of the degree of the CM contributions.

The CM contributions to SERS can be described in a similar manner with resonance Raman theory.13 When the incident energy is resonant with the energy difference between a metal state and a molecular state, the Raman polarizability R can be expressed as follows R ¼ AþBþC where the A-term denotes Franck
Condon contribution and the B- and C-terms refer to Herzberg
Teller contributions. According to the symmetry selection rules, the A-term contributes to the totally symmetric modes while the B- and C-terms enhance the nontotally symmetric modes. As for conventional SERS spectra of ABT, the appearance of b2 modes is usually ascribed to the C-term contribution, that is, the metal-tomolecule charge transfer resonance that is vibronically allowed by intensity borrowing from the π
π* molecular transition (1A1 f 1B2).36
38 Therefore, the extra intensity gain in the sphere-plane SERS on Au(111) might be explained by Franck
Condon A-term contribution for a1 modes and by the HertzbergTeller type vibronic C-term for b2 modes. This is in good agreement with the previous report by Lombardi et al.38 Incidentally, Tian et al. recently proposed that the appearance of b2 in SERS spectra of ABT is caused by a laser-induced oxidation reaction.39 However, this model fails to explain the crystal face dependent SERS spectra in this work; if the photoinduced chemical reaction is the origin of b2 modes, the Au(100)/ ABT-spectrum should become similar to the Au(111)/ABTspectrum after long-time exposure to the incident light. It is also difficult for this model to explain the extra intensity gain of the a1 modes at Au(111). The degree of CM contributions correlates with matching the excitation light energy and the energy difference between the 1719

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Figure 4. Potential dependence of sphere-plane SERS spectra of ABT at (a) Au(111) and (b) Au(100). The overall signal intensity is normalized by the 7a(a1) to focus on the ratios of b2 lines with respect to a1 lines. The cross-section spectra at 0.6, 0.2, and
0.2 V vs Ag/AgCl are also presented. Blue and green arrows indicate b2 modes and dimerization-related modes, respectively.

Fermi level of metal and molecular affinity levels. The energy difference can be changed by tuning the molecular LUMO level. Actually, this is the case for the spectral difference between ABT and MBT; for the benzenthiol derivatives, substitution of the functional group affects to the molecular states. The substitution of the amino group (ABT) to the methyl group (MBT) would give a positive shift of 0.37 eV to the LUMO level.40 This should be large enough to reduce the CM contributions in the MBT-SERS. The energy difference can be also varied with tuning the Fermi level of the metal substrate, which is achieved by electrochemical potential control in solution. This electrochemical method is indeed a standard technique in conventional SERS studies on rough surfaces. Figure 4 shows a potential dependence of the sphere-plane SERS spectra of ABT on Au(111) and Au(100). To focus on the ratios of b2 modes to a1 modes, the overall SERS intensity is normalized by the 7a(a1)-intensity. One can see that the intensity-potential profile for b2 lines revealed typical behavior of the metal-to-molecule charge transfer resonances.13 The threshold potential for the CM resonance was around
0.2 V for Au(111) and 0 V for Au(100). In a similar electrochemical experiment on a roughened Ag electrode,36 the threshold value is around
0.8 V for the same excitation energy, which is 0.6
0.8 V more negative than the value on Au facets. This difference can be well explained by the difference of the work function between Au and Ag.38 In the potential response of sphere-plane SERS spectra, as mentioned above, the threshold potential for the CM resonance was slightly different between the facets. This different

potential-induced behavior provides us detailed information on the interfacial electronic structures. When chemical bond formation occurs between molecules and the metal substrate, large dipoles are formed at the interface, resulting in shifting of the work function.41,42 The degree of the shift depends on the density and directions of the interfacial dipoles. In the present case, the surface density of ABT was experimentally 1.2 time higher at Au(100) than at Au(111). The molecular orientation was also different between the facets; for example, the relative intensity of 6a(a1) mode is higher at Au(100). In addition, the crystal face dependent potential response was seen in the ratio of b2 lines to a1 lines; all of b2 lines on the (100) were significantly weaker than those on the (111) in the entire potential region. Since the magnitude of the CM contributions is sensitive to the directions of transition dipoles of allowed molecular resonances,8 this result also suggests the difference of the molecular orientation between the facets. According to both of the 6a(a1)- and the b2-intensities, the molecular symmetry axis of ABT is presumably in standing-up configuration on Au(111). Such crystal face dependence in the adsorption geometry is not surprising because preferential adsorption sites or surface molecular density, which is sensitive to the crystal orientation, influences the molecular orientation.5,43,44 The spectral variations upon potential sweep were reproducible between
0.8 and þ0.4 V. When the potential goes to a potential positive of þ0.4 V, however, an irreversible spectral change was observed along with appearance of a broad peak around 1500 cm
1 and the πNH mode around 450 cm
1. In this 1720

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Nano Letters potential region, it has been reported that dimerization of ABT molecules is induced via the oxidation of ABT to the cation radical.45 The new peak around 1500 cm
1 is probably due to the intermolecular coupled νCdC vibrations. Presumably, this dimerization is relevant to the suggestion by Tian’s group.39 In summary, quantitative SERS spectroscopy becomes possible when uncontrollable variables for signal enhancement are reduced by using the sphere-plane type plasmonic cavity on a well-defined metal
molecular interface. The magnitude of the EM enhancement is affected by surface features on a nanometric or submicrometric scale. On the other hand, the magnitude of the CM enhancement is sensitive to atomic and molecular surface features. More specifically, the CM-activity of “SERS-hotspots” is closely related to the density and directions of interfacial dipoles formed at metal
molecular junctions. When the excitation energy is close to electronic resonances in the metal
molecular system, therefore, significant crystal-face dependence appears in SERS spectra. In the case of conventional SERS using a roughened metal substrate, the spectra presumably appear as a combination of contributions from multiple crystalline faces, resulting in extremely low reproducible spectral appearance. Conversely, when the excitation energy is far from the resonances, only the difference of the adsorption geometry can contribute to spectral appearance. In this study, both of the cases were demonstrated by ABT and MBT, respectively. Plasmon excitation at a planar metal surface is, in principle, possible by other methods such as tip-enhanced Raman spectroscopy (TERS)46 or Otto-type attenuated total reflection (ATR).47 However, quantitative control of the EM effect in these system is rather difficult; TERS is still technically difficult especially in electrochemical condition, and ATR is not sensitive because of the use of propagating plasmon polaritons. In contrast, the present method has several advantages in the practical in situ spectroscopic application and should open up a possibility of SERS spectroscopy that simultaneously probes geometric and electronic information of metal
molecular junctions.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: (K.I.) [email protected]; (K.U.) UOSAKI. [email protected].

’ ACKNOWLEDGMENT This research was supported by Grant-in-Aid Scientific Research on Priority Area “Strong Photon-Molecule Coupling Fields” (21020001), Grant-in-Aid for Young Scientists (B) (22750001), World Premier International Research Center (WPI) Initiative on Materials Nanoarchitechtonics, Global COE program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science), and MEXT Program for Development of Environmental Technology using Nanotechnology from Ministry of Education, Culture, Sports, Science, and Technology, Japan. The SEM measurements were supported by Hokkaido Innovation through NanoTechnology Support (HINTS, Nanotechnology Network Project supported by MEXT, Japan). ’ REFERENCES (1) Adams, D. M.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P. V.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyrle, M. E.; Miller, J. R.; Newton, M. D.;

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