Photoreduction of 4-Nitrobenzenethiol on Au by Hot Electrons

Feb 20, 2015 - Hot electrons generated plasmonically from a Ag substrate can reduce 4-nitrobenzenethiol (4-NBT) to 4-aminobenzenethiol (4-ABT). In ord...
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Photoreduction of 4‑Nitrobenzenethiol on Au by Hot Electrons Plasmonically Generated from Ag Nanoparticles: Gap-Mode SurfaceEnhanced Raman Scattering Observation Kwan Kim,*,† Jeong-Yong Choi,† and Kuan Soo Shin*,‡ †

Department of Chemistry, Seoul National University, Seoul 151-742, Korea Department of Chemistry, Soongsil University, Seoul 156-743, Korea

‡

ABSTRACT: Hot electrons generated plasmonically from a Ag substrate can reduce 4-nitrobenzenethiol (4-NBT) to 4aminobenzenethiol (4-ABT). In order to evaluate the effectiveness of hot electrons, we have carried out a surfaceenhanced Raman scattering (SERS) study by forming a nanogap system composed of a planar Au substrate and an Agcoated micrometer-sized silica bead, wherein 4-NBT was adsorbed first onto the Au substrate, and then Ag-coated silica beads, derivatized with 1-alkanethiols, were spread over the 4NBT layer: the distance between 4-NBT and a nanostructured Ag substrate was varied by the chain length of alkanethiol molecules. Although the planar Au substrate itself was ineffective in the reduction of 4-NBT, hot electrons usable in the reduction of 4-NBT were generated from the Ag-coated silica beads. The hot electrons generated by 514.5 nm radiation were more effective in the reduction of 4-NBT to 4-ABT than those generated by 632.8 nm radiation, although the nanogap was more SERS-active with the excitation at 632.8 nm than at 514.5 nm. The photoreduction efficiency of hot electrons nonetheless decreased exponentially with the distance they traveled from the Ag surface: the reduction capability at a distance of 2 nm apart is about one-fourth of that in contact situations.

1. INTRODUCTION Photoreactions of organic molecules occurring on the surfaces of Au and Ag substrates can be monitored by means of surfaceenhanced Raman scattering (SERS) spectroscopy.1−7 For instance, we can confirm by SERS that 4-nitrobenzenethiol (4-NBT) adsorbed on a nanostructured Ag substrate is subjected to a photoreaction upon visible laser irradiation (e.g., 514.5 and 632.8 nm light), although the identity of the photoreaction product has been the subject of argument among different groups of researchers.8−12 In fact, one group of researchers claimed that the product is 4-aminobenzenethiol (4-ABT) while another group insisted that the product is, instead, 4,4′-dimercaptoazobenzene (4,4′-DMAB).13−16 The origin of the argument was rooted in the similarity of the SERS spectra of 4-ABT and 4,4′-DMAB.17−23 In the normal Raman (NR) spectrum of 4-ABT, only totally symmetric vibrational bands are observed while in the corresponding SERS spectrum, nontotally symmetric lines are also observed.24−29 The latter SERS bands were attributed, by Osawa et al., to the b2-type bands appearing as a result of the metal-to-adsorbate electronic charge transfer.27 In line with this, Lombardi and Birke also explained the SERS spectrum of 4-ABT using the Herzberg− Teller surface selection rules.30 However, Huang et al. argued that they can be assumed to be the ag modes of 4,4′-DMAB produced from 4-ABT via catalytic coupling on metal nanostructures.18 Recently, we concluded, on the basis of the SERS spectra of 4-NBT and 4-ABT on Ag in icy environments © XXXX American Chemical Society

at liquid N2 temperature, that the nontotally symmetric lines in the SERS of 4-ABT are intrinsically due to 4-ABT itself.31 Accordingly, 4-NBT on Ag is assumed to be reduced photochemically to 4-ABT but not to oxidize further to 4,4′DMAB. The hot electrons generated plasmonically from Ag nanostructures must have acted as a reducing agent to convert 4-NBT on Ag to 4-ABT. The generation of hot electrons from a nanostructured Au substrate has also been reported in the literature.32−38 Mukherjee et al. claimed that under the illumination of a 632.8 nm laser light, H2 molecules adsorbed on Au dissociated because of hot electrons plasmonically generated from Au nanostructures.39 The 4-NBT molecules adsorbed on Au, however, did not undergo photoreaction by a 632.8 nm laser light. Au substrates are, usually, SERS-inactive under 514.5 nm excitation, and therefore it is not certain whether 4-NBT on Au can be reduced to 4-ABT by 514.5 nm radiation and thus monitored by SERS. In Ag substrates, hot electrons are plasmonically generated more readily at lower excitation wavelengths; however, it is yet not confirmed whether 4-NBT on Au can also be reduced by hot electrons generated from distantly located Ag nanostructures. Received: January 2, 2015 Revised: February 9, 2015

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mW Ar+ laser (Melles-Griot model 351MA520) or the 632.8 nm line from a 17 mW He/Ne laser (Spectra Physics model 127) was used as the excitation source.

In this work, we have attempted to obtain the Raman spectra of 4-NBT on a flat Au substrate by overlying Ag-coated, micrometer-sized silica beads thereon, wherein the Ag surface was derivatized in advance with 1-alkanethiols such that 4-NBT molecules were not in direct contact with the Ag surface. First, we confirmed that the nanogaps formed by a planar Au substrate and Ag-coated, micrometer-sized silica beads are more SERS-active under 632.8 nm excitation than under 514.5 nm excitation. Second, we confirmed that the hot electrons generated from Ag by 514.5 nm radiation are more effective than those generated by 632.8 nm radiation in their ability to reduce 4-NBT to 4-ABT on Au. Third, we demonstrated that the reduction capability of the hot electrons decreases exponentially with the distance they travel from the Ag surface; anyhow, the hot electrons are able to effectively transmit over a distance of, at least, ∼2 nm (equivalent to the chain length of 1hexadecanethiol) to reduce 4-NBT. Separately, we deduced that charge transfer to 4-ABT can also occur from a planar Au substrate based on the gap-mode SERS spectra of 4-ABT that exhibit the typical b2-type vibrational bands of 4-ABT.

3. RESULTS AND DISCUSSION Figure 1a and Figure 1b show the FE-SEM images of 1 μm sized silica beads taken before and after silvering them using

2. EXPERIMENTAL SECTION Chemicals. Au wire (99.999%), silver nitrate (99.8%), butylamine (99.5%), 4-mercaptobenzoic acid (4-MBA, 99%), 4nitrobenzenethiol (4-NBT, 80%), 4-aminobenzenethiol (4ABT, 97%), 1-butanethiol (1-BT, 99%), 1-decanethiol (1-DT, 98%), and 1-hexadecanethiol (1-HDT, 92%) were purchased from Sigma-Aldrich Co. and used as received. Silica beads (1 μm, 99%) were obtained from Alfa Aesar Co. Other chemicals, unless specified, were of reagent grade. Aqueous solutions were prepared using triply distilled water (resistivity greater than 18.0 MΩ·cm). Preparation of Nanogap Systems. To prepare Ag-coated silica beads, ethanol-cleaned silica was added to the silvering medium up to a final concentration of 0.10 mg·mL−1 (w/v, dried bead mass/ethanol) and then incubated for 50 min at 50 °C under vigorous shaking. As a silvering mixture, the concentrations of AgNO3 and butylamine were each maintained at 1 mM. After rinsing with ethanol, the Ag-coated silica beads were redispersed in ethanol under sonication for 5 min. Subsequently, 1 mL ethanolic solution of 10 mM 1-BT, 1-DT, or 1-HDT was added to 9 mL of ethanolic Ag-coated silica beads and later filtered out after 6 h. After rinsing the 1-BT (or 1-DT or 1-HDT)-adsorbed Ag-coated silica beads with ethanol, they were redispersed in 10 mL of ethanol under sonication.40,41 Macroscopically smooth Au substrates were prepared by resistive evaporation of titanium and gold at 1 × 10−6 Torr on cleaned glass slides. After the deposition of approximately 200 nm of gold, the evaporator was backfilled with nitrogen. The gold substrates were subsequently immersed overnight in 1 mM 4-MBA (or 4-NBT or 4-ABT) in ethanol. For the gapmode SERS measurements, 10 μL ethanolic solution of 1alkanethiol derivatized Ag-coated silica beads was dropped onto the 4-MBA (or 4-NBT or 4-ABT) adsorbed Au substrate. Instrumentation. UV−vis extinction spectra were obtained using a SCINCO S-4100 spectrometer. Atomic force microscope (AFM) images were obtained on a Digital Instruments Nanoscope IIIa system. Field-emission scanning electron microscopy (FE-SEM) images were obtained using a JSM6700F field-emission scanning electron microscope operated at 2.0 kV. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer. The 514.5 nm line from a 20

Figure 1. FE-SEM images of 1 μm sized silica beads recorded (a) before and (b) after silvering them using butylamine as the reductant of AgNO3.

butylamine as a reductant of AgNO3, respectively. The Agcoated silica beads themselves are highly SERS-active, not only at 514.5 nm excitation but also at 632.8 nm excitation. Figure 2a and Figure 2b show the SERS spectra of 4-MBA adsorbed on Ag-coated silica beads measured at 514.5 and 632.8 nm excitations, respectively; the laser power at the sampling position was the same at both excitations. The Ag-coated silica

Figure 2. SERS spectra of 4-MBA adsorbed on Ag-coated silica beads measured at (a) 514.5 nm and (b) 632.8 nm excitations. (c) Normal Raman spectrum of 4-MBA. B

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the excitation laser light was focused solely on the Au film, we were able to obtain their spectra by focusing the laser light on the Ag-coated silica bead. Hereafter, we will label the 1alkanethiol adsorbed Ag-coated silica beads as SiO2@Ag@Cn (n = 4, 10, or 16 for 1-BT, 1-DT, or 1-HDT, respectively). The bare Ag-coated silica bead is labeled as SiO2@Ag@Cn (n = 0). On this basis, the nanogap formed by spreading SiO2@Ag@Cn on the 4-MBA-adsorbed Au film will be labeled as SiO2@Ag@ Cn/4MBA/Au. Other nanogaps will be similarly denoted. The overall scheme of the preparation of a nanogap and the subsequent Raman spectral measurement is drawn in Scheme 1. Figure 4a and Figure 4b show the collective Raman spectra of SiO2@Ag@Cn/4MBA/Au (n = 0, 4, 10, 16) obtained using, respectively, 514.5 and 632.8 nm radiations as the excitation sources. Each spectrum is contributed not only by 4-MBA peaks but also by CH2-group associated peaks.42,43 The peaks in Figure 4b are about 20 times more intense than those in Figure 4a, reflecting the fact that the nanogaps formed by a planar Au substrate and Ag-coated micrometer-sized silica beads are about 20 times more SERS-active with the excitation at 632.8 nm than at 514.5 nm: the EF for the nanogap of SiO2@Ag@C0/4MBA/Au under the 632.8 nm excitation is estimated to be ∼105. This result is consistent with what was observed recently for 4-ABT sandwiched between a macroscopically smooth Au substrate and 20−80 nm sized Ag nanoparticles, in which the highest Raman signal was measured comparably at 632.8 and 568 nm excitation while the Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than at 632.8 nm excitation in agreement with the finite-difference time domain simulation.43 The peak intensities of the 4-MBA ring 8a band at ∼1600 cm−1 in Figure 4a and Figure 4b are depicted in Figure 4c as a function of the number of methylene groups (n) separating the Ag-coated silica bead and the 4-MBA monolayer on Au film.44 The SERS peaks decrease exponentially along the nanogap distance in conformity with the electromagnetic enhancement mechanism in SERS. Murray and Allara reported the Raman scattering from the monolayer as a function of the space layer thickness from a rough silver surface and observed a falloff of the enhancement by a factor of 10 with each subsequent 3.5−5 nm spacer thickness.45 Evidently, the SERS peaks are stronger at all distances with the excitation at 632.8 nm than at 514.5 nm. The 8a peak decreases in intensity to half its value as the distance increases from zero to 0.3 nm (C16 = 2.2 nm thick). The same observation is made for other peaks, suggesting that all peaks in Figure 4a and Figure 4b are electromagnetically enhanced ones. Figure 5a and Figure 5b show the collective Raman spectra of SiO2@Ag@Cn/4NBT/Au (n = 0, 4, 10, 16) obtained using 514.5 and 632.8 nm radiations as the excitation sources, respectively. Each spectrum is contributed not only by the peaks of the unreacted 4-NBT but also by the peaks of the photoproduct, i.e., 4-ABT, as well as the CH2-group associated peaks.8−12 All spectra in Figure 5a and Figure 5b were obtained after irradiation by the excitation laser light for 1 min: no spectral change occurred in the following minutes. The timecourse of the Raman spectral variation of the SiO2@Ag@Cn/ 4NBT/Au system was quite similar to that of silver 4nitrobenzenethiolate salt (Ag-4NBT). In fact, when Ag-4NBT is irradiated with an Ar+ laser, its Raman spectrum changes over time, resulting in the production of 4NBT-capped silver nanoparticles.46 The peak intensity of the NO2-stretching band at 1350 cm−1 in Figure 5a and Figure 5b is plotted as a function of the number of methylene groups (n) of SiO2@Ag@

beads were first soaked in 1 mM ethanolic 4-MBA solution for 6 h and washed with ethanol, after which a 10 μL aliquot was spread on a Si wafer for the Raman spectral measurement. All SERS peaks could be correlated with the normal Raman peaks of 4-MBA shown in Figure 2c. The Ag-coated silica beads were more SERS-active, by a factor of ∼7, under the 632.8 nm excitation than under the 514.5 nm excitation. This is in conformity with the electromagnetic enhancement mechanism in SERS, since the extinction of the Ag-coated bead increases from 300 to ∼600 nm and then remains constant up to ∼800 nm.39,40 On the basis of the peak intensity of the ring breathing band at 1587 cm−1 (ν8a), the enhancement factor (EF) at the 632.8.5 nm excitation is estimated to be ∼106; in this estimation, the roughness factor of Ag is assumed to be 1.37 by referring to the earlier publication.41 As described in the Experimental Section, atomically smooth Au films were prepared on glass slides by a vacuum deposition process. The AFM image in Figure 3a clearly shows that the Au

Figure 3. (a) AFM image of a vacuum-evaporated Au film on a glass slide and (b) optical microscope image of micrometer sized, Ag-coated silica beads spread on an Au film.

films are composed of atomically flat terraces. In order to evaluate the SERS activity of the nanogaps formed by a planar Au substrate and Ag-coated silica beads, we first allowed 4MBA molecules to adsorb fully onto the Au film and then 1alkanethiol derivatized, and Ag-coated silica beads were sparsely spread thereon. As shown in Figure 3b, the Ag-coated silica beads were easily visualized by an optical microscope. A number of Raman spectra can thus be measured by focusing the excitation laser light on a couple of well-isolated nanogaps. Although no 4-MBA Raman peaks were identifiable at all when C

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Figure 5. Raman spectra of SiO2@Ag@Cn/4NBT/Au (n = 0, 4, 10, 16) obtained using (a) 514.5 nm and (b) 632.8 nm radiations as the excitation sources. Vibrational modes due to 4-ABT are labeled as stars.

discrepancy between the full and dotted lines is greater in Figure 6a than in Figure 6b. This happens because the photoreduction of 4-NBT to 4-ABT occurs more easily with the excitation at 514.5 nm than at 632.8 nm. This is consonant with the observation of Fedruco et al.47 that the quantum yield of the photoreduction of CO2 at the roughened Ag/solution interface was as large as ∼0.5% at ∼500 nm, while it was negligibly small at ∼600 nm. In any case, the chemical species subjected to oxidation to maintain the charge neutrality is not certain at the moment. As in other systems,48 some surface Ag atoms might be oxidized to Ag+ by the holes generated simultaneously with hot electrons. Figure 7a shows the relative peak intensities of the ring 7a band at ∼1080 cm−1 and the NO2-stretching band at 1350 cm−1 in Figure 5a and Fignre 5b drawn versus the number of methylene groups (n) separating the Ag-coated silica bead and the 4-NBT monolayer on Au. More specifically, the filled circles

Figure 4. Raman spectra of SiO2@Ag@Cn/4MBA/Au (n = 0, 4, 10, 16) obtained using (a) 514.5 nm and (b) 632.8 nm radiations as the excitation sources. (c) Intensity variation of the 4-MBA ring 8a band versus the number of methylene groups (n) observed in (a) and (b). Vibrational modes due to 1-alkanethiols are labeled as circles. Error bars in (c) indicate standard deviations from the experiments performed in triplicate.

Cn/4NBT/Au in Figure 6a and Figure 6b, respectively (see the full lines). The dotted lines in Figure 6a and Figure 6b correspond to the NO2-stretching band intensities expected when 4-NBT has not been subjected to a photoreduction (deduced from the data for 4-MBA in Figure 4c). The D

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correspond to the relative intensities measured under the 514.5 nm excitation while the open circles correspond to those under the 632.8 nm excitation. The intensity ratio decreases with the increase in chain length. The ring 7a band must then be attributed more to 4-ABT than to 4-NBT, since the ratio would have remained constant if the band was entirely due to 4-NBT. The fact that the filled circles stay above the open circles in Figure 7a indicates that the photoconversion of 4-NBT to 4ABT occurs more readily with the excitation at 514.5 nm than at 632.8 nm. Since the ring 7a vibration is a totally symmetric ring breathing mode, its presence in the SERS spectrum of 4ABT can be attributed to the electromagnetic enhancement at the nanogap. Figure 7b shows the relative peak intensities of the 4-ABT ring 19b band at 1435 cm−1 with respect to the 4-NBT NO2stretching band at 1350 cm−1 in Figure 5a and Figure 5b represented versus the number of methylene groups (n) of SiO2@Ag@Cn/4NBT/Au. The filled circles correspond to the relative intensities measured under the 514.5 nm excitation, while the open circles correspond to those under the 632.8 nm excitation. The intensity ratio evidently decreases exponentially with the increase in chain length, indicating that the effect of hot electrons also decreases exponentially with distance. Once again, the filled circles stay above the open circles, since the photoconversion from 4-NBT to 4-ABT occurs more readily with the excitation at 514.5 nm than at 632.8 nm. The ring 19b band (as well as the ring 7a band) is identified even in the SiO2@Ag@C16/4NBT/Au system, indicating that hot electrons traveling over a length of 2.2 nm (from the nanostructured Ag surface corresponding to the thickness of C16) still have enough energy to reduce 4-NBT to 4-ABT. The 19b band is a typical 4-ABT b2-type band, known to originate from the chemical enhancement. The relative intensity variation of the 19b band in Figure 7b is somewhat more abrupt than that of the 7a band in Figure 7a. This would imply that the chemical enhancement portion decreases more rapidly than the electromagnetic enhancement one as the nanogap distance from the Ag surface increases. This is consistent with the theoretical interpretations of SERS, in that charge transfer is favored when the molecule is strongly attached to the surface, while the plasmon resonance contribution does not require a strong molecule−metal bond.49,50 We separately confirmed that no photoreduction takes place for 4-NBT adsorbed solely on a SERS-active Au substrate. This would be associated with the higher work function of Au (∼5.3 eV) than that of Ag (∼4.4 eV).51 We also measured the SERS spectra of 4-ABT adsorbed solely on a SERS-active Au substrate using 632.8 nm radiation as the excitation source, not only in ambient conditions but also in an icy environment at 77 K. As seen earlier for 4-ABT on Ag, the b2-type bands were identified clearly under both conditions, thus confirming that the b2-type bands were indeed the characteristic bands of 4-ABT. We also measured the Raman spectra of 4-ABT positioned in a nanogap between a flat Au substrate and Agcoated, micrometer sized silica beads. The Raman spectra of SiO2@Ag@Cn/4ABT/Au (n = 0, 4) measured in ambient conditions using 514.5 and 632.8 nm radiations as the excitation sources are shown in Figure 8a and Figure 8b, respectively. Owing to the strong scattering effect of the silica beads, however, the Raman spectra could not be measured in the icy environment. The b2-type bands are observed in all spectra depicted in Figure 8a and Figure 8b. Figure 8c and Figure 8d show the intensity ratios of the b2-type bands and the

Figure 6. (a) Intensity variation of the 4-NBT NO2-stretching band in Figure 5a and (b) that in Figure 5b represented versus the number of methylene groups (n). The dotted lines represent the NO2-stretching band intensities expected when 4-NBT has not been subjected to a photoreduction. See text.

Figure 7. (a) Relative peak intensities of the 4-ABT ring 7a band at ∼1080 cm−1 and (b) ring 19b band at 1435 cm−1, both measured with respect to the peak intensities of the NO2-stretching band at 1350 cm−1 in Figure 5a (see filled circles) and Figure 5b (see open circles). The relative peak intensities are represented versus the number of methylene groups (n). Error bars indicate standard deviations from the experiments performed in triplicate.

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Figure 8. Raman spectra of SiO2@Ag@Cn/4ABT/Au (n = 0, 4) measured using (a) 514.5 nm and (b) 632.8 nm radiations as the excitation sources. (c) Relative peak intensities of the b2-type bands measured with respect to the ring 7a band in (a) represented versus the number of methylene groups (n). (d) Similar ratios plotted using the data in (b). Error bars in (c) and (d) indicate standard deviations from the experiments performed in triplicate.

Since the silica beads used were 1 μm, the nanogaps formed were easily distinguished by an optical microscope. These nanogaps were initially characterized using 4-MBA as the model adsorbate owing to its photochemical stability. The EF for the nanogap of SiO2@Ag@C0/4MBA/Au under the 632.8 nm excitation was estimated to be ∼105; this nanogap was about 20 times more SERS-active with the excitation at 632.8 nm than at 514.5 nm. The 4-MBA SERS peaks decreased exponentially along the nanogap distance, in conformity with the electromagnetic enhancement mechanism in SERS. When there were 4-NBT molecules sitting in the nanogap, these were converted to 4-ABT even though 4-NBT was not in contact with the Ag nanostructure. Hot electrons generated plasmonically from Ag nanostructures flew to 4-NBT and reduced it to 4-ABT. The photoreduction of 4-NBT to 4-ABT occurred more easily with the excitation at 514.5 nm than at 632.8 nm. Hot electrons traveling over a length of 2.2 nm corresponding to the thickness of C16 still had enough energy to reduce 4-NBT to 4-ABT. Separately, presuming that the b2type bands of 4-ABT were associated with the chemical enhancement mechanism, the portion of chemical enhancement appeared to diminish more rapidly than that of the electromagnetic enhancement as the nanogap distance from the Ag surface was increased. This certainly implies that hot electrons generated plasmonically from Ag nanostructures must also contribute to the chemical enhancement of the b2-type bands of 4-ABT, in addition to the Au-to-4-ABT charge transfer contribution. In the near future, we will also conduct a gapmode Raman experiment for the SiO2@Au@C0/4NBT/Ag system in order to characterize more firmly the efficiency of hot electron generation in terms of the surface roughness of Ag substrate.

7a band in Figure 8a and Figure 8b, respectively, drawn against the number of methylene groups (n) of SiO2@Ag@Cn/4ABT/ Au. The b2-type bands are evidently more intense for SiO2@ Ag@C0/4ABT/Au than for SiO2@Ag@C4/4ABT/Au. This indicates that the effect of the chemical enhancement decreases as the distance of 4-ABT from the Ag surface increases. This, in turn, implies that hot electrons generated plasmonically from Ag nanostructures must contribute to the chemical enhancement of the b2-type bands of 4-ABT, though indirectly, by affecting the Fermi level of the Au substrate underneath. Otherwise, the b2-to-a1 intensity ratios would remain constant irrespective of the 4-ABT-to-Ag distance. On the other hand, it is worth recalling that as reported by Lombardi and Birke, charge-transfer contributions are more tied to the relative orientation of the molecule with respect to the surface.30

4. SUMMARY AND CONCLUSION In order to evaluate the photoreduction efficiency of the plasmonically generated hot electrons from Ag nanostructures, gap-mode SERS experiments were carried out. For that purpose, Ag-coated silica beads were initially prepared using butylamine as the reductant of silver nitrate. The Ag-coated silica beads themselves were highly SERS-active, showing a ∼106 fold enhancement for 4-MBA under the 632.8 nm excitation. The Ag-coated silica beads were subsequently derivatized with 1-alkanethiols to obtain four different hot electron generators labeled as SiO2@Ag@Cn (n = 0, 4, 10, 16). To fabricate a nanogap system, atomically smooth Au substrates were separately prepared by a vacuum evaporation method, and subsequently, 4-MBA, 4-NBT, or 4-ABT was selfassembled thereon. Although no Raman peaks of 4-MBA, 4NBT, or 4-ABT were observable in this stage, the peaks could be identified by spreading the Ag-coated silica beads onto them. F

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AUTHOR INFORMATION

Corresponding Authors

*K.K.: phone, +82-2-8806651; fax, +82-2-8891568; e-mail, [email protected]. *K.S.S.: phone, +82-2-8200436; fax, +82-2-8244383; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (Grants 2007-0056095 and 2012R1A2A2A01008004).

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