J. Phys. Chem. C 2007, 111, 3259-3265
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Effect of Silver Nanoplates on Raman Spectra of p-Aminothiophenol Assembled on Smooth Macroscopic Gold and Silver Surface Yuling Wang, Xiangqin Zou, Wen Ren, Weidong Wang, and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Graduate School of the Chinese Academy of Sciences, Changchun 130022, Jilin, People’s Republic of China ReceiVed: October 1, 2006; In Final Form: December 31, 2006
Recently, metal-molecule-metal junctions have been actively researched due to their potential applications in molecular electronics, and probing the structure of the molecules in the junctions is crucial for understanding electron-transfer properties. In this paper, self-assembled monolayers (SAMs) of p-aminothiophenol (p-ATP) are formed on a smooth macroscopic gold and silver surface, and colloidal silver nanoplates (∼100 nm edge length and 12 nm thickness) are assembled onto the SAM surface to form the silver nanoplates-moleculegold (silver) junctions for the first time. The resulting silver nanoplates-molecules-gold (silver) structures are studied with surface enhanced Raman scattering (SERS) at 514.5, 794.4, and 1064 nm excitations, respectively. Initially, there is very weak Raman signal of p-ATP on the smooth macroscopic gold and silver substrates, respectively, but Raman spectra are considerably enhanced by the presence of the silver nanoplates, which is due to the strong electromagnetic coupling between the localized surface plasmon (LSP) of silver nanoplates and the surface plasmon polariton (SPP) of the smooth gold and silver, respectively, so-called LSP-SPP coupling. The Raman spectra obtained are compared with that obtained on silver nanoplates deposited on glass at the specific excitation wavelengths. It was found that the enhancement factor (EF) of the junctions to the probe molecule is 3-9 times larger than that at silver nanoplates deposited on glass, indicating great potential in the research of molecular electronic device and biology by SERS.
1. Introduction Since the first discovery in 1974 that high-intensity Raman scattering of small molecules could be obtained on an electrochemically roughened silver surface by Fleischmann et al.,1 who attributed the high enhancement to the large number of the molecules on the roughened surface, and in 1977, when Jeanmaire et al.2 and Creighton et al.3 independently discovered that the enhancement of the Raman scattering is related to an intrinsic surface enhancement effect, marking the beginning phenomenon of surface enhanced Raman scattering (SERS),4 substantial interest has been focused on the research of mechanisms responsible for the extraordinarily large enhancement of Raman signals for adsorbates on the roughened metal surfaces. It has been widely accepted that there are two separate mechanisms to describe the overall SERS effect: the electromagnetic effect (EM) and the chemical effect (CM). The EM mechanism is based on the interaction of the transition moment of an adsorbed molecule with the electric field of a surface plasmon induced by the incoming light at the metal,5 whereas the CM is the interaction of the adsorbate adsorbed on metal and the metal, mostly from the first layer of the charge-transfer resonance between adsorbate and the metal.6 Recent studies on single molecule detection by SERS indicate that large enhancement on the order of 1014-1015 can be obtained at the junctions of two aggregated nanoparticles due to the coupling of the localized surface plasmon (LSP) of the metal nanoparticles produced by the hot spots.7 Based on this consideration, Keating et al. have investigated the heightened * Corresponding
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electromagnetic field between metal nanoparticles by fabricating junctions of metal-cytochrome c-metal sandwiches in solution.8 It was demonstrated that dramatic increases in SERS intensities of proteins were derived from the coupling of the LSP of the nanoparticles. Our group also found this phenomenon by fabricating junctions of metal nanoparticles-moleculesmetal nanoparticles on glass, and large enhancement is also obtained at the junctions.9 Except for the junctions between metal nanoparticles, Zheng et al. have found that the Raman spectrum of molecules positioned in the junction of silver nanoparticles and smooth macroscopic silver surface can also be enhanced, and the enhancement is attributed to the electromagnetic coupling of the LSP of the silver nanoparticles to the surface plasmon polariton (SPP) of the smooth silver.10 The coupling effect has been documented by research on a surface plasmon resonance (SPR) device as reported by Hutter et al.11 They have systematically studied the enhancement effect of the metal nanoparticles to SPR of the gold and silver film and attributed the enhancement to the coupling of the LSP-SPP.11 Later, Orendorff et al. fabricated the sandwich architecture of smooth gold-molecules-gold nanoparticles with different shapes,12 on which high enhancement factors (EFs) in the range of 107-109 were obtained and attributed to the LSP-SPP coupling. However, all these studies are on the LSP-SPP coupling of the same metals. Most recently, Kim and co-workers have investigated the coupling of silver-gold, silver-copper, and gold-copper by fabricating the junctions on the gold and copper substrates.13 It was found that the silver-copper coupling is more effective than that of gold-copper in the visible light. Not only for the theoretical research of the LSP-SPP effect by SERS, the metal-to-metal junctions also have great applications
10.1021/jp066444k CCC: $37.00 © 2007 American Chemical Society Published on Web 02/02/2007
3260 J. Phys. Chem. C, Vol. 111, No. 8, 2007 in molecular electronic devices14 and biology.15 Daniels et al. have fabricated the silver nanoparticles-molecules-silver mirror sandwich structure for SERS and found that the enhancement of Raman scattering in the sandwich structure is larger than that on roughened silver surface due to the plasmon coupling between the particles themselves as well as coupling between particles and the underlying continuous film.15a Because of the high enhancement of Raman scattering, they found that the substrate can be used to monitor the kinetics of Bacillus subtilis endospore germination via SERS.15b Recently, much effort has been devoted to the synthesis of silver nanoplates due to their unusual optical properties resulting in surface plasmon resonance peaks at relatively long wavelengths.16 However, SERS properties of the silver nanoplates are less investigated, probably due to the flat and thin shapes of the silver nanoplates. Zhang et al. have compared the SERS enhancement effect of silver nanospheres, triangular nanoplates, and nanowires and concluded that the nanospheres have the highest enhancement, the nanowires the middle enhancement, and the nanoplates the weakest enhancement at 514.5 nm excitation.17a However, they only considered the chemical effect and neglected the electromagnetic effect toward the enhancement. In our previous study, it was found that silver nanoplate aggregates induced by Cl- can produce strong Raman scattering enhancement and the obtained EF can reach as large as 4.5 × 105 at 1064 nm excitation.17b In fact, the SERS property of the silver nanoplates is dependent on the excitation wavelengths, and the electromagnetic enhancement can be enlarged by coupling of LSP of the silver nanoplates to SPP of smooth macroscopic gold or silver. Therefore, it was expected that the silver nanoplates can enhance the Raman scattering of the selfassembled monolayers (SAMs) on a smooth surface of gold or silver. In this paper, SERS spectra of p-aminothiophenol (p-ATP) SAMs sandwiched in silver nanoplates and smooth macroscopic gold and silver substrates have been investigated, respectively, and compared with that of silver nanoplates deposited on glass. It was found that the EF values at the junctions are several times larger than that at the silver nanoplates deposited on glass, which is attributed to the strong electromagnetic coupling of the LSP of silver nanoplates and the SPP of the metal surface at the junctions, indicating the great potential to study molecular electronic devices and biology by SERS. 2. Experimental Section 2.1. Chemicals. AgNO3, sodium citrate, NaBH4, 3-aminopropertrimethysiliane (3-APTMS), and p-ATP were purchased from Sigma-Aldrich Chemical Co. and used without further purification. The other chemicals were all reagent grade. 2.2. Synthesis of Silver Nanoplates. Silver nanoplates were synthesized according to a modified procedure developed by our group.17b Briefly, silver seeds were first synthesized according to our previous paper.17b From transmission electron microscopy (TEM) measurements, the silver seeds were nearly spherical with a diameter of 8.5 ( 3.5 nm and one surface plasmon resonance (SPR) band at 392 nm. Then, silver nanoplates were prepared by enlarging these small silver seeds in the presence of citrate. The method is similar to the reference method; the only difference was the concentration of silver seed, in which 60 µL of silver seeds was used in this system. These nanoplates were purified by centrifugation (10 000 rpm, 10 min) after aging for 24 h. Then all solid portions were collected and redispersed into pure water. 2.3. Fabrication of Silver Nanoplates on Glass, Gold, and Silver Substrate. The silver nanoplates deposited on glass were
Wang et al. prepared by depositing 10 µL of silver nanoplate colloid solution on 3-APTMS silanized glass to obtain the stable and highly SERS active silver nanoplate substrate. The obtained silver nanoplates/glass substrate was immersed into 0.1 mM p-ATP ethanol solution for 24 h to form a density monolayer on silver nanoplates. The sandwich structures of silver nanoplates/SAMs/ gold and silver nanoplates/SAMs/silver were prepared according to the following procedure: first, the gold and silver were mechanically polished with R-Al2O3 and rinsed in pure water three times and, then, the smooth gold and silver surface could be obtained. Second, the p-ATP SAMs on gold and silver were obtained by immersing the as-prepared smooth gold and silver into 0.1 mM p-ATP ethanol solution for 24 h to form a density monolayer. After washing with ethanol and water consecutively, the SAMs/gold and SAMs/silver were soaked in silver nanoplate sol for 12 h to assemble a monolayer of silver nanoplates. The obtained samples were washed with water and ethanol consecutively and left to dry in air, and then subjected to field emission scanning electron microscopy (FE-SEM) and Raman analysis. 2.4. Characterization of the Silver Nanoplates. The morphology of the silver nanoplates in solution was characterized by a JEOL JEM 2010 transmission electron microscope (TEM) operating at an acceleration voltage of 200 kV. FE-SEM images of the silver nanoplates on indium tin oxide (ITO) glass, gold, and silver were obtained using an XL30, ESEM-FEG field emission scanning electron microscope (FEI Co.). Tapping-mode atomic force microscopy (AFM) was conducted on glass substrate with a SPI3800N microscope (Seiko Instruments, Inc.). UV-vis-NIR spectra were measured in colloid and on a quartz slip using a Cary 500 UV-vis-NIR spectrophotometer. 2.5. SERS Measurements. SERS spectra were recorded on a T64000 Raman spectrometer by Jobin-Yvon Horiba, equipped with a charge coupled device (CCD) detector cooled by liquid nitrogen and with an Ar+ ion laser, which gives an excitation line of 514.5 nm, and a Ti:sapphire laser pumped by a Spectra Physics Model 2017 Ar+ laser, which gives an excitation line of 794.4 nm. Fourier transform (FT) SERS was conducted on a Nicolet-960 FT-Raman spectrometer equipped with an InGaAs detector and a Nd/VO4 laser (1064 nm) as an excitation source. It must be noted that the laser spot and penetration depth of the focused laser beam used are ∼1 µm and ∼16.5 µm, respectively, for 514.5 and 794.4 nm excitations,18a,b and ∼100 µm and ∼180 µm, respectively, for 1064 nm excitation;18b,c all the spectra are calibrated by the photon band of a silicon wafer at 514.5 and 794.4 nm and by the standard spectrum of sulfur at 1064 nm. SERS measurements were carried out with a BX40 Olympus microscope (100× objective, NA ) 0.95) on a JobinYvon T64000 Raman spectrometer at 514.5 and 794.4 nm excitations, and FT-SERS measurement was made on the view stage without a microscope. The laser spot under the microscope was determined by the equation R ) 1.22λ/(NA), where λ is the wavelength of the laser and NA is the numerical aperture of the lens. Then it was obtained to be 0.66 and 1.02 µm at 514.5 and 794.4 nm, respectively, which are approximately 1 µm in the calculations. The laser spot under 1064 nm is determined according to the instrumental parameter provided by the corporation. The laser power on the sample was 10, 50, and 300 mW at 514.5, 794.4, and 1064 nm, respectively. The spectra were obtained at 30 s accumulation two times at 514.5 and 794.4 nm, respectively. FT-SERS were recorded by averaging 512 scans with 4 cm-1 resolution. All these experimental parameters are taken into account for calculations of the enhancement factors.
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Figure 1. Representative TEM image of silver nanoplates (A) and UV-vis-NIR spectra (B) of the silver nanoplates in colloid (a) and on quartz slips (b). The excitation wavelengths used are indicated in the figure.
3. Results and Discussion 3.1. Morphology and Optical Absorption of Silver Nanoplates. Figure 1A shows the representative TEM image of the silver nanoplates whose absorption spectra in colloid and deposited on quartz slip are displayed in Figure 1B. The silver nanoplates are triangular with an edge diameter of about 100 ( 23 nm as shown in Figure 1A. According to the AFM and FE-SEM measurements (not shown), the thickness of the silver nanoplates is 12 ( 3 nm and the aspect ratio obtained is about 9. The adsorption spectrum of the colloid silver nanoplates exhibits three peaks as indicated by curve a in Figure 1B: the peak around 332 nm is attributed to the out-of-plane quadrupole resonance of nanoplates, the shoulder peak around 596 nm is due to the out-of-plane dipole resonance of nanoplates, and the strong peak at 927 nm in the near-infrared region is due to inplane dipole plasmon resonance.19 It is worth noting that the absorption spectrum of silver nanoplates on quartz slips (curve b in Figure 1B) has slightly changed in that the peak at 927 nm which is assigned to the in-plane dipole plasmon resonance is broadened, which may be induced by coupling of the silver nanoplates. Because the resonance is expected to occur at a wavelength longer than that of the LSP of individual particles,10,20 the different wavelengths used to excite the Raman scattering are also indicated in Figure 1B to satisfy the resonance coupling of the excitation and the absorption. 3.2. Morphology of Silver Nanoplates on Glass and p-ATP SAMs-Modified Gold and Silver Substrates. p-ATP was chosen as the interconnected molecule based on its bifunctional properties, which can attach to a metal surface through the metal-S bond and bind to silver nanoplates due to the pendent -NH2 group. Because the silver nanoplates are protected by sodium citrate, it is assumed that silver nanoplates can be assembled on p-ATP SAM-modified metal to form the junction structure of silver nanoplates-molecule-smooth metal. Figure 2 shows the representative FE-SEM images of silver nanoplates deposited on ITO glass and p-ATP SAM-modified gold (silver). Obviously, silver nanoplates are distributed fairly uniformly on gold and silver substrates as shown in Figure 2B,C, while some aggregations can be observed on glass as shown in Figure 2A, which may be induced by the evaporation of solvent at the airwater interface.21 Note that some silver nanoparticles (occupying 10% total in all) can be observed on the glass surface because
the colloid solution containing 10% silver nanoparticles was dropwise deposited on the glass, while on the gold and silver surface few of the silver nanoparticles can be observed because of the selective adsorption of the SAMs to the silver nanoplates. High magnifications of the FE-SEM images further indicate that the silver nanoplates are triangular with an edge length of about 100 ( 23 nm and fairly uniformly distributed on the gold and silver substrates. The surface coverage of the silver nanoplates on glass, gold, and silver substrates can be obtained to be approximately 45, 35, and 33 particles/µm2, respectively. It should also be mentioned that the surface coverage of the silver nanoplates on the gold and silver surface is higher than that obtained for spherical silver nanoparticles on the same metal surface as reported by Kim et al.,13 which is presumed to derive from the weaker electrostatic repulsion of the silver nanoplates than that of spherical silver nanoparticles, further demonstrating the selective adsorption of the SAMs to the silver nanoplates. 3.3. SERS Measurements and Discussion. To better interpretate the SERS spectra of p-ATP on different substrates, the normal Raman (NR) spectra of p-ATP have been first determined at 514.5, 794.4, and 1064 nm excitation wavelengths, respectively, as shown in curves a of Figure 3. Four strong and medium-strong bands can be observed at 1591, 1179, 1084, and 465 cm-1 at all specific excitation wavelengths, which are assigned to the vibrations of νCC, 8a (a1), δCH, 9a (a1), νCS, 7a (a1) and γCCC, 7a (a1), respectively, and are barely dependent on the excitation wavelengths.9,10,13,22 The frequencies and assignments for p-ATP vibrations at the specific excitation wavelengths have been given in Table 1. SERS spectra of p-ATP on the silver nanoplates deposited on glass at the specific excitation wavelengths are investigated as shown in curves b of Figure 3 for comparing the enhancements of the junctions to the p-ATP SAMs. It can be seen clearly that the Raman spectra of p-ATP at silver nanoplates deposited on glass have been enhanced at all specific excitation wavelengths. Four strong bands at 1582, 1437, 1392, and 1144 cm-1 and two medium-strong bands at 1188 and 1078 cm-1 at 514.5 nm excitation can be observed clearly, which are assigned to the b2 and a1 vibration modes of p-ATP, respectively. Four b2 modes were enhanced based on the charge transfer resonance mechanism as reported by Osawa et al.22 Only the strong a1 modes at 1593, 1184, 1080, and 390 cm-1 can be observed at 794.4 and 1064 nm excitations, whereas the b2 modes are weakly
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Figure 2. FE-SEM images of silver nanoplates deposited on ITO glass (A), p-ATP SAMs/gold (B), and p-ATP SAMs/silver (C). Inset shows the high magnification of the silver nanoplates on different substrates.
Figure 3. Raman and SERS spectra of neat solid p-ATP (a), p-ATP on silver nanoplates/glass (b), silver nanoplates-gold junction (c), and silver nanoplates-silver junction (d) at (A) 514.5, (B) 794.4, and (C) 1064 nm excitations, respectively.
enhanced at the near-infrared excitation, which may be due to the lesser extent of contribution of the Herzberg-Teller term in the near-infrared than in the visible region,10 controlled by the charge transfer resonance properties of p-ATP on a metal surface. Except for the spectra feature, the different intensities of the Raman spectra of p-ATP on the silver nanoplates at the
specific excitation wavelengths can also be observed as indicated in curves b of Figure 3, which may derive from the different enhancement effects of the silver nanoplates to the probe molecule at different excitation wavelengths. Because resonance between the incident radiation and the electronic absorption maxima should contribute to greater SERS enhancement for the
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TABLE 1: Frequencies (cm-1) and Assignments for p-ATP Vibrations on Different Substrates at Different Excitation Wavelengthsa silver nanoplates/glass solid
514.5 nm
794.4 nm
1591 1493 1425 1369 1288 1179 1126 1084 465 a
silver nanoplates-Au
1064 nm
794.4 nm
1580
1581
1433 1390
1430
1187 1143 1079
1178 1142 1078
1180 1142 1080
389
389
1593 1582 1470 1437 1392 1188 1144 1078
1180
1184
1082
1080
397
389
silver nanoplates-Ag
514.5 nm
1064 nm
514.5 nm
794.4 nm
1064 nm
assignment
1591
1591
1430 1388
1396
1439 1396
1183 1142 1078
1173 1139 1077
1180 1142 1078
392
389
νCC, 8a (a1) νCC, 8b (b2) νCC + δCH, 19b (b2) δCH + νCC, 3 (b2) νCC + δCH, 14b (b2) νCH, 7a (a1) δCH, 9a (a1) δCH, 9b (b2) νCS, 7a (a1) γCCC, 7a (a1) δCS
1591 1575 1442
Assignments for the SERS spectra of p-ATP from refs 9, 10, 13, and 22.
TABLE 2: EF Values of Silver Nanoplates on Glass and Junctions at the Specific Excitation Wavelengths for 7a at 1080 cm-1 EF value substrate
514.5 nm
794.4 nm
silver nanoplates/glass silver nanoplates-gold silver nanoplates-silver
(1.0 ( 0.5) × 10 (7.1 ( 3.3) × 104 (9.1 ( 4.3) × 104 4
nanostructures than those without appreciable absorption at the specific excitation wavelengths,12 and the resonance is expected to occur at a wavelength longer than that of the LSP of individual particles,10,20 the enhancement of silver nanoplates to p-ATP at 1064 nm is expected to be greater than that at 514.5 and 794.4 nm, which are in accordance with the optical properties of the maximal absorption of the silver nanoplates as shown in Figure 1B. To more quantitatively compare the enhancement effect of the silver nanoplates deposited on glass to the probe molecule at the specific excitation wavelengths, EF values have been estimated according to the literature,18a,23 and can be acquired via eq 1:
EF )
ISERS/Nads Ibulk/Nbulk
(1)
where ISERS is the intensity of a vibrational mode in the SERS spectrum of p-ATP and Ibulk is the intensity for the same vibrational mode in the NR spectrum from the solution phase or solid sample, both of which can be obtained via experiments. In this paper, νCS (7a1) at ∼1078 cm-1 was selected to determine the EF values because the enhancements to a1 vibration mode are mainly from the electromagnetic field effect. Nbulk is the molecule number of the solid p-ATP in the laser illumination volume, which can be determined via the laser spot and the penetration depth. According to the experiment condition, Nbulk can be calculated to be 7.4 × 1010 for 514.5 and 794.4 nm excitations and 1.8 × 1015 for 1064 nm excitation. Nads is the number of surface adsorbed p-ATP molecules within the laser spot, which can be estimated according to the method proposed by Murphy and co-workers:12
Nads ) NdAlaserAN/σ
(2)
where Nd is the number density of the silver nanoplates, Alaser is the area of the focal spot of the laser, AN is the silver nanoplate footprint area, and σ is the surface area occupied by an adsorbed p-ATP molecule (each p-ATP molecule occupies ∼0.20 nm2 on full coverage of Au and Ag assuming p-ATP molecules are adsorbed vertically on Au and Ag surface through a metal-S
1064 nm
(6.5 ( 3.0) × (5.0 ( 2.3) × 104 (2.7 ( 1.2) × 104 103
(2.4 ( 1.0) × 105 (7.7 ( 3.6) × 105 (9.4 ( 4.3) × 105
bond, indicating that σ can be adopted as ∼0.20 nm2/ molecule12,13). Nd and AN can be achieved from the FE-SEM images in Figure 2A, and Alaser can be obtained from the diameter of the laser spot (∼1 µm for 514.5 and 794.4 nm and ∼100 µm for 1064 nm).18 Then the Nads on the silver nanoplates deposited on glass can be calculated to be 7.6 × 105 for 514.5 and 794.4 nm excitations and 7.6 × 109 for 1064 nm excitation. Considering the fact that the intensity ratios of νCS at ∼1078 cm-1 in Figure 3 (curves a and b) were measured to be 0.1, 0.06, and 0.2 at 514.5, 794.4, and 1064 nm excitations, respectively, and taking into account the distrubutions of the sizes and the coverage, the EF values can be estimated to be (1.0 ( 0.5) × 104, (6.5 ( 3.0) × 103 and (2.4 ( 1.0) × 105 on silver nanoplates deposited on glass for 514.5, 794.4, and 1064 nm excitations, respectively, as shown in Table 2. It can be observed clearly that EF values at 1064 nm are higher than those at 514.5 nm, which are higher than those at 794.4 nm excitation, in accordance with the optical properties of the maximal absorption of the silver nanoplates as shown in Figure 1B. It should also be noted that the enhancement of the silver nanoplates on glass to p-ATP is mainly from the surface plasmon resonance of silver nanoplates and slightly from the localized surface plasmon resonance (LSP) of the interparticles. Raman spectra of p-ATP on the junctions of silver nanoplatesgold and silver nanoplates-silver at the specific excitation wavelengths are shown in curves c and d of Figure 3, respectively, and a large enhancement can be observed on these junctions. Considering the fact that there are weak Raman signals of p-ATP detected on the smooth gold and silver substrates, the enhancement must be SERS spectra, which are assumed to derive from the electromagnetic coupling of the LSP of the silver nanoplates to the SPP of the underneath gold or silver. The spectra features are similar to that at the silver nanoplates deposited on glass, whereas the intensity of the Raman spectra is much larger than that at the silver nanoplates deposited on glass. Therefore, the EF values on the junctions of silver nanoplates-gold and silver nanoplates-silver have also been estimated according to the similar method mentioned above and indicated in Table 2. Comparing the EF values on the different substrates, it can be seen clearly that the enhancement
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Figure 4. SERS spectra of p-ATP on electrochemically roughened gold (a) and silver (b) at (A) 514.5, (B) 794.4, and (C) 1064 nm excitations, respectively.
of the junctions to the coupling molecule is 3-9 times larger than that obtained at silver nanoplates deposited on glass, further indicating the strong electromagnetic coupling of the LSP of the silver nanoplates to the SPP of the metal. It should also be noted that Raman spectra of p-ATP on silver nanoplates/glass and in silver nanoplates/p-ATP/glass were compared, and because there is no electromagnetic coupling of silver nanoplates to glass, the Raman intensity of p-ATP is very weak. Raman spectra of coupling molecule 3-APTMS were also conducted on Ag nanoplates/3APTMS/glass and a weak Raman signal can be observed, further proving the absence of the coupling effect between silver nanoplates and glass. Noting that the polarization of the excitation is perpendicular to the connecting axis of silver nanoplates and smooth Au or Ag surface, it is expected that it will produce a larger electronic field when the polarization of the excitation is parallel to the connecting axis.24 Therefore, the enhancement of Raman scattering of the molecules must derive from the coupling of the LSP to SPP rather than from the coupling of the LSP-LSP of the silver nanoplates due to the uniform distrubution of silver nanoplates on Au or Ag surface. For these reasons, the differences of the enhancement on glass, smooth Au, and smooth Ag are distinct and meaningful because there is no LSP-SPP coupling between silver nanoplates and glass. Another point to be raised is that EF values on the three different substrates at 1064 nm excitation are larger than those obtained at 514.5 and 794.4 nm, which is assumed to derive from the resonance coupling of the excitation and the optical absorption of silver nanoplates because the optical absorption at 927 nm is stronger than that at 332 and 596 nm (indicated in Figure 1B). From curves c and d in Figure 3, one can note that the LSP-SPP coupling effect on the same junction at the
specific excitation wavelengths is different, which is controlled by the optical property of silver nanoplates because the variation of intensity of the Raman spectra of p-ATP on the junctions with the specific excitation wavelengths is similar to that on silver nanoplates deposited on glass. Therefore, the coupling of the LSP-SPP on a certain metal with the excitation wavelengths is controlled by the LSP of the nanostructures. Due to the different SPP properties of gold and silver, it is expected that the LSP-SPP effect between silver nanoplates-gold and silver nanoplates-silver is also different at the specific excitation wavelengths. Comparing the Raman spectra of p-ATP on the junctions of silver nanoplates-gold and silver nanoplates-silver at the specific wavelengths as shown in curves c and d of Figure 3, one can clearly observe that the intensity of Raman spectra on silver nanoplates-silver at 514.5 nm is better than that on silver nanoplates-gold, whereas at 794.4 nm it is reversed. This demonstrates that the electromagnetic coupling of silver nanoplates-silver is more effective at 514.5 nm, while silver nanoplates-gold coupling is more effective at 794.4 nm, further confirmed by the EF values indicated in Table 2. In contrast, the difference in the coupling effect at 1064 nm for silver nanoplates-gold and silver nanoplates-silver is weak as indicated in curves c and d of Figure 3C, which is also presumed to be controlled by the SPP of the metal surface. As reported by Schatz et al.,4,25 the imaginary part of the gold dielectric constant above 2.0 eV (about 620 nm) is rapid raised due to interband transitions, demonstrating that it is difficult to induce the Raman scattering enhancement on gold at wavelengths below 620 nm. Therefore, gold and silver have different dependences on the excitation wavelengths. Based on the consideration of electrodynamic resonance excitation,26 the present wavelength of 514.5 nm is more favorable for silver
SAMs of p-ATP on Gold and Silver Surfaces than for gold, whereas 794.4 nm is the reverse, which can be proved by the SERS spectra of p-ATP on the electrochemically roughened gold and silver surface under the same conditions at the specific excitation wavelengths as shown in Figure 4. Because the surface roughnesses of the gold surface and silver surface are similar, the intensity of SERS spectra of p-ATP on the surface is mainly controlled by the SPP of the metal. It can be observed clearly that the Raman scattering of p-ATP on the roughened silver has been enhanced more than that of gold at 514.5 nm and less than that at 794.4 nm. As for the excitation at 1064 nm, it is strange for the Raman enhancement at gold and silver substrates because silver is also sensitive to excitation at the near-infrared as shown in curve d of Figure 3C and curve b of Figure 4C. A detailed comparison of the enhancements for gold and silver at 1064 nm excitation needs to calculate the energy dissipation (damping) via the optical constants,27 which is left for another paper. Nevertheless, our results here clearly indicate that Raman scattering of coupling molecules can be enhanced greatly at 1064 nm excitation under the resonance coupling of the excitation and the optical absorption of silver nanoplates as well as the electromagnetic coupling of the LSP of silver nanoplates and SPP of the metal. 4. Conclusions The present investigation provides evidence that silver nanoplates-gold and silver nanoplates-silver junctions have distinct enhancement to the interconnected molecule at the specific excitation wavelengths, which was assumed to be from the strong electromagnetic coupling between the LSP of silver nanoplates and the SPP of the smooth macroscopic gold or silver, so-called LSP-SPP coupling. The EF values on the junctions are estimated to be as large as 104-105, which is 3-9 times larger than that on silver nanoplates deposited on glass. The coupling of the LSP-SPP on a certain metal with the excitation wavelengths is controlled by the LSP of the nanostructures, whereas on different metal surfaces it is controlled by the SPP of the metal, which may be dominated by the electrodynamic resonance excitations, confirmed by the SERS spectra of p-ATP on electrochemically roughened gold and silver surfaces at specific excitation wavelengths. Acknowledgment. The work was supported by the National Natural Science Foundation of China (Nos. 20575064 and 20427003). References and Notes (1) Fleischmann, M.; Hendra, P. J.; Mcquillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 1, 84. (3) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215. (4) Schatz, G. C. Acc. Chem. Res. 1984, 17, 370. (5) (a) Kahl, M.; Voges, E. Phys. ReV. B 2000, 61, 14078. (b) Shalaev, V. M.; Sarychev, A. K. Phys. ReV. B 1998, 57, 13265. (c) Moskovits, M.;
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