Characteristics of Nanogaps Formed by Planar Au and Pt

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Characteristics of Nanogaps Formed by Planar Au and Pt Nanoparticles Revealed by Raman Spectroscopy Kwan Kim,*,† Hyang Bong Lee,† 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: Raman peaks are barely detectable for molecules present on macroscopically smooth metal substrates. Raman peaks can be observed, however, by allowing Ag or Au nanoparticles to overlie the adsorbate molecules. For instance, no Raman peaks are detected for 4-aminobenzenethiol (4-ABT) on a flat Au substrate. Upon attaching Ag nanoparticles onto 4-ABT, however, Raman peaks are distinctly observed, not only with excitation at 632.8 nm but also with excitation at 514.5 nm, though the peaks at 632.8 nm are several tens of times more intense than those at 514.5 nm. When Au nanoparticles are attached, Raman peaks are negligibly observable with the excitation at 514.5 nm, though distinctly observed with the excitation at 632.8 nm. Interestingly, Raman peaks are observable even at 514.5 nm by attaching Pt nanoparticles onto 4-ABT on a flat Ag substrate to form Pt@4-ABT/Ag(flat). The larger the Pt nanoparticle is, the greater the electromagnetic field induced in the nanogap. Much the same observation is made for the Pt@4-ABT/Au(flat) system. However, the excitation wavelength dependence is opposite such that the higher intensity is observed in the order of excitations 632.8 > 568 > 514.5 > 488 nm for the Pt@4-ABT/Au(flat) system, while in the order of excitations 488 > 514.5 > 568 > 632.8 nm for the Pt@4-ABT/Ag(flat) system. The maximum enhancement factor (EF) at 632.8 nm excitation in the Pt@4-ABT/Au(flat) system is nonetheless only about one-half of the EF value observable at 632.8 nm excitation in the Pt@4-ABT/Ag(flat) system, suggesting that the planar Ag substrate should be far more effective than the planar Au substrate for the induction of SERS by virtue of Pt nanoparticles overlaid thereon.

1. INTRODUCTION Noble metallic nanostructures exhibit a phenomenon known as surface-enhanced Raman scattering (SERS) in which the scattering cross sections are dramatically enhanced for molecules adsorbed onto them.1 7 In recent years, it has been reported that even single molecule spectroscopy is possible by SERS, suggesting that the enhancement factor can reach as much as 1014 1015.3 5 According to theoretical studies, at least 8 10 orders of magnitude can arise from electromagnetic surface plasmon excitation, separately from a charge transfer mediated mechanism.7,8 Especially in conjunction with single-molecule SERS, an electromagnetic “hot spot” has been predicted to exist in large fractal aggregates of Ag particles. The junction of two aggregated Ag nanoparticles has also been claimed to be the “hot” site for SERS.11 As two particles approach each other, their transition dipoles are coupled in such a way that the enhanced electromagnetic fields around each particle can create a pattern of coherent interference.9 11 This implies that as the distance between the nanoparticles decreases the coupled plasmon resonance shifts to red; the enhanced electromagnetic field increases in the junction between the particles; and destructive interference of the fields occurs at other points in space. We recently found that high intensity Raman spectra could be obtained for organic adsorbates sandwiched between a planar Au substrate and a nanosized Ag or Au particle.12 14 The SERS signal must have derived from the electromagnetic coupling of r 2011 American Chemical Society

the localized surface plasmon of the Ag or Au nanoparticle with the surface plasmon polariton of the underlying Au metal under the illumination of a visible light.15 17 The coupling became more effective by increasing the size of Ag or Au nanoparticles at least with dimensions of under ∼100 nm, irrespective of the excitation wavelength. A slightly different observation was made concerning the effect of the kind of metal nanoparticle. When a Ag nanoparticle was used, the highest Raman signal was measured with the excitation at 568 nm, slightly larger than that at the 632.8 nm excitation. The Raman signals measured at 514.5 and 488 nm excitation were an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time-domain simulation. When a Au nanoparticle was used, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation. One critically weak point of SERS is that only the three coinage metals (Au, Ag, and Cu) and a few alkali metals (Na, K, and Li) can provide a large enhancement, which severely limits wider applications involving other metallic materials of both fundamental and practical importance such as Pt.18 20 Nevertheless, Ikeda et al. recently reported that significant enhancement of SERS intensities Received: July 29, 2011 Revised: September 15, 2011 Published: October 07, 2011 21047

dx.doi.org/10.1021/jp207247v | J. Phys. Chem. C 2011, 115, 21047–21055

The Journal of Physical Chemistry C Scheme 1. Schematic Diagram of a Nanogap, Formed by a Planar Au Substrate and a Pt Nanoparticle, in Which a Probe Molecule, 4-Aminobenzenethiol, Is Bound via the Thiolate Sulfur and Amine Group to Au and Pt, Respectively

of organic monolayers on a non-SERS-active substrate could be observed by means of gap-mode plasmonon excitation for the 4-chlorophenyl-isocyanide monolayer on Pt or Pd surfaces.21,22 We also found that very intense SERS spectra could be obtained even from adsorbates on flat Pt by overlaying Ag nanoparticles onto them.23 To our surprise, more intense Raman spectra were measured by overlaying Ag nanoparticles onto adsorbates assembled on a flat Pt substrate than on a flat Au substrate, at least under illumination by a short-wavelength laser at 488 and 514.5 nm. Then, how about the opposite case in which Pt nanoparticles were overlaid on a flat Ag substrate? Raman peaks could barely be identified for adsorbates assembled on a flat Ag substrate, but Raman peaks were distinctly detected by attaching Pt nanoparticles onto them. On one hand, a higher Raman signal was observed when larger Pt nanoparticles were attached, regardless of the excitation wavelength. On the other hand, a higher Raman signal was measured in response to the excitation at 488 nm, followed by the excitation at 514.5, 568, and 632.8 nm. This work is designed to examine the electromagnetic interaction of platinum with gold in a system composed of either a planar Au substrate and a Pt nanoparticle or a planar Pt substrate and an Au nanoparticle (see Scheme 1). The purpose of this work is to make a comparison between the electromagnetic interactions of Pt with Au and Ag. Gold and silver are two typical noble metals that can be used themselves, after a proper roughening process, as highly efficient SERS substrates. Platinum itself is not effective as a SERS substrate, but Pt is unequivocally useful in modern science and technology, especially in the area of catalysis, so that finding a couple able to induce a higher electromagnetic field on Pt is highly desirable. In this work, the Ag and Pt couple clearly turned out to be more effective than the Au and Pt couple at least in the ability to induce SERS for molecules situated in their nanogaps.

2. EXPERIMENTAL SECTION Gold foil (0.1 mm thick, 99.9%), silver foil (0.25 mm thick, 99.9%), platinum foil (0.1 mm thick, 99.99%), chloroauric acid trihydrate (HAuCl4 3 3H2O, 99.99%), silver nitrate (AgNO3, 99.999%), chloroplatinic acid hexahydrate (H2PtCl6 3 6H2O,

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37.50% Pt basis), sodium citrate (C6H5Na3O7 3 2H2O, 99.9%), sodium borohydride (NaBH4, 99%), L-ascorbic acid (C6H8O6, 99%), and 4-aminobenzenethiol (4-ABT, 97%) were purchased from Aldrich and used as received. All other chemicals used were reagent grade unless otherwise specified. Triply distilled water with a resistivity greater than 18.0 MΩ 3 cm (Millipore Milli-Q System) was used to prepare all aqueous solutions. As macroscopically smooth metal substrates, the Au, Ag, and Pt foils were polished to a mirror finish using 0.05 μm alumina powders. The polished foils were sonicated twice in water and then washed thoroughly with ethanol and cut into 0.8 1.0 cm2 pieces. The self-assembly of 4-ABT on Au, Ag, and Pt substrates was conducted in 1 mM ethanolic solution for 12 h. The metal substrates covered with a self-assembled monolayer of 4-ABT molecules were then washed thoroughly with ethanol to remove any physisorbed or excess 4-ABT molecules that might be left on metal substrates. Platinum sols were prepared according to the seed-mediated growth method reported by Bigall et al.24 Initially, small platinum seeds with a diameter of 5 7 nm were prepared. To accomplish this, 3 mL of a 0.2% solution of H2PtCl6 3 6H2O was added to 39 mL of boiling deionized water. After 1 min, 0.92 mL of 1% sodium citrate was added, followed by rapid injection of 0.46 mL of freshly prepared 0.08% solution of sodium borohydride containing 1% sodium citrate 30 s later. After 10 min, the sol solution was cooled to room temperature. The seeds obtained in this way were then used to prepare 26 nm Pt particles. Specifically, 1 mL of the platinum seed solution and 0.045 mL of 0.4 M H2PtCl6 solution were added consecutively to 30 mL of deionized water, after which 0.5 mL of 1.25% L-ascorbic acid solution containing 1% sodium citrate was added. The mixture was then slowly heated to the boiling point, after which it was allowed to boil for 30 min with stirring. TEM analysis revealed that the size of the Pt nanoparticles was 26 nm (vide infra). These 26 nm Pt particles were used as seeds to prepare 48 nm Pt particles. The reaction mixture for the 48 nm Pt particles was comprised of 26 mL of deionized water, 1 mL of 26 nm Pt solution, 0.045 mL of 0.4 M H2PtCl6, and a 0.5 mL mixture of 1% sodium citrate and 1.25% L-ascorbic acid. To prepare 68 (or 106 or 145) nm Pt particles, a reaction mixture composed of 29 mL of deionized water, 1 (or 0.25 or 0.25) mL of 48 (or 68 or 106) nm Pt solution, 0.045 mL of 0.4 M H2PtCl6, and a 0.5 mL mixture of 1% sodium citrate and 1.25% L-ascorbic acid was used. As before, the reaction mixture was boiled for 30 min with stirring, after which it was cooled to room temperature. The reaction products were then gathered by centrifugation at 1000 13 500 rpm depending on the nanoparticle size, after which they were washed three times with deionized water and then stored by dispersal in water. The Au and Ag sols were prepared by following the recipes of Lee and Meisel.25 Initially, approximately 100 mL of silver nitrate solution containing 17 mg of AgNO3 was brought to a boil. A solution of 1% sodium citrate (2 mL) was then added to the AgNO3 solution under vigorous stirring, after which boiling was continued for about 30 min. For Au sol, 100 mL of HAuCl4 (0.75 mM) solution and 4 mL of 1% (w/v) sodium citrate were used to obtain Au nanoparticles with sizes comparable to those of the Ag nanoparticles. The Au and Ag sols prepared by these processes were stable for several weeks, and both the Au and Ag nanoparticles were of nearly spherical shape with an average diameter of approximately 60 nm (vide infra). To attach Pt nanoparticles onto the pendent NH2 groups of 4-ABT on Au (or Ag), the 4-ABT-adsorbed Au (or Ag) substrate was soaked in Pt sol for 1 h. On the other hand, to attach Au 21048

dx.doi.org/10.1021/jp207247v |J. Phys. Chem. C 2011, 115, 21047–21055

The Journal of Physical Chemistry C (or Ag) nanoparticles onto the pendent NH2 groups of 4-ABT on Pt, the 4-ABT-adsorbed Pt substrate was soaked in Au or Ag sol for 1 h. Hereafter, the nanogap systems fabricated by these processes are referred to as Pt@4-ABT/Au(flat) or Pt@4-ABT/ Ag(flat) and Au@4-ABT/Pt(flat) or Ag@4-ABT/Pt(flat), respectively. After washing with water and ethanol consecutively, these nanogap systems were left to dry under vacuum for 2 h and then subjected to Raman spectral analyses. UV/vis spectra were obtained using a SCINCO S-4100 spectrometer. Transmission electron microscopy (TEM) images were acquired using a JEM-200CX transmission electron microscope at 160 kV. X-ray diffraction (XRD) of Pt nanoparticles was conducted using a Philips X’PERT-MPD diffractometer for a 2θ range of 5 80° at an angular resolution of 0.02° using Cu Kα (1.5419 Å) radiation. Field emission scanning electron microscopy (FE-SEM) images were obtained using a JSM-6700F field emission scanning electron microscope operating at 5.0 kV. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 488 and 514.5 nm lines from a 20 mW Ar+ laser (Melles-Griot model 351MA520), the 568 nm line from a 20 mW Ar+/Kr+ laser (Melles-Griot model 35KAP431), or the 632.8 nm line from a 17 mW He/Ne laser (Spectra Physics model 127) were used as the excitation source. Raman scattering was detected over 180° using a Peltier-cooled ( 70 °C) chargecoupled device (CCD) camera (400 600 pixels). The laser beam was focused onto a spot approximately 1 μm in diameter with an objective microscope at a magnification of 20. The data acquisition time was usually 30 s. The holographic grating (1800 grooves/mm) and the slit allowed the spectral resolution to be 1 cm 1. The Raman band of a silicon wafer at 520 cm 1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm 1. A three-dimensional finite-difference time-domain (3DFDTD) electrodynamics simulation was conducted using FDTD solutions (version 7.0.1) software provided by Lumerical Solutions, Inc.26 A nanostructure composed of a Pt nanoparticle and a flat gold surface was modeled as a single Pt sphere (20 150 nm) laid on a cubical gold substrate with dimensions of 400 nm 400 nm 150 nm. The gap between the sphere and the cuboid was fixed to be 1 nm. The dielectric constants of the platinum and gold were taken from the source program. The propagation directions of the plane waves (488 or 514.5 or 568 or 632.8 nm wavelength) were selected to be along the x- or z-axis. In each case, the electrical field was assumed to be polarized along the zaxis and the y-axis, respectively. Boundary conditions were imposed using the perfectly matched layer method. After computation of the local electrical field, the field intensity was evaluated for each mesh by integration and finally compared with the enhancement factor (EF) values estimated from the measured Raman spectra.

3. RESULTS AND DISCUSSION Characteristics of Pt, Ag, and Au Nanoparticles. The TEM images of Ag, Au, and Pt sol particles prepared in this work are shown in Figure 1(a). Most of them are spherical in shape. According to the histograms, the mean diameters of the Ag and Au nanoparticles are determined to be 62 ( 15 and 62 ( 10 nm, respectively. As shown in Figure 1(b), these Ag and Au particles exhibit very distinct surface plasmon absorption bands at 425 and 534 nm, respectively. On the other hand, the average sizes of the

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Figure 1. (a) TEM images of Ag, Au, and Pt sol particles (scale bar = 200 nm) and (b) their UV/vis extinction spectra in colloidal state. Magnified TEM images of 68 and 145 nm Pt nanoparticles are shown in the insets of (a).

Figure 2. FE-SEM images of Pt particles adsorbed on 4-ABT for 1 h on flat Au substrates: (a) 26, (b) 68, (c) 106, and (d) 145 nm sized Pt particles (scale bar = 500 nm).

four different Pt particles are determined to be 26 ( 2, 68 ( 10, 106 ( 12, and 145 ( 15 nm. According to the UV/vis spectra, the peak maximum associated with the surface plasmon resonance is red-shifted from 250 to 630 nm almost linearly in proportion to the size of Pt nanoparticles from 26 to 145 nm. SERS Characteristics of Pt@4-ABT/Au(flat) versus Pt@4-ABT/ Ag(flat). The polished Au and Ag foils are negligibly SERS active; therefore, no Raman peaks are detected when 4-ABT is selfassembled on these foils (see Figure 3(a)). As reported previously, Raman peaks are observable when Ag or Au nanoparticles are allowed to adsorb on the pendent NH2 groups of 4-ABT.15 17 This is not surprising since Ag and Au are noble metals both known to be highly SERS active. It will be favorable for these particles and substrates to couple electromagnetically, especially in the visible 21049


The Journal of Physical Chemistry C

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Figure 3. Raman spectra of (a) 4-ABT/Au(flat) and Pt@4-ABT/Au(flat), comprised of (b) 26 or (c) 68 or (d) 106 or (e) 145 nm diameter Pt particles, taken using 488 or 514.5 or 568 or 632.8 nm radiation as the excitation source (from bottom to top); the SERS intensities were the average of five different measurements. All spectra were taken while spinning at 3000 rpm to minimize the effect of inhomogeneous distribution of Pt nanoparticles, and all spectral intensities were normalized with respect to those of silicon wafers used for instrument calibration and also to the number of Pt nanoparticles adsorbed on 4-ABT on Au.

region. A surprising finding in our previous work was that even Pt nanoparticles could induce SERS for 4-ABT assembled on a planar Ag substrate.23 We found in this work that Pt nanoparticles can even induce SERS for 4-ABT assembled on a planar Au substrate. The excitation wavelength dependence of the SERS induction in Pt@4ABT/Au(flat) is, however, completely different from that in Pt@4ABT/Ag(flat). Figure 2 shows the FE-SEM images exhibiting the distribution of Pt particles adsorbed on 4-ABT on flat Au substrates. The average surface coverage computed for the adsorption of 26, 68, 106, and 145 nm Pt particles is 59.4, 6.15, 2.0, and 0.74 particles per μm2, respectively. Accordingly, the coverage is 4.0%, 2.8%, 2.2%, and 1.5%, respectively. Even when the coverage is 4.0%, the interparticle distance is more than three times larger than the Pt particles. Although not shown here, only featureless Raman spectra were obtained when Pt particles of 26 nm in size were attached to the 4-aminophenylsilane monolayers assembled on a silicon wafer. This indicates that Pt particles adsorbed onto amino groups neither are hot particles nor contribute to hot clusters in exhibiting electromagnetic enhancement. The interparticle interaction among Pt nanoparticles should then be far smaller than the interaction between a flat Au surface and Pt nanoparticles. On this basis, we measured the Raman spectra while spinning the Pt@4-ABT/Au(flat) samples at 3000 rpm to minimize any effect owing to the inhomogeneous distribution of the Pt nanoparticles. Figure 3 shows a series of Raman spectra taken for the Pt@4ABT/Au(flat) system comprised of 26 or 68 or 106 or 145 nm Pt particles, using 488 or 514.5 or 568 or 632.8 nm radiation as the

excitation source. All the spectra were normalized with respect to the absolute intensity of a silicon wafer at 520 cm 1, as well as to the number of Pt particles attached to 4-ABT. The Raman scattering intensity clearly depended on the size of the Pt nanoparticles, as well as on the excitation wavelength. In all cases, the highest intensity was observed at 632.8 nm excitation. Conversely, more intense Raman signals were obtained as the larger Pt particles were laid on the amine groups of 4-ABT. Accordingly, at 632.8 nm excitation, the 7a band of 4-ABT at ∼1080 cm 1, induced by the 145 nm Pt particles, was several hundreds of times stronger than that induced by the 26 nm Pt particles. One may argue that the number of 4-ABT molecules hidden below a 145 nm Pt particle is considerably larger than those below a 26 nm Pt particle, but it cannot be larger by more than 30 times. The electromagnetic coupling between a smooth Au substrate and a spherical Pt nanoparticle must then become stronger as the size of the Pt particle is increased. As reported previously, for the Pt@4-ABT/Ag(flat) system, more intense Raman signals were obtained as larger Pt particles were laid on the amine groups of 4-ABT,23 as in the Pt@4-ABT/ Au(flat) system. However, the highest intensity was observed at the 488 nm excitation rather than at the 632.8 nm excitation. The excitation wavelength dependence is thus completely opposite in the two systems. It appears that the surface plasmon polaritons of the planar Au and Ag substrates govern the excitation wavelength dependence more than the surface plasmons localized on Pt nanoparticles. For a more quantitative comparison, we have estimated the EF values per Pt nanoparticle as a function of the excitation wavelength, as well as the size of Pt nanoparticles. 21050



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Table 1. Experimental EFs per Pt nanoparticle in Pt@4ABT/Au(flat) and Pt@4-ABT/Ag(flat) Systems, As Well As Experimental EFs per Au or Ag Nanoparticle in the 60Au@4ABT/Pt(flat) or 60Ag@4-ABT/Pt(flat) System, Determined As a Function of Excitation Wavelength 488 nm

514.5 nm

568 nm

632.8 nm

20Pt@4-ABT/Au

-

60Pt@4-ABT/Au 100Pt@4-ABT/Au

-

-

3

13

18

45 77

75 2.0 102

150Pt@4-ABT/Au

-

65

2.7 102

9.7 102

20Pt@4-ABT/Ag

21

8

7

2

60Pt@4-ABT/Ag

8.1 102

2.8 102

2.6 102

83

100Pt@4-ABT/Ag

2.5 103

9.3 102

7.7 102

2.3 102

150Pt@4-ABT/Ag

3.5 10

1.4 10

4

1.0 10

2.3 103

4

4

60Au@4-ABT/Pt

-

23

92

1.7 102

60Ag@4-ABT/Pt

1.1 103

7.9 102

5.5 102

2.0 102

The EF values were estimated by comparing the SERS and normal Raman (NR) intensities of the 7a band of 4-ABT at ∼1080 cm 1. Initially, the number of 4-ABT molecules illuminated by the laser light used to obtain the NR spectrum was estimated by taking into account the facts that the size of the laser spot was ∼1 μm, its penetration depth into 4-ABT was ∼40 μm, and the density of 4-ABT in the solid state was 1.18 g/cm3:13 see the NR spectrum of 4-ABT in the bottom of Figure 8. Assuming that the sample volume was well represented by the product of the laser spot size and its penetration depth, the number of 4-ABT molecules illuminated was calculated to be 1.8 1011 (i.e., 3.0 10 13 mol). We subsequently estimated the number of 4-ABT molecules that could be present on the planar Au substrates. Invoking the fact that each 4-ABT molecule occupies an area of ∼0.20 nm2 at the full coverage limit, the number of 4-ABT molecules that can be illuminated by a 1 μm laser beam was calculated to be 3.9 106 (i.e., 6.5 10 18 mol).27 The ratio of the numbers of 4-ABT molecules illuminated in NR and SERS spectral measurements would then amount to 4.6 104 (i.e., 1.8 1011/3.9 106). The apparent EF values may then be obtained by dividing the latter value by the intensity ratio of the 7a band of 4-ABT in the NR and SERS spectra. Dividing further, the apparent EF values with the averaged number of Pt nanoparticles per μm2 on 4-ABT on a planar Au should result in EF values per Pt nanoparticle for the Pt@4-ABT/ Au(flat) systems. The EFs per Pt nanoparticle estimated in this way are collectively summarized in Table 1 (see also Figure 4(a)). It is evident that a higher EF is obtained with an increase in the size of Pt nanoparticles, as well as an increase in the excitation wavelength from 488 to 514.5 to 568 to 632.8 nm. Accordingly, the highest EF, ∼9.7 102, was obtained for the system composed of 145 nm sized Pt nanoparticle, (i.e., 145Pt@4-ABT/Au(flat)) at an excitation of 632.8 nm. The EF at 488 nm was more than 2 orders of magnitude smaller compared to that at 632.8 nm. For comparison, the EFs per Pt nanoparticle estimated for the Pt@4-ABT/Ag(flat) systems are also shown in Table 1: the previously reported values were separately confirmed in this work. First of all, the EF at 632.8 nm is seen to be an order of magnitude smaller than that at 488 nm. This obviously contrasts with what is observed in the Pt@4ABT/Au(flat) system. The surface plasmon polariton of the bottom substrate seems to be critically important for the induction of an enhanced electromagnetic (EM) field in the visible region by coupling with the surface plasmons localized on Pt nanoparticles. The planar Au substrate must favor an EM enhancement in the

Figure 4. (a) Experimental and (b) theoretical EFs per Pt nanoparticle in Pt@4-ABT/Au(flat) drawn as a function of the excitation wavelength as well as the size of Pt nanoparticles ((A) 26, (B) 68, (C) 106, (D) 145 nm). Experimental EFs were determined by referring to the SERS spectra shown in Figure 3. Theoretical EFs were predicted using the 3DFDTD method by taking into account the portions of the perpendicular and parallel polarizations under an actual experimental condition.

comparatively longer wavelength region, while the planar Ag substrate favors an EM enhancement in the shorter wavelength region.23 Another noteworthy point is that the maximum EF, ∼9.7 102, occurring at an excitation of 632.8 nm for the Pt@4ABT/Au(flat) system is about a quarter of the highest EF, ∼3.5 104, occurring at an excitation of 488 nm for the Pt@4-ABT/ Ag(flat) system. Furthermore, even at an excitation of 632.8 nm the EF in the 145Pt@4-ABT/Ag(flat) system is as much as 2.3 103, which is still more than 2 times larger than the maximum EF in the 145Pt@4-ABT/Au(flat) system. All these observations suggest that the planar Ag substrate must be more effective and have a potentially wider range of applications than the planar Au substrate for the induction of SERS by virtue of Pt nanoparticles overlaid thereon within the excitation wavelength range from 488 to 632.8 nm. FDTD Calculation. The FDTD method is an explicit time marching algorithm used to solve Maxwell’s curl equations on a discrete spatial grid.28 As described in the Experimental Section, we modeled a nanostructure composed of a single platinum sphere with a diameter of 20 150 nm on a cubical gold substrate of dimensions 400 nm 400 nm 150 nm. The gap distance between the Pt sphere and the Au cuboid was fixed at 1 nm. The intensity of the induced electrical field was extremely sensitive to the polarization direction of the incident radiation. With perpendicular polarization to the surface of the Au cuboid, a single maximum was located in the gap, but two maxima were present with parallel polarization. In either case, the “hot spot” was highly localized, so that most of the SERS signal must arise from a small area of the junction. First, the Raman scattering enhancement was assumed to scale as |E/Eo|4 in which E and Eo are the local and input fields, respectively.29 Moreover, to enable a better comparison with the experimental results, we computed the theoretical EF values by weighing the contribution of perpendicular and parallel polarized light in conformity with our experimental configuration.13 The EF values simulated in this way for 21051



Figure 5. (a) Schematic diagrams of (A) four 7 nm sized Pt particles in a tetrahedral shape with interparticle spacing of 0.1 nm or (B) one 11.1 nm-sized Pt particle, both of which are lying 1 nm above a flat Au cuboid. (b) |E/Eo|4 at the gap center computed under irradiation by perpendicularly polarized light, drawn as a function of the excitation wavelength.

the Pt@4-ABT/Au(flat) system are shown in Figure 4(b) as a function of the excitation wavelength and the size of the Pt nanoparticles. In agreement with the experiment, a higher EF value was computed for all excitations when larger Pt particles were laid on the flat Au substrate. For the 150 nm Pt, the highest EF value was computed under irradiation of 632.8 nm and then in the order of 568 > 514.5 > 488 nm, in agreement with the experiment. However, for smaller Pt particles, the higher EF values were computed when the wavelength of the incident radiation was 568 nm, after which they occurred in the order 632.8 > 514.5 > 488 nm. This contrasts with what was observed experimentally, in which higher EF values were obtained in the order 632.8 > 568 > 514.5 > 488 nm, irrespective of the size of the Pt particles. This discrepancy may be associated with the internal structure of the Pt nanoparticles. In an earlier study, XRD analysis revealed that Pt particles of 105 nm size appeared to consist of 7.2 nm seed particles.24,30 Hence, all Pt nanoparticles prepared here by a seedmediated growth method are also assumed to consist of smaller (∼7 nm in diameter) particles. The UV/vis absorption characteristics shown in Figure 1(b) would then be a result of electronic interactions between seed particles rather than a property of one homogeneous particle. In the FDTD calculation, we assumed that one bulky Pt particle lay on a planar Au substrate, which might not represent the actual situation. To determine what would happen if a bulky particle was actually comprised of smaller seed particles, we conducted an additional FDTD calculation by letting four 7 nm Pt seed particles (in a tetrahedral shape with interparticle spacing of 0.1 nm) lie 1 nm above a flat Au substrate (see Figure 5(a)). Figure 5(b) shows the |E/Eo|4 values computed at the gap center under irradiation by perpendicularly polarized light. Figure 5(b) also shows the EF

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Figure 6. (a) FE-SEM image (scale bar = 1 μm) and (b) Raman spectra of the 60Au@4-ABT/Pt(flat) system, taken using 488 or 514.5 or 568 or 632.8 nm radiation as the excitation source (from bottom to top); the SERS intensities were the average of five different measurements.

values computed using a single 11 nm Pt particle, where the volume of one 11 nm Pt particle is the same as the sum of the volumes of four 7 nm Pt seed particles. It is seen that for the fourparticle cluster the EF value at 632.8 nm is greater than that at 568 nm, while for the single Pt particle, the EF value at 632.8 nm is smaller than that at 568 nm. These findings suggest that at the longer excitation wavelengths the coupled electromagnetic interactions of individual Pt seed particles with an underlying Au substrate are collectively more effective than a single interaction between a bulky Pt particle and a flat Au substrate. In our earlier FDTD calculation of the Pt@4-ABT/Ag(flat) system, a higher Raman signal was computed in response to excitation at 488 nm, followed by excitation at 514.5, 568, and 632.8 nm, in agreement with experiment. One intriguing thing at that time was that the electromagnetic field enhancement observed was not in conformity with the UV/vis absorbance of ∼150 nm sized Pt nanoparticles, which suggested that higher EFs should be measured in the order of excitations at 632.8 > 568 > 514.5 > 488 nm. This discrepancy was understood by considering again that the 150 nm Pt particles were actually composed of 7 nm seed particles. A separate FDTD calculation suggested that the electromagnetic interactions of individual Pt seed particles with the underlying Ag substrate occurred collectively, being more effective at shorter excitation wavelengths.21 SERS Characteristics of Ag@4-ABT/Pt(flat) versus Au@4-ABT/ Pt(flat). To obtain further insight into the electromagnetic coupling between the localized surface plasmon of a metal nanoparticle and the surface plasmon polariton of a planar metal substrate, we also examined the SERS characteristics of the Au@4-ABT/Pt(flat) system, the opposite of the Pt@4-ABT/Au(flat) system.14 To see the effect of metal nanoparticle dependence, the SERS characteristics of the Au@4-ABT/Pt(flat) system were also compared with that of the Ag@4-ABT/Pt(flat) system. Figure 6(a) shows the FE-SEM image of the Au@4-ABT/ Pt(flat) system. The surface coverage of 62 nm sized Au nanoparticles 21052



Figure 7. Experimental EF (solid circles) and theoretical EF (open circles) values for the (a) 60Au@4-ABT/Pt(flat) and (b) 60Ag@4ABT/Pt(flat) systems.

adsorbed on the amine groups of 4-ABT is 7.1 particles per μm2. The interparticle distance is more than 4 times larger than the size of the Au nanoparticle so that any SERS peaks of 4-ABT must arise as a result of the Au nanoparticle-to-Pt(flat) substrate interaction. Figure 6(b) shows the Raman spectra of the Au@4-ABT/Pt(flat) system measured using 488 or 514.5 or 568 or 632.8 nm radiation as the excitation source. All the spectra were normalized with respect to the absolute intensity of a silicon wafer at 520 cm 1, as well as to the number of Au nanoparticles attached onto 4-ABT. No Raman signal was observed with the excitation at 488 nm. The Raman signal is observable at 514.5 nm, but still weak. The Raman intensity increases as the excitation wavelength is increased from 514.5 to 568 to 632.8 nm. The excitation wavelength dependence is evidently associated with the Au nanoparticle. The EF values per Au nanoparticle were determined to be 23, 92, and 168 corresponding to the excitation at 514.5, 568, and 632.8 nm, respectively. These values are also included in Table 1. It is noteworthy that the EFs of the 60Au@4ABT/Pt(flat) system are about two times greater than those of the 60Pt@4-ABT/Au(flat) system in which 60 nm sized Pt nanoparticles are adsorbed on 4-ABT on Au. Supposedly, if a Au nanoparticle as large as ∼150 nm is adsorbed onto 4-ABT on Pt(flat), the EF at 632.8 nm will be far larger than that, ∼9.7 102, found for the 145Pt@4-ABT/Au(flat) system.21 It is also informative to compare the EFs of the Au@4-ABT/ Pt(flat) system with those of the Ag@4-ABT/Pt(flat) system. As reported previously, the SERS intensity of 60Ag@4-ABT/Pt(flat) gradually increased as the excitation wavelength was decreased from 632.8 to 568, 514.5, and 488 nm.14 This is the opposite trend to that observed from the 60Au@4-ABT/Pt(flat) system. Specifically, the experimental enhancement factor per Ag nanoparticle was estimated to be as large as 7.9 102 under the illumination of 514.5 nm radiation, and it was even 2.0 102 at 632.8 nm.14 It is of note that the lowest EF per particle observable at 632.8 nm for the 60Ag@4-ABT/Pt(flat) system, 2.0 102, is rather close to the maximum EF observable for the 60Au@4ABT/Pt(flat) system, 1.7 102. It can then be concluded that the Ag nanoparticle must be more effective than the Au nanoparticle in inducing SERS for molecules adsorbed on weakly SERS-active metal substrates such as Pt.

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Figure 8. Raman spectrum of neat 4-ABT and those of the 60Pt@4ABT/Au(flat), 60Au@4-ABT/Pt(flat), 60Pt@4-ABT/Ag(flat), and 60Ag@4-ABT/Pt(flat) systems (from bottom to top), measured using 632.8 nm radiation as the excitation source; the SERS intensities were the average of five different measurements. Vibrational modes due to the b2-type bands of 4-ABT are labeled with an asterisk (*).

In Figure 7, the EF values calculated by the FDTD method are drawn as a function of the excitation wavelength, along with the experimentally determined values, for both the 60Au@4-ABT/ Pt(flat) and 60Ag@4-ABT/Pt(flat) systems. Although the experimental EF values are about 1 order of magnitude greater than the theoretical values, the theoretical trend of excitation wavelength dependence is well reproduced for both systems. We are confident that the Ag nanoparticle is demonstrably more effective than the Au nanoparticle at all excitation wavelengths for the induction of SERS at weakly SERS-active metal substrates. Electromagnetic Enhancement versus Charge-Transfer Enhancement. The NR spectrum of 4-ABT is shown in the bottom of Figure 8. The NR spectral pattern is independent of the excitation wavelength from 488 to 632.8 nm. The NR spectrum is seen to be quite featureless in the region of 1200 1500 cm 1. For reference, the Raman spectra measured for the 60Pt@4-ABT/ Au(flat) and 60Au@4-ABT/Pt(flat) systems are also reproduced in Figure 8: both are measured with the excitation at 632.8 nm. The latter two spectra are also quite featureless in the region of 1200 1500 cm 1. These spectral patterns contrast with those of the 60Pt@4-ABT/Ag(flat) and 60Ag@4-ABT/Pt(flat) systems, whose spectra are also reproduced in Figure 8. There are several peaks (denoted by an asterisk (*)), for instance, at ∼1143, ∼1390, and ∼1435 cm 1, that cannot be correlated to the NR peaks of 4-ABT. Earlier, these bands were assigned to the b2-type bands of 4-ABT (which are Raman inactive under normal conditions) and were attributed by Osawa et al. to arise in SERS spectra from the chemical (or charge-transfer) enhancement mechanism associated with the Herzberg Teller (vibronic) coupling.31 35 The involvement of the charge-transfer enhancement mechanism can be confirmed from the excitation wavelength dependence of the relative peak intensity of the b2 mode to the a1 mode. In both the 60Pt@4-ABT/Ag(flat) and 60Ag@4-ABT/Pt(flat) systems, the intensity of all the b2 bands was found to increase dramatically with respect to the 7a band as the excitation wavelength decreased, though weak at 632.8 nm excitation as in Figure 8.14,21 The absence of the b2-type bands in the 21053


The Journal of Physical Chemistry C spectra of the 60Pt@4-ABT/Au(flat) and 60Au@4-ABT/Pt(flat) systems (irrespective of excitation wavelength) suggests, in contrast, that the charge transfer between Au and 4-ABT is less likely, presumably due to the mismatch of the Fermi levels.36 38

4. SUMMARY AND CONCLUSION The highest Raman scattering intensity was observed at 632.8 nm excitation with the larger Pt particles being laid on the amine groups of 4-ABT on Au: specifically, the 7a band of 4-ABT at ∼1080 cm 1 induced by 145 nm Pt particles was several hundreds of times stronger (with the EF being as large as ∼9.7 102) than that induced by 26 nm Pt particles. Also, in the Pt@4-ABT/Ag(flat) system, the more intense Raman signals were obtained as the larger Pt particles were laid on the amine groups of 4-ABT. However, the excitation wavelength dependence was completely opposite: the highest intensity was observed at 488 nm excitation rather than at 632.8 nm excitation, suggesting that the surface plasmon polaritons of the planar Au and Ag substrates govern the excitation wavelength dependence more than the surface plasmons localized on Pt nanoparticles. To our surprise, even the EF at 632.8 nm in the Pt@4-ABT/Ag(flat) system was more than 2 times larger than the maximum EF in the Pt@4-ABT/Au(flat) system, suggesting that the planar Ag substrate must be more effective and have wider applicability than the planar Au substrate for the induction of SERS by virtue of Pt nanoparticles overlaid thereon. For comparison, the SERS characteristics of the Au@4-ABT/ Pt(flat) system, the opposite of the Pt@4-ABT/Au(flat) system, were also examined. In this case, the Raman intensity increased as the excitation wavelength was increased from 488 to 514.5 to 568 to 632.8 nm. It is noteworthy that the EF of the 60Au@4-ABT/ Pt(flat) system was about two times greater than that of the 60Pt@4-ABT/Au(flat) system. On the other hand, the SERS intensity of 60Ag@4-ABT/Pt(flat) gradually increased as the excitation wavelength was decreased from 632.8 to 568, 514.5, and 488 nm. This is the opposite trend to that observed from the 60Au@4-ABT/Pt(flat) system. Moreover, Ag nanoparticles appeared to be more effective than Au nanoparticles, irrespective of the excitation wavelengths, when inducing SERS for molecules adsorbed on weakly SERS-active metal substrates such as Pt. Finally, the involvement of the charge-transfer enhancement mechanism could be confirmed from the excitation wavelength dependence of the relative peak intensity of the b2 mode on the a1 mode. For the Pt@4-ABT/Au(flat) and Au@4-ABT/Pt(flat) systems, the b2-type bands were absent, while for the Pt@4-ABT/ Ag(flat) and Ag@4-ABT/Pt(flat) systems the intensity of all of the b2 bands was increased with respect to the 7a band as the excitation wavelength decreased, suggesting that the charge transfer between Au and 4-ABT, when compared to that between Ag and 4-ABT, should be less likely, presumably due to the mismatch of the Fermi levels. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: +82-2-8806651. Fax: +82-28891568 (K.K.). E-mail [email protected]. Tel.: +82-2-8200436. Fax: +82-2-8244383 (K.S.S.).

’ ACKNOWLEDGMENT This work was supported by National Research Foundation (NRF) of Korea Grant funded by the Korean Government

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(MEST) (No. 2011-0001218, 2011-0006737, 2010-0019204, 0409-20100172, and 2009-0072467).

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Characteristics of Nanogaps Formed by Planar Au and Pt

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