Excitation of Surface Plasmon Resonance in Composite Structures

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Excitation of Surface Plasmon Resonance in Composite Structures Based on Single-Layer Superaligned Carbon Nanotube Films Yinghui Sun,*,†,‡ Kai Liu,†,‡ Yimo Han,† Qunqing Li,† Shoushan Fan,† and Kaili Jiang† †

Department of Physics and Tsinghua−Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States

‡

S Supporting Information *

ABSTRACT: Surface-enhanced Raman scattering (SERS) provides valuable information on the vibrational modes of molecules and the physical mechanism of surface plasmon resonance (SPR). In this paper we study the localized SPR process in Ag- or Ag/ oxide-coated single-layer superaligned carbon nanotube (SACNT) films. Because of the unidirectional alignment of the carbon nanotubes in these films, the Raman signal is higher when the laser is polarized parallel to the aligned direction than when perpendicular to it. We investigated the polarization-dependent transmittance and Raman spectra for various Ag particle sizes and different oxide medium layers to study the localized SPR in these composite structures. These results systematically characterize the properties of SACNT film-based SERS substrates and clarify the origin of transmittance peaks.

1. INTRODUCTION Surface plasmons (SPs) are collective electron oscillations existing at the interface between any two materials where the real part of the dielectric function changes signs across that interface.1 At a metal−dielectric interface, SPs excited by coupled light cause electromagnetic field enhancement due to the large transient surface dipoles. SPs can propagate in a conductor at a dielectric−metallic interface through coupling of the electromagnetic fields to the oscillations of the electron plasma in the conductor. These propagating surface excitations are known as surface plasmon polaritons (SPPs), which are applicable in nanooptics and optical interconnects.2−4 SPs can also be localized in metallic nanoparticles or cavities to enhance the intensity of near field. Localized SPs are nonpropagating excitations of conductive electrons coupled to the electromagnetic field. When the intrinsic frequency of the metallic nanoparticles resonates with the excited field, localized SP resonance (LSPR) occurs, which is the main phenomenon giving rise to surface-enhanced Raman scattering (SERS).5−7 SERS is greatly applicable in sensitive detection8−10 and photothermal therapy.11−13 The SERS effect also provides a good platform for studying the LSPR process because of its easy characterization and strong signal. An effective SERS substrate is usually required with abundant “hot” sites to enhance local E-fields and with a large surface area to adsorb enough molecules to contribute to © 2013 American Chemical Society

the Raman signal. Recently, using nanowires or nanotubes as supporting materials of SERS substrates has attracted much attention, because these one-dimensional (1D) nanomaterials have extremely high surface-to-volume ratios and their nanoscale curvatures are favorable to form metal nanoparticles on.14−17 SERS substrates with silver or gold nanoparticles formed on nanostructures such as silver vanadate nanowires, carbon nanotubes (CNTs), and germanium nanowires have been found to be ultrasensitive for detecting various molecules, such as rhodamine 6G (R6G), trinitrotoluene (TNT), and 4mercaptophenol (4-MPH).14−17 Polarization-relevant SERS can help clarify the mechanism of LSPR in these substrates on the basis of 1D nanomaterials. However, these substrates are isotropic without polarization dependence because of their random or cross-stacked structures. Although metal-coated single or several aligned carbon nanotubes show polarized SERS signals, characterizing LSPR in these nanoscale materials is still challenging.18,19 In this paper we utilized macroscale Ag- or Ag/oxide-coated single-layer superaligned CNT (SACNT) films to study the LSPR process. Because of the unidirectional alignment of the Received: Revised: Accepted: Published: 23190

November 28, 2012 October 7, 2013 October 7, 2013 October 7, 2013 dx.doi.org/10.1021/jp3117165 | J. Phys. Chem. C 2013, 117, 23190−23197

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CNTs in the single-layer SACNT films, the Raman signal is higher when the polarization of the laser used is parallel to the direction of CNT alignment (parallel excitation) than when it is perpendicular (perpendicular excitation). The difference between Raman signals for parallel and perpendicular excitations can be further enhanced if a laser-trimmed singlelayer SACNT film is used as the supporting layer. We systematically investigate the polarization-dependent transmittance and Raman spectra for various Ag particle sizes and different oxide medium layers, identifying absorption or transmittance peaks and discussing them in detail to study the LSPR process in these composite structures.

2. EXPERIMENTAL SECTION First, single-layer SACNT films were directly drawn out in a dry state from SACNT arrays;20−23 we then adhered them to square frames to suspend the middle portions of the films. Silver nanoparticles or amorphous oxide layers (SiO2, Al2O3, or MgO) were deposited by e-beam evaporation on the suspended single-layer SACNT films, as well as on fused silica for comparison. The as-prepared Ag nanoparticles/SACNT (AgSACNT) or Ag nanoparticles/oxide layer/SACNT (Ag-oxideSACNT) composite films were transparent and free-standing. Transmittance measurements of the composite structures based on single-layer SACNT films were carried out using a PerkinElmer-Lambda950 (UV−vis−near-IR) spectrophotometer for wavelengths of 200−860 nm at intervals of 2 nm. The polarization of incident light can be tuned within a range of 0− 90° using an inserted polarizer. Raman spectra were measured under ambient conditions using a Renishaw microRaman spectroscopy system with a 514.5 nm argon-ion laser. The laser beam was focused by a 50× (numerical aperture (NA) = 0.75) objective lens, generating a spot diameter of about 2 μm. The samples were irradiated with a laser power no greater than 1.2 mW to avoid additional heating effects during the measurements. We used single-layer SACNT films made from the same SACNT array and selected detecting spots on CNT bundles with similar sizes in the Ag-SACNT layers to compare the measured Raman intensities, which guaranteed that the amount of material characterized was nearly the same for all cases. (See section 1 of the Supporting Information.) Scanning electron microscope (SEM) images were taken with an FEI Sirion 200 at an accelerating voltage of 10 kV. High-resolution transmission electron microscope (HRTEM) images were taken with an FEI Tecnai 20 at an accelerating voltage of 200 kV.

Figure 1. (a) Schematic of pure or oxide-coated single-layer SACNT film deposited with Ag nanoparticles. (b) Optical image of a macroscale suspended single-layer SACNT film with Ag nanoparticles. (c−f) TEM images of Ag-SACNT layers with different average nanoparticle sizes along the CNT axis. The sizes are (c) 15, (d) 19, (e) 27, and (f) 58 nm.

average sizes ± their standard deviations (μ ± σ) of the resultant Ag NPs along the axis of the CNTs are 15 ± 5, 19 ± 5, 27 ± 9, and 58 ± 30 nm, respectively. The smallest interparticle gaps are 9.0−11.3, 6.7−8.8, 3.9−8.1, and 2.1−7.6 nm, respectively (estimated from the high-resolution TEM images in Figure S1 of the Supporting Information). We analyzed some of the smallest gaps along the axis of the CNTs in the same focus plane, because Raman enhancement is more relevant to small interparticle gaps. 3.2. Transmittance and Raman Spectra of Ag-SACNT Layers. We found in our previous study that depositing Ag NPs on cross-stacked SACNT films can create effective SERS substrates and greatly enhance Raman signals.14 However, these films are isotropic without the polarization dependence of Raman signals because of their isotropic cross-stacked structures.25 In contrast, CNTs in single-layer SACNT films are aligned parallel to the drawing direction; thus, the films exhibit polarized absorption or emission of light.20−23 Figure 2a shows the transmittance spectra of the Ag NPs/SACNT composite structures, termed Ag-SACNT layer, when excited by polarized incident light. It is evident from these results that the transmittance is much higher when the polarization direction of the incident light is perpendicular to the direction of CNT alignment than when it is parallel. This behavior is caused by the polarized absorption of light by both the CNTs

3. RESULTS AND DISCUSSION 3.1. Structures of Ag-SACNT Layers. Figure 1a shows a schematic of a pure or oxide-coated single-layer SACNT film deposited with Ag nanoparticles (NPs). Figure 1b shows an optical image of our sample, where a SACNT film is suspended on metal frames. Parts c−f of Figure 1 show TEM images of Ag-SACNT composite structures with different average sizes of Ag NPs. By e-beam evaporation, Ag NPs were formed on the surface of the CNTs because of the weak interaction between Ag and the CNTs.14,24 The Ag particle size could be roughly tuned by changing the deposited nominal thickness of Ag, which was monitored by a quartz crystal oscillator. By increasing the nominal thickness of Ag, the Ag NPs nucleate and further enlarge on the CNTs. Therefore, a thicker deposited Ag film will produce larger NPs. According to our statistics, for the nominal thicknesses of 3, 5, 8, and 12 nm, the 23191

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Raman enhancement factors for G and D peaks are all around 2−3. However, the difference between the Raman signals shown in the upper panel of Figure 2b is small for practical applications. Using the intensity of the G peak as an example, the ratio of signals between the parallel and perpendicular directions is merely ∼1.4. This behavior may be caused by the presence of many large bundles in the SACNT films, which reduces the one-dimensional nature of the CNTs and thereby reduces the polarized excitation of the Raman signals. Our group has reported that laser trimming in air can improve the alignment of the CNTs by burning away the outermost CNTs of the large bundles, because larger bundles with small surface-to-volume ratios will absorb more laser energy and be heated to higher temperatures.26 In this paper we used laser trimming to process the single-layer SACNT films, reducing the number of CNTs perpendicular to the CNT axis. Therefore, the number of closely packed Ag NPs deposited on CNTs is also decreased in the perpendicular direction. Figure 2a compares the transmittances of the Ag-SACNT layers before and after laser trimming, showing that laser trimming enhances the transmittances of the Ag-SACNT layers in the parallel and perpendicular directions. However, the absorption bands in the transmittance curves are totally unchanged, which indicates that laser trimming did not affect the LSPR absorption mechanism. As shown in Figure 2b, the ratio of the Raman intensity of the G peak between the parallel and perpendicular directions increases to ∼2.8 after laser trimming. This change is caused by the reducing of large bundles of CNTs, which improves CNT alignment. Improving the CNT alignment benefits the 1-D alignment of Ag NPs because the CNTs are the framework for depositing the NPs. As described previously, when Ag NPs are deposited on CNTs before laser trimming, the polarized resonance absorption of light by Ag NPs is not obvious. Therefore, improving the CNT alignment allows us to better use polarized light to study the LSPR process in our system. 3.3. Dependence of Transmittance and Raman Spectra on Ag Particle Size. In addition to improving the alignment of CNTs in the SACNT films, we also attempted to control the size of the Ag NPs and their interparticle gaps in our samples. As we mentioned before, depositing different nominal thicknesses of Ag on the CNTs by e-beam evaporation is an easy way to roughly tune the size and interparticle gaps of the resultant NPs. Parts a and b of Figure 3 show the transmittance spectra of the Ag-SACNT films with different Ag particle sizes in two polarization directions. The peaks of the transmittance spectra when the light is polarized parallel to the direction of CNT alignment are different from those when the light is perpendicular to the direction. Figure 3a shows a broad absorption band at 290 nm (corresponding to energy of 4.3 eV) even without Ag, suggesting that it comes from the CNTs. Previous reports attributed this strong absorption band to the collective excitation of the π-plasmon of CNT.27−29 After depositing Ag on the CNTs, more peaks and dips emerge in the transmittance spectra. We identified the peak at 325 nm rather than the adjacent dip at ∼375 nm as the result of a genuine physical process, because the peak can also be unambiguously identified as a transmittance enhancement peak in the transmittance spectra of the Ag films deposited on the transparent fused-silica substrates. We will discuss the origin of this peak later. Upon comparing the transmittance spectra of the cross-stacked SERS substrates with different Ag particle

Figure 2. Transmittance and Raman spectra of an Ag-SACNT layer before and after laser trimming. (a) Comparison of transmittances when the light polarization is parallel and perpendicular to the direction of CNT alignment: solid curves, transmittances of AgSACNT layers prepared from as-drawn SACNT films; dashed curves, transmittances of those prepared from laser-trimmed SACNT films. The average Ag particle size is 19 nm. (b) Raman spectra of AgSACNT layers prepared from as-drawn and laser-trimmed SACNT films when laser polarization is parallel and perpendicular to the axis of the CNTs. (c) Comparison of Raman spectra of an Ag-SACNT layer and a SACNT-layer when the light polarization is parallel and perpendicular to the axis of the CNTs.

and the aligned Ag NPs. The origin of the peaks in the transmittance spectra will be discussed later. In the Raman spectra of the multiwalled CNTs (MWCNTs), we identify a G peak at ∼1580 cm−1 and a D peak at ∼1350 cm−1. As shown in the upper panel of Figure 2b, Raman signals are polarizationdependent; specifically, Raman signals are stronger when the polarization of the laser is parallel to the direction of CNT alignment, which we attribute to the polarized excitation of the Raman modes of the CNTs. The Raman signals are enhanced after Ag deposition, both for parallel and perpendicular excitations (Figure 2c). After Ag deposition, the intensities of the G and D peaks generated by parallel excitation are enhanced by 2.6× and 2.1×, respectively. Similarly, the intensities of the G and D peaks generated by perpendicular excitation are enhanced by 2.0× and 2.5×, respectively. The 23192

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absorption band red shifts as the average size of the NPs increases, a behavior consistent with other reports.32,33 In fact, the average size of the NPs is not fully responsible for the red shift of the LSPR energy. NP shape, coupling, and surrounding medium can also contribute to the change of LSPR energy. For example, the resonant frequency of coupled NPs often red shifts with decreasing interparticle separation.34,35 The dip (∼375 nm) next to the absorption band was formed due to the superposition of the π-plasmon dip of the CNT and the bulk plasmon peak of Ag.14 In Figure 3b, when the light polarization is perpendicular to the direction of CNT alignment, there is an absorption band near 224 nm (corresponding to energy of 5.5 eV), which is also attributed to the collective excitation of the π-plasmon of CNT. In low-dimensional systems, the nature of plasmon excitations depends on their polarization.36 The π-plasmon of CNT will split into two distinct contributions: a localized response, corresponding to a spatially confined plasmon perpendicular to the tube axis, and a dispersive response, corresponding to the plasmons propagating along the axis.28 Therefore, we conclude that the π-plasmon positions of the CNT near 4.3 and 5.5 eV correspond to the on-axis and crossed components, respectively.28 The observed peaks near 4.3 and 5.5 eV originate from the optical properties of graphite, corresponding to the maximum of the imaginary part of the dielectric function perpendicular to the graphite c-axis (parallel to the CNT axis), Im{ε⊥}, and the maximum of Im{−ε∥−1} (perpendicular to the CNT axis), respectively.27−29 ε⊥ and ε∥ are the dielectric functions for graphite perpendicular and parallel to the c-axis, respectively. In Figure 3b, the peak at 325 nm still exists and its position is nearly the same as that in Figure 3a. The LSPR absorption bands for Ag NPs are absent in the perpendicular excitation. This phenomenon can be explained as follows: the distances between adjacent Ag NPs are larger perpendicular to the CNT axis (Figure 1c−f), which weakens the interparticle coupling. This weak coupling, along with the difference in permittivity of the medium, may cause the absence of the LSPR absorption band in the transmittance spectra under perpendicular excitation, especially considering the LSPR absorption band is superimposed on the transmittance background of the CNTs. To investigate the enhancement of CNT Raman peaks in the Ag-SACNT layer and the effect of the size and interparticle gaps of the Ag NPs, we measured the Raman spectra of the composite structures with laser polarizations parallel to the direction of CNT alignment, as shown in Figure 3c. The intensities of the G and D peaks are distinctly enhanced because of the electric field enhancement induced by LSPR of the Ag NPs. The Raman intensity also increases with the increase of Ag NP sizes. These results can be understood by analyzing the microstructure of Ag-SACNT layers. The highresolution TEM images in Figure S1 of the Supporting Information show that, except for the limited number of small interparticle gaps between two adjacent NPs, other Ag NPs are far away from each other and can be treated as single NPs. For simplicity, the response behavior of NPs illuminated by a light can be modeled by metal spheres. When the wavelength of light is much larger than the diameter of a metal sphere, the distribution of the electric field outside the sphere can be calculated by assuming the simplified problem of a sphere in an electrostatic field. The outside electric field is enhanced near the sphere and becomes less enhanced as the distance from the sphere increases. Furthermore, when two

Figure 3. Optical transmittances of single-layer SACNT and AgSACNT layers with different Ag particle sizes when (a) the light polarization is parallel to the direction of CNT alignment and when (b) the light polarization is perpendicular to the direction of CNT alignment. The set of arrows in a indicates the central positions of the absorption band. (c) Raman spectra of Ag-SACNT layers with different Ag particle sizes. The laser polarization is parallel to the direction of CNT alignment. The laser power irradiating the samples is about 1.2 mW, and the exposure time is 20 s.

sizes (Ag-CNT grid),14 an extra absorption band appears at 450−600 nm. As the average Ag particle size increases incrementally from 15 to 58 nm, the central position of the absorption band shifts to longer wavelengths of 468, 498, 540, and 568 nm. Comparing the resonant energy for structures with different Ag particle sizes with the literature,30,31 we identify the absorption band at 450−600 nm to originate from excitation of LSPR of the Ag NPs. Sönnichsen et al. compared the plasmon resonance energy of Ag clusters with different nominal cluster diameters.30 The positions of the LSPR absorption band in our samples slightly deviate from those of Ag clusters on glass slides, because the shape of Ag NPs and the medium permittivity around them can influence their LSPR energy. For individual Ag nanospheres, the LSPR energy changes from 420 to 500 nm as the diameter increases from 40 to 90 nm.31 Considering the red shift of energy caused by the coupling of adjacent NPs and the different shapes of these NPs, the LSPR energy found in our experiment matches well with that in the literature. The central position of the LSPR 23193

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NPs are placed close to each other with a nanometer-sized gap and the polarization direction of the electric field is along the axis of the two particles, the local electric field in the gap can be further enhanced under the resonance condition, forming a “hot” spot for Raman scattering. Our system contains such paired NPs with small gaps as well as single NPs, each providing a unique Raman contribution. The local electric field enhancement factor and the number of particles/small gaps determine which of the two, the paired NPs with small gaps or single NPs, provides the larger contribution to the Raman signal. The SERS enhancement factor is proportional to (Elocal/ E0)4, where E0 is the strength of the incident E-field E0 and Elocal is the strength of the total local electric field Elocal in the presence of Ag microstructures.7,37 In order to differentiate the contributions of single NPs from the small gaps between NPs, we compared the enhancement factor of local electric field (Elocal/E0) of each case by the finite-difference time-domain (FDTD) method (see the Supporting Information), with considerations of different average sizes of Ag NPs and smallest interparticle gaps shown in Figure S1 of the Supporting Information. When the Ag NPs are small, the interparticle gaps are relatively large, and the electric field enhancement from both single NPs and interparticle gaps should be considered; when the Ag NPs become large, the electric field enhancement from the small interparticle gaps should dominate. It indicates that the Raman enhancement increases as the average Ag particle size increases, mostly due to the decrease of the smallest gap sizes, as shown in Figure 3c. It should be noticed that the Raman signal under our experimental conditions is enhanced only by 2−3 times. This small enhancement is due to the fact that the measurement takes an average of the signals within the laser-illuminated area, rather than measuring a single localized signal. On the other hand, CNTs are not exactly located at the spots of the largest electric field enhancement both for interparticle gaps and single NPs (as illustrated in Figure S3 of the Supporting Information). Furthermore, the intensity ratio of the D peak to the G peak (ID/IG) increases with the average Ag particle size. When the average Ag particle size incrementally increases from 15 to 58 nm, ID/IG increases as follows: 0.44, 0.61, 0.60, 0.63, and 0.78. This behavior indicates that Ag deposition might affect the lattice vibration mode of the CNTs. The enhanced intensity of the D peak has also been reported in other systems where Ag or Au is deposited on CNTs or graphene.19,38,39 The measured Raman spectra can be reproduced at many sites in each sample, and the data presented here represent the most typical cases. (See the Supporting Information for the detailed reproducibility of the Raman spectra.) 3.4. Origin of Transmittance Peaks near 325 nm. In order to clarify the origin of the transmittance peaks near 325 nm of the Ag-SACNT layers in Figure 3a,b, we deposited the same nominal thickness of Ag on transparent fused silica as we did on the Ag-SACNT layers. Figure 4 shows the transmittance spectra of the samples with different thicknesses of Ag films on fused silica (Ag-silica). There is a distinct transmittance enhancement peak at ∼323 nm for all thicknesses. This peak position is consistent with the transmittance peaks at 325 nm in the Ag-SACNT layers (Figure 3a,b). Therefore, we consider that the peaks at 325 nm are caused by a genuine physical process. This transmittance enhancement is similar to the enhanced transmission caused by periodic arrays of subwavelength holes in optically thick metallic films, which has been indexed as the bulk plasmon peak of Ag.40,41 In these

Figure 4. Transmittances of Ag-coated fused silica with different Ag thicknesses (3−50 nm).

studies the transmission intensity of the bulk plasmon peak decreases as the thickness of the Ag film increases,41 which is consistent with our results. In Figure 4, as the film thickness of Ag deposited on the fused silica increases, the peak transmission intensity at 323 nm gradually decreases and the peak sharpens. The peak at 323 nm (corresponding to energy of 3.8 eV) can also be attributed to the optical properties of Ag, which corresponds to the position where the imaginary part of the complex permittivity of Ag is minimum, Im{εAg(ω)}|min, and the real part is nearly zero, Re{εAg(ω)} ≈ 0.42,43 The behavior of the complex permittivity of Ag, εAg(ω), can induce the plasmon peak at 3.8 eV in the transmittance spectra or radiation spectra.44−46 In Figure 3a,b, the different intensities of the bulk plasmon peaks of Ag near 325 nm may be ascribed to the SACNTs on which the Ag NPs are deposited. The peak positions of the Ag bulk plasmon in Figure 3a are in the range of the broad absorption band of the CNT π-plasmon centered at 290 nm. This behavior means that either the Ag plasma oscillation can be distinctly reduced by the CNTs beneath the Ag NPs or the plasma radiation is absorbed by the CNTs. Therefore, the intensity of the Ag bulk plasmon peak at ∼325 nm is weaker when the light is parallel to the CNT layer. 3.5. Effect of Different Medium Layers on the LSPR Energy. The LSPR energy of metallic NPs is not only related to the size and interparticle gaps of the NPs but is also affected by the medium around them. To study the influence of the surrounding medium on the LSPR energy of the Ag NPs, we chose several typical amorphous oxides for use as medium layers between the CNTs and the Ag NPs. SiO2, MgO, and Al2O3 are transparent insulating layers commonly used in semiconductor devices. We deposited 20 nm of one of these materials on the SACNT single layer and then deposited 5 nm of Ag on top. TEM images are shown in Figure 5a−c. Figure 5d shows the transmittance spectra of samples with different medium layers. The absorption band of the π-plasmon of CNTs at ∼290 nm can still be seen. When the incident light polarization is parallel to the drawing direction of the SACNT films, we observe a set of absorption bands centered at 498 nm for SiO2, 678 nm for MgO, and 850 nm for Al2O3; these bands should originate from LSPR absorption by the Ag NPs. This behavior indicates that, although the thicknesses of the oxide medium layer and Ag are the same, the type of oxide used distinctly affects the position of the LSPR absorption bands of the Ag NPs, which may be attributed to the following two reasons. First and foremost, inserting a medium layer between the Ag and the CNTs affects the morphology of the Ag NPs, including 23194

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Figure 5. TEM images of an Ag-SiO2-SACNT layer (a), an Ag-MgO-SACNT layer (b), and an Ag-Al2O3-SACNT layer (c). The scale bars are 500, 200, and 500 nm, respectively. Insets are the high-resolution TEM images of Ag NPs. The scale bars in the insets are all 50 nm. (d) Transmittances of Ag-oxide-SACNT layers when the incident light polarization is parallel to the drawing direction of the SACNT film. (e) Raman spectra of Agoxide-SACNT layers when the laser polarization is parallel to the axis of the CNTs. These spectra are stacked.

additional peaks in the enhanced Raman spectra of the CNTs. This behavior indicates that Al2O3 may be used as a medium layer in SERS substrates based on SACNT films without introducing additional interference signals.

their size and shape. Table S2 in the Supporting Information shows statistical data of the average sizes (including standard deviations) of the Ag NPs for different medium layers. Although the nominal thickness of deposited Ag is the same, the average Ag NP sizes are slightly different because of the different wetting properties of Ag NPs on the medium layer. The Ag NPs on the Al2O3 layer have the largest average size. Note also that the sample with the SiO2 medium has more NPs with spherical shapes, while that with the Al2O3 medium has more NPs with irregular shapes, as shown in the insets of Figure 5a−c. Previous experimental work has shown that NP shape strongly influences the plasmon resonance energy.31,32 Spherically and triangularly shaped NPs of the same size have SPR wavelengths that differ by 100−200 nm, red-shifting in the case of the triangular particles.31 Second, the different dielectric properties of the medium may influence the local electrical field around the Ag NPs. The amorphous media used in this work, SiO2, MgO, and Al2O3, have refractive indexes of 1.44−1.46,47 1.63−1.66,48 and 1.65− 1.70,47 respectively. It is reported that when the refractive index of the surrounding medium changes from 1.0 to 1.44, the LSPR wavelength of individual Ag NPs red shifts by 50−100 nm.49 The change of refractive index from SiO2 to Al2O3 may induce the red shift of LSPR wavelength by tens of nanometers, which is partially responsible for the LSPR energy change. Therefore, the medium layer between the Ag and the CNTs may affect the system by changing both the dielectric properties of the medium around the Ag NPs and the morphology of the Ag NPs. When the laser polarization is parallel to the axis of the CNTs, we measured the effect of different medium layers on the Raman spectra of the CNTs, as shown in Figure 5e. Inserting SiO2 as a medium layer causes the highest enhancement of the Raman signal of the G peak of CNTs. We also notice that when SiO2 or MgO are used as the medium layer, a Raman peak corresponding to the medium used arises around 1000 cm−1. In contrast, Al2O3 does not induce

4. CONCLUSIONS We used Ag- or Ag/oxide-coated single-layer superaligned CNT films to study the localized SPR process. In this kind of substrate, the Raman signal is higher when the laser polarization is parallel to the direction of CNT alignment (parallel excitation) than when it is perpendicular (perpendicular excitation); this behavior is caused by the unidirectional alignment of the CNTs in a single-layer SACNT film. The polarization-dependent Raman signal can be further enhanced by laser trimming the single-layer SACNT films, which increases the anisotropic ratio of the Raman intensity from 1.4 for untrimmed films to 2.8. We also investigated the Raman signals and transmittance spectra of structures with various sizes of Ag nanoparticles and different oxide medium layers to study the LSPR process in these composite structures. We clarified the origin of the different peaks in the transmittance spectra, and found that amorphous SiO2 and Al2O3 are suitable medium layers in SACNT-based SERS substrates. These results systematically characterize the properties of carbon-nanotubebased SERS substrates and clarify the origin of transmittance peaks in these composites.

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ASSOCIATED CONTENT

S Supporting Information *

Selection of spots with the same amount of CNTs, highresolution TEM images of Ag-SACNT with different average Ag particle sizes, FDTD simulation of electric field distribution for different sizes of Ag NPs, reproducibility of the Raman spectra for different Ag NP sizes, SEM images of Ag-coated Si wafer with different Ag thicknesses, discussion about the transmittance spectra of Ag-coated fused silica, and statistical 23195

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average sizes of the Ag NPs for different medium layers. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.H.S. acknowledges valuable discussion with Bo Zeng and proof reading by Robert Tang-Kong. This work was financially supported by the National Basic Research Program of China (Grant 2012CB932301), NSFC (Grant 50825201), and Fok Ying Tung Education Foundation (Grant 111049). We thank Yongchao Zhai and Changqing Yin for their assistance with the experiments.

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