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3D Nanostructures of Silver NanoparticleDecorated Suspended Graphene for SERS Detection Li-Wei Nien, Miao-Hsuan Chien , Bo-Kai Chao, Miin-Jang Chen, Jia-Han Li, and Chun-Hway Hsueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11940 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on January 31, 2016
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3D Nanostructures of Silver Nanoparticle-Decorated Suspended Graphene for SERS Detection Li-Wei Nien,† Miao-Hsuan Chien,† Bo-Kai Chao,† Miin-Jang Chen,† Jia-Han Li,‡ and ChunHway Hsueh†,* †
Department of Materials Science and Engineering and ‡Department of Engineering Science and
Ocean Engineering, National Taiwan University, Taipei 10617, Taiwan *Corresponding Author: [email protected]
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ABSTRACT The silver nanoparticle-decorated suspended graphene was proposed and fabricated to increase the efficiency of surface-enhanced Raman scattering (SERS) mainly by the enhanced electric field resulting from exciting the localized surface plasmon resonance. The morphology of cavity under the graphene was controlled by the thickness of catalyst and the etching time in the metalassisted chemical etching process (MacEtch). The reflectance and ellipsometric spectra were examined to understand the optical behaviors of silver nanoparticle-decorated suspended graphene as functions of the etching time. For the samples treated with MacEtch, the Raman signals of graphene and p-mercaptoaniline were greatly enhanced due to the plasmonic cavity effect. Moreover, the graphene could increase the Raman intensity of the probed molecules by chemical enhancement. With the optimal etching time of 15 sec, the SERS signals reached the maximum that was 13~15 times larger than those without etching. The electric field enhancement profiles and the SERS enhancement factor were simulated by finite-difference time-domain method to characterize the field distribution around the silver nanoparticles and to verify the enhanced SERS phenomenon observed in measurements.
KEYWORDS: localized surface plasmon resonance, metal-assisted chemical etching, plasmonic cavity effect, surface-enhanced Raman scattering, finite-difference time-domain method
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INTRODUCTION The discovery of surface-enhanced Raman scattering (SERS) solves the problem of poor signal-collection efficiency of Raman spectroscopy. Different mechanisms, including electromagnetic enhancement (EM) and chemical enhancement (CE), have been explored to explain the characteristics and behaviors of SERS; however, none can completely explain all the observed phenomena. CE mechanism is referred to the charge transfer between the electron states of the metal and probed molecules, and it is considered as a short-range effect. However, it is complex and not well understood.1 On the other hand, EM mechanism, which is related to the excitation of localized surface plasmon resonance (LSPR) resulting from the electromagnetic waves interacting with the local field near the sub-wavelength metallic nanostructure, is the most widely accepted SERS mechanism to explain the enhanced Raman intensity.2 The properties of the metallic nanostructure, such as the composition, shape, size, orientation, degree of roughness, the gap between nanoparticles, and the surrounding conditions, play crucial roles in determining the behaviors of LSPR and the enhancement of Raman signals.3-7 Different kinds of SERS-active substrates have been proposed and studied. A tunable SERS substrate of Ag nanoparticle-decorated single-walled carbon nanotube network, which controls the enhancement of Raman signals by regulating the nanoparticle size and density, has attracted a lot of attention.8, 9 Compared to the conventional SERS substrates,10-12 the unique free-standing gold bowtie nanoantennas, in which two metallic triangular prisms separated by a small gap are suspended over the substrate through Si or SiO 2 posts, have been successfully fabricated recently.13, 14 It has been shown that the suspended nanostructure yields up to 2 orders of magnitude additional enhancement in SERS response than the non-suspended bowtie arrays because of the plasmonic cavity resonance effect and the reduction of the surrounding refractive
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index.15-17 However, the high cost of the fabrication process and the inefficiency in large-scale production, such as electron beam lithography and reactive ion etching, limit its development foreground and applications. Graphene, a two-dimensional material consisting of pure carbon element packed into a regular honeycomb lattice with a single-atom thickness, is an emerging and promising material in these days because of its superior electrical,18 thermal,19 and mechanical properties.20 Although the high optical transmission in the visible light region makes graphene not able to absorb the light efficiently to produce strong LSPR,21, 22 it has been demonstrated that graphene could be used as the SERS substrate to enhance Raman scattering of molecules by CE via the charge transfer between graphene and the adsorbate.23-25 Moreover, graphene is a potential host material to adsorb analytes for its characteristics of high affinity to aromatic molecules,26 and it resolves the low adsorption of the aromatic-functionalized analytes on the metal surface without the complex processing procedures.27-29 On the other hand, the suspended graphene has been fabricated,30 and greater resolution in transmission electron microscopy could be achieved by using the suspended graphene as the supporting structure.31-33 Metal-assisted chemical etching (MacEtch), which is an anisotropic wet etching method assisted by a metal catalyst (Ag, Au, Pt, and Pd, etc.) and was first proposed by Li and Bohn for silicon,34, 35 has attracted much attention in the field of light-emitting-diodes, chemical sensing, and solar cells due to the competitive advantages, such as high aspect ratio originating from the vertical structure of Si nanowires (SiNWs) and the rapid and low-cost fabrication process.36-38 In this technique, the Si substrate coated with metal catalyst is submerged in a dilute etching solution of HF and H 2 O 2 .39 The metal catalyzes the reduction of H 2 O 2 to oxidize Si, which would react with the etching solution to form H 2 SiF 6 at the metal-Si interface, and assists in
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etching of Si underneath.35 It has been found that the ratio of reactants in the solution would greatly affect the morphology and topography of the etched substrate.40 The purpose of the present study is to use an economic method to design metallic nanostructures on suspended graphene in the field of SERS detection by investigating experimentally and theoretically on a system of silver nanoparticle-decorated suspended graphene. A schematic diagram of the process to fabricate the silver nanoparticles as the SERSactive layer on the suspended graphene is shown in Figure 1a. By systematically examining the optical properties and the local electromagnetic field enhancement dependence on the suspended height, we could optimize and improve the performance of the Raman spectroscopy and other applications. Specifically, we compared the behaviors of SERS-active layer between two sets of suspended graphene, Set A and Set B, in which SiNWs were fabricated by depositing Ag catalyst with 5 and 10 nm thicknesses on Si substrate, respectively, to form agglomerated nanoparticles followed by MacEtch process and removal of Ag catalyst. The suspended height (i.e., the etching depth) was controlled by the etching time, which were 0, 5, 10, 15, 20, and 25 seconds in our studies.
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Figure 1. (a) Schematic diagram showing the process to fabricate silver nanoparticles as the SERS-active layer on suspended graphene supported by the silicon nanowires. Ag catalysts deposited on Si substrate for (b) Set A of 5 nm thickness and (c) Set B of 10 nm thickness to form agglomerated nanoparticles. The SERS-active layers on the suspended graphene for (d) Set A and (e) Set B with 10-sec etching. The dotted lines indicate the edge of graphene. (f) The particle-size distribution of the SERS-active layer on graphene. (g) The AFM mapping of Set B with 10-sec etching. EXPERIMENTAL AND THEORETICAL METHODS Fabrication of SiNWs by MacEtch. As-cut p-type (100) Si wafers (boron doped, 1-10 Ωcm) with a thickness of 500 µm were used in this work. Before Ag film deposition, Si substrate
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was treated and degreased by ultrasonication in acetone and ethanol sequentially for 10 minutes each, and then rinsed with deionized water. Different thicknesses (5 and 10 nm) of Ag films, which acted as the catalyst, were deposited on the Si wafer by electron beam evaporation at a background pressure below 1×10–5 Torr resulting in different morphologies of agglomerated Ag nanoparticles. Adopting a typical MacEtch process, the Si substrates deposited with Ag films were immersed in the etching solution of HF/H 2 O 2 /H 2 O (the molar ratio was defined by ρ = [HF]/([HF] + [H 2 O 2 ]) = 0.847 and [H 2 O] = 46.6 M) at room temperature (25 °C). The Si substrate underneath the Ag catalyst was preferentially etched and removed to form SiNWs because the Ag catalyst served as a local cathode to increase the reduction of the oxidants (H 2 O 2 ) and generate free holes (h+) at the interface between the metal and Si substrate. In this case, the etching depth was controlled by the etching time. After the MacEtch process, the Si substrate was immersed in nitric acid to remove the Ag catalyst. In the presence of SiNWs, cavities formed in between. Fabrication of Suspended Graphene. After fabricating SiNWs, graphene on the both sides of copper foil by chemical vapor deposition was transferred onto the top surface of SiNWs. The suspended graphene was formed above the cavities. The procedures of transferring graphene were as follows. First, polymethylmethacrylate (PMMA, 495K molecular weight) was spincoated onto the graphene and baked at 180 °C for 2 minutes as a support. Second, the copper foil was etched away by the method of floating the PMMA/Graphene/Cu on a solution of aqueous FeCl 3 (0.2 g/mL) for about 2 hours. Third, PMMA/Graphene was rinsed several times in deionized water with hydrochloric acid (< 10 %) to remove the residual ions on the surface of graphene. Finally, PMMA/Graphene was transferred onto the SiNWs and immersed into acetone solution (60 °C for 1 hour) to lift off PMMA.
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Morphology and Spectroscopy Characterization. The morphology and microstructure studies were performed using field emission scanning electron microscope (SEM, NOVA NanoSEM 450) operated at 10 kV and equipped with a secondary electron detector. The reflectance measurements were carried out by the microspectrophotometer, which was equipped with an optical microscope (ZEISS Imager.A2m) and a standard UV-vis spectrometer (Ocean Optics USB4000). The ellipsometric spectra were measured by the Elli-SE spectroscopic ellipsometer (Ellipso Technology) and the ellipsometric parameters, the amplitude component (Ψ) and the phase shift (∆), were recorded in the wavelength range from 400 to 800 nm. The ellipsometric parameters are related by: tan(Ψ)exp(i∆) = r p /r s , where r p and r s are the complex reflection coefficients for p and s polarizations, respectively. The SERS spectra were acquired using the UniRAM-Raman Microscope (Protrustech Co., Ltd) equipped with one TE cooled CCD of 1024×128 pixels and one diode-pumped solid-state laser at a wavelength of 532 nm. The spot size of the incident laser beam was set in position with a 50×, 0.5-NA (numerical aperture) microscope objective at a spatial resolution of about 2 µm and an output power of about 1 mW/cm2 at the exit of the objective. Measurements at different positions were performed for each sample to examine the reproducibility. The laser exposure time was fixed at 30 seconds for each measurement. FDTD Simulations. Lumerical FDTD Solutions, a commercial electromagnetic software based on the finite-difference time-domain (FDTD) method, was used to simulate the reflectance and the electromagnetic field intensity. An incident plane wave (of 400 to 800 nm wavelength) was illuminated from the top side to the surface of SERS-active layer on the suspended graphene. The morphologies of SERS-active layer and SiNWs structures with different etching time for simulations were imported from the SEM images and the measured SiNWs heights. The x-y in-
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plane dimensions of the simulation region were 500 nm × 500 nm, and the simulation domains in the z-direction were varied according to the different etching depths to ensure the convergence of the simulation results. The perfectly matched layers were used as the boundary condition to absorb waves leaving the simulation domain. The dielectric properties of Ag and Si used in simulations were taken from Palik’s handbook,41 and the graphene information was obtained from Falkovsky’s work.42 The mesh sizes in the SERS-active layer and suspended graphene varied from 0.5 to 2 nm, and automatic graded mesh was used in the region of SiNWs and Si substrate to ensure the numerical accuracy in consideration of the reasonable computation time. The simulation results of the near-field (local electric field) and the far-field (reflectance) were recorded. For near-field, the electric field enhancement was collected from the region containing the SERS-active layer and suspended graphene, and the top and the bottom of the collection region were defined, respectively, at 5 nm above the Ag nanoparticles and 5 nm below the suspended graphene. RESULTS AND DISCUSSION Ag catalysts with the thicknesses of 5 and 10 nm were deposited on Si substrate, respectively, for Set A and Set B (Figure 1b and c). Different thicknesses of Ag films would result in different morphologies of agglomerated Ag nanoparticles. Compared to Set A, Set B tended to form interconnected networks with smaller gap sizes between the Ag nanoparticles. After the MacEtch process, well-aligned SiNWs perpendicular to the Si substrate were formed, and the morphologies of Set A and Set B could be visualized, respectively, in the right of Figure 1d and e. From Figure 1d (Set A), the surface of Si substrate was covered with the meso- and macropores (cavities) with diameters slightly larger than the agglomerated Ag nanoparticles. However, the isolated and well-separated SiNWs were shown in Figure 1e (for Set B) resulting
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from a dense network of Ag catalysts in close proximity to each other. After graphene transfer and SERS-active layer deposition, the morphologies of Set A and Set B are shown in the left of Figure 1d and e (while the complete set of SEM images for different etching time are shown in Figures S1 and S2). The existence of suspended graphene, which acted as the support of metallic nanostructures by its superior mechanical property, could be verified by the wrinkle and the edge of graphene (indicated by the arrow and dotted lines in Figure 1d and e) and the different contrasts of underlying SiNWs. The distribution of SERS-active layer, agglomerated nanoparticles (as the white dots shown in Figure 1d and e) from 5 nm Ag deposition, was affected by the suspended graphene and underlying SiNWs. The Ag nanoparticles on the suspended graphene were less dense and more regular in shape than those directly on the Si substrate because of different wettabilities. The enlarged image of the SERS-active layer is shown in Figure S3a, and the topographic information from atomic force microscopy (AFM) is shown in Figure S3b. The density of SERS-active layer was approximately 1248 µm−2, and the coverage was about 43.3%. The Ag particle size distribution of the SERS-active layer on graphene is shown in Figure 1f. By fitting the distribution with a Gaussian curve, the diameter of silver nanoparticles was 20.6 ± 5.1 nm. The topography of the SERS-active layer on suspended graphene for Set B with etching of 10 s was revealed by AFM measurement and shown in Figure 1g. SiNWs could be observed in the uncovered region (see the top in Figure 1g). Moreover, the suspended graphene remained intact after the deposition of the SERS-active layer. The uneven surface of the suspended graphene partially reflected the topology of SiNWs underneath. The cross-section image of SiNWs is shown in Figure 2a. It could be observed that the SiNWs were vertical to the surface of Si substrate with a uniform height (i.e., the etching depth). The etching depths versus etching time are shown in Figure 2b for Set A and Set B. From Figure
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2b, the etching depth increased with the increasing etching time. By controlling the etching time from 0 to 25 sec, the etching depths varied from 0 to about 1000 nm. Moreover, it could be observed that the etching depths of Set A and Set B were similar at the same etching time.
Figure 2. (a) The cross-section image of SiNWs for Set B with etching time of 20 sec. (b) The suspended height (etching depth) as a function of etching time for Set A and Set B. The measured reflectance and the corresponding simulation spectra for Set A and Set B with different etching time are shown in Figure 3. For Set A (Figure 3a), the reflectance drops drastically for the first 5 sec of etching and decreases slightly for further etching. The formation of meso- and macropores (cavities) on the surface of Si substrate increases the possibility of
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multiple reflection and absorption in Si, and results in the huge change of reflectance in the initial stage. The smaller reflectance corresponds to the larger scattering and absorption in the system of silver nanoparticle-decorated suspended graphene. Compared to the measurements (Figure 3a), the simulated reflectance spectra (Figure 3c) show the same trend. Similar results occur for Set B (Figure 3b and d). The measured reflectance at the wavelength of the excitation laser for SERS measurements, 532 nm, versus the etching time is shown in Figure 3e for Set A and Set B. It could be observed that the reflectance decreases enormously from 43% to below 20% in the first 5 sec of etching. However, there is only a small change in the reflectance with longer etching time because of the limit of the light absorption and trapping effect in SiNWs.43 In addition, the reflectance of Set B was larger than Set A. The corresponding simulation results (Figure 3f) exhibit the similar reflection behaviors. The difference between the measurements and simulations shown in Figure 3 might result from the oxidation of Ag, the irregular SiNWs structure, and the assumption of a uniform height of SiNWs in simulations. On the other hand, the suspended graphene could also be identified by spectroscopic ellipsometry to characterize the optical behavior as shown in Figure S4. The Ψ amplitude in ellipsometric spectra at 532 nm wavelength increased with the increasing etching time which corresponded to the larger amplitude ratio of reflection coefficients in p and s polarizations (Figure S4c).
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Figure 3. The reflectance spectra for (a) Set A and (b) Set B at different etching time. The corresponding reflectance by FDTD simulations for (c) Set A and (d) Set B. (e) The measured reflectance at 532 nm wavelength as a function of etching time for Set A and Set B and (f) the corresponding reflectance by FDTD simulations.
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In the system of silver nanoparticle-decorated suspended graphene, the graphene could serve as not only the support of SERS-active layer by its superior mechanical property, but also a Raman probed molecule, which has a unique structure of electrons and phonons. Therefore, it is worth examining the dependence of Raman behaviors of suspended graphene on the etching time. The baseline-corrected Raman spectra of defect-containing graphene measured with a 532 nm laser for Set A and Set B are shown in Figure 4a and b, respectively, and the raw spectra are shown in Figure S5. There are two characteristic peaks of defect-free graphene, G band (1581 cm−1) and the 2D band (2681 cm−1), in Figure 4a and b. The G band is a first-order Raman scattering, which originates from the degeneration of iTO and iLO phonons at the Brillouin center (Γ point),44, 45 and the 2D band is a double resonance intervalley Raman scattering with two iTO phonons at the Κ point (zone boundary).46 In addition, D band (1345 cm−1) and D’ band (1619 cm−1), which exist in the disordered graphene or at the edges, could be observed. The Raman intensities of G band as functions of etching time are shown in Figure 4c for Set A and Set B. The intensity of G band initially increases with the etching time and reaches the maximum (about 8 to 11 times larger than those without etching) at 15 sec of etching because of the enhancement from plasmonic cavity effect.13 However, the Raman signal is weak at 20 sec of etching due to the departure of the suspended height (cavity) from the resonance condition of cavity effect. It is worth mentioning that the Raman intensity for the sample without etching (i.e., 0 sec of etching) is enhanced due to the LSPR of Ag nanoparticles. The suspended bilayer graphene, which is transferred together from the both sides of the copper foil, has the tendency to form the Bernal stacking structure during the graphene transfer process because Bernal-stacked graphene is the most stable state and has the lowest interaction energy between two graphene sheets compared to other stacking orders (AA stack or turbostratic
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structure).47 The configuration of graphene could be identified by the intensity ratio of 2D to G band, the shape and the full width at half-maximum (FWHM) of the 2D band in the Raman spectra.21, 48 The spectra in Figure 4a and b exhibit 2D bands with FWHM of ∼45 cm−1 and
2D/G integrated intensity ratio of ∼1.6, suggesting it is a Bernal-stacked bilayer system.49, 50 For
Bernal-stacked bilayer graphene, it is shown in Figure 4d that the 2D band (2681 cm−1) could be well fitted by four Lorentzian peaks (green lines), 2D 1B , 2D 1A , 2D 2A , 2D 2B .21, 48 The band diagram of double resonance process in bilayer graphene, which links the phonon wave vectors to the electronic band structure, is shown in the inset of Figure 4d.51 There are four virtual state
transitions: the laser-induced excitation and the recombination of electron-hole pairs between the valence band (π) and conductance band (π∗), and the electron-phonon scatterings with an opposite momentum (+q and −q). The double resonance would occur and could be observed as 2D band in the Raman spectra when the energy is conserved among these transitions.
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Figure 4. The baseline-corrected Raman spectra of graphene for (a) Set A and (b) Set B at different etching time. (c) The Raman signal intensities of G band (1581 cm−1) as functions of etching time for Set A and Set B. (d) The deconvoluted Raman spectra of 2D band (2681 cm−1). The green lines show the four Lorentzian peaks used to fit the data, and the red line is the fitting result. The inset shows the schematic energy band diagram of the double resonance process in bilayer graphene. P-mercaptoaniline (pMA), one of the common probes with the characteristic of selfassembled monolayer by aromatic thiol ligands, was chosen to evaluate the SERS performance of the silver nanoparticle-decorated suspended graphene. The Raman spectra for Set A and Set B covered with pMA, which were prepared by exposing the SERS-active layer (silver nanoparticles) to an aliquot of freshly prepared aqueous solution of pMA at 10−4 M in a plastic
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petri dish for 12 hours at room temperature and removing unbound pMA by deionized water, are shown, respectively, in Figure 5a and b. It could be observed that strong Raman peaks appear at 1076, 1141, 1190, 1303, 1391, 1435, 1473, and 1577 cm−1, which are in good agreement with the previous reports.52 For the samples treated with MacEtch, the Raman signals were enhanced compared to those without etching. The suspended graphene reduces the Si substrate effect and forms the cavities under the graphene to increase the local electric field by the plasmonic cavity effect.13 In addition, the suspended graphene could provide the extra enhancement by CE as the graphene-enhanced Raman scattering (GERS) substrate.24 To evaluate the contribution of the GERS effect to the overall enhancement in the silver nanoparticle-decorated suspended graphene system, the Raman spectra of pMA on different kinds of substrates were examined and the results are shown in Figure S6. The Raman signal of pMA on SERS+GERS substrate was slightly larger than those on SERS substrate (Figure S6a) due to the additional small amount of enhancement resulting from the charge transfer between graphene and the molecules. Compared to Ag on SiNWs (without graphene layer) as shown in Figure S6b, the Raman intensity of pMA on Ag nanoparticle-decorated suspended graphene was larger because the suspended graphene acts as a GERS platform in addition to providing plasmonic cavity induced by SiNWs. The spectral bands of pMA in Raman and SERS spectra have been classified into four different modes as a 1 , a 2 , b 1 , and b 2 modes,53 and the SERS spectra for pMA on Set A and Set B are dominated by a 1 and b 2 modes, which are assigned as in-plane vibrational modes. The bands at 1076, 1473, and 1577 cm−1 belong to the a 1 modes, whereas those at 1141, 1190, 1303, 1391, and 1435 cm−1 are attributed to the b 2 modes. Two vibration bands of pMA, 1076 and 1435 cm−1, for Set A and Set B with different etching time were investigated in the present study as shown in Figure 5c and d. The strongest band observed at 1435 cm−1 is assigned to [stretch (C–C) +
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bend (C–H)] b 2 mode, and the band at 1076 cm−1 is attributed to (C–S) stretching a 1 vibration. In addition to EM, the enhancement for the b 2 vibrational mode at 1435 cm−1 has been characterized and attributed to the charge transfer by CE in previous pMA SERS experiments, and Osawa et al. have proposed a model to interpret the Herzberg-Teller effect in the charge transfer processes.52 Therefore, it could be observed that the Raman intensity for 1435 cm−1 b 2 mode is larger and about twice of that at 1076 cm−1 a 1 mode. For Set A (Figure 5c), the trend of the dependence of Raman intensity for 1076 and 1435 cm−1 vibration bands in pMA on the etching time is the same as the G band (1581 cm−1) in graphene discussed in Figure 4c. The Raman signals increase first and reach the maximum (about 13 to 15 times larger than those without etching) at 15 sec of etching resulting from the large plasmonic cavity enhancement.13 However, the decreasing signal for 20 sec of etching is due to the departure of the suspended height from the resonance condition of cavity effect. Similarly, Set B (Figure 5d) has the same results.
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Figure 5. The Raman spectra (baseline offset) of 10−4 M pMA for (a) Set A and (b) Set B with different etching time. The Raman signal intensity of 1076 and 1435 cm−1 vibration bands in pMA as functions of etching time for (c) Set A and (d) Set B. The Raman intensities of characteristic vibration bands of 1076 and 1435 cm−1 as functions of the concentration of pMA for (e) Set A and (f) Set B with 15 sec of etching.
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The concentration-dependent Raman intensities of 1076 and 1435 cm−1 were examined for Set A and Set B with etching 15 sec and plotted in Figure 5e and f, respectively. The SERS intensity at 1076 and 1435 cm−1 decreases with the decreasing concentration of pMA from 10−4 to 10−8 M. The detection limit of pMA is as low as 10−8 M in our work. Moreover, it could be observed that there is no difference in the Raman intensity between 1076 and 1435 cm−1 bands when the concentration of pMA is below 10−7 M. Small possibility of charge transfer between pMA and silver nanoparticles by CE occurs under the condition of low densities of pMA on the surface of silver nanoparticles.54 Samples with the suspended graphene have better performance in SERS behaviors than those without MacEtch process as shown in Figures 4 and 5. It is worth exploring the electric field enhancement profiles by the FDTD method; i.e., |E|2/|E 0 |2 where E and E 0 are the electric field of the local area and the illumination wave, respectively, to understand the interaction between the electromagnetic wave and the SERS-active layer on the suspended graphene. The electric field enhancement profiles at 532 nm wavelength for Set B with etching 0 and 15 sec are shown, respectively, in Figure 6a and b, in which the geometries used for simulations were imported from SEM images with the heights of SiNWs taken from measurements. An example of converting the SEM images (Figure S7a and b) to the FDTD input files (Figure S7c and d) is shown in Figure S7. The electric field is enhanced around the edge of Ag nanoparticles for samples without etching (Figure 6a). However, the electric field for Set B with etching 15 sec (Figure 6b) is further enhanced because of the presence of SiNWs underneath the suspended graphene (as shown in Figure S7). The cross-section of electric field intensity enhancement profile indicated by the dotted line in Figure 6b is shown in Figure 6c. It could be observed that the enhanced electric field occurs around the Ag nanoparticles and shows the fringes in the
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cavities. The fringes were originated from the excitation of surface plasmon polaritons (SPPs) in the cavities, and had resonance modes characterized by different scatterings and field localizations.55 On the other hand, the theoretical SERS EM enhancement factor (EF SERS ) could be simplified and is proportional to the square of the electric field enhancement when the Stokes shift is small, such that56
(1)
where E’ represents the electric field at the Raman Stokes-shifted wavelength. Using FDTD simulations and eq 1, the normalized EF SERS at 532 nm wavelength as a function of etching time are shown in Figure 6d for Set A and Set B. The theoretical SERS EM enhancement factor increases to a maximum value (about 28 and 17 times larger than those without etching, respectively, for Set A and Set B) at 15 sec of etching with the help of coupling the SPP-induced field in cavities with the LSPR of Ag nanoparticles. However, there is a drop at 20 sec of etching because the SPP localized field could not efficiently concentrate in the region below Ag nanoparticles.57 The FDTD simulation result is consistent with the experimental findings. The slight difference between the experiment results (Figures 4c, 5c, and 5d) and the simulations (Figure 6d) might result from the imperfect nanostructures by the fabrication process and the extra enhancement from the charge transfer in CE that is not considered in simulations.
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Figure 6. The top view of E intensity enhancement profiles at 532 nm wavelength for Set B with etching time of (a) 0 and (b) 15 sec. The dotted line in (b) indicates the y-position of the E intensity enhancement profile shown in (c). The color bar in (c) is logarithmic-scale and limited to –2 to 2. (d) The normalized EF SERS for Set A and Set B at 532 nm wavelength as functions of etching time. CONCLUSIONS In this work, the Ag nanoparticles as SERS-active layer on the suspended graphene (Set A and Set B) were investigated and fabricated using an economic method. The suspended graphene served as (i) the support of metallic nanostructures, (ii) a Raman probed molecule, and (iii) the GERS substrate to provide CE enhancement for its extraordinary mechanical, electrical, and
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chemical properties. The suspended height beneath the graphene was controlled by the etching time in the MacEtch process. The dependence of the optical properties of silver nanoparticledecorated suspended graphene on the etching time was obtained from the reflectance and ellipsometric spectra. The reflectance obtained from the experiment and simulation both dropped drastically at the initial stage of etching as the meso- and macropores or SiNWs formed on the surface of Si substrate. The Raman signals of graphene and pMA in SERS spectra for Set A and Set B treated with MacEtch are significantly enhanced compared to those without etching due to the plasmonic cavity effect. In addition, the graphene could provide additional enhancement by CE between the probed pMA and Ag nanoparticles. The SERS signals increased with the increasing etching time and reached the maximum (about 13 to 15 times larger than those without etching) at 15 seconds of etching. Finally, the theoretical SERS EM enhancement factors for Set A and Set B, which were simulated by the FDTD method to analyze the effects of suspended structure on SERS enhancement, agreed well with the experiment results.
Supporting Information. The SEM images of SERS-active layers on Set A and Set B with different etching time, the information of SERS-active layer obtained from SEM and AFM, the ellipsometric spectra in Ψ for Set A and Set B with different etching time, the Raman spectra (without baseline correction) of graphene, the comparison of Raman spectra between SERS and GERS substrate, and an example of converting the SEM images to the FDTD input files. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENT The work was jointly supported by the Ministry of Science and Technology, Taiwan under Contract no. MOST 103-2221-E-002-076-MY3 and Excellent Research Projects of National Taiwan University under Project no. 104R8918. We are grateful to the National Center for HighPerformance Computing, Taiwan, for providing us with the computation time and facilities. Portion of experiments in this work were performed at the National Taiwan University NEMS Research Center and is gratefully acknowledged.
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Article
3D Nanostructures of Silver NanoparticleDecorated Suspended Graphene for SERS Detection Li-Wei Nien, Miao-Hsuan Chien , Bo-Kai Chao, Miin-Jang Chen, Jia-Han Li, and Chun-Hway Hsueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11940 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on January 31, 2016
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3D Nanostructures of Silver Nanoparticle-Decorated Suspended Graphene for SERS Detection Li-Wei Nien,† Miao-Hsuan Chien,† Bo-Kai Chao,† Miin-Jang Chen,† Jia-Han Li,‡ and ChunHway Hsueh†,* †
Department of Materials Science and Engineering and ‡Department of Engineering Science and
Ocean Engineering, National Taiwan University, Taipei 10617, Taiwan *Corresponding Author: [email protected]
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ABSTRACT The silver nanoparticle-decorated suspended graphene was proposed and fabricated to increase the efficiency of surface-enhanced Raman scattering (SERS) mainly by the enhanced electric field resulting from exciting the localized surface plasmon resonance. The morphology of cavity under the graphene was controlled by the thickness of catalyst and the etching time in the metalassisted chemical etching process (MacEtch). The reflectance and ellipsometric spectra were examined to understand the optical behaviors of silver nanoparticle-decorated suspended graphene as functions of the etching time. For the samples treated with MacEtch, the Raman signals of graphene and p-mercaptoaniline were greatly enhanced due to the plasmonic cavity effect. Moreover, the graphene could increase the Raman intensity of the probed molecules by chemical enhancement. With the optimal etching time of 15 sec, the SERS signals reached the maximum that was 13~15 times larger than those without etching. The electric field enhancement profiles and the SERS enhancement factor were simulated by finite-difference time-domain method to characterize the field distribution around the silver nanoparticles and to verify the enhanced SERS phenomenon observed in measurements.
KEYWORDS: localized surface plasmon resonance, metal-assisted chemical etching, plasmonic cavity effect, surface-enhanced Raman scattering, finite-difference time-domain method
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INTRODUCTION The discovery of surface-enhanced Raman scattering (SERS) solves the problem of poor signal-collection efficiency of Raman spectroscopy. Different mechanisms, including electromagnetic enhancement (EM) and chemical enhancement (CE), have been explored to explain the characteristics and behaviors of SERS; however, none can completely explain all the observed phenomena. CE mechanism is referred to the charge transfer between the electron states of the metal and probed molecules, and it is considered as a short-range effect. However, it is complex and not well understood.1 On the other hand, EM mechanism, which is related to the excitation of localized surface plasmon resonance (LSPR) resulting from the electromagnetic waves interacting with the local field near the sub-wavelength metallic nanostructure, is the most widely accepted SERS mechanism to explain the enhanced Raman intensity.2 The properties of the metallic nanostructure, such as the composition, shape, size, orientation, degree of roughness, the gap between nanoparticles, and the surrounding conditions, play crucial roles in determining the behaviors of LSPR and the enhancement of Raman signals.3-7 Different kinds of SERS-active substrates have been proposed and studied. A tunable SERS substrate of Ag nanoparticle-decorated single-walled carbon nanotube network, which controls the enhancement of Raman signals by regulating the nanoparticle size and density, has attracted a lot of attention.8, 9 Compared to the conventional SERS substrates,10-12 the unique free-standing gold bowtie nanoantennas, in which two metallic triangular prisms separated by a small gap are suspended over the substrate through Si or SiO 2 posts, have been successfully fabricated recently.13, 14 It has been shown that the suspended nanostructure yields up to 2 orders of magnitude additional enhancement in SERS response than the non-suspended bowtie arrays because of the plasmonic cavity resonance effect and the reduction of the surrounding refractive
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index.15-17 However, the high cost of the fabrication process and the inefficiency in large-scale production, such as electron beam lithography and reactive ion etching, limit its development foreground and applications. Graphene, a two-dimensional material consisting of pure carbon element packed into a regular honeycomb lattice with a single-atom thickness, is an emerging and promising material in these days because of its superior electrical,18 thermal,19 and mechanical properties.20 Although the high optical transmission in the visible light region makes graphene not able to absorb the light efficiently to produce strong LSPR,21, 22 it has been demonstrated that graphene could be used as the SERS substrate to enhance Raman scattering of molecules by CE via the charge transfer between graphene and the adsorbate.23-25 Moreover, graphene is a potential host material to adsorb analytes for its characteristics of high affinity to aromatic molecules,26 and it resolves the low adsorption of the aromatic-functionalized analytes on the metal surface without the complex processing procedures.27-29 On the other hand, the suspended graphene has been fabricated,30 and greater resolution in transmission electron microscopy could be achieved by using the suspended graphene as the supporting structure.31-33 Metal-assisted chemical etching (MacEtch), which is an anisotropic wet etching method assisted by a metal catalyst (Ag, Au, Pt, and Pd, etc.) and was first proposed by Li and Bohn for silicon,34, 35 has attracted much attention in the field of light-emitting-diodes, chemical sensing, and solar cells due to the competitive advantages, such as high aspect ratio originating from the vertical structure of Si nanowires (SiNWs) and the rapid and low-cost fabrication process.36-38 In this technique, the Si substrate coated with metal catalyst is submerged in a dilute etching solution of HF and H 2 O 2 .39 The metal catalyzes the reduction of H 2 O 2 to oxidize Si, which would react with the etching solution to form H 2 SiF 6 at the metal-Si interface, and assists in
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etching of Si underneath.35 It has been found that the ratio of reactants in the solution would greatly affect the morphology and topography of the etched substrate.40 The purpose of the present study is to use an economic method to design metallic nanostructures on suspended graphene in the field of SERS detection by investigating experimentally and theoretically on a system of silver nanoparticle-decorated suspended graphene. A schematic diagram of the process to fabricate the silver nanoparticles as the SERSactive layer on the suspended graphene is shown in Figure 1a. By systematically examining the optical properties and the local electromagnetic field enhancement dependence on the suspended height, we could optimize and improve the performance of the Raman spectroscopy and other applications. Specifically, we compared the behaviors of SERS-active layer between two sets of suspended graphene, Set A and Set B, in which SiNWs were fabricated by depositing Ag catalyst with 5 and 10 nm thicknesses on Si substrate, respectively, to form agglomerated nanoparticles followed by MacEtch process and removal of Ag catalyst. The suspended height (i.e., the etching depth) was controlled by the etching time, which were 0, 5, 10, 15, 20, and 25 seconds in our studies.
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Figure 1. (a) Schematic diagram showing the process to fabricate silver nanoparticles as the SERS-active layer on suspended graphene supported by the silicon nanowires. Ag catalysts deposited on Si substrate for (b) Set A of 5 nm thickness and (c) Set B of 10 nm thickness to form agglomerated nanoparticles. The SERS-active layers on the suspended graphene for (d) Set A and (e) Set B with 10-sec etching. The dotted lines indicate the edge of graphene. (f) The particle-size distribution of the SERS-active layer on graphene. (g) The AFM mapping of Set B with 10-sec etching. EXPERIMENTAL AND THEORETICAL METHODS Fabrication of SiNWs by MacEtch. As-cut p-type (100) Si wafers (boron doped, 1-10 Ωcm) with a thickness of 500 µm were used in this work. Before Ag film deposition, Si substrate
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was treated and degreased by ultrasonication in acetone and ethanol sequentially for 10 minutes each, and then rinsed with deionized water. Different thicknesses (5 and 10 nm) of Ag films, which acted as the catalyst, were deposited on the Si wafer by electron beam evaporation at a background pressure below 1×10–5 Torr resulting in different morphologies of agglomerated Ag nanoparticles. Adopting a typical MacEtch process, the Si substrates deposited with Ag films were immersed in the etching solution of HF/H 2 O 2 /H 2 O (the molar ratio was defined by ρ = [HF]/([HF] + [H 2 O 2 ]) = 0.847 and [H 2 O] = 46.6 M) at room temperature (25 °C). The Si substrate underneath the Ag catalyst was preferentially etched and removed to form SiNWs because the Ag catalyst served as a local cathode to increase the reduction of the oxidants (H 2 O 2 ) and generate free holes (h+) at the interface between the metal and Si substrate. In this case, the etching depth was controlled by the etching time. After the MacEtch process, the Si substrate was immersed in nitric acid to remove the Ag catalyst. In the presence of SiNWs, cavities formed in between. Fabrication of Suspended Graphene. After fabricating SiNWs, graphene on the both sides of copper foil by chemical vapor deposition was transferred onto the top surface of SiNWs. The suspended graphene was formed above the cavities. The procedures of transferring graphene were as follows. First, polymethylmethacrylate (PMMA, 495K molecular weight) was spincoated onto the graphene and baked at 180 °C for 2 minutes as a support. Second, the copper foil was etched away by the method of floating the PMMA/Graphene/Cu on a solution of aqueous FeCl 3 (0.2 g/mL) for about 2 hours. Third, PMMA/Graphene was rinsed several times in deionized water with hydrochloric acid (< 10 %) to remove the residual ions on the surface of graphene. Finally, PMMA/Graphene was transferred onto the SiNWs and immersed into acetone solution (60 °C for 1 hour) to lift off PMMA.
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Morphology and Spectroscopy Characterization. The morphology and microstructure studies were performed using field emission scanning electron microscope (SEM, NOVA NanoSEM 450) operated at 10 kV and equipped with a secondary electron detector. The reflectance measurements were carried out by the microspectrophotometer, which was equipped with an optical microscope (ZEISS Imager.A2m) and a standard UV-vis spectrometer (Ocean Optics USB4000). The ellipsometric spectra were measured by the Elli-SE spectroscopic ellipsometer (Ellipso Technology) and the ellipsometric parameters, the amplitude component (Ψ) and the phase shift (∆), were recorded in the wavelength range from 400 to 800 nm. The ellipsometric parameters are related by: tan(Ψ)exp(i∆) = r p /r s , where r p and r s are the complex reflection coefficients for p and s polarizations, respectively. The SERS spectra were acquired using the UniRAM-Raman Microscope (Protrustech Co., Ltd) equipped with one TE cooled CCD of 1024×128 pixels and one diode-pumped solid-state laser at a wavelength of 532 nm. The spot size of the incident laser beam was set in position with a 50×, 0.5-NA (numerical aperture) microscope objective at a spatial resolution of about 2 µm and an output power of about 1 mW/cm2 at the exit of the objective. Measurements at different positions were performed for each sample to examine the reproducibility. The laser exposure time was fixed at 30 seconds for each measurement. FDTD Simulations. Lumerical FDTD Solutions, a commercial electromagnetic software based on the finite-difference time-domain (FDTD) method, was used to simulate the reflectance and the electromagnetic field intensity. An incident plane wave (of 400 to 800 nm wavelength) was illuminated from the top side to the surface of SERS-active layer on the suspended graphene. The morphologies of SERS-active layer and SiNWs structures with different etching time for simulations were imported from the SEM images and the measured SiNWs heights. The x-y in-
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plane dimensions of the simulation region were 500 nm × 500 nm, and the simulation domains in the z-direction were varied according to the different etching depths to ensure the convergence of the simulation results. The perfectly matched layers were used as the boundary condition to absorb waves leaving the simulation domain. The dielectric properties of Ag and Si used in simulations were taken from Palik’s handbook,41 and the graphene information was obtained from Falkovsky’s work.42 The mesh sizes in the SERS-active layer and suspended graphene varied from 0.5 to 2 nm, and automatic graded mesh was used in the region of SiNWs and Si substrate to ensure the numerical accuracy in consideration of the reasonable computation time. The simulation results of the near-field (local electric field) and the far-field (reflectance) were recorded. For near-field, the electric field enhancement was collected from the region containing the SERS-active layer and suspended graphene, and the top and the bottom of the collection region were defined, respectively, at 5 nm above the Ag nanoparticles and 5 nm below the suspended graphene. RESULTS AND DISCUSSION Ag catalysts with the thicknesses of 5 and 10 nm were deposited on Si substrate, respectively, for Set A and Set B (Figure 1b and c). Different thicknesses of Ag films would result in different morphologies of agglomerated Ag nanoparticles. Compared to Set A, Set B tended to form interconnected networks with smaller gap sizes between the Ag nanoparticles. After the MacEtch process, well-aligned SiNWs perpendicular to the Si substrate were formed, and the morphologies of Set A and Set B could be visualized, respectively, in the right of Figure 1d and e. From Figure 1d (Set A), the surface of Si substrate was covered with the meso- and macropores (cavities) with diameters slightly larger than the agglomerated Ag nanoparticles. However, the isolated and well-separated SiNWs were shown in Figure 1e (for Set B) resulting
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from a dense network of Ag catalysts in close proximity to each other. After graphene transfer and SERS-active layer deposition, the morphologies of Set A and Set B are shown in the left of Figure 1d and e (while the complete set of SEM images for different etching time are shown in Figures S1 and S2). The existence of suspended graphene, which acted as the support of metallic nanostructures by its superior mechanical property, could be verified by the wrinkle and the edge of graphene (indicated by the arrow and dotted lines in Figure 1d and e) and the different contrasts of underlying SiNWs. The distribution of SERS-active layer, agglomerated nanoparticles (as the white dots shown in Figure 1d and e) from 5 nm Ag deposition, was affected by the suspended graphene and underlying SiNWs. The Ag nanoparticles on the suspended graphene were less dense and more regular in shape than those directly on the Si substrate because of different wettabilities. The enlarged image of the SERS-active layer is shown in Figure S3a, and the topographic information from atomic force microscopy (AFM) is shown in Figure S3b. The density of SERS-active layer was approximately 1248 µm−2, and the coverage was about 43.3%. The Ag particle size distribution of the SERS-active layer on graphene is shown in Figure 1f. By fitting the distribution with a Gaussian curve, the diameter of silver nanoparticles was 20.6 ± 5.1 nm. The topography of the SERS-active layer on suspended graphene for Set B with etching of 10 s was revealed by AFM measurement and shown in Figure 1g. SiNWs could be observed in the uncovered region (see the top in Figure 1g). Moreover, the suspended graphene remained intact after the deposition of the SERS-active layer. The uneven surface of the suspended graphene partially reflected the topology of SiNWs underneath. The cross-section image of SiNWs is shown in Figure 2a. It could be observed that the SiNWs were vertical to the surface of Si substrate with a uniform height (i.e., the etching depth). The etching depths versus etching time are shown in Figure 2b for Set A and Set B. From Figure
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2b, the etching depth increased with the increasing etching time. By controlling the etching time from 0 to 25 sec, the etching depths varied from 0 to about 1000 nm. Moreover, it could be observed that the etching depths of Set A and Set B were similar at the same etching time.
Figure 2. (a) The cross-section image of SiNWs for Set B with etching time of 20 sec. (b) The suspended height (etching depth) as a function of etching time for Set A and Set B. The measured reflectance and the corresponding simulation spectra for Set A and Set B with different etching time are shown in Figure 3. For Set A (Figure 3a), the reflectance drops drastically for the first 5 sec of etching and decreases slightly for further etching. The formation of meso- and macropores (cavities) on the surface of Si substrate increases the possibility of
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multiple reflection and absorption in Si, and results in the huge change of reflectance in the initial stage. The smaller reflectance corresponds to the larger scattering and absorption in the system of silver nanoparticle-decorated suspended graphene. Compared to the measurements (Figure 3a), the simulated reflectance spectra (Figure 3c) show the same trend. Similar results occur for Set B (Figure 3b and d). The measured reflectance at the wavelength of the excitation laser for SERS measurements, 532 nm, versus the etching time is shown in Figure 3e for Set A and Set B. It could be observed that the reflectance decreases enormously from 43% to below 20% in the first 5 sec of etching. However, there is only a small change in the reflectance with longer etching time because of the limit of the light absorption and trapping effect in SiNWs.43 In addition, the reflectance of Set B was larger than Set A. The corresponding simulation results (Figure 3f) exhibit the similar reflection behaviors. The difference between the measurements and simulations shown in Figure 3 might result from the oxidation of Ag, the irregular SiNWs structure, and the assumption of a uniform height of SiNWs in simulations. On the other hand, the suspended graphene could also be identified by spectroscopic ellipsometry to characterize the optical behavior as shown in Figure S4. The Ψ amplitude in ellipsometric spectra at 532 nm wavelength increased with the increasing etching time which corresponded to the larger amplitude ratio of reflection coefficients in p and s polarizations (Figure S4c).
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Figure 3. The reflectance spectra for (a) Set A and (b) Set B at different etching time. The corresponding reflectance by FDTD simulations for (c) Set A and (d) Set B. (e) The measured reflectance at 532 nm wavelength as a function of etching time for Set A and Set B and (f) the corresponding reflectance by FDTD simulations.
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In the system of silver nanoparticle-decorated suspended graphene, the graphene could serve as not only the support of SERS-active layer by its superior mechanical property, but also a Raman probed molecule, which has a unique structure of electrons and phonons. Therefore, it is worth examining the dependence of Raman behaviors of suspended graphene on the etching time. The baseline-corrected Raman spectra of defect-containing graphene measured with a 532 nm laser for Set A and Set B are shown in Figure 4a and b, respectively, and the raw spectra are shown in Figure S5. There are two characteristic peaks of defect-free graphene, G band (1581 cm−1) and the 2D band (2681 cm−1), in Figure 4a and b. The G band is a first-order Raman scattering, which originates from the degeneration of iTO and iLO phonons at the Brillouin center (Γ point),44, 45 and the 2D band is a double resonance intervalley Raman scattering with two iTO phonons at the Κ point (zone boundary).46 In addition, D band (1345 cm−1) and D’ band (1619 cm−1), which exist in the disordered graphene or at the edges, could be observed. The Raman intensities of G band as functions of etching time are shown in Figure 4c for Set A and Set B. The intensity of G band initially increases with the etching time and reaches the maximum (about 8 to 11 times larger than those without etching) at 15 sec of etching because of the enhancement from plasmonic cavity effect.13 However, the Raman signal is weak at 20 sec of etching due to the departure of the suspended height (cavity) from the resonance condition of cavity effect. It is worth mentioning that the Raman intensity for the sample without etching (i.e., 0 sec of etching) is enhanced due to the LSPR of Ag nanoparticles. The suspended bilayer graphene, which is transferred together from the both sides of the copper foil, has the tendency to form the Bernal stacking structure during the graphene transfer process because Bernal-stacked graphene is the most stable state and has the lowest interaction energy between two graphene sheets compared to other stacking orders (AA stack or turbostratic
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structure).47 The configuration of graphene could be identified by the intensity ratio of 2D to G band, the shape and the full width at half-maximum (FWHM) of the 2D band in the Raman spectra.21, 48 The spectra in Figure 4a and b exhibit 2D bands with FWHM of ∼45 cm−1 and
2D/G integrated intensity ratio of ∼1.6, suggesting it is a Bernal-stacked bilayer system.49, 50 For
Bernal-stacked bilayer graphene, it is shown in Figure 4d that the 2D band (2681 cm−1) could be well fitted by four Lorentzian peaks (green lines), 2D 1B , 2D 1A , 2D 2A , 2D 2B .21, 48 The band diagram of double resonance process in bilayer graphene, which links the phonon wave vectors to the electronic band structure, is shown in the inset of Figure 4d.51 There are four virtual state
transitions: the laser-induced excitation and the recombination of electron-hole pairs between the valence band (π) and conductance band (π∗), and the electron-phonon scatterings with an opposite momentum (+q and −q). The double resonance would occur and could be observed as 2D band in the Raman spectra when the energy is conserved among these transitions.
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Figure 4. The baseline-corrected Raman spectra of graphene for (a) Set A and (b) Set B at different etching time. (c) The Raman signal intensities of G band (1581 cm−1) as functions of etching time for Set A and Set B. (d) The deconvoluted Raman spectra of 2D band (2681 cm−1). The green lines show the four Lorentzian peaks used to fit the data, and the red line is the fitting result. The inset shows the schematic energy band diagram of the double resonance process in bilayer graphene. P-mercaptoaniline (pMA), one of the common probes with the characteristic of selfassembled monolayer by aromatic thiol ligands, was chosen to evaluate the SERS performance of the silver nanoparticle-decorated suspended graphene. The Raman spectra for Set A and Set B covered with pMA, which were prepared by exposing the SERS-active layer (silver nanoparticles) to an aliquot of freshly prepared aqueous solution of pMA at 10−4 M in a plastic
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petri dish for 12 hours at room temperature and removing unbound pMA by deionized water, are shown, respectively, in Figure 5a and b. It could be observed that strong Raman peaks appear at 1076, 1141, 1190, 1303, 1391, 1435, 1473, and 1577 cm−1, which are in good agreement with the previous reports.52 For the samples treated with MacEtch, the Raman signals were enhanced compared to those without etching. The suspended graphene reduces the Si substrate effect and forms the cavities under the graphene to increase the local electric field by the plasmonic cavity effect.13 In addition, the suspended graphene could provide the extra enhancement by CE as the graphene-enhanced Raman scattering (GERS) substrate.24 To evaluate the contribution of the GERS effect to the overall enhancement in the silver nanoparticle-decorated suspended graphene system, the Raman spectra of pMA on different kinds of substrates were examined and the results are shown in Figure S6. The Raman signal of pMA on SERS+GERS substrate was slightly larger than those on SERS substrate (Figure S6a) due to the additional small amount of enhancement resulting from the charge transfer between graphene and the molecules. Compared to Ag on SiNWs (without graphene layer) as shown in Figure S6b, the Raman intensity of pMA on Ag nanoparticle-decorated suspended graphene was larger because the suspended graphene acts as a GERS platform in addition to providing plasmonic cavity induced by SiNWs. The spectral bands of pMA in Raman and SERS spectra have been classified into four different modes as a 1 , a 2 , b 1 , and b 2 modes,53 and the SERS spectra for pMA on Set A and Set B are dominated by a 1 and b 2 modes, which are assigned as in-plane vibrational modes. The bands at 1076, 1473, and 1577 cm−1 belong to the a 1 modes, whereas those at 1141, 1190, 1303, 1391, and 1435 cm−1 are attributed to the b 2 modes. Two vibration bands of pMA, 1076 and 1435 cm−1, for Set A and Set B with different etching time were investigated in the present study as shown in Figure 5c and d. The strongest band observed at 1435 cm−1 is assigned to [stretch (C–C) +
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bend (C–H)] b 2 mode, and the band at 1076 cm−1 is attributed to (C–S) stretching a 1 vibration. In addition to EM, the enhancement for the b 2 vibrational mode at 1435 cm−1 has been characterized and attributed to the charge transfer by CE in previous pMA SERS experiments, and Osawa et al. have proposed a model to interpret the Herzberg-Teller effect in the charge transfer processes.52 Therefore, it could be observed that the Raman intensity for 1435 cm−1 b 2 mode is larger and about twice of that at 1076 cm−1 a 1 mode. For Set A (Figure 5c), the trend of the dependence of Raman intensity for 1076 and 1435 cm−1 vibration bands in pMA on the etching time is the same as the G band (1581 cm−1) in graphene discussed in Figure 4c. The Raman signals increase first and reach the maximum (about 13 to 15 times larger than those without etching) at 15 sec of etching resulting from the large plasmonic cavity enhancement.13 However, the decreasing signal for 20 sec of etching is due to the departure of the suspended height from the resonance condition of cavity effect. Similarly, Set B (Figure 5d) has the same results.
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Figure 5. The Raman spectra (baseline offset) of 10−4 M pMA for (a) Set A and (b) Set B with different etching time. The Raman signal intensity of 1076 and 1435 cm−1 vibration bands in pMA as functions of etching time for (c) Set A and (d) Set B. The Raman intensities of characteristic vibration bands of 1076 and 1435 cm−1 as functions of the concentration of pMA for (e) Set A and (f) Set B with 15 sec of etching.
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The concentration-dependent Raman intensities of 1076 and 1435 cm−1 were examined for Set A and Set B with etching 15 sec and plotted in Figure 5e and f, respectively. The SERS intensity at 1076 and 1435 cm−1 decreases with the decreasing concentration of pMA from 10−4 to 10−8 M. The detection limit of pMA is as low as 10−8 M in our work. Moreover, it could be observed that there is no difference in the Raman intensity between 1076 and 1435 cm−1 bands when the concentration of pMA is below 10−7 M. Small possibility of charge transfer between pMA and silver nanoparticles by CE occurs under the condition of low densities of pMA on the surface of silver nanoparticles.54 Samples with the suspended graphene have better performance in SERS behaviors than those without MacEtch process as shown in Figures 4 and 5. It is worth exploring the electric field enhancement profiles by the FDTD method; i.e., |E|2/|E 0 |2 where E and E 0 are the electric field of the local area and the illumination wave, respectively, to understand the interaction between the electromagnetic wave and the SERS-active layer on the suspended graphene. The electric field enhancement profiles at 532 nm wavelength for Set B with etching 0 and 15 sec are shown, respectively, in Figure 6a and b, in which the geometries used for simulations were imported from SEM images with the heights of SiNWs taken from measurements. An example of converting the SEM images (Figure S7a and b) to the FDTD input files (Figure S7c and d) is shown in Figure S7. The electric field is enhanced around the edge of Ag nanoparticles for samples without etching (Figure 6a). However, the electric field for Set B with etching 15 sec (Figure 6b) is further enhanced because of the presence of SiNWs underneath the suspended graphene (as shown in Figure S7). The cross-section of electric field intensity enhancement profile indicated by the dotted line in Figure 6b is shown in Figure 6c. It could be observed that the enhanced electric field occurs around the Ag nanoparticles and shows the fringes in the
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cavities. The fringes were originated from the excitation of surface plasmon polaritons (SPPs) in the cavities, and had resonance modes characterized by different scatterings and field localizations.55 On the other hand, the theoretical SERS EM enhancement factor (EF SERS ) could be simplified and is proportional to the square of the electric field enhancement when the Stokes shift is small, such that56
(1)
where E’ represents the electric field at the Raman Stokes-shifted wavelength. Using FDTD simulations and eq 1, the normalized EF SERS at 532 nm wavelength as a function of etching time are shown in Figure 6d for Set A and Set B. The theoretical SERS EM enhancement factor increases to a maximum value (about 28 and 17 times larger than those without etching, respectively, for Set A and Set B) at 15 sec of etching with the help of coupling the SPP-induced field in cavities with the LSPR of Ag nanoparticles. However, there is a drop at 20 sec of etching because the SPP localized field could not efficiently concentrate in the region below Ag nanoparticles.57 The FDTD simulation result is consistent with the experimental findings. The slight difference between the experiment results (Figures 4c, 5c, and 5d) and the simulations (Figure 6d) might result from the imperfect nanostructures by the fabrication process and the extra enhancement from the charge transfer in CE that is not considered in simulations.
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Figure 6. The top view of E intensity enhancement profiles at 532 nm wavelength for Set B with etching time of (a) 0 and (b) 15 sec. The dotted line in (b) indicates the y-position of the E intensity enhancement profile shown in (c). The color bar in (c) is logarithmic-scale and limited to –2 to 2. (d) The normalized EF SERS for Set A and Set B at 532 nm wavelength as functions of etching time. CONCLUSIONS In this work, the Ag nanoparticles as SERS-active layer on the suspended graphene (Set A and Set B) were investigated and fabricated using an economic method. The suspended graphene served as (i) the support of metallic nanostructures, (ii) a Raman probed molecule, and (iii) the GERS substrate to provide CE enhancement for its extraordinary mechanical, electrical, and
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chemical properties. The suspended height beneath the graphene was controlled by the etching time in the MacEtch process. The dependence of the optical properties of silver nanoparticledecorated suspended graphene on the etching time was obtained from the reflectance and ellipsometric spectra. The reflectance obtained from the experiment and simulation both dropped drastically at the initial stage of etching as the meso- and macropores or SiNWs formed on the surface of Si substrate. The Raman signals of graphene and pMA in SERS spectra for Set A and Set B treated with MacEtch are significantly enhanced compared to those without etching due to the plasmonic cavity effect. In addition, the graphene could provide additional enhancement by CE between the probed pMA and Ag nanoparticles. The SERS signals increased with the increasing etching time and reached the maximum (about 13 to 15 times larger than those without etching) at 15 seconds of etching. Finally, the theoretical SERS EM enhancement factors for Set A and Set B, which were simulated by the FDTD method to analyze the effects of suspended structure on SERS enhancement, agreed well with the experiment results.
Supporting Information. The SEM images of SERS-active layers on Set A and Set B with different etching time, the information of SERS-active layer obtained from SEM and AFM, the ellipsometric spectra in Ψ for Set A and Set B with different etching time, the Raman spectra (without baseline correction) of graphene, the comparison of Raman spectra between SERS and GERS substrate, and an example of converting the SEM images to the FDTD input files. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENT The work was jointly supported by the Ministry of Science and Technology, Taiwan under Contract no. MOST 103-2221-E-002-076-MY3 and Excellent Research Projects of National Taiwan University under Project no. 104R8918. We are grateful to the National Center for HighPerformance Computing, Taiwan, for providing us with the computation time and facilities. Portion of experiments in this work were performed at the National Taiwan University NEMS Research Center and is gratefully acknowledged.
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