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Using Metal-less Structures To Enhance the Raman Signals of Graphene by 100-fold while Maintaining the Band-to-Band Ratio and Peak Positions Precisely Yang-Chun Lee, En-Yun Wang, Yu-Lun Liu, and Hsuen-Li Chen* Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: In this study, we developed a reliable method to analyze the interference-enhanced Raman scattering (IERS) effect on graphene by considering the surface electric field (Efield), which can be calculated precisely by measuring the optical admittance of the thin-film assembly. Through accurate tuning of the optical properties of one-dimensional photonic crystals (1DPhCs), the strong and controllable interference effect allowed the surface E-field to be maximized and, thereby, to optimize the enhancement factors of the Raman scattering signals of graphene. Using this approach, we could enhance both the G and the 2D bands of graphene largely, uniformly, and equally, by about 180 times relative to those obtained on a silicon substrate. Under certain conditions, the Raman peak of graphene could even be enhanced by over 400 times. After transferring single-layer graphene (SLG) and few-layer graphene (FLG) onto various substrates, we found that the Raman spectra of both SLG and FLG on the 1D-PhCs substrate were enhanced without changing the band-to-band ratio or the peak positions of the main Raman bands of graphene. Without inducing any additional signal disturbance, this enhancement technique allowed us to maintain the accurate and precise informational features from the Raman spectra. The experimental enhancement factors in the coenhanced Raman spectra of graphene were higher than those previously obtained using the IERS effect. Moreover, the surface E-field of 1D-PhCs could be modulated by changing the incident angle of the excited light source, thereby allowing fine-tuning of the working wavelength. Thus, by controlling only the surface E-field, the Raman signals of graphene could be enhanced dramatically without any distortion on spectra. Accordingly, using 1D-PhCs and the optimized IERS effect is very helpful for fine structural characterization of graphene through conventional Raman spectroscopy.

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INTRODUCTION Graphene, a monolayer of carbon atoms packed into a perfect two-dimensional honeycomb crystal lattice, is receiving great attention because of its unique electrical and optical properties.1,2 An extremely high carrier mobility,3 a quantum Hall effect at room temperature,4 broadband absorption,5 and high sensitivity toward adsorption-specific molecules6 make graphene a useful material in many devices, including transistors,7,8 photodetectors,9,10 solar cells,11,12 and sensors.13 Because the structural quality of graphene is the key factor affecting the performance of graphene-based devices, rapid and accurate probing techniques should be developed to characterize and investigate the properties of graphene for various applications. Raman spectroscopy is a powerful technique for characterizing graphene in a rapid and nondestructive manner. The main Raman features of graphene are its G band (ca. 1580 cm−1) and 2D band (ca. 2680 cm−1). These two Raman signals are very sensitive to the fine structure of a graphene sample; therefore, they provide abundant information that can be used to identify the properties of graphene. For example, the shape and position of the 2D band, the shift of the G band, and the ratio of the integrated intensities of the G and 2D bands have been used to identify the quality and layer number of graphene;14−16 the blue shifts and the peak widths of G band have been used to © XXXX American Chemical Society

monitor the type or the concentration of dopants in graphene.17−19 In addition to these two major Raman peaks, the defect level in a graphene sample can be determined from the intensity of the disorder-induced D Raman feature of graphene (D band near 1350 cm−1) relative to that of the G band.20,21 Moreover, the intensity of the D band has been used to identify the edge structures.22,23 Although Raman spectra are very useful for identifying the properties of graphene samples, the monolayer of carbon atoms in graphene means that only a very small portion of incident light could be absorbed24 to generate the Raman scattering signal. Accordingly, the Raman signals from graphene are very weak; as a result, some fine structural characteristics, such as a low concentration of defects or dopants, would be difficult to observe. Therefore, enhancing the Raman scattering signals of graphene is necessary for any detailed Raman analysis of the fine structures on graphene. Several approaches toward enhancing Raman scattering signals have been developed using surface-enhanced Raman scattering (SERS),25 one of the most important analytic Received: October 30, 2014 Revised: December 20, 2014

A

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Chemistry of Materials techniques for characterizing molecules in the field of biochemistry. In SERS techniques, the major mechanism for increasing the Raman scattering signals of a molecule of interest is electromagnetic (EM) enhancement, which results from the enhanced local electric field (E-field) for the EM resonances in metallic structures.26 Regions featuring a high local E-field, known as “hot spots”, are usually located in the gaps between adjacent metallic nanostructures; here, the Raman scattering signals can be amplified dramatically, with an E4 dependence.27 The detection of molecules at ultralow concentrations, almost at the level of single-molecule detection, can be achieved using the SERS technique.28 Accordingly, the application of SERS to graphene is a promising method for enhancing its Raman scattering signals.29−34 Combining graphene with plasmonic metallic nanostructures has been used widely for SERS analyses of graphene. Gold (Au) or silver (Ag) nanoparticles (NPs) can be deposited on the surface of the transferred graphene through thermal evaporation30,31 or chemical synthesis.32 On the other hand, graphene can be transferred onto patterned metallic resonators to form graphene-covered metallic structures.33−35 Through strong EM resonance, the large increase in the local Efield around the hot spots in these metallic nanostructures results in enhanced light−graphene interactions when the graphene structures are close by. As a result, the Raman scattering of graphene can increase significantly when applying SERS techniques. Although the enhancement ratio of the Raman peaks of graphene can reach the order of approximately 103,35 the characterization of graphene through metallic nanostructurebased SERS remains poor because of several external disturbances on the graphene arising from the metallic nanostructures. For example, the deposition of metallic NPs onto the surface of graphene usually results in unavoidable damage to the graphene, increasing the intensity of the D band.36 In addition, because the remarkable SERS effect appears only when graphene is close to or even contacting the metallic structures, the strong interactions between the metal and the graphene will also influence the appearance of the Raman spectrum of the graphene, inducing band splits37 or shifting the positions of the peaks.38 As a result, it can be more difficult to analyze the natural properties of the graphene after introducing the metallic nanostructures required for SERS analysis. In addition to the external disturbances arising from the metallic nanostructures, another disadvantage of the SERS of graphene is inhomogeneous enhancement of the Raman signals, because enhanced local E-fields exist only in the gaps between adjacent metallic nanostructures. Thus, only the graphene species located near the hot spots would undergo effective increases in their signal intensities, and they would contribute an inordinate amount to the resulting bulk Raman scattering signals. This inhomogeneous enhancement of Raman signals would adversely affect spatial analyses of graphene (e.g., Raman mapping investigations). Therefore, metallic nanostructure-based SERS is not ideal for enhancing the Raman scattering signals of graphene. The thin film-based interference enhancement method known as interference-enhanced Raman scattering (IERS) can also be used to enhance the Raman scattering signals of graphene.39−41 Wang et al. were the first to study the IERS effect on the Raman G band of graphene positioned on a Si substrate featuring a SiO2 capping layer; they achieved a maximum enhancement factor of approximately 30 relative to that on silicon substrate by varying the thickness of the SiO2

layer to 269 nm and an enhancement factor of only 16 for 300 nm SiO2/Si substrate.39 Yoon et al. found that they could vary the Raman intensity ratio of the 2D band to G band of graphene by varying the thickness of the SiO2 buffer layer between the Si substrate and the graphene.40 Dyakov et al. studied the effect of the optical contrast of graphene samples on Si substrates featuring SiO2 or Al2O3 buffer layers of various thickness on the Raman G band intensity, achieving a maximum enhancement of nearly 50 times of only the G band intensity relative to that on silicon substrate in the presence of an optimized buffer layer.41 In these studies, the substrate coming into direct contact with the graphene could be the same as that in a graphene-based device to ensure modeling of the same interactions found in real applications. In addition, because the major enhancement mechanism of IERS is the interference effect, the Raman scattering signals can be enhanced uniformly over the whole area of the graphene sample; this feature is very helpful for spatial inspections of graphene samples. According to these characteristics, IERS appears to be a better technique than SERS for enhancing the Raman scattering signals of graphene. The highest enhancement ratio found previously through IERS for only the G band of graphene was reported in a study by Gao et al.42 They proposed a SERS- and interference-enhanced technique to improve the Raman scattering signal of graphene by using a substrate of Si capped with a surface-active metal and oxide double layers (Si/50 nm Ag/72 nm Al2O3). Although they claimed that the enhancement factor of G band should reach approximately 102 relative to that on silicon substrate, they did not obtain such an enhancement factor experimentally. Moreover, the negative influences of the metal layer on the Raman spectra of graphene were not completely avoidable. Therefore, to the best of our knowledge, using a pure interference effect to enhance the Raman signals has not been optimized systematically; in addition, enhancement of all of the Raman peaks of graphene has not been considered simultaneously when applying the IERS technique. In this study, we employed a systematic method to analyze the IERS effect of graphene by calculating the surface E-field of the substrates using thin-film optical theory.43 We have found that the surface E-field can be controlled well by preparing thin films from different materials at various thicknesses; therefore, the enhancement ratios of the Raman peaks of graphene can be predicted theoretically. By using the controlled optical properties of one-dimensional photonic crystals (1D-PhCs), which comprise quarter-wavelength layers of alternating high and low refractive indices, we found that the strong interference effect maximized the surface E-field and optimized the enhancement factors of the Raman scattering signals of graphene. The main Raman peaks of graphene could be enhanced equally and uniformly, with large enhancement factors, after transferring it onto the 1D-PhCs. Enhancing the Raman signals of graphene this way does not disturb the information present in the enhanced Raman spectra. Moreover, we have also demonstrated that changing the angle of the incident light can modulate the surface E-field of the 1D-PhCs, such that the working wavelength can be tuned readily. By controlling the surface E-field, the Raman spectra of graphene can be optimized while avoiding any uncertain effects arising from the enhancement technique. B

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EXPERIMENTAL METHODS

Preparation of Samples. To investigate the IERS effect through modulation of the surface E-field, three kinds of substrates were prepared: silicon, fused silica, and 1D-PhCs. The silicon and fused silica substrates were cleaned sequentially with acetone, isopropyl alcohol (IPA), and deionized water and then dried under a flow of N2. The 1D-PhCs were fabricated through deposition (sputtering system) of alternating TiO2 and SiO2 quarter-wavelength layers on a cleaned fused silica substrate with deposition rates of 0.70 and 0.58 Å per second, respectively. Two kinds of graphene, single-layer and few-layer graphene, were prepared using a conventional chemical vapor deposition (CVD) process.44 These two kinds of graphene were then transferred onto the various substrates (area: ca. 1 cm2) through a polymer-mediated transfer technique.45 Optical Characterization of Samples. The reflectance (R) and transmittance (T) spectra of the samples were measured using a Hitachi U4100 optical spectrometer. The absorbance (A) of graphene samples was calculated using the equation:

A (%) = 100 − R (%) − T (%)

(1)

Raman spectra were collected using a confocal Raman microscope (WITec, CRM200) equipped with an excitation laser operated at a wavelength of 532 nm. A laser power of 10 mW and a signal integration time of 100 s were adopted because of the very weak Raman signals of graphene on a silicon substrate.

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RESULTS AND DISCUSSION In past studies, the major mechanism for enhancement of Raman scattering signals has been EM enhancement, which can be approximated by an E4 dependence, due to enhanced absorption of the incident laser light and the enhanced spontaneous emission of the Raman scattered light.46 Therefore, the key factors when investigating the IERS effect for enhancing the Raman scattering signals of graphene are the Efield intensities of both the incident and the Raman-scattered light. Although the E-field within the graphene layer should be calculated by considering a thin-film stack featuring a graphene layer on the top surface (see the transfer matrix method in the Supporting Information), the characteristic matrix of the graphene layer is almost a unity matrix because graphene is so thin that the phase factor (δ, see equation S4 in the Supporting Information) is almost zero. The E-field within a graphene layer can be approximated by the surface E-field of the thin-film stack in the absence of graphene. This approximation is reasonable because the properties of the graphene sample, such as its number of layers or optical properties, would be unknown prior to Raman inspection. Therefore, the case can be simplified so that the IERS effect is analyzed by considering only the surface E-field of the thin-film assembly. Using the transfer matrix method, we found that the relationship between the optical admittance and the amplitude of the surface E-field could be expressed by the equation (see also equation S11 in the Supporting Information): Esurf =

4 (α + 1)2 + β 2

Figure 1. (a) Schematic representation of the alternating high- and low-refractive-index quarter-wavelength layers. (b) Calculated amplitudes of E-fields within a stack of five thin films and within silicon and glass substrates. (c) Calculated amplitudes of surface E-fields of quarter-wavelength thin-film assemblies featuring different numbers of TiO2/SiO2 layer pairs. Inset: Corresponding simulated reflectance spectra of the quarter-wavelength thin-film assembly.

substrate.43 In the case of quarter-wavelength dielectric stacks, the extinction coefficient of the dielectric materials is zero, such that the dielectric layer possesses only the real part of the optical admittance. Because the value of δr is π/2 for a quarterwavelength dielectric layer, the characteristic matrix can be simplified as follows (see also equation S3 in the Supporting Information): ⎡ i⎤ ⎢ 0 ± ⎥ y r⎥ ⎢ ⎢±iy 0 ⎥ ⎦ ⎣ r

(3)

where yr is the admittance of the dielectric layer. Accordingly, the optical admittance of a quarter-wavelength dielectric layer deposited on a fused silica substrate is

(2)

where α and β are the real and imaginary parts of the optical admittance, given by Y = α + iβ, of the thin-film assembly. Therefore, the value of Esurf can be maximized to reach 2 when α and β both approach zero. To achieve this condition, as displayed in Figure 1a, we considered a sample featuring alternating quarter-wavelength thicknesses of high- and lowrefractive-index dielectric layers stacked on a fused silica

Y=

yr 2 yfused silica

(4)

where yfused silica is the admittance of a bare fused silica substrate. Because the admittances of fused silica and the dielectric layer C

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field to the strong interference effect of the quarter-wavelength thin-film assembly or 1D-PhCs upon increasing the number of TiO2/SiO2 stack pairs. As displayed in the inset to Figure 1c, the phenomenon could be visualized through the enhanced reflectance of the photonic stop-band of these 1D-PhCs. On the basis of these features, we expected the surface E-field to be maximized when using 1D-PhCs with a top layer of SiO2. According to our analysis above, we suspected that enhancement of the surface E-field would be very useful for increasing the Raman scattering signals of graphene through the IERS effect. On the basis of the EM enhancement mechanism, however, the surface E-fields of the incident and Raman-scattered light should both be taken into account.40,46 As a result, the E-field differences in the wavelengths of the excited laser, the D band, the G band, and the 2D band in the Raman spectra of graphene should all be considered at the same time. In this study, we fixed the wavelength of the Raman excited laser at 532 nm; thus, the corresponding wavelengths of the Raman D, G, and 2D bands of graphene, located near 1350, 1580, and 2680 cm−1, respectively, were approximately 573, 581, and 612 nm, respectively. Because the large differences among these wavelengths might result in different surface Efield enhancement factors, through the IERS and SERS effects, for these Raman bands, we analyzed the amplitude of the surface E-field as a function of wavelength to investigate the enhancement of each Raman peak of graphene. Figure 2a displays the calculated surface E-field spectra of 1D-PhCs comprising five TiO2/SiO2 pairs having respective thicknesses of 58/86, 61/91, 67/97, 70/103, and 74 nm/108 nm. We observe that the peak value of the surface E-field could be adjusted by tuning the thicknesses of the TiO2 and SiO2 layers. To estimate the enhancement ratio of the Raman peaks of graphene through the IERS effect, we calculated the amplitude of the surface E-field of the 1D-PhCs at wavelengths of 532, 573, 581, and 612 nm, respectively. We also calculated the surface E-field amplitudes of the fused silica and silicon substrates as references because graphene-based devices were usually prepared on these two substrates.9−12,39−41 On the basis of the EM enhancements of both the incident and the Ramanscattered light, we used the following equation to estimate the enhancement ratios of the Raman D, G, and 2D bands of graphene:

are both real, the optical admittance of the thin-film assembly, therefore, consists of only the real part; in other words, the value of β in eq 2 is always zero in such a quarter-wavelength thin-film system. On the basis of such analysis, we suspected that a quarterwavelength thin-film assembly could be a candidate for maximization of the surface E-field, because its value of β would always be zero. To verify this assumption, we analyzed the optical admittance of quarter-wavelength stacks comprising common TiO2 (n = ca. 2.20) and SiO2 (n = ca. 1.46) films as the high- and low-refractive-index layers, respectively. Here, the quarter-wavelength layer of TiO2 is denoted as “H” (high) and the quarter-wavelength layer of SiO2 as “L” (low) at a wavelength of 532 nm. Table 1 displays the calculated optical Table 1. Calculated Optical Admittances of Silicon, Fused Silica, and TiO2/SiO2 Quarter-Wavelength Stacks (1−4 Layers) at a Wavelength of 532 nm number of layers

Re(y)

Im(y)

1 (H) 2 (HL) 3 (HLH) 4 (HLH L/2) 4 (HLHL) silicon fused silica

3.285 0.649 7.391 0.555 0.288 4.150 1.461

0 0 0 −1.350 0 −0.044 0

admittances of 1−4 layers of TiO2/SiO2 quarter-wavelength stacks having respective thicknesses of 61 and 91 nm; it also lists the optical admittances of the bare silicon and fused silica substrates (notably, the optical admittances of silicon and fused silica substrates were equal to their complex refractive indices). Table 1 also displays the optical admittance of a four-layer stack comprising three-quarter-wavelength layers and one 1/8wavelength layer (denoted as HLH L/2) on a fused silica substrate. Figure 1b presents the calculated amplitudes of the Efields within the five thin-film samples and the bare silicon and fused silica substrates. These simulations reveal three important features. First, the imaginary part of the optical admittance was zero in all of the quarter-wavelength conditions, as predicted above. If the thickness of the stack did not match a quarterwavelength (e.g., in the case of HLH L/2), the imaginary part existed and the amplitude of the surface E-field could not be maximized (Figure 1b). Second, the amplitude of the surface Efield of the quarter-wavelength thin-film assembly could be enhanced effectively only when the top layer was of a lowrefractive-index material (SiO2). When the top layer of the thinfilm assembly was a high-refractive-index material (TiO2), the optical admittance of the thin-film assembly increased; therefore, the amplitude of the surface E-field would decrease. Third, the surface E-field of the quarter-wavelength thin-film assembly depended strongly on the number of paired layers. We investigated this feature by calculating the surface E-field of quarter-wavelength stacks composed of different numbers of TiO2/SiO2 layer pairs. The top layer had to be the lowrefractive-index material, because the optical admittance would be small and the E-field would be large. Figure 1c presents the calculated reflectance spectra and the optical admittances. When five pairs were present, the optical admittance approached zero; therefore, the amplitude of the surface Efield would be increased and saturated at approximately double that of the incident light. We attribute this largest surface E-

enhancement =

2 E PhCs,532 2 Esub,532

×

2 E PhCs,Raman 2 Esub,Raman

(5)

where EPhCs is the amplitude of surface E-field of the 1D-PhCs and Esub is the amplitude of the surface E-field of the silicon or fused silica substrate. The former term is the enhanced interaction of the incident light and graphene; the latter is the enhancement of the Raman scattered light. Figure 2b and c displays the estimated enhancements of the D, G, and 2D bands on the 1D-PhCs relative to those on the fused silica and silicon substrates, respectively. We attributed the higher enhancement ratio of the Raman peaks of graphene on the 1D-PhCs relative to that on silicon to the amplitude of the surface E-field on bare fused silica being higher than that on silicon. As displayed in Figure 2b and c, the IERS effect of the 1D-PhCs strongly influenced the enhancement ratios of the Raman peaks as a result of the different amplitudes of the surface E-field. By controlling the amplitude of the surface Efield, the enhancement ratio could reach as high as approximately 500 times when compared to that on a silicon D

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Figure 3. Simulated and measured reflectance spectra of a 1D-PhCs substrate having TiO2/SiO2 layers with thicknesses of 70 and 103 nm, respectively.

spectra in Figure 3 means that the preparation of the sample could be monitored well through optical simulation. After preparing the 1D-PhCs, we transferred a single-layer graphene (SLG) onto the silicon, fused silica, and 1D-PhCs substrates. Figure 4a displays the absorbance spectrum of the SLG on fused silica obtained from the measured reflectance and transmittance spectra. The measured absorbance was approximately 2.4%, consistent with the well-known value of 2.3% per graphene layer predicted by Kuzmenko et al.24 Figure 4b presents a photograph of the transferred SLGs on these three substrates. The visible contrast of the SLG was highest on the 1D-PhCs, whereas the SLG was almost invisible on the silicon substrate. We suspect that the higher visibility on the 1D-PhCs arose from the enhanced interference effect, as predicted by the enhanced surface E-field in our previous analysis. Therefore, we expected that enhanced light−graphene interactions on the 1DPhCs would help to increase the Raman scattering signals of the graphene. To verify the IERS effect, we measured the Raman spectra of the transferred SLG on the silicon, fused silica, and 1D-PhCs substrates (Figure 4c). Although the Raman scattering signals were very weak for the SLG on the silicon substrate, the G and 2D bands of the graphene near 1580 and 2680 cm−1 could still be observed in each of these three samples. We also found that the Raman scattering signals of graphene were largely enhanced for the SLG on the 1D-PhCs substrate. In addition, because of the high quality of the SLG prepared in this study, its D band was too weak to be detected on the silicon and fused silica substrates; we could observe a very weak D band (ca. 1350 cm−1) for the graphene only on the 1D-PhCs substrate. Figure 4d displays the relative measured intensities of the G and 2D bands of the SLG on the silicon, fused silica, and 1D-PhCs substrates, respectively. Here, we normalized the relative intensities with respect to the intensity of the G band of the SLG on fused silica or silicon substrate. In this Article, we focus only on these two Raman peaks of graphene, because the intensity of the D band for graphene was almost zero on the silicon and fused silica substrates. The ratio of the G and 2D band intensities was approximately two for the SLG on the silicon and fused silica substrates. When we applied the 1DPhCs having the optimized surface E-field, the intensities of the G and 2D bands of the SLG both increased significantly. As displayed in Figure 4d, the measured enhancement ratio of the G band of SLG on 1D-PhCs was approximately 13 relative to that on fused silica, and reached 146 relative to that on silicon. In addition, the measured enhancement ratio of the 2D band of the SLG on the 1D-PhCs was approximately 12 relative to that

Figure 2. (a) Surface E-field spectra of 1D-PhCs featuring five TiO2/ SiO2 pairs having respective thicknesses of 58/68, 61/91, 67/97, 70/ 103, and 74 nm/108 nm. (b,c) Calculated enhancement factors of the G and 2D bands on the 1D-PhCs substrates relative to those on (b) fused silica and (c) silicon substrates.

substrate. Moreover, the D, G, and 2D bands of graphene could all be enhanced equally, by approximately 14 times, relative to that on a fused silica substrate, as well as by approximately 180 times relative to those on a silicon substrate. To investigate experimentally the IERS effect on the Raman scattering signals of graphene by optimizing the surface E-field, we prepared three kinds of substrates: silicon, fused silica, and a 1D-PhCs structure. We used a sputtering system to fabricate a 1D-PhCs structure featuring five pairs of TiO2/SiO2 layers having respective thicknesses of 70 and 103 nm. We chose this 1D-PhCs structure to obtain coenhanced Raman bands of graphene because our analysis predicted that the intensities of the D, G, and 2D bands would all be enhanced by the same ratio. Figure 3 displays the simulated and measured reflectance spectra of the 1D-PhCs structure comprising five pairs of TiO2/ SiO2 layers. Notably, the surface E-field spectra in Figure 2 are quite different from the reflectance spectra. Although the surface E-field spectra could not be observed directly using any type of optical spectroscopy, the good match in the reflectance E

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Figure 4. (a) Absorbance spectrum of SLG on fused silica. (b) Photograph of transferred SLGs on silicon, fused silica, and 1D-PhCs substrates. (c) Measured Raman spectra of transferred SLGs on silicon, fused silica, and 1D-PhCs substrates. (d) Measured relative intensities of the G and 2D bands of SLGs on silicon, fused silica, and 1D-PhCs.

Figure 5. (a) Absorbance spectrum of an FLG on fused silica. (b) Photograph of transferred FLGs on silicon, fused silica, and 1D-PhCs substrates. (c) Measured Raman spectra of transferred FLGs on silicon, fused silica, and 1D-PhCs substrates. (d) Measured relative intensities of the G and 2D bands of FLGs on silicon, fused silica, and 1D-PhCs.

on fused silica, and reached 150 relative to that on silicon. Although these enhancement ratios are slightly lower than those predicted in Figure 2b and c (ca. 14 relative to that on fused silica substrate; ca. 180 relative to that on silicon substrate), they were almost identical for both the G and the

2D bands. Therefore, the ratio of the intensities of the G and 2D bands remained at approximately two after introduction of the 1D-PhCs; thus, coenhanced Raman spectra of graphene, with the same enhancement ratio, could be achieved upon the 1D-PhCs substrate. F

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Chemistry of Materials We also used a few-layer graphene (FLG) sample to verify the coenhancement effect in the Raman spectra after transferring it onto silicon, fused silica, and 1D-PhCs substrates. Figure 5a displays the absorbance spectrum of the FLG on a fused silica substrate; the absorbance of the FLG (ca. 7%) is close to the value of three-layer graphene (6.9%).5,24 Figure 5b presents a photograph of the FLG transferred onto silicon, fused silica, and 1D-PhCs substrates. Again, the visibility of the FLG on the 1D-PhCs substrate is the highest among these three samples, suggesting that the light−graphene interaction might be enhanced on this substrate. Figure 5c displays the measured Raman spectra of the FLG on silicon, fused silica, and 1D-PhCs substrates. Again, the intensities of the signals in the Raman spectrum were enhanced dramatically by the 1D-PhCs substrate. In this case, we could observe the D band of graphene on both the fused silica and the 1D-PhCs substrates, arising from the relatively poor quality of the FLG. Although the D band of the graphene on the fused silica substrate was evident, its intensity was too weak to readily identify the defect level of the FLG. Therefore, the enhancement of the Raman scattering signals induced by the 1D-PhCs was very helpful for the characterization of graphene. Figure 5d displays the relative measured intensities of the G and 2D bands of the FLG on the silicon, fused silica, and 1D-PhCs substrates, respectively. Although the error bar in Figure 5d is larger than that in Figure 4d due to the poorer quality of the FLG, it still reveals that the ratio of the intensities of the G and 2D bands for the FLG on each of the three substrates was approximately 1.1, similar to the case for the SLG (Figure 4d). The enhancement ratio of the G band of the FLG on the 1D-PhCs substrate was approximately 10 times that on fused silica, and reached 120 times relative to that on silicon. The enhancement ratio of the 2D band of the FLG on the 1D-PhCs substrate was also approximately 10 times relative to that on fused silica, and reached 118 times relative to that on silicon. Thus, the G and 2D bands were coenhanced for both the SLG and the FLG samples, with the same enhancement ratio. Notably, the measured enhancement ratios were lower than the predicted ones, presumably because the amplitude of the E-field within the graphene (SLG or FLG) was slightly lower than that on the surface of the 1D-PhCs. Nevertheless, because graphene possesses universally low absorption over a wide spectral range, the coenhanced Raman spectra of graphene, with the same enhancement ratio, was still possible because the effect of absorption would be the same for both its G and 2D bands, as displayed in Figures 4d and 5d. In addition to coenhancement of the signals in the Raman spectra with the same enhancement ratio, as a result of modulation of the surface E-field, another important property of 1D-PhCs is the high variety of available thin-film materials that could prevent external disturbance of the Raman signals arising from different substrates. Thus, we investigated the effect of the substrates, silicon, fused silica, and 1D-PhCs, on the peak positions of the G and 2D bands of the transferred graphenes. Figure 6a and b displays the peak shifts of the G and 2D bands of the SLG and FLG, respectively, relative to their peak positions on the silicon substrate. The surface of 1D-PhCs substrate in this study was SiO2; therefore, it had the same surface properties as silicon (although there were a few nanometers of native oxide on the surface of silicon) and fused silica. Indeed, we observed almost no peak shifts for any of the three samples. As displayed in Figure 6, and according to ref 39, a large peak shift (>10 cm−1) can occur for graphene samples

Figure 6. Peak shifts of the G and 2D bands of (a) an SLG and (b) an FLG, relative to their peak positions on a silicon substrate.

coated onto metallic-based structures exhibiting the SERS effect.39 Therefore, this 1D-PhCs-based IERS technique appears to be a much better means of enhancing Raman spectra, based on the band-to-band ratios and peak positions both being unaffected by the nature of the substrate. Our experimental results confirm that the fine structures of graphene samples can be readily identified in their coenhanced Raman spectra, allowing identification of the natural properties of the graphene. Through the IERS effect, the Raman scattering signals can be enhanced up to 150 times for SLG and 120 times for FLG. To the best of our knowledge, these enhancement ratios are almost the highest ever obtained when using the IERS technique. The experiments described above were all performed under normal illumination of the exciting light source, with the surface E-field being modulated by the thicknesses of the layers in the thin-film assembly. We have found, however, that modulation of the surface E-field of 1D-PhCs can also be achieved by changing the incident angle of the light source, an important property for Raman spectroscopy performed under oblique incidence. Therefore, the working wavelength of 1D-PhCs substrates can be tuned readily. The detailed discussion can be found in the Supporting Information.

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CONCLUSIONS We have performed a systematic study of the IERS effect on graphene by simulating the surface E-field of the underlying substrates, using the tools of thin-film optics. The surface Efield could be calculated precisely in terms of the optical admittance of the thin-film assembly. To maximize the surface E-field, we found that both the real and the imaginary parts of the effective optical admittance on the underlying substrate should approach zero. 1D-PhCs, comprising quarter-wavelength layers of alternating TiO2 and SiO2 layers and a top layer of SiO2, could achieve this condition. We optimized the surface G

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Chemistry of Materials

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E-field so that the enhancement factors of both the Raman G and 2D bands of graphene could be predicted rationally. By increasing the amplitude of the surface E-field, the intensities of the G and 2D bands of graphene could be largely and simultaneously coenhanced, by approximately 14 times relative to those on a glass substrate and by approximately 180 times relative to those on a silicon substrate. Under some conditions, the intensities of the Raman peaks of graphene could be enhanced by even more than 400-fold. After transferring SLG and FLG onto silicon, glass, and 1D-PhCs substrates, we found that the Raman spectra of the SLG and FLG could both be enhanced on the 1D-PhCs substrate, without changing the intensity ratio or peak positions of the G and 2D bands. Therefore, without any additional signal disturbance, this enhancement technique could maintain the correct and precise information from the Raman spectra. The coenhanced Raman spectra of graphene, with enhancement factors as high as 150 relative to the signals for graphene on silicon, display among the highest degrees of experimental enhancement ever obtained using only an IERS effect. Moreover, we also demonstrated that changing the incident angle of the exciting light source could modulate the surface E-field of 1D-PhCs, allowing fine-tuning of the working wavelengths. By controlling the surface E-field, the Raman signals of graphene can be enhanced dramatically while avoiding any disturbance to the signals in the Raman spectra that arise from the enhancing technique. Thus, exploiting the optimized IERS effect of a 1D-PhCs substrate appears to be a very useful approach for fine structural characterization of graphene through conventional Raman spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

Calculation of the surface electric field and the additional discussions on the modulation of the surface E-field of 1DPhCs by changing the incident angle of the light source. 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 We thank the Ministry of Science and Technology, Taiwan, for supporting this study under contracts MOST-103-2221-E-002041-MY3 and MOST-103-2221-E-002-092-MY3, and the National Chung-Shan institute of Science and Technology, Taiwan, for supporting this study under contract CSIST-010V401 (103).

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REFERENCES

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DOI: 10.1021/cm504003t Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/cm504003t Chem. Mater. XXXX, XXX, XXX−XXX