Article pubs.acs.org/ac
Using Optical Anisotropy as a Quality Factor To Rapidly Characterize Structural Qualities of Large-Area Graphene Films Yu-Lun Liu,† Chen-Chieh Yu,† Cheng-Yi Fang,† Hsuen-Li Chen,*,† Chun-Wei Chen,† Chun-Chiang Kuo,∥ Cheng-Kai Chang,§ Li-Chyong Chen,‡ and Kuei-Hsien Chen∥,‡ †
Department of Materials Science and Engineering, ‡Center for Condensed Matter Science, and §Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan ∥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan S Supporting Information *
ABSTRACT: In this study, we find that the optical anisotropy of graphene films could be used as an alternative quality factor for the rapid characterization of large-area graphene films prepared through chemical vapor deposition. We develop an angle-variable spectroscopic method to rapidly determine the optical anisotropy of graphene films. Unlike approaches using Raman scattering spectroscopy, this optical anisotropy method allows ready characterization of the structural quality of largearea graphene samples without the application of high-intensity laser irradiation or complicated optical setups. Measurements of optical anisotropy also allow us to distinguish graphene samples with different extents of structural imperfections; the results are consistent with those obtained from using Raman scattering spectroscopy. In addition, we also study the properties of graphenebased transparent conductive films at wide incident angles because of the advantage of the optical anisotropic properties of graphene. The transmittance of graphene is much higher than that of indium tin oxide films, especially at large incident angles. raphene, the first developed example of a two-dimensional nanomaterial, has attracted much attention for its amazing electronic, optical, mechanical, and magnetic properties.1−5 Graphene differs from conventional semiconductors because of its nonexistent band gap and linear dispersion properties, making it useful for various optoelectronic devices.6,7 Graphene’s sp2-hydridized carbon atoms are packed in a honeycomb-like crystal lattice, forming a nearly ideal flat two-dimensional nanostructure, with potentially attractive properties for device integration. To date, most graphenebased materials have been developed for applications in nanoelectronics, optoelectronics, and spin electronics.8−11 Optoelectronic devices typically employ graphene-based materials as active layers, hole-injection layers, or flexible transparent electrodes; they will probably play important roles in future flexible devices.12 Organic photovoltaic devices (OPVs) incorporating graphene as a key electrode material have the main advantages of being cheap and flexible. Conventionally, indium tin oxide (ITO) is used as the transparent electrode, but the scarcity of its supply and its ceramic nature impose serious limitations on its applicability. The high transparency and high electrical conductivity of graphene sheets make these two-dimensional materials excellent candidates for replacing indium tin oxide (ITO) electrodes. Because the intrinsic sp2-hybridized structure favors transparency and conduction, graphene films could potentially be used at thicknesses of only several atomic layers.13−15
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© XXXX American Chemical Society
Recently, Li et al. proposed a method for preparing uniform graphene layers with large areas through chemical vapor deposition (CVD).14 This approach has solved the main problem of the traditional Scotch tape peeling method, which allows the preparation of only small-area graphene samples.1 Their surface-catalyzed process has opened up a new route for the scalable and repeatable preparation of graphene films. Combined with the transfer process, the preparation of largearea graphene films on various substrates has received much attention for potential application in practical devices.16 In addition, Bae et al. demonstrated an approach using a roll to roll process for the large-area preparation of graphene on transparent electrodes.17 Optoelectronic devices featuring graphene electrodes can be fabricated with fewer process restrictions. Electrodes based on two-dimensional nanomaterials have the potential to facilitate the development of novel devices; nevertheless, the optical properties of large-area graphene remain much less understood than their electronic properties.18,19 Because single-layer graphene has an optical absorption of approximately 2.3%, many researchers have focused on the optical properties of graphene films in the visible and ultraviolet (UV) regimes when developing related solar cells and detectors.9,20,21 Several different approaches have been Received: October 2, 2012 Accepted: December 31, 2012
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intensity laser irradiation and complicated optical facilities typically employed to measure the spectrum. Raman scattering is time-consuming when large-area detection is needed. The intensity and stability of the laser source, as well as the low inelastic scattering intensity, which is approximately 10−9 times lower than that of the incident laser light, should also be considered. The lower yield of the Raman signal may cause inconsistency between different detection points when the quality of large-area graphene films is needed to be determined. In this study, we employed optical anisotropy properties to characterize large-area, CVD-prepared, single-layer graphene. Without the need for high-intensity laser irradiation or complicated optical setups, we readily measured and calculated the optical anisotropy rapidly to characterize the structural qualities of large-area graphene samples prepared under various conditions, and the results were comparable to those obtained from Raman scattering spectroscopy.
developed to determine the optical constants (refraction index, extinction coefficient) of graphene films in different wavelength regimes.22,23 Ni and co-workers used contrast simulation to determine the optical constants of different layers of graphene.22 Blake et al. determined the optimal thickness of a SiO2 layer on a silicon wafer necessary to measure graphene samples with large reflection contrast.23 Furthermore, they found that graphene exhibited excellent transverse conductivity as a result of the peculiar structure of its two-dimensional planar system. An interesting optical phenomenon of this planar structure is that it induces high anisotropy of the optical constants. The first study of graphite’s anisotropy revealed that the extinction coefficient was zero when the electric field of incident light was perpendicular to the perfect carbon atom plane.24 In the early theoretical simulations, Kravets et al. also proposed no absorption in the z-axis of single-layer graphene (SLG) films.25 Here, we suggest an alternative characterization method for a two-dimensional nanostructure by measuring the optical anisotropy of graphene films. The main objective of this study is to provide a rapid, nondestructive, and low-cost method to identify large-area graphene films with different qualities. Because graphene possesses a two-dimensional flat planar structure, its absorptions of polarized light varied with incident angles. In a previous study, we demonstrated the strong influence of regioregularity (RR) on the optical anisotropy of hybrid poly(3-hexylthiophene) (P3HT)/6,6-phenyl-C61-butyric acid methyl ester (PCBM) films.26 We characterized the optical anisotropy of P3HT/PCBM films possessing different RRs and subjected them to different annealing processes. Unlike grazingincidence X-ray diffraction (GIXRD) analyses, optical anisotropic measurements are convenient for simultaneously studying polymer orientations and device anisotropic absorption.26 We found that the optical anisotropy of the polymer/fullerene blends was approximately 1.6 after they were subjected to the optimal annealing conditions. To the best of our knowledge, no previous studies have focused on the optical anisotropy of CVD-prepared graphene, and no methods related to optical anisotropy have been proposed to characterize the quality of graphene films. Key requirements for the characterization of graphene are the method is fast, nondestructive, and cost sparing. For these main purposes, spectroscopic analysis is a powerful and reliable technique for structural identification and determination. Raman scattering is a useful technology for determining the quality of graphene films and has been used previously to characterize the material properties of graphene films prepared under various conditions. It provides a direct and statistically sound procedure to confirm the quality, doping, and number of layers in the graphene application.27−39 Generally, there are three main peaks in the Raman spectra of graphene, including D, G, and 2D bands. As Kidanbi et al.38 mention, the D band to G band ratios can be used to identify the crystalline quality of CVD graphene under different growth parameters. This method is widely used for characterizing the defect density in a graphene surface nowadays. Besides, Yan et al.39 used the ratios of the 2D band to G band for identifying the numbers of graphene layers. Kim et al.37 used the relative position (D, G, and 2D) in Raman spectra to identify the carrier concentration in graphene samples. Therefore, Raman scattering is wellknown as an important technique for the characterization of graphene, but the Raman scattering spectrum can be used just to measure only the local part of the sample, requiring high-
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EXPERIMENTAL SECTION Preparation of CVD-Grown Graphene. A conventional CVD process similar to that reported by Li’s group was used to prepare the graphene samples.14 The polyfoil was cut into suitable pieces and placed on a quartz holder which was then placed in a furnace and heated at different temperatures (750− 970 °C). This process produced SLG films deposited on the surface of the copper foil. Hydrogen was then passed into the quartz tube to prevent oxidation of the copper foil. After the CVD growth process, a transfer method was used to transfer the graphene films onto the glass substrate (B270) through a polymer-mediated transfer technique,16 and the area of the graphene sample was ca. 4 cm2. Characterization. Raman scattering spectroscopy (JobinYvon LabRAM H800) was used to characterize graphene samples prepared under various CVD processing conditions. The spot size of light for measuring Raman spectroscopy was 10 μm2. A four-point probe system was used to measure the sheet resistances of the graphene films. The reflection and transmission spectra of the graphene films at different incident angles in the visible and near-infrared (IR) regimes were measured using a Hitachi U4100 spectrometer. The spot size for measuring reflection and transmission spectra was 25 mm2. The measurements of reflectance and transmittance were carried out five times to exclude the instability of the facilities. We calculated the optical anisotropy and added error bars for averaging the results.
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RESULTS AND DISCUSSION For the preparation of graphene films with different qualities, a conventional CVD process was used to prepare the graphene samples.14 First, a piece of copper polyfoil was used as a metal catalyst to synthesize a graphene film. The copper polyfoil was cut into suitable pieces and placed on a quartz holder, which was then placed into a furnace and heated at different temperatures from 750 °C (sample 5) to 970 °C (sample 1). Here we define the samples prepared under growth temperatures of 970, 920, 870, 820, and 750 °C as sample 1 to sample 5, respectively. Because of the low solubility of carbon in the copper foil, we could control the formation of SLG films on the surface. After the growth procedure, a polymer-mediated transfer technique was used to transfer the graphene films onto the glass substrates (B270). These processes produced B
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Figure 1. (a) Photographic image of the transferred graphene on a glass substrate. (b) Transmittance behavior and (c) surface morphology (5 μm × 5 μm) of CVD-grown SLG films on glass. (d) Thickness and surface roughness of CVD-grown SLG films featuring different qualities.
Table 1. Transmittance, Thickness, RMS Roughness, and Sheet Resistance of CVD-Grown SLG Films Featuring Different Qualities transmittance at 600 nm (%) thickness (nm) RMS roughness (nm) sheet resistance (Ω/□)
sample 1
sample 2
sample 3
sample 4
sample 5
97.50 1.093 0.531 0.554K ± 152
97.22 1.131 0.343 1.054K ± 193
97.17 1.251 0.492 1.426K ± 234
97.23 0.992 0.229 1.978K ± 365
97.21 1.208 0.706 2.679K ± 346
SLG films on the glass substrate with different structural qualities. For being applied as a transparent electrode, the basic requirement for a stacked graphene film is large area and high transmittance. Figure 1a displays the photographic image of CVD-prepared SLG films that we transferred onto the glass substrates having an area of approximately 4 cm2. Optical transmittance of the transferred graphene from the ultravisible to near-infrared region was measured with Hitachi U4100 spectrometer. Figure 1b displays the transmittance of the SLG films in the spectral regime from 240 to 2200 nm. The transmittance of all the SLG films was approximately 97% in the visible regime; this value was close to the ideal transmittance reported in the literature.17 As displayed in Figure 1b, we could not clearly identify the difference between different SLG films from the transmittance spectra. Figure 1c displays the AFM image of CVD-prepared SLG films, showing the uniform morphology of the sample surface. Moreover, we also determined the thickness and surface roughness of different samples in Figure 1d. The figure revealed an almost identical thickness of about 1 nm for all five samples. In
addition, the surface roughness also indicated that the five CVD graphene films under different growth conditions were almost the same. Table 1 lists the transmittance, thickness, surface roughness (root mean square, RMS), and sheet resistance of CVD-grown SLG films. From Table 1, we found that the transmittance at 600 nm, thickness, and RMS roughness varied slightly among the five samples, and the measurements could not be used to justify the difference between those samples. For a lack of proper methods to identify the sample qualities, some studies also took the sheet resistance as an important factor.33−35 Besides, the sheet resistance was also an important quality factor for graphene films. Here, the sheet resistance measured by a four-point probe was increased from 554 Ω/□ (sample 1) to 2679 Ω/□ (sample 5). We suspect this wide range of resistance would result from the induced defect/disorder at the different CVD process temperatures. The sheet resistance is a contact mode measurement, and the test electrodes must touch the sample for measuring the resistance. It may damage or add contaminations on the surface of ultrathin graphene films. However, the optical anisotropy is based on a noncontact C
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Figure 2. (a) Schematic diagram of Raman detection at different positions. (b) Raman spectra of samples 1−3 at different positions. (c) Average Raman spectra of SLG films featuring different qualities (sample 1 to sample 5).
These peaks are located around 1580 and 1360 cm−1, respectively.36,37 Imperfections in graphene films, such as structural defects, would also increase the intensity of the D band. The D band to G band ratio generally reveals the defect densities and grain sizes in graphene layers.37 The quality factor of a graphene film is often identified from its D band to G band ratio.29,37−39 The 2D band corresponds to a high-energy second-order mode of the D band, which is observed even in the absence of the D band. The 2D band is originated from a second-order mode of zone-boundary phononstwo phonons with opposite momentums in the highest optical branch near the K point in reciprocal space. The frequency of the 2D band is approximately 2 times that of the D band.27,29 As Ferrari mentions, this band is used when trying to count the numbers of graphene layers. There is a general band shift of the 2D band toward higher wavenumbers as the layer thickness increases, and the more pronounced change is in the band shape. On the other hand, Ferrari et al. also used the 2D band to G band ratio for identifying the numbers of graphene layers (single layer, bilayer, or multilayer graphene). It is worth noting that the 2D
optical measurement, and thus, it can provide a fast, uniform, and nondestructive method for graphene characterization. Next, we also used Raman scattering spectroscopy to characterize the CVD-prepared graphene films featuring different growth conditions. The instrument setup is displayed in Figure 2a, and we measured the Raman scattering intensity at different positions on the sample surface and averaged those signals. As displayed in Figure 2b, the five different positions on the sample surface represented the different Raman scattering signals from the centimeter-scale graphene film (sample 1 to sample 3). From Figure 2b, different signals generated from different detection points present inconsistent signals through Raman scattering. In addition, we also found that the uniformity of the Raman scattering signals slightly decreased as the growth temperature became lower. Parts b and c of Figure 2 reveal different signal intensities of the main peaks (e.g., D, G, and 2D bands) for the three different graphene films. The Raman spectra of the samples show the G band (due to bond stretching between pairs of sp2 carbon atoms) and the D peak (due to breathing modes of sp2 carbon atoms in rings). D
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Figure 3. (a) Schematic representation of angle-variable reflectance and transmittance under different types of polarized light. (b) Absorption coefficients under different types of polarization at different incident angles. (c−e) Comparisons of absorption coefficients of TE- and TM-polarized light at incident angles of (c) 20°, (d) 40°, and (e) 60°.
examine the optical anisotropy properties of graphene films with different numbers of stacked layers. First, TE-polarized light is used to calculate the in-plane optical absorption; next, the TM-polarized light is used to calculate the out-of-plane optical absorption of the graphene films. We employed the following equation reported by Bruna et al.40 to determine the anisotropic absorption properties of our graphene films:
band is sensitive to determine the graphene layers. As displayed in Figure 2c, the increasing defect densities of CVD-prepared SLG films were displayed by the different D band to G band ratios from sample 1 (ca. 0.24) to sample 5 (ca. 1.57), presumably arising from the increasing defect densities on the sample surface. Ferrari discusses in detail the effects of edge states, essentially stating that the defects give rise to the D band. The D band to G band ratio has been used for characterizing graphene qualities.29 Generally, the higher defect density for graphene induced the higher defect-related D-band peak, and this is the reason that the D band to G band ratios increased for graphene films with poorer quality.29 In this paper, we demonstrate that the optical anisotropy of graphene films can play an important role when identifying the sample quality of these two-dimensional nanomaterials. We used an optical spectrometer to measure the optical anisotropy of our three graphene films (sample 1 to sample 3). As displayed in Figure 3a, we could determine the optical anisotropy of these two-dimensional materials by measuring the transmission and reflection spectra for transverse electric (TE) and transverse magnetic (TM) polarized light at different incident angles. Here, we define the TE-polarized light as the direction of the electric field being perpendicular to the incident plane and parallel to the plane of the graphene surface at any incident angle. In this case, the TE-polarized light is also referred to as the in-plane incident light. The TM-polarized light is defined herein as the electric field parallel to the incident plane. In this paper, the TM-polarized light is also referred to as out-of-plane incident light. Using this approach, we could
⎧I ⎫ 4πk 1 1 ⎬ =− ln⎨ λ nd path ⎩ I0 (1 − R ) ⎭ cos θ ⎧ (1 − R ) ⎫ ⎬ = ln⎨ ⎩ T ⎭ nd
α=
where d path =
d cos θ
(1)
where α, n, k, and λ represent the absorption coefficient, refraction index, extinction coefficient, and incident wavelength, respectively. We used the reflectance (R) and transmittance (T) in Figure S-1 (Supporting Information) to calculate absorption coefficients at different incident angles (θ) while considering the effect of the effective optical path (dpath) at different incident angles. Parts b−e of Figure 3 display the absorption coefficient of CVD-prepared SLG films at different incident angles and polarizations. The absorption coefficients of graphene films for TE-polarized light remained almost constant upon changing the incident angle in Figure 3b. In contrast, the absorption coefficients for the TM-polarized light decreased considerably E
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Figure 4. Optical anisotropy of (a) CVD-grown SLG films (sample 1) in the visible regime and (b) five CVD-grown SLG films (sample 1 to sample 5) at various incident angles at a wavelength of 633 nm.
Figure 5. (a) Schematic representation of different absorption behaviors of CVD-grown SLG films with different polarizations. (b) Correlation between the optical anisotropy (left, 600 nm for 60°; right, 633 nm for 80°) and the D band to G band ratios from Raman scattering spectra.
value if the electric field of the polarized light were nearly perpendicular to the surface of the graphene film. As displayed in parts c−e of Figure 3, the difference in absorption coefficients between the TE- and TM-polarized light increased upon increasing the incident angle from 20° to 60°. To have a clear expression of the anisotropic absorption for a graphene layer, we define the ratio of absorption coefficients between the TE- and TM-polarized light (αTE/αTM) as the optical anisotropy for CVD-prepared graphene films at different incident angles. As displayed in Figure 4a, the optical anisotropy (αTE/αTM) of the one-layer graphene film in the visible regime increased from approximately 1.2 to 2.5 when the incident angle increased from 20° to 60°. From these result, the optical anisotropy properties appear to be in line with the structural defects in the carbon atom plane. A graphene layer
upon increasing the incident angle. Thus, the absorption coefficients of the graphene layer depended strongly on the degree of polarization, resulting in different absorption coefficients for different polarizations. If the electric field of the polarized light were parallel to the graphene surface, the absorption would be greater than that of other polarized light. In other words, the absorption coefficient would reach its largest value in the in-plane direction. If the electric field of the polarized light were not parallel to the graphene surface, however, the absorption coefficient would separate into two components: one parallel and the other vertical to the graphene surface. Upon increasing the incident angle, the absorption of the TM-polarized light decreased, because the angle between the electric field and the sp2-hybridized carbon plane became larger. The absorption coefficient would reach its minimum F
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Table 2. D Band to G Band Ratios, Optical Anisotropy, Transmittance, and Sheet Resistance of CVD-Grown SLG Films under Different Processing Conditions D band/G band ratio optical anisotropy, αTE/αTM(600 nm, 60°) optical anisotropy, αTE/αTM(633 nm, 80°) transmittance at 600 nm (%) sheet resistance (Ω/□)
sample 1
sample 2
sample 3
sample 4
sample 5
0.236 ± 0.081 2.51 ± 0.058 3.77 ± 0.082 97.50 0.554K ± 152
0.250 ± 0.063 2.04 ± 0.059 2.85 ± 0.081 97.22 1.054K ± 193
0.642 ± 0.206 1.85 ± 0.093 2.38 ± 0.110 97.17 1.426K ± 234
0.798 ± 0.234 1.65 ± 0.034 1.99 ± 0.079 97.03 1.978K ± 365
1.571 ± 0.830 1.34 ± 0.037 1.51 ± 0.055 97.01 2.679K ± 346
Table 3. Comparison between Scattering and Optical Anisotropy optical mechanism information detection area facility requirement energy density signal yield
Raman scattering
optical anisotropy
nonlinear optics (inelastic scattering) information abundance (structural justification) localized area laser (complicated) high, 104−108 (mW·cm−2) 10−9 (weak)
linear optics (transmittance and reflectance) single message (the 2D material only) entire region any light source (simple) low, 10−5 (mW·cm−2) 1 (strong)
and orange) represent the different polarized light (TE and TM). From the schematic diagram, the parallel absorption (αTE) in the CVD-prepared SLG films would decrease as the quality gets worse, presumably arising from structural imperfections. To investigate the correlation between the optical anisotropy and the structural quality of the graphene, we compared the D band to G band ratios from the Raman scattering spectra with the optical anisotropy in Figure 5b. Figure 5b reveals that the optical anisotropy decreased as the D band to G band ratio increased in the Raman scattering spectra. This phenomenon is readily rationalized by considering that structural defects in the two-dimensional nanostructure would affect the optical absorption of the in-plane direction. The in-plane optical absorption is related to the carrier density of the graphene plane; it is strongly affected by defects. From Figure 5b, we find the optical anisotropy was inversely related to the D to G ratios for different graphene qualities. Here we found the variation of the D band to G band ratios increased as the sample qualities got worse, but the measured result of optical anisotropy is with less fluctuation compared with the signal obtained from Raman scattering. The error bar of the optical anisotropy was much smaller than that obtained from Raman scattering measurement. The detection area of Raman scattering was just a few square micrometers, and thus, the differences in signals from point to point measurement was more obvious than that using optical anisotropy in large-area measurement, and this is the reason why the Raman scattering signal has a larger fluctuation. Moreover, this method provided the flexibility of using broadband wavelengths in the visible ranges when applying the anglevariable optical measurement (left, 600 nm for 60°; right, 633 nm for 80°). In contrast to the quality identification by Raman scattering, the data we show prove that optical anisotropy has the advantage of convenience when choosing different optical sources. Table 2 lists the values of optical anisotropy and D band to G band ratios. Sample 1 had the lowest D band to G band ratio (ca. 0.24) and the highest optical anisotropy (ca. 3.77) because of its nearly perfect graphene planar structure. At the lower growth temperature, the D band to G band ratios of samples 3−5 were much higher than that of sample 1 because of the increasing defect densities of the graphene layer. Because sample 5 was grown at the lowest temperature, its D band to G
features more free electrons in the in-plane than in the out-ofplane direction. Furthermore, in this study we found that the structural quality of graphene films significantly affected their optical anisotropy at different incident angles. Although infinite optical anisotropy has been found in perfect two-dimensional nanomaterials,25 large-area CVD-prepared graphene films typically possessed defects generated during the growth and transfer processes which undetermined their structural quality as two-dimensional nanomaterials.15,16,41 These structural defects of the plane decreased not only the conductivity in the plane but also the optical anisotropy. For larger incident angles, we used a He−Ne laser to characterize the optical anisotropy (αTE/αTM) of our CVD-prepared SLG films at incident angles from 20° to 80°. Figure 4b reveals that the optical anisotropy of sample 1 increased from 1.20 at an incident angle of 20° to 3.77 at 80°. Notably, the optical anisotropy of this graphene film is much greater than that of the P3HT/PCBM blend films that we examined previously.26 We also determined the optical anisotropy of graphene films with different structural qualities prepared through the CVD process. Figure 4b reveals that the optical anisotropy decreased significantly with decreasing quality of SLG films, especially when measured at large incident angles. We attribute the decreased optical anisotropy of SLG films to their poor structural quality arising from the different growth conditions. To investigate the relationship between the optical anisotropy and structural quality of two-dimensional nanomaterials, we used Raman scattering spectroscopy to determine the quality of the CVD-prepared SLG films prepared under various process conditions. Raman scattering is a very useful method for analyzing carbon-based materials; previous studies have demonstrated that the D band to G band ratio in Raman scattering spectra can be used as a quality factor when identifying graphene films.39,40 Typically, high-intensity laser light is irradiated only at certain areas of the graphene samples to obtain clear Raman scattering spectra. As a result, it is difficult to simultaneously measure the properties of an entire large-area CVD-prepared SLG film. In contrast, optical anisotropy is a very convenient tool for the large-area identification of CVD-prepared graphene films. Furthermore, increasing the defect densities of CVDprepared SLG films deteriorated the crystal quality. As displayed in Figure 5a, the arrows with different colors (green G
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Figure 6. Simulated (a) reflectance, (b) transmittance, (c) absorbance, and (d) normalized transmittance data for graphene (five-layer) and ITO (175 nm) films at various incident angles.
structural quality of large-area graphene samples without the need for high-intensity laser irradiation or the complicated optical setups of Raman scattering spectroscopy. Next, we used graphene to serve as a transparent conductive film owing to its optical anisotropy. We simulated the optical behavior of graphene films and ITO films when serving as transparent conductive films. We used the optical constants obtained from our experiments and calculation, combined with an optical thin-film model, to analyze and compare the feasibility of graphene- and ITO-based transparent conductive films at different incident angles. To obtain the same transmittance (ca. 80%) at normal incidence, we set the thickness of the graphene and ITO films at 1.67 nm (five-layer) and 175 nm, respectively, both on fused silica substrates. Figure 6 displays the simulated reflectance (R), transmittance (T), and absorbance (A) of the graphene- and ITO-based transparent electrodes at different incident angles and polarizations. The reflectances of the two transparent electrodes are almost identical in Figure 6a. We attribute the differences between the TE- and TM-polarized light primarily to the Brewster angle effect of the fused silica substrate below. Because of the increased reflectance of the TE-polarized light, the transmittance decreased upon increasing the incident angle for both the ITO and graphene films in Figure 6b. Accordingly, as a result of decreased reflectance, the transmittance of the TMpolarized light increased upon increasing the incident angle. The transmittance behaviors of the ITO and graphene films were different. The transmittance of a graphene film is higher than that of ITO at large incident angles because of the optical anisotropy of graphene films. Figure 6c displays the simulated
band ratio was larger than those of the other samples. The results are presumably because of the disorder of its twodimensional planar structure and the large amount of defects induced by the CVD process at a low growth temperature. Compared with other methods, optical anisotropy can be used as a simple quality factor for rapidly and nondestructively characterizing two-dimensional nanomaterials. In addition, we compare the differences between Raman scattering and optical anisotropy in Table 3. In previous research, Raman scattering was the major way to analyze the structure of graphene because of its abundant structural information. Although it offers many advantages, such as no contact and being nondestructive and fast, there are still some drawbacks. As we know, the signal yield of Raman scattering coming from the inelastic scattering mechanism was too weak to collect by a general optical setup compared with that of optical anisotropy. In other words, the instrumental cost of Raman scattering would be higher than that of an optical spectrometer. In addition, the most important issues are that the detection area of a single laser spot is too small for largescale sample identifications and the nonuniform signals between different detection points might increase the time and labor costs in industrial applications. Because of the weak yield of Raman scattering, high-intensity light is always needed in Raman measurements. The incident light intensity for Raman scattering measurement was much higher than that when measuring optical anisotropy. There are some researchers indicating that the light intensity would cause the deformation and destruction of graphene samples.37,42,43 Through measuring optical anisotropy, we can readily and rapidly determine the H
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absorbances of the ITO and graphene films at different incident angles and polarizations. The absorbances of ITO and graphene films for TE-polarized light were almost identical, presumably because the electric field of the incident light was always parallel to the substrate surface, regardless of the incident angle. For TM-polarized light, however, the electric field of the incident light gradually became perpendicular to the substrate surface upon increasing the incident angle. Because of its anisotropic absorption, the absorbance of a graphene layer decreased significantly as the incident angle increased. The absorbance of the graphene film for TM-polarized light reached approximately 2% at an incident angle of 80°. We also simulated the normalized transmittance defined as T/(1 − R). To simulate randomly polarized light, the normalized transmittance averaged the data from the TE- and TM-polarized light. Figure 6d reveals that the normalized transmittance of the graphene film was higher than that of the ITO film, especially at large incident angles. These results imply that optoelectronic devices using graphene as a transparent electrode could increase the total transmission light, even though the transmittances were the same for both graphene- and ITO-based electrodes at normal incidence.
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CONCLUSION
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ASSOCIATED CONTENT
ACKNOWLEDGMENTS We thank the National Science Council, Taiwan, for supporting this study under Contracts NSC-100-2628-E-002-031-MY3 and NSC- 100-2623-E-002-003-ET.
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In this study, we demonstrated that optical anisotropy could be an alternative factor for the rapid quality characterization of large-area graphene films. We developed an angle-variable spectroscopic method to determine the optical anisotropy of CVD-prepared graphene films. When the electric field of polarized light was not parallel to the graphene surface of sp2hybridized carbon atoms, the absorption coefficient could be separated into two components: one parallel and the other perpendicular to the graphene surface. Upon increasing the incident angle, decreased absorption of the out-of-plane light occurred because the angle between the electric field and carbon atom plane approached 90o. Without the need for highintensity laser irradiation or complicated optical setups, optical anisotropy measurements can be applied readily and rapidly to characterize the structural quality of large-area graphene samples prepared under various conditions; the measured optical anisotropy data were consistent with those obtained from Raman scattering spectroscopy. In addition, we also used graphene as a transparent conductive film at wide angles because of the advantage of its optical anisotropy. The transmittance of graphene films was greater than that of ITO films, especially in the large-incident-angle regime. These results imply that optoelectronic devices using graphene as the transparent electrode would exhibit an increase in the total amount of incident light.
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