Nondestructive Characterization of the Structural Quality and

Jul 14, 2014 - Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-a...
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Nondestructive Characterization of the Structural Quality and Thickness of Large-Area Graphene on Various Substrates Yu-Lun Liu,† Chen-Chieh Yu,† Keng-Te Lin,† En-Yun Wang,† Tai-Chi Yang,† Hsuen-Li Chen,*,† Chun-Wei Chen,† Cheng-Kai Chang,‡ Li-Chyong Chen,§ and Kuei-Hsien Chen§,∥ †

Department of Materials Science and Engineering, National Taiwan University, Taipei, 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei, 10617, Taiwan § Center for Condensed Matter Science, National Taiwan University, Taipei, 10617, Taiwan ∥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: We demonstrate an inspection technique, based on only one ellipsometric parameter, Ψ, of spectroscopic ellipsometry (SE), for the rapid, simultaneous identification of both the structural quality and thicknesses of large-area graphene films. The measured Ψ spectra are strongly affected by changes in the out-of-plane absorption coefficients (αTM); they are also correlated to the ratio of the intensities of the D and G bands in Raman spectra of graphene films. In addition, the electronic transition state of graphene within the UV regime assists the characterization of the structural quality. We also demonstrated that the intensities and shifts of the signals in Ψ spectra allow clear identification of the structural qualities and thicknesses, respectively, of graphene films. Moreover, this Ψ-based method can be further applied to graphene films coated on various substrates. In addition, mapping of the values of Ψ is a very convenient and useful means of rapidly characterizing both the structural quality and thickness of 2D materials at local areas. Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-area graphene.

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the development of rapid, in situ, nondestructive methods for characterization of the structural qualities and thicknesses of large-area graphene films during continuous-flow manufacturing is not only desired but also necessary. Many techniques have been investigated for characterizing the quality of graphene, including scanning electron microscopy, transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray photoelectron spectroscopy.8,14−18 Generally, expensive vacuum systems are necessary in these techniques, making them expensive, timeconsuming, and destructive, while also slowing throughput. Nondestructive and rapid optical characterization under ambient conditions would be preferable. Graphene films can be visualized by placing them onto 300 nm silicon dioxide (SiO2) films on Si substrates.19−21 The contrast of the reflective image changed when placing graphene layers of different thicknesses on the top of the substrate, as a result of optical interference. Subsequently, many researchers have attempted to increase the reflective contrast through the use of specially designed substrates.18,22,23 Although reflective microscopy is a

he discovery of graphene and related compounds has attracted much attention for their use in electronic and optoelectronic devices; it has also encouraged the development of new experimental techniques and theoretical calculations for preparing and characterizing two-dimensional (2D) materials.1,2 Because of its unique hexagonal structure, the mobility of mechanically exfoliated graphene sheets is approximately 200 000 cm2 V−1 s−1, thereby making it an exciting material for electronics and optoelectronics platforms.3 The attractive physical properties of graphene make it a potential material for use in many devices, including photodetectors, solar cells, and touch screens.4−6 Although exfoliated graphene sheets possess attractive material properties, the challenge remains to develop processes for preparing large-area graphene films with high-throughput. Accordingly, the development of efficient techniques for the characterization of graphene films is also an important industrial consideration. Although methods based on chemical vapor deposition (CVD) can be used to prepare large-area graphene films on catalyzed substrates, structural defects, including wrinkles and cracks, can be generated during thermal CVD processes and also in subsequent transfer procedures. These defects will degrade the electrical and mechanical properties of the as-prepared graphene films.7−13 Therefore, © 2014 American Chemical Society

Received: April 16, 2014 Accepted: July 14, 2014 Published: July 14, 2014 7192

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Analytical Chemistry

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simple method for characterization of the thicknesses of largearea samples, the contrast induced by graphene films on 300 nm SiO2/Si remains very low. For example, the contrast of an SLG film is only approximately 10%;19 therefore, a specially designed substrate generally should be used to increase the contrast.22,23 Although these techniques can be used to estimate the thicknesses of graphene films, the degrees of deformation and imperfection of graphene films cannot be directly perceived simultaneously. Most previous attempts at analyzing the structural qualities of CVD-prepared large-area graphene have relied on Raman scattering spectroscopy. Although Raman spectroscopy is a powerful technique for identifying the number of layers, the structural quality, the degrees of doping and disorder of a graphene sample, the high power density of the excitation laser, the small area of the measured spot, and the long duration required for large-area mapping remain obstacles when characterizing large-area graphene samples.24−26 At present, no related techniques can identify both the structural quality and thickness of large-scale graphene films nondestructively with high signal yields and high throughput. Graphene, a single layer of carbon atoms arranged hexagonally, possesses apparent optical anisotropy because of its unique flat, planar nanostructure.27 In a previous study, we demonstrated the different optical absorptions of graphene films measured using different types of polarized incident light (in-plane or out-of-plane); we found that the optical anisotropy was highly dependent on the structural quality of this 2D material.28 Graphene films feature more free electrons in the inplane direction than in the out-of-plane direction; accordingly, the absorption of the out-of-plane light decreases significantly as the angle between the incident electric field and the carbon atom surface approaches 90°. To clearly express the optical anisotropy, we define the optical anisotropy for large-area graphene films as the ratio of absorption coefficients (αTE/αTM) between the transverse electric (TE, in-plane) and transverse magnetic (TM, out-of-plane) polarized light. In a previous report, we also demonstrated that the structural quality of a large-area graphene film significantly affects the degree of optical anisotropy, especially at large incident angles.28 Spectroscopic ellipsometry (SE) is well established as a highly sensitive technique for characterizing ultrathin (from subnanometer to several nanometers) gate oxide layers in complementary metal-oxide-semiconductor (CMOS) devices.29 In SE, the amplitude ratio and phase difference of TM- and TEpolarized reflected light are collected and analyzed from a given sample. The measurements typically determine two ellipsometric parameters: the amplitude ratio (Ψ) and the phase difference (Δ) between the reflection coefficients of the TMand TE-polarized lights. Accordingly, SE has some major advantages when compared with the general R-T process. First, signal detection is based on the measurement of the amplitude ratio of the reflected light, rather than the absolute intensities of the reflected and transmitted light; such measurement would not be disturbed by fluctuations of the light source, resulting in measurements that are more highly reproducible than those from the R-T method. Second, because the R-T method measures the absolute intensity, it requires a calibrated standard to obtain quantitative results. A typical ellipsometer can accurately measure the parameters of Ψ and Δ with resolutions better than 0.02 and 0.01°, respectively, with no calibrations required for self-consistency.30

SE has been applied previously to determine the optical constants of graphene grown through various fabrication techniques, including epitaxial graphene (EG) grown through sublimation epitaxy on silicon carbide (SiC) or through catalyzed growth on metals.31,32 Although SE has many potential uses in graphene technology, neither determination of the structural qualities of large-area graphene by the SE method nor discussion focused on the analysis of large-area graphene films on different substrates (e.g., Cu, fused silica, Si), with the consideration of applicability and industrial scalability, have been reported. Recently, the direct growth of graphene films on metal, dielectric, and semiconductor substrates has become attractive for the development of graphene-based devices.33−35 As different synthetic methods have emerged, identification of the quality of graphene films grown on various substrates has become an important issue.36 In this study, we applied only a single ellipsometry parameter, Ψ, to verify the optical anisotropy of large-area graphene films on various substrates. Without the need for sample pretreatment or further optical modeling, we could directly identify the structural quality and thickness of largearea graphene films. In addition, our measurements of graphene on various substrates, including SiO2/Si, Cu, fused silica, and Si, agreed very well with simulation data based on a thin film model. In addition, mapping of this ellipsometric parameter allows characterization of the local regions of large-area graphene films. On the basis of this SE method, we could determine simultaneously both the structural quality and thickness of a graphene film.

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EXPERIMENTAL SECTION CVD-Graphene Preparation. Conventional low pressure chemical vapor deposition (LPCVD) processes were used to grow the large-area single-layer graphene (SLG) film on copper polyfoils (2 × 2 cm2). Copper polyfoils were first annealed at 1000 °C for 30 min under reduction processes. A mixture of CH4 and H2 was then introduced for graphene growth at various temperatures (800−950 °C). SLG films having different structural qualities were obtained by varying the growth conditions. After a 15 min growth process, the CH4 and H2 gases were shut off and Ar gas was introduced under a cooling process. The graphene films were then transferred onto different substrates (300 nm SiO2/Si, Cu film, fused silica, Si) using the general poly(methyl methacrylate) (PMMA)-based polymer-mediated method,9−12 and the process was illustrated in the Supporting Information (Figure S1). First, PMMA was spin-coated on the CVD-grown graphene films on Cu foil. Then, the underlying Cu foil was dissolved in an aqueous solution of iron nitrate (1 M) over a period of 3 h. After the Cu-etching process, the PMMA/graphene stack was washed with deionized water and subsequently transferred onto target substrates (SiO2/Si, fused silica, Cu, and pure Si). After transferring onto the target substrates, PMMA/graphene/ substrate was dried under ambient conditions. Finally, the PMMA layer on graphene was dissolved and washed away with acetone, leaving the graphene adhered to the target substrates. Measurements Characterization and Optical Simulation. Raman spectra of the graphene films were measured using a Raman microscope equipped with a He−Ne laser (633 nm) as the excitation source. A four-point probe measurement was performed on the transferred graphene films to measure the sheet resistance. SE was performed by using a J. A. Woollam M-2000 diode-array rotating compensator ellipsom7193

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Analytical Chemistry

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Figure 1. (a) Raman spectra of four graphene films (samples A−D) having different structural qualities. (b) Optical anisotropy of samples A−D. (c) Schematic representation of the variable-angle SE setup for anisotropic measurement of graphene films. (d) Measured and (e) simulated ellipsometric spectra (Ψ) of the SLG films having different qualities on 300 nm SiO2/Si substrates and the bare substrate at an incident angle of 70°.

eter with a Xe light source, over a wavelength range from the UV to the visible regime. The SE can be used to determine the optical constants for thin films, and the details are displayed in the Supporting Information (Figure S2). The spot size of the SE measurements was 300 × 300 μm2. A 100× objective lens and a position-calibrated sample stage were employed to perform repeated measurements and to develop the 3 × 3 mm2 mapping spectra. All measurements were performed at an incident angle of 70°. To analyze optical anisotropy, the anisotropic behavior was defined in terms of the ratios of different absorption coefficients (αTE/αTM). The absorption coefficients determined through R-T measurements were

compared with those determined using the SE method. Optical properties of graphene films were simulated using an optical thin-film model on various substrates (Cu, fused silica, Si),38 and a thin film model consisting of an air/graphene/substrate was used and also displayed in the Supporting Information (Figure S3).

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RESULTS AND DISCUSSION We fabricated graphene films of various structural qualities through variations in the temperature of the CVD process. Initially, we grew samples A−D on Cu polyfoils at growth temperatures of 800, 850, 900, and 950 °C, respectively, but 7194

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Figure 2. (a) Peak values in the UV and visible regimes of Ψ spectra of SLG samples having different qualities. (b) Measured peak values in the UV regime of Ψ spectra and of the D band-to-G band ratios in Raman spectra of SLG samples having different optical anisotropies. (c) Optical microscopy image of a graphene sample transferred onto a SiO2/Si substrate. (d) Corresponding Ψ map of the graphene sample, recorded at a wavelength of 270 nm and an incident angle of 70°. (e) Micro-Raman spectra recorded at the regions of the three colored rectangles in (d).

band-to-G band ratios, upon proceeding from sample A (ca. 0.41) to sample D (ca. 0.04). Thus, increasing the growth temperature led to an obvious decrease in the defect density on the graphene surface. From the measured reflectance and transmittance in Figure S4, Supporting Information, we could calculated the in-plane (TE) and out-of-plane (TM) absorption coefficients (α) to obtain the optical anisotropy (αTE/αTM) of the SLG films at different incident angles. Figure 1b reveals that the optical anisotropy of each SLG film increased significantly upon increasing the incident angle; in addition, the optical anisotropy increased upon improving the structural quality of the SLG film, especially when measured at larger incident angles. In previous studies, we found that imperfections in the 2D structures on the sample surfaces would also decrease the optical anisotropy of large-area graphene films.28 The correlation between the optical anisotropy and the structural

otherwise under the same experimental conditions. After the CVD process, we employed a general polymer-mediated method to transfer the graphene films onto different substrates (SiO2/Si, fused silica, Cu, pure Si),9−12 and the detailed processes were described in the Experimental Section. Figure 1a displays the Raman spectra of samples A−D. Although Raman spectroscopy can be used to identify carbon-based materials, it is difficult to use this approach to measure an entire large-area CVD-prepared SLG film. For this reason, we measured the signals at eight distinct points within an area of 1 cm2 and then averaged them to overcome the uncertainty of large-area characterization. The ratio of the D band to the G band is proportional to the defect density of a graphene surface; it can be used generally to characterize the quality of a graphene film.38−40 Figure 1a reveals a decrease in the defect densities of the CVD-prepared SLG films, as evidenced by decreasing D 7195

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Analytical Chemistry

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films. We fixed the thickness of each SLG film at 1 nm, with the optical constants and thicknesses of the SiO2 and Si substrates obtained from measured results. The simulated results in Figure 1e exhibiting different degrees of optical anisotropy are almost consistent with the experimental results (Figure 1d). Moreover, the right-hand side of Figure 1e displays expanded views of the calculated spectra for the SLG films having optical anisotropies of 1.6 and 3.1. The simulations suggested that higher optical anisotropy decreased the intensity of the Ψ peak, consistent with our experimental results. To investigate how the structural quality influenced the ellipsometric parameter Ψ, we compared the intensities of the signals in the Ψ spectra with the D band-to-G band ratios measured from four different CVD-prepared SLG films. Figure 2a reveals that the peak intensities in the Ψ spectra, in both UV and visible regimes, correlated significantly with respect to the structural quality of these SLG films. The SLG samples of lower structural quality exhibited higher peak intensities in their Ψ spectra as well as higher D band-to-G band ratios. Therefore, we could identify the structural quality of an SLG film directly from the intensity of its signals in the Ψ spectrum, without the need for further optical modeling calculations. In addition, we also observed an interesting phenomenon: the changes in intensity of the signals in the Ψ spectra, resulting from optical anisotropy, were more pronounced in the UV regime (decreasing from 89.14° to 67.62°) than in the visible regime (decreasing from 89.95° to 79.43°). This phenomenon can be applied to characterize the structural quality of graphene films. Figure S5, Supporting Information, reveals that the large change in the value of Ψ in the UV regime arose from the electronic π → π* transition, resulting in an obvious absorption peak of graphene at 4.6 eV (ca. 270 nm).27 Therefore, the intensity of the Ψ peak in the UV regime can be used to identify the structural quality of an SLG film, as described above. Figure 2b compares the peak values of Ψ spectra in the UV regime and the D band-to-G band ratios of our four different SLG samples. A decrease in the signal intensity in the Ψ spectrum correlated with a decrease in the D band-to-G band ratio, implying increased optical anisotropy and increased structural quality. Therefore, the peak values of Ψ spectra are very suitable to be applied for use as a quality factor for quantitative characterization of large-area graphene as shown in Table S1, Supporting Information. To determine the structural quality at local regions in a largearea graphene sample, we further improved the SE method by applying the functionality of large-area mapping of the ellipsometric parameters. By mapping the ellipsometric parameter Ψ, we could determine the structural quality of local regions in large-area graphene films. Figure 2c displays a standard optical microscopy image of sample A, the CVDprepared SLG film having the poorest quality. The surface of this transferred SLG on the SiO2/Si substrate appears nearly identical in every region. Measuring the local region indicated by the black square in Figure 2c, we applied Ψ mapping for large-area characterization of this graphene film. Figure 2d displays the Ψ map of the sample measured at a wavelength of 270 nm and an incident angle of 70°. The nonuniform Ψ mapping image confirmed that sample A had a nonuniform structure as a result of its growth at low temperature. To further inspect the results of Ψ mapping, we measured the micro-Raman spectroscopic behavior in the same local region of the graphene film of sample A. In the micro-Raman spectroscopic measurements in Figure 2d,e, with the results

quality of 2D materials can be used as a figure of merit for identifying large-area graphene films. Here, optical anisotropy provides a direct route for the rapid and large-area characterization of graphene films without the need for lengthy and complicated Raman spectroscopic measurements. To investigate the optical anisotropy of large-area graphene on opaque substrates, SE is a particularly attractive technique because it is very sensitive to differences between TE- and TMpolarized light. Figure 1c provides a schematic representation of a 2D graphene layer measured using SE. A well-collimated beam from a light source is passed through a polarizer to produce light of controlled polarization; this light interacts with the sample under identification and passes through an analyzer to give the ellipsometric parameters Ψ and Δ, which are abstracted from eq 1: tan Ψ × eiΔ = rTM /rTE

(1)

where rTM and rTE are the complex reflection coefficients of TM- and TE-polarized light, respectively; tan Ψ is the absolute amplitude ratio of rTM and rTE; and Δ is the phase difference. Although both Ψ and Δ spectra can reveal recognizable differences among SLG films, the peaks in Ψ spectra are typically sharper than those in Δ spectra and provide much more information for further identification. In this study, we employed only a single ellipsometric parameter, Ψ, to monitor both the structural qualities and thicknesses of SLG films on various substrates. To analyze the optical anisotropic properties of graphene films, we measured wavelength-dependent SE spectra of graphene films at an incident angle of 70°. Figure 1d displays the Ψ spectra of the SLG films of various structural qualities (samples A−D) on the 300 nm SiO2/Si substrates. Because of the interference effect, two obvious peaks are present in the Ψ spectra, one in the UV regime and the other in the visible regime. The intensities of these two signals in the Ψ spectra (rTM/rTE) were both greater than 45°, meaning that the value of rTM was larger than that of rTE. After we had transferred the graphene films onto the SiO2/Si substrate, the absorption and optical anisotropy of graphene films induced observable changes in their Ψ spectra, especially in the value of peak. The enlarged views of the spectra on the right-hand side of Figure 1d reveal that the positions of the signals in both the UV and visible regimes of the Ψ spectra were affected by the change in optical anisotropy of the graphene film. Because of the high extinction coefficients of the graphene layers transferred onto the surface of the SiO2 surface, the rTM and the value of Ψ peak both increased. An increase in optical anisotropy of a graphene film would decrease the extinction coefficient or absorption coefficient of TM-polarized light (αTM) at large incident angles; a decreased extinction coefficient would decrease the reflection coefficient of TM-polarized light (rTM), thereby resulting in a lower value of Ψ. Because of the very high sensitivity of SE method, the values of Ψ peaks provided direct information about the anisotropic graphene films on the SiO2/Si substrate that could not be discerned from the reflectance measurements in Figure S10, Supporting Information. Next, we applied an optical thin film model to simulate the Ψ spectra of the graphene films.37 As displayed in Figure 1e, we used an appropriate structural model to theoretically calculate Ψ spectra based on thin-film theory. The model we used consisted of a thin-film stack (air/graphene/substrate) and incorporated the different degrees of optical anisotropy (1.3, 1.6, 2.4, and 3.1, obtained from Figure 1b) of the real SLG 7196

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Figure 3. Simulated ellipsometric spectra (Ψ) of SLG samples of different qualities on (a) Cu, (b) fused silica, and (c) Si substrates. (d) Experimental and simulated data of the differences in values of (ΨS − ΨG) at a wavelength of 270 nm for substrates with and without SLG films.

layer onto these substrates. We also found that, when the optical anisotropy decreased gradually (i.e., an increase in the value of αTM), reflection from the TM-polarized light decreased, leading to a decrease in the value of Ψ. On the other hand, for fused silica, which exhibits low reflection, the Brewster angle of the substrate shifted to a larger incident angle after the transfer of a graphene layer. We attribute this shift of the Brewster angle to the increased effective index of refraction after coating the higher-refractive-index graphene layer onto the fused silica substrate. This phenomenon would also decrease the value of rTM at an incident angle of 70° as well as the value of Ψ. Therefore, the values of Ψ all decreased after adding a graphene layer onto these three types of substrates. To compare the differences between the substrates with and without the SLG films, one should determine the differences between the values of Ψ, defined as ΨS − ΨG, where the ΨS and ΨG are the values of Ψ of the substrate before and after coating onto a SLG film, respectively. Table S2, Supporting Information, lists the simulated values of Ψ of the SLGs on the various substrates at 270 nm. In Figure 3d, the filled points represent experimental data whereas the hollow ones are simulated data. The measured and simulated data are highly consistent, meaning that SLG films exhibiting different degrees of optical anisotropy on distinct substrates can be identified directly from their values of Ψ. Therefore, the value of Ψ in the UV regime provides a direct metrology technique for determining the structural quality of SLG films on various substrates. Moreover, the physical and electronic properties of graphene films are also affected dramatically by their thickness. Here, we demonstrate that both the structural quality and thickness of large-area graphene on SiO2/Si substrates can be identified through measurement of Ψ spectra using the SE method. We

displayed for the different colored spots, the region having the lowest values of Ψ (red rectangle) possessed the corresponding lowest D band-to-G band ratio, indicating a graphene of high structural quality. Moreover, the relatively large values of Ψ of the green and blue spots correspond with higher D band-to-G band ratios. Therefore, micro-Raman spectroscopy confirmed that the values of Ψ could be used to identify the local quality of the film. Although the lateral resolution of SE was restricted by the limits of the optical system (300 × 300 μm2), Ψ mapping appears to be an outstanding tool for rapidly analyzing optical anisotropy and structural quality, with a resolution of hundreds of micrometers, in local regions of 2D materials. During the fabrication of devices based on large-area graphene, characterization of the quality of the graphene on a variety of distinct functional substrates will be necessary to ensure effective device performance. Figure 3a−c displays simulated Ψ spectra of SLG films possessing various values of optical anisotropy (from 1.3 to 3.1) on Cu, fused silica, and Si substrates. The optical constants [refractive index (n), extinction coefficient (k)] of Cu, fused silica, and Si at a wavelength of 270 nm are 1.53 + 1.72i, 1.5, and 1.67 + 4.03i, respectively.41 These simulations reveal that the values of Ψ of the SLG films on the various substrates all decreased upon decreasing the optical anisotropy, especially in the UV regime. Because of the high absorption of graphene in the UV regime, the values of Ψ displayed obvious differences at wavelengths near 270 nm, allowing this parameter to be used to identify the optical anisotropy. Because of Brewster angle effects on the Cu, fused silica, and Si substrates, all of the values of Ψ in Figure 3a−c are less than 45°, meaning that the value of the rTM is less than the value of rTE. For Cu and Si, which are highly reflective, the degree of reflection decreased after transferring the absorbed graphene 7197

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Analytical Chemistry

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shifts for graphene films possessing the same value of optical anisotropy (3.1), but a different number of layers. To investigate the relationship between optical anisotropy and the thickness of CVD-prepared graphene films, Figure 4b summarizes the intensities and wavelengths of the relevant signals in the Ψ spectra. We observe that the peak intensity decreased upon increasing the optical anisotropy, whereas the peak shifted to longer wavelength upon increasing the number of layers in the graphene film; similar behavior appeared in the UV regime of the Ψ spectra. Thus, the intensities and shifts of the signals in Ψ spectra can be used to clearly identify the structural quality and thickness, respectively, of graphene films. Table 1 compares the performance of three common optical inspection techniques for the characterization of graphene. Although Raman spectroscopy provides abundant structural information, the very high intensity of the laser light source for the excitation of Raman signals might lead to structural damage of the graphene. The small detection area also limits the throughput and practical applications of Raman spectroscopy. The optical identification of graphene, based on the reflection contrast measured using an optical microscope, provides information regarding only the thickness of the graphene, and it requires particular substrates (SiO2, Al2O3, or other dielectric layers of specific thicknesses) for inspection. In contrast, in this study, we found that measurement of Ψ spectra using the SE technique offers information regarding both the thickness and structural quality of a graphene film, with a measurement time for large-area characterization that is much shorter than that required for Raman spectroscopy.

Figure 4. (a) Ellipsometric parameters (Ψ) of graphene films of various qualities and thicknesses (single layer, 1L; bilayer, 2L) on a 300 nm SiO2/Si substrate. (b) Intensities and positions of the signals in the Ψ spectra of graphene films of various optical anisotropies and thicknesses on SiO2/Si substrates.

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CONCLUSION Characterizing the structural quality and thickness of graphene films is important in both scientific research and industrial applications. In this paper, we demonstrate that the optical anisotropy and thickness of a graphene film can be determined rapidly through measurement of a single ellipsometric parameter, Ψ. We have also demonstrated that the measured values of Ψ correlate strongly to the D band-to-G band ratios found in the micro-Raman spectra of graphene films. In addition, we attribute the relatively large changes in the value of Ψ within the UV regime to the electronic transition state of graphene, thereby allowing characterization of the structural quality of graphene films. The presence of a dielectric SiO2 layer on a Si substrate led to a dramatic increase in the intensity of the Ψ peak, thereby increasing the detection limit of optical anisotropy. We have also demonstrated that the intensities and shifts of the signals in the Ψ spectra can provide a clear identification of the structural quality and thickness, respectively. Furthermore, this Ψ-based method can be applied to

simulated the Ψ spectra of single-layer (1L) and bilayer (2L) graphene films possessing different degrees of optical anisotropy (1.6 and 3.1). As displayed in Figure 4a, for both the 1L and 2L graphene films, the intensity of the Ψ peaks decreased upon increasing the optical anisotropic absorption, regardless of whether the measurement was made in the UV or visible regime. This result is consistent with the data in Figure 1d. Moreover, we observed an interesting feature: the position of the Ψ peak shifted to longer wavelength upon increasing the thickness of the graphene film, especially for the Ψ peak in the visible regime (ca. 470 nm). Small changes in the thickness of graphene films resulted in changes in the optical phase and reflectance ratio, which led to a red-shift of the Ψ peak. This phenomenon was more obvious in the longer wavelength (visible) regime. To further clarify the peak shifting in the Ψ spectra, Figure S6, Supporting Information, displays the peak

Table 1. Characteristics of Various Optical Techniques for Identifying the Thicknesses and Structural Properties of Graphene Films reflection contrast (optical microscope) light intensity detection area (spot size) throughput substrate information

low, 10−4 (mW cm−2) entire region (1 μm2 to several square centimeters) high, data from different spots collected simultaneously SiO2/Si or Al2O3/Si thickness and optical image

Raman scattering (micro-Raman spectroscopy) very high, 104−108 (mW cm−2) small region (from 1 to 500 μm2) low, 0.5−1 min per collection point SiO2/Si or Al2O3/Ag/Si abundant information (thickness, structural quality, doping, and edge structure of graphene) 7198

optical anisotropy (SE) low, 10−5 (mW cm−2) large-scale region (300 μm2 to several square centimeters) high, data from different spots collected simultaneously various substrates structural quality and thickness

dx.doi.org/10.1021/ac501557c | Anal. Chem. 2014, 86, 7192−7199

Analytical Chemistry

Editors' Highlight

graphene films coated upon various substrates. To the best of our knowledge, the direct application of Ψ spectra to determine the structural qualities of large-area graphene films on different substrates (Cu, fused silica, Si) has not been reported previously. In addition, mapping images of the values of Ψ are very convenient and useful for the rapid characterization of both the structural quality and thickness of 2D materials at local areas. Therefore, this SE-based method appears to be an excellent in-line, real-time technique for monitoring the growth of large-area graphene during CVD processes. In addition, this characterization method based on Ψ spectra has great potential for application in the mass production of graphene-based devices.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. 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 National Science Council, Taiwan, for supporting this study under contracts NSC-100-2628-E-002-023-MY2 and NSC-100-2628-E-002-031-MY3.

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dx.doi.org/10.1021/ac501557c | Anal. Chem. 2014, 86, 7192−7199