Article Cite This: J. Phys. Chem. C 2018, 122, 902−910
pubs.acs.org/JPCC
Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy Dong Ding,†,‡ Hiroki Hibino,§ and Hiroki Ago*,†,∥ †
Interdisciplinary Graduate School of Engineering Sciences and ∥Global Innovation Center (GIC), Kyushu University, Fukuoka 816-8580, Japan § School of Science and Technology, Kwansei Gakuin University, Hyogo 669-1337, Japan
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S Supporting Information *
ABSTRACT: We demonstrate that dark-field (DF) optical microscopy is a powerful tool to visualize grain boundaries (GBs) and grain structure of graphene grown by chemical vapor deposition (CVD). Copper oxide nanoparticles sparsely formed along the graphene GBs by postgrowth mild oxidation allow one to determine the position and structure of the GBs by the DF microscope. As DF imaging offers a much higher sensitivity than bright-field (BF) microscopy, some GBs were observed even without the postgrowth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize the grain structure of the as-grown graphene by DF microscopy. Moreover, DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced oxidation of the Cu surface below graphene in spite of the fact that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of DF optical microscopy in characterizing graphene and related two-dimensional materials grown on metal substrates.
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graphene layers on SiO2/Si substrate14 and for the quick inspection of graphene grain size on Cu substrate by a simple oxidation process without transfer.15,16 The gas barrier property of graphene allows one to visualize the graphene grains on Cu catalyst via preferential oxidation of the bare Cu (uncovered with graphene), giving discernible BF color contrast between bare Cu and Cu covered with graphene.16 BF optical microscopy can be also used to measure GBs existing in CVD graphene. The GB observation utilizes the higher chemical reactivity of GBs compared with the basal graphene planes. By exposing graphene/Cu foil to ultraviolet (UV) light in a moisture-rich environment or to oxygen plasma treatment,11,17 GBs can be detected by BF optical microscopy. These oxidation processes break the graphene GBs and enhance the oxidation of surface-exposed Cu. However, in order to observe such copper oxide formed along the graphene GBs, severe postgrowth oxidation is necessary because the width of Cu oxide lines needs to be wider than 100 nm due to the optical resolution of BF imaging which uses visible light. For example, the high-temperature oxidation (300 °C in air) was combined with the oxygen plasma treatment in order to observe GBs by BF mode, suggesting that only the oxygen plasma treatment is not sufficient to make the copper oxide
INTRODUCTION Graphene, a two-dimensional (2D) carbon sheet with singleatom thickness, has captured wide attention since the first report in 2004, because of its unique structure, excellent carrier transport, high mechanical flexibility, and optical transmittance, which promise applications in various fields.1 Recently, chemical vapor deposition (CVD) has been developed as one of the most promising methods to produce high-quality graphene in large scale using various metal catalysts, such as Cu,2 Ni,3 Co,4 Pt,5 and Rh.6 In particular, Cu catalyst has been widely used because of the successful production of large-area, uniform monolayer graphene films due to the self-limiting growth mechanism.2,7−9 However, there are still issues that need to be addressed for further development. The presence of grain boundaries (GBs) and wrinkles in CVD-grown graphene and the transfer process are critical issues for the improvement of the physical properties. Because graphene grains nucleate at random sites with various crystal orientations,10,11 graphene grown on Cu foil generally contains a number of GBs, leading to the degradation of the electrical and mechanical properties.12,13 Therefore, the understanding of the spatial distribution and length of graphene GBs is important to assess the physical property for realizing improved performance. Among various methods to characterize the CVD-grown graphene, optical microscopy is one of the most accessible, quick, and highly efficient methods. The bright-field (BF) microscope has been used to determine the number of © 2017 American Chemical Society
Received: October 14, 2017 Revised: December 2, 2017 Published: December 4, 2017 902
DOI: 10.1021/acs.jpcc.7b10210 J. Phys. Chem. C 2018, 122, 902−910
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Figure 1. (a, c) BF and (b, d) DF optical micrographs of merged graphene grains on Cu foil. Images a and b, c and d show identical positions. (e) DF image of the continuous graphene sheet grown on Cu. (Inset) Corresponding BF image. Samples a and b were oxidized at 250 °C for 5 s after CVD growth, while the samples c, d, and e were not subjected to the postoxidation after the CVD growth. Light blue arrows in e indicate grain boundaries in Cu foil.
(from room temperature to 1075 °C) stage, pure Ar (99.999%) was flown under ambient pressure. After reaching the target temperature, 2% H2 and 15−66 ppm of CH4 gases diluted in Ar were introduced for 30−120 min to grow graphene. Finally, the Cu foil was cooled down rapidly by taking out from the heating zone. For oxygen-free Cu growth, we heated it up in Ar with 5% H2 without preoxidation of the Cu, followed by reaction with CH4 at 1075 °C. Transfer of Graphene. The graphene sample grown on Cu foil was spin coated with poly(methyl methacrylate) (PMMA); then the Cu foil was chemically etched by APS. The back side of the Cu foil was removed by O2 plasma. After Cu etching, the PMMA/graphene stack was transferred onto a SiO2/Si wafer. Finally, the coated PMMA was removed by soaking in acetone. Characterization. BF and DF optical images were taken with a Nikon Eclipse ME600 with the CCD camera (Nikon DS-Fi1). A scanning electron microscope (SEM) (S-4800, Hitachi) and an atomic force microscope (AFM) (Nanoscope V, Bruker) were used to image the surfaces of Cu and graphene. Crystal orientations of as-grown graphene films were characterized by low-energy electron microscope (LEEM) equipment (Elmitec LEEM III). Crystal planes of Cu were measured by electron backscatter diffraction (EBSD) (TSL Solutions, OIM) attached to SEM (Ultra55, Zeiss). Raman spectra were measured with Nanofinder30 (Tokyo Instruments) using a 532 nm laser.
visible by BF optical microscopy.17 Accordingly, the graphene films suffer severe damage due to such strong oxidation processes, giving a strong Raman D band not only at the GBs but also inside the graphene grains.11,17 In contrast to BF optical microscopy, dark-field (DF) microscopy has not been widely utilized for analysis of graphene. DF imaging is employed to observe metal particles in biological and material sciences and also to improve the contrast in unstained samples.18,19 However, a few papers have reported the use of DF optical microscopy in CVD graphene. The presence of monolayer graphene, the ridge structure, the distribution of multilayer grains, and the formation of periodic ripples on Cu foil were imaged by DF microscopy.20−22 In this paper, we explore new applications of DF optical microscopy for characterizing CVD graphene without a transfer process. One application is GB visualization of CVD graphene grown on Cu foil. We found that DF imaging offers much higher sensitivity for observing graphene grain structures, e.g., structure and length of GBs and merged grains with different orientations, than the conventional BF mode of an optical microscope, thanks to the enhanced color contrast of Cu nanostructures under the DF mode. This provides a novel way of nondestructive observation of graphene GBs. Another application is investigation of the oxidation resistance of Cu under the protection of graphene. We found that in the case of CVD growth on large-grain graphene on oxygen-rich Cu, the oxygen atoms stored in Cu bulk lead to fast oxidation of the Cu surface just beneath graphene. Our findings broaden the applications of DF microscopy, contributing to the development of direct observation methods of 2D materials, such as graphene and hexagonal boron nitride (h-BN), that are synthesized on metal catalysts.
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RESULTS AND DISCUSSION Visualization of Graphene GBs Based on Copper Oxide Nanoparticles. Large graphene grains with lateral sizes around 100−500 μm were synthesized by ambient-pressure CVD at 1075 °C using CH4 on the Cu foil which was preoxidized in air, as reported previously.23 Although we can synthesize larger, millimeter-scale graphene grains, we tuned the grain size to submillimeter for observing multiple grains in the view area of our optical microscope. Figure 1a and 1b shows BF and DF optical images of two merged graphene grains taken after the postgrowth oxidation at 250 °C for a very short time (5 s) in air. In the BF image (Figure 1a), the bare Cu shows an orange color due to air oxidation of the surface-exposed Cu, while the area covered with graphene shows a bright metallic Cu color, indicating the Cu is protected from oxidation by graphene. In Figure 1a, the misaligned angle of two merged
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EXPERIMENTAL METHODS CVD Synthesis of Graphene. Commercially available Cu foil (Nilaco Co., 99.9% purity, 80 μm thick) was used as catalyst to grow graphene. The Cu foil was first cleaned by soaking it in aqueous ammonium persulfate (APS) etchant solution (0.2 M) for ∼3 min, followed by rinsing with deionized water. Oxygen-rich Cu foils were prepared by heating the surface-cleaned Cu foil at 250 °C for 1 min on a hot plate. Then the Cu foil was placed in the CVD chamber (quartz tube 45 mm diameter) to grow graphene. During the heating up step 903
DOI: 10.1021/acs.jpcc.7b10210 J. Phys. Chem. C 2018, 122, 902−910
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The Journal of Physical Chemistry C grains is ∼12°, but it was difficult to observe a GB possibly existing between these adjacent two grains. However, when we switched to the DF mode, the bright line was clearly observed along the interface between the grains, which is ascribed to the GB of monolayer graphene (yellow arrow in Figure 1b). This result presents that GB can be detected by DF more effectively than BF imaging. In some merged grains, graphene GBs were observed by the DF microscope even without the postgrowth oxidation process, as shown in Figure 1c and 1d. The BF image did not show any optical contrast due to the absence of the oxidation. However, the DF image clearly outlines the structure of graphene grains and visualizes the existence of GBs. In addition, multilayer graphene grains were observed with bright contrast at the center of monolayer grains. The previous DF observation only detected the presence of monolayer and multilayer graphene.20 It was suggested that the bright contrast of multilayer graphene in the DF images originates from Cu ripples formed during the release of the compressive strain in the CVD process.21 In this work, we newly propose that the presence of GBs existing between merged grains with different orientations can be clearly identified by DF optical measurement. As we will discuss later, this is due to the enhanced color contrast of Cu oxide nanoparticles. The presence of GBs can be confirmed not only in these isolated grains but also in a large graphene sheet by the DF microscope, as shown in Figure 1e. In contrast to the BF image that only shows linear dark lines corresponding to grain boundaries existing in the Cu foil (light blue arrows in Figure 1e inset), the DF image clearly demonstrates the existence of many GBs in the uniform graphene sheet (Figure 1e). We note that our observation of graphene GBs by the DF optical images offers a new method to visualize graphene grain structure, which has not been reported in previous work.22 As already mentioned, it is widely acknowledged that the oxidation occurs preferentially along GBs of graphene. In the case of graphene grown on Cu catalyst, it leads to the formation of copper oxide along the graphene GBs, which becomes visible by BF imaging, as discussed later. For comparison, we further oxidized the merged graphene grains with a misorientation angle of 11° for longer times, as shown in Figure 2. First, oxidation at 250 °C was performed for 1 s to distinguish graphene grains from bare Cu surface by the BF microscope (Figure 2b, top). However, no specific features were observed at the interface between the graphene grains. After prolonged oxidation for 5 min, the GB became partly visible by BF measurement (Figure 2d, top). On the other hand, the DF mode exhibited linear GB only after 1 s oxidation, as marked in Figure 2b (bottom). Further oxidation made this GB more apparent, improving the GB contrast with oxidation time. At the same time the DF images suggest the introduction of structural defects when the oxidation was continued to 30 s and 5 min (lower panels of Figure 2c and 2d and Supporting Information Figure S1), as the number of bright spots inside the grains increased with the oxidation time. These results indicate that DF microscopy has a much higher sensitivity than BF imaging. Thus, DF imaging can significantly reduce possible damage introduced by the postgrowth oxidation processes which are indispensable to conventional BF imaging of graphene GBs. To understand the mechanism of the DF observation of graphene GBs, we analyzed the GBs by AFM and SEM. Figure 3a is the DF optical image of the same sample as Figure 1e but was taken at a different location. Figure 3b and 3c shows the
Figure 2. Comparison of BF and DF images. (a) BF image taken after postgrowth oxidation at 250 °C for 1 s. (b−d) Magnified images of the square marked in a. Top and bottom images show BF and DF images, respectively. (c, d) Micrographs after prolonged oxidation. Scale bars in b−d are 50 μm. White oval indicates the graphene GBs.
AFM images measured at one of the graphene GBs. We found a linear array of nanoparticles sparsely distributed along the GB. SEM observation also supports this result (Figure 3d). The height of the nanoparticles was typically around 50−200 nm, as determined by AFM (Figure 3c, bottom). Raman analysis was performed for these nanoparticles, and it was found that they are copper oxide, highly likely Cu2O (see Figure S2). Moreover, these copper oxide nanoparticles were not found in the rest of the scanned area, suggesting that the oxidation occurs mainly along the graphene GBs, as illustrated in Figure 3e. Our DF observation is mainly based on Rayleigh scattering by Cu nanostructures, such as nanoparticles and ripples. Rayleigh scattering occurs when the dimensionless size parameter (α) is much smaller than 1 (α ≪ 1)24
α=
πDp λ
(1)
where Dp and λ are the particle diameter and wavelength of the light, respectively. Thus, generally, particles smaller than 100 nm give the Rayleigh scattering.24 Due to the high sensitivity of this scattering intensity, DF optical microscopy has been applied to study single-metal nanoparticles.25 In Rayleigh scattering, the intensity (I(θ)) of the scattered light is expressed by24 904
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Figure 3. Mechanism of GB visualization by DF imaging. (a) DF optical image of as-grown graphene taken without postgrowth oxidation. (b) AFM image of the area marked in a. (c) Zoomed Image of the square in b. Lower panel is the height profile of the nanoparticle shown in the upper panel. (d) SEM image of the other position. (e) Schematic illustration of the DF-based visualization method.
Figure 4. Grain structure analysis of monolayer graphene based on periodic Cu ripples. (a) DF and (b) AFM images of merged graphene consisting of two rotated grains with 13° misorientation. Inset in a shows the BF image of the same area. Lower panel of b is the height profile measured along the yellow line. (c) DF and (d) AFM images and height profile of other merged graphene. Images shown in a−d were measured without postgrowth oxidation. (e) BF and (f) DF images of merged grains with 27° misorientation angle. This sample was oxidized at 250 °C for 5 s. (g) Schematic illustrating the Cu ripple-assisted graphene grain visualization.
λ ⎛ πDp ⎞ I (θ ) = I 0 2 ⎜ ⎟ 8π ⎝ λ ⎠ 2
6
n n0
mode (Figures 2d and S1), in which wide and continuous defects were formed along the GBs. Our results demonstrate that DF optical microscopy offers an easier, faster, less destructive, and highly sensitive means to measure graphene GBs compared with the widely employed conventional BF optical microscope, AFM, and SEM. This is due to enhanced color contrast of copper oxide nanoparticles in the DF mode. For more statistical evaluation, we collected DF images for more than 50 merged graphene grains after each oxidation step. As shown in Figure S4, even after very mild oxidation (250 °C, 1 s), 24% of the graphene GBs can be recognized by DF measurement. This value increased up to ∼70% when the oxidation was continued to 5 min. The observed different oxidation resistance suggests that the stability of graphene GBs can vary depending on the misorientation angle of merged grains and the crystal orientation of underlying Cu lattice.25−28 Visualization of Graphene Grain Structure Based on Cu Ripples. In the graphene growth on Cu foil, the surface steps of the Cu experience step bunching to release the strain, resulting in the formation of Cu periodic ripples.20,21,29 Figure
2
() () n n0
−1
2
+2
(1 + cos2 θ ) (2)
where θ, I0, n, and n0 are the angle between the incident and the scattered light, incident light intensity, reflective index of nanoparticles, and reflective index of the medium, respectively. This equation indicates that the intensity of the scattered light is proportional to Dp6. Therefore, the scattered light intensity is very sensitive to the nanoparticle size, giving strong optical contrast in the DF image, in spite of the fact that the density of copper nanoparticles is low (see Figure 3). It is difficult to directly observe the presence of monolayer graphene by AFM (Figure 3b and 3c) and SEM (Figure 3d). Considering the large nanoparticle size, we speculate that the nanoparticles formed below graphene will break the GB partly with a size close to the nanoparticle diameter. However, most of the graphene GBs are supposed to be still connected considering the low nanoparticle density (see Figure 2b), being different from the heavy oxidation used to visualize GBs by BF 905
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and Cu grain (illustrated in Figure 4g). Previously, a polarized optical microscope has been used to observe the graphene GBs.30,31 However, in this case, the result was highly dependent on the polarizer angle.30,31 In our DF imaging, the optical contrast does not rely on the measured direction, as can be seen in Figure S6b. We randomly took DF images of the merged graphene grains having misaligned angles, and 89% of the GBs was identified from the DF color contrast. Some of the DF images are given in Figure S7. To confirm our assignment of graphene grain structure based on the DF optical micrograph, we measured LEEM for the merged grains, as shown in Figure 5. The LEEM is a powerful
S5 shows DF and AFM images of one hexagonal graphene grain grown over two neighboring Cu grains. As seen in Figure S5, the height and orientation of the periodic ripples of the Cu depends on the relative orientation of graphene lattice and the underlying Cu lattice, making clear the optical difference in the DF image. In this section we demonstrate that this is applicable to investigation of grain structure of monolayer graphene, which can be used as a complementary method of the previous nanoparticle-assisted DF imaging technique. Figure 4a shows the DF optical image of an as-grown graphene on Cu foil. Again, the DF image gives a much clearer image than the corresponding BF image (inset). The upper half of the graphene grain shows dark contrast, while the other half displays parallel strips with bright colors. AFM analysis (Figure 4b) of the identical grain revealed that the bright DF colors are associated with the periodic ripples21,29 with the height around 35 nm. On the other hand, the upper half shows smaller height modulation without periodicity. Since the BF image shows that there is no GB in the underlying Cu foil for the measured area, the different optical contrast seen in the DF image is not originated in the Cu boundary. From the AFM, it is clearly seen that the observed graphene consists of two merged grains with a 13° misaligned angle. The GB in this merged graphene can be drawn based on the recognizable morphology boundary (blue line in Figure 4b). Thus, the color contrast difference seen in Figure 4a is accounted for by the two merged grains with different crystal orientations. This indicates that DF microscopy offers another way of distinguishing graphene GBs. Figure 4c and 4d shows another example of the merged graphene. In addition to the bright contrast originated in multilayer graphene at the grain center, we observed different DF contrast marked as the white square in Figure 4c. As shown in Figure 4d, the directions of the Cu ripples differ at the marked area, which we think are the origin of the color contrast seen in the DF image, because the height difference is not so large in this case. Such color contrast cannot be seen in the SEM (Figure S6a), indicating the high sensitivity of DF optical microscopy to the morphological change of the underlying Cu catalyst. This method can be applied not only to the compact graphene grains looking like one grain but also to merged grains nucleated from different points. Figure 4e and 4f shows BF and DF images of two grains merged with an angle of 27°, respectively, measured after the postgrowth oxidation (250 °C, 5 s). Also, in this case, DF imaging allows optical determination of the graphene GB, while the BF image gives no information on the GB. As already discussed, Rayleigh scattering is very sensitive to nanostructures, thus making the DF imaging technique effective to investigate ripples appearing on the Cu surface. As shown in eq 2, the scattered light intensity is proportional to Dp6, so that we can distinguish a small change of the ripple height induced by overcoating graphene having different lattice orientations. In the cooling down step of the CVD, the thermally expanded Cu foil shrinks much more than graphene due to their different thermal expansion coefficients.21,29 The presence of a graphene layer on Cu modifies the strain induced in the Cu foil, especially at the Cu surface, leading to the formation of periodic ripples. These ripples are dependent on the relative orientation between the graphene lattice and the Cu lattice as well as the crystalline plane of the Cu surface. The graphene− Cu interaction depends on the relative orientation of the graphene lattice and the Cu lattice so that the formation of Cu ripples is also affected by orientations of both graphene grain
Figure 5. (a) BF and (b) DF images of merged graphene grains on Cu foil after oxidation at 250 °C for 5 s. Dotted lines in b indicate the GBs assigned from the DF image. (c) BF-LEEM image overlaid with colors determined by the DF-LEEM images. See Figure S8 for more comprehensive data. (d) Electron diffraction pattern taken at the points marked in a.
tool for the characterization of graphene, as reported previously.23,32,33 Figure 5a and 5b shows BF and DF images, measured after slight postgrowth oxidation, respectively. Being different from the BF image, the DF image shows the presence of three different rotated grains marked by the light blue dotted lines. Figure 5c is the BF-LEEM image highlighted with different colors based on the corresponding DF-LEEM images (see Figure S8 for more detailed analysis). Due to the limited image area, dozens of captured BF images are combined in Figure 5c. The electron diffraction patterns measured at points 1−3 marked in Figure 5a indicate that grains 2 and 3 are rotated against grain 1 by 30° and 4°, respectively. Another LEEM data of the merged grains is shown in Figure S9 together with the clear DF optical image. Our LEEM analyses support the high efficiency and reliability of DF optical microscopy in the visualization of graphene GBs. We demonstrate that by DF optical microscopy GBs existing in graphene can be effectively visualized based on the two principles: observation of copper oxide nanoparticles formed along GBs (Figures 1−3) and the color contrast originated in Cu ripples underneath graphene (Figures 4 and 5). The former needs mild oxidation to clearly visualize graphene GBs, while the latter does not require the postgrowth oxidation. We can also observe copper oxide nanoparticles for the GB recognized 906
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Figure 6. Exposure time dependence of optical micrographs of graphene grains grown on preoxidized Cu foil. (a, c, e, g) BF and (b, d, f, h) DF images. (i) EBSD image of the same area. As the bare Cu is heavily oxidized, electron diffraction was not obtained from the bare Cu, resulting in black background. All scale bars are 100 μm.
bare Cu by this BF image. In the corresponding DF image (Figure 6d) the graphene grain can be clearly recognized together with a number of bright spots that are seen only at the right part of the graphene grain (denoted as “G1”), separated by the Cu grain boundary. Bright spots were not observed at the left of the graphene grain (denoted as “G2”) as well as bare Cu under the magnification used here. Therefore, the graphene-coated area G1 is suggested to be oxidized even faster than bare Cu. After 4 days, the graphene grain became recognizable by BF imaging (Figure 6e). The hexagonal shape suggests the singlecrystalline nature of the measured graphene grain. In the DF image, the number of bright spots associated with copper oxide nanoparticles increased in the G1 area. However, at the bare Cu, bright spots were mostly perceived at the locations near the graphene edges (Figure 6f). Finally, after storage for 28 days, the BF color of the bare Cu turned to brown, making the graphene grain distinct (Figure 6g). The DF image of the bare Cu, on the other hand, became very bright (Figure 6h), indicating the deep oxidation being consistent with Figure S3f. However, the color contrasts of the graphene-covered area (both G1 and G2) did not show substantial change after 4 days. Figure 6i shows the EBSD image of the same area collected after 28 days. In the EBSD measurement, an electron diffraction pattern was not obtained from the bare Cu due to the formation of an amorphous copper oxide layer. However, the clear diffraction was obtained from the Cu covered with the graphene grains, which were assigned to Cu(110) and Cu(211) planes. This indicates that the Cu metal is preserved below graphene, signifying a good gas barrier property of the largegrain graphene. The above results demonstrate that initially the Cu below graphene (especially at G1) forms copper oxide nanoparticles; then the bare Cu starts to be oxidized. It is, thus, considered that at the beginning (first 4 days) graphene grown on Cu(110) accelerates the oxidation of the Cu foil more than bare Cu, instead of inhibiting oxidation. This suggests the oxidation rate of Cu is dependent on the crystalline plane of the Cu grain. However, after 28 days, the bare Cu suffered significant oxidation while the Cu below graphene did not show a large change. This indicates the protection of Cu foil from further oxidation by the large graphene grain which has no GBs inside. The quality of graphene was inspected after leaving in ambient at 40 °C with ∼40% humidity for 28 days (Figure 7a− h). The graphene grain in Figure 7a can be divided into three regions on the basis of Cu grain boundaries. Area 1: underlying Cu is well protected by graphene. Area 2: Cu is partially
by Cu ripples shown in Figure 4e and 4f after prolonged oxidation (Figure S10). As seen in Figure S10, the BF microscope image starts to detect graphene GB from 5 min, but the longer oxidation time (30 min) gives clearer information, while DF microscopy can detect copper oxide nanoparticles even after 1 min oxidation. This also illustrates the advantage of our DF optical microscopy for the investigation of GBs and grain structures of graphene. The present method is applicable not only to submillimeter- or millimeter-sized large graphene grains but also to smaller graphene grains with several micrometers. As seen from Figure 4c and 4d, DF imaging allows one to detect small grains with a lateral size of 5−10 μm. Thus, our method can be applied to CVD graphene samples with a wide range of grain sizes, showing the versatility in analyzing graphene grown on Cu. DF Microscopy for Imaging Gas Barrier Property of Graphene. Graphene is known as an excellent gas barrier film due to gas impermeability and chemical stability, making it an effective antioxidation barrier for metals, such as Cu and Cu/Ni alloy.34−38 As shown in Figure 2, Cu oxidation occurs preferentially at GBs of graphene and the oxidized area gradually expands with oxidation time.39 On the other hand, oxidation of Cu catalyst before or during graphene growth has been shown to significantly reduce the nucleation density of graphene, resulting in millimeter- or centimeter-sized singlecrystal graphene grains.14,40 Therefore, it is interesting to study the performance of large graphene grains as the gas barrier film for Cu foil, considering that such large graphene grains are free from graphene GBs. Moreover, as we already discussed, the enhanced optical contrast of copper oxide nanoparticles in the DF microscopy offers a good tool for the investigation of Cu oxidation. Large graphene grains were synthesized by using oxygen-rich Cu foil (see Experimental Methods for details). For the observation by optical microscopy, we controlled the grain size to 100−500 μm because millimeter grains are too large to capture multiple grains in one microscope image. Figure 6 shows the time evolution optical images of the graphene/Cu foil stored at 40 °C in air with ∼40% humidity. Just after the CVD the BF image (Figure 6a) shows uniform optical contrast except for Cu boundaries. On the other hand, in the DF image (Figure 6b) the surface of monolayer graphene grains looks slightly bright, indicating very weak oxidation even without postgrowth oxidation. After exposing to air for 1 day, the color of the whole BF image (Figure 6c) changed to yellow, indicating the uniform and slight oxidation of the Cu surface. The graphene grain is still difficult to be differentiated from 907
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Figure 7. (a) BF image of a graphene grain grown on Cu foil after 28 days of being stored at 40 °C, 40% humility. (b, c, and d) BF images of graphene taken at areas 1, 2, and 3 marked in a, measured after transfer on SiO2/Si wafer. (e, f, g) Raman mapping images (ID/IG intensity ratio) of the areas shown in b, c, and d. (h) Raman spectra of graphene collected at the marked positions in b−d. Color of the spectrum corresponds to the marked position with the same color. (i) Schematic of graphene grown on oxygen-rich (left) and oxygen-free (right) Cu foils. Dissolved oxygen atoms remaining in the oxygen-rich Cu after CVD results in the Cu oxidization assisted by graphene on the top. On the other hand, graphene can protect the underlying oxygen-free Cu from being oxidized by oxygen and water in air.
copper oxide nanoparticles. This is schematically shown in the left image of Figure 7i. The inhomogeneous oxidation below the graphene grain, seen in Figure 6i, indicates that the Cu oxidation is highly dependent on the Cu crystal orientation. Figure S11 shows the large-area EBSD image overlapped with the SEM image collected for the graphene/Cu foil after air oxidation for 28 days. The crystallographic planes of the Cu grains protected with graphene are assigned. We found that the severely oxidized areas of graphene-coated Cu have an index close to the (110) plane (green color in the EBSD). This is consistent with the previous reports that Cu(110) has a small activation energy for the formation of copper oxide28 and low oxygen gas pressure required for oxide nucleation because of the larger oxygen surface sticking coefficient.45 It is noted that we observed graphene GBs even without postgrowth oxidation process, as seen in Figure 1c−e. This also supports our assumption that oxygen dissolved in Cu foil contributes to the formation of copper oxide nanoparticles. Although the exact reason for the nanoparticle formation at the graphene GB is unclear, we speculate that the localized electronic state at the GB and a small amount of oxygen supplied from outside through the GB trigger the copper oxide nanoparticle formation. For comparison, we also synthesized graphene on the oxygen-free Cu foil without preoxidation before CVD. Figure S12 shows the BF and DF images taken randomly from the graphene grown on ∼1 cm × 1 cm sized Cu foil after 11 days of exposure in air (40 °C, ∼40%). The graphene-coated Cu was effectively protected, and we did not observe any bright spots originating in copper oxide nanoparticles at the graphenecovered areas in the DF images. Thus, the graphene acts as a
oxidized. Area 3: Cu surface is mostly oxidized. Figure 7b−d shows the BF optical images after the transfer on SiO2 (300 nm)/Si substrate using PMMA and Cu etching solution (APS). All three areas indicate the presence of a uniform graphene film, as confirmed by the Raman spectra shown in Figure 7h. All of the measured positions showed clear Raman signals, originated in monolayer graphene with the 2D band intensity being higher than the G band intensity. Moreover, the Raman D band was very weak, implying the graphene is of high quality, even after leaving 28 days on Cu foil. Raman mapping images of the relative intensity of the D band to the G band (ID/IG) are depicted in Figure 7e−g. Most of the scanned area shows a ID/ IG ratio lower than 0.1, suggesting the high quality of graphene.41 Therefore, the monolayer graphene suffers little influence during Cu oxidation in the present mild condition (40 °C with ∼40% humidity), and the observed oxidation behavior of Cu foil is not directly related to possible damage induced in the top monolayer graphene. Graphene is known to protect underlying metal films/foils from oxidation due to its gas barrier property.36−38,42 However, as shown in Figure 6, copper oxide nanoparticles appear even faster at the Cu(110) plane coated with graphene than uncovered, bare Cu surface. It is noted that we used the oxygen-rich Cu foil made by preoxidizing the pristine Cu foil at 250 °C for 1 min in air to synthesize large graphene grains by suppressing the nucleation density. Although the dissolved oxygen atoms in the preoxidized Cu are mostly consumed during graphene growth via releasing CO2 that suppresses the graphene nucleation,43,44 the remaining small amount of oxygen can diffuse out during the cooling down process after graphene growth. In this case, the overcoating graphene may confine the dissolved oxygen atoms, facilitating the formation of 908
DOI: 10.1021/acs.jpcc.7b10210 J. Phys. Chem. C 2018, 122, 902−910
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The Journal of Physical Chemistry C good gas barrier film, as illustrated in Figure 7i right, keeping the Cu foil metallic.
(4) Ago, H.; Ito, Y.; Mizuta, N.; Yoshida, K.; Hu, B.; Orofeo, C. M.; Tsuji, M.; Ikeda, K.; Mizuno, S. Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire. ACS Nano 2010, 4, 7407−7414. (5) Gao, L.; Ren, W.; Xu, H.; Jin, L.; Wang, Z.; Ma, T.; Ma, L. P.; Zhang, Z.; Fu, Q.; Peng, L. M.; et al. Repeated Growth and Bubbling Transfer of Graphene with Millimetre-Size Single-Crystal Grains Using Platinum. Nat. Commun. 2012, 3, 699. (6) Liu, M.; Zhang, Y.; Chen, Y.; Gao, Y.; Gao, T.; Ma, D.; Ji, Q.; Zhang, Y.; Li, C.; Liu, Z. Thinning Segregated Graphene Layers on High Carbon Solubility Substrates of Rhodium Foils by Tuning the Quenching Process. ACS Nano 2012, 6, 10581−10589. (7) Kobayashi, T.; Bando, M.; Kimura, N.; Shimizu, K.; Kadono, K.; Umezu, N.; Miyahara, K.; Hayazaki, S.; Nagai, S.; Mizuguchi, Y.; et al. Production of a 100-M-Long High-Quality Graphene Transparent Conductive Film by Roll-to-Roll Chemical Vapor Deposition and Transfer Process. Appl. Phys. Lett. 2013, 102, 023112. (8) Ago, H.; Ogawa, Y.; Tsuji, M.; Mizuno, S.; Hibino, H. Catalytic Growth of Graphene: Toward Large-Area Single-Crystalline Graphene. J. Phys. Chem. Lett. 2012, 3, 2228−2236. (9) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; et al. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−8. (10) Kim, K.; Lee, Z.; Regan, W.; Kisielowski, C.; Crommie, M. F.; Zettl, A. Grain Boundary Mapping in Polycrystalline Graphene. ACS Nano 2011, 5, 2142−2146. (11) Duong, D. L.; Han, G. H.; Lee, S. M.; Gunes, F.; Kim, E. S.; Kim, S. T.; Kim, H.; Ta, Q. H.; So, K. P.; Yoon, S. J.; et al. Probing Graphene Grain Boundaries with Optical Microscopy. Nature 2012, 490, 235−9. (12) Yazyev, O. V.; Chen, Y. P. Polycrystalline Graphene and Other Two-Dimensional Materials. Nat. Nanotechnol. 2014, 9, 755−67. (13) Ago, H.; Fukamachi, S.; Endo, H.; Solís-Fernández, P.; Mohamad Yunus, R.; Uchida, Y.; Panchal, V.; Kazakova, O.; Tsuji, M. Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides. ACS Nano 2016, 10, 3233−3240. (14) Zhou, H.; Yu, W. J.; Liu, L.; Cheng, R.; Chen, Y.; Huang, X.; Liu, Y.; Wang, Y.; Huang, Y.; Duan, X. Chemical Vapour Deposition Growth of Large Single Crystals of Monolayer and Bilayer Graphene. Nat. Commun. 2013, 4, 2096. (15) Liu, L.; Zhou, H.; Cheng, R.; Yu, W. J.; Liu, Y.; Chen, Y.; Shaw, J.; Zhong, X.; Huang, Y.; Duan, X. High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area Ab-Stacked Bilayer Graphene. ACS Nano 2012, 6, 8241−8249. (16) Wang, H.; Wang, G.; Bao, P.; Yang, S.; Zhu, W.; Xie, X.; Zhang, W. J. Controllable Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation. J. Am. Chem. Soc. 2012, 134, 3627−30. (17) Lee, J.-Y.; Lee, J.-H.; Kim, M. J.; Dash, J. K.; Lee, C.-H.; Joshi, R.; Lee, S.; Hone, J.; Soon, A.; Lee, G.-H. Direct Observation of Grain Boundaries in Chemical Vapor Deposited Graphene. Carbon 2017, 115, 147−153. (18) Sönnichsen, C.; Geier, S.; Hecker, N. E.; von Plessen, G.; Feldmann, J.; Ditlbacher, H.; Lamprecht, B.; Krenn, J. R.; Aussenegg, F. R.; Chan, V. Z. H.; et al. Spectroscopy of Single Metallic Nanoparticles Using Total Internal Reflection Microscopy. Appl. Phys. Lett. 2000, 77, 2949−2951. (19) Schultz, S.; Smith, D. R.; Mock, J. J.; Schultz, D. A. Single-Target Molecule Detection with Nonbleaching Multicolor Optical Immunolabels. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 996−1001. (20) Kong, X. H.; Ji, H. X.; Piner, R. D.; Li, H. F.; Magnuson, C. W.; Tan, C.; Ismach, A.; Chou, H.; Ruoff, R. S. Non-Destructive and Rapid Evaluation of Chemical Vapor Deposition Graphene by Dark Field Optical Microscopy. Appl. Phys. Lett. 2013, 103, 043119. (21) Kang, J. H.; Moon, J.; Kim, D. J.; Kim, Y.; Jo, I.; Jeon, C.; Lee, J.; Hong, B. H. Strain Relaxation of Graphene Layers by Cu Surface Roughening. Nano Lett. 2016, 16, 5993−5998.
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CONCLUSIONS We have demonstrated that DF optical microscopy is a powerful and effective tool to characterize the structures of graphene grown on Cu foil. The enhanced color contrast of copper oxide nanoparticles allows one to directly visualize graphene GBs on Cu foil after mild oxidation. Moreover, the grain structure of graphene can be also observed based on the periodic Cu ripples that are modified by the relative orientation of the Cu and graphene lattices. We also applied this technique to the evaluation of the gas barrier property of large graphene grains. The graphene showed good gas barrier protection, preventing the oxidation of the underlying Cu foil. However, when we used the preoxidized Cu, especially for the Cu(110) plane, the dissolved oxygen atoms induce the formation of copper oxide nanoparticles due to diffusion of dissolved oxygen to the Cu surface. The present work offers a new means to characterize graphene, facilitating the graphene research and industrial applications. This DF technique can be potentially applied to other 2D materials grown on metal catalysts, such as h-BN, widening the potential of this simple, quick, and nondestructive analysis method.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10210. Complementary optical images, Raman, SEM, LEED, and other related data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dong Ding: 0000-0001-7840-648X Hiroki Ago: 0000-0003-0908-5883 Present Address ‡
D.D.: The Furukawa Battery Co., Ltd., Kanagawa 240-0006, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant nos. JP15H03530, JP16H00917, and JP17K19036. We thank Y. Uchida for helping the EBSD measurement.
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REFERENCES
(1) Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192−200. (2) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (3) Chae, S. J.; Güneş, F.; Kim, K. K.; Kim, E. S.; Han, G. H.; Kim, S. M.; Shin, H.-J.; Yoon, S.-M.; Choi, J.-Y.; Park, M. H.; et al. Synthesis of Large-Area Graphene Layers on Poly-Nickel Substrate by Chemical Vapor Deposition: Wrinkle Formation. Adv. Mater. 2009, 21, 2328− 2333. 909
DOI: 10.1021/acs.jpcc.7b10210 J. Phys. Chem. C 2018, 122, 902−910
Article
The Journal of Physical Chemistry C (22) Wu, X.; Zhong, G.; Robertson, J. Nondestructive Optical Visualisation of Graphene Domains and Boundaries. Nanoscale 2016, 8, 16427−34. (23) Ding, D.; Solis-Fernandez, P.; Hibino, H.; Ago, H. Spatially Controlled Nucleation of Single-Crystal Graphene on Cu Assisted by Stacked Ni. ACS Nano 2016, 10, 11196−11204. (24) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed.; John Wiley & Sons, Inc.: New Jersey, 2006. (25) Grantab, R.; Shenoy, V. B.; Ruoff, R. S. Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene. Science 2010, 330, 946−948. (26) Platzman, I.; Brener, R.; Haick, H.; Tannenbaum, R. Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions. J. Phys. Chem. C 2008, 112, 1101−1108. (27) Yamamoto, S.; Andersson, K.; Bluhm, H.; Ketteler, G.; Starr, D. E.; Schiros, T.; Ogasawara, H.; Pettersson, L. G. M.; Salmeron, M.; Nilsson, A. Hydroxyl-Induced Wetting of Metals by Water at nearAmbient Conditions. J. Phys. Chem. C 2007, 111, 7848−7850. (28) Zhou, G.; Yang, J. C. Initial Oxidation Kinetics of Cu(100), (110), and (111) Thin Films Investigated by in Situ Ultra-HighVacuum Transmission Electron Microscopy. J. Mater. Res. 2005, 20, 1684−1694. (29) Zhang, H.; Zhang, Y.; Wang, B.; Chen, Z.; Zhang, Y.; Sui, Y.; Yu, G.; Jin, Z.; Liu, X. Stripe Distributions of Graphene-Coated Cu Foils and Their Effects on the Reduction of Graphene Wrinkles. RSC Adv. 2015, 5, 96587−96592. (30) Nguyen, V. L.; Shin, B. G.; Duong, D. L.; Kim, S. T.; Perello, D.; Lim, Y. J.; Yuan, Q. H.; Ding, F.; Jeong, H. Y.; Shin, H. S.; et al. Seamless Stitching of Graphene Domains on Polished Copper (111) Foil. Adv. Mater. 2015, 27, 1376−82. (31) Kim, D. W.; Kim, Y. H.; Jeong, H. S.; Jung, H. T. Direct Visualization of Large-Area Graphene Domains and Boundaries by Optical Birefringency. Nat. Nanotechnol. 2012, 7, 29−34. (32) Orofeo, C. M.; Hibino, H.; Kawahara, K.; Ogawa, Y.; Tsuji, M.; Ikeda, K.-i.; Mizuno, S.; Ago, H. Influence of Cu Metal on the Domain Structure and Carrier Mobility in Single-Layer Graphene. Carbon 2012, 50, 2189−2196. (33) Ogawa, Y.; Hu, B.; Orofeo, C. M.; Tsuji, M.; Ikeda, K.-i.; Mizuno, S.; Hibino, H.; Ago, H. Domain Structure and Boundary in Single-Layer Graphene Grown on Cu(111) and Cu(100) Films. J. Phys. Chem. Lett. 2012, 3, 219−226. (34) Lanza, M.; Wang, Y.; Sun, H.; Tong, Y.; Duan, H. Morphology and Performance of Graphene Layers on as-Grown and Transferred Substrates. Acta Mech. 2014, 225, 1061−1073. (35) Schriver, M.; Regan, W.; Gannett, W. J.; Zaniewski, A. M.; Crommie, M. F.; Zettl, A. Graphene as a Long-Term Metal Oxidation Barrier: Worse Than Nothing. ACS Nano 2013, 7, 5763−5768. (36) Singh Raman, R. K.; Chakraborty Banerjee, P.; Lobo, D. E.; Gullapalli, H.; Sumandasa, M.; Kumar, A.; Choudhary, L.; Tkacz, R.; Ajayan, P. M.; Majumder, M. Protecting Copper from Electrochemical Degradation by Graphene Coating. Carbon 2012, 50, 4040−4045. (37) Chen, S.; Brown, L.; Levendorf, M.; Cai, W.; Ju, S.-Y.; Edgeworth, J.; Li, X.; Magnuson, C. W.; Velamakanni, A.; Piner, R. D.; et al. Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy. ACS Nano 2011, 5, 1321−1327. (38) Weatherup, R. S.; D’Arsié, L.; Cabrero-Vilatela, A.; Caneva, S.; Blume, R.; Robertson, J.; Schloegl, R.; Hofmann, S. Long-Term Passivation of Strongly Interacting Metals with Single-Layer Graphene. J. Am. Chem. Soc. 2015, 137, 14358−14366. (39) Luo, D.; You, X.; Li, B.-W.; Chen, X.; Park, H. J.; Jung, M.; Ko, T. Y.; Wong, K.; Yousaf, M.; Chen, X.; et al. Role of Graphene in Water-Assisted Oxidation of Copper in Relation to Dry Transfer of Graphene. Chem. Mater. 2017, 29, 4546−4556. (40) Hao, Y.; Bharathi, M. S.; Wang, L.; Liu, Y.; Chen, H.; Nie, S.; Wang, X.; Chou, H.; Tan, C.; Fallahazad, B.; et al. The Role of Surface Oxygen in the Growth of Large Single-Crystal Graphene on Copper. Science 2013, 342, 720−3.
(41) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (42) Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: Corrosion-Inhibiting Coating. ACS Nano 2012, 6, 1102−1108. (43) Ding, D.; Solís-Fernández, P.; Yunus, R. M.; Hibino, H.; Ago, H. Behavior and Role of Superficial Oxygen in Cu for the Growth of Large Single-Crystalline Graphene. Appl. Surf. Sci. 2017, 408, 142− 149. (44) Zhang, H.; Zhang, Y.; Zhang, Y.; Chen, Z.; Sui, Y.; Ge, X.; Deng, R.; Yu, G.; Jin, Z.; Liu, X. Realizing Controllable Graphene Nucleation by Regulating the Competition of Hydrogen and Oxygen During Chemical Vapor Deposition Heating. Phys. Chem. Chem. Phys. 2016, 18, 23638−42. (45) Luo, L.; Kang, Y.; Yang, J. C.; Zhou, G. Effect of Oxygen Gas Pressure on Orientations of Cu2O Nuclei During the Initial Oxidation of Cu(100), (110) and (111). Surf. Sci. 2012, 606, 1790−1797.
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DOI: 10.1021/acs.jpcc.7b10210 J. Phys. Chem. C 2018, 122, 902−910
