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Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy Dong Ding, Hiroki Hibino, and Hiroki Ago J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10210 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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The Journal of Physical Chemistry

Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy Ding Dong,†,║ Hiroki Hibino,‡ and Hiroki Ago,*,†, §

†

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka 8168580, Japan ‡

School of Science and Technology, Kwansei Gakuin University, Hyogo 669-1337, Japan § Global Innovation Center (GIC), Kyushu University, Fukuoka 816-8580, Japan

Keywords: chemical vapor deposition, single-crystal graphene, dark filed, grain boundary, gas barrier.

║

Current address: The Furukawa Battery Co., Ltd., Kanagawa 240-0006, Japan

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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 post-growth mild oxidation allow to determine the position and structure of the GBs by the DF microscope. As the DF imaging offers a much higher sensitivity than the bright-field (BF) microscopy, some GBs were observed even without the post-growth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize grain structure of the as-grown graphene by the DF microscopy. Moreover, the DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced the oxidation of Cu surface below graphene in spite that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of the DF optical microscopy in characterizing graphene and related two-dimensional materials grown on metal substrates.


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INTRODUCTION Graphene, two-dimensional (2D) carbon sheet with single atom 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 selflimiting 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,1011

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 of the number of 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 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


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The 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 moisture-rich environment or to oxygen plasma treatment,11,

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GBs can be detected by the 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 optical resolution of BF imaging which uses visible light. For example, the hightemperature oxidation (300 ºC in air) was combined with the oxygen plasma treatment in order to observe GBs by BF mode, suggesting that the oxygen plasma is not sufficient to make the copper oxide visible by the BF optical microscopy.17 Accordingly, the graphene films suffer severe damage due to such strong oxidation processes, giving strong Raman D band not only at the GBs but also inside the graphene grains.11, 17 In contrast to the BF optical microscopy, dark-field (DF) microscopy has not been widely utilized for the analysis of graphene. The 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 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 the DF microscopy.20-22 In this paper, we explore new applications of the DF optical microscopy for characterizing CVD graphene without transfer process. One application is GB visualization of CVD graphene grown on Cu foil. We found that the DF imaging offers much higher sensitivity for observing graphene grain structures, e.g., structure and length of GBs and merged grains with different


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orientations, than the conventional BF mode of optical microscope, thanks to the enhanced color contrast of Cu nanostructures in 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 the fast oxidation of Cu surface just beneath graphene. Our findings broaden the applications of the 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.

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 ºC 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 heating up step (from room temperature to 1075 ºC) stage, pure Ar (99.999%) was flown under ambient pressure. After reaching the target temperature, 2% H2 and 15-66 ppm CH4 gases in Ar were introduced for 30 to 120 min to grow graphene. Finally, the Cu foil was cooling down rapidly by taking out from the heating zone. For oxygen-free Cu growth, we heated up in Ar with 5% H2 without pre-oxidation of the Cu, followed by reaction with CH4 at 1075 ºC. Transfer of graphene. The graphene sample grown on Cu foil was spin-coated with PMMA, then the Cu foil was chemically etched by APS. The back side of the Cu foil was removed by O2


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plasma. After the Cu etching, the PMMA/graphene stack was transferred onto a SiO2/Si wafer. Finally, the coated PMMA was removed by soaking in acetone. Characterizations. BF and DF optical images were taken with 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 lowenergy 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.

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 ºC using CH4 on the Cu foil which were pre-oxidized in air, as reported previously.23 Although we can synthesize larger, millimeter-scale graphene grains, we tuned the grain size to sub-millimeter for observing multiple grains in a view area of our optical microscope. Figure 1a,b shows BF and DF optical images of two merged graphene grains taken after the post-growth oxidation at 250 ºC for a very short time (5 sec) in air. In the BF image (Figure 1a), the bare Cu shows orange color due to air oxidation of the surface-exposed Cu, while the area covered with graphene shows bright metallic Cu color, indicating the Cu is protected from oxidation by graphene. In Figure 1a, the misaligned angle two merged grains is ~12°, but it was difficult to observe a GB possibly existing between these adjacent two grains.


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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 the BF imaging. In some merged grains, graphene GBs were observed by the DF microscope even without the post-growth oxidation process, as shown in Figure 1c,d. 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 the 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 demonstrate 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 shown 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


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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 misorientation angle of 11° for longer time, as shown in Figure 2. Firstly, the oxidation at 250 °C was performed for 1 sec to distinguish graphene grains from bare Cu surface by the BF microscope (Figure 2b upper). 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 the BF measurement (Figure 2d upper). On the other hand, the DF mode exhibited linear GB only after the 1 sec oxidation, as marked in Figure 2b lower panel. 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 sec and 5 min (lower panels of Figure 2c,d and Supplementary Information Figure S1), as the number of bright spots inside the grains increased with the oxidation time. These results indicate that the DF microscopy has a much higher sensitivity than the BF imaging. Thus, the DF imaging can significantly reduce possible damages introduced by the post-growth oxidation processes which are indispensable to the 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 with Figure 1e but was taken at a different location. Figure 3b,c shows the 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 lower panel). The 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


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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

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