Real-Time Optical Visualization of Graphene Defects and Grain

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Real-Time Optical Visualization of Graphene Defects and Grain Boundaries by Thermal Oxidation of Graphene-Coated Copper Foil Kyung Pyo Hong, Dohyeon Lee, Jae-Boong Choi, Yekyung Kim, and Hyeongkeun Kim ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00646 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Nano Materials

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Real-Time Optical Visualization of Graphene Defects and Grain Boundaries by Thermal Oxidation of GrapheneCoated Copper Foil Kyung Pyo Hong1, Dohyeon Lee2, Jae Boong Choi1, Yekyung Kim 3*, Hyeongkeun Kim2,*

1 School of Mechanical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 16419, Korea 2 Korea Electronics Technology Institute (KETI), 25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, 13509, Korea 3 Advanced Institutes of Convergence Technology, 145, Gwanggyo-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, 16229, Korea ABSTRACT: We have constructed a system that can visualize graphene defects in real-time by coupling a heater and an optical instrument. In this system, the phenomenon of copper oxidation was used to visualize defects in graphene. When heat is applied to graphene synthesized on a copper foil, oxygen in the atmosphere penetrates through the graphene grain boundaries and oxidizes the copper substrate. This system observes the entire process of copper oxidation in real-time and controls the rate of the visualization process by adjusting the temperature and time. A temperature-time graph for graphene defect visualization was plotted based on the experimental data, which indicates that it only requires 5 min at 225 °C for completion of the visualization process. Additionally, through scanning electron microscopy-energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses, it has been confirmed that copper oxidation occurs during the process of visualization of graphene defects. Compared to conventional optical visualization methods, this technique is faster and more convenient. This system is expected to enable easy and fast quality-inspection for mass production of graphene. KEYWORDS: Graphene, Copper foil, Oxidation, Optical visualization, Real-time

INTRODUCTION Graphene has a hexagonal honeycomb structure consisting of six sp2 bonded carbon atoms with three bonds attached to each carbon atom,1,2 which makes it a very stable material both physically and chemically.3 Graphene also exhibits excellent physical properties; for example, the electrical conductivity of graphene is 100 times higher than that of copper, mechanical strength is 200 times higher than that of steel, and thermal conductivity is twice as high as that of the diamond.4,5 In addition, because of its superior transparency and flexibility, graphene has attracted much attention as a new material that can overcome the limitations of existing technologies.6–8 However, the graphene’s excellence physical, chemical and electrical properties deteriorate at the graphene boundaries and defects.9,10

In order to commercialize graphene, not only highquality, large-area graphene production process, but also easy and quick quality-inspection technique is essential. Since graphene is only one-atom thick, the existing quality inspection methods require expensive equipment such as scanning electron microscopes (SEM), transmission electron microscopes (TEM), and Raman spectrometers.11–13 Furthermore, these methods require additional sample preparation processes and long times to examine the quality of graphene. Recently, an optical method that uses oxidation of a copper foil (substrate for graphene synthesis) has been studied to overcome the limitations of conventional graphene-inspection methods.14–17 However, this method also requires a long time to achieve complete oxidation of the copper foil. Because the copper oxidation step−the heating step−and the observation step are separated. That is, the graphene/Cu sample is pretreated to achieve copper oxidation, and then the pretreated sample is viewed with

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2 an optical microscope. To optimize the oxidation process in this two-step visualization method generally takes a lot of time. There are no such standard conditions for graphene visualization, especially for the temperature and heating time. The grain boundaries and the defects of graphene are highly influenced by various manufacturing conditions, such as copper foil type, annealing conditions, synthesis temperature and time, and so on. Copper oxidation conditions also vary depending on the graphene properties. Moreover, the specimen’s appearance and oxidation level are unknown until the sample is separated from the heater. When separated, there is a possibility that the sample was oxidized excessively or insufficiently for viewing by an optical microscope. Finding the optimum conditions for copper oxidation requires trial and error, testing many different temperatures and times.15–19 To overcome this problem, in this paper, a new system is described that can optically visualize the defects and grain boundaries of graphene. The difference of this visualization system is that the heating and observation processes are combined, hence in-situ, real-time visualization is possible. Also, the progress of copper oxidation beneath the single layer graphene can be observed using the proposed instrument set-up. Using this system, it is easy to inspect both large and localized areas of graphene. To evaluate the performance of the visualization process using the developed equipment, the results of the present study were compared with those obtained using other analytical tools.

EXPERIMENTAL SECTION System configuration. The graphene visualization system consists of three main components: optical observation, image processing, and sample heating. The optical component consists of three long-focus objective lenses (M Plan Apo Series, Mitutoyo, Japan) with magnifications of 10, 50, and 100X, and a coaxial halogen light to focus on the shape of the graphene grain boundaries (GGBs) at various magnifications. In addition, a 10X ocular eyepiece was used to enlarge the image to the final sample magnifications of 100, 500, and 1000X. The working distance of the objective lens is 33.5 to 13.0 mm, which is far enough to examine the sample on the stage without any interference. The image processing component is composed of a CCD camera (EyeCam, USA) with a resolution of 1600 × 1200 pixels and a PC to acquire images in real-time.20–22 EyeCam ToupView software was used for integrating the CCD camera with the PC. The sample heating component consists of a heating stage to oxidize the copper foil. The LTS 420 temperature-controlled stage (Linkam, England) was used to directly heat the copper foil. The heating stage can be heated up to 420 °C, and the temperature ramp profile can be set at heating rates of 0.01 to 50 °C/min. Measurement conditions. Since graphene is only few nanometers thick and very transparent, direct observation of its defects is very difficult. In general, the methods used for graphene inspection require the transfer of graphene to a specific substrate, followed by analysis using expensive equipment, e.g., SEM, TEM, and Raman

spectroscopy, as mentioned before. To reduce the cost and the time required for the process, we used an optical method to visualize graphene defects. In optical inspection methods, graphene defects are indirectly observed by oxidizing the copper-foil substrate used for graphene synthesis. Because copper is first oxidized along the graphene defects, the shape and the position of the graphene domains can be easily detected using an optical microscope.14–17 Figure 1a schematically shows the difference in the oxidation of a copper foil with and without graphene at elevated temperatures. The upper portion of the pristine copper foil, which is the surface exposed to the atmosphere, is easily oxidized, while that of the graphene-coated copper foil, which serves as a protection layer for copper oxidation, is not easily oxidized.18,23 When copper is covered with graphene, copper is oxidized over a long period of time when it is exposed only to air;15 however, heat and humidity can accelerate the oxidation. When heat is applied, the oxidation of copper starts from the GGBs where the thermal resistance is relatively low.15–19 Exposing copper foil to ultraviolet light in a humid atmosphere has been studied as an indirect method for observation of graphene defects.14 In another previous study, graphene defects were visualized by heating the copper foil on which the graphene was synthesized at temperatures of 150 to 200 °C.19 In the present study, the heating temperature was set between 125 and 225 °C and the experiments were conducted under ambient air conditions. To verify the performance of the new system and find general visualization trends of the graphene defects, ambient air conditions were used instead of using precisely controlled conditions. The overall graphene visualization process is shown in Figure 1b. The graphene sample was synthesized by rapid thermal-chemical vapor deposition (RT-CVD) on copper foil.24 The Cu-foil substrate was rapidly heated to around 1000 °C under H2 flow at 10 sccm and 550 mTorr. Once the temperature reached 1000 °C, the Cu foil was annealed for 300 s, after which CH4 was introduced into the chamber at 30 sccm for 600 s while maintaining the temperature for graphene growth. After the graphene growth, the chamber was cooled to room temperature. The graphene sample was fixed on a slide glass, and then the sample was inserted into a heating stage holder. The sample was heated up to a set temperature at the desired heating rate (0.01 to 50 °C/min) using a controller (T95, Linkam, England) connected to a PC. The applied heat from the heating stage was transferred to the copper foil through conduction. When the temperature of the copper foil without graphene reached 200 °C, the entire copper foil reacted with oxygen in the atmosphere. However, in the case of the graphene-coated copper foil, longer heating times or higher temperatures (exceeding 200 °C) were required for sufficient oxygen to diffuse through the GGBs. Finally, the copper oxidized along the graphene defects and the grain boundaries were observed using the optical inspection system described in this study.


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3 Figure 2. Optical images of the graphene-coated copper foil after heating at 360 °C for (a) 0, (b) 2, (c) 4, (d) 5, (e) 6, and (f) 7 min.

Figure 1. (a) Schematic showing the oxidation of copper foil with and without graphene after heating. (b) Mechanism and process for graphene visualization.

RESULTS AND DISCUSSION In the previously reported methods, graphene defects were visualized after heating the copper foil in air or oxidizing it by exposure to humid air.14–19 Therefore, there is a limitation in the visualization process, because observations are made after the copper oxidation is completed. In this study, the heating equipment and the optical system were integrated to examine and control the entire graphene-defect visualization process in real-time. To confirm the performance of the constructed visualization equipment, the temperature of the heater was increased to 360 °C, and oxidation of the copper foil along the GGBs were observed optically. When the graphene-coated copper foil was examined prior to heating, only the grain boundaries of copper were observed (Figure 2a); but as the temperature was increased, a faint orange line appears in the optical image (Figure 2b). As the temperature and the heating time are increased, the GGBs and the defects turn red (Figure 2d) and then dark green with complete oxidation of the copper foil (Figure 2e). When the heating time exceeds 7 min, the graphene surface as well as the defects are oxidized (Figure 2f). Owing to over-oxidation of the graphene surface by prolonged heat-exposure, the graphene itself is damaged and the shape of the graphene was not accurately observed at this stage. These results indicate that the system constructed in this study can visualize graphene defects and grain boundaries in real-time by heating the graphene-coated copper foil.

The graphene-coated copper foil was cut into 10 × 10 mm2 dimensions and placed on the heating stage. Then, the sample was heated at various temperatures, from 125 to 225 °C, to demonstrate the oxidation behavior of copper beneath the GGBs and defects. As shown in Figure 3, the visualization process was dependent on the sample exposure temperature and time. At 125 °C, the GGBs and defects were not observed, even after heating for a long time (Figure 3a), whereas the graphene contour became visible at 140 °C or higher. This indicates that the graphene layer acted as a barrier to copper oxidation at temperatures below 140 °C. The time required for complete visualization of graphene varied according to the temperature: approximately 81.0, 28.0, 9.2, 6.5, and 5.0 min were required at 140, 150, 175, 200, and 225 °C, respectively. The oxidized copper at the GGBs and defects are divided into two types: red oxide at relatively lower temperatures of 140 and 150 °C (Figure 3b-c) and dark green oxide at temperatures greater than or equal to 175 °C (Figure 3d-f). The color of the copper compounds varies according to the oxidation state of copper and the reacting material. Red Cu2O appears as the first form of oxidized copper at low temperatures, which is then converted into more stable forms such as black CuO or green CuCO3 when more energy is supplied.25–27

Figure 3. Optical images of the copper foil (for graphene visualization) after heating at (a) 125, (b) 140, (c) 150, (d) 175, (e) 200, and (f) 225 °C.



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4 Figure 4. Time required for completion of the visualization process as a function of the heating temperature.

Figure 4 shows the time required to visualize the graphene defects as a function of the heating temperature. The X-axis represents the temperature applied to the copper foil, and the Y-axis represents the heating time required for graphene defect visualization. The experimental results in Figure 4 were obtained using the results shown in Figure 3 for heating at 140, 150, 175, 200, and 225 °C. The regression fit line, the solid line in Figure 4, indicates a visualization threshold temperature of 134 °C – highlighted by the dashed line in the figure. Therefore, at 125 °C, which is lower than the visualization threshold, the GGBs and defects are not visible, even though the sample was exposed to heat for a long time. Furthermore, as shown in Figure 4, the regression line follows the shape of the –log(x) function, enabling the derivation of the optimal temperature for visualization. This time-temperature graph provides a reference curve for the visualization process in this study, including the threshold temperature, which indicates the minimum temperature for graphene visualization, and the required temperature based on the preferred visualization time. Verification of methodology. To visualize graphene defects and grain boundaries, this study used a method based on oxidation of graphene-coated copper foil. The oxidation state of copper was analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). Figure 5 shows the SEM-EDX and XPS results of the samples at different temperatures: 23 (unheated sample), 150, and 225 °C (heat-treated samples). These samples represent the three different states of copper present during the visualization process. The sample at 23 °C consists of a pristine state of copper, which is not yet oxidized (Figure 2a). The samples heattreated at 150 and 225 °C are composed of oxidized copper in different oxidation states represented by the colors red (Figure 3c) and dark green (Figure 3f). Graphene defects and grain boundaries are not evident in the SEM image of the unheated sample (Figure 5a), while they are clearly observed in the SEM images of the heattreated samples (Figure 5b-c). Furthermore, when the samples were heated to 225 °C, the GGB thicknesses increased, and the boundaries became apparent.17,19 The SEM-EDX results show that the amount of oxygen in the GGBs increases as the temperature is increased (Figure 5g). It appears that copper reacted with atmospheric oxygen during the visualization process, and more oxygen reacted at higher temperatures. The chemical state of oxygen was confirmed through XPS analysis, and the results are shown in Figure 5d-f. The O1s peak was analyzed instead of the Cu 2p3/2 peak, because the binding energies of copper (Cu) and copper oxide (Cu2O) are almost equal, 932 eV.27,28 Therefore, the O1s peaks of the three samples were compared to determine whether the copper foil was oxidized. Based on previous studies, the peak at 532 eV is attributed to a carbon-oxygen compound (C-O), and the peaks at 530 and 529 eV correspond to

Cu2O and CuO, respectively.28–30 Small peaks indicative of oxidized copper are observed in the XPS spectrum of the unheated sample (Figure 5d). The intensities of the Cu2O and CuO peaks increase gradually with the increase of the heating temperature (Figure 5e-f), indicating that copper underwent a phase-transformation to copper oxide.16,27,28 With the increase of the heating temperature to 225 °C, a peak corresponding to oxidized copper in the form of CuCO3 appears in the XPS spectrum (Figure 5f), which is the reason for the color change in the visualization process, as shown in Figure 3f. This XPS result validates our earlier speculations that the color change corresponds to different oxidation states of copper with the increase of the heating temperature and time. The quality of the heat-treated graphene samples (Figure 5a-c) were analyzed by Raman spectroscopy (Figure S2). The ID/IG ratio are 1.7, 1.7, and 2.1 for the graphene at 23(unheated), 150, and 225 °C (heat-treated), respectively. The graphene did not deteriorate by heat exposure using the constructed real-time visualization system, especially at the temperature of 150 °C. The similar consequence was observed by sheet resistance measurement (Figure S3). The sheet resistance was not increased much from unheated sample (236 Ω/□) to 150 °C-treated graphene (253 Ω/□), while the graphene treated at 225 °C showed 100 Ω/□ higher than unheated graphene. That is, the oxidation of copper takes place only at the graphene grain boundaries and defects already existed, and does not influence on graphene quality. Hence, a non-destructive inspection of graphene can be performed using the visualization system at a relatively low temperature.

Figure 5. SEM images of graphene on Cu foil at (a) 23, (b) 150, and (c) 225 °C. XPS spectra of graphene on Cu foil at (d) 23, (e) 150, and (f) 225 °C. (g) SEM-EDX results of graphene on Cu foil.

CONCLUSIONS A real-time visualization system was constructed by combining heating capabilities and observation process to


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5 optically visualize graphene synthesized on copper foil. The graphene defects and grain boundaries were visualized using a real-time visualization system through the color changes caused by copper oxidation as a result of heat treatment. The GGBs and defects were successfully observed by the visualization system. The time required for completion of the graphene visualization process was determined through experiments performed at various temperatures, and a temperature-time graph was plotted to determine the optimal oxidation conditions. Two oxidation states of copper were observed in the samples used for the graphene visualization process: a red colored state at the beginning of the process and a dark green colored state at high temperatures or long exposure times. Through SEM-EDX and XPS analyses of the heattreated graphene samples, it was confirmed that the degree of oxidation in the GGBs increased as the temperature increased, resulting in a color change. It has been shown from the results that a small difference in heating conditions can cause a large change in the degree of oxidation at high temperatures. In other words, the optimization process should be performed for all different samples. Using our hybrid system of heating and observation, it is possible to observe the process of the graphene visualization in real-time, and at the same time, the optimum time for graphene visualization can be obtained. In conclusion, the equipment constructed in this study provides a relatively simple and easy method for optical visualization of graphene defects and grain boundaries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI : Graphene synthesis condition, Raman spectra analysis, and sheet resistance data of graphene samples (PDF). Real-time visualization video (S1.avi)

AUTHOR INFORMATION Corresponding Authors * Tel: +82-031-888-9181, E-mail: [email protected] * Tel: +82-031-789-7166, E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Technology Innovation Program (No. 10067432, Equipment technology development for high-speed deposition of OLED encapsulating thin films by Time and Space Divided ALD) funded by the Ministry of Trade, Industry and Energy; and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015M3A7B4050452).

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Figure 0. Abstract image

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Real-Time Optical Visualization of Graphene Defects and Grain

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