Controlled Synthesis of Monolayer Graphene Toward Transparent Flexible Conductive Film Application
© The Author(s) 2010
Received: 24 June 2010
Accepted: 15 July 2010
Published: 28 July 2010
We demonstrate the synthesis of monolayer graphene using thermal chemical vapor deposition and successive transfer onto arbitrary substrates toward transparent flexible conductive film application. We used electron-beam-deposited Ni thin film as a synthetic catalyst and introduced a gas mixture consisting of methane and hydrogen. To optimize the synthesis condition, we investigated the effects of synthetic temperature and cooling rate in the ranges of 850–1,000°C and 2–8°C/min, respectively. It was found that a cooling rate of 4°C/min after 1,000°C synthesis is the most effective condition for monolayer graphene production. We also successfully transferred as-synthesized graphene films to arbitrary substrates such as silicon-dioxide-coated wafers, glass, and polyethylene terephthalate sheets to develop transparent, flexible, and conductive film application.
Graphene is a newly discovered carbon-based monolayer sheet consisting of honeycomb structures with two-dimensional arrays . It has been attracting great attention owing to its fascinating mechanical, electronic, and optical properties [1–3]. Fundamental and application research is being carried out intensively in order to determine the potential uses. Several approaches to synthesizing graphene have been developed, including mechanical and chemical exfoliation, thermal chemical vapor deposition (TCVD), the graphitization of silicon carbide, and the reduction of graphene oxides [4–8].
The TCVD method is considered an appropriate method for the large area synthesis of graphene [5, 9]. However, there is still a definite need for high-quality graphene production that also yields large area thickness uniformity. In addition, the synthesis mechanism remains poorly understood and requires further study [9, 10]. In terms of the industrial application aspects, its outstanding electrical conductivity in a plane and high elasticity makes graphene a powerful candidate material not only for various functional devices such as sensors and electronic elements but also for transparent flexible conductive (TFC) electrodes that can replace indium tin oxide film and therefore be used in flexible display, touch screens and flexible solar cells via transfer onto flexible and transparent receiving substrates . In this context, a large area synthesis with high thickness uniformity and successful transfer ability are essential and urgent topics in the realization of the aforementioned applications.
Here, we demonstrate the optimization of monolayer graphene synthesis using TCVD and the subsequent transfer onto arbitrary substrates toward TFC film application. We mainly investigated the effects of synthetic temperature and cooling rate on graphene thickness. We found that synthesis at 1,000°C and a cooling rate of 4°C/min was the most effective combination for monolayer graphene production. We also successfully transferred as-synthesized graphene films onto arbitrary substrates such as silicon-dioxide-covered wafers, glass, and polyethylene terephthalate (PET) polymer sheets for further development as TFC films.
After synthesis, we performed graphene transfer to a SiO2-covered Si wafer, glass plates, and PET sheets for further development. Here, we tried to carry out the transfer without a polymeric mediator such as polymethyl methacrylate film to prevent potential contamination during the removal of the polymeric films and thus preserve the clean surface of the as-synthesized graphene, as schematically shown in Fig. 2c. In brief, we first etched silicon dioxide using a 3M KOH solution at 75°C and obtained a catalytic Ni layer covered with graphene. The temperature of the solution was sensitive to removal of the SiO2 layer. Then, we further etched out the Ni layer using a 1M FeCl3 solution. When the Ni layer was completely removed, the graphene began floating on the solution and was discernable with the naked eye; we were thus able to scoop it up using receiving substrates. For optical microscopic observation, we used SiO2 (300-nm thick)-coated Si wafers since we can clearly determine the number of graphene layers by color differences (Fig. 2d).
We were able to obtain enhanced Raman profiles after the transfer compared to the pristine samples on Ni films, as shown in Fig. 2e. Raman spectroscopy is a useful and convenient tool to examine the structural property of graphenes [13, 14]. Here, we used a 532-nm excitation wavelength laser with 1 μm spot size and low power of less 3 mW (Horiba Aramis). The Raman spectra ranges of 1,000–3,000 cm−1 from as-synthesized and transferred samples are comparatively shown in Fig. 2e. We obtained intensive peaks around 1,350, 1,600, and 2,700 cm−1, which are known as the defect-induced D-band, the graphitic C–C vibration-related G-band, and the G′-band, which is an overtone of the D-band, respectively. In mono layer graphene, the G′-band shows a single Lorentzian profile below Raman frequency of 2,700 cm−1 and the intensity ratio of the G-band and the G′-band (IG/IG′) is below unity. As the number of graphene layers increases, the value of IG/IG′ decreases over unity and the line shape also becomes asymmetric.
Transmission electron microscopy (TEM, JEM2100 F) was used to reveal the detailed nanostructures of as-synthesized graphenes. The graphene samples for observation were prepared by direct transfer onto a TEM copper grid. For the characterization of the graphene-transferred-TFC film, the sheet resistance was measured by the four-point probe method, and transparency measurement was performed using incident light of 550-nm wavelength.
Results and Discussion
The optical image of the fastest cooled sample has very similar features as the 900°C-grown sample, which means that the carbon diffusion is insufficient, as shown in Fig. 4a. On the other hand, we can observe a mixture of monolayer and thick layer graphenes in the 6°C/min sample (Fig. 4b). The portion of thick graphene layer increased with decreasing cooling rate to 4°C/min (Fig. 4c), which implies that the enhanced surface diffusion of carbon might be responsible for the increased number of thick graphene spots through the Ostwald ripening phenomena in particle agglomeration. However, with a very low cooling rate of 2°C/min, it seemed that the precipitated carbon was sufficiently diffused and finally made some carbonaceous compound along the grain boundary, as shown in Fig. 4d.
We demonstrated a growth optimization procedure for monolayer graphene using TCVD and its subsequent transfer onto various substrates toward TFC film application. We investigated the effects of synthetic temperature and cooling rate on graphene structures and found that synthesis at 1,000°C and a cooling rate of 4°C/min were the most effective conditions for monolayer graphene production. We also fabricated graphene-based TFC films via the transfer of as-grown graphene films onto PET or glass substrates without the use of polymeric mediators.
This research was supported by Basic Science Research Program (No.2009-0089315) and Converging Research Center Program (No.2009-0083380) through the National Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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