The deviation of growth model for transparent conductive graphene
© Chan et al.; licensee Springer. 2014
Received: 1 September 2014
Accepted: 11 October 2014
Published: 20 October 2014
An approximate growth model was employed to predict the time required to grow a graphene film by chemical vapor deposition (CVD). Monolayer graphene films were synthesized on Cu foil at various hydrogen flow rates from 10 to 50 sccm. The sheet resistance of the graphene film was 310Ω/□ and the optical transmittance was 97.7%. The Raman intensity ratio of the G-peak to the 2D peak of the graphene film was as high as ~4 when the hydrogen flow rate was 30 sccm. The fitting curve obtained by the deviation equation of growth model closely matches the data. We believe that under the same conditions and with the same setup, the presented growth model can help manufacturers and academics to predict graphene growth time more accurately.
Graphene, comprising two-dimensional monolayer of sp2-bonded carbon atoms, has attracted substantial attention for use in transparent conductive electrodes (TCE), owing to its chemical stability and high optical transmittance from the ultraviolet to the infrared regions. Graphene as a TCE has a wide range of applications, including in solar cells, solid-state lighting, and detectors, owing to not only its higher optical transmittance but also its more favorable conductance[1–4] than those of traditional transparent conductive electrodes, such as indium tin oxide (ITO) and zinc oxide (ZnO). (Additionally, Furthermore, or Moreover), ITO is an expensive material, and it is unstable in chemical solution and cannot be utilized in a hydrogen-containing environment. ZnO-based thin film has also attracted interest for use in TCEs owing to its low cost, non-toxicity, and abundant constituent elements. However, the properties of ZnO-based thin films are not uniform or stable. The several ways to produce graphene include mechanical exfoliation, graphene oxide (GO), and chemical vapor deposition (CVD). Mechanical exfoliation can yield high-quality graphene from graphite but this method produces graphene over a small area of the order of only a few tens of micrometers. Graphene oxide can be formed by oxidizing graphite flakes; this method can produce large quantities of graphene whose electrical properties are, however, affected by the functional groups and various defects in the graphene. Nevertheless, CVD is a promising method for growing high-quality graphene over a large area using Cu foils. Li et al. were the first to grow graphene over a large area of the order of square centimeters on Cu foil by CVD using methane, and this method has become a standard approach to forming graphene films in recent years. Four-layered graphene exhibits a sheet resistance of about 350Ω/□, which represents a large step toward lower sheet resistance and a large increase in the range of graphene applications. Additionally, various methods have been proposed to optimize the properties of CVD graphene[10–14]. This work develops a simply derived graphene growth model to predict the growth time with various hydrogen flow rates.
Graphene films were grown by chemical vapor deposition (APCVD) on 25-μm-thick Cu foils (99.8%, Alfa-Aesar, item no. 13382) in a 3-in. quartz tube furnace under atmospheric pressure. Beforehand, electrochemical polishing (50% H3PO4 in deionized water of 100 mL) was utilized to smooth out the foil, and a voltage from 2 to 4 V was applied until the Cu foil glowed. Thereafter, the Cu foil was rinsed in a large amount of deionized water with sonication and then blow-dried with nitrogen gas. The Cu foil was placed in the reaction chamber, the Ar and H2 (1,000 and 2 sccm, respectively) gases were introduced into the chamber during temperature ramp-up. The Cu foil was annealed at 1,070°C for an hour. Then, 0.3 sccm of methane (purity, 99.99%) was used as a source of carbon to grow the graphene. The H2 flow rate was varied from 10 to 50 sccm prior to observation of the morphology of the graphene domains and the nucleation density. Following the growth process, the as-grown graphene/Cu foil was removed from the heating zone for rapid cooling. Polymethyl methacrylate (PMMA) was spin-coated on the as-grown graphene/Cu foil as a supporting layer to prevent any cracking during the transfer process. The graphene grew on both sides of the Cu foil. The graphene at the back of the foil was removed by floating on nitride acid solution (30% in deionized water) for 10 s. The Cu foil was etched away overnight using an ammonium persulfate solution (0.1 M) and then rinsed three times in deionized water. The PMMA/graphene was placed on the substrate, and the PMMA was then dissolved in hot acetone bath for 24 h. The residual PMMA was removed by annealing in air at 200°C for an hour and reduced to pristine graphene using an H2/Ar (7/20 sccm) mixture. The morphology and the nucleation density of the graphene domain were measured by scanning electron microscopy (SEM); The surface profile of Cu foils were measured by atomic force microscopy (AFM); the sheet resistance was measured using a four-probe stage; the Raman shift of the graphene was measured by Raman spectroscopy using a laser with a wavelength of 532 nm, the laser power at the focused spot was 2 mW, and the numerical aperture value was 0.75 on the sample with an area of 1 μm2.
Results and discussion
An approximate growth model of the synthesis of graphene by APCVD was developed. When the hydrogen flow rate was 30 sccm, the transmittance as a function of wavelength for single-layer graphene reached its maximum of 97.7% at λ =550 nm. The 2D/G ratio and the FWHM indicated that the graphene comprised a single layer of high quality. The lowest obtained sheet resistance of a single layer of graphene was about 310Ω/□. The results of the experiments closely matched the fitting curve. We believe that, under the same conditions and with the same experimental setup, the proposed growth model can help manufacturers and academics predict the growth time of graphene more accurately.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract nos. NSC 102-2633-E-008-001, NSC 103-2623-E-008-001-ET, and MOST 103-2221-E-008-101-MY2.
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