Bilayer graphene synthesis by plasma treatment of copper foils without using a carbon-containing gas

doi:10.1016/j.carbon.2014.05.087

Carbon

Volume 77, October 2014, Pages 823-828

https://doi.org/10.1016/j.carbon.2014.05.087 Get rights and content

Abstract

Bilayer graphene has been synthesized by using hydrogen plasma treatment of copper foils for 30 s at the temperature of 850 °C together with joule-heating treatment of the foils without using a carbon-containing gas such as methane in order to suppress the nucleation density of graphene. The effect of plasma provides active species of carbon atoms on copper substrate and a selective bilayer graphene formation of AB-stacking in a very short time. Carbon to be precipitated is delivered from the copper foil and/or the environment in the reaction chamber. The domain size of synthesized graphene, the controllability of a few layers and the electrical conductivity have been significantly improved compared with plasma chemical vapor deposition (CVD) using carbon-containing gas. The sheet resistance of bilayer graphene exhibits 951 Ω in average. The carrier mobility shows 1000 cm²/V s in maximum at room temperature. The sheet resistance of 130 ± 26 Ω has been attained after the doping by gold chloride solution.

Introduction

Graphene is ultimately thin atomic layer film in which carbon atoms are arranged in a honeycomb structure. Although the absolute number of electrons is small because of its thinness the conductivity is ensured by supplementing the high mobility of electrons [1], [2], [3]. Graphene is expected as a new transparent conductive film material having properties such as thinness and flexibility which indium tin oxide (ITO) films do not possess. Recently growth control of AB-stacked bilayer graphene with a high yield has been expected in the field of electronic device applications because of the appearance of band gap [4].

For the industrial application of graphene transparent conductive films, establishment of the synthesis method of high-quality and high-throughput is required. From the point of view of production technology for transparent conductive films, the synthesis of graphene by chemical vapor deposition (CVD) on transition metal substrates, in particular on copper, is most promising and controllable at the moment [5]. The transmittance of 90% of visible light and the sheet resistance of 30 Ω by four-layers stacking is the indicator of high-performance graphene synthesized by thermal CVD [6]. The demonstration of organic light-emitting diode (OLED) with graphene anode which has higher luminous efficiency than by using ITO has been reported [7].

For the mass production of graphene by such as roll-to-roll method the problem of thermal CVD is the thermal load on the apparatus given by the process temperature of higher than 1000 °C. It is also required a significant reduction of synthesis time. An attempt has been made to reduce the thermal load on the apparatus by direct joule heating of copper foil substrate and to demonstrate the roll-to-roll synthesis of graphene at 950 °C [8]. In this example the sheet resistance of 200 Ω and the transmittance of 97.1% have been reported. The winding speed of the substrate was a few millimeters per second, which is expected to be improved to establish the high throughput production for the industries.

We have attempted to develop a plasma-enhanced CVD of graphene to reduce the process temperature and the process time at the same time. By combining the low-temperature surface wave plasma CVD with the roll-to-roll transfer of copper film high throughput synthesis of graphene with winding speed of 5–10 mm/s was demonstrated [9], [10].

Current problem of the plasma CVD of graphene is the crystal size of 10 nm or smaller, which inhibits the electrical conductivity. By the large growth rate and high nucleation density of the plasma CVD, graphene growth in the two-dimensional direction is prevented, which causes the stacking of small flakes in multiple layers and deterioration of the controllability of graphene synthesis of less than several layers.

In this study, we attempt to expand the size of the graphene crystal and to improve the controllability of a few layers by reducing the concentration of the carbon source used for graphene synthesis which is expected to suppress the nucleation density. As an ultimate low concentration of carbon source, we utilize the trace amounts of carbon contained in the copper foil and/or supplied from the environment in the reaction chamber. AB-stacked bilayer graphene with 60% yield and disoriented bilayer graphene with 40% are synthesized on copper foil at 850 °C by hydrogen plasma treatment combining with a joule-heating in a very short time.

Section snippets

Copper foil pretreatment

Tough-pitch copper foils of 6.3 μm-thick were used for substrate of graphene synthesis. In many cases, the surface of as-received copper foil is coated by an anticorrosive treatment. Because an anticorrosive becomes the source of contaminations for synthesis of graphene it was removed carefully by dipping 5 wt.% H₂SO₄ for 1 min and washing copper foil by ion-exchanging water and shortly thereafter by nitrogen gas drying before heat treatment. An effect of removal process was examined by X-ray

Results and discussion

Fig. 1 shows the XPS survey scan spectrum of as-received copper foil and the copper foil after removal process of anticorrosive treatment. As shown in Fig. 1, N 1s peak contained in such as benzotriazole was observed from as-received copper foil [13]. On the other hand, N 1s peak was not observed from the copper foil after the removal process of the acid treatment (5 wt.% H₂SO₄). However, the peak of C 1s was observed slightly from both XPS spectrum.

Fig. 2 shows the Raman spectra of copper foils

Summary

AB-stacked bilayer graphene (60%) and disoriented bilayer graphene (40%) has been synthesized by hydrogen plasma treatment of copper foils for 30 s with joule-heating of the foils at 850 °C without using a carbon-containing gas such as methane. Carbon to be precipitated is delivered from the copper foil and/or the environment in the reaction chamber. The domain size of synthesized bilayer graphene and the electrical conductivity have been significantly improved compared with the plasma CVD using

Acknowledgement

The authors are grateful to Yoshinori Koga for fruitful discussions. This work was partially supported by “Basic research and development of high-quality graphene” funded by New Energy and Industrial Technology Development Organization (NEDO).

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Bilayer graphene synthesis by plasma treatment of copper foils without using a carbon-containing gas

Abstract

Introduction

Section snippets

Copper foil pretreatment

Results and discussion

Summary

Acknowledgement

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Nat Mater

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Nature

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Chem Mater

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Science

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Nat Nanotechnol

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Nat Photon

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Appl Phys Lett

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J Phys D: Appl Phys