S1-2
Synthesis of Graphene by Plasma Process
グラフェンのプラズマプロセス合成
Masataka Hasegawa(1,2), Ryuichi Kato (2), Masatou Ishihara(1,2), Takatoshi Yamada(1,2),
Yuki Okigawa(1,2)
長谷川雅考(1,2)、加藤隆一(2)、石原正統(1,2)、山田貴壽(1,2)、沖川侑揮(1,2)
1)

AIST Nanotube Research Center2)
TASC Graphene Division
1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan1)
産総研ナノチューブ応用研究センター
2)
TASCグラフェン事業部〒305-8565 茨城県つくば市東1-1-1 中央第5Graphene with high electrical conductivity has been synthesized by using hydrogen plasma treatment ofcopper foils for 30 seconds at the temperature of 850 ℃ together with joule-heating treatment of the foils
without using a carbon-containing gas such as methane in order to suppress the nucleation density ofgraphene. The domain size of synthesized graphene, the controllability of a few layers and the electricalconductivity 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/Vs in maximum at room temperature. The sheet resistance of 130±26 Ω
has been attained after the doping by gold chloride solution.

1. Introduction
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 theprocess temperature of higher than 1000 °C. It isalso required a significant reduction of synthesistime. An attempt has been made to reduce thethermal load on the apparatus by direct joule
heating of copper foil substrate and todemonstrate the roll-to-roll synthesis of graphene
at 950 °C. [1] While good properties (sheet
resistance of 200 Ω and the transmittance of97.1 %) have been reported, much higher
throughput and lower synthesis temperature havebeen expected to lower the cost and to improve
the quality of graphene films. We have attempted
to develop a plasma-enhanced CVD of graphene
to lower the synthesis temperature and theprocess time at the same time. [2][3] Current
problem of the plasma CVD of graphene is thecrystal size of 10 nm or smaller, which inhibits
the electrical conductivity. In this study, we
attempt to expand the size of the graphene crystaland to improve the controllability of a few layers
by reducing the concentration of the carbon
source used for graphene synthesis which isexpected to suppress the nucleation density.
2. Experimental
Tough-pitch copper foils of 6.3 μm-thick wereused for substrate of graphene synthesis. We

synthesize graphene on the condition of extremely
low-concentration carbon source, which is supplied
from copper foil substrate and/or environments in
the reaction chamber by microwave plasma CVD
apparatus, in particular, using hydrogen plasma
treatment. Hydrogen plasma treatment is performedin 30 sccm flow and 5 Pa for 30 seconds using asurface wave plasma excited by 2.45GHz
microwave.
3. Results and Discussion
We show a Raman spectrum for only hydrogenplasma treatment for 30 seconds without heat
treatment of copper foil in Fig. 1(a).[4] In this case,no peaks attributed to carbon related materials suchas graphene and amorphous carbon were observed.
Figure 1(c) shows a Raman spectrum from copper
foil subjected to hydrogen plasma treatment for 30
seconds subsequent to the treatment by the joule
heating at 1000 ℃. Although very weak intensitiesof G-band (1580 cm-1) and D-band (1350 cm-1)
were observed, 2D-band in the range of 2641 cm-1
to 2681 cm-1 was not observed, which indicates thatgraphene was not formed by at this temperature
because of an extremely small amount of carbon
supply accompanying with evaporation of copper
substrate. Figure 1(b) shows a Raman spectrum
from copper foil subjected to hydrogen plasma
treatment for 30 seconds at 850 ℃ subsequent to
the treatment by the joule heating at 850 ℃ .

Distinct G-band and 2D-band are observed with avery low intensity of D band which indicates lowdefect.

controllability of a few layers and the electricalconductivity have been significantly improved
compared with plasma chemical vapor deposition(CVD) using carbon-containing gas. The sheet
resistance of bilayer graphene exhibits 951 Ω inaverage. The carrier mobility shows 1000 cm2/Vs inmaximum at room temperature. The sheet
resistance of 130±26 Ω has been attained after thedoping by gold chloride solution.
6. Acknowledgement
This paper is partially based on results obtained
from a project supported by New Energy andIndustrial Technology Development Organization(NEDO).
References

Fig. 1. Raman spectrum of copper foil at room
temperature after hydrogen plasma treatment at eachtemperature. (a) Only hydrogen plasma treatment, (b)Hydrogen plasma treatment while heating 850 ℃, and(c) Hydrogen plasma treatment while heating 1000 ℃.

The sheet resistance of bilayer graphene exhibits
951 Ω in average. The carrier mobility shows 1000
cm2/Vs in maximum at room temperature.[5] Figure
2 shows the sheet resistance map of the synthesizedbilayer graphene film, which doped by goldchloride.[4] As shown in this figure, the sheet
resistance of the average in 6 × 6 mm2 was 130 Ωand the lowest sheet resistance was less than 100 Ω.

Fig. 2. Sheet resistance map of graphene film after wet
doping by gold chloride.

4. Summary
Graphene with high electrical conductivity has
been synthesized by using hydrogen plasma
treatment of copper foils for 30 seconds at thetemperature of 850 ℃ together with joule-heating
treatment of the foils without using acarbon-containing gas such as methane in order tosuppress the nucleation density of graphene. The
domain size of synthesized graphene, the[1] T. Kobayashi, et al., Appl. Phys. Lett. 102, 023112(2013).
[2] J. Kim, et al., Appl. Phys. Lett. 98, 091502 (2011).
[3] T.Yamada, et al, Carbon 50, 2615 (2012).
[4] R.Kato, et al., Carbon 77, 323 (2014)[5] Y. Okigawa, et al.,
http://dx.doi.org/10.1016/j.carbon.2014.10.029.