Vol.9 No.3 2017
18/74

Research paper : High quality and large-area graphene synthesis with a high growth rate using plasma-enhanced CVD (M. Hasegawa et al.)−131−Synthesiology - English edition Vol.9 No.3 (2017) 2DDG(a) H2-plasma only(b) Heated at 850 ℃+H2-plasma(c) Heated at 1000 ℃+H2-plasmaRaman shift (cm-1)Intensity (a. u.)30002500200015001000was measured at 36 points by a four probe method for sheet resistance using gold alloy probes at 1 mm mesh over the sample area of 6 × 6 mm2. The carrier mobility was estimated by Hall effect measurement in Van der Pauw geometry.[44] Next, the transferred graphene on the polymer substrate was immersed in an isopropyl alcohol solution of gold chloride (20 mol/ℓ), and dried. Figure 8 shows the Raman spectra of copper foil observed at room temperature after only the joule heating treatment in a hydrogen atmosphere. Although carbon-related signals in the Raman spectrum were not observed within the detection limit for the heat treatments lower than 300 °C, the spectra of copper foil subjected to the heat treatment at 400, 600, and 800 °C indicated the formation of amorphous carbon lms[45] on the surface. Since carbon-containing gas such as methane was not introduced, there should be alternative carbon sources such as one dissolved in the copper foil and/or one supplied from the environment inside the reaction chamber. The concentration of impurity carbon in the copper foil was examined by a combustion method, which has been estimated to be 5–31 ppm. The areal density of carbon atoms in graphene is 3.8×1015/cm2. If a graphene sheet with the highest impurity carbon concentration of 31 ppm is used, copper foil of 15 μm thickness is at least necessary to supply the carbon atoms to synthesize single layer graphene. Because the thickness of the copper foil in the present study was 6.3 μm, carbon atoms supplied from the environment in the reaction chamber must have been from an additional or main source of carbon atoms. The base pressure of the reaction chamber, which was evacuated by using oil-free turbo molecular pump system, was lower than 1.0 × 10-4 Pa. It was not clarified which was the main supplier of carbon, the copper foil or the environment in the reaction camber. In this paper, the discussion is based on both having the possibility. The copper foil substrate was treated by the joule heating treatment of temperatures up to 1000 °C in hydrogen atmosphere without supplying any carbon gas sources. Raman spectra were measured at room temperature after the cooling of the copper foil substrate in a hydrogen atmosphere. As shown in Fig. 8, however, we could not observe the Raman peaks which indicate the graphene formation on the copper surface. The peaks from amorphous carbon at 1350 cm-1 and 1580 cm-1 were lost by the heating at 1000 °C. It was considered that it was because heat treatment was conducted at temperature close to the melting point of copper (1085 °C) under low pressure, and the precipitated carbon atoms decomposed or were lost with the evaporation of the copper foil surface.Therefore, although amorphous carbon precipitation was observed at 400, 600, and 800 °C, there were no Raman signals on the copper substrate pretreated at the temperature between 25 °C and 300 °C, and 1000 °C.A Raman spectrum for only hydrogen plasma treatment for 30 s without heat treatment of copper foil is shown in Fig. 9(a). In this case, no peaks attributed to carbon related materials such as graphene and amorphous carbon were observed. Figure 9(c) shows a Raman spectrum from copper foil subjected to hydrogen plasma treatment for 30 s subsequent to the treatment by joule heating at 1000 °C. Although, the very weak G-band (1580cm-1) and the D-band (1350 cm-1) were observed, the 2D-band in the range of 2641–2681 cm-1 was not observed. This indicated that graphene was not formed at this temperature because an extremely small amount of carbon supply disappeared along with the evaporation of the copper foil substrate. Figure 9(b) shows Raman spectra from the copper foil substrate subjected to hydrogen plasma treatment for 30 s at 850 °C subsequent to the treatment by joule heating at 850 °C. Distinct G-band and 2D-band were observed with very low intensity of the D-band which indicated low defect. Then, since in Fig. 9(b) it was shown that the 2D-band has different line width and intensity distribution, we analyzed it in more detail. As shown in Fig. 10, two kinds of graphene which have different full width at half maximum (FWHM) of the 2D-band were observed. We analyzed the 2D-band at 46 points in 12 samples which were synthesized under the same conditions using the curve tting by a single Lorentzian peak and the sum of four single Lorentzian peaks as shown in Fig. 10(a) and (b), respectively, according to the fitting Fig. 9 Raman spectrum of the copper foil after hydrogen plasma treatment[14] (a) only hydrogen plasma, (b) hydrogen plasma while heating at 850 °C, and (c) hydrogen plasma while heating at 1000 °CCopyright (2014), with permission from Elsevier

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