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In this paper, we applied the Fourier Transformation as a notion to calculate the orientation of hexagonal graphene domains on Cu substrate. We developed that a hexagon function to describe the diffraction pattern of hexagonal graphene. Hexagonal graphene domains grown on Cu (111) has an average value of orientation surrounding 3° in the frequency domain. For transparent conducting electrode applications, optical and electrical properties of large-area graphene film (2cm2) was measured. The results demonstrate that graphene grown on Cu (111) was greater than graphene grown on polycrystalline Cu.
© 2015 Optical Society of America
1. Introduction
Graphene has been established as one of the most attracting materials by its unique properties including flexibility [1], heat spread ability [2], high mobility [3], transparency [4] and so further. Recent years, graphene displayed ability to be received in optoelectronic [5, 6] and large-area flexible electronics [7, 8]. The most common method for synthesizing large-area graphene is chemical vapor deposition (CVD) [9]. However, CVD graphene films on the growth substrate composed with thousands of domains with randomly orientations. Previous study indicated that graphene produced by CVD is polycrystalline, and as the scattering center at grain boundaries, this lead to degrade its performance [10]. In other words, domain boundaries increase the resistance of graphene films for all the optoelectronics dramatically. Two ways have been proposed to solve this problem, first method is to synthesize large graphene domains in order to decrease graphene boundaries [11]; second method is to manipulate the orientation of graphene domains in order to reduce the defect when domains merged to each other. Considering the epitaxy technique, Cu (111) is a suitable growth substrate to control the orientation of graphene domains owing to the lattice constant of Cu (111) and graphene are quite match [12–17]. In this study, we utilized Fourier Transformation (FT) to verify the orientation of hexagonal graphene domains. We grew hexagonal graphene domains on Cu (111) film using atmospheric-pressure chemical vapor deposition (APCVD). In the frequency domain, the far-field diffraction pattern of hexagonal graphene domains which grown on Cu (111) exhibited obvious conformity orientation. This method provides a simple way to certify the lattice facet of catalyst metal affects the orientation of graphene domains.
2. Experimental
Throughout the experiments, atmospheric pressure chemical vapor deposition (APCVD) was used to synthesize hexagonal graphene crystals on epitaxy Cu thin film and polycrystalline Cu foil at 1000 °C. The 1μm-thick Cu (111) thin film was deposited on c-plane sapphire by magnetron sputtering method at adjacent room temperature. The sputter power was 800W, the distance between Cu target and sapphire wafer was about 7.5 cm, and the sputtering pressure was maintained at 6.7 × 10−3 Torr. Prior to sputtering, the sapphire was rinsed in acetone, isopropyl alcohol (IPA) and deionized water with sonication for 5 min before being dried in nitrogen gas. We applied electroplating method to get peeled-off epitaxy Cu (111) foil [18]. The Cu (111) foils were annealed at 1000 °C for 30 min with a mixture gas (Ar/H2:1000/2 sccm) to remove the Cu oxide and increase the Cu grain size. The hydrogen flow rate of growth conditions for growing hexagonal graphene domains on Cu (111) film and Cu foil were 26 sccm and 30 sccm, respectively. The methane was used as the carbon source with 0.4 sccm of flow rate to get the graphene domains. After the growth process, a rapid cool down method was applied. For obtaining full coverage graphene films, the growth time was extended to 30 min.
Prior the wet etching, a 200-nm-thick of PMMA (poly-methyl methacrylate) was spin-coated on the top of graphene/copper and then baking it at 130°C for 5 min. The graphene grew on both sides of copper foil, the graphene at the back of the cooper foil was etched by floating on nitride acid solution (35% in deionized water) for 1 min. Then the copper foil was etched away overnight using an ammonium persulfate solution (0.1M) and then rinsed in deionized water over 24 hours. The PMMA/graphene films were transferred onto BK-7 glass, and the PMMA was dissolved in acetone bath for 24 hours. The residual PMMA was removed by annealing at 200°C in air for an hour and reduced to pristine graphene using an H2/Ar (7/20 sccm) mixture.
The image of graphene domain was observed by using scanning electron microscopy (SEM). Fourier transform analysis was employed to calculate the orientation of hexagonal graphene domain. We developed a model as a hexagon function for Fourier transform analysis to calculate the orientation of hexagonal graphene domains. The optical transmittance of full coverage graphene was measured by using Hitachi U-4100 spectroscopy. The carrier mobility and sheet resistance were measured using Hall measurement. The Raman mapping of graphene was measured by Raman spectroscopy using a laser with wavelength of 532 nm, the laser power at the focused spot and the numerical aperture were 2 mW and 0.75.
3. Results and discussion
Figure 1 displays the electron back scattering diffraction (EBSD) images of Cu film and Cu foil before and after annealing process. The Cu deposited on the sapphire substrate demonstrated that existed second orientation of (111) facet, as shown in Fig. 1(a). Although there were second orientation of (111) facet on the Cu film, but it is still a single lattice, and easy to form the same orientation after annealed in short time, as showed in Fig. 1(b). Contrary to the result of Cu (111) thin film, in Fig. 1(c), Cu foil illustrates variety facets of Cu before annealing process. Obviously, Fig. 1(d) displayed that the (200) facet dominates the primary facet of Cu foil, and the surface consist of randomly oriented grains.
In Fig. 2(a), graphene domains showed the inconsistency orientations on the Cu foil because of various lattice facets, contrary to the Cu foil, graphene domains grown on Cu (111) have a single orientation, as shown in Fig. 2(d). We applied the Fourier transform method to verify the graphene domains grown on Cu (111) has a single orientation, which can be attributed to the lattice of Cu (111) and graphene was intense matched. There are three steps for proving our approaches, step1: image of hexagonal graphene domains obtained by scanning electron microscopy (SEM); step2: filter the high frequency of each graphene domains, as show in Figs. 2(b) and 2(e); step3: Fourier Transform formula applied.
The following equations showed the hexagon function for Fourier transform analysis to calculate the orientation of hexagonal graphene domain. We defined Hgn(x) to express the hexagon function as y for simple calculation, as shown in Eq. (1).
The coordinates of hexagon were shown in Fig. 3(a), now, the three slope functions and ranges were sought such that.To express graphene grown on polycrystalline copper, as shown in Fig. 3(b), the rotation of hexagonal graphene is given by, Solving Eq. (4), (5) with Eq. (2) yields,The Fourier transform is defined by Eq. (7), where the is the spatial frequencyApparently, there are several orientations that displayed on the frequency domain as shown in Fig. 2(c), when graphene domains grown on Cu foil. Furthermore. Figure 2(f) displays the far-field diffraction pattern of hexagonal graphene domains that grown on Cu (111) has the convergence pattern on the frequency domain. However, it is not perfect for every hexagonal graphene domains by various factors such as temperature, diffuse length of carbon and etching effect of hydrogen. Based on those factors, we filtered the low-frequency data and saved the high-frequency part of Fig. 2(f), now it comes the Fig. 4. Obviously, we can observe the edge of diffraction pattern in the Fig. 4, and because the diffraction patterns are symmetrically, the error of the orientation could be easy calculated. We plotted three pair of lines to measure the orientation angel. Table 1 listed the deviation of orientation of hexagonal graphene domain grown on Cu (111). Overall, the calculation indicated that graphene domains have an average value of orientation around 2° to 3° under the synthesis parameter.
Fig. 4 The edge of far-field diffraction pattern of graphene domain which grown on Cu (111). The deviation of orientation can be targeted and measured.
Table 1. The deviation of orientation of hexagonal graphene crystals on Cu(111) facet.
For transparent conductive electrode applications, a large-area of graphene film is required. The graphene growth time was extended to 30 min to obtain the full coverage of large-area graphene. Raman spectroscopy has been used to probe the structural and electronic characteristics of graphite materials, providing general information of graphene fingerprints, such as defects (D band), in-plane vibration of sp2 carbon atoms (G band) and stacking orders (2D band). Figure 5 displays the graphene film which grown on a polycrystalline Cu foil and transferred onto a BK7 glass substrate. The Figs. 5(a) and 5(b) plot the intensity ratios of D and 2D peaks to the G peak, the D peak signal was occurred surrounding the merged position of graphene domain. Figure 5(c) gives the Raman spectra of graphene with four spots from Fig. 5(a), it can be clear seen that the intensity of D band was occurred. The defects may generated when graphene grains meet with a relative misorientation [19].
Fig. 5 (a) Raman image plotted by intensity ratio of D band to G band, and (b) 2D band to G band. (c) Raman spectra of graphene with four spots A, B, C and D.
The difference is very obvious for graphene grown Cu (111), Figs. 6(a) and 6(b) give the intensity ratios of D and 2D peaks to the G peak. The graphene grown on Cu (111) showed very low D peak or defect-free, indicated high quality of graphene even after the transfer process. The A spot of Fig. 6(c) displays the multi-layer graphene may also grow on Cu (111) by various factor such as pointed surface or pinhole. Overall, graphene film has more uniform properties when uses Cu (111) as the catalyst metal substrate, the result was similar to the previous study [12].
Fig. 6 (a) Raman image plotted by intensity ratio of D band to G band, and (b) 2D band to G band. (c) Raman spectra of graphene with four spots A, B, C and D.
Here, we showed electrical properties of large-scale graphene on BK7 glass. Figure 7 summarizes the average sheet resistance and average mobilities of large area graphene films, which calculated from eight samples. Graphene films were grown on polycrystalline Cu foil and Cu (111), the each size was 2cm2. Figure 7(a) demonstrates the transferred graphene film on the stage of Hall measurement. Error bars in the Fig. 7(b) give the graphene grown on Cu (111) has a lowest sheet resistance and carrier mobility were 354 Ω/□ and 1894 cm2Vs−1, respectively. The result values were greater than graphene grown on polycrystalline Cu foil, which was 630Ω/□ and 1238 cm2Vs−1, respectively. This is a certainly evidence due to the fact that, when the graphene domains merged to each other with misorientations, the boundary ripple produced. Those boundaries were be deemed as the carrier scatter center which increases the sheet resistance further.
Fig. 7 Comparison of sheet resistance and carrier mobility for graphene grown on Cu (111) and Cu foil.
In Fig. 8, the average transmittance of graphene grown on poly-Cu foil and Cu (111) foil were 97.35% and 97.52% in the visible light region, these results were calculated from ten samples. The inserted image showed that there are several double layers graphene and ripples on the Cu foil one. Besides, graphene grown on Cu (111) film was highly uniform without any double layer and ripples. The small fraction of double-layer graphene and the polymer residues during the transfer process were participated in the reduced transmittance for. It is difficult to dissolve the polymer residues on the graphene boundaries. This factor reduce the transmittance especially in short-wavelength region for the graphene grown on Cu foil, owing to the polymer absorbs the short-wavelength. Overall, Cu (111) as the catalyst substrate for growing graphene, is a suitable choice to replace the polycrystalline Cu foil.
Fig. 8 Transmittance of graphene films grown on Cu foil and Cu (111) film. The inserted OM images of graphene film were grown on Cu foil and Cu (111) thin film.
4. Conclusion
Fourier transformation was applied for observing the orientation of hexagonal graphene domains. The orientation of graphene domains displayed huge difference between using Cu (111) film and Cu foil. In the frequency domain, the far-field diffraction pattern of graphene grown on Cu (111) was highly coherence. We also measured the sheet resistance and carrier mobility with large area graphene films, the properties of graphene grown on Cu (111) was greater than graphene grown on Cu foil. The optical transmittance also showed slightly improvement by using Cu (111) as the catalyst metal for growing graphene film. We think this easy method can provide academics and factories to evaluate the parameters of synthesis graphene for large area transparent conducting electrode.
Acknowledgments
The authors would like to thank the National Science Council of Taiwan, for financially supporting this research under contract MOST 104-3113-E-008-004 and MOST 103-2221-E-008-101-MY2.
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