Graphene Research Institute, Sejong University, Seoul , Korea

Transcription

1 H 2 -free synthesis of monolayer graphene with controllable grain size by plasma enhanced chemical vapor deposition Yong Seung Kim a, b, Jae Hong Lee a, b, Sahng-Kyoon Jerng a, b, Eunho Kim a, c, Sunae Seo a, b, Jongwan Jung a, c a, b, *, Seung-Hyun Chun a Graphene Research Institute, Sejong University, Seoul , Korea b Department of Physics, Sejong University, Seoul , Korea c Institute of Nano and Advanced Materials Engineering, Department of Nano Science and Technology, Sejong University, Seoul Korea * Corresponding author: FAX: address: schun@sejong.ac.kr (S.H.Chun)

2 Abstract We report H 2 -free synthesis of high-quality graphene on Cu foil by plasma enhanced chemical vapor deposition. The advantage of this method is the easily controllable grain size of graphene by adjusting the plasma power. At a plasma power of 50~70 W, the grain size of graphene can be increased by a factor of 4 compared to that of graphene synthesized in H 2 -rich conditions. Raman spectra reveal that the quality of synthesized graphene is as good as that of best quality graphene grown under H 2 -rich environments. Moreover, the synthesis temperature of monolayer graphene can be reduced down to 650 C. Our results indicate that plasma enhanced chemical vapor deposition is an advantageous method not only for low-temperature synthesis but also for H 2 -free growth of graphene, which provides the ultimate advantage of the controllable synthesis of highquality graphene with reduced flammability.

3 1. Introduction Graphene is a two-dimensional carbon material that has been attracting great interest because of its unique electrical properties and high potential for applications [1-4]. So far, single-layer graphene has been made by a number of methods, including mechanical exfoliation [5], epitaxial growth on silicon carbide (SiC) at high temperature [6-7], and chemical vapor deposition on catalytic metal substrates [8-12]. Among them, the chemical vapor deposition (CVD) method has been widely used to obtain large-scale graphene with dimensions of up to 30 inches on transition metals [11]. However, due to the high growth temperature (~1000 C) required for thermal decomposition of hydrocarbons, plasma enhanced chemical vapor deposition (PECVD) has been utilized to achieve low-temperature growth of graphene by using reactive species generated in the plasma. This method has already been used for the low-temperature growth of carbon nanotubes (CNTs) with vertically aligned CNTs [13-17]. The advantages of PECVD are also effective for the synthesis of graphene, but the high electric field aligned perpendicular to the surface is not desirable because it prevents planar growth of graphene and results in vertically standing graphene sheets [18-19]. Recently, Nandamuri et al. firstly reported successful synthesis of single-layer graphene by eliminating this electric field on the surface via remote-plasma enhanced CVD [20]. Since then, various methods of plasma excitation, such as direct current (DC) discharge [21], radio-frequency (RF) discharge [22-23], and microwave (MW) plasma [24] are employed in the PECVD process to achieve large-area graphene growth at reduced substrate temperatures (~650 C).

4 All of these processes include hydrogen gas in the mixture for the synthesis of graphene, but the roles of plasma and hydrogen in the synthesis of PECVD graphene have not been discussed in detail. Also, in contrast with thermal CVD, the size of graphene grains and the growth parameters affecting their size are not studied in PECVD. In this work, we report the H 2 -free synthesis of graphene on Cu foil with controllable graphene grains by RF-PECVD. The growth parameters (such as plasma power, growth time, temperature, and the composition of the inlet gas) are systemically controlled to study their effects on the synthesis of graphene in an H 2 -free environment. Samples are characterized by Raman spectroscopy, scanning electron microscopy (SEM), and optical microscopy. When the graphene is synthesized in optimal conditions, high-quality monolayer graphene with enlarged grains is obtained from H 2 -free mixtures. The size of the graphene grains can be controlled systemically by varying the applied plasma power. Furthermore, the synthesis temperature of monolayer graphene is reduced to 650 C. The results are compared with those of graphene grown in H 2 -rich conditions in order to provide insights into the roles of plasma and hydrogen in the PECVD synthesis of graphene. 2. Experimental methods 2.1 Synthesis of graphene film The graphene films studied in this work were grown on 25 μm-thick Cu foils (Alfa Aesar, 99.8 %, #13382) by RF-PECVD. The chamber is equipped with an inductively coupled plasma reactor and pumped with a turbomolecular pump (Osaka Vacuum LTD,

5 TG1003), keeping the base pressure as low as ~10-7 Torr. Polycrystalline Cu foil was cut into 7x7 cm 2 pieces and mounted in the chamber without any pre-cleaning treatment. Five different stages were employed to synthesize graphene films on Cu foil using methane (CH 4 ) as a carbon source. The Cu substrate was heated to the growth temperature (650 ~ 830 C) at the heating rate of 3 C/s without any gas flow into the chamber. When it reached the target temperature, H 2 gas was introduced into the chamber at the flow rate of 40 standard cubic centimeters per minute (sccm). Hydrogen gas was discharged by an RF power of 50 W for 2 min to eliminate surface oxides on the copper foil. Then, the chamber was purged with Ar at the flow rate of 100 sccm for 2 min to remove residual hydrogen gas. During the graphene growth stage, RF plasma was generated for a specified growth time under a continuous flow of argon (or hydrogen, 40 sccm) and methane (1 sccm), while the pressure was kept at 10 mtorr. The plasma power was varied from 0 to 120 W, the substrate temperature from 650 to 830 C, and the growth time from 1 to 4 min. Subsequently, the sample was cooled down rapidly to room temperature at a cooling rate of 3 C/s by turning off the heating power, and then it was taken out for characterization. 2.2 Transfer of graphene film All the synthesized graphene films were transferred onto SiO 2 /Si substrates by etching the Cu foil in an aqueous solution of iron chloride (FeCl 3 ) [8]. Prior to wetetching, the surface of graphene/cu was spin-coated with poly-methyl methacrylate (PMMA, 950 A2), followed by baking at 150 C for 3 min. The purpose of the PMMA

6 coating is to protect graphene films from cracks and tears during the transfer process [9, 12]. When Cu foil was dissolved completely, the PMMA/graphene membrane was washed with deionized water and placed on the SiO 2 /Si substrate. Finally the PMMA film was dissolved and removed by acetone. 2.3 Characterization The surface morphologies of graphene films on Cu substrates are analyzed by SEM (Tescan, VEGA-3) and optical microscopy (Leica, DM2500M). The crystallinity and the structural information of synthesized graphene are obtained by micro-raman spectroscopy (Renishaw, invia system) using a nm laser. In order to estimate the uniformity of synthesized graphene, Raman spectra are measured at three different points in each graphene film. The averaged data of these points are presented in the plots of intensity ratios and the full width at half maximum (FWHM). The point-topoint variation of the Raman spectra is displayed as an error-bar. A minimal laser power of 2mW is used carefully during the measurements to avoid any damage or heating of the graphene films. 3. Results and discussion 3.1 Effect of synthesis time We firstly investigated the quality of graphene synthesized in an H 2 -free environment (Ar:H 2 :CH 4 = 40:0:1 sccm) for different growth times from 1 to 4 min,

7 while the plasma power and the growth temperature are fixed at 50 W and 830 C, respectively. Fig. 1(a) shows the Raman spectra of synthesized graphene, in which the three most prominent peaks are the D peak at ~1350 cm -1, the G peak at ~1580 cm -1, and the 2D peak at ~2680 cm -1. The D peak is a defect-induced Raman feature observed in the disordered sample or at the edge of the graphene [25-28]. The G peak is known to be associated with the doubly degenerate phonon mode at the Brillouin zone center, indicating sp 2 carbon networks in the sample [25-28]. The intensity ratio of the D peak to the G peak (I D /I G ) is less than 0.2, indicating a low defect density [the lower curve in Fig. 1(b)]. The 2D peak, which originates from a second-order Raman process, is widely used to determine the thickness of graphene [25-28]. For single-layer graphene, it is wellknown that the intensity ratio of the 2D peak to the G peak (I 2D /I G ) is higher than 2 [9, 25, 27-30], and the FWHM of the 2D band is close to 30 cm -1 (e.g., 24 ~ 30 cm -1 in mechanically exfoliated graphene and 31 ~ 34 cm -1 in thermal CVD graphene) [9, 25, 27-30]. In our graphene films, the ratio of I 2D /I G is much higher than two (3.5 ~ 4.0) [the upper curve in Fig. 1(b)] and the FWHM of the single Lorentzian 2D band is less than 30 cm -1 (27.5 ~ 29.5 cm -1 ) [Fig. 1(c)] regardless of the synthesis time. These results indicate that high-quality monolayer graphene can be synthesized in an H 2 -free environment by PECVD.

8 Fig. 1 - (a) Raman spectra of graphene synthesized at 830 C with Ar/CH 4 (40:1) mixtures for various synthesis times with a plasma power of 50 W. (b) Intensity ratios of the D and 2D peaks to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit. SEM images of graphene synthesized with different growth times are shown in Fig. 2(a-c). Nucleation of graphene domains with an average domain size of ~2 µm is observed when synthesis lasts for 1 min [Fig. 2(a)]. The optical image and the Raman maps, obtained from graphene transferred onto a SiO 2 substrate, clearly show graphene grains with dimensions of a few microns [Fig. 2(d-f)]. As the growth time increases, these domains continue to grow laterally [Fig. 2(b)] and coalesce into larger ones [Fig. 2(c)], resulting in a fully covering monolayer of graphene on the Cu surface when the growth time is longer than 3 min. The SEM image in Fig. 2(c) shows a continuous graphene film with wrinkles and non-uniform dark flakes similar to those in thermal CVD-grown graphene [9-10]. Our observations of the nucleation and coalescence process of graphene, which have not been reported by others in the PECVD of graphene, suggest that the synthesis mechanism of plasma-assisted graphene on Cu foil is similar to that of thermal CVD, a surface adsorption process of carbon.

9 Fig. 2 - SEM image of graphene on Cu foil synthesized at 830 C for (a) 1, (b) 2, and (c) 3 min in Ar/CH 4 (40:1) environments with a plasma power of 50 W. (d) Optical image and the Raman map of (e) G and (f) 2D peak intensity obtained from the graphene synthesized for 1 min after transferring it onto a SiO 2 substrate. 3.2 Effect of plasma power Fig. 3(a) shows the plasma power dependence of Raman spectra from synthesized graphene when the growth temperature and time are fixed at 830 C and 3 min, respectively. For the graphene film synthesized without RF plasma, the intensity of the D peak is as strong as that of the G peak and a notable disorder-induced D peak (~1620 cm -1 ) [25] is observed to the right of the G peak [Fig. 3(a)]. These peaks are reduced remarkably when the RF plasma assists during the graphene growth [Fig. 3(a)]. The low I D /I G (< 0.2) observed in plasma-assisted graphene suggests that the defect density is

10 reduced by the use of plasma [bottom of Fig. 3(b)]. This enhancement of Raman spectra can be attributed to the easier dissociation of methane by RF plasma into active species (CH x<4 ) which decompose more easily to solid carbon than methane does [16]. Due to the advantages of plasma, the graphene synthesized under RF plasma exhibits better crystallinity and a higher growth rate at relatively low temperatures. Fig. 3(d) shows a partially covering graphene film when it is synthesized for 3 min without RF plasma. On the other hand, a full covering of graphene is always observed when it is synthesized for 3 min with RF plasma [Fig. 2(c)]. Since graphene grains are not observable in the fully covered case, the graphene films synthesized for 1 min are used for the SEM image in Fig. 3(e-h). As the plasma power increases from 0 to 70 W, the FWHM of the 2D band decreases from 41 to 29 cm -1 [Fig. 3(c)] and the ratio of I 2D /I G increases from 2 to 4 [top of Fig. 3(b)] with an enlargement of the average size of graphene grains from 0.5 to 2 µm [Fig. 3(e, f, i), Fig. 2(a) for 50 W]. When the plasma power is higher than 70 W, the opposite results are observed for both grain size and Raman spectra. The average size of graphene grains reduces from 2 to 0.5 µm [Fig. 3(g-i)], the I 2D /I G decreases from 4 to 3 [top of Fig. 3(b)] and the FWHM of the 2D band increases from 30 to 35 cm -1 [top of Fig. 3(c)]. These observations indicate that the optimal plasma power for enhanced grain size and Raman spectra is 50 ~ 70 W. To the best of our knowledge, this plasma power dependent variation of grain size has not been reported previously in PECVD graphene. In thermal CVD, it has been well-known that atomic hydrogen plays a very important role in determining the dimensions of graphene grains because it acts not only as an activator of surface-bound carbon but also as an etching reagent that controls the size of the resulting graphene domains [31]. Considering this role of hydrogen in graphene

11 growth, the enlarged grain size under optimum plasma power could be attributed to the atomic hydrogen generated by RF plasma during the process of the dissociation of methane into active species. A theoretical model of inductively coupled plasma shows that more than 80 % of incoming CH 4 dissociates to other species, such as H, H 2, CH 3, C 2 H 4, C 2 H 6, etc., when the applied plasma power is higher than 50 W [16-17, 32-33]. The resulting mole fraction of atomic hydrogen is higher than 0.1 % in this condition. Although the actual intensity of H would be lower than the calculated one in our system because of the H 2 -free environment, it will increase with the plasma power and become high enough to affect the resulting size of graphene grains. Further studies are required to investigate the actual partial pressure of atomic hydrogen in an H 2 -free environment under various plasma powers.

12 Fig. 3 - (a) Raman spectra of graphene grown for 3 min at 830 C under an H 2 -free environment with various plasma powers. (b) Plasma power dependence of intensity ratio of the D and 2D peak to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit. The lines are guides for the eye. SEM images of graphene on Cu foil grown (d) for 3 min without plasma and (e-g) for 1 min with different plasma powers from 30 to 120 W. To observe the graphene grains, 1 min-grown graphene films are used for the SEM images. (i) Average size of graphene grains as a function of applied plasma power. 3.3 Effect of inlet gas composition

13 We synthesized graphene films in an H 2 -rich environment (Ar:H 2 :CH 4 = 0:40:1 sccm) in order to compare their grain size and quality with that of graphene grown in H 2 - free conditions (Ar:H 2 :CH 4 = 40:0:1 sccm). The plasma power and temperature are fixed at 50 W and 830 C, respectively. In the H 2 -rich environment, a similar growth process comprising nucleation and coalescence of graphene is observed in the SEM images [Fig. 4(a-c)]. However, the average size of graphene grains and the synthesis time required to produce a full covering of graphene are reduced to ~0.5 µm and 1 min, respectively. Fig. 4(d) shows the time dependent evolution of graphene grains in both H 2 -free and H 2 -rich environments. A 3 times higher growth rate and a 4 times reduced grain size are observed in H 2 -rich mixtures, which can be attributed to the higher partial pressure of atomic hydrogen generated by RF plasma from hydrogen molecules. The higher intensity of hydrogen leads to greater dissociation of methane by the hydrogen abstraction reaction (CH 4 + H CH 3 + H 2 ) and an increase in the growth rate of graphene. The Raman spectra in Fig. 4(e) show a high I 2D /I G (~3.4) and a low FWHM of the 2D band (~27.9 cm -1 ), similar to that of H 2 -free graphene, indicating that high crystallinity graphene can be obtained in both H 2 -free and H 2 -rich environments. Our observation suggests that hydrogen is not necessarily required in the PECVD of graphene.

14 Fig. 4 - SEM images of graphene films synthesized for (a) 20, (b) 30, and (c) 60 sec in H 2 /CH 4 (40:1) mixtures with a plasma power of 50 W. (d) Time dependence of average grain size of graphene synthesized under different inlet gas compositions. (e) Raman spectra of graphene films grown in H 2 -rich conditions. 3.4 Effect of growth temperature The effect of substrate temperature on graphene synthesis in an H 2 -free environment is investigated in a temperature range from 650 to 830 C. In this series of growth experiments, the plasma power and the synthesis time were maintained at 50 W and 3 min, respectively. Fig. 5(a-c) shows Raman spectra of graphene film grown at different substrate temperatures. As the temperature decreases, I D /I G increases from 0.2 to 1.3 and I 2D /I G decreases from 4.0 to 2.2 [Fig. 5(b)]. The higher intensity of the D peak [Fig. 5(a)] and the broader FWHM of the 2D band [Fig. 5(c)] at lower growth temperatures suggest

15 that there are gradual increments of disorder for graphene synthesized at lower temperatures. Interestingly, even at the substrate temperature of 650 C, the ratio of I 2D /I G is higher than 2 and a single-lorentzian 2D peak is observed, indicating that monolayer graphene is synthesized at this temperature. Fig. 5 - (a) Raman spectra of graphene films on SiO 2 /Si substrate grown for 3 min at various temperatures with a plasma power of 50 W and H 2 -free mixtures. (b) Intensity ratios of the D and 2D peaks to the G peak. (c) FWHM of 2D band of synthesized graphene. 4. Conclusion We presented the H 2 -free synthesis of high-quality monolayer graphene on Cu foil with a controllable grain size by PECVD. The Raman spectra of the synthesized

16 graphene were as good as those of graphene films grown under H 2 -rich environments, showing a high ratio of I 2D /I G (3.5 ~ 4.0) and a low FWHM of the 2D band (< 30 cm -1 ). The control over the grain size of synthesized graphene by adjusting the plasma power, achieved in this work for the first time, is the prominent advantage of the H 2 -free synthesis of graphene, which provides useful insights for understanding the growth mechanism of PECVD graphene and for optimization of the growth process to further improve the quality of graphene. The obtained graphene grains were 4 times larger than those grown in an H 2 -rich environment. We suggest that the variation in the size of graphene grains according to the plasma power and inlet gas composition could be attributed to the different amount of atomic hydrogen generated by the RF plasma. Since the actual pressure of atomic hydrogen in the H 2 -free environment is not known, further studies are required to fully understand the growth mechanism of plasma-assisted graphene. Our results suggest that PECVD is an advantageous method for the H 2 -free growth of graphene, providing the ultimate advantage of the controlled synthesis of graphene and reduced flammability. Acknowledgments This research was supported by the Priority Research Centers Program ( ), the Basic Science Research Program ( ), and the Center for Topological Matter in POSTECH ( ) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST).

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21 Figure Captions Fig. 1. (a) Raman spectra of graphene synthesized at 830 C with Ar/CH 4 (40:1) mixtures for various synthesis times with a plasma power of 50 W. (b) Intensity ratios of the D and 2D peaks to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit. Fig. 2. SEM image of graphene on Cu foil synthesized at 830 C for (a) 1, (b) 2, and (c) 3 min in Ar/CH 4 (40:1) environments with a plasma power of 50 W. (d) Optical image and the Raman map of (e) G and (f) 2D peak intensity obtained from the graphene synthesized for 1 min after transferring it onto an SiO 2 substrate. Fig. 3. (a) Raman spectra of graphene grown for 3 min at 830 C under an H 2 -free environment with various plasma powers. (b) Plasma power dependence of intensity ratio of the D and 2D peak to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit. The lines are guides for the eye. SEM images of graphene on Cu foil grown (d) for 3 min without plasma and (e-g) for 1 min with different plasma powers from 30 to 120 W. To observe the graphene grains, 1 min-grown graphene films are used for the SEM images. (i) Average size of graphene grains as a function of applied plasma power. Fig. 4. SEM images of graphene films synthesized for (a) 20, (b) 30, and (c) 60 sec in H 2 /CH 4 (40:1) mixtures with a plasma power of 50 W. (d) Time dependence of average grain

22 size of graphene synthesized under different inlet gas compositions. (e) Raman spectra of graphene films grown in H 2 -rich conditions. Fig. 5. (a) Raman spectra of graphene films on SiO 2 /Si substrate grown for 3 min at various temperatures with a plasma power of 50 W and H 2 -free mixtures. (b) Intensity ratios of the D and 2D peaks to the G peak. (c) FWHM of 2D band of synthesized graphene.