Article pubs.acs.org/cm
Study of Cooling Rate on the Growth of Graphene via Chemical Vapor Deposition Jihyung Seo,† Junghyun Lee,† A-Rang Jang,‡ Yunseong Choi,† Ungsoo Kim,† Hyeon Suk Shin,‡ and Hyesung Park*,† †
Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡ Department of Chemistry, Department of Energy Engineering, School of Natural Science, Low Dimensional Carbon Materials Center, Center for Multidimensional Carbon Materials, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea S Supporting Information *
ABSTRACT: The chemical vapor deposition (CVD) technique has become one of the most widely used methods in the synthesis/study of graphene owing to its capability in large-area and uniform synthesis with great potential in mass production. It is also well-known that single-layer graphene can be grown on copper-based catalytic substrates due to its low carbon solubility. However, fewlayer graphene patches are typically generated at grain boundaries or defect sites in the metal substrate, which lowers the overall qualities of graphene film. Various factors, often closely correlated, influence the CVD process, and thus the properties of graphene. In this work, we provide detailed analysis on the cooling rate in the CVD process and its effect on the general properties of graphene. Various configurations of cooling conditions, controlled by the speed of cooling rate, were examined. Its effects on several physical properties were investigated, and it is found that the cooling rate plays an important role in producing high-quality single-layer graphene. On the basis of our observations, synthesis of highquality, continuous, single-layer graphene with negligible few-layer patches can be successfully accomplished, which can promote the widespread industrial applications of CVD graphene.
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INTRODUCTION Since the first experimental demonstration, graphene has been considered as an attractive emerging nanomaterial due to its unique and remarkable physical properties,1−5 which are attributed to the hexagonal lattice structures composed of sp2 hybridization with tightly packed carbon atoms.1,2 With its oneatomic thickness in nature, graphene possesses high transmittance (∼97.7%) and superior flexibility.3,4 In addition, graphene exhibits a semimetallic property with a slight overlap between the valence and conduction band where the charge carriers act as massless Dirac Fermions, leading to outstanding carrier mobility.5−8 These distinctive features of graphene are utilized in a wide range of applications such as flexible displays, organic photovoltaics, and chemical sensors.9−11 Various synthesis routes have been explored, including mechanical/ chemical exfoliation, epitaxial growth on SiC, and chemical vapor deposition (CVD).5,12−17 The mechanical exfoliation method can produce high-quality graphene,5 which, however, is not appropriate from the commercialization perspective due to its limited control in the synthesis.16 The chemical exfoliation technique allows large-scale synthesis, but achieving high quality and layer control is relatively challenging.17 Epitaxial growth on SiC is suitable for large-scale production with highcrystalline graphene, but the associated high-cost process limits its practical applicability.16 On the other hand, the CVD © 2017 American Chemical Society
method is advantageous over the aforementioned approaches in several aspects with its capability for large-area, high-quality synthesis as well as layer controllability.18,19 In the CVD method, metal catalysts such as Cu and Ni have been commonly used as the growth template.20−22 Unlike Ni, Cu enables uniform monolayer graphene growth owing to the relatively lower carbon solubility of Cu at elevated temperatures.23 The growth mechanism of graphene on Cu is based on the surface reaction process, whereas that of Ni is dependent on the segregation process originating from the difference in carbon solubility.24 Once the surface of Cu is fully covered by the graphene, the growth ceases owing to the absence of catalytic sites exposed to the carbon source.22 In polycrystalline Cu, carbon atoms tend to be more dissolvable into the Cu film at the grain boundaries and defect sites.25,26 These accumulated carbon atoms become the source of few-layer graphene patches, where the dissolved carbon atoms are released to influence the graphene film formation as the synthesis temperature decreases during the cooling stage.22 In addition, hydrocarbons prefer to decompose on the grain boundaries with high activation energies, resulting in few-layer graphene patches.27 These fewReceived: October 17, 2016 Revised: April 26, 2017 Published: April 27, 2017 4202
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etching effect of graphene by H2 which also occurs during the cooling step. Weakly bonded carbon atoms are readily etched away by reacting with H2 at temperatures above 850 °C.33 In the following study, CVD graphene synthesized under various cooling rates is analyzed. Typical CVD parameters are used for the graphene growth as shown in Figure 1a,b, but with three
layer patches typically cause nonuniformity in the electronic properties of the CVD-grown graphene film.25 Various factors which can affect the graphene growth in the CVD process have been explored, such as carbon source flow rate, composition of precursor gas mixtures, the role of hydrogen during the cooling step, and the effect of cooling rate.26,28,29 In particular, the studies of cooling rate reported so far have primarily focused on the speed of cooling, i.e., “fast” or “slow” cooling. In this work, we thoroughly investigate various cooling rate parameters to provide detailed information on the effect of cooling rate on the overall properties of CVD-grown graphene, using copper as the growth template, through physical, optical, and electrical analysis. We find that appropriate control in the cooling condition during the CVD process can be a facile yet effective means to obtain high-quality single-layer graphene films with minimal few-layer patches, which can be achieved by simple manipulation of the cooling parameters.
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EXPERIMENTAL SECTION
Graphene Synthesis and Transfer. Graphene was synthesized on copper foil (25 μm in thickness) via low-pressure chemical vapor deposition as described elsewhere.30 First, copper foil was annealed at 1000 °C for 30 min under hydrogen environment (10 sccm). Subsequently, methane gas (30 sccm) was introduced, and the growth proceeded for another 30 min at 1000 °C. After the growth completed, the chamber was cooled down to room temperature under different cooling conditions. Once the temperature in the growth chamber reached the desired values of 1000, 900, and 800 °C (slow cooling), the furnace was immediately removed from the sample to initiate the fast cooling step. The as-synthesized graphene was transferred to the SiO2/Si substrate via poly(methyl methacrylate) (PMMA)-assisted transfer method as described elsewhere.31 In brief, PMMA was spincoated onto the graphene and dried in an oven to remove the solvent. Copper foil was then etched away by ferric chloride solution, and the PMMA/graphene stack was thoroughly rinsed. After being transferred to the target substrate, PMMA was finally removed by acetone. FET Device Fabrication. The CVD-grown graphene was transferred on the SiO2/Si substrate. The electrode pad for graphene field-effect transistor (FET) was patterned through the electron beam lithography process (NB3, NANOBEAM LTD), followed by the sequential deposition of titanium (5 nm) and gold (60 nm) using an E-beam evaporator (FC-2000, Temescal). The graphene channels were patterned by electron beam lithography and etched by oxygen plasma. The electrical properties of fabricated graphene FETs were measured using a vacuum probe station (CRX-4X, Lakeshore) with semiconductor characterization system (4200-SCS, Keithley). Characterization. The surface morphology of graphene was analyzed by optical microscope (OM, Eclipse LV150, Nikon), scanning electron microscope (SEM, S-4800, Hitachi), and atomic force microscope (AFM, DI-3100, Veeco). Optical transmittance was measured by UV−vis−NIR spectroscopy (Carry 5000, Agilent). Raman spectra were obtained from Alpha300R, WITec, with excitation wavelength at 532 nm. Sheet resistance of graphene film was measured by 4-point probe (CMT-SR2000N, Advanced Instrument Technology).
Figure 1. CVD synthesis parameters used in the graphene synthesis. (a) Temperature profile and gas precursor composition. (b) Three different cooling rates. Blue, red, and black lines indicate S1000, S900, and S800 conditions, respectively. (c) Schematic diagram for the fast cooling and change in the temperature profiles tabulated from part b.
distinctive cooling rates. In these conditions, the cooling stage is divided into two different zones, fast and slow cooling, where the temperature for starting fast cooling is varied as 1000, 900, and 800 °C, respectively (henceforth referred to as S1000, S900, and S800). For instance, in the S900 case, the growth system is slow-cooled from 1000 to 900 °C, and then fastcooled from 900 °C to room temperature. Fast cooling is achieved by moving the furnace away from the hot zone (Figure 1c), and the fast and slow cooling rates are estimated to be 4.5 and 0.4 °C/s, respectively. Morphological analysis of graphene films prepared under various experimental conditions was performed through OM, SEM, and AFM. As shown in Figure 2a,d, single-layer graphene is mostly formed in the S1000 sample without few-layer graphene patches. In this case, the CVD reactor rapidly cools down once the growth stage terminates at 1000 °C. As a result, thermal decomposition of CH4 and etching effect with H2 are
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RESULTS AND DISCUSSION Multiple reactions are involved in the cooling stage of the graphene CVD process.26 In the case of copper, formation of graphene on metal surface can be affected either by the decomposition of carbon precursors (CH4) or the released carbon atoms which have been dissolved inside the copper.26 The thermal decomposition of CH4 typically initiates around 900 °C,32 and the amount of carbon released from the copper catalyst is relatively small because of the low carbon solubility in the copper.22 Another commonly observed phenomenon is the
Figure 2. OM (a−c) and SEM (d−f) images of transferred graphene films prepared from different cooling rates. (a, d) S1000 sample showing uniform single-layer sheet with negligible few-layer patches. (b, e) S900 sample featured with few-layer patches. (c, f) Few-layer patches observed in parts b and e are removed by H2 in the graphene with the S800 case. 4203
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Chemistry of Materials minimized due to the short reaction time. Only a few carbon atoms are released from the copper substrate through the fast cooling step, which has a negligible effect on the formation of few-layer patches. In contrast, the slow cooling rate at temperatures above 900 °C (S900 sample) leads to enhanced CH4 thermal decomposition since the temperature required for the reaction is further maintained during the slow cooling period (1000 to 900 °C) unlike the S1000 case. Despite the presence of H2-mediated etching effect, the decomposition of CH4 is more pronounced owing to the higher partial pressure of CH4 over H2. Under the slow cooling period, the surface morphology of graphene can be affected by not only the carbon atoms released from the copper but also thermally decomposed CH4.28 The additional activated carbon atoms generated from the thermal decomposition prefer to nucleate at grain boundaries and contribute to pronounced nucleation densities, which subsequently results in the formation of few-layer patches on the single-layer graphene surface as shown in Figure 2b,e. In the slow cooling period above 800 °C (S800 sample), both the CH4 decomposition and H2-mediated etching play significant roles in the growth of graphene (Figure 2c,f). Under this condition, few-layer graphene patches are still likely to form because the decomposition of CH4 is expected to occur in the 1000−900 °C range, similar to the S900 case. However, edges of these few-layer patches, with their weakly bonded carbon atoms, reacted with H2 and eventually were etched away at temperatures below 900 °C. The morphology of graphene under different cooling rates was further analyzed by AFM (Figure 3). As the starting
Figure 4. (a) Optical transmittance of transferred graphene films on quartz substrates, synthesized under varying cooling conditions. (b) Transmittance values extracted at 550 nm incident radiation. S900 sample has lower transmittance than the other conditions.
graphene (∼97.7%),3 and the S900 sample shows ∼97.3% transmittance. These results confirm that the existence of fewlayer graphene patches in S900 is more pronounced than the other conditions. With this, additional specified cooling rate conditions were further investigated to provide more detailed analysis on the temperature effect in the CVD graphene during the cooling stage. As shown in Figure S1 in the Supporting Information, additional cooling rate conditions of S950, S850, and S750 were examined with fast cooling starting at 950, 850, and 750 °C, respectively, and the associated surface morphology profiles are presented in Figure S2 in the Supporting Information. In the case of S950, the activated carbon atoms seemed to participate in the formation of few-layer graphene patches during slow cooling between 1000 and 950 °C, although these few-layer patches were smaller in size and lower in concentration compared to those of the S900 case which is exposed to longer time under the activated carbon rich environment. S850 and S750 samples both exhibit monolayer graphene character with negligible few-layer graphene patches, similar to the case of S800 most likely due to the etching effect from the hydrogen. We also performed graphene growth with slow cooling from 1000 °C to room temperature (the S30 case) to identify the effect of completely slow cooling conditions on the surface morphology of graphene below 750 °C (Figure S3 in the Supporting Information). The S30 sample showed a surface morphology profile quite similar to those of the S850, S800, and S750 cases (Figure S4 in the Supporting Information), which implies that the slow cooling conditions below 850 °C
Figure 3. (a−c) AFM images of graphene films with different cooling rates. Surface roughness increases with increasing temperature for initiating the fast cooling.
temperature for fast cooling increases, graphene films possessed appreciably more wrinkles (Figure 3a−c). Since the thermal expansion coefficient of metal substrates is higher than that of graphene, metal substrates typically shrink more during the cooling step than the graphene that is grown on the metal surface, which results in the wrinkle formation: Rapid shrinkage of metal occurs, and wrinkles are formed at places where weak interactions between the graphene and metal substrate exist.34,35 Under a slow cooling environment, wrinkle formation in the graphene sheet becomes relatively small in general.36 For these reasons, the S1000 sample had the most wrinkles due to the abrupt decrease in temperature, and the relatively slower temperature drop in the S800 case showed less wrinkle formation while the S900 sample yielded an intermediate number of wrinkles. The corresponding roughness of graphene films transferred onto SiO2 with fast cooling starting at 1000, 900, and 800 °C is 1.74, 1.24, and 1.17 nm, respectively. Next, optical transmittance analysis on the graphene sheets was carried out to evaluate the existence of few-layer patches (Figure 4). Consistent with the morphological results, the transmittance of graphene films for S1000 and S800 samples is ∼97.6%, similar to that of mechanically exfoliated single-layer 4204
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Figure 5. (a) Raman spectra of graphene films with various cooling rates. (b) Zoom-in image of D peak. Among the three different cooling conditions, the S900 sample shows the highest D peak intensity, while the lowest D peak intensity is observed in S1000. (c) Raman mapping of the I(D)/I(G) ratio of graphene films synthesized from different cooling rates, showing the fewest defects in S1000.
Information). However, hydrogen-mediated etching of the graphene surface appeared to become activated from the H900 sample (Figure S8b,e in the Supporting Information), which became more evident for the case with an increasingly slow cooling time (H800, Figure S8c,f in the Supporting Information). To examine the quality of graphene with different cooling rate conditions, Raman spectroscopy was carried out on the astransferred graphene. Graphene has three distinctive Raman features, designated as the D, G, and 2D peaks at ∼1350, ∼1580, and ∼2700 cm−1, respectively.38 Figure 5a shows three such representative Raman peaks in all cases with a G peak at ∼1590 cm−1 indicating p-type doped characteristics arising from the etching-based wet-transfer process.39 In general, the 2D and G peaks provide information on the number of graphene layers, and the intensity ratios of the 2D to G peaks, shown in Figure 5a, exhibit primarily monolayer characteristics in all samples studied.38 The D peak, which represents the degree of disorder in the sp2 domain, is typically used as an indicator for defects where the hexagonal lattice structure is disrupted. As shown in Figure 5a,b, all three samples studied in this work show decent properties in the graphene film, but the D peak is more pronounced with the S900 case whereas that of the S1000 sample is almost negligible. Further analysis of Raman spectra on S950, S850, and S750 samples are presented in Figure S9 in the Supporting Information. As expected, a noticeable D peak was observed in the S950 case, similar to the S900 case, while the presence of the D peak in S850 and S750 cases was almost negligible. Figure S10 in the Supporting Information shows that Raman spectra for the S30 case also have a negligible D peak. Raman spectroscopy was also carried out on the graphene synthesized under two extreme precursor gas compositions, i.e., methane- or hydrogen-only, with various cooling rates. For the methane-only case, although a low D peak was observed for the M1000 case, the D peak intensity increased significantly as the starting temperature for the fast cooling decreased (Figure S11 in the Supporting Information).
do not have noticeable differences on the surface morphology of graphene under the presence of both methane and hydrogen gases. To better understand the role of each precursor gas element, methane and hydrogen, during the cooling stage, additional gas compositions under various cooling rates were investigated. Herein, two extreme cooling atmospheres were examined, i.e., methane- and hydrogen-only environments, while keeping the other CVD conditions the same as in the S1000, S900, and S800 cases. For the methane-only case, the hydrogen supply is terminated immediately after the growth stage under the different cooling rates as described in Figure S5 in the Supporting Information (henceforth referred to as M1000, M900, and M800). For the M1000 sample, no considerable changes in the surface morphology of graphene were observed compared with that of the S1000 case (Figure S6a,d in the Supporting Information). However, graphitic carbon-like structures on the monolayer background of graphene started to appear in the M900 case (Figure S6b,e in the Supporting Information), which became more pronounced for the case of M800 (Figure S6c,f in the Supporting Information). Hydrogen is known to act as a cocatalyst in the formation of graphene as well as an etching reagent to graphene,33 and graphitic carbon films, not graphene, are reported to form under the noncatalytic thermal decomposition of methane.37 Therefore, the above results suggest that graphitic carbon films are deposited under the methane-only cooling process likely due to the noncatalytic thermal decomposition of methane. Figure S7 in the Supporting Information illustrates the hydrogen-only cooling process under different cooling rates (henceforth referred to as H1000, H900, and H800). In these conditions, the hydrogenderived etching effect should be dominant during the cooling stage, which is expected to be more pronounced as the starting temperature for the fast cooling decreases. There seemed to be a negligible effect of the hydrogen gas on the surface of graphene for the H1000 case, possibly due to the short etching time at elevated temperatures (Figure S8a,d in the Supporting 4205
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Chemistry of Materials In particular, similarly high D and G peak intensities and a minimal 2D peak from the M800 case indicate the formation of amorphous carbon-like structures.40 Raman spectra of hydrogen-only samples all show negligible levels of D peaks as shown in Figure S12 in the Supporting Information. However, we note that the edge regions of etched graphene as in H800 clearly show increased levels of D peak intensity compared to that from the basal plane (Figure S13 in the Supporting Information). Raman mapping was further performed to evaluate the overall areal defect analysis on the graphene sheet. Figure 5c shows the intensity ratio mapping of I(D)/ I(G), which indicates that the overall quality of graphene is affected by the cooling rate, with the most defects in S900 and the fewest defects in S1000, consistent with the previous Raman spectra results. An additional mapping result of the I(2D)/I(G) intensity ratio is shown in Figure S14 in the Supporting Information, revealing the monolayer graphene character from all three cooling conditions (S1000, S900, and S800) with good uniformity. These results indicate that there is a finite window of temperature range which is responsible for the observed defectinduced low-quality graphene, and thus can be otherwise utilized as a useful guide to produce a high-quality monolayer graphene film. Furthermore, the composition of precursor gas elements during the cooling stage seems to have considerable impact for the synthesis of CVD graphene. Through controlling specified cooling rates, we observed that graphene films synthesized under the S1000 condition showed mostly singlelayer character with good physical properties, despite the presence of a somewhat higher level of wrinkles than samples from the other test conditions. We also found out that the slow cooling from 1000 to 900 °C caused few-layer patch formations and structural defects on graphene. On the basis of these observations, we propose that the combination of fast cooling from 1000 to 900 °C and slow cooling from below 900 °C in the presence of both methane and hydrogen precursor gas environments shall provide the desired physical properties in the CVD graphene with monolayer character. The electrical property of graphene sheets prepared under various cooling rates was investigated by conductivity and fieldeffect mobility measurements. The sheet resistance of astransferred graphene on SiO2, measured by a 4-point probe, was 304 ± 11, 427 ± 9, and 354 ± 17 Ω sq−1 for S1000, S900, and S800, respectively (Figure 6a). As previously mentioned, in the S900 case, an activated carbon rich environment under lasting high temperatures generates numerous few-layer features, and these randomly stacked graphene patches cause poor interlayer conduction.25 The S1000 sample, with negligible few-layer patches, shows better conduction than the others. In the S800 case, improved electrical quality is expected compared to that of the S900 sample since the few-layer patches are removed by H2 due to their weakly bound carbon atoms, leading to moderate conductivity.26 Similar to the observation from defect analysis of Raman spectroscopy, the highest conductivity was obtained in S1000 while the lowest value occurred in S900, which seemed to be closely related to the degree of defects in each condition. These results elucidate that the structural defects in graphene film are clearly related to the electrical properties, which originate from the defect-siteinduced charge carrier scattering.41 The electrical property of graphene synthesized with different gas compositions during various cooling rates was also investigated by a 4-point probe. For the methane-only case, the sheet resistance gradually
Figure 6. (a) Sheet resistance and (b) field-effect mobility of the transferred graphene on a SiO2/Si substrate with various cooling rate.
increased as the temperature for starting the fast cooling decreased likely due to the formation of amorphous carbon-like structures (Figure S15 in the Supporting Information). As the effect of hydrogen-mediated etching on graphene was enhanced, the overall sheet resistance also increased from H1000 to H800 samples (Figure S16 in the Supporting Information). Therefore, the dominant effect arising from each precursor gas element during the cooling stage seems to have an undesirable impact on the overall CVD process of graphene growth, thus leading to the poor quality of the graphene film. In order to better understand the electrical properties of graphene, we fabricated graphene-based FETs. The electron mobility was 1375, 983, and 1004 cm2 V−1 s−1 for S1000, S900, and S800, respectively (Figure 6b). The Dirac points in all devices exhibit p-doped characteristics resulting from the wettransfer process of graphene.42 Among the three test conditions, the S1000 sample showed the highest mobility value consistent with the results of sheet resistance measurements.
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CONCLUSIONS In this study, the effect of various cooling rates on the CVDgrown graphene synthesized from the copper growth template was investigated. In particular, the cooling stage was subdivided into various combinations of “fast” and “slow” cooling rates to examine the effect of each cooling rate condition to the overall properties in the as-synthesized graphene films. The three cases considered in this work, i.e., S1000, S900, and S800, show clear distinctions including different graphene surface morphologies and electrical/optical properties, which suggests that appro4206
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priate control in the cooling rate can play a significant role in the CVD graphene synthesis process toward obtaining highquality single-layer graphene films. The results in this study shall provide valuable information in the synthesis of highquality graphene and, hence, will have a beneficial influence on various applications of CVD graphene such as flexible displays, optoelectronics, and chemical sensors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04432. Temperature profile, gas precursor composition, OM and SEM images, Raman spectra, and sheet resistance values under various cooling conditions (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hyeon Suk Shin: 0000-0003-0495-7443 Hyesung Park: 0000-0002-7613-8706 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1D1A1A0105791). This work was also supported by the 2015 Research Fund (1.150102.01) of UNIST (Ulsan National Institute of Science and Technology).
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REFERENCES
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DOI: 10.1021/acs.chemmater.6b04432 Chem. Mater. 2017, 29, 4202−4208
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DOI: 10.1021/acs.chemmater.6b04432 Chem. Mater. 2017, 29, 4202−4208
