Graphene Thickness Control via Gas-Phase Dynamics in Chemical

Apr 27, 2012 - Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China,...
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Graphene Thickness Control via Gas-Phase Dynamics in Chemical Vapor Deposition Zhancheng Li, Wenhua Zhang, Xiaodong Fan, Ping Wu, Changgan Zeng,* Zhenyu Li,* Xiaofang Zhai,* Jinlong Yang, and Jianguo Hou Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Graphene has attracted intense research interest due to its exotic properties and potential applications. Chemical vapor deposition (CVD) on Cu foils has shown great promises for macroscopic growth of high-quality graphene. By delicate design and control of the CVD conditions, here we demonstrate that a nonequilibrium steady state can be achieved in the gas phase along the CVD tube, that is, the active species from methane cracking increase in quantity, which results in a thickness increase continually for graphene grown independently at different positions downstream. In contrast, uniform monolayer graphene is achieved everywhere if Cu foils are distributed simultaneously with equal distance in the tube, which is attributed to the tremendous density shrink of the active species in the gas phase due to the sink effect of the Cu substrates. Our results suggest that the gas-phase reactions and dynamics are critical for the CVD growth of graphene and further demonstrate that the graphene thickness from the CVD growth can be fine-tuned by controlling the gasphase dynamics. A similar strategy is expected to be feasible to control the growth of other nanostructures from gas phases as well.

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two-step growth in a multizone furnace is adopted to enable layer-by-layer epitaxy.19 The importance of gas-phase reactions and dynamics during the CVD growth of graphene is usually overlooked in the previous studies. Here, we demonstrate that the gas-phase dynamics can provide a flexible route to control the graphene thickness. By delicate design of the CVD growth, the gas-phase species are tuned to be in a nonequilibrium steady state along the CVD tube; that is, the gas composition varies systematically along the tube, and the active compounds from methane cracking increase in quantity downstream. As a consequence, if the gas phase reactions will make an important role in the graphene growth, we can tune the graphene thickness by only changing the position of Cu substrate in the CVD tube under the same growth conditions. The experimental results do confirm that graphene becomes thicker downstream, when it is grown at different positions independently. On the contrary, uniform monolayer graphene is achieved at every position if the Cu foils are distributed simultaneously with an equal distance in the tube. This is attributed to the relatively uniform composition of the gas phase due to the consumption of active species by the Cu foils. Our study strongly suggests that the gas-phase reaction is an important part of the CVD

raphene has attracted a lot of attention due to its unique fundamental properties1−3 and enormous potential for diverse applications such as in transistors,4,5 supercapacitors,6,7 as well as transparent electrodes.8 The production of highquality and large-scale graphene films is thus highly demanding. While mechanical exfoliation from graphite can provide highquality graphene, this technique is however limited by the small size and low yield. Recently, chemical vapor deposition (CVD) on Cu foils9−12 has already shown great promise, since largearea uniform monolayer9,12 and bilayer13 graphene can be achieved. Diversified hydrocarbon sources, such as gaseous methane, ethylene, liquid benzene, and ethanol, as well as solid polymers,14−17 have been used to grow graphene on Cu foils with the growth temperature ranging from approximately 1000 °C down to 300 °C. For typical CVD growth on Cu foils from methane, the reaction procedure is generally believed to be that the methane molecules absorb, dehydrogenate, nucleate, and grow into graphene on the Cu foils in sequence, with the Cu acting as a catalyst to reduce the barrier of dehydrogenation and coalescence.10 Within this picture, if the CVD process is limited by a surface reaction instead of mass transport (usually at low pressure), uniform monolayer graphene is achieved in a self-limiting behavior.9,10,18 Thicker graphene sample can also be obtained, if mass transport becomes much slower than the surface reaction (usually at atmospheric pressure),13,18 or if © 2012 American Chemical Society

Received: November 10, 2011 Revised: April 20, 2012 Published: April 27, 2012 10557

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can be analyzed if we have the kinetic data of all relevant reactions. Such an analysis has been performed by Rodat et al.22 A simple estimation based on their high-temperature data suggests that the methane conversion rate at a typical growth temperature (1000 °C) is in the order of magnitude of 1.0 s−1. If the residence time of the gas flow in the CVD tube is at such a time scale, the gas compounds in the tube are at a nonequilibrium steady state; that is, the gas composition varies for different positions along the tube, while it does not change with time. The residence time (or path) of the gas mixtures is getting longer downstream along the tube. Thus more and deeper cracking reactions happen, which leads to the monotonical increase of the produced active species downstream along the tube (Figure 1b). So the Cu substrate at different positions in the tube faces different chemical environments. As a consequence, the thickness of graphene may increase systematically downstream if gas phase reactions contribute directly to graphene growth. To verify this conjecture, we performed CVD growth by placing the Cu foils at seven different positions independently, adopting the growth parameters detailed in the Methods Section. The seven positions are equally distributed in the tube with a separation distance of 6 cm as depicted in Figure 2a. On basis of flow speed, pressure, and temperature, the residence time of the carbon source in the tube is about 0.6 s, at the same order of the reciprocal of the methane conversion kinetic rate constant.22 Figure 2b−h summarizes the results of the graphene growth at different positions. The graphene thickness increases gradually from position 1 downstream to position 7 from the color contrast of the optical photograph (Figure 2b). The optical transmittance spectra as depicted in Figure 2c show that the transmittance at 550 nm decreases for graphene grown from position 1 to position 7. The transmittance for graphene grown at position 1 is 97.2%, in agreement with that reported for monolayer graphene (97.1%),17 indicating monolayer thickness. The transmittance decreases from 96.6% to 93.4% when the growth location changes from position 2 to position 4. It is noted that 93.4% is close to the value of bilayer graphene (94.3%).17 Then the transmittance reduces significantly to 89.1% for graphene grown at position 5 and finally down to 78.5% at position 7. The optical spectra sustain that the graphene films become thicker and thicker downstream. To reveal the site-dependent microstructures, SEM and Raman characterizations were also conducted for the graphene films grown at different positions. The SEM image (Figure 2d) of graphene grown at position 1 indicates uniform thickness with wrinkles resolved. The Raman spectrum of this film (inset of Figure 2d) shows typical characteristics of monolayer graphene: The 2D band centered at ∼2670 cm−1 is symmetric and can be well-fitted by a single Lorentzian peak.23,24 The full width at half-maximum (fwhm) of the 2D band is ∼41 cm−1, and the intensity ratio of G band to 2D band (IG/I2D) is ∼0.46, which are similar to the values previously reported for the methane-derived monolayer graphene.9,15 The D band (∼1350 cm−1), a measure of defects in the graphene,25 is absent in the Raman spectrum, suggesting the high quality of graphene grown at position 1. For graphene grown at position 2 downstream to position 4, dark islands (predominantly the second layer) become obvious and increase in both size and density as indicated in Figure 2e− g, consistent with the slight decrease of the optical transmittance. A weak D peak arises in the corresponding Raman

mechanism for graphene growth and further demonstrates that graphene morphology can be fine-tuned at different sites in the CVD tube spontaneously by controlling the gas-phase dynamics.

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RESULTS AND DISCUSSION To thoroughly understand the mechanism of graphene growth from methane, we should take into account seriously the gasphase compounds in the CVD tube. Our density functional theory (DFT) calculation gives an endothermic reaction energy of about 4.8 eV for the first dehydrogenation step of methane in the gas phase. Although further decomposition steps have a similar large reaction energy, chain reactions are expected to proceed after the first dehydrogenation reaction. CH4 → CH3· + H·

CH4 + H· → CH3· + H 2 2CH3· → C2H6

C2H6 + H· → C2H5 + H 2 C2H6 + CH3· → C2H5· + CH4

C2H5· → C2H4 + H· C2H4 + H· → C2H5· C2H4 + CH3· → C2H3· + CH4 C2H3· → C2H 2 + H·

and so forth. To reveal how these gas-phase compounds evolve, we first check where their chemical equilibrium goes. At this stage, we do not take condensed-phase carbon into account. The gas-phase equilibrium of the methane pyrolysis has been intensively investigated at very high temperatures.20−22 It can be studied by minimizing the free energy of all relevant species. In this study, we include 15 species: H, H2, C, CH, CH2, CH3, CH4, C2, C2H, C2H2, C2H4, C2H6, C3, C4, and C5. Although more species should be included for a quantitative description, our model is expected to give a correct qualitative picture. The result is shown in Figure 1a. When the temperature is high,

Figure 1. (a) Calculated mole fractions of 15 species in gas-phase equilibrium from 500 to 1500 K. Only the species with an amount higher than 10−7 mol are shown. Total pressure is 20 Torr, H/C = 26:5 (corresponding to initial H2/CH4 = 3:5). (b) Schematic of the density distribution of the active species derived from methane cracking along the tube.

there is a non-negligible equilibrium mole fraction of CH3 radical. As an intermediate of the gas-phase chain reaction, its concentration can be much higher than the equilibrium value during graphene growth. Then an important question about gas phase dynamics is what is the time scale to reach such an equilibrium state. This 10558

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Figure 2. (a) Schematic of the CVD growth of graphene with Cu foils at seven different positions independently. (b, c) Photographs and optical transmittance spectra of the graphene films grown at the seven positions. The transmittances of light at 550 nm are also indicated in c. (d−h) Typical SEM images of graphene grown at positions 1−5, respectively. The scale bars are 5 μm. The insets of d−h show the corresponding Raman spectra.

Figure 3. (a, b) Photograph and optical transmittance spectra of graphene films grown independently at the seven positions using a quartz tube with the inner surface etched by HF. The transmittances of light at 550 nm are also indicated in b. (c) SEM image of graphene grown at position 3 using an etched quartz tube. The scale bar is 10 μm.

is higher than 2, the 2D band centered at ∼2714 cm−1 is not symmetric, and the fwhm of the 2D band is higher than 67 cm−1. Graphene films grown at position 6 and 7 show similar microstructures, but with higher thickness. The results reported here cannot be explained solely by surface reactions; gas-phase reactions have to be taken into account. The increase in graphene thickness downstream strongly suggests that the gas compounds in the tube are at a nonequilibrium steady state, and the density of active species derived from methane cracking increases downstream, which contribute to the increase in graphene thickness. The threedimensional microstructures grown at the rear side are possibly due to the deposition of large hydrocarbon radicals derived from the complicated chain reactions initialized by methane dehydrogenation. Since its thickness is dependent on the residence time or path of the carbon sources, graphene is expected to grow thicker if we could extend the residence time or path with other parameters unaltered. Therefore, we used the hydrofluoric acid (HF) to etch the inner wall of the quartz tube to make it much

spectra, which may be caused by the edge of the islands. The bilayer coverage is about 8% for graphene grown at position 2 and increases to about 60% at position 4. Both monolayer and bilayer features can now be resolved in the Raman spectrum for graphene grown at position 4 as depicted in the inset of Figure 2g. Bilayer graphene is very promising for application owing to its semiconducting behavior. It is noted that a bilayer coverage of 67% was obtained previously by a two-step CVD growth in a multizone tube furnace.19 We now demonstrate a convenient and flexible route to control the graphene thickness between monolayer and bilayer, which may be used to fine-tune the electronic and optical properties of graphene. The thickness increase of graphene downstream along the tube does not follow the layer-by-layer growth mode. Small multilayer patches can be seen on the bilayer islands for graphene grown at position 4 (Figure 2g). For graphene grown at position 5, the SEM image (Figure 2h) shows complicated three-dimensional microstructures rather than flat layer structures. The corresponding Raman spectrum (inset of Figure 2h) suggests few-layer graphene characteristics: IG/I2D 10559

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Figure 4. CVD growth of graphene with Cu foils at the seven different positions simultaneously. (a) Calculated mole fraction of 15 species in the gas-phase and solid-carbon-phase equilibrium. Only species with an amount higher than 10−7 mol are shown. Total pressure is 20 Torr, H/C = 26:5 (corresponding to initial H2/CH4 = 3:5). (b) Schematic of the density distribution of the active species along the tube when graphene are grown simultaneously at the seven positions. (c−e) Photograph, Raman spectra, and optical transmittance spectra of the grown graphene films, respectively. The transmittances of light at 550 nm are also indicated in e. (f) Typical SEM image of graphene grown at position 7. The scale bar is 5 μm.

trations of CH3 radical and several other hydrocarbon species will become much lower than that in Figure 1a. Therefore, the concentration of active carbon species in the gas phase will drop significantly after passing a Cu foil. If we put several Cu foils along the CVD tube, it is possible to maintain a relatively uniform low concentration of active carbon species (Figure 4b). At the same time, dehydrogenation of hydrocarbon becomes much easier on Cu surface compared to that in gas phase. As we have reported previously,27 on Cu(111) surface, the reaction energy for CHx (x = 1, ..., 4) dehydrogenation decreases to less than 1.5 eV, and the corresponding dissociation barrier is also lowered to less than 2.0 eV. Therefore, if Cu appears at early stage of the gas-phase reaction, graphene growth may be surface controlled, which leads to monolayer growth. In another experiment, we placed Cu foils at each of the seven positions shown in Figure 2a simultaneously using a tube without etching. As expected, the optical image (Figure 4c) shows macroscopic uniformity for all of the simultaneously grown graphene films. Their Raman spectra, optical transmittance spectra, and SEM image (Figure 4d−f) further reveal the formation of high-quality monolayer graphene for all of the samples. This is in strong contrast to the case of independent growth at the seven positions. We also repeated the simultaneous graphene growth at the seven different positions using an etched tube, and the results are shown in Figure S2 in the SI. Uniform monolayer graphene is also achieved for all seven samples. The growth mechanism is similar to the case without tube etching; that is, the density of the active species in the gas phase is extremely low due to the simultaneously placed Cu foils acting as a CH3 sink, so that the graphene growth is surface-controlled. To further confirm the critical role of the nonequilibrium (along the CVD tube) character of the gas phase, we grew graphene simultaneously at positions 4−7, that is, from the middle position to the last position downstream (Figure 5a) with an etched tube. The photograph (Figure 5b) suggests that the graphene grown at position 4 is much thicker than the others. Raman spectra and optical transmittance spectra (Figure

rougher [see Figure S1 in the Supporting Information (SI)]. By using this tube for CVD growth, the methane molecules in the flow have more chances to collide with the inner wall of the tube and thus elongate the residence path in the heating environment, which leads to higher density of dehydrogenated active species compared to the case without tube etching. As a consequence, the graphene thickness would increase at the same position. It is noted that the rapid and long-lifetime growth of carbon nanotube forests can be achieved when the dwell time of the carbon source gas was raised.26 We repeated the independent graphene growth at different positions as depicted in Figure 2a with an etched tube, and the results are displayed in Figure 3. The optical image, Raman spectra, and optical transmittance spectra clearly reveal thickness increasing downstream. The optical transmittance spectra further show that the film thickness is higher at the same growth position compared to those without tube etching, especially at the rear side (Figure 3b). For example, the optical transmittance at 550 nm for graphene grown at positions 3 and 7 is now 88.1% and 69.4%, respectively, which are much smaller than the values without tube etching (96% and 78.5%, respectively). The SEM images also confirm the thickness increase for graphene grown with an etched tube: The graphene grown at position 3 now shows complicated multilayer structures on the background of monolayer graphene (Figure 3c), in strong contrast to mainly monolayer graphene with 23% coverage of bilayer (Figure 2f) without tube etching. Thus this experiment confirms our conjecture that the density of active species derived from methane is increased using an etched tube and leads to higher graphene thickness. Until now, we just use Cu substrate as a probe to detect a local gas-phase environment. Meanwhile, strictly speaking, graphene growth on Cu foil will change the gas-phase composition itself. Our DFT calculation gives an adsorption energy of CH3 radical on Cu(111) as 1.50 eV. Therefore, the Cu surface can serve as a sink for CH3. If CH3 is trapped, the chain reaction of methane cracking in the gas phase will be prohibited. As shown in Figure 4a, if we include solid-state carbon in our chemical equilibrium calculation, the concen10560

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increasing for graphene grown independently at different positions downstream. On the other hand, uniform monolayer graphene is achieved everywhere if Cu foils are distributed simultaneously with equal distance in the tube. This is attributed to the significant density reduction for the active species in the gas phase due to the sink effect of the Cu foils. Our study provides deep insight on the CVD growth mechanism of graphene. At the same time, it also offers a new way to fine-tune the graphene thickness via controlling the gas-phase dynamics during CVD growth, which might also be applied to regulate the growth of other nanostructures from gas phases.

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METHODS Graphene Growth. The graphene growth was carried out in a split tube furnace (furnace length of 43 cm), where seven positions equally distributed with a neighboring distance of 6 cm are defined. Position 1 is 3 cm away from the furnace edge upstream, and position 7 is 4 cm away from the furnace edge downstream. The 25-μm-thick Cu foils (Alfa Aesar, item no. 13382, size of 1.5 cm × 2.8 cm) were heated to 1000 °C in a 100 sccm H2 flow and kept for 20 min. Then 50 sccm CH4 was introduced to the furnace with the H2 flow reduced to 30 sccm, while the total pressure was maintained between 18 and 20 Torr. The typical growth time was 30 min. After growth, the furnace was opened for fast cooling, and the as-grown graphene films were transferred onto Si substrate with a 300-nm-thick oxide capping layer for Raman and scanning electron microscopy (SEM) characterization11 or onto quartz substrates for optical transmittance measurement. Sample Characterization. Raman spectroscopy (French JY LABRAM-HR) with a laser excitation wavelength of 514.5 nm was used to characterize the thickness, quality, and uniformity of the grown graphene films at room temperature. Optical transmittance spectra were measured using a UV−visNIR spectrophotometers (Shimadzu DUV-3700). The morphology of graphene films was characterized using a field emission SEM (FEI Sirion 200) operated at 5 kV. Computational Modeling. Chemical equilibrium of N compounds is analyzed by minimizing the free energy G under the constrain of the mass conservation law

Figure 5. (a) Schematic of the simultaneous CVD growth of graphene with Cu foils at positions 4−7, using a quartz tube with the inner surface etched by HF. (b−d) Photograph, Raman spectra, and optical transmittance spectra of graphene films. The transmittances of light at 550 nm are also indicated in d.

5c and d) show clearly that the grown graphene is multilayer at position 4, mainly bilayer at position 5, and dominantly monolayer at positions 6 and 7. The growth pattern could be understood as follows: The density of active species increases along the tube downstream and reaches its maximum at position 4, then decays gradually due to the consumption of Cu foils at position 4 to position 7, thus leading to the observed different graphene thicknesses at different positions. This further illustrates that the graphene thickness is very sensitive to the density of active species in the CVD growth, which offers us a convenient way to control the structure and properties of graphene. To clarify the temperature effect on the graphene thickness variation, we also measured the temperature profile of tube in the furnace at a growth temperature of 1000 °C as shown in Figure S3 in the SI. The center of the tube is slightly hotter than the edge; for example, the temperature of position 4 is about 20 °C higher than that of positions 1 and 7. The temperature distribution with gas flow was also measured, and the temperature change induced by gas flow is smaller than 1 °C. Thus the temperature profile of the tube during growth is quite uniform and almost fixed. However, the graphene thickness distribution has been shown to vary significantly, depending on the number and locations of the simultaneously placed Cu foils, from thickness increasing downstream, to thickness decreasing downstream, and to thickness keeping constant everywhere. The diversification of the graphene thickness distributions thus cannot be explained by the temperature distribution and can only be attributed to the controllable distribution of the active species derived from the gas-phase dynamics. Graphene growth at 980 °C was also performed at different positions independently, and the results (Figure S4 in the SI) are similar to that for graphene grown at 1000 °C. This further excludes the temperature effect on the graphene thickness variation.

N

G=

N1

N2

i=1

i=1

∑ niμi = ∑ niGi0 + ∑ ni(Gi0 + RT ln(niP /nt)) i=1

where ni ≥ 0 is the mole number of compound i, nt is the total mole number of gas-phase compounds, T is temperature, and P is pressure. G0i values can be found in thermochemical tables.28 The constrained minimization problem is solved by sequential least-squares programming.29 DFT calculations are performed with the DMol3 package30,31 using the PBE exchangecorrelation functional.32

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ASSOCIATED CONTENT

S Supporting Information *

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Sample characterization, photograph of the tubes with and without inner wall etched, the characteristics of graphene films grown simultaneously at the seven positions using an etched tube, the temperature distribution of the tube in the furnace, and the SEM images of graphene grown at 980 °C. This material is available free of charge via the Internet at http:// pubs.acs.org.

CONCLUSION In summary, by delicate design and control of the CVD conditions, we show that a nonequilibrium steady state can be achieved in the gas phase along the CVD tube; that is, the active species from methane chain cracking increase monotonously in quantity downstream. This leads to thickness 10561

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(24) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Nano Lett. 2007, 7, 238−242. (25) Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rep. 2009, 473, 51−87. (26) Yasuda, S.; Futaba, D. N.; Yamada, T.; Yumura, M.; Hata, K. Nano Lett. 2011, 11, 3617−3623. (27) Zhang, W. H.; Wu, P.; Li, Z. Y.; Yang, J. L. J. Phys. Chem. C 2011, 115, 17782−17787. (28) Chase, M. W., Jr. NIST-JANAF Thermochemical Tables, 4th ed.; American Institute of Physics: New York, 1998. (29) Scipy package documentation. http://docs.scipy.org/doc/ (accessed Sept 18, 2011). (30) Delley, B. J. Chem. Phys. 1990, 92, 508−517. (31) Delley, B. J. Chem. Phys. 2000, 113, 7756−7764. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Z.); [email protected] (Z.L.); [email protected] (X.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Zhenyu Zhang for helpful discussions. This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. WK2340000011), NSFC (Grants Nos. 10974188, 91021018, 20933006, 11104258, 21173202, and 11034006), “One-hundred-person Project” of CAS, NKBRPC (Grant No. 2009CB929502), SRFDP (20113402110046), NCET, and CPSFFP (Grant No. 20100470837).

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