On the Role of Vapor Trapping for Chemical Vapor Deposition (CVD

Article pubs.acs.org/cm

On the Role of Vapor Trapping for Chemical Vapor Deposition (CVD) Grown Graphene over Copper Mark H. Rümmeli,*,†,‡ Sandeep Gorantla,§ Alicja Bachmatiuk,†,‡ Johannes Phieler,§ Nicole Geißler,§ Imad Ibrahim,§ Jinbo Pang,§ and Jürgen Eckert§,∥ †

IBS Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Daejon 305-701, Republic of Korea Department of Energy Science, Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea § IFW Dresden, Institute of Complex Materials, P.O. Box 270116, 01069 Dresden, Germany ∥ TU Dresden, Institute of Materials Science, 01062 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: The role of sample chamber configuration for the chemical vapor deposition of graphene over copper was investigated in detail. A configuration in which the gas flow is unrestricted was shown to lead to graphene with an inhomogeneous number of layers (between 1 and 3). An alternative configuration in which one end of the inner tube (in which the sample is placed) is closed so as to restrict the gas flow leads a homogeneous graphene layer number. Depending on the sample placement, either homogeneous monolayer or bilayer graphene is obtained. Under our growth conditions, the data show local conditions play a role on layer homogeneity such that under quasi-static equilibrium gas conditions not only is the layer number stabilized, but the quality of the graphene improves. In short, our data suggests vapor trapping can trap Cu species leading to higher carbon concentrations, which determines layer number and improved decomposition of the carbon feedstock (CH4), which leads to higher quality graphene. KEYWORDS: Graphene, CVD, molecular decomposition, gas kinetics

1.0. INTRODUCTION The isolation of graphene and the revelation of its many new and exciting properties have triggered massive global interest in this 2D material which consists of a single layer of carbon atoms arranged in a honeycomb lattice. In particular, graphene’s high carrier mobility and unique band structure make it a highly attractive material for future electronics.1−4 However, the implementation of single layer graphene in electronic devices requiring a band gap is not straightforward since it is intrinsically a semimetal. A band gap can be introduced by structuring the graphene sufficiently narrow into a ribbon5−7 or by using specialized substrates.8−10 Bilayer graphene also has exciting electronic properties.4 In bilayer graphene a band gap can be induced rather easily by the application of an electric field.11−13 Moreover the induced energy gap is tunable.14 Toward the goal of single- and bilayer graphene as a future material for electronics, significant efforts are in progress to fabricate them in large area and of suitable quality (electronic-grade). The most promising manufacturing routes are based on SiC decomposition and chemical vapor deposition (CVD). Of the CVD based routes used, the employment of copper foil as a catalyst/support is commonly © XXXX American Chemical Society

chosen over other catalytic metals due to the ease with which one can obtain a reasonably homogeneous single layer of graphene.4 Nonmetals can also be used.15,16 With nonmetals, growth can be catalytic17 or noncatalytic through the pyrolysis of methane18 when used in high concentrations and with extended growth times. In the specific case of copper foil as a catalyst, up to 3 layers can form,4 and thus, the technique, aside from monolayer graphene, also holds interest for bilayer graphene fabrication, (see for example refs 19−22). These works highlight a common parameter for the preferential formation of bilayer graphene over single-layer graphene, namely, that higher carbon concentrations are required during the CVD reaction to favor bilayer formation. This can be achieved by hydrocarbon partial pressure,23 hydrogen depletion, and cooling rate,20 substrate quality,19 and carbon feedstock choice.21 In terms of graphene quality, again, numerous parameters can affect the quality of CVD synthetic graphene. These include Received: May 23, 2013 Revised: November 4, 2013

A

dx.doi.org/10.1021/cm401669k | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

substrate type and purity, hydrogen, gas pressure, gas flow, and temperature.4,24−26 The setup of the Cu foil within a thermal CVD reactor is also of importance, particularly in how it affects the flow and the kinetics for the thermal decomposition of the feedstock (typically methane). This is highlighted in a work by Li et al. in which the Cu foil was wrapped to form enclosures.27 After a low pressure CVD reaction (using methane as the feedstock), upon close inspection, the graphene single crystals that formed on the inside were significantly larger than those obtained on the outside, reaching diameters of up to 0.5 mm. The authors argued this was due to the Cu vapor being in static equilibrium thus providing a lower pressure of unwanted species. No explanation as to how this occurs was provided. In another study by Zhang et al.,28 a vapor trapping setup was established by placing the Cu foil substrate inside an inner tube with one end sealed. The open end of the tube faced the gas flow. They found larger flower shaped graphene crystals from inside the inner tube as opposed to samples prepared outside the inner tube. The single crystals spanned up to 0.1 mm. Their investigations highlighted the importance of the local environment. In this work, we investigate the role of vapor trapping on graphene films grown over Cu using methane as the feedstock in a low pressure CVD reaction. The CVD parameters were fixed so as to focus on the role of the sample setup. In this case we employed an inner tube with one end closed in which the sample is loaded, similar to that used by Zhang et al.28 Unlike their study, in this instance, the closed end of the inner tube faced the gas flow. We also examined differences in the placement of the Cu foil inside such a half closed inner tube. For comparison equivalent studies using an inner tube with both ends open were conducted.

Each location was explored independently for both a single closed end and double open end inner tube. The oven length is 400 mm and has a stable temperature profile within 40 mm from the outer ends. Prior to synthesis the oven was heated up in a pure hydrogen flow of 16 sccm at a pressure of 1.5 mbar from room temperature to 1030 °C in 15 min, and then, the condition was maintained for another 15 min. After this, CH4 was introduced at a flow rate of 10 sccm in addition to the 16 sccm H2 flow. The pressure was maintained at 1.5 mbar. This reaction was maintained for 15 min and then the CH4 and H2 gas flow was stopped and the reactor evacuated (100 μm2 shows that most of the bi layer regions are non-AB stacked. The mapping also shows the homogeneity of the single layer regions and bilayer regions is mostly homogeneous. Moreover, it indicates the bilayer graphene does not originate from folding. To better ascertain the graphene layer number homogeneity and quality of the samples they were investigated in detail by measuring the Raman response of the samples at more than 50 locations for both the as-produced samples and after transfer on to Si/SiOx (area 4 × 4 mm). Analysis of the detailed spatial spectra revealed that there was little homogeneity for samples 3 and 4 (on both sides), which were produced in the open tube. The Raman spectroscopic investigations showed the presence of mono, bi, and higher order layers, probably, tri layer, in a random fashion for these samples. A representative example showing the mixture of layers from samples 3 and 4 is provided in Figure S1 of the Supporting Information. A statistical evaluation of the different layer coverage for samples 3 and 4 (both inside and outside) are provided in table S1 in the Supporting Information. Moreover, in general, the quality of the produced material from samples 3 and 4 was poor with G to D ratios of around 40. Evaluating the data it is difficult to observe any clear trends, except that the outside samples tend to have a higher percentage of bilayer and trilayer graphene as compared to the inside samples. However, for samples 1 and 2, formed in the half closed reactor tube clear trends could be observed, as shown in Figure 2. Note the data presented in Figure 2 are the averaged data from multiple locations for the samples after transfer to Si/SiOx. In Figure 2, panels a and b show the 2D mode full width at half-maximum (fwhm) and the

Figure 1. Typical examples of the Raman spectra observed for the investigated samples. The upper spectrum corresponds to that from monolayer graphene (black) while that of the lower spectrum (blue) corresponds to that from bilayer graphene with rotational stacking disorder.

graphene.4,31 The lower spectrum corresponds to bilayer graphene. The fact that the 2D/G mode is always larger than 1 (between 1.1 and 1.4) and that its profile remains almost Gaussian indicates that at least some of the bilayer graphene in the sample has rotational stacking disorder (i.e., non-AB “Bernal” stacking).32,33 This could be due to folded graphene having formed during the transfer process or to the sample actually being true bilayer with regions of having non-AB

Figure 2. Averaged Raman spectroscopic data for samples 1 and 2. (A) fwhm for the 2D mode, (B) 2D position, (C) 2D to G mode ratio, and (D) G to D ratio. C

dx.doi.org/10.1021/cm401669k | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

2D mode peak position, respectively. Both the inside samples (1 and 2) exhibit a lower fwhm (ca. 36 cm−1) and peak position (ca. 2688 cm−1) as compared to the outside samples, which have a fwhm of ca. 42 cm−1 and position of ca. 2691 cm−1. These differences are concomitant with monolayer and bilayer graphene, respectively. It is also worth noting the shift in G mode position (not shown) was also consistent with monolayer graphene and bilayer graphene. Examination of the 2G to G ratios show average values of ca. 1.7 for the inside samples and 1.3 for the outside samples, which correspond nicely to monolayer graphene and bilayer graphene with rotational stacking disorder. In terms of the quality of the samples as measured by the ratio between the G and D modes, the graphene produced on sample 1 (inside and outside) showed a markedly better quality (G/D ca. 10) than the graphene obtained from sample 2 (G/D ca. 4), suggesting spatial location plays a critical role on the graphene quality for both the graphene grown on the inside and outside of a Cu foil ring. We also conducted a statistical evaluation of the layer homogeneity of the sample surface for samples 1 and 2 and this data is provided in Table 1. Table 1. Estimates of Layer Homogeneity from Raman Spectroscopy and TEM Data for Samples 1 and 2a Raman spec. TEM a

1 inside

1 outside

2 inside

2 outside

85% ML 90% ML

90% BL 90% BL

85% ML 90% ML

80% BL 90% BL

Figure 3. Representative TEM images of mono- and bilayer graphene grown over Cu. (A) Shows the folded edge of monolayer graphene and (B) the corresponding HRTEM of monolayer graphene. (C) Shows the folded edge of a bilayer graphene and (D) the corresponding HRTEM of monolayer graphene. The insets in parts B and D show the corresponding FFT and the arrows point to a hole (vacuum) in the graphene layers facilitating one to count the layers.

Key: ML = monolayer, BL = bilayer. (The error is ±3%).

To support the Raman spectroscopic evaluations we also conducted detailed TEM investigations of the samples. Extensive studies of multiple regions over each specimen revealed the samples to be primarily monolayer with a fraction comprising bilayer graphene or predominantly bilayer graphene with a small fraction comprising monolayer graphene. Figure 2 shows characteristic micrograph pairs highlighting monolayer graphene (panels A and B) and bilayer graphene (panels C and D). In the left-hand panels of Figure 3, the folded edges of graphene are used to identify the layer number4 and in the right panels holes with discrete step edges enable one to identify the number of layers.34 A fuller set of TEM images for all the samples investigated for both sides of the Cu foil (inside and outside) are available in the Supporting Information in Figure S1. Statistical information on the number of layers measured over numerous spatial positions for each sample are provided in Table 1 and show excellent agreement with the Raman spectroscopic evaluations. Samples that were primarily bilayer were examined in detail to determine the layer stacking order. For the most part (∼ 50%) of the bilayer graphene exhibits AB Bernal stacking (panel A in figure 4) while the rest shows rotation stacking disorder as shown by Moiré patterns and multiple reflex spots in the Fourier transform of the micrographs (see panels B and C in Figure 4). These evaluations are fully consistent with the Raman spectroscopic data which indicate the produced bilayer graphene contains rotational stacking disorder.

is limited. In addition, the average number of graphene layers is less for sample 4 than sample 3. Sample 4 sits further downstream relative to sample 3 and so might be expected to have more layers forming, since the gas phase dynamics, as discussed by Li et al.35 indicate more decomposition occurs further downstream and hence more active carbon species should be available. We observe the opposite in our case (samples 3 and 4). We attribute this to the placement of the inner tube in the horizontal tube furnace with respect to the oven heating zone. In our study, the inner tube sits off center on the downstream side of the oven heating zone. Thus, we believe the zone with the highest concentration of active carbon species lies closer to sample 3 such that further downstream where sample 4 sits the active carbon species level is reduced. This is illustrated in Figure S4 in the Supporting Information. In addition, it may be that further downstream the active hydrogen species increase since H is an end-product of the methane decomposition (see D1 in the Supporting Information for discussion on the decomposition of methane). High H concentrations can lead to etching, however this argument is inconsistent with the data from samples 1 and 2 obtained in the half closed inner tube (see Discussion). In terms of the flow, calculation of the Reynolds number, R, yields a value well below 2000 indicating laminar flow conditions. Intuitively, one might anticipate that with laminar flow one would be more likely to obtain homogeneous graphene formation from samples 3 and 4. In practice, we obtain inhomogeneous layer formation and the reason for this is not clear. We propose the synthesis conditions lead to conditions at nucleation, which are suited to multilayer growth. In the case of the inner tube in which one end is sealed, the flow conditions are dramatically altered so that now with the closed end inhibiting the gas flow, the conditions inside the

4.0. DISCUSSION We start our initial discussion by looking at the differences in the as-produced graphene between the inner and outer reactor tubes. In the case of the open inner tube, inhomogeneous layer formation is found (1−3 layers) and the quality of the graphene D

dx.doi.org/10.1021/cm401669k | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 4. HRTEM images showing the different rotation stacking observed in the as grown bilayer graphene. (A) Shows the AB stacking, and (B), (C) shows a rotation stacking with rotation angle of 22° and 31° respectively. Insets: Fourier transform images taken from the micrographs showing the diffraction data. Additional reflexes for rotationally disordered graphene in panels B and C are easily observed.

inner tube are closer to static equilibrium. In this set up, first the gas must flow around the outside of the inner tube, which itself allows for greater thermal decomposition to occur (relative to the open inner tube). Thereafter, the species can only gently diffuse into the inner tube through the open end. We can anticipate that this elongated decomposition path leads to fewer active carbon species (see Supporting Information Figure S4) as compared to the open tube, and thus, the overall layer number is reduced. Moreover, since the vapor species are now nearly static more homogeneous graphene layer formation can occur. On the outside, where the Cu foil lies against the inner tube wall, the evaporated Cu species are trapped and so build up to a high concentration which could allow for a more efficient decomposition of CH4 and hence a higher C concentration exists on the outside of the sample relative to the inside. This high carbon concentration is sufficient to drive bilayer graphene formation. On the inside of the Cu foil, however, the Cu evaporated species can occupy a significantly larger volume and so the C formation is relatively less so that now only single layer graphene forms. This same argument can be applied to the outside samples 3 and 4, which also so a propensity for increased layer numbers as opposed to the inside of samples 3 and 4. Finally, in the case of samples 1 and 2, a clear difference between the two is observed in terms of the graphene quality, in that for sample 2, which sits near the open end of the tube, the G to D ratio is half of that observed for the graphene obtained from sample 1 (which resides at the back of the tube by the closed end). This is attributed the thermal and catalytic decomposition of methane being more complete for sample 1.

outcome as they affect the efficiency of feedstock decomposition and hence C availability. Higher carbon concentrations lead to bilayer formation in agreement with previous works. Moreover by restricting the flow and establishing a quasi-static equilibrium in gas species, we postulate one can achieve a more efficient methane decomposition, which in turn yields higher quality graphene. The results seem to highlight the importance of the CVD reactor chamber configuration and sample placement on graphene layer number, as well as, the need for efficient methane decomposition.

■

ASSOCIATED CONTENT

* Supporting Information S

Representative Raman spectroscopy mapping data, optical microscopy data, and TEM micrographs for the samples investigated. Discussion on the decomposition of methane and schematics of the methane decomposition in the furnace. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Institute of Basic Science (IBS) (EM1304). S.G. and I.I. thank the IFW Dresden for IKM fellowships. We are very grateful to Jochen Werner, Denys Makarov, and Luyang Han for technical support and discussion.

5.0. CONCLUSIONS In this study, the importance of local environment on the formation of graphene over Cu by low pressure CVD was investigated in detail by maintaining, in so as far as possible, the CVD synthesis parameters fixed with the exception of the Cu substrate placement and sample chamber configuration. Both sample placement and sample chamber configuration were shown to affect the graphene layer formation. Samples placed in an open ended sample chamber in which the gas flow is not hindered or restricted, yield inhomogeneous graphene films with the number of layers varying from 1 to 3. In the case of using a sample chamber (inner tube) with one end closed the layer number is homogeneous and can be either mono- or bilayer graphene depending on the sample placement. Our data suggest local variations in the concentration of sublimed Cu species and gas flow rate directly affect the layer number

■

REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Zhang, Y. B.; Tan, Y. W.; Stromer, H. L.; Kim, P. Nature 2005, 438, 201. (3) Rümmeli, M. H.; Rocha, C. G.; Ortmann, F.; Ibrahim, I.; Sevincli, H.; Boerrnert, F.; Kunstmann, J.; Bachmatiuk, A.; Pötschke, M.; Shiraishi, M.; Meyyappan, M.; Büchner, B.; Roche, S.; Cuniberti, G. Adv. Mater. 2011, 23, 4471. (4) Warner, J. H.; Schäffel, F.; Bachmatiuk, A.; Rümmeli, M. H. Graphene: Fundamentals and Emergent Applications; Elsevier: Waltham, MA, 2012. (5) Han, M. Y.; Ozyilmaz, B.; Zhang, Y.; Kim, P. Phys. Rev. Lett. 2007, 98, 206805. E

dx.doi.org/10.1021/cm401669k | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

(6) Chen, Z.; Lin, Y.-M.; Rooks, M. J.; Avouris, P. Phys. E 2007, 40, 228. (7) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Science 2008, 319, 1229. (8) Zhou, S. Y.; Gweon, G. H.; Fedorov, A. V.; First, P. N.; de Heer, W. A.; Lee, D. H.; Guinea, F.; Castro Neto, A. H.; Lanzara, A. Nat. Mater. 2007, 6, 770. (9) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951. (10) Sutter, P. W.; Flege, J.-I.; Sutter, E. A. Nat. Mater. 2008, 7, 406. (11) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; dos Santos, J. M. B. L.; Nilsson, J.; Guinea, F.; Geim, A. K.; Neto, A. H. C. Phys. Rev. Lett. 2007, 99, 216802. (12) Zhang, Y.; Tang, T.-T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Shen, Y. R.; Wang, F. Nature 2009, 459, 820. (13) Xia, F.; Farmer, D. B.; Lin, Y.-M.; Avouris, P. Nano Lett. 2010, 10, 715. (14) Park, C.-H.; Louie, S. G. Nano Lett. 2010, 10, 426. (15) Rümmeli, M. H.; Bachmatiuk, A.; Scott, A.; Börrnert, F.; Warner, J. H.; Hoffman, V.; Lin, J.-H.; Cuniberti, G.; Büchner, B. ACS Nano 2010, 4, 4206. (16) Chen, J.; Wen, Y.; Guo, Y.; Wu, B.; Huang, L.; Xue, Y.; Geng, D.; Wang, D.; Yu, G.; Liu, Y. J. Am. Chem. Soc. 2011, 133, 17548. (17) Scott, A.; Dianat, A.; Börrnert, F.; Bachmatiuk, A.; Zhang, S.; Warner, J. H.; Borowiak-Palen, E.; Knupfer, M.; Büchner, B.; Cuniberti, G.; Rümmeli, M. H. Appl. Phys. Lett. 2011, 98, 073110. (18) Sun, J.; Cole, M. T.; Lindvall, N.; Teo, K. B.; Yurgens, A. Appl. Phys. Lett. 2012, 100, 022102. (19) Liu, W.; Li, H.; Xu, C.; Khatami, Y.; Banerjee, K. Carbon 2011, 49, 4122. (20) Lee, S.; Lee, K.; Zhong, Z. Nano Lett. 2010, 10, 4702. (21) Wassei, J. K.; Mecklenburg, M.; Torres, J. A.; Fowler, J. D.; Regan, B. C.; Kaner, R. B.; Weiller, B. H. Small 2012, 8, 1415. (22) Nie, S.; Wu, W.; Xing, S.; Yu, Q.; Bao, J.; Pei, S.-S.; McCarty, K. F. New J. Phys. 2012, 14, 1367. (23) Bhaviripudi, S.; Jia, X. T.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2010, 10, 4128. (24) Younessi-Sinaki, M.; Matida, E. A.; Hamdullahpur, F. Int. J. Hydrogen Energy 2009, 34, 3710. (25) Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. ACS Nano 2011, 7, 6069. (26) Han, G. H.; Güneş, F.; Bae, J. J.; Kim, E. S.; Chae, S. J.; Shin, H.J.; Choi, J.-Y.; Pribat, D.; Lee, Y. H. Nano Lett. 2011, 11, 4144. (27) Li, X.; Magnuson, C. W.; Venugopal, A.; Tromp, R. M.; Hannon, J. B.; Vogel, E. M.; Colombo, L.; Ruoff, R. S. J. Am. Chem. Soc. 2011, 133, 2816. (28) Zhang, Y.; Zhang, L.; Kim, P.; Ge, M.; Li, Z.; Zhou, C. Nano Lett. 2012, 12, 2810. (29) Lin, Y.-C.; Jin, C.; Jen, S.-F.; Lee, J.-C.; Suenaga, K.; Chiu, P.-W. ACS Nano 2011, 5, 2362. (30) Boerrnert, F.; Bachmatiuk, A.; Gorantla, S.; Wolf, D.; Lubk, A.; Buechner, B.; Ruemmeli, M. H. J. Microsc. 2013, 249, 87. (31) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 18740. (32) Ni, Z.; Wang, Y.; Yu, T.; Shen, Z. Nano Res. 2008, 1, 273. (33) Poncharal, P.; Ayari, A.; Michel, T.; Sauvajol, J.-L. Phys. Rev. B 2008, 78, 113407. (34) Warner, J. H.; Rümmeli, M. H.; Ge, L.; Gemming, T.; Montanari, B.; Harrison, N. M.; Büchner, B.; Briggs, G. A. D. Nat. Nanotech. 2009, 4, 500. (35) Lee, Z.; Zhang, W.; Fan, X.; Wu, P.; Zeng, C.; Li, Z.; Zhai, X.; Yang, J.; Hou, J. J. Phys. Chem. C 2012, 116, 10557.

F

dx.doi.org/10.1021/cm401669k | Chem. Mater. XXXX, XXX, XXX−XXX