Nonisothermal Synthesis of AB-Stacked Bilayer Graphene on Cu Foils

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Non–Isothermal Synthesis of AB–Stacked Bilayer Graphene on Cu foils by Atmospheric Pressure Chemical Vapor Deposition Hai–bin Sun, Jun Wu, Yan Han, Jun-yong Wang, Fengqi Song, and Jian-guo Wan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

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The Journal of Physical Chemistry

Non–Isothermal Synthesis of AB–Stacked Bilayer Graphene on Cu foils by Atmospheric Pressure Chemical Vapor Deposition

Hai–Bin Sun, Jun Wu, Yan Han, Jun–Yong Wang, Feng–Qi Song and Jian–Guo Wan* National Laboratory of Solid State Microstructures, Collaborative Innovation Center for Advanced Microstructures, and Department of Physics, Nanjing University, Nanjing 210093, P. R. China * Corresponding author. E–mail: [email protected]

ABSTRACT: Since the discovery of Cu–catalyzed chemical vapor deposition (CVD), the preparation of large–area graphene films has been performed by the carbon precursor exposure under isothermal conditions. In this work, we report on a non–isothermal method to quickly synthesize the large–area AB–stacked bilayer graphene films (BGF) by atmospheric pressure CVD on the copper foils. The growth feature of the BGF is carefully studied by scanning electron microscopy, Raman spectroscopy and transmission electron microscopy. The results show that both cooling rate and CH4 flow rate play crucial roles on the BGF growth in the non–isothermal process. A phase diagram for the preparation of BGF is thereby derived from plenty of experiments. In addition, we find that bilayer graphene seeds grow into graphene islands at the initial growth stage and extend gradually to a continuous film. Accordingly, a possible growth mechanism combining with surface–catalyzed process and seed growth is proposed.

KEYWORDS: bilayer graphene films; atmospheric pressure chemical vapor deposition; non–isothermal growth

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1. INTRODUCTUION Graphene, a carbon atomic film of hexagonal arrangement, has been attracting enormous attention due to its excellent physical properties such as extremely high carrier mobility, ballistic transport, fractional quantum Hall effects and high optical transparency.1–5 However, the zero–bandgap of monolayer graphene hinders its applications for various devices. Recently, several groups have reported that the bandgap can be opened up to ~250 meV by a vertically external electric field in AB–stacked bilayer graphene films (BGF),6–7 which have potential applications for developing various electronic nanodevices such as tunnel field–effect transistors and pseudospintronics.8–9 The pristine BGF were produced by mechanical exfoliation

of

highly

oriented

pyrolytic

graphite

(HOPG).

But the

micrometer–size of the graphene sheets produced by HOPG limit its actual applications seriously. The preparation of large–area AB–stacked BGF can be realized by the chemical vapor deposition (CVD) method on the surface of transition metals. Two transition metals, i.e. Ni and Cu, are usually used as catalysts in the CVD process.10–11 For Ni, owing to its relatively high carbon solubility (typically ～1.3 atom % at 1000 ℃), a great deal of multilayer domains doggedly exist in the continuous graphene films synthesized in the segregation process.10, 12–14 To prepare BGF with higher quality, Cu–Ni alloy foils are used instead of Ni, but most as–prepared films are still non–uniform.15 Recently, Liu et al. obtained near–perfect AB–stacked BGF with high coverage prepared on engineered Cu–Ni alloy films.16 Comparatively, Cu has such small carbon solubility (typically ~0.0007 atom % at 1000 ℃) that it is widely used to prepare monolayer graphene films by self–limiting mechanism.11, 17–18 In order to prepare BGF on Cu foils, the self–limiting growth process has to be broken. So far some ways such as low–speed growth, epitaxial growth and low pressure synthesis have been developed for the preparation of BGF on Cu foils. For example, Li et al. prepared 410 µm

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bilayer graphene on Cu foils at a much low growth rate for three hours by surface–catalytic decomposition.19 Yan et al. fabricated the 67% coverage of AB–stacked BGF on monolayer graphene via layer–by–layer epitaxy process.20 Sun et al. synthesized the AB–stacked BGF on Cu foils by precisely tuning the total pressure.21 However, the scalable and economic production of high–quality AB–stacked BGF is still in its significant challenge. In this work, we report on the successful preparation of large–area high–quality AB–stacked BGF on Cu foils using another method. Based on the atmospheric pressure chemical vapor deposition (APCVD), a non–isothermal growth process is introduced to control the nucleation of bilayer graphene and AB–stacked BGF are well synthesized. We find that both the cooling rate and CH4 flow rate greatly influence the growth of the BGF. A phase diagram of the BGF growth under such non–isothermal process is obtained and a possible growth mechanism is proposed. Our results offer useful insights into the understanding of the gas–phase dynamics of graphene growth in the non–isothermal process, and make an important progress for controllable preparation of BGF based on the Cu–catalyzed CVD process.

2. EXPERIMENTAL METHODS 2.1 Graphene growth. All the syntheses were carried out in a tube furnace under the APCVD conditions. Cold–rolled 1cm x 1cm copper foils with a thickness of 25 µm (99.8% purity, Alfa Aesar Inc.) were used as catalysts. CH4 gases were used as the carbon source. Before APCVD, the copper foils were washed by an ultrasonic cleaner for 10 min in the solution of HCl/H2O (1:10), acetone, ethanol, and deionized water, respectively, and then were dried by nitrogen gas flow. The Cu foils were placed in a horizontal quartz tube and heated to 1000 ℃ at a rate of 20 ℃ /min in the mixtures of Ar and H2 with a total flow rate of 300 standard cubic centimeters per

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minute (sccm)，and then annealed at 1000 ℃ for 30 min to remove the absorbents and contaminations on the copper surface as well as enlarge the copper grain size. Subsequently, CH4 gases were added into the quartz tube for preparing the graphene films, meanwhile, the furnace was slowly cooled from 1000 ℃ to 900 ℃ with a certain cooling rate in the range of 5–50 ℃/min. Note that it is a non–isothermal growth process to prepare the graphene films on Cu foils. When the temperature dropped to 900 ℃, CH4 was turned off and the furnace was opened for fast cooling (~200 ℃/min) till room temperature under the protection of mixture of Ar and H2. The above preparation process can be summarized up as four stages: heating, annealing, growing under non–isothermal environment, and fast cooling, as shown in Figure 1. Since the change in the temperature is crucial to the preparation of the bilayer graphene, a precise sheathed thermocouple with a precision of 0.5 ℃ was used, which was placed as close to the sample as possible in order to monitor the change in the temperature of the sample precisely. The details of the setup can be found in Figure S1 of the supporting information.

Figure 1. Schematic illustration of graphene synthesis process and the four different stages. In stage 1 (Heating ) and stage 2 (Annealing), the Cu foils are heated to 1000 ℃ and annealed to smooth the surface with the mixed gases of H2 and Ar (300 sccm). In stage 3 (Growing),

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CH4 flows and the furnace is cooled down from 1000 ℃ to 900 ℃ with different cooling rates. Here, CH4 is decomposed catalytically and the carbon species are produced on the copper surface. In stage 4 (Cooling), the furnace is opened when it is cooled to 900 ℃, and the samples are cooled rapidly to room temperature in the same mixed gases of H2 and Ar.

2.2 Graphene Transfer. The graphene grew on both sides of the Cu foil. We only kept the upper graphene for measurements. In order to remove the bottom graphene, we first deposited a 300 nm Au film on it, and spin–coated a thin layer of poly(methyl methacrylate) (PMMA) (J&K Chem 35 PMMA,

9%

in

anisole)

on

the

upper

graphene.

A

multilayer

structure

(PMMA/Graphene/Cu/Graphene/Au) was thus formed. After that, we put the multilayer structure into the 1.0 M FeCl3 solution. The Cu foil was etched completely after two hours. The lower graphene/Au bilayer thus settled to the bottom due to the gravity, while the upper PMMA/Graphene floated on the surface of the solution. We then put the PMMA/Graphene film in deionized water for 30 min, and repeated this process three times. Later on, the film was scooped out from the deionized water by the target substrate and dried at 70 ℃ for one hour in the vacuum chamber. In the end, the PMMA layer was removed by the hot acetone. The above transfer process is illustrated in a schematic drawing, as shown in Figure S2.

2.3 Characterization. Graphene films were transferred onto the SiO2/Si wafers for optical microscope, scanning electron microscope (SEM) and Raman spectroscopy characterization. SEM images were observed with a Sirion 200 SEM at 5 kV. Raman spectroscopy analysis was carried out on Ntegra Spectroscopy Nanolaboratory (NT–MDT Co.) with an excitation wavelength of 633 nm and a 100× objective lens with 1 µm–diameter laser spots. Graphene films were also 5



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transferred to Cu grids for transmission electron microscopy (TEM) observations. TEM images and selected area electron diffraction (SAED) patterns were obtained on a Tecnai G2 F20 field emission gun TEM with an accelerating voltage of 200 kV.

3. RESULTS AND DISCUSSION 3.1 The preparation of BGF. An isothermal process usually results in monolayer graphene on Cu foils due to self–limiting growth process.11, 17 In this work, we introduce the non–isothermal process to break the self–limiting growth condition in order to prepare the BGF. Referring to the growth temperature of monolayer graphene,22 we controlled the growth temperature range from 1000 to 900 ℃ in the non–isothermal process. The optimized parameters, i.e. cooling rate of 20 ℃/min and CH4 flow rate of 1.2 sccm, were chosen. Figure 2(a) presents the optical image of a typical BGF transferred onto the SiO2/Si substrates. One clearly observes that a continuous film covers the whole substrate uniformly. The corresponding Raman spectroscopy shown in Figure 2(b) is coincident well with the characteristics of BGF. The D band at～1350 cm–1 is very weak, indicating that there are few defects in the film. Two prominent bands locating at ～1590 cm–1 and ～2650 cm–1 are exactly corresponding to the G and 2D bands of the graphene, respectively. The intensity ratio between 2D and G band (I2D/IG) is ～1.0, while the full width at half–maximum (FWHM) of the 2D band reaches ～ 55 cm–1. These characteristics strongly suggest the film is an AB–stacked BGF.20 In addition, we decomposed the asymmetric 2D band into four sub–bands with the Lorentz distribution, as shown in Figure 2(c). This is another signature of AB–stacked BGF.20, 23–24 In order to prove the quality of the BGF sufficiently, we also performed Raman mapping measurements over a 30 µm × 30µm area. The results are shown in Figures 2(d–f), which are corresponding to the Raman maps of D, G, and 2D bands, respectively. All the Raman maps 6


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show uniform color distributions except for several bright spots. These results indicate that the as–grown bilayer graphene is almost macroscopically uniform. To further evaluate the quality of the film, we performed TEM observations and SAED examinations on the graphene films suspended over the Cu grids. A folded or curled graphene membrane is clearly seen under low magnification with a sufficient defocus (Figures 3(a)). A high–resolution TEM image in Figure 3(b) shows some folded–edges in as–grown BGF. The SAED patterns in Figure 3(c) display a set of six–fold symmetry arrangement of carbon atoms, indicative of a single–crystal BGF. Moreover, the intensities of the outer–order diffractions are almost twice stronger than those of the inner–order diffractions (Figure 3(d)), indicative of AB–stacked BGF.15, 25 These observations agree well with the results of Raman spectroscopy, further confirming the formation of BGF with the AB–stacked order.

Figure 2. Characterization of a large–area BGF grown in APCVD with 1.2 sccm CH4 at a cooling rate of 20 ℃/min about 5min from 1000 ℃ to 900 ℃. (a) Optical image. (b) Raman spectra obtained from the blue circle–shape region in (a). (c) Enlarged 2D band in (b). The asymmetric 2D band can be decomposed into four sub–bands with the Lorentz distribution. (d–f) Raman maps of the D (1300–1400 cm–1), G (1550–1650 cm–1), and 2D 7



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(2600–2700 cm–1) bands, respectively.

Figure 3. Bright–field TEM image (a) and high–resolution TEM image (b) of the as–grown BGF. (c) Typical normal incident SAED patterns of the as–grown BGF taken from the blue circle–shape region in (a). (d) Intensity profiles along the indicated spots (from left to right) in (c).

3.2 The growth mechanism of BGF. To explore the growth mechanism of BGF, we carefully studied the initial nucleation and found that it played a critical role on determining the final layer number of the graphene film. In order to prepare the graphene nuclei at the initial growth stage, we precisely controlled the growth in a narrow temperature range of 1000–960 ℃ at a cooling rate of 20 ℃/min and 8


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with a CH4 flow rate of 1.2 sccm. When the growth temperature was dropped to 960 ℃, CH4 was turned off at once and the furnace was opened for fast cooling. Figures 4(a) and 4(b) present the SEM images of the as–prepared sample transferred onto 300 nm SiO2/Si substrates. Remarkably, plenty of small island–like sheets distributed in isolation, all of which possess respective orientations. Most of these islands exhibit hexagonal shapes, nevertheless, owing to the etching effect of H2 in the growth process,26 they tend to deform into round disks. In addition, one observes that some adjacent islands start to merge with each other to form larger sheets by weak van der Waals force.27

Figure 4. (a) SEM image of the initial graphene nuclei synthesized in the non–isothermal process at a cooling rate of 20 ℃/min and with 1.2 sccm CH4 about 2 min from 1000 ℃ to 960 ℃ by APCVD. (b) Enlarged SEM image of a typical graphene nucleus. (c) Raman spectra taken from the circle–shape region in (b). Here, the upper spectra are taken from the blue circle–shape region at the edge of the domain in (b) and the lower ones are taken from the red circle–shape region in the center of the domain in (b). (d) Distribution histograms of 2D–FWHM and I2D/IG for the Raman spectra at the edge region of the domains in (a). (e)

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Distribution histograms of 2D–FWHM and I2D/IG for the Raman spectra in the center region of the domains in (a). (f) Optical image of the sample synthesized on the Cu foil with 1.2 sccm CH4 at 980 ℃ for 2 min in the isothermal process at a fast cooling rate of ～ 200 ℃ /min. The insert is the corresponding Raman spectra.

From the SEM image in Figure 4(b), one can observe that the color contrast of the dark–gray hexagon is quite uniform, which is different from the inverted wedding cake structure reported in the bilayer graphene prepared by the underlayer growth mechanism in the isothermal process.28 This indicates that both the lower layer and upper layer of the initial seed may grow at the same velocity to form a larger domain. Raman spectroscopy analysis was subsequently carried out to examine the layer numbers of the graphene nuclei. We first chose the typical graphene domain in Figure 4(b) to measure the Raman spectra in both the center region and the edge region, as shown in Figure 4(c). A weak D band located at ～1324 cm–1 is observed, suggesting the presence of disordered defect states which come from the domain boundaries of the graphene.29 Two prominent bands are also observed at ～1590 cm–1 and ～2650 cm–1, corresponding to the G and 2D bands, respectively. The intensity ratio of I2D/IG is ～1.0 while the FWHM of the 2D band is ～60 cm–1. And then, we also observed the Raman spectroscopy at the center area of the graphene domain in Figure 4(b). In particular, the 2D band is wider and more asymmetric as compared to those of the monolayer graphene. These characteristics undoubtedly indicate the nuclei are AB–stacked bilayer graphene. In order to identify the growth mechanism, we further chose plenty of initial graphene domains and measured the Raman spectra in the center region and the edge region of different graphene domains, respectively. The statistical distribution histograms of both the FWHM of 2D band and the I2D/IG ratio are plotted in Figures 4(d) and 4(e). One observes that, whether the measurement spot locates in the center or at the edge of the domains, all the FWHM of 2D 10


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band are in the range of 50.0–60.0 cm–1 while all the I2D/IG ratios are in the range of 0.62–1.0. These statistical results strongly suggest that both the center region and the edge region of the domains are bilayer graphene,19–21 which should be ascribed to synchronous growth of both the upper and lower layers in the bilayer graphene seeds. We found that the bilayer graphene nuclei could be only prepared under the non–isothermal conditions. No graphene nucleus was produced if the preparation was performed under isothermal conditions. For comparison, we performed the preparations of graphene nuclei in an isothermal process at a constant growth temperature of 980 ℃ for 2 min, while all the other parameters remain unchanged. Figure 4(f) shows the optical image of the as–prepared sample and its corresponding Raman spectroscopy. It is clear that no detectable graphene image or Raman signal appears.

Figure 5. Schematic drawings for the growth process of bilayer graphene on Cu foils. (a) CH4 decomposition at 900–1000 ℃, followed by carbon atoms diffuse on Cu foils. (b–e) 11



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Side–view and 3D images of bilayer graphene nucleation on copper surface throughout the non–isothermal and isothermal growth process, respectively.

Based on the above results, we believe that non–isothermal growth process is crucial to the formation of bilayer graphene. We thereby propose a possible growth mechanism related to both surface–catalysis and seed growth under non–isothermal growth process as follows. Considering that the solubility of carbon in Cu is quite low (typically, 0.0007% atom at 1000 ℃), carbon atoms dissolving into the Cu foil is negligible in the cooling process. During the incubation, CH4 is decomposed catalytically by copper into various hydrocarbons such as C2H2 and C10H8 when the CH4 gases flow above the copper surface at high temperatures.30 These hydrocarbons are available for the formation of high–quality graphene on the copper surface as long as the temperature is above 900 ℃.21 A large number of hydrocarbons diffuse on the surface of the copper foils and gradually gather around the impurities on the copper.31,

32

Once the concentration of carbon atoms reaches a critical

supersaturation, initial graphene seeds will form spontaneously. These seeds prefer to form bilayer if the growth is under non–isothermal environment, which may be ascribed to the existence of certain time–dependent temperature gradient due to the continuous cooling.15, 33 By continually absorbing the carbon atoms on the copper surface, both the lower layer and upper layer of the bilayer graphene seeds grow at the same velocity, finally form a continuous film. Figures 5 (a), (b) and (d) present the schematic drawings for the BGF growth under the non–isothermal environment. This growth process is completely different from the usual isothermal growth process, in which the BGF growth is followed by an underlayer growth mechanism. 19, 28, 34 As shown in Figures 5(c) and (e), a monolayer graphene seed forms at the initial stage, and then the bilayer graphene grows through the open channels between the monolayer graphene and copper surface, the growth rate of the upper layer is faster than that

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of the lower layer. Once the upper layers between the two neighboring bilayer graphene meet with each other, the open channels will be shut, terminating the bilayer growth. Such underlayer growth mechanism seriously restricts the ability to produce continuous BGF. Moreover, both the hydrocarbon concentration and growth rate must be controlled to be small enough in order to achieve the successful preparation. Accordingly, we consider that the non–isothermal growth process is more advantageous for preparing large–area BGF with high quality.

Figure 6. (a–b) TEM images of the graphene synthesized with 2.0 sccm CH4 at a cooling rate of 20 ℃/min. The insert of (b) is the corresponding SAED patterns taken from the green circle–shape region in (b). (c–d) TEM images of the graphene prepared with 0.6 sccm CH4 at a slow cooling rate of 5 ℃/min. The insert of (d) is the corresponding SAED patterns taken from the blue square–shape region in (d).

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3.3 The influencing factors of BGF preparation. It is worth noticing that, at a fixed cooling rate, a high CH4 flow rate tends to produce poor quality of BGF due to the excessive CH4 breaking the growth equilibrium. For example, at a cooling rate of 20 ℃/min, the quality of BGF keeps fine with 1.2 sccm CH4 flow rate (shown in Figure 2). However, when the CH4 flow rate is increased to 2.0 sccm, the quality of BGF is seriously lowered, accompanied by the appearance of plenty of multilayer graphene spots beneath the BGF, as shown in Figures 6(a) and 6(b). Several sets of electron diffraction patterns shown in the inset of Figure 6(b) add weight to the emergence of multilayer graphene nuclei overlapping with the BGF, where these nuclei have individual crystallographic orientations. Too slow cooling rate is also disadvantageous for the synthesis of high–quality BGF. Figures 6(c) and 6(d) present the TEM images of the graphene films grown with 0.6 sccm CH4 flow rate at a cooling rate of 5 ℃/min. Many isolated dark dots coexist on the BGF. Electron diffraction patterns inserted in Figure 6(d) show a pearl ring with several sets of hexagonal patterns, indicating these dots are multilayer graphene.23

Figure 7. Phase diagram of BGF growth with different cooling rates and CH4 flow rates in the 14


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non–isothermal process. The region of growth temperature is 900–1000 ℃. Continuous bilayer graphene film on which multilayer islands coexist is synthesized using the preparation parameters in the upper–left region, discontinuous BGF is prepared using the preparation parameters in the middle striped region, and no–graphene film is obtained when the parameters are in the lower–right region. Inserts are the typical SEM images of the samples prepared at a fixed cooling rate of 10 ℃/min and with different CH4 flow rates (3.0, 0.8, and 0.6 sccm).

Based on the aforementioned results, we believe that both cooling rate and CH4 flow rate are crucial for synthesizing the BGF in the non–isothermal growth process. A phase diagram is helpful for better understanding the influence of those two factors on the growth of BGF. To generate it, we repeated lots of preparations using various CH4 flow rates and cooling rates at the non–isothermal growth environments when the temperature drops from 1000 to 900 ℃. Figure 7 plots out the phase diagram of BGF growth in such non–isothermal process. As we can see, when the cooling rates are tuned in the range of 5–50 ℃/min, the BGF are feasibly prepared if the CH4 flow rates are moderate accordingly (see the upper–left region of Figure 7). For example, at a cooling rate of 10 ℃/min, continuous large–area BGF can be synthesized if the CH4 flow rate is larger than 0.8 sccm, nevertheless, the quality of the film is low and many multilayer islands coexist on the surface of the BGF. When the preparation parameters locate at the red dashed line (e.g. at a cooling rate of 10 ℃/min and CH4 flow rate of 0.8 sccm), continuous BGF with better quality can be prepared (the coverage of BGF is larger than ~90%). Discontinuous bilayer graphene will appear if the preparation parameters are in the middle striped region of Figure 7 (e.g. at a cooling rate of 10 ℃/min and CH4 flow rate of 0.6 sccm). And no graphene forms when the preparation parameters are in the

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lower–right region of Figure 7. The inserts of Figure 7 show the typical SEM images of the samples prepared in different regions. In addition, the control over the cooling rate is also important for the quality of BGF. On one hand, slower cooling rate will cause massive carbon adatoms supersaturate on the copper surface, which induces the multilayer graphene nuclei to form synchronously beneath the bilayer graphene according to the underlayer growth mechanism. On the other hand, relatively faster cooling rate reduces the growth time of active carbon species and thus impedes the graphene nucleation on the copper surface. Finally, it is worthy noticing that, if the cooling rate is beyond the range of 50 ℃/min at the present experimental conditions, we can not synthesize graphene no matter what the CH4 flow rate is.

4. CONCLUSION We have demonstrated the successful preparation of large–area high–quality AB–stacked BGF on Cu foils by a non–isothermal method under APCVD conditions. The quality of the BGF depends on the cooling rate and CH4 flow rate. The suitable ranges of 5–50 ℃/min for the cooling rate and 0.5–4.0 sccm for the CH4 flow rate were obtained. Both parameters must be matched well so as to reach good growth equilibrium for the preparation of BGF with high–quality. Slower cooling rate and higher CH4 flow rate will cause the production of multilayer graphene on the BGF. A phase diagram for the preparation of BGF was thus derived from plenty of experiments. In addition, we found that a moderate temperature gradient was crucial for the growth of BGF, which can easily break down the self–limiting effect and prompt the formation of the initial bilayer graphene seeds. The bilayer graphene seeds nucleate at the initial growth stage and extend gradually to a continuous film. Accordingly, we proposed a possible growth mechanism of BGF based on the surface–catalyzed process and seed growth by this non–isothermal method. This work presents a feasible avenue to prepare the BGF and offers an insight into the understanding of

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growth mechanism under non–isothermal conditions, which is helpful for controllable preparation of BGF based on the Cu–catalyzed CVD process.

ACKNOWLEDGEMENTS This work was supported by the National Key Projects for Basic Research of China (Grant Nos. 2010CB923401, 2013CB922103), the National Natural Science Foundation of China (Grant No. 11134005), and the PAPD project.

Supporting Information Available: The setup for preparing the samples and the details of transfer process. This information is available free of charge via the Internet at http://pubs.acs.org.

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Nonisothermal Synthesis of AB-Stacked Bilayer Graphene on Cu Foils

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