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Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene by Edge Selective Etching with HO 2

Zhikai Qi, Haohao Shi, Mingxing Zhao, Hongchang Jin, Song Jin, Xianghua Kong, Rodney S. Ruoff, Shengyong Qin, Jiamin Xue, and Hengxing Ji Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03393 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Chemistry of Materials

Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene by Edge Selective Etching with H2O Zhikai Qi,1 Haohao Shi,2 Mingxing Zhao,3,4 Hongchang Jin,1 Song Jin,1 Xianghua Kong,5 Rodney S. Ruoff,6 Shengyong Qin,2 Jiamin Xue,3,4 and Hengxing Ji1* 1Department

of Materials Science and Engineering, CAS Key Laboratory of Materials for Energy Conversion,

iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), University of Science and Technology of China, Hefei, Anhui 230026, China. 2ICQD,

Hefei National Laboratory for Physical Sciences at Microscale, and Synergetic Innovation Center of

Quantum Information and Quantum Physics, CAS Key Laboratory of Strongly-Coupled Quantum Matter Physics, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 3School

of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China

4University

of Chinese Academy of Sciences, Beijing 100049, China

5School

of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China

6Center

for Multidimensional Carbon Materials, Institute of Basic Science, and Department of Chemistry, Ulsan

National Institute of Science and Technology, Ulsan 44919, Republic of Korea

*Correspondence and requests for materials should be addressed to H.X.J. (email: [email protected]).

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Abstract Bernal-stacked bilayer graphene is uniquely suited for application in electronic and photonic devices owing to its tunable band structure. Even though chemical vapor deposition (CVD) is considered to be the method of choice to grow bilayer graphene, the direct synthesis of high-quality large area Bernal-stacked bilayer graphene on Cu foils is complicated by overcoming the self-limiting nature of graphene growth on Cu. Here, we report a facile H2O-assisted CVD process to grow bilayer graphene on Cu foils, where graphene growth is controlled by injecting intermittent pulses of H2O vapor using a pulse valve. By optimizing CVD process parameters fully covered large area graphene with bilayer coverage of 77 ± 3.6% and high AB stacking ratio of 93 ± 3% can be directly obtained on Cu foils, which presents a hole concentration and mobility of 4.5  1012 cm-2 and 1100 cm2 V-1 s-1, respectively, at room temperature. The H2O selectively etches graphene edges without damaging graphene facets, which slows down the growth of the top layer and improves the nucleation and growth of a second graphene layer. Results from our work are important both for the industrial applications of bi-layer graphene and to elucidate the growth mechanism of CVD-graphene.


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1. INTRODUCTION Following the pioneering report by Andre Geim and Konstantin Novoselov in 2004, graphene has attracted worldwide attention due to its extraordinary physical and chemical properties, as well as its wide potential application in many fields.1-3 Yet, the absence of a bandgap greatly restricts the application of graphene in electronic devices such as tunneling field-effect transistors and tunable laser diodes.4 In contrast, Bernal (AB)-stacked bilayer graphene, where the bandgap can be precisely tuned by applying a vertical electrical field, is being explored as a promising candidate material for future electronic applications.5, 6 Today, chemical vapor deposition (CVD) on transition metals is considered to be the most important synthetic approach to produce graphene, since it allows high quality-control and also offers the possibility to tailor graphene properties at the atomic level.7 Currently, the large-scale production of high-quality monolayer graphene can be achieved using the CVD process,8, 9 but viable synthetic protocols need to be developed for the controlled growth of bilayer graphene by CVD. Bi- and multi-layer graphene are often grown by precipitation using Ni foils with a relatively high carbon solubility,10 but samples with uniform thickness are difficult to obtain using this process. Using Cu-Ni alloys to avoid rapid carbon precipitation, large-area (>3 in. × 3 in.) AB-stacked bilayer graphene film11 and single-crystal bilayer graphene with large grains (~ 300 m)12 have been obtained by optimizing the delicate balance between carbon precipitation from bulk Ni and the self-terminating growth on the Cu surface. Even so, due to the high corrosion resistance of Cu-Ni alloys, the transferred graphene often contains residual metal impurity, leading to significant reduction in quality. Commercially available Cu foil, especially in the form of a pocket,13 has also been employed to prepare large-area bilayer graphene films on the external surface of the Cu-pocket, where graphene growth occurs by the diffusion of carbon atoms through the Cu bulk.14 However, the complicated post-treatment required to transfer graphene from the pocket surface, makes it non-viable for industrial-scale production.15 Efforts have also been made to counter the self-limiting effect in graphene growth, so as to directly form bilayer graphene on Cu foils.16-22 For example, bilayer graphene has been obtained by van der Waals epitaxial growth of a second graphene layer on the surface of an existing graphene layer,23, 24 but due to the lack of an active Cu surface, the growth of the second layer can be extremely slow. A low-pressure CVD (LPCVD) process based on the adsorption-diffusion mechanism, has been reported to generate bilayer graphene with lateral sizes up to 540 m (growth time of ~ 6 h),25 but it has been observed that the fast growth of the first (top) graphene layer greatly ACS Paragon Plus Environment

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suppresses the expansion of the underlying second layer. Therefore, preparation techniques need to be urgently developed to directly grow bilayer graphene on Cu foils by enhancing the growth rate of the second layer. In this context, previous studies on carbon nanotube growth have shown that H2O vapor promotes the CVD synthesis of long aligned carbon nanotubes by selectively etching amorphous carbon and graphitic carbon.26 Moreover, the equilibrium constants (lgKP = +2.06) of reaction H2O(g) + C(s)  CO(g) + H2(g) is larger than that (lgKP = -2.10) of reaction 2H2(g) + C(s)  CH4(g) but far smaller that (lgKP = +15.92) of reaction O2(g) + C(s)  CO2(g) at the temperature of 1300 K,27 indicating that the capability of etching graphite is in the order of H2 < H2O < O2. Then, it is also known that H2O can etch defective graphene edges, causing graphene to be detached from the substrate.28, 29 Therefore, injecting H2O vapor during the CVD growth process is expected to overcome the selflimiting nature of graphene growth on Cu, and also serve to balance the growth rate of the top and bottom graphene layers by improving the diffusion of active C species at the graphene/Cu interface.25 However, a continuous flow of H2O vapor can break the balance between the adsorption and desorption of active C species on Cu surface and lead to the loss of adsorbed species, which is detrimental to the formation of bilayer graphene. In this regard, the development of pulsed CVD techniques, capable of regulating growth at the time scale of < 1 s, has opened new opportunities to explore the fast nucleation and growth kinetics of graphene on various substrates at high temperatures.30, 31 Herein, we report a novel LPCVD process to grow high-quality bilayer films on Cu foil by injecting H2O pulses during graphene growth. We observe that a precise regulation of the partial pressure and interval of the H2O pulse and the CH4 flow rate, leads to the growth of large bi-layer graphene films with a bilayer coverage of 77 ± 3.6% and a high AB stacking ratio of 93 ± 3%. Our results also indicate that the growth of bi- and multi-layer graphene on Cu foil follows the edge-nucleation and adsorption-diffusion growth mechanism, where H2O injection, while preventing the expansion of the top layer graphene by etching the edges, promotes the nucleation and growth of biand multi-layer graphene.


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2. RESULTS AND DISCUSSION

Figure 1. a) CVD system equipped with a pulse valve for H2O injection. b) CVD graphene growth process with H2O pulse. c) Variation of the partial pressure of H2O with respect to the pulse width. d) Gas partial pressure (H2 or CH4) as a function of the flow rate when using a mass flow controller. We set up a LPCVD system equipped with a pulse valve for the growth of graphene (Figure 1a), with the help of which, the Cu substrate can be exposed to intermittent pulses of H2O (steam) during graphene growth (Figure 1b, see Experimental Section for details). The pressure fluctuation in the reactor is measured in real-time to monitor the graphene growth process. Under 10 sccm H2, the gas pressure in the reactor is 16.2 Pa, and the addition of 0.5 sccm CH4 increases the gas pressure to 17.4 Pa (a pressure increment of 1.2 Pa). The injection of H2O pulse results in the abrupt increase of gas pressure, which then plateaus with time (Figure 1b), even though the duration of a H2O pulse after being injected is only ~15 s, with a full width at half maximum of 5 s. The pulse sequences are modulated by changing the pulse width (typically a few milliseconds) and the pulse interval (duration between successive pulses) using a programmable logic controller (PLC). To quantitatively study the relationship between H2O pulse and graphene growth, we have plotted the partial pressure of H2O pulse with respect to the pulse width; the results are shown in Figure 1c.


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Figure 2. SEM images of graphene grown on Cu foils by CVD (a) without and (b) with H2O pulse. c) Typical optical image of a large graphene film transferred onto a SiO2/Si substrate. d) Coverage statistics for different thicknesses in (c) showing that 77 ± 3.6% of the area is covered by bilayer graphene. e) Raman spectra taken from the marked graphene regions in (c). f) Transmittance spectra of mono- (blue) and bi-layer (red) graphene; inset shows mono- and bi-layer graphene films transferred onto SiO2 substrates. g) Changes in graphene’s conductivity () as a function of gate voltage (Vg) at 300K. Inset: Optical image of one of our field-effect transistor devices. h) Changes in hall resistance (RH) as a function of the magnetic field (B) at 300 K under Vg of 0 V. We synthesized graphene films on Cu foils using our modified CVD system (Figure 1a) with CH4 as the carbon source (see Experimental Section for details). The scanning electron microscopy (SEM) image in Figure 2a confirms that in the absence of H2O, a uniform monolayer graphene film is formed, resulting from the self-limited growth of graphene on Cu.32 In contrast, in the presence of H2O pulses during growth, bi- and multi-layer graphene domains are formed on Cu (Figure 2b), indicating that H2O plays an important role in growing bilayer graphene. To evaluate the quality of the synthesized graphene samples, we transferred a large graphene film onto a SiO2/Si substrate using the PMMA method (see Experimental Section for details). The optical image in Figure 2c displays graphene with varying contrast resulting from different number of layers; a bilayer coverage of 77 ± 3.6% (Figure 2d) is estimated. Raman spectra in Figure 2e, taken from different regions of the graphene film, are characterized by intense G (1580 cm-1) and 2D bands (2665 cm-1), where the intensity ratio of the 2D band to the G band (I2D/IG) decreases with layer number. All the spectra show negligible intensity D-bands (1360 cm-1) confirming the high quality of the graphene film obtained by H2O-assisted CVD.33 Furthermore, we have evaluated the average layer number of graphene by optical transmittance. Figure 2f shows that the graphene film by H2O-assisted CVD has a transmittance of 95.4% at 550 nm, which is in accordance with the optical transmittance of bilayer graphene.34 The sheet resistance (Rs) of graphene film grown by our process, measured by the van der Pauw method, is found to be 362.8 ± 58.4 .


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Moreover, electronic transport measurement was carried out to evaluate the electronic quality of the synthesized bilayer graphene. Multiterminal Hall bar devices (inset in Figure 2g) were fabricated by using electron-beam lithography (see Experimental Section for details). Figure 2g shows the graphene’s conductivity with respect to gate voltage (Vg) measured at 300 K, which indicates that graphene is p-doped with a Dirac voltage of about 40 V mainly due to the PMMA residual. Figure 2h presents the Hall resistance (RH) as a function of magnetic field (B) measured at T = 300 K and Vg = 0 V, which provides insight into the hole concentration of 4.5 × 1012 cm-2 and the Hall mobilities of 668 cm2 V-1 s-1 consistent with the field-effect mobility under the same conditions (Figure S1). Also, Figure S1b shows the device’s hole mobility reaches to 1100 cm2 V-1 s-1 at 300 K and Vg of 40 V, which is comparable to that of bilayer graphene obtained by the micromechanical cleavage of natural graphite.35

Figure 3. a) SEM image of the graphene film transferred onto a TEM grid (marked by red dashed lines). b) High resolution TEM image taken from the folded edge of graphene showing bilayer regions. c) SAED patterns of ABstacked graphene film. Inset: Diffraction peak intensity profile along the yellow dashed line. d) Stacking ratio statistics for the graphene sample shown in (a). e) STM image of graphene film across the edge of the second layer. Height profile shows that the separation between two neighboring layers is 0.34 nm. f – h) Atomic resolution STM images of the marked areas in (e) with scan parameters: (f, g) Vsample = 0.1 V, Itunnel = 3 nA and (h) Vsample = 0.1 V, Itunnel = 0.5 nA. The thickness and stacking order of graphene obtained by H2O-assisted CVD were evaluated by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and scanning tunneling microscopy (STM). Figure 3a shows a typical SEM image of the as-grown graphene film transferred onto a TEM grid. The highresolution TEM image in Figure 3b shows a folded graphene edge, where the two individual layers of the bilayer are clearly identified. The graphene areas marked by yellow dots in Figure 3a yield a set of diffraction spots with 6-fold symmetry (Figure 3c).24 Quantitative intensity analysis of the line profiles of diffraction patterns (see inset in Figure 3c) reveals that the intensities of the outer peaks from equivalent planes {1 2 10} are ~ two times those ACS Paragon Plus Environment

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of the inner peaks from {1 1 00}, typical of AB-stacked bilayer graphene.24 We analyzed 200 diffraction patterns acquired from the graphene samples to evaluate the AB-stacking ratio of graphene. The SAED patterns of bilayer graphene obtained from different positions (Figure S2) show different orientations, indicating that the obtained graphene film by H2O-assisted CVD is polycrystalline (Figure S2). And we find that 93 ± 3% of the graphene film consists of AB-stacked layers (Figure 3d). Figure 3e shows a 200 nm × 200 nm STM image covering the boundary between two graphene domains on Cu, where a step height difference of 0.34 nm is measured between two neighboring graphene surfaces, corresponding to the thickness of one layer of graphene.36 The atomically resolved STM image in Figure 3f reveals a typical hexagonal honeycomb structure with an inter-atomic distance of 0.14 nm, which is assigned to monolayer graphene grown on Cu.8 In sharp contrast, the STM image in Figure 3g shows a typical triangular pattern, where the distance between two neighboring carbon atoms on the same sublattice is 0.25 nm, which matches well with the lattice spacing of AB-stacked bilayer graphene.36 Figure 3h illustrates that the atomic lattice is uninterrupted at the boundary between the monolayer and AB-stacked bilayer graphene domains, indicating that the top layer of the bilayer graphene (Figure 3h, left) extends continuously over the monolayer graphene domain (Figure 3h, right).12 There are two possible structures for the bilayer graphene domains. The first is where a monolayer graphene island sits on a continuous monolayer graphene film, and the edge of the graphene island is exposed to air. The second possible structure for a bilayer graphene domain can be described as a continuous monolayer graphene that fully covers a monolayer graphene island, but in this case, the edge of the monolayer graphene island is underneath the top graphene layer (inset of Figure 3h). Our STM measurement suggests that the bilayer graphene obtained in this study has the second structure (inset of Figure 3h); this configuration was further confirmed by depositing a layer of Al2O3 by atomic layer deposition (Figure S3, see Supporting Information for details).37


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Figure 4. a) Dependence of graphene morphology on the partial pressures of CH4 and H2O pulse. The pulse interval and growth time were fixed at 10 s and 5 min, respectively. b) SEM images of the as-grown graphene formed by H2O-assisted CVD while varying the pulse interval from 1 s to 180 s. The partial pressures of H2, H2O and CH4 were fixed at 50.0 Pa, 2.3 Pa, 2.2 Pa, respectively, and the growth time was 30 min. The yellow arrows indicate the graphene wrinkles to present the graphene covered regions more clearly. All scale bars represent 20 m. Thus, our experimental results clearly show that the H2O injection during CVD is able to generate AB-stacked bilayer graphene on Cu substrates (Figure 2). To investigate the H2O-assisted CVD process in detail, we considered the key parameters in the nucleation and growth of bilayer graphene, namely, the partial pressures of H2O and CH4, and the pulse interval. We first investigated the influence of partial pressure of H2O pulse by comparing the growth of graphene at varying partial pressures of H2O pulse in the range 0.7 – 6.5 Pa, and that of CH4 in the range 2.2 – 4.1 Pa. Figure 4a shows the corresponding series of SEM images of graphene on Cu (growth time: 5 min). When the partial pressures of the H2O pulse and CH4 are, respectively, 0.7 Pa and 2.2 Pa, the coverage and domain size of graphene formed on Cu are similar to those (Figure S4) obtained without H2O pulse, indicating the negligible ACS Paragon Plus Environment

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effect of H2O on the graphene growth under these conditions. As the partial pressure of CH4 is increased to 4.1 Pa with that of H2O at 0.7 Pa, the Cu surface is fully covered by monolayer graphene (left column in Figure 4a). The graphene wrinkles originate from the shrinkage of Cu and expansion of graphene when the furnace is cooling down as a result of positive and negative thermal expansion coefficients of Cu and graphene, respectively.32 On the contrary, at 3.2 Pa of CH4 partial pressure (middle row in Figure 4a), when increasing that of H2O pulses from 0.7 to 2.3 Pa, flower-like bilayer graphene domains are formed on the Cu substrate. However, if the partial pressure of the H2O pulse is further increased to 3.3 Pa, the bilayer graphene domains nearly disappear and a number of etched grooves are seen on the monolayer graphene. Further increasing the partial pressure of H2O pulse (6.5 Pa) leads to various irregular graphene patches due to the significant etching effect of H2O. Based on these results, we conclude that H2O pulse with partial pressures in the 2.3 – 3.3 Pa (marked by green box) is favorable to bilayer graphene growth. Our results thus demonstrate that there is a competition between graphene growth and etching during the water assisted-CVD process, and under optimal partial pressures of H2O pulse and CH4, the growth of bilayer graphene is promoted by etching of graphene edges by H2O. Next, we studied the effect of pulse interval by carrying out a series of growth experiments at fixed partial pressures of H2O (2.3 Pa) and CH4 (2.2 Pa), and growth time (30 min), while varying the pulse interval from 1 to 180 s, as shown in Figure 4b. For the pulse interval of 1 s, no graphene is formed on the Cu substrate. When the pulse interval is increased to 10 s, star-shaped monolayer graphene islands are observed, indicating that the etching effect of H2O is stronger at lower H2O pulse intervals. Interestingly, at a higher pulse interval of 30 s, ~91% of the Cu substrate is covered by multilayer graphene containing bilayer graphene domains with lateral sizes up to 75 μm, probably due to an optimal balance between graphene growth and H2O etching processes. A very long pulse interval (180 s) gives rise to a graphene film with very low bilayer coverage, indicating that if a monolayer of graphene completely covers the Cu substrate, the nucleation and growth of the second layer is inhibited, since the H2O pulse (2.3 Pa) fails to etch and damage the completely grown graphene facet (Figure S5, see Supporting Information for details). Thus, it appears that the opposite processes of graphene growth and etching occur simultaneously in H2Oassisted LPCVD, and a dynamic balance between the two processes can be achieved by regulating the pulse interval, to selectively form bilayer graphene.


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Figure 5. a) Comparison of graphene growth by CVD process without and with H2O pulse; the samples were transferred onto SiO2/Si substrates. The black dashed lines mark the bi- and multi-layer graphene rings, and the white dashed arrows show the anisotropic growth of graphene. b) Optical image of the 12C/13C graphene sample transferred onto SiO2/Si substrate. c) Raman spectra of isotopically labeled mono- and bi-layer graphene regions identified by the corresponding colored spots in (b). Scanning Raman maps of the intensities of (d) G12 and (e) G13 bands for the sample in (b). The cyan dashed-line arrows represent the growth of graphene. Both insets show closeup views of the marked areas. f) Cross-section of the first and second graphene layers labeled by 12C and 13C. g) Schematic illustration showing the edge-nucleation and adsorption-diffusion mechanism of AB-stacked bilayer graphene growth in a LPCVD process with H2O pulse. The experimental results presented in Figure 4 prove that the partial pressures of H2O pulse and CH4, and the pulse interval, collectively influence the formation of bilayer graphene. To investigate the mechanism of bilayer graphene growth induced by H2O pulse, we compared graphene flakes grown on Cu foils for varied pulse intervals (5 and 15 min) under fixed partial pressures of H2 (16.5 Pa), H2O pulse (2.3 Pa) and CH4 (0.5 Pa) as well as growth time (30 min), as shown in Figure 5a. The optical image of graphene by CVD without H2O shows a monolayer graphene domain with the lateral size of ~ 250 m. In contrast, injecting H2O pulses forms graphene grains with hexagonal concentric patterns composed of multilayer graphene (marked by the black dashed lines in Figure 5a), where the periodic patterns appear to match the change in pulse interval for a given growth duration, indicating that H2O pulses promote new nucleation underneath the edges of the growing graphene. Besides, the domain sizes of the monolayer graphene decrease to ~175 m and ~125 m at pulse intervals of 15 min and 5 min, respectively, ACS Paragon Plus Environment

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showing that H2O pulses slow down the growth of the monolayer (top) graphene (Figure S6). Also, we found that the second layer graphene domains almost show the same orientations (Figure S7 and S8). Moreover, we calculated the growth rates of the graphene obtained at different conditions, and found that without H2O, the growth rate of monolayer graphene is 1098 ± 182 m2/min (PH2 = 16.5 Pa and PCH4 = 0.5 Pa). When H2O with partial pressure of 2.3 Pa was injected in the CVD system, the growth rates of the top layer of the bilayer graphene were suppressed to 929 ± 182 and 462 ± 123 m2/min at pulse interval of 15 and 5 min, respectively (Figure 5a). At the meantime, the growth rates of the bottom layer of the bilayer graphene are 128 ± 22 and 264 ± 27 m2/min at the pulse interval of 15 and 5 min (Figure 5a), respectively. These control experiments indicate that more frequent dozing of H2O suppresses the growth of top layer but accelerates the growth of bottom layer of the bilayer graphene. To further investigate the growth mechanism of the second layer of graphene, we employed carbon isotopelabeling to individually study the formation of the first and second layer.38 During graphene growth, 12CH4 and 13CH

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were introduced alternately, starting with 0.5 sccm of 12CH4 for 5 min, followed by 0.5 sccm of 13CH4 for 5

min; this process was continued for 25 min, during which, the injection of a H2O pulse of ~2.3 Pa coincided with the change in isotopic CH4 precursor (see Supporting Information for details). The graphene sample so obtained was transferred onto a SiO2/Si wafer; Figure 5b shows an optical image of the sample, where the variation in color contrast clearly indicates the presence of bi- or multi-layer regions along with a monolayer. Specific Raman bands in Figure 5c are labeled as G13 (1528 cm−1) and 2D13 (2577 cm−1), or G12 (1580 cm−1) and 2D12 (2665 cm−1), which, respectively, correspond to the G and 2D-bands of 13C, and 12C graphene regions, marked by the corresponding colored spots in Figure 5b. Figure 5d and e show, respectively, the scanning Raman maps of the G-band intensities for 12C and 13C (acquired from the graphene film in Figure 5b). We note that the first graphene layer maintains isotopically distinct regions, corresponding to the periodic etching of graphene by intermittent H2O pulses. Also, the narrower bands in isotopic graphene (the close-up views in Figure 5d and e) indicate the slower growth rate of the second layer graphene, as compared to that of the first layer, and also show unequivocally, that active C radicals are able to diffuse at the interface between the top graphene layer and the Cu substrate. In other words, the simultaneous injection of both H2O and 13CH4 (or 12CH4) results in both graphene etching and the nucleation of bior multi-layer graphene, from both kinds of C species on the Cu surface. At the following injection of 13CH4 (or 12CH

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the injected C species can diffuse to the edge of the newly formed second layer graphene, so as to expand

the bilayer graphene, as illustrated in Figure 5f. ACS Paragon Plus Environment

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Based on the evidence shown above, we propose a possible atomic mechanism of bilayer graphene growth, as illustrated in Figure 5f. Prior to the first H2O pulse, graphene growth follows the classic 2D film growth mechanism,39 where active carbon species generated from the surface adsorption and decomposition of CH4 migrate and attach to the edges of the growing graphene layer, resulting in domain growth.25 However, as a result of the strong coupling interaction between graphene and the Cu substrate, only a very small fraction of the carbon species can diffuse across the graphene/Cu interface to reach the edge of the adlayer, which restricts the lateral expansion of the underlying second layer. Upon injecting H2O vapor, graphene growth is abruptly arrested due to the pyrogenic reaction of C and H2O, which occurs at energetically favorable sites, namely, at the graphene edges,28, 40, 41

which reduces the interaction of graphene and Cu surface, resulting in a quasi-detachment of the first layer.

Meanwhile, H2O reacts with the surface-adsorbed C species and lowers down its concentration that reduces the growth of graphene. After the H2O pulse, the concentration of surface-adsorbed C species increases, and produce new nuclei that can be near an existing graphene layer but underneath. Next, the concentration of the surfaceadsorbed C species diffusing to the graphene edges easily reaches a critical supersaturation level; this supersaturation can be partly lowered by diffusion at the graphene/Cu interface and reaches the nuclei to form bilayer graphene. When the supersaturation at the graphene edges is depleted at the growth front of the bilayer, the top layer graphene gradually returns to its original growth rate, before the next pulse is injected. Thus, the H2O pulse effectively delays the expansion of the top graphene layer and triggers the formation of bilayer graphene nuclei, and a large bilayer graphene film can be obtained by optimizing the balance between edge-nucleation and adsorption-diffusion growth. Plus, after each H2O pulse, new nuclei that are next to the existing single-layer graphene can also be formed that initiates the formation of new single-layer graphene domain. 3. CONCLUSIONS In summary, Bernal-stacked bilayer graphene was grown on Cu using a H2O-assisted LPCVD process. The injection of H2O was modulated by a pulse valve, and the regulation of partial pressure and interval of H2O pulse were carried out using a PLC. The pressure in the CVD reactor was recorded in real time to monitor the graphene growth process. We investigated the influence of the CH4 flow rate, as well as the partial pressure and interval of H2O pulse, on the nucleation and growth of graphene, to obtain optimal parameter sets for the growth of large-area bilayer graphene films. The graphene samples synthesized using the optimized protocol showed a bilayer coverage of 77 ± 3.6% and AB stacking ratio of 93 ± 3%. A mechanism based on edge-nucleation and adsorption-diffusion ACS Paragon Plus Environment

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growth is proposed to explain the formation of bilayer graphene. Following H2O injection, the growth of the top layer graphene is delayed, during which, the nucleation and growth of the second layer is promoted; both these processes originate from H2O etching at graphene edges. We believe that our work will pave the way to the largescale synthesis of bilayer graphene for industrial applications and will also contribute to understanding the growth mechanism of CVD-derived bi- and multi-layer graphene on Cu.


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4. Experimental Section 4.1 Graphene Synthesis. H2O-assisted CVD process was used for the growth of bi- or multilayer graphene, in which, CH4 and H2 served as carbon source and reducing gas, respectively. Cu foils (Alfa Aesar, 99.8% purity, 25 m thick) were treated using a previously reported method,42 after which, the one end of the Cu foil was pressed and held in place by two flat quartz plates, and then inserted into a hot wall tube furnace (OTF-1200x, Hefei Kejing Co., Ltd). The reaction chamber was evacuated to ~ 0.1 Pa, and for a typical growth process, the substrate was heated to 1040 ºC at the rate of 25 ºC/min under vacuum and then annealed at this temperature under 10 – 50 sccm of H2 (pressure varying from 16 – 67 Pa) for 30 min. Before graphene growth, H2O pulse with partial pressure of 6.5 Pa and pulse interval of 10 s was injected into the reactor for 2 min to mildly oxidize the growth substrate and passivate the catalyst, so as to reduce the nucleation density of graphene.33 Graphene growth was typically achieved at 1040 ºC under a flow of 50 sccm H2 and 0.2 – 2 sccm CH4 for 5 – 30 min, during which, appropriate number of H2O pulses were injected. After growth, the pulse valve and furnace were turned off, and the substrate was cooled to room temperature under H2 atmosphere. 4.2 Graphene Transfer. A thin supporting polymer film (Sigma Aldrich 182265, PMMA, 30 mg mL-1 in chlorobenzene) was spin-coated (3000 rpm, 40 s) on the freshly prepared graphene on the Cu substrate. The PMMA/graphene/Cu sheet was floated on the etchant ((NH4)2S2O8, 0.5 M) for more than 4 h to thoroughly remove Cu catalyst. Next, the PMMA/graphene film was rinsed several times with DI water to wash off chemical residues and transferred onto target substrates such as SiO2 (285 nm)/Si wafers and TEM grids. Finally, the supporting PMMA film was removed by hot acetone. 4.3 Graphene Device Fabrication and Electrical Measurements. SiO2/Si substrates covered with bilayer graphene samples (obtained using PMMA transfer method) were annealed in vacuum at ~ 500 °C for 6h using a tube furnace to remove the absorbed molecules and impurities and then employed to fabricate devices. The sourcedrain electrodes (10/25 nm Ti/Au) were patterned on the sample by electron-beam lithography and thermal deposition, and each of hall bar devices has a transport channel of 2.5 m in length and 2 m in width. For electrical measurement, both the Ti/Au electrodes and the p++ Si back gate were wire bonded using Al wires. 4.4 Characterization. Optical microscopy (RX50M Nikon Eclipse LV100ND, Japan) was used to identify wrinkles, domain sizes, coverage, as well as layer number, for graphene transferred onto SiO2/Si wafers. SEM images of graphene samples were obtained with a JSM-2100F (JEOL Ltd.) operated at 10.0 kV and TEM was ACS Paragon Plus Environment

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performed using a JEM-ARM200F (JEOL Ltd.) at an accelerating voltage of 200 kV. STM measurements were carried out in a home-built UHV STM system at 298 K with base pressure < 1E-10 Torr. Raman spectra of graphene on SiO2/Si substrate were measured using Renishaw inVia spectrometer with a 532 nm laser and 50× objective lens. UV-vis spectra of graphene on SiO2 were collected on a Shimadzu Solid 3700 spectrometer. Sheet resistance of graphene films (2 cm × 2 cm) on PET substrates were measured by four-probe method using Keithley SCS 4200 semiconductor characterization system. The transport characterization of graphene device was carried out by using Keithley 2612B SourceMeter for FET measurement, Keithley 6221 DC and AC Current Source and Model SR830 DSP Lock-In Amplifier for Hall measurement in vacuum (2 Torr) at temperatures, T, from 2 to 300 K and in magnetic ﬁelds, B, up to 9 T.


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ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: Details of method and additional figures (PDF) AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Notes Authors declare no competing financial interests. ACKNOWLEDGEMENTS We appreciate funding support from the Natural Science Foundation of China (21373197), support from the 100 Talents Program of the Chinese Academy of Sciences, USTC Startup, Fundamental Research Funds for the Central Universities (WK2060140003), and iChEM. X.H.K. thanks the Natural Science Foundation of China (21503064) and the Anhui Provincial Natural Science Foundation for financial support (Grant 1508085QE103).


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