A Ternary Alloy Substrate to Synthesize Monolayer Graphene with

Jan 13, 2017 - Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dali...
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A Ternary Alloy Substrate to Synthesize Monolayer Graphene with Liquid Carbon Precursor Wei Gan,†,# Nannan Han,‡,# Chao Yang,†,# Peng Wu,† Qin Liu,† Wen Zhu,† Shuangming Chen,† Chuanqiang Wu,† Muhammad Habib,† Yuan Sang,† Zahir Muhammad,† Jijun Zhao,*,‡ and Li Song*,† †

National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230029, PR China ‡ Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, PR China S Supporting Information *

ABSTRACT: Here we demonstrate a ternary Cu2NiZn alloy substrate for controllably synthesizing monolayer graphene using a liquid carbon precursor cyclohexane via a facile CVD route. In contrast with elemental metal or bimetal substrates, the alloy-induced synergistic effects that provide an ideal metallic platform for much easier dehydrogenation of hydrocarbon molecules, more reasonable strength of adsorption energy of carbon monomer on surface and lower formation energies of carbon chains, largely renders the success growth of monolayer graphene with higher electrical mobility and lower defects. The growth mechanism is systemically investigated by our DFT calculations. This study provides a selective route for realizing high-quality graphene monolayer via a scalable synthetic method by using economic liquid carbon supplies and multialloy metal substrates. KEYWORDS: graphene, chemical vapor deposition, alloy, growth mechanism, density functional theory

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resulting in randomly stacked nonuniform multilayers. Interestingly, taking the advantages of both Cu and Ni, Cu− Ni binary alloy has been recently used as a catalytic substrate for graphene growth. By optimizing the Cu−Ni alloy component, inch-sized single-crystalline graphene and bilayer graphene were synthesized via the suppressing nucleation mechanism on the Cu−Ni substrates.11 Many other binary alloys, such as Ni−Au,12 Ni−Mo13 and Ni−Ti,14 have also been used as metallic catalysts for graphene synthesis. In spite of those significant achievements in the synthesis of graphene on the binary alloys, the synthetic routes used are high cost or impossible to scale up, which poses another great challenge for graphene’s applications in real optical and electrical devices. Therefore, it is still highly desirable to develop suitable substrates for producing high-quality graphene via a scalable synthetic method by using economic and environmentally friendly carbon supplies and ideal substrates. Notably, liquid carbon sources represent one of the economic and environmental precursors for CVD-grown

raphene, a stable two-dimensional (2D) material built of a single layer of sp2-hybridized carbon atoms, has attracted wide interest for promising applications in the fields of electronics, photonics and optoelectronics.1−4 However, such bright future would not be fully realized before large-scale graphene with high quality is available at low cost. Therefore, significant effort has been devoted to preparing graphene since the first monolayer graphene was mechanically exfoliated from bulk graphite.5 Unfortunately, the mechanical exfoliation method, which can provide high-quality single-layer graphene, is time-consuming and unsuitable for device applications. So far, chemical vapor deposition (CVD) is generally considered as one of the best methods to synthesize large-area, high-quality graphene films at low cost. In principle, the quality and performances of CVD-grown graphene is largely correlated to the growth conditions, such as substrate, carbon source, temperature and pressure.6−8 For example, single-layer graphene was grown on Cu foil using methane via low-pressure CVD, which can be explained by the surfacemedicated mechanism owing to the negligible carbon solubility.9 In contrast to Cu, Ni substrate shows more superior catalytic activity to boost carbon supply from the carbon precursors.10 In addition, the graphene growth on Ni is mainly cooling-induced segregation of the dissolved carbon atoms, © 2017 American Chemical Society

Received: September 11, 2016 Accepted: January 13, 2017 Published: January 13, 2017 1371

DOI: 10.1021/acsnano.6b06144 ACS Nano 2017, 11, 1371−1379

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Figure 1. Tophology analysis and Raman spectroscopy of as-obtained graphene samples. (a−c) Optical microscope images of graphene transferred onto SiO2/Si substrate. (d−f) Raman spectrum and (g−i) Raman mapping images of I2D/IG for the graphene samples grown on Cu, CuNi and Cu2NiZn substrates.

used as the growth substrate (pure Cu and CuNi alloy were also used as substrates for comparison). The detailed material synthesis and characterizations were described in the Methods and Figure S1. Optical microscopy was employed to analyze the topology of the prepared samples. Figure 1a−c shows the optical images of transferred-graphene samples synthesized on Cu, CuNi alloy and Cu2NiZn alloy at 1000 °C for 5 min. The graphene films can be clearly visualized by transferred on Si substrate (with 300 nm thick SiO2) due to its light interference. It is noteworthy that the color contrast for the graphene grown on CuNi alloy is highly inhomogeneous, while those on Cu and Cu2NiZn alloy exhibit homogeneous color, indicating the better uniformity and continuity of CVD-graphene by using Cu and Cu2NiZn substrates. Raman spectroscopy was employed to characterize the obtained graphene samples, as shown in Figure 1d−f. Three main Raman peaks at around 1350, 1600, and 2700 cm−1 are observed, which correspond to the D, G and 2D modes of graphene, respectively.17 Obviously, the intensity of G peak for both graphene/Cu and graphene/CuNi is much higher than that of 2D peak (Figure 1d and 1e), indicating a typical multilayer feature. On the contrary, the Raman spectrum of graphene/Cu2NiZn in Figure 1f exhibits much higher intensity of 2D peak than G peak (∼4.8 I2D/IG). In addition, the 2D mode shows a symmetric peak centered at ∼2678 cm−1 with a full width at half-maximum of ∼33 cm−1. This strongly suggests a typical feature of monolayer graphene for the samples grown on the Cu2NiZn alloy substrate.18 Besides, the absence of D

graphene due to their low price and high security as compared to gaseous sources,15,16 i.e., methane, ethylene or propylene. However, the graphene growth with liquid carbon sources is still inferior to the noncontrollable nucleation and the nonuniform thickness due to the fast decomposition process and unstable carbon concentration. A facile approach to address the above issues is to nucleate carbon atoms in a controlled manner to form the sp2-hybridized carbon network on a rationally designed substrate. Herein, we demonstrate a ternary alloy substrate to controllably synthesize high-quality and largescale monolayer graphene using a liquid carbon precursor cyclohexane (C6H12) via a facile CVD route. As revealed by first-principles calculations, the success of graphene growth on the ternary Cu−Ni−Zn alloy substrate is mainly attributed to much easier dehydrogenation of hydrocarbon molecules, more optimized adsorbed-positions of carbon monomer, and lower formation energies of carbon chains in contrast with elemental or bimetal substrates. This study will provide a facile way to prepare high-quality monolayer graphene for many specific applications, such as large-scale electronics, optics and flexible devices.

RESULTS AND DISCUSSION For our CVD growth, a commercial liquid cyclohexane (C6H12) was selected as a typical liquid carbon source, which can be easily tuned the decomposition rate and the carbon concentration by adjusting temperature and pressure in the reaction. A ternary Cu2NiZn alloy with simple treatment was 1372

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Figure 2. Evaluation of the unformity of the graphene samples. (a) Optical transmittance measurements of graphene films transferred to quartz substrate. The transmittance at 550 nm is about 90.1%, 92.7% and 97.3% for Cu, CuNi and Cu2NiZn substrates, respectively. (b) AFM image and profile analysis of the graphene grown on Cu2NiZn alloy.

peak in Figure 1f indicates the high quality of graphene with much lower defect density as compared to the graphene grown on Cu substrate (Figure 1d). To further confirm the uniformity, we performed I2D/IG Raman mapping on the samples, as shown in Figure 1g−i. In contrast to the large color range recorded on graphene/CuNi (Figure 1h), Figure 1i reveals that most areas over the graphene/Cu2NiZn sample exhibit very homogeneous color distribution, reflecting the uniform layer distribution of monolayer graphene. On the other hand, Figure 1g suggests the feature of uniform multilayer graphene synthesized on Cu foil.19 Meanwhile, Raman characterizations were used to compare the CVD graphene with different growth time (Figure S2). We chose Cu and Cu2NiZn substrates because of the uniformity of graphene samples. With increase in growth time, there is a gradual blue shift in the 2D band and an obvious increase in the intensity ratio of G to 2D peak (IG/I2D) on the graphene/Cu samples, indicating the increase of layer numbers. On the contrary, the Raman spectrum of graphene/Cu2NiZn keeps low IG/I2D ratio with negligible shift of 2D peak. On the basis of our experimental observation, monolayer graphene can quickly cover the surface of Cu2NiZn alloy in 3 min.20 More interestingly, the number of graphene layers on the ternary alloy does not increase with the growth time increasing. Optical transmittance measurements were performed on the inch-sized graphene which was transferred on quartz substrate.21 Figure 2a shows transmittance of the graphene samples grown on various substrates for 5 min. It is estimated that the transmittance at 550 nm is around 92.7% for graphene/Cu and 90.1% for graphene/CuNi sample, respectively. While the value increases to 97.3% for graphene grown on Cu2NiZn substrate, strongly demonstrating the feature of monolayer graphene. Atomic force microscope (AFM) analysis further confirms the thickness of 0.55 nm (approximately a monolayer) for the graphene/Cu2NiZn samples, as shown in Figure 2b. The obtained graphene was transferred to lacey carboncoated grids for transmission electron microscopy (TEM) analysis (Figure 3). The high-resolution TEM (HRTEM) image in Figure 3a confirms that the graphene grown on Cu2NiZn alloy is monolayer, which is distinct with the multilayers grown on Cu or CuNi substrates. Figure 3b shows the typical selected area electron diffraction (SAED) patterns recorded on the graphene/Cu2NiZn samples, revealing

Figure 3. Confirming the distinctive hexagonal structure of the asgrown graphene samples (a) TEM image (upper) and the corresponding SAED patterns (down) of graphene grown on the ternary Cu2NiZn alloy substrate, as compared with those grown on (b) Cu and (c) CuNi substrates.

the distinctive hexagonal structure of monolayer graphene.22 On the contrary, the graphene films grown on Cu and CuNi are proved to be multilayer with more defects, as shown in Figure 3c−f. In short, the TEM observations confirm that graphene synthesized on Cu2NiZn is a monolayer crystal with highquality (more SAED sets were shown in Figure S3), in agreement with the above Raman results. The electrical properties of graphene films grown on Cu2NiZn alloy were also investigated, compared with the samples grown on Cu or CuNi substrates. The graphene fieldeffect transistors (FETs) with back-gate were fabricated on highly n-doped silicon substrate with a 300 nm thick SiO2 layer, which is schematically shown in Figure 4a. More fabrication details can be found in the experimental part and Figure S4. Figure 4b shows a typical optical image of the as-made graphene FET device. The homogeneous color contrast in the image implies that the graphene channel is very flat and uniform. Figure 4c reveals the output Ids−Vds curve of the FET device with graphene/Cu2NiZn. The linearity of the curve indicates ohmic contact between the metal electrodes and graphene.23 Besides, the graphene FET exhibits strong gatetunable Ids−Vds characteristics under various gate voltage (Vg), ranging from −30 to 30 V. The slope of the Ids−Vds curve decreases as the gate voltage increases from −30 to 20 V. But when the gate voltage further increases to Vg = 30 V, the slope slowly increases and the amplitude is almost the same as Vg = 1373

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Figure 4. FET based on graphene grown on Cu2NiZn alloy. (a) Schematic of the device based on graphene. (b) Optical image of the as-made device. (c) Output curve of the device at different back-gate voltages. The linear curves show good ohmic contacts. (d) Transfer characteristics of the as-made device.

To gain atomistic insights into the growth behavior of graphene on Cu2NiZn (111) and Cu (111) surfaces, we simulated the dehydrogenation of carbon source and nucleation processes using density functional theory (DFT) calculations. On the basis of the XRD pattern of the Cu2NiZn alloy (Figure S6), we can see that the crystal surface of Cu2NiZn foil is (111). Thus, the Cu2NiZn (111) alloy surface was mimicked by a slab model with random distribution of metal elements, and the same-sized slab model was also considered for Cu (111) (Figure S7). Note that, the distribution of Cu, Ni and Zn atoms in the supercell model of Cu2NiZn (111) surface has only negligible effect on graphene as shown in Figure S7 of Supporting Information. After optimization, the equilibrium distances (h) between monolayer graphene and Cu2NiZn (Cu) surfaces is 3.45 Å (3.33 Å), and the corresponding binding energy is −0.07 eV/atom (−0.08 eV/atom), suggesting that the graphene sheet interacts weakly with both Cu2NiZn and Cu substrates. In typical CVD synthesis of graphene,30−32 the first stage is the dehydrogenation process of hydrocarbon molecules (e.g., CH4 and C2H4) on the substrates, where the catalysis activity of substrates is significant. The second stage is the nucleation of graphene, in which carbon clusters with different size and shape are formed on substrate. The last stage is the continuous growth of nucleus. In this stage, the migration of carbon atoms is crucial. On the basis of the above processes, we systematically studied the advantages of Cu2NiZn substrate over Cu to synthesize graphene. In our experiment, liquid cyclohexane was taken as the carbon source. According to the mass spectra analysis and previous data (Table S1 and Figure S8), the dominant species after heating is C2H4 (in fact, our control experiments showed that pure C2H4 was not suitable as pristine precursor for

10 V. A possible reason is that the graphene FET changes from p-type to n-type when Vg exceeds 20 V. From Ids−Vg curve in Figure 4d, we can clearly see that the charge neutrality point (CNP) is located at Vg ∼ 20 V, implying that graphene on Cu2NiZn is highly p-doped.24 This means that the graphene/Cu2NiZn will exhibit p-type at Vg < 20 V and n-type at Vg > 20 V, which is quite different from the exfoliated graphene with CNP at nearly Vg = 0 V.25 To rule out the possible influence from O2, H2O or other chemical species adsorbed on graphene, we pump the testing system to 2.1 × 10−3 Pa for more than 24 h; but we can hardly see the CNP shift toward Vg = 0. Hence, we suggest that this p-type characteristic originates from the CVD process, similar to previous reports.26 In contrast to the bipolar characteristic of the exfoliated graphene, graphene/Cu2NiZn FET exhibits very strong unipolar characteristic which is desperately required in a complementary metal oxide semiconductor (CMOS) logic circuit.27,28 On the basis of the transfer curve in Figure 4d, we can extract a field-effect mobility of μ ∼ 2300 cm2/(V s) using the formula:29 μ = [dIds/dVbg ] × [L /(WCgVds)]

where L = 5 μm is the channel length, W = 5 μm is the channel width and Cg = 1.15 × 10−4 F/m2 is the capacitance of SiO2 (Cg = ε0εr/dox; ε0 = 8.85 × 10−12 F/m; εr = 3.9 for SiO2; dox = 300 nm). For comparison, the field-effect mobility of graphene grown on Cu and CuNi alloy estimated from Figure S5 is 530 cm2/(V s) and 1113 cm2/(V s), respectively. Obviously, the electrical properties can be significantly enhanced by using ternary Cu2NiZn alloy as growth-substrate with regard to either Cu or CuNi substrate. 1374

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profile is presented in Figure 6. The final hydrocarbon product (e.g., Figure 5c) after the former dehydrogenation step was set as the initial species for the successive dehydrogenation step (e.g., Figure 5d), since the dissociated H atom has only little effect on the geometry of the species.33 From our DFT calculation, the dehydrogenation barriers of the four steps are 1.48, 0.49, 1.58 and 1.44 eV, respectively. Among them, the dehydrogenation barrier from C2H3 to C2H2 is particularly low, because that C2H2 is a molecule with close-shell electronic configuration and is more stable than C2H3 and C2H free radicals. To reveal the role of alloying elements (Ni and Zn), the fourstep dehydrogenation process of C2H4 on Cu (111) was also simulated (see Figure 6 and Figure S9) and the corresponding energy barriers are 1.49, 0.71, 1.70 and 1.20 eV, respectively. Note that the adsorption states of the dissolved H atom and the hydrocarbon species on Cu2NiZn (Figure 5) and Cu surfaces (Figure S9) are rather similar to each other. However, the dehydrogenation barriers on Cu2NiZn surface are lower than that on Cu (111) surface, implying an easier dehydrogenation of C2H4 due to alloying effect. As displayed in Figure 6, all dehydrogenation steps during the whole process are endothermic and the relative energies of every states on Cu2NiZn (111) surface are systematically lower than those on Cu (111) surface. Specially, the relative energy of final state (1.45 eV) on the alloy surface is much lower than that on Cu (111) of 2.49 eV, implying that carbon dimer adsorbed on alloy surface is more stable. Moreover, the largest reaction barrier in the third stage on Cu2NiZn surface is 0.12 eV lower than that on Cu surface. That is to say, Cu2NiZn substrate has stronger catalytic efficiency to dehydrogenate C2H4. After dehydrogenation, the retained C2Hx species are stabilized by metal passivation, which can be seen by the calculated binding energies of C2Hx on metal surfaces. As listed in Table 1, the

growing graphene monolayer in contrast to liquid cyclohexane, possibly because of its fast decomposition rate and uncontrollable carbon concentration). Here in order to simplify the simulation, we theoretically investigate the detailed dehydrogenation process of dominant species C2H4 on Cu2NiZn (111) surface. During the entire dehydrogenation process, there are three intermediates, i.e., C2H3, C2H2 and C2H. Then the final product is a C2 dimer. The initial, transitional and final configurations in the four dehydrogenation steps on Cu2NiZn surface are shown in Figure 5 and the corresponding energy

Table 1. Binding Energies (eV) of C2Hx (x = 0−4) on Cu2NiZn (111) and Cu (111) Substrates

Figure 5. Dehydrogenation processes of C2H4 on Cu2NiZn (111) surface. (a−c), (d−f), (g−i) and (j−l) correspond to the four steps of reaction, respectively. The blue, red, green, gray and white balls represent Ni, Cu, Zn, C and H atoms, respectively.

Ealloy ECu

C2H4

C2H3

C2H2

C2 H

C2

1.200 0.737

4.074 3.529

4.761 4.748

5.518 4.800

7.476 7.438

binding energies of C2Hx on Cu2NiZn surface increase with less number of hydrogen atoms (x) and are larger than those on Cu (111) surface at every states. Again, such stronger interaction between C2Hx and Cu2NiZn surface facilitates the dehydrogenation. To further elucidate the effect of alloying elements of Ni and Zn, we analyzed the adsorption interaction of a carbon monomer on Cu (111) and Cu2NiZn (111) surfaces in terms of charge density difference and formation energy of carbon monomer, which is defined as Eform = Etot − Esub − EC

where Etot, Esub and EC are energies of the total system, the substrates and one carbon atom, respectively. As shown in Figure 7a−c, the strengths of charge transfer between carbon monomer and different pure metal surfaces are distinct, that is, slightly stronger with Ni than that with Cu, and negligible with Zn. Consistent trend of Eform on pure Zn, Cu and Ni surfaces is obtained: 0.68 eV on Ni (111) < 2.68 eV on Cu (111) < 3.31 eV on Zn (0001). In the case of Cu2NiZn (111) surface, we considered three possible adsorption environments for the

Figure 6. Energy profile of the dehydrogenation processes of C2H4 on Cu (111) (black lines) and Cu2NiZn (111) surfaces (blue lines). The initial, transitional and final states marked by purple letters correspond to the atomic structures in Figure 5 and Figure S7. 1375

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exploited by considering chain and sp2 network configurations composed of pentagons and hexagons. The considered carbon cluster isomers are summarized in Figure S11, and Figure 8

Figure 7. Charge density difference between carbon monomer and different substrates: (a) pure Ni (111); (b) pure Cu (111); (c) pure Zn (0001) (d) Cu2NiZn (111) alloy; (e) Cu2NiZn (111) alloy with one Zn atom in the top layer replaced by Ni atom; (f) Cu2NiZn (111) alloy with one Ni atom in the top layer replaced by Zn atom. Atoms in the corners of the black triangles are in the top layer. The blue, red, green and gray balls represent Ni, Cu, Zn and C atoms, respectively. The isosurface is 0.015 e/Å3.

Figure 8. Formation energies of carbon chains (black line) and networks (red line) on Cu2NiZn (111) surface with different sizes. As representative cluster structures, the inset figures show the atomic structures of chains with three and seven carbon atoms respectively, and sp2 network with 15 carbon atoms.

carbon monomer, that is, the three nearest neighboring metal atoms on the top layer being Cu−Ni−Zn, Cu−Ni−Ni, Cu− Zn−Zn, as displayed in Figure 7d−f. Again, Ni atoms have noticeable charge transfer with carbon monomer, while almost no charge transfer is found for Zn atoms. Accordingly, formation energy for the carbon monomer on the slab model of Figure 7d, and 7e, and f are 1.39, 0.93 and 2.05 eV, respectively. Clearly, Eform strongly relies on the environment of carbon monomer and reduces with the increasing number of Ni atoms. As a consequence of such alloying effect, the calculated Eform on ternary alloy lies between those on Ni (111) and Cu (111). To demonstrate the advantage of ternary alloy substrate with inclusion of Zn atoms as compared to CuNi binary alloy, migration of C monomer from outer surface to subsurface on Cu/CuNi/Cu2NiZn substrates are investigated by comparative DFT calculations, and the results are presented in Figure S10 of Supporting Information. Considering all possible surface sites for C monomer, we find that C monomer prefers to stay on the outer surface after relaxation in the cases of Cu and CuNi substrates. However, it would penetrate into the subsurface without an energy barrier at some sites on Cu2NiZn substrate. The easy migration of C monomer, caused by alloying Zn atoms, facilitates the solution of carbon atoms in substrate, which is helpful for the growth of graphene since graphene is formed by the separation of the solute carbon atoms.32 As demonstrated by previous experiments, the high carbon solubility and the strong interaction between carbon and Ni substrate lead to the growth of multilayer graphene.34−36 On the other hand, the weak interaction between carbon and Cu substrate makes it hard to control the experimental conditions for achieving large-size and uniform monolayer graphene. Hence, the present Cu2NiZn substrate with moderate strength of adsorption interaction is an ideal compromise and suitable for the high-quality growth of monolayer graphene. To understand the second stage of graphene nucleation, the metal-supported carbon clusters with up to 15 atoms were

shows the formation energies of CN clusters on Cu2NiZn surface. The most stable configuration of CN clusters switches from one-dimensional (1D) chains to two-dimensional (2D) networks at a critical size of N = 13, which is close to the critical size of N = 12 on Ni (111) reported by Gao et al.37 as well as that on Cu (111) from the present calculations (Figure S12). As displayed in Figure S13, the formation energy of very small CN clusters (N ≤ 3) on Cu2NiZn surface is rather lower than that on Cu (111) surface. For larger clusters with more than four atoms, the formation energies on the two substrates become comparable. Therefore, at the early stage of nucleation, the formation of small carbon clusters is easier on Cu2NiZn substrate than that on Cu substrate. After nucleation, the continuous growth of graphene nucleus is investigated in aspect of the migration barrier of carbon atom by climbing nudged elastic band (CI-NEB) method.38 The migration paths are given in Figure S14. A typical migration barrier on Cu2NiZn substrate is calculated to be 0.3 eV, while it is 0.1 eV on Cu surface. Clearly, the alloying Ni and Zn atoms increase the diffusion barrier and reduces the mobility of carbon monomer on surface. This lower mobility provides carbon atoms better chance to reach the stable positions, which in turn results in the growth of high-quality graphene with fewer defects.

CONCLUSION In conclusion, we have demonstrated a ternary alloy to synthesize monolayer graphene with large-scale and high crystalline via a low-cost CVD route by using liquid carbon source. The Cu2NiZn alloy surface with moderate adsorption interaction of carbon species exhibits a high efficiency for quickly catalyzing C2H4, easy formation tendency of smaller carbon clusters, and low mobility of carbon atoms, which can prevent the agglomeration of large carbon clusters from forming inhomogeneous graphene multilayers. Our study provides a low-cost route to selectively synthesize monolayer 1376

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and 2.57 Å, respectively. To search the transitional states of dehydrogenation, CI-NEB method was employed.

graphene in large scale for further exploring its potential applications for electronics, optics, and so on.

ASSOCIATED CONTENT

METHODS

S Supporting Information *

CVD Growth of Graphene. Graphene was grown on 25-μm thick Cu foils, CuNi foils (the atomic ratio is 1:1) and Cu2NiZn foils (to uniformly alloy Zn atoms, a commercial CuNi foil was carefully deposited an around 5 nm Zn thin layer, and then annealed at 900 °C for 2 h under H2/Ar flow) in a low pressure CVD system. A typical growth procedure is as follows. The foils (2 × 2 cm2) were washed by HCl/H2O (1:10), triple rinsed by isopropyl alcohol and dried by N2 blow. The dried foils were loaded into a 5 cm-diameter CVD fused quartz tube, then the system was pumped down to a vacuum of 30 mTorr in 30 min and refilled with 200 sccm H2/Ar mixture (15% H2 balanced in Ar). We heated the substrates to 1000 °C in 60 min and annealed for 30 min under the protection of H2/Ar. Then, the cyclohexane steam was introduced into the CVD system for the graphene growth in 1000 °C with a pressure of 300−600 mTorr without H2/Ar. The growth procedure kept 3−20 min, and then the substrates were quickly pulled out of the high-temperature zone to cool down to room temperature under H2/Ar protection. Transfer Process. The process of transferring the graphene to the target substrates involves spin coating a poly(methyl methacrylate) (PMMA) film onto the graphene-grown substrates. The alloy foils were etched away in HNO3/H2O (1:4), resulting in a free-standing PMMA/graphene membrane floating on the surface of the etchant bath. Then the PMMA/graphene film was transferred onto the target substrates (for example, SiO2/Si). After air drying, the PMMA layer was dissolved with acetone and the substrate was rinsed with isopropyl alcohol to yield a graphene film on the substrate. Characterizations. JEM-2100F field emission transmission electron microscopy (TEM) with an acceleration voltage of 200 kV was used to collect high-resolution TEM images. UV−vis−NIR absorption spectra were recorded with a PerkinElmer Lambda 950 UV−vis−NIR spectrophotometer. Raman spectra were detected by a Renishaw RM3000 Micro-Raman system with a 514.5 nm Ar laser. Atomic force microscopy (AFM, Veeco Multimode V) was performed at room temperature. Device Fabrications and Electrical Measurements. In order to determine the electronic properties of graphene grown on different substrates (Cu, CuNi, Cu2NiZn) by CVD, we fabricated several backgated graphene FETs on highly n-doped silicon substrate with a 300 nm thick SiO2 layer. The fabrication process began with electron-beam lithography (EBL) and reactive ion etching (RIE) to predefine the CVD graphene into 9 × 5 μm2 square. The 10 nm/100 nm Ti/Au was deposited as electrodes. After all the fabrication process, the devices were annealed at 200 °C under the 50 sccm of Ar to remove the impurities as well as improve the contact between the metal electrodes and graphene. The electrical measurements were performed on a probe station equipped with a vacuum pump and a semiconductor characterization system (combined two Keithley 2450). More than 50 devices were tested for calculating the mobility. Theoretical Modeling and Calculations. Density functional theory (DFT)39 calculation was performed by using Vienna ab initio simulation package (VASP). To describe the ion-electron interactions, the projector augmented wave (PAW)40 potentials were adopted. The exchange-correlation interactions were described by the generalized gradient approximation (GGA)41 parametrized by Perdew, Burke and Ernzerhof (PBE). To account for the van der Waals (vdW) interactions, we employed the dispersion-corrected DFT-D342 scheme. The kinetic energy cutoff for planewave basis was set to 500 eV. The Brillouin zones of the supercells were sampled by uniform k-point meshes with spacing of 0.03/Å. All atomic structures were fully optimized until the forces were smaller than 0.02 eV/Å and energy change smaller than 10−4 eV. A three-layer slab model with the vacuum region larger than 15 Å was used to simulate the metal surface. During the calculation, the bottom layer of the metal slab was fixed to mimic a semi-infinite solid. We adopted the experimental in-plane lattice parameters of Cu (111) and Cu2NiZn (111) surfaces, i.e., 2.55

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06144. CVD growth procedure, Raman results, SAED data, Device fabrication process, FET results of graphene on Cu and CuNi, XRD pattern of Cu2NiZn substrate, Atomic structure of graphene on Cu2NiZn and Cu substrate, Mass spectra for cyclohexane decomposition, Dehydrogenation processes of C2H4 on Cu substrate, Situations for the migration of C monomer from outer surface to subsurface on different substrates, Atomic configurations of CN clusters on Cu2NiZn and Cu surfaces, Formation energies of carbon chains and networks on Cu surface with different sizes, Formation energies of the most stable CN clusters on Cu2NiZn and Cu surfaces, Migration paths and energy barriers of carbon monomer on Cu2NiZn and Cu surfaces, Mass spectra analysis data for cyclohexane decomposition (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Li Song: 0000-0003-0585-8519 Author Contributions #

W.G., N.N.H. and C.Y. contributed equally to this work. L.S. designed the experiments. W.G. and C.Y. performed the experiments. N.N.H. and J.J.Z. performed the simulations. P.W., Q.L., W.Z., S.M.C., C.Q.W., M.H., Y.S. and Z.M. partially contributed to sample preparations and TEM characterizations. W.G., N.N.H., J.J.Z. and L.S. analyzed the data and cowrote the paper. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the 973 Program (No. 2014CB848900), NSFC (No. U1532112, 11375198, 11574280, 11134005, 11574040) and the Fundamental Research Funds for the Central Univeristies of China (DUT16-LAB01, WK2310000053). L.S. thanks the recruitment program of global experts, the CAS Hundred Talent Program. We thank Supercomputing Center of Dalian University of Technology for computing resource, Mass Spectrometry endstations at NSRL and the helps from the USTC Center for Micro and Nanoscale Research and Fabrication. REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Banszerus, L.; Schmitz, M.; Engels, S.; Goldsche, M.; Watanabe, K.; Taniguchi, T.; Beschoten, B.; Stampfer, C. Ballistic Transport Exceeding 28 μm in CVD Grown Graphene. Nano Lett. 2016, 16, 1387−1391. 1377

DOI: 10.1021/acsnano.6b06144 ACS Nano 2017, 11, 1371−1379

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DOI: 10.1021/acsnano.6b06144 ACS Nano 2017, 11, 1371−1379