Surface Engineering of Copper Foils for Growing Centimeter-Sized

Feb 1, 2016 - ... states by bend resistance measurements in graphene cross junctions. J Du , J Y Li , N Kang , Li Lin , Hailin Peng , Zhongfan Liu , H...
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Surface Engineering of Copper Foils for Growing Centimeter-Sized Single-Crystalline Graphene Li Lin,†,⊥ Jiayu Li,‡,∥,⊥ Huaying Ren,†,∥ Ai Leen Koh,§ Ning Kang,‡ Hailin Peng,*,† H. Q. Xu,*,‡ and Zhongfan Liu*,† †

Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China § Stanford Nano Shared Facilities, Stanford University, Stanford, California 94305, United States ∥ Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The controlled growth of high-quality graphene on a large scale is of central importance for applications in electronics and optoelectronics. To minimize the adverse impacts of grain boundaries in large-area polycrystalline graphene, the synthesis of large single crystals of monolayer graphene is one of the key challenges for graphene production. Here, we develop a facile surfaceengineering method to grow large single-crystalline monolayer graphene by the passivation of the active sites and the control of graphene nucleation on copper surface using the melamine pretreatment. Centimeter-sized hexagonal single-crystal graphene domains were successfully grown, which exhibit ultrahigh carrier mobilities exceeding 25 000 cm2 V−1 s−1 and quantum Hall effects on SiO2 substrates. The underlying mechanism of melamine pretreatments were systematically investigated through elemental analyses of copper surface in the growth process of large single-crystals. This present work provides a surface design of a catalytic substrate for the controlled growth of large-area graphene single crystals. KEYWORDS: surface engineering, large single-crystal graphene, passivation, active sites graphene nucleation.18−21 In terms of carbon supply, the utilization of argon-diluted carbon source and high ratio of hydrogen to methane was proven to effectively reduce the number of nucleation seed.22 On the other hand, recent works reveal the strong correlation of copper surface morphology with the high density of active sites, such as atomic steps and point defects and copper grain boundaries (Cu-GBs), commonly seen in commercial polycrystalline copper foil.20 Consequently, great efforts have been concentrated on reducing the number of active sites by minimizing the impurity or defect density on copper foils. Numerous methods, such as electrochemical polishing and longtime annealing at high temperature, have been employed for producing a flat copper surface and thus growing large single-crystal graphene.22,23 In addition, millimeter-sized single-crystal graphene has been successfully grown

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he main challenge for large scale integration of graphene into electronics and optoelectronics is the scalable production of large-area and high-quality graphene.1−7 Inspired by previous works,8,9 chemical vapor deposition (CVD) method has been successfully developed for large-area growth of high-quality graphene on transition metals, especially copper foil.10−12 However, the CVD-grown graphene exhibits a polycrystalline film with the typical grain size of a few tens of micrometers and thus contains a high density of grain boundaries.13,14 In comparison with mechanically exfoliated graphene, the mechanical and electronic properties of CVDgrown graphene are potentially degraded across the grain boundaries.15−17 Thus, the synthesis of large-area graphene without any grain boundary, that is, single-crystal graphene, is highly desirable. The key point to the growth of large-area graphene single crystals is to suppress the nucleation density of graphene grain and to increase its grain size. The nucleation density of graphene is highly dependent on two important parameters, the carbon supply for graphene growth and the active sites for © XXXX American Chemical Society

Received: January 4, 2016 Accepted: February 1, 2016

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DOI: 10.1021/acsnano.6b00041 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Growth of large single-crystal graphene on Cu surface with melamine pretreatments. (a) Schematic illustration of active-sitespassivated growth strategy to suppress the nucleation density of graphene grains, demonstrating that the accumulation of carbon−nitrogen complex at Cu-GBs is the key to suppress the nucleation density. Growth behavior of graphene on untreated copper and pretreated copper are illustrated in left and right one, respectively. (b−d) SEM images of graphene domains formed on untreated Cu foil (b) and (c), and pretreated Cu foil (d). The H2:CH4 ratio is set to 200 for (b), and 800 for (c) and (d), respectively. The inset in (b) is the high-magnification SEM image of (b). (e) Nucleation preference of graphene on untreated Cu foil (black) and on pretreated Cu foil (red) as a function of H2:CH4 ratio. (f) Graphene nucleation density as a function of H2:CH4 ratio on untreated Cu foil (black) and on pretreated Cu foil (red). The pretreatment time of melamine is set to 10 min. (g) Optical image of large single-crystal graphene on Cu foil after exposed to O2. (h) Optical image of centimeter-sized graphene domains on Si/SiO2 substrate. (i) Contrast-enhanced photograph of wafer-sized, continuous monolayer graphene film on 4 in. Si/SiO2 substrate.

passivation of active sites on copper surface. Notably, the commercial polycrystalline copper foils are used for growing graphene in our paper, which is easy for batch production from industrial viewpoints. Besides atomistic steps, point defects on copper foils, the surface of copper foil consists of different grain orientations and thus has massive grain boundaries.10 Thus, the Cu-GBs would be taken as a representative kind of active sties on copper surface in the following discussion. In the common CVD growth of graphene, these active sites serve as nucleation center of graphene grains, exhibiting a higher density of graphene nuclei than that on flat surfaces owning to its lower nucleation barrier.20,33,34 Thus, to passivate these active centers, melamine was first introduced onto the polycrystalline copper foil before the initiation of graphene growth, through sublimation and subsequent transportation of melamine (Figure S1). The active Cu-GBs would catalyze the decomposition of melamine and the subsequent formation of carbon- and nitrogen-containing species at the Cu-GBs. The existence of these compounds would passivate the Cu-GBs, making it inactive in the following graphene nucleation stage. Consequently, graphene can only nucleate on the flat Cu surface rather than Cu-GBs, where the nucleation barrier is particularly high, leading to an overall suppressed nucleation density. The scanning electron microscopy (SEM) images of graphene samples synthesized at different hydrogen-to-methane (H2:CH4) ratios, shown in Figure 1b and Figure 1c, clearly reveal a high nucleation density and accumulation of graphene nuclei at the Cu-GBs, which are commonly observed in

on the smooth copper surface by using the enclosure or tube structure of copper.24,25 However, all the above methods need complex or careful treatment processes of the copper surface, and unlikely to be scalable, as a roll-to-roll process, for largescale growth of graphene in regard to commercial considerations. Recently, centimeter-sized single-crystal graphene was synthesized on the oxygen-rich copper,26 in which oxygen is supposed to passivate active sites of copper surface, but the mechanism of the suppressed graphene nucleation density still remains unclear.27−30 As we know, melamine has been widely used to synthesize C3N4 or other carbon- and nitrogencontaining species.31 When the strong interaction between nitrogen and copper is considered, melamine containing rich nitrogen atoms can be easily assembled on copper surface.32 Herein, we report a convenient method to achieve the growth of large single crystals of graphene, by passivating the active sites on copper surface with melamine. The domain size of the as-prepared single crystals of graphene with carrier mobility exceeding 25 000 cm2 V−1 s−1 can be as large as one centimeter. Our study provides a new surface design of copper substrate for deliberate control of graphene nucleation density and more importantly offers a convenient pathway for scalable growth of high-quality graphene with reduced boundaries, benefiting the large-scale integration of graphene-based electronic and optoelectronic devices.

RESULTS AND DISCUSSION Figure 1a illustrates the active-sites-passivated growth strategy of large-area graphene single crystals. This process relies on the B

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ACS Nano conventional CVD growth of graphene. Note that the H2:CH4 would determine the amount of active carbon for nucleation and growth of graphene on copper foil, which affects the nucleation density, by altering the competing processes between formation of graphene (pyrolysis of hydrocarbons and nucleation) and the etching effect of hydrogen on graphene.18,19 In fact, on one hand, graphene can nucleate at both the flat surface and the Cu-GBs, with a high carbon supply (low H2:CH4 ratio) suggesting a high possibility of nucleation at sites even with higher nucleation barrier. Remarkably, the accumulation of graphene nuclei at the Cu-GBs was usually observed, indicating a relatively large numbers of active sites there (Figure S3 and Figure S4). On the other hand, with lower carbon source, graphene prefers to nucleate at the Cu-grain boundary, in which nucleation barrier difference determines the possibility of nucleation at different sites on copper foil (Figure 1c and Figure S5). After the pretreatment of melamine, both a remarkable reduction of nucleation density and a sharp change of nucleation preference of graphene occur on the pretreated copper foil (Figure 1d and Figure S6). Due to the passivation of active sites, graphene only nucleates at the flat Cu surface, leading to the overall lower nucleation density (Figure S7). To statistically evaluate the effect of melamine pretreatment on the growth of graphene, the ratio of graphene nucleated on Cu-GBs to the total number of graphene nuclei as a function of H2:CH4 ratio is shown in Figure 1e, exhibiting a clear reduction of this percentage after the pretreatment, which is even more prominent at low carbon supply. Additionally, we varied the time of melamine pretreatment to further control the graphene nucleation. The time of melamine pretreatment was found to provide a tuning parameter for further suppressing the graphene nucleation density (Figure S9). In terms of the combination of the active sites passivation and the nucleation density control, we have achieved the growth of centimeter-sized single-crystal graphene on melamine-pretreated copper foil. The nucleation density can be reduced from 2 mm−2 to 5 × 10−3 mm−2 by increasing H2:CH4 ratio (Figure 1f, Figure S10 and Figure S11). The graphene nucleus can grow into large-area single crystal during the prolonged growth periods. Figure 1g shows the typical optical image of large-area single-crystal graphene on the surface of copper foil. The graphene domain became optically visible by quick heating the copper in air for 5 min at 200 °C after graphene growth. Moreover, centimeter-sized hexagonal singlecrystals of graphene were successfully transferred onto a SiO2/ Si substrate (Figure 1h), showing clearly an optical contrast. Several large-area single crystals of graphene can eventually coalesce into wafer-sized, continuous monolayer graphene film, which can be transferred onto the SiO2/Si substrate (Figure 1i). Transmission electron microscopy (TEM) and selective area electron diffraction (SAED) were conducted to confirm the single-crystal nature of the as-grown large graphene domains (around 1 mm), which are transferred onto the amorphous carbon-covered TEM grid. The SEM image of graphene on TEM grid shows that the graphene domain retained its structural integrity and exhibited the uniform thickness recognized from the contrast (Figure 2a and Figures S12a,b). SAED patterns were extensively analyzed, which are obtained from 20 individual regions across the whole domain (Figure 2b and Figures. S12d−h). The relative angle of graphene lattices extracted from the SAED patterns showed less than 1.5° rotation of the graphene lattice direction across the whole area,

Figure 2. Structural characterization of large single-crystal graphene domain. (a) SEM image of 1 mm graphene domain transferred onto an amorphous carbon-covered TEM grid. (b) Typical SAED patterns taken from different positions in (a). Inset: intensity profile of diffraction pattern along the red dashed line. (c) Histogram of angle distribution from extensive SAED patterns within the graphene domain. (d) Aberration-corrected highresolution TEM image of the graphene domain, showing a perfect carbon lattice with 6-fold symmetry without any structural defects.

indicating its single-crystal character (Figure 2c). The monolayer nature of the as-grown graphene is confirmed by analyzing line profiles of diffraction patterns (inset of Figure 2b). The atomic structure of the single-crystal graphene suspended on the lacey-carbon TEM grid was imaged with aberration-corrected and monochromated TEM. The 6-fold symmetry of the graphene lattice was typically observed without any detectable defects, vacancies, and dislocations (Figure 2d). Optical microscope (OM) image and Raman spectra indicated that the single crystals of graphene were mostly monolayer with only few multilayer domains inside and exhibited no detectable defect-related D band (Figure S13 and Figure S14).35 In addition, further XPS and Raman mapping characterization of as-grown graphene transferred onto Si/SiO2 exclude the possibility of incorporation of nitrogen atoms into graphene lattices (Figure S15 and Figure S16). In addition to the structural characterization, the electronic quality of large single crystals of graphene was evaluated by transport and magneto-transport measurements. Standard Hall bar devices were fabricated on large-sized graphene crystals transferred onto 300 nm thick SiO2/Si substrates (Figure 3a, inset). Figure 3a shows a typical plot of resistance of graphene as a function of gate voltage (Vg) at room temperature. We further extracted the field-effect mobility from the R (Vg) curve and obtained the room temperature mobility as high as 16 000 cm2 V−1 s−1, using the nonlinear fitting method.36 The statistics of carrier mobilities and Dirac point positions of 29 devices (Figure 3b), indicating a high-quality of our pristine samples. We also fit the measured transfer curves using a linear fitting (Figure S17).37,38 Figure 3c shows the transfer curve of one device at 1.9 K with mobility of 23 800 cm2 V−1 s−1 using the nonlinear fitting method. These carrier mobility values of the C

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Figure 3. Electronic properties of large single-crystal graphene domain. (a) Typical plot of resistance of graphene as a function of gate voltage (Vg) at room temperature. Inset (left): plot of conductivity as a function of gate voltage at room temperature. Inset (right): the OM image of the measured standard Hall bar devices. (b) Statistics of carrier mobilities and Dirac point positions of 29 devices using nonlinear fitting. (c) Typical plot of resistivity of graphene as a function of gate voltage at 1.9 K. Inset: plot of conductivity as a function of gate voltage at 1.9 K. (d) Temperature dependence of Hall mobilities of graphene. Inset: plot of longitudinal resistance (Rxx) and Hall resistance (Rxy) as functions of magnetic field B at 150 K. (e) Plot of longitudinal resistance (Rxx, red) and Hall resistance (Rxy, blue) of graphene as functions of gate voltage at 1.9 K and at low magnetic field of 3 T. (f) Rxx as a function of magnetic field (B) at a fixed gate voltage (−15 V). Inset: amplification of (f), with B scanning from 0 to 2 T.

a high quality that comparable to the mechanically exfoliated graphene.37 To get a better understanding of the underlying mechanism of melamine pretreatments, auger electron spectroscopy (AES) was utilized for the spatially resolved elemental analyses of copper surface at stages of growing large single-crystals. As shown in Figure 4a, the whole growth process of graphene on melamine-pretreated copper foil is composed of three stages: melamine pretreatment, graphene nucleation, and sequential growth. Figure 4b shows a SEM image of the copper foil consisting of Cu-GBs and flattened surfaces. Figure 4e reveals the corresponding single-point AES analysis from the Cu-GB sites and the flattened surface at the first stage, just after the introduction of melamine, respectively. The sites at Cu-GBs (red point), exhibited bright contrast in SEM image, which has a higher nitrogen and carbon content than that of the flattened copper surface (green point), indicating the existence of carbon- and nitrogen-containing compound at Cu-GBs. The N

as-grown graphene on SiO2 are among the best results of the CVD-grown graphene on SiO2.26,29 The Hall measurements of the same device, shown in Figure 3d, indicate the clear relationship of graphene Hall mobility with the test temperature, with extracted Hall mobility of around 25 000 cm2 V−1 s−1 at 1.9 K.39,40 And low temperature tests are also done using 3 He/4He dilution refrigerator, which also confirm the high mobilities of graphene samples (Figure S18). In addition, low temperature magneto-transport measurements of the graphene devices fabricated on SiO2/Si substrates show well-developed plateaus at filling factors 2, 6, 10, 14, 18, at quite low magnetic field of 3 T, indicating a high electronic quality (Figure 3e).29,41,42 Shubnikov−de Haas oscillations of Rxx were also observed at magnetic fields below 700 mT (Figure 3f), whereas Rxy began to reveal well-developed plateaulike structures at higher magnetic fields coinciding with the Rxx minima. These observations further confirmed that large-area graphene crystal grown on melamine-pretreated copper foil has D

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ing a passivation effect of these active sites for additional graphene nucleation. The nitrogen concentration decreased from ∼14% to ∼7% at this stage, indicating the gradual release of the compound. With low carbon supply, after the nucleation stage, the decreasing area of uncovered copper surface and continuous consumption of active carbon species at the growth frontier together make new nucleation extremely difficult,46,47 especially when the carbon- and nitrogen-containing compound still exists at the active sites. Thus, nucleation density would be significantly suppressed at this stage and would not be changing afterward. At the third stage, when graphene domain grows across the Cu-GBs, the SEM image and elemental analysis reveal the disappearance of carbon- and nitrogen-containing compound from these active sites covered by graphene, at which nitrogen element was undetectable (Figures 4d,g). This observation indicates that these compounds would totally vanish from copper foils, after complete coverage of graphene domain on the Cu-GBs, which was also confirmed by the AES mapping results (Figure S21). The gradual release of these compounds presumably originates from the reaction with hydrogen from the atmosphere or released by graphene growth, a very common reaction of these compounds at high temperature.31 X-ray photoelectron spectroscopy (XPS) was also conducted to confirm the presence and release of the compounds in the entire growth process. At the first stage, the peak of nitrogen atom was prominent with atomic percent concentration of ∼15% (Figure 4h), consistent with the AES results. During the growth of graphene, this concentration decreased to a certain level; then, after the full coverage of graphene on copper, no detectable nitrogen peak appears in the XPS results, indicating a complete vanishment of the carbon- and nitrogen-containing compound. Furthermore, detailed characterizations by XPS and Infrared spectroscopy (IR) of the carbon- and nitrogencontaining compound were presented in Figure S19. In brief, the presence of carbon- and nitrogen-containing compound results in the passivation of the active sites of Cu surfaces at the nucleation stage, leading to the reduced nucleation density, whereas the gradual release of these compounds allows for the full coverage of graphene on copper foil.

Figure 4. Underlying mechanism of melamine pretreatments. (a) Schematic illustration of the stages of growing large single-crystal graphene. (b−d) SEM image of the Cu foil with Cu-GBs and flat surfaces at stage I (b), stage II (c), and stage III (d), respectively. The scale bar of each image is 100 μm. The carbon- and nitrogencontaining compound exhibit a bright contrast. (e−g) Single-point AES analysis of the sites marked in different colors shown in SEM images of (b), (c), and (d). The red, green, blue, and purple points are corresponding to the Cu-GBs, flat surface, the Cu-GBs covered by graphene and flat surface covered by graphene, respectively. (e), (f), and (g) are corresponding to (b), (c), and (d), respectively. The portions of carbon and nitrogen peaks are shown. (h) Low resolution survey of X-ray photoelectron spectroscopy (XPS) spectra of the Cu foil before graphene growth (green), during graphene growth (black), and after the full-coverage growth of graphene (red). The portion of nitrogen peak is shown. The intensity of nitrogen peak is normalized according to the intensity of carbon peak, which is not shown.

1s peak of Cu-GBs is centered at 382 eV with nitrogen atomic percent concentration of ∼14% (Figure 4c). The carbon peak obtained from the flat Cu surface might be caused by the absorbed carbon from the ambient air, after the copper foil was taken out from the vacuum chamber. The spatial accumulation of carbon- and nitrogen-containing compound was also confirmed by the AES map of the nitrogen and carbon peaks (Figure S20). Thus, we believe that the presence of carbon- and nitrogen-containing species at the Cu-GBs induced by melamine pretreatments leads to the passivation effect. This effect makes the Cu-GBs or other kinds of active sites (atomic steps, dislocations, point defects, and their clusters) on copper foils not available for graphene nucleation, presumably owing to the interaction between the compounds and Cu-GBs. As we know, copper is self-limited catalysis in graphene growth, due to the dramatic reduction of its catalytic ability after being covered by graphene.10,43 The carbon- and nitrogen-containing compound might produce an analogous effect on active sites by covering them. In fact, these compounds have been also used in silicon-based electronics to inhibit the reaction between copper and silicon functioning as an isolating layer, which also presents hints of the explanations of the passivating effect.44,45 At graphene nucleation stage, the SEM image and elemental analysis (Figures. 4c,f) shows the formation of graphene can only be possible on the flattened surface, whereas carbon- and nitrogen-containing compound still exists at Cu-GBs, suggest-

CONCLUSION In summary, we have demonstrated a novel active-sitespassivated strategy to suppress the graphene nucleation density and achieved the growth of centimeter-sized single-crystal graphene by using melamine pretreatments on copper foil. The presence of carbon- and nitrogen-containing compounds at the active sites is the key of the passivation effect. Electronic measurements demonstrate that our single crystals of graphene on SiO2 substrate exhibit excellent electronic properties with the carrier mobility up to 25 000 cm2 V−1 s−1. This work demonstrates a novel surface design of a catalytic substrate for graphene growth in a controlled manner, which would pave the way for large-scale growth of high-quality graphene with reduced boundaries, promoting the large-scale integration of functional electronic and optoelectronic devices from graphene. EXPERIMENTAL SECTION Graphene Growth. Commercially available copper foil (Alfa-Aesar #46365, #46986) was electrochemically polished to clean and flatten the surface. Note that, the electrochemically polishing is not prominent in controlling the nucleation density E

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evaporator (Kurte J. Lesker AXXIS) and followed by a standard metal lift-off technique. Characterization. The morphology and structure of the grown graphene on Cu foil or on dielectric substrate were characterized with optical microscopy (Olympus BX51), SEM (Hitachi S-4800, acceleration voltage 5−30 kV) and Raman spectrum (Horiba, LabRAM HR-800, 514 laser wavelength, 100× objective). The graphene domain on amorphous carboncovered TEM grid was characterized by TEM (FEI Tecnai T20, acceleration voltage 200 kV). Aberration-corrected TEM studies were performed using an FEI 80−300 Environmental Titan operated in monochromated mode at 80 kV. The element analysis of Cu foil at stages of entire growth process was characterized by AES and XPS (Kratos Analytical AXISUltra with monochromatic Al Kα X-ray). The fabricated devices were characterized by AFM (Veeco dimension 3100), before electrical characterization. Electrical characterization at room temperature was carried out in a vacuum probe station (Lakeshore TTP-4), using Keithly Semiconductor Characterization System (Model 4200-SCS). Electrical transport and magneto transport-measurements at low temperatures were performed using a lock-in amplifier (Stanford Research 830) at 17 Hz with a 10−100 nA source current, and a 3He/4He dilution refrigerator (Oxford, Triton 200), respectively.

of graphene grains as the melamine treatment. Graphene was synthesized on Cu foil in a low pressure CVD (LPCVD) system equipped with a 1 in. diameter quartz tube. The Cu foil was loaded into the hot center of furnance, whereas 1 g melamine powder (C3N6H6, 98% purity, J&K Scientific) was placed upstream at a location 40 cm away from the hot center (Supporting Information). For the graphene growth process, the system was heated to 1020 °C under a H2 flow of 100 cm3 per min (sccm), corresponding to 110 Pa. The annealing was carried out at 1020 °C for 1 h to eliminate the surface oxygen and contamination. Before introducing CH4, melamine powders were heated with a heating tape to 120 °C for the gradual sublimation, with 100 sccm pure Ar as carrier gas for the transportation of the melamine downstream to the Cu foil at the hot center with a corresponding pressure 150 Pa. To be noted here, there would be no prominent reduction of nucleation density, if hydrogen is used as carrier gas, maybe caused by the reaction between hydrogen and carbon- and nitrogen-containing compound. The melamine was used with exposure time ranging from 3 to 15 min. Subsequently, the methane and hydrogen were introduced into the CVD system for the graphene growth at 1020 °C under different H2:CH4 ratios (10−6000) with the corresponding chamber pressure ranging from 10 to 600 Pa. Note that, the nucleation density can be reduced to less than 10−2 mm−2 for growth of centimeter single-crystal graphene at the H2:CH4 ratio = 5000, with PCH4 = 0.1 Pa, a total pressure 600 Pa. After growth, the system was cooled down to room temperature while still under the same H2 and CH4 flow. Graphene Transfer. The individual graphene domains or continuous films were transferred onto SiO2/Si substrates, using poly(methyl methacrylate) (PMMA)-assisted wet or dry method for Raman spectroscopy characterizations and electrical device fabrication. The graphene was grown on both sides of the copper foils, and one side of the graphene used for characterization was spin-coated with PMMA and baked at 150 °C for 5 min (Supporting Information). Then, the other side of the sample was exposure to O2 plasma for 3 min to remove the graphene. Subsequently, the 1 M Na2S2O8 solution was applied to etch the copper away. Then the free-standing PMMA/ graphene membrane floating on the surface of the etching solution was washed with deionized water three times, and then transferred onto target substrate. After drying, the PMMA was dissolved by acetone yielding graphene domains or continuous film on substrate. In “dry transfer” method, after rinsed by deionized water, the PMMA/graphene was subsequently washed by isopropanol and then dried in air for 12 h before it was placed onto the target substrates. Transport Property Measurements. The graphene samples were transferred onto SiO2/Si substrates with predefined markers for alignments, then followed by heat cleaning and atomic force microscope (AFM) imaging to check the flatness of the samples. After that, the graphene samples were etched into Hall bar geometry by using a PMMA etch mask (PMMA 950 K A2 @ 4000 rpm) from electron beam lithography (EBL) (Raith 150 s) and reactive ion etching (RIE) O2 etching (Trion technology minilock III). After the samples patterned, a second time AFM imaging was performed to ensure that the channel regions were free of winkles and residues. Finally, after using EBL to design a PMMA mask (PMMA 950 K A4 @ 4000 rpm), the samples were contacted with 5 nm Ti and 90 nm Au by using the electron-beam

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00041. Materials and Methods, Supplementary Text, Supplementary Figures S1−S21, and additional References. (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (H. Peng) *E-mail: [email protected] (H.Q. Xu) *E-mail: zfl[email protected] (Z. Liu) Author Contributions ⊥

L. Lin and J.Y. Li contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank J.C. Zhang and Y.J. Liu for the help in sample transfer. This work was financially supported by the National Basic Research Program of China (Nos. 2013CB932603, 2012CB933404, 2014CB932500, 2014CB932500, 2011CB921904, 2012CB932703), the National Natural Science Foundation of China (Nos. 51432002, 51520105003, 21173004, 21222303, 51121091, 11374019, 91221202, 91421303, 61321001 and 51362029), National Program for Support of Top-Notch Young Professionals, and Beijing Municipal Science & Technology Commission (No. Z151100003315013, Z141103004414103, Z131100003213016). Part of this work was performed at the Stanford Nano Shared Facilities. REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. F

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DOI: 10.1021/acsnano.6b00041 ACS Nano XXXX, XXX, XXX−XXX