Letter pubs.acs.org/NanoLett
Controlled Folding of Single Crystal Graphene Bin Wang,† Ming Huang,†,∥ Na Yeon Kim,†,∥ Benjamin V. Cunning,† Yuan Huang,† Deshun Qu,‡ Xianjue Chen,† Sunghwan Jin,† Mandakini Biswal,† Xu Zhang,† Sun Hwa Lee,† Hyunseob Lim,† Won Jong Yoo,‡ Zonghoon Lee,†,∥ and Rodney S. Ruoff*,†,§,∥ †
Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea SKKU Advanced Institute of Nano-Technology (SAINT), Department of Nano Science and Technology, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Republic of Korea § Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Folded graphene in which two layers are stacked with a twist angle between them has been predicted to exhibit unique electronic, thermal, and magnetic properties. We report the folding of a single crystal monolayer graphene film grown on a Cu(111) substrate by using a tailored substrate having a hydrophobic region and a hydrophilic region. Controlled film delamination from the hydrophilic region was used to prepare macroscopic folded graphene with good uniformity on the millimeter scale. This process was used to create many folded sheets each with a defined twist angle between the two sheets. By identifying the original lattice orientation of the monolayer graphene on Cu foil, or establishing the relation between the fold angle and twist angle, this folding technique allows for the preparation of twisted bilayer graphene films with defined stacking orientations and may also be extended to create folded structures of other two-dimensional nanomaterials. KEYWORDS: Graphene, fold, twisted bilayer, single crystal The folding of “graphitic sheets” can be traced back to the 1990s, where it occurred as a result of friction between scanning probe microscopy tips and the surface of highly oriented pyrolytic graphite.12 More recently, the occurrence of graphene folds was observed on exfoliated graphene flakes13−16 and CVD-grown graphene.17 For instance, by using atomic force microscopy (AFM) with a wear resistant tip, Rode et al. folded monolayer graphene flakes and studied the tribology properties of the resulting tBLG.18 However, all of these studies have been limited to small folded graphene structures or unintentional folds. The fabrication of larger tBLG with controlled twist angle by folding is thus an interesting topic. By using a large-area single crystal monolayer graphene film grown on a Cu(111) substrate which has been transferred to a tailored substrate having a hydrophobic region and a hydrophilic region, a “self-folding” was achieved that used film delamination from the hydrophilic region of the substrate when immersed in water. The withdrawal of the substrate from water resulted in macroscopic folded graphene (FG) with good uniformity over the millimeter scale. This process was capable
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raphene in which two layers are stacked in either AB form, or with a defined twist angle between the layers, possesses properties distinct from single layer graphene.1−5 ABstacked bilayer graphene possesses a tunable band gap and has been demonstrated to be a promising high-mobility channel material for tunneling transistors.6 By having a twist angle (θ) between two monolayers, twisted bilayer graphenes (tBLG) with different band structures can be obtained, which show novel properties such as θ-dependent interfacial conductivity,7 van Hove singularities,8,9 and tunable chiral properties.10 Due to the difficulty of obtaining tBLG by chemical vapor deposition (CVD), it has been obtained by transferring two CVD-grown single crystal monolayer graphenes.10,11 However, to synthesize large-scale tBLG in this manner with a defined twist angle, the crystal orientation of the graphene should be known prior to transfer, and several accurate alignment procedures are required, including the alignment between graphene and the transfer support, and then the alignment between the supports during stacking. An alternative approach to producing tBLG structures is through the controlled folding of graphene. By using just one, continuous single crystal graphene sample rather than two separate graphene samples, the alignment between the two graphene sheets (here, layers) is simplified, resulting in an easier production of tBLG. Also, the folding method can in principle make very large tBLG sheets. © XXXX American Chemical Society
Received: October 25, 2016 Revised: February 15, 2017
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Figure 1. Schematic of the folding process of a monolayer graphene film, including: substrate modification, folding driven by delamination from only the hydrophilic surface region, and the removal of polymers (PMMA and then PC).
a substrate was created with half its surface hydrophilic (the pristine substrate surface which was covered by photoresist) and the other half hydrophobic (modified by the self-assembled monolayer, SAM, of trichloro(1H,1H,2H,2H-perfluorooctyl)silane), as confirmed by the water contact angle measurements shown in Figure S3. (ii) A PMMA-coated graphene−Cu foil was pressed onto the modified substrate so that the substrate surface contacted the PMMA layer. This assembly was then immersed in aqueous ammonium persulfate to etch away the Cu foil. The region of the PMMA/graphene film that initially adhered to the hydrophilic region of the surface became detached during immersion (Figure S4), while the other half remained adhered to the hydrophobic region of the surface. This adhesion was explained in a previous report.22 After replacing the etchant with pure water, the assembly was slowly withdrawn from the solution (with the assembly oriented as shown in Figure 1: Folding), and the floating PMMA/graphene folded over the adhered PMMA/graphene on the hydrophobic region of the substrate as shown in the schematic in Figure 1. (iii) The PMMA layers on both sides of the FG were then removed. First, a PC film was spin-coated on top of the assembly to provide mechanical support to the PMMA/FG after it was removed from the substrate. The entire assembly was then floated on concentrated aqueous sodium hydroxide that etched away the SiO2 layer on the wafer, detaching the film from the substrate. The film was then floated onto acetic acid to etch away the bottom layer of PMMA. The film was then transferred onto a target substrate, and finally the PC/PMMA layer on top was removed by dissolution in chloroform. Through this multistep process, a FG film, mostly free of polymer coating, was obtained on a target substrate as verified by X-ray photoemission spectroscopy (XPS) measurements (Figure S5). The carbon 1s core level spectrum shows a large peak assigned to CC bonding from graphene and two weak peaks due to the CO/C−O bonds attributed to the polymers. The peak characteristic of the PC/PMMA residue is small,22,23 suggesting the removal of most polymer (PC and PMMA) residues.
of creating many FG each with a deliberately chosen twist angle. Cu(111) foils were first fabricated and used to grow single crystal monolayer graphene.19,20 Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were used to investigate the structure of this monolayer (Figures S1 and S2, see Supporting Information, SI) that is highly orientated as a result of heteroepitaxial growth on the Cu(111) foil; By increasing the growth time, the hexagonal graphene domains merged to form a continuous monolayer film. The graphene film was transferred onto a TEM grid for SAED evaluation at 10 regions in an area of about 1 mm2. The patterns have a lower diffraction intensity of the outer {112̅0} reflections than that of the inner {101̅0} reflections, consistent with the monolayer nature of the graphene film.21 The orientation of the graphene lattice obtained from the SAED patterns shows less than 0.1° divergence throughout this area, demonstrating that the film is very close to being a single crystal. Raman spectroscopy was also used to characterize the graphene as well as each of the FG films described below. By using this single crystal monolayer graphene, the folding was conducted as shown schematically in Figure 1, with details in the Methods (SI). There are three steps: (i) the modification of a substrate surface to have one region that is hydrophilic and one that is hydrophobic with a well-defined border between the two, and the subsequent transfer of poly(methylmethacrylate) (PMMA) coated graphene to that substrate, (ii) the folding of the PMMA-coated graphene film on the substrate by delaminating the film that was in contact with the hydrophilic region of the surface, and (iii) the removal of the PMMA layer (and a layer of polycarbonate (PC) that is also used) to obtain FG. Details of the three steps are as follows. (i) Half a rectangular piece of SiO2-on-Si (SiO2/Si) wafer was covered with photoresist using photolithography, and the wafer was then exposed to trichloro(1H,1H,2H,2Hperfluorooctyl)silane vapor at reduced pressure to make the uncovered region hydrophobic. After dissolving the photoresist, B
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Figure 2. Folded graphene film (FG). (a) Optical image of the FG on a SiO2/Si substrate showing the fold (edge). (b) SEM image of the FG suspended on a TEM grid. (c) TEM images of the in-plane and the (d) folded edge regions of FG. (e) AFM measurement of a FG edge showing the 2D image and the height profiles and also the related 3D height image.
Figure 3. Defined folding of graphene with a stacking angle of ∼0° by first identifying the initial orientation of graphene using low-energy electron diffraction (LEED) measurement. (a) Images showing the process to identify the initial orientation of graphene on Cu foil: (i) a graphene/Cu foil mounted on the LEED stage, (ii) the corresponding LEED pattern of the graphene on Cu foil obtained by using a beam energy of 70 eV with the foil edge from (i) overlaid relative to the diffraction pattern, and (iii) diagram of the real graphene lattice on copper corresponding to the LEED pattern in (ii) and the Cu edge in (i), allowing a lattice−substrate angle, α, to be used for determining the substrate fold angle. (b) SAED pattern of the FG showing a 0° twist angle between two layers, and the diffraction intensity profile suggesting a bilayer graphene that is close to AB stacking. (c) Highresolution image of the FG, where a single layer graphene (SLG) region is identified among the bilayer graphene (BLG). (d) Raman spectra of 0° twist angle FG as compared with a CVD-grown AB-stacked bilayer graphene.
To observe the geometry of the fold in our FG, we used AFM to image the fold topography. For comparison, a monolayer graphene film transferred onto a SiO2/Si wafer (PMMA transfer method) was also measured by AFM. The height of the monolayer graphene is about 1 nm as shown in Figure S6 (for a discussion of AFM measurement of heights of graphene on various substrates, please see ref 25). After folding, the average thickness of the FG in the “in-plane” region was measured to be about 2 nm based on line profiles (shown on the 2D image in Figure 2e).
An optical micrograph of a FG showing the fold region is shown in Figure 2a. For characterization purposes, we define two distinct regions: the in-plane region characterized by intimate van der Waals contact between the two graphene planes, and the fold region where the radius of curvature prohibits parallel stacking.24 FG was also transferred onto a TEM grid and remained suspended over the grid holes without breakage (Figure 2b). The in-plane area was characterized by HRTEM, where a clear Moiré pattern was observed (Figure 2c). C
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of graphene can be determined, defined as α, to select an appropriate substrate fold angle. This graphene was then folded to give an expected twist angle of 0°. The SAED pattern of the resulting FG sample is shown in Figure 3b and shows the expected 0° twist angle between the two layers. Moreover, the diffraction intensity of the outer spots is higher relative to the inner spots, suggesting the sample is bilayer graphene that is AB-stacked.11 The high-resolution TEM image (Figure 3c) further shows the lattice structure. No Moiré pattern of this FG sample is observed as expected for AB-stacked bilayer graphene. The Raman spectrum of this FG was also compared to a CVDgrown AB-stacked bilayer graphene domain in Figure 3d, where the position and full width at half-maximum intensity (FWHM) of the 2D bands are closely matched, indicating AB-stacked structure of the FG. It is noted that the twist angle through the entire tBLG film is not consistently 0°. By analyzing four SAED patterns distributed across the tBLG film (Figure S9a), an average value of 0.35° with standard error of 0.26° was obtained. The second approach to the controlled folding was achieved without knowing the initial orientation of graphene on Cu. As shown in Figure S8b, multiple rectangular monolayer graphene films with the same orientation were obtained by precisely dividing large single crystal graphene grown on a Cu foil. These pieces of graphene on Cu were separately transferred onto the substrates with different fold angles, thus resulting in different twist angles in the resultant folded graphene. By establishing the relation between fold angles and twist angles, any defined twist angle can be achieved through folding along a certain fold angle. As examples, six samples with fold angles of 90°, 95°, 100°, 105°, 110°, and 115° are shown in Figure 4a. It is noted that the FG samples shown here were folded in 5° increments
The fold geometry for pristine graphene on a flat substrate is dependent on a number of factors, including the fold length, substrate adhesive forces, and the mechanical properties of graphene.24 In our samples, we have a number of other factors affecting the fold geometry, mainly due to the processing technique (e.g., solvent residues, polymer residues, air). These are revealed further by 2D line profiles taken along the three lines indicated in Figure 2e. Along line 1, we observe the fold as a hump in the profile, with a maximum height of 5.0 nm, which becomes planar bilayer graphene about 50 nm from the fold. In contrast, line profiles 2 and 3 show asymmetric fold geometry indicative of a disordered fold that may be attributed to interstitial contaminants arising from the solution processing. The HRTEM image of the fold (Figure 2d) confirms the observation of a disordered fold from the AFM topography. We do observe some atomic order near and parallel to the folded edge; however, the areas on either side largely appear amorphous which we attribute to interstitial contaminants, and a wrinkled fold topography. We also evaluated the yield of intact tBLG after the folding and transfer process. A typical SEM image is depicted in Figure S7. Limited cracks and wrinkles were observed over the ≈0.3 mm2, suggesting a high yield of intact bilayer graphene by this folding method. Two-layer graphene films in which the two neighboring graphene layers are rotated by a single angle throughout the entire film differ from a Bernal AB-stacked bilayer and also from a stacked polycrystalline double-layer graphene where the bilayer has many regions with different rotations due to the relatively small grain size. In our samples, we have observed that the twist angle between two graphene monolayers can be both controlled and uniform over a large region by two different approaches. In the first approach, a defined twist angle was achieved by referencing the lattice orientation of graphene to a straight edge of its Cu(111) foil growth substrate. By changing the angle between the hydrophilic/hydrophobic boundary and the edge of the substrate used for folding (the fold angles), an appropriate fold angle can be chosen that will result in the desired twist angle. Various fold angles can be defined on the SAM-modified substrates through a lithographic process as shown in Figure S8a, in which many patterns were defined on a SiO2/Si wafer, with the angle between the fold and the vertical substrate edge increasing from 90° to 115° in 1° increments. To demonstrate this procedure, AB-stacked bilayer graphene (0° twist angle) was selected as a target twist angle. The lattice orientation of the initial monolayer graphene was identified before folding by using low-energy electron diffraction (LEED) measurement. This approach is possible using LEED instrumentation as the sample stage edge is aligned to the charge coupled device that captures the resulting diffraction pattern. By measuring the angle between a straight edge of the copper foil and the sample stage edge, the lattice orientation of graphene on the copper foil can be obtained. Figure 3a depicts this process schematically. A graphene/Cu foil was mounted on the LEED sample stage, and a straight edge of the foil was referenced to the edge of the stage as highlighted in Figure 3a-i. After sample alignment, the LEED pattern was extracted from the graphene on Cu foil (Figure 3a-ii). As the first-order diffraction reflections of the LEED pattern are the same “direction” as the real lattice of graphene, the real graphene lattice can be overlaid on the Cu foil (Figure 3a-iii),26 and the angle between the Cu foil edge and one of the lattice directions
Figure 4. Folded graphene films with controlled stacking orientations. (a) Optical image of FG samples with the fold lines indicated. (b) SAED patterns, (c) Moiré patterns, and (d) Raman spectra of the FG samples with controlled stacking orientations. The spectra were normalized to the intensity of the 2D bands. Inset in d are enlarged spectra in the range 50−200 cm−1. D
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Nano Letters rather than 1° in Figure S8a to show the capability of this folding method. Every 5° difference in fold angle should produce a 10° difference in the twist angle. In this way it was possible to obtain bilayer FG with a desired rotation between the two layers. Figure 4b shows the SAED patterns of the FGs with designed fold angles. The twist angles measured for samples denoted 90°, 95°, 100°, 105°, 110°, and 115° are 15.3°, 25.5°, 25.2°, 16.0°, 6.0°, and 4.2°, with a progressive angular difference between them of around 10°, as expected. The hexagonal symmetry of the graphene atomic lattice means the maximum angle between the nearest diffraction spots is 30°, resulting in angles of 25.2°, 16.0°, and 6.0°, rather than 34.8°, 44.0°, and 54.0°. The relationship is given in Table S1, demonstrating that the stacking orientation of the FG is controllable. HRTEM and Raman measurements were used to study these FG samples, and the results are given in Figure 4c−d. A moiré pattern is generated by the overlay of two graphene layers with the repeat distance of the periodic pattern becoming smaller with increasing twist angle. This observation agrees well with the stacking model of two graphene monolayers shown in Figure S10. Raman spectra of the six FG samples, pristine monolayer graphene, and also AB-stacked bilayer graphene are shown in Figure 4d. The Raman spectra for the FGs are consistent with twisted (but not folded) bilayer graphene as reported previously.27 For instance, the increased intensity of the graphene G mode and the appearance of the (ZO’)L mode (“layer breathing vibration”) for samples with twist angles around 16.0° are characteristic for CVD-grown twisted bilayer graphene domains, with rotational angles around 15°, as reported previously.28 Our TEM and Raman observations of samples made by both folding approaches demonstrate producing FGs with a defined rotational orientation between two stacked graphene layers by either (i) controlling the fold angle of single crystal graphene after knowing the initial orientation of graphene on Cu foil or (ii) by establishing the relation between the fold angle and the twist angle from studies of a few FG samples. The uniformity of the stacking orientation throughout the entire FG film was investigated as an important character of the as-folded samples. An FG sample with a twist angle of ∼16° is shown as an example in Figure 5. The FG region covers an area of about 2 mm2 and is indicated by the red line (Figure 5a). SAED patterns were used to determine the twist angle of each region. Four patterns collected from an area across roughly 1 mm2 indicate a twist angle of 16.2°, with a standard deviation of 0.38° (Figure 5b). The small spread in twist angle values at different positions is likely due to wrinkling of the top graphene layer during the folding process. It is possible that such wrinkles form when the top layer “snaps” to the bottom layer and adheres. Raman spectra were also collected from several spots across this FG, as labeled by the “+” marks (Figure 5c). In contrast to a folded poly crystalline graphene film (Figure S11) for which the Raman spectra varied significantly across the film, the spectra here are very similar, further supporting an essentially uniform stacking of the two layers across the whole FG. A Raman spectrum of pristine monolayer graphene shows the typical characteristics of I2D/IG ∼ 2, Pos2D ∼ 2681 cm−1 (FWHM ∼30.8 cm−1), in contrast with the ones for FG (twist angle, θ ∼ 16.2°) I2D/IG ∼ 0.4, Pos2D ∼ 2685 cm−1 (FWHM ∼37.5 cm−1), where I is intensity, and Pos is the peak position. The specific characteristics of the 2D peak for FG are mapped
Figure 5. Uniformity of stacking orientation throughout a FG sample folded at 105°. (a) SEM image of a FG sample transferred onto a TEM grid. The graphene coverage is highlighted by a red line. (b) SAED patterns collected from the labeled regions in panel a distributed over a roughly 1 mm2 area showing comparable twist angles of about 16°. (c) Raman spectra obtained from the labeled “+” regions in panel a as compared with a spectrum of monolayer graphene. The spot size is 0.25 μm. (d) Raman maps of the region indicated on the gold grid. The first inset in panel d is the optical image. TEM and Raman measurements suggest that the FG is uniform in its stacking configuration throughout the measured area.
in Figure 5d. The maps show little deviation of the 2D band characteristics from the spectra shown in Figure 5c, further indicating good uniformity. The FGs were also evaluated by measuring the carrier mobility of the samples with twist angles of ∼0° (FG-0°), ∼15° (FG-15°), and ∼30° (FG-30°), respectively. As analyzed from SAED patterns (Figure S9), the measured average twist angles of FG-0°, FG-15°, and FG-30°, are 0.35°, 14.63°, and 29.55°, with standard errors of 0.26°, 0.25°, and 0.26°, respectively. Three-terminal back-gated graphene field-effect transistor devices (GFETs) were fabricated on silicon wafers with a 90 nm thick oxide layer on the surface. Typical gate-dependent conductance plots and the statistics of the carrier mobility are shown in Figure S12 and Table S2. As a result, the highest fieldeffect mobility of FG-0°, FG-15°, and FG-30° were 6645 cm2 V−1 s−1, 4341 cm2 V−1 s−1, and 4728 cm2 V−1 s−1, respectively, while the average values were 2788 cm2 V−1 s−1, 2180 cm2 V−1 s−1, and 1749 cm2 V−1 s−1. The values are comparable to ABstacked bilayer GFETs prepared by a polymer transfer method,11 suggesting good quality of the tBLG samples. In conclusion, we have demonstrated a technique to allow folding of a CVD-grown single crystal monolayer graphene film to produce twisted bilayer graphene with good uniformity in the millimeter scale and with a defined twist angle between the two layers. This method of folding enables research to be carried out on graphene folds and twisted bilayer graphene with specific stacking orientations. This technique could also be used to fabricate fold structures and twisted bilayers of other twodimensional nanomaterials. E
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b04459. Methods, SEM and TEM measurements of the single crystal monolayer graphene film, contact angle measurement of the modified substrate used for folding, photographs of the steps in the folding process, XPS spectrum of the FG, AFM measurement of the monolayer graphene, SEM image of the FG, alignment between graphene/Cu foil and the substrate for controlled folding, SAED patterns of FG-0°, FG-15°, and FG-30°, stacking model of two graphene monolayers with gradually changing rotational angles, Raman spectra of FG fabricated from polycrystalline monolayer graphene, electrical characterization of FG-based GFETs, the relation between the fold angle and the measured twist angle of FG samples, and statistics of carrier mobility from the GFET devices (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ruoffl
[email protected]. ORCID
Bin Wang: 0000-0001-9576-2646 Xianjue Chen: 0000-0002-4757-7152 Won Jong Yoo: 0000-0002-3767-7969 Zonghoon Lee: 0000-0003-3246-4072 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R019-D1). We thank The Sixth Element Materials Technology Co., Ltd for kindly providing the polycrystalline monolayer graphene on polycrystalline Cu foil.
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REFERENCES
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