Building Large-Domain Twisted Bilayer Graphene with van Hove

Building Large-Domain Twisted Bilayer Graphene with van Hove Singularity Zhenjun Tan,†,§,∥ Jianbo Yin,†,∥ Cheng Chen,‡,∥ Huan Wang,† Li Lin,† Luzhao Sun,†,§ Jinxiong Wu,† Xiao Sun,†,§ Haifeng Yang,‡,⊥ Yulin Chen,‡ Hailin Peng,*,† 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 ‡ Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom § Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, P. R. China ⊥ State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, P. R. China S Supporting Information *

ABSTRACT: Twisted bilayer graphene (tBLG) with van Hove Singularity (VHS) has exhibited novel twist-angle-dependent chemical and physical phenomena. However, scalable production of high-quality tBLG is still in its infancy, especially lacking the angle controlled preparation methods. Here, we report a facile approach to prepare tBLG with large domain sizes (>100 μm) and controlled twist angles by a clean layer-by-layer transfer of two constituent graphene monolayers. The whole process without interfacial polymer contamination in two monolayers guarantees the interlayer interaction of the π-bond electrons, which gives rise to the existence of minigaps in electronic structures and the consequent formation of VHSs in density of state. Such perturbation on band structure was directly observed by angle-resolved photoemission spectroscopy with submicrometer spatial resolution (micro-ARPES). The VHSs lead to a strong light−matter interaction and thus introduce ∼20-fold enhanced intensity of Raman G-band, which is a characteristic of high-quality tBLG. The as-prepared tBLG with strong light−matter interaction was further fabricated into high-performance photodetectors with selectively enhanced photocurrent generation (up to ∼6 times compared with monolayer in our device). KEYWORDS: twisted bilayer graphene, interlayer coupling, van Hove singularity, photocurrent enhancement vapor deposition (CVD) growth of tBLG,20−24 suffer from limited domain size and uncontrollable twist angles, which have obstructed its applications. On the other hand, for folding or consecutively transferring of graphene monolayers,14,18 it is difficult to completely avoid the polymer interlayer contamination. A scalable approach to prepare high-quality tBLG with large domain size and θ-dependent VHSs is therefore critical, which requires minimal crystalline defects and well coupling within both large graphene layers. In this work, we developed a facile method to fabricate largedomain tBLGs with controllable twist angles and interlayer coupling by a clean layer-by-layer transfer of large singlecrystalline monolayers. Our fabrication process avoided the introduction of interlayer polymer contamination, enabling

T

wisted bilayer graphene (tBLG), in which one graphene monolayer rotates by a twist angle (θ) relative to the other, has expanded the electronic versatility of two-dimensional carbon system due to its unique θ-dependent band structure.1−3 The interlayer electronic interaction perturbs the band structure of each graphene layer and creates new, θ-dependent van Hove singularities (VHSs) in the density of state (DOS),4 which have been evidenced by the observation of scanning tunneling spectroscopy5−7 and Landaulevel spectroscopy.8 The emergence of VHSs in tBLG leads to intriguing θ-dependent physical and chemical phenomena9−11 such as the enhanced optical absorption,12,13 Raman G-band resonance,14−17 chiral optical property,18 selectively enhanced photocurrent generation,19 and selectively enhanced photochemical reactivity with benzoyl peroxide.20 Despite the knowledge of electronic and optical characteristics of tBLG, the production of high quality tBLG with controlled twist angle and interlayer coupling is still in its infancy. Current preparation methods, such as direct chemical © 2016 American Chemical Society

Received: March 24, 2016 Accepted: May 10, 2016 Published: May 10, 2016 6725

DOI: 10.1021/acsnano.6b02046 ACS Nano 2016, 10, 6725−6730

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Figure 1. (a) Schematic of the preparation process and the band structure evolution of tBLG from monolayer graphene. (b) The optical image of as-prepared large-domain tBLG with controlled angles on SiO2/Si substrates. Scale bar: 100 μm. (c) Low-magnification TEM image of the tBLG domain. Two graphene monolayer domains and stacked part are rendered artificial red, blue, and purple, respectively. Scale bar: 100 μm. (d−f) The corresponding SAED patterns with two constituent monolayers and tBLG as labeled by red, blue, and purple circle in (c).

strong interlayer interaction of π-bond electrons of two monolayers at different twist angles. Such interlayer coupling at overlapped Dirac cones in artificially stacked tBLG leads to the emergence of VHS in DOS, which is directly observed by micro-ARPES. In addition, the as-prepared high-quality largedomain tBLG exhibits typical θ-dependent Raman G-band resonances and selectively enhanced photocurrent generation, which are associated with the presence of VHSs.

for 2 h to enhance the interlayer electronic interaction of two monolayers. (iii) After the copper is removed by Na2S2O8 solution, the PMMA/tBLG film is transferred to a substrate such as heavily doped silicon with 90 nm SiO2, following which the PMMA is removed by acetone, leaving tBLG on the substrate. As shown in Figure 1b, tBLG with large domain size over 100 μm and controlled twist angles (e.g., 5°, 10°, 15°, 25°, 30°) can be prepared. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were conducted to examine the lattice orientations of the as-prepared tBLG domains transferred on a TEM grid (Figure 1c). Both of the two constituent square monolayers as labeled by artificial red and blue color exhibit clear hexagonal diffraction spots, implying good crystalline nature as shown in Figure 1d,e. According to the orientation between the diffraction spots and the edges of domain, the type of terminal edges of square monolayer can be determined. As shown in Figure 1c−e, one pair of square edges is along the armchair direction of graphene lattice, while another pair is generally along the zigzag direction. SAED of tBLG (purple color in Figure 1c) shows two sets of hexagonal diffraction spots which can be ascribed to the stacking of the two constituent monolayers. The twist angle measured from the stacked pattern (Figure 1f) is ∼10°, which agrees well with the orientation between linear edges of the two square monolayer domains. In addition to the twist angle control in large-domain tBLG, our transfer method guaranteed that two monolayer graphene domains are stacked free of interfacial polymer contamination. Such clean interface gives rise to strong interlayer interaction of

RESULTS AND DISCUSSION The high-quality tBLG with VHSs is artificially transferred and stacked from two large-domain monolayer graphene crystals. Square single-crystalline monolayer with domain sizes over 100 μm on copper foil is first grown by CVD method (details in Figures S1−S3). The crystal orientation of the monolayer was identified by the regular shape of the square domain and further determined by anisotropic etching process (Figure S4). To prepare high-quality large-domain tBLG from graphene monolayers, the three-step process was illustrated as follows (Figures 1a and S5): (i) poly(methyl methacrylate) (PMMA, 966 K 4% ethyl lactate) is spin-coated onto the monolayer graphene domains grown on copper foil. After baking at 150 °C for 5 min, the PMMA/graphene/copper foil is etched by a Na2S2O8 solution to remove copper and then rinsed by deionized (DI) water. (ii) As-obtained transparent PMMA/ graphene film is stacked onto another monolayer graphene domain on copper foil, monitored by an optical microscope to guarantee the specific twist angle between two constituent graphene monolayers (more details in Figure S6). The asprepared PMMA/tBLG/copper foil is baked again at 150 °C 6726


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graphene, where Δk from over- and underlayer equals zero. The more detailed comparison can be seen in Figure S8. The VHSs of tBLG lead to a selectively enhanced light− matter interaction in tBLG. When the energy of the incident photons matches the energy interval between two VHSs from conduction and valence bands (ℏω ≈ 2EVHS), the photoexcited electrons transit between the fine band structures around the minigaps (as illustrated on top-right panel of Figure 1a), which gives rise to the resonant enhancement of Raman G-band intensity. As a fingerprint of tBLG, this phenomenon has been previously observed on tBLG by several groups. 20−22 Accordingly, we conducted micro-Raman spectroscopy to evaluate our as-fabricated large-domain tBLG samples (Figure 3). As shown in Figure 3a,c, the blue and red colors in the

π-bond electrons of graphene monolayers, which is directly unraveled by micro-ARPES (Figure 2). The tBLG sample on

Figure 2. (a) 2D real-space micro-ARPES map of the tBLG. (b) Stacking plots of constant-energy contours at different binding energies of as-prepared tBLG. (c) The ARPES spectrum of asprepared tBLG across two adjacent Dirac cones. The right curve is energy spectrum density curve (EDC) integrated from the spectrum. VHSs are indicated by red arrows. (d) Constant energy contour (Eb = 0.45 eV), showing adjacent Dirac points from two graphene layers, respectively. The right part is an illustration of the twisted angle in twisted bilayer graphene, which is estimated to be around 19.5°. Figure 3. (a) The tBLG domain on SiO2 (90 nm)/Si under the illumination of a laser with a matched wavelength of 633 nm. (b) The Raman spectrum for the tBLG domain in panel a under illumination of 633 nm (1.96 eV) laser compared with typical monolayer and AB-stacked graphene. Inset is the corresponding Gband intensity mapping image. (c) The tBLG domain on SiO2 (90 nm)/Si illuminated with a matched laser wavelength of 532 nm. (d) The Raman spectrum tBLG in (c) under illumination of 532 nm (2.33 eV) laser. Inset is the corresponding G-band intensity mapping image.

single crystalline Cu substrate is observed by the contrast in intensity of photoelectrons (Figure 2a, details in Figure S7). The stacking plots of the band contours at different binding energies clearly describe the two sets of Dirac linear dispersions (Figure 2b). Around the intersection of two Dirac cones, the electron dispersion shows an opening as indicated by red arrow in Figure 2b. This fine structure is further revealed as a minigap structure, which is revealed by faint intensity in the band dispersion cutting across two adjacent Dirac cones (labeled by red arrows in left panel of Figure 2d) and the dip in the DOS plot (right panel of Figure 2d). This fine structure originates from the interlayer electronic interaction,6 which leads to the formation of VHSs in the DOS. Figure 2d shows the constant energy contour (Eb = 0.45 eV), which contains two adjacent Dirac points from the two graphene layers, respectively. A schematic illustration of the twist angle in the tBLG is shown in the right part. A twist angle of ∼19.5° between two graphene layers is determined after the Γ position was carefully located. Compared with the theory prediction (θ = 3aEVHS/2πℏvf),15 the measured twist angle ∼19.5° perfectly matches the aforementioned energy interval ∼3.4 eV (Figure S8). For tBLG built by our method, not only are the twist angle and energy interval matched with the result, but also clear VHS can be observed in the electron dispersion. The typical built tBLG band structure is quite different from AB stacked bilayer

bilayer domain are clearly observed, presumably originated from selectively enhanced absorption of the light with specified wavelengths.25 Under the illumination of 633 nm (1.96 eV) laser, the tBLG exhibits ∼20-fold of enhanced Raman G-band compared with the intensity of 2D-band (Figure 3b). Such an enhancement implies the strong interlayer interaction of πbond electrons and the emergence of VHSs. The Raman mapping of this enhanced G-band intensity exhibits a uniform feature with a domain size over 100 μm, which confirms the high quality of large-domain tBLG. On the basis of the theoretical calculations,15 the tBLG domain with resonant wavelength at 633 nm has a twist angle of ∼10°. A similar enhancement was observed on the tBLG domain under the illumination of the laser with 532 nm as the resonant wavelength (Figure 3c,d). These wavelength selective resonant 6727


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ACS Nano enhancements can be well explained by the θ-dependent physical properties of tBLG.3−5,15 The large-domain high-quality tBLG with VHSs can be used for the fabrication of high-performance photodetectors due to its enhanced light−matter interaction. The 10° tBLG domain and the adjacent graphene monolayer on SiO2 (90 nm)/Si were etched into a strip and embedded into a two-terminal device (Figures 4a,b, and S9). The typical transfer curve in Figure 4c

light−matter interaction introduced by VHSs in the DOS of tBLG. The effect of VHSs on the photoexcitation process can be quantitative described by joint density of state (JDOS) which is defined as28 JDOS(ω) =

1 4π 3

∫ δ[Ec(k) − Ev(k) − ℏω] dk

where v and c denote valence and conduction bands, respectively. A theoretical calculation29 predicted that the JDOS exhibits an abrupt increase which is associated with the VHS in DOS. This abrupt increase leads to an enhanced photoexcitation process and then enhanced photocurrent generation, holding great promise in the application of highsensitivity photodetection.

CONCLUSION In summary, we have successfully prepared large-domain-sized tBLG with controllable twist angle by a clean layer-by-layer transfer of the constituent graphene monolayers. The interlayer interaction of π-bond electrons of graphene monolayers is evidenced by the formation of minigap band structures and VHSs, which are directly observed by micro-ARPES. The ∼20fold enhanced intensity of Raman G-band further confirms the successful preparation of tBLG. Due to the strong light−matter interaction, the as-prepared tBLG holds promising potential in photodetection and shows a ∼6-fold enhanced photocurrent generation. Our method facilitates the study of θ-dependent properties and the optoelectronic applications of tBLG. Figure 4. (a) Schematic illustration of the device of tBLG for enhanced photocurrent generation, in which the channel is composed of both tBLG area and monolayer area. (b) The SEM image of the tBLG photodetector with a twist angle of ∼10°. Bilayer channel is on the left side of the dotted line, while monolayer area is on the right. (c) The typical transfer curve of the tBLG device. (d) Raman G-band mapping image of the device with 633 nm laser excitation. (e) In situ photocurrent mapping image with 633 nm incident laser of 200 μW. (f) The corresponding linescanning photocurrent of the photodetector.

EXPERIMENTAL SECTION Growth of Single Crystal Graphene. Chemical vapor deposited (CVD) single crystal graphenes with square shape were grown on copper foil (99.8% purity, 25 μm thick, Alfa Aesar) in a low pressure chemical vapor deposition system consisting of a horizontal tube furnace (Lindberg Blue M TF55035KC-1) equipped with a 1-in.diameter quartz tube. Two stacking polished copper foils or rolled-up copper foils were loaded in the hot center of the furnace. After the copper foils were annealed at 1035 °C without gas pumped into them (∼1 Pa) for 60 min, 200 sccm H2 (∼320 Pa) and 1 sccm CH4 (∼12 Pa) were introduced for 4 min to grow single crystal graphene domains. Finally, the sample was pulled out from the high-temperature zone to room temperature with a magnet without changing the gas flow. Characterization. The samples were characterized with optical microscopy (Olympus DX51), SEM (Hitachi S-4800; operating at 1 kV), Raman spectroscopy (Horiba, LabRAM HR-800), TEM (FET Tecnai F30), Nano-ARPES (performed at the Spectro Microscopy beamline (3.2L) at ELETTRA Synchrotron, Trieste, Italy). Measurement for Devices. The electrical measurements were performed by Keithley SCS-4200. The photoelectrical measurements were performed by a scanning photocurrent microscopy. Scanning photocurrent measurements were performed using a confocal optical microscopy equipped with motorized stage (alignment accuracy ≤0.1 μm); 532 and 633 nm were used as wavelength of laser sources. The chopper-modulated (∼1 kHz) laser beams were focused to 0.7−1 μm full-width at half-maximum (fwhm) spot on the device using a ×100 objective. The short-circuit photocurrents were then measured by preamplifier and lock-in amplifier. When the laser spot was scanned over the device, the induced photocurrents and beam positions were recorded and displayed simultaneously with the assistance of a computer, which communicated with lock-in amplifier and motorized stage (with device on it). All the electrical and photoelectrical measurements were performed in air at room temperature.

confirms the electrical reliability of the device. Since the energy interval of the two VHSs (2EVHS) in the 10° tBLG domain is ∼1.96 eV,15 it exhibits an enhanced Raman G-band under the illumination of 633 nm laser (Figure 4d). Under this matched laser, the net photocurrent without source-drain bias and gate bias is measured by scanning photocurrent microscopy, in which the photocurrent is recorded while scanning a focused 633 nm laser with ∼1 μm spot size on the device. The corresponding photocurrent mapping image is shown in Figure 4e. The photocurrent is generated mainly at graphene/metal junction rather than graphene channel, which is caused by charge-transfer doping underneath the contact and adjacent regions.26,27 Although there is still charge transfer between monolayer and tBL graphene (two different regions in channel) during the excitation process, it is considered far less than the effective segregation at metal junction. Thus, the enhanced photocurrent is only observed at tBLG/metal junction. Remarkably, the photocurrent on tBLG is ∼5-fold larger than that on graphene monolayer in the same device (Figure 4f). A significant photocurrent enhancement of ∼6-fold was also observed on a photodetector based on 13° tBLG domain (Figure S10). These enhancements can be attributed to strong 6728


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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/acsnano.6b02046. Materials and methods, supplementary text and additional experimental data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: zfl[email protected]. Author Contributions ∥

Zhenjun Tan, Jianbo Yin, and Cheng Chen contributed equally to this work. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the National Basic Research Program of China (Nos. 2014CB932500, 2013CB932603, 2012CB933404), the National Natural Science Foundation of China (Nos. 21173004, 51520105003, 51432002, 21222303, and 51362029), the National Program for Support of Top-Notch Young Professionals, and Beijing Municipal Science & Technology Commission (Z131100003213016, Z151100003315013). We are also grateful to Dr. Alexei Barinov for his kind help and support during our beamtime at the spectramicroscopy beamline of Elettra synchrotron. REFERENCES (1) Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H. Graphene Bilayer with a Twist: Electronic Structure. Phys. Rev. Lett. 2007, 99, 256802. (2) Ni, Z.; Wang, Y.; Yu, T.; You, Y.; Shen, Z. Reduction of Fermi Velocity in Folded Graphene Observed by Resonance Raman Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 235403. (3) Shallcross, S.; Sharma, S.; Kandelaki, E.; Pankratov, O. A. Electronic Structure of Turbostratic Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 165105. (4) Li, G. H.; Luican, A.; dos Santos, J.; Castro Neto, A. H.; Reina, A.; Kong, J.; Andrei, E. Y. Observation of Van Hove Singularities in Twisted Graphene Layers. Nat. Phys. 2010, 6, 109. (5) Luican, A.; Li, G.; Reina, A.; Kong, J.; Nair, R. R.; Novoselov, K. S.; Geim, A. K.; Andrei, E. Y. Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers. Phys. Rev. Lett. 2011, 106, 126802. (6) Brihuega, I.; Mallet, P.; González-Herrero, H.; Trambly de Laissardière, G.; Ugeda, M. M.; Magaud, L.; Gómez-Rodríguez, J. M.; Ynduráin, F.; Veuillen, J. Y. Unraveling the Intrinsic and Robust Nature of van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis. Phys. Rev. Lett. 2012, 109, 196802. (7) Yan, W.; Liu, M.; Dou, R.-F.; Meng, L.; Feng, L.; Chu, Z.-D.; Zhang, Y.; Liu, Z.; Nie, J.-C.; He, L. Angle-Dependent van Hove Singularities in a Slightly Twisted Graphene Bilayer. Phys. Rev. Lett. 2012, 109, 126801. (8) Wang, Z. F.; Liu, F.; Chou, M. Y. Fractal Landau-Level Spectra in Twisted Bilayer Graphene. Nano Lett. 2012, 12, 3833. (9) Havener, R. W.; Liang, Y.; Brown, L.; Yang, L.; Park, J. Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene. Nano Lett. 2014, 14, 3353. (10) Patel, H.; Havener, R. W.; Brown, L.; Liang, Y.; Yang, L.; Park, J.; Graham, M. W. Van Hove Singularities and Excitonic Effects in the 6729


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Building Large-Domain Twisted Bilayer Graphene with van Hove

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