Modulating the Electronic Properties of Monolayer Graphene Using a

Modulating the Electronic Properties of Monolayer Graphene Using a Periodic QuasiOne-Dimensional Potential Generated by HexReconstructed Au(001) Xiebo Zhou,†,‡ Yue Qi,‡ Jianping Shi,†,‡ Jingjing Niu,§,⊥ Mengxi Liu,‡ Guanhua Zhang,∥ Qiucheng Li,‡ Zhepeng Zhang,‡ Min Hong,†,‡ Qingqing Ji,‡ Yu Zhang,†,‡ Zhongfan Liu,‡ Xiaosong Wu,*,§,⊥ and Yanfeng Zhang*,†,‡ †

Department of Materials Science and Engineering, College of Engineering, ‡Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, and §State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing 100871, People’s Republic of China ⊥ Collaborative Innovation Center of Quantum Matter, Beijing 100871, People’s Republic of China ∥ State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China S Supporting Information *

ABSTRACT: The structural and electronic properties of monolayer graphene synthesized on a periodically reconstructed substrate can be widely modulated by the generation of superstructure patterns, thereby producing interesting physical properties, such as magnetism and superconductivity. Herein, using a facile chemical vapor deposition method, we successfully synthesized high-quality monolayer graphene with a uniform thickness on Au foils. The hex-reconstruction of Au(001), which is characterized by striped patterns with a periodicity of 1.44 nm, promoted the formation of a quasi-one-dimensional (1D) graphene superlattice, which served as a periodic quasi-1D modulator for the graphene overlayer, as evidenced by scanning tunneling microscopy/spectroscopy. Intriguingly, two new Dirac points were generated for the quasi-1D graphene superlattice located at −1.73 ± 0.02 and 1.12 ± 0.12 eV. Briefly, this work demonstrates that the periodic modulation effect of reconstructed metal substrates can dramatically alter the electronic properties of graphene and provides insight into the modulation of these properties using 1D potentials. KEYWORDS: graphene, scanning tunneling microscopy/spectroscopy, atomic structure, new Dirac points, Au(001)

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physical studies. Various paths have been reported previously, such as cutting graphene into graphene nanoribbons,17,18 chemical doping, and top gating.19−21 Louie et al. predicted theoretically that both two-dimensional (2D) and one-dimensional (1D) periodic potentials could be used to modulate the group velocities of charge carriers, thereby inducing the generation of new Dirac points. The energy differences between the new and original Dirac points were predicted to be inversely proportional to the superlattice period (L) according to theoretical calculations.22,23

raphene has attracted considerable attention because of its exceptional electronic, thermal, and optical properties and its high potential for applications such as high-frequency transistors, transparent electrodes, touch screens, and photodetectors.1−6 Several preparation methods have been developed for producing monolayer graphene, including mechanical exfoliation from graphite,1 thermal evaporation from silicon carbide (SiC) crystals,7−9 chemical reduction of graphene oxide,10 and chemical vapor deposition (CVD). Among these, CVD has been shown to be superior for synthesizing graphene layers with a large area, large domain size, and uniform thickness on Cu and Ni substrates.2,11−16 Tuning the energy band of graphene has been a top focus in current research, with significance for both applications and © 2016 American Chemical Society

Received: April 15, 2016 Accepted: August 1, 2016 Published: August 1, 2016 7550

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Figure 1. STM characterization of the quasi-1D graphene superlattices formed on hex-reconstructed Au(001) facets on Au foils. (a) Atomic model of the hex-reconstruction of the Au(001) face, characterized by a striped pattern (∼1.44 nm periodicity) and a schematic of two neighboring reconstructed Au(001) domains with a relative rotational orientation of 90°. (b) Large-area STM image of two reconstructed Au(001) domains (−1.02 V, 1.00 nA; 300 K; 150.00 nm × 150.00 nm) after graphene CVD. (c) Further magnification of the black rectangular region in (b), showing the 90° rotation of stripes from different domains (−1.00 V, 0.93 nA; 300 K; 25.00 nm × 25.00 nm). (d) Atomically resolved STM image showing the formation of a striped graphene superlattice (−0.02 V, 4.03 nA; 300 K, 10.00 nm × 10.00 nm) over the similarly striped hex-reconstructed Au(001) pattern. Inset: typical honeycomb lattice of graphene. (e) Height profile along the red line in (d), showing a period of ∼1.41 ± 0.03 nm between neighboring graphene stripes, which is nearly equivalent to that of the striped hex-Au(001).

inversely proportional to the superlattice period (L). Notably, traditional graphene ripples and wrinkles exhibit irregular shapes and random spatial distributions, and the effect of the new Dirac points on the global properties of graphene remains unknown. As reported previously, periodic 1D graphene moirés are difficult to realize on weakly coupled high-index metal surfaces because high-index metal surfaces become unstable at the high temperatures necessary for graphene growth. The reconstructed low-index surface of Au(001) may serve as a good template because the top layer of atoms changes from a 1 × 1 square lattice to a quasi-hexagonal (hex) lattice under appropriate annealing conditions, and a quasi-1D stripe-like reconstruction pattern evolves with a period of ∼1.44 nm.31 In this work, we successfully synthesized monolayer graphene on hex-reconstructed facets of Au(001). With the assistance of high-resolution scanning tunneling microscopy (STM), we found that the formed graphene presented a quasi1D striped superlattice resembling that of the reconstructed Au(001) substrate. Two new Dirac points were detected at −1.73 and 1.12 eV by scanning tunneling spectroscopy (STS). These new Dirac points were mediated by the 1D periodic potential effect arising from the graphene superlattice. Thus, this work demonstrates that the electronic properties of graphene can be modulated by 1D periodic potentials, resulting in the evolution of new Dirac points in the energy band.

However, the direct manufacture of nanoscale periodic potentials for application to graphene remains challenging. As an alternative method, by constructing heterostructures of graphene with other layered materials, graphene superlattices have been generated by the lattice mismatch effect in specific systems, such as graphene on hexagonal boron nitride (hBN).24,25 New Dirac points were detected in graphene/h-BN heterostructures in which the two composite layers possessed zero or slight rotation, wherein the graphene superlattices served as periodic potential for charge carriers.23,24 Accordingly, the transport properties of electrons in graphene were strongly altered, showing additional resistance peaks in graphene-based field-effect transistor (FET) devices.26,27 Several fundamental physical issues have been elucidated since, such as the splitting of flat Laudau level bands into Hofstadter minibands by a hierarchy of self-similar minigaps.25,26,28 Reports of new Dirac points generated by 1D-shaped periodic modulations, or moiré patterns, are rare. Hu’s group very recently constructed a 1D graphene superlattice by growing graphene on a high-index reconstructed Cu(410)−O surface using low-pressure CVD (LP-CVD).29 A 1D moiré superlattice (L ∼ 2.6 nm) was obtained that could tune the electronic properties of graphene by generating two new Dirac points located at ±900 meV. This result was in good agreement with the previous theoretical calculations.24 Subsequently, Lee’s group reported the availability of individual 1D graphene ripples within graphene/h-BN/Cu foils, which also resulted in the generation of new Dirac points in the electronic energy band of graphene.30 Furthermore, the energy difference between the new and original Dirac points was, as predicted, 7551

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Figure 2. Orientation of the graphene lattice (along the zigzag direction) relative to that of the striped graphene superlattice (30° rotated). (a−c) Atomically resolved STM images of various graphene domains on hex-reconstructed Au(001): −0.08 V, 5.19 nA, 5.00 nm × 5.00 nm; −0.10 V, 5.19 nA, 10.00 nm × 10.00 nm; and −0.10 V, 4.78 nA, 10.00 nm × 10.00 nm. (d) Corresponding 2D FFT pattern of (a), showing diffraction spots from graphene and the striped superlattice in yellow and blue circles, respectively. (e) Simulation of the stacking order of graphene on hex-reconstructed Au(001) with the graphene zigzag direction rotated by 30° relative to that of the graphene striped pattern. (f) Statistical distribution of the relative rotation angles.

RESULTS AND DISCUSSION A simplified sphere model is presented in Figure 1a that describes the hex-reconstruction of Au(001). This hexreconstruction was reported to be caused by the rearrangement of the first layer of Au(001) atoms into a more closely packed hexagonal lattice resembling that of Au(111), with the evolution of striped superstructure patterns (Figure 1a).31 In the schematic model, the vectors a1 and a2 represent the lattice vectors of the hex-reconstructed layer, whereas b1 and b2 represent the lattice vectors of Au(001); |a1| = |a2| = 0.276 nm and |b1| = |b2| = 0.288 nm. According to the literature,31 a1 has two possible directions: [110] and [−110]. The symmetry difference and lattice mismatch between the first two layers of Au(001) (hex-Au(001) and 1 × 1 Au(001)) resulted in the formation of a large-period striped pattern (red shadowed area) with a periodicity of ∼1.44 nm between adjacent stripes. The orientation of the stripes is well-aligned with that of a1. In Figure 1a, a1 is aligned along [−110] but could also be aligned along [110]. Therefore, two types of reconstructed Au(001) domains should be produced that exhibit orientations rotated by 90° relative to each other, as schematically presented in the rightmost panel of Figure 1a. An atmospheric-pressure CVD (AP-CVD) system was selected to synthesize graphene directly on Au foils according to a previously reported growth method.32 X-ray photoelectron spectroscopy (XPS) was performed to confirm the formation of graphene on the Au foils (Figure S1). Interestingly, after graphene growth, the Au foils were found to be mainly composed of (001) facets, as shown in Figure S2 in the

Supporting Information. Figure 1b is a large-scale STM image of graphene on hex-reconstructed Au(001), in which a dashed line distinguishes between the different graphene domains of A and B. A magnified view of the region on the boundary between the two neighboring domains (black rectangular area in Figure 1b) reveals the same striped patterns in both regions but with a relative rotational angle of 90° (Figure 1c). Within the specific domain, this long-range ordering was perfectly preserved and exhibited good coincidence with that of the hex-reconstructed Au(001). To provide more solid proof of the existence of hexAu(001) underneath the graphene layer, low-energy electron diffraction data are also supplied in the Supporting Information (Figure S3). To further verify the formation of graphene layers on the hex-reconstructed Au(001) surface, atomically resolved STM images are presented in Figure 1d and the inset thereof, showing the characteristic honeycomb structures of the graphene lattice. A large-period modulation is also seen in the magnified image, as characterized by the slightly curved, striped patterns. According to the section-view depth profile analysis shown in Figure 1e, this superlattice has a height of ∼0.05 nm and a periodicity of ∼1.41 ± 0.03 nm. Thus, graphene can be considered to imitate the surface morphology of the striped Au(001) hex-reconstruction. Notably, a similar phenomenon was reported by Stensgaard et al. for a graphene/ Pt(001) system, in which the graphene superlattice’s periodicity and orientation were similar to those of the hex-reconstructed Pt(001).33 In general, the periodicity and shape of the graphene superlattice and the domain orientations of the graphene itself 7552

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Figure 3. Orientations of the graphene lattices in two neighboring graphene domains and related electron-scattering effects at the domain boundary. (a) Large-area STM image of the graphene grain boundary crossing two neighboring hex-Au(001) domains (−0.03 V, 8.36 nA; 300 K, 50.00 nm × 50.00 nm). (b) Magnified STM image of the left graphene domain marked “L” within the rectangle shown in (a) (−0.02 V, 13.47 nA; 300 K, 8.00 nm × 8.00 nm). (c) Magnified STM image of the right graphene domain “R” (−0.02 V, 4.43 nA; 300 K, 8.00 nm × 8.00 nm). (d) Magnified STM image on the graphene domain boundary “G′′ (−0.02 V, 13.47 nA; 300 K, 10.00 nm × 10.00 nm). (e) Electronscattering effect at the domain boundary over the rectangle in (d) (−0.02 V, 14.23 nA; 300 K, 4.00 nm × 4.00 nm). (f) Corresponding 2D FFT pattern of (e) showing the formation of the graphene reciprocal lattice (GRL).

30° angle relative to the direction of the striped superlattice (or the [−110] direction of hex-Au(001) and the a1 direction in Figure 1a). Notably, a similar 30° rotation between the graphene lattice and the hex-reconstructed Au was also reported for graphene on Au(111).35 Nevertheless, further theoretical calculations are highly desired to fully understand the effects of hex-Au(001) on the stacking geometry of graphene. As mentioned before, the graphene lattice is usually rotated by 30° relative to the striped graphene superlattice and the hexreconstructed Au(001) striped pattern. The hex-reconstructed Au(001) layer is typically composed of domains with relative rotations of 90°.31 Therefore, it is reasonable to deduce that many graphene domain boundaries should exist on the hexreconstructed Au(100) surface. Additionally, the rotational angles of the graphene domains should be 30° rather than 90° because of the hexagonal symmetry of graphene. This was also demonstrated by selected area electron diffraction (SAED) analysis of a transferred graphene sample on a Cu transmission electron microscopy (TEM) grid (Figure S4). These graphene domain boundaries are also observable in atomically resolved STM images. As presented in Figure 3a, a boundary can be imaged as a bright white, crooked stripe, which occurs on one hex-reconstructed Au(001) domain and extends to the neighboring domain. To reconfirm the existence of an actual graphene domain boundary rather than the reconstructed Au(001) boundary, magnified STM images (Figure 3b,c) were obtained from both sides of the boundary, and both revealed characteristic honeycomb lattices and

are closely related to those of the hex-reconstructed Au(001) substrate. Therefore, the striped graphene pattern can be considered a quasi-1D graphene superlattice. Further investigation revealed that the zigzag direction of most graphene domains has a constant rotational angle of 30° relative to that of the striped pattern. Additional atomically resolved STM images related to this phenomenon are displayed in Figure 2a−c. In Figure 2a, the atomic lattice of graphene is fitted by several regular red hexagons. The zigzag direction and the striped superlattice direction of graphene are rotated by 30° relative to each other. The statistical results, obtained by measuring more than 39 different areas using different tip conditions, demonstrate that ∼70% of the graphene domains present a similar relationship with the substrate (Figure 2f). A 2D fast Fourier transform (FFT) analysis of Figure 2a was also performed, as shown in Figure 2d. Graphene has a lattice constant (agraphene = 0.246 nm)34 much smaller than that of the striped superlattice (∼1.41 nm), and their symmetries are quite different. Therefore, the 2D FFT spots can reliably distinguish between graphene (six spots arranged hexagonally) and the striped superlattice (two spots in the center) (Figure 2d). These data reconfirm the preferential orientation of graphene (rotated by 30°) relative to the hex-reconstruction of the Au(001) substrate. Based on the 30° relationship between the zigzag and stripe directions of graphene, the specific stacking order for graphene on hex-Au(001) can be determined, as indicated by the simplified simulation shown in Figure 2e. The graphene (along the zigzag direction) is demonstrated to preferentially align at a 7553

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Figure 4. Electronic properties of the quasi-1D graphene superlattice by STM/STS. (a) STM image of graphene on mostly hex-reconstructed Au(001) (1.5 V, 0.70 nA; 78 K, 35.00 nm × 35.00 nm). (b,c) dI/dV spectra (1.5 V, 0.70 nA; 78K, Vrms = 10 mV, f = 932 Hz) of graphene captured from the regions with and without striped patterns in (a), with selected locations indicated by triangles and circles at intervals of ∼5.00 nm. (d) Magnification of dI/dV spectra (1.5 V, 0.70 nA; 78 K, Vrms = 10 mV, f = 932 Hz) for graphene with and without striped modulations as seen in (a), corresponding to the top and bottom spectra, respectively.

interesting. Theoretical work has already predicted that both 1D and 2D periodic potentials could generate new Dirac points in graphene by renormalizing the group velocity for states with k, the wavevector of the Bloch state defined relative to the Dirac point.22 Compared to 2D periodic potentials, a 1D periodic potential can completely reduce the group velocity to zero, with k perpendicular to the direction of the potential.22 However, the challenges of generating such nanoscale 1D periodic potentials have greatly retarded related experimental explorations. Figure 4a shows an STM image of graphene on hexreconstructed Au(001). Notably, some striped patterns are missing in the lower part of the image, presenting areas of uniform black contrast (called “black areas” in the figure). This phenomenon is occasionally observed experimentally (Figure S5) and is likely attributable to imperfections in the hexreconstruction of Au(001).40 However, this sample area provides an excellent opportunity to confirm the tuning effect of quasi-1D modulations on the electronic properties of graphene. To improve the reliability of the STS data, two sets of locations (marked along the arrows in Figure 4a with an interval of ∼5 nm)perpendicular and parallel to the stripe directionwere collected, as shown in Figure 4b,c, respectively. Comparing the spectra reveals that two dips (located at approximately −1.73 ± 0.02 and 1.12 ± 0.12 eV) appear at every point in the striped regions (marked with a solid arrow) but are absent (as indicated by dashed arrows) in the “black areas” possessing no obvious striped modulations (Figure 4b,c).

periodic striped patterns. Moreover, the graphene lattices from orthogonal domains present a relative rotational angle of 30°. This finding further confirms the formation of monolayer graphene flakes across neighboring hex-Au(001) domains. Domain boundaries are typically reported as negatively impacting the transport properties of graphene because of the strong electron-scattering effect.36−38 As presented in Figure 3d, this electron-scattering pattern appears near the domain boundary, as indicated by the rectangle. Magnifying this region (Figure 3e) reveals a lattice larger than that of graphene and a periodicity of ∼0.38 nm. The 2D FFT pattern corresponding to Figure 3e shows six bright outer points characteristic of the graphene reciprocal lattice and a pair of inner points corresponding to the K and K′ points of the Brillouin zone (BZ) of graphene (Figure 3f). This finding reflects the electronscattering effect from the graphene domain boundary.39 Furthermore, the distance from the inner to the outer points is 2.69 nm−1, corresponding to a value of ∼0.37 nm in real space, in good agreement with the measured value (∼0.38 nm) shown in Figure 3e. This value is approximately the Fermi wavelength of graphene: λF = 3a/2 = 0.369 nm (instead of λF/2 with agraphene = 0.246 nm). Therefore, the scattered waves are confined within the C−C bond of graphene, and only weak coupling occurs between the graphene and the Au substrate. This result agrees with those of previous reports, in which the scattering wavelengths were reported to be ∼0.370 nm for graphene on SiO237 and ∼0.362 nm on Ag(111).39 At this stage, investigating the ability of the striped graphene superlattice (arising from the hex-reconstructed Au(001)) to modulate the electronic properties of graphene would be 7554

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Figure 5. Macroscopic properties of CVD graphene synthesized from Au foils. (a,b) SEM images of full-coverage, as-grown graphene (25.00 μm × 25.00 μm; 25.00 μm × 25.00 μm). (c) OM image of graphene transferred on a SiO2/Si substrate (110.00 μm × 110.00 μm), showing a uniform color contrast. (d) TEM image confirming the monolayer nature of the CVD graphene. (e) Comparative Raman spectra of graphene before and after sample transfer, reconfirming its monolayer nature. (f) Raman mapping (2D/G) image of graphene transferred on SiO2/Si (20.00 μm × 25.00 μm). (g) Hall mobility of transferred graphene. (h) Optical image of a Hall bar structure of graphene.

mapping data of graphene on hex-reconstructed Au(001) and Au(001) areas (Figure S6 of the Supporting Information). Apart from the STM/STS measurements, other characterizations, such as scanning electron microscopy (SEM), optical microscopy (OM), TEM, and Raman spectroscopy, were also performed to reveal the macroscopic properties of CVD-grown graphene on Au foils, which is mainly composed of reconstructed Au(001). As presented in Figure 5a, nearly fullcoverage monolayer graphene was synthesized on the Au foil via CVD. The naked substrate regions for incomplete samples are usually characterized by regions of white contrast (Figure 5b). The uniform film thickness is further confirmed by the uniform OM contrast of the graphene transferred onto a SiO2/ Si substrate (Figure 5c). Additionally, as evidenced by the TEM section-view analysis in Figure 5d, the graphene primarily exists as a monolayer. To further confirm the thickness of the graphene layer, Raman spectral measurements of graphene on both Au foils and SiO2/Si substrates were also collected (Figure 5e). Notably, the characteristic peaks of graphene are significantly suppressed by the large background signal of the Au substrate. However, after being transferred onto a Si/SiO2 substrate, the typical 2D (2684 cm−1) and G bands (1589 cm−1) can be clearly identified with a 2D/G intensity ratio of ∼1.4 and a narrow full width at half-maximum (fwhm) of the 2D peak of ∼38 cm−1, suggesting the monolayer nature of the graphene. The strong D bands are ascribed to the existence of abundant grain boundaries, as mentioned in Figure 3. Furthermore, Raman mapping of the ratio of 2D to G peaks (Figure 5f) shows a relatively uniform contrast, again indicating the rather high uniformity in the thickness of the synthesized sample. Based on these characterizations, it can be inferred that the CVD graphene synthesized on Au foils mainly exists as a monolayer thickness and has high sample quality. In addition, compression strain should exist in the graphene/Au foil system, and its influence on the electronic properties of graphene is discussed in Figure S7 of the Supporting Information.

Before the origin of these dips is discussed, it is necessary to confirm full coverage of the area with graphene. As indicated by the gray-shaded area in Figure 4b,c, the dI/dV signals are very similar near the Fermi energy. For clarity, magnified spectra ranging from −0.5 to +0.5 eV are presented, showing the characteristic Dirac cone-like feature of graphene at ∼0 eV on both the striped and “black” areas (Figure 4d).41 This nondoped graphene feature can be explained based on the weak interfacial coupling between graphene and Au(001), as mentioned previously in Figure 3e. According to theoretical predictions, these two dips in the dI/dV spectra correspond to two new Dirac points arising from the tuning effect of the quasi-1D potential generated by the graphene superlattice, which should be located on the boundary of the mini-BZ.22 Because the linear energy dispersion between energy (ε) and momentum (κ) for graphene is ε = ℏνFκ, where ℏ is the reduced Plank constant (1.05 × 10−34 j·s), νF is the Fermi velocity (106 m/s), κ = π/d, and d = ∼1.41 nm (Figure 1e), the two new Dirac points should be located at ±1.46 eV. Notably, this calculation is in good agreement with the experimental results: −1.73 ± 0.02 and 1.12 ± 0.12 eV, respectively. The small deviation is attributed to the inhomogeneity of the velocities of electrons and holes; in the current system, these velocities are 1.18 × 106 m/s for electrons and 0.77 × 106 m/s for holes when applying ε = −1.73 ± 0.02 eV for electrons and 1.12 ± 0.12 eV for holes in the formula. This inhomogeneity is most likely induced by both nextnearest-neighbor hopping and next-nearest-neighbor interlayer hopping effects, as described in recent literature.24,42 To summarize, the quasi-1D Au reconstruction (∼1.44 nm in period) promotes the formation of a quasi-1D graphene superlattice (∼1.41 ± 0.03 nm in period) and leads to the generation of two new Dirac points in the energy band of graphene. Moreover, the modulation effect is very sensitive to the substrate reconstruction, as indicated by the dI/dV 7555

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ACS Nano The transferred graphene film was also used to construct a Hall bar device for measuring the Hall mobility (Figure 5g,h). The carrier mobility of the sample was estimated to be ∼625 cm2·V−1·s−1, nearly 10 times higher than the value reported for graphene synthesized on Au(111).32 Considering the existence of numerous grain boundaries and the related electronscattering phenomenon, this change in the transport properties seems reasonable.

XPS data are presented in Figure S1 to demonstrate the existence of graphene on Au. Electron backscattered diffraction data are shown in Figure S2 and reflect the (001)-facet-textured feature of Au foils after annealing. Low-energy electron microscopy data are presented in Figure S3 to confirm the existence of the hexreconstruction of Au(001) under the graphene layer. SAED data are presented in Figure S4 to show that most of the graphene has a constant angle relative to the Au(001) substrate. More STM data are displayed in Figure S5, illustrating the imperfect hex-reconstruction of Au(100). Furthermore, dI/dV mapping data are supplied in Figure S6 to show the sensitivity of graphene to modulation by Au(001) reconstruction. Finally, Raman mapping data are provided in Figure S7 to indicate the existence of compression strain in graphene/Au foils (PDF)

CONCLUSION In summary, we verified that CVD-grown graphene on hexAu(001) behaves similar to a quasi-1D striped superlattice with a periodicity of 1.41 ± 0.03 nm, as mediated by the same striped reconstruction of the substrate. STM characterizations demonstrated that the orientation of the graphene lattice has a constant rotational angle (30°) relative to the striped superlattice. We also report that the striped graphene superlattice induced by the hex-reconstruction of Au(001) can act as a quasi-1D periodic potential, modifying the electronic structure of graphene to the extent of generating two new Dirac points, as confirmed by STM/STS. In this regard, this work should have profound implications with regard to tuning the electronic properties of graphene by applying a 1D periodic potential. We anticipate that this work will inspire more intensive investigations of the unique properties of such 1D systems.

AUTHOR INFORMATION Corresponding Authors

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

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (Grants Nos. 2016YFA0200103, 2012CB921404, 2012CB933404, and 2013CB932603), the National Natural Science Foundation of China (Grants Nos. 51472008, 51290272, 51222201, 21201012, 51121091, 51072004, and 51201069), the Beijing Municipal Science and Technology Planning Project (No. Z151100003315013), and the ENN Energy Research Institute.

METHODS Growth Procedures. Au foils (8 mm × 8 mm, 0.025 mm thick, 99.99 wt % metal basis) were purchased from Alfa Aesar. In a typical CVD reaction, a piece of Au foil was loaded into a quartz tube (outer diameter: 1 in., 1 in. = 2.54 cm), which was pumped down to a base pressure of 1 Pa and flushed repeatedly with H2 gas to guarantee a favorable growth atmosphere. Then, the sample was heated to 980 °C under flowing H2 gas (30 sccm) and annealed at 980 °C for the required duration of 10 h. For the growth of graphene, the Au foil was ramped to 950 °C and kept at 950 °C for 30 min in H2 (30 sccm) and Ar (200 sccm) to remove any residual carbon, followed by the introduction of CH4 (1.5 sccm) to initiate the growth of graphene. After a growth time of 10 min was maintained, the Au sample was quickly taken out of the high-temperature zone and quickly cooled to room temperature under a flow of H2 and CH4. Characterizations. The samples were characterized by OM (Olympus BX51), SEM (Hitachi S-4800, 2 kV), XPS (Kratos Analytical AXIS-Ultra with monochromatic Al Kα X-ray), Raman and photoluminescence (Renishaw, Invia Reflex, excitation light of 514 nm in wavelength), and TEM (FEI Tecnai G2 F20, acceleration voltage 200 kV) techniques. Ultrahigh vacuum (UHV) low-temperature STM/STS characterizations were conducted under a base pressure lower than 10−10 mbar. The STS spectra were measured at ∼78 K by recording the output of a lock-in system with the manually disabled feedback loop. A modulation signal of 10 mV at 932 Hz was selected under tunneling conditions of 1.20 V and 0.20 nA. Prior to the STM observations, the CVD graphene/Au foil samples were loaded into a UHV system and then degassed at approximately 800 K for several hours under a base pressure lower than 10−9 mbar. Angleresolved photoemission spectroscopy and microprobe low-energy electron microscopy data were acquired at the X-ray photoemission electron microscopy end station of the 09U (Dreamline) beamline of the Shanghai Synchrotron Radiation Facility.

REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (2) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706−710. (3) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, J.; Kim, K. S.; Ö zyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-To-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. (4) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. Graphene Photonics and Optoelectronics. Nat. Photonics 2010, 4, 611−622. (5) Balandin, A. A. Thermal Properties of Graphene and Nanostructured Carbon Materials. Nat. Mater. 2011, 10, 569−581. (6) Novoselov, K. S.; Fal'ko, V.; Colombo, L.; Gellert, P.; Schwab, M.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192−200. (7) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; Heer, W. A. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312, 1191−1196. (8) Ohta, T.; El Gabaly, F.; Bostwick, A.; McChesney, J. L.; Emtsev, K. V.; Schmid, A. K.; Seyller, T.; Horn, K.; Rotenberg, E. Morphology of Graphene Thin Film Growth on SiC (0001). New J. Phys. 2008, 10, 023034. (9) Zhou, S. Y.; Gweon, G.-H.; Fedorov, A. V.; First, P. N.; de Heer, W. A.; Lee, D.-H.; Guinea, F.; Castro Neto, A. H.; Lanzara, A.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02548. 7556

DOI: 10.1021/acsnano.6b02548 ACS Nano 2016, 10, 7550−7557

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ACS Nano Substrate-Induced Bandgap Opening in Epitaxial Graphene. Nat. Mater. 2007, 6, 770−775. (10) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as A Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (11) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (12) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Large Area, Few-layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 2009, 9, 30−35. (13) Gao, L.; Guest, J. R.; Guisinger, N. P. Epitaxial Graphene on Cu (111). Nano Lett. 2010, 10, 3512−3516. (14) Gao, L.; Ren, W.; Xu, H.; Jin, L.; Wang, Z.; Ma, T.; Ma, L.-P.; Zhang, Z.; Fu, Q.; Peng, L.-M.; Bao, X. H.; Cheng, H.-M. Repeated Gowth and Bubbling Transfer of Graphene with Millimetre-Size Single-Crystal Grains Using Platinum. Nat. Commun. 2012, 3, 699. (15) Choubak, S.; Biron, M.; Levesque, P. L.; Martel, R.; Desjardins, P. No Graphene Etching in Purified Hydrogen. J. Phys. Chem. Lett. 2013, 4, 1100−1103. (16) Liu, X.; Fu, L.; Liu, N.; Gao, T.; Zhang, Y.; Liao, L.; Liu, Z. Segregation Growth of Graphene on Cu−Ni Alloy for Precise Layer Control. J. Phys. Chem. C 2011, 115, 11976−11982. (17) Han, M. Y.; Ö zyilmaz, B.; Zhang, Y.; Kim, P. Energy Band-Gap Engineering of Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. (18) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors. Science 2008, 319, 1229−1232. (19) Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752−1758. (20) Coletti, C.; Riedl, C.; Lee, D. S.; Krauss, B.; Patthey, L.; von Klitzing, K.; Smet, J. H.; Starke, U. Charge Neutrality and Band-Gap Tuning of Epitaxial Graphene on SiC by Molecular Doping. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 235401. (21) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S.; Waghmare, U.; Novoselov, K.; Krishnamurthy, H.; Geim, A.; Ferrari, A.; Sood, A. A. K. Monitoring Dopants by Raman Scattering in A Electrochemically Top-Gated Graphene Transistor. Nat. Nanotechnol. 2008, 3, 210−215. (22) Park, C.-H.; Yang, L.; Son, Y.-W.; Cohen, M. L.; Louie, S. G. Anisotropic Behaviors of Massless Dirac Fermions in Graphene Under Periodic Potentials. Nat. Phys. 2008, 4, 213−217. (23) Park, C.-H.; Yang, L.; Son, Y.-W.; Cohen, M. L.; Louie, S. G. New Generation of Massless Dirac Fermions in Graphene Under External Periodic Potentials. Phys. Rev. Lett. 2008, 101, 126804. (24) Yankowitz, M.; Xue, J.; Cormode, D.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Jacquod, P.; LeRoy, B. J. Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2012, 8, 382−386. (25) Ponomarenko, L.; Gorbachev, R.; Yu, G.; Elias, D.; Jalil, R.; Patel, A.; Mishchenko, A.; Mayorov, A.; Woods, C.; Wallbank, J.; Mucha-Kruczynski, M.; Piot, B. A.; Potemski, M.; Grigorieva, I. V.; Novoselov, K. S.; Guinea, F.; Fal’ko, V. I.; Geim, A. K. Cloning of Dirac Fermions in Graphene Superlattices. Nature 2013, 497, 594− 597. (26) Hunt, B.; Sanchez-Yamagishi, J.; Young, A.; Yankowitz, M.; LeRoy, B. J.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Ashoori, R. C. Massive Dirac Fermions and Hofstadter Butterfly in A van der Waals Heterostructure. Science 2013, 340, 1427−1430. (27) Yang, W.; Chen, G.; Shi, Z.; Liu, C.-C.; Zhang, L.; Xie, G.; Cheng, M.; Wang, D.; Yang, R.; Shi, D.; Watanabe, K.; Taniguchi, T.; Yao, Y.; Zhang, Y.; Zhang, G. Epitaxial Growth of Single-Domain Graphene on Hexagonal Boron Nitride. Nat. Mater. 2013, 12, 792− 797.

(28) Dean, C.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P. Hofstadter’s Butterfly and The Fractal Quantum Hall Effect in Moiré Superlattices. Nature 2013, 497, 598−602. (29) Lin, C.; Huang, X.; Ke, F.; Jin, C.; Tong, N.; Yin, X.; Gan, L.; Guo, X.; Zhao, R.; Yang, W.; Wang, E.; Hu, Z. Quasi-OneDimensional Graphene Superlattices Formed on High-Index Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 085416. (30) Jang, W.-J.; Lee, M. W.; Kim, H.; Park, S.; Jung, S. J.; Lee, S.; Song, Y. J.; Kahng, S.-J. Observations of New Dirac Points in OneDimensionally-Rippled Graphene on Hexagonal BN Using Scanning Tunneling Spectroscopy. J. Phys. Chem. C 2015, 119, 19535−19538. (31) Hammer, R.; Sander, A.; Förster, S.; Kiel, M.; Meinel, K.; Widdra, W. Surface Reconstruction of Au (001): High-Resolution Real-Space and Reciprocal-Space Inspection. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 035446. (32) Oznuluer, T.; Pince, E.; Polat, E. O.; Balci, O.; Salihoglu, O.; Kocabas, C. Synthesis of Graphene on Gold. Appl. Phys. Lett. 2011, 98, 183101. (33) Nilsson, L.; Andersen, M.; Bjerre, J.; Balog, R.; Hammer, B.; Hornekær, L.; Stensgaard, I. Preservation of The Pt (100) Surface Reconstruction after Growth of a Continuous Layer of Graphene. Surf. Sci. 2012, 606, 464−469. (34) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Controlling the Electronic Structure of Bilayer Graphene. Science 2006, 313, 951−954. (35) Nie, S.; Bartelt, N. C.; Wofford, J. M.; Dubon, O. D.; McCarty, K. F.; Thürmer, K. Scanning Tunneling Microscopy Study of Graphene on Au (111): Growth Mechanisms and Substrate Interactions. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 205406. (36) Tsen, A. W.; Brown, L.; Levendorf, M. P.; Ghahari, F.; Huang, P. Y.; Havener, R. W.; Ruiz-Vargas, C. S.; Muller, D. A.; Kim, P.; Park, J. Tailoring Electrical Transport Across Grain Boundaries in Polycrystalline Graphene. Science 2012, 336, 1143−1146. (37) Koepke, J. C.; Wood, J. D.; Estrada, D.; Ong, Z.-Y.; He, K. T.; Pop, E.; Lyding, J. W. Atomic-scale Evidence for Potential Barriers and Strong Carrier Scattering at Graphene Grain Boundaries: A Scanning Tunneling Microscopy Study. ACS Nano 2013, 7, 75−86. (38) Yazyev, O. V.; Louie, S. G. Electronic Transport in Polycrystalline Graphene. Nat. Mater. 2010, 9, 806−809. (39) Kiraly, B.; Iski, E. V.; Mannix, A. J.; Fisher, B. L.; Hersam, M. C.; Guisinger, N. P. Solid-Source Growth and Atomic-Scale Characterization of Graphene on Ag (111). Nat. Commun. 2013, 4, 2804. (40) Höfer, A.; Duncker, K.; Kiel, M.; Wedekind, S.; Widdra, W. Adsorption of α-Sexithiophene on Au (001): Molecule-Induced Partial Lifting of the Substrate Reconstruction. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 075414. (41) Deshpande, A.; Bao, W.; Miao, F.; Lau, C. N.; LeRoy, B. J. Spatially Resolved Spectroscopy of Monolayer Graphene on SiO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 205411. (42) Meng, L.; He, W.-Y.; Zheng, H.; Liu, M.; Yan, H.; Yan, W.; Chu, Z.-D.; Bai, K.; Dou, R.-F.; Zhang, Y.; Liu, Z.; Nie, J.-C.; He, L. StrainInduced One-Dimensional Landau Level Quantization in Corrugated Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 205405.

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DOI: 10.1021/acsnano.6b02548 ACS Nano 2016, 10, 7550−7557