Toward Single-Layer Uniform Hexagonal Boron NitrideâGraphene

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Towards Single-layer Uniform Hexagonal Boron Nitride –Graphene Patchworks with Zigzag Linking Edges Yabo Gao, Yanfeng Zhang, Pengcheng Chen, Yuanchang Li, Mengxi Liu, Teng Gao, Donglin Ma, Yubin Chen, Zhihai Cheng, Xiaohui Qiu, Wenhui Duan, and Zhongfan Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl4021123 • Publication Date (Web): 12 Jun 2013 Downloaded from http://pubs.acs.org on June 13, 2013

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Towards Single-layer Uniform Hexagonal Boron Nitride –Graphene Patchworks with Zigzag Linking Edges Yabo Gao 1, Yanfeng Zhang 1,2*, Pengcheng Chen 3, Yuanchang Li 4, Mengxi Liu 1

, Teng Gao1, Donglin Ma1, Yubin Chen 1, Zhihai Cheng 3, Xiaohui Qiu 3, Wenhui Duan 4, 5, Zhongfan Liu 1*

1

Center for Nanochemistry (CNC), Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, People’s Republic of China

2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China 3

National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

4

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, People's Republic of China

5

Institute for Advanced Study, Tsinghua University, Beijing 100084, People's Republic of China *Corresponding Author: [email protected]; [email protected]

1 Environment ACS Paragon Plus

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Abstract The atomic layer of hybridized hexagonal boron nitride (h-BN) and graphene has attracted a great deal of attention after the pioneering work of P. M. Ajayan et al. on Cu foils because of their unusual electronic properties.1 However, many fundamental issues are still not clear, including the in-plane atomic continuity as well as the edge type at the boundary of hybridized h-BN and graphene domains. To clarify these issues, we have successfully grown a perfect single-layer h-BN-graphene (BNC) patchwork on a selected Rh(111) substrate, via a two-step patching growth approach. With the ideal sample, we convinced that at the in-plane linking interface, graphene and h-BN can be linked perfectly at an atomic scale. More importantly, we found that zigzag linking edges were preferably formed, as demonstrated by atomicscale STM images, which was also theoretically verified using density functional theory (DFT) calculations. We believe the experimental and theoretical works are of particular importance to obtain a fundamental understanding of the BNC hybrid and to establish a deliberate structural control targeting high-performance electronic and spintronic devices.


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Pristine graphene, a honeycomb lattice of sp2 hybridized carbon atoms, is a zero-gap semimetal, which therefore needs a suitable bandgap opening for traditional electronics.2 Great efforts have been made for the bandgap engineering by nanoribbon3,4 and nanomesh (antidot lattices)5–7 fabrication, chemical modification,8 and electrostatic gating9, etc. A very recent innovation along this direction is to create a hybrid structure of graphene and its structural analogue, a single-layer hexagonal boron nitride (h-BN).1,10 Aside from the bandgap opening of graphene, first principle calculation results suggest that this kind of h-BN-graphene (BNC) hybrid possesses unusual physical properties, such as magnetism,11 unique thermal transports,12 and robust half-metallic behavior.13,14 A recent theoretical calculation also reports a ubiquitous gap opening for the periodic BN-embedded graphene hybrid lattice (named as mosaic graphene).15 The hybrid films consisting of small h-BN and graphene domains were initially synthesized on Cu foils via a chemical vapor deposition (CVD) method using mixed methane and ammonia borane (NH3-BH3) as precursors. The obtained films had a small bandgap (~18 meV) opening.1 In the one-step growth process, the film underwent spontaneous phase separation into h-BN and graphene domains, which is consistent with the theoretical prediction.16 Very recently, the lateral heterostructure of graphene and h-BN was also prepared using a patterned regrowth procedure. Monolayer graphene was first synthesized on Cu foils, and patterned by using photolithography (PL) and reactive ion etching (RIE). h-BN was then grown on the surface to form monolayers.17 A similar work, which employed a different etching method and a different growth sequence of h-BN and graphene, was also performed to achieve such lateral heterostructures on Cu and Ni foils.18 However, investigating the structural details of the BNC hybrids, for example the atomic continuity and the


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edge structures at the boundary of h-BN and graphene domains, is impossible because of the highly-corrugated nature of Cu and Ni foils.19 In the present study, we prepared the hybrid BNC films on a selected single crystalline Rh(111) substrate via a two-step patching growth approach under UHV conditions. Using an in-situ high-resolution scanning tunneling microscope (STM), we revealed their atomic-scale continuity and edge structure (armchair or zigzag) at the linking boundary of the two analogues. To understand the physical origin of the perfect patching of the BNC hybrid, we also performed DFT theoretical calculations, including geometry optimizations, binding energy calculations, and energy band calculations. The growth procedure of the BNC hybrid is schematically shown in Fig. 1a. Discrete h-BN islands were first grown on Rh(111). Graphene was then patched onto the uncovered surface until a complete hybrid monolayer evolved. This growth process was named as a two-step patching growth process. By controlling the growth time of h-BN, the area ratio of graphene domains in BNC hybrids can be easily modulated from 6.6% to 62% (corresponding to the surface ratio of h-BN) (see Supplementary Fig. S1). Figure 1b is a typical SEM image of the hybrid with h-BN and graphene showing bright and dark contrasts, respectively. This spontaneous formation of individual h-BN and graphene domains can be further verified by X-ray photoemission spectroscopy (XPS) measurements. Figure 1c shows the peaks of the B, N, and C 1s spectrum of the BNC hybrid, which are located at 190.10, 397.57, and 284.38 eV, respectively, well consistent with the published results (see supplementary Fig. S2).1,17 When imaged via high-resolution STM (Figs. 1d and e), h-BN and graphene exhibit remarkably different superstructures on Rh(111), which greatly facilitates the


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identification of each analogue as well as the bonding nature at the interface. The atomically resolved STM image (Fig. 1d) of h-BN on Rh(111) shows a periodic nanomesh superstructure over the atomic lattice, which is consistent with the published results.20,21 This kind of superstructure arises from the coincidence lattice of (13×13) B-N on (12×12) Rh(111) with a period of ~3.2 nm.20,21 In fact, different STM contrasts of this superstructure can be obtained under different scanning voltage and current conditions of STM (see supplementary Fig. S3). As for graphene on Rh(111), the STM morphology presents a typical triangular superlattice (Fig.1e), which originates from the coincidence lattice of (12×12) C-C on (11×11) Rh(111) with a period of ~3.0 nm, the same as that of h-BN. Similarly, various STM contrasts can be obtained at different tunneling conditions (see Supplementary Fig. S4). The different superstructure contrasts for h-BN and graphene can be explained by the different spatial occupations of B, N and C atoms on Rh(111), which correspond to disparate overlayer-substrate interactions.21,25 Figure 1f displays a representative STM image focusing on the boundary of hBN (right) and graphene (left) domains on Rh(111). The circled regions (with a missing of an h-BN moiré) showing a depressed height of ~0.264 nm were identified to be the bare Rh(111) substrate, as evidenced by the atomically-resolved Rh(111) lattice. Thus, the neighboring h-BN can be assigned to be monolayer. The sectionview analysis across the two domains reveals a similar apparent height(Fig. 1g along L1), which illustrates that graphene and h-BN can be positioned in the same plane. This in-plane patching growth is highly characteristic over the whole Rh(111) substrate. From a large-scale STM image shown in Fig. 2a, the boundary linking hBN and graphene can be noticed to spread continuously over several Rh(111) terraces. Along the boundary, h-BN and graphene possess a very sharp interface with each


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other (Figs. 2a, b). With the zoom-in STM images focused at the boundary (Figs. 2c, d), both superstructures and atomic lattices can be simultaneously resolved. The atomic rows of two analogues are noticed to be perfectly linked to each other, so that an atomic-scale seamless connection is evolved. These experimental observations strongly suggest that the two-step patching growth in UHV conditions resulted in a perfect monolayer hybrid of h-BN and graphene on Rh(111). It is amazing to achieve such an atomic-scale seamless patching in the hybrid film, considering the non-negligible lattice mismatch, the overlayer-substrate distances, as well as the moiré corrugations of graphene and h-BN. Firstly, it is proposed that the slight difference in crystalline lattice (1.7%) of graphene (0.246 nm) and h-BN (0.250 nm) may be accommodated via in-plane bond length variations. Secondly, graphene and h-BN may present a similar apparent height on Rh(111). This can be visually proved by STM data, where the apparent heights of hBN and graphene were both about 0.270 nm. First principle calculations were also carried out to estimate the distance between graphene or h-BN and Rh(111), and a similar value of ~0.215 nm was obtained. Apparently, almost no height matching barrier exists for the seamless patching growth of BNC on Rh(111). Lastly, both h-BN and graphene have corrugated surfaces of ~0.17 and ~0.16 nm due to the superstructure formation, as estimated from the STM height profiles (Figs. 3a, b). DFT calculations, which considered the effect of interfacial van der Waals forces, were also conducted (Figs. 3c, d). It is worthy of mentioning that, the inclusion of van der Waals interaction in the DFT calculations can generate more reasonable surface contour data to match up to the experimental results (see Supplementary Fig.S5).26 As seen from the section views in Fig. 3e, the calculated surface corrugation values are 0.124 nm for h-BN/Rh(111) and 0.118 nm for


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graphene/ Rh(111). This similarity in surface roughness ensures an in-plane seamless connection of h-BN and graphene domains, and these are in agreement with our experimental result. By contrast, without considering the van der Waals forces in geometry optimizations, earlier DFT calculations showed much different corrugations for h-BN and graphene (~0.055 nm vs.~0.16 nm) on Rh(111).23,25 This remarkable difference would lead to significant bonding strains and defects at the linking boundary, which cannot be used to account for the current experimental data. In brief, h-BN and graphene with a similar crystalline lattice, a consistent overlayer-substrate distance and a highly-comparable surface corrugation are the ideal combination for achieving the perfect patching growth. DFT calculations were also carried out to understand the energetic origin of the preferable formation of BNC hybrid. The binding energies of h-BN, graphene, and BNC hybrids were calculated using atomic models of (12×12) C-C on (11×11) Rh(111), (13×13) B-N on (12×12) Rh(111), and BNC overlayer on (12×12) Rh(111) with the inclusion of three Rh(111) layers, and by considering the geometric optimization of the bottom Rh layers (Fig. 4a). For BNC hybrids, a (13×13) overlayer on (12×12) Rh(111) supercell was selected to simplify the calculation. The two supercells follow the lattice mismatch between h-BN and Rh(111), and the presence of a slight strain in the graphene sheet is inevitable.24, 27 Generally, the binding energy (Eb) per Rh atom can be expressed as Eb = [E(layer)+E(sub)-E(layer-sub)]/n, (1) where E(layer), E(sub), and E(layer-sub) denote the energy of the overlayer, the substrate and the total energy of the system, respectively. n represents the number of atoms in the substrate unit cell.


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The binding energies of graphene and h-BN per Rh atom are calculated to be 0.36 and 0.37 eV, respectively (Fig. 4a). For BNC hybrids, the binding energies depend on the specific configurations, e.g., the bonding type at the linking boundary of the two analogues, the domain size of each component, and the embracing type. DFT calculations have been performed on many BNC hybrids with different domain sizes and bonding types in this work. The bonding type variations include pure B or N zigzag edges, armchair edges, and mixed zigzag and armchair edges. It is intriguing to find that the binding energies per Rh atom for the BNC hybrid are always larger than 0.39 eV. Two examples of the BNC hybrids, namely, graphene embracing BN (hBN@G) and BN embracing graphene (G@h-BN), are shown in Fig. 4b, and the binding energies are 0.41 and 0.46 eV, respectively. Obviously, the BNC hybrids on Rh(111) always have larger binding energies than those of pure graphene and h-BN. Therefore, from the energetic point of view, hBN and graphene domains prefer to link with each other to form a BNC hybrid during the patching growth process. This issue can be further proved by examining the initial growth of graphene in the second step of the growth process. Graphene is noticed to preferably grow from the edge of h-BN domains, forming nearly compact islands are formed (Fig. 4c)(see supplementary Figs. S6). Moreover, the patching can be atomically perfect, as shown by the sequential zoom-in images on the linking boundaries (Figs. 4d, e). The reason why h-BN and graphene can be bonded together is considered that the preexisting h-BN edge, having a large number of dangling bonds, can only be saturated by attaching graphene, and this patching is simultaneously mediated by the underlying Rh(111) lattice. Another important issue for the novel hybrid is the identification of the edge structures at the boundary linking h-BN and graphene domains, since recent


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theoretical studies indicate that different edge structures may lead to disparate physical properties of BNC hybrids. For instance, BNC hybrids with zigzag edges exhibited half metallicity and spin polarization effects.14, 28–30 The most direct way for edge type identification is from the atomically resolved images of the linking boundaries. Figs. 5a, b, Figs. 5c, d, and Figs. 5e, f present the large-scale and atomically-resolved images of linking edges, corresponding to typical armchair, zigzag, and the transition from zigzag to armchair types as guided by the fitted ball models, respectively. A more effective method is established that the edge type identification relies simply on moiré-scale STM images. An orientation correlation between the superstructure unit cell and the atomic chains at the linking boundary is addressed in Supplementary material of Figs. S7-S10. The orientation of the zigzag edge is parallel to the orientation of the shorter diagonal of the unit cell of rhombus h-BN superstructure, whereas the orientation of the armchair edge is parallel to that of the shorter diagonal. The proportion of different edge structures over the whole hybrid BNC film on Rh(111) was calculated by combining the two methods. The proportion of the zigzag edge vs. armchair edge is 77.64% : 22.36% (Fig.5g). This result strongly suggests that zigzag edges are more preferred at the hybrid boundaries than armchair edges during the two-step patching growth. Note that, this experimental result is slightly different from previous theoretical predictions.14,

30

In one of the reported

results, the suspended hybrid (h-BN embracing graphene with atomic number n>100) with a zigzag linking edge has a slightly higher binding energy than that of the armchair edge14, which hence would result in a nearly equal occurrence probability for the two types of edges. This discrepancy is believed to be mediated by the contribution of the Rh(111) substrate.23


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Generally, the N atom is repelled from Rh(111), whereas the B atom is attracted by the surface. Therefore, for an armchair edge, the N and B atoms suffer from opposite forces by the underlying Rh(111) substrate. This phenomenon may induce bond distortion on the adjacent carbon atoms close to the boundary edge. As a result, the armchair edges are not energetically preferable in BNC hybrids. This preliminary interpretation has been supported by our DFT calculations. As shown in Fig. 5h, for a BNC hybrid, which consists of nearly equal B, N, and C atoms, the armchair, N-C zigzag, and B-C zigzag edges have binding energies of 0.39, 0.41, and 0.45 eV, respectively. Obviously, the zigzag edges with either B-C or N-C links are more energetically favorable than the armchair edges, which is in agreement with experimental observations.

In conclusion, we succeeded in growing nearly seamless BNC hybrids on a deliberately selected substrate of Rh(111), and have achieved atomically resolved structures of the boundaries linking graphene and h-BN, which show perfect atomicscale continuity as convinced by both experimental and DFT theoretical results. More importantly, by virtue of the perfect BNC system, we found that the formation of zigzag linking edge is more preferred than that of armchair during the patching growth. Therefore, this work is considered to be of particular importance both for an elaborate understanding of the BNC hybrid and for its precise structural modulation for high-performance electronic and spintronics devices.

Experimental method Omicron VT-STM and LT-STM systems were used for the growth and STM characterizations. Rh(111) substrates were pretreated via Ar+ sputtering and post-


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annealing at 600 °C under UHV conditions to remove surface impurities. The hybrid h-BN and graphene film was grown on Rh(111) by heating to 600 °C and exposing it to vaporized ammonia borane (NH3-BH3) and ethylene in a sequential process. All STM images were obtained either at room temperature (Figs. 1, 2) or at a low temperature of 77 K. Hitachi 4800 SEM was employed to characterize the distribution and area ratio of h-BN and graphene in the hybrid film. Meanwhile, the hybridized monolayer film can be removed out of the vacuum chamber for X-ray photoemission spectroscopy measurements because of its inert nature. The total energies, forces, geometry optimizations, and the localized density of states were calculated using the Vienna ab initio simulation package (VASP)31 within the framework of the density-functional theory (DFT). The Perdew-BurkeErnzerhof32 generalized gradient approximation and the projector-augmented wave33 potential with cut-off energy of 400 eV were used to describe the exchangecorrelation energy and the electron-ion interaction, respectively. We included van der Waals (vdW) interaction with the DFT-D2 method to describe the interaction between graphene or BN and Rh accurately.34 Geometry optimization calculations were performed on the z-coordinate of B and N atoms with a (13×13) B-N on (12×12) Rh geometry, as well as that of C atoms with a (12×12) C-C on (11×11) Rh(111) geometry. Only Г point was used to show the Brillouin zone due to the large supercell. All the geometries were optimized without any symmetry constraint until the residual force on each atom was less than 0.05 eV/Å.


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15 Zhao, R. Q.; Wang, J. Y.; Yang, M. M.; Liu, Z. F.; Liu, Z. R. J. Phys. Chem. C 2012, 116, 21098-21103. 16 Martins, J. d. R. & Chacham H. L. ACS Nano, 2011, 5, 385-393. 17 Levendorf, M. P.; Kim, C. J.; Brown, L.; Huang, P. Y.; Havener, R. W.; Muller, D. A.; Park J. W. Nature 2012, 488, 627–632. 18 Liu, Z.; Ma, L. L.; Shi, G.; Zhou, W.; Gong, Y. J.;Lei, S. D.; Yang, X. B.; Zhang, J. N.; Yu, J. Q.; Hackenberg, K. P.; Babakhani, A.; Idrobo, J. C.; Vajtai, R.; Lou, J.; Ajayan, P. M. Nat. Nanotech. 2013, 8, 119–124. 19 Zhang, Y. F.; Gao, T.; Gao, Y. B.; Xie, S. B.;Ji, Q. Q.; Yan, K.; Peng, H. L.; Liu, Z. F. ACS Nano 2011, 5, 4014–4022. 20 Corso, M.; Auwarter, M.; Muntwiler, M.; Tamai, A.; Greber, T.; Osterwalder, J. Science 2004, 303, 217-220. 21 Berner, S.; Corso, M.; Widmer, R.; Groening, O.; Laskowski, R.; Blaha, P.; Schwarz, K.; Goriachko, A.; Over, H.; Gsell, R.; Schreck, M.; Sachdev, H.; Greber, T.; Osterwalder, J. Chem. Int. Ed. 2007, 46, 5115 –5119. 22 Sicot, M.; Leicht, P.; Zusan, A.; Bouvron, S.; Zander, O.; Weser, M.; Dedkov, Y. S.; Horn, K.; Fonin, M. ACS Nano 2012, 6, 151–158. 23 Laskowski, R.; Blaha, T.; Schwarz, K. Phys. Rev. Lett. 2007, 98, 106802. 24 Laskowski, P.; Blaha P. J. Phys.: Condens. Matter 2008, 20, 064207. 25 Wang, B.; Caffio, M.; Bromley, C.; Früchtl, H.; Schaub, R. ACS Nano 2010, 4, 5773-5782. 26 Stradi, D.; Barja, S.; Dıaz, C.; Garnica, M.; Borca, B.; Hinarejos, J. J.; SanchezPortal, D.; Alcamı´, M.; Arnau, A.; Vazquez de Parga, A. L.; Miranda, R.; Martin, F. Phys. Rev. Lett. 2011, 106, 186102. 27 Grimme, S. J. Comput. Chem. 2006, 27, 1787-1799.


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Supporting Information Figure S1-10. This material is available free of charge via internet at http://pubs.acs.org.

Corresponding Author: Email: (Z.F.L) [email protected] Notes: The author declare no competing financial interest.

Acknowledgements. This work was financially supported by the Ministry of Science and Technology of China (Grants Nos. 2013CB932603, 2012CB933404, 2012CB921404, 2011CB921903, 2012CB933001), the National Natural Science Foundation of China (Grants Nos. 51290272, 21073003, 51222201, 51121091, 51072004, 21173058).


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Figure 1 (a) Schematic diagram of the patching growth. (b) SEM morphology of hybridized domains. (c) XPS spectra of B, N, C 1s core levels. (d-e) (VT = -0.002 V, IT =23.00 nA; -0.011 V, 6.30 nA) Atomicallyresolved STM images of pure h-BN and graphene on Rh(111), respectively. (f) (-0.200 V, 33.30 nA) Zoomed-in STM image at the boundary linking h-BN and graphene. (g) Cross-sectional profiles along L1 and L2 in (f) showing the apparent corrugations spanning over two analogues. 209x106mm (300 x 300 DPI)

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Figure 2 (a-b) (VT= -0.002 V, IT= 23.00 nA; -0.222, 12.64 nA) Large-scale STM images of the linking edge. (c, d) (-0.002 V, 23.00 nA) Sequential zoomed-in images of (a) showing h-BN and graphene domains perfectly connected to each other. 104x103mm (300 x 300 DPI)


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Figure 3 (a, b) (-0.002 V, 23.00 nA) Atomically resolved STM images of h-BN and graphene on Rh(111) and their height profiles along the marked lines. (c, d) Contour maps of h-BN and graphene obtained via theoretical calculations by taking the interfacial vdW type interactions into account. (e) Calculated line profiles showing the roughness of both materials on Rh(111), respectively. 115x108mm (300 x 300 DPI)


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Figure 4 (a) DFT calculations of the binding energies (Eb) of graphene, h-BN, and BNC hybrids. (b) Two typical BNC hybrids of BN@G and G@BN. (c) (VT= -0.800 V, IT= 0.02 nA) Large-scale STM images showing the preferred linking of graphene to preexisting h-BN domains. (d) (-0.700 V, 0.02 nA) and (e) (-0.400 V, 0.02 nA) Sequential zoomed-in of (c). 56x47mm (300 x 300 DPI)


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Figure 5 (a, b) (VT= -0.002 V, IT= 23.00 nA) Atomically-resolved STM images on a armchair linking edge. (c, d) (-0.081 V, 0.01 nA) STM images on a zigzag linking edge. (e, f) (-0.050 V, 0.02 nA) Transition from zigzag to armchair linking edges. (g) Experimental statistics of boundaries linking h-BN and graphene with zigzag and armchair types. (h) DFT calculations of Eb for B-C zigzag, N-C zigzag, and armchair linking edges. 145x133mm (300 x 300 DPI)


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Toward Single-Layer Uniform Hexagonal Boron NitrideâGraphene

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Toward Single-Layer Uniform Hexagonal Boron NitrideâGraphene