Quasi-Freestanding Monolayer Heterostructure of Graphene and

Subscriber access provided by TEXAS A&M INTL UNIV

Communication

Quasi-freestanding Monolayer Heterostructure of Graphene and Hexagonal Boron Nitride on Ir (111) with a Zigzag Boundary Mengxi Liu, Yuanchang Li, Pengcheng Chen, Jingyu Sun, Donglin Ma, Qiucheng Li, Teng Gao, Yabo Gao, Zhihai Cheng, Xiaohui Qiu, Ying Fang, Yanfeng Zhang, and Zhongfan Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl502780u • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 5, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Quasi-freestanding Monolayer Heterostructure of Graphene and Hexagonal Boron Nitride on Ir (111) with a Zigzag Boundary Mengxi Liu1, †,Yuanchang Li2, †, Pengcheng Chen2, Jingyu Sun1, Donglin Ma1, Qiucheng Li1, Teng Gao1, Yabo Gao1, Zhihai Cheng2, Xiaohui Qiu2, Ying Fang2, Yanfeng Zhang1,3,*, & Zhongfan Liu1,* 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

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

3

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

These authors contributed equally to this work.

*Correspondence and requests for materials should be addressed to Y. F. Zhang & Z. F. Liu (E-mail: [email protected]; [email protected])

1

ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: In-plane heterostructure of hexagonal boron nitride and graphene (h-BNG) has become a focus of graphene research owing to its tunable bandgap and intriguing properties. We report herein the synthesis of a quasi-freestanding h-BN-G monolayer heterostructure on a weakly-coupled Ir (111) substrate, where graphene and h-BN possess distinctly different heights and surface corrugations. An atomically sharp zigzag type boundary has been found to dominate the patching interface between graphene and h-BN, as evidenced by high-resolution STM investigation as well as density functional theory (DFT) calculation. Scanning tunneling spectroscopy (STS) studies indicate that the graphene and h-BN tend to exhibit their own intrinsic electronic features near the patching boundary. The present work offers a deep insight into the h-BN-graphene boundary structures both geometrically and electronically together with the effect of adlayer-substrate coupling.

Keywords: STM, graphene, h-BN, In-plane heterostructure, boundary type

2


Page 2 of 22

Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Introduction The monolayer heterostructure of hexagonal boron nitride (h-BN) and graphene (G) has attracted recent attentions due to its predicted fascinating properties such as bandgap opening and magnetism,1-7 which hence has ignited the experimental growth attempts of such in-plane two-dimensional (2D) hybrid materials.8-15 The first pioneering experiment was performed by L. Ci et al., who obtained few layer films with randomly-distributed graphene and h-BN domains on Cu foils and observed a small bandgap opening (18 meV).8 Several follow-up attempts have been made by growing graphene onto the bare regions of photolithographically-patterned h-BN monolayer, offering a possibility for fabricating atomically thin electronic devices.9,10 It has also been demonstrated very recently that a continuous monolayer of h-BN-G hybrid can be formed by a heteroepitaxial growth of h-BN from the fresh graphene domain edge on Cu foils.11, 12 Scanning tunneling microscopy (STM) has demonstrated its great power for investigating the atomic and electronic structures at the h-BN and graphene linking boundary.13-15 It is found that on Ru (0001), graphene and h-BN tend to evolve into hBC2N alloys at the interface under ultrahigh vacuum (UHV) conditions due to the chemical reaction of the existing graphene with the introduced borazine precursors for h-BN growth.13,14 In contrast, it is recently revealed that on Rh (111), graphene and hBN could patch seamlessly into a hybrid monolayer, with a preferred zigzag type boundary.15 It is worth noting that, this kind of substrates, such as Ru (0001), couple strongly with graphene by π-d orbital hybridization, leading to downward shift of 3


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

graphene π bands away from the Fermi level, or decay of the intrinsic electronic structure of graphene.16-18 In this regard, the influence of h-BN to the electronic property of graphene is hard to be identified on such h-BN-G heterostructures. Therefore, a weakly-coupled system is highly desirable to exclude the substrate electronic doping effect. In this work, we employed Ir (111) single crystal as the weakly-coupled growth substrate19 and successfully obtained the h-BN-G monolayer heterostructure under UHV condition by using a sequential two-step growth technique. It was found that graphene and h-BN with different moiré periods and corrugations were able to form a seamless monolayer with preferred zigzag type boundary structures. The h-BN-G monolayer on Ir (111) was quasi-freestanding, as evidenced by the observed intrinsic electronic structures of both graphene and h-BN from dI/dV spectra. A sharp transition of electronic states across the domain boundary was also observed with the absence of new hybridized electronic states.

Results and discussion The atomic-scale observations of the respective features of graphene and h-BN on Ir (111) are the prerequisite for identifying the two analogues in h-BN-G heterostructure. Hereby, we first grew monolayer graphene and h-BN on Ir (111), respectively, and characterized their different morphologies. The Ir (111) single crystal (MaTeck GmbH, 99.99 % purity) was processed by standard method involving several cycles of sputtering and oxygen adsorption.15 Graphene and h-BN were formed by exposing the Ir (111) to ethylene (C2H4) and ammonia borane (BH3NH3) at 800~1000 K, respectively. The detailed growth conditions are illustrated in 4


Page 4 of 22

Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Supporting Information Figure S1 and Figure S2. The STM images in Figures 1a, 1b show that graphene on Ir (111) is featured by a periodic superstructure with a period of ~2.5 nm, which is consistent with the published results.20-23 The superstructure arises from a coincidence lattice of 10×10 CC/9×9 Ir (111) as inferred from the atomically resolved STM image (Figure 1c) and DFT calculation (Figure 1g), both of which display nearly the same moiré contrast. DFT calculations (black curve in Figure 1i) show an average distance of ~0.42 nm from graphene layer to the Ir (111) plane, as well as a tiny graphene corrugation of 0.02 nm. This tiny corrugation is also in good agreement with the result gained from STM height profile (black curve in Figure 1j). It is worth noting that, the layer corrugation of graphene on Ir(111) is much smaller than that on Rh (111) (~0.16 nm)15or Ru (0001) (~0.15nm),24 strongly indicative of a weak adlayer-substrate interaction between graphene and Ir (111). On the other hand, the STM images of hBN grown on Ir (111) in Figures 1d-f exhibit periodic superstructures with a period of ~3.2 nm. Zoom-in STM image in Figure 1f confirms that the superstructure arises from the coincidence lattice of 13×13 B-N/12×12 Ir(111). This superstructure agrees well with the reported result of h-BN/Ir (111).25 Likewise, height profiles of simulated morphology and STM image of h-BN on Ir (111) all indicate that h-BN is positioned ~0.38 nm away from the Ir(111) plane with a corrugation of ~0.14 nm (red curves in Figures 1i, 1j). Obviously, although related theoretical calculations have suggested a quite weak interaction between h-BN and Ir (111),26, 27 the layer corrugation of h-BN is much greater than that of graphene on Ir (111) (~0.02 nm). Therefore, it would be 5


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interesting to see how graphene and h-BN can link together to form in-plane heterostructure on Ir (111), since the individual domains of graphene and h-BN possess distinct surface corrugations and adlayer-substrate distances on Ir (111). Figure 2a shows the schematic illustration of our sequential two-step growth approach for achieving h-BN-G heterostructure on Ir(111) in a sequence of graphene and then h-BN. The sub-monolayer graphene islands were grown on Ir(111) using C2H4 as the precursor, followed by patching h-BN onto the remaining bare substrate with ammonia borane as precursor. In this case, the graphene was embedded into the continuous h-BN monolayer, labeled by G@h-BN. The surface feature of pre-grown graphene islands was examined by atomic-scale STM imaging prior to h-BN growth. As shown in Figure 2b, hexagon-like islands are predominant due to the intrinsic symmetry of graphene. In particular, the graphene islands are of zigzag termination as evidenced by the following two facts. First, the edge direction stays in parallel with the direction of graphene moiré pattern. Second, the zigzag orientation of the atomic lattice is in line with the direction of moiré pattern (as indicated by the arrows in the atomically-resolved STM image in Figure 2b inset). Actually, this can be taken as a general rule for identifying the edge type of as-grown h-BN-G heterostructure. After the patching growth of h-BN, graphene and h-BN domains are connected together, exhibiting different moiré contrasts in STM image with periods of ~2.5 nm and ~3.2 nm, respectively (Figure 2c). Obviously, the pre-grown graphene island maintains the initial hexagonal shape and a sharp interface is formed between graphene and h-BN on Ir(111), which is different from the reported mixed h-BC2N formation on Ru 6


Page 6 of 22

Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(0001).13, 14 Similarly, an h-BN-G heterostructure is also obtained on Ir (111) with h-BN being embedded into the continuous graphene phase (See Figure 2d, h-BN@G). In this case, the pre-grown h-BN islands exhibit triangular shapes (Figure 2e), which are characterized with zigzag type edges (inset in Figure 2e). Notably, the triangular h-BN islands keep almost unchanged after the second graphene growth (Figure 2f). The superstructure and even the atomic lattice of h-BN can be clearly identified by highresolution STM images which are distinct from that of graphene regions, indicative of no atomic mix at the patching interface (Supporting Information Figure S3). The Xray photoelectron spectra (XPS) of the as-grown samples display B, C, and N 1s core level peaks, further verifying the formation of h-BN-G heterostructure (Supporting Information Figure S4). To further understand the formation process of the h-BN-G heterostructure, the initial patching behavior of h-BN on a pre-grown hexagonal graphene domain was examined by decreasing the h-BN growth time to 30 sec. It is clearly seen in Figures 2g, 2h that the h-BN domain grows preferentially along the edge of graphene domain. A sharp boudary between graphene and h-BN can be identified by their different moiré contrasts even at sub-monolayer coverage. Figure 2i depicts the schematic visualization of the h-BN and graphene interfacial region. The edge-dominated patching growth results observed in our h-BN-G system is consistent with the reported theoretical calculations.15 In fact, a similar phenomenon has also been observed on Cu foil, which was called as heteroepitaxial growth: h-BN preferred to grow from fresh edges of graphene with atomic lattice coherence to form a 7


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

seamless interface.11, 12 In the previous papers, the type of boudary linking graphene and h-BN has been deeply studied on different metal substrates. A zigzag type boundary of h-BN-G is reported to be preferentially evolved on both singlecrystalline Rh (111)15 and Ru (0001)28 via UHV chemical vapor deposition (CVD) and on polycrystalline Cu foils12 via atmosphere pressure CVD methods. To further identify the boundary type of hBN-G heterostructures on Ir (111), atomically-resolved STM images have been randomly captured at the interface areas linking graphene and h-BN. Three typical boundaries and their magnified STM images are listed in Figures 3a-f. Obviously, the graphene and h-BN domains can be easily distinguished from each other in STM images due to their different moiré contrasts. Most boundaries are of zigzag type with no atomic mix as illustrated in Figure 3a, c, which are indicated by the fitted atomic models in Figure 3b, d. Note that, the B atoms or N atoms in h-BN cannot be identified due to the limitation of STM in element identification. Therefore, the atomic models in Figure 3b, d are used for showing the zigzag linking type, not for marking the B-C or N-C linking bonds. More STM images of zigzag boundary type at the interface are demonstrated in Supporting Information Figures S5-S10. Statistically, for the sampling scanning of more than 100 areas, only one armchair type boundary was observed, as shown in Figure 3e, f. The interface with armchair linking also shows no atomic mix on both graphene and h-BN domians. As a consequence, the presented two-step growth approach in this work is favorable for growing h-BN-G heterostructures with predominant zigzag boundaries, which is very attractive for 8


Page 8 of 22

Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

investigating the intriguing properties such as half metallicity and spin polarization.4, 6, 7

DFT calculations of the interfacial corrugations also confirmed that a seamless zigzag boundary with either B-C or N-C linkage can form on Ir (111) (Figure 3g), with the B-C bonds being pulled down more seriously than the N-C bonds. More details of the simulated model is discribed in Supporting Information Figure S11. The B-C linkage is hence expected to be more stable than the N-C linkage on Ir (111), consistent with the fact that the B-C linkage possessed a larger binding energy than that of the N-C linkage for the h-BN-G on Rh(111).15 It is noted that, the interface height undulation is not revealed in this work because of the resolution limit of STM. Detailed atomic force microscopy (AFM) studies are ongoing towards a clearer identification of the interface lattice arrangement as well as the height undulation. The above observations suggest that graphene and h-BN can overcome the apparent height, surface corrugation and moiré period differences to generate a seamless zigzag type boundary on Ir (111). The patching window of graphene and h-BN is rather wide on Ir (111), which is different from the reported h-BN-G systems on Ru (0001) and Rh (111).13-15 The electronic characteristic at the boundary area of h-BN and graphene is certainly an interesting issue. Of particular importance to us is whether the electronic states of graphene and h-BN interfere with each other at the interfacial region. To answer the questions, scanning tunneling spectroscopy (STS) studies were conducted, as shown in Figure 4. For the pure graphene monolayer on Ir (111), the Dirac point 9


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

locates at 0 eV, indicating the absence of doping effect of Ir (111) substrate to the asgrown graphene (Figure 4a). For the as-grown monolayer h-BN on Ir (111) (Figure 4b), a nominal bandgap of ~5 eV is obtained by STS measurements , which is quite close to that of intrinsic h-BN (5.9 eV). The reduced band gap of h-BN (5 eV) is considered to be mediated by the weak interaction from the metal substrate.25. Figure 4c shows a hexagonal graphene island embedded in h-BN with a sharp boundary. Sequential line scan dI/dV data were obtained across a zigzag boundary from graphene to h-BN regions, as shown in Figure 4d. Each spectrum was labeled with a letter corresponding to the spatial position in Figure 4c. It can be clearly seen that, at the location A far from the boundary, the spectrum appears to be a V-shape with the Dirac point locating at ~0 eV, which is typical of an electronically freestanding graphene. In contrast, on h-BN domain, the dI/dV curves at locations D, E and F all exhibit a flat background close to zero from -0.8 eV to 0.8 eV, suggesting the insulating nature of h-BN. Interestingly, when the STM tip scan approaches to the boundary from graphene side, the V-shaped dI/dV curve becomes broadened prominently (curves B and C in Figure 4d). This remarkable decrease of graphene conductance would be attributed to the edge effect of graphene island.29 Besides the most frequently observed rectilinear linking boundaries between graphene and h-BN domains (as mentioned above), some tortuous boundaries can also be occasionally seen (Figure 5a). The tortuous boundaries are actually made up of a number of short zigzag boundaries, as can be clearly seen in the magnified STM image in Figure 5b and the schematic model in Figure 5c. Such tortuous moiré 10


Page 10 of 22

Page 11 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

formation may not influence the seamless patching at the linking boundaries, which has been comfirmed by the large-scale STM image and corresponding fast Fourier transform image in Supporting Information Figure S12. Moreover, graphene wrinkles are occasionally observed at the step edges of Ir (111). Figure 5d presents a typical wrinkle with an apparent height of ~1.3 nm (Figure 5e), much lower than that of the wrinkles formed in conventional CVD graphene (~3-10 nm).30,31 On the top of the bumped wrinkle (Figure 5f), honeycomb graphene lattices can be clearly resolved. These corrugated graphene wrinkles could contribute to tuning the electronic states of graphene via creating a pseudo-magnetic field.32 In summary, we have synthesized a quasi-freestanding in-plane h-BN-G heterostructure on a weakly-coupled Ir(111) single crystal surface via a two-step growth approach, regardless of the disparate moiré period, layer corrugation and adlayer-substrate distance of graphene and h-BN. The transition from graphene to hBN domains can be atomically sharp in the h-BN-G heterostructure from both chemical composition and local electronic states. The presented work will help us understand the fundamental issues of h-BN-G heterostructure, including the growth mechanism, the boundary structures and the electronic interference at the boundaries.

METHODS DFT Calculations. We performed the density-functional theory (DFT) calculations using the Vienna ab-initio simulation package (VASP)33 with the Perdew-BurkeErnzerhof

34

exchange-correlation. The projector-augmented wave35 potential with a 11


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cutoff energy of 400 eV was used to describe the electron-ion interaction. In order to consider the interaction between graphene/h-BN and Ir accurately, van der Waals (vdW) interaction was included through the DFT-D2 method.36 The parameters for Ir were used as that of Au, namely, 40.62 J nm6/mol for the dispersion coefficient (C6), and 1.772 Å for the vdW radius (R0).37 Only Г point of the Brillouin zone was considered due to the large supercell. All the geometries were optimized without any symmetry constraint until the residual force on each atom was less than 0.06 eV/Å. STM/STS measurements. Omicron LT-STM/STS systems were used for the characterization of samples with a base pressure better than 10-10mbar. The local differential conductance (dI/dV) spectra were measured at 77K by recording the output of a lock-in system with the manually-disabled feedback loop. A modulation signal of 5 mV, 952 Hz was selected under a tunneling condition of 1 V, 20pA.

ASSOCIATED CONTENT Supporting Information Figure S1−12. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: (Y. Z.) [email protected]; (Z. L.) [email protected]. Notes The authors declare no competing financial interest. 12


Page 12 of 22

Page 13 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 51222201, 51290272, 11304053, 51121091) and the Ministry of Science and Technology of China (Grants 2013CB932603; 2011CB921903, 2012CB921404, 2012CB933404, 2011CB933003).

REFERENCES 1. Rocha Martins, J.; Chacham, H. ACS Nano 2010, 5, (1), 385-393. 2. Shinde, P. P.; Kumar, V. Phys. Rev. B 2011, 84, (12), 125401. 3 Zhao, R.; Wang, J.; Yang, M.; Liu, Z.; Liu, Z. J. Phys. Chem. C 2012, 116, (39), 21098-21103. 4. Ramasubramaniam, A.; Naveh, D. Phys. Rev. B 2011, 84, (7), 075405. 5. Bhowmick, S.; Singh, A. K.; Yakobson, B. I. J. Phys. Chem. C 2011, 115, (20), 9889-9893. 6. Jiang, J.; Wang, J.; Wang, B. Appl. Phys. Lett. 2011, 99, (4), 043109. 7. Pruneda, J. Phys. Rev. B 2010, 81, (16), 161409. 8. Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z.; Storr, K.; Balicas, L. Nat. Mater. 2010, 9, (5), 430-435. 9. Liu, Z.; Ma, L.; Shi, G.; Zhou, W.; Gong, Y.; Lei, S.; Yang, X.; Zhang, J.; Yu, J.; Hackenberg, K. P. Nat. Nanotechnol. 2013, 8, (2), 119-124. 10. Levendorf, M. P.; Kim, C.; Brown, L.; Huang, P.; Havener, R.; Muller, D.; Park, J. Nature 2012, 488, (7413), 627-632. 13


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11. Han, G.; Rodriguez-Manzo, J.; Lee, C.; Kybert, N.; Lerner, M.; Qi, Z.; Dattoli, E.; Rappe, A.; Drndic, M.; Johnson, A. ACS Nano 2013, 7, (11), 10129-10138. 12. Liu, L.; Park, J.; Siegel, D.; McCarty, K.; Clark, K.; Deng, W.; Basile, L.; Idrobo, J.; Li, A.; Gu, G. Science 2014, 343, (6167), 163-167. 13. Sutter, P.; Cortes, R.; Lahiri, J.; Sutter, E. Nano Lett. 2012, 12, (9), 4869-4874. 14. Lu, J.; Zhang, K.; Liu, X.; Zhang, H.; Sum, T.; Neto, A.; Loh, K. Nat. Comm. 2013, 4, 2681. 15. Gao, Y.; Zhang, Y.; Chen, P.; Li, Y.; Liu, M.; Gao, T.; Ma, D.; Chen, Y.; Cheng, Z.; Qiu, X.; Duan, W.; Liu, Z. Nano Lett. 2013, 13, 3439-3443. 16. Sánchez-Barriga, J.; Varykhalov, A.; Scholz, M.; Rader, O.; Marchenko, D.; Rybkin, A.; Shikin, A.; Vescovo, E. Diam. Relat. Mater. 2010, 19, (7), 734-741. 17. Enderlein, C.; Kim, Y.; Bostwick, A.; Rotenberg, E.; Horn, K. New J. Phys. 2010, 12, (3), 033014. 18. Wang, B.; Caffio, M.; Bromley, C.; Früchtl, H.; Schaub, R. ACS Nano 2010, 4, (10), 5773-5782. 19. Usachov, D.; Fedorov, A.; Vilkov, O.; Adamchuk, V.; Yashina, L.; Bondarenko, L.; Saranin, A.; Grüneis, A.; Vyalikh, D. Phys. Rev. B 2012, 86, (15), 155151. 20. Coraux, J.; N’Diaye, A.; Busse, C.; Michely, T. Nano Lett. 2008, 8, (2), 565-570. 21. N’Diaye, A.; Coraux, J.; Plasa, T.; Busse, C.; Michely, T. New J. Phys. 2008, 10, 043033. 22. Coraux, J.; N’Diaye, A.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Heringdorf, F.; Gastel, R.; Poelsema, B.; Michely, T. New J. Phys. 2009, 11, 023006. 14


Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

23. Voloshina, E.; Fertitta, E.; Garhofer, A.; Mittendorfer, F.; Fonin, M.; Thissen, A.; Dedkov, Y. Sci. Rep. 2013, 3, (1072). 24. Martoccia, D.; Willmott, P.; Brugger, T.; Björck, M.; Günther, S.; Schlepütz, C.; Cervellino, A.; Pauli, S.; Patterson, B.; Marchini, S. Phys. Rev. Lett. 2008, 101, (12), 126102. 25. Schulz, F.; Drost, R.; Hämäläinen, S.; Demonchaux, T.; Seitsonen, A.; Liljeroth, P. Phys. Rev. B 2014, 89, (23), 235429. 26. Laskowski, R.; Blaha, P. Phys. Rev. B 2010, 81, (7), 075418. 27. Laskowski, R.; Blaha, P.; Schwarz, K. Phys. Rev. B 2008, 78, (4), 045409. 28. Lu, J.; Gomes, L.; Nunes, R.; Castro Neto, A. and Loh, K. Nano Lett. 2014, DOI: 10.1021/nl501900x. 29. Phark, S.; Borme, J.; Vanegas, A. L.; Corbetta, M.; Sander, D.; Kirschner, J. Nanoscale Res. Lett. 2012, 7, (1), 1-4. 30. Liu, M.; Zhang, Y.; Chen, Y.; Gao, Y.; Gao, T.; Ma, D.; Ji, Q.; Zhang, Y.; Li, C.; Liu, Z. ACS Nano 2012, 6, (12), 10581-10589. 31. Zhang, Y.; Gao, T.; Gao, Y.; Xie, S.; Ji, Q.; Yan, K.; Peng, H.; Liu, Z. ACS Nano 2011, 5, (5), 4014-4022. 32. Levy, N.; Burke, S.; Meaker, K.; Panlasigui, M.; Zettl, A.; Guinea, F.; Neto, A.; Crommie, M. Science 2010, 329, (5991), 544-547. 33. Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, (1), 558. 34. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys.Rev. Lett. 1996, 77, (18), 3865. 35. Blöchl, P. E. Phys. Rev. B 1994, 50, (24), 17953. 15


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

36. Grimme, S. J. Comput. Chem. 2006, 27, (15), 1787-1799. 37. Medeiros, P. V.; Gueorguiev, G. K.; Stafström, S. Phys. Rev. B 2012, 85, (20), 205423.

16


Page 16 of 22

Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. STM observation and DFT simulation of pure graphene and h-BN monolayer on Ir (111). (a-c) STM images of graphene at moiré and atomic scales (a, b, c: VT= -0.138, -0.002, -0.002 V; IT= 5.361, 11.492, 4.874 nA). (d-f) STM images of h-BN at moiré and atomic scales (d, e, f: VT= -0.037, -0.004, -0.002 V; IT= 1.284, 2.751, 52.267 nA,). (g, h) DFT calculations of the surface corrugations and the adlayer-substrate distances of graphene and h-BN on Ir (111), respectively. (i, j) Comparison of the theoretical and experimental data of adlayer corrugations of graphene and h-BN on Ir (111), as derived from the crosssectional views of the height mapping in (g, h) and the STM images in (b, e), respectively. Scale bars: (a) 8 nm; (b) 2.5 nm; (c) 1.5 nm; (d) 15 nm; (e) 3 nm; (f) 2 nm. 208x192mm (150 x 150 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Two-step sequential growth of h-BN-G heterostructure. (a) Schematic of the sequential growth for G@h-BN heterostructure. (b) STM image of pre-grown graphene island on Ir (111) with the atom-resolved image (inset) showing the zigzag orientation of the domain (VT = -0.194, IT = 2.501 nA; inset: VT= -0.002, IT = 40.953 nA). (c) STM image of G@h-BN heterostructure (VT = –0.002, IT = 1.879 nA). (d) Schematic of the sequential growth steps for h-BN@G heterostructure. (e) STM image of the pre-grown h-BN island on Ir (111) with the atomically-resolved image (inset) showing the zigzag domain edge (VT = -0.122, IT = 2.134 nA; inset: VT = -0.002, IT = 25.429 nA). (f) STM image of h-BN@G heterostructure (VT = -0.152, IT = 1.820 nA). (g, h) Sequential zoom-in images of an h-BN moiré row growing along the edge of an existing graphene domain (g, h: VT = -0.211, -0.021 V; IT = 1.820, 1.820 nA). (i) Schematic view of the growth boundary between graphene and h-BN. Scale bars: (b) 7 nm; (c) 8 nm; (e) 15 nm; (f) 10 nm; (g)10 nm; (h) 3nm. 160x114mm (300 x 300 DPI)


Page 18 of 22

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3.Edge type identification of the h-BN-G heterostructure by atomically-resolved STM investigation. (a, b) Sequential zoom-in STM images at the interface of h-BN-G heterostructure showing zigzag type boundary formation (a, b: VT = -0.004, -0.004 V; IT = 2.750, 2.750 nA). The boundary and edge type are marked by dotted lines and fitted net models, respectively. (c, d) Another example of the zigzag type edge formation between graphene and h-BN (c, d: VT = -0.002, -0.002 V; IT =1.655, 1.504 nA). (e, f) Sequential zoom-in STM images of the rarely observed armchair edge (e, f: VT = -0.002, -0.002 V; IT = 3.250, 3.250 nA). (g) DFT calculation of the corrugation at the interface region of B-C and N-C linkages. Scale bars: (a) 3 nm; (b) 1.2 nm; (c) 2.5 nm; (d) 1.2 nm; (e) 4nm; (f) 1.5 nm. 99x83mm (300 x 300 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Electronic transition at the interface of h-BN and graphene.(a, b) STM images and typical dI/dV curves of graphene and h-BN on Ir (111), respectively (a, b: VT = -0.100, -0.152V; IT = 2.432, 2.750 nA). (c, d) STM image of G@h-BN heterostructure (VT = -0.183 V; IT = 2.134 nA) and the point-to-point dI/dV curves measured along the marked locations across the linking boundary. Scale bars: (a) 6 nm; (b) 9 nm; (c) 10 nm. 87x95mm (300 x 300 DPI)


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. (a, b) Sequential zoom-in STM images showing the tortuous linking interface between graphene and h-BN evidenced at moiré and atomic scales (a, b: VT = -0.007, -0.007 V; IT = 1.655, 1.655 nA). (c) Schematic of the tortuous boundary between graphene and h-BN at atomic scale. (d, e) Large-scale STM image and corresponding three-dimensional map of a graphene wrinkle on the step of Ir (111) (VT = -0.016 V; IT = 2.860 nA). (f) Atomic-scale STM image on graphene wrinkle showing honeycomb graphene lattice (VT = -0.016 V; IT = 2.860 nA). Scale bars: (a) 2 nm; (b) 1.2 nm; (d) 10 nm; (f) 1 nm. 71x42mm (300 x 300 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

54x33mm (300 x 300 DPI)


Page 22 of 22

Quasi-Freestanding Monolayer Heterostructure of Graphene and

Recommend Documents