Strong Adlayer–Substrate Interactions “Break” the Patching Growth of

Strong Adlayer−Substrate Interactions “Break” the Patching Growth of h‑BN onto Graphene on Re(0001) Yue Qi,† Nannan Han,∥ Yuanchang Li,§ Zhepeng Zhang,† Xiebo Zhou,‡ Bing Deng,† Qiucheng Li,† Mengxi Liu,† Jijun Zhao,*,∥ Zhongfan Liu,*,† and Yanfeng Zhang*,†,‡ †

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 ‡ Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China § National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ∥ Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: Hetero-epitaxial growth of hexagonal boron nitride (h-BN) from the edges of graphene domains or vice versa has been widely observed during synthesis of in-plane heterostructures of h-BN-G on Rh(111), Ir(111), and even Cu foil. We report that on a strongly coupled Re(0001) substrate via a similar two-step sequential growth strategy, h-BN preferably nucleated on the edges of Re(0001) steps rather than on the edges of existing graphene domains. Statistically, one-third of the domain boundaries of graphene and h-BN were patched seamlessly, and the others were characterized by obvious “defect lines” when the total coverage approached a full monolayer. This imperfect merging behavior can be explained by translational misalignment and lattice mismatch of the resulting separated component domains. According to density functional theory calculations, this coexisting patching and non-patching growth behavior was radically mediated by the strong adlayer−substrate (A−S) interactions, as well as the disparate formation energies of the attachment of B−N pairs or B−N lines along the edges of the Re(0001) steps versus the graphene domains. This work will be of fundamental significance for the controllable synthesis of in-plane heterostructures constructed from two-dimensional layered materials with consideration of A−S interactions. KEYWORDS: graphene and hexagonal boron nitride heterostructures, ultra-high-vacuum scanning tunneling microscopy/spectroscopy, preferable nucleation, edges of Re steps and graphene domains tures.8,9,20−25 A pioneering work involved synthesizing a hBN-G hybrid directly on Cu foils via a facile chemical vapor deposition (CVD) method by concurrently introducing methane (CH4) and ammonia borane (NH3−BH3) precursors.20 Several follow-up attempts were also accomplished by growing graphene (or h-BN) on the bare regions of photolithographically patterned h-BN (or graphene) monolayers for the fabrication of atomically thin electronic devices.8,9 Very recently, the growth of h-BN-G heterostructures on Cu foils was also realized via a similar two-step sequential growth

R

ecently, two-dimensional (2D) layered materials (i.e., graphene,1 hexagonal boron nitride (h-BN),2,3 and transition metal dichalcogenides (TMDCs))4−7 have been used as the basic building blocks for constructing heterostructures with distinct optical and electrical properties.8−11 Among these structures, the lateral heterostructures of h-BN and graphene (h-BN-G) have been reported to be capable of inducing attractive physical properties, i.e., bandgap opening, magnetism, and good thermal transport.12−16 Similarly, greatly enhanced optoelectronic properties were also achieved by using semiconducting TMDC lateral heterostructures.17−19 Notably, the in-plane h-BN-G heterostructure is by far the most representative system for exploring the synthetic routes and formation mechanisms of in-plane 2D heterostruc© 2017 American Chemical Society

Received: November 17, 2016 Accepted: January 21, 2017 Published: January 21, 2017 1807

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Figure 1. STM morphology and DFT simulation of graphene/Re(0001). (a) STM image of graphene islands or nanoclusters synthesized at ∼550 °C for ∼10 min. (b) “Missing graphene moiré” evolved inside the graphene moirés. (c,d) STM images of the triangular- and hexagonalshaped graphene nanoclusters composed of three and seven graphene moirés. (e) STM image of large-domain graphene derived by annealing the lowtemperature deposited sample of (a) at ∼630 °C for ∼10 min. (f) Corresponding atomic-resolution STM image featuring a periodic moiré superstructure (∼2.48 nm in period) and a coincidence lattice of 10×10 C−C/9×9 Re(0001), which agrees well with the DFT calculations in (g). (h) Contour map of a graphene moiré based on DFT calculations with consideration of the interfacial vdW interaction. (i) Calculated (red) and experimental (black) height profiles showing the roughness of graphene/Re(0001) (∼0.133 and ∼0.141 nm on average, respectively). Scanning conditions: (a) VT = −0.020 V, IT = 2.58 nA; (b) −0.002 V, 2.58 nA; (c) −0.002 V, 1.94 nA; (d) −0.002 V, 2.13 nA; (e) −0.017 V, 4.29 nA; (f) −0.002 V, 3.33 nA.

method, wherein h-BN preferentially grew from the edges of existing graphene domains.21,22 Concurrently, the construction of h-BN-G heterostructures was also realized under ultra-high-vacuum (UHV) conditions on single-crystal substrates via a similar CVD method.22,24,26−28 Scanning tunneling microscopy/spectroscopy (STM/STS) measurements were then utilized to unravel the atomic-scale morphology, interface continuity, and interface electronic properties.14,29−31 On a strongly interacting Ru(0001) substrate, the interface between h-BN and graphene was analogous to a buffer zone, composed of either phase-separated graphene and h-BN domains or substitutional B−C−N alloy phases.23 Particularly, a “heart-shaped” dislocation, mainly for releasing the lattice-mismatch-induced interface strain, was detected at the heterostructure interface on Ru(0001).26 On a weakly coupled Rh(111) substrate,32 a seamlessly patched hBN-G heterostructure was also characterized with dominant zigzag-type linking edges.24 More intriguingly, quasi-freestanding h-BN-G heterostructures were also realized on weakly coupled Ir(111)27 and Pt(111),33 wherein the intrinsic electronic properties of the adlayers were completely maintained. The common trait of the existing systems involves growth that is mainly dominated by a graphene edge heteroepitaxial growth mechanism, even on the relatively strongly interacting Rh(111) and Ru(0001) substrates reported so far.23,24,26 However, the substrate modulation effect is negligible.

In contrast, how the heterostructures form on a strongly interacting substrate, which might be different from weakly interacting substrates, is far from being clearly understood. Herein, we chose the seldom-used Re(0001) as the substrate for the growth of graphene and h-BN and investigated their patching behavior. Re(0001) was reported to be the most strongly interacting substrate for graphene synthesis,34,35 and the growth of h-BN and in-plane h-BN-G heterostructures have never been attempted on such a substrate. This study offers a great opportunity to unravel whether a graphene heteroepitaxial mechanism still operates for the growth of h-BN-G, as well as to explore the morphologies and electronic properties of graphene and h-BN on Re(0001) through on-site STM/STS characterization techniques. Density functional theory (DFT) calculations were also performed to simulate the morphologies of h-BN and graphene on Re(0001), the adlayer−substrate (A− S) interactions, and the initial attaching preference of B−N pairs or B−N lines to the respective edges of the Re steps and graphene domains. Furthermore, the current system was also compared to previously reported systems (on Rh(111)24 and Ir(111)27) to achieve an in-depth understanding of lateral heterostructures synthesis.

RESULTS AND DISCUSSION Detailed observations of the respective growth behaviors, as well as the atomic-scale morphologies of graphene and h-BN on Re(0001) were the premise for the heterostructure synthesis. As in our previous works,24,27,36 these two structural analogues 1808

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Figure 2. STM morphology and DFT simulation of h-BN/Re(0001). (a,b) STM morphology of irregular h-BN superstructure synthesized at ∼650 °C for ∼10 min. STM image in (b) was captured from the region marked with a square in (a). (c,d) STM images of the uniform superstructures formed at ∼730 °C for ∼10 min. The h-BN/Re(0001) superstructure (∼3.00 nm in period) arose from a coincidence lattice of 12×12 B−N/11×11 Re(0001). (e−h) “Heart-shaped” moiré structures evolved from the merging of two nanomesh “pores” (observed from low-temperature-deposited samples). (i,j) Structural model of 12×12 B−N/11×11 Re(0001), and the contour map obtained via DFT calculations (with vdW interactions). (k) Calculated (red) and experimental (black) height profiles showing similar roughnesses of ∼0.152 and ∼0.153 nm, respectively. Scanning conditions: (a) −0.162 V, 2.35 nA; (b) −0.005 V, 10.12 nA; (c,d) −0.031 V, 11.13 nA; (e,f) −0.018 V, 6.91 nA; (g) −0.088 V, 3.23 nA; (h) −0.002 V, 5.36 nA.

were synthesized by exposing Re(0001) to ethylene (C2H4) and ammonia borane (BH3NH3) at 550−750 °C under UHV conditions, respectively. The growth methods are schematically represented in Figure S1. The formation of graphene and h-BN on Re(0001) was verified by XPS, with the C 1s peak located at ∼284.38 eV and B, N 1s peaks at ∼190.10 and ∼397.57 eV, respectively (Figure S2a,b). These results agree with the published data for graphene and h-BN on different metal substrates.24,37 The STM technique was then used for detailed structural characterization. After graphene synthesis at ∼550 °C for ∼10 min, some irregularly shaped islands were observed on the metal surface, showing graphene-moiré (bright-spot)-like contrasts with a unique period of ∼2.48 nm (Figure 1a). The close-up STM image shows clear atomic lattices with a lattice constant of ∼0.246 nm (Figure 1b), again indicating the formation of graphene. Moreover, inside the uniform graphene moirés, “missing” graphene moirés were also occasionally noticeable, presenting dark spot-like contrasts (indicated by the dashed circle in Figure 1b). On such depressed contrast regions, defective or distorted graphene lattices could still be connected with the normal moiré regions (Figures 1b and S3a,b). Notably, under a relatively low growth temperature (∼550 °C), compact graphene nanoclusters were anchored on the surfaces, as representatively shown in Figure 1c,d, possessing typical triangular and hexagonal shapes, respectively. Graphene nanoclusters with different shapes, such as trapezoid and irregular hexagons are also displayed in Figure S3c−f. Based on these results, it can be inferred that the growth of graphene at this relatively low temperature proceeds with the unit of a graphene moiré or even a moiré defect. This growth indicates that the rate-limiting step in the growth process probably relates to the diffusion of graphene nanoclusters, as modulated

by a concurrent effect from both the low growth temperature and strong graphene−Re(0001) interface interaction. To improve the surface diffusion of the graphene nanoclusters, the low-temperature (∼550 °C)-deposited sample was further annealed at ∼630 °C for ∼10 min. Intriguingly, the small graphene nanoclusters disappeared, accompanied by the presence of large graphene domains with an edge length of several tenths of nanometers. According to statistics, the graphene coverage remained nearly unchanged throughout the annealing process (Figure 1e). The corresponding atomically resolved STM image in Figure 1f concurrently exhibits perfect graphene moirés and atomic lattices. This moiré pattern actually arises from a coincidence lattice of (10×10) C−C/ (9×9) Re(0001), which also agrees well with the DFT calculations in Figure 1g. This coincidence can be calculated from the following equation:36 D=

(1 + δ)a 2(1 + δ)(1 − cos θ ) + δ 2

where D, θ, a, and δ denote the moiré period of graphene, rotation angle between the graphene and Re(0001) lattices, lattice constant of graphene (∼0.246 nm), and lattice mismatch (∼11.4%) between graphene and Re(0001), respectively. Based on the STM results, the calculated rotation angle between the adlayer and substrate was 0°, highly indicative of an epitaxial growth behavior of graphene on Re(0001). Furthermore, the moiré pattern usually presented periodic undulations (Figure 1f) due to the different spatial occupations of C on Re(0001), along with different graphene−Re(0001) interactions, which was further confirmed by the DFT calculations of the contour map (Figure 1h), with consideration of the interfacial vdW interactions. Obviously, the moiré-scale contour map matches well with the STM image, which is also justified by analysis of their height profiles, revealing similar values of ∼0.133 and 1809

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ACS Nano ∼0.141 nm (Figure 1i), respectively. Notably, the surface corrugation of graphene on Re(0001) was larger than that on Rh (111) (∼0.118 nm), suggesting likely stronger graphene− Re(0001) interface interaction.24,34,35 On the same Re(0001) substrate, h-BN was successfully synthesized. Irregular h-BN networks (∼3.00 nm in period) can be achieved after growth at ∼650 °C for ∼10 min (Figure 2a). Surprisingly, on the irregular superstructure regions, the atomic lattice was still continuous with a lattice constant of ∼0.250 nm, which indicates the formation of h-BN/Re(0001) (Figure 2b). Similar to h-BN/Ru(0001),26 “heart-shaped” moiré structures were also frequently observed on such samples, where two nanomesh “pores” overlapped and intersected at an angle of 60° (Figures 2e−h and S4). According to previous study,26 heptagon-pentagon pair-like defects should be generated at the overlapping h-BN regions. However, after sample annealing at ∼730 °C for ∼10 min, very uniform, periodic “nanomesh-like” superstructures evolved and presented the same “nanomesh” morphology as that of hBN on Ru(0001) and Rh(111) (Figure 2c).38,39 This superstructure is proposed to arise from the coincident lattice of (12×12) B−N/(11×11) Re(0001), as inferred from the atomically resolved STM images (Figures 2d and S5) and corresponding DFT calculations (Figure 2i). Obviously, the contour map derived from the DFT calculations agrees well with that of the STM morphology (Figure 2j). Moreover, according to the same equation mentioned above, the rotation angle (θ) between the h-BN and Re(0001) lattices was calculated to be ∼0°, which suggests an epitaxial growth mode of h-BN/Re(0001). Further height profile analyses of the DFT map and STM image (red and black lines in Figure 2k, respectively) revealed a similar surface corrugation of ∼0.152 and ∼0.153 nm, respectively, which are much larger than that of h-BN/Rh(111) (∼0.124 nm).24 This possibly indicates a relatively stronger modulation effect of Re(0001) with respect to that of Rh(111). At this stage, both graphene and h-BN could be synthesized on Re(0001) substrate at a reasonable growth temperature. Considering the strong graphene−Re(0001) and h-BN− Re(0001) interactions, it is interesting to know, under such intense substrate modulation condition, whether these two types of materials can be patched seamlessly in a monolayer, similar to that reported previously for h-BN-G on Ru(0001),23,26 Rh(111),24 and Ir(111).27 A two-step growth strategy was also utilized in this work for such exploration, i.e., via the sequential growth of graphene and then h-BN, as schematically illustrated in Figure 3a (more details are shown in Figure S6a). The reversal growth sequence was also tried; however, after the sub-monolayerh-BN growth, the formation of rhenium carbide on the uncovered Re(0001) regions inhibits the subsequent graphene growth, as shown in Figure S6c,d. From a global view of the formed interfaces linking graphene and h-BN in the STM images, nearly onethird of the interfaces are patched seamlessly (Figure 3b,c), while the others are characterized by obvious “defect lines” when the total coverage of the two-component materials approaches a monolayer (Figure 3d,e). After higher-temperature annealing treatment (∼800 °C for ∼30 min), even though the “defect lines” could be largely repaired, some moiréscale defects remained (Figure S7a−d) due to the pristine lattice mismatch of ∼1.8% between graphene and h-BN, the translational misalignment between the adjacent domains and the strong A−S interaction. In addition, the spatially resolved

Figure 3. Two-step sequential growth of h-BN-G heterostructure on Re(0001). (a) Schematic representation of the growth strategy. (b) Continuous interface identified by the different moiré morphologies of h-BN and graphene. (c) Magnified STM image revealing the perfect coherence of the two types of lattices at the interface, as well as the zigzag-edge type. (d,e) Discontinuous interface featuring a “defect line”, as evidenced by the sequential zoom-in image. Scanning conditions: (b) −0.002 V, 1.03 nA; (c) −0.002 V, 1.41 nA; (d) −2.671 V, 1.06 nA; (e) −1.269 V, 0.68 nA.

STS characterizations at the interfaces were also shown in Figure S7e. The V-shaped dI/dV curve of graphene becomes broadened when approaching to the boundary from the graphene side. At the same time, the band gap of h-BN is reduced when approaching to the boundary from the h-BN side. Further band gap reduction occurs at the defective interface. Additionally, a sharp one-dimensional interface can be identified in the seamless boundary because the graphene moiré humps (bright regions) always face the h-BN nanomesh “pores” (dark regions) (Figure 3b). With a high-resolution STM image, it is also possible to distinguish the edge types of the lateral heterostructures and explore their attractive physical properties. According to recent theoretical studies, h-BN-G heterostructures with abrupt zigzag edges can exhibit half metallicity and spin polarization effects,14,29−31 and the topological defects at the h-BN and graphene interface can abnormally enhance the thermal conductance.40 According to the literature, zigzag-type boundaries preferentially evolve on Ir(111),27 Ru(0001),26 Rh(111),24 and Ni(111)41 under the same UHV-CVD synthesis route, as well as on polycrystalline Cu foil via an atmospheric pressure CVD strategy.22 Intriguingly, the zigzag linking is also distinguishable for h-BN-G on Re(0001) based on the following two facts: (1) the edge direction aligns well with the direction of graphene moiré (green arrows in Figure 3b), and (2) the zigzag orientation of the atomic lattice is also in line with the direction of graphene moiré pattern (atomic models indicated in Figure 3c). Note that the B or N atoms in h-BN could not be identified due to the limitation of STM in element identification. Therefore, the fitted atomic models cannot be used to distinguish the interfacial B−C or N−C linking bonds. However, the arrows in Figure 3c indicate nearly the same lattice orientations between the h-BN and graphene lattices, which should be the premise for the lattice coherence between h-BN and graphene. Statistically, almost no armchair-type interface was visible among more than 100 scanned areas. This observation strongly suggests that zigzag linking is preferred over an armchair one for the patching growth of h-BN onto graphene on Re(0001), which is consistent with the published results on Ru(0001), Rh(111), and Ir(111).24,26,27 1810

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Figure 4. Growth of h-BN from or away from the edges of the existing graphene domains. (a) Initial growth of h-BN along the edges of the existing graphene domains with a unit of a moiré row. (b) More universally observed free graphene edges after h-BN growth. The h-BN and graphene domains were obviously separated by “gaps” with an average width of ∼3 nm. (c,d) Schematics of the subsequent h-BN growth from or away from the edges of the existing graphene domains. (e,f) and (g,h) DFT calculated formation energies (FEs) for double B−N pairs and double B−N lines nucleating on the respective edges of the Re steps and graphene domains, respectively. (i,j) DFT calculated FEs for one and two B−N pairs on the Re terraces. The FEs of one, two, and three B−N pairs and B−N lines are summarized in Tables 1 and 2. Scanning conditions: (a) −0.426 V, 1.33 nA; (b) −0.020 V, 1.33 nA.

distributed graphene and h-BN domains are expected to merge with each other, leading to the formation of a monolayer h-BNG heterostructure. Herein, defects or discontinuous interfaces should readily evolve due to the different lattice constants of the two materials, as well as the translational misalignment between the component domains. This phenomenon was indeed observed in the current system, in sharp contrast to previous reports of the formation of continuous heterostructure interfaces on some weakly coupled substrates mainly mediated by a graphene edge hetero-epitaxial growth mechanism.22,26,27 In order to achieve a deeper understanding of the special growth behaviors, DFT calculations of three cases, i.e., h-BN nucleation at the edges of the Re steps, on the Re terraces and at the edges of the existing graphene domains were also performed to derive their formation energies (FEs). For different B−N pairs, several attachment sites were considered, as shown in Figure S10, and the most stable configurations for one or two B−N pairs are presented in Figure 4e,f,i,j. Clearly, along the Re edges and on the Re terraces, the B−N pairs preferred to grow separately. Whereas along the graphene edges, the B−N pairs first formed hexatomic rings with carbon atoms in graphene. The FEs for the attachment of one, two and three B−N pairs at the edges of the Re steps, on the Re terraces and at the edges of the graphene domains are summarized in Table 1 (within Figure 4). The FE per B−N pair was defined as

For the imperfectly patched interfaces, the dashed yellow and solid blue lines in Figure 3d,e label the discontinuous and continuous segments, respectively. This anomalous interface condition is considered to have been induced by another probable channel. To clarify the formation mechanism of the two types of interfaces shown in Figure 3b,d, the growth time of the second deposited h-BN was deliberately reduced to catch the intermediate growth states. At the initial growth stage, h-BN was occasionally observed to be capable of nucleating at the edges of the existing graphene domains, with a unique unit of one h-BN moiré (Figure S8c) or a moiré row (Figure 4a). This growth behavior is very similar to the hetero-epitaxial growth of h-BN onto graphene on Cu foil,21 Ru(0001),26 and Ir(111).27 After more than 100 areas were scanned, only two presented hBN nucleation on the edges of the graphene domains. On the other hand, individual h-BN islands more frequently evolved on the bare edges of Re steps, as shown in Figure S8a,b (h-BN growth for only ∼3 min). Moreover, by extending the growth time to ∼5 min (∼90% coverage), the domain size of h-BN increased, and obvious “gaps” between the h-BN and graphene domains accordingly evolved with a width of ∼3 nm (Figures 4b and S9). Obviously, this gap value is very close to the period of an h-BN moiré. In this regard, it can be inferred that the edges of the bare Re steps overrode the edges of the existing graphene domains to template the nucleation and subsequent growth of h-BN. This is different from the graphene edge hetero-epitaxial growth mechanism on Cu foils,21,22 Ir(111),27 Rh(111),24 and even Ru(0001)23,26 substrates. The schematics of the two typical growth processes are shown in Figure 4c,d, presenting the attaching and merging growth of h-BN with graphene, respectively. Specifically, the second deposited h-BN preferably nucleates on the edges of the bare Re steps (Figure 4d) than along the edges of the existing graphene domains (Figure 4c). In the mechanism in Figure 4d, the randomly

Eform = (Etot − E basal − EBNn)/n

where Etot, Ebasal, and EBN represent the energies of the total structure, basal substrates, and one B−N pair in the monolayer h-BN and n is the number of B−N pairs. The obtained FEs were ∼ −0.68, ∼0.67, and ∼1.70 eV/B−N for one B−N pair at the edges of the Re steps, on the Re terraces, and at the edges of the graphene domains, respectively. For two B−N pairs, the corresponding FEs were ∼ −0.21, ∼0.64, and ∼1.33 eV/B−N, and for three B−N pairs, the FEs were ∼0.08, ∼0.66, and ∼1.15 1811

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ACS Nano eV/B−N, respectively. These energies indicate that the edges of the Re steps, the Re terraces, and the edges of the existing graphene domains exhibited decreased nucleation activity of hBN at the initial stage of h-BN growth. Moreover, the schematics for attaching one, two, and three B−N lines to the respective edges of the Re steps and graphene domains are presented in Figures 4g,h and S11. Note that, according to our calculations, the N atoms (blue spheres) in the B−N pairs were easier to attach to the respective edges of the Re steps and graphene domains than the B atoms (red spheres), as shown in Figure S10b. Hence, here, we only considered the case that N atoms attached to the edges. As summarized in Table 2 (within Figure 4), the FEs for one B−N line on the respective edges of the Re steps and graphene domains were ∼0.49 and ∼0.66 eV/B−N, respectively. For two B−N lines, the FEs were ∼0.07 and ∼0.22 eV/B−N, and the values were ∼ −0.06 and ∼0.03 eV/B−N for three B−N lines, respectively. Compared with the FEs for attaching B−N pairs, the same trend was found for the B−N lines at the initial stage of h-BN growth, that is, h-BN more preferably nucleated on the edges of the Re steps than the edges of the graphene domains. Hence, at the initial stage of h-BN growth onto the submonolayer graphene on Re(0001), the edges of the Re steps have higher priority over the edges of existing graphene domains to bond with h-BN. In this regard, “defect lines”, as a consequence of the imperfect merging of neighboring graphene and h-BN domains, would naturally form at the linking interfaces of graphene and h-BN, as experimentally observed in Figure 3d,e. Radically, the preferential growth of h-BN to the edges of the Re steps rather than the edges of the existing graphene domains was induced by the relatively strong h-BN−Re(0001) and graphene−Re(0001) interactions. In return, such strong interface interactions should also enhance the difficulty of the coalescence of neighboring component domains toward the formation of a perfect hybrid monolayer. From a theoretical point of view, these interface interactions are further evidenced by DFT calculations of the binding energies (BEs) of different systems (Figure 5a). The BEs were calculated using atomic models of (10×10) C−C/(9×9) Re(0001) and (12×12) B− N/11×11 Re(0001) with the inclusion of four Re (0001) layers and by considering the geometric optimization of the bottom Re layers. Generally, the BE (Eb) per Re atom can be expressed as Eb =

Figure 5. Strong interface interaction for h-BN and graphene on Re(0001). (a,b) Binding energies (BEs) and A−S distances for graphene and h-BN on Ir(111), Rh(111) and Re(0001). (c,e) Re 4f spectra of graphene/Re and h-BN/Re, respectively. New components appear at ∼41.5 eV (Re 4f7/2) and ∼43.9 eV (Re 4f5/2) for graphene on Re and ∼41.1 eV (Re 4f7/2) and ∼43.5 eV (Re 4f5/2) for h-BN on Re. (d,f) Single-point STS data for graphene and h-BN on Re(0001) at the two typical regions of “valley” (V) and “ridge” (R) locations for graphene or h-BN moiré, respectively. The typical “V”-type STS signal is almost invisible for graphene/ Re(0001), and the band gap of h-BN is reduced to ∼3.5 or ∼3.3 eV (corresponding to “R” or “V” locations, respectively). Scanning conditions: (inset in d) −0.002 V, 11.49 nA; (inset in f) −0.031 V, 11.13 nA.

BN-covered samples exhibited two new components at higher BEs (4f7/2, ∼41.5 eV; 4f5/2, ∼43.9 eV for graphene-covered Re and 4f7/2, ∼41.1 eV; 4f5/2, ∼43.5 eV for h-BN-covered Re), compared with the intrinsic Re 4f core level (Re 4f7/2, ∼40.3 eV; 4f5/2, ∼42.7 eV) (Figure 5c,e). The occurrence of these new higher BE components also implies strong A−S interactions, according to published references on adlayer− metal interactions, such as strong C−Mo and C−Re interactions.33,42−46 This deduction was further confirmed by STS measurement of the electronic properties of graphene and h-BN on Re(0001) (Figure 5d,f). As reported previously, strong A−S interactions could result in electronic doping effects from substrates to adlayers through modifying their local density of states,28,34,47 such as graphene/Rh(111)48 and h-BN/Cu foil.49 For graphene/Ru(0001), the strong interface interaction was mainly induced by the π-d orbital hybridization, as evidenced by the downward shift of graphene π-bands from the Fermi level.47 Similarly, the tunneling spectra for graphene/Re(0001) at both the strongly and weakly coupled locations (marked by “V” and “R” in Figure 5d, respectively) lose the typical Diraccone-like electronic properties of graphene, which implies the existence of strong π-d orbital hybridization between graphene and Re. Moreover, the doping effect from Re(0001) on the local density of states of h-BN is also observable from the dramatic decrease of the band gap from the bulk value (normally ∼5.9 eV) to ∼3.5 or ∼3.3 eV (“ridge” or “valley” locations, respectively) (Figure 5f).36,50,51 Briefly, the large electronic property difference between the strongly coupled system and that of the weakly coupled systems (such as quasifreestanding h-BN and graphene on Ir(111)27 and Pt(111)33) is a straightforward evidence for strong A−S interactions.

[E(layer) + E(sub) − E(layer − sub)] n

where E(layer), E(sub), and E(layer − sub) denote the energy of the overlayer, substrate, and total system, respectively, and n represents the number of atoms in the unit cell of the substrate. As a result, the BEs of graphene and h-BN per Re atom were calculated to be ∼0.54 and ∼0.59 eV (Figure 5a), respectively, which are much higher than that for graphene and h-BN on Rh(111) and Ir(111) (∼0.36, ∼0.37 eV and ∼0.32, ∼0.39 eV, respectively). The descendant A−S distances on Ir(111), Rh(111), and Re(0001) (∼0.420, ∼0.215, and ∼0.205 nm for graphene; ∼0.380, ∼0.215, and ∼0.194 nm for h-BN, respectively) shown in Figure 5b also suggest incremental A− S interactions. Moreover, the relatively strong interface interactions between the two component materials and Re(0001) were also examined with various characterization techniques. XPS measurements revealed that Re 4f peaks from graphene or h-

CONCLUSION In summary, we established a very special in-plane heterostructural model of h-BN-G on Re(0001) for exploring the key 1812

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Research Fund Program of State Key Laboratory of Coal-based Low-carbon Energy (ENN Group Co., Ltd.), Langfang 065001, China.

factors that dominate the in-plane patching growth of 2D layered materials, such as graphene and h-BN. Both the experimental and DFT calculation results indicate that h-BN much more favors to grow from the edges of the Re steps than from the edges of the existing graphene domains, leading to the coexistence of perfectly patched h-BN-G heterostructures and separated domains. This complex growth behavior is attributed to the strong modulation effect from the metal substrate. In this regard, this work serves as a fundamental reference for the synthesis of in-plane or vertically stacked heterostructures based on 2D layered materials because of its special insight into the effects of the A−S interactions.

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METHODS DFT Calculations. The total energies, forces and equilibrium geometries were calculated using the Vienna ab initio simulation package (VASP)52 within the framework of DFT. The Perdew− Burke−Ernzerhof53 generalized gradient approximation and the projector-augmented wave54 potential were used to describe the exchange-correlation energy and electron−ion interaction, respectively.55 The cutoff energy for the planewave basis set was 450 eV. Sample Preparations. An ultra-high-vacuum molecular beam epitaxial (UHV-MBE) chamber was used for the on-site growth of hBN. The Re(0001) substrate was pretreated via Ar+ sputtering and post-annealing at ∼600 °C under UHV conditions to remove surface impurities. The heterostructure film was synthesized by exposing Re(0001) to ethylene and vaporized ammonia borane (NH3−BH3) in a sequential process at ∼630 and ∼730 °C, respectively. Additionally, the sample was also removed out of the vacuum chamber for X-ray photoemission spectroscopy measurements due to its inert nature against atmospheric conditions. STM/STS Measurements. An Omicron LT-STM/STS system was used for sample characterization with a base pressure better than 10−10 mbar. All STM images were obtained at room temperature. The local differential conductance (dI/dV) spectra were measured at 78 K inside another LT-STM/STS system.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07773. Detailed XPS and STM characterizations of h-BN, graphene, and h-BN-G, including Figures S1−S11 and Tables S1 and S2 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: zfl[email protected]. *E-mail: [email protected]. ORCID

Zhongfan Liu: 0000-0003-0065-7988 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported by the National Key Research and Development Program of China (2016YFA0200103), the National Natural Science Foundation of China (Nos. 51290272, 51472008, 51432002, 50121091, and 21201012), the National Basic Research Program of China (Nos. 2013CB932603, 2012CB933404, 2014CB921002), the Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (No. KF201601), and the Open 1813

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