Hexagonal Boron Nitride–Graphene Core–Shell Arrays Formed by

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Hexagonal Boron Nitride–Graphene Core-Shell Arrays Formed by Self-Symmetrical Etching Growth Chenxiao Wang, Junlai Zuo, Lifang Tan, Mengqi Zeng, Qiqi Zhang, Huinan Xia, Wenhao Zhang, Yingshuang Fu, and Lei Fu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07718 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Hexagonal Boron Nitride–Graphene Core–Shell Arrays Formed by Self-Symmetrical Etching Growth Chenxiao Wang,§,† Junlai Zuo,§,† Lifang Tan,§,† Mengqi Zeng,§,† Qiqi Zhang,† Huinan Xia,‡ Wenhao Zhang,‡ Yingshuang Fu,‡ and Lei Fu*,† †

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China

‡

School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, P. R. China _____________________________________________________ ABSTRACT: The synthesis and integration of core–shell materials have been extensively explored in three-dimensional nanostructures, while they are hardly ever extended into the emerging two-dimensional (2D) research field. Herein, demonstrated by graphene (G) and hexagonal boron nitride (hBN) and via a sequential chemical vapor deposition method, we succeed for the first time in synthesizing 2D h-BN–G core–shell arrays (CSA), which possess extremely high uniformity in shapes, sizes and distributions. Each of the core–shell unit is composed of G ring-shaped shell internally filled with h-BN circular core. In addition, we perform simulations to further explain the selfsymmetrical etching growth mechanism of the h-BN–G CSA, demonstrating its potential to be used as an efficient synthetic method suitable for other 2D CSA systems. ____________________________________________________ Since they were first synthesized in early 1990s,1,2 core–shell materials have triggered a great research upsurge because of the potential in expanding their pristine properties and emergent applications of materials, which originally appeared as three-dimensional (3D) concentric core–shell nanocrystals. For example, through constructing core–shell systems, the fluorescence quantum yield of semiconductors can be improved because the surface trap states are passivated. In addition, the shell ensures effective protection against environmental changes and degradation for the core material.3 Beyond traditional 3D semiconductor nanocrystals, the emerging two-dimensional (2D) materials have provided a new perspective and infinite imagination to the scientific community since the isolation of graphene (G). Alike the exploration of 3D semiconductors, initially researchers extensively studied the synthesis and properties of individual 2D materials. Later, the rational combination of two different 2D materials has drawn significant attention, which includes the construction of 2D core–shell structures. However, as we know, 2D core–shell structures have yet to be experimentally achieved. Drawing lessons from the fabrication of 3D core–shell systems, people embarked on using structural analogues as shell layers to regulate the properties of 2D core materials. For example, hexagonal boron nitride (h-BN) and graphene are considered as good candidates for fabricating 2D core– shell structures owning to their structural similarity4 and

distinct electronic properties. It has been theoretically predicted that through in-plane connecting with an h-BN shell layer of different geometrical shapes, the band gap of graphene can be modified, which is important for its application in electronics.5 Thus explorations to the synthesis of h-BN–G core–shell layers with controllable geometry possess a significant value. Besides the elaborate construction of individual functional structures, the integration of small components into orderly arrays is a fairly advanced and even more vital for their applications in practical fields (e.g. plasmonics, catalysis, electronic and so on). Array structures contribute to pattern formation and mass production of functional materials, and will accelerate the integration of electrical and optical devices. Thus far, the controllable synthesis of arrays of 3D core–shell nanoparticals6 (or nanorods,7 nanotubes8) has been extensively studied. Although many single-component arrays of graphene or h-BN crystals have been successfully obtained,9,10 the efficient synthesis of 2D core–shell arrays (CSA) remains unexplored. Herein, via regulating the self-symmetrical arrangement and lateral etching growth of h-BN in uniform graphene films in a sequential CVD process, we succeed for the first time in synthesizing 2D h-BN–G arrays, in which all the building blocks exhibit core–shell shapes constituted by h-BN circular cores and graphene ring-like shells. Thus, it’s the first experimentally obtained 2D h-BN–G CSA. The as-obtained h-BN–G CSA can be achieved over millimetre-scale large area, and the sizes and distributions of the h-BN–G core– shell units exhibit extremely high uniformity. This continuous synthetic method perfectly inherits the advantages of traditional self-assembly methods, thus processes both high efficiency and controllability. We are confident that the presented approach will open up new territory for 2D core– shell structures synthesis and promote the application of 2D functional structures in integrated devices to a great extent. Scheme 1 demonstrates the formation process of h-BN–G CSA. Cu foil on W substrate was put in a quartz tube and heated up to 1100 °C in Ar/H2 atmosphere to form smooth liquid surface, then CH4 was introduced as the carbon source to grow graphene film on the melted Cu surface. By using

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Scheme 1. Schematic for the formation process of h-BN–G CSA.

liquid metal as the catalyst, large-area uniform graphene films could be obtained (Figure S2),11 which is a crucial premise for the further large-area self-assembly of h-BN. After that, the temperature was reduced to achieve the solidification of liquid Cu with the coating of graphene. Then the growth of h-BN was carried out. During this process, the formation of core–shell ordered structure involved the adsorption and self-symmetrical arrangement of h-BN building blocks (such as borazine) on graphene platform, and subsequent in situ etching and substitution of graphene by hBN building blocks. Finally, the growth of h-BN led to both cores and shells formation, thus the h-BN–G CSA was successfully synthesized. The morphology and uniformity of the as-grown h-BN–G CSA were evaluated, as illustrated in Figure 1. A typical optical image of the h-BN–G CSA is shown in Figure 1a. One can see that the ordered hexagonal packed structure of the CSA is maintained over the entire 80,000 µm2 area, in which both the shapes and sizes of the core–shell units possess excellent consistency. Larger area of CSA at a millimetre scale is shown in Figure S3. Moreover, a scanning electron microscope image of the h-BN–G CSA is shown in Figure S4. To exhibit the regularity of the CSA, we statistically studied over 100 randomly collected data from Figure 1a. The statistical distributions of the inner diameters (Dinner) and outer diameters (Douter) of the core–shell structures are illustrated in Figure 1b. The average values of the Dinner and Douter are 7.95 and 12.01 µm with the standard deviations of 0.16 and 0.15 µm, respectively. Thus the deviation rates are as low as 2.0% and 1.2%. Amazingly low deviation rates reveal the ultra-high consistency of the core– shell units. Furthermore, in order to demonstrate the regularity of the arrays, we employed a statistical study on the included angles among each three adjacent circles and the centre distances between adjacent circles, and the results are shown in Figure 1c. The average value of the included angles is exactly the ideal value for hexagonal packed structures, 60.0°, and the deviation rate is only 4.0%. For centre distances, the average value is 12.20 µm, almost equals that of Douter (the difference between them is smaller than the accidental error). This is also consistent with ideal hexagonal

Figure 1. Statistics of the h-BN–G CSA. (a) Large-area optical image of the CSA from which the data was taken. (b) Statistical distributions of the inner and outer diameters. (c) Statistical distributions of the included angles and center distances.

packed structure. The above-mentioned statistical analysis suggests that the as-obtained h-BN–G CSA possesses admirable consistency and order. What’s more, the periodicity of the CSAs and the diameter of each core–shell unit can be regulated by changing the amount of the borazine precursor and the growth time of h-BN (Figure S5 and S6). Raman spectra were employed to characterize the composition of the core–shell structure. The typical Raman spectra of three points located in the core, shell and gap areas are shown in Figure 2a and b. Inside the core area the dominant Raman peak is at 1,369 cm–1, which is consistent with the characteristic peak of the E2g symmetry vibration mode of h-BN.12 This indicates that the circular core is composed of pure h-BN. The peak in 1,448 cm–1 is from the third-order transverse optic mode of the Si in SiO2/Si substrate.13 The peak intensities of the spectra for the core area and the substrate in Figure 2a are magnified 15 times to make the peaks more visible. In the shell and outside gap, the peaks located at 1,348 cm–1 are attributed to graphene’s D band. The intensity ratio (ID/IG) at the shell area is larger than that at the gap area, which might be attributed to the slight B or N doping and oxidation. The existence and layer numbers of the graphene are further confirmed in Figure 2b. In the outside gap area, the intensity ratio (I2D/IG) is about 2.0, in consistent with that of monolayer graphene. While in the shell area the 2D peak intensity decreased and the intensity ratio (I2D/IG) reduced to less than 1.0, indicating the layer number and defect density of graphene may both increase in the process of shell formation. The flat spectrum line in the core region reveals no intermix of graphene within the h-BN core. Also, the X-ray photoelectron spectroscopy was conducted to confirm the existence and bonding of the corresponding B, N and C elements (Figure S7 and Table S1). Raman mapping was employed to further characterize the distribution of h-BN and graphene in the CSA. Figure 2c

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Figure 3. Schematic showing the self-assembly mechanism. (a) Molecular structures and electrostatic potential distributions of h-BN building blocks. (b) Electrostatic force-induced auto-organization of h-BN building blocks on the graphene basal plane and the formation of the arrays.

Figure 2. (a,b) Raman spectra showing the components of an h-BN–G core–shell unit. (c) Raman mapping of the intensity of 2D peak for a core–shell circular unit. (d) Raman mapping of the intensity of E2g peak for the h-BN core area.

shows the intensity map of graphene 2D peaks of a core–shell unit. In the circular core the 2D peak intensity is nearly zero, indicating that no graphene exists in the cores, while next to the core there is a visible circular ring with lower 2D peak intensities than those in the outside area, revealing the existence of few-layered graphene shell.14 The intensity map of h-BN E2g peaks in the core area is shown in Figure 2d. The bright area in Figure 2d is consistent with the dark area in Figure 2c, and the similar intensities in the core reveal the uniformity of h-BN. Moreover, atomic force microscopy (AFM) and scanning Kelvin microscopy (SKM) were conducted to clarify the 2D h-BN–G core–shell structure (Figure S8 and S9). In order to further identify the structures and electronic properties of the core and shell, transmission electron microscopy (TEM) and scanning tunnelling microscopy (STM) were both employed (Figure S10 and S11). From selected area electron diffraction (SAED) patterns, we found that the h-BN core and graphene gap were both highly crystalline. The as-derived interplanar spacings are in consistence with those of h-BN and graphene, respectively. While in the shell area, there are several sets of diffraction patterns with the interplanar spacing of 0.210 nm, showing that the shell might be few-layered graphene. From highresolution TEM image, the structure of multilayer graphene shell can be clearly observed. From the atomic resolution STM images, the atom arrangements of the h-BN core and graphene shell are displayed. The corresponding dI/dV–V curves reveal the electronic properties of the core and shell, which agree well with the reported work.15 What is worth noting is that, the artfully set growing procedure, in which graphene is grown on liquid Cu substrate

followed by the solidification of Cu for h-BN growth, is a key cause for the ordered arrangement of h-BN–G core–shell units. Owing to the structural isotropy and smooth nature of liquid surface, graphene film grown on liquid Cu exhibits large-area uniformity and single-layer consistency (demonstrated in Figure S2)11, which could serve as a perfect platform for the electrostatic interaction and self-arrangement of h-BN building blocks. What’s more, when the liquid Cu is solidified with a graphene film covered on it, the crystal form of its surface is uniform and can be assigned to (101) face, as confirmed by the electron backscattered diffraction (EBSD) characterization in Figure S12. Thus, the uniform graphene on the isomorphous Cu surface could serve as a perfect platform for the self-symmetrical etching growth of h-BN. In order to demonstrate the unique and decisive role of the uniform monolayer graphene gown on the Cu surface plays in h-BN–G CSA synthesis, a comparative experiment was carried out on re-solidified Cu–W substrate. Due to the nonuniformity of graphene film grown on solid Cu surface, only irregular h-BN could form (Figure S13). Our proposed mechanism, that is the h-BN–G CSAs are formed by self-symmetrical etching and lateral regrowth of hBN, is based on the experimental results. AFM and Raman characterizations confirmed the in-plane core-shell structure. The highly ordered 2D CSA should origin from that of the adsorbed h-BN unit array on the graphene film, which has been verified by the controllability of the periodicity of the CSA and the diameters of the core–shell structure via adjusting the precursor amount and the growth time. We further confirmed the as-proposed self-symmetrical adsorbing and etching process through theoretical simulation. The interaction of h-BN building blocks (take borazine molecules as an example) adsorbed on the graphene surface was simulated. Molecular structures and the electrostatic potential (ESP) maps of borazine molecules are determined. The results reveal an anisotropic electrostatic potential distribution at boron edges and nitrogen edges. The calculated ESP Vs(r) on the molecular surface of these precursor molecules are presented in Figure 3a. The larger

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the molecule is, the more anisotropic the electrostatic potential distribution will be. In each instance, ESP Vs(r) is positive over the nitrogen edges, while it’s negative over the boron edges. Thus, the directivity of the static electric field will lead to electrostatic interactions among the building blocks and direct the oriented movements of them into an ordered structure. We further employed structural optimization to a system in which borazine molecules are randomly adsorbed on a graphene basal plane,16 as schematically represented in Figure 3b. During simulation, the distances between a building block and its neighbouring ones became closer to the average value (e.g. from 4.039 and 2.105 Å to 3.623 and 2.381 Å, respectively) (see Figure S14). Also, the total energy of the unorderly adsorbed borazinegraphene system is 52 meV higher than that of the orderly adsorbed system (Figure S15). As a result, the original disordered BN building blocks adsorbed on the large-area uniform and smooth17 graphene platform would tend to selforganize themselves to form an ordered structure. As demonstrated by the thermodynamical calculation, the following decomposition of the hydrogen-rich h-BN building blocks would lead to chemical etching of graphene and lateral growth of h-BN to form h-BN–G CSA. Such an etching effect of borazine on graphene has ever been predicted in the previous work.18 When each seven graphene hexagonal single-rings were substituted by seven BN rings, the total energy of the system achieved a reduction of 45.5 eV (Figure S16), which means that this reaction is thermodynamically supported. Previous works support this conclusion as well.19 The formation of graphene shell is surmised to be due to the curling and stack-up of graphene film forced by rapid in-plane growth of h-BN. In summary, we have demonstrated a novel experimental pathway to obtain the h-BN–G arrays with ultra-uniform shapes, sizes and ordered arrangements, that is, the first selfassembled h-BN–G CSA. Each of the naturally formed units in the arrays with unique core–shell shapes own the potential to serve as an electronic device with its properties remaining to be explored. We are confident that the presented approach, with its considerable controllability and efficiency, is going to open new territory for the precise and large-scale synthesis of more 2D ordered arrays with complex and functional material structures, and will facilitate their application in 2D integrated systems and devices.

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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Natural Science Foundation of China (Grants 21673161, 21473124).

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Supporting Information Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions §

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These authors contributed equally to this work.

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