Van der Waals epitaxy and characterization of hexagonal boron nitride nanosheets on graphene
© Song et al.; licensee Springer. 2014
Received: 9 May 2014
Accepted: 21 July 2014
Published: 28 July 2014
Graphene is highly sensitive to environmental influences, and thus, it is worthwhile to deposit protective layers on graphene without impairing its excellent properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection. In this research, we demonstrated the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by chemical vapor deposition, using borazine as the precursor to h-BN. The h-BN nanosheets had a triangular morphology on a narrow graphene belt but a polygonal morphology on a larger graphene film. The h-BN nanosheets on graphene were highly crystalline, except for various in-plane lattice orientations. Interestingly, the h-BN nanosheets preferred to grow on graphene than on SiO2/Si under the chosen experimental conditions, and this selective growth spoke of potential promise for application to the preparation of graphene/h-BN superlattice structures fabricated on SiO2/Si.
Graphene has attracted global research interests across a wide range of applications [1, 2]. However, graphene is highly sensitive to extraneous environmental influences. Thus, it was deemed worthwhile to deposit protective layers over graphene without impairing its properties. Hexagonal boron nitride (h-BN), a well-known dielectric material, may afford the necessary protection for graphene [3, 4].
As an analogue of graphene, h-BN shows a minimal lattice mismatch with graphene of about 1.7%, yet has a wide band gap [5–8] and lower environmental sensitivity [3, 4]. Hence, h-BN proves to be a promising dielectric material, or substrate, for two-dimensional electronic devices and especially for those based upon the use of graphene [9–13]. Graphene, partially covered by h-BN protective layers, may display promising electronic characteristics of graphene with much lower environmental sensitivity.
Recently, chemical vapor deposition (CVD) synthesis of h-BN on Ni [14–16] or Cu [13, 17–19] substrates has been further investigated. For the following applications in graphene electronic devices, h-BN can be acquired by etching of the catalyst substrates and a transfer technique. Nevertheless, the transfer process brings inevitable contamination or even destruction, and it is difficult to determine the position and the coverage ratio of h-BN on graphene. Considering this problem, we pay attention to the catalyst-free CVD growth of h-BN on graphene, which promises direct application in graphene electronic devices and may obviate the need for a transfer process.
It has been demonstrated that van der Waals epitaxy by catalyst-free CVD can be a promising route for the growth of topological heterostructures [20–22]. Moreover, the surface of graphene is atomically flat and without dangling bonds, which makes graphene a promising template for the van der Waals epitaxy of other two-dimensional materials. Compounds with 1:1 B/N stoichiometry are often selected as h-BN precursors for CVD, and borazine (B3N3H6) could be a promising choice as it would produce BN and hydrogen, which are both environmentally friendly.
In this research, the van der Waals epitaxy of h-BN nanosheets on mechanically exfoliated graphene by catalyst-free low-pressure CVD, using borazine as the precursor to h-BN, was demonstrated. The h-BN nanosheets preferred to grow on graphene rather than on SiO2/Si and tended to exhibit a triangular morphology when grown on a narrow graphene belt. The h-BN nanosheets grown on graphene were highly crystalline, albeit with various in-plane lattice orientations.
h-BN nanosheets were synthesized in a fused quartz tube with a diameter of 50 mm. Graphene was transferred onto silicon oxide/silicon (SiO2/Si) wafers by mechanical exfoliation from highly oriented pyrolytic graphite (HOPG, Alfa Asear, Ward Hill, MA, USA). The h-BN precursor (borazine) was synthesized by the reaction between NaBH4 and (NH4)2SO4 and purified according to our previous reports [23, 24]. The temperature for the CVD growth of h-BN nanosheets was set to 900°C. Before the growth of h-BN, with the tube heated to 900°C, graphene grown on SiO2/Si was first annealed for 60 min in an argon/hydrogen flow (Ar/H2, 5:1 by volume, both gases were of 99.999% purity from Pujiang Co., Ltd, Shanghai, China) of 180 sccm to remove pollutants remaining on the graphene after mechanical exfoliation. During the growth process, borazine, in a homemade bubbler, was introduced to the growth chamber by another Ar flow of 2 sccm, while the Ar/H2 flow remained unchanged. The typical growth time was 5 min, while the pressure was 10 to 100 Pa. After the growth process, the tube was rapidly cooled to room temperature.
Raman spectroscopy was performed in a Thermo DXR with 532-nm laser excitation (Thermo Fisher Scientific, Waltham, MA, USA). Atomic force microscopy (AFM) (Dimension Icon, Bruker, Karlsruhe, Germany) and scanning electron microscopy (SEM) (Nova NanoSEM 320, FEI Co., Hillsboro, OR, USA) were used to observe the thickness and morphology of the h-BN nanosheets. X-ray photoelectron spectroscopy (XPS) (AXIS Ultra, Kratos Analytical, Ltd, Manchester, UK) was conducted to analyze the chemical composition of the films. The h-BN nanosheets with the graphene substrate were transferred to transmission electron microscopy (TEM) grids for further characterization. Both morphology images and selected area electron diffraction (SAED) patterns of the h-BN nanosheets were obtained by field emission high-resolution transmission electron microscopy (HRTEM) (Tecnai G2 20, FEI Co.).
Results and discussion
This result possibly originated from the minimal lattice mismatch between h-BN and graphene, and the small amount of defects remaining in the graphene after mechanical exfoliation and high temperature annealing, and these would enable the h-BN to nucleate on graphene and grow thereafter. This selective growth phenomenon promises potential applications for graphene/h-BN superlattice structures fabricated on SiO2/Si.
The h-BN nanosheets exhibited a polygonal morphology with some nanosheets becoming isolated islands on the graphene, while others with different thicknesses joined and became stacked, as shown in Figure 2c. Moreover, the h-BN nanosheets tended to exhibit a triangular morphology on the much narrower graphene belt, as shown in Figure 2b. This result is similar to van der Waals epitaxial growth of MoS2 on graphene  and perhaps originates from the higher boundary effect of the narrower graphene belt after mechanical exfoliation . Besides, the triangular h-BN nanosheets on graphene showed different in-plane orientations from each other.
Raman spectroscopy provided a useful means of gleaning information about the lattice vibration modes of graphene and h-BN. After being transferred to SiO2/Si by the Scotch tape mechanical exfoliation method, the graphene was generally aligned with the (002) lattice plane parallel to the surface of the SiO2/Si wafer [1, 2].
According to previous reports , the gas-phase nucleation for h-BN was absent at growth temperatures lower than 1,000°C; hence, the growth of h-BN nanosheets on graphene was dominated by the surface nucleation during our CVD process at 900°C. Moreover, the surface topography of the substrate is vital to the surface nucleation . Consequently, the nucleation of the h-BN nanosheets on the graphene substrate was regulated by the surface morphology of graphene in our work. Additionally, the atomic scale defects, dislocations, and steps for the graphene substrate were inevitable during the mechanical exfoliation process due to the strong interlayer binding of graphite , and the atomic-level defects, dislocations, and steps of the substrates would serve as the nucleation centers for CVD growth, for the curved sp2 π bonds in the graphene defects, dislocations, and steps were more reactive than the planar graphene regions [21, 32]. In our work, a small number of defects for the graphene substrates were proved by the weak D peak of Raman spectra in Figure 3. The atomic defects offer additional bond sites to the carbon atoms, making them energetically preferred for nucleation. During the CVD growth, the atomic-level defects of graphene could effectively cause nucleation of the h-BN on the graphene. Subsequently, with an increased amount of precursor, the h-BN nanosheets could grow on the surface of graphene through weak van der Waals interactions.
We have pointed out the reason for the nucleation of the h-BN on graphene. In fact, the deposition of h-BN nanosheets on graphene was performed as instantaneous nucleation followed by three-dimensional growth in our catalyst-free CVD growth. Similar results of three-dimensional growth in certain situations have been proved by previous reports [21, 32]. As discussed above, energy optimization is of great importance to the nucleation of h-BN, and the defects, dislocations, and steps of graphene are energetically preferred. During the CVD growth of h-BN on graphene, the above energetically preferred regions of graphene would be covered or remedied by h-BN layers with a certain domain size. As an alternative, the edges of the as-grown h-BN layers and the regions near the defects of graphene turned energetically preferred for nucleation of new h-BN layers, which both favor the vertical or three-dimensional growth of h-BN nanosheets on the graphene.
In summary, we have demonstrated the van der Waals epitaxy of h-BN nanosheets on graphene by catalyst-free CVD, which may maintain the promising electronic characteristics of graphene. The h-BN nanosheets tended to have a triangular morphology on a narrow graphene belt, whereas they had a polygonal morphology on a much larger graphene film. The B/N ratio of the h-BN nanosheets on graphene was 1.01, indicative of an almost stoichiometric composition of h-BN. The h-BN nanosheets preferred to grow on graphene rather than on SiO2/Si, which offered the promise of potential applications for the preparation of graphene/h-BN superlattice structures. The h-BN nanosheets on graphene had a high degree of crystallinity, except for various in-plane lattice orientations. The synthesis of h-BN nanosheets on multilayer graphene has been studied, and h-BN nanosheets on few-layer and even monolayer graphene will be synthesized in future work. This may satisfy certain application requirements for topological heterostructures and graphene-related electronic devices.
This work was financially supported by projects from the Natural Science Foundation of China (Grant Nos. 11104303, 11274333, 11204339, 61136005, and 50902150), Chinese Academy of Sciences (Grant Nos. KGZD-EW-303, XDA02040000, and XDB04010500), the Open Foundation of State Key Laboratory of Functional Materials for Informatics (Grant No. SKL201309), the National High-tech R & D Programme (Grant No. 2012AA7024034), and the National Science and Technology Major Projects of China (Grant No. 2011ZX02707). We thank the anonymous reviewers for their helpful suggestions which have improved the manuscript.
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