Synthesis of vertically aligned boron nitride nanosheets using CVD method
Graphical abstract
Highlights
► The synthesized boron nitride nanosheets (BNNSs) are vertically aligned and very thin. ► No electrical field is applied in the CVD process. ► The thin BNNSs show a low turn-on field of 6.5 V μm−1 and emit strong UV light.
Introduction
Recently, 2D materials have attracted great attention since the appearance of the graphene which has been found to possess excellent quantum transport and mechanical properties [1]. Hexagonal boron nitride is comprised of layered structures as the isoelectric analog of graphite. Compared with graphite carbon, BN has excellent mechanical properties and thermal conductivity and is much more thermally and chemically stable, which makes BN a better candidate for composite materials in hazardous environments [2], [3]. Boron nitride is a semiconductor with a wide band gap near 6 eV, in contrast to the semimetallic nature of graphite. Recent researches have revealed that BN is of high promise in application of ultraviolet (UV) light emission devices [4], [5]. As the main member of BN nanomaterials, single or few layers of boron nitride have interesting properties and potential applications. Terrones et al. found that BN nanoribbons with zigzag edges can behave as metals, thus exhibiting excellent electron field emission properties [6]. Yu et al. detected that boron nitride nanosheets were superhydrophobicity [5]. Ultrathin BN nanosheets protruding from BN fibers and ultrathin BN nanosheets protruding from Si3N4 nanowires had been found to exhibit excellent field emission properties [7], [8].
Up to now, several methods have been proposed to obtain graphene analogs of boron nitride, including micromechanical cleavage a bulk BN crystal [9], unwrapping multiwalled BN nanotubes through plasma etching [10], sonication [11], [12] of BN particles or using a high-energy electron beam [13], [14], solid phase reaction [15] and chemical vapor deposition [16], [17]. Vertically aligned boron nitride nanosheets were prepared on silicon substrates from gas mixture of BF3–N2–H2 through microwave plasma chemical vapor deposition (MPCVD) technique previously. Yu et al. proposed that the etchant of F atoms and the electrical field were the main reasons for the formation of vertically aligned BNNSs [5].
In this paper, we report the synthesis of 2D boron nitride nanosheets protruding from BN films on silicon substrates from gas mixture of BCl3–NH3–H2–N2 by chemical vapor deposition technique. No electrical field was applied. The structure and morphology of the products were systematically investigated by scanning electron microscopy, transmission electron microscopy and Fourier transformation infrared spectroscopy. The chemical composition was studied by energy dispersive spectroscopy and X-ray photoelectron spectroscopy. The optical property of boron nitride nanosheets studied via cathodoluminescence spectra reveals strong cathodoluminescence emission in the ultraviolet range, and this indicates that the present novel BN nanosheets are highly promising for application in optical devices. A turn-on electric field of 6.5 V μm−1 has been observed in the as-prepared BN nanosheets.
Section snippets
Experimental
The growth of the BNNSs protruding from BN films was performed in a quartz tube type conventional CVD system. A silicon monocrystal wafer as the substrate was placed in the center of the quartz tube. The furnace was first flushed with pure nitrogen gas for 30 min and then the furnace was heated under N2 + 5% H2 gas (200 ml min−1). When the temperature of the substrate reached 1000 °C, NH3 and BCl3 were separately introduced into the tube. The gas flow rates of N2 + 5% H2, BCl3, NH3 were 200, 15, 150 ml
Results and discussion
The SEM images of the obtained sample are shown in Fig. 1a and b. The substrate surface is homogeneously covered by a thick layer of nanosheets, as shown in Fig. 1a. The high-magnification image (Fig. 1b) shows that parts of the nanosheets are vertically aligned on the substrate. The thickness of the aligned nanosheets is less than 10 nm. The inset image in Fig. 1b shows the chemical composition of the product. The elements of B, N, Si and O can be found from the spectrum. The Si signal comes
Conclusions
BNNSs protruding from BN films have been successfully synthesized using a simple CVD technique. Parts of the BNNSs are vertically aligned on the BN films with a graphite analogs structure. The thickness of the BNNSs is less than 10 nm. A strong cathodoluminescence emission in the ultraviolet range was detected from the product, and this indicates that the BNNSs are highly promising for application in optical devices. The FE characteristic of the product shows a low turn-on field of 6.5 V μm−1.
Acknowledgements
This work was financially supported by National Basic Research Program of China (2009CB930503), NSFC (Contract Nos. 50801042, 50823009), the Fund for the Natural Science of Shandong Province (ZR2010EM020, ZR2010EM049), and SRF for ROCS, SEM.
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