Controlled Growth Of Boron Carbonitride Nanosheets And Their Properties

Two-dimensional (2D) nanomaterial is a novel nanomaterial with unique structure. Since the discovery of graphene, increasing attention has been concentrated on the research of 2D nanomaterials due to their unique properties and potential applications in electronic devices. Hexagonal boron nitride (hBN) is similar to graphite in structure and possesses some properties superior to graphite. Therefor, many researchers interest in 2D BN nanomaterials. BN nanosheets have many properties complementary to graphite such as semiconducting, high stability at high temperature, and high chemical stability, and thus they will potentially applied in UV light emission nanoelectronic devices, high temperature catalyst supports, electron field emitter, and self-cleanning coatings. Boron carbonitride (BCN) materials prepared by doping BN with carbon possess improved conductivity and adjustable band structure, and thus have unique properties and applications.In this thesis, deposition of BCN films composed of 2D nanosheets on (001)-oriented Si wafers by microwave plasma chemical vapor deposition (MPCVD) were investigated. The BCN nanosheets were prepared by introducing CH4 as the carbon source based on the established deposition method for growing hBN nanosheets from the gas mixture of N2-BF3-H2. The growth behavior of the BCN nanosheets is much different from that of the BN nanosheets due to the introducing of CH4 into the reactive system. For examples, the suitable working pressure for the growth of BN nanosheets is 6 kPa and and they can not grow below 4 kPa, while after additional CH4, the proper working pressure for BCN nanosheets is 2-3 kPa. Moreover, the introduction of CH4 to the deposition system can accelerate the growth of BCN nanosheets. The effects of the process parameters on the morphology, structure, and property of the BCN nanosheets were systematically investigated, including working pressure, microwave power, dc bias, and the gas flow rate. The growth mechanism of BCN nanosheets was also investigated. The results show that the size, thickness, shape, and density of the nanosheets can be controlled by changing the growth conditions such as BF3/H2 ratio and CH4 flow rate. BCN nanosheets change from smooth ones to those with branches depending on the gas composition. SEM analysis indicates the length of the nanosheets is controlled in the range of 0.3-2.2μm, and the thickness is mostly smaller than 20 nm. TEM measurements suggest that the interlayer space of BCN nanosheets is 0.34 nm, close to the interplanar distance of the (002) plane of hBN and graphite. It also suggests the thickness of nanosheets is mainly less than 10 nm. The FTIR spectra and Raman spectra suggest that the carbon elements are well introduced in BCN nanosheets. As CH4 flow rate increases the photoluminescence peak shifts from 375 nm to 530 nm. Too high carbon concentration causes the BCN nanosheets to lose the light emission ability and may not be semiconducting.