| Boron nitride film is a kind of graphene analogue film with a honeycomb lattice structure, which is by composed of alternating boron and nitrogen atoms. Boron nitride film inherits the excellent performance mechanical, electrical, thermal and optical aspects of boron nitride bulk materials, and because of its special two-dimensional structure, it has more features and more extensive application prospects. To date BN film is still in the basic research stage. Based on the summary of preparation processes of boron nitride films, with Borazine used for boron nitride precursor, and Ni, Cu, and carbon materials used for substrates, a high degree of crystallinity, large boron nitride film of uniform thickness was successfully prepared by chemical vapor deposition（CVD） process. In this paper, we presented a method for the preparation and atomic layers control of BN films on Ni substrates, as well as the mechanisms of the growth of BN single crystal and thin film on Cu substrates, and boron nitride single crystals with a maximum size of about 20 μm were prepared on Cu substrates. BN films on graphene and carbon aerogel were prepared, and ultralight BN aerogels were obtained through process control, while the BN aerogels were actually BN films with three-dimensional structures. The optical, electrical and anti-oxidant properties of BN films were studied.BN films with thickness of several atomic layers were prepared on Ni substrates by CVD, and the effects of parameters such as temperature, time, dilution gas on the BN films were studied. BN films with various atomic layers were achieved, and the mechanism of the atomic layers control of BN films were discussed. Preparation of Ni substrate surface The optimized CVD conditions for BN film were as following: 1000 °C, atmospheric pressure, with 15 min growth time, diluted gas of Ar/H2 mixed gas, and the ratios of Borazine/H2/Ar were 2:45:225. BN films with thicknesses of 2 to 10 or more layers were achieved through process control, with the interlayer spacing of 0.34 ± 0.01 nm. The epitaxial growth and segregation mechanisms of BN films were observed, and the final thicknesses of the films could be composed of two steps, the epitaxial growth process and the diffusion-segregation process. During CVD process（epitaxial growth process）, the boron nitride films show epitaxial growth on Ni foil, meanwhile the B and N atoms dissolve in the Ni foil or the grain boundary. Subsequently, during the cooling stage of Ni foil after CVD（diffusion-segregation process）, the contraction of Ni foil forces the dissolved B and N atoms to diffuse, with segregation occurs at the topmost surface of Ni foil to form new h-BN layers. The impact of the extension of growth time on film thickness reflects the epitaxial growth mechanism, while the influence of the cooling rate on the thickness of BN films reflects the segregation mechanism. With the upgrading of the cooling rates, the thicknesses of the BN films were increased, but the thickness uniformity dropped. With Ni/Si substrate, the Si and Ni atoms reacted to form nickel silicide barrier at high temperature, preventing B and N atoms in the dissolution and diffusion of Ni substrate, ensuring epitaxial growth of BN films directly on the surface of Ni, and BN film with single atomic layer were achieved.Single-atomic-layer BN films and large scale BN single crystal were prepared on Cu substrates by CVD. The effect of CVD temperature, time, gas conditions and the state of subatrates on the synthesized BN films were studied. The optimized CVD conditions for BN film and single crystal were as following: polished copper substrate, 1000 °C, low pressure, dilute gases of Ar/H2 mixture, the ratio of Borazine/H2/Ar were 1:180:270. BN single crystal prepared at 1000 ° C exhibited a regular triangllar morphology with the maximum size of about 20 μm. Based on chemical thermodynamic principles, for boron nitride film on copper surface at high temperature, the boundary terminated with B atoms was unstable, while the boundary terminated with B atoms was stable, and therefore the boron nitride film tended to exhibit a triangular shape. The growth of boron nitride film on copper were mainly due to the epitaxial growth mechanism, with the extention of growth time（15 to 60 min）, the procedure of the nucleation, small domains and ultimately a continuous film were observed on Cu substrates. As hydrogen is the by-product of borazine decomposition, considering the impact of chemical equilibrium, with Ar/H2 mixed gases used as diluent gases, BN single crystal and films with good quality were prepared. The presence of hydrogen showed etching and modifications for the triangullar morphology of the BN single crystal and films. The polished or oxidation of copper substrate had a great impact on the growth of boron nitride products.BN domains were prepared on graphene substrates by CVD. The effect of CVD temperature and time on the synthesized BN films were studied. The optimized temperature for CVD growth of BN films on graphene was 900 °C. The growth of BN domains on graphene substrates can be divided into three processes, namely adsorption, nucleation and crystal growth processes. At higher temperatures, the kinetic energy of the BN elements or radicals were too high, and the adsorption process was inhibited on surface of the graphene substrates, the desorption was dominant as a result and leaded to the decrement of nucleation density. However, at lower temperature, the opposite happened.Graphene was ultra flat substrate of atomic scale which had no catalytic effect for the growth of BN films. With the prolongation of CVD time, the adsorption, nucleation and crystal growth process persisted in the initial stage of the CVD reaction. However, in the later stage, the coverage of BN domains on graphene substrates reached a certain percentage, thus it was difficult to achieve adsorption, nucleation and crystal growth processes in the uncovered areas of the substrate, and the vertical growth along the old nucleation centers turned dominant, as a result it was difficult to produce a continuous BN film on graphene substrate.With template-assisted CVD method, BN thin films were synthesized with carbon aero gels as templates in 900 °C, then the carbon aerogels templates were removed at oxidation temperature of 600 °C, and BN aerogels was obtained. The BN aerogels were actually BN films with special three-dimensional structures. The lattice structures of BN aerogels were similar to those of carbon materials, however with slightly lower crystallinity, which was because of the growth of the BN films without catalysis. An ultralight BN aero gel with a bulk density of 0.6 mg/cm3 was successfully prepared with a CVD reaction time of 30 min. The density is much lower than the bulk density of air（1.29 mg/cm3）. The ultralight BN aerogel had specific surface area of up to 1051 m2/g.The optical, electrical and anti-oxidant properties of BN films were studied. BN films had good transparency in the visible region, but with a strong absorption peak in the deep UV region at 203.0 nm. M onolayer BN film had a wide optical bandgap of 6.05 ± 0.03 e V. A resistance device was fabricated, and a four-terminal method was used to test the resistance of the BN films transferred onto the SiO2/Si surface. The BN films showed good insulation properties, indicating that they were with no doping cases and high quality. The BN films exhibited high temperature oxidation resistance with the upper limit temperature of 800 °C. Under the oxidation temperature over 800 °C, the E2g intensity of Raman spectra of of the oxidized BN films began to weaken, and the frequencies became higher, while the FWHM values became larger. The results were attributed to the oxygen doping and disorder of BN lattices caused by oxidation. BN aerogels had oxidation resistance at high temperatures far better than carbon aerogels. The oxidation of BN aerogels began at 800 °C, the oxidation process of BN aerogels contained two changes, the oxidation of BN and the sublimation of B2O3. However, according to the TG analysis, even if the oxidation temperature reach ed 1300 °C, the residue weight of the BN aerogels was still over 60%. BN aerogels were selective adsorbents, which exhibited superhydrophobic properties, while with good absorbency for organics（oil）. The BN aerogels could absorb up to 160 times their own weight in oil. The BN aerogels after oil absorption could be restored for reuse simply by burning them in air, and the restored BN aerogels showed equal absorption properties with the original BN aerogels.In this paper, the atomic layers control of boron nitride films were achieved, and large scale triangular boron nitride single crystal as well as ultralight boron nitride aerogel were prepared. In the future, we will explore the applications of boron nitride films in microelectronic devices especially in graphene devices, and prepare boron nitride aerogels with much larger scale to explore their further applications. |
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