Innovative synthesis and characterization of large h-BN single crystals : From bulk to nanosheets

Abstract : In the past decades, due to their exceptional chemical and thermal stabilities together with their electrical insulation properties, hexagonal boron nitride nanosheets (BNNSs) have become a promising support substrate for graphene and promoted the incentive of various van der Waals heterostructures. For such applications, BNNSs are generally obtained by Chemical Vapor Deposition (CVD) or exfoliation. In order to achieve high quality and large BNNSs, our group has proposed a novel synthesis strategy based on the Polymer Derived Ceramics (PDCs) route combined with sintering techniques: Spark Plasma Sintering (SPS) or Hot Isostatic Pressing (HIP). Since hexagonal boron nitride (h-BN) crystallization is a key point in the synthesis of high quality BNNSs, efforts have been led to understand the beneficial role of a promotor of crystallization (Li3N), adopting a suitable in situ dynamic approach. It has been established that Li3N does improve the crystallization level of the product, and lower the transformation temperatures from polyborazylene to h-BN. Then, we have further investigate the influence of the SPS sintering temperature (1200-1950°C) and of the crystal promoter content (Li3N, 0-10 wt.%) on BN growth. The tested SPS parameters strongly modify the size of the resulting h-BN flakes. For an optimal Li3N concentration of 5 wt.%, h-BN flakes larger than 200 μm2 (average flake area) have been obtained. A high degree of crystallinity and purity have been achieved, even if the very-sensitive cathodoluminescence technic indicated traces of impurities, probably due to surrounding graphite parts of the SPS. Few-layered BNNSs have been successfully isolated, through exfoliation process. As a final application purpose, further physical measurements have confirmed that SPS derived h-BN exhibits an interesting dielectric constant of 3.9 associated with a dielectric strength of 0.53 V/nm. Due to a very high compact character of SPS-derivative h-BN crystals, the post-exfoliation step is made very difficult, resulting in BNNSs of tens of microns lateral size. Therefore, we have studied another sintering procedure by HIP for the ceramization process. Through this combination, we aim to promote the size of h-BN single crystals, leading to larger size exfoliated BNNSs. Characterizations from bulk crystals to BNNSs have been carried out in three aspects: morphology, lattice structure and chemical composition. This novel attempt has provided us transparent and colorless h-BN single crystals with large lateral size, up to 2000 μm. Besides, BNNSs with high purity have also been confirmed. HIP, as a new ceramization process of PDCs, has to be considered as a promising way to obtain large h-BN single crystals and nanosheets for supporting graphene and 2D heterostructures.
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Yangdi Li. Innovative synthesis and characterization of large h-BN single crystals : From bulk to nanosheets. Materials. Université de Lyon, 2019. English. ⟨NNT : 2019LYSEI025⟩. ⟨tel-02137475⟩

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