Sciences & Société
Soutenance de thèse : Yangdi Li
Innovative synthesis and characterization of large h-BN single crystals: from bulk to nanosheets
Doctorant : Yangdi Li
Laboratoire INSA : MATEIS
Ecole doctorale : ED 34 Matériaux de Lyon
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 application, BNNSs are generally obtained by chemical vapor deposition (CVD) and exfoliation. In the first method, domain size, morphology and layer number remains to be challenges for the applications of CVD BNNSs. Secondly, obtaining BNNSs by exfoliation is greatly relevant to the quality of h-BN crystal source.
In order to achieve high quality large BNNSs, our group has proposed a novel synthesis strategy by the polymer derived ceramics (PDCs) route combined with sintering technics: spark plasma sintering (SPS) or High Isostatis Pressing.
Here we firstly present the in situ tests of PBN and crystallization promoter (Li3N) mixture, trying for a better understanding of the ceramization process. In our study, both pure PBN and PBN/Li3N mixture are characterized by thermal analysis and in situ XRD technique.
In a second part, we further investigate the influence of sintering temperature and crystallization promoter content on h-BN growth. As-sintered h-BN pellets and exfoliated BNNSs are characterized from both chemical and microstructural viewpoints. The effect of SPS temperature on both purity and crystallinity is also studied using cathodoluminescence. Few-layered BNNSs are successfully isolated by exfoliation. Furthermore, physical measurements also confirm that SPS derived h-BN exhibits a low dielectric constant.
In the end, we propose hot isostatic pressing (HIP) technique as a further improved method for ceramization process. Characterizations from bulk crystals to BNNSs are carried out in three aspects: morphology, lattice structure and chemical composition. This novel attempt provide us transparent BN single crystals with large lateral size. Besides, BNNSs with a high purity degree have also been confirmed.