Boron Nitride Nanosheets: Tackling the Intertness and Evolution of Moire Patterns in Van Der Waals Superstructures
Chen, Zhongfang (Consejero)
In this dissertation, I present a comprehensive study on boron nitride nanomaterials including computational study on experimental study on sonication treatment of boron nitride nanotubes in aqueous ammonia solution and oxidative etching boron nitride nanosheets using silver nanoparticle. Boron nitride nanotubes (BNNTs), the one-dimensional member of the boron nitride nanostructure family, are generally accepted to be highly inert to oxidative treatments and can only be covalently modified by highly reactive species. Conversely, we discovered that the BNNTs can be chemically dispersed and their morphology modified by a relatively mild method – simply sonicating the nanotubes in aqueous ammonia solution. The dispersed nanotubes were significantly corroded, with end-caps removed, tips sharpened, and walls thinned. The sonication treatment in aqueous ammonia solution also removed amorphous BN impurities and shortened BNNTs, resembling various oxidative treatments of carbon nanotubes. Importantly, the majority of BNNTs were at least partially longitudinally cut, or “unzipped”. Entangled and freestanding BN nanoribbons (BNNRs), resulting from the unzipping, were found to be ~ 5 – 20 nm in width and up to a few hundred nm in length. This is the first chemical method to obtain BNNRs from BNNT unzipping. This method was not derived from known carbon nanotube unzipping strategies but is unique to BNNTs because the use of aqueous ammonia solutions specifically targets the B-N bond network. Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. Therefore, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. We presented a facile, controllable, and scalable method to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. Two-dimensional (2D) van der Waals (vdW) superstructures, or vdW solids, are formed by the precise restacking of 2D nanosheet lattices, which can lead to unique physical and electronic properties that are not available in the parent nanosheets. Moiré patterns formed by the crystalline mismatch between adjacent nanosheets are the most direct features for vdW superstructures under microscopic imaging. In this chapter, we report the transmission electron microscopy (TEM) observation of hexagonal Moiré patterns with unusually large micrometer-sized lateral area (up to ~1 μm2) and periodicities (up to ~50 nm) from restacking of liquid exfoliated hexagonal boron nitride nanosheets (BNNSs). This observation was attributed to the long range crystallinity and the contaminant-free surfaces of these chemically inert nanosheets. Parallel-line-like Moiré fringes with similarly large periodicities were also observed. Our simulations and experiments unambiguously revealed that the hexagonal patterns and the parallel fringes were originated from the same rotationally mismatched vdW stacking of BNNSs and can be inter-converted by simply tilting the TEM specimen following designated directions.