Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Khan, Majharul Haque, Synthesis and application of few-layered hexagonal boron nitrade, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2016. http://ro.uow.edu.au/theses/4813
Hexagonal boron nitride nanosheets (BNNS) is one of the most widely studied twodimensional (2D) materials these days, due to their extraordinary properties and potential applications. For example, BNNS has been proposed as an ideal substrate for graphene based electronics because of its ultra-flatness and structural similarity to graphene. The synthesis of large-area, homogeneous, and few-layered BNNS, however, remains challenging. Among the various synthetic routes, atmospheric pressure – chemical vapour deposition (AP-CVD) is preferred on the grounds of its cost effectiveness and its ability to yield large-area BNNS from a single run. Maintaining the homogeneity and crystallinity of the nanosheets over a large surface area requires fine-tuning of variables in the AP-CVD procedure to their ideal levels.
Copper has been widely used as a catalyst for the growth of BNNS. A comparative study of BNNS growth on solid and melted copper confirms the advantages of melted copper. On the solid copper, the BNNS is largely inhomogeneous and tends to adopt a multilayered tetrahedral shape along the defects, while on the melted copper surface it is single crystalline over several microns, with the majority of nanosheets being monoor bi-layer thick. This difference is likely caused by the significantly reduced and uniformly distributed nucleation sites on the smooth melted surface, in contrast to the large amounts of unevenly distributed nucleation sites that are associated with grain boundaries and other defects on the solid surface.
Pretreatment to remove the residual hydrocarbon from ammonia borane, the most commonly used precursor, is necessary to remove the carbon contamination found during the growth. In addition, flattening the copper and tungsten (chosen due to good wettability of the liquid copper) substrates prior to growth and slow cooling around the copper melting point facilitate the uniform growth of BNNS. This is the result of a minimal temperature gradient across the copper substrate. Confining the growth inside alumina boats effectively minimizes etching of the nanosheets by silica nanoparticles originating from the commonly used quartz tube. The CVD grown h- BNNS on solid Cu surfaces adopts AB, ABA, AC´, and AC´B stacking orders, which are known to have higher energies than the most stable AA´ configuration. Electron ii energy loss spectroscopy (EELS) was found to be effective in determining the number of layers by means of the energy loss near edge structure (ELNES) of the N edge and by core loss edge quantification. These findings are instrumental for the fabrication of high-quality BNNS via CVD and would motivate studies on the stacking order dependent properties and performance of BNNS.
Compact nanoscale devices require ultra-thin coatings for protection. Hexagonal boron nitride has high thermal and chemical stability, is electrically insulating, and the hexagons are considered to be impermeable to water and oxygen. The reactivity of Cu in different environments with the presence of few-layered BNNS was therefore examined. Accelerated reaction of Cu directly below BNNS towards water was observed compared with oxygen, which was unexpected considering the chemical stability and insulating properties of BNNS. Density functional theory (DFT) calculations demonstrate that water dissociates spontaneously between the single BNNS layer and the Cu(111) support promoting the formation of Cu oxide underneath the BNNS. X-ray photoelectron spectroscopy (XPS) and EELS analysis studies reveal that there is no change in the chemical states of B and N atoms, indicating that h-BN catalyzes the reaction between Cu and water. Once fully covered by BNNS, Cu experiences a dramatically slowed reaction under salt water compared with bare Cu, which is likely due to the impermeability of the hexagons with respect to the ions. The efficacy of protection is closely related to the quality of the BNNS, where defects such as domain boundaries and wrinkles are detrimental. Future improvement in protection by BNNS lies in the minimization of defects or their patching up with protective atomic filler.