University of Wollongong Research Online University of Wollongong Thesis Collection 1954-2016 University of Wollongong Thesis Collections 2016 Synthesis and application of few-layered hexagonal boron nitrade Majharul Haque Khan University of Wollongong Follow this and additional works at: https://ro.uow.edu.au/theses University of Wollongong Copyright Warning You may print or download ONE copy of this document for the purpose of your own research or study. The University does not authorise you to copy, communicate or otherwise make available electronically to any other person any copyright material contained on this site. You are reminded of the following: This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part of this work may be reproduced by any process, nor may any other exclusive right be exercised, without the permission of the author. Copyright owners are entitled to take legal action against persons who infringe their copyright. 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For further information contact the UOW Library: [email protected] Institute for Superconducting and Electronic Materials SYNTHESIS AND APPLICATION OF FEW- LAYERED HEXAGONAL BORON NITRIDE MAJHARUL HAQUE KHAN Presented as part of the requirements for the award of the Degree of ‘Doctor of Philosophy’ of the University of Wollongong July, 2016 ABSTRACT Hexagonal boron nitride nanosheets (BNNS) is one of the most widely studied two- dimensional (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 mono- or 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 i 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. ii ACKNOWLEDGEMENTS All praise and gratitude to Allah, the Almighty, for giving me the strength and patience to complete this thesis work. During my PhD work, I received encouragement and assistance from a wide range of people. I wish to express my deep appreciation to my supervisors, Prof. Hua Kun Liu and Dr. Zhenguo Huang, for their encouragement, understanding, invaluable advice, and constant support during my study at the University of Wollongong. Gratitude is owed to Dr. Gilberto Casillas, who helped to perform the aberration- corrected TEM studies. His expertise and knowledge prove invaluable to my research. I am also indebted to Dr. Sina S. Jamali for his help with the corrosion analysis. I am thankful for technical assistance from Dr. Tony Romeo (SEM and Sputter Coater), Dr. Mitchell Nancarrow (Leica DM 6000 Optical Microscope and Struers Durascan-70 Hardness), Dr. Patricia Hayes (Raman and UV-vis Spectrophotometer), Dr. Germanas Peleckis (XRD), Dr. Michael Higgins, Dr. Yi Du (AFM), Dr. David R. G. Mitchell (EELS analysis), and Dr. Dongqi Shi (XPS). I am also grateful to Mr. Robert Morgan and Mr. John Wilton for their unfailing assistance to solve the mechanical and electrical problems of the CVD furnace that I encountered several times during my doctoral study. My special thanks go to Dr. Tania Silver for editing this thesis as well as publications. I am thankful for my International Postgraduate Tuition Award and University Postgraduate Award (3.5 years) from the University of Wollongong, Australia and Dr. Zhenguo Huang’s 6-months of financial support towards finishing my Ph.D. This thesis could not have been completed without the contribution from my parents and my brothers and sister. They continuously encouraged me to obtain this higher degree. Their unfailing support and advice helped me to accomplish this study. iii PUBLICATIONS 1. Majharul Haque Khan, Zhenguo Huang, Feng Xiao, Gilberto Casillas, Zhixin Chen, Paul J. Molino, & Hua Kun Liu, “Synthesis of Large and Few Atomic Layers of Hexagonal Boron Nitride on Melted Copper”, Scientific Reports, vol. 5, pp 1- 8, 2015 (doi:10.1038/srep07743). – (one of the top 100 read Scientific Reports articles of 2015) 2. Majharul Haque Khan, Gilberto Casillas, David R.G. Mitchell, Hua Kun Liu, Lei Jiang, & Zhenguo Huang, “Carbon- and Crack-free Growth of Hexagonal Boron Nitride Nanosheets and Their Uncommon Stacking Order”, Nanoscale, 8, 15926-15923, 2016 (doi: 10.1039/C6NR04734C). 3. Majharul Haque Khan, Sina S. Jamali, Andrey Lyalin, Gilberto Casillas, Paul J. Molino, Lei Jiang, Hua Kun Liu, Tetsuya Taketsugu, & Zhenguo Huang, “Atomically thin hexagonal boron nitride nanofilm for Cu protection: the importance of film perfection”, Advanced Materials, Accepted, 2016. 4. Feng Xiao, Sina Naficy, Gilberto Casillas, Majharul Haque Khan, Tomas Katkus, Lei Jiang, Hua Kun Liu, Huijun Li, and Zhenguo Huang, “Edge- Hydroxylated Boron Nitride Nanosheets as an Effective Additive to Improve the Thermal Response of Hydrogels”, Advanced Materials, 27, 7247-7247, 2015 (doi: 10.1002/adma.201502803). iv ABBREVIATIONS 1D One-Dimensional 2D Two-Dimensional AC-SECM Alternating Current - Scanning Electrochemical Microscopy