Synthesis and atomic resolution AC-TEM characterisation of graphene edges

Thesis

My primary goal was to perform an in depth characterisation of the atomic structure of graphene edges, which was fulfilled through the use of an aberration corrected transmission electron microscope (AC-TEM) – OJ2200 MCO that was enhanced with a double Wien slit monochromator to improve its spatial resolution to ∼ 80pm. The chemical vapour deposition grown graphene was characterised by various techniques, including the use of Scanning Electron Microscopy (SEM) and Raman spectroscopy to respectively determine the homogeneity and number of layers, and selected area electron diffraction (SAED) to distinguish the size of single crystal region. With this set up, I obtained images of graphene edges at atomic resolution that allowed bond length determination. Based on the acquired information and the application of the density functional theory, it was possible to correlate structural information with the bonding nature of graphene edges; allowing me to further determine the hydrogenation states of graphene with a single AC-TEM frame. Imaging with electron beam monochromation also allowed me to discover the theoretically predicted extended Klein edge structure, which is included as the fourth inherent periodic structure of graphene edges.

With the aid of an in situ heating holder, the temperature dependence of graphene edges were investigated over 350 frames: zig-zag edges are found to dominate at ≤ 400°C; and at temperatures above 600°C, the proportion of armchair and reconstructed 5-7 edges increase dramatically. This is because low temperatures allow contamination, which result in edges becoming etched at a more rapid rate that favours the formation of intrinsic zig-zag edges. The predominance of armchair and reconstructed zig-zag edges at high temperatures could be attributed to the evaporation of surface adsorbates, which resulted from higher thermodynamic stability.

Finally, the in situ growth of a second layer graphene was investigated. Extra layers of graphene was found to heterogeneously nucleate around gold nanoparticles and continuous to grow at 600°C. This study shed light on the growth mechanism of CVD. By bridging the dangling bonds from the edges of two layers of graphene, this structure was extended to form a closed edge graphene nanopore with diameter ranging from 1.4 – 7.4 nm, The closed edge nanopores in bilayer graphene are robust to back-filling even after exposure to atmospheric conditions for days - this opens new possibilities of nanopore fabrication routes for applications such as graphene sensors and DNA sequencing technology.

Authors: He, K

• DOI: 10.5287/ora-jnbk6ejxr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: in-situ heating; Graphene; Atomic Resolution; HRTEM; edges
• Subjects: Nanomaterials; Materials Science