Molecular microscopy on graphene

Abstract

The autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of nanostructures with exotic and desirable properties, synthesised from the bottom-up, on an industrial scale from metallic, semiconducting and molecular materials. The work of this thesis aims to address questions concerning molecular self-assembly on graphene. Firstly, techniques for fabricating graphene membranes for electron microscopy (EM) are outlined. The complete fabrication process is described, beginning with the growth of CVD graphene, followed by the transfer of graphene from chemical vapour deposition (CVD) foils to a transmission electron microscope (TEM) support, and finishing with the cleaning steps involved to produce pristene regions of graphene. Strategies to chemically functionalise graphene through covalent and non-covalent means are detailed, as well as methods to fabricate more specialised graphene TEM membranes consisting of stacked and sandwiched graphene layers. With the methods used to fabricate and modify graphene EM membranes described, attention is next focused on specific microscopy techniques developed in order to study organic materials that readily damage when exposed to the electron beam in an electron microscope. Strategies to mitigate the damage arising due to beam exposure are investigated for a range of different organic molecules, and the effects of using a range of detection devices are also studied. Next, the growth of two very similar overlayer systems on graphene are studied. Trimesic acid (TMA) and terephthalic acid (TPA) thin films are grown on both freestanding and CVD graphene substrates for a range of thicknesses, and the resulting structures are probed using a range of microscopy techniques. For TMA, van der Waals epitaxy results in two preferred orientations of the assembly structure that grows in a layer-by-layer Frank-van der Merwe fashion, up to a height of ≈ 20 nm. In stark contrast, TPA assembles into a 2D monolayer before rapidly transitioning to its bulk-like structure as further layers are deposited, following a layer-plus-island, or Stranski-Krastanov, growth mode. Continuing the investigations into the structure of self-assembling molecular films on graphene, a pair of porphyrin-based molecules of the octaethyl porphyrin (OEP) class are studied. A monolayer film of OEP molecules is deposited either side of a freestanding graphene membrane, and the resulting assembly structure is driven by a remote interaction across the graphene between the two OEP films. The remote interaction is shown to diminish on the length scale of two graphene layers. Finally, the structure and motion of individual metal nanoclusters (M-NCs) deposited on freestanding graphene is studied using high-resolution TEM. Computational routines involving cross-correlation techniques are developed in order to better study the dynamic behaviour of M-NCs in atomically-resolved time-series image sequences. The strategies developed provide a means for accurately studying more complex systems, structural changes, and chemical reactions at atomic-resolution and in real-time