Assembly mechanisms of CVD graphene investigated by scanning tunnelling microscopy

Abstract

In this thesis, the growth mechanism of graphene on a transition metal support is determined, and the epitaxial relationship investigated. The main technique used is low- temperature scanning tunnelling microscopy (STM), which is introduced in Chapter 2.

Epitaxial graphene synthesised on copper (foil and (110) single crystal), from the dehydrogenation of ethene, is investigated by STM and low energy electron diffraction (LEED) in Chapter 4. Despite the weak epitaxial relationship that exists, LEED uncovers two preferred orientations of the graphene over the copper. Further investigation reveals restructuring of the copper foil from a predominantly (100) orientation to (n10) facets. Structural feedback is found to exist, with the graphene growth inducing and stabilising faceting of the copper surface, and the facets in-turn playing an important role in the graphene growth mechanism. The preferred orientations, which are also seen on the single crystal, are most likely determined during nucleation and early stage growth, where it is expected that the interaction is stronger.

The growth mechanism for the formation of graphene from ethene is studied on a Rh(111) surface in Chapter 5. This is found to consist of two regimes, with the first revolving around the transformation from aliphatic hydrocarbons to aromatic intermediates. This occurs through the decomposition and condensation of ethene, resulting in the formation of one-dimensional polyaromatic hydrocarbons (1D-PAHs). The second regime is characterised by the transition from these 1D-PAHs, to the 2D graphene. The previously produced 1D-PAHs, decompose to form size-selective carbon clusters, with these clusters being the precursors to graphene condensation.

In Chapter 6, the conclusions of Chapter 5 are built upon through investigation into the effect of different hydrocarbon feedstocks on the graphene growth pathway. Benzene, tetracene, and perylene are the feedstocks examined. In all cases 1D-PAHs are formed, which decompose to clusters that subsequently condense to form graphene.