- Title
- Quantum chemical investigation of the growth and chemistry of graphene
- Creator
- Mitchell, Izaac
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Graphene offers a wide range of applications ranging from optics, electronics, computing and structural applications, however, to achieve the full use of graphene it must i) be grown on a large enough scale and to a high enough quality and ii) be modified in a controlled and useful manner that allows graphene's properties to be properly tuned and exploited. To understand how to control both the growth and modification of graphene both the thermochemistry and the kinetics of the mechanisms behind each must be understood. Towards understanding the growth of graphene the effects of hydrogen, a factor often ignored in thermochemical investigations, on the makeup of initial fragments during the chemical vapour deposition on two model catalytic substrates, Cu(111) and Ni(111) is investigated. It is found that the hydrogenation and structure of the fragments during early growth is determined by a complex balance between the hydrogen chemical potential and the metal-carbon interactions, leading to different behaviours on the two substrates and explaining why experimental growth on Ni(111) exhibits lower temperature growth conditions compared to Cu(111). Further, it is found that higher hydrogen chemical potentials can aid in the nucleation of graphitic ring structures through the ability to passivate the edges of fragments. For developing and understanding the modification of graphene in a controlled way the thermochemical effects of alkali metals (K, Na and Li) adsorbed on several transition metal substrates (Ni(111), Ni(100), Cu(111) and Cu(100)) on a graphene over-layers using epoxidation as a probe for C-C bond reactivity. It is found that the trend in increased bond reactivity in proximity to the alkali metals follows the trend of alkali metal atomic radius (K > Na > Li) and the trend in substrate adsorption strength (Ni > Cu). This trend of increased reactivity (which can be as high as 115.8 kJ mol-1) is attributed to distortions in the graphitic overlayer induced by the metal substrate pulling down on, and the alkali metal pushing up on, the graphitic overlayer and not to the electropositivity of the alkali metals or electron donation into the graphene π* orbitals. While thermochemical methods for understanding these processes are well established, methods for understanding the kinetics are underdeveloped due to the timescale problem, a limitation set on conventional dynamic calculations caused by the presence of high energy transition barriers. Thus in this thesis are described two new theoretical methods for overcoming these high energy barriers, the combination global reaction route mapping-on the fly kinetic Monte Carlo-density functional tight binding (GRRM-KMC) method and the combination metadynamics-density functional tight binding method (DFTB MTD). The GRRM-KMC method utilises the GRRM code, a code for finding the equilibria and transition states of a molecular system using anharmonic downward distortion following (ADDF) and which already uses density functional tight binding. The outputs from the GRRM are used to run a conventional on-the-fly kinetic Monte Carlo simulation, in essence overcoming the timescale problem by explicitly calculation the kinetics rather than rely on rare stochastic events. The BFTB MTD method involves modifying conventional molecular dynamics as implemented in the DFTB+ density functional tight binding code with time dependent biases via an interface with the PLUMED libraries. These biases can be used to run a metadynamics simulation to investigate the free energy surface of a molecular system, thus overcoming the timescale problem by discouraging the molecular dynamics from visiting areas of the free energy surface repeatedly and forcing the configuration out of low energy minima. Both these methods are then tested against the simple model systems of malonaldehyde, finding relative consistency with prior results. Further the GRRM-KMC is utilised to investigate the simple diffusion of carbon atoms on a Fe13 nanoparticle, an important step in understanding the kinetics of growth after the formation of initial active species and the DFTB MTD is utilised to investigate the diffusion of epoxide groups on graphene, an important factor in the timescale of modification experiments.
- Subject
- graphene; density functional theory; density functional tight binding; chemistry; plumed; metadynamics; thermochemistry; kinetics
- Identifier
- http://hdl.handle.net/1959.13/1395553
- Identifier
- uon:33900
- Rights
- Copyright 2019 Izaac Mitchell
- Language
- eng
- Full Text
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