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Low Temperature Thermionic Emission from Modified Diamond Surfaces

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Low Temperature Thermionic Emission from Modified Diamond Surfaces This electronic thesis or dissertation has been downloaded from Explore Bristol Research, http://research-information.bristol.ac.uk Author: Fogarty, Fabian Title: Renewable Energy – Low Temperature Thermionic Emission from Modified Diamond Surfaces General rights Access to the thesis is subject to the Creative Commons Attribution - NonCommercial-No Derivatives 4.0 International Public License. A copy of this may be found at https://creativecommons.org/licenses/by-nc-nd/4.0/legalcode This license sets out your rights and the restrictions that apply to your access to the thesis so it is important you read this before proceeding. Take down policy Some pages of this thesis may have been removed for copyright restrictions prior to having it been deposited in Explore Bristol Research. However, if you have discovered material within the thesis that you consider to be unlawful e.g. breaches of copyright (either yours or that of a third party) or any other law, including but not limited to those relating to patent, trademark, confidentiality, data protection, obscenity, defamation, libel, then please contact [email protected] and include the following information in your message: •Your contact details •Bibliographic details for the item, including a URL •An outline nature of the complaint Your claim will be investigated and, where appropriate, the item in question will be removed from public view as soon as possible. Renewable Energy – Low Temperature Thermionic Emission from Modified Diamond Surfaces Fabian Fogarty A dissertation submitted to the University of Bristol in accordance with the requirements for the award of the degree of Doctor of Philosophy in the Faculty of Science, School of Chemistry, February 2020. Word count ∼32k exc. prelim pages, references, appendices, tables and figures Fabian Fogarty University of Bristol Abstract Thermionic energy conversion holds great promise for effective and clean energy production. Diamond has a unique set of properties that distinguish it as an excellent potential thermionic emitter, a crucial component of thermionic energy converters. In particular, the hydrogen-terminated diamond surface exhibits a negative electron affinity, which allows for efficient emission of electrons. The hydrogen termination is not stable at elevated temperatures, which necessitates investigating alternative candidates such as titanium. This work is primarily concerned with the investigation of the electronic properties and surface stability of titanium containing diamond surface terminations to assess their potential as a thermionic emitter. Ab initio computational modelling of various titanium surface terminations showed that titanium nitride, titanium oxide, and titanium carbide all have the potential to exhibit negative electron affinities. The titanium nitride surface termination showed similar stability as the hydrogen termination, while the titanium oxide surfaces exhibited better stability. The titanium carbide terminations proved the most stable, with promising electronic properties. Titanium-oxide- and titanium-nitride-terminated diamond surfaces were prepared using techniques such as acid and plasma treatment, chemical vapour deposition, and e-beam evaporation. The fabricated surfaces were annealed at temperatures of up to 1000 °C and analysed primarily using X-ray and ultraviolet photoelectron spectroscopy. For most of the titanium surfaces, the oxygen or nitrogen desorbed by 1000 °C while the titanium did not, resulting in titanium-carbide-terminated diamond. Though these titanium carbide surfaces typically exhibited moderately higher work functions than the equivalent titanium oxide or nitride terminations, they still exhibited negative electron affinities. Therefore, titanium-carbide-terminated diamond surfaces were found to be stable at elevated temperatures while also exhibiting favourable electronic properties. Page I Fabian Fogarty University of Bristol Acknowledgments Firstly, I would like to thank Professor Paul May and Dr Neil Fox for the opportunity and resources that enabled me to conduct this research in the first place, as well as their support and guidance throughout this endeavour. I particularly appreciate Dr Mattia Cattelan and Dr James Smith for their patience and invaluable guidance in the diamond lab and the NanoESCA lab. I would also like to thank the whole diamond research group for their companionship and insights over the last three years. Max and Gary deserve a special mention for their very useful discussions on the finer points of English grammar during the writing of this thesis. Lastly, and most importantly, I am most grateful for the unconditional love and support of my wife, Martine, without whom I would have surely been lost to insanity some time ago. Page II Fabian Fogarty University of Bristol I declare that the work in this dissertation was carried out in accordance with the requirements of the University's Regulations and Code of Practice for Research Degree Programmes and that it has not been submitted for any other academic award. Except where indicated by specific reference in the text, the work is the candidate's own work. Work done in collaboration with, or with the assistance of, others, is indicated as such. Any views expressed in the dissertation are those of the author. SIGNED: ....................................................................... DATE: ............................................. Page III Fabian Fogarty University of Bristol Table of Contents Table of Contents ............................................................................. IV List of Figures and Tables ................................................................. IX List of Abbreviations ................................................................... XVIII Outline ........................................................................................... XIX 1 Introduction ................................................................................. 1 1.1 Climate Change ........................................................................................... 2 1.2 Thermionic Energy Conversion .............................................................. 3 History .............................................................................................................. 4 Background ...................................................................................................... 5 Challenges ........................................................................................................ 6 1.3 Diamond .................................................................................................. 8 Why Diamond for TEC devices? ..................................................................... 9 1.4 Doping .................................................................................................... 11 Common Dopants ............................................................................................ 11 Effect of Doping on Thermionic Emission .................................................... 12 1.5 Surface Terminations ............................................................................. 15 Hydrogen ......................................................................................................... 15 Oxygen ............................................................................................................ 16 Nitrogen .......................................................................................................... 16 Other Single-Element Terminations ............................................................. 17 Group I and II Metal Oxide Terminations .................................................... 18 Transition Metal Oxides ................................................................................. 19 Termination Summary ................................................................................... 19 Page IV Fabian Fogarty University of Bristol 2 Computational Modelling .......................................................... 20 2.1 Theoretical Background ......................................................................... 21 Born-Oppenheimer Approximation ............................................................. 22 Hartree-Fock Approximation ........................................................................ 22 Density Functional Theory (DFT)................................................................. 23 2.1.3.1 The Kohn-Sham Approach .................................................................... 24 2.1.3.2 Correlation Interaction .......................................................................... 24 2.1.3.3 Exchange Interaction ............................................................................. 24 Exchange-Correlation Energy (XC) Functionals .......................................... 25 2.1.4.1 Local and Semi-local Approximations .................................................. 25 2.1.4.2 Hybrid Functionals ................................................................................ 26 Basis Sets ........................................................................................................ 28 2.2 Experimental Details ............................................................................ 29 Geometry Optimization ................................................................................ 29 Adsorption Energy ........................................................................................
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