Renewable Energy – Low Temperature Thermionic Emission from Modified Diamond Surfaces

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.

Date of Award	29 Sept 2020
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Paul W May (Supervisor) & Neil A Fox (Supervisor)