Work function modification studies for energy applications
: from surface chemical functionalisation to plasmonic tuning

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

This thesis studies the modification of the surface work function from the perspective of its potential application to the field of energy generation. In this case, improving thermionic electron emission from diamond surfaces is the focus of this thesis, as a diamond based device can be used for power generation when implemented as a thermionic energy converter. The success of diamond-based thermionic energy converters strongly relies on the quality and stability of the diamond surface, which has to provide a low work function and a negative electron affinity. These characteristics, low work function and negative electron
affinity, are achieved by terminating the diamond surface with a monolayer of hydrogen. However, the poor stability of this surface at high temperatures, at which a thermionic energy converter would operate, is the main drawback for the successful development of diamondbased devices.

Thermionic emission from diamond surfaces was studied, culminating with the proposal of a new theoretical model for thermionic emission from hydrogenated diamond surfaces. This new model, to the best of the author's knowledge, is the first to successfully reproduce the behaviour of the thermionic emission current from hydrogenated diamond surfaces, including the decrease of emission current due to the desorption of hydrogen. As a consequence, the new model allows the extraction of additional information about the surface properties of diamond, like the hydrogen activation energy of desorption Ed or the Richardson constant A. Additionally, it is now possible to relate the components of the emission current with certain surface phases or chemical states of hydrogen on the
diamond surface. This finding led to the conclusion that, for <100> single crystal diamond, most of the emission at high temperatures comes from flat terraces with a C(100)-(2×1):H reconstruction. Therefore, in order to
improve thermionic emission from diamond, surface treatments should be applied to the surface to promote the formation of this surface phase. In addition, single crystal diamond proved to be a much better emitter than polycrystalline diamond by emitting 1.5 mA/cm2 as compared to 0.3 mA/cm2 emitted by the polycrystalline diamond.
An alternative method for the modification of the surface work function was investigated in order to assess its potential application to diamond surfaces. This method consists on the use of light excited plasmo-electric potentials. This phenomenon is quite novel, so the research on this thesis is more focused on increasing our understanding of the physics of this effect that on its immediate application to diamond surfaces. Hence, plasmo-electric potentials on
silver nanoplates deposited on indium-tin-oxide were analysed. By studying the transient behaviour of plasmo-electric potentials, it was found that they are surprisingly long (τ ≈ 100 s), which is unusual for a plasmonic-based effect, that normally operates on the picosecond regime. It was concluded that this was caused by the charge transport characteristics of the nanoplate-substrate interface. This is because plasmo-electric potentials are caused by the
transfer of electrons in or out the nanoplate, which modifies its electrochemical potential and in turn its work function. Moreover, it was found that plasmo-electric potentials are present on the same nanoplate with opposite signs, which is believed to be caused by impurities deposited on the surface of the nanoplate. Lastly, in order to apply plasmo-electric potentials to improve thermionic emission more research is needed to better understand the physics of this phenomenon. However, it was found that plasmo-electric potentials could have a strong relation with plasmon-assisted catalysis, which could open a new and exciting pathway for novel research.
Date of Award26 Sept 2017
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorNeil A Fox (Supervisor) & Thomas Bligh Scott (Supervisor)

Keywords

  • thermionic
  • plasmonics
  • diamond

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