Sustainable functionalization of diamond surface with tin, lithium, and oxygen for low work function applications

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The overarching goal and ambition of this research is to realise a potential solution to the chemical instability found with known functionalised diamond surfaces. A sub-set of these form a dipole that gives rise to a negative electron affinity property that promotes enhanced electron emission at thermionic temperatures and has numerous technological applications including concentrated solar thermal power generation.The prime focus of the research presented in this thesis is the use of tin (Sn), a very nontoxic, abundantly available, less expensive heavy metal, as an alternative termination of the diamond surface. The suitability of tin was established using the density functional theory (DFT) where Sn and tin monoxide (SnO) groups used as a termination on the diamond (100) surface have resulted in a large adsorption energy of -4.4 eV in half monolayer configuration (HML) with an electron affinity of –1.43 eV and up to -6.5 eV in HML configuration with an electron affinity up to −1.37 eV, respectively. The NEA occurs as a result of dipole formation on the surface due to the shift in the electron density toward or in the vicinity of surface carbon atoms, realised through the electrostatic potential and density of states calculations.Lithium nitride solution was deposited on the diamond surface with subsequent annealing at higher temperatures to gain an insight into the mobility of the Li atoms on the diamond. Li atoms were seen to move from the surface into the near surface bulk and then back to the surface through the temperatures of 650°C – 750°C – 850°C, at which point the LiO was completely desorbed from the surface. This was seen to affect the bulk properties of diamond.An air stable sub monolayer of SnO nanoclusters was formed “reliably” on the surface of diamond (100) using physical vapor deposition. SnO was seen to impart an NEA of -0.02 eV to the diamond surface along with a reduction in the WF by 1.8 eV. Inclusion of Li atoms into this structure resulted in Li atoms taking up oxygen from SnO and forming SnOx-LiO2 heterostructure with an NEA of – 0.42 eV and WF reduction by 2.3 eV. This interaction has also resulted in the increased stability of the Li on diamond surface. Only about 15% of Li was lost in the case of Li2O-SnOx termination on diamond (100) compared to 47% lost in case of LiO terminated diamond at similar elevated temperatures.
Date of Award21 Mar 2023
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorNeil A Fox (Supervisor) & David J Fermin (Supervisor)

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