Abstract
This thesis presents a comprehensive study of the surface electronic properties of scandium on the diamond surface, combining state-of-the-art density-functional theory (DFT) modelling and experimental techniques. The primary objective of this study is to engineer a robust diamond surface supported with a low work function (WF) and negative electron affinity (NEA) that is suitable for high-temperature electron emission applications.Using ab initio DFT calculations, the electronic and structural properties of Sc adsorption with up to 1 monolayer (ML) coverage bare, oxygenated, and nitrogenated diamond (100) surfaces were investigated. Our results showed that Sc adatoms preferentially adsorb on the O-terminated diamond surface which has the ketone structure (C=O groups). Specifically, an exceptionally large adsorption energy of –8.68 eV/atom for 0.25 ML coverage of Sc on the O-terminated diamond was calculated, being greater than the maximum value observed for all metal adsorbates within this framework. This value was also more than 1.5 times greater than that observed for the widely studied NEA surface of the hydrogenated diamond, suggesting higher thermal stability that may exceed 1000 °C experimentally. Moreover, Sc-adsorbed O-terminated, bare, and N-terminated surfaces exhibited significant NEA, with the most negative values of –3.73 eV, –3.02 eV, and –1.75 eV, respectively, for 0.25 ML coverage of Sc.
Experimental evidence for large NEA on the 0.25 ML Sc-adsorbed bare diamond (100) and (111) surfaces was reported through a systematic study utilising state-of-the-art surface-science techniques (i.e. X-ray and ultraviolet photoemission (XPS and UPS), real and reciprocal mapping by energy-filtered photoemission electron microscopy (EF-PEEM), and spot-profile analysis low-energy electron diffraction (SPA-LEED), respectively. The surface electronic states of Sc-terminated single-crystal diamond (100) and (111) surfaces were tuned controllably through a step-by-step annealing process up to 900 °C, leading to a high yield of secondary electron emissions and high thermal stability. The experimental investigation revealed low WF values of 3.22 eV and 3.52 eV for the (100) and (111) surfaces, respectively, with the corresponding NEA values of −1.45 eV and −1.13 eV which are the largest yet reported for a metal-diamond interface.
Finally, the 0.25 ML ScO-terminated nitrogen-doped diamond (NDD) microcrystalline surface was studied by evaluating their thermionic emission yield and surface stability at elevated temperatures. The highest emission current of 3.1 mA was achieved for the ScO-terminated NDD sample, exceeding five times that of the H-terminated one (0.586 mA). This was accompanied by an effective WF value of 1.14 eV for the ScO-terminated NDD surface determined through fitting to the Richardson model, which was lower than the value for the hydrogenated NDD surface (1.48 eV).
These complementary computational and experimental findings strongly suggest that Sc-terminated bare, oxidised and nitrogenated diamond surfaces, with their high NEA values, low WF values, and high thermal stability, could be the most promising candidates for thermionic electron-emission devices yet reported.
| Date of Award | 20 Jun 2023 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Bolashak International Scholarship |
| Supervisor | Paul W May (Supervisor) & Natalie Fey (Supervisor) |