This thesis presents the investigation of potential shallow n-type donors that are candidates to be used as therm ionic emitters for converting solar energy to electrical energy. Due to the various problems associated with current n-type dopants in diamond, the work has examined the use of Li-N codoping as a possible alternative doping strategy in chemical vapour deposition (CVD) of polycrystalline diamond films. Lithium nitride (Li3N) suspension in 1 % w Iv polyoxy in chloroform and N2 or NH3 gas were used as Li and N precursors, respectively, in preparing Li-N co-doped diamond films using a hot-filament CVD system. Using this system, high dopant concentrations of Li (-5x1019 cm-3 ) and N (-3x1020 cm- 3) atoms were successfully incorporated into the diamond films. The addition of Li atoms into N-doped diamond films improved the resistance of the diamond film from >200 MO to <50 MO. A resistance as low as 85 kO was recorded for Li-N co-doped diamond grown on a SCD type Ib substrate, however, these values and the overall electrical characteristics of the films were not sufficient for electronic devices. Ab initio calculations predicted that a LiN cluster with a Li: N ratio of 1:4 should produce shallow donor characteristics with an energy level -0.1 eV below the diamond conduction band minimum. The model only favours the formation of Li in substitutional sites rather than in interstitial sites due to its lower formation energy (4.88 eV and >10 eV, respectively). This model also suggests that N atoms might act as traps to pin down Liatoms and reduced their mobility in diamond lattice. The thermionic emission characteristics of Li-N co-doped diamond films were tested in Arizona State University, Arizona, USA. The films exhibited 121 ~A cm -2 current density at 900 K with a threshold temperature at 800 K. The effective work function of the emitters varied from 2.87 eV to 3.62 eV. The presence of a negative electron affinity (NEA) surface is found to be crucial for increasing the electron emission from diamond. H terminated diamond exhibits NEA and is usually prepared by exposing the diamond films in hydrogen plasma. However, desorption of H atoms between 925-1050K limits the application for these films for thermionic-based solar panels at higher temperature. Thus, various metaloxygen- terminated diamond surfaces were examined to determine their thermionic electron yield and stability at high temperatures. Cr-"O, Al-O and Ti-O terminated diamond surfaces exhibit NEA characteristics and were thermally stable at 650°C. However, only Cr-O terminated diamond showed true NEA characteristics while Al-O and Ti-O terminated diamond showed effective NEA.

This thesis presented a detailed research work on diamond materials. Chapter 1 is an overall introduction of the thesis. In the Chapter 2, the literature review on the physical, chemical, optical, mechanical, as well as other properties of diamond materials are summarised. Followed by this chapter, several advanced diamond growth and characterisation techniques used in experimental work are also introduced. Then, the successful installation and applications of chemical vapour deposition system was demonstrated in Chapter 4. Diamond growth on a variety of different substrates has been investigated such as on silicon, diamond-like carbon or silica fibres. In Chapter 5, the single crystalline diamond substrate was used as the substrate to perform femtosecond laser inscription. The results proved the potentially feasibility of this technique, which could be utilised in fabricating future biochemistry microfluidic channels on diamond substrates. In Chapter 6, the hydrogen-terminated nanodiamond powder was studied using impedance spectroscopy. Its intrinsic electrical properties and its thermal stability were presented and analysed in details. As the first PhD student within Nanoscience Research Group at Aston, my initial research work was focused on the installation and testing of the microwave plasma enhanced chemical vapour deposition system (MPECVD), which will be beneficial to all the future researchers in the group. The fundamental of the on MPECVD system will be introduced in details. After optimisation of the growth parameters, the uniform diamond deposition has been achieved with a good surface coverage and uniformity. Furthermore, one of the most significant contributions of this work is the successful pattern inscription on diamond substrates by femtosecond laser system. Previous research of femtosecond laser inscription on diamond was simple lines or dots, with little characterisation techniques were used. In my research work, the femtosecond laser has been successfully used to inscribe patterns on diamond substrate and fully characterisation techniques, e.g. by SEM, Raman, XPS, as well as AFM, have been carried out. After the femtosecond laser inscription, the depth of microfluidic channels on diamond film has been found to be 300~400 nm, with a graphitic layer thickness of 165~190 nm. Another important outcome of this work is the first time to characterise the electrical properties of hydrogenterminated nanodiamond with impedance spectroscopy. Based on the experimental evaluation and mathematic fitting, the resistance of hydrogen-terminated nanodiamond reduced to 0.25 MO, which were four orders of magnitude lower than untreated nanodiamond. Meanwhile, a theoretical equivalent circuit has been proposed to fit the results. Furthermore, the hydrogenterminated nanodiamond samples were annealed at different temperature to study its thermal stability. The XPS and FTIR results indicate that hydrogen-terminated nanodiamond will start to oxidize over 100ºC and the C-H bonds can survive up to 400ºC. This research work reports the fundamental electrical properties of hydrogen-terminated nanodiamond, which can be used in future applications in physical or chemical area.

Diamonds are formed at high pressure, temperature and low oxygen fugacity conditions in the Earth's deep mantle. However, during their residence in the mantle and transport to the surface by kimberlite magma, they may be exposed to deformation or oxidising conditions outside of their stability field, causing diamond corrosion and resorption processes. The infiltration of silicate-rich melts and hydrothermal fluids are key processes that cause this corrosion (oxidation). Understanding the processes involved in diamond transport and preservation is important for diamond exploration and to assess the potential diamond grade for economic geology of kimberlite deposits. However, a thorough understanding of variations in diamond grade and survival or preservation conditions in the Lunda province (LP) has not yet been developed. Here a diamond preservation index model has been developed. Chemical analyses and petrography both support the view that the observed variations in diamond grade or abundance within Lunda Province (NE Angola) are associated with oxygen fugacity (fO2), oxidation state, viscosity of kimberlite magma, speed of kimberlite emplacement. Variations in temperature and pressure can impose conditions that are sufficiently oxidising for diamond resorption or to form CO2. Several kimberlite pipes were sampled and it is known that they transported different amounts of diamond to the Earth's surface. In this study, both mantle xenoliths and kimberlite rocks from five kimberlite pipes (Catoca, Camatxia, Camagico, Caixepa and Camutue) from the Lucapa graben (NE Angola) were investigated, with the highest diamond concentrations occurring in LP. Alteration characteristics of kimberlite indicator minerals (KIMs) from the sampled deposits were investigated by optical microscopy, SEM, electron probe microanalysis (EMPA), XRF, XRD and 57Fe Mössbauer spectroscopy. The results of petrography and geochemical studies of peridotite, eclogite and kimberlite rocks during this project have revealed that the sampled kimberlite diamond deposits have experienced several alteration processes and the analysed diamond indicator minerals (olivine, garnet, spinel, ilmenite and pyroxene) reflect these alteration / redox reactions. The hydrothermal-metasomatic processes are associated with factors that controlled kimberlite emplacement. Mössbauer spectroscopy and EMPA results reveal that in the diamondiferous pipes of Camatxia, Caixepa and Camutue, iron redox ratios in ilmenites are considerably lower, indicating lower ƒO2 and consequently better diamond preservation conditions than for Catoca diamonds. The observed low diamond quality from Catoca pipe and the higher measured Fe3+/ΣFe ratio of Catoca indicator minerals agrees with a model which supports the use of ilmenite redox ratios to measure diamond and estimate the resorption on diamond. Results from Mössbauer and EMPA suggest that even with higher Fe3+/ΣFe Fe3+, and a lack of the original Mg-rich ilmenite and complex ilmenite formation, the presence or mineralization of diamond within Lucapa graben kimberlites cannot be ruled out.

This thesis reports research on point defects in synthetic single crystal diamond. The principal technique used for investigation is electron paramagnetic resonance, EPR. Nuclear magnetic resonance, NMR, and optical absorption spectroscopy have also been employed. Uniaxial stress has been used to investigate the properties and migration of defects under an applied perturbation. The use of field modulated rapid passage EPR, RP-ERP(FM), for quantitative measurements of neutral single substitutional nitrogen, N⁰_s, in diamond has been investigated. Optimisation of field sweep rate and experiment temperature have been shown to provide a factor of 5.6 improvent in signal to noise; a x25 speed up on previous conditions. The repeatability of RP-EPR(FM) has been investigated by repeated measurements of the same sample (containing N⁰_s). These results indicate a random error of ±2.5%; a 50% reduction compared with slow passage EPR, SP-EPR. Careful cavity characterisation has been utilised to enable reliable quantitative measurements of diamond samples that are large compared to an EPR cavity.

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