Amakubo, Suguru Frederick; (2017) Carbon Electronics: Nano-carbons for the development of radiation sensors, image intensifiers and medical sensors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Carbon nano-materials, both in sp2 (graphene like) and sp3 (diamond) con- figurations are renowned for their unmatched novel properties. In particular, its extremely high electrical conductivity, radiation hardness and electron amplification are widely coveted. This investigation aims to capitalise on the above by developing blood pressure sensors, radiation detectors and signal amplifiers from the said carbon nano-materials. Namely, carbon nanotubes (sp2 carbon) were integrated into a polymer host to form a composite. Where it has been found that by altering the surface functionalisations of carbon nanotubes (non-functionalised, -OH and -COOH) the electrical resistance of the composite could vary drastically as much as 1012Ω to 107Ω. This brings potential benefits in reduced production costs, reduced environmental damage and wider technological adoption of carbon composite based devices. Carbon nanotubes were then encased in a soft and biocompatible host, polydimethylsiloxane (PDMS), in order to fabricate an in vivo blood pressure sensor, exploiting its piezo-resistivity. Results have shown a successful and adequate degree of piezo-resistivity (109Ω to 106Ω for 2D and 4kΩ to 750kΩ for 3D compression) at the desired size-scale of 200µm and 4mm respectively. This is a size equivalent to that of the diameter of blood vessels in question. However, further investigation into re-miniaturisation is recommended for future works. Diamond (sp3 carbon), on the other hand, was used as a longlasting solution to neutron detection for a Trident nuclear submarine, HMS Artful. The investigation entailed a three-phase process of: α-particle detection, LiF conversion layer addition and neutron detection. Results has shown clear signs of α-particle and neutron detection with a device efficiency of 32.3% and 48.3% respectively, as well as γ-ray transparency and sufficient Q-factor between the signal peak and detection peak. Diamond was also used as a signal amplifier that has application as an image intensifier for night-vision goggles where it was found thatby altering the surface functionalisation of nano-diamonds (H, O and LiO) one could enhance or suppress the secondary electron emission effect. Additionally, it was found that the electrical gain from the said secondary electron emission has a strong dependence on the crystal structure of the diamond layer and in turn its growth conditions. Most notably, LiO functional group was found to be more resilient towards higher temperatures (800oC) and electron bombardments but fell short in the amount of electrical gain it generated in comparison to conventional functionalisations such as H. However, the X-ray photoelectron spectroscopy (XPS) results suggest that this may be due to the lack of LiO coverage and upon further investigation, LiO may potentially bode better if not surpass the gain performance of H.

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