Ahmad, R.K.; (2011) Diamond nanostructured devices for chemical sensing applications. Doctoral thesis , UCL (University College London).

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Abstract

Research in the area of CVD single crystal diamond plates of which only recently has been made commercially available saw significant advancements during the last decade. In parallel to that, detonation nanodiamond (DND) particles also now widely made accessible for requisition are provoking a lot of scientific investigations. The remarkable properties of diamond including its extreme hardness, low coefficient of friction, chemical inertness, biocompatibility, high thermal conductivity, optical transparency and semiconducting properties make it attractive for a number of applications, among which electronic and micro electrical-mechanical systems devices for chemical and biological applications are few of the key areas. A detailed knowledge of diamond devices at the prototypical stage is therefore critical. The work carried out encapsulated in this thesis describes the employment of the nanometer-scale diamond structures for the design, fabrication and testing of electronic devices and micro electrical-mechanical system (MEMS) structures for chemical sensing applications. Two major approaches are used to achieve engineering novelty. The first type being devices based on single crystal diamond substrates, which include state of the art δ-doped single crystal diamond Ion Sensitive Field Effect Transistor with an intrinsic layer capping the delta-doped layer for pH sensing and the fabrication and characterization of a triangular-face single crystal diamond MEMS. A comprehensive set of characterisations was systematically performed on the delta ISFET devices. Cyclic Voltammetry has been used to determine the devices’ potential window determining the limits of the applied potential for the Current-Voltage measurements. In solutions of different pH levels, an improved sensitivity of 55mV/pH compared to cap-less design in a previous study is taken as the salient figure of merit. Electrochemical Impedance Spectroscopy sheds some light on device performance in terms of flatband voltages and conduction pathways through circuit modelling. Improved ISFET characteristics such as lower flat-band voltage at 3.74V, simpler conduction paths and drain current saturation onsets show the chosen design is correct and advances delta-doped diamond ISFET research and development work. For the single crystal diamond cantilever, the theoretical modelling supports the triangular-face design to be a better option, generating 3x greater deflections in relation to the conventional rectangular-face design, when operated as a static mode sensor. Based on experimental characterisation methods such as Raman and Energy Dispersive Spectroscopy, the focusedion beam only milling technique inflicts minimum damage to the beam structure. In the second approach, a novel hybrid device idea was conceived and implemented using off-the-shelf silicon ISFETs and cantilevers with a coat of nanodiamond particles on the ‘active area’ surfaces of the respective devices. These nanodiamond-coated silicon devices exhibit high sensitivity for tracing threat signatures such as explosive precursors and analogues with the former in both liquid and vapour medium, and the latter in the vapour phase. The nanodiamond-gated ISFET shows a voltage response of a commendable maximum voltage shift of ~90 mV throughout 0 to 0.1M concentration range of NO2 - and ClO3 - solutions. In the vapour phase detecting 2,4-DNT, a sensitivity of ~20mV/0.4ppm is observed. The nanodiamond-coated silicon cantilever demonstrates a performance advantage of 7.4 Hz/ppb to 1.7 Hz/ppb in a previous study. Fourier Transform Infra-red spectroscopy was carried out on the nanodiamond surfaces hosted by potassium bromide (KBr) discs to ascertain the vapour chemisorption. With the fabrication technique simplified, commercialisation of these proof-of-concept devices should be less time consuming thus enabling quicker deployment of diamond-based surface sensing technology.

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