The processing of heteroepitaxial thin-film diamond for electronic applications

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Design of gas sensors using carbon nanotubes

PhD ThesisThis thesis is concerned with the structural, chemical and physical properties of carbon nanotubes (CNTs) and composites of CNTs with conductive polymers and DNA. The application of these composites in electronic sensors for volatile organic compounds VOCs, carbon monoxide and ozone gas was also investigated. CNTs are promising materials for a gas sensor because of their unique properties; small size, large specific surface area and high aspect ratio. However, the conductance of bare CNTs gives a small response to many analytes and therefore the formation of CNT composites with other polymeric materials to enhance the sensing performance was explored. The first part of the study is based on coating CNTs by polypyrrole and using these composites to detect volatile organic compounds (VOCs: methanol, ethanol, acetone and chloroform). Polypyrrole (Ppy)-coated CNTs were prepared by an in situ oxidative polymerization method with FeCl3:6H2O as the oxidant. TEM and AFM images showed a significant change in the diameter of the nanotubes upon polymerization from the mean value of 10 nm for multi walled carbon nanotubes (MWCNTs) to 60 nm after coating and from 5 nm to 50 nm for single-walled carbon nanotubes (SWCNTs). FTIR and Raman spectra indicated successful coupling between CNTs and polypyrrole. In addition, I-V characterization of two terminal nanotube devices and impedance spectroscopy demonstrated the change in the electrical properties of drop-cast CNT films after coating with polymer. The electrical current decreased after coating for both MWCNTs and SWCNTs (8 mA to 0.027 μA) and (10 mA to 88 μA) at an applied voltage of 2 V. Uncoated CNTs had a small analytical sensitivity (S), where S is ii defined as the percentage change in resistance upon exposure to analyte. For 12.9 kPa of methanol vapour, typically S < 1% for bare CNTs, while the sensitivity of the nanocomposites was typically S > 50% for 12.9 kPa of MeOH at room temperature. The sensing mechanism was found to be reversible and the temperature dependence could be analyzed using a simple extension of the Van’t Hoff equation. This suggests that the temperature dependence of the sensitivity is controlled by the enthalpy of adsorption on the composite. The second part of this study used CNTs/boron nitride nanotube (BNNTs) composites as an ozone gas sensor. Ozone is a powerful oxidant and polymer additives are not sufficiently robust for this application. CNT/BNNT films were prepared by drop-casting from equimolar solutions of BNNTs/methanol and CNTs/methanol. The electrical properties of drop-cast CNTs were changed after adding the insulating BNNTs; the electrical current decreased from 8 mA to 1 mA at applied voltage of 2 V. The sensitivity was improved from 18% to 50% for 80 ppm of ozone. However, the problem with the CNT ozone sensor was a long recovery time which can be 25 min or more, depending on the gas concentration. For CNTs/BNNTs the recovery time was shorter, but still lies between (2-17) min at room temperature. The third part of the study was related to detection of CO gas by CNTs/Ppy at room temperature. It has been shown that the sensitivity of CNTs is enhanced after polypyrrole coating: > 20% for SWCNTs/Ppy and < 2% for SWCNTs at 1923 ppm CO in air. Again, the sensitivity of these nanotube composites decreased with increased temperature according to an adsorption equilibrium model.The last part of this study evaluated DNA@CNT composites as a VOCs sensor. Three samples of CNTs/DNA were prepared with three different amounts of -DNA 2μL, 5 μL and 10 μL of (500 μg mL-1 ) which were added to 50μL (0.001 mg mL-1 ) of an aqueous dispersion of CNTs. DNA@CNT Films were drop cast across microelectrodes and from I-V measurements, it was found that the current (at a bias of 2 V) decreased after coating with increasing amounts of DNA from 8 mA (bare CNTs) to 4.5 mA to 2 mA and finally to 1 mA for the 50:10 sample. AFM and TEM images showed the DNA coats the CNTs and this suggests that tunnel junctions are introduced between CNTs which account for the drop in conductance. These junctions are also suggested to be the origin of the improved sensing response: DNA@CNT composites have good sensitivity for VOCs (MeOH, EtOH, C3H6O and CHCl3) and are more sensitive to methanol vapour than other VOCs. Further, DNA/CNTs films show a larger response to chloroform vapour at 21.08 kPa than CNTs/Ppy films at room temperature. Interestingly, the sensitivity of CNTs/DNA films increased as the temperature was raised; this suggests that another mechanism apart from adsorption/desorption is involved in their response. Although CNTs have been suggested as transducers in various gas sensors, they show a poor sensitivity (fractional change in resistance upon exposure to analyte). However by preparing composites of CNTs and less conductive materials, the analytical sensitivity can be greatly increased even though the conductivity of the composite is usually much less than of the bare CNTs.Iraqi ministry of higher educatio

Diamond nanostructured devices for chemical sensing applications

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

Deposição de filmes do diamante para dispositivos electrónicos

This PhD thesis presents details about the usage of diamond in electronics. It presents a review of the properties of diamond and the mechanisms of its growth using hot filament chemical vapour deposition (HFCVD). Presented in the thesis are the experimental details and discussions that follow from it about the optimization of the deposition technique and the growth of diamond on various electronically relevant substrates. The discussions present an analysis of the parameters typically involved in the HFCVD, particularly the pre-treatment that the substrates receive- namely, the novel nucleation procedure (NNP), as well as growth temperatures and plasma chemistry and how they affect the characteristics of the thus-grown films. Extensive morphological and spectroscopic analysis has been made in order to characterise these films.Este trabalho discute a utilização de diamante em aplicações electrónicas. É apresentada uma revisão detalhada das propriedades de diamante e dos respectivos mecanismos de crescimento utilizando deposição química a partir da fase vapor com filament quente (hot filament chemical vapour deposition - HFCVD). Os detalhes experimentais relativos à otimização desta técnica tendo em vista o crescimento de diamante em vários substratos com relevância em eletrónica são apresentados e discutidos com detalhe. A discussão inclui a análise dos parâmetros tipicamente envolvidos em HFCVD, em particular do pré-tratamento que o substrato recebe e que é conhecido na literatura como "novel nucleation procedure" (NNP), assim como das temperaturas de crescimento e da química do plasma, bem como a influência de todos estes parâmetros nas características finais dos filmes. A caracterização morfológica dos filmes envolveu técnicas de microscopia e espetroscopia.Programa Doutoral em Engenharia Eletrotécnic

Diamond Structures for Advanced Electronics

Although diamond is slowly becoming an advanced technology there is con- tradictory information and misunderstanding surrounding the fundamental electronic attributes of the material system. In particular, the properties of boron doped diamond for electronics on quantum length scales has yet to be fully understood or utilized within devices. In this thesis, new insight into the electronic band structure of boron doped diamond on nano and macro scales is found and novel planar boron doped nanowires are fabricated electronically probed and a new type of side gated diamond nanowire transistor conceived. High quality single crystal diamond with thin δ-shaped boron-doped epi- layers have been thought to offer a viable approach towards transistors that can operate at high speed, high power and high temperatures. δ-doping diamond has been conjectured to achieve high mobilities and carrier con- centrations, properties of real interest for electronic applications. Taking advantage of diamond’s thermal and electronic properties, thin films can be incorporated into realistic nanoscale devices more easily than the parent bulk system. Using angle-resolved-photoemission spectroscopy (ARPES), the electronic structure of bulk and thin films (≈ 2 nm) of boron-doped di- amond are uncovered. Surprisingly, the ARPES measurements do not reveal any significant differences for these systems, irrespective of their physical dimensionality. This suggests that it is possible to grow nearly atomic-scale structures whilst still preserving the properties of bulk diamond, facilitating the use of thin films diamond for devices which necessitate nearly atomic- scale components. Using a range of techniques such as Secondorary Ion Mass, Angle Resolved Photo-emission and Raman Spectroscopy we compare thin boron doped delta layers (BDDδl) and effectively infinite, thick bulk Boron doped di- amond. We see remarkably little electronic difference and hints of low dimensional transport in both films. Using photo-lithography and Reactive Ion Etching processes, macro scale devices are fabricated, these are charac- terized using Hall effect techniques. For the first time, lateral boron doped diamond nanowires are defined using electron beam lithography. These nanowires are then processed into a variety of novel transistor like devices, showing exciting emergent quantum properties as well as classical transistor like behaviour. In developing the techniques and methods to fabricate structures in diamond we find a variety of processes require optimisation and develop a skill base to handle small and sometimes fragile substrates and process them into devices

Electronic properties and microstructure of nanoparticulate silicon systems for diode applications

Includes bibliographical references.In printed electronics the use of semiconducting silicon nanoparticles allows more than the simple printing of conductive materials. It gives the possibility of fabricating robust and inexpensive, active components. This work presents the design, fabrication, and characterization of Schottky barrier diodes using silicon nanoparticulate composites. Within this work it could be shown, that silicon nanoparticles produced by high energy milling can be used to replace the pigment in water-based graphic inks, which on curing have unique semiconducting properties, arising from the transport of charge through a percolation network of crystalline silicon nanoparticles. In this thesis scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and mid-infrared scanning near-field optical microscopy (IR s-SNOM) were employed to investigate the micro-scale as well as the meso-scale structure of the printed particle networks and, more importantly the structure of the interface between particles. A close contact between lattice planes of different particles was observed, without the presence of a thick intervening oxide layer. Altogether, the results presented in this thesis suggest that highly doped silicon nanoparticles produced by high energy milling are suitable to be used for Schottky barrier diodes fabricated by screen printing. The saturation current of the diodes was about 0.11µA for reverse bias voltages up to 5V with an ideality factor of 10.6, and rectification ratios of approximately 10&#8308; were observed

Synthesis, characterisation and applications of diamond materials

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