Wireless Sensor Networks are ad-hoc networks that consist of sensor nodes that typically use low-power radios to connect to the Internet. The channels used by the low-power radio often suffer from interference from the other devices sharing the same frequency. By using multichannel communication in wireless networks, the effects of interference can be mitigated to enable the network to operate reliably. This thesis investigates an energy efficient multichannel protocol in Wireless Sensor Networks. It presents a new decentralised multichannel tree-building protocol with a centralised controller for ad-hoc sensor networks. The proposed protocol alleviates the effect of interference, which results in improved network efficiency, stability, and link reliability. The protocol detects the channels that suffer interference in real-time and switches the sensor nodes from those channels. It takes into account all available channels and aims to use the spectrum efficiently by transmitting on several channels. In addition to the use of multiple channels, the protocol reconstructs the topology based on the sensor nodes’ residual energy, which can prolong the network lifetime. The sensor nodes’ energy consumption is reduced because of the multichannel protocol. By using the lifetime energy spanning tree algorithm proposed in this thesis, energy consumption can be further improved by balancing the energy load in the network. This solution enables sensor nodes with less residual energy to remain functional in the network. The benefits of the proposed protocol are described in an extensive performance evaluation of different scenarios in this thesis.

Autonomous Underwater Vehicle (AUV) technology has reached a sufficient maturity level to be considered a suitable alternative to conventional Mine Countermeasures (MCM). Advantages of using a network of AUVs include time and cost efficiency, no personnel in the minefield, and better data collection. A major limitation for underwater robotic networks is the poor communication channel. Currently, acoustics provides the only means to send messages beyond a few metres in shallow water, however the bandwidth and data rate are low, and there are disturbances, such as multipath and variable channel delays, making the communication non-reliable. The solution this thesis proposes using a network of AUVs for MCM is the Synchronous Rendezvous (SR) method --- dynamically scheduling meeting points during the mission so the vehicles can share data and adapt their future actions according to the newly acquired information. Bringing the vehicles together provides a robust way of exchanging messages, as well as means for regular system monitoring by an operator. The gains and losses of the SR approach are evaluated against a benchmark scenario of vehicles having their tasks fixed. The numerical simulation results show the advantage of the SR method in handling emerging workload by adaptively retasking vehicles. The SR method is then further extended into a non-myopic setting, where the vehicles can make a decision taking into account how the future goals will change, given the available resource and estimation of expected workload. Simulation results show that the SR setting provides a way to tackle the high computational complexity load, common for non-myopic solutions. Validation of the SR method is based on trial data and experiments performed using a robotics framework, MOOS-IvP. This thesis develops and evaluates the SR method, a mission planning approach for underwater robotic cooperation in communication and resource constraint environment.

Nanodiamonds (ND) have been the subject of intense research in recent years, for they have unique physical properties normally associated with diamond, in addition to their rich surface chemistry and bio-compatibility. In this thesis, the electronic properties of intentionally boron-doped nanodiamond materials are studied. In chapter 5, the possibility of substitutional doping of NDs is investigated. The properties of boron-doped, detonation nanodiamonds (B-DND) are studied using electrical impedance measurements and spectral analysis, and are compared to un-doped detonation-NDs (DND). Activation energies from variable-temperature impedance spectroscopy are found to be lower in comparison to intrinsic NDs. Chapter 6 discusses the nucleation of high-pressure, high-temperature (HPHT) boron-NDs, as well as B-DNDs on silicon. By combining pH titration and ultra-sonication from solution, nucleation densities are measured using atomic force microscopy (AFM). It is found that for most samples, highly acidic solutions (pH~2) are ideal for high surface coverage. Chapter 7 describes the electrical properties and activation energies of boron-doped HPHT and detonation nanodiamonds. Thin films are aggregated on conductive silicon substrates, and are subjected to electrical impedance measurements in vacuum. Following vacuum annealing, electrical measurements showed activation energies comparable to highly boron-doped PE-CVD thin film diamond. Electrical conductivity and resistivity are also compared to literature. In chapter 8, aluminium-diamond Schottky-barrier diodes (SBD) are fabricated. HPHT nanodiamond films were used as both Ohmic contacts and as a source of dopant (boron), where aggregated nanodiamonds were subjected to PE-CVD film growth. Electrical (I-V) and capacitance-voltage (C- V) measurements are performed to study conduction mechanisms in fabricated devices. Resulting devices are found to have low carrier densities in the grown active layer (~1015 cm-3), which is desirable for SBDs. This is the first account of using doped-NDs as the source of low boron-doping in PE- CVD diamond films, paving the way for potentially economical nanoscale diamond electronic devices.

5G-and-beyond communication systems are expected to be in a heterogeneous form of multiple-antenna cellular base stations (BSs) overlaid with small cells. The fully-digital BS structures can incur significant power consumption and hardware complexity. Moreover, the wireless BSs for small cells usually have strict size constraints, which incur additional hardware effects such as mutual coupling (MC). Consequently, the transmission techniques designed for future wireless communication systems should respect the hardware structures at the BSs. For this reason, in this thesis we extend generic downlink precoding to more advanced hardware-informed transmission techniques for a variety of BS structures. This thesis firstly extends the vector perturbation (VP) precoding to multiple-modulation scenarios, where existing VP-based techniques are sub-optimal. Subsequently, this thesis focuses on the downlink transmission designs for hardware effects in the form of MC, limited number of radio frequency (RF) chains, and low-precision digital-to-analog converters (DACs). For these scenarios, existing precoding techniques are either sub-optimal or not directly applicable due to the specific hardware constraints. In this context, this thesis first proposes analog-digital (AD) precoding methods for MC exploitation in compact single-user multiple-antenna systems with the concept of constructive interference, and further extends the idea of MC exploitation to multi-user scenarios with a joint optimisation on the precoding matrix and the mutual coupling effect. We further consider precoding for wireless BSs with a limited number of RF chains, in the form of compact parasitic antenna array as well as hybrid analog-digital structures designed for large-scale multiple-antenna systems. In addition, with a reformulation of the constructive interference, this thesis also considers the low-complexity precoding design for the use of low-resolution DACs for a massive-antenna array at the BSs. Analytical and numerical results reveal an improved performance of the proposed techniques compared to the state-of-the-art approaches, which validates the effectiveness of the introduced methods.

Synthetic Aperture Radar (SAR) imagery has been used extensively within UK Defence and Intelligence for many years. Despite this, the exploitation of SAR imagery is still challenging to the inexperienced imagery analyst as the non-literal image provided for exploitation requires careful consideration of the imaging geometry, the target being imaged and the physics of radar interactions with objects. It is therefore not surprising to note that in 2017 the most useful tool available to a radar imagery analyst is a contextual optical image of the same area. This body of work presents a way to address this by adopting recent advances in radar signal processing and computational geometry to develop a SAR simulator called SARCASTIC (SAR Ray-Caster for the Intelligence Community) that can rapidly render a scene with the precise collection geometry of an image being exploited. The work provides a detailed derivation of the simulator from first principals. It is then validated against a range of real-world SAR collection systems. The work shows that such a simulator can provide an analyst with the necessary tools to extract intelligence from a collection that is unavailable to a conventional imaging system. The thesis then describes a new technique that allows a vibrating target to be detected within a SAR collection. The simulator is used to predict a unique scattering signature - described as a one-sided paired echo. Finally an experiment is described that was performed by Cranfield University to specifications determined by SARCASTIC which show that the unique radar signature can actually occur within a SAR collection.

In the rapidly growing field of integrated quantum devices, two particular areas of interest are the development of an on-chip cryogenic current comparator (CCC) for completing the metrological triangle and the development of integrated de- vices for fast qubit operations. This thesis aims to significantly further our understanding of a quantum pump, a device integral to the CCC and potentially critical for realising fast qubit operations. A quantum pump is a device that transfers a discrete number of electrons between two electrically isolated regions when a potential barrier is cyclically oscillated. Initially, quantum pumps were single electron turnstile devices, which were limited in operational frequency by the Coulomb potential of the turnstile. Modern quantum pumps, utilising a dynamic quantum dot in a 2-dimensional electron gas (2DEG), are not limited by frequency. The fast operation of these modern pumps makes them very promising devices for accurately measuring the electron charge and performing fast qubit operations. In this study, we address the technical challenges of measuring a Al- GaAs/GaAs quantum pump and detail the processing and measurement setup. One of the challenges is rectified current swamping pump current. We develop a model for the rectified current and investigate ways to suppress it. We then show how the accuracy of a quantum pump changes as a function of amplitude, temperature, and frequency, and develop a model towards explaining the changes.

In this thesis, integrated quantum devices defined using a split gate technique are studied experimentally. These integrated devices provide a novel platform to investigate the property of quantum systems, such as spin polarization, via non-local measurement. Information extracted from these integrated devices leads to a comprehensive understanding of the puzzling phenomenon such as the 0.7 anomaly. Meanwhile, these devices are possibly suitable for studying quantum entanglement because perturbation due to measurement is minimized in the non-local setup. Devices demonstrated here are also promising to be used as a building block such as quantum injector/detector or quantum bus (which is a information channel where quantum information can be transported coherently) for more complicated quantum systems. In the first experiment, a transverse electron focusing in n-type GaAs heterojunction is present where pronounced splitting of odd focusing peaks are observed. From the asymmetry of sub-peaks of the first focusing spin polarization is extracted directly, this provides direct evidence for intrinsic spin polarization in a quasi-one-dimensional system. Parameters which may affect transverse electron focusing are studied systemically. Changing the shape of the injector, thus tuning the adiabaticity of the injection process, can influence the presence of peak splitting or not, with the sharp (non-adiabatic) injector the peak splitting is absent while peak splitting is observed with the flat (adiabatic) injector. Adjusting the length of injector affects the spin polarization, the longer the channel the higher the spin polarization can be achieved. This highlights the role of exchange interaction which results in the spin polarization in the quasi-1D channel. Applying a dc source-drain bias leads to such a result, peak splitting is preserved with negative bias while it smears out with positive bias when the bias is above a particular value (0.5 mV in the experiment), this proves the existence of spin-gap. In the second experiment, the coupling between different quantum devices are investigated by using an integrated quantum device consisting of an QPC and electronic cavity, where the cavity is defined with the arc-shaped gate and an inclined reflector. Unique features such as the double-peak structure occurs in the 1D-2D transition regime of the arc-QPC and 5 fine oscillations associated with conductance plateaus and 0.7 anomaly are observed when the reflector voltage is sufficiently negative and these features smear out when the reflector voltage is less negative. The double-peak structure and fine oscillations are proved to arise from the coupling between the discrete states in the QPC and continuum cavity state by the manifestation of Fano resonance via tuning reflector voltage or small transverse magnetic field. In the third experiment, quantum interference in a double-cavity system is studied by magneoresistance measurement. An unique evolution of the line shape of the magnetoresistance are observed, the magnetoresistance has a Lorentzian shape, corresponding to ergodic and chaotic motion, when the injector conductance is sufficiently small and then alters into linear line shape arising from non-ergodic and regular motion when injector is opens a bit more and finally a Lorentzian shape when the injector opens even further. Apart from the line shape, the strength of the magnetoresistance is found to fluctuate with injector conductance, it is enhanced at conductance plateaus and weakens elsewhere. Such behaviours are likely to arise from both deformation of the arc-shaped potential barrier at the vicinity of injector and detector QPC as well as the non-uniform spatial distribution of the cavity state.

Si-based light emitting sources are highly demanded for applications in optoelectronic integration circuits. Unfortunately, Si has an indirect bandgap and thus a low efficiency in photon emission. On the other hand, III–V semiconductors have superior optical properties and are considered as strong candidates to achieve efficient light emitting sources on Si platforms via wafer bonding or monolithically epitaxy growth. III–V materials monolithically grown on Si substrate could introduce various types of defects including antiphase domain, threading dislocation, misfit dislocation. These defects must be dealt with satisfactorily in order to fulfill the potential of III–V/Si integration. In this thesis, buffer layers for InAs/GaAs quantum dots (QDs) monolithically grown Si substrate have been investigated. The buffer layer study is mainly focused on the different types of defect filter layers (DFLs). The measurements of atomic force microscopy, photoluminescence and transmission electron microscopy are carried out to investigate the effectiveness of each type of DFLs. The results of lasers and superluminescent diodes (SLDs) have been presented based on the studies of DFLs. In order to improve the performance of InAs/GaAs QDs grown on Si substrates, a GaAs buffer layer and DFLs have been used to reduce the defect density from ~1010 to 106 cm-2 after three sets of DFLs, which consists of strained layer superlattices (SLSs). In the thesis, the optimisation of DFLs has been carried out. Different types of DFLs are investigated in the Chapter 3, including InAs/GaAs QDs, InGaAs submonolayer QDs, InGaAs/GaAs SLSs and InAlAs/GaAs SLSs. DFLs made of InAlAs/GaAs SLSs show the strongest performance, based on the measurements of atomic force microscopy, photoluminescence and transmission electron microscopy. The high performance InAs/GaAs QDs lasers with low threshold current density (194 A/cm2 ) and high operating temperature (85 ̊C) has been obtained for the samples with optimised DFLs. In addition to III–V/Si lasers, III–V SLDs monolithically grown on silicon substrates would further enrich the silicon photonics toolbox, enabling low-cost, highly scalable, high-functional, and streamlined on-chip light sources. In this thesis, the first InAs/GaAs QD SLDs monolithically grown on a Si substrate have been demonstrated based on the similar growth structure of laser devices. The fabricated two-section InAs/GaAs QD SLD produces a close- 4 to-Gaussian emission spectrum of 114 nm centred at ∼1255 nm wavelength, with a maximum output power of 2.6 mW at room temperature. The optimisation of InGaAs/GaAs SLSs DFLs has been carried out in the Chapter 5. The optimisation includes introducing different growth methods into GaAs spacer layer between each set of DFL, indium composition and GaAs thickness in InGaAs/GaAs SLSs. The optimisation is examined by atomic force microscopy, photoluminescence and transmission electron microscopy. The laser device with optimised InGaAs/GaAs SLSs DFLs has a lower threshold current density, higher operating temperature and characteristic temperature. In conclusion, InAs/GaAs QDs lasers with low threshold current density and the first QDs SLDs monolithically grown on Si substrates have been demonstrated. InAlAs/GaAs SLSs DFLs have been proved that as considerable solution to reduce the threading dislocation density significantly. The optimisations of InGaAs/GaAs SLSs DFLs successfully improve the QDs laser performance which could also be used in III–V/Si monolithically integration. The III–V QDs lasers and SLDs monolithically grown on Si substrate are essential steps for Si photonics integration, which will fill the “holy grail” of opto-electronic integration circuits.

Spectroscopic measurements in the terahertz-frequency-range can offer insight into the picosecond dynamics, molecular conformation, and biological function of chemical systems. Despite the recent emergence of terahertz-frequency time-domain spectroscopy as a tool for the measurement of dry, solid samples, the investigation of liquid analytes is complicated by the strong attenuation of terahertz-frequency signals in aqueous environments. The integration of microfluidic systems with on-chip waveguides offers a potential solution as picosecond pulses confined to a waveguide can interact with nano- or microlitre liquid sample volumes over a distance of several millimetres, with significantly reduced attenuation compared to free-space techniques. Specifically, the single-wire planar Goubau line waveguide has attracted attention in recent years owing to the relatively large extent of the supported evanescent field, enabling sensitive interaction between a propagating electric field and nearby samples. In this work, the first on-chip microfluidic spectrometer, capable of measuring the complex permittivity of liquids in the terahertz-frequency range is introduced. The fabrication of planar Goubau line devices with integrated photoconductive switches for the generation and detection of terahertz-frequency electric fields is discussed in detail. Given the importance of maximising the signal-to-noise ratio in spectroscopic measurements, an investigation of the signals excited from these switches is conducted, and factors such as the pump-power, generating beam polarisation, and switch geometry are found to have a significant impact on signal generation efficiency and noise. In addition to problematic signal noise, waveguide geometries can introduce artefacts that complicate further analysis. To simplify later modelling of these structures, the sources of unwanted reflections and propagation modes are identified, and prevented by design. The integration of microfluidic systems with on-chip waveguides presents several interesting challenges. Intimate contact between the waveguide and analyte allows for sensitive measurement of the sample properties, yet the electronic circuitry required to generate and detect a probing terahertz field must be isolated from the risk of a short-circuit presented by the potentially conductive liquid. A device structure is proposed that simultaneously overcomes these design limitations, and comprises a geometry that can be accurately modelled. Given the lack of analytical models with which the planar Goubau line can be described, numerical modelling techniques are used to create an accurate simulation of the device structure. A method is then introduced that allows interpretation of experimental data, such that the complex permittivity of unknown liquid samples can be calculated. This new technique is used to measure the complex permittivity of a selection of well-studied polar alcohols, and the results are found to compare well to those available in literature. A free-space terahertz spectroscopy system is then used to measure liquid samples that have not been published in order to verify the results of the on-chip spectrometer when used to measure a wider range of liquid samples.

The development of ultra sensitive charge sensing devices such as single-electron transistors (SETs) for next-generation biomedical applications has received considerable attention in the past few years. In this thesis, a potential approach for the mass-fabrication of metallic SETs for ultra-sensitive biosensing applications --- an important prerequisit for early diagnosis of many serious diseases --- is investigated. Using the orthodox theory of Coulomb blockade it is shown that it is possible to engineer an SET system that can satisfy the requirements for a highly sensitive charge sensor operating at room temperature while using metallic electrodes rather than semiconductor structures. In this configuration, the SET design and fabrication process is simplified greatly by lifting the dependence of the system on the confinement energy of electrons in the quantum dots (QDs), as is the case in semiconductor SETs. In return, this makes the tunnel junction properties and the geometrical arrangement of the islands and electrodes far more critical in determining the maximum operating temperature of the device. Here, the geometrical requirements for such a sensitive device are studied theoretically whilst the tunnel junction properties are studied experimentally and then theoretically to provide a thorough assessment of the abilities of the proposed SET system. Atomic-layer deposition (ALD) has proven to be a highly reliable technique for depositing uniform thickness and reproducible thin metal-oxide films and particularly the Al2O3 ALD process is known to be `ideal' with highly reproducible properties. Here, a systematic study of the electronic properties of ALD deposited Al2O3 thin films in MIM structures was performed to assess the ALD techniques applicability to the mass fabrication of quantum tunneling junctions for metallic SET structures. The two most crucial material parameters relevant to the design of metallic SET tunnel barriers are studied in detail; the dielectric constant of the film that determines the junction capacitance, and the properties of the potential barrier that mediates electron tunneling. Photolithographic techniques were used to create electrodes with a wide range of characteristic lengths as to provide a wide range of impedances. Measurements and subsequent analysis show that a high consistency can be attained over large surface areas in the film properties, and that electrode coverage is very effective, showing promise for mass-fabrication applications. Further analysis of the measurements shows that small static distortions in the barrier can affect the symmetry of MIM diode IV characteristics operating in the direct tunneling regime and that under certain circumstances the effect of surface states can be observed in the tunneling conductance.