The aim of the project described in this thesis is to synthesise new functional molecules which interact with light for neurophysiological applications. In particular, I describe a family of amphiphilic porphyrins with large first hyperpolarisabilities which are used as SHG contrast agents and voltage-sensitive probes. In addition I detail a methodological microscopy tool and a novel caged form of a neuronal ion-channel antagonist. Chapter 1 introduces the key concepts underlying the use of dyes as SHG contrast agents. In particular it focuses on aspects of molecular design, covering both the amphiphilicity and nolinearity required by the target molecule. It covers quantification of the nonlinear properties of SHG stains, then surveys a number of examples which showcase the flexibility of SHG imaging as a biomedical technique. Chapter 2 describes a family of amphiphilic porphyrins with large first hyperpolarisabilities. Working from the structure-property relationships identified in Chapter 1, we fully characterise these dyes and demonstrate that they can be used in SHG imaging. We demonstrate that these molecules may also be tuned by complexation of a metal ion which can modulate their photophysical and solubility behaviour. Chapter 3 provides a description of how to determine the orientational distribution of dipolar dyes in a membrane by multiphoton microscopy. We measure the signal intensity of the dye in a model membrane system then find distributional moments which lead to the distribution itself. Chapter 4 explores whether off-axis contributions to the first hyperpolarisability tensor can significantly augment the dominant on-axis contribution from the main dipolar charge-transfer band. We synthesise and characterise a series of cis-donor cis-acceptor porphyrin compounds and explore their biophysical characteristics. Chapter 5 is the culmination of this project and after discussing method development, goes on to show how we measure the voltage sensitivity of an amphiphilic porphyrin SHG dye. We compare the archetypal porphyrin dye chromophore with three commercially available styryl dyes and demonstrate that our dye has greater sensitivity and a more rapid response. Chapter 6 describes a side project, the use of a photolabile cage to protect MK801, a neuronal ion-channel antagonist. By developing a water soluble photolabile cage using molecular design techniques, we are able to release MK801 in neurons with precise spatiotemporal control, allowing us to pinpoint the locus of two key neurophysiological processes.

A scalable spray processing technique was used to fabricate carbon nanotube (CNT)-based film electrodes and solid-state supercapacitors. The sprayed CNT-based electrodes comprised a randomly interconnected meso-porous network with a high electrical conductivity. Layer-by-layer (LbL) deposition of functionalised and oppositely charged single-wall carbon nanotubes (SWNTs) increased the electrode density and improved charging and discharging kinetics when compared with carboxylic functionalised only SWNT electrodes. The capacitance was further increased to 151 F g-1 at 2 mV s-1 and 120 F g-1 at 100 mV s-1 after vacuum and H2 heat treatments that removed the functional groups, and resulted in a hybrid microstructure of SWNTs and multi-layer graphene sheets from unzipped SWNTs. Flexible solid-state supercapacitors were fabricated by directly spraying multi-wall carbon nanotube (MWNT)-based aqueous suspensions onto both sides of a Nafion membrane and dried. A single cell with MWNT-only electrodes had a capacitance of 57 F g-1 per electrode at 2 mV s-1 and 44 F g-1 at 150 mV s-1. Cells with MWNT/ionomer electrodes showed a higher H+ mobility and a lower charge transfer resistance, and the capacitance increased to 145 F g-1 at 2 mV s-1 and 91 F g-1 at 150 mV s-1. Finally, MWNT/TiO2 nanoparticle/ionomer hybrid electrodes were used in the same solid-state supercapacitor configuration and provided a capacitance of 484 F g-1 per electrode at 5 mV s-1 and 322 F g-1 at 100 mV s-1. A qualitative model of the charge storage mechanism was developed, where TiO2 promoted H+ ions via redox reactions that fed protons into the proton-conducting ionomer coating over the MWNTs (in which the TiO2 was embedded), while electrons were readily conducted through the MWNT scaffold. This solid-state supercapacitor provided both attractive energy (31.8 Wh kg-1) and power (14.9 kW kg-1) densities, where such high energy density is difficult to achieve for MWNTs alone and such high power density is difficult for metal oxides alone, especially in the solid state.

This thesis presents work relating to the fabrication of novel thin film electrodes for energy storage applications, with a focus on low cost, nanostructured transition metal oxides, and electrode manufacture by atomised spray deposition. Iron oxide (FeO_x) nanowires were synthesised hydrothermally and combined with multi-walled carbon nanotubes (MWNT) in sprayed electrodes, which provided the necessary conductivity enhancement for effective energy storage. The spray processing technique allowed for facile control over the relative fraction of MWNTs in the sprayed electrodes. Optimised electrodes were investigated in a range of aqueous electrolytes, and the best energy storage behaviour occurred in Na₂SO₃ with a maximum capacitance from cyclic voltammetry of 312 Fg^-1 at a scan rate of 2 mVs^-1. The FeO_x/MWNT electrodes were investigated for their suitability as lithium-ion battery anodes and showed reasonable energy storage behaviour. Nickel oxide (NiO) electrodes were manufactured by hydrothermal synthesis and annealing followed atomised spray deposition. The performance of the NiO electrodes was enhanced though combination with aqueous graphene suspensions, produced in-house by ultrasonic exfoliation of graphite. The processing route used to combine the nanomaterials was considered and a co-synthesis route resulted in the best performing electrodes. Different substrates were investigated, as the most commonly used Ni-foam substrate reacted with the basic electrolytes necessary for electrochemical activity of NiO. NiO/graphene electrodes showed charge/discharge capacitances of up to 571 Fg^-1 at a current density of 10 Ag^-1, which was maintained at over 300 F/g at a very high current density of 100 Ag^-1. Asymmetric supercapacitor devices were constructed using various combinations of FeO_x, NiO, and commercial carbon black electrodes to extend the operating potential window beyond the ~1.23 V limit of symmetric aqueous-electrolyte devices. Power densities of over 20 kWkg^-1 were achieved for an FeO_x/MWNT-carbon device, which was comparable with current commercial carbon-only supercapacitors.

Silks are biomaterials made by spiders and silkworms, evolved for natural functions ranging from protection to predation. The research presented in this Thesis combines principles and methods from engineering, physics and biology to study the material properties of single silk fibres from a biological perspective. In particular, the factors that contribute to the variation in properties of single silk fibres are investigated. The first part of the Thesis focuses on silks made by silkworms. Whether naturally spun or forced reeled, the mechanical properties of these silks are sensitive to a range of environmental and processing conditions, such as humidity, stretching and reeling speed. The research presented in this section contributes to the understanding of how these applied conditions affect silk mechanical properties, which can be understood in terms of silk’s protein structure and biological context. The second section compares both silkworm and spider silk single fibres to other materials in terms of their sonic properties – how the materials propagate sound waves, whether following impact, or propagating vibrations. The results are discussed in the context of the silk’s natural function for impact resistance (silkworm cocoon or spider web) and vibrational signalling (spider silks). The Thesis ends with a discussion of how the presented techniques can be applied to help further our understanding of orb web function through studying spider silks. Overall, this interdisciplinary Thesis contributes to our understanding of the structure-property-function links of these fascinating biomaterials.

Solid-state dye-sensitized solar cells (ssDSCs) offer the possibility of high power conversion efficiencies (PCEs) of over 20%. However, after more than a decade of research, devices still barely reach over 7% PCEs. In this thesis, limitations to device performance are studied in detail, and solutions for future advancement are put forward. In the first part of the thesis, factors limiting charge generation are explored by studying the crystallization environment of mesoporous TiO2 self-assembled through block copolymers. It was found that the density and distribution of sub band gap states are a function of the synthesis conditions and critically affect the performance characteristics of the self-assembled titania used in ssDSCs. As a result, the self-assembled mesoporous oxide system presented in this thesis outperforms for the first time the conventional nanoparticle based electrodes fabricated and tested under the same conditions, with demonstrated PCEs of over 5%. In chapters 6, 7, and 8, the factors limiting the diffusion length and hence, the thickness of the fabricated devices, are carefully examined. Previous literature points towards insufficient pore-filling of the hole transporting material (HTM) as the main limiting factor. In chapter 6, a pore-filling study is shown where a new technique to evaluate the pore-filling fraction of the HTM in the conventional mesoporous metal oxide electrode is also presented and conclude that sufficient pore-filling of thick films can easily be achieved. Another usual strategy to extend the electron lifetime in the devices and thus, the charge diffusion length, involving thin film coatings of insulating metal oxides is examined in chapter 7, with satisfactory results for SnO2-based ssDSCs. The diffusion length can also be extended if the factors limiting the diffusion of charges through the device are identified and removed, as presented in chapter 8. Finally, a study on the stability of the ssDSC is presented in chapter 9. The developments achieved enable long term stability to be effectively targeted, and represent a key milestone towards commercial realization of ssDSCs.

This thesis studies the application of silver nanowire transparent conductors in PbS quantum dot photovoltaics. Silver nanowires were synthesized using a colloidal method and characterized using scanning electron microscopy. Nanowires were deposited on glass substrates by a stamp transfer process to generate a low density continuous network of conductive nanowires. This resulted in a highly conductive and transparent film appropriate for optoelectronic applications. Nanowire synthesis, deposition, and processing were optimised to produce transparent conductors suitable for thin film photovoltaics. These nanowire films were used to fabricate lead sulphide (PbS) colloidal quantum dot solar cells. In this structure, p-type PbS quantum dots form a junction with a n-type ZnO nanoparticle layer. A variety of fabrication and processing treatments were developed in order to reduce short-circuiting of devices and to enhance cell performance. Moderate nanowire density, improved ZnO adherence, slight device aging, and increased PbS film thickness proved to result in the highest quality devices. The champion device developed in this thesis achieved a power conversion efficiency of 2.2%.

This thesis studies quaternary chalcogenide nanocrystals and their photovoltaic applications. A temperature-dependent phase change between two distinct crystallographic phases of stoichiometric Cu₂ZnSnS₄ is investigated through the development of a one pot synthesis method. Characterisation of the Cu₂ZnSnS₄ nanocrystals was performed using absorption spectroscopy, transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). An investigation was conducted into the effects of using hexamethyldisilathiane (a volatile sulphur precursor) in the nucleation of small (<7nm), mono-dispersed and solution-stable quaternary Cu₂ZnSnS₄ nanocrystals. A strategy to synthesize high quality thermodynamically stable kesterite Cu₂ZnSnS₄ nanocrystals is established, which subsequently enabled the systematic study of Cu₂ZnSnS₄ nanocrystal formation mechanisms, using optical characterization, XRD, TEM and Raman spectroscopy. Further studies employed scanning transmission electron microscopy (STEM) energy dispersive x-ray (EDX) mapping to examine the elemental spatial distributions of Cu₂ZnSnS₄ nanocrystals, in order to analyse their compositional uniformity. In addition, the stability of nanocrystals synthesised using alternative ligands is investigated using Fourier transform infrared spectroscopy, without solution based ligand substitution protocol is used to replace aliphatic reaction ligands with short, aromatic pyridine ligands in order to further improve Cu₂ZnSnS₄ colloid stability. A layer-by-layer spin coating method is developed to fabricate a semiconductor heterojunction, using CdS as an n-type window, which is utilised to investigate the photovoltaic properties of Cu₂ZnSnS₄ nanocrystals. Finally, three novel passivation techniques are investigated, in order to optimise the optoelectronic properties of the solar cells to the point where a power conversion efficiency (PCE) of 1.00±0.04% is achieved. Although seemingly modest when compared to the performance of leading devices (PCE>12%) this represents one of the highest obtained for a Cu₂ZnSnS₄ nanocrystal solar cell, fabricated completely under ambient conditions at low temperatures.

Rapidly solidified nanoquasicrystalline Al₉₃Fe₃Cr₂Ti₂ at% alloy has previously shown outstanding mechanical performance and microstructural stability up to elevated temperatures. Despite this, no in-depth study had previously been performed assessing the active strengthening mechanisms, the long term microstructural stability and the effect of plastic deformation at elevated temperature to simulate the production methods utilised for engineering applications. The current project analysed eight bars consisting of a nanoquasicrystalline Al₉₃Fe₃Cr₂Ti₂ at% alloy matrix with varying amounts of pure Al fibres, produced through gas atomisation and warm extrusion. Microstructural characterisation and thermal analysis of the as-atomized powder was carried out to assess whether microstructural changed were likely to occur at the extrusion temperature. A microstructure made primarily of nanometre-sized icosahedral particles in an FCC-Al matrix was observed through a combination of SEM, TEM (and CBDP), EDX, XRD. Thermal analysis of the powders performed by DSC showed that no change was expected to occur at the extrusion temperature. Five bars were extruded during the course of this project: one bar of pure Al-Fe-Cr-Ti alloy, two composite bars with 10 vol% added pure Al and two bars with 20 vol% added Al. Three more bars were received from a previous project and analysed. Warm extrusion caused the powder particles to become well bonded and elongated in the extrusion direction introducing a preferred orientation in the FCC-Al grains. A bimodal distribution of grain size was observed after extrusion. Several low angle (5-15 &deg;) grain boundaries were also identified by EBSD along the extrusion direction. No obvious change in size or shape was observed by TEM in the icosahedral phase (a bimodal distribution of hard, incoherent precipitates was observed after extrusion), or any change in the amount of solutes in solid solution in the Al matrix. Mechanical properties at room temperature were tested by Vickers microhardness, quasi-static tensile tests, dynamic tensile tests and dynamic compression tests. A theoretical model correlating the microstructures observed with the various active strengthening mechanisms was applied in order to predict an estimate of the yield strength of the material produced. It was found that the strength of the Al₉₃Fe₃Cr₂Ti₂ alloy came primarily from a combination of the effect of the hard, incoherent nanoparticles, the small grain size and work hardening. The fibre addition to this alloy caused a linear decrease in mechanical strength with increasing vol% pure Al. This work represents the first quantitative estimate of which strengthening mechanisms are active and how much they influence the mechanical strength of Al₉₃Fe₃Cr₂Ti₂ alloy and composites. An understanding of the yield strength is essential as engineering components would only be safe to use within the elastic region. To investigate the thermal stability of the alloy and composites, thermal analyses involving DSC and long heat treatments (up to a maximum of 1000 hours) were performed at various temperatures along with microstructural characterisation by XRD, SEM and TEM and microhardness tests. No microstructural change was detected, however a 2-5% decrease in microhardness was observed. Compression tests were performed across a range of temperatures and strain rates to simulate the behaviour of these composites under typical conditions necessary to process them into useful engineering components. Phase changes occurring during plastic deformation at high temperature were investigated by XRD. The measured yield strength at 350 &deg;C was over 3x that of high strength 7075 T6 Al alloy showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase at 500 &deg;C into more stable intermetallic phases. Serrated flow was also observed in some of the tests. The high temperature compressive data was then used for the first time in a semi-quantitative analysis to determine which species in solid solution (Fe, Cr or Ti) was likely to cause the serrations. A dynamic strain ageing model, which calculates the diffusion coefficients at the minimum in ductility and strain rate sensitivity, suggested that the Ti in solid solution in the matrix could be the most likely candidate.

Thin film multiferroic oxides with co-existing ferroelectric and ferromagnetic ordering have attracted much interest in recent years, partly as a result of the enhancements achieved through the adoption of strained thin film geometries. This thesis presents work on two such thin film oxides; lanthanide substituted BiFeO₃ and Fe substituted PbTiO₃. Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi_1-x-yDy_xLa_yFeO₃ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal and rare-earth doped BiFeO₃ were determined from acoustic mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV. Bulk crystals and thin films of PbTi_1-xFe_xO₃ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pb_n+1(Ti_1-xFe_x)_nO_3n+1 forms spontaneously during pulsed laser deposition of PTFO on LaAlO₃ substrates. High-resolution transmission electron microscopy, x-ray difraction and x-ray photoemission spectroscopy were utilised to perform a structural and ompositional analysis, demonstrating that n&sime;8 and x&sime;0.33. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes, and the observation of second harmonic generation, establishes a non-centrosymmetric point group for Pb_n+1(Ti_0.67Fe_0.33)_nO_3n+1-&delta; consistent with ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO₃ and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

Topological insulators (TIs) are recently predicted, and much studied, new quantum materials. These materials are characterised by their unique surface electronic properties; namely, behaving as band insulators within their bulk, but with spin-momentum locked surface or edge states at their interface. These surface/edge crossing states are protected by the underlying time-reversal symmetry (TRS) of the bulk band structure, leading to a robust topological surface state (TSS) that is resistant to scattering from impurities which do not break TRS. Their surface band dispersion has a characteristic crossing at time reversal invariant momenta (TRIM) called a Dirac cone. It has been predicted that the introduction of a TRS breaking effect, through ferromagnetic order for instance, will open a band-gap in this Dirac cone. It can be seen that magnetic fields are not time reversal invariant by considering a solenoid. If time is reversed, the current will also reverse in the solenoid and so the magnetic field will also be reversed. So it can be seen that magnetic fields transform as odd under time reversal, the same will be true of internal magnetisation. By manipulating this gapped surface state a wide range of new physical phenomena are predicted, or in some cases, already experimentally observed. Of particular interest is the recently observed quantum anomalous Hall effect (QAHE) as well as, e.g., topological magneto-electric effect, surface Majorana Fermions and image magnetic monopoles. Building on these novel physical effects, it is hoped to open new pathways and device applications within the emerging fields of spintronics and quantum computation. This thesis presents an investigation of the nature of magnetic doping of the chalcogenide TIs Bi₂Se₃ and Bi₂Te₃ using 3d transition-metal dopants (Mn and Cr). Samples were grown by molecular beam epitaxy (MBE), an ideal growth method for the creation of high-quality thin film TI samples with very low defect densities. The grown films were investigated using a range of complementary lab-based and synchrotron-based techniques to fully resolve their physical structure, as well as their magnetic and electronic properties. The ultimate aim being to form a ferromagnetic ground state in the insulating material, which may be expanded into device applications. Samples of bulk Mn-doped Bi₂Te₃ are presented and it is shown that a ferromagnetic ground state is formed below a measured T_C of 9-13 K as determined by a range of experimental methodologies. These samples are found to have significant inhomogeneities within the crystal, a problem that is reduced in MBE-grown crystals. Mn-doped Bi₂Se₃ thin films were grown by MBE and their magnetic properties investigated by superconducting quantum interference device (SQUID) magnetometry and x-ray magnetic circular dichroism (XMCD). These reveal a saturation magnetisation of 5.1 &mu;_B/Mn and show the formation of short-range magnetic order at 2.5 K (from XMCD) with indication of a ferromagnetic ground state forming below 1.5 K. Thin films of Cr-doped Bi₂Se₃ were grown by MBE, driven by the recent observation of the QAHE in Cr-doped (Bi_1−xSb_x)₂Te₃. Investigation by SQUID shows a ferromagnetic ground state below 8.5 K with a saturation magnetisation of 2.1 &mu;_B/Cr. Polarised neutron reflectometry shows a uniform magnetisation profile with no indication of surface enhancement or of a magnetic dead layer. Further studies by extended x-ray absorption fine structure (EXAFS) and XMCD elucidate the electronic nature of the magnetic ground state of these materials. It is found that hybridisation between the Cr d and Se p orbitals leads to the Cr being divalent when doping on the Bi^3&plus; site. This covalent character to the electronic structure runs counter to the previously held belief that divalent Cr would originate from Cr clusters within the van der Waals gap of this material. The work overall demonstrates the formation of a ferromagnetic ground state for both Cr and Mn doped material. The transition temperature, below which ferromagnetic order is achieved, is currently too low for usable device applications. However, these materials provide a promising test bed for new physics and prototype devices.