In this work, a new condensed matter approach to the study of excitons based on crystalline thin films of the organic molecule phthalocyanine is introduced. The premise is inspired by a wealth of studies in inorganic semiconductor ternary alloys (such as AlGaN, InGaN, SiGe) where tuning compositional disorder can result in exciton localization by alloy potential fluctuations. Comprehensive absorption, luminescence, linear dichroism and electron radiative lifetime studies were performed on both pure and alloy samples of metal-free octabutoxy-phthalocyanine and transition metal octabutoxy-phthalocyanines, where the metal is Mn, Co, Ni, and Cu. Varying the ratios of the metal to metal-free phthalocyanines in all of these studies, as well as looking across a temperature range from 4 Kelvin up to room temperature is essential for quantifying the exciton wavefunction delocalization in crystalline thin films. A comparative study is performed across organic aromatic ringed molecules of different sizes in the same family: phthalocyanine, naphthalocyanine and tetra-phenyl porphyrin. In an analogy to nanocrystals and their size effects, variations in pi-conjugated ring sizes imply an altering in the number of delocalized electrons, impacting the wavefunction overlap between pi-pi orbitals along the perpendicular axis of neighboring molecules. Finally, complementary measurements that assess crystallinity of the in-house deposited thin films, including individual grain absorption, small angle x-ray scattering images, polarized microscope images and a new unique linear dichroism microscopy dual imaging/luminescence technique are also discussed.

Quantum confinement in small metal clusters leads to a bunching of states into electronic shells reminiscent of shells in atoms. The addition of ligands can tune the valence electron count and electron distribution in metal clusters. A combined experimental and theoretical study of the reactivity of methanol with AlnIm− clusters reveals that ligands can enhance the stability of clusters. In some cases the electronegative ligand may perturb the charge density of the metallic core generating active sites that can lead to the etching of the cluster. Also, an investigation is conducted to understand how the bonding scheme of a magnetic dopant evolves as the electronic structure of the host material is varied. By considering VCun+, VAgn+, and VAun+ clusters, we find that the electronic and atomic structure of the cluster plays a major role in determining how an impurity will couple to its surroundings.

Tandem solar cells are potentially much more efficient than the silicon solar cells that currently dominate the market but require materials with different bandgaps. This thesis presents work on silicon nanocrystals (Si-NC) embedded in silicon carbide (SiC), which are expected to have a higher bandgap than bulk Si due to quantum confinement, with a view to using them in the top cell of a tandem cell. The strong photoluminescence (PL) of precursor films used to prepare Si-NC in SiC (Si-NC/SiC) was markedly reduced upon Si-NC formation due to simultaneous out-diffusion of hydrogen that passivated dangling bonds. This cannot be reversed by hydrogenation and leads to weak PL that is due to, and limited by, non-paramagnetic defects, with an estimated quantum yield of ≤5×10^-7. Optical interference was identified as a substantial artefact and a method proposed to account for this. Majority carrier transport was found to be Ohmic at all temperatures for a wide range of samples. Hydrogenation decreases dangling bond density and increases conductivity up to 1000 times. The temperature-dependence of conductivity is best described by a combination of extended-state and variable-range hopping transport where the former takes place in the Si nanoclusters. Furthermore, n-type background doping by nitrogen and/or oxygen was identified. In the course of developing processing steps for Si-NC-based tandem cells, a capping layer was developed to prevent oxidation of Si-NC/SiC, and diffusion of boron and phosphorus in nanocrystalline SiC was found to occur via grain boundaries with an activation energy of 5.3±0.4 eV and 4.4±0.7 eV, respectively. Tandem cells with a Si-NC/SiC top cell and bulk Si bottom cell were prepared that exhibited open-circuit voltages V_oc of 900 mV and short-circuit current densities of 0.85 mAcm^-2. Performance was limited by photocurrent collection in the top cell; however, the V_oc obtained demonstrates tandem cell functionality.

Hybrid organic-inorganic solar photovoltaic (PV) cells capable of directly converting sunlight to electricity have attracted much attention in recent years. Despite evident technological advancements in the PV industry, the widespread commercialisation of solar cells is still being mired by their low conversion efficiencies and high cost per Watt. Perovskites are an emerging class of semiconductors providing a low-cost alternative to silicon-based photovoltaic cells, which currently dominate the market. This thesis develops a series of studies on “all-solid state perovskite solar cells” fabricated via vapour deposition which is an industrially-accessible technique, to achieve planar heterojunction architectures and efficient PV devices. Chapter 2 presents a general outlook on the operating principles of solar cells, delving deeper into the specific operational mechanism of perovskite solar cells. It also explores the usual methods employed in the fabrication of perovskite thin films. Chapter 3 describes the experimental procedures followed during the fabrication of the individual components constituting the device from the synthesis of the precursors to the construction of the functioning perovskite PV devices. Chapter 4 demonstrates pioneering work involving the dual-source vapour deposition (DSVD) of planar heterojunction perovskite solar cells which generated remarkable power conversion efficiency values surpassing 15%. These significant results pave the way for the mass-production of perovskite PVs. To further expand the range of feasible vapour deposition techniques, a two-layer sequential vapour deposition (SVD) technique is explored in Chapter 5. This chapter focusses on identifying the factors affecting the fundamental properties of the vapour-deposited films. Findings provide an improved understanding of the effects of precursor compositions and annealing conditions on the films. Chapter 5 concludes with a comparison between SVD and DSVD fabricated films, highlighting the benefits of each vapour deposition technique. Furthermore, hysteretic effects are analysed in Chapter 6 for the perovskite PV devices fabricated based on different structural configurations. An interesting discovery involving the temporary functioning of compact layer-free perovskite PV devices suggests the presence of a built-in-field responsible for the hysteresis of the cells. The observations made in this chapter yield a new understanding of the functionality of individual cell layers. Combining the advantages of the optimum vapour deposition technique established in Chapter 4 and Chapter 5, with the enhanced understanding of perovskite PV cell operational mechanism acquired from Chapter 6, an ongoing study on an “all-perovskite” tandem solar cell is introduced in Chapter 7. This demonstration of the “all-perovskite” tandem devices confirms the versatility of perovskites for a broader range of PV applications.

This thesis presents details of x-ray and neutron scattering experiments used to probe quantum magnets with strong spin-orbit interaction. The first of these systems are the three-dimensional iridate compounds, in which the three-fold co-ordination of IrO₆ octahedra has been theoretically hypothesized to stabilize anisotropic exchange between Ir⁴⁺ ions. This novel interaction between these spin-orbital entangled, J_eff=1/2 moments is described by a Hamiltonian first proposed by Kitaev, and would be the first physical realization of this Hamiltonian in a condensed matter system. This thesis details the determination of the structure of a new polytype within these compounds, the 'stripyhoneycomb' &gamma;-Li₂IrO₃. Furthermore, through resonant magnetic x-ray diffraction experiments on single crystals of &beta;-Li₂IrO₃ and &gamma;-Li₂IrO₃, an incommensurate, non-coplanar structure with counter-rotating moments is found. The counter-rotating moment structure is a rather counter-intuitive result, as it is not stabilizied by Heisenberg exchange between magnetic sites, however, the Kitaev exchange naturally accounts for this feature. As such, these experiments reveal, for the first time, systems which exhibit dominant Kitaev interactions. The ordering wavevector of both &beta;- and &gamma;-Li₂IrO₃ polytypes are found to be identical, suggesting that the same magnetic interactions are responsible for stabilizing magnetic order in both materials, despite their different lattice topologies. Following this, the spinel FeSc₂S₄ is considered. Here, despite the presence of strong exchange between Fe<sup>2+,/sup>, and the fact that these ions sit in a Jahn-Teller active environment, the system does not order in the spin or orbital degrees of freedom. A 'spin-orbital singlet' has been theoretically proposed to describe the groundstate of this system, and here inelastic neutron scattering (INS) is used to probe the resulting triplon excitations. This allows determination of microscopic parameters in the single ion and exchange Hamiltonians, and moreover experiments in external magnetic field reveal the true spin-and-orbital nature of these triplon excitations. Finally, Ba₃CoSb₂O₉, a physical realization of the canonical triangular antiferromagnet model is explored with INS and the high energy excitations from the 120 degree magnetic structure are found to display significant differences from those calculated by linear spin wave theory, suggesting the presence of quantum dynamics not captured in the 1/S linear spin wave expansion.

This thesis improves our understanding of the charge-transfer dynamics in organic materials employed in dye-sensitized and nanotube-thiophene solar cells. For the purpose of this work, a femtosecond transient absorption spectroscopy setup was built. Additionally, microsecond transient absorption spectroscopy was utilised to explore dynamics on a longer time-scale. In the first study, the dependence of dye regeneration and charge collection on the pore- filling fraction (PFF) in solid-state dye-sensitized solar cells (DSSCs) is investigated. It is shown that while complete hole transfer with PFFs as low as ~30% can be achieved, improvements beyond this PFF are assigned to a stepwise increase in the charge-collection efficiency in agreement with percolation theory. It is further predicted that the chargecollection efficiency saturates at a PFF of ~82%. The study is followed by an investigation of three novel hole-transporting materials for DSSCs with slightly varying HOMO levels to systematically explore the possibility of reducing the loss-in-potential and thus improving the device efficiency. It is shown that despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. Furthermore, it is demonstrated that the design of the HTM has an additional impact on the electronic density of states present at the TiO₂ electrode surface, and hence influences not only hole- but also electron-transfer from the sensitizer. Finally, a study on a polymer-single-walled carbon nanotube (SWNT) molecular junction is presented. Results from femtosecond spectroscopic techniques show that the polymer poly(3-hexylthiophene) (P3HT) is able to transfer charges to the SWNT within 430 fs. Addition of excess P3HT polymer leads to long-lived free charges making these materials a viable option for solar cells.

Spin properties of donor impurities in silicon have been investigated by electron spin resonance (ESR) techniques for more than sixty years. These studies gave us a contribution towards understanding some of the physics of doped semiconductor materials in general, which is the platform for much of our current technology. Despite the fact that donor electron and nuclear spins have been researched for so long, ESR studies of their properties are still giving us interesting insights. With the introduction of the concept of quantum information in the 1980s, some properties of donor spins in silicon, that were known from the fifties (such as long relaxations), have been reinterpreted for their potential application in this field. Since then, incredible experimental results have been achieved with magnetic resonance control, including manipulation and read-out of individual spins. However, some open questions are still to be answered before the realisation of a spin-based silicon quantum architecture will be achieved. Currently, ESR studies still contribute to help answering some of those questions. In this thesis, we demonstrate electrical and optical methods for donor charge state manipulation measured by ESR. Recent experiments have demonstrated that coherence time of nuclear spins may be enhanced by manipulating the state of donors from neutral to singly charged. We investigate electric field ionisation/neutralisation of arsenic donors in a silicon SOI device measured by ESR. Below ionisation threshold, we also measure the hyperfine Stark shift of arsenic donors spins in silicon. These results have, for instance, implications on how fast individual addressability of donor spins may be achieved in certain quantum computer architectures. Here, we also study optical-driven charge state manipulation of selenium impurities in silicon. Selenium has two additional electrons when it replaces an atom in the silicon crystal (i.e. double donor). The electronic properties of singly-ionised selenium make it potentially advantageous as spin qubit, compared to the more commonly studied group-V donors. For instance, we find here that the electron spin relaxation and coherence times of selenium are up to two orders of magnitude longer than phosphorus at the same temperature. Finally, we demonstrate that it is possible to bring selenium impurity in singly-charged state and subsequently re-neutralise them leaving a potential long-lived ⁷⁷Se nuclear spin.

Toughness enhancements can be induced in cubic-B1 transition metal nitride alloys by an increased occupation of the d-t2g metallic states. In this Licentiate Thesis I use density functional theory to investigate the mechanical properties of TiN and VN and of the ternaries obtained by replacing 50% of Ti and V atoms with M (M = V, Nb, Ta, Mo, and W) to form ordered structures with minimum number of inter-metallic bonds. The calculated values of elastic constants and moduli show that ternary alloys with high valence electron concentrations (M = Mo and W), have large reductions in shear moduli and C44 elastic constants, while retaining the typically high stiffness and incompressibility of ceramic materials. These results point to significantly improved ductility in the ternary compounds. This important combination of strength and ductility, which equates to material toughness, stems from alloying with valence electron richer dmetals. The increased valence electron concentration strengthens metal–metal bonds by filling metallic d-t2g states, and leads to the formation of a layered electronic configuration upon shearing. Comprehensive electronic structure calculations demonstrate that in these crystals, stronger Ti/V – N and weaker M – N bonds are formed as the valence electron concentration is increased. This phenomenon ultimately enhances ductility by promoting dislocation glide through the activation of an easy slip system.

A preparação de vetores sintéticos para terapia gênica envolve a encapsulação de DNA em pequenos volumes em altas concentrações e nessas condições sabe-se que as moléculas de DNA podem ser organizar formando estruturas líquido-cristalinas. Essas moléculas, uma vez incorporadas a célula devem difundir e vários trabalhos experimentais têm mostrado que essa difusão é limitada pelo tamanho dos fragmentos de DNA. É nesse contexto que soluções de DNA in vitro constituem um sistema modelo para o estudo de processos dinâmicos de difusão na fase isotrópica e na fase anisotrópica. Neste trabalho investigamos a difusão de fragmentos de DNA de 150 pares de base dispersos em água com concentrações variando no intervalo de 80 a 350 mg/ml, que se situam no domínio da fase isotrópica e da fase colestérica, passando uma região intermedi ária de coexistência de fases. Nessas soluções foi acrescentado sal, em concentração suciente para blindar as interações eletrostáticas repulsivas, de maneira que os fragmentos de DNA podem ser tratados como cilindros rígidos e não interagentes. A técnica de FRAP (Fluorescence Recovery After Photobleach) foi empregada para determinação do coeciente de difusão dos fragmentos de DNA que foram marcados com uma prova uorescente, que se intercala entre os nucleotídios por meio de uma ligação covalente. As imagens de FRAP foram analisadas segundo dois métodos diferentes para ajuste do perl de intensidade de luz na mancha de bleach, que pode ser dado por uma função gaussiana, no caso de um processo de difusão isotrópico, ou uma por uma função de Bessel modicada no caso de um processo de difusão anisotrópico. Na fase isotrópica a função gaussiana é a que melhor descreve o perl de intensidade de luz da mancha de bleach e o valor obtido para o coeciente de difusão dos fragmentos de DNA está em torno de 20 m2=s, que é um valor compatível com os valores encontrados na literatura para soluções de DNA com fragmentos de mesma dimensão na fase isotrópica. Na fase colestérica, a análise das imagens de FRAP mostrou que o perl de intensidade de luz é mais bem ajustado por uma função de Bessel modi- cada, permitindo a determinação dos coecientes, D1 e D2, que correspondem aos coecientes de difusão paralelo e perpendicular à direção de orientação molecular local, respectivamente. Nessa mesofase, os valores obtidos para o coeciente de difusão estão entre 101m2=s e 102m2=s, com uma anisotropia, D1=D2 = 12. Esses resultados são compatíveis com os obtidos para soluções de vírus fd na fase nemática. Também foram feitas medidas para amostras com composição que se situam na região intermediária entre a fase isotrópica e a fase colestérica e que podem apresentar duas ou até três fases em coexistência. A análise das imagens permitiu a identicação de processos de difus ão anisotrópico e isotrópicos, em uma mesma amostra, comprovando a coexistência de fases que não são identicadas apenas pela observação da textura em microscopia de luz polarizada. / Preparation of synthetic vectors for gene therapy applications implies encapsulation of DNA in small cavities in high concentrations and in these conditions DNA molecules self-organize forming liquid crystalline structures. Once incorporated to the cell, DNA must diuse in cytoplasm and many experimental work have demonstrated that such diusion is limited by the fragments size. In this context, DNA solutions can be regarded as a promising model system for investigating isotropic and anisotropic diusion. In this work we have investigated diusion of DNA fragments, (150 base pairs), dissolved in water, for concentrations varying from 80 mg/ml to 350 mg/ml, ranging the domains of isotropic and cholesteric phases, with an intermediary region of phase coexistence. Salt was added to the solution, in concentration high enough to screen repulsive electrostatic interactions allowing us to regard DNA fragments as non interacting rigid rods. FRAP (Fluorescence Recovery After Photobleach) technique was used to determine diusion coecients of DNA fragments marked with a uorescent probe, which binds covalently to the nucleotides. Analysis of FRAP images are performed following two possible ttings to the intensity prole in bleach zone, which can be a Gaussian function, for isotropic diusion, either a Bessel modied function if the diusion process is anisotropic. In the isotropic phase the Gaussian function was found to be more appropriate to describe the intensity prole in the bleach spot zone, resulting in a diusion coecient around 20 m2=s, which is compatible with values reported in literature for DNA solutions of fragments of the same dimension in the isotropic phase. Image analysis in the cholesteric phase has shown that light intensity prole of the bleach spot is best tted with a Bessel modied function, allowing us to determine two diusion coecients, D1 and D2, corresponding to the diusion coecients parallel and perpendicular to the local molecular orientation, respectively. In such mesophase the obtained values for the diusion coecients are around 101m2=s and 102m2=s, with an anisotropy; D1=D2 = 12. Such results are compatible with results obtained in the nematic phase of virus fd solutions. Solutions with concentrations lying in the intermediary zone presenting two or even three phase in coexistence were also investigated. By applying the two tting proles to the bleach spot it was possible to identify zones in the sample corresponding to isotropic diusion process and other zones presenting anisotropic diffusion process. This conrms the coexistence of isotropic and anisotropic phases, which cannot be identied only by optical polarized microscopy.

O objetivo principal deste trabalho é o de investigar relações entre mapeamentos de modelos de spin em modelos de percolação e a existência de algoritmos de cluster capazes de simular de forma eficiente o modelo. Apresentamos um mapeamento do modelo de Blume-Capel em um modelo de percolação que permite reobter um algoritmo proposto anteriormente por nós através de uma prova de balanço detalhado, o que abre a possibilidade de descrevermos todo o diagrama de fases do modelo em termos de propriedades dos clusters formados. Isto é particularmente interessante, já que o modelo possui um ponto tricrítico, nunca antes analisado em termos de propriedades de percolação. Encontramos também um mapeamento para o modelo de Ashkin-Teller, e através dos algoritmos de cluster resultantes investigamos a possibilidade de existência de uma fase de Baxter Assimétrica. Analisamos também questões relacionadas ao comportamento de tamanho finito de sistemas que apresentam transições de fase de primeira ordem assimétricas. Finalmente, o algoritmo de cluster desenvolvido para o modelo de Blume-CapeI é também generalizado: de forma a podermos aplicá-lo ao estudo do modelo de Blume-Emery-Griffiths. / The main goal of this work is to investigate relations between mappings of spin models into percolation models and the possibility of devising an efficient cluster algorithm to simulate the model. We present a mapping of the Blume-Capel model into a percolation model that results in a cluster algorithm proposed previously by us through a detailed balance proof, enabling us to describe the whole phase-diagram in terms of cluster properties. This is particularly appealing, since the model has a tricritical point, a feature not yet analysed in terms of percolation properties. We present also a mapping for the Ashkin-Teller model, and using the obtained cluster algorithms we analyse the possibility of existence of the Asymmetric Baxter phase. We also address questions related to the finite-size behavior of systems in asymmetric first-order phase transitions. Finally, the cluster algorithm developed for the Blume-Capel model is generalized to the study of the Blume-Emery-Griffiths model.