The investigation in this thesis focuses on the synthesis of cation-ordered perovskite phases by introducing anion vacancies into the structure. Complex cation-ordered phases Ba2YMO5 and Ba3YM2O7.5 (M = Fe, Co) have been synthesized using ceramic or citrate gel methods under flowing argon. Close inspection reveals that the structures are constructed from Y2M2O102 basic units which consist of two YO6 octahedra and two MO4 tetrahedra in a rock-salt type arrangement. In the structure of Ba2YMO5 (M = Fe, Co), the neighbouring Y2M2O102 units are connected with an equivalent one in the yz-plane with YO6 octahedra sharing an apex. In the structure of Ba3YM2O7.5 (M = Fe, Co), the basic units are connected to each other by the M2O7 dimers via a chain of Y – O – M – O – M – O – Y bonds. Complex cation ordering can be achieved by carefully controlling the anion vacancies and selecting the cations with different ionic radii. The anion vacancies present in Ba2YMO5 (M = Fe, Co) (space group P21/n) allow the intercalation of anions like O2- and F- into the lattice. The fluorination of Ba2YCoO5 leads to the formation of a new orthorhombic phase Ba2YCoO5F0.42 (space group Pbnm) in which the inserted fluoride ions are distributed in a disordered manner. In contrast, the topochemical oxidation of Ba2YFeO5 leads to the formation of a new orthorhombic phase Ba2YFeO5.5 (space group Pb21m), in which Fe4+ centres are located in 4-coordinate tetrahedral sites and 5-coordinate pyramidal sites, respectively. The polar structure of Ba2YFeO5.5 is confirmed by the observation of second-harmonic generation activity and pyroelectric behaviour. Ba2YFeO5.5 also exhibits a combination of ferromagnetic and antiferromagnetic behaviours at low temperature. LaCa2Fe2GaO8 adopts a six-layer structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the four-layer brownmillerite structure (space group Pbma). The chains of tetrahedra in the structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion in which the twisting direction of the chains of tetrahedra alternates in adjacent tetrahedral layers. LaxSr2-xCoGaO5+δ (0.5 < x < 1) adopts brownmillerite structures which consist of octahedral and tetrahedral layers with mixed valence of Co2+/Co3+. The members with x = 0.5, 0.6 and 0.7 adopt structures with I2mb space group symmetry, in which all the tetrahedra twist in the same direction. The members with x = 0.8, 0.9 and 1.0 adopt structures with Imma space group symmetry, in which the chains of the tetrahedra twist in a disordered manner. A change in the Co3+ spin state from high spin (HS) to low spin (LS) is observed as the La/Sr ratio increases. The change of the Co3+ spin state can be rationalized on the basis of internal chemical pressure.

No description available.

Les cellules solaires comprenant comme matériau actif une pérovskite organométallique hydride sont apparues récemment comme une technologie de haute performance et bon marché pour la conversion de l’énergie solaire. Les efforts mobilisés pendant cette thèse se sont concentrés sur un composant clé des cellules solaires à pérovskite, le transporteur de trous, qui assure à la fois les performances et la stabilité des cellules. Les avantages des polymères semi-conducteurs pour cette fonction se retrouvent dans leur stabilité, leurs bonnes propriétés de transport de charge et leur caractère filmogène. Ainsi, la stabilité des cellules solaires à pérovskite planes a pu être améliorée en intégrant un polyvinylcarbazole fonctionnalisé, tout en conservant de bonnes efficacités. Le potentiel de polyélectrolytes à base de PEDOT a pu être évalué dans les cellules de type inverses. Et enfin, une nouvelle méthode de dopage par un polyélectrolyte à base d’imidazole a démontré des efficacités remarquables, supérieures à 20%. / Perovskite solar cells based on hybrid organometal perovskite recently appeared as a cost-effective and highly efficient technology for the conversion of sunlight. Efforts undertaken during this PhD thesis focused on one component of the perovskite solar cells, the hole transport material, which rules both, performance and stability of the devices. Advantages of semiconducting polymers result in their thermal and chemical stability, their good charge transport properties and their ability to form homogeneous thin films. Thereby, through synthesis of novel polyvinylcarbazole and incorporation into devices, stability of planar perovskite solar cells has been enhanced while conserving good efficiency. The potential of PEDOT-based polyelectrolytes has been investigated in inverted perovskite solar cells. Finally, a new strategy to efficiently dope hole transporting materials has been demonstrated through the introduction of N-heterocyclic imidazolium-based polyelectrolytes. Thereby, efficiency of solar cells has been promoted to over 20%.

Uma tecnologia importante para reduzir a quantidade de poluentes atmosféricos lançados na atmosfera, é a utilização de catalisadores, que convertem os gases altamente poluentes como o NO, para outros gases inofensivos ao meio ambiente. Neste trabalho, os óxidos do tipo perovskitas La2CuO4, LaNiO3, LaMnO3, La1,4Sr0,6CuO4, La0,7Sr0,3NiO3 e La0,7Sr0,3MnO3 foram preparados através do método da co-precipitação, caracterizados por difração de raios-X, redução a temperatura programada, fisissorção de nitrogênio e análise química, e, posteriormente avaliados frente as reações de redução de NO com CO, e decomposição direta de NO. Ambas as reações foram testadas nas temperaturas de 400oC e 500oC e o tempo de reação foi de 7 à 10 horas. Através dos ensaios catalíticos verificou-se que o catalisador La2CuO4 apresentou a maior atividade para a reação de redução, e quando substituiu-se parcialmente o lantânio pelo estrôncio, houve uma melhora significativa para todos os catalisadores. As análises de DRX indicaram que, mesmo após os ensaios catalíticos, a estrutura cristalina dos catalisadores foi preservada. / A important technology to reduce the atmospheric pollution is the use of catalysts, to transform high pollutant as NO in other inoffensive gases to the environment. In this work, the perovskite oxides La2CuO4, LaNiO3, LaMnO3, La1,4Sr0,6CuO4, La0,7Sr0,3NiO3 e La0,7Sr0,3MnO3 were prepared through co-precipitation method and characterized by X-ray diffraction and temperature programmed reduction, nitrogen physsisorption and subsequent valued on the reduction of NO by CO and the direct decomposition of NO. These reaction were tested at 400oC and 500oC temperatures and times of reaction between 7 and 10 hours. Through the catalytic tests the La2CuO4 catalyst shown the best activity to the reduce reaction, and when the La is partially substituted by strontium all the catalyst showed a better significant for all the catalysts. The XRD analysis shown that the catalytic structure of the catalysts were preserved after the catalytic test yet.

Hybrid perovskites are a new class of semiconductor which have proven to be an ideal material for making thin film solar cells. They have the advantages of flexibility, low cost, and easy processing, whilst achieving efficiencies competitive with monocrystalline silicon. Many of the properties which make them ideal for solar cells are also applicable to light emitting devices, and there is now increasing interest in their application for light emitting diodes (LEDs) and lasers. This thesis aims to use a range of novel spectroscopy techniques to investigate the origin of these favourable properties, and to exploit these properties to produce high performance distributed feedback lasers. A detailed understanding of the origins of the excellent properties of hybrid perovskites is of crucial importance in the search for new variations with improved performance or lowered toxicity. This thesis uses Kelvin probe, air photoemission, and resonant ultrasound spectroscopy to probe deeply into the underlying physics of hybrid perovskite single crystals and devices. Using these techniques, we are able to produce detailed maps of the energy levels in a common perovskite solar cell, and we also gain strong insight into the underlying strains and instabilities in the perovskite structure that give rise to their elastic properties. The strong light emission of hybrid perovskites is then exploited to produce high quality distributed feedback lasers emitting in the green and infrared part of the spectrum. These lasers are observed to have superior stability, good thresholds, and many interesting beam parameters owing to their high refractive index. We explore a wide range of processing methods in order to achieve the lowest lasing threshold and the best stability. Finally, we investigate the properties of low dimensional perovskites and investigate their potential in optoelectronic applications.

Lead halide perovskites are a class of soft ionic semiconductors characterized by strong excitonic absorption and long carrier lifetimes. Recent studies suggest that electrons and holes in these materials interact with longitudinal optical phonons to form large polarons on subpicosecond time-scales. The same interaction is responsible for hot electron cooling via phonon emission and is thought to be screened by large polaron formation resulting in the long-lived hot electrons observed in methylammonium lead iodide perovskite. Time-resolved two-photon photoemission is used to follow the initial hot electron cooling and large polaron formation dynamics in single-crystal cesium lead bromide perovskite at 80 K and 300 K. The initial relaxation rates are found to be weakly temperature-dependent and are attributed to the cooling of unscreened hot electrons by the emission of longitudinal optical phonons. The large polaron formation times, however, are inferred to be approximately three times faster at 300 K. The decrease in polaron formation time with temperature is correlated with the broadening in phonon linewidths, suggesting that disorder can assist large polaron formation. In addition, the initial electron relaxation is faster than large polaron formation explaining the absence of long-lived hot electrons in cesium lead bromide perovskite as opposed to methylammonium lead iodide perovskite where the two processes are competitive. The second part of this thesis focuses on the strong light-matter interaction in nanowire waveguide geometries of single-crystal lead halide perovskites which are well known for their emission tunability and low lasing thresholds under pulsed optical excitation. Using fluorescence microscopy, it is found that the luminescence from single-crystal cesium lead bromide perovskite nanowires is dominated by sub-bandgap modes called exciton-polaritons, i.e. hybridized exciton-photon states. A one-dimensional exciton-polariton model reproduces the observed modes at the bottleneck of the lower polariton branch with a Rabi splitting of about 200 milli-electron volts. As the power density increases under continuous excitation, the exciton-polaritons undergo Bose-stimulated scattering and a super-linear increase in mode intensity is observed. This is the first demonstration of continuous-wave lasing in lead halide perovskite nanowires and reveals an inherently strong light-matter interaction in lead halide perovskites that can be used for continuous-wave optoelectronic applications. These findings corroborate the role of dynamic screening in unifying these two regimes of carrier-carrier interactions responsible for the strong absorption and subsequent carrier protection. We also demonstrate the viability of lead halide perovskite nanowires for future optoelectronics.

No description available.

by Yueng Hiu Wa. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaf 56). / Text in English; abstracts in English and Chinese. / by Yueng Hiu Wa. / Acknowledgements --- p.i / Abstract --- p.ii / 論文摘要 --- p.iii / Table of contents --- p.iv / List of Figures --- p.v / List of Tables --- p.vi / Chapter Chapter I --- Introduction / Chapter 1.1 --- Development of magnetoresistance materials --- p.1 / Chapter 1.1.1 --- Magnetoresistance (MR) --- p.1 / Chapter 1.1.2 --- Giant magnetoresistance (GMR) --- p.1 / Chapter 1.1.3 --- Colossal magnetoresistance (CMR) in rare-earth manganites ..… --- p.4 / Chapter 1.2 --- Our research motivation --- p.5 / Chapter 1.2.1 --- Scope of this thesis --- p.7 / Chapter Chapter II --- Instrumentation / Chapter 2.1 --- Vacuum annealing system --- p.8 / Chapter 2.2 --- X-ray diffraction (XRD) --- p.11 / Chapter 2.3 --- Rutherford backscattering spectroscopy --- p.13 / Chapter Chapter III --- Oxygen diffusion / Chapter 3.1 --- Introduction --- p.15 / Chapter 3.2 --- Theory --- p.16 / Chapter 3.3 --- Experimental --- p.21 / Chapter 3.4 --- Result and discussion --- p.24 / Chapter Chapter IV --- Vacuum annealing effect in La0.67Ca0.33Mn03 thin films / Chapter 4.1 --- Introduction --- p.31 / Chapter 4.2 --- Principles of resonant RBS and ion channeling --- p.32 / Chapter 4.2.1 --- Resonant RBS and its application / Chapter 4.2.2 --- Ion channeling and its application / Chapter 4.3 --- Experimental --- p.39 / Chapter 4.4 --- Data analysis and discussion --- p.48 / Chapter 4.5 --- Conclusion --- p.50 / Chapter Chapter V --- Conclusion

This thesis investigates some of the physics of semiconducting low-dimensional structures at low temperatures, as well as investigating the effects of defects on low-dimensional structures and magnetic fields on charge recombination in organic photovoltaics (OPVs). The applications of the systems studied vary from photodetectors, to LEDs, lasers and solar cells. We synthesised Cs2CuCl4 and CsCuCl3 nanoparticles. By varying the ratio of coordination solvents in the synthesis we can control the composition and morphologies of the fabricated nanoparticles, including dots, rods and wires. These nanocrystals showed broadband green emission upon excitation with sub-300 nm radiation, which we showed was due to emission from an intra-band Cu(II) defect. We also fabricated a variety of alkylammonium lead iodide materials. We found that hexylammonium lead iodide and dodecylammonium lead iodide emit broadband red light at low temperatures due to a Frenkel defect (f-centre). Additionally, we see evidence of biexciton emission in dodecylammonium lead iodide films at high excitation densities below temperatures of 225 K. The films in this study were seen to adopt two coexistent phases of dodecylammonium lead iodide, both a monoclinic P121/a structure and an orthorhombic Pbca structure, at room temperature. These films were used as the gain medium in a biexciton vertical cavity laser. This device was constructed from a distributed Bragg reflector coated with dodecylammonium lead iodide, a poly(methylmethacrylate) spacer layer, and an evaporated silver mirror. The onset of lasing was seen at 5.6 × 1018 excitations/cm3 at 75 K. Finally, the modulation of open-circuit voltage and short-circuit current by applied magnetic field in OPV devices made from PIDTPhanQ and PC(71)BM was investigated. We saw that magnetic fields influenced the recombination of charge-transfer (CT) states, and simulations indicated that the formation of CT states had a significant contribution from the bimolecular recombination of free charges. The simulations also showed that singlet CT state lifetimes were much smaller than triplet CT state lifetimes.

Pressureless-sintered lanthanum gallate (LaGaO3) ceramics have been analyzed using X-ray diffractometry (XRD), scanning electron and transmission electron microscopy (SEM and TEM), and electron-backscatter diffraction (EBSD). Transformation-induced twin domains are generated by solid-state phase transition upon cooling from the rhombohedral (r, space group R c) to orthorhombic (o, space group Pnma) symmetry at 145oC. Four types of transformation twins {101}, {121},