Nano-size effects on opto-electronic, structural and vibrational properties of vanadium and tungsten oxides produced by laser and ultrasonic spray pyrolysis techniques

The thesis reviews two pyrolysis techniques – ultrasonic spray pyrolysis (USP) and laser pyrolysis (LP). The two techniques - USP at the Physics LP at the National Laser Centre, CSIR in Pretoria, South Africa – were designed and assembled by the candidate for the purpose of producing two related materials - VO2 and WO3. The two smart materials find applications in energy regulation for air conditioning alternatives, gas sensing for pollution control purposes, recording industry and computer memory. The thesis shows the pursuit for small particles of these materials in order to see the change in their properties at nano-scale. Novel structures that were not expected were foundnanowires and nanotips of WO3 and nanobelts and nano-ribbons of VO2. The confinement of the 700 cm-1 and 800 cm-1 optical phonons is reported in WO3 nanowires, enhanced thermochromism of VO2 nano-structures with a hysteresis width of 80oC is presented, the 145 cm-1 phonon splitting is reported in VO2 nanoribbons and attributed to surface phonons as a greater portion of atoms become surface atoms at nanoscale. A number of theoretical models have been proposed in order to explain some inexplicable phenomena: the new solid-vapor-solid growth mechanism of the nanowires, a modified phonon confinement model to suit phononconfinement in nanoribbons geometry, a model to relate the hysteresis width in the thermochromism of VO2 to the ribbon thickness and grain size based on martensitic type of transformations and a simple “charge-up” model to predict how hot the laser-aerosol interaction zone gets at various laser power settings. More questions have been unearthed and these are also addressed and the way forward is proposed

Fabrication and Characterization of Hybrid Metal-Oxide/Polymer Light-Emitting Diodes

Hybrid metal-oxide/polymer light-emitting diodes (HyLEDs) are a novel class of electronic devices based on a combination of electroluminescent organic and charge-injecting metal-oxide components. These devices employ air-stable electrodes, such as ITO and Au, and are therefore well suited for fabrication of encapsulation-free light-emitting devices. The current work is intended to provide an insight into operating mechanisms and limitations of the HyLEDs, and, on the basis of this knowledge, aims at modifying the device architecture in order to improve the performance. The choice of optically transparent metal-oxide charge-injection layers appears to be critical in this respect in order to optimize the electron-hole balance within the polymer layer. Starting from the original device architecture, ITO/TiO2/F8BT/MoO3/Au, which uses ITO as a cathode and Au as an anode, we follow different approaches, such as the use of dipolar self-assembled monolayers and nanoscale structuring of the electron-injecting interface, pursuing the goal of enhancing electron injection into the emissive layer. However, substitution of the electron-injecting layer of TiO2 with ZrO2 is demonstrated to be the most efficient of the approaches employed herein. Further, optimization of the device utilizing the latter metal oxide is demonstrated in terms of deposition and post-deposition treatment of the electron-injecting and electroluminescent layers. Substrate temperature during spray pyrolysis deposition of the electron-injecting layer is found to have a strong influence on the HyLED performance, as well as the precursor solution spraying rate and the layer thickness. On the other hand, post-deposition annealing of the polymer layer is shown to improve the device efficiency and brightness significantly, possible explanations lying in enhancement in polymer luminescence efficiency and formation of a more intimate contact between the electron-injecting and the active polymer layers. Combining electron-transporting (TiO2 and ZnO) and hole-blocking (Al2O3 and ZrO2) materials into a single electron-injecting layer is demonstrated to be an effective strategy of enhancing efficiency in the HyLEDs. The search for a hole-injecting electrode alternative to the conventionally used MoO3/Au leads to the device employing the PEDOT:PSS/VPP-PEDOT system, which though resulting in a poorer device efficiency, provides route for fabrication of vacuum deposition-free organic light-emitting devices. Finally, the HyLED architecture is demonstrated to offer better stability than the conventional architecture using LiF/Al as a cathode. It is hoped that the current work provides a better understanding of the requirements for fabrication of encapsulation-free organic light-emitting devices