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

摘要

The thesis reviews two pyrolysis techniques – ultrasonic spray pyrolysisud(USP) and laser pyrolysis (LP). The two techniques - USP at the Physics LP atudthe National Laser Centre, CSIR in Pretoria, South Africa – were designed andudassembled by the candidate for the purpose of producing two relatedudmaterials - VO2 and WO3. The two smart materials find applications in energyudregulation for air conditioning alternatives, gas sensing for pollution controludpurposes, recording industry and computer memory. The thesis shows theudpursuit for small particles of these materials in order to see the change in theirudproperties at nano-scale. Novel structures that were not expected were foundnanowiresudand nanotips of WO3 and nanobelts and nano-ribbons of VO2. Theudconfinement of the 700 cm-1 and 800 cm-1 optical phonons is reported in WO3udnanowires, enhanced thermochromism of VO2 nano-structures with audhysteresis width of 80oC is presented, the 145 cm-1 phonon splitting isudreported in VO2 nanoribbons and attributed to surface phonons as a greaterudportion of atoms become surface atoms at nanoscale. A number of theoreticaludmodels have been proposed in order to explain some inexplicableudphenomena: the new solid-vapor-solid growth mechanism of the nanowires, audmodified phonon confinement model to suit phononconfinement in nanoribbonsudgeometry, a model to relate the hysteresis width in theudthermochromism of VO2 to the ribbon thickness and grain size based onudmartensitic type of transformations and a simple “charge-up” model toudpredict how hot the laser-aerosol interaction zone gets at various laser powerudsettings. More questions have been unearthed and these are also addressed and the way forward is proposed.