Nano-size effects on opto-electronic, structural and vibrational properties of vanadium and tungsten oxides produced by laser and ultrasonic spray pyrolysis techniques
Date
2010-04-15T13:30:16Z
Authors
Mwakikunga, Bonex Wakufwa
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Abstract
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.