Two-dimensional colloidal crystals, as photonic band gap materials, have a wide
variety of interesting and valuable industrial applications as photonic materials as well
as simple models to study the basic processes of the atomic model such as phase
transition, stability, crystallisation, ordering, and nucleation and growth. Recently,
colloidal photonic crystals have got a great interest as templates for the fabrication of
two-dimensional (2D) arrays for lithography applications. The dependence of these
applications efficiency upon the quality of colloidal ordering during self-assembly
process was the motivation for many researchers to perform several investigations in
this field. These efforts have been oriented to develop a detailed explanation for
mechanisms that took place during the complex self-assembly process of colloids,
which may lead to developing a more controllable method to fabricate these structures
with better quality. However, we are not yet able to fully understand what exactly
happens during the colloidal self-assembly process. Hence, we are still unable to
optimise processing conditions and so exploiting the fascinating colloidal optical
characteristics is still limited till now.
Several techniques have been used to fabricate highly ordered two-dimensional
monolayer photonic crystals such as dip coating, electrophoretic deposition, selfassembly
at the gas/liquid interface and electric-field induced assembly. However,
these techniques have many drawbacks such as the incompatibility to scale-up from
laboratory-scale tests to industrial scale mass fabrication. Also, inability to control the
thickness of the final film limits the use of these techniques as fabrication methods for
uniform colloidal crystals. On the other hand, spin coating was found to be more
feasible due to its advantages over other techniques. Spin coating offers a cheap,
simple and straightforward technique for the fabrication of two and three-dimensional
colloidal crystals. Spin coating provides easy control of the uniformity, domain size
and thickness of the fabricated thin films through tuning the operating parameters such
as spinning speed, acceleration rate, solids content and solvent volatility. However,
the short duration of the process (5-30 s) and rapidly rotating sample (1000-10000
rpm) makes in situ studies challenging and as such we do not yet fully understand
colloidal self-assembly, so are unable to optimise processing conditions effectively.
I have developed a laser scattering setup, which facilitates collecting laser
scattering patterns diffracted by silica colloids in real time during the spin coating
process. Tracking the development of these scattering patterns in real time may help
to discover in details the stepwise evolution of the geometrical arrangements of
monolayer colloidal crystals (MCCs) during the self-assembly process. Monitoring
the colloidal self-assembly mechanisms may help to produce better quality colloidal
crystals with a minimum defects density and also may help pave the way to fabricate
complete three-dimensional photonic band gaps colloidal crystals with valuable
photonic industrial applications.
This work aims to study the critical factors affecting the degree of ordering of
colloids as they self-assemble through the development of the in situ laser scattering
experimental techniques. In addition, samples are investigated with scanning electron
microscopy (SEM) to characterise the impact of each factor on the colloidal thin films
morphology produced. Further understanding of colloidal self-assembly will allow
processing conditions to be optimised so that highly uniform, long range and defectfree
colloidal thin films may be easily fabricated.
