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Title: Fabrication and characterisation of novel polymeric and colloidal films for reflective and antireflective coatings
Author: Mohamed, Mahmoud
ISNI:       0000 0004 5989 2955
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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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.
Supervisor: Howse, Jonathan Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available