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Title: Control of aluminium anodization to regulate the optical properties of porous aluminium oxide membranes
Author: Orsi, Alice
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Porous alumina anodized (PAA) membranes are widely used as templates for electrodeposition of conductive materials or directly as photonic material [1-3] due to the regular distribution of pores when anodized under defined temperature, electrolyte and voltage. This thesis investigated PAA nucleation and development under constant voltage in order to develop a periodic anodizing procedure to control pore diameter and interpore distance along the pore main axis. The periodic change in the geometrical parameters modifies the optical properties of the membranes and visible light interacts with the porous membrane when the periodicity of the structure is of the same order as the light wavelength. Two porous structures with reflectance peaks in the visible range were realized and their optical properties were studied. Branched membranes presented alternated layers of branched pore and main pore layers of controllable thicknesses. The reflectance spectra was modelled as a Bragg stack whose thicknesses and refractive indices of the alternating layers were obtained from ellipsometrical and SEM measurements of PAA membranes. The second structure was defined as necked membrane, as it presented periodic enlargements and restrictions of pore diameter along the pore main axis without branching. Reflectance spectra showed a single peak in the visible range whose position could be controlled by the anodizing temperature. Reflectance spectra showed by branched and necked samples were considered promising with regards to applications as interleaved reflectors in reflective displays. The presence of selective reflective layers interleaved in a stack display reduces light losses due to undesired absorbance of colour layers and more freedom in display design. [4] 1. Wang, B., et al., Preparation of photonic crystals made of air pores in anodic alumina. Nanotechnology, 2007. 18: p. 1. 2. Zheng, W.J., et al., Modulation of Transmission Spectra of Anodized Alumina Membrane Distributed Bragg Reflector by Controlling Anodization Temperature. Nanoscale Research Letters, 2009. 4(7): p. 665. 3. Zheng, W.J., et al., Distributed Bragg reflector made of anodic alumina membrane. Materials Letters, 2009. 63(8): p. 706. 4. Kitson, S., et al., Bright color reflective displays with interlayer reflectors. Optics Express, 2011. 19(16): p. 15404.
Supervisor: Riley, Jason Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available