| dc.description.abstract | In this thesis a number of metal nanostructures have been investigated for structural colour at sub-micron scales. These structures have utilised localised surface plasmon resonance (LSPR) to produce near-field interactions between components, generating sharp spectral features in reflectance. The use of phase change materials (PCMs) have also been investigated, for dynamic tuning of colour after fabrication of the structures, with a view towards display applications.
An aluminium nanodisc-nanohole array structure is explored for colour generation in reflection. The structure consists of a square dielectric pillar array, with aluminium coating the top of the pillar and the backreflector to produce the disc and hole components. Simulation is carried out by the finite-difference time-domain (FDTD) method, with varying geometry parameters to determine the colour gamut of the structure. The strong LSPR of the disc and hole interact in a Fano resonance condition, resulting in sharp spectral features in the visible region. A 5 mm square array with 400 nm pitch and 200 nm diameter pillars have been fabricated by electron beam lithography (EBL), with close correlation between simulated and measured spectra. A diffraction effect has been observed, following a first order diffraction profile with grating constant of 400 nm, consistent with this sample. The limited viewing angle presented by this structure leads research towards alternative materials, capable of colour generation at smaller dimensions.
A hybrid metal nanostructure consisting of Au discs on a polymer thin film and Ag backreflector has been investigated for vibrant structural colour in reflectance with a minimal unit cell size. The structure benefits from the strong LSPR of Au nanodiscs, high reflectivity of Ag and the optical cavity produced by the transparent polymer layer generating a strong Fano resonance that results in near-zero reflection in a narrow wavelength range. A cyclical colour trend is observed as polymer layer thickness is varied, with a repetition periodicity every 180 nm thickness change. The dependency of optical path length in the polymer, and material component composition on the colour produced is examined. Arrays measuring 100 x 100 µm, of 150 nm diameter discs with a 300 nm pitch have been fabricated using EBL. Assessment of the polymer film by ellipsometry revealed that the fabrication process caused negligible damage, and good agreement is found between arrays simulated by FDTD and those fabricated.
A thin film stack consisting of layers of Indium Tin Oxide (ITO) with an intermediate Vanadium Oxide (VO2) layer with an Ag backreflector is explored for dynamic structural colour. Compared with other phase change materials (PCMs), such as Germanium Antimony Telluride (GST), VO2 can be considered as a lower power consumption alternative. It has been overlooked in the visible region, due to its smaller refractive index change below 700 nm. The sensitivity of the visible reflectance spectrum to the change in phase of a 30 nm VO2 layer is shown to increase after it is incorporated in a thin film stack, with comparable performance other phase change materials. An improved maximum ΔR is observed, with 20% recorded in structures containing GeSbTe (GST) and AgInSbTe (AIST) increasing to 30% in a 10-30-25-100 nm ITO-VO2-ITO-Ag thin film stack. CIE separation is also shown to increase in this VO2 stack configuration. Inclusion of a top ITO layer is also shown to improve the chromaticity change on phase transition.
Finally, these VO2 thin films have been combined with Au nanoparticle arrays. Several nanoparticle shapes have been investigated, in positions on-top of the VO2 layer, and imbedded within it, to determine colour change potential of a combined nanostructure architecture. Change in reflectance of the thin film VO2 stack, as well as the modification LSPR response of the disc component as the surrounding media is altered, combine to increase the colour change of the overall structure. Imbedded particles are shown to result in the decrease of the maximum ΔR to 20%. However, this architecture results in the largest separation in CIE values of any architecture examined, with particle shape having little impact on the colour change observed. | en |
