UC Irvine

This dissertation presents the study of lithographically patterned nanostructured mesoscale optical materials with potential applications in biomedical devices, chemical sensing, and energy storage. We describe the fabrication and characterization of two types of nanostructured mesoscale surfaces: periodic nanocone arrays and diffraction gratings. The first part of the dissertation focuses on our research on periodic nanocone arrays. Using a straight-forward fabrication process, periodic nanocone arrays are created through oxygen plasma etching of hexagonally close-packed polystyrene bead monolayers on polymer surfaces. The periodicity and the height of the nanocone arrays are controlled by the polystyrene bead diameter and the overall etching time, due to the differential etching rates of the bead and the polymer surface underneath. These nanocone arrays exhibit broadband antireflectivity over a wide spectral range (450 – 800 nm) at various angles of incidence (8° ≤ θ ≤ 67.5°). In Chapter 2, we present the interesting electrochemically modulated optical properties shown by periodic nanocone arrays composed of the electroactive polymer poly(3,4-ethylenedioxythiophene) (PEDOT). Electrochemical modulation of the oxidation state of PEDOT nanocone arrays was used to change both its optical absorption (electrochromism) and reflection (electroreflectivity). In Chapter 3, we demonstrate that nanocone arrays modified with a nanostructured zinc oxide (ZnO) thin film on the surface exhibited a very low broadband reflectivity of less than 0.1% at a wide range of incident angles. The ultra-antireflective ZnO-coated nanocone array surfaces also exhibited an enhanced photoreactivity for the oxidative degradation of methylene blue, suggesting their potential as self-cleaning antireflective surfaces. The final chapter of this dissertation shifts focus to mesoscale diffraction gratings. Electrodiffractive and electrochromic properties were investigated from a novel two-component composite nanostructured electrodeposited grating that incorporated both ZnO and either tungsten oxide (WO3) or PEDOT. The sequential electrodeposition of these two materials through a photopatterned photoresist layer onto a fluorine-doped tin oxide (FTO) coated glass substrate created a grating structure that exhibited optical diffraction that could be modulated electrochemically. While the nanostructured ZnO is non-absorbing, its high refractive index and large surface area redirect light into the electrochromic grating and create a four-fold enhancement in the grating’s electrodiffractive response.