Study on electrically tunable colloidal photonic crystal for reflective display application

Abstract

In this dissertation, electrically tunable colloidal photonic crystal is mainly discussed with particular focus on the reflective display device. In chapter 1, introductive review of electrically tunable photonic crystal device is presented. A brief description of tunable photonic crystals and band-gap formation of colloidal photonic crystal is mentioned. Also the requirements for commercially adoptable reflective display and current status of colloidal photonic crystals are reviewed. The basic driving properties of polystyrene based tunable colloidal photonic crystal and the evaluation set-up is also compactly described. The remainder of the dissertation is composed of three chapters which mention the property enhancement of electrically tunable colloidal photonic crystals intensively. In chapter 2, the effects of adopting high refractive index nanoparticle in tunable photonic crystal are discussed. Simulated results showing relationships between bandwidth and peak wavelength of the crystalline colloidal arrays of various refractive index nanoparticles in water obtained from the FDTD method. And the tunable reflectance of polystyrene core/titania shell (PS/TiO2) nanostructures with narrow size distribution under the electric field is described. PS/TiO2 structure was prepared to make the materials with lower density, higher reflective index for fast response and high reflectance intensity. Then the optoelectric behavior of highly concentrated and mildly charged TiO2 nanoparticle colloids under external electric field is reported. Reflectance intensity change at the certain photonic bandgap and its irreversible switching characteristics are observed and discussed. Through computer aided numerical analysis simulation, the gradient spacing between charged particles are predicted which seems singularly appropriated the experimental results. Also the experimental evidence that the charged nanoparticle can play the role as a major charge carrier in non-aqueous liquid is described. Its short range moving and stacking on electrode cause the capacitive current flow and charge accumulation like ions in liquid state. In chapter 3, the color tuning behavior and switching stability of an electrically-tunable colloidal photonic crystal system were studied with particular focus on the electrochemical aspects to explore the reason why switching stability is decreased. The number of effective color tuning cycle was limited due to generation of unwanted ions by electrolysis of the water medium during electrical switching. By introducing larger electrochemical potential window electrodes, such as conductive diamond-like carbon or boron-doped diamond, the switching stability was appreciably enhanced through reducing the number of ions generated. In chapter 4, the switching stability enhancement of an electrically-tunable colloidal photonic crystal system was studied. To overcome this drawback, functional interlayer was introduced on the ITO electrode surface to prevent the ion generation and/or diffusion. Modification of the ITO-coated glass electrode surface to include a passivation layer increased the switching reliability

however, this methodology resulted in a reduction of the color tuning range. The thin ion blocking layer of over-coating on the ITO surface prevented the ion diffusion from electrode to colloidal particles and allowed increased number of cycles of stable color-tuning switching from red to green over 800 times, which is the best result ever reported for a tunable photonic crystal whose Δλ can be manipulated more than 100 nm.