Structural Color Manipulation Using Tunable Photonic Crystals with Enhanced Switching Reliability

Abstract

535wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.de tuning in the whole visible regime through large changes in the particle-particle separation originating from the electrophoretic movement of the charged particles while maintaining the ordered crystal structure under the external fi elds. [ 10 ] In spite of these merits, there is no report that considered these systems applicable for reversible switching for practical application.Although de-ionized water is the most suitable dispersion medium for producing high quality periodic crystalline structures with enhanced refl ectance in these systems, the electrolysis of water molecules at the indium tin oxide (ITO) surface produces signifi cant amounts of H + and OH − ions as by-products at the cathode and anode, respectively. Therefore, the diffusion of those ions into colloidal arrays substantially limits the tuning stability and range because this phenomena induces decreased thickness of electrical double layer on the surface of the colloidal nanoparticles. To solve this problem, ITO surface was modifi ed by various dielectric insulator passivation layers, including SiO 2 , SiN, polyimide, and parylene that can block electrochemical reactions at the electrodes (unpublished results). However, this methodology resulted in a substantial reduction of the color tuning range due to decreases in voltage applied to the colloidal dispersion because of the large amount of additional impedance introduced by the passivation layer. Considering the volume resistivity of these commonly used passivation materials (∼10 16 Ω m), required thickness of the passivation layer should be at the sub-nm scale in order not to decrease the voltage applied signifi cantly in colloidal dispersion. Preparation of such a thin and uniform fi lm without any defected area is extremely diffi cult to achieve.Here we report our effort for achieving enhanced tuning stability of the colloidal photonic crystal composed of long-range ordered crystalline colloidal arrays through modifying the ITO electrode with ion exchange resins. The thin layer of over-coating on the ITO surface 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 photo nic crystal whose Δλ can be manipulated more than 100 nm.Figure 1 (a) depicts the device structure of a unit pixel and the concept of improving the stability of photonic structural color tuning through blocking ion diffusion from both electrodes by coating the electrode surface with a layer of ion-exchange resin. A colloidal dispersion composed of monodisperse 138 nm-diameter polystyrene (PS) nanoparticles dispersed in deionized water was injected between two electrodes. The strong negative charge of the nanoparticles and their thick electrical double layer induce the self-assembly of the long-range ordered colloids through electrostatic repulsion, which dominates the

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