Active color switching in an inorganic photonic omission glass

The interactions of light with nanostructures can cause interesting macroscopic optical effects.  One is that materials with regular, periodic (or even statistically quasi-periodic) structure can preferentially reflect narrow bands of the visible spectrum. This gives rise to physical color for the macroscopic object, rather than one that is defined by a chemical absorption process. Physical color is found in nature in the color of butterfly wings and beetle shells, for example. This effect can also be synthesized in artificial materials with well-defined periodicity. In recent work, we have demonstrated nanostructured photonic glass materials where the physical color can be tailored in self-assembled electrodes. We use this as a strategy for trapping light in nanostructured, composite photoelectrodes used for driving chemical reactions with light. These electrodes are fabricated by using atomic layer deposition (ALD) of ultra-thin films (< 50 nm) in a polymer-inorganic nanocomposite. The properties of the ALD film (thickness, refractive index) and the composite (relative fraction of polymer [pores] and inorganic particles) determine the optical properties of the structure, including its physical color. Here, we describe methods to use the same basic structure for materials with actively switchable physical color. In simulations, we show that changing the refractive index of the empty space in the material (from air [n=1.0] to water [n=1.33] to glycerol [n=1.47] or higher, for example) can shift the peak of reflectance by up to 150 nm in the same material. This approach can activate reversible color changes in the same nanostructure, from blue to orange-red, for example. Based on these simulations and experimental characterization of these photonic glass materials, we describe the integration of this active color changing functionality into experimental systems. We will describe an approach to the activation of color change in these materials, based on the flow of immiscible liquids of varying refractive index. We will then propose other potential mechanisms for activating changes to the reflectance based on the electrochemically active interface.