Quasi‐amorphous colloidal structures exhibiting angle‐independent tunable photonic colors in response to the electric stimuli. Moderately polydisperse colloidal Fe3O4@SiO2 nanoparticles dispersed in organic solvents exclusively form quasi‐amorphous photonic materials at sufficiently high concentrations, and which reversibly reflect incident light in visible region in response to the relatively low bias voltages.

We report a facile way of fabricating hybrid organic/inorganic photonic gels by selective swelling and subsequent infiltration of SiO(2) into one type of lamellar microdomain previously self-assembled from modest-molecular-weight block copolymers. Transparent, in-plane lamellar films were first prepared by assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), and subsequently the P2VP domains were swollen with a selective solvent, methanol. The swollen structures were then fixated by synthesizing SiO(2) nanoparticles within P2VP domains. The resulting frozen photonic gels (f-photonic gels) exhibited strong reflective colors with stop bands across the visible region of wavelengths.

Polystyrene‐block‐poly(2‐vinyl pyridine) (PS‐b‐P2VP) block copolymer photonic gels are fabricated that exhibit controllable optical hysteresis in response to a cyclic pH sweep. The optical hysteresis is tuned by controlling the ion‐pairing affinity between various anions and the protonated pyridinium ions on the P2VP block, which is highly dependent on the hydration energy of the ions, the dielectric constant of the solvent, and the ionic strength of the medium. The pH coercivity defining the magnitude of hysteresis of the photonic gels could be varied from 0.26 to 7.4. Photonic gel films with strong optical hysteresis can serve as wet photonic memory films where information can be cyclically recorded and erased at least 15 times and maintained for at least 96 h. The memory colors can be further tuned by selection of the copolymer molecular weight.

Materials that display hysteresis, such as ferromagnetic or ferroelectric materials, have been extensively investigated for their potential engineering applications towards electromagnetic memory devices and switches. While similar hysteretic volume phase transitions are observed in many soft organic materials, including hydrogels [1] and biomaterials, [2] it has not been well exploited but is often considered as problematic because it complicates the calibration procedure and affects signal reproducibility in many conventional applications of hydrogels in sensors and drug delivery.[3] However, such hysteretic volume phase transitions of hydrogels can be utilized in a positive way when they are combined with other functional materials and carefully tuned to exhibit strong bistability. For example, Kim et al. recently reported wet photonic gel memory pixels by combining the hysteresis of hydrogels with photonic crystals.[4] Block copolymer photonic gels consisting of alternating glassy polystyrene (PS) and swellable poly(2-vinyl pyridine) (P2VP) gel layers showed pH-dependent photonic stop bands. The optical responses of the photonic gels varied with the direction of pH changes. This hysteretic optical response of photonic gels was able to be tuned and further optimized by controlling ion-pairing affinity between the protonated pyridine groups and their counteranions.Extending the previous work, herein we report electrochemically controllable nonvolatile photonic pixels and their applications towards e-papers. Our approach is clearly different from the nonvolatile photonic ink demonstrated by Ozin and co-workers [5] in the sense of using hydrogel hysteresis instead of using redox reaction of metallopolymers. Electroactive photonic pixels were achieved by coupling the hysteretic optical properties of PS-b-P2VP block copolymer photonic gels to the electrochemical cells, where the pH gradient can be tuned. Similar to other conventional hydrogels, [6] our photonic gels show volume expansion and contraction in response to the applied electric field, and accordingly exhibit various photonic colors. [7] We assumed that electroactive nonvolatile photonic gels can be achieved when the volume transition of gel layers was tuned to exhibit strong hysteresis with changes of electrochemically driven pH gradient.The electroactive photonic pixel comprises a simple electrochemical cell consisting of two transparent electrodes separated by 1 mm thick spacer and electrolytes (Figure 1 a). The block copolymer photonic gel films were coated on the working electrode. The block copolymer photonic gel films were prepared as previously described. [4,[7][8] Briefly, in-plane oriented lamellar films were first prepared by spin casting Figure 1. Schematic structure of photonic pixel and proposed mechanisms for gel swelling and deswelling by an electrically induced pH gradient.