Coexistence of Electrochromism and Bipolar Nonvolatile Memory in a Single Viologen

Viologens are fascinating redox-active organic compounds that have been widely explored in electrochromic devices (ECDs). However, the combination of electrochromic and resistive random-access memory in a single viologen remains unexplored. We report the coexistence of bistate electrochromic and single-resistor (1R) memory functions in a novel viologen. A high-performance electrochromic function is achieved by combining viologen (BzV2+2PF6) with polythiophene (P3HT), enabling a "push-pull" electronic effect due to the efficient intermolecular charge transfer in response to an applied bias. The ECDs show high coloration efficiency (ca. 1150 ± 10 cm2 C-1), subsecond switching time, good cycle stability (>103 switching cycles), and low-bias operation (±1.5 V). The ECDs require low power for switching the color states (55 μW cm-2 for magenta and 141 μW cm-2 for blue color). The random-access memory devices (p+2-Si/BzV2+2PF6/Al) exhibit distinct low and high resistive states with an ON/OFF ratio of ∼103, bipolar and nonvolatile characteristics that manifest good performances, and "Write"-"Read"-"Erase" (WRE) functions. The charge conduction mechanism of the RRAM device is elucidated by the Poole-Frenkel model where SET and RESET states arise at a low transition voltage (VT = ±1.7 V). Device statistics and performance parameters for both electrochromic and memory devices are compared with the literature data. Our findings on electrochromism and nonvolatile memory originated in the same viologen could boost the development of multifunctional, smart, wearable, flexible, and low-cost optoelectronic devices.

Color-Switchable Subwavelength Organic Light-Emitting Antennas

Future photonic devices require efficient, multifunctional, electrically driven light sources with directional emission properties and subwavelength dimensions. Electrically driven plasmonic nanoantennas have been demonstrated as enabling technology. Here, we present the concept of a nanoscale organic light-emitting antenna (OLEA) as a color- and directionality-switchable point source. The device consists of laterally arranged electrically contacted gold nanoantennas with their gap filled by the organic semiconductor zinc phthalocyanine (ZnPc). Since ZnPc shows preferred hole conduction in combination with gold, the recombination zone relocates depending on the polarity of the applied voltage and couples selectively to either of the two antennas. Thereby, the emission characteristics of the device also depend on polarity. Contrary to large-area OLEDs where recombination at metal contacts significantly contributes to losses, our ultracompact OLEA structures facilitate efficient radiation into the far-field rendering transparent electrodes obsolete. We envision OLEA structures to serve as wavelength-scale pixels with tunable color and directionality for advanced display applications.

Facile Preparation of Oxygen-Vacancy-Engineered MoOx Nanostructures for Photoreversible Switching Systems

Photochromic materials have attracted increasing attention. Here, we report a novel photo-reversible color switching system based on oxygen-vacancy-engineered MoO_x nanostructures with water/N-methyl-2-pyrrolidone (NMP) as solvents. In this work, the system rapidly changed from colorless to blue under UV irradiation (360-400 nm) and slowly recovered its colorless state under visible light irradiation. The obtained oxygen vacancy-engineered MoO_x nanostructures exhibited good repeatability, chemical stability, and cycling stability. Upon UV light irradiation, H⁺ was intercalated into layered MoO_x nanostructures and the Mo⁶⁺ concentration in the H_xMoO_x decreased, while the Mo⁵⁺ concentration increased and increased oxygen vacancies changed the color to blue. Then, it recovered its original color slowly without UV light irradiation. What is more, the system was highly sensitive to UV light even on cloudy days. Compared with other reported photochromic materials, the system in this study has the advantage of facile preparation and provides new insights for the development of photochromic materials without dyes.

Robust Scalable-Manufactured Smart Fabric Surfaces Based on Azobenzene-Containing Maleimide Copolymers for Rewritable Information Storage and Hydrogen Fluoride Visual Sensor

Functionalized materials with reversible color switching are highly attractive in many application fields, especially as rewritable media for information storage. It is critical yet challenging to develop a cost-effective strategy for the fabrication of stimulus-responsive chromogenic systems. Herein, we present a versatile dip-coating approach to fabricate robust smart textile with acid/base-driven chromotropic capability. Owing to the introduction of novel maleimide-based copolymers bearing azobenzene derivative moieties, smart textiles possess rapid color switching between yellow and orange-red, which is triggered by acid-base stimulations with the resulting reversible protonation/deprotonation of maleimide moieties. As a proof of concept of the application of the smart textile for high-performance rewritable media, various rewritable elaborate patterns can be fast trifluoroacetic acid-printed/triethylamine-erased (within 20 s) with excellent cycling stability and long legible duration (>30 days). Meanwhile, the smart textile can be employed as a visual sensor for the detection of hydrogen fluoride gas leakage. It is highlighted that the as-prepared robust smart textiles with superhydrophobic surfaces have excellent antifouling properties and chemical/mechanical stabilities, which can tolerate harsh environmental conditions and repetitive mechanical deformation. The robust smart textiles with simple low-cost large-scale production may find more advanced potential applications besides information storage and sensors demonstrated.

Photoresponsive and Polarization-Sensitive Structural Colors from Cellulose/Liquid Crystal Nanophotonic Structures

Cellulose nanocrystals (CNCs) possess the ability to form helical periodic structures that generate structural colors. Due to the helicity, such self-assembled cellulose structures preferentially reflect left-handed circularly polarized light of certain colors, while they remain transparent to right-handed circularly polarized light. This study shows that combination with a liquid crystal enables modulation of the optical response to obtain light reflection of both handedness but with reversed spectral profiles. As a result, the nanophotonic systems provide vibrant structural colors that are tunable via the incident light polarization. The results are attributed to the liquid crystal aligning on the CNC/glucose film, to form a birefringent layer that twists the incident light polarization before interaction with the chiral cellulose nanocomposite. Using a photoresponsive liquid crystal, this effect can further be turned off by exposure to UV light, which switches the nematic liquid crystal into a nonbirefringent isotropic phase. The study highlights the potential of hybrid cellulose systems to create self-assembled yet advanced photoresponsive and polarization-tunable nanophotonics.

UV/NIR-Light-Triggered Rapid and Reversible Color Switching for Rewritable Smart Fabrics

Remote, rapid, and ink-free printing/erasure on fabrics has great potential to revolutionize specialized clothing in numerous applications including fashion/aesthetic and security fields, but the construction of such smart fabrics has not been realized due to underlying obstacles in obtaining suitable photoreversible color-switching systems (PCSS). To address this problem, we have prepared TiO_{2- x} nanorods as photocatalytic and photothermal component. With redox dyes as reversible color indicators and hydroxyethyl cellulose (HEC) as polymer matrix, TiO_{2- x}/dye/HEC-based PCSS is coated on poly(dimethylsiloxane)-treated cotton fabric. Under 365 nm light irradiation, discoloration occurs in 180 s, resulting from the efficient photocatalytic reduction of the dye. On the contrary, when the colorless fabric is irradiated by 808 nm light, recoloration occurs in a very short time (∼100 s), far lower than the traditional heating mode (30-8 min at 90-150 °C). This rapid recoloration should be attributed to the localized high temperature (164.3-184.5 °C) induced by photothermal effect of TiO_{2- x}. Particularly, when TiO_{2- x}/dye/HEC-based PCSS is extended to coat commercial clothes (such as T-shirts), red/green/blue figures/letters can be rapidly and remotely printed by UV-light pen and then erased by near-infrared light, with high cycle stability. Therefore, such rewritable smart fabric represents an attractive alternative to regular clothes in meeting the increasing aesthetic or camouflage needs.

Double-Sided Electrochromic Device Based on Metal-Organic Frameworks

Devices displaying controllably tunable optical properties through an applied voltage are attractive for smart glass, mirrors, and displays. Electrochromic material development aims to decrease power consumption while increasing the variety of attainable colors, their brilliance, and their longevity. We report the first electrochromic device constructed from metal organic frameworks (MOFs). Two MOF films, HKUST-1 and ZnMOF-74, are assembled so that the oxidation of one corresponds to the reduction of the other, allowing the two sides of the device to simultaneously change color. These MOF films exhibit cycling stability unrivaled by other MOFs and a significant optical contrast in a lithium-based electrolyte. HKUST-1 reversibly changed from bright blue to light blue and ZnMOF-74 from yellow to brown. The electrochromic device associates the two MOF films via a PMMA-lithium based electrolyte membrane. The color-switching of these MOFs does not arise from an organic-linker redox reaction, signaling unexplored possibilities for electrochromic MOF-based materials.

Color Depth Modulation and Resolution in Phase-Change Material Nanodisplays

The demonstration of non-volatile color-depth modulation in novel phase change nanodisplays allowing for continuous "grayscale" images with ultrahigh resolution and low dimensionality is described. These results hold promise for a new generation of bistable, ultrahigh-resolution, and flexible display technologies, while allowing for other potential applications in nanophotonics and optoelectronics.

Color-switchable, emission-enhanced fluorescence realized by engineering C-dot@C-dot nanoparticles

This paper reports the preparation and properties of color-switchable fluorescent carbon nanodots (C-dots). C-dots that emit dark turquoise and green-yellow fluorescence under 365 nm UV illumination were obtained from the hydrothermal decomposition of citric acid. Dark green fluorescent C-dots were obtained by conjugating prepared C-dots to form C-dot@C-dot nanoparticles. After successful conjugation of the C-dots, the fluorescence emission undergoes a blue-shift of nearly 20 nm (∼0.15 eV) under UV excitation at 370 nm. The C-dots emit goldenrod, green-yellow, and gold light under excitation at 455 nm, which shows that the prepared C-dots are color-switchable. Furthermore, conjugation of the C-dots results in enhanced, red-shifted absorption of the π-π* transition of the aromatic sp(2) domains due to the conjugated π-electron system. N incorporation in the carbon structure leads to a degree of dipoles for all the aromatic sp(2) bonds. The enhanced absorption in a wide range from 226 to 601 nm indicates extended conjugation in the C-dot@C-dot structure. The time-resolved average lifetimes for the three different types of C-dots prepared in this study are 7.10, 7.65, and 4.07 ns. The radiative rate (reduced decay lifetime) increases when the C-dots are conjugated in the C-dot@C-dot nanoparticles, leading to the enhanced fluorescence emission. The fluorescence emission of the C-dot@C-dot nanoparticles can be used in applications such as flow cytometry and cell imaging.

Luminescence color switching of supramolecular assemblies of discrete molecular decanuclear gold(I) sulfido complexes

A series of discrete decanuclear gold(I) μ(3)-sulfido complexes with alkyl chains of various lengths on the aminodiphosphine ligands, [Au(10){Ph(2)PN(C(n)H2(n+1))PPh(2)}(4)(μ(3)-S)(4)](ClO(4))(2), has been synthesized and characterized. These complexes have been shown to form supramolecular nanoaggregate assemblies upon solvent modulation. The photoluminescence (PL) colors of the nanoaggregates can be switched from green to yellow to red by varying the solvent systems from which they are formed. The PL color variation was investigated and correlated with the nanostructured morphological transformation from the spherical shape to the cube as observed by transmission electron microscopy and scanning electron microscopy. Such variations in PL colors have not been observed in their analogous complexes with short alkyl chains, suggesting that the long alkyl chains would play a key role in governing the supramolecular nanoaggregate assembly and the emission properties of the decanuclear gold(I) sulfido complexes. The long hydrophobic alkyl chains are believed to induce the formation of supramolecular nanoaggregate assemblies with different morphologies and packing densities under different solvent systems, leading to a change in the extent of Au(I)-Au(I) interactions, rigidity, and emission properties.