including the use of liquid crystals, [7] suspended particles, [8] photonic crystals, [9] and chromogenic materials driven by temperature, [10] light, [11] mechanical force, [6,12] magnetic field, [13] and electrical field. [14] However, it still faces great challenges in practical uses of the smart optical materials, especially in smart windows, for example, due to the complicated process, high cost, and low efficiency for the preparation and fabrication of such smart windows. Therefore, it is highly desirable to develop simple and effective strategies to design new smart optical materials with rapid responsive time, large transmittance modulation, and good repeatability, which may open up new opportunities for applications in large-area smart windows.Wrinkling (or buckling) is a ubiquitous phenomenon that occurs in both natural [15] and artificial systems, [12a,16] with dimensions across a wide range of length scale from meters down to nanometers. Artificial wrinkles can be generated by introducing compressive stresses to a skin layer/elastomer substrate bilayer film through various approaches, such as thermal stress, [17] mechanical stress, [18] capillary force, [19] light-driven photoisomerization, [20] and osmotic pressure. [21] Owing to their spontaneous nature, versatility, and tunability or even total reversibility in response to external stimuli, surface wrinkling has provided an effective platform to create smart, controllable, and responsive surface topography and subsequently to dynamically tune functionality, which inspired various important applications including flexible electronic devices, [22] tunable diffraction gratings, [23] microlens arrays, [24] dry adhesives, [25] dynamic scaffolds, [26] and optical devices. [27] Since mechanical force as an external stimuli has many advantages such as simple and independent control of the amount, direction, and timing of strain, [28] some recent studies have demonstrated that mechanoresponsive surface wrinkles offer an effective and repeatable strategy to dynamically tune optical properties on-demand by changing the surface topography in responsive to mechanical force. For example, the light transmittance of mechanoresponsive surface wrinkles for smart windows has been dynamically tuned by means of micro-and nanopillar arrays on wrinkled elastomers, [28,29] harnessing Preparation of surface wrinkles on a skin layer/elastomer substrate bilayer film has attracted extensive attention because of their unique and broad applications in flexible electronic devices, tunable diffraction gratings, and smart windows. However, it still remains a great challenge to develop a general strategy for fabricating mechanoresponsive surface wrinkles using various skin layer materials with wide tunability of wrinkles' wavelength and amplitude, large optical modulation range, rapid optical switching rate, and high stability. Here, a general, simple, and cost-effective strategy is reported, through uniaxially stretching and subsequently releasing skin layer/elastomer substrate...

Dynamic manipulation of optical properties through the reversible assembly of plasmonic nanoparticles offers great opportunities for practical applications in many fields. The previous success, however, has been limited to Au nanoparticles. Reversible assembly and plasmonic tuning of Ag nanoparticles (AgNPs) have remained a significant challenge due to difficulty in finding an appropriate surface agent that can effectively stabilize the particle surface and control their interactions. Here, we overcome the challenge by developing a limited-ligand-protection (LLP) strategy for introducing poly­(acrylic acid) with precisely controlled coverage to the AgNP surface to not only sufficiently stabilize the nanoparticles but also enable effective control over the surface charge and particle interaction through pH variation. The as-synthesized AgNPs can be reversibly assembled and disassembled and accordingly display broadly tunable coupling of plasmonic properties. Compared to the Au-based system, the success in the reversible assembly of AgNPs represents a significant step toward practical applications such as colorimetric pressure sensing because they offer many advantages, including broader spectral tuning range, higher color contrast, a one-pot process, and low materials and production cost. This work also highlights LLP as a new avenue for controlling the interparticle forces, their reversible assembly, and dynamic coupling of physical properties.

Visible-light-responsive reversible color-switching systems are attractive to many applications because visible light has superior penetration and causes far less damage to organic molecules than UV. Herein, we report that self-doping of SnO nanocrystals with Sn red-shifts their absorption to the visible region and simultaneously produces oxygen vacancies, which can effectively scavenge photogenerated holes and thus enable the color switching of redox dyes using visible light. Wavelength-selective switching can also be achieved by coupling the photocatalytic activity of the SnO NCs with the color-switching kinetics of different redox dyes. The fast light response enables the further fabrication of a solid film that can be repeatedly written on using a visible laser pen or projection printing through a photomask. This discovery represents a big step forward towards practical applications, especially in areas in which safety issues and photodamage by UV light are of concern.

Photoreversible color switching that can change colors with fast response and high stability is urgently desired in color-on-demand applications. Yet, developing such materials has long been a significant challenge. In this work, a strategy based on the integration of TiO nanoparticle (NP) photocatalytic color switching of redox dyes and poly(vinyl alcohol) gel matrix could produce robust and flexible photochromic gels (FPGs) that exhibit fast light-responsive time and high photoreversible stability. Benefited by the soft network structures and monomeric form of redox dyes in the FPG maintained by poly(vinyl alcohol) and ethylene glycol molecules, as well as enhanced photoreductive activity of TiO NPs modified by both surface ligands and oxygen vacancies, the FPG exhibits long photoreversible switching cycles (≥50 times), decoloration in a short period of less than 8 s upon UV illumination, and recoloration in 16 min in ambient air and rapidly in 140 s upon near-infrared light illumination. Consequently, the excellent photoreversible color switching of the FPGs is highly applicable as both self-erasing rewritable media and colorimetric oxygen indicators. We believe that the current systems represent a big step forward toward practical applications, such as time-sensitive information storage, colorimetric oxygen sensor, and potentially many other technologies.

The dynamic dual-stimuli-responsive surface wrinkles on a bilayer film with high bistability are unattainable and attractive for the applications of smart windows and optical displays. Here, we report a new strategy in developing moisture and temperature dual-responsive surface wrinkles on the polyvinyl alcohol/polydimethylsiloxane (PVA/PDMS) bilayer film by rationally designing the modulus changes of the PVA skin layer upon moisture and temperature. By optimizing the thickness of the PVA layer to 4.5 μm, the as-prepared surface wrinkles show long-awaited properties, such as fast response time, excellent reversibility without degradation of optical contrast, and high light transmittance modulation, which greatly outperforms the reported surface wrinkles. Moreover, the surface wrinkles on the bilayer film remain highly bistable without additional energy consumption for more than five months in ambient room conditions both in the opaque and transparent states. These promising dual-stimuli-responsive surface wrinkles on bilayer films hold great promises for various applications triggered by moisture and temperature, such as smart windows and rewritable optical displays.