Electrochromism, an emerging energy conversion technology, has attracted immense interest due to its various applications including bistable displays, optical filters, variable optical attenuators, optical switches, and energy-efficient smart windows. Currently, the major drawback for the development of electrochromism is the slow switching speed, especially in inorganic electrochromic materials. The slow switching speed is mainly attributed to slow reaction kinetics of the dense inorganic electrochromic films. As such, an efficient design of nanostructured electrochromic materials is a key strategy to attain a rapid switching speed for their real-world applications. In this review article, we summarize the classifications of electrochromic materials, including inorganic materials (e.g., transition metal oxides, Prussian blue, and polyoxometalates), organic materials (e.g., polymers, covalent organic frameworks, and viologens), inorganic-organic hybrids, and plasmonic materials. We also discuss the electrochromic properties and synthesis methods for various nanostructured inorganic electrochromic materials depending on structure/morphology engineering, doping techniques, and crystal phase design. Finally, we outline the major challenges to be solved and discuss the outlooks and our perspectives for the development of high-performance nanostructured electrochromic materials.

Static plasmonic metal–insulator–nanohole (MIN) cavities have been shown to create high chromaticity spectral colors for display applications. While on–off switching of said devices has been demonstrated, introducing active control over the spectral color of a single cavity is an ongoing challenge. Electrochromic oxides such as tungsten oxide (WO3) offer the possibility to tune their refractive index (2.1–1.8) and extinction (0–0.5) upon ion insertion, allowing active control over resonance conditions for MIN based devices. In combination with the dynamic change in the WO3 layer, the utilization of a plasmonic superstructure allows creation of well-defined spectral reflection of the nanocavity. Here, we employ inorganic, electrochromic WO3 as the tunable dielectric in a MIN nanocavity, resulting in a theoretically achievable resonance wavelength modulation from 601 to 505 nm, while maintaining 35% of reflectance intensity. Experimental values for the spectral modulation result in a 64 nm shift of peak wavelength with high reproducibility and fast switching speed. Remarkably, the introduced device shows electrochemical stability over 100 switching cycles while most of the intercalated charge can be regained (91.1%), leading to low power consumption (5.6 mW/cm–2).

Replacing a single atom of a host semiconductor nanocrystal with a functional dopant can introduce completely new properties potentially valuable for "solotronic" information-processing applications. Here, we report successful doping of colloidal CdSe quantum dots with a very small number of manganese ions-down to the ultimate limit of one. Single-particle spectroscopy reveals spectral fingerprints of the spin-spin interactions between individual dopants and quantum-dot excitons. Spectrally well-resolved emission peaks are observed that can be related to the discrete spin projections of individual Mn ions. In agreement with theoretical predictions, the exchange splittings are enhanced by more than an order of magnitude in these quantum dots compared to their epitaxial counterparts, opening a path for solotronic applications at elevated temperatures.

We present LEDs based on WS2 monolayers in a p–n architecture, generating red electroluminescence with luminance up to 50 cd m−2.

Despite recent advances in hydrogel electrolytes for flexible electrochemical energy storage, ion conductors still exhibit some major shortcomings including low ionic conductivity and short lifetimes. As such, for applications in electrochromic batteries, a transparent, highly conductive electrolyte based on a dimethyl-sulfoxide (DMSO) modified polyacrylamide (PAM) hydrogel is being developed and implemented in a dual-ion Zn 2+ /Al 3+ electrochromic device consisting of a Zn anode and WO 3 cathode.Gelation in a DMSO : H 2 O mixed solvent leads to highly increased electrolyte retention in the hydrogel and prolonged life time for ionic conduction. The hydrogel-based electrochromic device offers a specific charge capacity of 16.9 mAh cm À2 at a high current density of 200 mA cm À2 while retaining 100% coulombic efficiency over 200 charge-discharge cycles. While the DMSO-modified electrolyte shows ionic conductivities up to 27 mS cm À1 at room temperature, the formation of DMSO : H 2 O nanoclusters enables ionic conduction even at temperatures as low as À15 C and retention of ionic conduction over more than 4 weeks. Furthermore, the electrochromic WO 3 cathode gives the device a controllable absorption with up to 80% change in transparency. Based on low-cost, earth abundant materials like W (tungsten), Zn (zinc) and Al (aluminum) and a scalable fabrication process, the introduced hydrogel-based electrochromic device shows great potential for next-generation flexible and wearable energy storage systems.

Plasmochromics, the interaction of plasmons with an electrochromic material, have spawned a new class of active plasmonic devices. By introducing electrochromic materials into the plasmon's dielectric environment, plasmons can be actively manipulated. We introduce inorganic WO 3 and ion conducting LiNbO 3 layers as the core materials in a solid-state plasmochromic waveguide (PCWG) to demonstrate light modulation in a nanoplasmonic waveguide. The PCWG takes advantage of the high plasmonic loss at the high field located at the WO 3 /Au interface, where the Li + ions are intercalated into a thin WO 3 plasmon modulating layer. Through careful PCWG design, the direction for ion diffusion and plasmon propagation are decoupled, leading to enhanced modulation depth and fast EC switching times. We show that at a bias voltage of 2.5 V, the fabricated PCWG modulator achieves modulation depths as high as 20 and 38 dB for 10 and 20 μm long devices, respectively.

The presented work demonstrates an innovative method to overcome electrolyte restrictions for electrodeposited tungsten oxide (WO3) electrochromic electrodes. By self-assembly of a phosphonic acid protection layer on top of the WO3 electrode, the cycle life of a WO3 electrode in aqueous electrolytes of potassium (KCl) and lithium chloride (LiCl) is dramatically enhanced. Based on the hydrophobic nature of the self-assembled monolayer (SAM), the modification allows for ion intercalation while it prevents etching of the electrode. The cycle life of a WO3 electrode in 1 M KCl increased from under 100 to over 1000 cycles between −0.6 and 0.6 V versus Ag/AgCl. Furthermore, the current–voltage cycling and simultaneous optical transparency measurements show that a WO3 electrode having a self-assembled monolayer of an n-dodecylphosphonic acid exhibits no degradation through detachment of the electrochromic material. Our results suggest that SAM modification of electrochromic oxides is a promising new route toward long lifetime electrochromic devices even in hostile electrolyte environments.

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