Advances in electrochromic device technology through the exploitation of nanophotonic and nanoplasmonic effects.

Research regarding electrochromic (EC) materials, such materials that change their color upon application of an electrochemical stimulus, has been conducted for centuries. However, most recently, increasing efforts have been put into developing novel solutions to utilize these on-off switching materials in advanced nanoplasmonic and nanophotonic devices. Due to the significant change in dielectric properties of oxides such as WO3, NiO, Mn2O3 and conducting polymers like PEDOT:PSS and PANI, EC materials have transcended beyond simple smart window applications and are now found in plasmonic devices for full-color displays and enhanced modulation transmission and photonic devices with ultra-high on-off ratios and sensing abilities. Advancements in nanophotonic ECDs have further decreased EC switching speed by several orders of magnitude, allowing integration in real-time measurement and lab-on-chip applications. The EC nature of such nanoscale devices promises low energy consumption with low operating voltages paired with bistability and long lifetimes. We summarize these novel approaches to EC device design, lay out the current short comings and draw a path forward for future utilization.

On-chip high ion sensitivity electrochromic nanophotonic light modulator.

Since the discovery of electrochromism, the prospect of employing various electrochromic materials for smart window glass, variable reflectivity mirrors, and large-area displays has been the main drive for such an intriguing phenomenon. However, with advances in nanofabrication and the emergence of improved electrochromic materials offering reversible large changes in dielectric properties upon electrically induced redox reactions, the application strategies are starting to encompass the field of nanophotonics and nanoplasmonics. Herein, a novel strategy is proposed and demonstrated for offering both ultrahigh light modulation depth and high sensitivity ion detection in a single nanophotonic waveguiding platform. By using WO3 to ionically-drive dynamic light control via modulating the refractive index and the losses within the waveguide at ±1.5 V, ultrahigh optical modulation depth of 106, rapid response speed of <0.56 s, long cyclic life, and very sensitive Na+ ion detection ability in 1 mM-1 M concentration, are achieved within a volume of a few µm3. It is envisioned that our introduction of such a multifunctional electrochromic nanophotonic waveguide platform will stimulate and promote further efforts toward fundamental research on technologically promising on-chip integrated next-generation nanophotonic and nanoplasmonic devices for various niche applications.

Nanoscale All-Solid-State Plasmochromic Waveguide Nonresonant Modulator.

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 WO3 and ion conducting LiNbO3 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 WO3/Au interface, where the Li+ ions are intercalated into a thin WO3 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.

Plasmochromic Nanocavity Dynamic Light Color Switching.

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).

Electrochemical Stability Enhancement of Electrochromic Tungsten Oxide by Self-Assembly of a Phosphonate Protection Layer.

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.

WS2 monolayer-based light-emitting devices in a vertical p-n architecture.

2D semiconductors represent an exciting new material class with great potential for optoelectronic devices. In particular, WS2 monolayers are promising candidates for light-emitting devices (LEDs) due to their direct band gap with efficient recombination in the red spectral range. Here, we present a novel LED architecture by embedding exfoliated WS2 monolayer flakes into a vertical p-n layout using organic p- and inorganic n-supporting layers. Laser lithography was applied to define the current path perpendicular to the WS2 flake. The devices exhibit rectifying behavior and emit room temperature electroluminescence with luminance up to 50 cd m-2 in the red spectral range.

Correction: Rechargeable Zn2+/Al3+ dual-ion electrochromic device with long life time utilizing dimethyl sulfoxide (DMSO)-nanocluster modified hydrogel electrolytes.

[This corrects the article DOI: 10.1039/C9RA06785J.].

Giant Excitonic Exchange Splittings at Zero Field in Single Colloidal CdSe Quantum Dots Doped with Individual Mn2+ Impurities.

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 Mn2+ 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.