Abstract
Plasmonic structural colors have recently received a lot of attention. For many applications there is a need to actively tune the colors after preparing the nanostructures, preferably with as strong changes in the optical response as possible. However, to date, there is a lack of systematic investigations on how to enhance contrast in electrically induced color modulation. In this work we implement electrochromic films with plasmonic metasurfaces and compare systematically organic and inorganic materials, with the primary aim to maximize brightness and contrast in a reflective color display. We show nanostructures with good chromaticity and high polarization-insensitive reflectivity (-90%) that are electrochemically stable in a nonaqueous solvent. Methods are evaluated for reliable and uniform electropolymerization of the conductive polymer dimethylpropylenedioxythiophene (PProDOTMe2) on gold. The resulting organic films are well-described by Lambert-Beer formalism, and the highest achievable contrast is easily determined in transmission mode. The optical properties of the inorganic option (WO3) require full Fresnel models due to thin film interference, and the film thickness must be carefully selected in order to maintain the chromaticity of the metasurfaces. Still, the optimized fully inorganic device reaches the highest contrast of approximately 60% reflectivity change for all primary colors. The switching time is about an order of magnitude faster for the organic films (hundreds of ms). The bistability is very long (hours) for the inorganic devices and comparable for the polymers, which makes the power consumption essentially zero for maintaining the same state. Finally, we show that switching of the primary colors in optimized devices (both organic and inorganic) provides almost twice as high brightness and contrast compared to existing reflective display technologies with RGB subpixels created by color filters.
Highlights
To date, there is a lack of systematic investigations on how to enhance contrast in electrically induced color modulation
Structural color generation was achieved by a combination of Fabry−Peŕ ot modes and plasmonic effects using a triple layer consisting of a metal mirror, an insulator with varied thickness, and a thin top metal film with nanoholes.[3,27]
We have shown primary colored metasurfaces with high reflectivity that are electrochemically stable in a nonaqueous electrolyte
Summary
Structural color generation was achieved by a combination of Fabry−Peŕ ot modes (cavity resonances) and plasmonic effects using a triple layer consisting of a metal mirror, an insulator with varied thickness, and a thin top metal film with nanoholes.[3,27] The structures were prepared over large areas by colloidal lithography. The WO3 thickness that gives optimal contrast on a given metasurface was instead selected based on Fresnel models excluding the nanoholes (Figures S6 and S7) Such calculations do not include the plasmonic contribution to the coloration for blue and green samples, they enabled us to predict which thickness of WO3 would make the thin film interference effects of all layers (four in total) favorable. In order to provide colors, electrophoretic displays and LCDs introduce color filters to generate RGB subpixels (Figure 7A) Such displays do exist, but they are generally not found on the market, which arguably is due to their poor image quality in terms of brightness and contrast.[10] For instance, E-ink markets the Triton, yet such e-reader devices are not found among suppliers (at the time of writing).[10] The metasurfaces are already red, green, or blue, as prepared, and their reflectivity is high as well as independent of polarization. The image quality will be good enough for many applications, especially image display in bright environments
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