In a paper titled “Scalable electrochromic nanopixels using plasmonics” published in Science Advances, the researchers describe electrochromic nanoparticles with a multilayered plasmonic composite architecture filled with a dielectric spacer. The nanoparticles-on-mirror (NpoM) as they call them (because the particles are coated onto a metallic mirror), strongly confine light within the individual gaps to the underlying mirror, producing extremely localized cavity resonances and precisely tuned colour scattering independent and insensitive to the angle and polarization of incident light.
The electrochromic eNPoMs consist of colloidal gold nanoparticles (Au NPs) encapsulated in a conductive polymer shell (polyaniline or PANI for short), which are then cast onto a planar gold mirror. The shell thickness obtained through the encapsulation process defines the critical gap spacing to the underlying mirror as well as the spacing between neighboring particles (which the authors say reduces their optical coupling and inhibits aggregation). That is why even when designed at large scale, each individual NPoM acts as an independent active nanopixel.
By switching the charge state of the entire PANI shell and changing its redox state (sweeping a voltage from −0.2 to 0.6 V across the Au mirror as the working electrode), the authors were able to rapidly shift the resonant scattering colour of the eNPoM across wavelength ranges in excess of 100nm. They calculated that each active nanopixel only requires about 0.2 fJ of energy for each 1 nm shift in wavelength while achieving commercial video rates (over 50Hz), and these dynamics were proven to scale from the single nanoparticle level to multi-centimetre scale ultra-thin films.
The researchers had their prototypes pixel cells running for over three months at power densities below 300μW/cm2, or about 10 times lower than for commercial e-paper, they point in the paper. Meanwhile, because each nanoparticle acts as an individual pixel, theoretical pixel densities reach in excess of 109 pixels per inch, they argue, although differentiating addressable areas would require larger cell constructs and circuitry. The researchers also expect their electrochromic plasmonic pixels to be compatible with flexible substrates for large-scale roll-to-roll manufacturing on polymer films.