Smallest Pixels Ever Created for Color-Changing Buildings

Nanopixels formed from gold nanoparticles encapsulated in a conductive polymer shell. (Source: NanoPhotonics Cambridge, H.-H Jeong, J. Peng)

The smallest pixels yet created – a million times smaller than those in smartphones, made by trapping particles of light under tiny rocks of gold – could be used for new types of large-scale flexible displays, big enough to cover entire buildings. The colour pixels, developed by a team of scientists led by the Uni­versity of Cambridge, are compatible with roll-to-roll fabrication on flexible plastic films, drama­tically reducing their production cost.

It has been a long-held dream to mimic the colour-changing skin of octopus or squid, allowing people or objects to disappear into the natural background, but making large-area flexible display screens is still prohi­bitively expensive because they are constructed from highly precise multiple layers. At the center of the pixels developed by the Cambridge scientists is a tiny particle of gold a few billionths of a metre across. The grain sits on top of a reflective surface, trapping light in the gap in between. Surrounding each grain is a thin sticky coating which changes chemically when elec­trically switched, causing the pixel to change colour across the spectrum.

The team of scientists, from different disci­plines including physics, chemistry and manu­facturing, made the pixels by coating vats of golden grains with an active polymer, poly­aniline, and then spraying them onto flexible mirror-coated plastic, to dramatically drive down production cost. The pixels can be seen in bright sunlight and because they do not need constant power to keep their set colour, have an energy per­formance that make large areas feasible and sustainable. “We started by washing them over aluminized food packets, but then found aerosol spraying is faster,” said Hyeon-Ho Jeong from Cambridge’s Cavendish Labora­tory.

“These are not the normal tools of nano­technology, but this sort of radical approach is needed to make sustainable techno­logies feasible,” said Jeremy J. Baumberg of the Nano­Photonics Centre at Cambridge’s Cavendish Labora­tory, who led the research. “The strange physics of light on the nanoscale allows it to be switched, even if less than a tenth of the film is coated with our active pixels. That’s because the apparent size of each pixel for light is many times larger than their physical area when using these resonant gold archi­tectures.”

The pixels could enable a host of new application possi­bilities such as building-sized display screens, archi­tecture which can switch off solar heat load, active camouflage clothing and coatings, as well as tiny indicators for coming internet-of-things devices. The team are currently working at improving the colour range and are looking for partners to develop the tech­nology further. (Source: U. Cambridge)

Reference: J. Peng et al.: Scalable electrochromic nanopixels using plasmonics, Sci. Adv. 5, eaaw2205 (2019); DOI: 10.1126/sciadv.aaw2205

Link: NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK

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