Cheap plasmonics to enable full-colour fast switching e-paper

Cheap plasmonics to enable full-colour fast switching e-paper

Technology News |
Looking into ways to design high-resolution low cost display panels, researchers from Sandia National Laboratories and the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology have re-visited the concept of e-paper by combining inexpensive electrochromic polymers with plasmonic structures.
By Julien Happich


Prof. A. Alec Talin and his colleagues presented their findings in the journal Nature Communications with the paper “High-contrast and fast electrochromic switching enabled by plasmonics”, showing a candidate technology for the cheap manufacture of thin full colour displays with resolutions two orders of magnitude higher than today’s high-definition displays, while boasting switching speeds in the range of milliseconds.

Instead of having to lay multiple layers and colour-specific electrochromic polymers sandwiched with dedicated control electrodes, Talin relied on Au and Al metallic nanoslit arrays (serving as the plasmonic structures) conformally coated with two ordinary electrochromic polymers, PANI and PolyProDOT-Me2. The arrays of vertical nanoscale slits (each slit only 60nm deep and 250nm wide with a pitch of 500nm) are perpendicular to the direction of the incoming light. When light hits the aluminium nanoslits, it is converted into surface plasmon polaritons (SPPs), which are electromagnetic waves containing frequencies of the visible spectrum that travel along the dielectric interfaces – here, of aluminium and electrochromic polymer.

Schematic diagram of a plasmonic electrochromic electrode incorporating (a) Au-nanoslit array and (b) reference planar electrochromic electrode. The pitch of the Au-nanoslit array is 500 nm. (c) Chemical structures of PANI in the reduced and oxidized form. SEM images of the fabricated Au-nanoslit electrode (d) before and (e) after deposition of a PANI to a thickness d≈15 nm. (f) Magnified SEM image from e. Scale bars, 300 nm (d,e). Scale bar, 100 nm (f).

The plasmonic structure would turn into a deep black by simply applying a tiny electric current across the top of the slit, cutting off the entering light and the SPPs within milliseconds. When the current was flicked off, light frequencies passed through the slits and instantly turned on the pixel.

Here, because the pitch of the slits determines the wavelengths of the light being transmitted down through the array, by changing the nanoslit patterns, the researchers were able to demonstrate a whole array of switchable colours using the same electrochromic polymers.

(c,d) Optical transmission spectra of PolyProDOT-Me2-coated Al-nanoslit structures with respective values of slit period P=240, 270, 300, 330, 360 and 390 nm, along with corresponding optical micrographs of device areas imaged in transmission. Transmission spectra and micrographs for (c) ON and (d) OFF states of the polymer are displayed, respectively.

The paper concludes that using such simple plasmonics considerably simplifies the fabrication process and could easily be extended to large areas for mass production, using a flexible substrate through techniques such as roll-to-roll nanoimprint lithography or nanotransfer printing.

For their experiments, the researchers created colour pixels about 10×10μm each, but Talin pointed eeNews Europe to an earlier paper “An Integrated Electrochromic Nanoplasmonic Optical Switch” published in Nano Letters, demonstrating that a single slit device could effective switch light on or off.

“However, in order to use the slits array to define colour, several slits with regular spacing on the order of the optical wavelength are necessary, which would require dimensions of around 1 micron or larger” Talin wrote in an email exchange.

When asked if he is envisaging the commercialization of such high definition colour electrochromic displays, either through IP licensing, or through a spin-off company, Talin answered: “Currently neither myself nor any of my co-authors are actively pursuing commercialization of our plasmonic-electrochromic display concept. However, we would be excited to engage any company interested in pursuing this technology, including IP licensing. Although I have considered several possible commercial names for the plasmonic-electrochromic displays, none have been trademarked at this point”.

Talking about the technological transfer necessary to take these findings from the lab to commercial success, the researcher said: “The principal technological barriers to widespread adoption, in my opinion, include the demonstration of a reflective version which uses ambient light, then the integration of pixel arrays with drivers, replacing the liquid electrolyte we used in our paper with a solid polymer electrolyte or inorganic electrolyte and the use of a roll-to-roll compatible manufacturing method, such as nanoimprint lithography, to fabricate the nanoslit arrays”.

“None of these represent barriers that require new scientific breakthroughs, rather mostly engineering and development” he concluded.


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