British researchers create building blocks for new high-resolution display technology
Phase-change film 'sandwich' shows potential for thin, flexible nano-pixel displays
A team led by scientists at Oxford University has shown the possibility of combining optical and electrical control in ultrathin phase-change films to create pixels just a few hundred nanometers across. They think their research, published in this week's Nature, could pave the way for extremely high-resolution and low-energy thin, flexible displays for applications such as 'smart' glasses, synthetic retinas, and foldable screens.
Phase-change materials (materials that can change from an amorphous to a crystalline state) such as the alloy germanium antimony tellurium (GST) have for years been used in optical storage media such as rewritable DVDs. More recently, such materials have been investigated as candidates for the next generation of electrically operated non-volatile memories. In this latest study, the researchers found that by sandwiching a seven nanometer thick layer of GST between two layers of a transparent electrode made from indium tin oxide (ITO) they could use a tiny current to 'draw' images within the sandwich 'stack'. The layers of the GST sandwich are created using a sputtering technique where a target is bombarded with high-energy particles so that atoms from the target are deposited onto another material as a thin film.
Initially still images were created using an atomic force microscope but the team went on to demonstrate that such tiny 'stacks' can be turned into prototype pixel-like devices. These 'nano-pixels' - just 300 by 300 nanometres in size - can be electrically switched 'on and off' at will, creating the coloured dots that would form the building blocks of an extremely high-resolution display technology.
Whilst the work is still in its early stages, the Oxford team has filed a patent on the discovery with the help of Isis Innovation, Oxford University's technology commercialisation company. Isis is now discussing the displays with companies who are interested in assessing the technology, and with investors.
"We didn't set out to invent a new kind of display," said Harish Bhaskaran of Oxford University's Department of Materials, who led the research. "We were exploring the relationship between the electrical and optical properties of phase change materials and then had the idea of creating this GST 'sandwich. We found that not only were we able to create images in the stack but, to our surprise, thinner layers of GST actually gave us better contrast. We also discovered that altering the size of the bottom electrode layer enabled us to change the colour of the image.'
He added: "Because the layers that make up our devices can be deposited as thin films they can be incorporated into very thin flexible materials - we have already demonstrated that the technique works on flexible Mylar sheets around 200 nanometres thick."
Peiman Hosseini of Oxford University's Department of Materials, first author of the paper, said: "One of the advantages of our design is that, unlike most conventional LCD screens, there would be no need to constantly refresh all pixels, you would only have to refresh those pixels that actually change. This means that any display based on this technology would have extremely low energy consumption."
The research suggests that flexible paper-thin displays based on the technology could have the capacity to switch between a power-saving 'colour e-reader mode', and a backlit display capable of showing video. Such displays could be created using cheap materials and, because they would be solid-state, promise to be reliable and easy to manufacture.
David Wright of the Department of Engineering at the University of Exeter, co-author of the paper, said: "Along with many other researchers around the world we have been looking into the use of these GST materials for memory applications for many years, but no one before thought of combining their electrical and optical functionality to provide entirely new kinds of non-volatile, high-resolution, electronic colour displays - so our work is a real breakthrough."
A report of the research, entitled 'An optoelectronic framework enabled by low-dimensional phase change films' by Hosseini et al is published in Nature 511, 206-211 (10 July 2014) doi:10.1038/nature13487