"Nano-pixels" hold huge potential for flexible, low-power, high-res screens
By Darren Quick
July 10, 2014
The Retina displays featured on Apple's iPhone 4 and 5 models pack a pixel density of 326 ppi, with individual pixels measuring 78 micrometers. That might seem plenty good enough given the average human eye is unable to differentiate between the individual pixels, but scientists in the UK have now developed technology that could lead to extremely high-resolution displays that put such pixel densities to shame.
Led by Oxford University scientists, a research team has created a prototype device that features pixels just 30 x 30 nanometers in size. The high resolution potential of the technology was discovered while the team was exploring the link between the electrical and optical properties of phase change materials (PCMs) that can switch from an amorphous to a crystalline state.
By sandwiching a seven-nanometer thick layer of the PCM Germanium-Antimony-Tellurium (Ge2Sb2Te5 or GST) between two layers of transparent electrodes made of indium tin oxide (ITO), the scientists discovered they could "draw" still images within the sandwich "stack" using an atomic force microscope. They then found that the "nano-pixels" could be switched on and off electronically, creating colored dots that could be used as the basis for an extremely high-resolution display.
"We didn't set out to invent a new kind of display," said Professor 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' made up of layers just a few nanometers thick. 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 color of the image."
But extremely high-resolution isn't the only impressive quality of the technology. The layers that make up the GST sandwich are created using a sputtering technique, which would allow them to be deposited as thin films on extremely thin and flexible substrates.
"We have already demonstrated that the technique works on flexible Mylar sheets around 200 nanometres thick," said Professor Bhaskaran. "This makes them potentially useful for 'smart' glasses, foldable screens, windshield displays, and even synthetic retinas that mimic the abilities of photoreceptor cells in the human eye."
Additionally, like the power-saving E-Ink displays used in eReaders, all the pixels don't need to be constantly refreshed, as is the case with LCD screens. This means that only those pixels that actually change would need to be refreshed, significantly lowering energy consumption.
Furthermore, the scientists believe displays based on the technology could switch between a color e-Reader mode that saves power by relying on reflected light, and a video display mode that makes use of a backlight.
The team believes displays made using the technology would be easy to manufacture from inexpensive materials, while also being reliable as they would be solid-state. Amongst the wide variety of potential applications, the team highlights the technology's benefits for use in smart glasses, where the nano-pixels would allow even images projected at a larger size would offer very high resolution.
"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," says Professor David Wright from the Department of Engineering at the University of Exeter, who is co-author of a paper detailing the technology. "But no one before thought of combining their electrical and optical functionality to provide entirely new kinds of non-volatile, high-resolution, electronic color displays – so our work is a real breakthrough."
Although the work is still in its early stages, the team has filed a patent for the technology and is fielding discussions on the displays with companies and investors through Isis Innovation, Oxford University's technology commercialization company.
The team's paper appears in the journal Nature.
Source: Oxford University