A new discovery will make it possible to create pixels just a few hundred nanometers across that 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.

A team led by Oxford University scientists found that by sandwiching a seven-nanometer-thick layer of a phase-change material called GST between two layers of a transparent electrode, they could use a tiny current to draw images within the sandwich “stack.”

GST is the alloy Ge2Sb2Te5 (Germanium-Antimony-Tellurium) sandwiched between electrode layers made of indium tin oxide (ITO).

Such tiny stacks can be turned into prototype pixel-like devices. These ‘nano-pixels’ — just 300 by 300 nanometers in size — can be electrically switched on and off at will, creating the colored dots that would form the building blocks of an extremely high-resolution display technology.

A report of the research is published in Nature.

“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,” said said Professor Harish Bhaskaran. “We also discovered that altering the size of the bottom electrode layer enabled us to change the color of the image.”

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.

“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 nanometers thick,” said 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.”

One of the advantages of the 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 (static pixels remain as they were). 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 “color 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. The tiny nano-pixels make it ideal for applications, such as smart glasses, where an image would be projected at a larger size since even enlarged, they would offer very high resolution.

Support was provided by UK’s Engineering and Physical Sciences Research Council (EPSRC).

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Abstract of Nature paper

The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states—amorphous and crystalline—in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young’s modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge₂Sb₂Te₅) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent ‘smart’ glasses, ‘smart’ contact lenses and artificial retina devices.