LIQUID-CRYSTAL displays are a familiar and ubiquitous technology. But if Harish Bhaskaran ofOxford University is right, their days may be numbered. The essential feature of LCDs is thatthe pixels in them switch between amorphous and crystal-like phases, which changes theiroptical properties. In a paper in this week's Nature, Dr Bhaskaran and his colleagues describesomething similar in a solid material. At the least, that would stop the messy abstract-impressionist patterns which happen when an LCD is dropped too hard. At most, it might openup a new range of applications, from clothes that change colour to dimmable windscreens.
Solid phase-change materials are already used to store data in optical memory disks. They arealso being considered for use in memory chips, because the switch between amorphous andcrystalline states alters their electrical properties in ways that can store electronic bits of data.Dr Bhaskaran, though, has shown that thin enough films of the right sort of material can bemade to change colour, too.
This property would make them suitable both for displays that rely on reflected light (so-calledelectronic paper) and the older, backlit sort that rely on transmitted light. The resulting displayswould be thin and could be flexible if printed on the right material—increasing the range ofapplications they might be used in. And they would consume little power, since energy need beused only when a pixel has to be flipped from one phase to another.
The researchers' material of choice is an alloy of germanium, antimony and tellurium. Both thecrystalline and the amorphous phases of this substance are stable at any temperature adevice is likely to experience, and thin films of it are more or less transparent. The powerneeded to effect the phase change could be fed to individual pixels by electrodes made ofindium tin oxide, which is also transparent.
The colour of a pixel would depend not only on its phase, but also on its thickness, whichwould affect the way light waves being reflected within it interfere with one another, cancellingout some frequencies while amplifying others. (The effect is similar to the creation of colours bya thin layer of oil on a puddle.) Generally, the alloy layer needs to be thinner than 20nanometres for that to happen.
To demonstrate their idea, the researchers sprayed films of their alloy onto pieces of silicon,quartz and plastic. They then used a device called an atomic-force microscope, which has atip a few nanometres across, to apply appropriate electric currents in a grid pattern acrossthe film's surface. This grid mimicked an of pixels, creating a stable pattern. The result,as their picture of a Japanese wave shows, is a recognisable image—if not, yet, a perfect one.
Adding the indium-tin-oxide electrodes is a more complicated process, but to show it can bedone in principle, Dr Bhaskaran has made a single pixel this way. Whether his idea will get offthe lab bench and into the shops remains to be seen. It is by no means the only suggestionaround for a new generation of display screens. But it looks plausible.