2015年经济学人 显示屏 新的电子图案显示方法（在线收听）

Display screens

Going through a phase

A new way to create electronic images

Wave of the future?

LIQUID-CRYSTAL displays are a familiar and ubiquitous technology. But if Harish Bhaskaran of Oxford University is right, their days may be numbered.

The essential feature of LCDs is that the pixels in them switch between amorphous and crystal-like phases, which changes their optical properties.

In a paper in this week's Nature, Dr Bhaskaran and his colleagues describe something 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 open up 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 are also being considered for use in memory chips, because the switch between amorphous and crystalline 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 be made to change colour, too.

This property would make them suitable both for displays that rely on reflected light (so-called electronic paper) and the older, backlit sort that rely on transmitted light.

The resulting displays would be thin and could be flexible if printed on the right material—increasing the range of applications they might be used in.

And they would consume little power, since energy need be used 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 the crystalline and the amorphous phases of this substance are stable at any temperature a device is likely to experience, and thin films of it are more or less transparent.

The power needed to effect the phase change could be fed to individual pixels by electrodes made of indium tin oxide, which is also transparent.

The colour of a pixel would depend not only on its phase, but also on its thickness, which would affect the way light waves being reflected within it interfere with one another,

cancelling out some frequencies while amplifying others. (The effect is similar to the creation of colours by a thin layer of oil on a puddle.)

Generally, the alloy layer needs to be thinner than 20 nanometres 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 a tip a few nanometres across,

to apply appropriate electric currents in a grid pattern across the film's surface. This grid mimicked an array 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 be done in principle, Dr Bhaskaran has made a single pixel this way.

Whether his idea will get off the lab bench and into the shops remains to be seen.

It is by no means the only suggestion around for a new generation of display screens. But it looks plausible.