Nano-pixels: a display revolution in the making?

How the thickness of the bottom ITO layer (t) affects the pixel’s reflected color. Credit: Oxford University

Phase change flexible films in reflective mode created in the Advanced Nanoscale Engineering Laboratory at the University of Oxford, exploiting thin film interference effects. Photo Credit: Isis Innovation

Today's pixels, even the smallest ones, are measured in µm. As an example a very high resolution smart phone with a 400dpi density has a pixel size of about 50µm (the iPhone pixels are 76µm). That's good enough since our eye resolution is less than 300dpi! Why would you want to have more than your eye can see?

Well, can you see the billion of stars in our galaxy? No, your eye (and mine is even worse) has not enough resolution. But if you get your eye behind a telescope all of a sudden you can see many more stars! The telescope does the trick. Now suppose that you take a magnifying lens and you place it in front of your cell phone screen. Are you going to see an image with many more details that you see with your naked eye? Actually no (or very few more indeed) since the difference between what you see without the magnifying lens (300 dpi) and what you see with it (400dpi) is not that much (remember that for our brain to really appreciate the difference you would need to move to a resolution at least twice better...).

The problem of pixel size vs resolution gets worse as the screen estate gets smaller. If you consider a Google Glass, the size of the screen is probably 10 times smaller than the one of your smartphone screen. But the density of pixels is basically the same, constrained by the 50µm minimum size. To let you view a virtually larger screen Google Glass use a magnifying lens. This works well in the sense that you perceive the screen as being larger than the one you have on your smartphone but the resolution drops dramatically, which means the quality of the image is poor.

The only way to increase the quality of the image is to increase the resolution, hence to decrease the pixel size. Unfortunately, today’s technology does not support that.

Here is where the news coming from the Oxford University can change the rules of the game.

A team of researchers discovered that by inserting a 7 nm layer of a phase change material made of germanium, antimony and tellurium, called for short GST, between two electrode layers made of indium tin oxide (ITO) they can get pixels 300nm x 300nm in size that can be switched on and off, each one a coloured dot making up an incredibly dense display. The pixel size (if you do the math) is 27,777 times smaller than the one of the smallest pixels used today in LCD screens. However, notice that this does not imply a resolution 27,777 times bigger: you need to take into account the separation between pixels that as pixels get smaller weights more and more. However, depending of the pixels layout architecture, you should get an increase of three orders of magnitude. Not bad!

Interestingly, the researchers found out that by varying the size of the bottom electrode they can change the color of the pixel. The process to create the layer relies on bombarding a substance containing the atoms needed. This makes some atoms bounce off and they end up deposited on the thin film used as substrate, like a Mylar sheet 200 nm thick.

This makes the new screen usable for smart glasses, foldable screens, windshield displays and synthetic retinas.

The screen does not need to be refreshed, if a pixel has to display the same color it does not need to be activated periodically, it keeps its colour, like an e-ink pixel. This lead to lower power consumption. Also, the material involved, as well as the manufacturing process, are cheap and that should lead to low cost products. When? End of this decade is my pick.

Author - Roberto Saracco

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