Episode 4: Tunable electronic paper developed at human visual resolution

In this podcast episode, MRS Bulletin’s Laura Leay interviews Kunli Xiong from Uppsala University in Sweden about his development of metapixels as small as 560 nm, conducive for small video displays that can be located close to the human eye. Instead of using emissive pixels, Xiong uses electronic paper made up of tungsten trioxide nanodisks. By tuning the diameter and spacing of the nanodisks, certain wavelengths of light can be selectively reflected, leading to a color display. The degree of light reflection can be modulated by making use of the material’s electrochromic properties. This work was published in a recent issue of Nature. 

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. As the pixel density of digital video displays increases, pixels are getting smaller. Research has led to a new way of producing metapixels as small as 560 nanometers. This advance paves the way for small displays that can be located close to the eye, perhaps within a virtual reality headset where resolution could be good enough to blend the virtual world seamlessly with reality. It could also lead to video displays that, compared to current technology, are more comfortable to look at for extended periods and so could improve eye health for those of us that spend more and more time using video devices. Instead of using emissive pixels, the new development uses electronic paper made up of tungsten trioxide nanodisks. By tuning the diameter and spacing of the nanodisks, certain wavelengths of light can be selectively reflected, leading to a color display. The degree of light reflection can be modulated by making use of the material’s electrochromic properties. The small size of the pixels means that the pixel count is similar to the number of photoreceptors in the human eye, making it ideal for development of virtual reality devices.

KUNLI XIONG: Because I will use a totally different method compared to traditional VR devices, I need to have some material with very thin thickness to give me very high resolution.

LAURA LEAY: That was assistant professor Kunli Xiong from Uppsala University in Sweden. The tungsten trioxide layer is around 100 nanometers thick and the refresh rate is much higher than traditional electronic papers, achieving in excess of 25 Hertz. And, because energy is only consumed when the pixels are changed, offers much lower energy consumption when compared to emissive displays. To create the metapixels, the nanostructures in the tungsten trioxide layer were patterned using electron beam lithography. Once the smallest dimension for the three primary color metapxiels was determined, they could be merged to create a full color display.  

KUNLI XIONG: Quite a big challenge I solved is: I merged the RGB pixels together. To use LED technology to create a very small lamp or pixel is not hard; some people have already shown that it can reach to 100 nm. But, for LED, you have to deposit different materials to get the RGB colors and then as the lamp gets super small, you have to merge the small RGB lamps together. This becomes super hard because you have to sputter in three different materials and then, let’s say, slip in some substrate, and then try to merge them perfectly aligned on the substrate. It’s super hard but here we show a very good image where we can perfectly merge the RGB colors or CMY colors together.

LAURA LEAY: Further research will concentrate on the stability of the device and continue to improve the optical properties as well as control of pixel intensity. The research will also consider how to scale the technology over a larger area; examples from the initial proof-of-concept were around a few millimeters across. This development has enormous potential for a future where smart lenses and augmented reality are commonplace.

KUNLI XIONG: When the display screen has revolutionary increase in resolution or decrease in size, and becomes closer and closer to your eyes, it changes a lot our lifestyle. If you imagine in the future people wear smart glass or smart lens in front of your eyes and everything you see is digitalized or interacting with you, it will interact your life and how you think. I hope this technology can help a lot of people, especially for online education or online operation, and also that people can totally release their imagination to build their own world. 

LAURA LEAY: This work was published in a recent issue of Nature. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.