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For Phase-change Materials, A New Use In Digital Displays

Materials Science: Inorganics long used for data storage
show promise for digital displays 

by Mitch Jacoby
July 11, 2014 | A version of this story appeared in Volume 92, Issue 28

Credit: Isis Innovation/Oxford U.
Controlling the structure and thickness of a device containing flexible films and transparent electrodes imparts tunable color variations.
The colorful, flexible films seen here, which may be used in future electronic displays, feature a nanometer thick film of a phase changing inorganic material.
Credit: Isis Innovation/Oxford U.
Controlling the structure and thickness of a device containing flexible films and transparent electrodes imparts tunable color variations.

By simultaneously exploiting a material’s electronic, optical, and structural properties, researchers in the U.K. have taken a first step toward developing a new type of electronic display technology. The advance may lead to low-cost, flexible, color display devices that outperform today’s technology in terms of resolution, power consumption, and response time (Nature 2014, DOI: 10.1038/nature13487). Future applications may include smart eyeglasses and contact-lens-type displays.

At the heart of the study lies a well-known phase-change material, an alloy of germanium, antimony, and tellurium—Ge2Sb2Te5 (GST). Phase-change materials respond to stimuli such as heat or an electric pulse by switching between two solid states—amorphous and crystalline. This type of structural change, which has been studied in detail, underpins data storage in rewritable CDs, DVDs, and other types of commercial data storage devices.

The change in these materials’ lattice structure is accompanied by other changes—for example, in optical absorption, refractive index, and electrical conductivity. As it turns out, when a material such as GST is incorporated into a device designed to capitalize on these simultaneous changes, the material functions as an advanced display medium.

Oxford University materials scientists Harish Bhaskaran and Peiman Hosseini didn’t set out to invent a new type of display, Bhaskaran says. Rather, the team, which includes the University of Exeter’s C. David Wright, hit upon the display idea while probing unexplored relationships between GST’s structural, electrical, and optical properties.

To test GST’s usefulness as a display medium, the group sandwiched the material between a pair of transparent conductors made of indium tin oxide (ITO) and made several unexpected discoveries. For example, they found that by electrically pulsing a sandwich containing a GST layer measuring just a few nanometers in thickness, they could create high-contrast images. They also observed that reducing the GST layer’s thickness improved contrast. In addition, the group discovered that by controlling GST’s crystallinity, and tuning the thickness of one of the ITO layers, they could create a rich color palette.

In a subsequent series of tests, the team fashioned a large array of 300 x 300 nm pixels and used a scanning probe tip to demonstrate that the pixels could be switched individually. They formed such arrays on reflective and transparent substrates supported on flexible surfaces, all of which are key steps in making ultra-high-resolution foldable display devices.

Before this technology can enter the display market, several issues need to be addressed, says Dirk J. Broer of Eindhoven University of Technology, in the Netherlands. Commenting in the same issue of Nature, Broer notes, for example, that the device must be fully integrated electronically, with each pixel controlled in perfect timing with microscopic transistors. The devices must also be able to produce an even broader range of colors.

Broer adds, “If and when these issues are appropriately addressed, this new display concept may find use in applications that are outside the reach of current display technologies.”



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