Imagine an extremely thin film of lightweight flexible material which, when placed flat on your table, is transformed as though by magic into a high-definition screen on which you can read a map or play a videogame. And that you then pick up the same rectangle of film and place it against the glass of a window and watch a movie on it, or a TV program.
Before long something similar may well be normal: a group of researchers from the Universities of Oxford and Exeter have invented a Phase-change film made of a Germanium Antimony Tellurium alloy (Ge2Sb2Te5 or more simply GST) which can react to an external stimulus such as heat, light or electricity by switching between two different solid states with different optical and electronic properties.
In particular, when a GST film reacts to an electric current it passes from an amorphous state to a crystalline state, changing its refractive index and color. The electronic impulses are transmitted to the GST film by two layers of indium oxide and tin, which act as transparent electrodes.
At the actual state of research, the achievable dimensions of such display screens are relatively small, but this is to some extent compensated by the possibility of using lithographic techniques to structure the film (whose thickness can be set anywhere between 5 and 50 nanometers) in pixelated form, where the pixels too are measured in nanometers: this offers a higher degree of resolution than the vast majority of other existing technologies.
The passage to and fro between one state and another in this material happens incredibly quickly (less than 100 nanoseconds), resulting in image-refreshing 20,000 times faster than today’s latest television screen technologies. Thus we are dealing with doubly improved visual quality, due both to the degree of High Definition and of Refresh Speed… quicker image refreshing in moving images means less flicker and therefore less eye fatigue.
GST screens can also have another advantage: the dual stability of its material means that one image can be maintained for long periods even when the display is in stand-by. This characteristic could be exploited to dramatically reduce energy consumption in functions not requiring quick image refreshment, such as e-book devices: to display a fixed image would consume zero power.
There are of course various problems to be overcome before this film can be produced on an industrial scale, such as perfecting the connections between the electronic components which ensure that each pixel receives the right electrical impulse at the right moment, and extending the range of colors and especially the range of greys… this last a particularly difficult issue with phase-changing films.
This Oxford and Exeter University study paves the way for a new class of ‘smart’ devices that will be both electrically and optically active, and its results are exciting because they represent the first application of electro-optical properties in phase-changing materials.
Beyond the spectacular potential results of the ability of Phase-change materials (PCMs) to contribute to the development of screen technology based on ultra-thin sensitive optoelectronic film, these exciting materials offer many other thrilling possibilities thanks to their ability to store data and to perform arithmetic and logical operations, and they could also potentially mimic the functionality of photoreceptor cells in the human eye, thus opening up new fields of research in human–machine interaction, multi-functional glasses, contact lenses and synthetic retina devices.
This research project, bearing the title An optoelectronic framework enabled by low-dimensional phase-change films was carried out by Peiman Hosseini, C. David Wright and Harish Bhaskaran, and was published in Nature in July 2014.