Alloy primed for phase change from CD to TV

CDs and DVDs might be in terminal decline, but one of their key alloys may get a new lease of life in flexible displays and electronics. Researchers in the UK have electrically switched between phases of the Ge₂Sb₂Te₅ or GST alloy, changing its colours to form pictures, including on flexible films.

‘It’s non-volatile – once you change the phase it actually stays,’ says University of Oxford’s Harish Bhaskaran, who led the work. ‘That’s good for energy consumption, not just in displays but in a host of other applications that can utilise such optoelectronic effects.’ In displays alone the technology could enable artificial retinas, semi-transparent ‘smart’ glasses and contact lenses, the scientists suggest.  

GST is used for rewritable CDs and DVDs, where laser pulses switch it between amorphous and crystalline phases, changing its reflectivity. Bhaskaran previously encountered such phase change materials at IBM Research in Zurich, Switzerland, where they studied triggering the transition – which also affects the alloy’s resistance – using electrical heating for computer memory. ‘I was curious about what would happen at the interface of optical and electronic properties,’ Bhaskaran tells Chemistry World.

Bhaskaran’s team has now laid foundations for uniting these usually properties that are usually handled separately. The researchers sandwiched GST between transparent indium tin oxide (ITO) electrodes, one of whose thickness they vary to determine the colours the film switches between. The thin GST filling – around 7 nm thick – provides high contrast levels and can be switched with little electrical power.

Starting from uniform coloured films, the scientists used an atomic force microscope (AFM) to apply current and create pictures, mimicking a pixelated array. They’ve made such images on opaque silicon, transparent quartz and transparent, flexible biaxially oriented polyethylene terephthalate. But the AFM can only switch the material in the amorphous-to-crystalline direction, so Bhaskaran’s team constructed single pixel ‘crossbar’ devices that allow back-and-forth electronic switching. 

Delia Milliron, who studies phase change memory materials at University of Texas, Austin, US, likes how optical disc materials have been ‘cleverly repurposed to create images with nanoscale pixels’.  ‘I am excited to see what new technologies will be developed to take advantage of these visually striking advances,’ she adds.

The next step is a full prototype display that does not require an AFM, which Bhaskaran hopes to make by the end of 2015. ‘To go from here to the next stage requires quite a lot of investment,’ he admits.