Transparent electrical conductors would be a huge asset in many fields of optoelectronics. Circuits made of such conductors could control liquid crystal displays in optical devices without the need for unsightly conventional chips. However, most clear substances are electrical insulators.

Now practical invisible circuits are a step closer thanks to the material developed by Hayashi’s group. Based on calcium oxide and aluminium oxide, its crystal lattice consists of ‘cages’ that have a positive charge. When the researchers heated these crystals in hydrogen, they found that hydride ions – which have a single negative charge – became trapped in the cages. The crystals were still clear after cooling, and electrical tests showed that they were still insulators.

But when the team shone ultraviolet light onto the substance, they found that its electrical conductivity leapt by a factor of 109, and remained at this level even after the light was switched off. The researchers realized that this effect would allow invisible circuits to be created: by shining ultraviolet light onto the material through a mask, electrical ‘wires’ and electrodes could be forged while the unexposed regions between them would remain insulating.

To establish the origin of the effect, Hayashi and co-workers measured the electrical properties of the material as it was heated and cooled. They found that its conductivity dropped sharply at a temperature of 320°C, but returned to its original level upon cooling. But at a temperature of 550°C, the material released its trapped hydrogen – and permanently lost its sensitivity to light.

This led Hayashi and colleagues to believe that the material owes its electrical characteristics to its trapped hydride ions. The researchers suggest that the ultraviolet light makes these ions eject their ‘extra’ electrons, which are then attracted to the positively charged empty cages in the crystal. But this attraction is so weak that the electrons can hop from cage to cage, creating a ‘sea’ of electrons that allows the crystal to conduct electricity.

As well as applications in optoelectronics, Hayashi’s group believes that the new material could be used to develop high-density optical memory. They also speculate that a similar approach could work for other ‘main group’ metal oxides.