In 2015, Ali Javey, Professor of Electrical Engineering and Computer Science at UC Berkley published a paper in Science demonstrating that monolayer semiconductors are capable of emitting bright light under the right conditions.
Now, a group of researchers from the same institution succeeded in breaking through a number of fundamental barriers in utilising LED technology on monolayer semiconductors and built a bright-light emitting device that is millimetres wide and fully transparent when turned off.
“The materials are so thin and flexible that the device can be made transparent and can conform to curved surfaces,” said Der-Hsien Lien, a postdoctoral fellow at UC Berkeley and a co-author on the paper in the journal Nature Communications.
At first pass, the device was no thicker than a human hair, whereas now its width spans several millimetres, which gives promise to “invisible” displays on walls and windows, and even more fanciful applications like light-emitting tattoos.
While regular LEDs require two contact points – one for injecting negatively charged particles, and another for the opposite – the new device circumvents it by way of a clever engineering trick.
In a feat of ingenuity, the Berkley team laid the semiconductor monolayer on an insulator, and placed electrodes both on the monolayer and underneath the insulator, which allowed them to apply an AC signal across the insulator.
Right at the moment when the AC signal reverts in polarity, both positive and negative charges are present at the same time in the semiconductor, which produces a spike of bright light.
The researchers have also shown the mechanism to work just as well in four different monolayer materials, each of which emits a different colour of light.
For now, the team estimates their invention to be about 1 percent efficient (which is difficult to do in systems such as the present one), compared to 25 to 30 percent efficiency seen in commercial LEDs.
“A lot of work remains to be done and a number of challenges need to be overcome to further advance the technology for practical applications,” Javey said. “However, this is one step forward by presenting a device architecture for easy injection of both charges into monolayer semiconductors.”