Scientists at the University of California Berkeley (UCB) have developed a new light emission device that is full transparent, with a light emitting layer that is just three atoms thick. The group sees applications in walls and in windows as well as light emitting tattoos. Their study was published March 26 in the journal Nature Communications. Der-Hsien Lien, a postdoctoral fellow at UC Berkeley said,
“The materials are so thin and flexible that the device can be made transparent and can conform to curved surfaces”.
The device was developed in the laboratory of Ali Javey, professor of Electrical Engineering and Computer Sciences at Berkeley. In 2015, Javey’s lab published research in the journal Science showing that monolayer semiconductors are capable of emitting bright light, but stopped short of building a light-emitting device. The new work in Nature Communications overcame fundamental barriers in using LED technology on monolayer semiconductors, allowing for such devices to be scaled from sizes smaller than the width of a human hair up to several millimeters. That means that researchers can keep the thickness small, but make the lateral dimensions (width and length) large, so that the light intensity can be high.
Commercial LEDs consist of a semiconductor material that is electrically injected with positive and negative charges, which produce light when they meet. Typically, two contact points are used in a semiconductor-based light emitting device; one for injecting negatively charged particles and one injecting positively charged particles. Making contacts that can efficiently inject these charges is a fundamental challenge for LEDs, and it is particularly challenging for monolayer semiconductors since there is so little material to work with.
The Berkeley research team engineered a way to circumvent this challenge by designing a new device that only requires one contact on the semiconductor. By laying the semiconductor monolayer on an insulator and placing electrodes on the monolayer and underneath the insulator, the researchers could apply an AC signal across the insulator. During the moment when the AC signal switches its polarity from positive to negative (and vice versa), both positive and negative charges are present at the same time in the semiconductor, creating light.
The researchers showed that this mechanism works in four different monolayer materials, all of which emit different colours of light.
This device is a proof-of-concept, and much research still remains, primarily to improve efficiency. Measuring this device’s efficiency is not straightforward, but the researchers think it’s about 1% efficient. Commercial LEDs have efficiencies of around 25% to 30%.
The concept may be applicable to other devices and other kinds of materials, the device could one day have applications in a number of fields where having invisible displays are warranted. That could be an atomically thin display that’s imprinted on a wall or even on human skin.
