Atomically Thin Light-Emitting Device

Photograph of a packaged, 3 mm × 2 mm, device in the on state. (Source: UC Berkeley / NPG)

UC Berkeley engineers have built a bright-light emitting device that is milli­meters wide and fully transparent when turned off. The light emitting material in this device is a monolayer semi­conductor, which is just three atoms thick. The device opens the door to invisible displays on walls and windows – displays that would be bright when turned on but see-through when turned off – or in futuristic appli­cations such as light-emitting tattoos, accor­ding to the researchers.

“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. The device was developed in the labora­tory of Ali Javey, professor of Electrical Engi­neering and Computer Sciences at Berkeley. In 2015, Javey’s lab created monolayer semi­conductors that are capable of emitting bright light, but stopped short of building a light-emitting device. The new work overcame funda­mental barriers in uti­lizing 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 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 colors 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 percent efficient. Commer­cial LEDs have efficiencies of around 25 to 30 percent. 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.

“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.” (Source: UC Berkeley)

Reference: D.-H. Lien et al.: Large-area and bright pulsed electroluminescence in monolayer semiconductors, Nat. Commun. 91229 (2018); DOI: 10.1038/s41467-018-03218-8

Link: A. Javey Research Group, Electrical Engineering and Computer Sciences, University of California, Berkeley, USA

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