Engineers have built a bright light-emitting device that is millimeters wide and fully transparent when turned off. The light-emitting material in the device is a monolayer semiconductor that is just three atoms thick. Researchers at University of California, Berkeley used LED technology on monolayer semiconductors to create a design that can be scaled from sizes smaller than the width of a human hair up to several millimeters. The thickness of the device can be minimal but the width and length of the device can be large, so that the light intensity can be high even if the device is very thin. Probes inject positive and negative charges in the light-emitting device, which is transparent under the campanile outline, producing bright light. Courtesy of Javey Lab. “The materials are so thin and flexible that the device can be made transparent and can conform to curved surfaces,” said researcher Der-Hsien Lien. Typically, two contact points are used in a semiconductor-based LED, but the new device requires only one contact point on the semiconductor. Researchers laid the semiconductor monolayer on an insulator and placed electrodes on the monolayer and underneath the insulator. This arrangement allowed researchers to 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, and light is emitted. The team demonstrated this mechanism in four different monolayer materials, all of which emit different colors of light. This device is a proof-of-concept, and research still remains to be done, primarily to improve efficiency. Measuring this device’s efficiency is not straightforward, but the researchers think it is about 1 percent efficient. Commercial LEDs have efficiencies of around 25 to 30 percent. The concept on which the device is based could be applicable to other devices and other kinds of materials. The device could one day be used for invisible displays on walls and windows that would be bright when turned on but see-through when turned off. An atomically thin display could be imprinted on a wall or even on human skin, in the form of a light-emitting tattoo. “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. However, this is one step forward by presenting a device architecture for easy injection of both charges into monolayer semiconductors,” professor Ali Javey said. The research was published in Nature Communications (doi:10.1038/s41467-018-03218-8).