A bright-light-emitting device that is millimeters wide and fully transparent was developed. The light-emitting material in this device is a monolayer semiconductor that is three atoms thick, allowing it to conform to curved surfaces.
Monolayer semiconductors are capable of emitting bright light. This 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 the thickness can be kept small but the lateral dimensions (width and length) can be kept 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 that 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.
Researchers 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 mono-layer 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 mechanism works in four different monolayer materials, all of which emit different colors of light.
The concept may be applicable to other devices and other kinds of materials, making the device suitable for applications in which having invisible displays is warranted; for example, an atomically thin display that is imprinted on a wall or even on human skin.