A new generation of LEDs emit light and also respond to it. They are made of a thin film of nanorods less than 5 nm in diameter, each of which is made of three types of semiconductor material. One of the three emits and absorbs visible light. The other two control how charge flows through the first material.

The LED functions by switching back and forth from emitting to detecting at a rate that is three orders of magnitude faster than standard display refresh rates.

A laser stylus writes on a small array of multifunction pixels made by dual-function LEDs than can both emit and respond to light.The work so far has been done with red LEDs. The research team, from the University of Illinois at Urbana-Champaign and Dow Electronic Materials in Marlborough, Massachusetts, is now investigating three-color displays and working on ways to improve their light-harvesting capabilities.

A display made of these LEDs could be programmed to respond to light signals in several different ways.

For example a display could automatically adjust its brightness in response to ambient light conditions and do that on a pixel-by-pixel basis. The entire display on a tablet could therefore adjust to moving shadows and sunlight to maintain a steady contrast.

Since pixels could react to an approaching finger, they also could be used in an interactive display that responds to gestures or that recognizes objects. And they could also respond to a laser stylus.

One property is that these LEDs can convert light to electricity like a solar cell.

"Imagine your cell phone just sitting there collecting the ambient light and charging. That's a possibility without having to integrate separate solar cells. We still have a lot of development to do before a display can be completely self-powered, but we think that we can boost the power-harvesting properties without compromising LED performance, so that a significant amount of the display’s power is coming from the array itself," says project leader Moonsub Shim.

In another possible application, nanorod LED displays can interact with each other as large parallel communication arrays. Communication between single LEDs would be slower than device-two-device technologies like Bluetooth. However Bluetooth is a serial interface, only sending one bit at a time. Two LED arrays facing each other could communicate with as many bits as there are pixels in the screen.

The various applications are demonstrated in this video.