A laser stylus writes on a small array of multifunction pixels composed of dual-function LEDs than can both emit and respond to light. (Image credit: Moonsub Shim)
A laser stylus writes on a small array of multifunction pixels composed of dual-function LEDs than can both emit and respond to light. (Image credit: Moonsub Shim)

Nanorod LEDs with Dual Functions Could Enable Multifunctional Displays

Dual function LEDs can both emit and respond to light. One application these LEDs are used for is a laser stylus writing on a small array of multifunction pixels. Thanks to new LED arrays that can both detect and emit light cell phones, computer monitors and other devices could soon charge themselves using ambient light and be controlled with touchless gestures.

The LEDs are made of tiny nanorods arrayed in a thin film and could be used in new multitasking devices and interactive functions. The journal Science recently published the study done by researchers at the University of Illinois at Urbana-Champaign and Dow Electronic Materials in Marlborough, Massachusetts.

Moonsub Shim, study leader and a professor of materials science and engineering at the U. of I. explained that these LEDs could enable displays to do something completely different than what current displays are capable of. This would move them well beyond simply displaying information to become fully interactive devices. That can become the foundation for new and interesting electronic designs.

The tiny nanorods each measure less than 5 nanometers in diameter and are made of three types of semiconductor material. One type absorbs and emits visible light. The other two control how charge flows through the first material. The LEDs can sense, emit, or respond to light by virtue of this combination.

The nanorod LEDs are able to perform both functions by quickly switching between emitting and detecting. They switch so fast that they appear to stay on continuously. Their refresh rates are in fact three orders of magnitude faster than that of standard displays. The LEDs are also near-continuously absorbing and detecting light at the same time, and a display manufactured with the LEDs can be programmed to respond to light signals in different ways.

A display could for example automatically adjust brightness in response to ambient light conditions. Shim noted that if you were sitting outside reading your tablet, it would detect the brightness and adjust it for individual pixels. When a shadow falls across the screen, that part will be dimmer and it will be brighter where it is in the sun, so a steady contrast can be maintained.

The researchers demonstrated pixels that respond to an approaching finger, as well as pixels that automatically adjust brightness. These could be integrated into interactive displays that recognize objects, or respond to touchless gestures. Some arrays respond to a laser stylus and these could form the basis of tablets, smart whiteboards, or other surfaces for drawing or writing with light. The LEDs do not only respond to light, but can also convert it to electricity.

Shim also explained that the LEDs respond to light like a solar cell. This means that interaction between users and devices or displays can be enhanced, and the displays can actually also be used to harvest light. A cellphone will be able to collect the ambient light and charge without having to integrate separate solar cells.

Although there is still a lot of development to do before a display can be completely self-powered, the researchers think they can boost the power harvesting properties without compromising LED performance. This would result in a significant amount of the display’s power coming from the array itself.

The nanorod LED displays can also interact with each other as large parallel communication arrays. It would be slower than technologies such as Bluetooth, but those technologies are serial, meaning they can only send one bit at a time. Two LED arrays communicating with each other could do so with as many bits as there are pixels in the screen.

Study coauthor Peter Trefonas, a corporate fellow in Electronic Materials at the Dow Chemical Company explained that users interface with their electronic devices through their displays, and a display’s appeal is determined by the user’s experience of manipulating and viewing information. The new LED materials’ bidirectional capability could enable devices to respond intelligently to external stimuli in new ways.

All demonstrations were done with arrays of red LEDs. The researchers are now working on methods to pattern three-color displays with blue, red and green pixels. They are also working on ways to boost the light-harvesting capabilities by changing the composition of the nanorods.