Two-way LEDs that emit and detect light could be used in displays that support gesture control and charge themselves with ambient light. The LEDs, created by researchers at the University of Illinois at Urbana-Champaign and Dow Electronic Materials in Marlborough, Massachusetts, are made of tiny nanorods arrayed in a thin film.
Moonsub Shim, a professor of materials science and engineering at the U. of I. and the leader of the study, said: “These LEDs are the beginning of enabling displays to do something completely different, moving well beyond just displaying information to be much more interactive devices. That can become the basis for new and interesting designs for a lot of electronics.”
The nanorods measure less than 5 nanometers in diameter and are made of three types of semiconductor material. One type emits and absorbs visible light. The other two semiconductors control how charge flows through the first material. The combination allows the LEDs to emit, sense and respond to light.
The nanorod LEDs switch back and forth from emitting to detecting so fast that, to the human eye, the display appears to stay on continuously. The LEDs are virtually continuously detecting and absorbing light and a display made of the LEDs can be programmed to respond to light signals in a number of ways.
For example, a display could automatically adjust brightness in response to ambient light conditions – on a pixel-by-pixel basis.
Shim explained: “You can imagine sitting outside with your tablet, reading. Your tablet will detect the brightness and adjust it for individual pixels. Where there’s a shadow falling across the screen it will be dimmer, and where it’s in the sun it will be brighter, so you can maintain steady contrast.”
The researchers demonstrated pixels that automatically adjust brightness, as well as pixels that respond to an approaching finger, which could be integrated into interactive displays that respond to touchless gestures or recognise objects.
They also demonstrated arrays that respond to a laser stylus, which could be the basis of smart whiteboards, tablets or other surfaces for writing or drawing with light. And the researchers found that the LEDs not only respond to light, but can convert it to electricity as well.
Shim said: “The way it responds to light is like a solar cell. So not only can we enhance interaction between users and devices or displays, now we can actually use the displays to harvest light.
“So imagine your cellphone 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.”
In addition to interacting with users and their environment, nanorod LED displays can interact with each other as large parallel communication arrays. It would be slower than device-to-device technologies like Bluetooth, Shim said, but those technologies are serial – they can only send one bit at a time. Two LED arrays facing each other could communicate with as many bits as there are pixels in the screen.
Study co-author Peter Trefonas, a corporate fellow in Electronic Materials at The Dow Chemical Company, said: “We primarily interface with our electronic devices through their displays, and a display’s appeal resides in the user’s experience of viewing and manipulating information. The bidirectional capability of these new LED materials could enable devices to respond intelligently to external stimuli in new ways. The potential for touchless gesture control alone is intriguing, and we’re only scratching the surface of what could be possible.”
The researchers did all demonstrations with arrays of red LEDs. They are now working on methods to pattern three-color displays with red, blue and green pixels, as well as working on ways to boost the light-harvesting capabilities by adjusting the composition of the nanorods.
This work was supported by a collaborative research effort between the Dow Chemical Company and the University of Illinois. The National Science Foundation also supported this work.
Source: University of Illinois