Nanorods Emit and Detect Light, Could Lead to Displays That Communicate via Li-Fi

A laser stylus writes on a small array of multifunction pixels made by dual-function LEDs than can both emit and respond to light.
Gif: Moonsub Shim/University of Illinois/IEEE Spectrum

Ever since the 2015 Consumer Electronics Show, quantum dots have been in a market struggle to displace light-emitting diodes (LEDs) as a backlight source for liquid crystal displays (LCDs).

Now an advance by a team of researchers from the University of Illinois at Urbana–Champaign, the Electronics and Telecommunications Research Institute in South Korea, and Dow Chemical may turn the display market on its head by eliminating the need for backlights in LCD devices. They have produced a LED pixel out of nanorods capable of both emitting and detecting light.

In the video below, you can get a further description of how the nanorods manage to both detect and emit light as well as some pretty attractive future applications, like mobile phones that can “see” without the need of a camera lens or communicate with each other using Light Fidelity (Li-Fi) technology.

“It’s very different from the use of quantum dots as backlight,” explained Moonsub Shim, a professor at the University of Illinois at Urbana–Champaign and coauthor of the paper, in an interview with IEEE Spectrum. “In backlight displays you have white light that gets diffused into a plane, and color filters create your red, green, and blue pixels. But here the individual pixels would be these nanorods that would emit a specific color, so you wouldn’t need a backlight.”

In research described in the journal Science, the international team of researchers mixed three types of semiconductors to produce engineered nanorods. “The nanorods contain three different semiconductor materials,” explains Shim. “The first semiconductor, which is attached at the tips of the nanorod, is the quantum dot that emits and absorbs visible light.” The other two semiconductors are the main body of the rod and the shell around the quantum dot. These components facilitate and control the flow of electrons (negative charges) and holes (positive charges) to and from the quantum dot.

The semiconductor materials in the rod and the shell each have a bandgap in which no electron states can exist as well as band alignment. With these two semiconductors in contact with the quantum dot, the nanorods are extremely efficient at both emitting and detecting light.

For example, in light-emission operation, which is induced electrically as in normal LEDs, the rod body delivers electrons efficiently to the quantum dot and blocks the holes from flowing in the wrong direction. The shell does the same thing, but for holes. In the light-detecting mode, after a photon creates an electron-hole pair, the rod body extracts the electrons and again prevents holes from going in the wrong direction. At the same time, the shell extracts holes and blocks electrons.

“If you want to operate an LED or a photodetector or a photovoltaic, you have to either inject electron holes, positive and negative charges into this quantum dot for light emission, or extract those charges out,” explained Shim. “Once the photon hits the quantum dot, it produces an electron-hole pair. So, what these two additional components—the main body of the rod and the shell around the quantum dot—do is facilitate the process of moving the charges to the right place, controlling the flow of the charges.”

Shim and his coauthor Nuri Oh, who is now a postdoctoral fellow at the University of Pennsylvania, believe these nanorods open up a host of new display capabilities.

“Our proof-of-concept devices show how LEDs can play multifunctional roles from light detection to data communication, ranging from visible light communication, such as Li-Fi, to energy harvesting/self powered displays,” said Oh in an email interview with IEEE Spectrum.

One of the key benefits of the nanorod-based LED pixel is that the response time of light emission and detection is quite fast, according to Shim. “The photo response time is about three orders of magnitude faster than a typical video refresh rate,” he said. “These LEDs can turn on and off so fast your eyes can’t tell. So to your eyes it’s constantly on.”

The first challenge the researchers had in making these nanorods was perfecting the fabrication process, which was detailed in research conducted by the same team back in 2014 and described in Nature Communications. “We realized at that point that we had engineered a certain band structure so that these different components were not only efficient at light emission (LED operation, electroluminescence) but also that you can detect light,” added Shim.

Shim concedes that is possible to use a LED lightbulb as a light-emitting device or as a light-detecting device. Even more, for thin film inorganic semiconductors this is a relatively easy thing to do, and, in fact, sort of similar to what the researchers have done here at the individual nanorod level. But because the researchers have made the LED pixel from this colloidal nanorod, it can be processed in solution and in turn be used to make large arrays of LEDs.

“An LED lightbulb, it’s pretty big—even a LED chip is on the order of millimeters—so in order to make a very efficient LED that also detects photons well from these inorganic thin film materials, you need a wafer to make these things, and typically wafers are pretty small and there’s no way to scale up beyond that scale,” argues Shim. “However, in principle with these materials you can spin coat them from solution onto any substrate. You can even think about flexible displays, large area screens where these things remain efficient both for light emission and detection.”

Significant engineering challenges remain. So far they have only demonstrated this technology on a single-color, red-emitting LED. For displays they need to create green and blue as well.

The next challenge is how to pattern all three different colors. While solution processing means you can spin coat the material, it also means that the material is soluble. That makes it difficult to use conventional lithography or patterning techniques that involve solvents.

“You would have to come up with different approaches, which we’re also working on to pattern three different color arrays of these things on a single substrate, or single plane,” added Shim.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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