The University of Illinois-led team fashioned quantum dot heterostructures that can both detect and emit light, creating an array of pixels that could be turned on with the action of a laser pointer. [Image: Moonsub Shim, University of Illinois]
A team of scientists from the United States and Korea, led by Moonsub Shim of the University of Illionis, Urbana-Champaign, has developed heterostructures of semiconductor nanorods and luminescent quantum dots that can serve as both photovoltaic sensors and light-emitting diodes (LEDs), depending on bias current direction (Science, doi: 10.1126/science.aal2038). The team found that these “double-heterojunction nanorods” (DHNRs) can switch from light-emitting and light-harvesting modes at a rate orders of magnitude faster than typical display refresh rates—and far faster than the human eye can detect.
The nanorods’ dual personality could, according to the researchers, form the foundation for a new generation of “multitasking” interactive displays. Such displays could, even while providing an apparently uninterrupted view to the user, be harvesting and analyzing ambient light in the background, allowing automatic control of display brightness, touch-free response to gestures, and even direct, massively parallel two-way communication between displays.
At the heart of the vision are tiny semiconducting nanorods of cadmium sulfide (CdS), each several tens of nanometers long, capped on both ends with luminescent quantum dots consisting of cadmium selenide (CdSe), a smaller-bandgap semiconductor. Then, the CdSe tips are encased in another larger-bandgap semiconductor, zinc selenide (ZnSe). The result is a dumbbell-shaped double heterostructure in which the CdSe tips act as radiative recombination centers for light emission, and the heterojunctions between those tips and the higher-bandgap CdS and ZnSe allow for efficient shunting of electrons and holes to and from CdSe quantum dots, depending on the bias current direction.
The team then took these nanorod dumbbells and encased them in a thin film, and used the film to create a 10-by-10-pixel LED array on a patterned indium-tin-oxide substrate. Under a forward current bias, the researchers found, the DHNRs, as expected, served as highly efficient quantum emitters at low voltage. Under zero or reverse bias, however, the nanorods became photodetectors; the team was able to “write” a signal to the array using an external laser pointer, and use external light to create a measurable photocurrent.
The researchers further demonstrated that the tiny devices can toggle between light-harvesting and light-emitting mode in only around 6.5 microseconds. That response time is sufficiently fast, according to the scientists, that a device using the DHNRs, operating under an A/C bias, could “detect external light sources while appearing to be continuously emitting bright light.”
The team created a number of proof-of-concept devices to show off the nanorods in action, including an array of DHNR-enabled LEDs whose brightness automatically adjusts with changes to ambient light intensity, and a “touchless” user interface that detects and responds to the shadow of the user’s finger. Shim, in a press release, characterized that the new nanorod LEDs as “the beginning of enabling displays to do something completely different, moving well beyond just displaying information to be much more interactive devices.”
Toward “intelligent” displays?
That vision includes much more than simply a smartphone that can brighten up when you step into the shadows, however. For example, Shim notes that, since the DHNRs can in principle do double-duty as light-emitters and photovoltaics, one can imagine a smartphone that collects ambient light in the background and self-charges even as it is being used. The ability to use a laser stylus to “write” to arrays of dual-function LEDs could allow the creation of new, light-activated electronic whiteboards.
And, as the much-heralded Internet of Things gathers steam, Shim and his colleagues believe that the DHNR technology could offer the prospect for displays linked to autonomous sensors that can pass detailed, massively parallel information to one another, automatically and in real time. “The bidirectional capability of these new LED materials could enable devices to respond intelligently to external stimuli in new ways,” says one of the study coauthors, Peter Trefonas of the Dow Chemical Company. “We’re only scratching the surface of what could be possible.”