Small, easy-to-produce particles called quantum dots could soon take the place of more expensive single-crystal semiconductors in advanced electronic devices in solar panels, camera sensors and medical imaging tools. Although quantum dots have begun to penetrate the consumer market, in the form of quantum dot TVs, they have been hampered by long-standing uncertainties about their quality. Now, a new measurement technique developed by Stanford University researchers can finally dispel these doubts.
"Traditional semiconductors are monocrystals, grown in vacuum under special conditions, which we can do in large quantities, in a flask, in a laboratory and we have shown that they are as good as the best single crystals," said David Hanifi, student graduated in chemistry at Stanford and co-author of the author of the work written on this work, published March 1
Researchers focused on how efficiently quantum dots re-emit the light they absorb, a measure that reveals the quality of semiconductors. While previous attempts to calculate the efficiency of quantum dots suggested high performance, this is the first measurement method to prove with confidence that they could compete with single crystals.
This work is the result of a collaboration between the laboratories of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, the Distinguished Professor of Nanoscience and Nanotechnology Samsung at the University of California, Berkeley , who is a pioneer in quantum dot research and senior author of the paper. Alivisatos emphasized how the measurement technique can lead to the development of new technologies and materials that require an accurate knowledge of the efficiency of our semiconductors.
"These materials are so efficient that existing measurements were not able to quantify how effective they are. This is a leap forward," Alivisatos said. "It could one day allow applications that require materials with luminescence efficiency well over 99 percent, most of which have not yet been invented."
Between 99 and 100
Being able to give up the need for expensive manufacturing equipment is not the only advantage of quantum dots. Even before this work, there were signs that quantum dots could approach or exceed the performance of some of the best crystals. They are also highly customizable. Changing their size changes the wavelength of light they emit, a useful feature for color-based applications like tagging biological samples, televisions or computer monitors.
Despite these positive qualities, the small size of the quantum dots means that it could require billions of them to do the work of a single great crystal perfect. Doing so many of these quantum dots means more chances for something to grow incorrectly, more chances for a defect that can hinder performance. The techniques that measure the quality of other semiconductors previously suggested that quantum dots emit more than 99 percent of the absorbed light, but this was not sufficient to answer questions about their potential for defects. To do this, the researchers needed a more suitable measurement technique to accurately assess these particles.
"We want to measure emissions efficiencies in the realm from 99.9 to 99.999 percent because, if semiconductors are able to re-emit as light every photon absorbs, you can make science really fun and make devices that didn't exist before," he said. said Hanifi
The researchers' technique involved controlling the excess heat produced by energized quantum dots, rather than evaluating only the emission of light because the excess of heat is a signature of inefficient emissions. This technique, commonly used for other materials, had never been applied to measure quantum dots in this way and was 100 times more accurate than what others have used in the past. They discovered that groups of quantum dots reliably emitted about 99.6 percent of the absorbed light (with a potential error of 0.2 percent in both directions), which is comparable to the best monocrystalline emissions.
"It was surprising that a film with many potential flaws is as good as the most perfect semiconductor that can be made," said Salleo, who co-authored the document.
Contrary to concerns, the results suggest that quantum dots are surprisingly tolerant to defects. The measurement technique is also the first to resolutely resolve the way in which the different structures of quantum dots compare one another: quantum dots with exactly eight atomic layers of a special coating material emit the faster light, a higher quality indicator. The shape of these points should guide the design of new materials for light emission, said Alivisatos.
Completely new technologies
This research is part of a collection of projects within an Energy Frontier Research Center funded by the Department of Photonics. Led by Jennifer Dionne, associate professor of materials science and engineering at Stanford, the focus of the center is to create optical materials, materials that influence the flow of light, with the highest possible efficiencies.
A further step in this project is also developing more precise measures. If researchers can establish that these materials achieve efficiencies at or above 99.999 percent, this opens up the possibility of technologies we've never seen before. These could include new incandescent dyes to improve our ability to look at atomic scale biology, luminescent cooling and luminescent solar concentrators, which allow a relatively small set of solar cells to absorb energy from a large area of solar radiation. That being said, the measures they have already established are a milestone, probably to encourage a more immediate impulse in research and quantum dot applications.
"People who work on these quantum materials have been thinking for more than a decade that points could be as efficient as monocrystalline materials," Hanifi said, "and now we finally have the evidence."
A more stable light comes from intentionally "crushed" quantum dots
David A. Hanifi et al. Redefining the almost unitary luminescence in quantum dots with quantum yield of the photothermal threshold, Science (2019). DOI: 10.1126 / science.aat3803