Stanford Researchers Redefine Quantum Dots to be Near-Perfect

Miniature, easy-to-produce particles, known as quantum dots, may soon replace more costly single crystal semiconductors in progressive electronics found in camera sensors, solar panels, and medical imaging tools. Although quantum dots have started to penetrate the consumer market—in the form of quantum dot TVs—they have been hindered by long-standing doubts about their quality. Currently, a new measurement method formulated by scientists at Stanford University may, at last, dissolve those uncertainties.

A close-up artist’s rendering of quantum dots emitting light they have absorbed. (Image credit: Ella Marushchenko)

“Traditional semiconductors are single crystals, grown in vacuum under special conditions. These we can make in large numbers, in flask, in a lab and we’ve shown they are as good as the best single crystals,” said David Hanifi, graduate student in chemistry at Stanford and the paper’s co-lead author. The research findings are published in the March 15th issue of Science.

The scientists concentrated on how effectively quantum dots re-emit the light they absorb, one revealing measure of semiconductor quality. While earlier attempts to work out quantum dot efficiency implied at high performance, this is the first measurement technique to positively reveal they could contend with single crystals.

This research is the outcome of a partnership between the labs of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California, Berkeley, who is a forerunner in quantum dot research and the paper’s co-senior author. Alivisatos stressed how the measurement method could result in the creation of new technologies and materials that require an insight into the efficiency of semiconductors to a meticulous degree.

These materials are so efficient that existing measurements were not capable of quantifying just how good they are. This is a giant leap forward. It may someday enable applications that require materials with luminescence efficiency well above 99 percent, most of which haven’t been invented yet.

Paul Alivisatos, Study Co-Senior Author, Samsung Distinguished Professor of Nanoscience and Nanotechnology, University of California, Berkeley.

Between 99 and 100

Being able to forgo the need for costly fabrication equipment is not the only plus point of quantum dots. Even earlier work, there were hints that quantum dots could approach or exceed the performance of some of the finest crystals. They are also extremely modifiable. Altering their size alters the wavelength of light they discharge, a beneficial feature for color-based applications such as tagging biological samples, computer monitors, or TVs.

In spite of these positive qualities, the small size of quantum dots means that it may require billions of them to perform the work of one large, perfect single crystal. Making that many quantum dots means more likelihood of something to grow erroneously, more chances for a flaw that can hinder performance. Methods that compute the quality of other semiconductors formerly suggested quantum dots emit more than 99% of the light they absorb but that was not sufficient to answer questions about their potential for flaws. To do this, the scientists required a measurement method well suited to exactly assessing these particles.

We want to measure emission efficiencies in the realm of 99.9 to 99.999 percent because, if semiconductors are able to reemit as light every photon they absorb, you can do really fun science and make devices that haven’t existed before.

David Hanifi, Study Co-Lead Author and Graduate Student in Chemistry, Stanford University.

The scientists’ method required checking for extra heat created by energized quantum dots, instead of just assessing light emission because extra heat is a signature of ineffective emission. This method, typically used for other materials, had never been used to measure quantum dots in this manner and it was 100 times more precise than what others have applied previously. They learned that groups of quantum dots consistently emitted about 99.6% of the light they absorbed (with a potential inaccuracy of 0.2% in either direction), which is comparable to the finest single-crystal emissions.

“It was surprising that a film with many potential defects is as good as the most perfect semiconductor you can make,” said Salleo, who is paper’s co-senior author.

Contrary to apprehensions, the results indicate that the quantum dots are extremely defect-tolerant. The measurement method is also the first to decisively resolve how various quantum dot structures compare to each other—quantum dots with exactly eight atomic layers of a special coating material discharged light the fastest, an indicator of higher quality. The shape of those dots should facilitate the design for new light-emitting materials, said Alivisatos.

Entirely new technologies

This study is part of an assortment of projects within a Department of Energy-funded Energy Frontier Research Center, called Photonics at Thermodynamic Limits. Headed by Jennifer Dionne, associate professor of materials science and engineering at Stanford, the center’s goal is to develop optical materials—materials that impact the flow of light—with the maximum possible efficiencies.

The following step in this project would be to develop even more precise measurements. If the scientists can establish that these materials attain efficiencies at or above 99.999%, that opens up the opportunity for technologies the market has never seen before. These could include new glowing dyes to improve the ability to study at biology at the atomic scale, luminescent cooling and luminescent solar concentrators, which enable a moderately small set of solar cells to absorb energy from a large area of solar radiation. That being said, the measurements they have already proven are a milestone of their own. This would likely inspire a more instant boost in quantum dot research and applications.

“People working on these quantum dot materials have thought for more than a decade that dots could be as efficient as single crystal materials,” said Hanifi,” and now we finally have proof.”

Noah D. Bronstein of Lawrence Berkeley National Laboratory, and Brent A. Koscher of Lawrence Berkeley National Laboratory and UC Berkeley are also paper’s co-lead authors. Other co-authors are Zach Nett, Adam M. Schwartzberg and Lorenzo Maserati of Lawrence Berkeley National Laboratory; Joseph K. Swabeck of Lawrence Berkeley National Laboratory and UC Berkeley; Kaori Takano of JXTG Nippon Oil & Energy Corporation in Japan; Koen Vandewal of Hasselt University in Belgium; and Yoeri van de Burgt of Eindhoven University of Technology in the Netherlands. Salleo is also an affiliate of the Precourt Institute for Energy and the Wu Tsai Neurosciences Institute.

This research received funding from the Department of Energy, the European Research Council (ERC), and JXTG Nippon Oil & Energy.

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