Quantum dot measurement method promises never-before-seen tech

Technology News | March 21, 2019

By Rich Pell

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Quantum dots are seen as having the potential to replace more expensive single-crystal semiconductors in advanced electronics found in solar panels, camera sensors, and medical imaging tools. Although quantum dots have begun to break into the consumer market, their use has been hampered by long-standing uncertainties about their quality.

Now, say the researchers, their new measurement technique may finally dissolve those doubts and provide a boost in quantum dot research and applications.

“Traditional semiconductors are single crystals, grown in vacuum under special conditions,” says David Hanifi, graduate student in chemistry at Stanford and co-lead author of a paper on the research. “These [quantum dots] we can make in large numbers, in flask, in a lab and we’ve shown they are as good as the best single crystals.”

In their work, the researchers focused on how efficiently quantum dots re-emit the light they absorb (image) – a telltale measure of semiconductor quality. While previous attempts to determine quantum dot efficiency hinted at high performance, this, say the researchers, is the first measurement method to confidently show they could compete with single crystals.

The measurement technique, say the researchers, could lead to the development of new technologies and materials that require knowing the efficiency of their semiconductors to a painstaking 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,” says Paul Alivisatos, the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California, Berkeley, who is a pioneer in quantum dot research and co-senior author of the paper. “It may someday enable applications that require materials with luminescence efficiency well above 99 percent, most of which haven’t been invented yet.”

While offering great performance potential, quantum dots are so small that it may take billions of them to do the work of one large, perfect single crystal – which means more chances for something to grow incorrectly during their manufacture, and more chances for a defect that can hamper performance.

Techniques that measure the quality of other semiconductors previously suggested quantum dots emit over 99% of the light they absorb but that was not enough to answer questions about their potential for defects. To do this, say the researchers, they needed a measurement technique better suited to precisely evaluating these particles.

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

Rather than only assessing light emission of quantum dots, the researchers’ technique involved checking for any excess heat produced by energized quantum dots – a signature of inefficient emission. This technique, commonly used for other materials, had never been applied to measure quantum dots in this way, say the researchers, and it was 100 times more precise than what others have used in the past.

They found that groups of quantum dots reliably emitted about 99.6% of the light they absorbed (with a potential error of 0.2% in either direction), which is comparable to the best single-crystal emissions.

“It was surprising that a film with many potential defects is as good as the most perfect semiconductor you can make,” says Alberto Salleo, professor of materials science and engineering at Stanford and co-senior author of the paper.

The results, say the researchers, suggest that the quantum dots are strikingly defect tolerant. The measurement technique is also the first to firmly resolve how different quantum dot structures compare to each other – quantum dots with precisely eight atomic layers of a special coating material emitted light the fastest, an indicator of superior quality. The shape of those dots should guide the design for new light-emitting materials, say the researchers.

A next step in the project is to develop even more precise measurements. If, say the researchers, they can determine that these materials reach efficiencies at or above 99.999%, that opens up the possibility for technologies that have never been seen before. These could include new glowing dyes to enhance the ability to look at biology at the atomic scale, luminescent cooling, and luminescent solar concentrators that allow a relatively small set of solar cells to take in energy from a large area of solar radiation.

For more, see “Redefining near-unity luminescence in quantum dots with photothermal threshold quantum yield.”