Researchers measure near-perfect performance in low-cost semiconductors

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A new measurement technique developed by researchers at Stanford University may finally dissolve doubts over the uncertainty that surrounds quantum dot quality.

"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, lead author of the study.

The researchers focused on how efficiently quantum dots re-emit the light they absorb, one tell-tale measure of semiconductor quality. While previous attempts to figure out quantum dot efficiency hinted at high performance, this is the first measurement method to confidently show they could compete with single crystals, according to the team of researchers.

The hope is that this measurement technique could lead to the development of new technologies and materials that require knowing the efficiency of our 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," said Paul Alivisatos, the Samsung Distinguished Professor of Nanoscience and Nanotechnology at the University of California, Berkeley, who is senior author of the paper. "It may someday enable applications that require materials with luminescence efficiency well above 99 per cent, most of which haven't been invented yet."

The researchers' technique involved checking for excess heat produced by energised quantum dots, rather than only assessing light emission because excess heat is a signature of inefficient emission. This technique, commonly used for other materials, had never been applied to measure quantum dots in this way and it was 100 times more precise than what others have used in the past, the researchers said.

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

Contrary to concerns, the results 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, said Prof Alivisatos.

"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."