By Josh Perry, Editor
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Researchers at Stanford University (Calif.) developed a new measurement technique to determine the quality of quantum dots, which they believe could soon replace single-crystal semiconductors in advanced electronics.

A close-up artist’s rendering of quantum dots emitting light they’ve absorbed.
(Ella Marushchenko/Stanford University)

Quantum dots, according to a report from the university, are easier to produce in quantity than single-crystal semiconductors, which require vacuums and other special conditions to grow. One researcher noted that quantum dots can be grown in large numbers in a flask in the lab.

Prior to the current measurement techniques, the biggest concern limiting the use of quantum dots in the consumer market was about the quality. Researchers at Stanford and the University of California, Berkeley, demonstrated the efficiency with which quantum dots reemit the light that they absorb, which was a measure of semiconductor quality.

“Even prior to this work, there were signs that quantum dots could approach or surpass the performance of some of the best crystals,” the article continued. “They are also highly customizable. Changing their size changes the wavelength of light they emit, a useful feature for color-based applications such as tagging biological samples, TVs or computer monitors.”

The tiny size of the dots requires a significant amount to do the same work of a large crystal, which caused concern over the likelihood of defects in the process. To measure efficiency, researchers checked on the excess heat produced by energized dots, which proved to be 100 times more precise than previous measurement techniques.

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

This demonstrated that concerns about potential defects were unfounded. The measurements also showed that different quantum dot structures could impact efficiency, as those with eight atomic layers of a special coating emitted light faster.

The research was recently published in Science. The abstract read:

“A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible.

“To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty.

“This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.”