A new technique for precisely measuring quantum dot (QD) performance, developed by researchers at Stanford University, showed that groups of QDs reliably emit about 99.6 percent of the light they absorb, with a potential error of 0.2 percent in either direction. The measurement technique focuses on how efficiently QDs re-emit the light they absorb. The results suggest that, contrary to long-standing concerns, QDs are defect-tolerant and their emission 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,” said professor Alberto Salleo. A close-up artist’s rendering of quantum dots emitting light they’ve absorbed. Courtesy of Ella Marushchenko. To evaluate QD performance, the Stanford team developed a technique for measuring luminescence efficiency with sufficient accuracy below one part per thousand. Because excess heat is a signature of inefficient emission, the team’s approach involved checking for excess heat produced by energized QDs, rather than only assessing light emission. The researchers used photothermal deflection spectroscopy to measure very small nonradiative decay components in QD photoluminescence. This allowed them to tune the synthesis of cadmium selenide/cadmium sulfide (CdSe/CdS) quantum dots so that the external luminescent efficiencies exceeded 99.5 percent, indicating nearly complete suppression of nonradiative decay channels. The new measurement technique also shows how different QD structures compare to each other. QDs with precisely eight atomic layers of a special coating material emitted light the fastest, an indicator of superior quality. The shape of these dots should guide the design for new light-emitting materials, said professor Paul Alivisatos. According to the researchers, their technique, which is commonly used for other materials but has never been applied to measuring QD efficiency, is 100 times more precise than measurement techniques used for QDs in the past. A next step in this project will be to develop even more precise measurements. If the researchers can determine that quantum dots can reach efficiencies at or above 99.999 percent, this could lead to new technologies such as glowing dyes to examine biological samples at atomic scale, or luminescent cooling and luminescent solar concentrators, which would allow a relatively small set of solar cells to take in energy from a large area of solar radiation. The QD measurements already established by the team could encourage a more immediate boost in QD 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 and now we finally have proof,” said researcher David Hanifi. The research was published in Science (http://dx.doi.org/10.1126/science.aat3803).
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