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Researchers measure near perfect performance in low cost semiconductors



The reproduction of quantum dots that absorb light from a near-by artist. Picture credits: Ella Marushchenko

Small, easy-to-produce particles, called quantum dots, can soon replace more expensive single crystal semiconductors in advanced electronics in solar cells, camera sensors, and medical imaging equipment. Although quantum dots have begun to penetrate the consumer market ̵

1; in the form of quantum dot television – they have been hampered by years of uncertainty about their quality. Now, a new measurement technique developed by Stanford University researchers can finally resolve these doubts.

"Traditional semiconductors are monocrystals grown in vacuum under special conditions, which we can produce in large quantities, in bottles, in a laboratory, and have been shown to be as good as the best single crystals," he said David Hanifi, chemistry student at Stanford University and co-author of the article on this work, published March 15 in Science .

Researchers focused on how efficiently quantum dots re-emit their absorbed light, a telltale measure of semiconductor quality. While previous attempts to determine quantum dot efficiency indicated high performance, this is the first measurement method that confidently shows that they can compete with single crystals.

This work is the result of collaboration between the laboratories of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, the renowned Samsung professor of nanoscience and nanotechnology at the University of California, Berkeley, pioneer of quantum dot research and senior Author of the newspaper. Alivisatos emphasized how metrology could lead to the development of new technologies and materials that require accurate knowledge of the efficiency of our semiconductors.

"These materials are so efficient that the existing measurements were unable to quantify their quality, which is a big leap forward," said Alivisatos. "It may one day enable applications that require materials with a luminescent efficiency well in excess of 99 percent, most of which have not yet been invented."

Between 99 and 100

Abandoning the Need For expensive fabrication equipment is not the only advantage of quantum dots. Even before this work, there were signs that quantum dots could reach or surpass the performance of some of the best crystals. They are also very customizable. Changing their size changes the wavelength of the light they emit, a useful feature for color-based applications such as marking biological samples, televisions or computer monitors.

Despite these positive properties, the small size of quantum dots can cost them billions of dollars to do the work of a large, perfect single crystal. Making so many of these quantum dots means more chances for false growth, more for a defect that can affect performance. Techniques that measure the quality of other semiconductors have previously suggested that quantum dots emit more than 99 percent of the light they absorb. However, this was not enough to answer questions about their potential for defects. For this, the researchers needed a measurement method that was more suitable for an accurate assessment of these particles.

"We want to measure emission efficiencies in the range of 99.9 to 99.999 percent, because when semiconductors can emit any photon they absorb again as light." You can really enjoy science and make devices that did not said Hanifi.

The researchers' technique was to test the excess heat generated by the quantum energy dots, rather than just evaluating the light emission, because the excessive heat is excessive heat signature of inefficient emissions used for other materials was never used to measure quantum dots in this way and was 100 times more accurate than those used in the past, and they found that groups of quantum dots reliably accounted for about 99.6 percent of them absorbed light emitted (with a potential error of 0.2 percent in both R This 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 produce," said Salleo, who is co-author of the work.

Contrary to the concerns, the results indicate that the quantum dots are conspicuously defect-tolerant. The measurement method is also the first to determine how different quantum dot structures are compared: quantum dots with exactly eight atomic layers of a specific coating material emitted the fastest light, an indicator of superior quality. The shape of these points should determine the design of new light-emitting materials, said Alivisatos.

Completely New Technologies

This research is part of a collection of projects funded by a Department of Energy Energy Frontier Research Center, called Photonics at Thermodynamic Limits. Led by Jennifer Dionne, a professor of materials science and engineering at Stanford, the center's goal is to create optical materials that influence the flow of light with the greatest possible efficiency.

A next step in this project is developing more accurate measurements. If researchers find that these materials can reach efficiencies of over 99.999 percent, it opens the door to technologies we've never seen before. These might include new fluorescent dyes that study biology at the atomic level, luminescence cooling, and luminescent solar concentrators that allow a relatively small set of solar cells to absorb energy from a wide range of solar radiation. All in all, the measurements they have already made are a milestone for them, which should boost the research and application of quantum dots even more directly.

"The people who work on these quantum dot materials have more than they thought For decades, dots could be as efficient as single crystal materials," said Hanifi, "and now we finally have proof."


Explore Further:
More stable light comes from deliberately "squashed" quantum dots

Further information:
David A. Hanifi et al., Redefinition of luminescence near unity in quantum dots with photothermal threshold quantum yield, Science (2019). DOI: 10.1126 / science.aat3803

Magazine Reference:
science

Provided by:
Stanford University


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