Researchers measure near-perfect performance in low-cost semiconductors

"Traditional semiconductors are single crystals, grown under vacuum in special conditions, we can manufacture them in large numbers, in flasks, in a laboratory and we have shown that they are as efficient as the best single crystals", said David Hanifi, a graduate student. in chemistry at Stanford and co-lead author of the paper written on this work, published March 15 in Science.

Researchers have focused on how efficiently quantum dots re-emit the light they absorb, a revealing measure of semiconductor quality. While previous attempts to understand the effectiveness of quantum dots alluded to high performance, this measurement method is the first to show in a confident way that they could compete with single crystals.

This work is the result of a collaboration between the laboratories of Alberto Salleo, professor of materials science and engineering at Stanford, and Paul Alivisatos, distinguished Samsung professor of nanoscience and nanotechnology at the University of California. 39, University of California at Berkeley, a pioneer of quantum technology. dot research and lead author of the article. Alivisatos emphasized how measurement technology can lead to the development of new technologies and new materials that require in-depth knowledge of the efficiency of our semiconductors.

"These materials are so effective that existing measurements were not able to quantify their quality – it's a huge step forward," said Alivisatos. "This may someday allow applications requiring materials with luminescence efficiencies of well over 99%, most of which have not yet been invented."

Between 99 and 100

Ability to forgo the need for expensive manufacturing equipment is not the only benefit of quantum dots. Even before this work, there were signs that quantum dots could approach or surpass the performance of some of the best crystals. They are also highly customizable. Changing their size changes the wavelength of the light emitted, a useful feature for color-based applications such as marking biological samples, TVs or screens. ;computer.

Despite these positive qualities, the small size of the quantum dots means that it can take billions to do the work of a single great crystal perfect. Doing so much of these quantum dots means more chances that something is growing incorrectly, more risk of failure that can affect performance. Techniques that measure the quality of other semiconductors previously suggested that quantum dots emitted more than 99% of the light they absorbed, but this was not enough to answer questions about their potential for defects. To do this, the researchers needed a measurement technique better suited to the accurate evaluation of these particles.

"We want to measure emission efficiencies in the range of 99.9 to 99.999%, because if the semiconductors are able to restore under the light each photon they absorb, you can do a really fun science and create devices that did not exist before, "Hanifi said.

The researchers' technique consisted in checking the excess heat produced by excited quantum dots, instead of evaluating only the light emission, because this excess of heat is the sign of an inefficient emission. This technique, commonly used for other materials, had never been applied to measure quantum dots in this way and was 100 times more accurate than that used by others. They found that groups of quantum dots reliably emit about 99.6% of the light absorbed (with a potential error of 0.2% in both directions), which is comparable to better monocrystalline emissions.

"It was surprising that a movie with many potential flaws was as good as a perfect semiconductor you could create," said Salleo, co-author of the article.

Contrary to the concerns, the results suggest that quantum dots are remarkably fault tolerant. The measurement technique is also the first to clearly define how the different quantum dot structures compare to each other – quantum dots with exactly eight atomic layers of a special coating material emitting light as quickly as possible, which is a high quality indicator. The shape of these points should guide the design of new materials emitting light, said Alivisatos.

Totally new technologies

This research is part of a set of projects within an Energy Frontier research center funded by the Ministry of Energy, called Photonics at Thermodynamic Limits. Led by Jennifer Dionne, an associate professor of materials science and engineering at Stanford, the center aims to create optical materials, materials that affect the flow of light, with maximum efficiency.

Another step in this project is to develop even more precise measurements. If researchers can determine that these materials achieve efficiencies equal to or greater than 99.999%, this opens the way to technologies never seen before. These could include new incandescent dyes to enhance our ability to examine atomic-scale biology, luminescent coolers, and luminescent solar concentrators, which allow a relatively small set of solar cells to absorb light. Energy of a large area of ​​solar radiation. That said, the measurements they have already established are an important milestone, likely to encourage a more immediate acceleration of research and applications for quantum dots.

"People working on these quantum dot materials have been thinking for more than a decade that points could be as effective as single-crystal materials," said Hanifi, "and we finally have the proof."

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