Small, lightweight particles for production, called quantum dots, may soon replace expensive crystalline semiconductors in advanced electronics found in solar panels, camera sensors, and medical imaging tools. Although quantum dots have begun to break into the consumer market – in the form of quantum dot televisions – they have been hampered by long-standing uncertainty about their quality. Now, a new measurement technique developed by researchers at Stanford University may finally dissolve these doubts.
"Traditional semiconductors are individual crystals that grow in vacuum under special conditions, which can be produced in large numbers, in a bottle, in the laboratory, and we have shown that they are as good as individual crystals," says David Hanify, a graduate student in chemistry at Stanford University and author of the paper. , Published on March 15 science.
The researchers focused on how efficient quantum dots change the light they absorb, one measure of semiconductor quality. While previous attempts to understand quantum point efficiency hinted at high performance, this is the first measurement method to show with confidence that they can compete with individual crystals.
This work is the result of collaboration between the laboratories of Alberto Salleo, professor of materials science and engineering at Stanford University, and Paul Alivisatos, a distinguished Samsung professor of nanotechnology and nanotechnology at the University of California, Berkeley, who is a quantum pioneer and research senior author of the paper. Alivisatos stressed how the measurement technique could lead to the development of new technologies and materials that require knowing the efficiency of our semiconductors to a strict level.
"These materials are so effective that existing solitude could not quantify their success, it's a huge leap forward," said Elibisatus. "It may someday allow applications that require materials with luminescent efficiency over 99 percent, most of which have not been invented yet."
Between 99 and 100
The ability to forego the need for costly production equipment is not the only advantage of quantum dots. Even before this work, there were signs that quantum dots could approach or exceed the performance of some of the best crystals. They are also customizable. Changing their size changes the wavelength of light they emit, a useful feature for color-based applications such as labeling biological samples, televisions or computer monitors.
Despite these positive features, the small size of the quantum dots means that it may take billions of them to do the work of a large, perfect single crystal. Making so many of these quantum dots means more chances for something to grow incorrectly, the more chances are damaged it can hinder performance. Techniques that measure the quality of other semiconductors previously suggested quantum dots emit over 99 percent of the light they absorb but that was not enough to answer questions about their potential defects. To this end, the researchers needed a more appropriate measurement technique for accurate evaluation of these particles.
"We want to measure emissions efficiency in the range of 99.9 to 99.999 percent because if the semiconductors are capable of being considered as any photon light they absorb, you can make science really fun to make devices that did not exist before," Hanifee said.
The researchers' technique included testing for excess heat produced by quantum energy points, rather than just estimating light emissions because excess heat is the signature of inefficient emission. This technique, commonly used for other materials, has never been applied to quantifying quantum dots in a way that was 100 times more accurate than others have used in the past. They found that groups of quantum dots reliably emitted 99.6 percent of the light they absorbed (with a potential error of 0.2 percent in both directions), which is comparable to the best crystal emission.
"It was surprising that a film with many potential flaws is as good as the most perfect semiconductor you can make," said Salleo, who is a co-author of the article.
Contrary to the concerns, the results indicate that the quantum dots are surprisingly striking. The measurement technique is also the first to resolutely resolve how various quantum dot structures compare to each other quantum dots with exactly eight atomic layers of special coating material emitted by the fastest light, an excellent quality indicator. The shape of these points should guide the design for new light emitting materials, Alivisatos said.
Completely new technologies
This study is part of a collection of projects within the Department of Energy and Energy-funded Frontier Research Center, called Photonics at Thermodynamic Limits. Under the guidance of Jennifer Dion, Professor of Materials Science and Engineering at Stanford University, the center's goal is to create optical materials – materials that influence the flow of light – with the highest efficiency.
The next step in this project is to develop even more accurate measurements. If researchers can determine that these materials reach efficiencies above 99.999 percent, this opens the possibility to technologies we have never seen before. These can include new glowing colors to enhance our ability to look at atomic scale biology, luminous cooling and sunshine concentrations that allow a relatively small group of solar cells to take energy from a large area of solar radiation. All this being said, the measurements they have already established are their own landmark, and may encourage a more immediate push in quantum research and research applications.
"People who work on these quantum materials have thought for more than a decade that the points can be as efficient as single crystal materials," Hanifee said, "and now we have finally proved."
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David A. Hanifi et al., Re-definition of Enlightenment Near Unity in Quantum Points with quantum photothermal yield threshold, science (2019). DOI: 10.1126 / science.aat3803