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Mid-infrared quantum dots offer route to cheaper sensing

13 Sep 2023

University of Chicago manufacturing method improves on current QD performance.

A team at the University of Chicago (UChicago) has demonstrated a new way to obtain mid-infrared emission from colloidal quantum dots (QDs), potentially opening up new applications for the sources.

Colloidal QDs are semiconductor nanocrystal materials that offer a promising route to synthesizing light sources in bulk quantities, through wet-chemistry solution-processing techniques.

Their electroluminescence in the visible range can already be highly efficient and cost effective, but other wavelengths have to date proven more challenging, especially the mid-infrared region.

The UChicago lab of Philippe Guyot-Sionnest (PGS Lab), which specializes in the study of nanocrystal quantum dots created by colloidal synthetic chemistry, has now developed a QD with much improved emission characteristics in the mid-IR, and published the findings in Nature Photonics.

"A cost-effective and easy-to-use method to make infrared light with quantum dots could be very useful," commented Xingyu Shen from the PGS Lab, and the discovery could ultimate lead to significantly cheaper infrared lights and lasers, or to new mid-IR applications.

The current work builds on previous research in the PGS Lab into manufacture and performance of quantum dot emitters, including efforts to improve the nanoparticle size distribution and the development of nanocrystal quantum dot infrared detectors, potentially able to rival commercial devices at a fraction of the cost.

In 2022 the group demonstrated the first mid-IR colloidal quantum dot LEDs, based on mercury telluride (HgTe), a material with semiconductor properties and stability that are conducive to emission. Such QDs "are poised to disrupt the very high cost/gram of infrared imaging, with exciting and new fabrication processes," noted the team at that time.

One of the best applications for quantum dots

For its new research the project studied the manufacturing technique used to create its colloidal QDs and the methods used to stimulate them. It took inspiration from the established cascade approach to laser emission, in which electrons travel through a series of distinct energy levels and emit a portion of energy as light at each level.

Such a cascade technique has never been achieved with colloidal quantum dots until now, according to the PGS Lab, which created a black "ink" of the HgTe nanocrystals, spread it on a substrate and sent an electrical current through.

"We thought it would be likely to work, but we were really surprised by how well it worked," said Philippe Guyot-Sionnest. "Right away, from the first time we tried it, we saw light."

The QDs emitted 5-micron light at a quantum efficiency of 4.5 percent, approaching that of commercial epitaxial cascade quantum well light-emitting diodes, according to the team's Nature Photonics paper. With further optimization, the cascade approach could surpass existing methods.

"We're very excited for the possibilities," said Guyot-Sionnest. "I think it's one of the best examples of a potential application for quantum dots. Many other applications could be achieved with other materials, but this architecture really only works because of the quantum mechanics. I think it's pushing the field forward in a really interesting way."

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