February 27, 2024

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Scientists create non-toxic quantum dots for shortwave infrared image sensors

Scientists create non-toxic quantum dots for shortwave infrared image sensors
Zoom in / Vials of quantum dots with emission gradually varying from violet to dark red.

Shortwave infrared (SWIR) light sensors are desirable in a wide range of applications, particularly in the service robotics, automotive, and consumer electronics sectors. SWIR-tunable colloidal quantum dots show promise for such sensors since they can be easily integrated into CMOS, but their widespread market use is hampered by the fact that most of them contain toxic heavy metals such as lead or mercury. Now a team of scientists has manufactured quantum dots from non-toxic materials and tested them in a laboratory-scale photodetector, according to one of the researchers. Recent paper Published in Nature Photonics.

“SWIR light for sensing and imaging is of paramount importance because of its unique properties,” the authors wrote. “It is safe for the eyes; it can penetrate fog, haze and other weather conditions, enabling imaging under adverse weather conditions for automotive, environmental sensing and remote sensing applications; the presence of nightglow under night in the SWIR range enables passive night vision; and optical imaging combined with Infrared spectroscopy enables machine vision, bioimaging, and food and process quality inspection,” among other applications.

As mentioned earlier, a quantum dot is a small semiconductor grain with a diameter of a few tens of atoms. Billions of dollars can be put on the head of a pin, and the smaller the better. At those small scales, quantum effects kick in and give the dots superior electrical and optical properties. They glow brightly when light is shined on them, and the color of this light is determined by the size of the quantum dots. Larger dots emit redder light; Small dots emit bluer light. So you can tailor quantum dots to specific frequencies of light just by changing their size.

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Once thought impossible to make, quantum dots have become a popular component in computer monitors, television screens, and LEDs, among other uses. For example, quantum dots allow TV manufacturers to fine-tune emitted colors, producing more accurate colors over a wider range, all while using less electricity. It is useful as substitute Organic dyes used to label reactive agents in fluorescent-based biosensors have been incorporated into glass windows to essentially turn those windows into photovoltaic cells, potentially harvesting small amounts of solar energy to offset home energy costs.

In 2013, German physicists built… Empirical Eq Maxwell's demon with a pair of interacting quantum dots. In 2015, scientists Making quantum “pee dots.” From recycled urine and used to image vital mouse cells. Future applications may include incorporating quantum dots into flexible electronics, small sensors, and solar cells or using them in encrypted quantum communications systems.

Yongjie Wang (BIST) and Julian Schreyer (QURV) hold a sample solution of quantum dots, a SWIR photodetector, and an image sensor.
Zoom in / Yongjie Wang (BIST) and Julian Schreyer (QURV) hold a sample solution of quantum dots, a SWIR photodetector, and an image sensor.

The authors of this latest paper hail from the Barcelona Institute of Science and Technology (BIST) and Corve Technologies in Spain. The BIST team was looking for ways to manufacture nanocrystals of silver bismuth telluride for photovoltaic devices, and noticed that silver telluride was one of the byproducts. Silver telluride had ideal properties for colloidal quantum dots, most notably tunability. So the team changed tack and developed a process to make quantum dots from silver telluride.

The resulting quantum dots had a good size distribution and were tunable over a wide spectral range, including SWIR. The next step was to integrate those quantum dots into a lab-scale photodetector. Backing away from a typical device setup has been a challenge since one of the devices shines light from the bottom of most lab-scale devices, whereas CMOS-integrated CQD kits include a bright light from the top, with the CMOS electronics at the bottom. The first attempt was only moderately successful because the resulting photodiode did not perform as expected in the SWIR range.

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BIST researchers redesigned the sensor with an additional insulating layer to fix the problem, producing a more effective SWIR sensor. They then collaborated with Korf scientists to build a proof-of-concept SWIR image sensor made of non-toxic quantum dots operable at room temperature. They were able to take images of moving silicon wafers under SWIR light and look inside opaque plastic bottles under visible light. The next step is to re-engineer the layer array to improve photodiode performance, as well as explore other surface chemistry.

Natural Photonics, 2024. DOI: 10.1038/s41566-023-01345-3 (About digital IDs).

Non-toxic quantum dots for shortwave CMOS infrared image sensors for consumer electronics.