A proof-of-concept lidar system from Institut Català de Física i Óptica (ICFO) demonstrates the potential to use SWIR colloidal quantum dots (CQDs) for automotive applications and consumer electronics. The researchers built the SWIR lidar using CQDs made from silver telluride (Ag₂Te), an environmentally friendly material that is Restriction of Hazardous Substances (RoHS)-compliant.

The properties of SWIR make it an attractive option for several applications, including 3D imaging and machine vision, as well as lidar and electronics. SWIR light experiences less scattering in the atmosphere, compared to visible light, allowing greater penetration depth, especially in adverse weather conditions. SWIR is considered “eye-safe,” and enables continuous sensing and ranging during day and night.

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Ag₂Te CQD SWIR photodiodes are shown on the left and solution-processed Ag₂Te QDs are shown on the right. Courtesy of ICFO/Jordi Cortés.

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However, SWIR photodetectors are made with expensive materials that are difficult to manufacture. This has limited the applications that use SWIR light mostly to specialized areas like scientific instrumentation and military operations.

Heavy metal-free CQDs have emerged as an alternative material for the development and manufacture of SWIR optoelectronics. Ag₂Te CQDs show device performance comparable to CQDs made with toxic heavy-metals like lead or mercury. But to meet the rigorous demands of sensing and lidar applications, Ag₂Te CQD performance needs to improve in three areas: dark current, linear dynamic range, and response speed.

The ICFO researchers developed a new method to create Ag₂Te CQDs that addresses the high dark current, limited linear dynamic range, and slow response speed exhibited by these materials. The team optimized the synthesis of CQDs to eliminate surface defects, which can reduce efficiency, by performing surface engineering of Ag₂Te CQDs with tight-binding thiols.

Although the researchers implemented surface engineering to improve colloidal stability, carrier lifetime, and photoluminescence quantum yield (PLQY), this strategy alone was not enough. “Initially, the device performance was not very satisfactory,” researcher Yongjie Wang said. “It wasn’t until we applied a silver nitrate post-treatment to our quantum dot thin film that we saw major improvements, suggesting that this optimization approach was promising.”

The researchers engineered doping on CQD films by introducing a new cationic ligand of silver nitrate (AgNO₃) to further optimize the QD stack of the photodiode.

Using the new approach, the researchers developed SWIR photodiode devices made with CQDs that achieved a dark current density of less than 500 amps per square centimeter (500nA/cm², an external quantum efficiency at 1400 nm of 30%, a linear dynamic range of more than 150 decibels (dB), and a time response as fast as 25 nanoseconds (ns).

“At the beginning of the project, we didn’t expect such a significant leap in the final device performance,” Wang said.

Leveraging the improved performance of the SWIR photodiodes, the researchers built a proof-of-concept SWIR lidar using CQDs made from nontoxic materials that are compliant with the RoHS directive. To the best of the team’s knowledge, this is the first SWIR lidar made with nontoxic CQDs.

The device measured distances over 10 m with decimeter resolution, showcasing the potential of Ag₂Te CQDs for lidar.

The engineering strategy for enhancing Ag₂Te CQDs could help move the development of SWIR optoelectronic devices forward. The strategy leverages the cost-effectiveness and manufacturing advantages of CQDs, while enhancing their performance as an environmentally friendly alternative to heavy-metal CQDs.

Future research will focus on achieving faster response times, higher quantum efficiency, and more reliable operation under realistic temperatures and humidity levels. This work could be a key step in advancing the widespread adoption of SWIR light in consumer electronics and the automotive sector.

The research was published in Advanced Materials (www.doi.org/10.1002/adma.202500977).

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