Infrared heavy metal-free quantum dots provide sensitive and fast sensors for eye-catching rider applications

The frequency regime in shortwave infrared (SWIR) has very unique properties ideal for some applications, such as not easily affected by atmospheric scattering and being “eye guardian”. These include photodetection and range (LIDAR), methods for determining range and distance using lasers, space localization and mapping, imaging of bad weather for monitoring and car safety, environmental monitoring, and more.

However, SWIR light is currently limited to niche areas, including scientific instrumentation and military use. This is mainly because the SWIR photo sector relies on materials that are expensive and difficult to manufacture. Over the past few years, colloidal quantum dots (solution-treated semiconductor nanocrystals) have emerged as an alternative to mainstream home appliances.

Although toxic heavy gold (such as lead and mercury) is usually used, quantum dots can also be made with environmentally friendly materials such as Telluride Silver (AG2TE). In fact, silver teluride colloid quantum dots show device performance comparable to their toxic counterparts. However, they are still in the early stages and require some challenges to be addressed before they can be used in real-world applications.

Currently, ICFO researchers Yongjie Wang, Hao Wu, Carmelita Rodà, Dr. LuchengPeng, Dr. Nima Taghipour and Dr. Miguel Dosil, led by ICREA Professor Gerasimos Konstantatos, demonstrated a new way to create silver terrode quantum dots to address these challenges. The team also developed the first proof-of-concept swir lidar using colloidal quantum dots made of non-toxic materials, measuring distances of over 10 meters at a resolution of decimeters.

Published in Advanced Materials, this study is a critical step towards practical, cost-effective, and environmentally friendly LIDAR systems for the consumer and automotive markets.

Overcoming the challenges of non-toxic colloidal quantum dots for SWIR photodetection

Silver Terurido colloid quantum dots traditionally face three challenges: High current, limited linear dynamic range, and response speed.

Dark currents are small currents that flow through the photo sector even when there is no light. High currents increase noise and limit sensitivity to weak signals.

For LIDAR applications, this ultimately limits the ability to detect objects in the distance. This is because distance and atmospheric interference lead to more signal attenuation. Linear dynamic range refers to the range of minimum detectable light intensity and maximum detectable light intensity. The wider range, the higher the contrast of the scene that the SWIR detector can sense and visualize.

Finally, the response speed of the photodetector measures how quickly it can respond to changes in incident light intensity. Fast response promotes, among other things, accurate distance measurements and optical communication.

ICFO researchers have significantly improved all three features compared to previous records reported in natural photonics just a year ago. Specifically, we achieved a dark current density of less than 500 Na/cm2, external quantum efficiency at 1400 nanometers of 30%, LDR over 150 dB, and a fast time response of 25 nanoseconds.

These successful results encouraged the construction of a proof of concept swir lidar using colloidal quantum dots made of materials compliant with the restrictions of the Hazardous Substances Directive for the first time. The device measures distances over 10 meters with decimeter resolution, and introduced the promising potential of silver Telluride colloidal quantum dots for Lidar applications.

“At the start of the project, we didn’t expect a major leap in the performance of the final device,” recalls Dr. Yongjie Wang, the first co-author of the paper. The team began by optimizing the synthesis of quantum dots to eliminate surface defects. This tends to reduce efficiency. However, this strategy alone was not sufficient.

“Initially, the device performance was not very satisfying, suggesting that this optimization approach was promising until the application of post-treatment treatment of silver to quantum dot thin films, which saw a massive improvement,” the researchers add.

The proposed engineering strategy will advance the development of SWIR optoelectronic devices by taking advantage of the cost-effectiveness and manufacturing benefits of colloidal quantum dots, significantly improving performance as an environmentally friendly alternative. Future research will focus on achieving faster response times, higher quantum efficiency, and more reliable operations under realistic temperature and humidity conditions.

These advancements, including this study, are the ultimate goal: to bring one step closer to the widespread adoption of SWIR light in home appliances.