The realm of computer vision is continuously evolving, pushing the boundaries of what technology can achieve. One of the most promising advancements in this field is the utilization of shortwave infrared (SWIR) light. SWIR light, although invisible to the human eye, has the potential to revolutionize industries such as service robotics, automotive, and consumer electronics. Its unique properties allow for reliable operation under adverse conditions like bright sunlight, fog, haze, and smoke. Additionally, SWIR light enables the detection of material properties through molecular imaging.
Colloidal quantum dots (CQD) have emerged as a promising technology platform for SWIR image sensors. These nanometric semiconductor crystals can be integrated with CMOS and provide access to the SWIR range. However, a significant roadblock has prevented the mass-market adoption of this technology. Many CQDs contain heavy metals like lead or mercury, making them subject to regulations by the Restriction of Hazardous Substances (RoHS) directive. This European directive restricts the use of heavy metals in commercial consumer electronic applications, posing a challenge to the widespread use of SWIR colloidal quantum dot technology.
In a groundbreaking study published in Nature Photonics, researchers at ICFO have addressed this challenge by developing high-performance infrared photodetectors and an SWIR image sensor based on non-toxic colloidal quantum dots. The team, led by ICREA Professor Gerasimos Konstantatos, achieved this breakthrough by synthesizing size-tunable, phosphine-free silver telluride (Ag2Te) quantum dots. These dots exhibit quantum-confined absorption akin to their heavy-metal counterparts while being free from toxic elements.
The researchers initially set out to synthesize silver bismuth telluride (AgBiTe2) nanocrystals to enhance the performance of photovoltaic devices. However, they unexpectedly obtained silver telluride (Ag2Te), which showed strong and tunable quantum-confined absorption. Recognizing its potential for SWIR photodetectors and image sensors, the team focused on developing a new process to synthesize phosphine-free versions of silver telluride quantum dots. By using different phosphine-free complexes, they successfully obtained quantum dots with controlled size distribution and excitonic peaks across a broad spectrum.
The newly synthesized quantum dots exhibited remarkable performance, with distinct excitonic peaks exceeding 1,500nm. To validate their potential for practical applications, the researchers fabricated a simple laboratory-scale photodetector using the obtained phosphine-free quantum dots. Initially, the device exhibited low performance in sensing SWIR light, prompting the implementation of a buffer layer. This adjustment significantly enhanced the photodetector’s performance, resulting in a SWIR photodiode capable of detecting a spectral range from 350nm to 1,600nm with exceptional figures of merit.
Building upon the success of the non-toxic quantum dot-based photodetector, the researchers teamed up with Qurv, an ICFO spin-off, to demonstrate its potential by constructing a SWIR image sensor. By integrating the photodiode with a CMOS-based read-out integrated circuit (ROIC) focal plane array (FPA), the team achieved a proof-of-concept image sensor that operates at room temperature. Testing the imager, the researchers successfully captured images of silicon wafers’ transmission under SWIR light and visualized the contents of opaque plastic bottles.
The development of this heavy-metal-free quantum dot technology has significant implications for various industries. Access to the SWIR range using cost-effective technology opens up a range of applications, including improved vision systems for the automotive industry, enabling safe driving under adverse weather conditions. Furthermore, the SWIR band around 1.35-1.40 µm provides an eye-safe window, ensuring minimal background light interference for long-range light detection and ranging (LiDAR), three-dimensional imaging, augmented reality, and virtual reality applications.
With these groundbreaking advancements, the researchers are committed to further enhancing the performance of SWIR photodiodes through engineering and stack layer optimization. Additionally, they aim to explore new surface chemistries for the Ag2Te quantum dots, aiming to improve both their performance and thermal/environmental stability. By overcoming the limitations of heavy metals in quantum dots, this technology holds the key to unlocking the full potential of SWIR light in various high-volume markets.