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<(From left) Professor Jung-Yong Lee, Ph.D. candidate Yun Hoo Kim and Dr. Byeongsu Kim>

In a significant breakthrough for quantum technology, KAIST researchers have developed an advanced avalanche multiplication technology using colloidal quantum dots (CQDs). This innovation addresses key limitations in avalanche photodiode (APD) devices, which are crucial for applications such as quantum computing, night vision, autonomous vehicles, and space observation. *Avalanche Photodiode Devices: APD devices using crystalline semiconductors have long been employed to detect extremely faint light. However, these conventional devices suffer from high thermal noise, requiring cryogenic cooling, and lack materials with high detection efficiency in the infrared spectrum. These challenges have restricted their practicality for quantum communication and infrared sensing applications.

EE research team led by Professor Jung-Yong Lee has successfully developed an avalanche charge multiplication technology utilizing colloidal quantum dots (CQDs). This technology achieves 85-fold electron generation from a single infrared photon absorption, surpassing the limitations of conventional techniques. *Avalanche Charge Multiplication: A signal amplification method where electrons in a semiconductor, subjected to a strong electric field, gain kinetic energy and collide with adjacent atoms, generating additional electrons.

Colloidal quantum dots (CQDs), chemically synthesized semiconductor nanoparticles, are emerging as promising candidates for next-generation infrared sensors due to their solution processability and facile bandgap tunability. Unlike tranditional crystalline semiconductors, CQDs possess a unique energy structure that effectively suppresses thermal noise. However, their low charge mobility and high charge recombination rates due to surface bonds often degrade charge extraction efficiency.

The research team addressed these challenges by applying a strong electric field to accelerate electrons, gaining kinetic energy and generating additional electrons through a cascading process in neighboring quantum dots. This led to an 85-fold signal amplification under infrared irradiation at room temperature and the developed CQD-based photodetector achieved a specific detectivity of over 1.4×10¹⁴ Jones, surpassing the sensitivity of standard night vision devices by tens of thousands of times.

<Figure 1. Schematic illustration of avalanche charge amplification mechanisms

in quantum dot devices and the specific detectivity performance of the quantum dot-based avalanche photodiode.>

Infrared photodetectors are essential for a wide range of applications, including autonomous vehicles, quantum computing, and advanced medical imaging. However, traditional quantum dot-based technologies have long been limited by low sensitivity and high noise.

This research represents a paradigm-shifting technological advance, establishing a strong foundation for South Korea to lead the global quantum technology market by securing core innovations in quantum sensing and infrared detection.

Dr. Byeongsu Kim, the first author of the study, emphasized the groundbreaking nature of this work: “The quantum dot avalanche device is an entirely novel research area, never previously reported. This foundational technology has the potential to drive ventures that will lead the global markets in autonomous vehicles, quantum computing, and medical imaging.”

Dr. Byeongsu Kim from KAIST’s Information and Electronics Research Institute, Dr. Sang Yeon Lee from IMEC, and Dr. Hyunseok Ko from the Korea Institute of Ceramic Engineering and Technology contributed as co-first authors of the study. Their findings was published in Nature Nanotechnology on December 18, under the title: “Ultrahigh-gain colloidal quantum dot infrared avalanche photodetectors” (DOI: 10.1038/s41565-024-01831-x).

This research was supported by the National Research Foundation of Korea, including Nano-Material Technology Development Program, Strategic Research Laboratory Program for Future Displays, and Individual Basic Research Program.

Carbon dioxide (CO2) is a major respiratory metabolite, and continuous monitoring of CO2 concentration in exhaled breath is not only an important indicator for early detection and diagnosis of respiratory and circulatory system diseases, but can also be widely used for monitoring personal exercise status. KAIST researchers succeeded in accurately measuring CO2 concentration by attaching it to the inside of a mask.

EE Professor Seunghyup Yoo’s research team in the School of  Electrical Engineering have created a breakthrough wearable CO₂ sensor. This new device enables real-time breath monitoring while maintaining low power consumption and high-speed performance.

Traditional non-invasive CO₂ sensors have been hampered by their bulky size and high power requirements. While optochemical sensors that use fluorescent molecules offer promising advantages in terms of size and weight, they also face a significant challenge: the fluorescent dyes tend to degrade over time when exposed to light. This instability has limited their practical use in wearable healthcare devices. As these optochemical sensors work by measuring changes in fluorescence intensity, which decreases with CO₂ concentration, the key to their effectiveness lies in accurately detecting these fluorescence variations over a sufficiently long period of time.

To address these challenges, the research team engineered a low-power CO₂ sensor that incorporates an organic photodiode that surrounds an LED. This design greatly enhances light collection efficiency and minimizes the exposure of fluorescent molecules to excitation light for a given level of signal. As a result, the device achieves power consumption of just 171 μW, which is substantially lower than the several milliwatts consumed by existing optochemical CO₂ sensors.

The research team further elucidated the photodegradation path of fluorescent molecules used in CO₂ sensors. They uncovered the cause of the increase in error rates over time in photochemical sensors, and proposed an optical design method to mitigate these errors.

Building on these insights, the research team developed a sensor that significantly reduces errors caused by photodegradation, a persistent issue with previous photochemical sensors. Impressively, the new sensor maintains continuous functionality for up to nine hours—far surpassing the 20-minute lifespan of existing technologies—and can be reused multiple times simply by replacing the CO₂-detecting fluorescent film.

The newly developed sensor, which is lightweight (0.12 g), thin (0.7 mm), and flexible, was effectively integrated inside a face mask. It boasts fast response times and high resolution, enabling it to monitor respiratory rates by distinguishing between inhalation and exhalation in real-time.

Professor Seunghyup Yoo commented, “The low power consumption, high stability, and flexibility of the developed sensor make it highly suitable for wearable devices. It holds great potential for the early diagnosis of various conditions, including hypercapnia, chronic obstructive pulmonary disease, and sleep apnea.” He also highlighted its utility in environments with high dust levels or where masks are worn for extended periods, such as during seasonal changes, noting its potential to alleviate the side effects caused by rebreathing.

This groundbreaking research was conducted with the involvement of Minjae Kim, an undergraduate student from Department of Materials Science and Engineering, and Dongho Choi, a doctoral student from the School of Electrical Engineering, as joint first authors, and published in the online version of Cell’s sister journal, Device, on the 22nd of last month. (Paper title: Ultralow-power carbon dioxide sensor for real-time breath monitoring) DOI: https://doi.org/10.1016/j.device.2024.100681

This study was supported by the Ministry of Trade, Industry and Energy’s Materials and Components Technology Development Project, the National Research Foundation of Korea’s Original Technology Development Project, and the KAIST Undergraduate Research Participation (URP) Project.