Jeonbuk National University (JBNU) PhD candidate Kim Jeong-hyun (Division of Advanced Materials Engineering (Electronic Materials Engineering) and Dept. of JBNU-KIST Industry-Academia Convergence Research) announced on the 7th that he has developed a wearable optical sensor built by stacking only inorganic semiconductors without any organic materials. The sensor can accurately measure heart rate and blood oxygen saturation (SpO2) simply by attaching it to the wrist.

Most conventional optical sensors widely used in wearable devices such as smartwatches are based on organic materials. Organic materials are easy to make thin and flexible, but they are vulnerable to moisture and heat, which limits their longevity. In addition, distinguishing light of different wavelengths typically requires separate optical filters, which increases device size and power consumption. In contrast, inorganic semiconductors such as silicon are stable and high-performing, but stacking different materials in multiple layers has been difficult, making their application to wearables challenging until now.

The research team solved this dilemma by heterointegrating monolayer-thickness two-dimensional semiconductors — two-dimensional semiconductors: next-generation semiconductor materials arranged with a thickness on the order of a single atomic layer, i.e., a few nanometers or less — (WS2·MoS2) onto silicon (a three-dimensional semiconductor).

This 3D heterointegrated structure allows red light to be detected by the upper 2D semiconductor layer and near-infrared light by the underlying silicon layer, respectively, without a separate optical filter.

The team also focused on the tiny atomic defects that naturally arise during the heterojunction formation of the two semiconductors (WS2·MoS2). When water molecules weakly adsorb onto these defects, excess electrons remaining in the semiconductor are removed, and the photoluminescence (Photoluminescence: the phenomenon in which a material absorbs photon energy and is excited to a higher energy state, then returns to a stable state by re-emitting light of a specific wavelength; PL) properties were found to become significantly enhanced.

As a result, the intensity was increased by up to 1,140%.

The rise in PL intensity also indicates that the semiconductor interface is correspondingly clean and efficiently separates charges. In fact, for the photodetector the photocurrent (Photocurrent: the current that flows in a circuit when light illuminates a material such as a metal or semiconductor, caused by electrons emitted via the photoelectric effect) reached 16.4 μA, up to 982% higher than for a monolayer semiconductor, and the responsivity (Responsivity: the ratio of the electrical output generated—current or voltage—to the input light intensity (optical power)) reached 97 A/W. These values far exceed the minimum requirements for operating wearable heart rate and SpO2 sensors.

Finally, the device was applied to a wrist-mounted wearable patch and successfully measured heart rate and SpO2 in real time. The developed sensor achieved a signal-to-noise ratio per unit area 23 times higher than previously reported wearable PPG devices (Signal-to-noise ratio: the ratio of the power of the desired useful signal to the power of background noise), and it precisely captured changes in oxygen saturation associated with apnea and its resumption.

The research team stated, “This study demonstrates a case in which the chronic limitations of organic-material-based wearable optical sensors—reliability and wavelength selectivity—were overcome by inorganic semiconductor heterojunctions and defect engineering. By controlling the interface at the atomic level to simultaneously enhance emission and photoelectric properties, this work can contribute to the development of next-generation noninvasive healthcare sensors.”

 

The study was carried out by JBNU Division of Advanced Materials Engineering PhD candidate Kim Jeong-hyun under the supervision of Professor Han-eol Lee, Department of Materials Science and Engineering, GIST, and Professor Tae-hoon Lee, Department of Energy Engineering, DGIST. The research was supported by the Ministry of Science and ICT Mid‑Career Research Program, the Gwangju Metropolitan City local government–university collaborative regional innovation project, the Ministry of Education PhD Student Research Incentive Program, and the POSCO Cheongam Foundation POSCO Science Fellowship.

The research results were published in the latest issue of eScience (Impact Factor: 52.9), a top international journal in engineering.

<Paper>

Spectrally decoupled, signal-boosted photodetection via vertical 2D/3D heterointegration for non-invasive biosignal monitoring, eScience, 2026, 100634, ISSN 2667-1417, https://doi.org/10.1016/j.esci.2026.100634.

○ Paper Title and Author Information 

 - Journal: eScience (Impact factor: IF: 52.9, JCR top 0.5%, 2026)

 - Paper Title: Spectrally Decoupled, Signal-Boosted Photodetection via Vertical 2D/3D Heterointegration for Non-Invasive Biosignal Monitoring 

 - Author information: Kim Jeong-hyun (first author, JBNU Division of Advanced Materials Engineering), Yoon Sun-joo (co-author, JBNU Division of Advanced Materials Engineering), Lee Sang-yeop (co-author, Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH)), Cho Chang-kyu (co-author, JBNU Division of Advanced Materials Engineering), Cho Joo-hyung (co-author, Bio-Medical Standards Center, Korea Research Institute of Standards and Science (KRISS)), Kim Seok (co-author, POSTECH Department of Mechanical Engineering), Lee Yoon-kyung (co-author, JBNU Division of Advanced Materials Engineering), Tae-hoon Lee (co-corresponding author, Department of Energy Engineering, DGIST), Han-eol Lee (corresponding author (lead), Department of Materials Science and Engineering, GIST; Next-Generation Energy Research Institute; Graduate School of Semiconductor Specialization)