A team of Ajou scientists has developed a near infrared ray (NIR) photodetector capable of detecting deterioration and damage in human muscle structures. The new device is expected to have many applications, including medical imaging equipment that requires highly sensitive and efficient sensors.

The team, led by (Dept. of Materials Science and Engineering / Graduate Dept. of Energy Systems, pictured), has developed a new image sensing device using alternating current photovoltaic effects, which offer high detectivity and rapid rise/decay time. The development was introduced in an article entitled, “Broadband alternating current photovoltaic effect: An application for high-performance sensing and imaging body aches” in the August 29 online issue of Nano Energy (IF = 16.602). The team includes Prof. Park Ji-yong yong (Dept. of Physics / Graduate Dept. of Energy Systems Research), Prof. Kim Sang-wan (Dept. of Electrical and Computer Engineering), and Dr. Mohit Kumar (first author).

Photodetectors, which convert light into electric signals, have so far been associated with solar energy cells. But these devices have much broader application, as they are essential not only in new and renewable energy, but also smartphones, the Internet of Things (IoT), and optical communications. Lately, there has also been growing interest in developing medical applications as well, as photodetectors can help make diagnosis and ongoing monitoring of patients far easier. Industries and academia alike have been particularly interested in photodetector technology that can facilitate the imaging of damage to muscles inside the human body. Photodetectors are already being used in assessing patients with advanced-degree burns.

Photodetector researchers, in turn, have been paying attention to NIRs, as they are expected to minimize further damage to the body by light and enable the detector to sense at greater depths of internal tissues and organs. As a result, there has been great demand for high-performance photodetectors capable of detecting even the slightest changes in NIR signals. Such photodetectors would also have to be highly energy-efficient to be installed on portable medical diagnostic devices.

Compound semiconductors have been the typical choice for NIR-based photodetectors so far. These compound semiconductors, including gallium-arsenic, however, are quite expensive, have low levels of detectivity in the NIR range, and have not performed well thus far. Most critically, they fail to have the high level of detectivity necessary for medical devices to identify and image damage inside the human body.

Prof. Seo’s team has sought to overcome this by laying a quality titanium oxide (TiO2) nanofilm onto a silicone (Si) substrate and controlling the surface characteristics. The researchers have also succeeded in developing a photodetector structure that maximizes optical responsivity using silver nanorays. 

As a result, the team was able to create a high-performance photodetector promising a much higher switching ratio (≈1E4), detectivity (≈1E11 Jones), and response time (~ 120 μs) than available today. The resulting photodetector is superior in all areas of performance over the NIR photodetectors available today.

The alternating photovoltaic current enables the photodetector to generate energy from the rapidly flickering incident rays on its own, eliminating the need to connect it to an outside power source. The photodetector, moreover, generates photovoltaic currents that are nearly 39,000 times greater than direct photovoltaic currents typical of solar cells, making it very well-suited to medical imaging.

Prof. Seo’s team has also identified the operating mechanism of the photodetector using Kelvin probe force and electrostatic force microscopy. The alternating incident rays generate inhomogeneous absorption-induced imbalanced carriers inside the semiconductor. The resulting quasi-Fermi level splitting and realignment, the researchers demonstrated, ensures fast response and high detectivity by the photodetector.

Prof. Seo: “We successfully imaged internal changes in the muscular tissues of the fingers by scan-imaging them with our photodetector.” He explained, “With this device, we will be able to detect [the locations of] body aches and pain and image them at ultra-high speed. We expect the photodetector will be applied to scanning and diagnosing various other types of changes occurring not just in muscles, but in other parts of the body as well, particularly for pediatric patients.”

The team’s research project was made possible in part thanks to the Program for the Future New Material and Original Technology Development Program and the Basic Research Support Program for Experienced Researchers, both from the Ministry of Science and ICT and the National Research Foundation of Korea. A patent application on the invention is now pending.