HKU Engineering scholar demonstrates the smallest all-printed infrared photodetectors to date

University of Hong Kong (HKU)

A research team led by Professor Leo Tianshuo Zhao from the Department of Electrical and Electronic Engineering at the Faculty of Engineering, University of Hong Kong (HKU), has developed the world’s smallest fully printed infrared photodetectors, which are an innovative room-temperature nano-printing platform that overcomes the limitations of traditional silicon-based technology.

Near-infrared (NIR) technology is essential for applications such as autonomous systems, biomedical sensing, and high-speed optical communications. However, conventional silicon-based CMOS technology cannot directly detect NIR wavelengths. Current solutions rely on expensive processes to grow IR-absorbing materials and bond them to silicon circuits, adding complexity and limiting sensor miniaturisation.

To address this challenge, Professor Zhao’s research team has developed a nano-printing platform to demonstrate the world’s smallest fully printed infrared photodetectors—marking a significant advancement in optoelectronic device fabrication.

The versatile nano-printing platform assembles solution-phase colloidal nanocrystals (NCs) and modifies their properties in situ, enabling the construction of advanced optoelectronic devices. The research, conducted in collaboration with Professor Ji Tae Kim from the Korea Advanced Institute of Science and Technology, represents a major step forward in high-resolution, multi-layer printing for electronic device integration.

Using electrohydrodynamic printing (EHDP) combined with surface chemical treatment, the team achieved unprecedented precision—printing silver nanocrystal lines as narrow as 70 nanometers and creating dense films with conductivity comparable to bulk silver, all without high-temperature processing. Most notably, the researchers demonstrated sub-10-micrometre all-printed IR photodiodes for the first time.

“This powerful tool opens new possibilities for printed electronics and heterogeneous integration. First synthesised 30 years ago, colloidal NCs have shown great potential through in-depth fundamental studies, but limited device fabrication and integration with Si technologies have hindered their wider applications in optoelectronics. While the library of NCs provides ideal printable inks, this nano-printing method offers a viable approach to building NC devices from the bottom up,” said Professor Zhao.

The leading author of this paper, Mr Zhixuan Zhao, further elaborated: “Compared to existing techniques that require post-printing sintering, this innovation introduces a room-temperature chemical treatment immediately after printing, enabling high conductivity and customizable material properties without damaging sensitive materials or substrates. This approach also supports layer-by-layer printing, making it possible to achieve the smallest printed IR detectors to date.”

Professor Kim added, “This work offers a novel solution to reduce thermal budgets for IC chip interconnection and integration, which could attract significant interest from the semiconductor industry.”

The team is now advancing this technology by leveraging its high resolution and tunable functionality for broader applications, including optical metasurfaces, biosensors, and hybrid electronics.