Infrared detectors play an important role in military and aerospace fields including guidance, remote sensing, and reconnaissance. At present, infrared detectors are mainly based on bulk semiconductor materials such as mercury cadmium telluride (HgCdTe), indium gallium arsenic (InGaAs), and indium antimonide (InSb). However, these materials need to be fabricated on a lattice-matched substrate by a high-cost epitaxially grown method and be integrated with readout circuits through complex flip-chip bonding technology, restricting the further improvement of imaging array scale and resolution. Thus, it is significant to develop new material systems to replace traditional bulk semiconductor materials, so as to achieve low-cost, large-scale, and high-resolution infrared detectors.

The colloidal quantum dots (CQDs), as new semiconductor nanocrystal materials, can achieve precise band-gap regulation in a wide spectrum due to the quantum confinement effect. Besides, CQDs can be synthesized on a large scale and at a low cost by liquid-phase chemical method. Furthermore, the liquid phase processing technology of CQDs enables direct on-chip electrical coupling with silicon readout circuits without the need for flip-bonding. Therefore, CQD materials have gained wide attention and made significant progress in infrared detection and imaging. Among them, mercury chalcogenide CQDs have been proven to have a wide range of infrared detection bands including short-wave, mid-wave, and long-wave infrared bands. Besides, two-color or multi-color band detection, focal plane array imaging, and infrared-to-visible upconverters based on mercury chalcogenide CQDs have been studied and exhibited excellent device performance. Although infrared optoelectrical detection technology based on mercury chalcogenide CQDs has been widely studied, there is a lack of review to summarize the recent works. Hence, it is important to summarize the existing research and propose the future development direction.

First, according to the absorption process of CQDs, the infrared detectors based on mercury chalcogenide CQDs can be divided into interband and intraband transition. The device performance including cut-off wavelength, detectivity, external quantum efficiency (EQE), and responsivity are summarized and compared, as shown in Figs. 2-6 and Table 1. In 2011, Guyot-Sionnest professor from the University of Chicago first reported the interband transition mid-wave infrared photodetector based on the mercury telluride (HgTe) CQDs, exhibiting the detectivity of 10⁹ Jones. In 2014, the same team developed an intraband transition mid-wave infrared detector based on mercury selenide (HgSe) CQDs with a detectivity of 8.5×10⁸ Jones at 80 K. On this basis, since 2020, the group from the Beijing Institute of Technology carried out systematic research on infrared detectors based on mercury chalcogenide CQDs and made breakthroughs in two-color or multi-color band detection. In 2022, the team developed a CQDs single-band short-wave infrared imaging and fused-band imaging (short-wave and mid-wave infrared) dual-mode detector capable of detecting, separating, and fusing photons from various wavelength ranges using three vertically stacked CQD homojunction. The dual-mode detectors showed a detectivity of up to 8×10¹⁰ Jones at the fused-band mode and 3.1×10¹¹ Jones at the single-band mode, respectively.

Infrared-to-visible upconverters converting low-energy infrared light to higher-energy visible light without bringing in complicated readout integrated circuits have triggered enormous excitement. In 2022, the group from the Beijing Institute of Technology reported the upconverters using HgTe CQDs as the sensing layer and extended the operation spectral ranges to short-wave infrared bands for the first time (Fig. 7). Besides, mercury chalcogenide CQDs play an important role in improving the resolution of infrared focal plane array (FPA) imagers because the pixel pitch is only determined by the readout circuit array. In 2016, the research team at the University of Chicago reported the first HgTe CQD mid-wave infrared FPA imagers with EQE of 0.30%, detectivity of 1.46×10⁹ Jones, and noise equivalent temperature difference (NETD) of 2.319 K at the temperature of 95 K (Fig. 8). In 2022, the research team of Sorbonne University in France prepared photoconductive HgTe CQD FPA imagers of 1.8 μm through spin coating technology with 640×512 and pixel pitch of 15 μm (Fig. 8). On this basis, the group from the Beijing Institute of Technology continued to innovate in the field of mercury chalcogenide CQD FPA imagers. In 2022, a new device architecture of a trapping-mode detector was proposed and successfully utilized for HgTe CQD FPA imagers. The complementary metal oxide semiconductor (CMOS)-compatible HgTe CQD FPA imagers exhibit low photoresponse non-uniformity (PRNU) of 4%, dead pixel rate of 0%, high EQE of 175%, and high detectivity of 2×10¹¹ Jones for extended short-wave infrared bands (cut-off wavelength is 2.5 μm) @ 300 K and 8×10¹⁰ Jones for mid-wave infrared bands (cut-off wavelength is 5.5 μm) @ 80 K (Fig. 9). Furthermore, high-resolution single-color images and merged multispectral images from ultraviolet to short-wave infrared bands were obtained by using direct optical lithography for FPA imagers based on HgTe CQDs (Fig. 10). The performance of mercury chalcogenide CQDs-based FPA imagers is summarized, as shown in Table 1. In the end, the problems faced and the ongoing research trends in this field are discussed.

In the past decade, there have been great breakthroughs in mercury chalcogenide CQDs-based infrared detectors from single-pixel detectors to FPA imagers. In summary, the physical properties of mercury chalcogenide CQDs such as carrier mobility and device performance including response speed, infrared detection band range, detectivity, and photoresponse uniformity still need to be improved, so as to promote the development of mercury chalcogenide CQDs-based infrared detectors.