Special Issue “Application of Advanced Quantum Dots Films in Optoelectronics”

Colloidal quantum dots (CQDs) have been extensively investigated in recent decades. As an alternative to bulk semiconductors, the energy gaps of CQDs can be tuned by not only the CQDs’ composition but also their size. So far, CQDs have been used in a wide range of applications including both light emission and detection. This Special Issue aims to provide a forum for researchers to share their current research findings and to promote further research into CQD-based optoelectronics, including experimental characterization and theoretical calculations. For this Special Issue, we intend to collect 10 original articles on the most recent works on CQD light-emitting diodes, infrared photodetectors, and X-ray sensors.

In the work by Zhang et al. [1], mercury telluride CQDs (HgTe CQDs) were introduced as a new option for infrared detection. Limited by their energy gaps, early studies on PbS CQDs mainly focused on solar cells or near-infrared detection. The emergence of HgTe CQDs greatly extended the spectral sensing ranges of colloidal nanomaterials to mid-infrared and THz ranges. The synthesis, device physics, photodetection mechanism, and multispectral imaging of HgTe CQDs have been discussed. Moreover, nanomaterials are subject to quantum confinement effects. Therefore, they should have a smaller energy state density, which might lead to a reduced dark current and a higher operation temperature. Novel low-dimensional materials with easy fabrication processes and excellent photoelectronic properties provide a possible solution for room temperature infrared photodetectors. Li et al. summarized the preparation methods and characterization of several low-dimensional materials (PbS, PbSe, and HgTe, new two-dimensional materials) and the room temperature infrared photodetectors based on them [2].

For traditional infrared semiconductors, focal-plane arrays (FPAs) have to be fabricated via complicated flip-bonding methods, leading to expensive infrared imagers. Emerging materials, such as inorganic–organic metal halide perovskites, organic polymers, and colloidal quantum dots, have been proposed to develop CMOS-compatible optoelectronic imagers. In the work by Bi et al. [3], the fabrication methods and key figures of merit for FPAs were discussed.

Besides imaging, spectral sensing is of great importance. Unlike HgTe CQDs, HgSe CQDs show a narrowband photoresponse via intraband transitions. Zhao et al. investigated an intraband mid-infrared photodetector based on HgSe colloidal quantum dots (CQDs) [4]. The size, absorption spectra, and carrier mobility of HgSe CQD films were all experimentally studied. Wen et al. reported a simulation study of a microspectrometer fabricated by integrating an intraband HgSe CQD detector with a distributed Bragg reflector (DBR) [5]. Intraband HgSe CQDs possess a unique narrowband absorption and optical response, which makes them an ideal material platform to achieve high-resolution detection for infrared signatures such as molecular vibration.

Integrating CQD films with functional optical structures is an effective way to provide functionalities beyond the CQDs’ intrinsic properties. Infrared detectors with polarization sensitivity could extend the information dimension of the detected signals and improve the target recognition ability. However, traditional infrared polarization detectors with epitaxial semiconductors usually suffer from a low extinction ratio, complexity in structure, and high cost. In the work by Zhao et al. [6], a simulation study of CQD infrared detectors with a monolithically integrated metal wire grid polarizer and an optical cavity was conducted. The polarization selectivity of HgTe CQDs with resonant-cavity-enhanced wire grid polarizers was studied in both shortwave and midwave infrared regions. Besides the high extinction ratio, the optical-cavity-enhanced wire grid polarizer could also significantly improve light absorption by a factor of 1.5, which leads to higher quantum efficiency and better spectral selectivity. With a meta-lens as a light concentrator, an over-20-times enhancement in light absorption can be achieved [7].

Besides infrared, X-ray detectors also demonstrate a wide range of applications. Halide perovskite has remarkable optoelectronic properties, such as high atomic number, large carrier mobility–lifetime product, high X-ray attenuation coefficient, and simple and low-cost synthesis process, and has gradually developed into a next-generation X-ray detection material. In the work by Tan et al., the fabrication methods of halide perovskite film and the development progress of halide-perovskite-based X-ray detectors were introduced [8].

Emerging infrared upconversion imaging devices can directly convert low-energy infrared photons into high-energy visible-light photons; thus, they hold promise in accomplishing pixel-less high-resolution infrared imaging at low cost. In the work by Rao et al., the recent advances and progress of infrared-to-visible upconversion devices were summarized [9]. To further improve the performance of infrared upconverters, one possible direction is to upgrade single-color upconverters to multicolor upconverters, which can correlate the color of emitted light with infrared light intensity, temperature, pressure, or biosignals. The key component, a multicolor light-emitting diode, was discussed by Ma et al. [10].