The discovery and synthesis of colloidal quantum dots (QDs) were awarded the 2023 Nobel Prize in Chemistry. QDs, as a novel class of materials distinct from traditional molecular materials and bulk materials, have rapidly emerged in the field of optoelectronic applications due to their unique size-, composition-, surface-, and process-dependent optoelectronic properties. More importantly, their ultra-high specific surface area allows for the application of various surface chemical engineering techniques to regulate and optimize their optoelectronic performance. Furthermore, three-dimensionally confined QDs can achieve nearly perfect photoluminescence quantum yields and extended hot carrier cooling times. Particularly, their ability to be colloidally synthesized and processed using industrially friendly solvents is driving transformative changes in the fields of electronics, photonics, and optoelectronics.

Heavy-metal-free QDs. The review article in the field of heavy-metal-free QDs is from Soochow University of China^[5]. It introduces the development of wide-bandgap and heavy-metal-free QDs for blue QLEDs, addressing the limitations of cadmium- and lead-based QDs due to toxicity concerns. The study explores various QD materials, including ZnSe-based, InP-based, and carbon dots, highlighting their synthesis methods, surface modifications, and core/shell structures to enhance optical performance. It discusses challenges in blue QLEDs, such as low efficiency and stability, and presents strategies like ligand engineering, doping, and innovative device architectures to improve performance. The review provides insights into optimizing QDs for display and lighting technologies, emphasizing the need for environmentally friendly and high-efficiency alternatives to conventional QDs.

Perovskite (CsPbI₃). The article in the field of perovskite QDs is from Jilin University of China^[4]. It introduces the quantum confinement effect in CsPbI₃ QD light-emitting diodes (QLEDs) and its impact on charge transport, exciton dynamics, and emission efficiency. The study explores the trade-offs between QD size and device performance, revealing that smaller QDs enhance efficiency but suffer from charge losses, while larger QDs exhibit higher brightness and stability at high current densities. The research employs advanced spectroscopic and structural analysis to elucidate the relationship between QD size and electronic properties. These findings provide insights into optimizing QLED design for scalable, high-performance optoelectronic applications, balancing efficiency, charge transport, and device stability to enhance next-generation display and lighting technologies.

This Special Topic crystallizes the remarkable momentum in QD optoelectronics research, providing a platform to showcase cutting-edge discoveries and interdisciplinary breakthroughs in this rapidly advancing field. We would like to thank the authors for contributing their articles for this Special Topic. We also thank many anonymous reviewers whose feedback and comments ensure the high quality of the journal. We hope that this JoS Special Topic on QD optoelectronics will provide a useful cross section of the recent progress on the development of the QD synthesis and devices.

In this Special Topic, we have selected four typical types of QD materials and their optoelectronic applications, including 4 Research Articles and 1 Review, to introduce the latest research advances in QD materials and optoelectronic fields.

The second article is from The University of Electro-Communications of Japan^[2]. It introduces a novel solution-phase ligand exchange (SPLE) approach to synthesize stable p-type PbS QD inks using inorganic ligands for high-efficiency solar cells. By incorporating tin (II) iodide (SnI₂) into PbX₂ ligand solutions, the study achieves a controlled transition from n-type to p-type PbS QDs, optimizing their energy levels and carrier dynamics. Compared to traditional layer-by-layer ligand exchange methods, the new approach reduces interfacial defects and enhances device stability. The resulting solar cells achieve a power conversion efficiency of 10.93%, outperforming conventional devices. This work provides a scalable and efficient strategy for producing stable QD inks, advancing next-generation optoelectronic applications.

Cadmium selenide (CdSe). The article in the field of CdSe is from Shandong University of China^[3]. It introduces the layer-dependent optical and dielectric properties of CdSe semiconductor colloidal quantum wells (CQWs), which are crucial for next-generation optoelectronic applications. The study investigates CdSe CQWs with monolayer numbers ranging from 2 to 7, synthesized via thermal injection and atomic layer deposition. Using spectroscopic ellipsometry and first-principles calculations, it reveals a decrease in bandgap from 3.1 to 2.0 eV as thickness increases due to quantum confinement effects. The analysis further explores the refractive index, extinction coefficient, and exciton binding energy variations with layer number, demonstrating their impact on device performance. The findings provide fundamental insights for optimizing CQWs in applications such as photodetectors, solar cells, and quantum emitters.

Lead sulfide (PbS). The first article in the field of PbS QDs is from Soochow University of China^[1]. It introduces a new synthesis approach for lead chalcogenide/lead chalcohalide (PbYX/PbY) core/shell nanostructures, focusing on the effect of halogen precursors on PbS nanocrystals. The study successfully fabricates PbS/Pb₃S₂X₂ core/shell structures, enhancing their optical properties, including a photoluminescence quantum yield increase from 49.3% to 72.0%. Notably, Pb₃S₂Br₂ nanocrystals are synthesized for the first time. Structural evolution from PbS to Pb₃S₂X₂ is confirmed through XRD and XPS analyses. Optical properties, including absorbance and fluorescence lifetime, are significantly influenced by halogen incorporation. This work provides valuable insights into the integration of PbYX and PbY materials, paving the way for further exploration of chalcohalide-based optoelectronic applications.