Progress on the surface ligand engineering of lead sulfide colloidal quantum dots

Colloidal quantum dots (CQDs) are zero-dimensional semiconductor materials with solution-processable properties. These materials have attracted considerable attention in the research and development of new photodetectors, photovoltaic cells, light-emitting diodes, and chemical sensors. The large exciton Bohr radius and Debye length and the considerable quantum size effect make lead sulfide one of the typical hotspots in CQDs. Owing to the large specific surface area and abundant surface dangling bonds, the surface ligand of CQDs greatly influences their physical and chemical properties. Surface ligand engineering can be used to realize the functional design and performance improvement of quantum dot semiconductor devices. This article reviews the research progress in the surface ligand engineering of lead sulfide (PbS) CQDs, focusing on the influence of surface ligands on their conductive properties and chemical activity. To improve the surface passivation and carrier mobility, PbS CQDs are developed from short-chain organic ligands to inorganic ligands represented by metal chalcogenide complexes (MCC), especially by the introduction of monovalent halogen atomic ligands, through surface ligand engineering. We further introduce the liquid-phase ligand-exchange technology. Compared with film-level ligand exchange, liquid-phase ligand exchange favors complete ligand replacement and one-step deposition of quantum dot solids using a colloidal stable nanoparticle ink. With further detailed research, the air stability of PbS CQDs can be improved. This improvement will not only lay a solid foundation for the application of PbS CQDs in optoelectronic devices but also provide opportunities for the development of room-temperature chemical sensors. In the last part, we discuss the chemical activity of PbS CQDs and their application for gas sensing. CQDs are ideal gas-sensitive materials owing to their large surface area and abundant active sites for gas adsorption, and the surface states formed by gas adsorption considerably affect the physical and chemical properties of the CQDs. Meanwhile, the CQDs have excellent film-forming properties at room temperature. Hence, they can be coated on a silicon substrate by simple and controllable methods such as spin coating or spraying. In summary, the surface ligand engineering of CQDs is an important strategy to develop new semiconductor functional devices. The optoelectronic properties and chemical activity of PbS CQDs indicate sufficient scope for design and regulation with various types, components, and introduction methods of the surface ligand. Great breakthroughs have been made with regard to PbS CQDs in both stability research and low-cost mass production over the last decade. Despite various challenges, the basic research on PbS CQDs for photoelectric and chemical sensing is ongoing. To improve the design ideas and fabrication methods of CQD functional devices, it is necessary to use methods based on theoretical calculations and microcharacterization techniques to reveal the effect of surface ligands on CQDs. The understanding of surface science and device physics of CQDs will drive the utilization of the semiconductor quantum effect. In the future, great breakthroughs are expected for the use of PbS quantum dots in the fields of infrared imaging, spectral analysis, gas sensing, and biochemical sensing.