Colloidal quantum dots (QDs) are semiconductor nanocrystals that have unique size-tunable optoelectronic properties and are suitable for wet processing. QD research aims to answer fundamental questions about the chemical and physical properties of nanoscale materials and use these tools for technological applications ranging from bio-imaging to quantum optics. At the core of this field is a set of synthetic, processing and analytical methods designed to produce QDs in uniform ensembles that meet the highest performance standards. This Primer reviews QD fabrication methods with a focus on the applications of QDs in printed optoelectronics and quantum optics. After outlining the current state-of-the-art QD syntheses, the experimental and computational analysis of QDs is discussed. These topics are then connected to the methodologies, processes and concepts required for developing QD-based photodetectors, light-emitting devices and quantum optics applications. Special attention is paid to challenges in reproducibility and current limitations of the field, such as the need to balance non-restricted material composition with high performing technology while achieving long-term stability in QD devices under operating conditions. Finally, the ongoing advancement in QD synthesis, precise atomic-level analysis and computational methodologies are highlighted as key drivers towards rational QD design, particularly in understanding how structural changes under loading impact QD properties.

One of the greatest challenges in the field of semiconductor nanomaterials is to make trap-free nanocrystalline structures to attain a remarkable improvement of their optoelectronic performances. In semiconductor nanomaterials, a very high number of atoms is located on the surface and these atoms form the main source of electronic traps. The relation between surface atom coordination and electronic structure, however, remains largely unknown. Here, we use density functional theory to unveil the surface structure/electronic property relations of zincblende II-VI CdSe model nanocrystals, whose stoichiometry and surface termination agree with recent experimental findings. On the basis of the analysis of the surface geometry and the recent classification of the ligand surface coordination in terms of L-, X-, and Z-type ligands, we show that, contrary to expectations, most under-coordinated "dangling" atoms do not form traps and that L- and X-type ligands are benign to the nanocrystal electronic structure. On the other hand, we find clear evidence that Z-type displacement induces midgap states, localized on the 4p lone pair of 2-coordinated selenium surface atoms. We generalize our findings to the whole family of II-VI metal chalcogenide nanocrystals of any size and shape and propose a new schematic representation of the chemical bond in metal chalcogenide nanocrystals that includes explicitly the coordination number of surface atoms. This work results in a detailed understanding of the formation of surface traps and provides a clear handle for further optimization of colloidal nanocrystals for optoelectronics applications.