Ligand exchange on nanocrystals: fundamental mechanisms, regulation of nanocrystal properties, self-assembly, and advanced applications
Colloidal nanocrystals (NCs) represent essential building blocks for emerging next-generation technologies. However, long-chain organic ligands introduced during synthesis are electrically insulating, which creates substantial bottlenecks for charge transport in NC-based assemblies. In this review, we systematically summarize the well-established framework for NC surface coordination chemistry (X-, L-, and Z-type ligands) and sort out the existing mechanistic understandings regarding the thermodynamic equilibria and kinetic pathways that govern ligand displacement. Based on these fundamentals, we systematically evaluate ligand-exchange strategies including single-phase and two-phase solution methods, with an emphasis on their capabilities for atomic-level surface engineering. Importantly, we discuss how ligand engineering modulates interparticle interactions (e.g., van der Waals, electrostatic and magnetic forces) to direct the self-assembly of NCs into superlattices with strong interfacial electronic coupling. We further summarize the diverse applications of ligand-engineered NC assemblies in energy storage, catalysis and optoelectronics. To address the existing challenges regarding structural stability and scalable fabrication, we highlight future research directions, such as achieving uniform ligand exchange at the atomic scale, in situ characterization of exchange kinetics, and machine learning-assisted inverse design. Progress in these areas will facilitate the rational design of functional nanomaterials based on precisely tailored NCs.