Quantum dot capping agents are functional molecules that modulate the surface chemistry of quantum dots (QDs) through coordination or physical adsorption. The core functions include stabilizing colloidal dispersion, suppressing surface defects, regulating optical properties, and endowing functional interfaces. The choice of capping agents directly affects the stability, fluorescence quantum yield, and subsequent application compatibility of QDs.
Thiol capping agents form stable monolayers via strong thiol coordination to surface metal atoms. The high binding energy of sulfur-metal bonds can effectively passivate the surface dangling bonds and reduce non-radiative recombination. However, thiols are susceptible to photo-oxidation and thermal degradation; thus, the stability needs to be enhanced by molecular chain length or conjugated structure design.
Phosphorus-containing capping agents coordinate to the surface of QDs via lone pair electrons of the phosphorus atom. The weak coordination characteristics allow dynamic ligand exchange, which is suitable for precise control of size and morphology. They serve dual roles as solvents in high-temperature synthesis, however, they may introduce lattice strain or environmental toxicity.
Carboxylic acid capping agents stabilize QD surfaces through the coordination of carboxyl groups, and the binding strength can be adjusted via pH, which is suitable for aqueous or polar solvent systems. However, carboxylic acid ligands are easily desorbed at high temperatures or strong polar media, thus, long alkyl chains are required to balance coordination stability and dispersibility.
Amine capping agents are adsorbed on the surface of QDs through the coordination or hydrogen bonding of amino groups, and their weak coordination characteristics are conducive to the regulation of surface charge modulation and functionalization. However, their basicity may induce aggregation or luminescent quenching.
Ammonium salt capping agents stabilize QD colloids by electrostatic interaction, which are suitable for polar solvent systems. Cationic headgroups suppress Ostwald ripening via electrostatic attraction to negatively charged surfaces. However, it is easy to decompose at high temperatures, so it is necessary to combine other ligands to improve thermal stability.
Composite capping agents integrate multiple functional groups (e.g., thiol-carboxyl, amine-phosphine) for multifunctionality, such as enhancing colloidal stability and light stability simultaneously or endowing targeted biocompatibility. The design must balance competitive adsorption and steric effects among ligands.