Metal-Dictated Reactivity of Z-Type Ligands to Passivate Surface Defects on CdSe Nanocrystals

Accessing semiconductor nanocrystals free from surface defects is an outstanding challenge in the design of materials with targeted properties. Despite the established importance of Z-type ligand surface passivation to eliminate defects, the optical and electronic properties of nanocrystals vary depending on the nanocrystal composition and Z-type ligand identity. In this work, a series of Cd-, Zn-, and Pb-based non-native Z-type ligands with the formula MX₂ (X = undecylenate or chloride) were employed to elucidate Z-type ligand characteristics that result in surface passivation of undercoordinated surface ions to eliminate trap states from CdSe nanocrystals. First, CdSe nanocrystals were reacted with N,N,N',N'-tetramethylethylene-1,2-diamine (TMEDA) to remove native Cd(oleate)₂ Z-type ligands from the surface, resulting in undercoordinated surface chalcogen ions. After subsequent reaction with M(UDA)₂, ligands bound to the surface were quantified by NMR spectroscopy, and in parallel, the impact of Z-type ligands on the nanocrystal optical properties was monitored using photoluminescence spectroscopy. We find that Cd- and Zn-based Z-type ligands exhibit similar reactivity with the nanocrystal surface via NMR spectroscopy, yet Cd(UDA)₂ passivation results in an 800% PL increase while Zn(UDA)₂ passivation yields a 13% increase in photoluminescence intensity. Nanocrystals reacted with Pb-based Z-type ligands have lower surface coverage, as quantified by NMR spectroscopy, and lead to only a marginal increase of nanocrystal photoluminescence intensity (60%). These data indicate that the metal identity of the Z-type ligand has a profound impact on the reactivity and resulting electronic structure of the postsynthetically modified nanocrystal. This work provides a framework for achieving defect-free CdSe nanocrystals.