Binary Amine−Phosphine Passivation of Surface Traps on CdSe Nanocrystals

Surface traps, such as electron and hole traps, quench the photoluminescence (PL) of semiconductor nanocrystals. We find a binary ligand system that effectively passivates those surface traps on bare CdSe nanocrystals, thereby making the nanocrystals highly luminescent. Zinc-blende CdSe nanocrystals are prepared by colloidal synthesis, and their optical properties are monitored by varying the amounts of propylamine (PA) and tributylphosphine (TBP) in chloroform at room temperature. The starting CdSe nanocrystals that are mostly covered with fatty acid carboxylates show very low quantum efficiency with a multiexponential PL decay. Addition of excess PA induces blueshifts in both absorption and emission spectra with a slight increase in quantum efficiency and PL lifetime, whereas that of TBP reduces the PL intensity. Surprisingly, addition of both PA and TBP makes nanocrystals emit light with an ∼50% quantum efficiency and a nearly single-exponential PL decay, regardless of the sequence of ligand addition. ...