Binary Amine-Phosphine Passivation of Surface Traps on CdSe Nanocrystals

Abstract

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 similar to 50% quantum efficiency and a nearly single-exponential PL decay, regardless of the sequence of ligand addition. We further characterize the chemical species dissolved into the Solution after amine-phosphine passivation by using X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry (ESIMS). The XPS data indicate that fatty acid carboxylates are covered on the surface of as-prepared nanocrystals and both amine and phosphine remove carboxylates from the surface. The ESI mass spectra identify the chemical species dissolved into the solvent by ligand. Taken all together, our results suggest that primary amine and tertiary phosphine dissolve surface adatoms into the solution and cooperatively passivate surface dangling bonds on CdSe nanocrystals, thereby reproducibly yielding bright CdSe nanocrystals.