Ligand-dependent blinking of zinc-blende CdSe/ZnS core/shell nanocrystals

Blinking of zinc-blende CdSe/ZnS core/shell nanocrystals are studied as a function of surface passivating ligands. Organic-soluble CdSe/ZnS core/shell nanocrystals are prepared by colloidal synthesis free of trioctylphosphine oxide and converted into water-soluble ones by ligand exchange with three different hydrophilic thiols, 2-aminoethanethiol, 3-mercapto-1-propanol, and 3-mercaptopropionic acid. The zinc-blende lattice structure is confirmed by powder X-ray diffraction, the size and shape distributions are visualized by high-resolution transmission electron microscopy, and hydrodynamic size distributions of water-soluble nanocrystals are determined by dynamic light scattering. Ligand-dependent optical properties, such as the absorption and emission spectra as well as the photoluminescence lifetime, are obtained in both solution and glass substrate to characterize the effects of ligand on the bright state of nanocrystals. The time trace of blinking is recorded for single nanocrystals in polymer film. Both on- and off-time distributions are fit to a power law. The off-time exponents are clustered at 1.67+/-0.05, whereas the on-time exponents are scattered in the range of 1.71-2.25. The thiolate conjugation on the surface zinc atom greatly reduces the on-time duration, suggesting that the rate of photoinduced charge separation from the bright (on) to the dark (off) state increases as the number of surface hole-trap states increases.