Photogenerated excitons in plain core CdSe nanocrystals with unity radiative decay in single channel: the effects of surface and ligands.

A systematic and reproducible method was developed to study the decay dynamics of an exciton, a photogenerated electron-hole pair, in semiconductor nanocrystals in solution. Results revealed that the excitons in plain core CdSe nanocrystals in either zinc-blende or wurtzite or mixed lattice structures could be reproducibly prepared to decay radiatively in unity quantum yield and in single channel. The single-channel lifetime was found to increase monotonically by increasing size of the CdSe nanocrystals, with zinc-blende ones increasing in a relatively slow pace. Surface inorganic stoichiometry was found to be a sensitive parameter to affect the exciton decay dynamics for all crystal structures with different sizes. Excess Se (Cd) sites on the surface were found to induce short (long) lifetime channels for the excitons. Both types of traps reduced the quantum yield of the radiative decay of the excitons, and the hole traps associated with Se sites were nearly not emissive. With optimal surface inorganic stoichiometry, primary amines were identified as "ideal" organic ligands for CdSe core nanocrystals to achieve unity radiative decay of excitons in single channel in comparison to other types of neutral ligands commonly applied in the field.