Investigating the influence of ligands on the surface-state emission of colloidal CdSe quantum dots

Semiconductor based light generation is of enormous contemporary interest, given that a large fraction of global energy is used for lighting. White-light semiconductor colloidal quantum dots may find application in future solid state lighting technologies. These dots possess two inherent emission bands, a narrow emissive band attributed to a quantum confined exciton, and a broad emission associated with surface trapping. White light CdSe colloidal semiconductor nanocrystals passivated with phosphonic acids were synthesized by a hot-injection method. Aliquots of this sample are then ligand exchanged with amine and thiol ligands. These samples are embedded in polystyrene films, and a series of temperature dependent photoluminescence measurements are performed. The spectral width as a function of temperature is plotted for all samples. These data are then analyzed in terms of three models. The results suggest that surface line shape broadness may be tied to strong electron-phonon coupling and is largely ligand dependent. The amine and phosphonic acid passivated samples showed large temperature dependence over the range studied, whereas the thiol passivated sample had a lower dependence. This is tentatively explained in terms of hole delocalization in the case of thiol passivation.