Relations between the photoluminescence efficiency of CdTe nanocrystals and their surface properties revealed by synchrotron XPS

The surface of colloidally prepared CdTe nanocrystals capped with thioglycolic acid has been studied by photoelectron spectroscopy with tunable synchrotron radiation excitation. Colloidally prepared CdTe nanocrystals possess photoluminescence properties which depend on the growth conditions. Two samples of the same particle size prepared at different net growth rates were investigated. The nanocrystals which were slowly grown in a dynamic equilibrium of growth and dissolution present a much higher photoluminescence quantum yield. We have performed a photoemission study in order to reveal if there are structural differences at the nanocrystal surface which might explain the observed differences in the photoluminescence efficiency. Significant differences are indeed observed in high-resolution spectra of the Te 4d and Te 3d level. In contrast to the highly luminescent nanocrystals, the lowly luminescent particles present a large amount of Te surface sites. A structure model is proposed which allows explanation of the differences in the luminescence efficiency.