Ligand Mediated Surface Reconstruction of Photoluminescent CdTe Quantum Dots
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Enhancement of photoluminescence (PL) is observed for light-shielded dodecylamine-capped colloidal CdTe quantum dots (CdTe/DDA QDs) dispersed in toluene after washing and recapping. The PL quantum yield (QY) increases from 19% to 25% in 25 hours, indicating gradual reconstruction of the surface. This enhancement is contrasted with PL quenching of light-shielded CdTe QDs recapped with octadecylamine (CdTe/ODA), for which the QY decreases from 17% to 15% in 26 hours. The differences in PL evolution are explained by the presence of ligand-imposed energy barriers on the order of kT and > kT for surface reconstruction of the DDA- and ODA-capped surfaces, respectively. This is supported by the observation of enhancement of the PL QY from 19% to 27% in 26 hours for CdTe/DDA and 17% to 21% for CdTe/ODA QDs that are exposed to short flashes of 400 nm light. The kinetics are related to interactions between amine alkyl chains by vibrational spectroscopy. The results demonstrate the active involvement of the amine capping layer in NC surface reconstruction. A model is proposed that explains this form of surface reconstruction as a transition from a state in which the rigid capping layer imposes disorder on the NC, to a state in which the NC dictates the configuration of the capping layer. It is suggested that illumination locally heats the capping layer of QDs in which the exciton decays non-radiatively, so that the effect is similar to earlier reports of luminescence temperature antiquenching in amine-capped CdSe QDs.