Photoluminescence Enhancement of CdSe and CdSe-ZnS Nanocrystals by On-Surface Ligand Modification

We have investigated the spectroscopic properties of CdSe and CdSe-ZnS nanocrystal quantum dots (QDs) in the presence of the electron acceptor tetracyanoethylene (TCNE) in organic solution. Our results indicate that TCNE reacts with the n-alkylamine capping ligands at the nanocrystals surface, generating cyanovinylamine products that remain bound to the surface of the QDs, substantially increasing their emission efficiency. The further addition of an excess of TCNE causes a decrease of the luminescence intensity, most likely because of an electron-transfer quenching process from the photoexcited nanocrystals to the electron accepting TCNE molecules. The TCNE-induced emission enhancement is observed also for strongly luminescent CdSe-ZnS core-shell QDs. This approach enables a post-synthetic adjustment of the luminescence efficiency of amine-capped QDs. We also show that the threshold-dependent response of the QD emission on the TCNE concentration can be used to encode AND and XOR boolean logic operations