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 tetracyanoethene (TCNE) in organic solution. Our results indicate that TCNE reacts with the n-alkylamine capping ligands at the surfaces of nanocrystals to generate (cyanovinyl)amine products that remain bound to the surfaces of the QDs, substantially increasing their emission efficiency. Further addition of an excess of TCNE caused a decrease in the luminescence intensity, most likely because of an electron-transfer quenching process from the photoexcited nanocrystals to the electron-accepting TCNE molecules. TCNE-induced emission enhancement was also observed for the strongly luminescent CdSe–ZnS core–shell QDs. This approach enables a post-synthetic adjustment of the luminescence efficiency of amine-capped QDs. We have also shown that the threshold-dependent response of the QD emission on the TCNE concentration can be used to encode NAND and XOR Boolean logic operations