Size-optical characteristics of CdSe/ZnS quantum dots modified by thiol stabilizers

CdSe quantum dots stabilized with oleic acid were obtained by the colloidal method and their optical properties were studied. The average nanoparticle size, determined by the exciton absorption peak, was 3.7 nm. By growing a ZnS shell onto the surface of CdSe nanoparticles, CdSe/ZnS core-shell hybrids are synthesized. In this case, the quantum yield of luminescence increases from 19 to 76%. It was shown that during the growth process, the size of the CdSe core decreases to 2.8 nm due to the replacement of surface cadmium ions by zinc ions. As a result, the absorption and emission peaks of CdSe/ZnS quantum dots shift to the short-wavelength region. The total (hydrodynamic) particle size increases due to formation of the ZnS shell. The obtained nanoparticles are used in surface modification reactions by replacing the stabilizer. For efficient replacement molecules containing a terminal thiol group were selected – dodecanethiol-1 and dihydrolipoic acid dodecyl ester, capable of forming strong bonds with the surface of the nanoparticles. Ligand exchange reactions were carried out under mild conditions using an excess amount of a thiol stabilizer. Successful replacement of the stabilizers was established by IR spectroscopy. It was also determined that some of the oleic acid molecules remain unsubstituted. It is shown that surface modification leads to a change in the hydrodynamic size of quantum dots, due to the difference in the lengths of the molecules used in the reactions. The optical properties of modified quantum dots were investigated. It was found that the attachment of thiol stabilizers to the surface leads to a decrease in their luminescence intensity.