PbS Nanocrystals Functionalized with a Short-Chain, Ionic, Dithiol Ligand

Ligand exchange from oleic acid-capped PbS nanocrystals was used to prepare sodium 2,3-dimereaptopropanesulfonate (DT) functionalized, water-soluble PbS nanocrystals. Oxygen-free aqueous solutions of the PbS-DT nanocrystals were observed to be stable for more than five months. In aerated solutions, the PbS-DT nanocrystals were observed to be stable to precipitation for more than 24 h in the dark, with precipitation accelerated by illumination. Under both light and dark conditions, the PbS nanocrystals prepared with the chelating DT ligand were observed to be more stable to precipitation than nanocrystals prepared with the monothiol analogue sodium 3-mercaptopropanesulfonate (NIT). The greater stability of the PbS-DT nanocrystals is argued to be due to the formation of oligodisulfides that remain partially bound to the nanocrystal surface thereby inhibiting precipitation. Support for this comes from the fact that the PbS-DT nanocrystals were not observed to substantially accelerate the photooxidation of excess DT ligand, in contrast to the behavior of the PbS-MT nanocrystals in the presence of excess ligand. Due to the compact size of the DT ligand, thin films of PbS-DT nanocrystals were observed to exhibit much greater photoconductivity than PbS nanocrystals capped with the longer chain oleic acid ligand. Responsivities of greater than 1 A/W in the near-infrared were achieved with thin films of PbS-DT NCs on interdigitated array electrodes.