Ionically-Functionalized Lead Sul de Nanocrystals
Moody, Ian Storms
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Moody, Ian Storms
Lead sulfide nanocrystals (PbS-NCs) are an important class of semiconductor nanomaterials that are active in the near-infrared and exhibit unique properties distinct from their bulk analogues, notably, size tunability of the band gap and solution processability. One factor influencing PbS-NC properties is the presence of an organic ligand shell, which forms the interface between the nanocrystal core and its environment. The specific focus of this dissertation is how ionic functionalization of the ligand shell alters the physical and chemical properties of the resulting PbS-NC/ligand complex. Short-chain ligands can improve photoconductivity in PbS-NC thin films, but there are few solution-based preparations available. Chapter II demonstrates how ionic groups can enable functionalization of PbS-NCs with two short- chain thiol ligands - sodium 3-mercaptopropanesulfonate (MT) and sodium 2,3-dimercaptopropanesulfonate (DT) - via a solution-phase exchange procedure. Despite a structural similarity, DT-functionalized PbS-NCs (PbS-DT) are more stable to oxidation than MT-functionalized PbS-NCs (PbS-MT). The relative stabilities are explained in terms of different binding modes to the nanocrystal surface (bidentate vs. monodentate) and oxidation pathways (intermolecular vs. intramolecular). Toxicology studies on nanomaterials have been limited by the availability of water-soluble samples with systematically controlled structures. As examples of such materials, PbS-DT and PbS-MT nanocrystals are studied in Chapter III for their toxicological impacts on embryonic zebrafish. PbS-DT solutions induce less toxicity than PbS-MT solutions, which is explained in terms of the relative stabilities of the nanocrystal solutions. Finally, Chapter IV investigates the hitherto unexplored effects of ionic functionalization on the optical/electrical properties of PbS-NC thin films, with an emphasis on understanding how counter ions affect the photoconductivity of PbS-DT thin films. Films containing small counter ions exhibit increased dark conductivity and responsivity with time under an applied bias, whereas films containing larger or multivalent counter ions show a suppression of this behavior. These results are discussed in terms of ion motion and ion-assisted carrier injection at the PbS-NC/electrode interface. This dissertation includes previously published and unpublished co-authored material.