Molecular Interactions at Cadmium Selenide Nanocrystal Surfaces
Creators: Chen, Peter
Publisher: name: Columbia University
Year: 2017
Subjects: Chemistry
Cadmium selenide
Nanocrystals

description: The synthesis of n-alkylamine-bound CdSe-NH2Rʹ nanocrystals from carboxylateterminated CdSe-Cd(O2CR)2/HO2CR requires the removal of acidic impurities prior to the addition of primary amine. Otherwise, the formation and subsequent tight binding of n-alkylammonium carboxylate ion pairs prevents quantitative removal of carboxylate species. Dimethylcadmium and diethylzinc were used as reagents to deprotonate acidic impurities, which either causes methylation (with a surface density of 0.04−0.22 nm−2) and photoinduced reduction of the nanocrystal core or X-type ligand exchange with ethyl species, respectively. The acid-scavenged nanocrystals could be completely isolated from displaced carboxylate ligands (≤ 0.01 carboxylates nm-2). In addition to traditional selective precipitation procedures, gel permeation and silica chromatography were investigated as alternative purification methods for the isolation of CdSe-NH2Rʹ. Both demonstrated no improvement compared to the more convenient precipitation process. Thin films fabricated from CdSe-NH2C4H9 show little to no grain growth upon thermal annealing at 250 ºC, maintaining domains (~10 nm) despite complete desorption of n-butylamine from the nanocrystal surface above 150 ºC. Despite no passivation of the surface and a high density of grain boundaries, thin film transistors of CdSe-NH2C4H9 fabricated on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with an average electron mobility of 12 ± 1 cm2 V-1s-1, a low threshold voltage hysteresis (4.0 ± 0.6 Vth), and an on/off ratio of 8x104. Colloidal dispersions of amine bound nanocrystals (CdSe−NH2Rʹ) are indefinitely stable at amine concentrations of 0.1 M or higher and slowly aggregate at lower concentrations. Dissociation and evaporation of the amine ligands in 4-ethylpyridine, tri-n-butylphosphine, or molten tri-n-octylphosphine oxide solution results in nanocrystal aggregation. Greater stability can be achieved using dimethyl-n-octadecylphosphine as the L-type ligand, yielding soluble CdSe- PMe2C18H37 nanocrystals with a phosphine coverage of 1.8 nm-2. CdSe-PMe2C18H37 is the first stable nanocrystal sample bound solely by neutral phosphines. Z-type rebinding was investigated with metal oleate species (Mn+(O2CR)n, M = Cd2+, Zn2+, Pb2+, In3+), and a relative binding affinity of these complexes can be established. Rebinding of metal oleate species at 25 ºC yield lower coverages, yet can reach saturation upon heating to 100 ºC. The rebinding of cadmium chloride to aggregated CdSe-PBu3 stabilizes the particle and aids in their redissolution. L-type ligand exchange and subsequent Z-type rebinding was employed towards the synthesis of a new model compound passivated by dimethyl-n-octadecylphosphine and cadmium trifluoroacetate ligands, CdSe-Cd(O2CCF3)2/PMe2C18H37, which is characterized by UV-Vis, 1H, 19F, and 31P NMR spectroscopies. The findings of this dissertation demonstrate the importance of ion-pair species in the colloidal stabilization of colloidal nanocrystal systems. It also indicates the utility of stoichiometric, amine and phosphine-bound CdSe-L to act as both reporter complexes and as a clean reactive reagent for synthesis of novel CdSe-MX2/L systems to study the molecular interactions at nanocrystal surfaces.