Molecular control of the nanoscale: Effect of phosphine-chalcogenide reactivity on CdS-CdSe nanocrystal composition and morphology

摘要

We demonstrate molecular control of nanoscale composition, alloying, and morphology (aspect ratio) in CdS-CdSe nanocrystal dots and rods by modulating the chemical reactivity of phosphine-chalcogenide precursors. Specific molecular precursors studied were sulfides and selenides of triphenylphosphite (TPP), diphenylpropylphosphine (DPP), tributylphosphine (TBP), trioctylphosphine (TOP), and hexaethylphosphorustriamide (HPT). Computational (DFT), NMR (~(31)P and ~(77)Se), and high-temperature crossover studies unambiguously confirm a chemical bonding interaction between phosphorus and chalcogen atoms in all precursors. Phosphine-chalcogenide precursor reactivity increases in the order: TPPE < DPPE < TBPE < TOPE < HPTE (E = S, Se). For a given phosphine, the selenide is always more reactive than the sulfide. CdS _(1-x)Se _x quantum dots were synthesized via single injection of a R 3PS-R 3PSe mixture to cadmium oleate at 250 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV/Vis and PL optical spectroscopy reveal that relative R _3PS and R _3PSe reactivity dictates CdS _(1-x)Se _x dot chalcogen content and the extent of radial alloying (alloys vs core/shells). CdS, CdSe, and CdS _(1-x)Se _x quantum rods were synthesized by injection of a single R 3PE (E = S or Se) precursor or a R _3PS-R _3PSe mixture to cadmium-phosphonate at 320 or 250 °C. XRD and TEM reveal that the length-to-diameter aspect ratio of CdS and CdSe nanorods is inversely proportional to R 3PE precursor reactivity. Purposely matching or mismatching R _3PS-R _3PSe precursor reactivity leads to CdS _(1-x)Se _x nanorods without or with axial composition gradients, respectively. We expect these observations will lead to scalable and highly predictable "bottom-up" programmed syntheses of finely heterostructured nanomaterials with well-defined architectures and properties that are tailored for precise applications.