Molecular Control of the Nanoscale: Effect of Phosphine-Chalcogenide Reactivity on CdS-CdSe Nanocrystal Composition and Morphology

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 (P-31 and Se-77), 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. CdS1-xSex quantum dots were synthesized via single injection of a R3PS-R3PSe mixture to cadmium oleate at 250 degrees C. X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV/Vis and PL optical spectroscopy reveal that relative R3PS and R3PSe reactivity dictates CdS1-xSex dot chalcogen content and the extent of radial alloying (alloys vs core/shells). CdS, CdSe, and CdS1-xSex quantum rods were synthesized by injection of a single R3PE (E = S or Se) precursor or a R3PS-R3PSe mixture to cadmium-phosphonate at 320 or 250 degrees C. XRD and TEM reveal that the length-to-diameter aspect ratio of CdS and CdSe nanorods is inversely proportional to R3PE precursor reactivity. Purposely matching or mismatching R3PS-R3PSe precursor reactivity leads to CdS1-xSex 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.