Topics in colloidal nanocrystals: synthesis and characterization, polymorphism, and self-assembly
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The first step in the utilization of the unique and potentially technologically impactful size- and shape-dependent properties of nanocrystals is to develop reproducible methods to synthesize monodisperse nanocrystals. The development of the solventless synthetic approach showed a new pathway towards the synthesis of high quality rodshaped and triangular prism-shaped rhombohedral NiS nanocrystals. The nanorods and nanoprims both form via the thermal decomposition of the nickel thiolate single-source precursor. The length on the nanorods was dependent on growth conditions, varying between 15 and 50 nm. The solventless approach was also capable of producing triangular nanoprisms in a 1:1 ratio with the nanorods, depending on the overall concentration of the reactant species. We have demonstrated that synthetic pathway chosen can also dictate the shapes and phases accessible to the nanocrystal product with our comparisons between the solution-phase and solventless syntheses of nickel sulfide and copper sulfide. The solution-phase approach to nickel sulfide resulted only in polydisperse cubic Ni3S4 nanocrystals, which were a reaction byproduct in the solventless synthesis, shaped either as quasi-spheres or quasi-cubes. The stoichiometery of the copper sulfide product synthesized in solution was dependent on the Cu:S reactant molar ratio, with either CuS (covellite) and Cu1.8S (digenite) forming. CuS, Cu1.8S and Ni3S4 differ from the Cu2S and NiS nanocrystals obtained by solventless decomposition of metal thiolate single source precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and morphology for nickel sulfide. CuS nanodisks self-assembled into well-ordered columnar phases. Sterically-stabilized CdS nanorods were observed to self-align into networks of stripes several micrometers long when dropcast from dispersions at sub-monolayer coverage. The nanorods assemble side-by-side with their long axes parallel to the stripe direction. Nanorods 3 to 5 nm in diameter, with aspect ratios ranging from 4 to 12 were found to form stripe patterns. We propose that interparticle attractions between the rods drive the formation of the stripes, and that the rods are trapped in the stripe networks by solvent evaporation dynamics.