Templating the synthesis of compound semiconductor nanostructures using microemulsions and lyotropic liquid crystals.

摘要

In this work, novel synthetic strategies have been developed that employ templates to control the size, shape, and composition of compound semiconductor nanostructures at room temperature. The templates were formed by self-assembly of a poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) amphiphilic block copolymer in the presence of a polar solvent and a less polar one. PEO-PPO-PEO block copolymers, due to their amphiphilic character, exhibit very rich structural polymorphism when dissolved in selective solvents of different polarity, and can attain a number of microstructures such as spheres, cylinders, and lamellae. Using these self-assembled structures as nanoreactors, a variety of semiconductor nanocrystals were obtained that acquired the geometry of the template.; Luminescent quantum dots were synthesized by a microemulsion-gas contacting technique that exploited the spherical nanodroplets of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were grown by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a PEO-PPO-PEO amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The microemulsion-encapsulated nanocrystals exhibit size-dependent luminescence and their photoluminescence spectra remain unchanged over a period of several months. The ability of the technique to tune the chemical composition of the nanocrystals was demonstrated with the growth of CdSe quantum dots, which was realized after substituting diethylzinc with dimethylcadmium.; Lyotropic liquid crystals of the PEO-PPO-PEO/heptane/formamide self-assembled system were employed to control the shape and morphology of semiconductor nanostructures. ZnSe quantum dots, nanowires and nanodisks were grown inside the spherical, cylindrical, and planar nanodomains of the cubic, hexagonal, and lamellar liquid crystalline phases of the above ternary system, respectively. The phase behavior of the ternary system at room temperature was investigated in detail to identify the compositions corresponding to the phase regions of interest. (Abstract shortened by UMI.)