Synthesis of doped semiconductor nanostructures using microemulsions and liquid crystals as templates
Semiconductor nanocrystals, also known as quantum dots (QDs), are a relatively new class of materials with unique size-dependent optical properties that enable the use of these materials in a variety of applications, including fluorescent labels for biomolecules, illumination and display technologies and photovoltaics. When the size of the QD is smaller than the mean separation of an optically excited electron-hole pair, or exciton, size-dependent fluorescence is observed as their emission peak shifts to larger wavelengths with increasing size. Doping of QDs with transition metals enables the tuning of their optoelectronic properties, leading to emission wavelengths longer than their bulk emission. The doping of QDs has recently garnered significant attention because it allows for the ability to tune the QD emission without changing its size.
Currently, the most common method for synthesizing QDs involves the injection of organometallic precursors into hot coordinating solvents. To obtain monodisperse nanocrystals with this technique, instantaneous injection of the reactants, uniform nucleation over the entire reactor volume and perfect mixing are required. These conditions are difficult to achieve in practice, and even more difficult in a scaled-up reactor system necessary for commercial applications. The use of microemulsions as templates can enable the synthesis of semiconductor nanocrystals of uniform size and shape, and allow for scalability. The template used in this work consists of para-xylene as the continuous phase, water as the dispersed phase, and a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO37-PPO56-PEO37) block copolymer as the surfactant, with the reactants dissolved in the aqueous dispersed phase. Microemulsions formed by this technique, exhibit very slow droplet to droplet coalescence kinetics and allow for the growth of particles with narrow size distribution.
A microemulsion template was used to synthesize Mn-doped ZnSe QDs using zinc-acetate and manganese-acetate as reactants which are dissolved in the aqueous dispersed phase. The microemulsion was placed in a reactor and hydrogen selenide gas was bubbled through the solution. A single ZnSe QD formed in each droplet of the microemulsion via an irreversible reaction between the precursors and coalescence of the resulting nuclei. The size of the nanocrystals was controlled to be between 5 and 8 nm by adjusting the initial concentration of zinc-acetate in water. The quantum confinement threshold for ZnSe is 9 nm and the bulk emission of ZnSe is 460 nm. The as-grown particles initially exhibit a size-dependent emission peak, attributed only to ZnSe, with a wavelength less than 460 nm. An emission peak at 585 nm, attributed to Mn2+ ions, appears after a few days in storage and increases substantially with time, eventually reaching a plateau. This indicates that ZnSe QDs are formed first and Mn2+ ions slowly diffuse into their lattice. The synthesis method employed in this work allows for a detailed study of dopant incorporation into ZnSe nanocrystals as a function of time. The time evolution of the intensity and the ratio of the ZnSe and Mn2+ emission peaks were studied as a function of dopant salt concentration in the precursor solution. A model was developed to describe the Mn2+ incorporation into the ZnSe nanocrystal by assuming that the Mn2+ to ZnSe emission intensity ratio is proportional to the amount of Mn2+ incorporated in the ZnSe lattice.
To enable the use of the doped QDs in applications, a procedure was developed for extraction of the QDs from the template, capping with hydrophilic ligands, and stabilization in an aqueous solution. Experiments were also performed to accelerate the Mn2+ incorporation in the ZnSe lattice. A ZnSe layer was grown over the initial QDs and was found to substantially increase the fluorescence emission intensity. Additionally the synthesis technique was expanded to use liquid crystals as templates with the purpose of growing Mn-doped ZnSe nanostructures, such as nanodiscs or nanowires, which have potential applications in nanoelectronics.