(348k) Template-Assisted Synthesis, Extraction, Annealing, and Surface Capping of Zinc Selenide Quantum Dots

Semiconductor nanocrystals or quantum dots are exciting nanomaterials due to their size-tunable and narrow photoluminescence emission spectra, broad excitation by any wavelength smaller than the emission wavelength, high quantum yields, and excellent photostability. Quantum dots have applications in biological sensing, light-emitting devices, lasers, and solar cells. As quantum dot based discoveries become mature for commercial exploitation, the development of scalable synthesis and post-processing techniques for these materials is a significant challenge that must be addressed.

The most common synthesis technique for quantum dots utilizes small batch reactors in which nanocrystals grow as function of time, after a swift injection of organometallic precursors into a hot coordinating solvent. Scale up is limited by poor mixing of the precursors in larger batches and by non-uniform nucleation of particles, which can lead to broad size distributions. The high cost, flammability and toxicity of organometallic precursors are additional concerns associated with this operator-intensive synthesis method.

A novel technique that employs stable microemulsions and liquid crystals as templates for the synthesis of quantum dots has been developed [1,2]. The technique can be easily scaled up for commercial production and enables precise control of particle size and size distribution. The typical components of a water-in-oil microemulsion employed for quantum dot synthesis are water, p-xylene, and a polyethylene oxide - polypropylene oxide - polyethylene oxide (PEO-PPO-PEO) amphiphilic block copolymer as the surfactant. The technique enables the synthesis of a single quantum dot in each droplet of the microemulsion, whose size is controlled by the concentration of a precursor dissolved in the aqueous dispersed phase.

Zinc selenide quantum dots can be grown by reacting zinc acetate diluted in the aqueous dispersed phase with hydrogen selenide gas that is bubbled through the microemulsion at room temperature [2]. Quantum dot size can be precisely controlled by selecting the appropriate concentration of zinc acetate in the aqueous dispersed phase and allowing complete conversion of the reactant to particles. A single quantum dot is obtained in each droplet due to coalescence of all nuclei and clusters formed inside the droplet. The size distribution of the quantum dots is controlled by the size distribution of the droplets of the microemulsion, which is typically narrow. This technique can be scaled up very easily and is less operator-intensive when compared to the commonly used batch processing technique that employs injection of precursors into hot ccordinating solvents.

To prepare quantum dots for commercial applications, a technique for extracting them from the microemulsion templates has been developed. Subsequent annealing of the extracted quantum dots in hot hexadecylamine and trioctylphosphine has been used to improve their crystalline quality and maximize their fluorescence emission intensity. Capping of the zinc selenide quantum dots with a zinc sulfide shell was used to further stabilize their surface and maximize their quantum yield. The synthesis and post-processing steps combine the scalability of template-assisted methods with the high crystalline quality of quantum dots associated with processing in hot coordinating solvents. The fluorescence emission intensity of the quantum dots increased with each processing step and was maximized by tuning the annealing temperature and time.

The ZnSe quantum dots and ZnSe/ZnS core-shell structures prepared by using this technique can be subsequently capped with bifunctional molecules, such as mercaptocarboxylic acids, and stabilized in water for use in biological sensing applications. They can also be capped with thiols or other molecules that have a hydrophobic tail and stabilized in non-polar solvents. Extension of the technique to the synthesis and post-processing of Mn-doped ZnSe quantum dots will also be discussed.

1. Karanikolos, G.N., et al., Synthesis and Size Control of Luminescent ZnSe Nanocrystals by a Microemulsion-Gas Contacting Technique. Langmuir, 2004. 20(3): p. 550-553.

2. Karanikolos, G.N., et al., Water-based synthesis of ZnSe nanostructures using amphiphilic block copolymer stabilized lyotropic liquid crystals as templates. Nanotechnology, 2006(13): p. 3121.