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Synthesis of Doped Zinc Selenide Quantum Dots Using Microemulsions as Templates

Tracy Heckler, Qi (Grace) Qiu, Jun Wang, and T. J. Mountziaris. Chemical Engineering, University of Massachusetts, Amherst, MA 01003

The doping of II-VI Quantum Dots (QDs) with transition metals is attracting significant attention because of the possibility of tuning their emission wavelength to longer wavelengths than the corresponding bulk gap emission and for the development of magneto-optical QDs for spintronic applications.

In this presentation we will discuss the synthesis of ZnSe:Mn and ZnSe:Cu QDs in a microemulsion template, their extraction from the template, surface modification, and stabilization in water. The synthesis route is based on the templated growth of ZnSe nanocrystals reported by Karanikolos et al. [1-3]. The template consists of a microemulsion having p-xylene as the continuous phase, water as the dispersed phase, and a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer as the surfactant. This template is very stable and has very slow droplet-droplet coalescence kinetics, thus eliminating particle aggregation and allowing precise control of particle size. The precursors for Zn, Mn, and Cu were acetate salts of these metals that were dissolved in the aqueous dispersed phase. The concentration of Zn acetate in the dispersed phase determines the final size of the QDs and the concentration of Mn or Cu acetate determines the dopant concentration in the nanocrystal. The Se precursor was hydrogen selenide gas diluted in hydrogen and was bubbled through the microemulsion.

The fluorescence spectra of the resulting QDs exhibit a blue-violet peak attributed to ZnSe gap emission and either a yellow-orange peak or a green peak attributed to Mn2+ or Cu2+, respectively. The evolution of the intensity and ratio of the ZnSe:Mn peaks was studied as a function of dopant salt concentration in the precursor solution and time. The results indicate that ZnSe QDs are formed first and Mn slowly diffuses into the ZnSe lattice.

To make the doped ZnSe QDs useful as multi-color tags for biological imaging and sensing applications, a procedure was developed that allows QD removal from the microemulsion template, capping with functional hydrophilic ligands, and stabilization in an aqueous solution. The microemulsion-based synthesis technique is easy to scale up and employs less expensive chemicals compared to the common QD synthesis technique that involves hot injection of organometallic precursors in a coordinating solvent.

References

1. G. N. Karanikolos, P. Alexandridis, G. Itskos, A. Petrou, and T. J. Mountziaris, Langmuir 20, 550 (2004).

2. G.N. Karanikolos, P. Alexandridis, R. Mallory, A. Petrou, and T. J. Mountziaris, Nanotechnology, 16, 2372-2380 (2005)

3. G.N. Karanikolos, N.-L. Law, R. Mallory, A. Petrou, P. Alexandridis, and T. J. Mountziaris, Nanotechnology, 17, 3121-3128 (2006).