(524a) Microwave-Assisted Templated Synthesis of II-VI Semiconductor Nanocrystals

Semiconductor nanocrystals are interesting nanomaterials whose tunable optical and electronic properties make them ideal for applications in biological sensing and imaging, light-emitting devices, displays, and solar cells. [1] Nanocrystals of II-VI semiconductors, such as CdSe and ZnSe, and core/shell structures based on them, such as CdSe/ZnS and ZnSe/ZnS, have attracted significant attention because of their unique optoelectronic properties. [2] The commercial exploitation of these materials requires the development of synthesis techniques that are scalable, economical, and environmentally friendly, while enabling precise control of the size, shape and size distribution of the nanocrystals. The most common synthesis technique for II-VI nanocrystals employs small batch reactors in which nanocrystals grow as a function of time following a rapid injection of organometallic precursors into a hot coordinating solvent. [2] The limitations of this process for large-scale commercial exploitation stem from the incomplete mixing of the precursors in large batches that can lead to non-uniform nucleation and broad particle size distributions, from the high cost, flammability, and toxicity of the organometallic precursors, and from its operator-intensive nature.

Templated synthesis techniques for II-VI nanocrystals have distinct advantages over other methods, including more precise control of particle size, shape, and size distribution and easier scalability for commercial applications. Stable microemulsions and liquid crystals, formed by self-assembly of an amphiphilic block copolymer in the presence of a polar and a less polar solvent, have been successfully employed as templates for the synthesis of II-VI nanocrystals [3-6]. The nanocrystals are formed by reacting a group II precursor dissolved in the dispersed phase of the template with a group VI hydride gas. Water-in-oil microemulsions and liquid crystals have emerged as the most promising templates, because they employ inexpensive water-soluble metal salts and eliminate the need for using more expensive and pyrophoric organometallic compounds. [5] To eliminate the toxic and flammable group VI hydride gases, a microwave-assisted templated synthesis route is being developed.  The technique employs bursts of microwaves to selectively heat the aqueous dispersed droplets of water-in-oil microemulsions that contain the water-soluble precursors of the group II and VI elements, thus leading to nucleation and formation of a single nanocrystal inside each droplet. In proof-of-principle experiments, luminescent CdS nanocrystals were synthesized by exposing water-in-oil microemulsions containing cadmium acetate and thiourea in their aqueous dispersed phase to bursts of microwaves. Nanocrystals grown by the microwave-assisted route exhibited higher quantum efficiency when compared to nanocrystals grown by the original templated technique. Results from experiments aiming at optimizing the microwave-assisted templated synthesis technique and expanding it to other II-VI nanocrystals will be presented.

[1] A.P. Alivisatos, “Semiconductor Clusters, Nanocrystals, and Quantum Dots”, Science 271(5251), 933-937 (1996).

[2] C.B. Murray, C.R. Kagan and M.G. Bawendi, “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies”, Annual Review of Materials Science,30, 545–610 (2000).

[3] G.N. Karanikolos, P. Alexandridis, G. Itskos, A. Petrou and T.J. Mountziaris, “Synthesis and Size Control of Luminescent ZnSe Nanocrystals by a Microemulsion-Gas Contacting Technique”, Langmuir, 20(3), 550-553 (2004).

[4] G.N. Karanikolos, P. Alexandridis, R. Mallory, A. Petrou and T.J. Mountziaris, “Templated synthesis of ZnSe nanostructures using lyotropic liquid crystals”, Nanotechnology,16 (10), 2372-2380 (2005).

[5] G.N. Karanikolos, N.L. Law, R. Mallory, A. Petrou, P. Alexandridis and T.J. Mountziaris, “Water-based synthesis of ZnSe nanostructures using amphiphilic block copolymer stabilized lyotropic liquid crystals as templates”, Nanotechnology,17 (13), 3121-3128 (2006).

[6] G.N. Karanikolos, P. Alexandridis and T.J. Mountziaris, “Growth of ZnSe and CdSe nanostructures in self-assembled block copolymer-stabilized templates,” Materials Science and Engineering B: Advanced Functional Solid-State Materials, 152(1-3), 66-71 (2008).