TEM and HRTEM images of CuCr2Se4 nanocrystals with an average size of 25 nm.
The controlled synthesis of inorganic nanocrystals (NCs) has generated much interest in recent years, motivated in large part by the unusual electronic, optical and magnetic properties that are exhibited in this size regime. The solution-based synthesis of a wide range of magnetic NCs of different morphologies has been reported, including those of transition metals (Fe, Co, Ni), alloys (FePt, CoPt, etc), and oxide materials (ferrites, perovskites, etc). Their properties have been studied quite extensively, both from the viewpoint of fundamental understanding and applications, ranging from magnetic resonance imaging, drug delivery, biosensing, and nanoelectronic devices. These studies notwithstanding, the synthesis and properties of a number of other nanocrystalline magnetic materials, such as those based on the rare earths and chalcogenides, remain largely unexplored. Of particular interest are the chromium-based spinel chalcogenides, ACr2X4 (A= Cu, Cd, Hg, Fe, Co; X = S, Se, Te), which are ferro/ferrimagnetic insulators, semiconductors, or even metals that display unique properties in the bulk. As in the case of the standard magnetic systems, the utility of the chalcospinels can be augmented if they can be synthesized as colloidal nanocrystals with highly controlled dimensions.
Room temperature ferromagnetism with a Curie temperature of 430 K makes CuCr2Se4 an interesting system of study amongst the chalcospinels. Solution-based synthesis of CuCr2Se4 and other chalcospinels has, however, been limited by the complexity involved in the synthesis; particularly since they cannot be precipitated from aqueous solution. Furthermore, it is difficult to synthesize the stoichiometric compound and/or the desired spinel phase because of the volatility of Se and the availability of a number of oxidation states of chromium. Recently, there have been reports of solution-based solvothermal and microwave synthesis of CuCr2Se4 crystals. While the phase-pure material has been stabilized, these reactions result in the formation of relatively large crystallites or agglomerates with a broad size and shape distribution. We have developed a facile synthesis process for size-controlled CuCr2Se4 nanocrystals (∼15-30 nm, see Figure) that are nearly monodisperse and readily form a colloidal suspension. The process involves the thermal decomposition and reaction of the metal-acetylacetonate precursors with selenium in a high boiling organic solvent mixture of oleylamine (OLA) and 1-octadecene (ODE). The coordinating solvents play the dual role of forming an organo-Se intermediate and in selectively adsorbing on the surface of the nucleated product to passivate and control their size.
Y-H. A. Wang, N. Bao, L. Shen, P. Padhan, and A. Gupta, J. Am. Chem. Soc., in print.