CaWO₄ is highly interesting because of its luminescent properties [1]. The title compound has been extensively used as blue emitting phosphor (433 nm) in oscilloscopes and X-ray-detection [2]. Nanoscale CaWO₄ may play an essential role in future luminescent thin film devices.

Different methods are known to synthesize luminescent CaWO₄ nanocrystals, e.g. hydrothermal and microemulsion techniques [3] or microwave heating in aqueous solutions [4, 5]. The demerit of these methods is the need for a strong stabilizing agent.

We have firstly developed a microwave-assisted synthesis of CaWO₄ nanocrystals in Ionic Liquids (ILs). Aim of this novel synthetic approach is to combine the advantages of ILs and microwave heating, to realise nanoscale phosphors with a high crystallinity and quantum yield [6]. Due to the polarity of ILs, intense absorption of microwaves leads to instantaneous heating. This measure results in very high reaction rates as well as in a rapid nucleation and crystallisation.

Here, non-agglomerated and resuspendable CaWO₄ nanocrystals with an average diameter of 60 nm were synthesized. The factors affecting the size, morphology and crystallinity of the product, such as reaction temperature, concentration of the reactants and type of tungstate precursor, were studied. It was possible to recycle the Ionic Liquid after completion of the reaction. Additionally, the quantum efficiency was measured and compared with bulk material [7].

[1] S.H. Yu, M. Antonietti, H. Còlfen, M. Giersig, Angew. Chem. Int. Ed. 41 (2002), 2356.

[2] G. Blasse, B.C. Grabmaier, Luminescent materials, 1994, Springer Verlag Berlin.

[3 L. Sun, M. Cao, Y. Wang, G. Sun, C. Hu, J. Crystal Growth 289 (2006) 231.

[4] J.T. Kloprogge, M.L. Meier, L.V. Duong, R.L. Frost, Mater. Chem. Phys. 88 (2004) 438.

[5] J.H. Ryu, J.W. Yoon, K.B. Shim, Electrochem. Sol. Stat. Let. 8 (2005) D15.

[6] G. Bühler, C. Feldmann, Angew. Chem. Int. Ed., 118, (2006) 1.

[7] M. Mai, C. Feldmann, in progess.

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1. Synthesis of luminescent YVO₄:Eu nanocrystals in ionic liquids.
2. Ionic Liquid based Approach to Nanoscale Functional Materials