Acknowledgements

We thank Prof. V. Presser (Saarland University and Institute of New Materials, Saarbrücken) for giving us the possibility to use the electrospinning apparatus. The collaboration Saarbrücken-Nancy was supported by the EU-INTERREG IVa project "GRMN." This work is supported by Volkswagen Foundation and DFG project Ko 2323/8, which is gratefully acknowledged.
