1. Z. E. Elektronenmikroskopie, Universität Ulm, 89069 Ulm, Germany
2. School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, UK
3. IPICYT, Av. Venustiano Carranza 2425-A, Col. Bellas Lomas, San Luis Potosí 78210, México
4. Université Catholique de Louvain, Unité PCPM, Place Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium
5. Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, N.Y. 12180-3590, USA