1. Advanced Technology Institute, Electronic Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom
2. Niels Bohr Institute and Nanoscience Center, Universitetsparken 5, 2100 Kobenhavn, Denmark
3. Riso DTU, Materials Research Division, Frederiksborgvej 399, P.O. Box 49, DK-4000 Roskilde, Denmark