1. Department of Physics; University of Surrey; Guildford Surrey GU2 7XH UK
2. Department of Electrical and Electronic Engineering; University of Surrey; Guildford Surrey GU2 7XH UK
3. School of Material Science and Engineering; Hubei University; Wuhan 430062 P. R. China
4. Materials Research Institute and School of Physics and Astronomy; Queen Mary University of London; Mile End Road London E1 4NS UK
5. Joule Physics Laboratory; School of Computing, Science & Engineering; University of Salford; Salford M5 4WT UK
6. Radboud University; Institute for Molecules and Materials; FELIX Laboratory; Toernooiveld 7c 6525 ED Nijmegen The Netherlands