1. Semiconductor Materials Division, Department of Physics, Chemistry and Biology-IFM, Linköping University, S-58183 Linköping, Sweden
2. Nanoscale Engineering Division, Department of Physics, Chemistry and Biology-IFM, Linköping University, S-58183 Linköping, Sweden
3. Department of Chemistry, Shantou University, 515063 Guangdong, People's Republic of China
4. Departamento de Física & I3N, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
5. Department of Physics, University of Helsinki, P.O. Box 64, FI-00014 Helsinki, Finland