1. Department of Physics and Materials Sciences Centre, Philipps-Universität Marburg, Marburg 35032, Germany
2. Department of Physics, COMSATS University Islamabad-Lahore-Campus, Pakistan
3. Department of Physics, Faculty of Science, Ain Shams University, Cairo 11566, Egypt
4. School of Physics, University of the Witwatersrand, 2050 Johannesburg, South Africa
5. Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
6. Centre for Nanoscience and Nanotechnology, University of Bath, Claverton Down, Bath BA2 7AY, UK