1. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA.
2. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA.
3. Department of Physics and Astronomy, School of Natural Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK.
4. National Graphene Institute, University of Manchester, Manchester M13 9PL, UK.
5. Department of Basic Sciences and Humanities, University of Engineering and Technology, New Campus, GT Road Lahore, Kala Shah Kaku, Pakistan.
6. Department of Physics, Shahid Rajaee Teacher Training University, 16875-163 Lavizan, Tehran, Iran.
7. Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium.
8. Department of Chemistry, School of Natural Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK.