1. Department of Physics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
2. Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Ensenada, Baja California, Código Postal 22800, Mexico
3. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109
4. Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701