1. Materials Department, University of California, Santa Barbara, California 93106, USA
2. King Abdulaziz City for Science and Technology, Riyadh 11442, Saudi Arabia
3. Department of Physics and Helsinki Institute of Physics, University of Helsinki, 00014 Helsinki, Finland
4. Department of Applied Physics, Aalto University, 02150 Espoo, Finland
5. Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA