1. Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
2. Department of Materials Design and Innovation, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
3. School of Physics and Electronics, Central South University, Changsha 410083, China
4. School of Physics and Electronics, Shandong Normal University, Jinan 250014, China
5. Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
6. Center for Integrated Nanostructure Physics, Institute for Basic Science, Suwon 16419, Korea