1. Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, United States
2. Department of Electrical and Computer Engineering, University of California, Riverside, California 92521, United States
3. School of Physics and Electronics, Hunan Key Laboratory for Supermicrostructure and Ultrafast Process, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, P.R. China
4. Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia