1. Device Research Laboratory, Department of Electrical Engineering, University of California, Los Angeles, California 90095, United States
2. Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, United States
3. Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
4. Advanced Light Source Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States.