1. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronics Science and Technology of China, Chengdu 610054, People's Republic of China
2. Department of Microelectronics, Fudan University, Shanghai 200433, People's Republic of China
3. School of Electrical and Electronic Engineering, Nanyang Technology University, Singapore 639798, Singapore
4. SUNY College of Nanoscale Science and Engineering, State University of New York, Albany, New York 12203, USA