1. State Key Laboratory of Electronic Thin Films and Integrated Devices, and School of Optoelectronic Science and Engineering; University of Electronic Science and Technology of China (UESTC); Chengdu Sichuan 610054 China
2. Department of Electronic and Electrical Engineering; University College London; Torrington Place London WC1E 7JE United Kingdom
3. Institute of Fundamental and Frontier Sciences; University of Electronic Science and Technology of China; Chengdu Sichuan 610054 China
4. Department of Electrical & Computer Engineering, and Center for Nanoscale Science & Engineering; University of Kentucky; Lexington KY 40506 USA