1. State Key Laboratory of Electronic Thin Films and Integrated Devices School of Optoelectronic Science and Engineering University of Electronic Science and Technology of China (UESTC) Chengdu 610054 P. R. China
2. State Key Laboratory of Powder Metallurgy Powder Metallurgy Research Institute Central South University Changsha 410083 P. R. China
3. Department of Electrical Engineering City University of Hong Kong 83 Tat Chee Avenue Kowloon Hong Kong 999077 P. R. China
4. School of Physics and Electronics and Hunan Key Laboratory for Super‐Microstructure and Ultrafast Process Central South University Changsha 410083 P. R. China