1. State Key Laboratory of Traction Power, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
2. Department of Bioengineering, University of California, Los Angeles, Los Angeles, California 90095, United States
3. School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China