1. Key Laboratory of Optoelectronic Technology & Systems Ministry of Education, and International R & D Center of Micro‐Nano Systems and New Materials Technology Chongqing University Chongqing 400044 P. R. China
2. Suzhou Institute of Nano‐Tech and Nano‐Bionics Chinese Academy of Sciences Suzhou 215123 P. R. China
3. Department of Electrical and Computer Engineering National University of Singapore Singapore 117583 Singapore