1. Key
Laboratory of Organic Optoelectronics and Molecular Engineering of
the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China
2. Center
for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, P.R. China
3. Key
Laboratory of Opto-Electronics Technology, Ministry of Education,
College of Electronic Science and Technology, Faculty of Information
Technology, Beijing University of Technology, Beijing 100022, P.R. China