1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA
2. State Key Laboratory of Polymer Materials Engineering; Polymer Research Institute at Sichuan University; Chengdu 610065 China
3. Department of Applied Physics; Chongqing University; Chongqing 400044 China
4. Institute of Microsystems Physics and School of Physics and Electronics; Henan University; Kaifeng 475004 P. R. China
5. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST); Beijing 100083 China