1. Nanobiology Laboratory, Bio-X Life Science Research Center, College of Life Science and Biotechnology, Shanghai Jiaotong University, Shanghai 200030, China, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China, Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China, and Department of Chemistry and Centre for Materials and Nanoscience, University of NebraskaLincoln, Lincoln, Nebraska 68588