1. Wuhan
National Laboratory for Optoelectronics, School of Optical and Electronic
Information, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. Department
of Chemistry and Biochemistry, and California NanoSystems Institute, University of California, Los Angeles, California 90095, United States
3. Department
of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States