1. Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, and State Key Laboratory of Chemo/Biosensing and Chemometrics; Hunan University; Changsha Hunan 410082 P. R. China
2. Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of the Ministry of Education; Wuhan University; Wuhan Hubei 430072 P. R. China
3. Department of Chemistry and Biochemistry and California NanoSystems Institute; University of California at Los Angeles; Los Angeles CA 90095 USA