1. State Key Laboratory
of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China
2. Key Laboratory for Micro-Nano Physics and
Technology of Hunan Province, School of Physics and Microelectronic
Science, and State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China