1. School of Materials Science and Engineering, Yunnan University, Kunming 650091, P.R.China
2. Kunming Institute of Physics, Kunming 650223, P.R.China
3. Suzhou Institute of Nano-Tech and Nano-Bionics(SINANO), Chinese Academy of Sciences, Suzhou 215123, P.R.China
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAP, P.R.China