1. Xi’an University of Posts and Telecommunications, Xi’an 710121, P.R. China
2. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710000, P.R. China
3. Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, PR China