1. School of Science, Department of Applied Chemistry; Xi'an Jiaotong University; No. 28 Xianning West Road 710049 Xi'an P.R. China
2. Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic and Information Engineering; Xi'an Jiaotong University; No. 28 Xianning West Road 710049 Xi'an P.R. China
3. School of Science; Xi'an University of Post & Telecommunications; No. 563 Chang'an South Road 710121 Xi'an P.R. China