1. School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710126, P.R. China
2. School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China
3. Key Laboratory of Ecophysics and Department of Physics, School of Science, Shihezi University, Xinjiang 832003, P.R. China
4. Department of Materials and Engineering, University of Washington, Seattle, Washington 98195-2120, United States