1. School of Physics and Electronic Information, Yan’an University, Yan’an 716000, China
2. Network Information Center, Yan’an University, Yan’an 716000, China
3. Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong 999077, China
4. Department of Materials Physics, Saveetha School of Engineering, Saveetha Institute of Medical and Technical Sciences (SIMTS), Thandalam, Chennai 602105, India