1. School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 China
2. School of Material Science and Engineering and State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 China
3. School of Physics Science and Technology; Lanzhou University; Lanzhou 730000 China
4. School of Material Science and Engineering; Xi'an University of Technology; Xi'an 710048 China
5. State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology; School of Microelectronics; Xidian University; Xi'an 710071 China
6. Frontier Institute of Science and Technology; Xi'an Jiaotong University; Xi'an 710049 China
7. School of Physics; Southern University of Science and Technology; Shenzhen 518055 China
8. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons; Forschungszentrum Jülich; D-52425 Jülich Germany