1. State Key Laboratory for Mechanical Behaviour of Materials and School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710049, China
2. School of Microelectronics, Xi'an Jiaotong University, Xi'an 710049, China
3. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, D-52425 Jülich, Germany