1. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China
2. College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
3. University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Hong Kong, China
4. The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, China