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 Yunnan China
3. University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Hong Kong, China
4. Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore
5. Shenzhen Research Institute, The Hong Kong Polytechnic University, Shenzhen, P. R. China