1. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong SAR, China and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong SAR, China
2. Hefei National Laboratory for Physical Sciences at Microscale and Department of Nanomaterials and Nanochemistry, University of Science & Technology of China, Hefei, China
3. School of Materials Science and Engineering, Nanyang Technological University, Singapore