1. Department of Materials Science and Engineering City University of Hong Kong 83 Tat Chee Avenue Kowloon Hong Kong SAR 999077 P. R. China
2. Shenzhen Futian Research Institute City University of Hong Kong Shenzhen 518048 P. R. China
3. Nanomanufacturing Laboratory (NML) Shenzhen Research Institute City University of Hong Kong Shenzhen 518057 P. R. China
4. Department of Physics The Hong Kong University of Science and Technology City University of Hong Kong Clear Water Bay Hong Kong SAR 999077 P. R. China