1. Department of Physics, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong
2. SDU NanoSYD, Mads Clausen Institute, University of Southern Denmark, Alsion 2, Sønderborg DK-6400, Denmark
3. Department of Physics, College of Science, Shantou University, Guangdong 515063, China
4. Department of Materials Science and Engineering, City University of Hong Kong, 83 Tat Chee Ave., Kowloon, Hong Kong