1. Department of PhysicsThe Chinese University of Hong Kong Shatin Hong Kong
2. Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of ChinaSchool of Physics and TechnologyWuhan University Wuhan Hubei 430072 China
3. Department of Materials Science and EngineeringCity University of Hong Kong Kowloon Hong Kong
4. School of Physical SciencesUniversity of Science and Technology of China Hefei Anhui 230026 China