1. Materials Interfaces CenterShenzhen Institutes of Advanced TechnologyChinese Academy of Sciences Shenzhen 518055 P. R. China
2. School of Advanced MaterialsShenzhen Graduate SchoolPeking University Shenzhen 518055 P. R. China
3. Department of Materials Science and EngineeringCity University of Hong Kong Tat Chee Avenue Kowloon Hong Kong China
4. Department of Physics and Department of Materials Science and EngineeringCity University of Hong Kong Tat Chee Avenue Kowloon Hong Kong China