1. Department of Mechanical and Biomedical Engineering; City University of Hong Kong; 83 Tat Chee Avenue Kowloon Hong Kong
2. School of Materials Science and Engineering; Georgia Institute of Technology; 771 Ferst Drive Atlanta 30332-0245 GA USA
3. State Key Laboratory for Physical Chemistry of Solid Surfaces; Department of Chemistry; Xiamen University; Xiamen 361005 P.R. China
4. Semiconductor Photonics Research Center; Department of Physics; Xiamen University; Xiamen 361005 P.R. China