1. Department of Materials Science and Engineering; South University of Science and Technology of China; Shenzhen 518055 P. R. China
2. Department of Mechanical and Materials Engineering; University of Nebraska-Lincoln; Lincoln NE 68588 USA
3. Department of Chemistry; South University of Science and Technology of China; Shenzhen 518055 P. R. China
4. Department of Chemistry and the Materials Research Center; the Argonne-Northwestern Solar Energy Research Center; Northwestern University; 2145 Sheridan Road Evanston IL 60208 USA
5. Department of Materials Science and Engineering; University of California; Los Angeles CA 90095 USA
6. State Key Laboratory of Luminescent Materials and Devices; South China University of Technology; Guangzhou 510640 P. R. China