1. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology (NCNST); Beijing 100083 P. R. China
2. College of Optoelectronic Engineering; Shenzhen University; Shenzhen 518060 P. R. China
3. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA