1. Wuhan National Laboratory for Optoelectronics; Huazhong University of Science and Technology; Wuhan 430074 China
2. School of Materials Science and Engineering; Huazhong University of Science and Technology; Wuhan 430074 Hubei China
3. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China