1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States
2. Guangdong Engineering Research Center of Optoelectronic Functional Materials and Devices, Institute of Opto-Electronic Materials and Technology, South China Normal University, 510631 Guangzhou, China
3. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, 100083 Beijing, China