1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States
2. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China
3. Department of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China
4. Department of Electrical Engineering, University of California, Riverside, California 92521, United States