1. Beijing
Institute of Nanoenergy and Nanosystems, National Center
for Nanoscience and Technology (NCNST), Chinese Academy of Sciences, Beijing 100083, China
2. Department
of Physics and Key Laboratory of Artificial Micro- and Nano-structures
of Ministry of Education, Wuhan University, Wuhan 430072, China
3. State
Key Laboratory for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
4. School
of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States