1. Department
of Applied Physics, Nanjing University of Science and Technology, Nanjing,
Jiangsu 210094, P. R. China
2. Hefei National Laboratory for
Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China
3. Division of Materials Science, NanYang Technological University, 50 NanYang Avenue,
639798 Singapore