1. Beijing National Laboratory
for Molecular Sciences (BNLMS) (The State Key Laboratory of Rare Earth
Materials Chemistry and Applications), College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, P. R. China
2. Electron Microscopy Laboratory and State Key Laboratory for Mesoscopic Physics, College of Physics, Peking University, Beijing 100871, China
3. College of Engineering, Peking University, Beijing 100871, P. R. China