1. Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 P. R. China
2. School of Microelectronics Xidian University Xi'an 710071 P. R. China
3. Department of Chemistry and 4D LABS Simon Fraser University Burnaby British Columbia V5A 1S6 Canada
4. Department of Mechanical Engineering National University of Singapore Singapore 1176081 Singapore
5. School of Physics State Key Laboratory of Crystal Materials Shandong University Jinan 250100 P. R. China