1. College of Materials Science and Opto-Electronic Technology and Key Laboratory of Vacuum Physics; University of Chinese Academy of Sciences; Beijing 100049 P. R. China
2. Beijing Key Laboratory of Function Materials for Molecular & Structure Construction; School of Materials Science and Engineering; University of Science and Technology Beijing; Beijing 100083 P. R. China
3. Department of Mechanical and Materials Engineering; Nebraska Center for Materials and Nanoscience; University of Nebraska; Lincoln NE 68588-0656 USA
4. State Key Laboratory for Superlattices and Microstructures; Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 China
5. Beijing National Laboratory for Molecular Sciences; Key Laboratory of Organic Solids; Institute of Chemistry; Chinese Academy of Sciences; Beijing 100190 China