1. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P. R. China
2. Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
3. X17C of NSLS, CARS, University of Chicago, Upton, New York 11793
4. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D.C. 20015