1. Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609
2. State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China
3. Department of Physics, Hampton University, Hampton, Virginia 23668
4. College of Material Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China