1. Department of Physics, Philipps-Universität Marburg, 35037 Marburg, Germany
2. Department of Chemistry, Purdue University, West Lafayette, Indiana, 47907, USA
3. Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
4. State Key Laboratory of Superhard Materials, Jilin University, Changchun, Jilin, 130012, China
5. Davidson School of Chemical Engineering, Purdue University, West Lafayette, Indiana, 47907, USA