1. Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry; City University of Hong Kong; Hong Kong SAR 999077 P. R. China
2. Department of Physics; Chemistry and Biology (IFM); Linköping University; Linköping 58183 Sweden
3. Division of Physics and Applied Physics; School of Physical and Mathematical Sciences; Nanyang Technological University; 21 Nanyang Link Singapore 637371 Singapore
4. Department of Physics and Astronomy; and Collaborative Innovation Center of IFSA (CICIFSA); Shanghai Jiao Tong University; Shanghai 200240 P. R. China
5. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
6. Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P. R. China
7. Institute of Material Research and Engineering; A*-STAR; 2 Fusionopolis Way Singapore 138634 Singapore
8. Department of Physics; Clarendon Laboratory; University of Oxford; Parks Road Oxford OX1 3PU UK