1. Department of Physics and Materials Science and Center of Super-Diamond and Advanced Films, City University of Hong Kong, HKSAR, China
2. Department of Engineering Physics, Polytechnique Montréal, Montreal, Quebec H3A 3A7, Canada
3. Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
4. Department of Mechanical and Materials Engineering and Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B8, Canada
5. State Key Laboratory in Ultra-Precision Machining Technology, Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, HKSAR, China
6. Vacuum Electronics Ltd., London, Ontario N6K 5A2, Canada