1. School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, U.K.
2. EPSRC Centre for Diamond Science and Technology, Coventry CV4 7AL, U.K.
3. School of Engineering, Cardiff University, Cardiff CF10 3AT, U.K.
4. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, U.K.