Study of the optimal conditions for NV- center formation in type 1b   diamond, using photoluminescence and positron annihilation spectroscopies
  We studied the parameters to optimize the production of negatively-charged nitrogen-vacancy color centers (NV-) in type~1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV-. Concentrations as high as 2.3 x 10^18 cm^-3 of NV- have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV-. This study brings insight into the production of a high concentration of NV- in diamond, which is of utmost importance in ultra-sensitive magnetometry and quantum hybrid systems applications. 
J. Botsoa, T. Sauvage, M.-P. Adam, P. Desgardin, E. Leoni, B.   Courtois, F. Treussart, and M.-F. Barthe
cond-mat.mtrl-sci quant-ph