Ultrahigh nitrogen-vacancy center concentration in diamond

High concentration of negatively charged nitrogen-vacancy (NV−) centers was created in diamond single crystals containing approximately 100 ppm nitrogen using electron and neutron irradiation and subsequent thermal annealing in a stepwise manner. Continuous-wave electron paramagnetic resonance (EPR) was used to determine the transformation efficiency from isolated N atoms to NV− centers in each production step and its highest value was as high as 17.5%. Charged vacancies are formed after electron irradiation as shown by EPR spectra, but the thermal annealing restores the sample quality as the defect signal diminishes. We find that about 25% of the vacancies form NVs during the annealing process. The large NV− concentration allows observing orientation dependent spin-relaxation times and also determining the hyperfine and quadrupole coupling constants with high precision using electron spin echo (ESE) and electron-nuclear double resonance (ENDOR). We also observed the EPR signal associated with the so-called W16 centers, whose spectroscopic properties might imply a nitrogen dimer-vacancy center for its origin.