Identification of NV centers in synthetic fluorescent nanodiamonds and control of defectiveness of crystallites using electron paramagnetic resonance

A 100 nm synthetic diamond particle with a large ($>$ 4 ppm) amount of nitrogen vacancy (NV) centers has been studied. The latter exhibit lines associated with forbidden $\Delta m_s$ = 2 and allowed $\Delta m_s$ = 1 transitions in the electron paramagnetic resonance (EPR) spectra of the ground state of the NV$^{(-)}$ center. The luminescence intensity of particles in the range 550–800 nm increases with an increase in the irradiation dose of 5 MeV electrons and correlates with the integrated intensity of the peak EPR line with a $g$-factor $g$ = 4.27. This value is used to estimate the concentration of NV$^{(-)}$ centers and to select diamond powders with the highest fluorescence intensity. The dependence of the EPR signal intensity of the $\Delta m_s$ = 2 transition of the NV$^{(-)}$ center on the microwave power that increases before decaying rather well characterizes the crystal quality of the local environment of the centers under study in these particles. The intensity of the $x$, $y\Delta m_s$ = 1 transition (at $\sim$ 281.2 mT, 9.444 GHz) turns out to be sensitive to changes in particle size in the submicron range and the appearance of near-surface defects obtained during mechanical processing.