Synthetic high-pressure high-temperature (HPHT) microdiamonds demonstrate strong resonant-like enhancement of the NV− photoluminescence (PL) at unexpectedly low concentration of the substitutional nitrogen atoms (NS) of 90 ppm, which is significantly smaller than the ∼250 ppm characteristic for conventional HPHT synthesis. The EPR, Raman, and luminescent spectroscopies, including time-resolved PL, provide evidence that this is due to competition between the increase in PL with increasing concentration of the NV− centers and nonradiative deactivation of these centers by defects induced by the NS in the diamond crystal lattice. It is suggested that the intensity of the NV− center emission can be optimized by the appropriate choice of nitrogen doping concentration which does not substantially disrupt the diamond crystal lattice.
