It is remarkable that none of the diamonds revealed strong red fluorescence of NV centers, neither after sole LPHT annealing, nor after irradiation and subsequent LPHT annealing. Red fluorescence is a very
characteristic feature of as-grown commercial CVD diamonds, which always contain dispersed nitrogen (C—defects) in concentrations of at least a fraction of ppm. The more so when the diamonds are doped
with nitrogen to a concentration of a few ppm. The absence of the red fluorescence of NV centers in the present case is definitely the result of HPHT and LPHT annealing. Both treatments are effective in aggregation of nitrogen and in destruction of NV defects. Yet, the presence of minor concentration of NV centers can be detected in nitrogen-containing diamonds after any treatment, including HPHT and LPHT heating at extremely high temperatures. Since NV centers are very effective in luminescence, they may produce detectable fluorescence even when in small concentrations.