Photoluminescence from NV Centres in 5 nm Detonation Nanodiamonds: Identification and High Sensitivity to Magnetic Field.

The content of nitrogen-vacancy (NV) colour centres in the nanodiamonds (DNDs) produced during the detonation of nitrogen-containing explosives was found to be 1.1 ± 0.3 ppm. This value is impressive for nanodiamonds of size < 10 nm with intentionally created NV centres. The concentration was estimated from the electron paramagnetic resonance as determined from the integrated intensity of the g = 4.27 line. This line is related with "forbidden" ∆m = 2 transitions between the Zeeman levels of a NV centre's ground triplet state. Confocal fluorescence microscopy enables detection of the red photoluminescence (PL) of the NV colour centres in nanoscale DND aggregates formed from the 5-nm nanoparticles. Subwavelength emitters consisting of NV with sizes a few times smaller than the diffraction-limited spot are clearly distinguished. We have further observed an abrupt drop in the PL intensity when mixing and anti-crossing of the ground and excited states spin levels in NV occurs under an applied external magnetic field. This effect is a unique quantum feature of NV centres, which cannot be observed for other visible domain light-emitting colour centres in a diamond lattice.