Anomalous Formation of Irradiation-Induced Nitrogen-Vacancy Centers in 5 nm-Sized Detonation Nanodiamonds

Nanodiamonds containing negatively charged nitrogen-vacancy (NV–) centers are versatile room-temperature quantum sensors in a growing field of research. Yet, knowledge regarding the NV– formation mechanism in very small particles is still limited. This study focuses on the formation of the smallest NV–-containing diamonds, 5 nm detonation nanodiamonds (DNDs). As a reliable method to quantify NV– centers in nanodiamonds, half-field signals in electron paramagnetic resonance (EPR) spectroscopy are recorded. By comparing the NV– concentration with a series of nanodiamonds from high-pressure high-temperature (HPHT) synthesis (10–100 nm), it is shown that the formation process in 5 nm DNDs is unique in several aspects. NV– centers in DNDs are already formed at the stage of electron irradiation, without the need for high-temperature annealing, an effect related to the very small particle size. Also, the NV– concentration (in atomic ratio) in 5 nm DNDs surpasses that of 20 nm-sized nanodiamonds, which contradicts the observation that the NV– concentration generally increases with particle size. This can be explained by the 10 times higher concentration of substitutional nitrogen atoms in the studied DNDs ([NS ≈ 1000 ppm]) compared to the HPHT nanodiamonds ([NS ≈ 100 ppm]). Upon electron irradiation at a fluence of 1.5 × 1019 e–/cm2, DNDs show a 12.5-fold increment in the NV– concentration with no sign of saturation reaching 1 out of about 80 DNDs containing an NV– center. These findings can be of interest for the creation of defects in other very small semiconductor nanoparticles beyond NV-nanodiamonds as quantum sensors.