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 ([N_S≈ 1000 ppm]) compared to the HPHT nanodiamonds ([N_S≈ 100 ppm]). Upon electron irradiation at a fluence of 1.5 × 10¹⁹e^-/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.