Synthesis of Loose Nanodiamonds Containing Nitrogen-Vacancy Centers for Magnetic and Thermal Sensing

The production of fluorescent nanodiamonds (NDs) with specific color centers and exhibiting a controlled size and purity represents an important challenge. In the context of quantum technologies they may indeed provide useful probes for magnetic or thermal sensing with extreme performance and biocompatibility. While detonation and grinding are the most used large-scale production methods, they do not allow for an accurate and flexible control over the defect content of the nanoparticles. In this work we report on the synthesis of loose NDs by high-power plasma chemical vapor deposition (CVD) without the need of a seeded substrate for nucleation. The as-grown NDs, dispersed in solution, exhibit high crystalline quality and low impurity background. A tunable amount of nitrogen-vacancy (NV) centers is introduced thanks to moderate N2 additions during growth. Irradiation post-treatments using He+ and H+ ion beams followed by annealing are used to increase NV luminescence or create specific defects such as the helium-vacancy (HeV) center observed for the first time in a nanoscale material. Hydrogen-related impurities are, however, evidenced in electron paramagnetic resonance (EPR) spectroscopy and are likely to play a role in the preferential stabilization of the neutral charge state of the NV defect. Thanks to this synthesis approach, large-scale production of CVD NDs with a wide tunability of their chemical composition is achieved, paving the way to their use as a platform for exploring quantum sensing applications.