A single route is often taken to fabricate such fluorescent nanoparticles. It consists of irradiating substitutional nitrogen-containing diamond nanocrystals, produced by the diamond industry, with electron or ion beams to create vacancies in the crystal lattice. Isolated substitutional nitrogen atoms then trap a moving vacancy during annealing to form a fluorescent NV centre. Unfortunately, the efficiency and yield of this route are low due to amorphization and the loss of moving vacancies to the surface during irradiation and annealing.

The top-down processing of diamond microcrystals, which are less prone to amorphization and vacancy loss, provides a more industrially scalable route. However, in this case two barriers have to be surmounted – the difficulties of irradiating large amounts of material and converting microdiamonds into nanocrystals while keeping both fluorescence properties and crystal structure intact.

In a recent study, which was published in Nanotechnology, researchers in France and Germany have explored with success this alternative route to producing homogeneous samples of pure and very small fluorescent diamond nanoparticles with high yield. The fabrication procedure starts with the irradiation of finely controlled micron-size diamonds and requires subsequent milling and purification steps. In this novel process, substitutional nitrogen-containing microdiamonds with defined atomic composition were irradiated using a high-energy electron beam and then annealed at high temperature (800 °C) to create the desired photoluminescent centres in an intact diamond lattice. An original two-step milling protocol was designed to convert the fluorescent microdiamond into very small (<10 nm) round-shape nanoparticles of highly pure sp3 diamond with very bright and stable photoluminescent centres.

Such a fine fabrication process can now be used for the large-scale production of fluorescent diamond nanoparticles. You can vary and tailor their properties via the composition of the starting material to answer the needs of future applications.