A cheaper path to nanodiamonds

Rebecca C. Jernigan, rebecca.jernigan@laurin.com

They don’t photobleach, they don’t blink, and they are very bright and biologically inert. So why are fluorescent nanodiamonds still not widely used in research? Because they are exceptionally difficult and expensive to produce, making them impractical for many applications.

Researchers at Université d’Evry, in collaboration with scientists from the Physics Institute of the University of Stuttgart in Germany and from the Material Centre of Mines-ParisTech and Technology University of Belfort-Montbéliard, have developed a process to create fluorescent nanodiamonds that could change the way they are viewed and used. According to the investigators, milling the particles from diamond microcrystals preserves their unique properties, making it possible to produce them industrially on a large scale.

Left: High-resolution transmission electron microscopy reveals the form of a diamond nanoparticle that was produced by milling. Courtesy of M. Sennour and A. Thorel, Material Centre of Mines-ParisTech. Center: A diamond nanoparticle is viewed using atomic force microscopy. Right: A nanosize diamond fluoresces brightly. Courtesy of F. Jelezko and G. Balasubramanian, Physics Institute, Stuttgart, Germany.

Up to now, fluorescent nanodiamonds could only be created by irradiating substitutional nitrogen-containing diamond nanocrystals, obtained as expensive and poorly characterized industrial wastes of the diamond industry, with electron or ion beams to create vacancies in the crystal lattice. The particles then are annealed, causing the substitutional nitrogen atoms to trap moving vacancies and to create a fluorescent nitrogen-vacancy center. However, amorphization and the loss of moving vacancies to the surface of the diamond during processing greatly reduce the efficiency and yield of this procedure.

Top-down processing of fluorescent diamond microcrystals, which are more stable than their smaller counterparts, theoretically could provide an industrially scalable procedure. But the practicality of irradiating large amounts of material – and the problem of converting microdiamonds to nanocrystals without compromising their fluorescence or structure – had kept the plan from being realized.

The investigators surmounted these problems by creating a two-step process to mill the diamonds. They used perfect synthetic diamond crystals – measuring ~0.2 mm – that had been irradiated with a high-energy electron beam and annealed at 800 °C. The crystals were mechanically ground using a jet milling system, which used a stream of inert gas to energetically knock them together. Once the diamonds were small enough that the machine’s exit filter could no longer contain them, the particles were transferred to a planetary mill, where they were mixed with dense tungsten carbide beads to continue being ground. The process produced rounded nanodiamonds as small as 4 nm with bright, stable photoluminescent centers.

Researcher Dr. Patrick Curmi of Université d’Evry said that mass production of fluorescent nanodiamonds could lead to breakthroughs in a wide range of fields. The particles could be used as single-photon sources for quantum information processing, as high-resolution magnetometers in nanotechnology and electronics, and as labels to track organic and nonorganic materials.

For more information about fluorescent nanodiamonds, see “Twinkle, Twinkle, Little Diamond” at photonics.com.