May 27, 2009
Microcrystal processing yields fluorescent nanodiamonds
Fluorescent diamond nanoparticles present outstanding photophysical properties. They are very bright and possess long-term non-bleaching, non-blinking photoluminescence in the red/NIR region. Thanks to these unique properties, multiple applications based on their use are foreseen in physics, material science, biochemistry and biology. However, until recently, the production of nanoparticles was limited to the laboratory.
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.
About the author
The work was carried out at the French INSERM Institute and Evry University, France; the Stuttgart University, Germany; the Mines-ParisTech Materials Centre, France; and the UMR 5060, CNRS, UTBM, Belfort, France. This work was supported by the European Commission under the Nano4Drugs consortium. Dr Jean-Paul Boudou is a CNRS scientist currently working on the fabrication and application of fluorescent nanodiamonds in the Laboratoire Structure-Activité des Biomolécules Normales et Pathologiques INSERM/UEVE Unit U829. Dr Patrick Curmi, MD, PhD is research director at INSERM, France. He was the scientific leader of the European Nano4Drugs project. He is currently the director of the INSERM/UEVE Unit U829. Dr Fedor Jelezko is currently group leader: "Single defects in diamond" at 3. Physical Institute of Stuttgart University. Prof. Joerg Wrachtrup is director of the 3. Physical Institute at Stuttgart University. Drs Rolf Reuter and Gopalakrischnan Balasubramanian are currently research fellows at 3. Physical Institute of Stuttgart. Dr Pascal Aubert is researcher in the Laboratoire d'étude des Milieux Nanométriques at the University of Evry. He works mainly on microstructural and mechanical properties of nanomaterials. Mohamed Sennour is senior scientist at Mines-ParisTech. He is the operational engineer of the Evry Transmission Electron Microscopy joint facility at Evry. Dr Alain Thorel is research professor at Mines-ParisTech. He is the leader of the Microstructure-Interface-Ceramic group, and at the head of the Transmission Electron Microscopy joint facility (Mines-ParisTech, Inserm, Genopole) at Evry. Dr E Gaffet is research director at CNRS, currently leader of the Nanomaterials Research Group (NRG / UMR CNRS 5060), aiming to study (since 1987) far from equilibrium phase transitions (kinetics and thermodynamics approaches) induced by mechanical processing.