Fluorescent nanodiamonds derived from HPHT with a size of less than 10 nm
Résumé
The fabrication of fluorescent nanodiamonds by the electron irradiation of a high-pressure high-temperature microdiamond followed by annealing and fragmentation has a number of advantages over other fabrication approaches. High energy electron irradiation of micron-sized diamonds is a safe and convenient method to create vacancies within the lattice, thereby allowing for simple reactor designs. Well-defined annealing conditions facilitate vacancy migration and its subsequent capture by substitutional nitrogen (Ns) atoms, while avoiding the formation of unwanted coke on the surface of the diamond. In addition, microdiamonds offer a long vacancy migration path, which significantly increases the probability of vacancy trapping by nitrogen. In this report, we show that the fragmentation of irradiated and annealed microdiamonds creates round ultrasmall nanodiamonds composed of perfectly crystallized cubic-diamond nanocrystals, with fluorescent centers inside the nanocrystal core. Atomic force microscopy and confocal fluorescence microscopy demonstrate that approximately 30% of diamond nanocrystals with a size of less than 10 nm are fluorescent and have a remarkably long spin decoherence time (2.7 μs for a 7 nm diamond nanocrystal). The presence of a high content of non-fluorescent ultrasmall nanodiamonds can be explained by the limited N concentration and its heterogeneous distribution in the initial raw high-pressure high-temperature diamond. The remarkably long spin decoherence time of the ultrasmall fluorescent nanodiamonds may be due to surface cleaning and nanodiamond fabrication procedures, which result in a low number of spin impurities in and around the nanocrystal.