  This work explores the possibility of increasing the density of negatively
charged nitrogen-vacancy centers [NV-] in nanodiamonds using nitrogen-rich type
Ib diamond powders as the starting materials. The nanodiamonds (10 - 100 nm in
diameters) were prepared by ball-milling of microdiamonds, in which the density
of neutral and automatically dispersed nitrogen atoms [N0] was measured by
diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). A
systematic measurement for the fluorescence intensities and lifetimes of the
crushed monocrystalline diamonds as a function of [N0] indicated that the [NV-]
increases nearly linearly with [N0] at 100 - 200 ppm. The trend, however,
failed to continue for nanodiamonds with higher [N0] (up to 390 ppm) but poorer
crystallinity. We attribute the result to a combined effect of fluorescence
quenching as well as the lower conversion efficiency of vacancies to NV- due to
the presence of more impurities and defects in these as-grown diamond
crystallites. The principles and practice of fabricating brighter and smaller
fluorescent nanodiamonds (FNDs) are discussed.
