Doping of carbon nanoparticles with impurity atoms is central to their application. However, doping has proven elusive for very small carbon nanoparticles because of their limited availability and a lack of fundamental understanding of impurity stability in such nanostructures. Here, we show that isolated diamond nanoparticles as small as 1.6 nm, comprising only ∼400 carbon atoms, are capable of housing stable photoluminescent colour centres, namely the silicon vacancy (SiV). Surprisingly, fluorescence from SiVs is stable over time, and few or only single colour centres are found per nanocrystal. We also observe size-dependent SiV emission supported by quantum-chemical simulation of SiV energy levels in small nanodiamonds. Our work opens the way to investigating the physics and chemistry of molecular-sized cubic carbon clusters and promises the application of ultrasmall non-perturbative fluorescent nanoparticles as markers in microscopy and sensing.

up ȁ80% of observed meteorite falls, but so far very few asteroid spectral analogs have been identified for these meLaboratory results from a simulation of possible optical effects of impact melting and repeated crystallization on asteroi-teorites. dal surfaces are presented. Quick melting and crystallizationThe S-asteroids are widespread in the inner belt and of surface materials were simulated by impulse laser treatment appear to consist mostly of olivine, orthopyroxene, and of powdered materials in vacuum. The mafic materials used metal. They resemble OCs spectrally but are characterized in this study were: ordinary chondrite (OC) Elenovka L5, car-by steep red continua unlike those of the OCs, and their bonaceous chondrite Allende CV3, terrestrial olivine, clinopy-spectrally derived mineralogies are far outside the OC roxene, and olivine-clinopyroxene mixture 1 : 1. The reflectance range (Gaffey et al. 1993a). Moreover, the character of the spectra of powdered samples before and after laser treatment rotational spectral variations of surfaces of two large were recorded in the range 0.3-25 m. The laboratory laser S-asteroids, 8 Flora and 15 Eunomia, appear to be inconsisalteration produced a reduction in overall reflectance and spectent with the primitive undifferentiated OC composition tral contrast, a noticeable increase in spectral slope, the shifts (Gaffey 1984, Gaffey andOstro 1987). Consequently, the in the wavelength positions of absorption band centers, and a majority of investigators are inclined to interpret S-type decrease in band area ratio. The continuum slope of the ordinary chondrite Elenovka after laser treatment is comparable asteroids as composed of various differentiated assemto that of S-type asteroids. Laser impulse alteration raises the blages which are the products of melting in the deep interispectrally derived olivine/orthopyroxene ratio of Elenovka ors of asteroid parent bodies. These materials are similar from 2.8 to 5.4. This value is far outside the OC range, but to stony-iron meteorites and some achondrites (Gaffey falls within the olivine/orthopyroxene range for S-asteroids. 1993a, Hiroi et al. 1993a).The results suggest that quick melting and subsequent crystalliThus, the source of ordinary chondrites remains unclear. zation can enhance the spectral similarity between S-asteroids On the basis of some spectral resemblance between the and ordinary chondrites.

Nanometer-size presolar diamonds from the Efremovka CV3 chondrite were physically separated into several grain size fractions by ultracentrifugation. The coarsest size fraction is the most enriched in carbon-12; the others have broadly similar carbon isotopic compositions. Measurement of noble gases shows that their concentration decreases with decreasing grain size. This effect is attributed to ion implantation. Such an episode could occur in the envelope of a supernova that produced the diamonds, or in interstellar space; in either case, ions with energies above a certain threshold pass completely through the smaller diamond grains without being captured. Concentrations of nitrogen show only minor variations with grain size, indicating a different mechanism of incorporation into the diamonds.