How small nanodiamonds can be? MD study of the stability against graphitization

How small nanodiamonds can be is a crucial question for biomedical applications. To answer this question, we present here molecular dynamic simulations of the annealing of very small diamond clusters (diameter between 0.3 and 1.3 nm) of various shape in vacuum and in the presence of oxygen. Isothermal cycles of 30 ps were carried out at 500, 1000, 1500, and 2000 K with 10 ps ramps between them. Predominantly {100} faceted diamond clusters as small as 1 nm (~ 250 atoms) survive these short anneals up to 1500 K. Longer anneals at 1500 K, as well “accelerated” MD at very high temperatures, indicate that the diamond core is still preserved when thermal equilibration is reached. The primary effect of oxygen seems to be the saturation of threefold-coordinated surface carbon atoms and the etching of lower coordinated ones. Oxygen accelerates the graphitization somewhat but does not affect the critical size. Our result means that nanodiamonds with a core of only 0.8 nm can be kinetically stable up to 1500 K. This is significantly less than the lower limit of the thermodynamic stability (~ 1.9 nm).