Formation of multiwall fullerenes from nanodiamonds studied by atomistic simulations

The high-temperature annealing of nanodiamonds with sizes typical of ultradisperse diamonds is studied with atomistic simulations using a recent and accurate classical reactive potential. At 3000 K, the complete transformation of the particles into carbon onions made of five to seven concentric fullerenes occurs according to a three-step mechanism: (i) formation of two to three graphitic shells at the surface, (ii) transformation of the diamond core into an amorphous sp(2) carbon, and (iii) reorganization of the core into concentric fullerene layers. At lower temperatures, the transformation stops at step (i) and the final structure is made of a diamond core surrounded by a few fullerene shells. The analysis of the internal pressure of the diamond core reveals that this state is metastable.