Three-stage transformation pathway from nanodiamonds to fullerenes.

The dynamics of structure evolution of nanodiamonds ranging from 22 to 318 atoms of various shapes is studied by density functional tight-binding molecular dynamics. The spherical and cubic nanodiamonds can be transformed into fullerene-like structures upon heating. A number of the transformed fullerenes consist of pentagons and hexagons only. Others contain squares, heptagons, and octagons. One simulated fullerene is an isomer of C(60). The temperature of the transformation depends on the size, shape, and orientation of initial cluster. To be transformed into onion-like fullerenes, the spherical nanodiamonds should have 200 atoms or more, while the cubic ones require 302 atoms or more. The time-resolved energy profiles of all the transformations clearly reveal three-stage transformation character. During the first stage, the energy reduces quickly due to converting sp(3) carbon with dangling bond at the surface into sp(2) one, and the formation of partial sp(2) envelope wrapping the cluster. For the second stage, energy decreases slowly. The remaining interior carbon atoms come to the surface through the hole in the sp(2) envelope, and similar amount of sp(3) and sp(2) atoms coexist. The third stage involves the closure of holes, accompanied by the detachment of C(2) molecules and carbon chains from the edges. The energy decreases relatively fast in this stage. The proposed three-stage transformation pathway holds for all the simulations performed in this work, including those with the instant heating.