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 $s{p}^2$ 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.

Figure 1

(Color online) Minimum energy path for the diamond conversion to rhombohedral graphite ($d_{\text{cc}}$ is the length of the dissociating C-C bond). Symbols: DFT results of Fahy et al. (Ref. 18); dotted line: REBO2 potential (Ref. 16); dashed line: AIREBO potential (Ref. 17); dashed-dotted line: EDIP potential (Ref. 12) (data taken from Ref. 20); full line: LCBOPII potential (Ref. 19).

Figure 2

(Color online) $6\text{Å}$-thick slices as a function of time (a: 0 ps, b: 50 ps, c: 100 ps, d: 200 ps, e: 800 ps, and f: 1600 ps) illustrating the graphitization mechanism of a 3 nm UDD at 3000 K. Atoms are colored according to their coordination (red: twofold, blue: threefold, gray: fourfold, yellow: other).

Figure 3

Time evolution of the potential energy and coordination distribution during graphitization. Insets focus on the first 250 ps of simulation (only the threefold fraction is pictured): horizontal dotted lines highlight the presence of two plateau for the threefold fraction at $t=50\text{ps}$ and $t=90\text{ps}$.

Figure 4

(Color online) Carbon onion structure as identified by a cluster analysis of five to seven shortest path rings. (a) Cross section of the onion, (b) external ${\text{C}}_{1134}$, (c) ${\text{C}}_{739}$, (d) ${\text{C}}_{405}$, (e) ${\text{C}}_{179}$, and (f) internal ${\text{C}}_{42}$ [only threefold atoms are considered in the analysis, the remaining $s{p}^3$ atoms are shown with large sphere in Fig. 4a].

Figure 5

(Color online) $6\text{Å}$-thick slices of the final structures at (a) 2500 K and (b) 3000 K from (NVT) MC simulations starting from a roughly spherical nanodiamond containing $N=5851$ atoms with diameter 4.5 nm. Same color code as in Fig. 2

Figure 6

(Color online) Fluctuations in the diamond core pressure $P_{dc}$ (in GPa) as determined during MC simulations at 2500 K for the two nanoaggregates ND2425 and ND5851. Dashed line gives the average value.