Nonequilibrium Route to Nanodiamond with Astrophysical Implications

Nanometer-sized diamond grains are commonly found in primitive chondritic meteorites, but their origin is puzzling. Using evidence from atomistic simulation, we establish a mechanism by which nanodiamonds form abundantly in space in a two-stage process involving condensation of vapor to form carbon onions followed by transformation to nanodiamond in an energetic impact. This nonequilibrium process is consistent with common environments in space and invokes the fewest assumptions of any proposed model. Accordingly, our model can explain nanodiamond formation in both presolar and solar environments. The model provides an attractive framework for understanding noble gas incorporation and explains all key features of meteoritic nanodiamond, including size, shape, and polytype. By understanding the creation of nanodiamonds, new opportunities arise for their exploitation as a powerful astrophysical probe.

Figure 1

(a) Cross sectional view (5 Å thick) of a molecular dynamics simulation of a carbon onion impacted onto a (001) diamond substrate with an incident energy of $0.5\mathrm{eV}/\mathrm{atom}$. (b) Same as (a) but with an incident energy of $1.5\mathrm{eV}/\mathrm{atom}$. (c) Same as (a) but with an incident energy of $3\mathrm{eV}/\mathrm{atom}$. (d) TEM images of crystals within nanodiamond residue (Ref. 6) exhibit an angular shape consistent with the conical impact zones seen in the simulations.

Figure 2

Time and energy dependence of the $s{p}^3$ fraction. (a) The effect of impact energy on the time dependence of $s{p}^3$ bond formation, using a 1.85 Å cutoff to define neighbors. (b) The energy-window effect for optimum diamond formation. The optimal energy range shifts to slightly higher values on a tetrahedral amorphous carbon ($ta$-C) substrate of lower hardness than diamond.

Figure 3

(a) Final structure of the slab impact simulation employing a ten-layer slab consisting of randomly stacked graphenelike sheets. On a very rapid time scale, the graphene sheets transform to a hexagonal-type nanocrystal. (b) Debye calculation of the diffraction intensity for the resulting nanocrystal in (a). Vertical lines denote diffraction reflections for hexagonal (2H) diamond [33]; showing the nanocrystal in this instance is lonsdaleite. (c) Conical region of the nanodiamond resulting from the impact of a carbon onion with an impact energy of $1.5\mathrm{eV}/\mathrm{atom}$ [Fig. 1b]. Green and blue circles denote three- and fourfold coordination, respectively. (d) Debye calculation for the conical structure in (c). Vertical lines denote the first three diffraction peaks of cubic (3C) diamond [33].