Simulation of nanodiamond and nanographite formation from molten carbon in the presence of hydrogen

Hydrogen plays a significant role in the formation of nanodiamond, terminating diamond surfaces, and removing $s{p}^2$-bonded atoms from the surface during chemical-vapor deposition diamond growth. However, there are only few calculations that simulate nanodiamond development directly and even less that do so in a hydrogen-containing environment. Recently, nanoscale graphitic layers embedded in amorphous carbon were observed experimentally. We report here on results from a comprehensive study of nanodiamond and nanographite formation from molten carbon in the presence of hydrogen under varied conditions of external pressure and cooling rate. We find that hydrogen-free nanodiamond crystals are precipitated more readily at increased melt densities and cooling rates, whereas slower cooling rates permit formation of graphitic layers.

Figure 1

(Color online) A-C:H structures with different content of hydrogen atoms. The black, gray, and white balls (purple, yellow, and white online) are the carbon atoms with four, three, and two C-C bonds (excluding C-H bonds), respectively. Hydrogen atoms are represented by large gray (light blue online) balls.

Figure 2

Radial distribution function of the hydrogenated amorphous carbons containing 25 hydrogen atoms generated at 3.9 g/cc with the intermediate cooling rate (solid line) compared with that for C-C bonds only (dashed line). Additional peaks of the complete radial distribution located at $\sim 1.13$ and $\sim 1.86\text{Å}$ represent the distances from H atoms to nearest and second-nearest carbon atoms, respectively.

Figure 3

Radial distribution function of the hydrogenated amorphous carbon samples generated at 3.9 g/cc containing 5 hydrogen atoms (solid line) and containing 25 hydrogen atoms (dashed line).

Figure 4

(a) Density of states of the hydrogenated amorphous carbon samples at 3.9 g/cc, containing 5 hydrogen atoms (solid line) and 25 hydrogen atoms (dashed line). In (b) a magnified part of the density of states near the band gap is shown.

Figure 5

(Color online) A diamond cluster containing 22 carbon atoms found in the sample with 10 hydrogen atoms that are generated at intermediate cooling rate.

Figure 6

Radial distribution function of a diamond cluster inside hydrogenated amorphous carbon, containing ten hydrogen atoms generated with intermediate cooling rate (solid line), compared with the radial distribution function of pure diamond (dashed line).

Figure 7

Angular distribution function of diamond cluster inside hydrogenated amorphous carbon, containing ten hydrogen atoms generated at 3.9 g/cc with fast cooling rate (solid line), compared with the angular distribution function of pure diamond (dashed line).

Figure 8

Density of states of diamond cluster inside hydrogenated amorphous carbon, containing ten hydrogen atoms generated at 3.9 g/cc with intermediate cooling rate (solid line), compared with the density of states of pure diamond (dashed line).

Figure 9

(Color online) (a) A graphitic configuration containing ten hydrogen atoms generated at 3.9 g/cc with slow cooling rate. (b) A single damaged graphene sheet. Color coded as in Fig. 1.

Figure 10

Radial distribution function of hydrogenated graphite containing 25 hydrogen atoms generated at 3.9 g/cc with the slow cooling rate (dashed line) compared with the partial radial distribution function of C-C bonds only (solid line).

Figure 11

Angular distribution function of hydrogenated graphite containing 25 hydrogen atoms generated at 3.9 g/cc with the slow cooling rate.