Computational details

In the second stage of calculation the structures of amorphous carbon located between two layers of diamond were studied. Three different samples with a density of 3.5 g/cc, initially arranged as a perfect diamond crystal, were constructed, their sizes being 2 $\times$ 2$\times$ 6 (192 atoms), 2 $\times$ 2$\times$ 7 (224 atoms) and 2 $\times$ 2$\times$ 8 (256 atoms). The interesting feature of these simulations was that the 32 upper and 32 lower atoms of each sample were frozen, i.e. the motion of these atoms was forbidden. The initial geometry of the samples is shown in Fig. 8.6.

Figure 8.6: Initial configuration of the samples used in the second stage of the simulations; white balls represent the frozen atoms, grey balls represent the moving atoms.

The central layers were heated up to temperatures of 14000-30000 K. The outer frozen diamond layers strived to return the atoms in the intermediate layers to their initial positions. Therefore these atoms could not move as quickly as the atoms in the centre of the sample. By this way a temperature gradient from the edges to the center of the sample was created. Once the liquid phase reached equilibrium (see Fig.8.7), the central layers were cooled to the room temperature of 300 K at a cooling rate of 10 K/fs.

Figure 8.7: The total energy of the third sample (256 atoms) showed in the process of heating to 23000 K, the system reaches equilibrium in 1500 MD steps (0.75 ps).

The size of the frozen layers in all three samples was the same, therefore only the height of the hot layers varied from 4 (128 atoms) to 6 (192 atoms). It turned that the size of the moving layers as well as the temperature of heating affected the results of the simulations.