The small cube of diamond inside amorphous carbon


The samples of diamond cluster surrounded by amorphous carbon (see Fig.\ref{orensample}) were generated at different temperatures. This samples contains 216-512 carbon atoms at density of 3.55 g/cc. Initially, atoms were arranged as perfect diamond. Then an inner cubic cluster of 1-4 diamond unit cells was frozen, i.e. the motion of atoms in the cluster was forbidden. The remaining outer envelope was heated. Once the liquid phase reached equilibrium, the temperature was immediately decreased to room temperature. The strucrure of each sample depends on the heating temperature. The sample with 216 atoms (8 of them are frozen diamond) id drawn on the picture below. Red balls are a fourfold atoms, blue balls are a threefould atoms, yellow balls are a twofold atoms, large green balls are a frozen diamond unit cell (3.55 X 3.55 X 3.55 Angstrom).





The percentage of the sp3 bonded atom and the band gaps of the samples with 216 atoms built at different temperature are shown in the table below.


Temperature
percentage of sp3
band gap of the whole sample
band gap of the central diamond atom
12000 K
95
0.8 eV
5.5 eV
13000 K
70

0.3 eV

3.7 eV
14000 K
50

0

0.5 eV
15000 K
57

0.15 eV (?)

1.2 eV
17500 K
49

0

0
20000 K
52

0

0
22500 K
54

0

1.1 eV


The local density of states of inner diamond atoms was calculated. The results don't shows the quantum confinement effects. The local density of states of the frozen diamond atoms is close to this of diamond with band gap of 5.5 eV (see Fig. below, when densities of states of all 8 frozen diamond atoms of the sample with the temperature of heating of 12000 K are drawn.)



As the edge of the sample is approached, the band gap becomes thinner. The figure 2 shows the band gap of the atoms which are in diamond but close to amorphous shell. Here we can see the new states appears in the gap. As the temperature of heating increases (percentage of sp3 bonded atoms decrease), the size of inner diamond cluster (frozen diamond + nearest atoms) decreases. At high heating temperature if the sample inner diamond cluster is small all diamond atoms are close to the amorphous envelope, therefore their band gap is thinner than band gap of diamond.