The main goal with the present work has been to study the possibility and thermodynamical stability for a sequential termination with either carboxylic groups (COOH), or amino groups (NH2), from an initially H-terminated diamond (111), or diamond (100)-2x1 surface. When sequentially substituting the H species with COOH groups, the total energy of adsorption onto the diamond (100)-2x1 surface was observed to drop from -5.45 eV (6.25%) to -26.22 eV (50%). It was not possible to cover the surface with COOH species at a higher surface coverge. For the diamond (111) surface, the corresponding adsorption energy was calculated as -5.10 eV (6.25%) to -20.03 eV (50%). The values in parentheses are the COOH surface coverages. These values show that it is energetically preferable to terminate both types of surface planes to 50%. For NH2, it was observed possible to terminate both types of surface planes up to 100% coverage. The total adsorption energies went from -3.83 eV (6.25%) to -45.84 eV (100%) for the diamond (100)-2x1 surface, and from -3.61 eV (6.25%) to -39.91 eV (100%) for the diamond (111) surface.
In order to follow the individual bond energy variations with variations in surface coverage, the averaged adsorption energies have also been calculated. As expected, the lowest COOH coverage resulted in the energetically most preferable adsorption energies [(-5.45 eV for diamond (100)-2x1, and -5.10 eV for diamond (111)]. The corresponding situation for the NH2 group was identical for the diamond (111) surface only, with the lowest surface leading to the most preferable adsorption situation (-3.61 eV for the first NH2 group). For the situation with diamond (100)-2x1, a continuous decrease in average adsorption energy was obtained when going from the lowest surface energy of 6.25 % (-3.83 eV) up to 43.75% (-3.95 eV). Hence, there is a thermodynamically preference for diamond (100)-2x1 to be terminated with NH2 groups for higher concentration up to 43.75%.
Partial Density of States were calculated with the purpose to analyse the COOH-induced surface electronic properties, The results showed that NH2 groups will contribute to the shift of the LUMO or HOMO energy levels. As a result, this will lead to a decrease in the HOMO-LUMO gaps, being valid for both diamond surface planes. Moreover, partially filled states were observed in the HOMO-LUMO gaps for COOH-terminated diamond (100)-2x1 surface, as well as for both COOH- and NH2-terminated diamond (111) surfaces. These specific types of surface terminations thereby display surface conductivities, which were not observed for 100% H-terminated surfaces.