Graphitization effects on diamond surfaces and the diamond graphite interface

Graphitic layers have previously been conjectured to play an active role in diamond nucleation by Lambrecht et al. and may also be involved in a mechanism for homoepitaxial diamond growth since the surfaces of diamond may partially graphitize under high-temperature conditions typical of growth processes. Recent molecular dynamics simulations of the diamond {111} surface, briefly reviewed and discussed here, indicate a progressive graphitization with increasing temperature which is strongly facilitated by any kind of surface perturbation or roughness such as step-like adsorbates. Here we show specifically that also twin boundaries promote graphitization. The process of debonding of the surface layer which is a simple displacive motion of the outer layer is also shown to be closely related to the delamination of the tetrahedrally bonded icosahedral C-100 molecule into two concentric C-20 and (fullerene-like) C-80 fragments. In contrast, the tetrahedrally bonded icosahedral C-300 molecule which contains one more concentric shell, does not spontaneously graphitize into a bucky onion (consisting of concentric C-80 and C-240 fullerenes) although the latter has lower energy. Progressive graphitization at a surface towards deeper lagers before the top layer is delaminated can occur under certain conditions and then may lead to graphite/diamond prism plane interfaces similar to those previously investigated in connection with nucleation. The structural stability of the prism plane interface between graphite and diamond is re-investigated here. While the initial calculations with a classical potential underestimated the interface energy: the structural stability of the models previously presented is confirmed by the present quantum mechanical simulations.