Surface free energy of diamond nanocrystals - a molecular dynamics study of its size dependence

The dependency of the surface free energy (SFE) of diamond nanocrystals on particle size was studied by means of molecular dynamics (MD) and DFT simulations. It was demonstrated how to avoid the ambiguities in calculating the surface area of very small crystallites by expressing the particle size in terms of the number of atoms which we called the number of atoms convention (NAC) rather than in units of length. The NAC method was applied to a set of models terminated with either (100) or (111) crystal faces. The MD simulations were done for two widely used potentials, i.e. Tersoff and AIREBO. Both potentials show appreciable changes in surface free energy with decreasing crystal size but in opposite directions. In the limit of an infinite crystal both tested potentials give the energy of the (100) surface to be more than two times higher than that of the (111) surface. Also the absolute figures calculated from the AIREBO potential are twice larger than those from the Tersoff potential. DFT simulations of the selected small particles confirmed the MD calculations based on the AIREBO results for the (111) surface but for the (100) surface the values were considerably smaller. The surface free energy value of nanoparticles is not constant - it depends on particle shape and varies with particle size.