Computational study on surface energy of amorphous DLC with respect to hybridization state of carbon and potential functions

Abstract

In this study, molecular dynamics (MD) simulations were performed to measure the surface energy of amorphous carbon (a-C) in relation to its hybridization state. Liquid quenching method was used to prepare the a-C samples. To control the sp2/sp3 ratio of a-C, two different approaches were attempted, i.e., changing the quenching rate and the density of carbon atoms. The results showed that the quenching rate did not yield any appreciable change in sp2/sp3 ratio, whereas the number of sp3 bonding was proportional to the density. With the controlled sp2/sp3 ratio in a-C, its surface energy was measured using two interactive potentials, i.e., Tersoff and Airebo, where the coordination cutoff length was set to 1.75 Å. In both potentials, the measured surface energy showed a non-monotonous correlation with the hybridization state. Even though the two potentials produced very similar value of maximum surface energy of a-C, they had different trends of surface energy with sp3% (=the percentage of sp3 hybridized atoms with respect to total number of atoms in the sample) – the measured surface energy increased with sp3% under Tersoff potential but decreased with sp3% under Airebo potential. This trend of surface energy with carbon hybridization state can be interpreted by the combined effects of non-polar interaction (or dispersive component) induced by sp3 bonds and polar interactions by dangling bonds. In case of Tersoff potential, the dominant factor for the surface energy value was either the dispersive or the polar components depending on the level of sp3% in the sample, whereas the surface energy under Airebo potential was dominantly affected by the number of triple dangling bonds (sp0).