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

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 sp ² /sp ³ 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 sp ² /sp ³ ratio, whereas the number of sp ³ bonding was proportional to the density. With the controlled sp ² /sp ³ 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 sp ³ % (=the percentage of sp ³ hybridized atoms with respect to total number of atoms in the sample) – the measured surface energy increased with sp ³ % under Tersoff potential but decreased with sp ³ % 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 sp ³ 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 sp ³ % in the sample, whereas the surface energy under Airebo potential was dominantly affected by the number of triple dangling bonds (sp ⁰ ).