Nanostructural interpretation for elastic softening of amorphous carbon induced by the incorporation of silicon and hydrogen atoms

Abstract

First-principles molecular dynamics simulation is used to investigate the elastic softening of amorphous carbon on the incorporation of silicon and hydrogen atoms, and the mechanisms responsible for this phenomenon are discussed from the viewpoint of atomic structure. With increasing silicon incorporation, it is found that the bulk moduli of silicon-incorporated amorphous carbon (a-C:Si) and silicon-incorporated hydrogenated amorphous carbon (a-C:Si:H) decrease, whereas the total number of sp3-bonded atoms increases. This is explained on the basis of interatomic bond structures such as: increasing silicon incorporation reduces the number of interatomic (both single and double) bonds between carbon atoms while increasing the number of interatomic bonds between silicon and carbon atoms. Furthermore, for a given density and silicon content, it is found that the bulk modulus of the a-C:Si structure is greater than that of the a-C:Si:H structure, though their interatomic bond structures are similar. The reduced bulk modulus with incorporated hydrogen atoms is found to be due to enhanced internal displacement, which can be understood as atomic displacement in deformed structures: that is, hydrogen-terminated atoms are not bound by interatomic bonds originating from the hydrogen atoms, whereas atoms that bond only to carbon or silicon are bound by all the interatomic bonds.