Using molecular dynamics simulations with a reactive force field (ReaxFF), we generate models of amorphous carbon (a-C) at a wide range of densities (from 0.5 g/cc to 3.2 g/cc) via the "liquid-quench" method. A systematic study is undertaken to characterize the structural features of the resulting a-C models as a function of carbon density and liquid quench simulation conditions: quench rate, type of quench (linear or exponential), annealing time and size of simulation box. The structural features of the models are investigated in terms of pair correlation functions, bond-angles, pore-size distribution and carbon hybridization content. Further, the influence of quench conditions on hybridization/graphitization is investigated for different stages of the simulation. We observe that the structural features of generated a-carbon models agree well with similar models reported in literature. We find that in the low-density regime, 2 effects play an important role in determining the pore size distribution and the structures are predominantly anisotropic. Whereas, at densities larger than 1.0 g/cc, the structures are spacefilling and differences exist only in terms of carbon hybridization. The rate of structural evolution (pore size and hybridization) during the quench process is observed to be dependent on the quench type, rate and the annealing time.
IntroductionCarbon shows remarkable versatility since it exists in various chemical and structural forms. On one hand, crystalline and ordered phases such as graphene, diamond, carbon nanotubes, etc., confer an extraordinary range of properties. On the other, equally important is the plethora of amorphous structures of carbon (denoted as a-C and alternately referred to as disordered carbon) existing in a wide range of densities ranging from low-density, char-like carbon to high-density diamond-like and tetrahedral amorphous carbon (denoted as ta-C [1]), with varied structural and chemical features. Correspondingly, this confers a-C with a wide variety of properties and applications, ranging from low-conductivity heat-shield ablators [2,3] in the low-density regime, to high-hardness, chemically inert and optically transparent coatings [4,5], magnetic storage applications [6] among others [5] for diamond-like amorphous carbon films.The term "amorphous carbon" can be attributed as an umbrella term to carbon at a large range of densities ranging from char-like carbon (~ 0.2 to 0.5 g/cc [3]) to high-density, diamond-3 like carbon (~3.2 g/cc [5]). Since there is no well-defined order for amorphous carbon, it has been a challenge to characterize and fully understand their structure [7]. It is in this regard that models of a-C structure generated by computer simulation techniques become very useful in understanding complex structure-property relations and optimize desired properties.