Generation of amorphous carbon models using liquid quench method: A reactive molecular dynamics study

Abstract

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.

An Atomistic Carbide-Derived Carbon Model Generated Using ReaxFF-Based Quenched Molecular Dynamics

“…This is in agreement with prior QMD studies of amorphous carbons [53,55,71]. Shown in Figure 2a, a quench rate of 100 K ps −1 generates a highly amorphous structure with many stringy domains and few rings.…”

“…While a fraction of pores is in the subnanometer diameter range, Figure 3 shows similar quantities of pores up to 2.5 nm in size. This emergence of larger pores with decreasing quench rate has been observed in previous QMD studies [55,71,92]. It bears repeating that quench rate in simulation lacks a direct physical analog; therefore, this trend cannot directly be attributed to any features of synthesis via halogen etching despite how physically realistic the generated structures may be.…”

“…This allows ReaxFF to largely approach the accuracy of first-principles calculations, but at significantly (several orders of magnitude) reduced computational cost, although, we note more costly than other reactive and non-reactive force fields [52]. The use of ReaxFF for the formation of CDC structures is limited, with only a single recent study [71] examining the formation of amorphous carbon structures using QMD. This study examined the carbon hybridization, ring formation, and pore size distribution as a function of density and quench rate, and demonstrated the potential that ReaxFF could be effective in generating model CDCs; however, the study was not explicitly focused on the formation of experimentally relevant CDCs.…”

“…Our combined computational and experimental approach directly compares experimentally-derived structural information to properties of models derived from simulation. Here, we examine the effects of quench rate on CDC structure, including a slower rate than was used in prior ReaxFF studies [71]. We also investigate the role of a novel post-quenching compression step as a means to adjust pore size distribution.…”

Structural prediction of graphitization and porosity in carbide-derived carbons

“…However, graphitic stacking as seen in experiments was not observed and there was no link between the experiments and the high MD temperatures used for the annealing. Recently, Ranganathan et al [39] used a two-stage MD methodology commencing with quenching followed by high temperature annealing. Even though the potential they used (ReaxFF [40]) includes a long-range attractive term, stacking of layers was not observed.…”