Atomistic Multi-Lattice Kinetic Monte Carlo (KMC) Modeling of Hyperthermal Oxidation of Multi-Layer Graphene

An atomic scale multi-lattice Kinetic Monte Carlo (KMC) model is developed to understand defect generation in multi-layered graphene, instigated by the adsorption and diffusion of epoxy groups on the surface. Sun et al. have reported low energy barrier (0.5 eV to 0.6 eV) reaction pathways for epoxy groups to form point defects through the formation of surface groups such as lactone-ethers and ether-lactone-ethers. A key feature of our model is that it uses this approach to initiate defects from a defect free surface. The expansion of these point defects is simulated through several reactions that produce CO molecules from the edges of defects. Schmitt has collected and listed several reactions for the formation of CO from different types of defect edge sites, and the majority of these reactions are adopted in the present work. Simulations based on our KMC model reveal details of the pitting process that are dependent on the temperature and pressure. Our simulations reveal a change in defect shape from circular to a branched as the temperature increases from 1300K to 2000K. We also note that the increase in carbon removal rate with temperature is more significant than the increase in the carbon removal rate with pressure, and we report these rates for each temperature and pressure combination. In this work we also elucidate the role of epoxy diffusion in defect formation. We find that the contribution of epoxy groups in forming CO at the edges of defects increases as the temperature increases due to increased epoxy adsorption and diffusion at higher temperatures. The atomistic KMC model can easily reach milliseconds of physical simulation time on a laptop computer, and thus, it can be efficiently used as a tool to understand how various other atomistic reactions affect defect formation in multi-layer graphene.