H2-dependent attachment kinetics and shape evolution in chemical vapor deposition graphene growth. (24th July 2017) - Meca, Esteban
Abstract: Experiments on graphene growth through chemical vapor deposition (CVD) involving methane (CH4 ) and hydrogen (H2 ) gases reveal a complex shape evolution and a non-monotonic dependence on the partial pressure of H2 ( p H 2 ). To explain these intriguing observations, we develop a microkinetic model for the stepwise decomposition of CH4 into mobile radicals and consider two possible mechanisms of attachment to graphene crystals: CH radicals to hydrogen-decorated edges of the crystals and C radicals to bare crystal edges. We derive an effective mass flux and an effective kinetic coefficient, both of which depend on p H 2, and incorporate these into a phase field model. The model reproduces both the non-monotonic dependence on p H 2 and the characteristic shapes of graphene crystals observed in experiments. At small p H 2, growth is limited by the kinetics of attachment while at large p H 2 growth is limited because the effective mass flux is small. We also derive a simple analytical model that captures the non-monotone behavior, enables the two mechanisms of attachment to be distinguished and provides guidelines for CVD growth of defect-free 2D crystals.