Inside the CVD “Black Box”: A Physics-Driven Exploration of Reactor Conditions during Graphene Growth

Though more than a decade has passed since the first report of the chemical vapor deposition (CVD) of graphene on metal substrates, the CVD reactor resembles something of a black-box. The process flow conditions, and, temperature and reactant distribution profiles are poorly understood which causes significant repeatability and reproducibility issues. This also affects studies on the growth mechanism of graphene and the synthesis of completely defect free products. To address these challenges, the open source computational fluid dynamics toolbox OpenFOAM is used for the first systematic exploration of reactor conditions for the synthesis of single crystal and single layer graphene (SLG) in a tube reactor that is typically used in the laboratory. It is found that the reactor flow conditions are in the turbulent regime, and, the temperatures of the 50-100 μm thick copper foils on which graphene is grown are not uniform but have large temperature gradients with a sensitivity to the system pressure, the total mass flow rate and the process gas composition that increases with decreasing foil thickness. Furthermore, both the foil and gas temperatures can be manipulated by modifying the mass flow rates of Ar and H2 in the process gas mix. Based on this new-found knowledge, a process window was identified where the amount of Ar gas in the reactor was optimized to mimic cold-wall like conditions inside the hot-wall reactor and that favored the synthesis of pristine SLG films (while hotter reactor conditions produced films with numerous unwanted adlayers).