The role of hydrogen in oxygen-assisted chemical vapor deposition growth of millimeter-sized graphene single crystals

Involving oxygen in the traditional chemical vapor deposition (CVD) process has proven a promising approach to achieve large-scale graphene single crystals (GSCs), but its many relevant fundamental aspects are still not fully understood. Here we report a systematic study on the role of hydrogen in the growth of millimeter-sized GSCs using enclosure-like Cu structures via the oxygen-assisted CVD process. Results show that GSCs have different first layer growth behaviors on the inside and outside surfaces of a Cu enclosure when the H₂ environment is varied, and these behaviors will consequently and strongly influence the adlayer formation in these GSCs, leading to two entirely different growth modes. Low H₂ partial pressure (P_H2) tends to result in fast growth of dendritically shaped GSCs with multiple small adlayers, but high P_H2 can modify the GSC shape into hexagons with single large adlayer nuclei. This difference of adlayers is attributed to the different C diffusion paths determined by the shapes of their host GSCs. On the basis of these observations, we developed an isothermal two-step method to obtain GSCs with significantly improved growth rate and sample quality, in which low P_H2 is first set to accelerate the growth rate followed by high P_H2 to restrict the adlayer nuclei. Our results prove that the growth of GSCs can reach a reasonable optimization between their growth rates and sample quality by simply adjusting the CVD H₂ environment, which we believe will lead to more improvements in graphene synthesis and fundamental insight into the related growth mechanisms.