While the authors only show the difference with increasing thickness, they do not explain why the best results were obtained with a substrate at 150 µm rather than the substrate at 250 µm. The authors did mention that the increasing thickness within the substrate corresponds to micro-strain decrease, which was told to be crucial for single-layer graphene growth. Nguyen et al. used a quartz cap to cover the substrate to minimize impurities issuing, as they said, from the bulk of the substrate as well. .