Figure shows diagram of the CVD reaction of graphene from methane and hydrogen -numbered steps are explained in the text for clarity. First, the reactants are transported by convection in the gas flow, step 1, and thermally activated, steps 2. Then they are transported by gas diffusion from the main gas stream through the stationary boundary layer, step 3. These subsequently are adsorbed on the surface of the substrate Figure : Classification of CVD methods . (step 4) and/or diffused into the bulk of the substrate (step 5), depending on the carbon's solubility and the physical properties of the substrate used. In substrates with low carbon solubility, the growth step of graphene occurs at the surface where CH4 catalytic decomposition and adsorption of carbon atoms forming graphene film occur. On the contrary, a substrate of high carbon solubility sees carbon atoms diffracted into the bulk, while the graphene growth step occurs during the cooling process, where carbon precipitates into the surface. The kinetics of formation, therefore calls for the fast cooling in order to suppress the nucleation and growth of graphene multi-layers. During surface processes, step 6, catalytic decomposition of reactive species besides surface migration to the attachment sites, occurs along with other heterogeneous reactions. The by-products are then desorbed from the substrate, step 7, following the film growth. The final steps are diffusing the byproducts through the boundary layer to the main gas stream, step 8, to be convected by force to the exhaust system, step 9 . [21]