Atmospheric contaminations deposit on the graphene surface when it is stored. If not removed, they hinder the oxidation of graphene. More importantly, depending on the coverage of contaminations, they make it challenging to achieve reproducible results. We found it to be an extremely critical step. Therefore, graphene was cleaned by heating under a reducing atmosphere of H2/Ar mixture at 600 ℃. While a lower temperature (500 ºC) was also effective, improved reproducibility was observed when heating to at least 600 ºC. Storage of CVD graphene also tends to generate CuO on the Cu foil, which makes the surface rough. Heating under a reduced atmosphere helped smoothen the surface (Figure ). This step was also crucial to obtain a smooth Cu/graphene surface, which was important for successful membrane fabrication. It should be noted that this step can be avoided when pore formation is implemented immediately after the synthesis of graphene.