As-synthesized graphene was characterized by Raman spectroscopy. Figure shows a spectrum where the two characteristic peaks of graphene, G, and 2D, could be observed at 1589 and 2678 cm -1 , respectively. The D peak at ~1350 cm -1 , attributed to lattice defects, was negligible, confirming that high-quality graphene can be synthesized on a low-cost Cu foil. A 2D/G peak intensity ratio of ~2.6 confirms that the graphene was a single layer. High-resolution aberrationcorrected transmission electron microscopy (AC-HRTEM) further confirmed that graphene was a single layer (Figure ). Raman mappings (48 × 80 µm 2 area) of peak intensity ratio for D/G (ID/IG: 0.05±0.02, Figure ) and 2D/G (I2D/IG: 2.15±0.35, Figure ) confirmed uniformity at this length scale. Characterization of graphene at several locations along the 55 cm long synthesis zone in the CVD reactor confirmed macroscopic uniformity (Figure ). This established that large-area high-quality graphene, needed for synthesizing pinhole-free membranes, can be prepared in large area CVD furnace by developing techniques to treat cheap Cu. ). i-n, Optical microscope images of as-received (i, j) and acid-treated (k, l) Cu foil before (i, k) and after graphene growth (j, l). Profilometry mapping images of as-received (m) and acid-treated (n) Cu foil after graphene growth. o, Raman spectrum of graphene transferred on Si/SiO2 wafer. p, AC-HRTEM image of single-layer graphene synthesized in this work. q,r, Raman mappings of D and G peak intensity ratio (ID/IG, q) and 2D and G peak intensity ratio (I2D/IG, r) across an area of 48 × 80 µm 2 .