▎ 摘　　要

Although graphene-based materials have been projected to be impervious even to Nature's smallest molecule, He, so far graphene-based barriers have exhibited a rather high water vapor transmission rate (WVTR) of 10-1 - 10- 2 g/m2 center dot day. This disparity stems from the abundant nano- and micron-scale defects in graphene basal planes, which typically function as shortcuts or highways for water diffusion through the tortuous layers of graphenebased sheets. Here WVTR of 4.1 x 10-5 g/m2 center dot day and lag time of 1 month (730 h) are demonstrated in a tortuously nanostructured thin film solely consisting of reduced graphene oxide (rGO) sheets, approaching the WVTR toward the theoretical impermeability. Effective design principles proven for this ultrathin super-barrier are (i) tortuous nanostructure realized by random vertical order of the oppositely charged graphene oxide (GO) layers, (ii) 100 % surface coverage, (iii) greater than 35 mu m lateral width of the building block GO, (iv) suppressed defect density during production of GO, (v) surface hydrophobization, and (vi) interlayer distance narrowing down to 0.36 nm. These design principles consistently follow one simple rule: blockage of all possible water diffusion pathways. Also, it is proven here that such impermeability can be actualized by scalable layer-bylayer assembly process, which can shed a light on mass-produced super-barriers for flexible electronics in the near future.