Fig. 3: Stress relaxation of the membrane on a water droplet.

a Membrane floating on the water droplet is under stress; the associated stress is inversely proportional to the radius of curvature (R) and the membrane thickness (d). The contact angles (ϕ) between the water droplet and PET substrate decreased b from ~73.4° for untreated PET c to ~19.4° for oxygen-plasma-treated PET. Accordingly, the radius of curvature increased from 3.1 to 13.9 nm. Thus, we can transfer much thinner membranes with a clean surface and a wide area on oxygen-plasma-treated PET substrates.