▎ 摘　　要

We have developed a new preparation method for graphene oxide (GO) film capable of long-term storage beyond five years. A contamination-free film several micron-meters in size is prepared. We report that low-voltage scanning transmission electron microscopy produces a good image of a GO film with mono-atomic thickness for a single protein molecule, NADH-ubiquinone dehydrogenase, 20 nm in size, without staining.