We begin our discussion with the TEM results in Fig. 1. Lower magnification TEM imaging and electron diffraction confirms the successful preparation, i.e. a coverage of ca. 10% of the holes in the Quantifoil TEM grids with single- and few-layer graphene oxide membranes (Fig. 1a–c). Fig. 1d shows an atomically resolved exposure of single-layer GO. With the present setting of spherical aberration and defocus, dark con- trast in this image can be directly interpreted in terms of atomic structure. This image (and many similar ones) show small patches of clean, defect free graphene (indicated by cir- cles), while most of the area is covered by adsorbates or func- tional groups with no obvious structure. The TEM images confirm that GO membranes consist of ordered (graphene- like) segments along with functionalized areas that alternate on a few-nm scale, in agreement with previous models [12,19] and experimental studies [21,22].