  Using the normal incidence x-ray standing wave technique as well as low
energy electron microscopy we have investigated the structure of
quasi-freestanding monolayer graphene (QFMLG) obtained by intercalation of
antimony under the $\left(6\sqrt{3}\times6\sqrt{3}\right)R30^\circ$
reconstructed graphitized 6H-SiC(0001) surface, also known as zeroth-layer
graphene. We found that Sb intercalation decouples the QFMLG well from the
substrate. The distance from the QFMLG to the Sb layer almost equals the
expected van der Waals bonding distance of C and Sb. The Sb intercalation layer
itself is mono-atomic, flat, and located much closer to the substrate, at
almost the distance of a covalent Sb-Si bond length. All data is consistent
with Sb located on top of the uppermost Si atoms of the SiC bulk.
