  The behavior of single layer van der Waals (vdW) materials is profoundly
influenced by the immediate atomic environment at their surface, a prime
example being the myriad of emergent properties in artificial heterostructures.
Equally significant are adsorbates deposited onto their surface from ambient.
While vdW interfaces are well understood, our knowledge regarding atmospheric
contamination is severely limited. Here we show that the common ambient
contamination on the surface of: graphene, graphite, hBN and MoS2 is composed
of a self-organized molecular layer, which forms during a few days of ambient
exposure. Using low-temperature STM measurements we image the atomic structure
of this adlayer and in combination with infrared spectroscopy identify the
contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms.
Through its ability to self-organize, the alkane layer displaces the manifold
other airborne contaminant species, capping the surface of vdW materials and
possibly dominating their interaction with the environment.
