  This Chapter deals with single layers of carbon (graphene) and hexagonal
boron nitride on transition metal surfaces. The transition metal substrates
take the role of the support and allow due to their catalytic activity the
growth of perfect layers by means of chemical vapor deposition. The layers are
sp2 hybridized honeycomb networks with strong in plane sigma and weaker pi
bonds to the substrate and to the adsorbates. This hierarchy in bond strength
causes anisotropic elastic properties, where the sp2 layers are stiff in plane
and soft out of plane. A corrugation of these layers imposes a third hierarchy
level in bond energies, with lateral bonding to molecular objects with sizes
between 1 and 5 nanometer. This extra bond energies are in the range of thermal
energies kT at room temperature and are particularly interesting for
nanotechnology. The concomitant template function will be discussed. The
peculiar bond hierarchy also imposes intercalation as another property of sp2
layer systems. Last but not least sp2 layer systems are particularly robust,
i.e. survive immersion into liquids, which is a promise for sp2 layers being
useful outside ultra high vacuum. The Chapter shortly recalls the synthesis,
describes the atomic and electronic structure, is followed by a discussion of
properties like intercalation and the use of sp2 layers on metals as tunneling
junctions or as templates. The Chapter ends with an Appendix that summarizes
the basics of atomic and electronic structure of honeycomb lattices. Of course
the Chapter does not cover all aspects of sp2 single layers. Topics like free
standing layers, edge structures of ribbons, topological defects, or mechanical
and chemical properties were not covered.
