  One of the keys behind the success of the modern semiconductor technology has
been the ion implantation of silicon, which allows its electronic properties to
be tailored. For similar purposes, heteroatoms have been introduced into carbon
nanomaterials both during growth and using post-growth methods. However, due to
the nature of the samples, it has been challenging to determine whether the
heteroatoms have been incorporated into the lattice as intended, with direct
observations so far being limited to N and B dopants, and incidental Si
impurities. Furthermore, ion implantation of these materials is more
challenging due to the requirement of very low ion energies and atomically
clean surfaces. Here, we provide the first atomic-resolution imaging and
electron energy loss spectroscopy (EELS) evidence of phosphorus atoms
incorporated into the graphene lattice by low-energy ion irradiation. The
measured P L-edge response of an single-atom EELS spectrum map shows excellent
agreement with an ab initio spectrum simulation, conclusively identifying the P
in a buckled substitutional configuration. Our results demonstrate the
viability of phosphorus as a lattice dopant in $sp^2$-bonded carbon structures
and provide its unmistakeable fingerprint for further studies.
