  Growth of two-dimensional metals has eluded materials scientists since the
discovery of the atomically thin graphene and other covalently bound 2D
materials. Here, we report a two-atom-thick hexagonal copper-gold alloy, grown
through thermal evaporation on freestanding graphene and hexagonal boron
nitride. The structures are imaged at atomic resolution with scanning
transmission electron microscopy and further characterized with spectroscopic
techniques. Electron irradiation in the microscope provides sufficient energy
for a phase transformation of the 2D structure--atoms are released from their
lattice sites with the gold atoms eventually forming face-centered cubic
nanoclusters on top of 2D regions during observation. The presence of copper in
the alloy enhances sticking of gold to the substrate, which has clear
implications for creating atomically thin electrodes for applications utilizing
2D materials. Its practically infinite surface-to-bulk ratio also makes the 2D
CuAu particularly interesting for catalysis applications.
