  The large-scale production of graphene monolayer greatly relies on epitaxial
samples which often display stress-relaxation features in the form of wrinkles.
Wrinkles of graphene on Ir(111) are found to exhibit a fairly well ordered
interconnecting network which is characterized by low-energy electron
microscopy (LEEM). The high degree of quasi-hexagonal network arrangement for
the graphene aligned to the underlying substrate can be well described as a
(non-Poissonian) Voronoi partition of a plane. The results obtained strongly
suggest that the wrinkle network is frustrated at low temperatures, retaining
the order inherited from elevated temperatures when the wrinkles interconnect
in junctions which most often join three wrinkles. Such frustration favors the
formation of multi-lobed wrinkles which are found in scanning tunneling
microscopy (STM) measurements. The existence of multiple lobes is explained
within a model accounting for the interplay of the van der Waals attraction
between graphene and iridium and bending energy of the wrinkle. The presented
study provides new insights into wrinkling of epitaxial graphene and can be
exploited to further expedite its application.
