  Most two-dimensional (2D) materials experimentally studied so far have
hexagons as their building blocks. Only a few exceptions, such as PdSe2, are
lower in energy in pentagonal phases and exhibit pentagons as building blocks.
While theory has predicted a large number of pentagonal 2D materials, many of
them are metastable and their experimental realization is difficult. Here we
report the successful synthesis of a metastable pentagonal 2D material, the
monolayer pentagonal PdTe2, by symmetry-driven epitaxy. Scanning tunneling
microscopy and complementary spectroscopy measurements are used to characterize
the monolayer pentagonal PdTe2, which demonstrates well-ordered low-symmetry
atomic arrangements and is stabilized by lattice matching with the underlying
Pd(100) substrate. Theoretical calculations, along with angle-resolved
photoemission spectroscopy, reveal monolayer pentagonal PdTe2 is a
semiconductor with an indirect bandgap of 1.05 eV. Our work opens an avenue for
the synthesis of pentagon-based 2D materials and gives opportunities to explore
their applications such as multifunctional nanoelectronics.
