  Materials with topological flat bands can serve as a promising platform to
investigate strongly interacting phenomena. However, experimental realization
of ideal flat bands is mostly limited to artificial lattices or moir\'e
systems. Here we report a general way to construct one-dimensional (1D) flat
bands in phosphorene nanoribbons (PNRs) with pentagonal nature: penta-hexa-PNRs
and penta-dodeca-PNRs, wherein the corresponding flat bands are directly
verified by using angle-resolved photoemission spectroscopy. We confirm that
the observed 1D flat bands originate from the electronic 1D sawtooth and Lieb
lattices, respectively, as revealed by the combination of bond-resolved
scanning tunneling microscopy, scanning tunneling spectroscopy, tight-binding
models, and first-principles calculations. Our study demonstrates a general way
to construct 1D flat bands in 1D solid materials system, which provides a
robust platform to explore strongly interacting phases of matter.
