  Monolayer PbSe has been predicted to be a two-dimensional (2D) topological
crystalline insulator (TCI) with crystalline symmetry-protected Dirac-cone-like
edge states. Recently, few-layered epitaxial PbSe has been grown on the SrTiO3
substrate successfully, but the corresponding signature of the TCI was only
observed for films not thinner than seven monolayers, largely due to
interfacial strain. Here, we demonstrate a two-step method based on molecular
beam epitaxy for the growth of the PbSe-CuSe lateral heterostructure on the
Cu(111) substrate, in which we observe a nanopore patterned CuSe layer that
acts as the template for lateral epitaxial growth of PbSe. This further results
in a monolayer PbSe-CuSe lateral heterostructure with an atomically sharp
interface. Scanning tunneling microscopy and spectroscopy measurements reveal a
four-fold symmetric square lattice of such monolayer PbSe with a quasi-particle
band gap of 1.8 eV, a value highly comparable with the theoretical value of
freestanding PbSe. The weak monolayer-substrate interaction is further
supported by both density functional theory (DFT) and projected crystal orbital
Hamilton population, with the former predicting the monolayer's anti-bond state
to reside below the Fermi level. Our work demonstrates a practical strategy to
fabricate a high-quality in-plane heterostructure, involving a monolayer TCI,
which is viable for further exploration of the topology-derived quantum physics
and phenomena in the monolayer limit.
