  Monolayer group-IV monochalcogenides (MX, M = Ge, Sn, Pb; X = S, Se, Te) are
a family of novel two-dimensional (2D) materials that have atomic structures
closely related to that of the staggered black phosphorus lattice. The
structure of most monolayer MX materials exhibits a broken inversion symmetry,
and many of them exhibit ferroelectricity with a reversible in-plane electric
polarization. A further consequence of the noncentrosymmetric structure is that
when coupled with strong spin-orbit coupling, many MX materials are promising
for the future applications in non-linear optics, photovoltaics, spintronics
and valleytronics. Nevertheless, because of the relatively large exfoliation
energy, the creation of monolayer MX materials is not easy, which hinders the
integration of these materials into the fast-developing field of 2D material
heterostructures. In this Perspective, we review recent developments in
experimental routes to the creation of monolayer MX, including molecular beam
epitaxy and two-step etching methods. Other approaches that could be used to
prepare monolayer MX are also discussed, such as liquid phase exfoliation and
solution phase synthesis. A quantitative comparison between these different
methods is also presented.
