γ‑GeSe: A New Hexagonal Polymorph from Group IV–VI Monochalcogenides

The family of group IV–VI
monochalcogenides has an atomically
puckered layered structure, and their atomic bond configuration suggests
the possibility for the realization of various polymorphs. Here, we
report the synthesis of the first hexagonal polymorph from the family
of group IV–VI monochalcogenides, which is conventionally orthorhombic.
Recently predicted four-atomic-thick hexagonal GeSe, so-called γ-GeSe,
is synthesized and clearly identified by complementary structural
characterizations, including elemental analysis, electron diffraction,
high-resolution transmission electron microscopy imaging, and polarized
Raman spectroscopy. The electrical and optical measurements indicate
that synthesized γ-GeSe exhibits high electrical conductivity
of 3 × 105 S/m, which is comparable to those of other
two-dimensional layered semimetallic crystals. Moreover, γ-GeSe
can be directly grown on h-BN substrates, demonstrating a bottom-up
approach for constructing vertical van der Waals heterostructures
incorporating γ-GeSe. The newly identified crystal symmetry
of γ-GeSe warrants further studies on various physical properties
of γ-GeSe.