  The two-dimensional (2D) semiconductor indium selenide (InSe) has attracted
significant interest due its unique electronic band structure, high electron
mobility and wide tunability of its band gap energy achieved by varying the
layer thickness. All these features make 2D InSe a potential candidate for
advanced electronic and optoelectronic applications. Here, we report on the
discovery of new polymorphs of InSe with enhanced electronic properties. Using
a global structure search that combines artificial swarm intelligence with
first-principles energetic calculations, we identify polymorphs that consist of
a centrosymmetric monolayer belonging to the point group D$_{3d}$, distinct
from the well-known polymorphs based on the D$_{3h}$ monolayers that lack
inversion symmetry. The new polymorphs are thermodynamically and kinetically
stable, and exhibit a wider optical spectral response and larger electron
mobilities compared to the known polymorphs. We discuss opportunities to
synthesize these newly discovered polymorphs and viable routes to identify them
by X-ray diffraction, Raman spectroscopy and second harmonic generation
experiments.
