  The structural, electronic, and optical properties of flat and buckled AlN
monolayers are investigated using first-principles approaches. The band gap of
a flat AlN monolayer is changed from an indirect one to a direct one, when the
planar buckling increases, primarily due to diminishing sp$^2$ overlapping and
bond symmetry breaking in the conversion to sp$^3$ bonds. The sp$^3$
hybridization thus results in a stronger $\sigma\text{-}\pi$ bond rather than a
$\sigma\text{-}\sigma$ covalent bond. The calculations of the phonon band
structure indicates that the buckled AlN monolayers are structurally and
dynamically stable. The optical properties, such as the dielectric function,
the refractive index, and the optical conductivity of an AlN monolayer are
evaluated for both flat systems and those impacted with planar buckling. The
flat AlN monolayer has outstanding optical characteristics in the Deep-UV and
absorbs more effectively in the UV spectrum due to its large band gap. The
results reveal that optical aspects are enhanced along different directions of
light polarization, with a considerable shift in the optical spectrum from
Deep-UV into the visible range. Additionally, depending on the polarization
direction of the incoming light, increased planar buckling enhances the optical
conductivity in both the visible and the Deep-UV domains. The ability to modify
the optical and electronic properties of these essential 2D materials using
planar buckling technique opens up new technological possibilities,
particularly for optoelectronic devices.
