A First-Principles Investigation of InSiN₂ Monolayer: A Novel Two-Dimensional Material with Enhanced Stability and Tunable Vibrational and Electronic Properties

The recent synthesis of large-scale, high-quality MoSi2N4 and WSi2N4 monolayers using a bottom-up approach has led to the emergence of a new family of two-dimensional (2D) materials. This development has paved the way for exploring various 2D crystals with the general formula of MSi2N4. Although previous studies have predominantly focused on systems containing transition metals, it is intriguing to consider the extension of these structures to other metallic elements. Motivated by the growing interest in this class of materials owing to their novel properties, we designed an InSiN2 (In2Si2N4) monolayer and investigated its vibrational, electronic, optical, mechanical, and piezoelectric properties, using first-principles methods. Verification of the dynamical stability of the considered nanosheet is accomplished through phonon dispersion analysis and ab initio molecular dynamics simulations. The electronic structure calculations reveal that the InSiN2 monolayer is a direct bandgap semiconductor with a gap energy of 2.38 eV, in contrast to the indirect bandgap MoSi2N4 monolayer. The optical response calculations, including many-body effects, highlight significant light absorption within the visible spectrum with bound excitons. The mechanical properties of the InSiN2 nanosheet within an elastic regime are investigated in terms of in-plane stiffness, Poisson's ratio, and ultimate tensile strength, and the obtained results point out its rigid and ductile nature. The piezoelectric calculations demonstrate that InSiN2 surpasses the MoSi2N4 monolayer in in-plane piezoelectric performance. The phonon spectrum analyses show that the InSiN2 nanosheet remains stable over a wide strain range, maintaining structural integrity up to 7% tensile and 5% compressive strain, accompanied by substantial phonon mode shifts. In addition, the variations in electronic and vibrational properties are investigated under equibiaxial strain. The Raman-active phonon modes exhibit softening (hardening) under tensile (compressive) strain. Direct-to-indirect band gap transitions are also observed within the specified strain range, accompanied by bandgap variation. The robust stability and tunability of vibrational and electronic properties via strain engineering make the InSiN2 monolayer a promising material for strain-modulated optoelectronic applications.