First-principles investigation on the structural, vibrational, mechanical, electronic, and optical properties of MSi₂Z₄ (M: Pd and Pt, Z: N and P) monolayers

The recent synthesis of two-dimensional (2D) layered crystals of MoSi2N4 and WSi2N4 has received significant attention due to their novel properties and potential applications. The inclusion of silicon during chemical vapor deposition growth enhances the ambient stability of transitionmetal nitrides and enables large-scale growth in monolayer limit. The transition metal and nitrogen elements can potentially be replaced with other metal and pnictogen atoms, which can constitute a new and large 2D family. In this respect, based on first-principles calculations, we design MSi(2)Z(4) (M: Pd and Pt, Z: N and P) monolayers and investigate their structural, vibrational, mechanical, electronic, and optical properties. The dynamical, thermal, and mechanical stabilities of the proposed systems are confirmed through phonon band dispersion calculations, ab initio molecular dynamics simulations, and elastic tensor analyses, respectively. The computed Raman and infrared spectra of the nanosheets reveal that the frequency and the intensity of the most prominent peaks are specified by the type of pnictogen atom in the structures. The mechanical response of the considered systems in the elastic regime is investigated in terms of in-plane stiffness (Y2D) and the Poisson's ratio (nu). The obtained values of Y2D indicate that MSi2N4 monolayers are stiffer than MSi2P4 nanosheets and the nu values are found to be within the same range and close to the ductile-brittle transition limit. The electronic structure investigation shows that while MSi2N4 monolayers are quasidirect wide-band-gap semiconductors, the MSi2P4 monolayers are semiconductors with an indirect narrow band gap. Additionally, these monolayers have isotropic optical spectra and, depending on the type of pnictogen atom, significant optical absorption peaks within the infrared, visible, and ultraviolet regions can be attained. Additionally, it is found that MSi2N4 monolayers possess multiple bound excitons with strong exciton binding when electron-hole interactions are taken into account. Our results not only expand the class of 2D MSi(2)Z(4) crystals but also offer valuable insights on the physical properties of the designed systems and suggest them as promising materials in diverse optoelectronic applications.