Polymorphic Janus ZrSC monolayer: a first-principles study of structural, mechanical, vibrational, and electronic properties.

Two-dimensional Janus materials, characterized by their asymmetric surface composition, offer unique opportunities for tailoring electronic and mechanical properties. Among these, the hypothetical ZrSC monolayer, composed of zirconium sandwiched between sulfur and carbon layers, represents an underexplored system with potential to merge the mechanical durability of carbides with the electronic tunability of chalcogenides. Using first-principles density functional theory (DFT) with van der Waals corrections and Hubbard-U adjustments, we systematically investigate the structural, mechanical, vibrational, and electronic properties of its two polymorphs: the trigonal prismatic 1H and octahedral 1T phases. Our results establish the 1H phase as the thermodynamically stable ground state with a cohesive energy of 6.99 eV per atom, a moderate Young's modulus of 70.7 N m-1, and a tunable semiconductor with an indirect bandgap of 1.63 eV that is highly responsive to mechanical strain. Phonon spectra confirm its dynamical stability, while the 1T phase exhibits significant imaginary frequencies, indicating metastability. Strain engineering induces semiconductor-to-semimetal transitions in the 1H phase, highlighting its potential for flexible electronics and spintronics. We conclude that the 1H ZrSC monolayer is a promising candidate for next-generation 2D devices, offering a versatile platform that bridges mechanical robustness with electronic adaptability, and providing a clear roadmap for experimental synthesis and device integration.