Robust Spin Splitting and Strain-Controlled Optical Response in Monolayer CrC2N4 for Valleytronic and Optoelectronic Applications

Abstract

The atomically thin CrC₂N₄ has recently emerged as a promising two-dimensional semiconductor, yet its spin−orbit-coupled (SOC) valley physics and strain-tunable nanoscale optical response remain largely unexplored. Here, we investigated the electronic, valley, charge-transfer, and optical properties of pristine and biaxially strained monolayer CrC₂N₄ using first-principles calculations. Total-energy comparison among nonmagnetic, ferromagnetic, and antiferromagnetic configurations confirmed a nonmagnetic ground state, corroborating that time-reversal symmetry was preserved while spatial inversion symmetry was broken. The monolayer exhibited a direct band gap of 2.33 eV at the K/K′ valleys. SOC produced valley-contrasting out-of-plane spin polarization, yielding a valence band spin splitting of 80.7 meV and a conduction band spin splitting of 11.6 meV. This asymmetry was enforced by the orbital angular momentum character of the band edges, the valence edge carrying |m_l| = 2 and the conduction edge being dominated by an m_l = 0 state for which the first-order SOC contribution vanishes by symmetry. Orbital-resolved analysis showed that the edge states were governed by Cr-d and N-p hybridization, while Bader analysis indicated polar-covalent bonding through charge transfer toward N atoms. Biaxial strain from −4% to +4% tuned the band gap from 2.749 to 1.759 eV and drove an indirect-to-direct gap transition, with phonon dispersion and ab initio molecular dynamics calculations confirming dynamical and thermal stability at both strain limits. Tensile strain enhanced the Berry curvature from 30.85 to 42.35 bohr² and shifted the optical response across the visible−near-infrared region for the strain range. These results highlight monolayer CrC₂N₄ as a promising platform for strain-engineered nanoscale valleytronic and optoelectronic device applications.