Single-layer CrI ₂ as a magnetic semiconductor: a detailed first-principles study

The study of magnetic monolayer (ML) structures has garnered increasing attention due to their potential for unveiling diverse physical phenomena in two-dimensional (2D) systems, as well as their promising applications in spintronics, optoelectronics, and magnonics. In this work, we present a comprehensive density functional theory investigation of the physical properties of monolayer chromium diiodide (CrI₂). As a starting point, we revisit the bulk CrI₂ system and show that its structural and vibrational properties, including Raman and infrared (IR) spectra, are highly sensitive to the treatment of van der Waals interactions, as modeled by different dispersion correction schemes. We then examine in detail the structural, electronic, magnetic, vibrational, and thermodynamic properties of the CrI₂ ML. Our results indicate that the antiferromagnetic (AFM) configuration corresponds to the ground state, with a magnetic moment of 3.8<i>μ</i>_B per Cr atom. The ML exhibits semiconducting behavior with an indirect band gap of 0.64 eV, and effective masses of -1.25<i>m</i>_e and 0.24<i>m</i>_e for the valence band maximum and the conduction band minimum, respectively. Additionally, we evaluate the magnetocrystalline anisotropy energy and identify key energy extrema in the AFM ML. A complete characterization of the vibrational modes at the Γ point is also provided, detailing their Raman and IR activity and atomic displacement patterns across different frequency ranges. The interplay of low-dispersive valence bands, semiconducting behavior, and robust antiferromagnetism suggests that CrI₂ MLs are promising candidates for next-generation spintronic and optoelectronic applications, offering both fundamental insights and technological potential.