Investigation of vibrational and optoelectronic properties of a RuICl Janus monolayer: A density functional theory study

Janus two-dimensional (2D) materials have attracted increasing attention due to their intrinsic out-of-plane asymmetry, which leads to unconventional electronic, optical, and vibrational properties. Here, we present a comprehensive first-principles investigation of the structural, electronic, optical, vibrational, and thermodynamic properties of the Janus RuICl monolayer using density functional theory within the GGA–PBE approximation. The optimized structure is found to be energetically and dynamically stable, as confirmed by cohesive energy calculations and phonon dispersion relations without imaginary frequencies. The electronic band structure reveals an indirect band gap of approximately 1.62 eV, originating mainly from the hybridization between Ru-d and halogen-p orbitals. Optical calculations show strong anisotropy and intense absorption in the ultraviolet region. Infrared and Raman spectra exhibit characteristic IR-active modes in the far-infrared region and dominant Raman-active A modes around 170 cm -1 , providing clear spectroscopic fingerprints of the Janus phase. Thermodynamic analysis indicates good thermal stability over a wide temperature range, with the heat capacity approaching the classical Dulong–Petit limit at high temperatures. These results establish the Janus RuICl monolayer as a promising candidate for future optoelectronic and spectroscopic applications.