ψ-BCN monolayers as emerging 2D materials: effects of hydrogen passivation on structure, stability, and functionality

Two-dimensional (2D) materials with non-hexagonal topologies provide a promising platform for investigating unconventional electronic, optical, and mechanical properties. Using density functional theory (DFT), we investigate the structural, vibrational, electronic, and optical properties of hydrogenated ψ-type boron-carbon-nitride (ψ-BCN) monolayers. While the pristine ψ-BCN compound exhibits metallicity originating from delocalized π-states near the Fermi level, hydrogen passivation induces site-specific electronic transitions. Passivation at the boron site (ψ-B_HCN) results in an indirect band gap of ∼0.48 eV, whereas passivation at the carbon site (ψ-BC_HN) significantly widens the gap to ∼3.58 eV. In contrast, hydrogen passivation at nitrogen atoms (ψ-BCN_H) leads to lattice dynamical instability, as evidenced by pronounced imaginary frequencies in the phonon dispersions. Phonon spectra further reveal significant interactions between optical and acoustic branches, alongside high-frequency localized stretching modes in the stable ψ-B_HCN and ψ-BC_HN phases. Analysis of the dielectric function indicates pronounced polarization anisotropy in the pristine and ψ-B_HCN structures, while ψ-BC_HN shifts the response toward isotropy with absorption extending into the ultraviolet. These findings demonstrate how lattice topology and site-specific hydrogen passivation jointly control the stability and functionality of ψ-BCN monolayers, underscoring their potential for advanced opto-electronic applications.