ψ-BCN Monolayers as Emerging 2D Materials: Effects of Hydrogen Passivation on Structure, Stability, and Functionality

Abstract

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-BHCN) results in an indirect band gap of ~0.48 eV, whereas passivation at the carbon site (ψ-BCHN) significantly widens the gap to ~3.58 eV. In contrast, hydrogen passivation at nitrogen atoms (ψ-BCNH) 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-BHCN and ψ-BCHN phases. Analysis of the dielectric function indicates pronounced polarization anisotropy in the pristine and ψ-BHCN structures, while ψ-BCHN 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.