High-Dielectric 2D Bismuth Oxides with Large Bandgaps: The Role of 6s² Lone Pair Hybridization

Large-bandgap and high-dielectric 2D dielectrics are crucial for transistor performance because they maximize gate-to-channel capacitive coupling, yet such high-quality materials remain scarce. This study employed first-principles calculations to predict 18 distinct 2D bismuth oxides (BiOs). It is demonstrated that the 6s² lone pairs in 2D BiOs form benign positive feedback between the bandgap and dielectric constant. The hybridization of 6s² and 6p_z orbitals is key to setting the bandgap, revealing a significant negative linear relationship between the bond length and energy gap. In particular, due to the stereochemical activity of 6s² that enhances the ionic contribution, these materials are capable of sustaining a high dielectric constant value surpassing 24, even within bandgaps wider than 4 eV. This discovery enhances the theoretical understanding of 2D materials exhibiting large bandgaps and high dielectric constants, providing deeper insights into the impact of lone pairs on 2D materials.