Exploring the high dielectric performance of Bi₂SeO₅: from bulk to bilayer and monolayer

Bi₂SeO₅ has garnered considerable attention as a van der Waals (vdW) layered dielectric material featuring excellent electrical insulation properties. However, the related theoretical understanding of the dielectric properties of atomically thin films is still lacking. Here, we conducted the first-principles calculations to determine the dielectric performance of Bi₂SeO₅, showing a high average dielectric constant (ε) of >20 ranging from bulk to bilayer and monolayer. Besides, the conduction and valance band offsets between the monolayer Bi₂SeO₅ and bilayer Bi₂O₂Se were calculated to be greater than 1 eV, suggesting that monolayer Bi₂SeO₅ works well as the dielectric for atomically thin Bi₂O₂Se. Unlike h-BN or other two-dimensional (2D) vdW insulators, ε of Bi₂SeO₅ is dominated by its ionic component and remains nearly constant as the thickness decreases, demonstrating an ultralow equivalent oxide thickness (EOT) of 0.3 nm for its monolayer form. Moreover, the high ε of monolayer Bi₂SeO₅ survives under tensile or compressive strains up to 6%, which greatly facilitates its integration with various 2D semiconductors. Our work suggests that Bi₂SeO₅ ultrathin films can serve as excellent atomically flat encapsulation and dielectric layers for high-performance 2D electronic devices.