Direct Synthesis of Atomically Thin High-κ Bi₂SeO₅ and Derivatives as a Single-Crystalline Dielectric

Two-dimensional (2D) materials hold great potential for next-generation electronics. However, the synthesis of compatible dielectrics with a high dielectric constant (κ) presents a critical bottleneck for realizing high-performance field-effect transistors (FETs). Here, we report the direct synthesis of atomically thin Bi₂SeO₅ single-crystalline by a flux-assisted growth (FAG) method. Leveraging the confined thin molten flux, Bi₂SeO₅ flakes with thicknesses down to 3.9 nm, lateral sizes up to ∼0.5 mm, and a dielectric constant of ∼15 are achieved. Furthermore, two novel 2D dielectrics of Bi₂Se_1-xNa_yO_5-z (BSO-Na) and Bi₂Se_1-xO_5-yI_z (BSO-I) with maximum lateral sizes of 0.7 and 0.2 mm are obtained by a one-step heterogeneous atom doping strategy, enabling dielectric tunability. In MoS₂ FETs, FAG-grown Bi₂SeO₅ enables superior electrostatic control, achieving an on/off ratio exceeding 10⁷ and a minimum subthreshold swing of 62.7 mV/dec. This work presents a scalable method for producing high-quality high-κ dielectrics and a broadly applicable strategy for the compositional engineering of dielectric systems.