This study presents the synthesis and characterization of LaOBr, a rare-earth oxyhalide with a high dielectric constant (ε₀ ≈ 9) and a wide bandgap of 5.3 eV, suitable for use as a high-κ dielectric in two-dimensional (2D) van der Waals heterostructures. The researchers developed a high-temperature flux growth method to produce large, stoichiometric LaOBr crystals, which were then exfoliated into 2D flakes. These flakes were used as dielectrics in van der Waals field-effect transistors (FETs) and as encapsulants for excitonic devices based on 2D materials. The LaOBr dielectric exhibited low leakage currents, high breakdown voltage (8 MV/cm), and a low interface defect concentration, making it an ideal material for high-performance 2D electronic devices. The study also demonstrated that LaOBr can be used as a gate dielectric to control excitonic features in 2D materials, showing the modulation of excitonic species in MoSe₂ by electronic gating. The results highlight the potential of LaOBr as a versatile high-κ dielectric for 2D heterostructures, offering a promising alternative to traditional high-κ oxides like Al₂O₃. The work underscores the importance of van der Waals heterostructures in advancing 2D materials-based electronics and optoelectronics.This study presents the synthesis and characterization of LaOBr, a rare-earth oxyhalide with a high dielectric constant (ε₀ ≈ 9) and a wide bandgap of 5.3 eV, suitable for use as a high-κ dielectric in two-dimensional (2D) van der Waals heterostructures. The researchers developed a high-temperature flux growth method to produce large, stoichiometric LaOBr crystals, which were then exfoliated into 2D flakes. These flakes were used as dielectrics in van der Waals field-effect transistors (FETs) and as encapsulants for excitonic devices based on 2D materials. The LaOBr dielectric exhibited low leakage currents, high breakdown voltage (8 MV/cm), and a low interface defect concentration, making it an ideal material for high-performance 2D electronic devices. The study also demonstrated that LaOBr can be used as a gate dielectric to control excitonic features in 2D materials, showing the modulation of excitonic species in MoSe₂ by electronic gating. The results highlight the potential of LaOBr as a versatile high-κ dielectric for 2D heterostructures, offering a promising alternative to traditional high-κ oxides like Al₂O₃. The work underscores the importance of van der Waals heterostructures in advancing 2D materials-based electronics and optoelectronics.