Electric Field Tunable Dipolar Excitons and Excitonic Insulator in α-In₂Se₃

Two-dimensional (2D) ferroelectrics can exhibit strong polarization down to a single-atomic-layer and can also host excitons with large binding energies and dipole-induced charge separation, opening pathways to exploring fundamental physics and applications, arising from the coupling between photoexcitation and electric polarization. However, excitonic properties in 2D ferroelectric semiconductors remain largely unexplored, especially from first-principles perspective. Here, we employ time-dependent density functional theory calculations with optimally tuned, screened, and range-separated hybrid functionals to examine excitonic properties in α-In₂Se₃ monolayer and bilayers. We reveal dipolar charge-separated excitons with long lifetimes, resulting from both the intrinsic in-plane and out-of-plane polarizations. Additionally, we demonstrate that an external electric field can modulate the electron and hole wavefunctions, with tunable exciton energy and oscillator strength. Furthermore, in bilayer α-In₂Se₃ with 2H or 3R stacking, out-of-plane polarization significantly reduces the quasiparticle bandgap without much enhanced electronic screening, leading to an excitonic insulator phase. The excitonic insulator and excitonic properties are further tunable by an electric field, offering a means to manipulate their behavior and phase transitions. Our study provides novel insights for future research of excitons and excitonic insulators in 2D ferroelectric materials.