Electrically Switchable Single-Photon Emission from Carbon Defects in hBN via a Ferroelectric In ₂ Se ₃ Substrate

van der Waals heterostructures, formed by stacking 2D materials, offer a route to programmable quantum light sources. Using first-principles calculations, we show that a ferroelectric In 2 Se 3 substrate enables large, reversible tuning of absorption and zero-phonon lines (ZPLs) of carbon-related single-photon emitters (C B C N, C 2 C N, and V N C B ) embedded in hBN by stacking and polarization switching. Polarization reversal induces ZPL shifts up to 0.3 eV and, for V N C B, switches the charge state ( q = 0 → +1). These effects arise from strong, defect-site-dependent electrostatics at the hBN/In 2 Se 3 interface and are robust to stacking variations. Our results establish ferroelectric substrates as an electrical knob for engineering spectrally tunable and spin-configurable 2D quantum emitters, easing the way for ultrathin, integrable single-photon sources.