Interface Excitons in van der Waals Sandwich Heterostructures.
Exciton engineering in van der Waals heterostructures (vdWHs) is essential for next-generation optoelectronics, yet they normally require near-perfect stacking and are highly sensitive to moiré potentials. Here, we demonstrate a polarity-engineering strategy using a γ-InSe/transition metal dichalcogenide/γ-InSe sandwich heterostructure. The out-of-plane spontaneous polarization of γ-InSe intrinsically breaks interfacial inversion symmetry, giving rise to interface excitons (IFXs) that exhibit a linear Stark effect with an ultrasmall dipole moment of 0.15 e·nm. First-principles calculations and Kelvin probe force microscopy reveal asymmetric interfacial charge transfer governed by γ-InSe's polarity. Transient spectroscopy shows nonmonotonic relaxation dynamics, including a characteristic signal reversal that indicates pre-existing interfacial charge states. Our results establish that exciton dipole moments, interlayer coupling, and relaxation dynamics can be precisely tuned through material polarity and thickness. Polarity engineering thus provides a versatile and robust route to control excitonic properties in vdWHs, offering expanded design strategies for advanced excitonic and optoelectronic devices.