Interlayer excitons (IXs), formed in type-II van der Waals (vdW) heterostructures where electrons and holes reside in adjacent monolayers, have attracted increasing interest due to their spatially indirect nature, long lifetime, strong Coulomb binding, and unique out-of-plane dipole moment. These features make IXs a promising platform for exploring many-body physics and realizing next-generation excitonic devices. This review systematically presents the formation mechanisms, identification methods, and external modulation strategies of interlayer excitons in two-dimensional materials.First, we analye the prerequisites for the IX formation, emphasizing the role of band alignment, interlayer charge transfer, and momentum mismatch. Recent studies have also revealed that direct interlayer absorption is an alternative pathway for IX generation. For identification, we summarize multiple optical techniques, including photoluminescence (PL), photoluminescence excitation (PLE), transient absorption (TA), and electro-absorption (EA). These techniques can detect IX energy positions, binding energies, and recombination pathways. However, distinguishing interlayer excitons from defect-bound or momentum-indirect excitons remains challenging in experiment due to spectral overlap and measurement-dependent explanation.Then, we review five primary external modulation methods: electric field, strain, magnetic field, twist angle, and optical cavities. Electric fields can realize fast, reversible tuning of exciton energy levels, especially for excitons with large dipole moments. Strain provides nanoscale spatial control and can reshape local potential landscapes. Magnetic fields affect the spin-valley configurations and allow access to exciton polarization dynamics. Moiré engineering via twist angles introduces periodic potential landscapes, yielding moiré-trapped IXs and novel hybrid exciton–polaritons. Optical cavities enhance exciton radiative recombination via light–matter coupling and open up possibilities for strong coupling regimes. We further discuss additional strategies such as substrate-induced screening, dielectric environment, probe-induced local stress, and ferroelectric gating, all of which enrich the modulation toolbox.To facilitate cross-comparison, we present a comprehensive summary table comparing different modulation approaches in terms of tuning targets, dimensionality, efficiency, dynamic responsiveness, and implementation complexity.Finally, we discuss emerging applications of IXs in optoelectronic and quantum devices. Their tunable emission and long-lived nature make them suitable for exciton-based memory, logic, lasers, and reconfigurable photonic circuits. With the development of material synthesis, interface engineering, and hybrid integration, interlayer excitons are evolving from basic quasiparticles to programmable excitonic elements in chip-scale photonics and quantum information technologies.
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