Proximity-Induced Interlayer Excitons
- Proximity-induced interlayer excitons are electron–hole pairs residing in adjacent layers, exhibiting unique optical and electronic signatures due to layer proximity effects.
- They form via type-II band alignment, tunneling, and hybridization in 2D materials, demonstrating tunable lifetimes and exciton–phonon interactions.
- These excitons enable novel optoelectronic device concepts by allowing precise control over binding energies, oscillator strengths, and permanent dipole moments.
Proximity-induced interlayer excitons are Coulomb-bound electron–hole pairs whose constituents reside in different adjacent layers and whose existence, generation, fine structure, or optical activity is created or strongly modified by layer proximity. In atomically thin van der Waals bilayers this proximity is expressed through type-II band alignment, interlayer tunneling, layer hybridization, dielectric screening, moiré modulation, and permanent out-of-plane dipoles; in intrinsic multilayers and layered perovskites it can arise from adjacent inorganic sheets within the same crystallographic unit cell, which supply a genuine layer degree of freedom to the excitonic spectrum (Brotons-Gisbert et al., 2024, Das et al., 2019, Grenzer et al., 21 Apr 2026). The term therefore covers both spatially indirect excitons that are weakly allowed or dark in the centrosymmetric limit and layer-hybridized excitons that retain substantial optical dipole while acquiring interlayer character (Hsu et al., 2019, Sponfeldner et al., 2021, Thodika et al., 26 Sep 2025).
1. Definitions, taxonomy, and material realizations
The basic distinction is between intralayer and interlayer excitons. Intralayer excitons place electron and hole in the same monolayer or inorganic sheet and typically exhibit large oscillator strength; interlayer excitons place the electron and hole in different layers, acquire a permanent out-of-plane dipole , and usually have reduced electron–hole overlap, smaller oscillator strength, and longer radiative lifetimes. In transition-metal dichalcogenide heterobilayers this distinction is commonly tied to type-II alignment, whereas in homobilayers and few-layer crystals it can emerge from layer-selective confinement or carrier-species-specific hybridization (Brotons-Gisbert et al., 2024, Ovesen et al., 2018, Das et al., 2019).
A second taxonomy concerns how “interlayer” the exciton actually is. In conventional spatially indirect interlayer excitons, the electron and hole are predominantly localized in different layers; in layer-hybridized excitons, one carrier remains layer-confined while the other is extended across both layers. Commensurate H-type WSe/MoSe and WS/MoS realize the latter situation through valence-band hybridization at , yielding excitons that combine a large optical transition dipole with a large out-of-plane electric dipole (Hsu et al., 2019). Naturally 2H-stacked bilayer MoS provides another nonstandard case: the observed interlayer exciton at has strong oscillator strength and is described as a hole state delocalized across the two layers bound to an electron localized in one of the layers (Sponfeldner et al., 2021).
The same language extends beyond TMDs. In few-layer 2H-WSe, the band-edge states at are effectively confined to alternating layers, producing bright direct (quasi-)intralayer excitons and lower-energy interlayer dark excitons; the bright states are organized by the coupled spin, valley, and layer rule 0 (Das et al., 2019). In the 1 Ruddlesden–Popper perovskite (PEA)2PbI3, the two Pb–I octahedral sheets in one crystallographic unit cell double the intralayer fine structure and enable spatially indirect interlayer excitons within the same unit cell (Grenzer et al., 21 Apr 2026). Proximity-induced interlayer excitons have also been identified in magnetic WSe4/CrI5 stacks and in bilayer armchair graphene nanoribbons, where stacking, hybridization, and screening again determine whether the lowest excitations are intralayer, interlayer, or hybridized (Thodika et al., 26 Sep 2025, Rocha et al., 15 Jul 2025).
2. Microscopic routes by which proximity produces interlayer excitons
In many van der Waals heterobilayers, the minimal route is type-II band alignment. The review literature summarizes the corresponding exciton energy as 6, with 7 set by the interlayer spacing (Brotons-Gisbert et al., 2024). In MoSe8–WSe9, the type-II offset places the conduction-band minimum in MoSe0 and the valence-band maximum in WSe1, so optical excitation of an intralayer exciton is followed by hole tunneling and formation of an interlayer exciton (Ovesen et al., 2018). In twisted WSe2/MoS3, femtosecond photoemission momentum microscopy shows that interlayer excitons are dominantly formed on the sub-50 fs timescale via interlayer tunneling at the 4 valleys, followed by slower formation through dark 5-valley excitons and phonon cooling (Schmitt et al., 2021).
A second route is proximity-controlled exciton conversion rather than direct optical creation. In Janus-based heterobilayers such as MoS6@WSSe, the intrinsic out-of-plane electric field of the Janus layer tunes the type-II offsets and hence the energy separation 7 between bright intralayer and dark interlayer excitons. When 8 is resonant with strong in-plane optical phonons, first-order exciton–phonon scattering can efficiently convert bright intralayer excitons into lower-lying interlayer excitons without an external gate (Torun et al., 2022). For MoS9@WSSe in AB stacking, the reported 0 eV matches the transverse optical-like 1 modes near 2 meV, and the projected exciton–phonon couplings reach 3–4 meV (Torun et al., 2022).
A third route is interlayer hybridization. In commensurate H-type WSe5/MoSe6 and WS7/MoS8, momentum alignment and symmetry-allowed interlayer hopping hybridize the valence-band edges while the relevant conduction state remains layer-confined. The resulting two-level valence Hamiltonian,
9
produces hybridized hole states whose excitons inherit both optical brightness and an electric dipole. Experimentally extracted parameters are 0 meV and 1 meV in H-type WSe2/MoSe3, and 4 meV and 5 meV in H-type WS6/MoS7 (Hsu et al., 2019).
Intrinsic layered materials supply yet another route. In few-layer WSe8, symmetry suppresses conduction-band hopping at 9 while strong spin–orbit splitting keeps the valence states effectively layer-localized, so adjacent-layer Coulomb binding yields lower-energy interlayer dark excitons without requiring a heterointerface (Das et al., 2019). In (PEA)0PbI1, structural inequivalence of the two adjacent inorganic layers generates a layer-dependent offset of band edges, furnishing interlayer transitions whose electron–hole probability density lies on adjacent layers (Grenzer et al., 21 Apr 2026).
3. Fine structure, selection rules, and optical activity
The fine structure of proximity-induced interlayer excitons is often inseparable from that of nearby intralayer states. In (PEA)2PbI3, polarization-resolved low-temperature photoluminescence at 4 K resolves four bright intralayer transitions at 5 eV, 6 eV, 7 eV, and 8 eV, each with 9 meV uncertainty. These form two closely spaced 0-like pairs associated with the two inorganic layers, with intralayer 1 anisotropy of 2–3 meV and an 4 centroid offset of 5 meV. A weaker doublet at 6 eV and 7 eV, about 8 meV above the bright intralayer states and split by 9 meV, is assigned to interlayer excitons (Grenzer et al., 21 Apr 2026). Static 0+BSE places the interlayer manifold higher, at 1 meV above the bright intralayer states, and predicts small oscillator strengths; distortion-induced mixing with bright intralayer excitons is proposed to account for the observed brightening and nearly orthogonal in-plane polarizations (Grenzer et al., 21 Apr 2026).
Few-layer WSe2 supplies a complementary fine-structure limit in which interlayer states are intrinsically dark. In bilayer WSe3, the lower-energy 4 is interlayer and dark, whereas the higher-energy 5 is intralayer and bright; the calculated emission energies differ by about 6 meV, and the bright state has about two orders of magnitude larger radiative rate when 7 (Das et al., 2019). More generally, an 8-layer film contains two doubly degenerate bright 9 excitons, distributed over even or odd layers, while the remaining 0 1 bands are interlayer and dark (Das et al., 2019).
Layer-hybridized excitons modify these selection rules by redistributing the oscillator strength between branches. In the H-type bilayers analyzed by Tang and co-workers, the optical dipoles scale as 2 and the electric dipoles as 3 (Hsu et al., 2019). Accordingly, H-type WSe4/MoSe5 shows a WSe6-derived doublet split by 7 meV with near-8 spectral weight, while H-type WS9/MoS0 shows a corresponding doublet split by 1 meV; in R-type or twisted bilayers the doublets disappear because the interlayer hopping at 2 is symmetry-forbidden or momentum-suppressed (Hsu et al., 2019).
Coupling between interlayer and intralayer excitons can itself become a spectroscopic observable. In naturally 2H-stacked bilayer MoS3, reflectivity-derived optical susceptibility reveals an avoided crossing between an interlayer exciton branch and the intralayer B exciton with positive coupling 4 meV, and a weaker avoided crossing with the intralayer A exciton with negative coupling 5 meV (Sponfeldner et al., 2021). In magnetic WSe6/CrI7 and CrI8/WSe9/CrI00, a robust interlayer magnetic exciton appears near 01 eV between the WSe02 A and B excitons, with oscillator strength nearly twenty times larger than the intralayer CrI03 Frenkel excitons and dominant out-of-plane dipole character (Thodika et al., 26 Sep 2025).
4. Spatiotemporal formation, localization, and transport
The temporal sequence of interlayer exciton formation has been established most clearly in TMD heterostructures. In MoSe04–WSe05, a 06 fs Gaussian pump resonant with the MoSe07 08 intralayer exciton creates coherent intralayer polarization that decays within hundreds of femtoseconds via radiative emission and exciton–phonon scattering; interlayer hole tunneling then converts the population to interlayer excitons on a sub-picosecond timescale, with near-complete transfer by 09 ps (Ovesen et al., 2018). The interlayer photoluminescence at 10 eV surpasses intralayer emission by around 11 ps, and by 12 ps the equilibrium intensity ratio 13 at 14 K (Ovesen et al., 2018). In WSe15/MoS16, the trARPES signature is an interlayer-exciton peak at 17 eV above the WSe18 valence-band maximum, a characteristic three-peak momentum fingerprint at the mini-Brillouin-zone 19 points, and a wavefunction with approximately 20 of its probability within one moiré unit cell (Schmitt et al., 2021).
The spatial structure of interlayer excitons is not universally localized. Real-space calculations for reconstructed H-type MoSe21/WSe22 show that at a twist deviation of 23 from 24, where the moiré period is 25 nm and the reconstructed potential minima are about 26 meV, the ground-state interlayer-exciton center-of-mass wavefunction extends over multiple moiré cells and reaches the computational domain limit (Figueiredo et al., 2024). At 27 there is a mixture of extended and localized states, whereas at 28 the lowest states become localized within a single moiré unit cell and some excited states form quasi-1D quantum-wire-like modes (Figueiredo et al., 2024). Cryogenic photoluminescence supports this crossover: near 29, the steady-state line at 30 eV is Lorentzian with FWHM 31 meV and dominates at long delays, while earlier-time lines have FWHM values of 32, 33, and 34 meV (Figueiredo et al., 2024).
Transport at finite density is likewise proximity-sensitive. A microscopic theory for MoSe35–WSe36 derives the interaction energy per exciton as 37, where the repulsive dipolar term competes with attractive fermionic exchange and a Coulomb-hole shift (Erkensten et al., 2022). Above 38, the resulting drift term dominates over ordinary diffusion and drives highly nonlinear propagation; for 39, the model yields 40 around 41 ns without spacer, and up to 42 at early times with hBN spacers (Erkensten et al., 2022). Counter to the naive expectation that weaker environmental screening should speed transport, the theory finds that free-standing samples propagate more slowly than hBN-encapsulated ones because stronger screening reduces the magnitude of the attractive Coulomb-hole term, leaving more dipolar repulsion uncompensated (Erkensten et al., 2022).
5. Theoretical descriptions
Most microscopic descriptions start from a Bethe–Salpeter treatment of correlated electron–hole pairs. In the perovskite case, the exciton eigenproblem is written as
43
or equivalently 44, with oscillator strength
45
Within this framework, layer-doubled intralayer manifolds and weak interlayer manifolds emerge naturally once the two-layer unit cell and symmetry lowering are included (Grenzer et al., 21 Apr 2026). In Janus heterobilayers, the same BSE amplitudes are projected onto phonon modes through
46
so that the interconversion rate follows Fermi’s Golden Rule and is maximal when 47 (Torun et al., 2022).
For TMD heterostructures, an explicit two-body Dirac formulation has also been developed. Starting from monolayer massive Dirac Hamiltonians, one constructs a four-band interlayer-exciton Hamiltonian and couples it to a proximity-modified interaction
48
whose dielectric kernel includes the top, bottom, and barrier media, as well as polarization effects in the transition-metal and chalcogen layers (Donck et al., 2018). This formalism predicts interlayer binding energies on SiO49 of roughly 50–51 meV for several TMD pairs at 52 nm, larger average in-plane electron–hole separation than in intralayer excitons, and a linear Stark shift 53 for the interlayer peak (Donck et al., 2018).
Effective Hamiltonians remain useful when the spectroscopy is dominated by a few interacting branches. In bilayer MoS54, a coupled-oscillator susceptibility model,
55
combined with intensity asymmetries at avoided crossings, determines both magnitude and sign of the inter-exciton couplings (Sponfeldner et al., 2021). At the many-body level, transport and condensation problems often reduce to density-dependent bosonic descriptions. For interlayer excitonic insulators, an explicitly solvable mean-field model uses an interlayer interaction 56 and predicts that including proximity-enhanced overlap in 57 permits an interlayer excitonic-insulator phase up to 58 nm for 59, whereas neglecting the overlap restricts the phase to 60 nm (Trushin, 2022).
6. Control modalities, device concepts, and unresolved issues
Because interlayer excitons carry a permanent dipole, electric fields are a primary control axis. In MoS61/WSe62 on LiNbO63, Rayleigh-mode surface acoustic waves generate in-plane and out-of-plane piezoelectric fields with 64, enabling contactless and selective manipulation of intralayer and interlayer species (Sun et al., 5 Nov 2025). The interlayer excitons IX65 and IX66, emitting near 67 nm and 68 nm with a robust splitting of 69 meV across twist angles from 70 to 71, respond linearly through 72 with extracted dipoles 73 and 74, whereas intralayer excitons exhibit the quadratic Stark effect 75 (Sun et al., 5 Nov 2025). The same acoustoelectric fields strongly quench IX photoluminescence, with 76 at 77 K and 78 dBm for IX79, evolving to near-complete quenching at 80 K (Sun et al., 5 Nov 2025).
Nanoscale electrostatic trapping uses the same dipolar physics in a more localized form. A nanopatterned graphene gate above MoSe81/WSe82 creates a sharp reduction of the out-of-plane electric field beneath a 83–84 nm effective hole, producing a trap potential 85 for the interlayer exciton (Shanks et al., 2021). The free IX tunes with slope 86 eV/(V/nm), implying 87 nm and 88, while the trapped IX tunes with 89 of that slope because the local field is reduced (Shanks et al., 2021). At maximum trapping bias the trap depth is 90–91 meV with lateral FWHM 92 nm; the corresponding harmonic approximation gives 93 meV, a ground-state energy of 94 meV, and a ground-state spatial width of 95 nm (Shanks et al., 2021). The trapped state also shows low-power saturation at 96 nW and an increased lifetime 97 ns relative to 98 ns (Shanks et al., 2021).
Stacking and dimensionality broaden the design space further. In bilayer armchair graphene nanoribbons, vertical stacking produces both type-I and type-II alignments; in 699 homobilayers the bright intralayer exciton lies at 00 eV and an interlayer exciton at 01 eV retains up to 02 of the maximum absorption, while in 6-10AGNR heterobilayers the lowest exciton is interlayer at 03 eV with oscillator strength 04 of 05 and a room-temperature radiative lifetime of 06s (Rocha et al., 15 Jul 2025). More speculative many-body control follows from proximity-engineered single-particle dispersions: if either the electron or hole acquires a Mexican-hat dispersion, the interlayer-exciton dispersion evolves from parabolic to quartic and eventually to a ring minimum at finite momentum, opening routes to liquid–solid transitions and finite-momentum condensates under external-field tuning (Skinner, 2016).
Several recurring misconceptions are corrected by the present literature. In (PEA)07PbI08, the low-energy multiplet is consistently explained by exchange, symmetry lowering, octahedral distortions, and layer doubling, without invoking Rashba splitting or exciton–polaron physics as the primary origin; Rashba-like features can appear as a consequence of the same distortions that brighten the interlayer manifold (Grenzer et al., 21 Apr 2026). In reconstructed MoSe09/WSe10, atomic reconstruction does not inevitably localize interlayer excitons; for sufficiently shallow reconstructed potentials, the ground state is laterally extended over multiple moiré cells (Figueiredo et al., 2024). In WSe11/CrI12, by contrast, the interlayer magnetic excitons are robust across bilayer and trilayer stackings, whereas the proximity-induced modifications of the WSe13 A and B excitons depend sensitively on fine details of interface alignment and hybridization (Thodika et al., 26 Sep 2025). Taken together, these results place proximity-induced interlayer excitons at the intersection of interfacial band engineering, lattice symmetry, many-body screening, and nonequilibrium dynamics, rather than under a single universal mechanism.