Type-II van der Waals Heterostructures

• Type-II van der Waals heterostructures are layered assemblies of dissimilar 2D crystals with staggered band alignment that spatially separates electrons and holes.
• They enable long-lived interlayer excitons and efficient charge transfer by leveraging band offsets, interlayer hopping, and interface electrostatics.
• They underpin applications in excitonic solar cells, photodetectors, and tunable devices exhibiting topological and multifunctional behavior.

Type-II van der Waals heterostructures (vdWHs) are layered assemblies of dissimilar two-dimensional crystals in which the conduction-band minimum (CBM) and valence-band maximum (VBM) reside in different constituent layers. This staggered alignment spatially separates electrons and holes across the interface, making type-II vdWHs a central platform for interlayer excitons, charge-transfer photophysics, and carrier-separation-based optoelectronics. Within this class, the decisive descriptors are not limited to isolated-layer band edges: interlayer hopping, dielectric screening, interface dipoles, momentum matching of the relevant valleys, and the possibility of direct, indirect, or inverted regimes all enter the final heterostructure physics (Latini et al., 2016, Wang et al., 2020, Zhang et al., 2023).

1. Definition, classification, and physical significance

In the standard semiconductor classification, type-I vdWHs confine both band edges in the same layer, type-II vdWHs place the CBM and VBM in different layers, and type-III vdWHs realize a broken-gap alignment. For type-II systems, the energetic preference for electrons and holes to occupy different monolayers suppresses electron–hole recombination and extends exciton lifetimes. This is the basis for long-lived interlayer excitons, photovoltaic charge separation, and photodetection based on built-in asymmetry (Xu et al., 2018, Latini et al., 2016).

In transition-metal dichalcogenide (TMDC) heterobilayers, the type-II condition is particularly consequential because it generates interlayer valley excitons, also termed charge-transfer excitons, with the electron and hole confined to opposite layers. Reported advantages include ultrafast charge transfer, long-lived interlayer excitons, reduced electron–hole overlap, and suppressed valley mixing, which is why type-II TMDC bilayers are treated as promising systems for optoelectronics and valleytronics (Wang et al., 2020).

Type-II alignment is not a rare special case. In a first-principles survey of 990 commensurate TMD vdWHs built from 45 monolayers, type-II alignment comprised 60.1% of the analyzed stacks, while type-III alignment accounted for about 10% (Lee et al., 23 Jun 2025). A separate catalog of 51 $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$-based vdWHs classified 15 systems as type-II, alongside 19 type-I, 2 type-III, and 5 type-H cases, with the remaining entries corresponding to Ohmic and Schottky contacts (Tho et al., 2022). These surveys indicate that staggered alignment is a dominant rather than exceptional regime in contemporary vdWH design.

2. Band alignment beyond Anderson’s rule

A standard starting point is Anderson’s rule: align the vacuum levels of the isolated monolayers and infer the heterostructure type from the relative CBM and VBM positions. This construction is explicitly used for MX$_2$ heterobilayers, where most combinations are classified as Anderson type II, and for many systems it provides a useful first qualitative map of staggered, straddling, and broken-gap lineups (Xu et al., 2018). The same logic is used as a preliminary screening tool in $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$ heterostructures, although direct heterostructure calculations are required for final classification because interlayer coupling and interface dipoles can invalidate isolated-layer lineups (Tho et al., 2022).

The principal limitation of Anderson’s rule in vdWHs is that it neglects the interfacial dipole generated by charge redistribution across the vdW gap. In the generalized linear-response framework, the crucial quantity is the dipole step $eV_{\rm h}$, which shifts the two constituent band structures approximately as rigid electronic blocks. For approximately $10^3$ vdWHs, a generalized linear-response model using only two isolated-layer descriptors—the charge neutrality level offset and the sum of the monolayer bandgaps—reproduced DFT lineups with $r^2 \sim 0.9$ across type-I, II, and III stacks (Lee et al., 23 Jun 2025). The same study showed that Anderson and midgap models are limiting cases that become unreliable once charge spillage across the interface is appreciable, especially near the broken-gap regime.

For pure type-II systems, the charge-spillage pathway is absent, and the alignment is governed primarily by the screened charge-neutrality-level offset rather than by the broken-gap correction (Lee et al., 23 Jun 2025). This suggests that even when type-II classification is unchanged, accurate band offsets still require interface electrostatics. The representative type-II bilayer $2\mathrm{H}$-CrS$_2$/$2\mathrm{H}$-CrTe$_2$ illustrates this point: its heterostructure develops a dipole step of about $_2$0, and the resulting band shifts are largely explained by that step rather than by vacuum-level alignment alone (Lee et al., 23 Jun 2025).

A more explicit interfacial decomposition was given for monolayer InSe/bilayer WS$_2$1, where two mechanisms were separated: quasichemical-bonding (QB) interaction and interface dipole. The interface dipole was quantified by a differential charge density

$_2$2

and the corresponding potential step was reported as $_2$3, with a dipole moment $_2$4. In that system, QB shifts were valley dependent, while the dipole shifted the bands more rigidly (Zhang et al., 2023). For practical screening of robust momentum-matched type-II vdWHs, the same work proposed retaining candidates with $_2$5, $_2$6, $_2$7, and $_2$8 (Zhang et al., 2023).

3. Interlayer coupling, momentum matching, and excitonic structure

Type-II alignment by itself only states that electrons and holes prefer different layers; it does not specify how strongly the corresponding states hybridize or whether the optical transition is direct in momentum space. In 2H-stacked TMDC heterobilayers, symmetry analysis at the $_2$9 valley shows that the interlayer hopping in the conduction band vanishes, $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$0, while the dominant interlayer coupling channel is in the valence band. A four-level $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$1 model fitted to first-principles bands yielded interlayer valence-band hopping $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$2 for SeMoSe/SeWSe, $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$3 for SeMoSe/SWSe, and $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$4 for SeMoSe/SeWS, with corresponding hybridization degrees $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$5, $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$6, and $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$7 (Wang et al., 2020). These values establish that type-II alignment can range from a nearly pure charge-transfer limit to strongly layer-hybridized valence states.

This distinction controls exciton formation. In SeMoSe/SWSe, the strong interlayer polarization suppresses hopping so effectively that the interlayer exciton approaches the pure type-II charge-transfer limit. In SeMoSe/SeWSe, spin-allowed hybridization of valence states produces two excitonic species, whereas in SeMoSe/SeWS the stronger hybridization yields four excitonic species in which the hole is layer hybridized while the electron remains confined in one layer (Wang et al., 2020). Type-II alignment is therefore compatible with a spectrum of interlayer-coupling regimes rather than a single excitonic archetype.

A second axis is momentum matching. In many monolayer TMD heterostructures, the relevant edges lie at $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$8, so a small twist angle or lattice mismatch can make the interlayer transition momentum indirect. A distinct design strategy is to select materials whose relevant edges are both at $(\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4$9. In multilayer InSe/TMD interfaces, the conduction-band minimum of InSe and the valence-band maximum of the TMD both occur at $eV_{\rm h}$0, so a type-II interface supports radiative interlayer transitions irrespective of lattice constant mismatch, rotational/translational alignment, or whether the constituents are direct- or indirect-gap semiconductors individually (Ubrig et al., 2019). The reported interlayer transition energies span roughly $eV_{\rm h}$1–$eV_{\rm h}$2 (Ubrig et al., 2019).

Monolayer InSe/bilayer WS$eV_{\rm h}$3 provides a complementary case in which the isolated components already satisfy the zone-center condition: InSe has its CBM at the I point and bilayer WS$eV_{\rm h}$4 its VBM at the I point. After stacking, the vdWH becomes a direct-gap semiconductor with $eV_{\rm h}$5, while the VBM remains mainly on bilayer WS$eV_{\rm h}$6 and the CBM mainly on InSe, preserving a momentum-matched type-II configuration (Zhang et al., 2023).

The excitonic problem in type-II vdWHs also requires nonlocal screening. For MoS$eV_{\rm h}$7/WSe$eV_{\rm h}$8 and MoS$eV_{\rm h}$9/hBN/WSe$10^3$0, a first-principles framework combining the quantum electrostatic heterostructure (QEH) model, $10^3$1, and a generalized 2D Mott–Wannier model was used to compute band alignment and exciton binding energies. The effective exciton equation was written as

$10^3$2

and interlayer exciton binding energies of up to about $10^3$3 were found, decreasing monotonically as the layers are separated further (Latini et al., 2016). The same study showed that hBN primarily tunes the excitons rather than the fundamental type-II ordering, because spacer thickness strongly modifies interlayer binding while only modestly affecting band-edge alignment (Latini et al., 2016).

Field response adds another layer of control. For six type-II TMD bilayers, interlayer excitons were modeled with a bilayer Keldysh potential and their dissociation under in-plane electric field was computed using exterior complex scaling. In the weak-field regime, WS$10^3$4/WSe$10^3$5 supported the fastest dissociation rates among the six structures, and dissociation rates in vdWHs were found to be significantly larger than in the monolayer counterparts (Kamban et al., 2020). This makes interlayer excitons simultaneously long lived and electrically dissociable, a combination of direct relevance for photocurrent extraction.

4. Representative material systems

The breadth of type-II vdWHs is best seen through concrete material realizations spanning TMDCs, group-III–VI chalcogenides, nitrides, and chemically dissimilar bilayers.

System	Type-II characteristic	Selected quantitative features
MoS$10^3$6/WSe$10^3$7; MoS$10^3$8/hBN/WSe$10^3$9	VBM on WSe$r^2 \sim 0.9$0, CBM on MoS$r^2 \sim 0.9$1	Interlayer exciton binding energies up to about $r^2 \sim 0.9$2 (Latini et al., 2016)
SeMoSe/SeWSe, SeMoSe/SWSe, SeMoSe/SeWS	Janus-engineered type-II TMDC bilayers	$r^2 \sim 0.9$3; $r^2 \sim 0.9$4 (Wang et al., 2020)
InSe/Te	Vertical p–n type-II junction	Work functions $r^2 \sim 0.9$5 and $r^2 \sim 0.9$6; offset about $r^2 \sim 0.9$7 (Qin et al., 2020)
InSe/TMD multilayers	$r^2 \sim 0.9$8-$r^2 \sim 0.9$9 type-II interfaces	Radiative interlayer transitions over roughly $2\mathrm{H}$0–$2\mathrm{H}$1 (Ubrig et al., 2019)
MoSi$2\mathrm{H}$2N$2\mathrm{H}$3/ZnO	Indirect type-II	VBM at $2\mathrm{H}$4 from ZnO; CBM at $2\mathrm{H}$5 from MoSi$2\mathrm{H}$6N$2\mathrm{H}$7; gap $2\mathrm{H}$8 (Ng et al., 2021)
MoSi$2\mathrm{H}$9N$_2$0/InSe; MoSi$_2$1N$_2$2/WSe$_2$3	Type-II excitonic-solar-cell candidates	$_2$4; PCE $_2$5 (Tho et al., 2022)
SiH/CdCl$_2$6	Type-II with strong internal potential difference	$_2$7, $_2$8, $_2$9, HSE gap $2\mathrm{H}$0 (Priydarshi et al., 2023)

Across these examples, the layer character of the band edges is the defining criterion, but the microscopic mechanisms differ. In MoSi$2\mathrm{H}$1N$2\mathrm{H}$2/ZnO, stronger charge redistribution relative to MoSi$2\mathrm{H}$3N$2\mathrm{H}$4/GaN was associated with the work-function mismatch and supported the staggered alignment (Ng et al., 2021). In SiH/CdCl$2\mathrm{H}$5, the type-II condition is directly reflected in the real-space localization of the HOMO/VBM on SiH and the LUMO/CBM on CdCl$2\mathrm{H}$6, together with a plane-averaged potential drop $2\mathrm{H}$7 and charge transfer from SiH to CdCl$2\mathrm{H}$8 (Priydarshi et al., 2023). In $2\mathrm{H}$9-based catalogs, type-II cases are explicitly targeted for excitonic solar cells because the CBM and VBM reside dominantly in different sub-monolayers (Tho et al., 2022).

5. Device manifestations and application domains

The most direct device consequence of type-II alignment is rectifying and photovoltaic behavior driven by the built-in field. A vertical multilayer InSe–Te vdWH forms a type-II p–n junction in which isolated InSe is n-type and isolated Te is p-type. Under $_2$0, the measured $_2$1–$_2$2 curve over $_2$3 showed forward current $_2$4, reverse current lower than pA, and forward rectification ratio $_2$5. Under weak illumination at $_2$6 and $_2$7, the same device exhibited $_2$8, $_2$9, $_2$00, $_2$01, and response times of $_2$02 rise and $_2$03 decay. At zero bias, it remained a self-powered photodetector with responsivity $_2$04, detectivity $_2$05, and broadband response from $_2$06 to $_2$07 (Qin et al., 2020).

Type-II alignment is equally central to excitonic solar cells. In the $_2$08 catalog, the power conversion efficiency was estimated as

$_2$09

with $_2$10 for the integrated AM1.5G spectrum. The standout type-II systems were MoSi$_2$11N$_2$12/InSe and MoSi$_2$13N$_2$14/WSe$_2$15, which combined small conduction-band offsets of $_2$16 and $_2$17 with predicted PCEs of $_2$18 and $_2$19, respectively (Tho et al., 2022). These values were highlighted as competitive with state-of-the-art silicon solar cells in the same study.

Broad-spectrum optoelectronics provides a different use case. For $_2$20-aligned InSe/TMD interfaces, the experimentally observed interlayer photoluminescence lies below all intralayer optical transitions, strengthens as temperature decreases, and blue-shifts with increasing excitation power, consistent with interlayer excitons having a dipolar character. Because the transition is direct in $_2$21-space at $_2$22, the emission remains robust against lattice mismatch on the order of $_2$23 and against rotational misalignment, which substantially relaxes fabrication constraints compared with $_2$24-valley TMDC heterobilayers (Ubrig et al., 2019).

Type-II alignment can also be embedded in multifunctional designs. In SiH/CdCl$_2$25, the same internal potential difference that produces staggered band edges was connected to visible-range absorption with a first absorption peak at about $_2$26, photocatalytic water splitting with redox levels at $_2$27 and $_2$28, out-of-plane piezoelectricity with $_2$29 under biaxial strain and $_2$30 under vertical strain, and a tunnel field-effect transistor concept with reported on-state current about $_2$31 and subthreshold swing below $_2$32 (Priydarshi et al., 2023). The central claim of that study was that the heterostructure’s multifunctionality mainly depends on the potential difference between the constituent monolayers.

6. Misconceptions, non-type-II interfacial phenomena, and frontier directions

A common misconception is that any vdWH exhibiting interfacial transport or optical switching is therefore a type-II heterostructure. This is not generally correct. In a molecular/inorganic vdWH built from a spin-crossover metal–organic framework and few-layer graphene or WSe$_2$33, the dominant mechanism is strain transfer associated with the high-spin/low-spin volume change of the molecular layer. The paper explicitly states that the system is not framed as a type-II band-aligned heterostructure study; rather, it is a straintronics platform in which the adjacent 2D crystal is mechanically modulated (Boix-Constant et al., 2021). Likewise, BN/silicene heterostructures were analyzed in terms of Dirac-cone preservation and silicene–silicene interlayer coupling through BN, with no explicit type-I/II/III classification or charge-separation-driven heterojunction behavior (Wu et al., 2021).

Another frontier is the inverted type-II regime. In moiré superlattices of massive-Dirac vdW heterobilayers, an interlayer bias can tune the system from a normal regime into an inverted type-II regime. In that regime, registry-dependent interlayer hybridization determines whether a local region is a topological insulator or a normal insulator, producing a topological mosaic with helical modes at TI/NI phase boundaries (Tong et al., 2016). Here, type-II alignment is not merely a charge-separation condition; it becomes the precursor to electrically switchable topological order.

Interlayer-exciton spectroscopy contains its own controversy. The microscopic mechanisms of self-trapped states were described as controversial in a study of type-II MoS$_2$34/h-BN-based vdWHs. There, coupling to interface optical phonons produced a binding-energy correction

$_2$35

and two kinds of self-trapped interlayer excitons were identified: type I with increasing binding energy and red-shifted spectra in the tens of meV range, and type II with decreasing binding energy, proposed as a possible explanation for blue-shift and broad linewidth at low temperature (Deng et al., 2021). This shows that even after the band alignment is fixed as type II, the excitonic phenomenology can remain nontrivial and materials-specific.

Current design practice increasingly combines band-offset criteria with explicit interface and momentum-space descriptors. One route screens for robust momentum-matched type-II vdWHs using band offsets and valley separations large enough to survive QB and dipole corrections (Zhang et al., 2023). Another route uses the generalized linear-response model, in which the charge neutrality level offset and the sum of the isolated-layer bandgaps dominate the alignment physics across types I–III (Lee et al., 23 Jun 2025). This suggests a broader transition from isolated case studies toward descriptor-based, high-throughput type-II vdWH engineering.