---
title: Type-II van der Waals Heterostructures
url: https://www.emergentmind.com/topics/type-ii-van-der-waals-heterostructures-vdwhs
type: topic
---

# Type-II van der Waals Heterostructures

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 [1612.05736] [2009.01394] [2306.12821].

## 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 [1804.02518] [1612.05736].

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 [2009.01394].

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% [2506.18850]. 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 [2206.11765]. 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 [1804.02518]. 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 [2206.11765].

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 [2506.18850]. 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 [2506.18850]. 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 \(0.225\ \mathrm{eV}\), and the resulting band shifts are largely explained by that step rather than by vacuum-level alignment alone [2506.18850].

A more explicit interfacial decomposition was given for monolayer InSe/bilayer WS\(_2\), where two mechanisms were separated: quasichemical-bonding (QB) interaction and interface dipole. The interface dipole was quantified by a differential charge density
\[
\Delta \rho(x,y,z)=\rho_{vdWH}-\rho_{BL\text{-}WS_2}-\rho_{InSe},
\]
and the corresponding potential step was reported as \(AV=0.071\ \mathrm{eV}\), with a dipole moment \(u(z)=+0.163\ \mathrm{Debye}\). In that system, QB shifts were valley dependent, while the dipole shifted the bands more rigidly [2306.12821]. For practical screening of robust momentum-matched type-II vdWHs, the same work proposed retaining candidates with \(VBO>0.2\ \mathrm{eV}\), \(CBO>0.2\ \mathrm{eV}\), \(AVBM>0.1\ \mathrm{eV}\), and \(ACBM>0.1\ \mathrm{eV}\) [2306.12821].

## 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 \(K\) valley shows that the interlayer hopping in the conduction band vanishes, \(t_{cc}=0\), while the dominant interlayer coupling channel is in the valence band. A four-level \(k\cdot p\) model fitted to first-principles bands yielded interlayer valence-band hopping \(t \approx 46\ \mathrm{meV}\) for SeMoSe/SeWSe, \(t \approx 5\ \mathrm{meV}\) for SeMoSe/SWSe, and \(t \approx 60\ \mathrm{meV}\) for SeMoSe/SeWS, with corresponding hybridization degrees \(P_H \approx 91\%\), \(1\%\), and \(73.5\%\) [2009.01394]. 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 [2009.01394]. 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 \(K/K'\), 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 \(\Gamma\). In multilayer InSe/TMD interfaces, the conduction-band minimum of InSe and the valence-band maximum of the TMD both occur at \(\Gamma\), 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 [1912.10345]. The reported interlayer transition energies span roughly \(1.0\)–\(1.6\ \mathrm{eV}\) [1912.10345].

Monolayer InSe/bilayer WS\(_2\) 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\(_2\) its VBM at the I point. After stacking, the vdWH becomes a direct-gap semiconductor with \(E_g=0.91\ \mathrm{eV}\), while the VBM remains mainly on bilayer WS\(_2\) and the CBM mainly on InSe, preserving a momentum-matched type-II configuration [2306.12821].

The excitonic problem in type-II vdWHs also requires nonlocal screening. For MoS\(_2\)/WSe\(_2\) and MoS\(_2\)/hBN/WSe\(_2\), a first-principles framework combining the quantum electrostatic heterostructure (QEH) model, \(G_0W_0\), and a generalized 2D Mott–Wannier model was used to compute band alignment and exciton binding energies. The effective exciton equation was written as
\[
\left[-\frac{\nabla_{2D}^2}{2\mu_\textrm{ex}} + W({\bf r}_\parallel)\right]F({\bf r}_\parallel)=E_\textrm{b}F({\bf r}_\parallel),
\]
and interlayer exciton binding energies of up to about \(0.3\ \mathrm{eV}\) were found, decreasing monotonically as the layers are separated further [1612.05736]. 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 [1612.05736].

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\(_2\)/WSe\(_2\) 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 [2002.00824]. 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\(_2\)/WSe\(_2\); MoS\(_2\)/hBN/WSe\(_2\) | VBM on WSe\(_2\), CBM on MoS\(_2\) | Interlayer exciton binding energies up to about \(0.3\ \mathrm{eV}\) [1612.05736] |
| SeMoSe/SeWSe, SeMoSe/SWSe, SeMoSe/SeWS | Janus-engineered type-II TMDC bilayers | \(t \approx 46,\ 5,\ 60\ \mathrm{meV}\); \(P_H \approx 91\%,\ 1\%,\ 73.5\%\) [2009.01394] |
| InSe/Te | Vertical p–n type-II junction | Work functions \(4.02\ \mathrm{eV}\) and \(4.35\ \mathrm{eV}\); offset about \(0.35\ \mathrm{eV}\) [2001.07407] |
| InSe/TMD multilayers | \(\Gamma\)-\(\Gamma\) type-II interfaces | Radiative interlayer transitions over roughly \(1.0\)–\(1.6\ \mathrm{eV}\) [1912.10345] |
| MoSi\(_2\)N\(_4\)/ZnO | Indirect type-II | VBM at \(\Gamma\) from ZnO; CBM at \(K\) from MoSi\(_2\)N\(_4\); gap \(1.60\ \mathrm{eV}\) [2112.14526] |
| MoSi\(_2\)N\(_4\)/InSe; MoSi\(_2\)N\(_4\)/WSe\(_2\) | Type-II excitonic-solar-cell candidates | \(CBO=0.03,\ 0.05\ \mathrm{eV}\); PCE \(20.5\%,\ 22.0\%\) [2206.11765] |
| SiH/CdCl\(_2\) | Type-II with strong internal potential difference | \(VBO=3.1\ \mathrm{eV}\), \(CBO=0.8\ \mathrm{eV}\), \(\Delta V=2.36\ \mathrm{eV}\), HSE gap \(1.18\ \mathrm{eV}\) [2306.02048] |

Across these examples, the layer character of the band edges is the defining criterion, but the microscopic mechanisms differ. In MoSi\(_2\)N\(_4\)/ZnO, stronger charge redistribution relative to MoSi\(_2\)N\(_4\)/GaN was associated with the work-function mismatch and supported the staggered alignment [2112.14526]. In SiH/CdCl\(_2\), the type-II condition is directly reflected in the real-space localization of the HOMO/VBM on SiH and the LUMO/CBM on CdCl\(_2\), together with a plane-averaged potential drop \(\Delta V=2.36\ \mathrm{eV}\) and charge transfer from SiH to CdCl\(_2\) [2306.02048]. In \((\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4\)-based catalogs, type-II cases are explicitly targeted for excitonic solar cells because the CBM and VBM reside dominantly in different sub-monolayers [2206.11765].

## 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 \(V_g=10\ \mathrm{V}\), the measured \(I\)–\(V\) curve over \(V_{ds}=\pm 3\ \mathrm{V}\) showed forward current \(\sim 3.7\ \mu\mathrm{A}\), reverse current lower than pA, and forward rectification ratio \(>10^7\). Under weak illumination at \(V=-2\ \mathrm{V}\) and \(400\ \mathrm{nm}\), the same device exhibited \(R=0.45\ \mathrm{A/W}\), \(D^\*=1.1\times 10^{13}\ \mathrm{Jones}\), \(EQE=136.5\%\), \(LDR=93.6\ \mathrm{dB}\), and response times of \(600\ \mu\mathrm{s}\) rise and \(800\ \mu\mathrm{s}\) decay. At zero bias, it remained a self-powered photodetector with responsivity \(170\ \mathrm{mA/W}\), detectivity \(>10^{12}\ \mathrm{Jones}\), and broadband response from \(300\) to \(1000\ \mathrm{nm}\) [2001.07407].

Type-II alignment is equally central to excitonic solar cells. In the \((\mathrm{Mo,W})\mathrm{Si}_2\mathrm{N}_4\) catalog, the power conversion efficiency was estimated as
\[
\eta = \frac{0.65\,(E_g-\Delta E_c-0.3)\int_{E_g}^{\infty} P(\hbar\omega)\, d(\hbar\omega)} {\int_0^{\infty} P(\hbar\omega)\, d(\hbar\omega)},
\]
with \(P_{\text{solar}} \approx 1000\ \mathrm{W/m}^2\) for the integrated AM1.5G spectrum. The standout type-II systems were MoSi\(_2\)N\(_4\)/InSe and MoSi\(_2\)N\(_4\)/WSe\(_2\), which combined small conduction-band offsets of \(0.03\ \mathrm{eV}\) and \(0.05\ \mathrm{eV}\) with predicted PCEs of \(20.5\%\) and \(22.0\%\), respectively [2206.11765]. 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 \(\Gamma\)-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 \(k\)-space at \(\Gamma\), the emission remains robust against lattice mismatch on the order of \(\sim 15\%\) and against rotational misalignment, which substantially relaxes fabrication constraints compared with \(K\)-valley TMDC heterobilayers [1912.10345].

Type-II alignment can also be embedded in multifunctional designs. In SiH/CdCl\(_2\), the same internal potential difference that produces staggered band edges was connected to visible-range absorption with a first absorption peak at about \(2.5\ \mathrm{eV}\), photocatalytic water splitting with redox levels at \(-4.44\ \mathrm{eV}\) and \(-5.67\ \mathrm{eV}\), out-of-plane piezoelectricity with \(e_{33}=-7.75\ \mathrm{pC/m}\) under biaxial strain and \(e_{33}=-0.76\ \mathrm{pC/m}\) under vertical strain, and a tunnel field-effect transistor concept with reported on-state current about \(1300\ \mu\mathrm{A}/\mu\mathrm{m}\) and subthreshold swing below \(60\ \mathrm{mV/dec}\) [2306.02048]. 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\(_2\), 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 [2110.02990]. 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 [2103.04212].

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 [1608.00115]. 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\(_2\)/h-BN-based vdWHs. There, coupling to interface optical phonons produced a binding-energy correction
\[
\Delta E_b = \Delta E_s - \Delta E_c,
\]
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 [2112.08603]. 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 [2306.12821]. 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 [2506.18850]. This suggests a broader transition from isolated case studies toward descriptor-based, high-throughput type-II vdWH engineering.

Source: https://www.emergentmind.com/topics/type-ii-van-der-waals-heterostructures-vdwhs