A reflection on ‘Type-I van der Waals heterostructure formed by MoS₂ and ReS₂ monolayers’

Abstract

Type-I van der Waals heterostructures, in which both the conduction and valence band edges of one material lie within the bandgap of an adjacent layer, offer unique opportunities for engineering charge carrier confinement and enhancing light emission in two-dimensional systems. In our original work (M. Z. Bellus, M. Li, S. D. Lane, F. Ceballos, Q. Cui, X. C. Zeng and H. Zhao, Nanoscale Horiz., 2017, 2, 31–36, https://doi.org/10.1039/C6NH00144K), we demonstrated a type-I heterostructure formed by monolayer MoS₂ and ReS₂, verified through both first-principles calculations and time-resolved spectroscopy. Since then, growing interest in type-I band alignment has led to the discovery of a broad range of new type-I systems through theoretical predictions and experimental methods. Moreover, dynamic tuning of band alignment via vertical electric fields or strain has enabled reversible transitions between type-II and type-I configurations. Applications of type-I heterostructures in light-emitting and photodetection devices have also been experimentally explored. Looking ahead, we anticipate continued development of type-I heterostructures with enhanced light-emitting performance and their integration into complex multilayer stacks with mixed band alignments to realize novel optoelectronic and quantum devices.

In our first article in Nanoscale Horizons, we reported the experimental demonstration of the first type-I van der Waals (vdW) heterostructure formed by monolayer semiconductors (https://doi.org/10.1039/C6NH00144K; cover article).¹

Two-dimensional (2D) materials derived from layered vdW crystals offer unprecedented opportunities for controlling electron motion at the atomic scale—an essential goal in condensed matter and materials physics, with far-reaching implications for modern electronic and optoelectronic technologies. While electrons are confined within a single layer in an individual 2D material, stacking different 2D layers into heterostructures enables interlayer transport and charge redistribution, which can be precisely tuned through engineered band alignments, stacking sequences, and twist angles. A key advantage of vdW heterostructures is the absence of lattice-matching constraints, allowing for a wide range of material combinations. This flexibility opens new pathways for creating artificial materials with tailored properties, potentially transforming materials science and technology.

While the earliest studies of vdW heterostructures focused on semi-metallic graphene,^2,3 since 2014, increasing attention has shifted toward semiconducting vdW heterostructures, particularly those involving transition-metal dichalcogenides (TMDs).^4–7 In these systems, band alignment serves as one of the most powerful tuning knobs for engineering their physical properties. Depending on the band structures of the constituent materials, such heterostructures typically exhibit either type-I or type-II alignment.¹ In a type-I (also known as straddling gap) configuration [Fig. 1(a) and (b)], both the conduction band minimum (CBM) and valence band maximum (VBM) reside within the narrower-gap material. Consequently, electrons and holes photoexcited in the wider-gap layer transfer into the narrower-gap layer [Fig. 1(a)]. In contrast, carriers generated in the narrower-gap material remain confined, as neither electrons nor holes possess sufficient energy to transfer to the wider-gap layer [Fig. 1(b)]. In comparison, type-II (or staggered gap) alignment [Fig. 1(c) and (d)] occurs when the CBM and VBM are located in different materials. In this case, excitation in either layer leads to spatial separation of electrons and holes across the interface, with only one type of carrier transferring to the adjacent layer.

Fig. 1 Schematic illustration of band alignment between two semiconductors, showing type-I configurations in panels (a) and (b), and type-II configurations in panels (c) and (d). Reproduced from ref. 1 with permission from the Royal Society of Chemistry.

Early studies showed that most 2D semiconducting heterostructures exhibit type-II band alignment.^4–7 For example, all six combinations formed from the four most commonly studied TMDs, MX₂ (M = Mo, W; X = S, Se), display type-II alignment. This configuration promotes charge separation and interlayer exciton formation, resulting in prolonged recombination lifetimes—features advantageous for applications such as photovoltaics and photodetectors. In sharp contrast, no type-I heterostructures had been reported at the time. The simultaneous confinement of electrons and holes within the same layer in type-I structures is beneficial for light-emitting applications. Furthermore, the demonstration of this alternative band alignment expands the design space of vdW systems, enabling the construction of multilayer heterostructures with sophisticated and tunable band landscapes.

In our original paper, we reported the first experimental demonstration of a type-I semiconducting vdW heterostructure.¹ Based on the band structures of monolayer MoS₂ and ReS₂, a type-I band alignment was expected, with both the CBM and VBM located in the ReS₂ layer. This expectation was confirmed by our first-principles calculations, which revealed conduction and valence band offsets of 210 meV and 130 meV, respectively [Fig. 2(a)].

Fig. 2 (a) Type-I band alignment of MoS₂/ReS₂ predicted by first-principles calculations. (b) Optical microscopy image of the fabricated heterostructure. (c) Key results from transient absorption measurements revealing interlayer electron and hole transfer dynamics that confirm the type-I band alignment. Reproduced from ref. 1 with permission from the Royal Society of Chemistry.

Experimentally, we fabricated the heterostructure by stacking exfoliated MoS₂ and ReS₂ monolayers onto a Si/SiO₂ substrate, as shown in Fig. 2(b). The type-I alignment was confirmed through transient absorption measurements.⁸ Specifically, by photoexciting MoS₂ and time-resolving the carrier populations in both layers, we observed that the signal from MoS₂ decayed while that from ReS₂ rose, both with similar time constants of approximately 1 ps [Fig. 2(c)]. This behavior is consistent with the transfer of both electrons and holes from MoS₂ to ReS₂, as illustrated in Fig. 1(a). It also rules out the type-II scenario depicted in Fig. 1(c), in which only electrons would transfer while holes remain in MoS₂, leading to a long-lived signal in that layer (which was absent). Furthermore, when ReS₂ was photoexcited and MoS₂ was probed, no transient absorption signal was detected, indicating the absence of both electron and hole transfer from ReS₂ to MoS₂ [Fig. 1(b)]. This further rules out the hole transfer pathway expected in type-II alignment [Fig. 1(d)]. Together with additional experimental evidence,¹ these results unambiguously establish that the MoS₂/ReS₂ heterostructure exhibits a type-I semiconducting band alignment.

Following this publication, our group has continued to investigate type-I heterostructures.

Using similar sample fabrication and ultrafast pump–probe measurement techniques, we demonstrated new type-I heterostructures based on monolayer semiconductors, including WSe₂/MoTe₂ ⁹ and WSe₂/PtSe₂.¹⁰ Given that type-I heterostructures composed entirely of monolayer materials remain rare compared to their type-II counterparts, one effective strategy for realizing type-I alignment is to incorporate multilayer or bulk materials as the narrower-gap component, as these often exhibit smaller bandgaps than their monolayer counterparts. Using this approach, we identified type-I configurations involving monolayer WS₂ with bulk black phosphorus (BP)¹¹ and with trilayer PdSe₂.¹²

By further exploiting the thickness-dependent band structure of 2D semiconductors, we recently achieved simultaneous type-I and type-II interfaces within a single device by stacking a monolayer WS₂ flake onto a MoSe₂ flake containing regions of varying thickness. Photoluminescence and transient absorption measurements revealed a type-II interface in regions where monolayer MoSe₂ is in contact with monolayer WS₂, and type-I interfaces where multilayer MoSe₂ interfaces with monolayer WS₂. The coexistence of both interface types in a single device offers new opportunities for designing sophisticated 2D heterostructures with finely tunable photocarrier behavior.¹³

The concept of using different types of band alignment to control carrier dynamics also extends to lateral junctions. Similar to vertical vdW stacks, most lateral junctions studied to date exhibit type-II alignment. In a subsequent Nanoscale Horizons article, we reported a type-I lateral junction formed within a MoSe₂ monolayer.¹⁴ In that experiment, a portion of the MoSe₂ monolayer was covered with a hexagonal boron nitride (h-BN) flake. Photoluminescence measurements showed that the optical bandgap in the covered region is larger than in the uncovered region due to the change in the dielectric environment, resulting in a type-I alignment. Transient absorption microscopy performed at the junction confirmed ultrafast exciton transport across the junction. Overall, our reports of type-I vertical and lateral junctions in Nanoscale Horizons^1,14 revealed new horizons for harnessing artificial vdW materials with precisely controlled carrier properties.

We are pleased to see that, since our initial reports, growing interest in type-I vdW heterostructures has sparked a surge of new studies in the field. These efforts span theoretical predictions, experimental demonstrations, and the development of device applications based on type-I band alignment.

The importance of type-I interfaces in vdW materials has stimulated numerous theoretical and computational efforts to predict new candidates. When the electron affinities (i.e., the energy of the CBM relative to the vacuum level) and bandgaps of the individual materials are known, the band alignment of a heterostructure can be inferred by aligning their vacuum levels—an approach known as Anderson's rule.¹⁵ Using this rule and large computational databases of 2D material band structures, researchers have systematically classified possible heterostructures by their band alignment types.¹⁶ More refined yet simple and effective models have also been proposed.¹⁷ While these models provide a valuable starting point, accurate determination of band alignment and interlayer hybridization requires first-principles calculations, such as DFT, which can account for interlayer coupling effects. Such efforts have identified a wide range of type-I interfaces, including MoS₂/PtSe₂,¹⁸ MS₂/VS₂ (M = Mo, W),¹⁹ MoS₂/SnS₂,²⁰ Mg(OH)₂/VS₂,²¹ BSe/SiC,²² and TiO₂/WTe₂.²³

First-principles calculations have also revealed that certain type-II heterostructures can be dynamically converted to type-I by applying a vertical electric field. The field modifies the band structures of both layers, and in cases where the initial band offsets are relatively small, this field-induced shift can be sufficient to transform a type-II interface into a type-I configuration. Recent theoretical studies have identified several candidate systems in which this transition can be achieved using experimentally accessible electric field strengths, including MS₂/VS₂ (M = Mo, W),¹⁹ BP/P₄O₁₀,²⁴ h-BN/SnS₂,²⁵ GeS/arsenene,²⁶ GaSe/SnX₂ (X = S, Se),²⁷ and SiAs₂/GeAs₂.²⁸ A similar transition from type-II to type-I alignment can also be induced by lattice strain. By altering the relative band positions of the constituent materials, strain engineering provides an additional dynamic control knob. For example, the InSe/InTe heterostructure has been predicted to undergo such a transition under appropriate strain conditions.²⁹

Accompanying these theoretical efforts, several notable experimental reports have emerged on new type-I heterostructures. In addition to the transient absorption technique used in our work—which directly time-resolves photocarrier transfer dynamics—a powerful approach for identifying type-I band alignment is based on photoluminescence (PL) enhancement. This method is particularly effective when the narrower-gap material exhibits relatively strong PL. A significant increase in PL yield from the narrower-gap layer within the heterostructure, compared to that of the individual material, provides compelling evidence that both electrons and holes photoexcited in the wider-gap layer transfer to the narrower-gap layer. Moreover, the absence of charge separation—which would otherwise lead to substantial PL quenching—strongly supports the exclusion of a type-II configuration.

Using this PL-based method, along with other techniques, several new type-I heterostructures have been experimentally confirmed, including WX₂/MoTe₂ (X = S, Se),^30–32 MoS_2(1−x)Se_2x/WS₂,³³ PbI₂/WS₂,^34,35 MoSe₂/FePS₃,³⁶ CsPbBr₃/WS₂,³⁷ and pentacene/MoSe₂.³⁸ Furthermore, the electric-field-induced transition from type-II to type-I alignment has been experimentally demonstrated in MoSe₂/WS₂.³⁹ In addition to these vertical heterostructures, type-I lateral heterostructures have also been experimentally demonstrated, including PtSe₂/MoSe₂ ⁴⁰ and CdS_xSe_1−x alloys.⁴¹

A general feature of 2D semiconductors is that their CBM decreases and VBM increases with increasing thickness. This trend provides a strategy for creating type-I homojunctions by stacking two thin layers of the same 2D material with different thicknesses. This approach has recently been demonstrated in monolayer–multilayer WSe₂ systems⁴² and is likely to be broadly applicable to other 2D semiconductors.

As more type-I heterostructures are discovered, their applications in optoelectronic devices have been increasingly explored. The confinement of both electrons and holes within a single layer facilitates radiative recombination, which is advantageous for light-emitting devices. As discussed above, PL enhancement has been observed in many type-I systems.^30–38 The potential of type-I heterostructures for photodetection has also been demonstrated experimentally. For instance, type-I band alignment can enhance photodetection efficiency by suppressing recombination losses in the wider-gap layer while leveraging the superior carrier transport properties of the narrower-gap layer, as shown in systems such as InSe/Te,⁴³ WS₂/PtS₂,⁴⁴ and SnSe₂/SnS₂.⁴⁵ Moreover, the integration of anisotropic layers into type-I heterostructures offers a promising route toward polarization-selective photodetectors.⁴⁶

Studies of type-I heterostructures have also stimulated growing interest in another class of interfaces, known as type-III or broken-gap alignments. In this configuration, the CBM of one layer lies below the VBM of the other. Examples of experimentally or theoretically identified type-III heterostructures include phosphorene/SnX₂ (X = S, Se),⁴⁷ BP/MoS₂,⁴⁸ and SnSe₂/MoTe₂.⁴⁹ A unique feature of type-III interfaces is that, upon contact, electrons occupying a valence band that lies above the CBM of the adjacent layer can transfer across the interface. This leads to ground-state charge transfer, resulting in p-type and n-type doping of the two layers, respectively. Such charge redistribution can significantly alter the electronic and optical properties of the constituent materials, as recently demonstrated in WSe₂/α-RuCl₃.⁵⁰

Finally, another important aspect of our original publication is the introduction of ReS₂ to the material library of vdW heterostructures. ReS₂ exhibits several intriguing properties, including pronounced in-plane anisotropy, weak interlayer coupling, a small bandgap in the near-infrared range, and strong spin–orbit coupling. Its potential as a functional component in vdW heterostructures⁵¹ has spurred further investigations into its charge transport characteristics,⁵² photocarrier dynamics,^53–55 and nonlinear optical responses.^56,57

Building on the progress in type-I heterostructures, several promising future directions have emerged. A key avenue is the exploration of new type-I systems, particularly those involving narrower-gap materials with strong photoluminescence, which are highly desirable for light-emitting applications. Additionally, integrating type-I interfaces with type-II or type-III alignments in sophisticated multilayer architectures offers exciting opportunities to engineer complex charge carrier and exciton dynamics. Such hybrid structures could enable novel functionalities, including spatially controlled carrier confinement, directional energy flow, and tunable recombination pathways—paving the way for advanced optoelectronic and quantum devices based on artificial vdW materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

I acknowledge support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under awards (DE-SC0020995).

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