Signatures of moiré-trapped valley excitons in MoSe2/WSe2 heterobilayers.

Moiré is More: Access to New Properties of Two-Dimensional Layered Materials

<sec>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.</sec><sec>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.</sec><sec>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.</sec><sec>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.</sec><sec>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.</sec>

The fine-tuning of exciton transition and valley polarization process in two-dimensional materials have drawn tremendous research interest due to their rich valley-contrasting physics. Here, we demonstrate highly tunable exciton and valley characteristics in monolayer and bilayer WS2 through coupling to AlGaN with different doping levels. A notable redshift in exciton energy is observed by interfacing WS2 with n-type AlGaN. More interestingly, an interlayer exciton peak emerges as a result of the formation of type-II band alignment in bilayer WS2. Both the interlayer and intralayer exciton energies are tunable by the twist angle of bilayer WS2. A high valley polarization of 82.2% is achieved in monolayer WS2 at 13 K by coupling with n-type AlGaN, due to the faster exciton decay rate through electron-phonon interaction and the reduced intervalley scattering by doping-induced carrier screening. The valley polarization of interlayer exciton is higher than that of the intralayer exciton, due to the suppressed intervalley scattering resulting from the reduced electron-hole interaction. This work has presented a facile and efficient technique to modulate the excitonic properties of 2D materials. The reported high valley polarization in monolayer WS2 and the discovery of interlayer exciton in bilayer WS2 will trigger innovative study in valley exciton physics and facilitate emerging valleytronic applications.

Moiré superlattices provide a powerful way to engineer the properties of electrons and excitons in two-dimensional van der Waals heterostructures1–8. The moiré effect can be especially strong for interlayer excitons, where electrons and holes reside in different layers and can be addressed separately. In particular, it was recently proposed that the moiré superlattice potential not only localizes interlayer exciton states at different superlattice positions, but also hosts an emerging moiré quasi-angular momentum (QAM) that periodically switches the optical selection rules for interlayer excitons at different moiré sites9,10. Here, we report the observation of multiple interlayer exciton states coexisting in a WSe2/WS2 moiré superlattice and unambiguously determine their spin, valley and moiré QAM through novel resonant optical pump–probe spectroscopy and photoluminescence excitation spectroscopy. We demonstrate that interlayer excitons localized at different moiré sites can exhibit opposite optical selection rules due to the spatially varying moiré QAM. Our observation reveals new opportunities to engineer interlayer exciton states and valley physics with moiré superlattices for optoelectronic and valleytronic applications. Stacked 2D materials can host excitons with distinct valley selection rules due to the spatial variation of the moiré pattern. The authors demonstrate this via optical spectroscopy, opening a route for control of optoelectronic devices.

Since the successful exfoliation of graphene, two-dimensional (2D) materials have attracted a lot of scientific interest due to their electronic, chemical, and mechanical properties. Due their reduced dimensionality, these 2D materials exhibit superior mechanical and optoelectronic properties when compared to their bulk counterparts. Within the family of 2D materials, the ultrathin transition metal dichalcogenides (TMDs) such as Tungsten diselenide and Molybdenum disulphide have gained significant attention due to their chemical versatility and tunability. Furthermore, it is possible to leverage the distinct characteristic properties of these 2D materials, which are held together by van der Waals forces, by stacking different 2D layers on top of each other resulting in van der Waals (vdW) heterostructures. Due to the absence of feasible methods to effectively deform the crystal structures of these 2D materials and vdW heterostructures, their mechanical properties have not been thoroughly understood. The atomistic simulations can effectively capture the material behavior at the nanoscale level and help us not only not only understand the mechanical properties of these materials but also aid in the development of tailored processes to tune the material properties for the design of novel metamaterials. Using atomistic simulations, we develop the process - property relationships which can guide the direction of experimentation efforts, thereby making the process of discovering and designing new metamaterials efficient. In this work, we have used laser shock nanostraining technique which is a scalable approach to modulate the optomechanical properties of 2D materials and vdW materials for practical semiconductor industry applications. The deformation mechanisms of 2D materials such as graphene, boron nitride (BN) and TMDs such as WSe2 and MoS2 are examined by employing a laser shocking process. We report studies on crystal structure deformation of multilayered WSe2 and monolayer graphene at ultra-high strain rate using laser shock . The laser shocking process generates high pressure at GPa level, causing asymmetric 3D straining in graphene and a novel kinked-like locking structure in multilayered WSe2. The deformation processes and related mechanical behaviors in laser shocked 2D materials are examined using atomistic simulations. Moire heterostructures can be obtained by introducing a twist angle between these 2D layers, which can result into vdW materials with different properties, thereby adding an additional degree of freedom in the process-property design approach. We were able to successfully create a tunable stain profile in 2D materials and vdW heterostructures to modulate the local properties such as friction, and bandgap by controlling the level of laser shock, twist angle between the 2D layers and by applying appropriate laser shock pressure . We thus extend this knowledge to further explore the pathways of strain modulation using a combination of laser shocking process, moire engineering, and strain engineering in 2D materials consisting of graphene, BN, and MoS2 and to develop the process - property relationships in vdW materials. In summary, this research presents a systematic understanding of the effect of laser shocking process on the van der Waals materials and demonstrates the modulation of mechanical and opto-electronic property using laser nanostraining approach. This understanding provides us with opportunities for deterministic design of 2D materials with controllable properties for semiconductor and nanoelectronics applications.

Since the discovery of graphene in 2004, there has been a great interest in two-dimensional (2D) materials from both the academic community and the semiconductor industry. In this work, we study various 2D materials and 2D heterostructures, aimed towards large-area device fabrication. Through detailed experimental work and extensive first-principle calculations, we determined the lowest energy structure for the interface of graphene formation on the C-face of SiC. The lowest energy structure contains > 1 monolayer of Si at the interface, forming an adatom-on-adlayer structure. Low-energy electron microscopy (LEEM) was employed to study properties of 2D heterostructures such as graphene–WSe2 and graphene–MoS2. Work function differences from the layers were extracted and band alignments were obtained, from which the nature of the contact at the interface was revealed. The electrical contact was found to be dependent on the constituent 2D layers of the heterostructures, as well as on the doping of the 2D layers. Finally, we consider simulation of devices made with 2D materials. We focus on interlayer tunneling field-effect transistors (TFETs) using 2D materials as the drain and source electrodes. By employing the first-principles density-functional-theory (DFT) wavefunctions, in the Bardeen tunneling formalism, we develop a “DFT-Bardeen” method that permits the computation of current-voltage characteristics in interlayer TFETs with reliable values for the magnitude of the currents. This method allows incorporation of differing materials into the source and drain electrodes, i.e. with different crystal structure, lattice constants, and/or band structure. Large variations in tunneling current were found, depending on the 2D materials being used. It is shown that the DFT-Bardeen method takes into account effects that are beyond simple lateral-momentum conservation, including the detailed symmetry and form of the wavefunctions. Predicted values for the tunneling current, including the subthreshold swing and the ON current, are compared with benchmark values for low-power digital applications.

Van der Waals heterostructures integrated by two-dimensional (2D) transition metal dichalcogenides (TMDs) hold great potential for engineering promising exciton phenomena, particularly interlayer excitons (IXs) featuring extended lifetimes and permanent out-of-plane electric dipole moments. While electrically controllable IX emission has been extensively explored in TMD heterobilayers, reconfiguring their band alignment to achieve selected exciton states and exploit these states for novel excitonic functionalities remains an urgent challenge. Here, we demonstrate dynamic exciton control by electrically tunable band alignment transitions between type-II and type-I configurations in WS2/(iso-BA)2PbI4heterostructures, enabling reversible conversion between charged interlayer (IX⁻) and charged intralayer (X⁻) excitons. By tailoring these exciton states, we achieve two critical functionalities: (i) 7 μm ON/OFF control of exciton transport via transition between delocalized IX⁻ (ON) and confined X⁻ (OFF) states; (ii) Valley polarization switching with a 16.3 ON/OFF ratio, leveraging distinct spin-valley configurations of IX⁻ (unpolarized) and X⁻ (polarized). Unlike twist-angle-dependent TMD heterobilayers, this platform operates independently of stacking alignment, establishing a broadly hybrid 2D system as a practical paradigm towards programmable nanophotonic circuits.

The emergence of two-dimensional (2D) materials has intrigued a great deal of research on novel physical phenomena and various functional applications due to their particular crystal structures and reduced dimensionality. Unlike 3D materials, bulks of 2D materials can be easily thinned to atomic thickness by mechanical exfoliation and atomically thin 2D materials can be arbitrarily stacked to form vertical van der Waals (vdW) heterostructures, which could inherit the unique characteristics of the constituent layers and even exhibit new properties not possessed by them. Particularly, when type-Ⅱ vdW heterostructures are excited, the positive and the negative charges would reside in the different layers after the charge transfer but be limited within a short distance due to the strong quantum confinement effect of 2D materials, leading to strong Coulomb interaction and the formation of interlayer excitons (IXs). IXs are generally equipped with orientated dipole moment and long lifetime, making them ideal media for the future interconnects between optical transmission and electronic computation. Currently, studies on IXs are mainly focused on the vdW heterostructures formed by monolayers of transition-metal dichalcogenides (TMDs). Here, I will first talk about vdW heterostructures formed by 2D perovskites and TMDs for studying IXs. Stacking different kinds of 2D perovskites and TMDs, formation of IXs is confirmed by excitation-power-, temperature-, electric-field-dependent and time-resolved photoluminescence (PL) studies. Notably, robust IX emission can be observed regardless of the stacking sequence and geometric alignment of the constituent layers, showing great advantages over the TMD/TMD vdW heterostructures, which require special twist-angle and thermal annealing. Then, I would like to give a brief introduction on widely tuning the IX emission energy by changing the layer number of the 2D perovskite or the TMD, which shed light on the application of 2D perovskite/TMD vdW heterostructures in broad-spectrum optoelectronics. Furthermore, I would like to next talk about how the selection of organic chains in 2D perovskites influences the properties of the IXs. By using chiral 2D perovskites, the IX emission shows substantial circular polarization and the polarization direction is only related to the chirality of the molecules regardless of the excitation scheme or any other external field, which could open up new passages for controlling valley- or spin-polarization of IXs. By introducing molecules with different dielectric constants, the dielectric screening strength in the vdW heterostructure is changed and hence the binding energy of the IXs is also modified, which offers great opportunities for exploiting tunable excitonic devices and studying exciton condensation. Finally, I will also introduce IX as a non-destructive tool for probing the local phase transition at the surface of the 2D perovskite (BA)2PbI4 flakes. By spatially PL mapping of the (BA)2PbI4/WSe2 heterostructure, two different IX species can be observed to distinguish the low-temperature and the high-temperature phase respectively.

The international road map of semiconductors (ITRS) lists 2D materials as possible future materials for electronic devices [ITRS]. Among them, the semiconducting transition metal dichalcogenides (TMDC) like MoS2 or WS2 are the most promising ones. As the miniaturization of electronic devices continues and the sub-5 nm gate limit is reached, direct source-to-drain tunneling will become a problem in Si devices with conventional architecture. 2D TMDC as channel material for FET might overcome this issue in digital CMOS. The larger carrier effective masses of most TMDC result in reduced direct source-to-drain tunneling, while they also yield a large density of states and hence an increased ballistic current in this extreme dimensions [Fiori]. Additionally, the lower in-plane dielectric constants of TMDC enable a better vertical electrostatic control over the channel [Desai]. Still, TMDC will have to compete against other approaches, e.g. multiple-gate transistors [Schwiertz]. Beyond that, other properties of TMDC turn them interesting for future devices, such as their strong absorbance across the visible spectrum for optoelectronic devices [Lotsch]. The first photodetectors, electromluminescent p-n diodes and photovoltaic cells have already been shown [Lopez, Cheng, Tsai]. The excellent mechanical properties additionally enable the realization of different kinds of flexible devices [Schwiertz]. And last but not least, the integration of 2D materials into conventional 3D semiconductor concepts would be another application. For example, the strain originating from the large difference in lattice constants in a 3D heterostructure is often a challenge. Inserting 2D materials might be a solution. Due to the fully terminated surface of a TMDC monolayer, the binding to another material is of van-der-Waals type. Hence, TMDC can be stacked with materials with huge differences in lattice constants without leading to considerable strain or to interface trap densities [Vogel]. First devices with a combination of 2D and 3D materials have already been investigated [Krishna, Lee]. Large-scale fabrication of TMDC is still a challenge to be overcome. Up to now, the most frequently used techniques are either exfoliation of natural layered crystals or the growth via chemical vapor deposition (CVD). Exfoliation is a very time-consuming process with relatively low yield and reproducibility. In CVD, different gaseous precursors decompose on a substrate and react with each other. Thermally evaporated MoO3 and S are commonly used as precursors for the fabrication of MoS2 monolayers via CVD. This technique allows the deposition of monolayered crystallites with a size in the range of micrometers [Dumcenco]. The process is carried out in small experimental reactors, and a uniform deposition on a whole wafer is very challenging. The use of metalorganic precursors is one possibility to enter an industrial scale of fabrication. The development of III/V and II/VI semiconductors has shown that metalorganic chemical vapor deposition (MOCVD) makes controllable and reproducible processes feasible, which are easily scalable and hence suitable for large-area deposition. Additionally, the reactors are well-developed, and a future integration of 2D materials and other semiconductors is within reach. First publications show very good results on the deposition of MoS2 and WSe2 via MOCVD [Kang, Eichfeld]. But despite these works and first simulations of the growth kinetics [Nie], little is known about the growth mechanism. For this reason, we have started to investigate the deposition of 2D MoS2 on an AIXTRON MOCVD reactor.The experiments are carried out in a horizontal hot-wall MOCVD reactor in a 10 × 2 inch configuration. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as Mo and S sources, respectively. In preliminary experiments, the carrier gas based transport of the precursors into the reactor and possible growth conditions are tested. Their results are used to develop an initial growth process. This process leads to a uniform, wafer-scale deposition of MoS2 on various substrate types such as sapphire (0001), Si (111), and AlN and GaN templates on sapphire substrates. The deposited films mainly consists of MoS2 bilayers and exhibit a very high initial nucleation density. Further investigations of the influence of the growth temperature, the carrier gas composition and the pretreatment of the substrates are carried out. With optimization of these growth parameters, a crystal growth process closer to thermodynamical equilibrium can be achieved. This results in the formation of triangular crystals on sapphire substrates which are also reported from CVD processes and exhibit higher crystal quality [Dumcenco]. Additional experiments are conducted to investigate the nucleation of the films and to further tune nucleation density and lateral growth rate in order to deposit wafer-scale monolayered MoS2 films.

Moiré patterns with a superlattice potential can be formed by vertically stacking two layered materials with a relative twist or lattice constant mismatch. In transition metal dichalcogenide-based systems, the moiré potential landscape can trap interlayer excitons (IXs) at specific atomic registries. Here, we report that spatially isolated trapped IXs in a molybdenum diselenide/tungsten diselenide heterobilayer device provide a sensitive optical probe of carrier filling in their immediate environment. By mapping the spatial positions of individual trapped IXs, we are able to spectrally track the emitters as the moiré lattice is filled with excess carriers. Upon initial doping of the heterobilayer, neutral trapped IXs form charged IXs (IX trions) uniformly with a binding energy of ~7 meV. Upon further doping, the empty superlattice sites sequentially fill, creating a Coulomb staircase: stepwise changes in the IX trion emission energy due to Coulomb interactions with carriers at nearest-neighbour moiré sites. This non-invasive, highly local technique can complement transport and non-local optical sensing techniques to characterize Coulomb interaction energies, visualize charge correlated states, or probe local disorder in a moiré superlattice.

Growing twisted bilayer graphene at small angles

Interlayer excitons in heterobilayers of transition-metal dichalcogenides (TMDCs) have generated enormous interest due to their permanent vertical dipole moments and long lifetimes. However, the effects of mechanical strain on the optoelectronic properties of interlayer excitons in heterobilayers remain relatively uncharacterized. Here, we experimentally demonstrate strain tuning of Γ-K interlayer excitons in molybdenum disulfide and tungsten diselenide (MoS2/WSe2) wrinkled heterobilayers and obtain a deformation potential constant of ∼107 meV/% uniaxial strain, which is approximately twice that of the intralayer excitons in the constituent monolayers. We further observe a nonmonotonic dependence of the interlayer exciton photoluminescence intensity with strain, which we interpret as being due to the sensitivity of the Γ point to band hybridization arising from the competition between in-plane strain and out-of-plane interlayer coupling. Strain engineering with interlayer excitons in TMDC heterobilayers offers higher strain tunability and new degrees of freedom compared to their monolayer counterparts.

Two dimensional (2D) layered materials are receiving great attention due to various chemical functionality and anisotropic properties which are related to the crystalline arrangement of atoms. Their biocompatibility along with their structural, electrical and mechanical properties created interest in various fields of science and technology. The materials are flexible, transparent, light weight and have high stability. 2D layered materials are highly electrically and thermally conductive in nature. Additionally, phase boundaries and dynamic behavior can be observed in the same materials. It was expected that these layered 2D materials can be used as electrocatalyst to study their interactions with various molecules by electrochemical methods. The 2D materials based electrode have shown promising electro-catalytic activity, higher sensitivity, with selectivity, more stability and biocompatibility when used in chemical and biosensor applications. Interestingly, various kind of layered 2D materials can be easily synthesized without any sophisticated instruments. In this review, we have discussed about unique features of layered materials and various electrochemical exfoliation methods reported to prepare 2D layered materials (For example; hexagonal boron nitride (hBN), molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten dsulfide (WS2) and tungsten diselenide (WSe2)) with fewer defects at a relatively low cost. In addition, the use of 2D layered materials for electrochemical sensor fabrication and detection of pesticides were highlighted with their applications in real-world samples. Since, pesticides are highly toxic even at very low concentrations, it is very important to identify and detect their concentration level in fruits, vegetables and food to ensure human health and safety. For this purpose, 2D layered materials based electrochemical sensors have been demonstrated as a promising alternative method. The keep increasing demand for wearable/flexible sensor may utilize the simple electrochemical synthetic methods and electrode modification strategies to prepare electrochemical transducers for on-site pesticides detection in food samples with relatively low-cost.

ConspectusThe electronic properties of atomically thin van der Waals (vdW) materials can be precisely manipulated by vertically stacking them with a controlled offset (for example, a rotational offset─i.e., twist─between the layers, or a small difference in lattice constant) to generate moiré superlattices. In recent years, the application of this "twistronics" concept to interfacial electrochemistry has unveiled unique pathways for tailoring the electrochemical reactivity. This Account provides an overview of our work that leveraged a suite of structural characterization methods, such as interferometric four-dimensional scanning transmission electron microscopy, dark-field transmission electron microscopy, and scanning tunneling microscopy, along with nanoscale electrochemical measurement techniques, namely, scanning electrochemical cell microscopy (SECCM), to uncover and dissect the profound impact of electrode electronic structure, controlled by interlayer twist, on interfacial electron transfer kinetics. At the heart of our findings is the discovery that moiré engineering enables the isolation of thermodynamically unfavorable stacking configurations, or topological defects, that substantially increase the standard electron transfer rate constant at the solid-liquid interface beyond what has been measured on conventional, nontwisted two-dimensional (2D) materials. This enhancement in interfacial reactivity can be attributed to the localization of a high density of electronic states within these particular sites in the superlattice, a similar effect to that which occurs upon incorporation of physical defects or vacancies in an electrode material but instead using an atomically pristine surface with a highly tunable structure. Throughout our studies, understanding the nuances of the relationship between the preimposed moiré twist angle and the observed electron transfer kinetics has heavily relied on the interrogation of additional factors such as spontaneous superlattice reconstruction and three-dimensional localization of electronic states, illustrating the importance of combining electrochemical measurements with both nanoscale structural probes and theoretical modeling for designing and optimizing moiré-engineered electrodes. The insight afforded by our efforts in this space continues to deepen our understanding of the fundamental mechanisms governing electron transfer at electrochemical interfaces at large and also points to the revolutionary prospect of twistronics for advancing electrochemical technologies. While our electrochemical studies have, so far, focused largely on graphene-based moiré materials, we also offer a perspective on the promise of transition metal dichalcogenide (TMD)-based moirés as candidates for highly versatile (photo)electrode surfaces. Accordingly, we provide a discussion of our studies on the structural relaxation observed in moiré superlattices of TMDs, and we summarize our work combining SECCM with field-effect electrostatic gating of TMDs to deconvolute the influences of material conductivity and intrinsic electron transfer kinetics from the overall electrochemical response of a semiconducting 2D material. Overall, this body of work establishes a distinctive foundation for the design of a wide range of materials with tailored properties that can provide crucial insights into interfacial charge transfer chemistry─potentially serving as platforms for sensing, energy conversion, and electrocatalysis─in addition to the emergent exotic correlated electron physics that originally ignited intense interest in moiré twistronics.

Transition metal dichalcogenides (TMDCs) heterostructure with a type II alignment hosts unique interlayer excitons with the possibility of spin-triplet and spin-singlet states. However, the associated spectroscopy signatures remain elusive, strongly hindering the understanding of the Moiré potential modulation of the interlayer exciton. In this work, we unambiguously identify the spin-singlet and spin-triplet interlayer excitons in the WSe2/MoSe2 heterobilayer with a 60° twist angle through the gate- and magnetic field-dependent photoluminescence spectroscopy. Both the singlet and triplet interlayer excitons show giant valley-Zeeman splitting between the K and K' valleys, a result of the large Landé g-factor of the singlet interlayer exciton and triplet interlayer exciton, which are experimentally determined to be ∼10.7 and ∼15.2, respectively, which is in good agreement with theoretical expectation. The photoluminescence (PL) from the singlet and triplet interlayer excitons show opposite helicities, determined by the atomic registry. Helicity-resolved photoluminescence excitation (PLE) spectroscopy study shows that both singlet and triplet interlayer excitons are highly valley-polarized at the resonant excitation with the valley polarization of the singlet interlayer exciton approaching unity at ∼20 K. The highly valley-polarized singlet and triplet interlayer excitons with giant valley-Zeeman splitting inspire future applications in spintronics and valleytronics.