Transition Metal Dichalcogenide Heterostructures Research Articles

Moiré Synergy: An Emerging Playground by Coupled Moirés.

Moiré superlattices of tunable wavelengths and the further developed coupled-moiré systems, by artificially assembling two-dimensional (2D) van der Waals (vdW) materials as designed, have brought up a versatile toolbox to explore fascinating condensed matter physics and their stimulating physicochemical functionalities. In this Perspective, we briefly review the recent progress in the emerging field of moiré synergy, highlighting the synergetic effects arising in distinct multi-moiré heterostructures of graphene and transition metal dichalcogenides (TMDCs). A spectrum of coupled-moiré configurations, the advanced characterization, and the exploitation efforts on the moiré-moiré interactions will be discussed. Finally, we look out for urgent challenges to be conquered in the community and some potential research directions in the near future.

Transition Metal Dichalcogenides Heterostructures Nanoribbons

The integration of two-dimensional van der Waals (vdW) heterostructures breaks through the constraints of lattice matching and symmetry, offering substantial opportunities in the development of advanced materials. After the width is reduced, one-dimensional vdW heterostructures show rich band structures and strong spin–orbit coupling behaviors, widening the applications in optoelectronics and spintronics. However, the synthesis of one-dimensional vdW heterostructure nanoribbons has rarely been reported yet. Herein, we report a general approach to realizing transition metal dichalcogenide (TMD) heterostructure nanoribbons for the first time by unzipping self-assembled TMD heterostructure nanoscrolls. As demonstrated by MoS2/WS2 nanoribbons, the obtained TMD heterostructure nanoribbons with alternating stacked layers possess flat edge structures and high quality. Meanwhile, this strategy can be extended to create diverse vdW TMD nanoribbons, demonstrating its versatility. Our work thus shows great potential to prepare TMD heterostructure nanoribbons with various compositions and stacking modes, and new structures can be customized with targeted properties.

Confined-state physics and signs of fermionization of moiré excitons in WSe2/MoSe2 heterobilayers

We revisit and extend the standard bosonic interpretation of interlayer excitons (ILX) in the moiré potential of twisted heterostructures of transition-metal dichalcogenides. In our experiments, we probe a high quality MoSe2/WSe2 van der Waals bilayer heterostructure via density-dependent photoluminescence spectroscopy and reveal strongly developed, unconventional spectral shifts of the emergent moiré exciton resonances. The observation of saturating blueshifts of successive exciton resonances allow us to explain their physics in terms of a model utilizing fermionic saturable absorbers. This approach is strongly inspired by established quantum-dot models, which underlines the close analogy of ILX trapped in pockets of the moiré potential, and quantum emitters with discrete eigenstates.

Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure.

Transition metal dichalcogenide heterostructures provide a versatile platform to explore electronic and excitonic phases. As the excitation density exceeds the critical Mott density, interlayer excitons are ionized into an electron-hole plasma phase. The transport of the highly non-equilibrium plasma is relevant for high-power optoelectronic devices but has not been carefully investigated previously. Here, we employ spatially resolved pump-probe microscopy to investigate the spatial-temporal dynamics of interlayer excitons and hot-plasma phase in a MoSe2/WSe2 twisted bilayer. At the excitation density of ∼1014 cm-2, well exceeding the Mott density, we find a surprisingly rapid initial expansion of hot plasma to a few microns away from the excitation source within ∼0.2 ps. Microscopic theory reveals that this rapid expansion is mainly driven by Fermi pressure and Coulomb repulsion, while the hot carrier effect has only a minor effect in the plasma phase.

Phase engineering of layered anode materials during ion-intercalation in Van der Waal heterostructures

Transition metal dichalcogenides (TMDs) are a class of 2D materials demonstrating promising properties, such as high capacities and cycling stabilities, making them strong candidates to replace graphitic anodes in lithium-ion batteries. However, certain TMDs, for instance, MoS2, undergo a phase transformation from 2H to 1T during intercalation that can affect the mobility of the intercalating ions, the anode voltage, and the reversible capacity. In contrast, select TMDs, for instance, NbS2 and VS2, resist this type of phase transformation during Li-ion intercalation. This manuscript uses density functional theory simulations to investigate the phase transformation of TMD heterostructures during Li-, Na-, and K-ion intercalation. The simulations suggest that while stacking MoS2 layers with NbS2 layers is unable to limit this 2H → 1T transformation in MoS2 during Li-ion intercalation, the interfaces effectively stabilize the 2H phase of MoS2 during Na- and K-ion intercalation. However, stacking MoS2 layers with VS2 is able to suppress the 2H → 1T transformation of MoS2 during the intercalation of Li, Na, and K-ions. The creation of TMD heterostructures by stacking MoS2 with layers of non-transforming TMDs also renders theoretical capacities and electrical conductivities that are higher than that of bulk MoS2.

Exciton polariton interactions in Van der Waals superlattices at room temperature

Monolayer transition-metal dichalcogenide (TMD) materials have attracted a great attention because of their unique properties and promising applications in integrated optoelectronic devices. Being layered materials, they can be stacked vertically to fabricate artificial van der Waals lattices, which offer unique opportunities to tailor the electronic and optical properties. The integration of TMD heterostructures in planar microcavities working in strong coupling regime is particularly important to control the light-matter interactions and form robust polaritons, highly sought for room temperature applications. Here, we demonstrate the systematic control of the coupling-strength by embedding multiple WS2 monolayers in a planar microcavity. The vacuum Rabi splitting is enhanced from 36 meV for one monolayer up to 72 meV for the four-monolayer microcavity. In addition, carrying out time-resolved pump-probe experiments at room temperature we demonstrate the nature of polariton interactions which are dominated by phase space filling effects. Furthermore, we also observe the presence of long-living dark excitations in the multiple monolayer superlattices. Our results pave the way for the realization of polaritonic devices based on planar microcavities embedding multiple monolayers and could potentially lead the way for future devices towards the exploitation of interaction-driven phenomena at room temperature.

Layer-engineered interlayer charge transfer in WSe2/WS2 heterostructures

The layer thickness determines the electronic structure of two-dimensional (2D) materials, leading to different band alignments, which are crucial for the transition metal dichalcogenides heterostructures. Here, we investigated the heterostructure of WSe2/WS2 with different layer thicknesses by steady-state and transient absorption spectroscopy. We observed different ultrafast charge transfer behaviors in 1L-WSe2/2L-WS2 and 2L-WSe2/2L-WS2 few-layer heterostructures. We demonstrate that the layer thickness determines the sequence of intralayer exciton relaxation and interlayer charge transfer. The valley transfer of the band edge induced by the layer thickness can effectively mediate the hot carrier transfer time and interlayer exciton lifetime. These provide us a deeper understanding of carrier dynamics in 2D indirect bandgap semiconductor heterostructures.

Robustness of Momentum-Indirect Interlayer Excitons in MoS2/WSe2 Heterostructure against Charge Carrier Doping

Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics.

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS2 from the edges of mechanically exfoliated multilayer flakes of WSe2 or NbxMo1-xS2. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS2 on the exfoliated flakes. For the WSe2/MoS2 sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the NbxMo1-xS2/MoS2 in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS2. The formation of a staggered gap band alignment of NbxMo1-xS2/MoS2 is also supported by first-principles calculations.

Electronic Structures and NLO Properties of a Series of TMDs Lateral‐Core–Shell Heterostructures Quantum Dots

AbstractThe electronic structures and nonlinear optical (NLO) responses of eight triangle group‐VI transition metal dichalcogenides (TMDs) lateral‐core–shell heterostructures quantum dots (QDs) [MaX2MbX2 and MXa2MXb2; M (Ma, Mb) = Mo and W; X (Xa, Xb) = S and Se] have been explored by using quantum chemistry method. Frontier molecular orbital (FMOs) analyses indicate that the triangular frameworks with more active W and/or Se atoms at shell have smaller energy gaps than those with Mo and/or S atoms, and the active sites of the ultra‐small TMD heterostructures mainly locate at the edge and/or the corner of the frameworks. Generally speaking, active atoms at the shell are beneficial to the enhancement of the frequency‐dependent first hyperpolarizabilities compared with the active atoms in the cores. First hyperpolarizability density analyses show that the local contributions of the first hyperpolarizability in x direction is mainly located at the left and right corners, while that in y direction is mainly located at the bottom edge and top corner, confirming the important role that the shell of the TMDs lateral‐core–shell heterostructure QDs play in its NLO response. Further, the electronic transitions contributed to the SHG responses also occur at the shell of these triangular frameworks.

Photo‐Carrier Lifetime in Binary and Ternary Heterostructures of Transition Metal Dichalcogenides

Binary and ternary transition metal dichalcogenide (TMD) heterostructures, MSe2 and MSeS (where M = Mo or W), are investigated as future building blocks for high‐performance device applications with a tailorable electronic bandgap. For light‐emitting device applications, investigating the decay time of the material is a prerequisite. Herein, time‐resolved photoluminescence (TRPL) of binary and ternary TMD heterostructures is measured. A tri‐exponential function is adopted to fit the TRPL data. Short, medium, and long decay times are <50 ps, ≈400 ps, and >400 ps, respectively. Except for monolayer MoSe2 and MoSeS, only two PL decay time components are observed with a large contribution from short decay time photocarriers (>80%). Bilayer TMDs exhibit a much shorter PL decay time, which could be attributed to the indirect nature of excitons and different dielectric environments, compared with that of monolayer TMDs. Using the center‐of‐mass (CM) method, a PL decay time map is successfully obtained for binary and ternary TMD heterostructures. This allows the qualitative visualization of the decay time distribution within the heterostructure or among other materials. This technique could be applied to assess the decay time of materials used in high‐performance light‐emitting devices.

Ion-gel-based light-emitting devices using transition metal dichalcogenides and hexagonal boron nitride heterostructures

Fabrication of high-performance optoelectronic devices is an important aspect of the application research of transition metal dichalcogenides (TMDCs). In this study, heterostructures of TMDCs and hexagonal boron nitrides (hBN) were successfully fabricated into light-emitting devices. Monolayer and artificially stacked homobilayer WS2 were prepared on hBN, respectively. They were then deposited with electrodes and covered by the ion gels to function as light-emitting devices. Both devices showed clear electroluminescence (EL) with voltages of ∼3 V. In monolayer device, a symmetric EL peak was observed with suppressed inhomogeneity. The bilayer device showed spectra that agreed with the natural bilayer samples. These results indicate the enhancement of the optical performance of TMDCs and the heterostructure could expand the potential of TMDC-based light-emitting devices.

Tailoring Polymorphic Heterostructures of MoS2–WS2 (1T/1T, 2H/2H) for Efficient Hydrogen Evolution Reaction

Earth-abundant and inexpensive transition metal dichalcogenides (TMDCs) with existing polymorphisms (metallic 1T phase and semiconducting 2H phase) have been proposed as alternatives to noble metals (e.g., Pt, Ir, and Ru) to achieve an efficient hydrogen evolution reaction (HER). Although the 1T phase of TMDCs (1T-TMDCs) is essential as an HER catalyst, practical application in the HER has not been realized owing to the lack of any large-scale production of the 1T-TMDC and 1T/1T-TMDC heterostructure fabrication method. Here, polymorphic TMDC–TMDC heterostructures at a 4 in. wafer scale is reported for 1T-MoS2/1T-WS2 vertical heterostructures (1T/1T-MWH) or 2H-MoS2/2H-WS2 vertical heterostructures (2H/2H-MWH) under cold plasma conditions and process temperature. Simultaneous ion-bombardment onto substrates induces a few nanosized grain boundaries with discontinuous films, resulting in exposed edges that act as catalytically active sites. The electrocatalytic performances of the prepared polymorph (1T or 2H phases of MoS2 and WS2) and polymorphic heterostructures of 1T/1T-MWH and 2H/2H-MWH are compared. 1T/1T-MWH shows the highest electrocatalytic performance owing to its metallic 1T phase and heterostructures containing alloy structures at the heterointerface. Moreover, the nanosized grains of 1T/1T-MWH preserve their original phase after 1000 HER cycles, proving their robustness and durability.

Exotic states in moiré superlattices of twisted semiconducting transition metal dichalcogenides

Moiré superlattices formed by van der Waals materials with small lattice mismatch or twist angle open an unprecedented approach to generate flat bands that don’t exist in the “parent” materials, which provides a controllable platform for exploring quantum many body physics. Owing to the wide angle range for the existence of flat bands, as well as the valley-spin-locking band structure and the excellent optical properties, twisted semiconducting transition metal dichalcogenides (TMDs) heterostructures have recently attracted lots of attention. In this review, we discuss the exotic states discovered in the twisted TMDs heterostructures, including Mott insulator, generalized Wigner crystals, topological non-trivial states, and moiré excitons, how to manipulate these exotic states and related mechanisms, and finally some perspectives on the opportunities and challenges in this field.

Trends in energy and charge transfer in 2D and integrated perovskite heterostructures.

Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.

Observation of positive trions in α-MoO3/MoS2 van der Waals heterostructures.

Mono-layer transition metal dichalcogenides (TMDCs) have emerged as an ideal platform for the study of many-body physics. As a result of their low dimensionality, these materials show a strong Coulomb interaction primarily due to reduced dielectric screening that leads to the formation of stable excitons (bound electron-hole pairs) and higher order excitons, including trions, and bi-excitons even at room temperature. van der Waals (vdW) heterostructures (HSs) of TMDCs provide an additional degree of freedom for altering the properties of 2D materials because charge carriers (electrons) in the different atomically thin layers are exposed to interlayer coupling and charge transfer takes place between the layers of vdW HSs. Astoundingly, it leads to the formation of different types of quasi-particles. In the present work, we report the synthesis of vdW HSs, i.e., α-MoO3/MoS2, on a 300 nm SiO2/Si substrate and investigate their temperature-dependent photoluminescence (PL) spectra. Interestingly, an additional PL peak is observed in the case of the HS, along with A and B excitonic peaks. The emergence of a new PL peak in the low-energy regime has been assigned to the formation of a positive trion. The formation of positive trions in the HS is due to the high work function of α-MoO3, which enables the spontaneous transit of electrons from MoS2 to α-MoO3 and injection of holes into the MoS2 layer. In order to confirm charge transfer in the α-MoO3/MoS2 HS, systematic power and wavelength-dependent Raman and PL studies, as well as first-principle calculations using Bader charge analysis, have been carried out, which clearly validate our mechanism. We believe that this study will provide a platform towards the integration of vdW HSs for next-generation excitonic devices.

Twist-angle-dependent momentum-space direct and indirect interlayer excitons in WSe2/WS2 heterostructure.

Interlayer excitons (ILEs) in the van der Waals (vdW) heterostructures of type-II band alignment transition metal dichalcogenides (TMDCs) have attracted significant interest owing to their unique exciton properties and potential in quantum information applications. However, the new dimension that emerges with the stacking of structures with a twist angle leads to a more complex fine structure of ILEs, presenting both an opportunity and a challenge for the regulation of the interlayer excitons. In this study, we report the evolution of interlayer excitons with the twist angle in the WSe2/WS2 heterostructure and identify the direct (indirect) interlayer excitons by combining photoluminescence (PL) and density functional theory (DFT) calculations. Two interlayer excitons with opposite circular polarization assigned to the different transition paths of K-K and Q-K were observed. The nature of the direct (indirect) interlayer exciton was confirmed by circular polarization PL measurement, excitation power-dependent PL measurement and DFT calculations. Furthermore, by applying an external electric field to regulate the band structure of the WSe2/WS2 heterostructure and control the transition path of the interlayer excitons, we could successfully realize the regulation of interlayer exciton emission. This study provides more evidence for the twist-angle-based control of heterostructure properties.

Signature of Cascade Transitions between Interlayer Excitons in a Moiré Superlattice.

A moiré superlattice in transition metal dichalcogenides heterostructure provides an exciting platform for studying strongly correlated electronics and excitonic physics, such as multiple interlayer exciton (IX) energy bands. However, the correlations between these IXs remain elusive. Here, we demonstrate the cascade transitions between IXs in a moiré superlattice by performing energy- and time-resolved photoluminescence measurements in the MoS_{2}/WSe_{2} heterostructure. Furthermore, we show that the lower-energy IX can be excited to higher-energy ones, facilitating IX population inversion. Our finding of cascade transitions between IXs contributes to the fundamental understanding of the IX dynamics in moiré superlattices and may have important applications, such as in exciton condensate, quantum information protocols, and quantum cascade lasers.

Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe2-WSe2 lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe2-WSe2 heterojunctions is 3 nm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe2 to MoSe2.

Ultrafast Exciton and Trion Dynamics in High‐Quality Encapsulated MoS2Monolayers

The extreme confinement and reduced screening in monolayer transition metal dichalcogenides (TMDs) leads to the appearance of tightly bound excitons which can also couple to free charges, forming trions, owing to strong Coulomb interactions. Low temperatures and encapsulation in hexagonal boron nitride (hBN) can narrow the excitonic linewidth, approaching the regime of homogeneous broadening, mostly dominated by the radiative decay. Ultrafast spectroscopy is a perfect tool to study exciton formation and relaxation dynamics in TMD monolayers. However, high‐quality hBN‐encapsulated structures have usually lateral sizes of the order of a few micrometers, calling for the combination of high spatial and temporal resolution in pump–probe experiments. Herein, a custom broadband pump–probe optical microscope is used to measure the ultrafast dynamics of neutral and charged excitons in high‐quality hBN‐encapsulated monolayer MoS2at 8 K. Neutral excitons exhibit a narrow linewidth of 7.5 meV, approaching the homogeneous limit, which is related to the fast recombination time of ≈130 fs measured in pump–probe. Moreover, markedly different dynamics of the trions over the neutral ones are observed. The results provide novel insights on the exciton recombination processes in TMD monolayers, paving the way for exploring the ultrafast behavior of excitons and their many‐body complexes in TMD heterostructures.