Transition Metal Dichalcogenide Heterojunctions Research Articles

Programmable Interfacial Band Configuration in WS2/Bi2O2Se Heterojunctions.

van der Waals heterojunctions based on transition-metal dichalcogenides (TMDs) offer advanced strategies for manipulating light-emitting and light-harvesting behaviors. A crucial factor determining the light-material interaction is in the band alignment at the heterojunction interface, particularly the distinctions between type-I and type-II alignments. However, altering the band alignment from one type to another without changing the constituent materials is exceptionally difficult. Here, utilizing Bi2O2Se with a thickness-dependent band gap as a bottom layer, we present an innovative strategy for engineering interfacial band configurations in WS2/Bi2O2Se heterojunctions. In particular, we achieve tuning of the band alignment from type-I (Bi2O2Se straddling WS2) to type-II and finally to type-I (WS2 straddling Bi2O2Se) by increasing the thickness of the Bi2O2Se bottom layer from monolayer to multilayer. We verified this band architecture conversion using steady-state and transient spectroscopy as well as density functional theory calculations. Using this material combination, we further design a sophisticated band architecture incorporating both type-I (WS2 straddles Bi2O2Se, fluorescence-quenched) and type-I (Bi2SeO5 straddles WS2, fluorescence-recovered) alignments in one sample through focused laser beam (FLB). By programming the FLB trajectory, we achieve a predesigned localized fluorescence micropattern on WS2 without changing its intrinsic atomic structure. This effective band architecture design strategy represents a significant leap forward in harnessing the potential of TMD heterojunctions for multifunctional photonic applications.

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe2-WS2-WSe2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

The Role of Electron–Electron Interaction in Charge Transport Calculations through Transition Metal Dichalcogenides Heterojunctions

2D materials have raised a lot of interest in the last decades, due to their novel and diverse properties, particularly for energy technology applications where materials such as molybdenum disulfide are known for their outstanding catalytic ability. While intensive research is devoted to 2D transition metal dichalcogenide (TMDCs) semiconductor characterization, comprehensive understanding of metal/semiconductor heterojunctions is still lacking especially for junctions containing 2D metallic monolayers other than graphene. Using wave‐packet propagation, charge transport of two electrons through heterojunctions is simulated, in order to assess the influence of the electron–electron interaction on the charge transport efficiency. TMDCs with similar structure for both metal and semiconductor monolayers are used, leading to a high‐quality metal contact. It is found that charge transport is more efficient in systems containing a chromium disulfide semiconductor monolayer compared to systems with MoS2 or tungsten disulfide. This trend in the efficiency is the same with and without electron–electron interaction, demonstrating the validity of the qualitative input provided by models that lack electron–electron interactions. Nevertheless, for all systems, the interaction increases charge transmission probability from the metal to the semiconductor, due to electronic repulsion that pushes one of the charge carriers forward across the interface.

Exciton Dynamics in MoS2-Pentacene and WSe2-Pentacene Heterojunctions.

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS2-pentacene and WSe2-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS2 and WSe2 monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS2 to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS2-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS2 on SiO2. The B-exciton decay in WSe2 shows faster hole transfer rates for WSe2-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe2 also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions.

An ultrafast photodetector driven by interlayer exciton dissociation in a van der Waals heterostructure.

Ultrafast photodetectors based on two-dimensional materials suffer from low responsivities and high dark currents. Interlayer exciton dissociation in type-II vertical heterojunctions of transition metal dichalcogenides is a viable mechanism for achieving higher responsivities with picosecond response times. Here, we propose a novel device concept based on these structures, with potential for self-powered photodetector applications characterized by an unprecedented trade-off between speed and responsivity with zero dark current. In order to assess the realistic performance to be expected in the proposed device, we have purposely devised a simulation approach able to provide a detailed investigation of the physics at play, while showing excellent predictive capabilities when compared with experiments on interlayer exciton transport available in the literature. The proposed high-performance photodetectors with tunable responsivities are at reach with available fabrication techniques and could help in paving the way towards monolithically integrated artificial neural networks for ultrafast machine vision in speed sensitive applications.

Layer and material-type dependent photoresponse in WSe2/WS2 vertical heterostructures

Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe2/WS2 vertical heterostructures as well as in pristine WS2 and WSe2 layers. For WS2, we find that photocurrents increase with increasing layer thickness, whereas for WSe2 the layer dependence is more complex and depends on both the layer number and applied bias (Vb ). We further find that photocurrents in the WSe2/WS2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe2/WS2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices.

Crossover between thermo- and field-assisted carrier injection in staggered pn heterojunction of MoTe2 and ReS2

In semiconductor devices, understanding and controlling the properties of metal–semiconductor and semiconductor–semiconductor junctions is vital. A wide variety of heterojunctions using transition metal dichalcogenides (TMDs) have been fabricated and characterized. Rectification of the drain-source current (Ids) has been observed from p/n-TMD junctions, which is responsible for thermionic emission. In this study, we investigated diode behaviors of MoTe2 and ReS2 heterostructures, whose built-in potential ranged between 0.18 and 0.37 eV; however, we observed that the device showed a crossover between thermionic and Fowler-Nordheim (F-N) tunneling for a carrier injection mechanism. Thermionic emission existed within a narrow temperature and bias windows. Our experimental and theoretical analyses revealed that the carrier injection can be controlled by modulating the potential barrier height, temperature, and applied strength. For instance, at 300 K under a low drain-source voltage Vds, the carriers were thermally injected. Subsequently, the injection mechanism quickly shifted to the thermal field and F-N tunneling as Vds increased. Furthermore, when the temperature was lower than 220 K, the field-enhanced tunneling of the carrier prevailed regardless of Vds magnitude. This result expands the insight into the charge transport mechanism in staggered TMD heterojunctions.

Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions.

Two-dimensional transition metal dichalcogenides (2D-TMDCs) have gained attention for their promise in next-generation energy-harvesting and quantum computing technologies, but realizing these technologies requires a greater understanding of TMDC properties that influence their photophysics. To this end, we discuss here the interplay between TMDC microstructure and defects with the charge generation yield, lifetime, and mobility. As a model system, we compare monolayer-only and monolayer-rich MoS2 grown by chemical vapor deposition, and we employ the TMDCs in Type-II charge-separating heterojunctions with semiconducting single-walled carbon nanotubes (s-SWCNTs). Our results suggest longer lifetimes and higher yields of mobile carriers in samples containing a small fraction of defect-rich multilayer islands on predominately monolayer MoS2. Compared to the monolayer-only heterojunctions, the carrier lifetimes increase from 0.73 μs to 4.71 μs, the hole transfer yield increases from 23% to 34%, and the electron transfer yield increases from 39% to 59%. We reach these conclusions using a unique combination of microwave photoconductivity (which probes only mobile carriers) along with transient absorption spectroscopy (which identifies spectral signatures unique to each material and type of photoexcited quasiparticle, but does not probe mobility). Our results highlight the substantial changes in photophysics that can occur from small changes in TMDC microstructure and defect density, where the presence of defects does not necessarily preclude improvements in charge generation.

Ultrathin Quasibinary Heterojunctioned ReS2/MoS2 Film with Controlled Adhesion from a Bimetallic Co-Feeding Atomic Layer Deposition.

Heterojunctioned transition-metal dichalcogenide (TMD) films with regulatable interface adhesion have shown broad application prospects in the design of advanced materials and the manufacturing of novel functional devices. To date, the controlled fabrication of TMD heterojunctions or heterojunction-rich films with tailorable thickness and composition has proved challenging. Herein, a bimetallic co-feeding atomic layer deposition (ALD) system was developed capable of fulfilling these requirements. In the co-feeding ALD fabrication, by adjusting the Re/Mo ratio, 3-layered quasibinary heterojunctioned ReS2/MoS2 films with adjustable composition and grain size were prepared. Moreover, the measurements between atomic force microscopy Si tip coated with the ReS2/MoS2 films and films on the substrate indicate that the adhesion force can be regulated from 13.5 to 136.3 nN. Further experimental data and theoretical analysis show that the adhesion force between the coated tip and films possesses a positive correlation with the "tip-film unanimity" in composition.

Lateral Monolayer MoSe2 -WSe2 p-n Heterojunctions with Giant Built-In Potentials.

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p-n junctions, 2D p-n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light-emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high-quality p-n heterojunctions between molybdenum diselenide (MoSe2 ) and tungsten diselenide (WSe2 ) by one-step chemical vapor deposition method are reported. These p-n heterojunctions exhibit high built-in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self-powered photodetectors. The simple one-step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.

Excitons in strain-induced one-dimensional moiré potentials at transition metal dichalcogenide heterojunctions.

The possibility of confining interlayer excitons in interfacial moiré patterns has recently gained attention as a strategy to form ordered arrays of zero-dimensional quantum emitters and topological superlattices in transition metal dichalcogenide heterostructures. Strain is expected to play an important role in the modulation of the moiré potential landscape, tuning the array of quantum dot-like zero-dimensional traps into parallel stripes of one-dimensional quantum wires. Here, we present real-space imaging of unstrained zero-dimensional and strain-induced one-dimensional moiré patterns along with photoluminescence measurements of the corresponding excitonic emission from WSe2/MoSe2 heterobilayers. Whereas excitons in zero-dimensional moiré traps display quantum emitter-like sharp photoluminescence peaks with circular polarization, the photoluminescence emission from excitons in one-dimensional moiré potentials shows linear polarization and two orders of magnitude higher intensity. These results establish strain engineering as an effective method to tailor moiré potentials and their optoelectronic response on demand.

Photoinduced charge transfer in transition metal dichalcogenide heterojunctions – towards next generation energy technologies

We review recent strides in understanding and manipulating photoinduced charge transfer in heterojunctions between 2D transition metal dichalcogenides and other semiconductors, with implications for a broad array of energy applications.

Direct Synthesis of a Self-Assembled WSe2 /MoS2 Heterostructure Array and its Optoelectrical Properties.

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next-generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self-assembled WSe2 /MoS2 heterostructures through facile solution-based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe2 wires are selectively stacked over the MoS2 wires at a specific angle, which enables the formation of parallel- and cross-aligned heterostructures. The realized WSe2 /MoS2 -based p-n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W-1 and response speed of 16 µs. As a feasible application, a WSe2 /MoS2 -based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution-based growth of hierarchical structures with various alignments could offer a method for the further development of large-area electronic and optoelectronic applications.

Metal‐Guided Selective Growth of 2D Materials: Demonstration of a Bottom‐Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe2 and n-type MoSe2 is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

Lateral and vertical heterostructures in two-dimensional transition-metal dichalcogenides [Invited

Heterostructures (HSs) of two-dimensional (2D) transition-metal dichalcogenides (TMDs) offer a plethora of opportunities in materials science, condensed-matter physics, and device engineering. The out-of-plane van der Waals interaction of 2D TMDs with surrounding environments enables the synthesis of HSs on virtually any substrate. This unmatched quality gives TMD HSs a superior edge in applications such as flexible optoelectronics in which III-V HSs are still struggling with lattice-mismatch issues. 2D TMDs can be vertically stacked or lateral stitched to form vertical (i.e., out-of-plane) or lateral (i.e., in-plane) heterojunctions, respectively. Motivated by the critical impact of synthesis methods on the progress of this field, in this article, we have reviewed the state-of-the-art synthesis techniques employed for the creation of lateral and vertical junctions between heterogenous TMD films. At the end of this article, we have also briefly reviewed the spectroscopic characterization of TMD heterojunctions.

High performance tunnel field effect transistors based on in-plane transition metal dichalcogenide heterojunctions

In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green’s functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e. WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analyzed. Our results indicate that, by fine-tuning the design parameters, those devices can yield extremely low subthreshold swings (<5 mV/decade) and ION/IOFF ratios higher than 108 at a supply voltage of 0.3 V, making them ideal for ultra-low power consumption.

Spin polarized vertical transport in stacked TMDCs hetero-junctions

Electronic and transport properties of stacked metal/semiconductor and metal/metal heterojunctions of transition metal dichalcogenides (TMDCs) are studied in this paper. MoS2, MoSe2, WS2, and WSe2 are considered for the semiconducting material, while NbS2, NbSe2, CoS2, and CoSe2 are considered for the metallic contact. Our density functional theory-based calculations reveal a strongly spin-polarized conductance for the studied systems. Furthermore, our results predict that the band gap of the semiconducting TMDC shifts above the Fermi energy of the connected system, leading to a diode-like semiconductor/metal contact. Moreover, the quantum conductance calculations for these junctions, carried out within the Landauer–Büttiker formalism for ballistic transport, shows that even the metal/metal heterojunctions have a diode-like character near the Fermi level. This interesting behaviour is discussed in terms of the misalignment between electronic states on each of the materials comprising the junction.

Band-offset-induced lateral shift of valley electrons in ferromagnetic MoS2/WS2 planar heterojunctions

Low-energy coherent transport and a Goos-Hänchen (GH) lateral shift of valley electrons in planar heterojunctions composed of normal MoS2 and ferromagnetic WS2 monolayers are theoretically investigated. Two types of heterojunctions in the forms of WS2/MoS2/WS2 (type-A) and MoS2/WS2/MoS2 (type-B) with incident electrons in the MoS2 region are considered in which the lateral shift of electrons is induced by band alignments of the two constituent semiconductors. It is shown that the type-A heterojunction can act as an electron waveguide due to electron confinement between the two WS2/MoS2 interfaces which cause the incident electrons with an appropriate incidence angle to propagate along the interfaces. In this case, the spin- and valley-dependent GH shifts of totally reflected electrons from the interface lead to separated electrons with distinct spin-valley indexes after traveling a sufficiently long distance. In the type-B heterojunction, however, transmission resonances occur for incident electron beams passing through the structure, and large spin- and valley-dependent lateral shift values in propagating states can be achieved. Consequently, the transmitted electrons are spatially well-separated into electrons with distinct spin-valley indexes. Our findings reveal that the planar heterojunctions of transition metal dichalcogenides can be utilized as spin-valley beam filters and/or splitters without external gating.

Strain distributions and their influence on electronic structures of WSe2-MoS2 laterally strained heterojunctions.

Monolayer transition metal dichalcogenide heterojunctions, including vertical and lateral p-n junctions, have attracted considerable attention due to their potential applications in electronics and optoelectronics. Lattice-misfit strain in atomically abrupt lateral heterojunctions, such as WSe2-MoS2, offers a new band-engineering strategy for tailoring their electronic properties. However, this approach requires an understanding of the strain distribution and its effect on band alignment. Here, we study a WSe2-MoS2 lateral heterojunction using scanning tunnelling microscopy and image its moiré pattern to map the full two-dimensional strain tensor with high spatial resolution. Using scanning tunnelling spectroscopy, we measure both the strain and the band alignment of the WSe2-MoS2 lateral heterojunction. We find that the misfit strain induces type II to type I band alignment transformation. Scanning transmission electron microscopy reveals the dislocations at the interface that partially relieve the strain. Finally, we observe a distinctive electronic structure at the interface due to hetero-bonding.

Scalable and Self-Aligned Growth of Two-Dimensional Transition Metal Dichalcogenides Lateral Heterojunctions for Optoelectronic Applications

Recent development in atomically thin layered materials has provided a new possibility for studying two-dimensional (2D) physics. Among them, the monolayer transition metal dichalcogenides (TMDs) with direct bandgaps in the near-infrared to the visible region and with unique electrical and mechanical properties have attracted great research interests and have shown the application potential in optoelectronics. Furthermore, the TMDs heterojunction not only can form the p-n junction as the basic building block for logic circuits, also can provide a new route for band engineering which is of scientific and technology important for exploring next generation electronics. However, the difficulty on precise spatial and geometry control during synthesis or preparation is still an obstacle for 2D materials to integrate into real circuit applications. Herein, we demonstrate a simple spatially controlled process for synthesizing self-aligned monolayer lateral TMDs heterojunctions array in centimeter scale. The selective growth from metal pad can help on in-situ location and geometry control for forming the heterojunction devices without patterning process. Meanwhile, this etching-free growth method provides a clean and high quality lateral junction. Our WSe2-MoS2 p-n junction exhibits clear current rectification effect and electroluminescence (EL) from the interface under forward bias > 3 V. Detailed EL mechanism shall be discussed. Our self-aligned growth can be easy applied in large scale device fabrication or integrate into circuit design for achieving optoelectronic applications, and also provide a better platform for fundamental studies.