Transition Metal Dichalcogenide Heterostructures Research Articles

Interface engineered WS2/ZnS heterostructures for sensitive and reversible NO2 room temperature sensing

The inherent high surface areas and unique semiconducting properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) make them attractive for the sensing applications. However, the challenges of sensitivity, recoverability and stability still need to be overcome for the rapidly developing fields of TMD based gas sensors. With the general advantages of 2D nanomaterials, construction of 2D TMD based heterostructure is an effective approach for improving their sensing performance by the synergistic effects of hybridization. Herein, we design a novel tungsten disulfide (WS2)/zinc sulfide (ZnS) based TMD heterostructure to realize quick and reversible nitrogen dioxide (NO2) detection at room temperature (25 °C). Specifically, the engineered 2D/0D WS2/ZnS p–n heterostructure gives a desired response of 32.5 to 5 ppm NO2 with a fast response speed of 4 s, which is dramatically enhanced compared with that of pure WS2 nanosheets. More importantly, WS2/ZnS heterostructure based gas sensor can achieve full recovery state after each sensing cycle without using external energies, showing an excellent recyclability at room temperature. The optimal sensor also exhibits low detection concentration of 10 ppb, high selectivity as well as outstanding long-term stability. Such impressive features originating from the synergistic effects of WS2/ZnS heterostructure have been discussed in detail from the geometrical, electronic and chemical effects. These outstanding sensing properties of WS2/ZnS heterostructure sensor imply the possibility of using this novel nanostructure in highly efficient NO2 room temperature sensing applications.

Strain-Induced Metastable Topological Networks in Laser-Fabricated TaS2 Polytype Heterostructures for Nanoscale Devices.

The stacking of layered materials into heterostructures offers diverse possibilities for generating deformed moiré states arising from their mutual interaction. Here we report self-assembled two-dimensional nanoscale strain networks formed within a single prismatic (H) polytype monolayer of TaS2 created in situ on the surface of an orthorhombic 1T-TaS2 single crystal by a low-temperature laser-induced polytype transformation. The networks revealed by scanning tunneling microscopy (STM) take on diverse configurations at different temperatures, including extensive double stripes and a twisted 3-gonal mesh of connected 6-pronged vertices. The resulting phase diagram can be understood to be a consequence of thermally driven minimization of discommensurations between the H and 1T layers. Nontrivial dislocation defects of embedded 2- and 4-gonal structures are shown to be associated with local inhomogeneous strains. The creation of metastable heterostructures by laser quench at cryogenic temperatures in combination with STM manipulation of local strain demonstrates nanoscale control of topological defects in transition metal dichalcogenide heterostructures may be utilized in the fabrication of nanoscale electronic devices and neural networks.

Rational Kinetics Control toward Universal Growth of 2D Vertically Stacked Heterostructures.

The rational control of the nucleation and growth kinetics to enable the growth of 2D vertical heterostructure remains a great challenge. Here, an in-depth study is provided toward understanding the growth mechanism of transition metal dichalcogenides (TMDCs) vertical heterostructures in terms of the nucleation and kinetics, where active clusters with a high diffusion barrier will induce the nucleation on top of the TMDC templates to realize vertical heterostructures. Based on this mechanism, in the experiment, through rational control of the metal/chalcogenide ratio in the vapor precursors, effective manipulation of the diffusion barrier of the active clusters and precise control of the heteroepitaxy direction are realized. In this way, a family of vertical TMDCs heterostructures is successfully designed. Optical studies and scanning transmission electron microscopy investigations exhibit that the resulting heterostructures possess atomic sharp interfaces without apparent alloying and defects. This study provides a deep understanding regarding the growth mechanism in terms of the nucleation and kinetics and the robust growth of 2D vertical heterostructures, defining a versatile material platform for fundamental studies and potential device applications.

Tunable Control of Interlayer Excitons in WS2/MoS2 Heterostructures via Strong Coupling with Enhanced Mie Resonances.

Strong Coulomb interactions in monolayer transition metal dichalcogenides (TMDs) produce strongly bound excitons, trions, and biexcitons. The existence of multiexcitonic states has drawn tremendous attention because of its promising applications in quantum information. Combining different monolayer TMDs into van der Waals (vdW) heterostructures opens up opportunities to engineer exciton devices and bring new phenomena. Spatially separated electrons and holes in different layers produce interlayer excitons. Although much progress has been made on excitons in single layers, how interlayer excitons contribute the photoluminescence emission and how to tailor the interlayer exciton emission have not been well understood. Here, room temperature strong coupling between interlayer excitons in the WS2/MoS2 vdW heterostructure and cavity‐enhanced Mie resonances in individual silicon nanoparticles (Si NPs) are demonstrated. The heterostructures are inserted into a Si film‐Si NP all‐dielectric platform to realize effective energy exchanges and Rabi oscillations. Besides mode splitting in scattering, tunable interlayer excitonic emission is also observed. The results make it possible to design TMDs heterostructures with various excitonic states for future photonics devices.

Electron-Driven In Situ Transmission Electron Microscopy of 2D Transition Metal Dichalcogenides and Their 2D Heterostructures.

Investigations on monolayered transition metal dichalcogenides (TMDs) and TMD heterostructures have been steadily increasing over the past years due to their potential application in a wide variety of fields such as microelectronics, sensors, batteries, solar cells, and supercapacitors, among others. The present work focuses on the characterization of TMDs using transmission electron microscopy, which allows not only static atomic resolution but also investigations into the dynamic behavior of atoms within such materials. Herein, we present a body of recent research from the various techniques available in the transmission electron microscope to structurally and analytically characterize layered TMDs and briefly compare the advantages of TEM with other characterization techniques. Whereas both static and dynamic aspects are presented, special emphasis is given to studies on the electron-driven in situ dynamic aspects of these materials while under investigation in a transmission electron microscope. The collection of the presented results points to a future prospect where electron-driven nanomanipulation may be routinely used not only in the understanding of fundamental properties of TMDs but also in the electron beam engineering of nanocircuits and nanodevices.

Band Alignment of MoTe2/MoS2 Nanocomposite Films for Enhanced Nonlinear Optical Performance

AbstractBand alignment is a key issue for the optoelectronics based on 2D layered transition metal dichalcogenides (TMDs) heterostructures. Herein, band alignment of MoTe2/MoS2 mixed heterostructure is measured with high‐resolution X‐ray photoelectron spectroscopy. The MoTe2/MoS2 heterostructure belongs to type‐II heterostructure with the conduction band offset of 0.46 eV and the valence band offset of 0.9 eV. The stronger saturable absorption is observed in MoTe2/MoS2 heterostructure film compared with that of pure MoTe2 and MoS2 nanofilms at the same condition. An energy‐level model combined with Runge–Kutta algorithm is used to understand the enhancement mechanism. It is found that the interlayer transition from MoTe2/MoS2 heterojunction plays an important role in the nonlinear optical enhancement. Meanwhile, band structure of MoTe2/MoS2 heterostructure is calculated by the first principles. The contributions of the MoTe2 and MoS2 to the heterojunction are almost equal and the valence band maximum and conduction band minimum exist in MoTe2 and MoS2 separately. This structure can form the interlayer carriers easily. The results suggest that the band alignment of TMDs paves the way for the type‐II heterostructure for enhanced nonlinear response in the development of optical modulator, ultrafast laser mode lockers, saturable absorbers, and optical switches.

Sputtering and Electron Beam Irradiation of WS2/MoS2and MoS2/WS2Heterostructures for Enhanced Photoresponsivity

In the present study, we proposed heterostructures of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> formed by radio frequency (RF) magnetron sputtering and electron beam irradiation (EBI) for obtaining enhanced photoresponsivity. Each amorphous transition metal dichalcogenide (TMD) was sequentially deposited and EBI was performed on the as-deposited TMD heterostructures. After EBI, the atomic rearrangement of the heterostructures was studied. Moreover, the depth profile of the synthesized heterostructures was investigated using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). We achieved a photoresponsivity of 5.1 A/W in EBI-treated TMD heterostructures, which was further enhanced by three orders as compared with that obtained in EBI-treated TMD single structures.

Toward heterostructured transition metal hybrids with highly promoted electrochemical hydrogen evolution.

It is essential to precisely develop low-cost and sustainable electrocatalysts for the hydrogen evolution reaction. Herein, we explore a robust and controllable hydrothermal approach to synthesize defect-rich MoS2 nanoflakes on exfoliated MoS2 and WS2. Such well-designed hetero-structural hybrids of MoS2/exfoliated MoS2 and MoS2/exfoliated WS2 exhibit dramatically promoted electrochemical activity and high stability. The as-grown MoS2 nanoflakes hybridized on exfoliated MoS2 and WS2 generate abundant active edge sites (rich in basal defects) and unsaturated sulfur atoms, resulting in highly enhanced electrocatalytic performance. This is expected to pave the way towards a significant improvement in transition metal dichalcogenide heterostructures as electrocatalysts.

Modulation of the electronic properties of two-dimensional MoTe2/WSe2 heterostructure by electrical field

The structure and electronic properties of two-dimensional MoTe2/WSe2 van der Waals heterostructure under external electric field have been investigated by first-principles calculation. The interlayer distance between MoTe2 and WSe2 is 3.613 Å and the binding energy per unit is −0.183eV, which indicated that MoTe2 is bound to WSe2 via van der Waals interaction. The interlayer distance can be modified by external electric field and the band gap of MoTe2/WSe2 heterostructure continuously decreases with increasing external electric field, eventually a transition from semiconductor to metal is observed, particularly, the band alignment of the MoTe2/WSe2 heterostructure can be effectively tuned from intrinsic type-II to type-I. Applying external electric field along +z direction and −z direction has different effects on the band gap due to the intrinsic spontaneous polarization in MoTe2/WSe2 heterostructure. Our study indicates that the external electric field can significantly tune the band offsets and modify the band alignment between MoTe2 and WSe2. The present study would be helpful for application of such transition-metal dichalcogenides heterostructures in nano- and optoelectronics.

Electrically and Optically Tunable Responses in Graphene/Transition-Metal-Dichalcogenide Heterostructures.

Heterostructures involving layered two-dimensional (2D) transitionmetal dichalcogenides (TMDCs) are not only fundamentally interesting to explore emerging properties at atomically thin limit, but also technically important to achieve novel optoelectronic devices. However, achieving tunable optoelectronic properties and clarifying interlayer processes (charge transfer, energy transfer) in 2D heterostructures have remained part of the key challenges so far. Here, by fabricating heterostructures of graphene and monolayer TMDCs (n-type MoS2 and p-type WSe2), we demonstrate both electrically and optically tunable responses of the heterostructures, revealing the critical interface processes between graphene and TMDCs. In MoS2/graphene heterostructures, electron transfer from MoS2 to graphene is observed, and gate-tunable interface relaxation induces the electrically controlled photoluminescence (PL), whereas in WSe2/graphene heterostructures, electron transfer from graphene to WSe2 is observed, and the PL is tuned by carrier density, which can be controlled by the gate voltage. The interlayer process can also be modulated by laser intensity, which enables photoinduced doping on graphene and optically tunable electrical characteristics of graphene. Combining the tunable Fermi level of graphene and strong light-matter interaction of monolayer TMDCs, our demonstrations are important for the design of multifunctional and efficient optoelectronic devices with TMDC/graphene heterostructures.

Moiré Intralayer Excitons in a MoSe2/MoS2 Heterostructure.

Spatially periodic structures with a long-range period, referred to as a moiré pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra- and interlayer excitons of transition metal dichalcogenide heterostructures. Here, we report on the impact of the moiré pattern formed in a MoSe2/MoS2 heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe2 exciton and trion inemission and (for the exciton)absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.

Ultrafast dynamics in van der Waals heterostructures.

Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-to-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this Review, we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of long-lived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from timescales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic, valleytronic and spintronic devices.

Two-dimensional MoS2-MoSe2 lateral superlattice with minimized lattice thermal conductivity

Single-layer transition metal dichalcogenides (TMDCs) are showing promising thermoelectric applications due to their superior stability and electronic transport properties. Unfortunately, the intrinsic high lattice thermal conductivity prevents their further improvement of thermoelectric performance. Motivated by recent experimental synthesis of two-dimensional TMDC heterostructures and superlattices, we propose to minimize the lattice thermal conductivity of single-layer MoS2 and MoSe2 using the lateral superlattice (LS) as building blocks. First-principles calculations with the phonon Boltzmann transport equation reveal a remarkably low lattice thermal conductivity of MoS2-MoSe2 LS due to the enhanced anharmonic phonon scattering as compared to the individual single-layer. We also show that the strong phonon anisotropy of MoS2-MoSe2 LS is primarily ascribed to the out-of-plane quadratic acoustic branch and orientation-dependent anharmonic scattering. Our calculations clearly demonstrate the advantages of LS structure in minimizing the lattice thermal conductivity of single-layer TMDCs and also accelerate their related applications in the field of renewable energy.

Controllable epitaxial growth of MoSe2–MoS2 lateral heterostructures with tunable electrostatic properties

Recently, two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have attracted much attention due to their promising applications in the fields of electronics and optoelectronics. Controllable growth of TMDC heterostructures stimulates new interest by tuning their optical and electronic properties. Herein, large-scale lateral MoSe2–MoS2 and MoSe2(1−x)S2x–MoS2 heterostructures have been synthesized through one-step epitaxial ambient-pressure chemical vapor deposition method and we found that the growth time plays an important role in the formation of lateral heterostructures. Lateral MoSe2–MoS2 heterostructures have been systematically characterized by using atomic force microscopy, Raman spectroscopy and photoluminescence spectroscopy. Corresponding surface potential and charge distributions of MoSe2–MoS2 heterostructures have been investigated by employing Kelvin probe force microscopy. We found that the electrostatic properties of MoSe2–MoS2 heterostructures can be effectively tuned by injecting charges through conductive atomic force microscopy. Our results pave a new route for constructing 2D lateral heterostructures toward electronic and optoelectronic applications.

Performance Enhancement of SPR Biosensor Based on Phosphorene and Transition Metal Dichalcogenides for Sensing DNA Hybridization

Two-dimensional (2-D) materials have attracted a lot of attention for using in surface plasmon resonance (SPR) biosensors. High sensitivity and figure of merit are two desirable parameters in sensor performance analysis. Using heterostructures of phosphorene, graphene, and transition metal dichalcogenides is proposed to enhance the sensitivity of biosensor. Although the proposed biosensor can be used for detection of different analytes with a wide refractive index range, DNA hybridization is the sensing target in this paper. Sensing parameters of different biosensor configurations based on a few-layer black phosphorus (BP) and other 2-D materials are compared with the conventional Au-based SPR sensor and the highest sensitivity of 187°/RIU for the structure consists of 10-layer BP and monolayer WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is obtained. The suggested sensor structure provides the sensitivity more than two times of the conventional biosensor. This high sensor performance may have potential applications in medical diagnosis and biochemical detection.

Interlayer excitons in transition metal dichalcogenide heterostructures

Starting from the single-particle Dirac Hamiltonian for charge carriers in monolayer transition metal dichalcogenides (TMDs), we construct a four-band Hamiltonian describing interlayer excitons consisting of an electron in one TMD layer and a hole in the other TMD layer. An expression for the electron-hole interaction potential is derived, taking into account the effect of the dielectric environment above, below, and between the two TMD layers as well as polarization effects in the transition metal layer and in the chalcogen layers of the TMD layers. We calculate the interlayer exciton binding energy and average in-plane interparticle distance for different TMD heterostructures. The effect of different dielectric environments on the exciton binding energy is investigated and a remarkable dependence on the dielectric constant of the barrier between the two layers is found, resulting from competing effects as a function of the in-plane and out-of-plane dielectric constants of the barrier. The polarization effects in the chalcogen layers, which in general reduce the exciton binding energy, can lead to an increase in binding energy in the presence of strong substrate effects by screening the substrate. The excitonic absorbance spectrum is calculated and we show that the interlayer exciton peak depends linearly on a perpendicular electric field, which agrees with recent experimental results.

Synthesis of transition metal dichalcogenides and their heterostructures

As compared to the heterostructures based on bulk materials, the heterostructures based on transition metal dichalcogenides (TMDCs) have the advantages of atomically sharp hetero-interface and freedom from lattice-match restriction, which make them promising for nanoscale electronic and photonic devices. In this paper, we systematically investigate the synthesis of TMDC heterostructures using two-step processes via chemical vapor deposition (CVD). We found that if a MoS2 growth at 730 °C is followed by a WSe2 growth at 875 °C, the selenium atoms in the precursor will replace the sulfur atoms in MoS2 during the second step and form MoSe2/WSe2 lateral heterostructures. On the other hand, if WSe2 growth is followed by MoS2 growth, WSe2/MoS2 lateral heterostructures were formed and no ion-exchange was observed. This result indicates that in the two-step processes, low-temperature growth followed by high-temperature growth may result in ion-exchange in the first material, while high-temperature growth followed by low-temperature growth results in epitaxy growth along the edge without ion-exchange. In addition, we have investigated the synthesis of MoS2 and WSe2 on sapphire substrates. In contrast to MoS2 grown on SiO2/Si substrates where the domains are randomly oriented, the domain edges of MoS2 grown on sapphire substrates are mostly aligned to the primary directions, which can significantly reduce the amount of grain boundaries when the domains are merged together to form continuous films. This finding will be critical to enable the fabrication of large-scale electronic devices with high-performance and uniform distributions across the wafers based on these materials.

Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation.

Hybrid quantum dot (QD)/transition metal dichalcogenide (TMD) heterostructures are attractive components of next generation optoelectronic devices, which take advantage of the spectral tunability of QDs and the charge and exciton transport properties of TMDs. Here, we demonstrate tunable electronic coupling between CdSe QDs and monolayer WS2 using variable length alkanethiol ligands on the QD surface. Using femtosecond time-resolved second harmonic generation (SHG) microscopy, we show that electron transfer from photoexcited CdSe QDs to single-layer WS2 occurs on ultrafast (50 fs to 1 ps) time scales. Moreover, in the samples exhibiting the fastest charge transfer rates (≤50 fs) we observed oscillations in the time-domain signal corresponding to an acoustic phonon mode of the donor QD, which coherently modulates the SHG response of the underlying WS2 layer. These results reveal surprisingly strong electronic coupling at the QD/TMD interface and demonstrate the usefulness of time-resolved SHG for exploring ultrafast electronic-vibrational dynamics in TMD heterostructures.

Surface Engineering of Two‐Dimensional Materials

AbstractTwo‐dimensional (2D) materials have attracted intensive interest because of their unique optical and electronic properties, their potential applications in next generation electronic devices, as well as serving as an ideal platform to explore new physics or chemistry at the 2D limit. As an atomically thin layered material, their atomic/electronic structures and device performances are susceptible to the various surfaces presented. In this regard, plenty of surface modification approaches have been applied to enrich their functionalities and complement their limitations. In this review, we summarize the state‐of‐the‐art progress on the surface engineering of graphene and transition metal dichalcogenides (TMDCs). We begin with the defect and single‐atomic incorporation in graphene lattice and surface transfer doping of graphene, followed by the bandgap and mobility engineering, and the construction of versatile heterostructures of TMDCs. Although this review mainly focuses on graphene and TMDCs, it is not an exhaustive list of the experimental results, but it rather summarizes the established knowledge in surface engineering and pinpoints the most promising future trends. These surface engineering approaches can also be applied to other 2D materials, such as phosphorene and antimonene.

Morphology Engineering in Monolayer MoS2‐WS2 Lateral Heterostructures

AbstractIn recent years, heterostructures formed in transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties beyond the individual components. Atomically thin TMD heterostructures, such as MoS2‐WS2, MoS2‐MoSe2, MoS2‐WSe2, and WSe2‐WS2, are synthesized so far via chemical vapor deposition (CVD) method. Engineering the morphology of domains including size and shape, however, still remains challenging. Here, a one‐step CVD strategy on the morphology engineering of MoS2 and WS2 domains within the monolayer MoS2‐WS2 lateral heterostructures through controlling the weight ratio of precursors, MoO3 and WO3, as well as tuning the reaction temperature is reported. Not only can the size ratio in terms of area between WS2 and MoS2 domains be easily controlled from less than 1 to more than 20, but also the overall heterostructure size can be tuned from several to hundreds of micrometers. Intriguingly, the quantum well structure, a WS2 stripe embedded in the MoS2 matrix, is also observed in the as‐synthesized heterostructures, offering opportunities to study quantum confinement effects and quantum well applications. This approach paves the way for the large‐scale fabrication of MoS2‐WS2 lateral heterostructures with controllable domain morphology, and shall be readily extended to morphology engineering of other TMD heterostructures.