Transition Metal Dichalcogenide Heterostructures Research Articles

Efficient photoelectrodes based on two-dimensional transition metal dichalcogenides heterostructures: from design to construction

Efficient photoelectrodes based on two-dimensional transition metal dichalcogenides heterostructures: from design to construction

Physical properties of novel electronic states related to flat band in twisted two-dimensional quantum materials

Two-dimensional (2D) materials can exhibit novel quantum phenomena and be easily tuned by the external environment, which has made them one of the most attractive topics in condensed matter physics during the recent decades. The moiré superlattice induced by varied stacking geometry can further renormalize the material band structure, resulting in the electronic flat bands. With the help of external fields, one can tune the electron-electron correlated interaction in these flat bands, even control the overall physical properties. In this paper we review the recent researches of novel properties in twisted 2D materials (graphene and transition metal dichalcogenide heterostructure), involving strong correlation effect, unconventional superconductivity, quantum anomalous Hall effect, topological phase, and electronic crystals. We also discuss some open questions and give further prospects in this field.

A rigorous computational framework employing coupled-mode theory for assessing lasing with transition metal dichalcogenide bilayers in the nanoscale

A whispering-gallery mode nanophotonic laser cavity having as active medium a transition-metaldichalcogenide (TMD) bilayer is examined. The proposed system is analysed and designed utilizing a strict and rigorous computational framework based on the coupled-mode theory. Our framework is capable of accurately and efficiently handling the gain properties of two-dimensional materials, such as contemporary TMD monolayers, multilayers, and heterostructures. The presented lasing cavity exhibits an adequately low pump threshold and light emission in the order of milliwatts is predicted. Exploiting the capabilities of the developed framework, we were in position to efficiently design the cavity as well as to estimate quantitative lasing parameters such as the pumping threshold and the lasing frequency.

Unveiling the layer-dependent electronic properties in transition-metal dichalcogenide heterostructures assisted by machine learning.

The electronic properties of layered two-dimensional (2D) transition-metal dichalcogenide (TMD) van der Waals (vdW) heterostructures are strongly dependent on their layer number (N). However, extremely large computational resources are required to investigate the layer-dependent TMD vdW heterostructures for every possible combination if N varies in a large range. Fortunately, the machine learning (ML) technique provides a feasible way to probe this problem. In this work, based on the density functional theory (DFT) calculations combined with the ML technique, we effectively predict the layer-dependent electronic properties of TMD vdW heterostructures composed of MoS2, WS2, MoSe2, WSe2, MoTe2, or WTe2, in which the layer number varies from 2-10. The cross-validation scores of our trained ML models in predicting the bandgaps as well as the band edge positions exceed 90%, suggesting excellent performance. The predicted results show that in the case of a few-layer system, the number of layers has a significant effect on the electronic properties. The bandgap and band alignment could be dramatically changed from bilayer to triple-layer heterostructures. However, with the increase of the number of layers, the electronic properties change, and some general trends can be summarized. When the layer number is larger than 8, the properties of the TMD heterostructures tend to be stable, and the influence of the layer number decreases. Based on these results, our work not only sheds light on the understanding of the layer-dependent electronic properties of multi-layer TMD vdW heterostructures, but also provides an efficient way to accelerate the discovery of functional materials.

Lateral transition-metal dichalcogenide heterostructures for high efficiency thermoelectric devices.

Monolayer transition-metal dichalcogenides (TMDC) have emerged as promising candidates for thermoelectric applications due to their large effective mass and low thermal conductivity. In this article, we study the thermoelectric performance of lateral TMDC heterostructures within a multiscale quantum transport framework. Both n-type and p-type lateral heterostructures are considered for all possible combinations of semiconducting TMDCs: MoS2, MoSe2, WS2, and WSe2. The band alignment between these materials is found to play a crucial role in enhancing the thermoelectric figure-of-merit (ZT) and power factor far beyond those of pristine TMDCs. In particular, we show that the room-temperature ZT value of n-type WS2 with WSe2 triangular inclusions is five times larger than that of the pristine WS2 monolayer. We also show that p-type MoSe2 with WSe2 inclusions has a room-temperature ZT value which is two times larger than that of the pristine MoSe2 monolayer. The peak power factor values, calculated here, are the highest reported amongst gapped monolayers at room temperature. Hence, monolayer lateral TMDC heterostructures open new avenues to develop ultra-efficient, planar thermoelectric devices.

Density functional theory study of graphene adhesion on WX2 (X = S and Se) monolayer: Role of atom vacancy and atomic reorganization defects

AbstractDefects usually play an important role in the modification of the properties of materials. In this investigation, atom vacancy and atomic reorganization defects in various heterostructures obtained using different pristine (or defect‐free) and defective transition metal dichalcogenides (TMDCs) with pristine and defective graphene have been studied using density functional theory (DFT) calculation. Results reveal that: (i) the contact of pristine and defective graphene with various pristine and defective TMDCs is energetically stable, (ii) the stability of these heterostructures driven by dispersion interaction, (iii) the presence of defect significantly influences the work function of the resulting heterostructure, (iv) the pristine graphene/pristine TMDCs heterostructures are metallic in nature with large Schottky barrier (ΦSBH), (v) the heterostructures involving defective graphene are direct band gap semiconductors, (vi) the heterostructures involving defective WS2 are also direct band gap semiconductors, and (vii) the SW defective graphene with pristine WS2/WSe2 forms type‐II heterojunction.

Coherent exciton-exciton interactions and exciton dynamics in a MoSe2/WSe2 heterostructure

Coherent coupling between excitons is at the heart of many-body interactions with transition metal dichalcogenide (TMD) heterostructures as an emergent platform for the investigation of these interactions. We employ multi-dimensional coherent spectroscopy on monolayer MoSe\textsubscript{2}/WSe\textsubscript{2} heterostructures and observe coherent coupling between excitons spatially localized in monolayer MoSe$_2$ and WSe$_2$. Through many-body spectroscopy, we further observe the absorption state arising from free interlayer electron-hole pairs. This observation yields a spectroscopic measurement of the interlayer exciton binding energy of about 250 meV.

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.

Trapping interlayer excitons in van der Waals heterostructures by potential arrays

Transition metal dichalcogenide heterostructures can host interlayer excitons (IXs), which consist of electrons and holes spatially separated in different layers. IXs possess permanent dipoles and have proven to offer a wealth of novel physics. We develop a discrete, random-walk model which includes annihilation and repulsion interactions among IXs. Using this model, we simulate the trapping of IXs in traps of different depths, densities, and shapes. Our results show that dipole-dipole interactions play an important role in regulating IX trapping. The effects of dipole interactions can be mitigated with small, deep traps which are realizable with atomic defects and moire potentials.

Voltage-controlled long-range propagation of indirect excitons in a van der Waals heterostructure

Indirect excitons (IXs), also known as interlayer excitons, can form the medium for excitonic devices whose operation is based on controlled propagation of excitons. A proof of principle for excitonic devices was demonstrated in GaAs heterostructures where the operation of excitonic devices is limited to low temperatures. IXs in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by high binding energies making IXs robust at room temperature and offering an opportunity to create excitonic devices operating at high temperatures suitable for applications. However, a characteristic feature of TMD heterostructures is the presence of moir\'e superlattice potentials, which are predicted to cause modulations of IX energy reaching tens of meV. These in-plane energy landscapes can lead to IX localization, making IX propagation fundamentally different in TMD and GaAs heterostructures and making uncertain if long-range IX propagation can be realized in TMD heterostructures. In this work, we realize long-range IX propagation with the $1/e$ IX luminescence decay distances reaching 13 microns in a MoSe$_2$/WSe$_2$ heterostructure. We trace the IX luminescence along the IX propagation path. We also realize control of the long-range IX propagation: the IX luminescence signal in the drain of an excitonic transistor is controlled within 40 times by gate voltage. These data show that the long-range IX propagation is possible in TMD heterostructures with the predicted moir\'e superlattice potentials.

Enhanced Emission from Interlayer Excitons Coupled to Plasmonic Gap Cavities.

The emergence of interlayer excitons (IEs) from atomic layered transition metal dichalcogenides (TMDCs) heterostructures has drawn tremendous attention due to their unique and exotic optoelectronic properties. Coupling the IEs into optical cavities provides distinctive electromagnetic environments which plays an important role in controlling multiple optical processes such as optical nonlinear generation or photoluminescence enhancement. Here, the integration of IEs in TMDCs into plasmonic nanocavities based on a nanocube on a metallic mirror is reported. Spectroscopic studies reveal an order of magnitude enhancement of the IE at room temperature and a 5-time enhancement in fluorescence at cryogenic temperatures. Cavity modeling reveals that the enhancement of the emission is attributed to both increased excitation efficiency and Purcell effect from the cavity. The results show a novel method to control the excitonic processes in TMDC heterostructures to build high performance photonics and optoelectronics devices.

Enhanced out-of-plane piezoelectric effect in In2Se3 /transition metal dichalcogenide heterostructures

The two-dimensional material $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, which possesses both out-of-plane ferroelectricity and piezoelectricity in monolayer, opens opportunity for the smart material integration in a micro-electro-mechanical system. However, the piezoelectric response decreases with the decreasing thickness of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, hindering the application of few-layer $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ in electromechanical transformation. In this work, we report a strategy to enhance the out-of-plane piezoelectric coefficients by constructing ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$/transition metal dichalcogenides (TMDs) heterostructures. Such a strategy shows great piezoelectric enhancement compared with monolayer ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, and the largest ${e}_{31}$ of $2.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{C}/\mathrm{m}$ is achieved in $\mathrm{Mo}{\mathrm{S}}_{2}/{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$. A phenomenological model is further constructed to connect the piezoelectric coefficients with the polarizations of the heterostructures. Furthermore, the induced dipole moment strongly affects the band structure of heterostructures, yielding tunable light absorption. The proposed ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$/TMDs heterostructures represent an innovative strategy for the development of future efficient piezoelectric and are attractive for deployment in electromechanical systems.

Spin/Valley Coupled Dynamics of Electrons and Holes at the MoS2-MoSe2 Interface.

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure MoS2-MoSe2 to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, ∼30 and <1 ns-1, respectively, and show that this difference relates to the disparity in the spin-orbit splitting in conduction and valence bands of TMDs. Our work provides insights into the spin/valley dynamics of photoexcited carriers unaffected by complex excitonic processes and establishes TMD heterostructures as generators of spin currents in spin/valleytronic devices.

Criteria for Assessing the Interlayer Coupling of van der Waals Heterostructures Using Ultrafast Pump-Probe Photoluminescence Spectroscopy.

van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDCs) provide an excellent paradigm for next-generation electronic and optoelectronic applications. However, the reproducible fabrications of vdW heterostructure devices and the boosting of practical applications are severely hindered by their unstable performance, due to the lack of criteria to assess the interlayer coupling in heterostructures. Here we propose a physical model involving ultrafast electron transfer in the heterostructures and provide two criteria, η (the ratio of the transferred electrons to the total excited electrons) and ζ (the relative photoluminescence variation), to evaluate the interlayer coupling by considering the electron transfer in TMDC heterostructures and numerically simulating the corresponding rate equations. We have proved the effectiveness and robustness of two criteria by measuring the pump-probe photoluminescence intensity of monolayer WS2 in the WS2/WSe2 heterostructures. During thermal annealing of WS2/WSe2, ζ varies from negative to positive values and η changes between 0 and 4.5 × 10-3 as the coupling strength enhanced; both of them can well characterize the tuning of interlayer coupling. We also design a scheme to image the interlayer coupling by performing PL imaging at two time delays. Our scheme offers powerful criteria to assess the interlayer coupling in TMDC heterostructures, offering opportunities for the implementation of vdW heterostructures for broadband and high-performance electronic and optoelectronic applications.

Performance Optimization of Monolayer 1T/1T’-2H MoX2 Lateral Heterojunction Transistors

Recently, 2-D transition metal dichalcogenides (TMDs) lateral heterojunction field-effect transistors (FETs) have been demonstrated experimentally, in which metallic TMDs were used for the source/drain. In this work, we systematically investigate the contact property and device performance of monolayer 1T/1T $^{\prime }$ -2H MoS2, MoSe2, and MoTe2 FETs. Schottky barrier (SB) heights are extracted from density functional theory calculations, and nonequilibrium Green’s function transport simulations have been performed to study device characteristics. Our simulation results show that the inherent SB strongly affects the overall performance of these devices. Here, we optimize the performance of TMD lateral heterojunction FETs by using two different approaches. First, we have improved the electrostatic control by scaling equivalent oxide thickness and gate underlap, which boosts both ON- and OFF-state characteristics, making the device suitable for high-performance applications. On the other hand, moderate doping has been used in the gate underlap region, improving ON/OFF current ratio with negligible impacts on ON-state characteristics, and this approach is more preferred for low-power applications. This study reveals that 1T $^{\prime }$ -2H MoTe2 FET shows the highest ON current ( $\sim 1$ mA/ $\mu \text{m}$ ) among the three with a reasonably small subthreshold swing (80 mV/dec) if properly scaled, while 1T-2H MoS2 FET exhibits the highest ${I} _{\scriptscriptstyle{\text{ON}}}/{I} _{\scriptscriptstyle{\text{OFF}}}$ (~107) when ohmic contact is established with moderate doping in the gate underlap region. This study not only provides physical insight into the electronic devices based on novel TMD heterostructures but also suggests engineering practice for device performance optimization in experiments.

Bilayer Wigner crystals in a transition metal dichalcogenide heterostructure.

One of the first theoretically predicted manifestations of strong interactions in many-electron systems was the Wigner crystal1-3, in which electrons crystallize into a regular lattice. The crystal can melt via either thermal or quantum fluctuations4. Quantum melting of the Wigner crystal is predicted to produce exotic intermediate phases5,6 and quantum magnetism7,8 because of the intricate interplay of Coulomb interactions and kinetic energy. However, studying two-dimensional Wigner crystals in the quantum regime has often required a strong magnetic field9-11 or a moiré superlattice potential12-15, thus limiting access to the full phase diagram of the interacting electron liquid. Here we report the observation of bilayer Wigner crystals without magnetic fields or moiré potentials in an atomically thin transition metal dichalcogenide heterostructure, which consists of two MoSe2 monolayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states at symmetric (1:1) and asymmetric (3:1, 4:1 and 7:1) electron doping of the two MoSe2 layers at cryogenic temperatures. We attribute these features to bilayer Wigner crystals composed of two interlocked commensurate triangular electron lattices, stabilized by inter-layer interaction16. The Wigner crystal phases are remarkably stable, and undergo quantum and thermal melting transitions at electron densities of up to 6×1012 per square centimetre and at temperatures of up to about 40kelvin. Our results demonstrate that an atomically thin heterostructure is a highly tunable platform for realizing many-body electronic states and probing their liquid-solid and magnetic quantum phase transitions4-8,17.

Crystal phases of charged interlayer excitons in van der Waals heterostructures

Throughout the years, strongly correlated coherent states of excitons have been the subject of intense theoretical and experimental studies. This topic has recently boomed due to new emerging quantum materials such as van der Waals (vdW) bound atomically thin layers of transition metal dichalcogenides (TMDs). We analyze the collective properties of charged interlayer excitons observed recently in bilayer TMD heterostructures. We predict strongly correlated phases—crystal and Wigner crystal—that can be selectively realized with TMD bilayers of properly chosen electron-hole effective masses by just varying their interlayer separation distance. Our results can be used for nonlinear coherent control, charge transport and spinoptronics application development with quantum vdW heterostuctures.

Twisted angle modulated structural property, electronic structure and carrier transport of MoS2/AlN(0001) mixed-dimensional van der Waals heterostructure

The emerging mixed-dimensional heterostructures of transition metal dichalcogenides with wide bandgap semiconductors of piezoelectric characteristics have shown excellent performance with novel pressure-modulation in flexible devices. Meanwhile, the recent introduction of “twist” at the interface of van der Waals (vdW) systems brought much flexibility in the manipulation of electrical and optical properties, giving rise to new physics. In this work, twisted angles ranging from 0° to 150° were introduced to the interface of the newly MoS2/AlN(0001) vdW mixed-dimensional heterostructures. Although all systems form type-I band alignment, the interfacial coupling can be evidently modulated by the twisted angles, leading to twisted angle-dependent structural characteristics, electronic structure, and carrier transport with a period of 60°. The most intriguing is that a large manipulating scale of band offset and carrier transmission window (intensity) can be achieved by specifying the rotation, suggesting an effective route to design the electronic structure. It was further revealed that twisted angle modulated interfacial coupling gives rise to a diversity of resonant states’ distribution in the momentum space. Our study not only systemically investigated the newly MoS2/AlN(0001) vdW mixed-dimensional heterostructure, but also proposed a novel route to effectively manipulate the properties of mixed-dimensional heterostructure layer by layer.

Spontaneous interlayer compression in commensurately stacked van der Waals heterostructures

Interest in layered two dimensional materials, particularly stacked heterostructures of transition metal dichalcogenides, has led to the need for a better understanding of the structural and electronic changes induced by stacking. Here, we investigate the effects of idealized heterostructuring, with periodic commensurate stacking, on the structural, electronic, and vibrational properties, when compared to the counterpart bulk transition metal dichalcogenide. We find that in heterostructures with dissimilar chalcogen species there is a strong compression of the inter-layer spacing, compared to the bulk compounds. This compression of the heterostructure is caused by an increase in the strength of the induced polarization interaction between the layers, but not a full charge transfer. We argue that this effect is real, not due to the imposed commensurability, and should be observable in heterostructures combining different chalcogens. Interestingly, we find that incommensurate stacking of Ti-based dichalcogenides leads to the stabilization of the charge density wave phonon mode, which is unstable in the 1T phase at low temperature. Mixed Ti- and Zr-heterostructures are still unstable, but with a charge density wave or a ferroelectric instability.

1D p–n Junction Electronic and Optoelectronic Devices from Transition Metal Dichalcogenide Lateral Heterostructures Grown by One‐Pot Chemical Vapor Deposition Synthesis

AbstractLateral heterostructures of dissimilar monolayer transition metal dichalcogenides provide great opportunities to build 1D in‐plane p–n junctions for sub‐nanometer thin low‐power electronic, optoelectronic, optical, and sensing devices. Electronic and optoelectronic applications of such p–n junction devices fabricated using a scalable one‐pot chemical vapor deposition process yielding MoSe2‐WSe2 lateral heterostructures are reported here. The growth of the monolayer lateral heterostructures is achieved by in situ controlling the partial pressures of the oxide precursors by a two‐step heating protocol. The grown lateral heterostructures are characterized structurally and optically using optical microscopy, Raman spectroscopy/microscopy, and photoluminescence spectroscopy/microscopy. High‐resolution transmission electron microscopy further confirms the high‐quality 1D boundary between MoSe2 and WSe2 in the lateral heterostructure. p–n junction devices are fabricated from these lateral heterostructures and their applicability as rectifiers, solar cells, self‐powered photovoltaic photodetectors, ambipolar transistors, and electroluminescent light emitters are demonstrated.