Layered Transition Metal Dichalcogenides Research Articles

Engineering Symmetry Breaking in Twisted MoS2-MoSe2 Heterostructures for Optimal Thermoelectric Performance.

Engineering symmetry breaking in thermoelectric materials holds promise for achieving an optimal thermoelectric efficiency. van der Waals (vdW) layered transition metal dichalcogenides (TMDCs) provide critical opportunities for manipulating the intrinsic symmetry through in-plane symmetry breaking interlayer twists and out-of-plane symmetry breaking heterostructures. Herein, the symmetry-dependent thermoelectric properties of MoS2 and MoSe2 obtained via first-principles calculations are reported, yielding an advanced ZT of 2.96 at 700 K. The underlying mechanisms reveal that the in-plane symmetry breaking results in a lowest thermal conductivity of 1.96 W·m-1·K-1. Additionally, the electric properties can be significantly modulated through band flattening and bandgap alteration, stemming directly from the modified interlayer electronic coupling strength owing to spatial repulsion effects. In addition, out-of-plane symmetry breaking induces band splitting, leading to a decrease in the degeneracy and complex band structures. Consequently, the power factor experiences a notable enhancement from ∼1.32 to 1.71 × 10-2 W·m-1·K-2, which is attributed to the intricate spatial configuration of charge densities and the resulting intensified intralayer electronic coupling. Upon simultaneous implementation of in-plane and out-of-plane symmetry breaking, the TMDCs exhibit an indirect bandgap to direct bandgap transition compared to the pristine structure. This work demonstrates an avenue for optimizing thermoelectric performance of TMDCs through the implementation of symmetry breaking.

Experimental investigation on the electrodeposited nickel-based dichalcogenides for the efficient overall water splitting

Layered transition metal dichalcogenides are effective electrocatalysts for water splitting due to their unique properties such as high electrical conductivity, active site density, and catalytic properties. This study focused on the deposition of Ni-based dichalcogenides (NiX2, where X = S, Se, Te) onto a carbon cloth substrate using a simple one-step electrodeposition method at room temperature with different deposition times (10, 15, and 20 minutes). The optimized NiS2-15, NiSe2-15, and NiTe2-15 electrocatalysts showed excellent bifunctional electrocatalytic activity for both hydrogen evolution reaction and oxygen evolution reaction in alkaline media. The overpotential values for hydrogen evolution were ∼56 mV, ∼37 mV, and ∼75 mV for NiS2-15, NiSe2-15, and NiTe2-15, respectively, while the overpotential values for oxygen evolution were ∼220 mV, ∼170 mV, and ∼210 mV, respectively. The electrocatalysts maintained stability for 24 hours during the evolution reactions. The density functional theory calculations suggested that NiSe2 had a lower ΔGH* value due to its smaller bandgap and inherent metallic characteristics, which is consistent with the experimental results. These findings provide a new direction for designing NiX2 (X = S, Se, Te) nanosheets for electrocatalysis applications.

Cycle-to-cycle Switching Endurance Variability in Vertically Aligned Nanocrystalline Molybdenum Disulfide: Computational Insights

Abstract We propose a model to depict the endurance test results of a resistive switching device with vertically oriented layers of nanocrystalline transition metal dichalcogenide layers with respect to the substrate. We aim to relate and understand the endurance test results in the literature with our model, which is tested using density functional theory simulations. We conclude a relation between endurance test results and skin effect dominated alternating electric current resistance and surface resistance. These results are important to manipulate and understand the reliability and accuracy of cycle-to-cycle variations in the endurance test results.

Temperature-dependent local structure and lattice dynamics of 1T-TiSe[formula omitted] and 1T-VSe[formula omitted] probed by X-ray absorption spectroscopy

The local atomic structure and lattice dynamics of two isostructural layered transition metal dichalcogenides (TMDs), 1T-TiSe2 and 1T-VSe2, were studied using temperature-dependent X-ray absorption spectroscopy at the Ti, V, and Se K-edges. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra, employing reverse Monte Carlo (RMC) simulations, enabled tracking of the temperature evolution of the local environment in the range of 10–300 K. The atomic coordinates derived from the final atomic configurations obtained using the RMC method were used to calculate the partial radial distribution functions (RDFs) and the mean-square relative displacement (MSRD) factors for the first ten coordination shells around the absorbing atoms. Characteristic Einstein frequencies and effective force constants were determined for TiSe, TiTi, VSe, VV, and SeSe atom pairs from the temperature dependencies of MSRDs. The obtained results reveal differences in the temperature evolution of lattice dynamics and the strengths of intralayer and interlayer interactions in TiSe2 and VSe2.

Effect of layered transition metal dichalcogenide hybrid nanomaterials on the piezoelectric performance of non‐solvent induced phase separated polyvinylidene fluoride

AbstractLayered transition metal dichalcogenides (TMDs) with high aspect ratios enhance the alignment of polymer chains and induce a preferred orientation of the polymeric crystallites when incorporated into polyvinylidene fluoride (PVDF). In addition to offering an effective charge‐transfer mechanism, TMDs give PVDF more rigidity and piezoelectric qualities. This work reports the non‐solvent induced phase separation (NIPS) introduced while developing the PVDF/MoS2 composites. During the NIPS, the PVDF chains become phase‐separated, which induces high polarization in the PVDF matrix. Phase‐separated PVDF/MoS2 composites show high porosity and charge distribution attributed to the enhanced piezoelectric output voltage. While the neat PVDF demonstrated very feeble output voltage generation, the hybrid composite containing 2 wt.% of MoS2/ZnO facilitated almost 20 times higher performance (peak‐to‐peak voltage of 2.4 V). This work yielded a phase‐separated composite that finds uses in energy harvesting, sensors, and actuators, among other fields.Highlights NIPS creates high‐porosity composites with improved charge distribution. Layered TMDs improve charge‐transfer mechanism and PVDF's electrical properties. 2 wt.% MoS2/ZnO exhibits nearly 20 times higher voltage generation than neat PVDF.

Optimizing Gold‐Assisted Exfoliation of Layered Transition Metal Dichalcogenides with (3‐Aminopropyl)triethoxysilane (APTES): A Promising Approach for Large‐Area Monolayers

Optimizing Gold‐Assisted Exfoliation of Layered Transition Metal Dichalcogenides with (3‐Aminopropyl)triethoxysilane (APTES): A Promising Approach for Large‐Area Monolayers

Highly Intrinsic Carrier Mobility in Tin Diselenide Crystal Accessed with Ultrafast Terahertz Spectroscopy

Highly Intrinsic Carrier Mobility in Tin Diselenide Crystal Accessed with Ultrafast Terahertz Spectroscopy

Tuning the electrocatalytic activity of MoS2 nanosheets via the in situ hybridization with ruthenium and graphene network

The quest for highly efficient, stable, and non-precious electrocatalysts as alternatives to Pt in the hydrogen evolution reaction (HER) at ambient temperature is paramount. Two-dimensional (2D) layered transition metal dichalcogenides (TMDs), notably MoS2, have emerged as promising noble-metal-free candidates. However, MoS2′s limited active edge sites and sluggish electron transfer kinetics present challenges in achieving efficient HER activity under both acidic and basic conditions. In this study, we introduce a composite catalyst composed of 2D MoS2 nanosheets enveloped by graphene networks, with uniform dispersion of ruthenium (Ru) nanoparticles within the extended 2D structure. This ternary hybrid catalyst exhibits excellent catalytic activity, with a low onset overpotential of 60 mV at 10 mA cm−2, a small Tafel slope of 38 mV dec-1, and remarkable durability during continuous 10-hour operation at 100 mV potential in 1.0 M KOH, without substantial activity loss. The conductive graphene support effectively prevents the restacking of exfoliated MoS2 nanosheets, enhancing mass transport and charge transfer kinetics. Ru incorporation induces the partial phase transformation of MoS2 from its trigonal (2H) to octahedral (1 T) phase, concurrently generating sulfur vacancies. The induced phase transformation not only activates the inert basal planes of MoS2 but also reduces the energy barrier for the adsorption/desorption of H* intermediates. Beyond the structural advantages, the intrinsic synergistic effects, primarily related to electronic structure modulation and defect engineering, contribute significantly to the enhanced electrochemical HER performance. In summary, this study not only introduces an efficient, cost-effective alternative to Pt-based electrocatalysts but also lays the foundation for designing other TMD-based hybrid catalysts, characterized by abundant exposed active sites and reduced barriers for hydrogen binding energy, thereby advancing electrochemical hydrogen production.

Direct Assessment of Defective Regions in Monolayer MoS2 Field-Effect Transistors through In Situ Scanning Probe Microscopy Measurements.

Implementing two-dimensional materials in field-effect transistors (FETs) offers the opportunity to continue the scaling trend in the complementary metal-oxide-semiconductor technology roadmap. Presently, the search for electrically active defects, in terms of both their density of energy states and their spatial distribution, has turned out to be of paramount importance in synthetic transition metal dichalcogenides layers, as they are suspected of severely inhibiting these devices from achieving their highest performance. Although advanced microscopy tools have allowed the direct detection of physical defects such as grain boundaries and point defects, their implementation at the device scale to assess the active defect distribution and their impact on field-induced channel charge modulation and current transport is strictly restrained. Therefore, it becomes critical to directly probe the gate modulation effect on the carrier population at the nanoscale of an FET channel, with the objective to establish a direct correlation with the device characteristics. Here, we have investigated the active channel in a monolayer MoS2 FET through in situ scanning probe microscopy, namely, Kelvin probe force microscopy and scanning capacitance microscopy, to directly identify active defect sites and to improve our understanding of the contribution of grain boundaries, bilayer islands, and defective grain domains to channel conductance.

Anomalous redshift in interlayer exciton emission with increasing twist angle in WSe2/MoSe2 heterostructures

Van der Waals heterostructures utilizing semiconducting transition metal dichalcogenide (TMDC) monolayers have surfaced as compelling candidates due to their intriguing optical characteristics, which can be effectively controlled by the manipulation of the stacking twist angle. This study investigates the intricate correlation between twist angle, band offset, and interlayer exciton emission within twisted WSe2/MoSe2 heterostructures. Our findings suggest a crucial influence of monolayer stacking order on the band offset and the dipole orientation in twisted heterostructures that leads to either blueshift or redshift in emission energy. Herein, we fabricate heterobilayers with twist angles varying from 1 ∘ to 56 ∘ and observe an anomalous redshift energy of 100 meV in the interlayer exciton emission. Additionally, photoluminescence excitation spectroscopy measurements highlight the systematic twist angle dependence of intralayer exciton resonances, indicating significant angle dependent effects on individual monolayer bandgaps and on the interlayer coupling strength. Our fundamental study of exciton resonances provides comprehensive insights into the nuanced interplay between twist angle, dipole orientation, and dielectric asymmetry, providing a deeper understanding of the factors governing the optical properties of layered TMDC heterostructures.

From 2D to 3D: WS2/MoS2 heterostructure in-situ anchored on Ti3C2Tx MXene with enhanced ion/electron migration and sodium storage at −20 ℃

Layered transition metal dichalcogenides (TMDs), featuring robust electrochemical activity, high capacity, and large interlayer spacing, have attracted widespread interest in research on sodium storage. However, the limitations in dynamics and structure expansion lead to degraded cycling life and rate capability, especially in cold climate with low temperatures. Generally, heterostructure engineering, combined with conductive matrix, gives a promise towards enhancing electronic transportation, accelerating ionic diffusion, and alleviating large structure change of layered TMDs upon sodium storage. Herein, a robust WS2/MoS2 heterostructure, rendering ultrafast charge transfer, is presented, and further hybridization with layered conductive Ti3C2Tx MXene results in three-dimensional crosslinked nanoarchitectonics, leading to electrochemical performance at 25 °C and -20 °C, which shows only 0.03 % capacity degradation per cycle up to 2800 cycles at 2.0 A g−1 (25 °C). Strikingly, the desired WS2/MoS2/Ti3C2Tx exhibits a high capacity of 293.7 mAh g−1 (100 cycles) at 0.1 A g−1 even at −20 °C. Besides, density functional theory calculations further manifest the decreased sodium diffusion barrier and facilitated charge-transfer dynamics in the designed heterostructure with Ti3C2Tx. More importantly, this work not only represents the correlation between structure and performance for target electrode, but also provides a deep insight for the rational design of heterostructure/Ti3C2Tx MXene for next-generation high-performance electrode materials.

Emerging Frontiers of 2D Transition Metal Dichalcogenides in Photovoltaics Solar Cell

AbstractTwo‐dimensional (2D) transition metal dichalcogenides (TMDCs) have gained much attention due to their excellent electronic and optoelectronic properties. The reasonable band gap, higher light‐matter interaction, hundreds of categories as well as wafer‐scale growth and Si‐compatible fabrication etc. proof their immense potential for next‐generation solar cells. Over the past years, a variety of specific device structure are investigated including multi‐field tunable 2D TMDCs‐based photovoltaic solar cell since its electronic structure is easily tunable. Their work function distributes in a wide range which facilitate interfacial modulation and optimize the electron/hole transfer layer in perovskite‐/organic‐based photovoltaic solar cell. In this review, the recent developments of TMDCs in photovoltaic solar cell are comprehensively described. First, the energy diagram of traditional semiconductor and 2D TDMCs are discussed. Then, 2D TMDCs‐based photovoltaic solar cell is introduced considering the homojunction, heterojunction and Schottky‐junction as well as their multi‐field tunability. Third, the role of 2D TMDCs layer as photosensitive layer and modifying layer in silicon‐based solar cells, as well as their applications in perovskite‐/organic‐based solar cells are summarized considering their role of electron/hole transfer layer or active layer. Lastly, the prospect for future materials, device and process trends and practical applications of TMDCs‐based photovoltaic solar cell are presented.

Atomic layer deposition of two-dimensional layered zirconium sulfide

Zirconium disulfide (ZrS2) is a two-dimensional (2D) layered transition metal dichalcogenide (TMDC) and has important implications in optoelectronics and energy storage. Despite some research efforts reported to date for synthesizing 2D ZrS2, there still lacks a method to grow large-scale continuous ZrS2 thin films. In this study, we developed a low temperature vapor-phase process for controllably growing zirconium sulfide (ZrSx) films via atomic layer deposition (ALD), using tetrakis (dimethylamido)zirconium (TDMA-Zr) and H2S as the precursors in the range of 75–300 °C. We verified the self-limiting growth behavior of the ZrSx ALD using an in-situ quartz crystal microbalance (QCM). Furthermore, we characterized the resultant ZrSx films using a suite of instruments, including scanning electron microscopy (SEM), synchrotron X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HR-TEM). The results showed that the growth per cycle (GPC) of the ZrSx films varies with temperature and decreases from ∼1.2 Å/cycle at 75 °C to ∼0.3 Å/cycle at 300 °C. The as-deposited films are mainly amorphous in all the temperatures and have an S/Zr ratio varying from ∼2 to 1. Very interestingly, our in situ annealing HR-TEM analyses revealed that the resultant films could transform from an amorphous layer into a layer of crystalline nanoparticles of 2D ZrS2 with increasing temperatures up to 1000 °C. We also confirmed that the as-deposited ZrSx films are electrochemically active and show promising capability as an anode material for lithium-ion batteries (LIBs). Thus, this study represents a new ALD process for ZrSx, which may have important applications in multiple areas.

Toward Broadband Photodetection: Band Alignment and Interlayer Charge Transfer in 2D Transition Metal Dichalcogenides/3D-Ga2O3 Hybrid-Dimensional Heterostructures.

Recently, various transition metal dichalcogenides (TMDs)/Ga2O3 heterostructures have emerged as excellent candidates for the development of broadband photodetection, exhibiting various merits such as broadband optical absorption, efficient interlayer carrier transfer, a relatively simple fabrication process, and potential for flexibility. In this work, vertically stacked MoSe2/Ga2O3, WS2/Ga2O3, and WSe2/Ga2O3 heterostructures were experimentally synthesized, all exhibiting broadband light absorption, spanning at least from 200 to 800 nm. The absorption coefficients of these TMDs/Ga2O3 heterostructures are significantly improved compared to those of individual Ga2O3 films. The superior performance can be attributed to the type-I band alignment and efficient interlayer carrier transfer, which result from various band offsets along with the different doping conditions of the TMD layers, leading to distinct photoluminescence (PL) emission properties. Through a detailed analysis of the excitation-power-dependent PL spectra, we offer an in-depth discussion of the interlayer carrier transfer mechanism in the TMDs/Ga2O3 heterostructures. Regarding interlayer coupling effects, the shift of the EF of TMD layers plays a crucial role in modulating their trion emission properties. These findings suggest that these three TMDs/Ga2O3 heterostructures have great potential in broadband photodetection, and our in-depth physical mechanism analysis lays a solid foundation for a new device design.

Orientation Dependent Interlayer Coupling in Organic–Inorganic Heterostructures

AbstractOrganic–inorganic 2D heterostructures combine the high optical absorption of organic molecules with exciton‐dominated optical properties in layered transition metal dichalcogenides (TMDs) such as MoS2. Critical to the interaction and the optical response in such hybrid systems is the electronic band alignment at the interface between the two species. Here, the coupling of monolayers of perylene derivatives is investigated with bilayer MoS2. In particular, variation in the perylene orientation on the MoS2 surface is identified using Raman spectroscopy and scanning probe microscopy. Low‐temperature optical spectroscopy reveals orientation‐dependent interlayer exciton formation. Furthermore, power‐dependent photoluminescence measurements provide insight into the modified interlayer charge transfer in these heterostructures. A saturation of interlayer states is found under high excitation power when the perylene molecules are in a perpendicular orientation to the surface that leads to electron accumulation in the MoS2, whereas parallel alignment of the perylene molecules leads to enhanced populations of organic–inorganic interlayer excitons. This work provides insights into the optimization of organic–inorganic heterostructures, with particular relevance to applications for optoelectronic and excitonic devices.

Reducing contact resistance of MoS2-based field effect transistors through uniform interlayer insertion via atomic layer deposition.

Molybdenum disulfide (MoS2), a semiconducting two-dimensional layered transition metal dichalcogenide (2D TMDC), with attractive properties enables the opening of a new electronics era beyond Si. However, the notoriously high contact resistance (RC) regardless of the electrode metal has been a major challenge in the practical applications of MoS2-based electronics. Moreover, it is difficult to lower RC because the conventional doping technique is unsuitable for MoS2 due to its ultrathin nature. Therefore, the metal-insulator-semiconductor (MIS) architecture has been proposed as a method to fabricate a reliable and stable contact with low RC. Herein, we introduce a strategy to fabricate MIS contact based on atomic layer deposition (ALD) to dramatically reduce the RC of single-layer MoS2 field effect transistors (FETs). We utilize ALD Al2O3 as an interlayer for the MIS contact of bottom-gated MoS2 FETs. Based on the Langmuir isotherm, the uniformity of ALD Al2O3 films on MoS2 can be increased by modulating the precursor injection pressures even at low temperatures of 150 °C. We discovered, for the first time, that film uniformity critically affects RC without altering the film thickness. Additionally, we can add functionality to the uniform interlayer by adopting isopropyl alcohol (IPA) as an oxidant. Tunneling resistance across the MIS contact is lowered by n-type doping of MoS2 induced by IPA as the oxidant in the ALD process. Through a highly uniform interlayer combined with strong doping, the contact resistance is improved by more than two orders of magnitude compared to that of other MoS2 FETs fabricated in this study.

Synthesis of NbSe2/Bilayer Nb‐Doped WSe2 Heterostructure from Exfoliated WSe2 Flakes

Forming heterostructures of 2D metals and semiconductors using chemical vapor deposition (CVD) has significant potential to effectively reduce contact resistance in electronic devices. However, semiconducting transition metal dichalcogenide (TMD) layers in metal–semiconductor heterostructures are currently restricted to monolayers despite the superior mobility and density of states in bilayer TMDs. Herein, NbSe2/bilayer Nb‐doped WSe2 metal/semiconductor heterostructure from exfoliated WSe2 flakes are synthesized first. The exfoliated WSe2 bulk crystals on an Nb‐coated substrate are heated to 950 °C under a flow of selenium vapor, and then the NbSe2/bilayer Nb‐doped WSe2 heterostructures are formed. Statistics on the number of Nb‐doped WSe2 layers grown on a 1 cm × 1 cm CVD substrate shows that 65% of the Nb‐doped WSe2 layers are grown as bilayers. X‐ray photoelectron spectroscopy, optical microscopy, and transmission electron microscopy clearly clarify the number of Nb‐doped WSe2 layers and heterostructure of NbSe2/bilayer Nb‐doped WSe2. Electrical measurements using Cr contacts show that bilayer Nb‐doped WSe2 displays 7‐ and 10‐times higher mobility and on/off ratio than monolayer Nb‐doped WSe2. The mobility and on/off ratio are further doubled in NbSe2/bilayer Nb‐doped WSe2 contact compared to Cr/bilayer Nb‐doped WSe2 contact, attributed to a clean interface in vertical stack heterostructure, enhancing electrical performance.

Predicted multiple charge density wave phases in monolayer 1T-NbO2

Layered transition-metal dichalcogenides, such as NbSe2, have been extensively studied for almost half a century due to their intriguing properties, such as charge density wave (CDW) and superconductivity. Can the layered transition-metal dioxide, such as NbO2, be stable and exhibit CDW, given that it has the same crystal structure and electronic configuration as NbSe2? Here, we use first-principles calculations to predict that 1T-NbO2 is possibly stable at high temperatures, but it would undergo two CDW transitions with 12×12 and 13×13 periodicities at low temperatures. Both CDW transitions are accompanied by a metal-semiconductor transition. Notably, the 13×13 CDW phase of NbO2 possesses localized magnetic moments and hosts a Mott insulating state. This work offers a fresh outlook on studying CDW and Mott transition in low-dimensional oxide materials.

A high-efficiency programmable modulator for extreme ultraviolet light with nanometre feature size based on an electronic phase transition

AbstractThe absence of efficient light modulators for extreme ultraviolet (EUV) and X-ray photons considerably limits their real-life application, particularly when even slight complexity of the beam patterns is required. Here we report on a novel approach to reversible imprinting of a holographic mask in an electronic Wigner crystal material with a sub-90-nm feature size. The structure is imprinted on a sub-picosecond timescale using EUV laser pulses, and acts as a high-efficiency diffraction grating that deflects EUV or soft X-ray light. The imprinted nanostructure is stable after the removal of the exciting beams at low temperatures, but can be easily erased by a single heating beam. Modelling shows that the efficiency of the device can exceed 1%, approaching state-of-the-art etched gratings, but with the benefit of being programmable and tunable over a large range of wavelengths. The observed effect is based on the rapid change of lattice constant upon transition between metastable electronically ordered phases in a layered transition metal dichalcogenide. The proposed approach is potentially useful for creating tunable light modulators in the EUV and soft X-ray spectral ranges.

Unraveling the mechanism of vanadium self-intercalation in 1T-VSe2: atomic-scale evidence for phase transition and superstructure model for intercalation compound

Self-intercalation is an efficient strategy for tailoring the property of layer structured materials like transition metal dichalcogenides (TMDCs), while the associated kinetics and mechanism remain scarcely explored. In this study, we investigate the atomic-scale dynamics and mechanism of vanadium (V) self-intercalation in multi-layer 1T-VSe2 using in situ high resolution scanning transmission electron microscopy. The results reveal that the self-intercalation of V induces structural transformation of pristine VSe2 into three V-enrich intercalated compounds, i.e. V5Se8, V3Se4 and VSe. The self-intercalated V follows an ordered arrangement of 2×2 , 2×1 , and 1×1 within the interlayer octahedral sites, corresponding to an intercalation concentration of 25%, 50% and 100% in V5Se8, V3Se4 and VSe, respectively. The V intercalants induced lattice distortions to the host 1T-VSe2 such as the dimerization of neighboring lattice V is observed experimentally, which are further supported by density functional theory (DFT) calculations. Finally, a superstructure model generalizing the possible structures of self-intercalated compounds in layered TMDCs is proposed and then validated by the DFT determined formation energy landscape. This study provides comprehensive insights on the kinetics and mechanism of the self-intercalation in layered TMDC materials, contributing to the precise control for the structure and stoichiometry of self-intercalated TMDC compounds.