Two-dimensional Transition Metal Dichalcogenides Research Articles

Multiple conformal-contact transfer of large-area crack-free transition metal dichalcogenide stacks

Abstract Atomically-thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as an ideal platform for both physics investigation and device applications. By stacking different layers into homo- or hetero-structures, an extra degree of freedom is involved in further tuning their properties, thereby boosting scenarios in twistronics, moiré photonics and optoelectronics. However, interfacial imperfections such as contaminations and cracks, frequently occur during the layer stacking sequence and accumulate layer by layer, greatly degenerating the interface quality. In this study, we developed a multiple conformal-contact transfer method to construct TMD stacks with crack-free intrinsic interfaces. The design of a deformable buffer layer is crucial to guarantee the conformal contact and intact transfer of each layer, contributing to the successful construction of centimetre-scale TMD stacks up to 8 layers. Precise control over spatial location and interlayer twist angle is also feasibly achieved, evidenced by the stacking-dependent interlayer exciton (IE) effects in WS2-WSe2 heterostructures. This work provides a facile and precise approach for architecting 2D stacks with perfect interfaces, which will further accelerate the customized design for their device functionalization.

Electrically Controlled Excitons, Charge Transfer Induced Trions, and Narrowband Emitters in MoSe2-WSe2 Lateral Heterostructure.

Controlling excitons and their transport in two-dimensional (2D) transition metal dichalcogenide heterostructures is central to advancing photonics and electronics on-chip integration. We investigate the controlled generation and manipulation of excitons and their complexes in monolayer MoSe2-WSe2 lateral heterostructures (LHSs). Incorporating graphene as a back gate and edge contact in a field-effect transistor geometry, we achieve the precise electrical tuning of exciton complexes and their transfer across interfaces. Photoluminescence and photocurrent maps at 4 K reveal the synergistic effect of the local electric field and interface phenomena in the modulation of excitons, trions, and free carriers. We observe spatial variations in the exciton and trion densities driven by exciton-trion conversion under electrical manipulation. Additionally, we demonstrate controlled narrow-band emissions within the LHS through carrier injection and electrical biasing. Density functional theory calculation reveals significant band modification at the lateral interfaces. This work advances exciton manipulation in LHS and shows promise for next-generation 2D quantum devices.

Atomic catalysis meets heterostructure synergy: Unveiling the trifunctional efficacy of transition Metal@WS2/ReSe2

Integrating and advancing renewable energy storage and conversion technologies such as water splitting and rechargeable air-based batteries face significant challenges in finding appropriate catalysts that offer both high activity and low cost. This paper successfully designed efficient, cost-effective, and eco-friendly catalysts that meet the requirements for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) catalytic activity by employing two-dimensional (2D) transition metal dichalcogenides (TMDs) interface engineering combined with a single-atom doping strategy. First, the heterostructure was constructed, and its optimal layer spacing (3.13 Å) was determined by calculating the most negative binding energy. These materials were theoretically evaluated using density functional theory (DFT), demonstrating that transition metal (TM) can all be firmly attached to the S1 site of the WS2/ReSe2 Z-scheme heterostructure with favorable binding energies and excellent electronic properties. By comparing the catalysts doped with different metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), the three functional catalysts with the best performance were finally selected: Mn@WS2/ReSe2, Ni@WS2/ReSe2, and Cu@WS2/ReSe2. In particular, Mn@WS2/ReSe2 exhibited exceptional promise as multifunctional catalysts, with overpotentials for the HER/OER/ORR of 0.05/0.29/0.35 V, respectively. The superior catalytic performance of this WS2/ReSe2 Z-scheme heterostructure was attributed to the introduction of TM atoms and the synergistic interaction between rhenium and the TM atoms. This interaction facilitated more efficient electron transfer between the catalyst and the reactants, enhancing the overall catalytic activity. This research is a successful attempt to combine interface engineering with single-atom catalysis (SACs). It provided valuable insights into the design of next-generation multifunctional electrocatalysts, offering a novel approach to address the growing energy demands in an environmentally benign and economically viable way.

Phase-Engineered Transition Metal Dichalcogenides for Highly Efficient Surface-Enhanced Raman Scattering.

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an attractive avenue to construct new surface-enhanced Raman scattering (SERS) substrates. Herein, 2D WS2 and MoS2 monolayers with high-purity distorted octahedral phase (1T') are prepared for highly sensitive SERS detection of analytes (e.g., rhodamine 6G, rhodamine B and crystal violet). 1T'-WS2 and 1T'-MoS2 monolayers show the detection limits of 8.28 × 10-12 and 8.57 × 10-11 M for rhodamine 6G, with the enhancement factors of 4.6 × 108 and 3.9 × 107, respectively, which are comparable to noble-metal substrates, outperforming semiconducting 2H-W(Mo)S2 monolayers and most of the reported non-noble-metal substrates. First-principles density functional theory calculations show that their Raman enhancement effect is mainly ascribed to highly efficient interfacial charge transfer between the 1T'-W(Mo)S2 monolayers and analytes. Our study reveals that 2D TMDs with semimetallic 1T' phase are promising as next-generation SERS substrates.

Phase Transformation on Two-Dimensional MoTe2 Films for Surface-Enhanced Raman Spectroscopy.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently become attractive candidate substrates for surface-enhanced Raman spectroscopy (SERS) owing to their atomically flat surfaces and adjustable electronic properties. Herein, large-scale 2D 1T'- and 2H-MoTe2 films were prepared using a chemical vapor deposition method. We found that phase structure plays an important role in the enhancement of the SERS performances of MoTe2 films. 1T'-MoTe2 films showed a strong SERS effect with a detection limit of 1 × 10-9 M for the R6G molecule, which is one order of magnitude lower than that of 2H-MoTe2 films. We demonstrated that the SERS sensitivity of MoTe2 films is derived from the efficient photoinduced charge transfer process between MoTe2 and adsorbed molecules. Moreover, a prohibited fish drug could be detected by using 1T'-MoTe2 films as SERS substrates. Our study paves the way to the development and application of high-performance SERS substrates based on TMD phase engineering.

2D sheets of MoSe2 for waveguide based second harmonic generation

Two-dimensional transition metal dichalcogenides show a strong second-order nonlinear response, which can only be exploited if the interaction between light and 2D material is maximized as it happens in waveguiding structures. Such an approach also allows the addition of a second order nonlinear response to linear integrated optics components by just placing a sheet of transition metal dichalcogenides material on top. Here, we analyze the second harmonic generation in MoSe2 sheets combined with silicon nitride ridge and thin-film waveguides for both modal phase matching and quasi-phase-matching. The calculated second harmonic generation efficiency is comparable with that in lithium niobate waveguides and can further be enhanced by increasing the number of MoSe2 layers on top of the waveguides. This also holds true for flux correlated photons generated by spontaneous parametric down conversion as we show by quantitative analysis.

Humidity tolerant enhanced hydrogen gas sensing using MoSe[formula omitted]-WSe[formula omitted] heterostructures: An experimental and computational insights

In recent times, air pollution’s threat to humanity highlights the urgent need for advanced sensors to monitor harmful gases, essential for industrial regulation, gas leak detection, and air quality surveillance. Two-dimensional transition metal dichalcogenides (TMDCs) has garnered noteworthy attention as potential materials for gas sensing. This paper investigates the synthesis and characterization of a heterostructure composed of molybdenum diselenide with tungsten diselenide (MoSe2-WSe2) nanomaterials using a liquid phase exfoliation technique and it’s H2 sensing performance. The material characterisations confirmed the successful exfoliation into a hexagonal sheet-like, nanocrystalline MoSe2-WSe2 nanostructure. The study further assessed the sensor’s response to H2 gas, for concentrations of 5–25 ppm at room temperature, comparing the performance of MoSe2-WSe2 sensor with a pristine WSe2 sensor. The MoSe2-WSe2 sensor outperformed the pristine WSe2 sensor with a response of 59.57%, rapid response times and recovery times (16 s and 30 s respectively), low detection limit of 5.55 ppm, good repeatability, and high durability (30 days). Additionally, the impact of humidity was evaluated at 25 ppm H2 (at relative-humidity from 40% to 90%). The hydrophobic nature of MoSe2-WSe2 (CA = 141.4°) aligns with the first principle studies, showing almost no change in bandgap when exposed to humidity. These findings emphasize the potential of MoSe2-WSe2 heterostructure sensors for detecting H2 in humid conditions, filling a gap in research and advancing gas sensing technology for environmental safety.

N-P type charge compensatory synergistic effect for Schottky and efficient photoelectrocatalysis applications: Doping mechanism in (Nb/Re)@WS2/graphene heterojunctions.

Band gap engineering based on doped two-dimensional (2D) transition metal dichalcogenides (TMDs) has shown great potential in the design and development of new nano photoelectronic devices and their application in photoelectrocatalysis. However, there are two key issues that are difficult to take into account, namely the impurity levels induced by dopant atoms appear in the forbidden band of the doping system, which can become the recombination center of photogenerated carriers, thereby reducing the photocatalytic efficiency. Compared with the carrier mobility of the corresponding doped systems, that of intrinsic 2D TMDs is too low. Understanding the doping mechanism of heteroatoms in these systems and designing corresponding crystal structures rationally is important for solving these problems. In this study, the crystal structures of co-doped monolayer WS2 with Nb and Re atoms were designed using density functional theory, and doping systems with graphene (high carrier mobility) were assembled into a heterostructure using the concept of heterorecombination. The N-P type co-doping of Nb and Re atoms retained the continuous band characteristics of the original monolayer WS2 while also providing the high carrier mobility of graphene, yielding an excellent multipurpose material for manufacturing high-speed Schottky devices and efficient water-splitting H evolution catalysts.

Two-dimensional ASEP model to study density profiles in CVD growth

The growth of two-dimensional (2D) transition metal dichalcogenides using chemical vapor deposition has been an area of intense study, primarily due to the scalability requirements for potential device applications. One of the major challenges of such growths is the large-scale thickness variation of the grown film. To investigate the role of different growth parameters computationally, we use a 2D asymmetric simple-exclusion process (ASEP) model with open boundaries as an approximation to the dynamics of deposition on the coarse-grained lattice. The variations in concentration of particles (growth profiles) at the lattice sites in the grown film are studied as functions of parameters like injection and ejection rate of particles from the lattice, time of observation, and the right bias difference between the hopping probabilities towards right and towards left) imposed by the carrier gas. In addition, the deposition rates at a given coarse-grained site is assumed to depend on the occupancy of that site. The effect of the maximum deposition rate, i.e., the deposition rate at a completely unoccupied site on the substrate, has been explored. The growth profiles stretch horizontally when either the evolution time or the right bias is increased. An increased deposition rate leads to a step-like profile, with the higher density region close to the left edge. In 3D, the growth profiles become more uniform with the increase in the height of the precursor with respect to the substrate surface. These results qualitatively agree with the experimental observations.

Multiple electromechanical coupling in wrinkled monolayer MoS2

Abstract The electromechanical coupling of two-dimensional (2D) transition metal dichalcogenides (TMDs) is crucial for the design of highly efficient optoelectronic devices. However, achieving multiple electromechanical coupling effects in one 2D material remain a major challenge. Here, we investigate the coexistence of energy funneling, piezoelectricity, and flexoelectricity in wrinkled monolayer TMDs through the atomic-bond-relaxation approach. We find that the periodic undulation strain induced by wrinkles can lead to multiple electromechanical coupling properties. The synergistic interaction of energy funneling, piezoelectric, and flexoelectric effects can result in spatially orthogonal charge separation and transport, as well as the suppression of recombination during charge separation and transport processes. Our study provides a new route for the design of 2D material based optoelectronic devices.

Impact of monolayer WS2 surface properties on the gate dielectrics formation by atomic layer deposition

Two-dimensional transition metal dichalcogenides (2D TMDs), such as MoS2 and WS2, have emerged as promising channel materials for future generation transistors. However, carbon-based surface contaminants pose a significant challenge in the formation of high-quality metal–oxide–semiconductor gate stacks for 2D TMDs. Carbon-based surface contaminants are known to be present even on directly grown 2D TMDs that have not been in contact with polymers. These organic contaminants affect precursor adsorption during atomic layer deposition (ALD) of gate dielectrics on 2D TMDs and as such the 2D-dielectric interface. This study examines the effectiveness of predeposition annealing in mitigating carbon-based contaminants while maintaining the integrity of a directly grown WS2 monolayer on a SiO2 substrate. We show that a WS2 monolayer on a SiO2/Si substrate remains stable during vacuum annealing at temperatures up to 400 °C. Water contact angle measurements and x-ray photoelectron spectroscopy confirm that the surface concentration of carbon starts to decrease at 150 °C. Thermal anneal improves the surface coverage of Al2O3 for both conventional chemisorption-based ALD and physisorbed-precursor-assisted ALD processes by facilitating more effective Al2O3 nucleation on the WS2 monolayer. The impact of predeposition anneal on the Al2O3 growth behavior in both processes can be explained by changes in surface contaminant levels. Our results underscore the importance of surface pretreatment in dielectric deposition on 2D TMDs and demonstrate that predeposition anneal is an effective method to enhance ALD-based dielectric deposition on directly grown 2D TMDs.

Two-Dimensional ZrS2 and HfS2 for Making Sub-10 nm High-Performance P-Type Transistors.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been recognized as reliable candidates for future sub-10 nm physical gate length field-effect transistors (FETs). However, the device performance of 2D P-type devices is far inferior to that of N-type devices, which seriously hinders the development of complementary metal-oxide-semiconductor (CMOS) integrated circuits. Herein, we presented that two new 2D TMDC channel materials, ZrS2 and HfS2, can realize high-performance P-type MOSFETs through first-principles quantum transport simulations. Different from the 2D MoS2 and WSe2, the continuous in-plane p-orbitals at the valence band edge of 2D ZrS2 and HfS2 lead to a small hole effective mass of 0.24 m0. As a result, 2D ZrS2 and HfS2 P-type MOSFETs with 10 nm gate length possess an on-state current (Ion) as high as 2000 μA/μm. Moreover, even when the gate length shrinks to 5 nm, the Ion can also reach ∼1500 μA/μm with the energy delay product ranging from 3 × 10-30 to 1 × 10-29 Js/μm, which are better than many other 2D P-type MOSFETs like MoS2 and WSe2. Our work demonstrates that 2D ZrS2 and HfS2 are competitive channel materials for constructing future sub-10 nm P-type high-performance FETs.

Scalable fabrication of vertically arranged Bi2Se3 crossbar arrays for memristive device applications

Despite the unique advantages of the memristive switching devices based on two-dimensional (2D) transition metal dichalcogenides, scalable growth technologies of such 2D materials and wafer-level fabrication remain challenging. In this work, we present the gold-assisted large-area physical vapor deposition (PVD) growth of Bi2Se3 features for the scalable fabrication of 2D-material-based crossbar arrays of memristor devices. This work indicates that gold layers, prepatterned by photolithography processes, can catalyze PVD growth of few-layer Bi2Se3 with 100-folds larger crystal grain size in comparison with that grown on bare Si/SiO2 substrates. We also present a fluid-guided growth strategy to improve growth selectivity of Bi2Se3 on Au layers. Through the experimental and computational analyses, we identify two key processing parameters, i.e., the distance between Bi2Se3 powder and the target substrate and the distance between the leading edges of the substrate and the substrate holder with a hollow interior, which plays a critical role in realizing large-scale growth. By optimizing these growth parameters, we have successfully demonstrated cm-scale highly-selective Bi2Se3 growth on crossbar-arrayed structures with an in-lab yield of 86%. The whole process is etch- and plasma-free, substantially minimizing the damage to the crystal structure and also preventing the formation of rough 2D-material surfaces. Furthermore, we also preliminarily demonstrated memristive devices, which exhibit reproducible resistance switching characteristics (over 50 cycles) and a retention time of up to 106 s. This work provides a useful guideline for the scalable fabrication of vertically arranged crossbar arrays of 2D-material-based memristive devices, which is critical to the implementation of such devices for practical neuromorphic applications.

Device simulation study of multilayer MoS2 Schottky barrier field-effect transistors

Molybdenum disulfide (MoS2) is a representative two-dimensional layered transition-metal dichalcogenide semiconductor. Layer-number-dependent electronic properties are attractive in the development of nanomaterial-based electronics for a wide range of applications including sensors, switches, and amplifiers. MoS2field-effect transistors (FETs) have been studied as promising future nanoelectronic devices with desirable features of atomic-level thickness and high electrical properties. When a naturallyn-doped MoS2is contacted with metals, a strong Fermi-level pinning effect adjusts a Schottky barrier and influences its electronic characteristics significantly. In this study, we investigate multilayer MoS2Schottky barrier FETs (SBFETs), emphasizing the metal-contact impact on device performance via computational device modeling. We find thatp-type MoS2SBFETs may be built with appropriate metals and gate voltage control. Furthermore, we propose ambipolar multilayer MoS2SBFETs with asymmetric metal electrodes, which exhibit gate-voltage dependent ambipolar transport behavior through optimizing metal contacts in MoS2device. Introducing a dual-split gate geometry, the MoS2SBFETs can further operate in four distinct configurations:p - p,n - n,p - n, andn - p. Electrical characteristics are calculated, and improved performance of a high rectification ratio can be feasible as an attractive feature for efficient electrical and photonic devices.

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Layer Number and Stacking Engineering of MoS2 Crystals for High-Performance Polarization-Sensitive Photodetector.

The layer and stacking engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) gives rise to novel phenomena and multiapplications; thus, TMDs have garnered considerable attention. However, the precisely customized fabrication of stacked 2D materials to date is largely limited to the lack of effective and controllable growth strategies, prone to the unpredictable stacking orders and randomly distributed nucleation sites. Here, we devise an optimized chemical vapor deposition approach for modulating the MoS2 single crystals from monolayer to multilayer with diverse stacking configurations. Significantly, the phototransistor based on monolayer MoS2 single crystal exhibits an ultrasensitive performance with a high photoresponsivity (R) of 3.3 × 104 A W-1 and a remarkable detectivity (D*) of above 1.7 × 1014 Jones at 405 nm light illumination. Ultralow-frequency and angle-resolved polarized Raman spectroscopy is used to systematically uncover the delicate interlayer interactions and crystallographic anisotropy. Moreover, the polarization-sensitive photodetectors using 1-3L MoS2 show a layer number-dependent anisotropic performance, with dichroism ratios of 1.36, 1.44, and 1.52. This work offers a promising method to not only enable the fabrication of new customized layer-, stacking-, and twist-2D materials but also provides the foundation for the development of advanced polarization-sensitive and optoelectronic devices based on stacking transitions.

Exchange-enhanced spin-orbit splitting and its density dependence for electrons in monolayer transition metal dichalcogenides

We show that spin-orbit splitting (SOS) in monolayers of semiconducting transition metal dichalcogenides (TMDs) is substantially enhanced by an electron-electron interaction. This effect, similar to the exchange enhancement of the electron g factor, is most pronounced for conduction band electrons (in particular, in MoS2), and it has a nonmonotonic dependence on the carrier sheet density n. That is, the SOS enhancement is peaked at the onset of filling of the higher-energy spin-split band by electrons, n*, which also separates the regimes of slow (at n<n*) and fast (for n>n*) spin and valley relaxation of charge carriers. Moreover, this density itself is determined by the enhanced SOS value, making the account of exchange renormalization important for the analysis of the spintronic performance of field-effect transistors based on two-dimensional TMDs. Published by the American Physical Society 2024

Quasi-Zero-Dimensional Source/Drain Contact for Fermi-Level Unpinning in a Tungsten Diselenide (WSe2) Transistor: Approaching Schottky-Mott Limit.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) known for their exceptional electrical and optical properties have emerged as promising channel materials for next-generation electronics. However, as strong Fermi-level pinning (FLP) between the metal and the 2D TMDC material at the source/drain (S/D) contact decides the Schottky barrier height (SBH), the transistor polarity is fixed to a certain type, which remains a challenge for the 2D TMDC field-effect transistors (FETs). Here, a S/D contact structure with a quasi-zero-dimensional (quasi-0D) contact interface, in which the dimensionality reduction effect alleviates FLP, was developed to gain controllability over the polarity of the 2D TMDC FET. As a result, conventional metal contacts on the WSe2 FET showed n-type characteristics due to strong FLP (pinning factor of 0.06) near the conduction band, and the proposed quasi-0D contact enabled by the Ag conductive filament on the WSe2 FET exhibited p-type characteristics with a SBH very close to the Schottky-Mott rule (pinning factor of 0.95). Furthermore, modeling of Schottky barriers of conventional contacts, one-dimensional (1D) contacts, and quasi-0D contacts revealed that the SBH of the quasi-0D contact is relatively less subject to interface dipoles that induce FLP, owing to more rapid decaying of dipole energy. The proposed contact in this study provided a method that progressed beyond the alleviation of FLP to achieve controllable polarity. Moreover, reducing the contact dimensionality to quasi-0D will enable high compatibility with the further scaled-down nanoscale device contact structure.

Comparative analysis of thermoelectric properties in bulk 2H and monolayer MoS2: a first-principles study at high temperatures

The pursuit of high-efficiency heat-to-electricity conversion is one of the indispensable driving forces toward future renewable energy production. The two-dimensional (2D) transition metal dichalcogenide, such as molybdenum disulfide (MoS2), is at the forefront of research due to its outstanding heat propagation features and potential applications as a thermoelectric material. Using the first-principles density functional theory coupled with the semi-classical Boltzmann transport equation within the constant relaxation time approximation, we present the thermoelectric and energy transport in the bulk 2H and monolayer MoS2 material system. In order to advance the underlying physics, we calculate several crucial transport parameters such as electrical conductivity, electronic thermal conductivity, Seebeck coefficient, and power factor as a function of the reduced chemical potential for different doping types and temperatures, in addition to the electron energy dispersion relation of the material system. Our comprehensive study employs the Shankland interpolation algorithm and the rigid band approximation to attain a high degree of accuracy. This thorough investigation reveals the high Seebeck coefficient of 1534 and 1550 μ V/K at 500 K for the bulk 2H and monolayer MoS2, respectively. Furthermore, the ultrahigh power factor values of 9.21 × 1011 and 3.69 × 1011 Wm −1 K −2 s −1 are shown at 800 K in the bulk 2H and monolayer MoS2, respectively. Based on the power factor results, our in-depth analysis demonstrates that the bulk 2H MoS2, when compared to monolayer MoS2, exhibits great potential as a promising semiconducting thermoelectric material for advanced high-performance energy device applications.

Exciton dissociation in two-dimensional transition metal dichalcogenides: Excited states and substrate effects

Exciton dissociation in two-dimensional transition metal dichalcogenides: Excited states and substrate effects