MoS2 Transition Metal Dichalcogenides Research Articles

Charge Critical Phenomena in a Field Heterostructure with Two-Dimensional Crystal

The charge properties and regularities of mutual influence of the electro-physical parameters in a metal (M)/insulator (I)/two-dimensional crystal heterostructure were studied. In one case, the transition metal dichalcogenide (TMD) MoS2 was considered as a two-dimensional crystal, and in another the Weyl semi-metal (WSM) ZrTe5, representative of a quasi-two-dimensional crystal was chosen for this purpose. By self-consistently solving the electrostatic equations of the heterostructures under consideration and the Fermi–Dirac distribution, the relationship between such parameters as the concentration of charge carriers, chemical potential, and quantum capacitance of the TMD (WSM), as well as the capacitance of the I layer and the interface capacitance I–TMD (WSM), and their dependence on the field electrode potential, have been derived. The conditions for the emergence of charge instability and the critical phenomena caused by it are also determined.

High-throughput ab initio design of atomic interfaces using InterMatch

Forming a hetero-interface is a materials-design strategy that can access an astronomically large phase space. However, the immense phase space necessitates a high-throughput approach for an optimal interface design. Here we introduce a high-throughput computational framework, InterMatch, for efficiently predicting charge transfer, strain, and superlattice structure of an interface by leveraging the databases of individual bulk materials. Specifically, the algorithm reads in the lattice vectors, density of states, and the stiffness tensors for each material in their isolated form from the Materials Project. From these bulk properties, InterMatch estimates the interfacial properties. We benchmark InterMatch predictions for the charge transfer against experimental measurements and supercell density-functional theory calculations. We then use InterMatch to predict promising interface candidates for doping transition metal dichalcogenide MoSe2. Finally, we explain experimental observation of factor of 10 variation in the supercell periodicity within a few microns in graphene/α-RuCl3 by exploring low energy superlattice structures as a function of twist angle using InterMatch. We anticipate our open-source InterMatch algorithm accelerating and guiding ever-growing interfacial design efforts. Moreover, the interface database resulting from the InterMatch searches presented in this paper can be readily accessed online.

The influence of MoS2 on perpendicular magnetic anisotropy and spin–orbit torque of Pt/Co/Pt multilayer

Two-dimensional (2D) materials provide a wide choice for the next-generation spintronic devices at the post-Moore era. Here, we reveal the effect of the 2D transition metal dichalcogenide MoS2 on the perpendicular magnetic anisotropy (PMA) and spin–orbit torque (SOT) of the Pt/Co/Pt multilayer. MoS2 can distinctly enhance the PMA of Pt/Co/Pt multilayer, which shows no obvious dependence on the thickness of MoS2. Moreover, the magnetization of monolayer MoS2/Pt/Co/Pt can be switched under a small external magnetic field around 300 Oe and maintained up to 4000 Oe, verifying the enhanced PMA by MoS2. Interestingly, the SOT efficiency can also be increased about 9.65% by monolayer MoS2, which can be attributed to the spin absorption at the MoS2/Pt interface. Comparing the SOT behaviors of devices with different MoS2 thickness, the monolayer MoS2 is proved to be an ideal candidate for high-performance SOT devices.

Development of a highly sensitive voltammetric sensor for the detection of folic acid by using MoS2 and ionic liquid-modified carbon paste electrode.

Sensitive analytical determination of folic acid is important in clinical laboratories due to its versatile biological functions. A simple folic acid sensor was successfully fabricated based on two-dimensional transition metal dichalcogenide MoS2 modified carbon ionic liquid paste electrode (MoS2-CILPE). The electrochemical properties of the fabricated electrode were investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and chronoamperometry. The fabricated sensor displayed excellent electroactivity towards folic acid using CV. Under optimal conditions (0.1 M PBS (pH 7.0)), the DPV oxidation peak current was proportional to folic acid concentration in the range from 5.0 μM to 100.0 μM with an estimated limit of detection of 1.0 μM and limit of quantification of 5.0 μM. The ability of the sensor for routine analyses was demonstrated by the detection of folic acid present in folic acid tablets and urine samples with appreciable recovery values.

2D/2D Mo0.74W0.26S2/Ti3C2 MXene for microwave absorption with broad effective absorption band

Transition metal dichalcogenide MoS2 is considered as a type of dielectric loss dominated electromagnetic wave absorbing material owing to the high specific surface area, layered structure, and lightweight. Introduction of interfaces will improve the electromagnetic wave absorbing performance. Here, the phase engineering of MoS2 is realized through W doping since of the microstrain effect, resulting in the appearance of 2H/1T MoS2 phase interface. Furthermore, MoS2/Ti3C2 MXene interface is realized through the construction of MoS2/Ti3C2 MXene heterostructure, leading to obvious improvements in electromagnetic wave absorbing. Because of the simultaneous introduction of 2H/1T MoS2 phase interface and the MoS2/Ti3C2 MXene interface, the microwave reflection loss can reach −45.2 dB with broad effective absorption bandwidth (<−10 dB) of 7.1 GHz (7.8–14.9 GHz) at the same thickness of 3 mm. The results shed light on enhancing electromagnetic wave absorbing performance by phase engineering as well as construction of two dimensional material/two dimensional material heterostructures, thereby introducing multiple interfaces.

Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices.

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS2 and WS2. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

"Dead" Exciton Layer and Exciton Anisotropy of Bulk MoS2 Extracted from Optical Measurements.

Excitons (electron-hole pairs bound by the Coulomb potential) play an important role in optical and electronic properties of layered materials. They can be used to modulate light with high frequencies due to the optical Pauli blocking. The properties of excitons in 2D materials are extremely anisotropic. However, due to nanometre sizes of excitons and their short life times, reliable tools to study this anisotropy are lacking. Here, we show how direct optical reflection measurements can be used to evaluate anisotropy of excitons in transition metal dichalcogenides MoS2. Using focused beam spectroscopic ellipsometry, we have measured the polarized optical reflection of bulk MoS2 for two crystal orientations: c-axis being perpendicular to the surface from which reflection is measured and c-axis being parallel to the surface from which reflection is measured. We found that for the parallel configuration the optical reflection near excitonic transitions is strongly affected by the presence of the exciton "dead" layer such that the excitonic reflection peaks become the excitonic dips due to light interference. At the same time, the optical reflection for the perpendicular orientation is not significantly altered by the exciton "dead" layer due to large anisotropy of exciton properties. Performing simultaneous Fresnel fitting for both geometries, we were able to evaluate exciton anisotropy in layered materials from simple optical measurements.

Magnetization dynamics and spin pumping in Heusler compound Co2FeSi interfaced with MoS2

We present a detailed investigation of spin pumping from full Heusler compound Co2FeSi (CFS) to single-layer two-dimensional (2D) transition metal dichalcogenide (TMD) MoS2 (molybdenum disulfide) excited by ferromagnetic resonance. Microwave-induced spin pumping has been identified by a significant enhancement of Gilbert damping in Si/SiO2/MoS2/CFS heterostructures as compared to bare Si/SiO2/CFS samples. The CFS thickness dependent spin mixing conductance indicates an efficient spin transparent interface of MoS2/CFS. In addition, the spin-to-charge conversion process has been quantified using inverse spin Hall effect (ISHE) measurements. In ISHE measurements, a highly symmetric Lorentzian DC voltage signal in Si/SiO2/MoS2/CFS heterostructures is observed in comparison with a highly asymmetric signal in bare Si/SiO2/CFS samples, establishing a clear sign of FMR induced spin pumping. The results in this work are expected to aid in the understanding of the spin angular momentum transfer phenomenon at the interface of the 2D TMDs/ferromagnet bilayer system.

A critical assessment of the role of ionic surfactants in the exfoliation and stabilization of 2D nanosheets: The case of the transition metal dichalcogenides MoS2, WS2 and MoSe2

Transition metal dichalcogenides (TMDs), like other two-dimensional layered materials beyond graphene, have gained enormous interest in recent years owing to their distinct electronic and optical properties, and potential applicability in areas such as sensing, nanoelectronics and catalysis. Surfactant-assisted exfoliation is commonly used to prepare aqueous dispersions of TMD nanosheets, but a clear picture of the TMD and surfactant features that influence the dispersion process is still lacking. In this work, we present a systematic study of the dispersibility of MoS2, WS2 and MoSe2 in aqueous medium using a cationic (cetyltrimethylammonium bromide, CTAB) and an anionic (sodium cholate, SC) dispersant, in a wide concentration range (seven orders of magnitude) and resorting to a carefully controlled sonication-centrifugation procedure. We present detailed, high precision dispersibility curves (concentration of dispersed TMD versus concentration of surfactant used), together with zeta potential and pH measurements, allowing insight into the influence of the type of metal and chalcogen, surfactant charge and surfactant concentration, on the effectiveness of the exfoliation and stabilization. The metal (Mo vs. W) influences the dispersibility at low surfactant concentrations, while the chalcogen (S vs. Se) plays a more significant role as the surfactant concentration is increased, alongside the surfactant charge. Structural characterization by scanning electron microscopy (SEM), Raman spectroscopy and atomic force microscopy (AFM) shows that the methodology applied yields well-exfoliated nanosheets with controlled mean lateral dimension (≈ 100 nm) and thickness (≤5 layers). Finally, the type of ionic surfactant (cationic vs. anionic) and its concentration play a pivotal role in the profile of the dispersibility curves, leading us to propose two types of master curves with distinct regions of phase behavior.

Ultrabroadband Photodetector Based on Ferromagnetic van der Waals Heterodiode

AbstractThe discovery of 2D ferromagnetic (FM) van der Waals (vdW) semiconductors with narrow bandgap and p‐type transport behavior makes them promising for infrared photodetection. Here, a 2D vdW heterodiode uncooled long‐wave infrared (LWIR) photodetector by stacking a p‐type 2D FM material CrSiTe3 on top of an n‐type transition metal dichalcogenides (TMDs) MoS2 is reported. A good rectification ratio >102 and an ultra‐broadband photoresponse from 0.52 to 10.6 μm are demonstrated. The photoresponsivity of up to 20.7 A W−1 and the external quantum efficiency (EQE) of up to 4031.7% are obtained under a 1310 nm laser at a −1 V bias in ambient, which indicates that these CrSiTe3–MoS2 p–n junction devices have a good photovoltaic response. Meanwhile, the photovoltaic responsivity up to 0.15 A W−1, EQE up to 29.7%, and fast response with a decay time of 2.8 μs at 637 nm are demonstrated. In addition, the room temperature mid‐wave infrared (MWIR) specific detectivity and LWIR specific detectivity of CrSiTe3–MoS2 is 6.1 × 109 cm Hz1/2 W−1 and 1.93 × 109 cm Hz1/2 W−1, respectively. These observations open up possibilities for developing high‐sensitive infrared detection based on the valley optical selection rule and LWIR image technology.

Transition metal dichalcogenides boost the performance of optical fiber SPR sensors

We show the active role that the transition metal dichalcogenide MoS2 plays in enhancing the sensing performance of optical fiber sensors based on surface plasmon resonance (SPR). This is proved experimentally suggesting that the optical fiber SPR sensor with the MoS2 has an about 24 percent increase in the bulk refractive index sensitivity compared to the one without the MoS2. Also, we investigate the surface sensing performance of the sensor using self-assembly monolayers of polyelectrolytes with well-defined thickness and refractive index.

Multidimensional Imaging Reveals Mechanisms Controlling Multimodal Label-Free Biosensing in Vertical 2DM-Heterostructures.

Two-dimensional materials and their van der Waals heterostructures enable a large range of applications, including label-free biosensing. Lattice mismatch and work function difference in the heterostructure material result in strain and charge transfer, often varying at a nanometer scale, that influence device performance. In this work, a multidimensional optical imaging technique is developed in order to map subdiffractional distributions for doping and strain and understand the role of those for modulation of the electronic properties of the material. As an example, vertical heterostructures comprised of monolayer graphene and single-layer flakes of transition metal dichalcogenide MoS2 were fabricated and used for biosensing. Herein, the optical label-free detection of doxorubicin, a common cancer drug, is reported via three independent optical detection channels (photoluminescence shift, Raman shift, and graphene enhanced Raman scattering). Non-uniform broadening of components of multimodal signal correlates with the statistical distribution of local optical properties of the heterostructure. Multidimensional nanoscale imaging allows one to reveal the physical origin for such a local response and propose the best strategy for the mitigation of materials variability and future device fabrication, enabling multiplexed biosensing.

Heteroatoms (Si, B, N, and P) doped 2D monolayer MoS2 for NH3 gas detection.

2D transition metal dichalcogenide MoS2 monolayer quantum dots (MoS2-QD) and their doped boron (B@MoS2-QD), nitrogen (N@MoS2-QD), phosphorus (P@MoS2-QD), and silicon (Si@MoS2-QD) surfaces have been theoretically investigated using density functional theory (DFT) computation to understand their mechanistic sensing ability, such as conductivity, selectivity, and sensitivity toward NH3 gas. The results from electronic properties showed that P@MoS2-QD had the lowest energy gap, which indicated an increase in electrical conductivity and better adsorption behavior. By carrying out comparative adsorption studies using m062-X, ωB97XD, B3LYP, and PBE0 methods at the 6-311G++(d,p) level of theory, the most negative values were observed from ωB97XD for the P@MoS2-QD surface, signifying the preferred chemisorption surface for NH3 detection. The mechanistic studies provided in this study also indicate that the P@MoS2-QD dopant is a promising sensing material for monitoring ammonia gas in the real world. We hope this research work will provide informative knowledge for experimental researchers to realize the potential of MoS2 dopants, specifically the P@MoS2-QD surface, as a promising candidate for sensors to detect gas.

Excited States Calculations of MoS2@ZnO and WS2@ZnO Two-Dimensional Nanocomposites for Water-Splitting Applications

Transition metal dichalcogenide (TMD) MoS2 and WS2 monolayers (MLs) deposited atop of crystalline zinc oxide (ZnO) and graphene-like ZnO (g-ZnO) substrates have been investigated by means of density functional theory (DFT) using PBE and GLLBSC exchange-correlation functionals. In this work, the electronic structure and optical properties of studied hybrid nanomaterials are described in view of the influence of ZnO substrates thickness on the MoS2@ZnO and WS2@ZnO two-dimensional (2D) nanocomposites. The thicker ZnO substrate not only triggers the decrease of the imaginary part of dielectric function relatively to more thinner g-ZnO but also results in the less accumulated charge density in the vicinity of the Mo and W atoms at the conduction band minimum. Based on the results of our calculations, we predict that MoS2 and WS2 monolayers placed at g-ZnO substrate yield essential enhancement of the photoabsorption in the visible region of solar spectra and, thus, can be used as a promising catalyst for photo-driven water splitting applications.

Significantly improved photo carrier injection by the MoS2/ZnO/HNP hybrid UV photodetector architecture

A unique ultraviolet (UV) photodetector architecture of MoS2/ZnO/HNP is explored and significantly increased photocurrent (Iph) of 4 × 10−3 A is demonstrated under the 385 nm illumination of 54.9 mW/mm2 at 10 V. This leads to the superior device performance factors: i.e., photoresponsivity of 21,111 mA/W, detectivity of 7.9 × 1011 jones and EQE of 6,800 %. These device performance factors confirm that the proposed photodetector configuration is superior as compared to the similar versions of ZnO-based UV photodetectors. The hybrid MoS2/ZnO/HNP architecture is constructed by the 2-D transition metal dichalcogenide MoS2 nanoflakes, wide-bandgapZnO quantum dots (QDs) and plasomonic PdAg hybrid nanoparticles (HNPs). The increased Iph is related to the hot carrier injection through the strongly improved localized surface plasmon resonance (LSPR) by the PdAg HNPs and additional photo carriers from the 2-D MoS2 nanoflakes on top of the original photo-induced carriers in the ZnO QD layer. The PdAg HNPs are fabricated by a unique two-step solid-state dewetting (SSD) technique, having the core-shelled PdAg NPs with the high-density small background Ag NPs. This HNP configuration offers very high-density hot spots and significantly improved LSPR. The photocurrent enhancement of MoS2/ZnO/HNP architecture is systematically studied by the finite-difference time-domain (FDTD) simulations.

A biodegradable polymer-assisted efficient and universal exfoliation route to a stable few layer dispersion of transition metal dichalcogenides

Quasi two-dimensional transition metal dichalcogenides (2D-TMDs) with 1–5 layers have been obtained by developing an effective and highly efficient bio-based polymer-solvent system by mild ultrasonication. The hydroxypropyl cellulose (HPC)/1-Butanol (BuOH) system was determined to be an universal polymer-solvent system to effectively exfoliate the bulk transition metal dichalcogenides (TMDs) MoS2, TiS2, WS2, NbS2 and MoSe2. The above process is significant as it is a room temperature liquid phase exfoliation method that leads to a stable dispersion of a few layer 2D-TMD having significant proportion (>50%) of exfoliated material which remains stable in suspension for several months.

Deep Learning for Rapid Analysis of Spectroscopic Ellipsometry Data

High‐throughput experimental approaches to rapidly develop new materials require high‐throughput data analysis methods to match. Spectroscopic ellipsometry is a powerful method of optical properties characterization, but for unknown materials and/or layer structures the data analysis using traditional methods of nonlinear regression is too slow for autonomous, closed‐loop, high‐throughput experimentation. Herein, three methods (termed spectral, piecewise, and pointwise) of spectroscopic ellipsometry data analysis based on deep learning are introduced and studied. After initial training, the incremental time for inferring optical properties can be a thousand times faster than traditional methods. Results for multilayer sample structures with optically isotropic materials are presented, appropriate for high‐throughput studies of thin films of phase‐change materials such as GeSbTe (GST) alloys. Results for studies on highly birefringent layered materials are also presented, exemplified by the transition metal dichalcogenide MoS2. How the materials under test and the experimental objectives may guide the choice of analysis methods are discussed. The utility of our approach is demonstrated by analyzing data measured on a composition spread of GeSbTe phase‐change alloys containing 177 distinct compositions, and identifying the composition with optimal phase‐change figure of merit in only 1.4 s of analysis time.

Electrochemical Visualization of Gas Bubbles on Superaerophobic Electrodes Using Scanning Electrochemical Cell Microscopy.

Electrocatalytic gas evolution reactions, where gaseous molecules are electrogenerated by reduction or oxidation of a species, play a central role in many energy conversion systems. Superaerophobic electrodes, usually constructed by their surface microstructures, have demonstrated excellent performance for electrochemical gas evolution reactions due to their bubble-repellent properties. Understanding and quantification of the gas bubble behavior including nucleation and dynamics on such microstructured electrodes is an important but underexplored issue. In this study, we reported a scanning electrochemical cell microscopy (SECCM) investigation of individual gas bubble nucleation and dynamics on nanoscale electrodes. A classic Pt film and a nonconventional transition-metal dichalcogenide MoS2 film with different surface topologies were employed as model substrates for both H2 and N2 bubble electrochemical studies. Interestingly, the nanostructured catalyst surface exhibit significantly less supersaturation for gas bubble nucleation and a notable increase of bubble detachment compared to its flat counterpart. Electrochemical mapping results reveal that there is no clear correlation between bubble nucleation and hydrogen evolution reaction (HER) activity, regardless of local electrode surface microstructures. Our results also indicate that while the hydrophobicity of the nanostructured MoS2 surface promotes bubble nucleation, it has little effect on bubble dynamics. This work introduces a new method for nanobubble electrochemistry on broadly interesting catalysts and suggests that the deliberate microstructure on a catalyst surface is a promising strategy for improving electrocatalytic gas evolution both in terms of bubble nucleation and elimination.

Direct probing of phonon mode specific electron\u2013phonon scatterings in two-dimensional semiconductor transition metal dichalcogenides

Electron–phonon scatterings in solid-state systems are pivotal processes in determining many key physical quantities such as charge carrier mobilities and thermal conductivities. Here, we report direct probing of phonon mode specific electron–phonon scatterings in layered semiconducting transition metal dichalcogenides WSe2, MoSe2, WS2, and MoS2 through inelastic electron tunneling spectroscopy measurements, quantum transport simulations, and density functional calculation. We experimentally and theoretically characterize momentum-conserving single- and two-phonon electron–phonon scatterings involving up to as many as eight individual phonon modes in mono- and bilayer films, among which transverse, longitudinal acoustic and optical, and flexural optical phonons play significant roles in quantum charge flows. Moreover, the layer-number sensitive higher-order inelastic electron–phonon scatterings, which are confirmed to be generic in all four semiconducting layers, can be attributed to differing electronic structures, symmetry, and quantum interference effects during the scattering processes in the ultrathin semiconducting films.

Strain engineering 2D MoS2 with thin film stress capping layers

We demonstrate a method to induce tensile and compressive strain into two-dimensional transition metal dichalcogenide (TMDC) MoS2 via the deposition of stressed thin films to encapsulate exfoliated flakes. With this technique we can directly engineer MoS2 strain magnitude by changing deposited thin film stress, therefore allowing variable strain to be applied on a flake-to-flake level. These thin film stressors are analogous to SiN x based stressors implemented in industrial complementary metal-oxide-semiconductor (CMOS) processes to enhance Si mobility, suggesting that our concept is highly scalable and may be applied for large-scale integration of strain engineered TMDC devices. We choose optically transparent stressors to allow us to probe MoS2 strain through Raman spectroscopy. Combining thickness dependent analyses of Raman peak shifts in MoS2 with atomistic simulations, we can explore layer-by-layer strain transfer. MoS2 on conventional substrates (SiO2, MgO) show strain transfer into the top two layers of multilayer flakes with limited strain transfer to monolayers due to substrate adhesion. To mitigate this limitation, we also explore stressors on van der Waals heterostructures constructed of monolayer (1L) MoS2 on hexagonal boron nitride (h-BN). This concept frees the 1L-MoS2 allowing for a 0.85% strain to be applied to the monolayer with a corresponding strain induced bandgap change of 75 meV. By using thin films with higher stress, strain may be engineered to be even higher. Various stressors and deposition methods are considered, showing a stressor material independent transfer of strain that only depends on stressor film force with negligible defects induced into MoS2 when thermal evaporation is used.