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We have prepared nanocrystals of MoS2 across a range of length scales by heating single crystals of the molecular precursor (NH4)2Mo3S13·H2O. Rod-shaped crystals of the polysulfide precursor retain their original morphology after heating at temperatures up to 1000 °C and undergo complete conversion to MoS2 while acting as a template for the confined formation of MoS2 nanocrystals. This solid state transformation proceeds with the release of gaseous species without blowing the crystals apart and leads to formation of pores embedded into a nanocrystalline assembly of the templated nano-MoS2. The obtained assemblies of MoS2 nanocrystals have the exact same shape of the original rod-shaped (NH4)2Mo3S13·H2O crystals indicative of a pseudomorphic shape-retentive process. Such crystal-shaped nanocrystal assemblies show electrical conductivity values similar to a bulk MoS2 single crystal with electron carrier concentration of 1.5 × 1014 cm–3 and mobility of 7 cm2/(V s). The nanocrystals of MoS2 were grown at temperatures ranging from 450 to 1000 °C, and the sizes, shapes, morphologies, and their orientations can be engineered as a function of heating rate, soaking time, and temperature. These findings suggest a unique process for constrained templated nanocrystal growth from an organized molecular precursor structure with control of bulk morphology, size distribution, and orientation of nanocrystallites. [ABSTRACT FROM AUTHOR]

Mass spectrometry (MS)-based glycoproteomics research requires highly efficient sample preparation to eliminate interference from non-glycopeptides and to improve the efficiency of glycopeptide detection. In this work, a novel MoS2/Au-NP (gold nanoparticle)-L-cysteine nanocomposite was prepared for glycopeptide enrichment. The two-dimensional (2D) structured MoS2 nanosheets served as a matrix that could provide a large surface area for immobilizing hydrophilic groups (such as L-cysteine) with low steric hindrance between the materials and the glycopeptides. As a result, the novel nanomaterial possessed an excellent ability to capture glycopeptides. Compared to commercial zwitterionic hydrophilic interaction liquid chromatography (ZIC-HILIC) materials, the novel nanomaterials exhibited excellent enrichment performance with ultrahigh selectivity and sensitivity (approximately 10 fmol), high binding capacity (120 mg g-1), high enrichment recovery (more than 93%), satisfying batch-to-batch reproducibility, and good universality for glycopeptide enrichment. In addition, its outstanding specificity and efficiency for glycopeptide enrichment was confirmed by the detection of glycopeptides from an human serum immunoglobulin G (IgG) tryptic digest in quantities as low as a 1:1250 molar ratio of IgG tryptic digest to bovine serum albumin tryptic digest. The novel nanocomposites were further used for the analysis of complex samples, and 1920 glycopeptide backbones from 775 glycoproteins were identified in three replicate analyses of 50 μg of proteins extracted from HeLa cell exosomes. The resulting highly informative mass spectra indicated that this multifunctional nanomaterial-based enrichment method could be used as a promising tool for the in-depth and comprehensive characterization of glycoproteomes in MS-based glycoproteomics. [ABSTRACT FROM AUTHOR]

In this paper, we selected a less studied high-speed dispersive homogenizer as a shear-exfoliating device and selected NMP which matches the surface energy of MoS2 as a solvent to prepare few-layer MoS2 nanosheets. The effects of operating parameters on the concentration of few-layer MoS2 nanosheets were systematically studied. The results showed that the change of operating conditions has a direct influence on the exfoliation effects. The concentration of MoS2 nanosheets was 0.96 mg/mL in pure NMP under the optimized conditions. The concentration reached 1.44 mg/mL, and the highest yield was 4.8% after adding sodium citrate. Particularly, their lateral size is about 50-200 nm, in which almost 65% of MoS2 nanosheets are less than four layers and 9% are monolayer. It was verified that the as-used exfoliation method is simple and highly efficient. [ABSTRACT FROM AUTHOR]

The development of denitrification catalysts which can reduce nitrate and nitrite to dinitrogen is critical for sustaining the nitrogen cycle. However, regulating the selectivity has proven to be a challenge, due to the difficulty of controlling complex multielectron/proton reactions. Here we report that utilizing sequential proton-electron transfer (SPET) pathways is a viable strategy to enhance the selectivity of electrochemical reactions. The selectivity of an oxo-molybdenum sulfide electrocatalyst toward nitrite reduction to dinitrogen exhibited a volcano-type pH dependence with a maximum at pH 5. The pH-dependent formation of the intermediate species (distorted Mo(V) oxo species) identified using operando electron paramagnetic resonance (EPR) and Raman spectroscopy was in accord with a mathematical prediction that the pKa of the reaction intermediates determines the pH-dependence of the SPET-derived product. By utilizing this acute pH dependence, we achieved a Faradaic efficiency of 13.5% for nitrite reduction to dinitrogen, which is the highest value reported to date under neutral conditions. [ABSTRACT FROM AUTHOR]

The deep understanding of nucleation and growth mechanisms is fundamental for the precise control of the size, layer number, and crystal quality of two-dimensional (2D) transition-metal dichalcogenides (TMDs) with the chemical vapor deposition (CVD) method. In this work, we present a systematic spectroscopic study of CVD-grown MoS2, and two types of MoS2 flakes have been identified: one type of flake contains a central nanoparticle with the multilayer MoS2 structure, and the other is dominated by triangular flakes with monolayer or bilayer structures. Our results demonstrate that two types of flakes can be tuned by changing the growth temperature and carrier-gas flux, which originates from their different nucleation mechanisms that essentially depends on the concentration of MoO3–x and S vapor precursors: a lower reactant concentration facilitates the 2D planar nucleation that leads to the monolayer/bilayer MoS2 and a higher reactant concentration induces the self-seeding nucleation which easily produces few-layer and multilayer MoS2. The reactant-concentration dependence of nucleation can be used to control the growth of MoS2 and understand the growth mechanism of other TMDs. [ABSTRACT FROM AUTHOR]

Wear occurs between two rubbing surfaces. Severe wear due to seizure under high pressure leads to catastrophic failures of mechanical systems and raises wide concerns. In this paper, a kind of synthetic oil-soluble ultrathin MoS2 sheets is synthesized and investigated as lubricant additives between steel surfaces. It is found that, with the ultrathin MoS2 sheets, the wear can be controlled under the nominal pressure of about 1 GPa, whereas the bearable nominal pressure for traditional lubricants is only a few hundred megapascals. It is found that when wear is under control, the real pressure between the asperities agrees with the breaking strength of ultrathin MoS2. Therefore, it is believed that, because of the good oil solubility and ultrasmall thickness, the ultrathin MoS2 sheets can easily enter the contact area between the contacting asperities. Then, the localized seizure and further wear are prevented because there will be no metal-to-metal contact as long as the real pressure between the asperities is below the breaking strength of ultrathin MoS2. In this way, the upper limit pressure the lubricant can work is dependent on the mechanical properties of the containing ultrathin two-dimensional (2D) sheets. Additionally, ultrathin MoS2 sheets with various lateral sizes are compared, and it is found that sheets with a larger size show better lubrication performance. This work discovers the lubrication mechanism of ultrathin MoS2 sheets as lubricant additives and provides an inspiration to develop a novel generation of lubricant additives with high-strength ultrathin 2D materials. [ABSTRACT FROM AUTHOR]

Ternary Fe2O3-MoS2-Cu2O nanocomposites were fabricated via electrodeposition and hydrothermal method. The as-prepared photocatalytic films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results indicated that MoS2 and Cu2O particles were successfully deposited onto the surface of Fe2O3 particles. MoS2 and Cu2O coloading achieved a synergetic effect on the improvement of the photoelectrochemical performance of Fe2O3 film. The highest photocurrent density was achieved on Fe2O3-MoS2-Cu2O film, which was 20, ~5.5, and 2 times those of Fe2O3, Fe2O3-MoS2, and Fe2O3-Cu2O films, respectively. The excellent photoelectrocatalytic performance was attributed to the Z-scheme electron transfer mechanism, which results in the fast charge transfer and strong redox ability on the ternary composite. This work provides a promising Z-scheme ternary semiconductor for environmental purification and water oxidation. [ABSTRACT FROM AUTHOR]

Cd-based semiconductor quantum dots (QDs) with size-tunable luminescence and high quantum yield have become the most promising electrochemiluminescence (ECL) emitters. However, their unavoidable biotoxicity limited their applications in bioassays. Here, the nontoxic and economical MoS2 QDs prepared by chemical exfoliation from the bulk MoS2 were first investigated as new ECL emitters, and then the possible luminescence mechanism of MoS2 QDs was studied using ECL-potential curves and differential pulse voltammetry (DPV) methods in detail. With MoS2 QDs as the ECL emitters and triethylamine (TEA) as the efficient coreactant, a practical and label-free aptasensor for lipopolysaccharide (LPS) detection was constructed based on aptamer recognition-driven target-cycling synchronized rolling circle amplification. Comparing to conventional stepwise reactions, this target-cycling synchronized rolling circle amplification achieved more efficient signal amplification and simpler operation. The developed assay for LPS detection demonstrated a wide linear range of 0.1 fg/mL to 50 ng/mL with limit of detection down to 0.07 fg/mL. It is worth mentioning that MoS2 QDs with stable ECL emission exhibited a great application potential in ECL bioanalysis and imaging as a new type of excellent emitter candidates. [ABSTRACT FROM AUTHOR]

High-volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultrathin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor-phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metal-organic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)bis(dimethylamido)molybdenum and diethyl disulfide, we deposit MoS2 films from -1 nm to -25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-level uniformity are demonstrated on 50 mm quartz wafers. Process chemistry and impurity incorporation from precursors are also discussed. This low-temperature and fast process highlights the opportunities presented by metal-organic reagents in the controlled synthesis of TMDs. [ABSTRACT FROM AUTHOR]

In this work, ultrathin MoS2 nanosheets (MoS2 NSs) with defect-rich structure were prepared and used as catalysts for the reduction of p-nitropheonol. Also, thanks to the defect sites, noble metal (Au, Ag, Pd, Pt) nanoparticles (NPs) can be successfully deposited on the MoS2 NSs via a facile and efficient one-step photochemical reduction process without the assistance of a stabilizer. The morphology and crystal structure of as-synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 physisorption mearurements. Catalytic results demonstrate that as-prepared MoS2 NSs and noble metal modified MoS2 NSs (NM/MoS2 NSs) effectively catalyze the reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) with NaBH4. The Pd/MoS2 NSs shows the best catalytic performance, exhibiting an apparent rate constant (κapp) of 0.386 min-1 even with moderate chemical stability. Interestingly, the activity of MoS2 NSs is comparable to and even higher than previously studied noble metal-based catalysts. Moreover, the good reusability without significant activity loss suggests that MoS2 NSs can be a strong candidate for sustainable chemical catalysis. The origins of the high catalytic activity are also discussed and a probable reaction mechanism is suggested. The present work opens up new opportunities for the future design and fabrications of versatile MoS2-based composite materials. [ABSTRACT FROM AUTHOR]

MoS x serving as a hydrogen evolution reaction electrocatalyst is known for its morphology sensitive characteristic. The low temperature thermo-decomposition method provides an easy and energy saving pathway to produce highly active MoS x on carbon paper substrates. However, during the precursor solution drying process, the dynamics of liquid wetting behavior dominates the morphology of the precursor salt and eventually the morphology of MoS x . As a result, here, for the first time, by carefully pairing the substrate hydrophobicity and solvent polarity, the cohesive force between solvent molecules and adhesive force between solvent and carbon substrate can be tuned, and thus the MoS x morphology can be controlled. Pairing hydrophilic carbon paper with DMF + H2O mixing solvent results in a relatively strong adhesive force, as a result, we are able to lower the overpotential required at the benchmark current density, 10 mA/cm², to as low as 0.160 V and boost the current density to 40 mA/cm² at -+0.2 V vs RHE. This mainly results from the low charge transfer resistance and the well wrapped MoS x on carbon paper fiber structure. Furthermore, this well wrapped MoS x on hydrophilic carbon paper was proved to be comparably stable for constant voltage electrolysis operation. [ABSTRACT FROM AUTHOR]

Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T'-ReSe2 regardless of crystal thickness, indicating weak interlayer coupling effects on the band gap characteristics. Our results pave the way for polarization-sensitive applications, such as optical logic circuits operating in the infrared spectral region. [ABSTRACT FROM AUTHOR]

We report on the facile preparation of versatile MoS2-thiobarbituric acid conjugates, which, in addition to excellent electrochemical behavior, can serve as nanosheet platforms for further functionalization in a multitude of applications. We show that chemically exfoliated MoS2 was extensively modified with up to 50% surface coverage, while maintaining its metallic character, and that the strategy can be extended to MoSe2, WS2, and WSe2. The covalent functionalization endowed the materials not only with good aqueous dispersibility, but also with improved hydrogen evolution reaction (HER) activity, as well as promise in the oxidative detection of DNA nucleobases in solution. [ABSTRACT FROM AUTHOR]

Recent advances in the field of two-dimensional (2D) transition metal dichalcogenide (TMD) materials have indicated that atomic layer deposition (ALD) of the metal oxide and subsequent sulfidation could offer a method for the synthesis of large area two-dimensional materials such as MoS2 with excellent layer control over the entire substrate. However, growing large area oxide films by ALD with sub 1 nm nucleation coalescence remains a significant challenge, and the necessary steps are unexplored. In this work, we demonstrate the necessary process improvements required to achieve sub 1 nm nucleation control by characterization of nucleation domains formed by oxide deposition. Synthesis of the TMD MoS2 from sulfidation of oxide deposited by both thermal ALD from (tBuN)2(NMe2)2Mo and O3 and plasma enhanced ALD (PEALD) from (tBuN)2(NMe2)2Mo and remote O2 plasma was performed. Large uniform MoS2 areas were achieved by optimizing the effects of various growth process conditions and surface treatments on the ALD nucleation and growth of Mo-oxide and the postsulfidation of MoS2. In addition to insights into the control of the oxide deposition, film chemistry analysis during a multistep sulfidation based on less toxic sulfur as compared to H2S was performed for several temperature profiles revealing sulfur incorporation and molybdenum reduction at low temperatures but higher temperatures required for 2H crystal structure formation. The knowledge gained of the ALD, PEALD, and postsulfidation was leveraged to demonstrate tunable film thickness and centimeter-scale monolayer growth. Material quality can be studied independently of the MoS2 layer count as demonstrated by the control of the monolayer photoluminescence intensity by the temperature ramp rate during sulfidation. [ABSTRACT FROM AUTHOR]

By selectively promoting heterogeneous nucleation/growth of MoS2 on graphene monolayer sheets, edge-oriented (EO) MoS2 nanosheets with expanded interlayer spacing (∼9.4 Å) supported on reduced graphene oxide (rGO) sheets were successfully synthesized through colloidal chemistry, showing the promise in low-cost and large-scale production. The number and edge length of MoS2 nanosheets per area of graphene sheets were tuned by controlling the reaction time in the microwave-assisted solvothermal reduction of ammonium tetrathiomolybdate [(NH4)2MoS4] in dimethylformamide. The edge-oriented and interlayer-expanded (EO&IE) MoS2/rGO exhibited significantly improved catalytic activity toward hydrogen evolution reaction (HER) in terms of larger current density, lower Tafel slope, and lower charge transfer resistance compared to the corresponding interlayer-expanded MoS2 sheets without edge-oriented geometry, highlighting the importance of synergistic effect between edge-oriented geometry and interlayer expansion on determining HER activity of MoS2 nanosheets. Quantitative analysis clearly shows the linear dependence of current density on the edge length of MoS2 nanosheets. [ABSTRACT FROM AUTHOR]

We suggest a unimer-assisted exfoliation method for the exfoliation of van der Waals two-dimensional (2D) materials such as graphene, MoS2, and h-BN and show that the micellar size is a critical parameter for enhancing the exfoliation efficiency. To explain the effectiveness of the unimers in the exfoliation, the influence of the micellar size of a biocompatible block copolymer, Pluronic F-68, is evaluated in view of the yield and thickness of exfoliated 2D flakes. By the addition of water-soluble alcohols, the surfactants exist in the form of a unimer, which facilitates the intercalation into the layered materials and their exfoliation. The results showed that the high exfoliation efficiency could be achieved by controlling the micellar size mostly to be unimers; the average yield rate of MoS2 exfoliation was 4.51% per hour, and the very high concentration of 1.45 mg/mL was obtained by sonication for 3 h. We also suggested the dielectrophoresis technique as a method for forming a film composed of 2D flakes for diverse applications requiring electrical signals. The unimer-assisted exfoliation method will be substantially utilized to achieve highly concentrated aqueous dispersion solutions of 2D materials. [ABSTRACT FROM AUTHOR]

We report herein investigation on crystallization of amorphous molybdenum sulfide a-MoSx induced by electron and laser beam resulting in formation of crystalline molybdenum disulfide c-MoS2. This crystallization occurred in situ during transmission electron microscopic and Raman analyses of a-MoSx material. It was also found that a-MoSx to c-MoS2 phase transformation was not fully beneficial for H2-evolving catalytic performance. c-MoS2 showed better robustness but significantly lower catalytic performance. Furthermore, c-MoS2 was less tolerant to oxidation stress, as the one caused by photogenerated holes within the light harvester, compared with a-MoSx catalyst. Thus, a-MoSx is a better candidate for implementation within photocatalysts for overall solar water-splitting application. [ABSTRACT FROM AUTHOR]

We report the sulfur evaporation rate controlled screw dislocation driven growth of two-dimensional MoS2 that contains unprecedented atomic step density. Screw dislocation assisted growth of atomic thin MoS2 on amorphous SiO2 or a crystalline and conducting Si substrate paves the way to form spiral morphology of two-dimensional materials without the need for single crystalline miscut substrates to initiate the line defects. The unique spiral morphology promoted by screw dislocation is typically observed at a high nucleation rate induced by an abrupt increase in sulfur concentration. Screw dislocation assisted spiral MoS2 growth leads to a high step density with an ∼5 fold increase in the total edge length. Statistical analysis from detailed atomic force microscope phase imaging of flat and spiral MoS2 flakes reveals the linear increase in total edge length from ∼470 nm to ∼2325 nm with respect to variation in the number of steps available at the spiral structure in the range of 2-8. High resolution transmission electron microscopy imaging reveals the local atomic structure of the ledge that separates the individual layers in spiral MoS2 structure and aids the development of growth mechanism. The optimized spiral growth of MoS2 provides countless active sites for hydrogen energy generation applications. Hydrogen evolution performance of as-grown MoS2 on p-type Si substrates decorated with high ledge density provides active adsorption sites leading to hydrogen evolution at a lower potential with a higher current density. The developed strategy of increasing the sulfur evaporation rate to induce a spike in the nucleation rate promotes screw dislocation driven growth of MoS2 with spiral morphology. Such sulfur evaporation rate controlled spiral growth is important for surface engineering of two-dimensional materials to harvest the benefits of active edge sites for hydrogen evolution, catalyst, and sensor applications. [ABSTRACT FROM AUTHOR]

To prepare atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) single atomic crystals with highly tunable photoluminescence (PL) is of special importance in 2D photonics and optoelectronics. However, to control the PL in 2D TMDs atomic crystals is still a big challenge due to the atomically thin 2D nature which induces far more complicated and variable PL behaviors than normal semiconductors. In this report, triangular and tetragonal MoS2 atomic crystals are prepared simultaneously on one single growth substrate for the first time in a single chemical vapor deposition process. Through manipulating the growth dynamics to reduce nucleation density and enhance lateral growth with mixture precursor and other growth parameters, both the morphology and PL of the MoS2 atomic crystals can be efficiently modulated. The PL of the optimized triangular MoS2 atomic crystals can be over one to two orders higher than the PL of other triangular morphology modulated by different growth dynamics. In addition, position-dependent PL and a gradient decrease in PL intensity from the center toward edge region are found in both triangular and tetragonal MoS2 atomic crystals, which show 1-2 orders of magnitude of decrease in the PL and imply the strong influences of atomic structure disorders and edges considering the high-resolution transmission electron microscopy observations and first-principles calculations. These results indicate a facile way to tune the PL of TMDs by morphology and atomic structure. [ABSTRACT FROM AUTHOR]

Whether an alloy is random or ordered can have profound effects on its properties. The close chemical similarity of W and Mo in the two-dimensional semiconductors MoS2 and WS2 has led to the expectation that WxMo1-xS2 is a random alloy. Here we report that triangular monolayer flakes of WxMo1-xS2 produced by sulfurization of MoO3/WO3 are not only nonrandom, but also anisotropic: W and Mo form atomically thin chains oriented parallel to the edges of the triangle, especially around x ∼ 0.5, as resolved by aberration-corrected transmission electron microscopy. First-principles calculations reveal that the binding energies of striped and random alloys are nearly identical but that phase segregation at the growth edge favors one metal over another depending on the local sulfur availability, independent of the composition deeper inside the monolayer. Thus, atomically thin striping is kinetically driven and controlled by fluctuations that couple the local chemical potentials of metals and chalcogenide. Considering the nearly identical electronic properties but very different atomic masses of Mo and W, the resulting striped alloy is electronically isotropic, but vibrationally anisotropic. Phonon anomalies associated with the stripe ordering are predicted, as is an anisotropic thermal conductivity. More generally, fluctuation-driven striping provides a mechanism to produce in-plane subnanometer superlattices within two-dimensional crystals, with broad implications for controlling the electronic, optical, and structural properties of these systems. [ABSTRACT FROM AUTHOR]

Molybdenum sulfide (MoS2) is widely recognized for its catalytic activities where the edges of the crystals turn over reactions. Generating sulfur defects on the basal plane of MoS2 can improve its catalytic activity, but generally, there is a lack of model systems for understanding metal-centered catalysis on the basal planes. Here, we synthesized a new phase of substoichiometric molybdenum sulfide (s-MoSx) on a sulfur-enriched copper substrate. The basal plane of s-MoSx contains chemically reactive Mo-rich sites that can undergo dynamic dissociative adsorption/desorption processes with molecular hydrogen, thus demonstrating its usefulness for hydrogen-transfer catalysis. In addition, scanning tunneling microscopy was used to monitor surface-directed Ullmann coupling of 2,8-dibromo-dibenzothiophene molecules on s-MoSx nanosheets, where the 4-fold symmetric surface sites on s-MoSx direct C–C coupling to form cyclic tetramers with high selectivity. [ABSTRACT FROM AUTHOR]

Layered hexagonal boron nitride (h-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS2 can induce ∼6.5 × 1011 cm-2 electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS2 on h-BN was found to be ∼5 × 1013 cm-2 at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS2/h-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal-insulator transition at a much lower charge density of ∼1.0 × 1012 cm-2 (T = 25 K). The reduced effective Schottky barrier height in MoS2/h-BN is attributed to the decreased effective work function of MoS2 arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO2. [ABSTRACT FROM AUTHOR]

Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The critical 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temperature-composition (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available experiments, and the present data provide quantitative insight into the controlled growth of 2D layered MoS2. [ABSTRACT FROM AUTHOR]

It is well-known that excitonic effects can dominate the optical properties of two-dimensional materials. These effects, however, can be substantially modified by doping free carriers. We investigate these doping effects by solving the first-principles Bethe-Salpeter equation. Dynamical screening effects, included via the sum-rule preserving generalized plasmon-pole model, are found to be important in the doped system. Using monolayer MoS2 as an example, we find that upon moderate doping, the exciton binding energy can be tuned by a few hundred millielectronvolts, while the exciton peak position stays nearly constant due to a cancellation with the quasiparticle band gap renormalization. At higher doping densities, the exciton peak position increases linearly in energy and gradually merges into a Fermi-edge singularity. Our results are crucial for the quantitative interpretation of optical properties of two-dimensional materials and the further development of ab initio theories of studying charged excitations such as trions. [ABSTRACT FROM AUTHOR]

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated. [ABSTRACT FROM AUTHOR]

By implementing four-wave mixing (FWM) microspectroscopy, we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe2. We reveal their dephasing times T2 and radiative lifetime T1 in a subpicosecond (ps) range, approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature of 6 K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At the nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse toward the dark states but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer intervalley exciton dynamics, revealing an initial polarization degree of around 30%, constant during the initial subpicosecond decay, followed by the depolarization on a picosecond time scale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing us to investigate the neutral exciton, the charged one, or both transitions at the same time. In the latter, we observe the exciton-trion beating in the coherence evolution indicating their coherent coupling. [ABSTRACT FROM AUTHOR]

Monolayers of transition metal dichalcogenides are interesting materials for optoelectronic devices due to their direct electronic band gaps in the visible spectral range. Here, we grow single layers of MoS2 on Au(111) and find that nanometer-sized patches exhibit an electronic structure similar to their freestanding analogue. We ascribe the electronic decoupling from the Au substrate to the incorporation of vacancy islands underneath the intact MoS2 layer. Excitation of the patches by electrons from the tip of a scanning tunneling microscope leads to luminescence of the MoS2 junction and reflects the one-electron band structure of the quasi-freestanding layer. [ABSTRACT FROM AUTHOR]

Controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS2/WS2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods. [ABSTRACT FROM AUTHOR]

Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.16 ± 0.04 eV) and WS2 (2.38 ± 0.06 eV) in the materials as measured on the heterostructure regions and the general type II band alignment for the heterostructure, which shows an interfacial band gap of 1.45 ± 0.06 eV. [ABSTRACT FROM AUTHOR]

C60 and single-layer MoS2 nanocomposites were facilely prepared via a combined solvent transfer and surface deposition (STSD) method by templating C60 aggregates on 2D MoS2 nanosheets to construct hybrid van der Waals heterojunctions. The electronic property of the hybrid nanomaterials was investigated in a direct charge transport diode device configuration of ITO/C60-MoS2 nanocomposites/Al; rewritable nonvolatile resistive switching with low SET/RESET voltage (∼3 V), high ON/OFF resistance ratio (∼4 × 10³), and superior electrical bistability (>104 s) of a flash memory behavior was observed. This particular electrical property of C60-MoS2 nanocomposites, not possessed by either C60 or MoS2 nanosheets, was supposed to be due to the efficiently established C60/MoS2 p-n nanojunction, which controls the electron tunneling via junction barriers modulated by electric-field-induced polarization. Thus, our 2D templating method through STSD is promising to massively allocate van der Waals p-n heterojunctions in 2D nanocomposites, opening a window for important insights into the charge transport across the interface of organic/2D-semiconductors. [ABSTRACT FROM AUTHOR]

Research into layered transition metal dichalcogenides (TMDCs), most notably those of molybdenum and tungsten disulfides, has become extensive, involving fields as diverse as optoelectronics, spintronics, energy storage, lubrication, and catalysis. The modification of TMDCs by transition metal doping can improve their performance in such applications and hence extend their potential for technological applications. This review concerns the synthetic strategies that have been used to incorporate transition metals into TMDCs and the applications of the resultant materials and relevant computational studies on the predicted properties of the doped materials. [ABSTRACT FROM AUTHOR]

Atomic-level visualization of the intercalation of layered materials, such as metal chalcogenides, is of paramount importance in the development of high-performance batteries. In situ images of the dynamic intercalation of Li ions into MoS2 single-crystal electrodes were acquired for the first time, under potential control, with the use of a technique combining laser confocal microscopy with differential interference microscopy. Intercalation proceeded via a distinct phase separation of lithiated and delithiated regions. The process started at the atomic steps of the first layer beneath the selvedge and progressed in a layer-by-layer fashion. The intercalated regions consisted of Li-ion channels into which the newly inserted Li ions were pushed atom-by-atom. Interlayer diflusion of Li ions was not observed. Deintercalation was also clearly imaged and was found to transpire in a layer-by-layer mode. The intercalation and deintercalation processes were chemically reversible and can be repeated many times within a few atomic layers. Extensive intercalation of Li ions disrupted the atomically flat surface of MoS2 because of the formation of small lithiated domains that peeled off from the surface of the crystal. The current-potential curves of the intercalation and deintercalation processes were independent of the scan rate, thereby suggesting that the rate-determining step was not governed by Butler--Volmer kinetics. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se) hold great potential in optoelectronics and photovoltaics. To achieve efficient light-to-electricity conversion, electron-hole pairs must dissociate into free charges. Coulomb interaction in MX2 often exceeds the charge transfer driving force, leading one to expect inefficient charge separation at a MX2 heterojunction. Experiments defy the expectation. Using time-domain density functional theory and nonadiabatic (NA) molecular dynamics, we show that quantum coherence and donor-acceptor delocalization facilitate rapid charge transfer at a MoS2/MoSe2 interface. The delocalization is larger for electron than hole, resulting in longer coherence and faster transfer. Stronger NA coupling and higher acceptor state density accelerate electron transfer further. Both electron and hole transfers are subpicosecond, which is in agreement with experiments. The transfers are promoted primarily by the out-of-plane Mo-X modes of the acceptors. Lighter S atoms, compared to Se, create larger NA coupling for electrons than holes. The relatively slow relaxation of the "hot" hole suggests long-distance bandlike transport, observed in organic photovoltaics. The electron-hole recombination is notably longer across the MoS2/MoSe2 interface than in isolated MoS2 and MoSe2, favoring long-lived charge separation. The atomistic, time-domain studies provide valuable insights into excitation dynamics in two-dimensional transition metal dichalcogenides. [ABSTRACT FROM AUTHOR]

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6]3+/2+ and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination. [ABSTRACT FROM AUTHOR]

We report a new strategy for fabricating 2D/2D low-resistance ohmic contacts for a variety of transition metal dichalcogenides (TMDs) using van der Waals assembly of substitutionally doped TMDs as drain/source contacts and TMDs with no intentional doping as channel materials. We demonstrate that few-layer WSe2 field-effect transistors (FETs) with 2D/2D contacts exhibit low contact resistances of ~0.3 kΩ µm, high on/off ratios up to >109, and high drive currents exceeding 320 µA µm-1. These favorable characteristics are combined with a two-terminal field-effect hole mobility µFE ≈ 2 x 10² cm² V-1 s-1 at room temperature, which increases to >2 x 10³ cm² V-1 s-1 at cryogenic temperatures. We observe a similar performance also in MoS2 and MoSe2 FETs with 2D/2D drain and source contacts. The 2D/2D low-resistance ohmic contacts presented here represent a new device paradigm that overcomes a significant bottleneck in the performance of TMDs and a wide variety of other 2D materials as the channel materials in postsilicon electronics. [ABSTRACT FROM AUTHOR]

We present a novel metal-insulator-semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS2 for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (I-V) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p-n junction and the metal-semiconductor diodes made of layered materials. The I-V characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination. [ABSTRACT FROM AUTHOR]

Emerging two-dimensional semiconductor materials possess a giant second order nonlinear response due to excitonic effects while the monolayer thickness of such active materials limits their use in practical nonlinear devices. Here, we report 3300 times optomechanical enhancement of second harmonic generation from a MoS2 monolayer in a doubly resonant on-chip optical cavity. We achieve this by engineering the nonlinear light-matter interaction in a microelectro-mechanical system enabled optical frequency doubling device based on an electrostatically tunable Fabry-Perot microresonator. Our versatile optomechanical approach will pave the way for next generation efficient on-chip tunable light sources, sensors, and systems based on molecularly thin materials. [ABSTRACT FROM AUTHOR]

We demonstrate electrical control of the A-exciton interband transition in mono- and few-layer MoS2 crystals embedded into photocapacitor devices via the DC Stark effect. Electric field-dependent low-temperature photoluminescence spectroscopy reveals a significant tuneability of the A-exciton transition energy up to ~16 meV from which we extract the mean DC exciton polarizability (βN) = (0.58 ± 0.25) x 10-8 Dm V-1. The exciton polarizability is shown to be layer-independent, indicating a strong localization of both electron and hole wave functions in each individual layer. [ABSTRACT FROM AUTHOR]

A cold-wall chemical vapor deposition method for growing MoS2 that allows independent control over all deposition parameters is described. Ar carrier gas flow rate and pressure, substrate temperature, and the temperatures of the individual solid-source precursors can all be independently varied during growth onto 100-nm-thick SiO2 films on Si substrates. Individually optimizing each deposition parameter enables formation of islanded, single-layer MoS2 films with excellent run-to-run repeatability in coverage and uniformity over several square millimeter areas. Auger electron spectroscopy (AES) and Rutherford backscattering spectrometry were used to quantify the elemental composition of the films. Film morphology was analyzed using scanning electron microscopy (SEM). This analysis indicates that excess Mo deposits on the SiO2 substrate without incorporating into the stoichiometric single-layer MoS2 clusters imaged by SEM. In addition, the amount of S detected using AES is nearly identical to the amount required to form the MoS2 islands. [ABSTRACT FROM AUTHOR]

We report a systematic study of coherent spin precession and spin dephasing in electron-doped monolayer MoS2. Using time-resolved Kerr rotation spectroscopy and applied in-plane magnetic fields, a nanosecond time scale Larmor spin precession signal commensurate with g-factor |g0| ≃ 1.86 is observed in several different MoS2 samples grown by chemical vapor deposition. The dephasing rate of this oscillatory signal increases linearly with magnetic field, suggesting that the coherence arises from a subensemble of localized electron spins having an inhomogeneously broadened distribution of g-factors, g0 + Δg. In contrast to g0, Δg is sample-dependent and ranges from 0.042 to 0.115. [ABSTRACT FROM AUTHOR]

We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap Δ of monolayer MoS2 directly from the device transfer curves and find Δ ≈ 2.4-2.7 eV. In the ambipolar injection regime, we observe electroluminescence due to exciton recombination in MoS2, originating from the region close to the hole-injecting contact. Both the observed transport properties and the behavior of the electroluminescence can be consistently understood as due to the presence of defect states at an energy of 250-300 meV above the top of the valence band, acting as deep traps for holes. Our results are of technological relevance, as they show that devices with useful optoelectronic functionality can be realized on large-area MoS2monolayers produced by controllable and scalable techniques. [ABSTRACT FROM AUTHOR]

We present results on photoexcited carrier lifetimes in few-layer transition metal dichalcogenide MoS2 using nondegenerate ultrafast optical pump-probe technique. Our results show a sharp increase of the carrier lifetimes with the number of layers in the sample. Carrier lifetimes increase from few tens of picoseconds in monolayer samples to more than a nanosecond in 10-layer samples. The inverse carrier lifetime was found to scale according to the probability of the carriers being present at the surface layers, as given by the carrier wave function in few layer samples, which can be treated as quantum wells. The carrier lifetimes were found to be largely independent of the temperature, and the inverse carrier lifetimes scaled linearly with the photoexcited carrier density. These observations are consistent with defect-assisted carrier recombination, in which the capture of electrons and holes by defects occurs via Auger scatterings. Our results suggest that carrier lifetimes in few-layer samples are surface recombination limited due to the much larger defect densities at surface layers compared with the inner layers. [ABSTRACT FROM AUTHOR]

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin-orbit coupling (SOC) and interlayer coupling in different structural symmetries. Such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS2 blocks. [ABSTRACT FROM AUTHOR]

Artificial semiconductors with manufactured band structures have opened up many new applications in the field of optoelectronics. The emerging two-dimensional (2D) semiconductor materials, transition metal dichalcogenides (TMDs), cover a large range of bandgaps and have shown potential in high performance device applications. Interestingly, the ultrathin body and anisotropic material properties of the layered TMDs allow a wide range modification of their band structures by electric field, which is obviously desirable for many nanoelectronic and nanophotonic applications. Here, we demonstrate a continuous bandgap tuning in bilayer MoS2 using a dual-gated field-effect transistor (FET) and photoluminescence (PL) spectroscopy. Density functional theory (DFT) is employed to calculate the field dependent band structures, attributing the widely tunable bandgap to an interlayer direct bandgap transition. This unique electric field controlled spontaneous bandgap modulation approaching the limit of semiconductor-to-metal transition can open up a new field of not yet existing applications. [ABSTRACT FROM AUTHOR]

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ~ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications. [ABSTRACT FROM AUTHOR]

We report the synthesis of ion-exchangeable molybdenum sulfide chalcogel through an oxidative coupling process, using (NH4)2MoS4 and iodine. After supercritical drying, the MoSx amorphous aerogel shows a large surface area up to 370 m2/g with a broad range of pore sizes. X-ray photoelectron spectroscopic and pair distribution function analyses reveal that Mo6+ species undergo reduction during network assembly to produce Mo4+-containing species where the chalcogel network consists of [Mo3S13] building blocks comprising triangular Mo metal clusters and S22- units. The optical band gap of the brown-black chalcogel is ~1.36 eV. The ammonium sites present in the molybdenum sulfide chalcogel network are ion-exchangeable with K+ and Cs+ ions. The molybdenum sulfide aerogel exhibits high adsorption selectivities for CO2 and C2H6 over H2 and CH4. The aerogel also possesses high affinity for iodine and mercury. [ABSTRACT FROM AUTHOR]

Ultrasmooth hexagonal boron nitride (h-BN) can dramatically enhance the carrier/phonon transport in interfaced transition metal dichalcogenides (TMDs), and amplify the effect of quantum capacitance in field-effect gating. All of the current processes to realize h-BN-based heterostructures involve transfer or exfoliation. Rational chemistries and process techniques are still required to produce large-area, transfer-free, directly grown TMDs/BN heterostructures. Here, we demonstrate a novel boron-oxygen chemistry route for oxide-assisted nucleation and growth of large-area, uniform, and ultrathin h-BN directly on oxidized substrates (B/N atomic ratio = 1:1.16 ± 0.03 and optical band gap = 5.51 eV). These intimately interfaced, van der Waals heterostructures of MoS2/h-BN and WS2/h-BN benefit from 6.27-fold reduced roughness of h-BN in comparison to SiO2. This leads to reduction in scattering from roughness and charged impurities, and enhanced carrier mobility verified by an increase in electrical conductivity (5 times for MoS2/h-BN and 2 times for WS2/h-BN). Further, the heterostructures are devoid of wrinkles and adsorbates, which is critical for 2D nanoelectronics. The versatile process can potentially be extrapolated to realize a variety of heterostructures with complex sandwiched 2D electronic circuitry. [ABSTRACT FROM AUTHOR]

Two-dimensionallayered materials like MoS2have shown promise for nanoelectronicsand energy storage, both as monolayers and as bulk van der Waals crystalswith tunable properties. Here we present a platform to tune the physicaland chemical properties of nanoscale MoS2by electrochemicallyinserting a foreign species (Li+ions) into their interlayerspacing. We discover substantial enhancement of light transmission(up to 90% in 4 nm thick lithiated MoS2) and electricalconductivity (more than 200×) in ultrathin (∼2–50nm) MoS2nanosheets after Li intercalation due to changesin band structure that reduce absorption upon intercalation and theinjection of large amounts of free carriers. We also capture the firstin situ optical observations of Li intercalation in MoS2nanosheets, shedding light on the dynamics of the intercalationprocess and the associated spatial inhomogeneity and cycling-inducedstructural defects. [ABSTRACT FROM AUTHOR]

Two-dimensional(2-D) materials including graphene and transitionmetal dichalcogenides (TMDs) are an exciting platform for ultrasensitiveforce and displacement detection in which the strong light–mattercoupling is exploited in the optical control of nanomechanical motion.Here we report the optical excitation and displacement detection ofa ∼ 3 nm thick MoS2resonator in the strong-couplingregime, which has not previously been achieved in 2-D materials. Mechanicalmode frequencies can be tuned by more than 12% by optical heating,and they exhibit avoided crossings indicative of strong intermodecoupling. When the membrane is optically excited at the frequencydifference between vibrational modes, normal mode splitting is observed,and the intermode energy exchange rate exceeds the mode decay rateby a factor of 15. Finite element and analytical modeling quantifiesthe extent of mode softening necessary to control intermode energyexchange in the strong coupling regime. [ABSTRACT FROM AUTHOR]