Defect-free MoS2 Research Articles

Memristive feature and mechanism induced by laser-doping in defect-free 2D semiconductor materials

Memristors as non-volatile memory devices have gained numerous attentions owing to their advantages in storage, in-memory computing, synaptic applications, etc. In recent years, two-dimensional (2D) materials with moderate defects have been discovered to exist memristive feature. However, it is very difficult to obtain moderate defect degree in 2D materials, and studied on modulation means and mechanism becomes urgent and essential. In this work, we realized memristive feature with a bipolar switching and a configurable on/off ratio in a two-terminal MoS2 device (on/off ratio ~100), for the first time, from absent to present using laser-modulation to few-layer defect-free MoS2 (about 10 layers), and its retention time in both high resistance state and low resistance state can reach 2 × 104 s. The mechanism of the laser-induced memristive feature has been cleared by dynamic Monte Carlo simulations and first-principles calculations. Furthermore, we verified the universality of the laser-modulation by investigating other 2D materials of TMDs. Our work will open a route to modulate and optimize the performance of 2D semiconductor memristive devices.

CVD growth of large-area, continuous, and defect-free MoS2 multilayer films from solution-cast seed nanoflakes

Defects in transition metal dichalcogenides are among the primary source of their chemical reactivity, including their response to water. The CVD growth of defect-free continuous molybdenum disulfide (MoS2) films has been particularly challenging. In this work, we demonstrate the growth of large-area continuous films of MoS2 using solution-cast MoS2 seed nanoflakes, both with and without the assistance of alkali halide. The MoS2 seed nanoflakes serve as nucleation sites for the in-plane growth of few-layer MoS2, which is accompanied by localized vertical growth of MoS2 particles. For films grown with the combination of seed and NaCl, very low oxygen content was observed together with the stoichiometric ratio of Mo: S being close to the ideal value of 1: 2. These films show humidity-insensitive electrical response over a wide interval (5%–95% RH). This is attributed to very low concentrations of edge and basal-plane defects, leading to low water dissociation and restricted proton transport.

Regulating Local Atomic Environment around Vacancies for Efficient Hydrogen Evolution.

Defect engineering is essential for the development of efficient electrocatalysts at the atomic level. While most work has focused on various vacancies as effective catalytic modulators, little attention has been paid to the relation between the local atomic environment of vacancies and catalytic activities. To face this challenge, we report a facile synthetic approach to manipulate the local atomic environments of vacancies in MoS2 with tunable Mo-to-S ratios. Our studies indicate that the MoS2 with more Mo terminated vacancies exhibits better hydrogen evolution reaction (HER) performance than MoS2 with S terminated vacancies and defect-free MoS2. The improved performance originates from the adjustable orbital orientation and distribution, which is beneficial for regulating H adsorption and eventually boosting the intrinsic per-site activity. This work uncovers the underlying essence of the local atomic environment of vacancies on catalysis and provides a significant extension of defect engineering for the rational design of transition metal dichalcogenides (TMDs) catalysts and beyond.

Synergistic Production of Pyruvic Acid and Propionic Acid over Defect-Rich MoS2

This work reported the coupling reaction of lactic acid (LA) to produce pyruvic acid (PyA) and propionic acid (PA) simultaneously over defect-rich MoS2 nanosheets. The process intensification linked to dehydrogenation of LA and hydrodeoxygenation of LA brings about the advantage that is not supplied additionally with oxygen and hydrogen, achieving a green and safe route. The defected structure is characterized by high-resolution transmission electron microscopy (HRTEM) and positron annihilation lifetime spectroscopy (PALS), which promotes the catalytic performance for the coupling reaction of LA. To fully understand the importance of the defected structure, the catalytic performance of defect-free MoS2 is also evaluated. It is found that the PyA/PA ratio is less than 1 and acetaldehyde selectivity is relatively high, suggesting that the produced hydrogen from the dehydrogenation of LA is insufficient for hydrodeoxygenation of LA over defect-free MoS2, and the difference of hydrogen comes from the side reactions of decarbonylation and decarboxylation of LA. Unlike the defect-free MoS2, the dehydrogenation of LA over the defect-rich MoS2 can provide sufficient hydrogen for hydrodeoxygenation of LA, so that the side reactions are drastically weakened. The widened interplanar spacing can fully expose the defect and the LA reactant molecule is more accessible to defective sites, which can endow a better catalytic activity. Surface hydrophilicity can improve adsorption and transfer, and better activity is observed over the defect-rich MoS2 and widened defect-rich MoS2. At 300 °C and a LA LHSV of 2.6 h–1 under an inert atmosphere, the catalyst offers a satisfactory total selectivity to pyruvic acid and propionic acid of about 85%, and the molar ratio of pyruvic acid/propionic acid is 1.1.

Brittle and ductile behavior in monolayer MoS2

Using first-principles calculations, we study the response of monolayer MoS2 with pentagon–heptagon (5|7) pair defect-dominated grain boundary (GB) to an in-plane tension. Brittle or ductile behaviors can occur depending on the GB tilt angle (θ). Brittle failure is dominated by step-by-step dissociation of bonds at GBs, while ductile failure is characterized by a series of reaction and glide events of dislocations. Surprisingly, as θ = 9.40°, 5|7 dislocation defects are annihilated by stretching to form defect-free MoS2. Moreover, the failure strength increases with increasing 5|7 dislocation defect and can be explained by the continuum disclination dipole theory.

Single Transition Metal Atom Bound to the Unconventional Phase of the MoS2 Monolayer for Catalytic Oxygen Reduction Reaction: A First-Principles Study.

Supported single-atom catalysts (SACs) have received a lot of attention due to their super-high atom utilization and outstanding catalytic performance. However, the instability of the supported transition-metal (TM) atoms hampers their widespread applications. Exploration of an appropriate substrate to stabilize the supported single atom is crucial for the future implementation of SACs. In recent years, two-dimensional materials have been proposed as possible substrates due to their large specific surface areas, but their chemically inert surfaces are difficult to stabilize TM atoms without defecting or doping. Herein, by means of systematic first-principles calculations, we demonstrate that the defect-free MoS2 monolayer in the unconventional phase (1T') can effectively immobilize single TM atoms owing to its unique electrophilic property as compared to the conventional 2H phase. As a prototype probe, we investigated oxygen reduction reaction (ORR) catalyzed by a total of 21 single TM atoms stabilized on 1T'-MoS2 and successfully screened out two candidates, Cu and Pd@1T'-MoS2, which have a low overpotential of 0.41 and 0.32 V respectively, outperforming most of the previously reported ORR catalysts. Furthermore, we reveal that the adsorption energy of the ORR intermediate, *OH, provides an excellent descriptor to assess the ORR activity, which is further determined by the d-band center of the supported TM adatoms, thus being a great advantage for future design of stable and high-performance SACs.

Synergetic role of charge transfer and strain engineering in improving the catalysis of Pd single-atom-thick motifs stabilized on a defect-free MoS2/Ag(Au)(111) heterostructure

Single-atom-thick-PdN magic nanomotifs are demonstrated to be stabilized on defect-free MoS2 overlayer supported on Ag(111) via the synergetic role of charge transfer among the Pd active sites, leading to superior catalytic performance to SAC-Pd1.

Piezo-potential induced molecular oxygen activation of defect-rich MoS2 ultrathin nanosheets for organic dye degradation in dark

As a new concept, single-and odd-numbered layer of molybdenum disulfide (MoS2) is a piezocatalyst semiconductor. Herein, we report the created odd numbered layer of defect-rich MoS2 (DR-MoS2) with high sulfur (S) vacancy concentration (S/Mo ratio-0.83) which can strongly interact with adsorbed oxygen molecules for highly efficient generation of reactive oxygen species (ROS) in the piezocatalytic degradation process. DR-MoS2 presents far superior piezocatalytic activity for Eriochrome Black T dye degradation which is 2.14 and 18.12 times higher than that of defect-free MoS2 (DF-MoS2) and bulk MoS2. The abundant native S vacancies induced the enhanced carrier density (electron density) of DR-MoS2 is to be 4.6 × 1017 cm–3 and is roughly 15 times higher than that the 2.9 × 1016 cm–3 of DF-MoS2. The S vacancies induced the increased flat-band potential and the vast majority of charge density strengthens the electron transfer dynamics and increases the surface band bending, which drive the charge carrier separation efficiently along with piezopolarization. More importantly, Mo active sites created by S vacancy defects can facilitate the two-electron reduction of molecular oxygen activation by introducing Mo4+/Mo6+ redox couple. In addition, based on the experimental results, a tentative mechanism was proposed to explain the origin of the piezocatalytic enhancement in DR- MoS2.

Biological interactions of biocompatible and water-dispersed MoS2 nanosheets with bacteria and human cells

Two dimensional materials beyond graphene such as MoS2 and WS2 are novel and interesting class of materials whose unique physico-chemical properties can be exploited in applications ranging from leading edge nanoelectronics to the frontiers between biomedicine and biotechnology. To unravel the potential of TMD crystals in biomedicine, control over their production through green and scalable routes in biocompatible solvents is critically important. Furthermore, considering multiple applications of eco-friendly 2D dispersions and their potential impact onto live matter, their toxicity and antimicrobial activity still remain an open issue. Herein, we focus on the current demands of 2D TMDs and produce high-quality, few-layered and defect-free MoS2 nanosheets, exfoliated and dispersed in pure water, stabilized up to three weeks. Hence, we studied the impact of this material on human cells by investigating its interactions with three cell lines: two tumoral, MCF7 (breast cancer) and U937 (leukemia), and one normal, HaCaT (epithelium). We observed novel and intriguing results, exhibiting evident cytotoxic effect induced in the tumor cell lines, absent in the normal cells in the tested conditions. The antibacterial action of MoS2 nanosheets is then investigated against a very dangerous gram negative bacterium, such as two types of Salmonellas: ATCC 14028 and wild-type Salmonella typhimurium. Additionally, concentration and layer-dependent modulation of cytotoxic effect is found both on human cells and Salmonellas.

Defect engineering activating (Boosting) zinc storage capacity of MoS2

Rechargeable aqueous zinc ion batteries are particularly attractive for large-scale application due to their low cost, environmental friendliness and safety. However, the development of zinc ion batteries is seriously impeded by the limited choice of suitable cathode materials owing to their low reversibility and slow diffusion of divalent zinc cations in cathodes. Herein, we report defect engineered MoS2-x nanosheets as cathode for Zn ion cells. These MoS2-x nanosheets show a preferential insertion of Zn ions into sulfur vacancies, allowing a much greater capacity to be obtained compared to pure MoS2. Combined STEM analysis, electrochemical study, and theoretical modeling reveal that numerous edge sites and sulfur vacancies act as preferential intercalation sites for the zinc ions, leading to a much greater capacity than that of defect-free MoS2. After 1,000 cycles at a specific current of 1,000 mA/g, the defect engineered MoS2-x electrode can still deliver a reversible capacity of 88.6 mAh/g, accounting for 87.8% of the initial capacity. The MoS2-x electrode displays excellent electrochemical performance, making it a highly promising cathode material for future zinc ion batteries. It is believed that our strategy to use the chemistry of defects to unlock the electrochemical activity of known materials can be applied to other materials for practical divalent/multivalent batteries.

A Reactive Molecular Dynamics Study of Atomistic Mechanisms During Synthesis of MoS2 Layers by Chemical Vapor Deposition

ABSTRACTTransition metal dichalcogenide (TMDC) monolayers like MoS2 are promising materials for future electronic applications. Large-area monolayer MoS2 samples for these applications are typically synthesized by chemical vapor deposition (CVD) using MoO3 reactants and gas-phase sulfur precursors. Recent experimental studies have greatly improved our understanding of reaction pathways in the CVD growth process. However, atomic mechanisms of sulfidation process remain to be fully elucidated. In this work, we present quantum-mechanically informed and validated reactive molecular dynamics (RMD) simulations for CVD synthesis of MoS2 layers using S2 precursors. Our RMD simulations clarify atomic-level reaction pathways for the sulfidation of MoO3 surfaces by S2, which is a critical reaction step for CVD synthesis of MoS2 layers. These results provide a better understanding of the sulfidation process for the scalable synthesis of defect-free MoS2 and other TMDC materials.

High-Yield Production of MoS2 and WS2 Quantum Sheets from Their Bulk Materials.

Mass production of two-dimensional quantum sheets (2D QSs) is highly desired to fully exploit their properties. Herein, we present a general strategy for the high-yield production of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) QSs by a sequential combination of salt-assisted ball-milling and sonication-assisted solvent exfoliation of their bulk materials. Such a strategy enables reproducible production of intrinsic and defect-free MoS2 and WS2 QSs with exceedingly high yields of 25.5 and 20.1 wt %, respectively. By precipitation-redispersion treatment, the QSs can be redispersed in a wide range of solvents with redispersion concentration up to 20 mg/mL or even higher. Remarkable nonlinear absorption saturation is demonstrated in the QSs-poly(methyl methacrylate) (PMMA) hybrid thin film with loading content of merely 0.1 wt %. Our method provides an avenue toward mass production and full exploration of 2D QSs.

Few-layer MoS2 as nitrogen protective barrier

We report experimental and theoretical investigations of the observed barrier behavior of few-layer MoS2 against nitrogenation. Owing to its low-strength shearing, low friction coefficient, and high lubricity, MoS2 exhibits the demeanor of a natural N-resistant coating material. Raman spectroscopy is done to determine the coating capability of MoS2 on graphene. Surface morphology of our MoS2/graphene heterostructure is characterized by using optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, density functional theory-based calculations are performed to understand the energy barrier performance of MoS2 against nitrogenation. The penetration of nitrogen atoms through a defect-free MoS2 layer is prevented by a very high vertical diffusion barrier, indicating that MoS2 can serve as a protective layer for the nitrogenation of graphene. Our experimental and theoretical results show that MoS2 material can be used both as an efficient nanocoating material and as a nanoscale mask for selective nitrogenation of graphene layer.

Large-scale production of defect-free MoS2 nanosheets via pyrene-assisted liquid exfoliation

Two dimensional (2D) layered MoS2 nanosheets are gaining increasing attention due to their attractive properties associated with myriads potential applications. However, large-scale production of high-quality MoS2 nanosheets in a simple manner remains a big challenge. Herein, we present a facile approach for scalable production of defect-free MoS2 nanosheets by direct ultrasonication exfoliation of bulk MoS2 crystals in ethanol in the presence of pyrene. Our findings indicate that the pyrene can not only serve as an exfoliating agent to significantly accelerate the exfoliation process but also endow the resulted MoS2 nanosheets with excellent solution dispersibility and stability. The obtained MoS2 nanosheets are highly crystalline and exhibit superior electrical properties as compared with the bulk MoS2. In addition, the MoS2 nanosheets can also be employed as nanofillers to enhance the mechanical properties of polymers. Moreover, this method can also be extended to exfoliation of other 2D layered transition metal dichalcogenide as demonstrated by the exfoliation of bulk WS2 crystals in this study. This work thus brings more opportunities for the investigation and applications of 2D layered inorganic nanosheets.

Defect-free MoS2 nanosheets: Advanced nanofillers for polymer nanocomposites

The preparation of defect-free MoS2 nanosheets is a key challenge and essential for practical applications. Herein the dodecanethiol was firstly performed as the antioxidant and surface modifier to produce the defect-free MoS2 by direct ultrasonication of bulk MoS2 in N,N-dimethylformamide. Incorporating defect-free MoS2 into polyethylene obviously improved the properties of PE/MoS2 nanocomposites. For crystallization under quiescent condition, the half crystallization time (t0.5) of nanocomposites containing 0.2wt% MoS2 was reduced by 87.0% compared to that of neat PE. A 54.3°C increase in the temperature of maximum weight loss (Tmax) was observed by inclusion of as low as 0.7wt% defect-free MoS2 nanosheets. In addition, the uniformly distributed MoS2 can considerably improve the mechanical properties of composites. These observations suggest that the robust nature, dramatic barrier action of defect-free MoS2 and the strong nanosheets/matrix interfacial adhesion would be the motivation to improve the performance of the polymeric nanocomposites.

Monolayers of MoS2 as an oxidation protective nanocoating material

First-principle calculations are employed to investigate the interaction of oxygen with ideal and defective MoS2 monolayers. Our calculations show that while oxygen atoms are strongly bound on top of sulfur atoms, the oxygen molecule only weakly interacts with the surface. The penetration of oxygen atoms and molecules through a defect-free MoS2 monolayer is prevented by a very high diffusion barrier indicating that MoS2 can serve as a protective layer for oxidation. The analysis is extended to WS2 and similar coating characteristics are obtained. Our calculations indicate that ideal and continuous MoS2 and WS2 monolayers can improve the oxidation and corrosion-resistance of the covered surface and can be considered as an efficient nanocoating material.

Density Functional Theory Studies of Chemisorption and Diffusion Properties of Ni and Ni−Thiophene Complexes on the MoS2 Basal Plane

First principles calculations based on spin-polarized density functional theory and the generalized gradient approximation have been used to study the chemisorption and diffusion of Ni atoms and clusters and a Ni−thiophene complex on defect-free MoS2(0001). The calculations employ slab geometry and periodic boundary conditions. We have identified several possible adsorption configurations of Ni atoms on the surface. The most stable configuration corresponds to adsorption at 3-fold hollow sites. By increasing the Ni coverage, several types of clusters can be formed that have a lower binding energy per Ni atom than single adsorbed Ni atoms. Minimum energy pathways for the diffusion of Ni atoms between selected pairs of local minima have been determined. The results indicate the existence of relatively large barriers for hopping with values between 20 and 44 kcal/mol. Additional calculations have been performed to analyze the adsorption of the thiophene molecule on bare and Ni-covered MoS2 basal planes. Ni a...