Defective MoS2 Research Articles

Tuning Electronic and Functional Properties in Defected MoS2 Films by Surface Patterning of Sulphur Atomic Vacancies.

Defects are inherent in transition metal dichalcogenides and significantly affect their chemical and physical properties. In this study, surface defect electrochemical nanopatterning is proposed as a promising method to tune in a controlled manner the electronic and functional properties of defective MoS₂ thin films. Using parallel electrochemical nanolithography, MoS₂ thin films are patterned, creating sulphur vacancy-rich active zones alternated with defect-free regions over a centimetre scale area, with sub-micrometre spatial resolution. The patterned films display tailored optical and electronic properties due to the formation of sulphur vacancy-rich areas. Moreover, the effectiveness of defect nanopatterning in tuning functional properties is demonstrated by studying the electrocatalytic activity for the hydrogen evolution reaction.

Regulating iron center by defective MoS2 for superior Fenton-like catalysis in water purification: The key role of surface interaction and superoxide radical in accelerating metal redox-cycling

Transition metals exhibit high reactivity for Fenton-like catalysis in environmental remediation, but how to save consumption and reduce pollution is of great interest. In this study, rationally designed defect-engineered Fe@MoS2 (Fe@D-MoS2) was prepared by incorporating reactive iron onto structural defects generated from the chemical acid-etching, aiming to improve the energetic consumption of the catalyst in Fenton-like applications. Morphological and structural properties were elucidated in details, the Fenton-like reactivity was evaluated with five phenolic contaminants for oxidant activation, radical generation and environmental remediation. Compared to Fe@MoS2, Fe@D-MoS2 exhibited a 18.9-fold increase in phenol degradation (0.09 versus 1.79 min−1). Quenching experiments, electron paramagnetic resonance tests and electrochemical measurements revealed the dominant sulfate and superoxide radicals. Rendered by strong metal-substrate surface and electronic interactions from regulated chemical environment and coordination structure, the inert ≡ Fe(III) was reduced to the reactive ≡ Fe(II) accompanied by the ≡ Mo(IV) oxidation to ≡ Mo(V) in MoS2 lattice, with adjacent sulfur serving as the key electron transfer bridge. Therefore, this work shows that the incorporation of reactive centers is able to boost two-dimensional sulfide materials for environmental catalysis applications.

Molten glass-mediated conditional CVD growth of MoS2 monolayers and effect of surface treatment on their optical properties

In the rapidly developing field of optoelectronics, the utilization of transition-metal dichalcogenides with adjustable band gaps holds great promise. MoS2, in particular, has garnered considerable attention owing to its versatility. However, a persistent challenge is to establish a simple, reliable and scalable method for large-scale synthesis of continuous monolayer films. In this study, we report the growth of continuous large-area monolayer MoS2 films using a glass-assisted chemical vapor deposition (CVD) process. High-quality monolayer films were achieved by precisely controlling carrier gas flow and sulfur vaporization with a customized CVD system. Additionally, we explored the impact of chemical treatment using lithium bistrifluoromethylsulfonylamine (Li-TFSI) salt on the optical properties of monolayer MoS2 crystals. To investigate the evolution of excitonic characteristics, we conditionally grew monolayer MoS2 flakes by controlling sulfur evaporation. We reported two scenarios on MoS2 films and flakes based on substrate-related strain and defect density. Our findings revealed that high-quality monolayer MoS2 films exhibited lower treatment efficiency due to substrate-induced surface strain. whereas defective monolayer MoS2 flakes demonstrated a higher treatment sensitivity due to the p-doping effect. The Li-TFSI-induced changes in exciton density were elucidated through photoluminescence, Raman, and x-ray photoelectron spectroscopy results. Furthermore, we demonstrated treatment-related healing in flakes under variable laser excitation power. The advancements highlighted in our study carry significant implications for the scalable fabrication of diverse optoelectronic devices, potentially paving the way for widespread real-world applications.

Novel structure and excellent nonlinear optical performance nanocomposite derived from flower-like MOF grown on MoS2

Molybdenum disulfide (MoS2) is a nonlinear optical material with graphene-like structure. In order to further improve the nonlinear optical performance of MoS2 and expand the application fields of MoS2, we report a simple in-situ self-growth method to functionalize MoS2 nanosheets with metal-organic framework (MOF) for the first time to pursuit excellent reverse saturable absorption performance and optical limiting performance. Firstly, defective MoS2 nanosheets was obtained by chemical etching with H2O2 and cysteamine was inserted into the sulfur vacancies under ultrasound to obtain MoS2–NH2. Then 5,10,15,20-Tetra (4-carbomethoxyphenyl) porphyrin (TCPP) as MOF active growth sites were grafted on MoS2–NH2 by amido linkage to obtain MoS2-TCPP in mild conditions. Finally, flower-like porphyrin-based CuMOF was grown on MoS2-TCPP by solvothermal method to obtain MoS2-CuMOF with stable structure. The third order nonlinear optical performance of MoS2-CuMOF were studied by Z-scan technique. It was found that the third order nonlinear absorption coefficient of MoS2-CuMOF was significantly improved compared with MoS2. Under the same energy of laser irradiation, the third order nonlinear absorption coefficient of MoS2-CuMOF nanocomposite in solution was 6 × 10−10 m/W about 2.3 times of the pristine MoS2 (2.6 × 10−10 m/W). After blending the above materials with polymethyl methacrylate (PMMA) to prepare a solid device, the third order nonlinear absorption coefficient of MoS2-CuMOF/PMMA was 18 × 10−10 m/W about 3.6 times of the MoS2/PMMA (5 × 10−10 m/W). And MoS2-CuMOF/PMMA had better optical limiting ability than MoS2/PMMA. The significant improvement in the performance of the composites can be attributed to the energy transfer between MoS2 and CuMOF, and porphyrin aggregation was inhibited by PMMA after the preparation of devices. Energy transfer can be proved by fluorescence spectroscopy. This work not only provided an example for the covalent functionalization of MoS2, but also offered a new method for designing materials with excellent nonlinear optical performance.

Defect engineering of 1T′ MX 2 (M = Mo, W and X = S, Se) transition metal dichalcogenide-based electrocatalyst for alkaline hydrogen evolution reaction

The alkaline electrolyzer (AEL) is a promising device for green hydrogen production. However, their energy conversion efficiency is currently limited by the low performance of the electrocatalysts for the hydrogen evolution reaction (HER). As such, the electrocatalyst design for the high-performance HER becomes essential for the advancement of AELs. In this work, we used both hydrogen (H) and hydroxyl (OH) adsorption Gibbs free energy changes as the descriptors to investigate the catalytic HER performance of 1T′ transition metal dichalcogenides (TMDs) in an alkaline solution. Our results reveal that the pristine sulfides showed better alkaline HER performance than their selenide counterparts. However, the activities of all pristine 1T′ TMDs are too low to dissociate water. To improve the performance of these materials, defect engineering techniques were used to design TMD-based electrocatalysts for effective HER activity. Our density functional theory results demonstrate that introducing single S/Se vacancy defects can improve the reactivities of TMD materials. Yet, the desorption of OH becomes the rate-determining step. Doping defective MoS2 with late 3d transition metal (TM) atoms, especially Cu, Ni, and Co, can regulate the reactivity of active sites for optimal OH desorption. As a result, the TM-doped defective 1T′ MoS2 can significantly enhance the alkaline HER performance. These findings highlight the potential of defect engineering technologies for the design of TMD-based alkaline HER electrocatalysts.

A comparative study on the linear scaling relations for the diffusion of S-vacancies on MoS2 and WS2.

This work uses density functional theory (DFT) calculations and kinetic Monte Carlo (kMC) simulations to compare the diffusion of S-vacancies on defective MoS2 and WS2, two structures that are often discussed as catalysts. Similar to what has been discussed for MoS2, the vacancy diffusion barriers on WS2 also follow Brønsted-Evans-Polanyi (BEP) type linear scaling relations. The vacancy diffusion kinetics is discussed at the example of a large vacancy cluster consisting of 37 unoccupied sites in direct vicinity and how its structure changes with time. Using barriers estimated via linear scaling relations as input for the kMC simulations yields results that qualitatively agree with results calculated self-consistently at DFT level. As the diffusion barriers for WS2 are significantly higher than those for MoS2, the vacancy diffusion on WS2 is poorly described by the linear scaling relations derived from MoS2 and vice versa. This work further shows that one needs DFT level barriers of about 40% of all S-vacancy diffusion processes on a material to derive sufficiently reliable linear scaling relations. This means that computational costs for future studies may be reduced by only explicitly computing one fraction of the diffusion barriers while estimating the remaining ones via linear scaling. However, in this case, one would lack information about the partition function of the transition states, which are needed for calculating the rate constants. Thus, we have also proposed a scheme to estimate the contribution of the partition functions based only on the initial state's vibrational modes.

Defect engineered magnetism induction and electronic structure modulation in monolayer MoS2

The electronic, magnetic, and optical characteristics of a defective monolayer MoS2 were examined by employing density functional theory (DFT)-based first-principles calculations. The effects of several defects on the electrical, magnetic, and optical properties, including Mo vacancies, MoS3 vacancies, and the substitution of a single Mo atom by two S atoms were studied in this work. Our first-principles calculations revealed that different types of defects produced distinct energy states within the band gap, leading to a band gap reduction after the introduction of various types of defects, which caused a change from semiconducting to metallic behavior. The spin-up and spin-down states were separated in the case of MoS3 vacancy. The total magnetization was ∼ −0.83 μB/cell, and the absolute magnetization was ∼ 1.23 μB/cell. Moreover, spin-up states had a 0.45 eV band gap, whereas spin-down states were metallic. Consequently, it can be promising for spin filter applications. It was disclosed that the broadband part of the electromagnetic spectrum has a high absorption coefficient, which is necessary for applications including impurity detection, photodiodes, and solar cells. Designing spintronic and optoelectronic devices will benefit from the modification of the electrical, optical, and magnetic properties by defect engineering of MoS2 monolayers presented here.

Investigation of dual atom doped single-layer MoS2 for electrochemical reduction of carbon dioxide by first-principle calculations and machine-learning

The exploration of efficient electrocatalysts for carbon dioxide reduction reaction (CO2RR) with viable activity and superior selectivity remains a great challenge. The efficiency of CO2RR over traditional transition metal-based catalysts is restricted by their inherent scaling relationships, so breaking this scaling relationship is the key to improving the catalytic performance. In this work, inspired by the recent experimental progress in the synthesis of dual atom catalysts (DACs), we reported a rational design of novel DACs with two transition metal atoms embedded in defective MoS2 with S vacancies for CO2 reduction; 21 metal dimer systems were selected, including six homonuclear catalysts (MoS2-M2, M = Cu, Fe, Ni, Mn, Cr, Co) and 15 heteronuclear catalysts (MoS2-M1M2). First-principles calculations showed that the MoS2-NiCr system not only breaks the linear relationship of key intermediates but also possesses a low overpotential of 0.58 V and superior selectivity in the process of methane generation, which can be used as a promising catalyst for methane formation from CO2 electroreduction. Notably, by combining random forest regression machine learning study, we found that the CO2RR activity of DACs is essentially controlled by some fundamental factors, such as the distance between metal centers and the number of outer electrons in the metal atoms. Our findings provide profound insights for the design of efficient DACs for CO2RR.

Efficient and selective Lead(II) removal within a wide concentration range through chemisorption and electrosorption coupling process via defective MoS2 electrode

Developing method for efficient lead(II) removal from aqueous solutions over a wide concentration range is urgent but challenging. In this work, defective molybdenum disulfide (MoS2) was served as electrode coating to capture lead(II) over a wide range from aqueous solution via chemisorption and electrosorption coupling (CEC) process. The distinct adsorption behaviors of lead(II) on defective MoS2 electrode were theoretically and experimentally investigated. In a wide concentration range (1 ∼ 100 mg/L), the removal efficiency of lead(II) from aqueous solutions can be close to ∼ 100 %. Adsorption capacities for lead(II) removal at initial concentration of 8.37 and 18.71 mg/L were 64.99 and 159.98 mg/g, respectively. After applying voltage, adsorption capacity and removal efficiency were enhanced almost 8 folds due to strengthened migration of ions, electrostatic interaction and electric double layer. Besides, lead(II) was completely desorbed from defective MoS2 electrode within 60 min, and defective MoS2 electrode by CEC process maintained excellent regeneration ability even after 10 cycles. Defective MoS2 electrode has shown superior properties for lead(II) removal, such as (i) great specific capacitance and (ii) low charge transfer resistance toward the enhanced adsorption capacity and diffusion for lead(II) in the electrical double layer, compared with MoS2. Extensive computations and characterizations of defective MoS2-lead(II) interaction demonstrated that the active unsaturated S sites were indispensable for lead(II) adsorption, which was beneficial for chemisorption. Based on the strong interaction with lead(II), defective MoS2 electrode showed high-selectivity lead(II) adsorption with co-existence metal ions. Overall, defective MoS2 as electrode coating by CEC has a great potential for the practical application in removing heavy metal ions from aquatic environment.

Effect of five typical vacancy defects on the tribological behaviors of MoS2 sheet: A molecular dynamics study

The tribological behavior of the defective MoS2 sheet has been investigated under different normal loads by molecular dynamics simulations. Five typical vacancy defects have been introduced to estimate the anti-friction and anti-wear properties of defective MoS2 sheets. The existence of vacancy defects increases the local friction of the MoS2 sheet and accelerates wear failure. The degree of vacancy defect influence on the anti-friction and anti-wear properties of MoS2 is as follows: VMo < VS < VS2 < VMoS3 < VMoS6. The atomic frictional deviation of the a-C tip and the potential energy surfaces of the defective MoS2 sheet have been analyzed to reveal the potential mechanisms of the atomic pinning effect. The enhancements of atomic pinning and energy barrier induced by vacancy defects are the main contributions for the increased local friction in the MoS2 sheet. The S atomic vacancies in the bottom S layer have been further considered. The top layer S atom tends to transfer and fill the bottom S vacancy under a certain normal load of 130nN. The transformation of vacancy defect further increases the surface friction of defective MoS2 sheets.

Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide.

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d5/2 core peak at 230.1 eV emerges in the annealed MoS2 associated with nonstoichiometric MoSx (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 1014 cm-2, we observe a defect peak at ∼1.72 eV (referred to as LXD) at room temperature in the photoluminescence (PL) spectrum. The LXD peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS2 are influenced by sulfur vacancies at room and low temperatures.

Diffusion of the catalysis-relevant S-vacancies on defective 2H-phase MoS2

BackgroundS-vacancies play an essential role in various MoS2-catalyzed processes. Knowing that the size and shape of the S-vacancies affect the catalytic properties, this work investigates the diffusion of S-vacancies along different directions and whether these diffusion processes lead to the formation of specific surface patterns on a defective MoS2 monolayer. MethodsA first-principles-based kinetic Monte Carlo model is used to study how the S-vacancies of a vacancy cluster, i.e., a region where all surface S atoms are removed, migrate when the diffusion of the S-vacancies is limited to the [100], [120] and [1¯2¯0] directions, respectively. Significant findingsAt up to 600 K, the diffusion processes transform a vacancy cluster into a pattern of single S-atoms surrounded by six S-vacancy sites. Breaking this pattern requires high-barrier diffusion steps, only accessible at high temperatures where defective MoS2 sublimates. We have further found that the diffusion along the considered directions is very different, with the diffusion along the [120] direction being the slowest. In general, when diffusion is confined to a specific direction, the transformation of the vacancy cluster is slower than in the case with no constraints. This finding may help stabilize vacancy clusters on a catalyst.

Microscale spectroscopic mapping of defect evolution and filling in large-area growth of monolayer MoS2

Defects in monolayer transition metal dichalcogenides are known to be problematic in that they usually deteriorate optical and electronic properties, thus acting as a constraint in nanodevice applications. In this regard, understanding defect-related strain and charge distributions is intriguing to identify the inhomogeneous energy landscape of two-dimensional materials. Herein, simultaneous Raman and photoluminescence results unveil how strain and charge carriers evolve over the large-area growth of monolayer MoS2. Unexpectedly, spatially-resolved correlation analysis between the in-plane and out-of-plane phonon frequencies of MoS2 reveals that both compressive strain and charge carriers increase as the morphology evolves from a triangular flake to a partially coalesced film and, finally, to a fully covered film, indicating that the number of sulfur vacancies increases with surface coverage. Theoretically calculated formation energies of sulfur vacancies support the evolution scenarios with respect to morphological variations. Photoluminescence mappings of excitons and trions provide further quantitative information on electron concentrations that are consistent with Raman correlation data. Notably, chemical treatment of defective MoS2 with a p-type dopant leads to a relaxation of compressive strain and a decrease in excess electrons, where in particular, the p-doping effects are more prominent in the fully covered film region than in the partially coalesced region.

Substitutional Coinage Metals as Promising Defects for Adsorption and Detection of Gases on MoS2 Monolayers: A Computational Approach.

Defective molybdenum disulfide (MoS2) monolayers (MLs) modified with coinage metal atoms (Cu, Ag and Au) embedded in sulfur vacancies are studied at a dispersion-corrected density functional level. Atmospheric constituents (H2, O2 and N2) and air pollutants (CO and NO), known as secondary greenhouse gases, are adsorbed on up to two atoms embedded into sulfur vacancies in MoS2 MLs. The adsorption energies suggest that the NO (1.44 eV) and CO (1.24 eV) are chemisorbed more strongly than O2 (1.07 eV) and N2 (0.66 eV) on the ML with a cooper atom substituting for a sulfur atom. Therefore, the adsorption of N2 and O2 does not compete with NO or CO adsorption. Besides, NO adsorbed on embedded Cu creates a new level in the band gap. In addition, it was found that the CO molecule could directly react with the pre-adsorbed O2 molecule on a Cu atom, forming the complex OOCO, via the Eley-Rideal reaction mechanism. The adsorption energies of CO, NO and O2 on Au2S2, Cu2S2 and Ag2S2 embedded into two sulfur vacancies were competitive. Charge transference occurs from the defective MoS2 ML to the adsorbed molecules, oxidizing the later ones (NO, CO and O2) since they act as acceptors. The total and projected density of states reveal that a MoS2 ML modified with copper, gold and silver dimers could be used to design electronic or magnetic devices for sensing applications in the adsorption of NO, CO and O2 molecules. Moreover, NO and O2 molecules adsorbed on MoS2-Au2s2 and MoS2-Cu2s2 introduce a transition from metallic to half-metallic behavior for applications in spintronics. These modified monolayers are expected to exhibit chemiresistive behavior, meaning their electrical resistance changes in response to the presence of NO molecules. This property makes them suitable for detecting and measuring NO concentrations. Also, modified materials with half-metal behavior could be beneficial for spintronic devices, particularly those that require spin-polarized currents.

A potential function of MoS2 based on machine learning

MoS2 has been used as a non-toxic and economical thermoelectric material, which attract the interest of researchers. In terms of computational simulation, system energy is the most basic and important property of microscopic materials. Although the first principles based on density functional theory (DFT) can calculate the energy accurately, the calculation is limited by time and scale. The potential function based on existing data can make up for this shortage. This study uses the Beller-Parrinello (BP) method to construct the machine-learning potential of the Mo-S system. The potential function is trained by the atomic energy network (aenet) software package, and is able to be used in bulk structure and two-dimensional structure. We predicted the energy of structures under different strains and conduct molecular dynamics (MD) simulation to predict the energy change during the simulation process. In addition, we calculated the defect formation energy of MoS2. The results of ANN were very close to the calculations of DFT. This potential function with high accuracy can greatly shorten the period of exploring the defect structure of MoS2 and it provides a powerful tool for exploring the large-scale defective MoS2.

Facile and Scalable Mechanochemical Synthesis of Defective MoS2 with Ru Single Atoms Toward High-Current-Density Hydrogen Evolution.

Designing a facile strategy to prepare catalysts with highly active sites are challenging for large-scale implementation of electrochemical hydrogen production. Herein, a straightforward and eco-friendly method by high-energy mechanochemical ball milling for mass production of atomic Ru dispersive in defective MoS2 catalysts (Ru1 @D-MoS2 ) is developed. It is found that single atomic Ru doping induces the generation of S vacancies, which can break the electronic neutrality around Ru atoms, leading to an asymmetrical distribution of electrons. It is also demonstrated that the Ru1 @D-MoS2 exhibits superb alkaline hydrogen evolution enhancement, possibly attributing to this electronic asymmetry. The overpotential required to deliver a current density of 10mA cm-2 is as low as 107mV, which is much lower than that of commercial MoS2 (C-MoS2 , 364mV). Further density functional theory (DFT) calculations also support that the vacancy-coupled single Ru enables much higher electronic distribution asymmetry degree, which could regulate the adsorption energy of intermediates, favoring the water dissociation and the adsorption/desorption of H*. Besides, the long-term stability test under 500mA cm-2 further confirms the robust performance of Ru1 @D-MoS2 . Ourstrategy provides a promising and practical way towards large-scale preparation of advanced HER catalysts for commercial applications.

Effect of molybdenum disulfide doping with substitutional nitrogen and sulfur vacancies on lithium intercalation

Molybdenum disulfide (MoS2) nanomaterials were synthesized by rapid decomposition of ammonium tetrathiomolybdate in argon or ammonia at 600 °C and 700 °C. The change of environment had no effect on the morphology, but it affected the composition and electronic structure of the nanomaterial. The use of gaseous NH3 in the synthesis led to the incorporation of nitrogen and the formation of sulfur vacancies in the MoS2 lattice. An electrochemical study showed that this dual lattice modification significantly reduced the irreversible capacity in the first cycle and improved the lithium capacity and structural stability of MoS2 in the voltage range of 2.5–1.1 V. After 65 cycles of the operation of lithium-ion battery, the specific capacity of the defective MoS2 was 189 mAh g–1, which is higher than the theoretical capacity of ideal MoS2. An increase in electrical conductivity and a decrease in charge transfer resistance determined by using electrochemical impedance spectroscopy were associated with a decrease in the band gap in MoS2 due to the substitutional nitrogen atoms and sulfur vacancies, as shown by density functional theory calculations. In addition, these defects create new sites for lithium adsorption and increase the intercalation voltage.

Cluster Formation Effect of Water on Pristine and Defective MoS2 Monolayers.

The structure and electronic properties of the molybdenum disulfide (MoS2) monolayer upon water cluster adsorption are studied using density functional theory and the optical properties are further analyzed with the Bethe-Salpeter equation (BSE). Our results reveal that the water clusters are electron acceptors, and the acceptor tendency tends to increase with the size of the water cluster. The electronic band gap of both pristine and defective MoS2 is rather insensitive to water cluster adsorbates, as all the clusters are weakly bound to the MoS2 surface. However, our calculations on the BSE level show that the adsorption of the water cluster can dramatically redshift the optical absorption for both pristine and defective MoS2 monolayers. The binding energy of the excitons of MoS2 is greatly enhanced with the increasing size of the water cluster and finally converges to a value of approximately 1.16 eV and 1.09 eV for the pristine and defective MoS2 monolayers, respectively. This illustrates that the presence of the water cluster could localize the excitons of MoS2, thereby greatly enhance the excitonic binding energy.

Constructing a square-like copper cluster to boost C–C coupling for CO2 electroreduction to ethylene

By DFT computations, we proposed Cu5@MoS2—an anchored Cu5 cluster on defective MoS2—as a promising catalyst for CO2-to-ethylene conversion. With distinctive square-like sites, Cu5@MoS2 demonstrates remarkable activity and selectivity, particularly in alkaline conditions.

Using ternary steric hindrance synergy of a defective MoS2 monolayer to manipulate the electrocatalytic mechanism toward nitric oxide reduction: a first-principles and machine learning study

The complex competing electroreduction reaction pathways in the nitric oxide reduction reaction are guided in the expected direction by taking advantage of the innate steric hindrance of defective MoS2.