Sulfur Vacancies In MoS2 Research Articles

Atomic-scale perspective on individual thiol-terminated molecules anchored to single S vacancies in MoS2

Sulfur vacancies in MoS2 on Au(111) have been shown to be negatively charged as reflected by a Kondo resonance. Here, we use scanning tunneling microscopy to show that these vacancies serve as anchoring sites for thiol-based molecules (CF3−3P−SH) with two distinct reaction products, one of them showing a Kondo resonance. Based on comparisons with density-functional theory (DFT) calculations, including a random structure search and computation of energies and electronic properties at a hybrid exchange-correlation functional level, we conclude that both anchored molecules are charge neutral. We propose that one of them is an anchored intact CF3−3P−SH molecule while the other one is the result of catalytically activated dehydrogenation to CF3−3P−S with subsequent anchoring. Our investigations highlight a perspective of functionalizing defects with thiol-terminated molecules that can be equipped with additional functional groups, such as charge donor or acceptor moieties, switching units, or magnetic centers. Published by the American Physical Society 2024

Bead-like Co/C@MoS2Composites connected by Multi-Walled carbon nanotubes for high-efficiency electromagnetic wave absorption

This study presents a novel approach to synthesize a composite material composed of bead-like Co/C nanoparticles anchored on MWCNTs, encapsulated by MoS2 nanosheets through a combination of carbonization and hydrothermal treatment using ZIF-67 as the precursor. The resulting composite demonstrates exceptional electromagnetic wave (EMW) absorption performance, with a minimum reflection loss (RL) of −63 dB at a thickness of 2.44 mm and an effective absorption bandwidth (EAB) of 7.2 GHz when RL is below 10 dB together with an expanded EAB of 12.3 GHz (d = 5 mm).The superior absorption capabilities arise from a well-thought-out design of material composition and structure. The synergy of Co/C and MWCNTs enhances ferromagnetic resonance, conductivity, and polarization loss, while sulfur vacancies in MoS2 act as polarization centers, facilitating dipole polarization and efficient conversion of electromagnetic wave energy. The core–shell structure and heterogeneous interfaces introduce various energy loss mechanisms, including multiple polarizations, scattering effects, and dielectric loss. This work introduces a facile method for synthesizing a composite material with outstanding EMW absorption performance. The synergistic combination of unique material components and the carefully engineered structure make this composite a promising candidate for advanced applications in electromagnetic wave absorption technology.

Defect-engineering of liquid-phase exfoliated 2D semiconductors: stepwise covalent growth of electronic lateral hetero-networks.

Two-dimensional (2D) in-plane heterostructures display exceptional optical and electrical properties well beyond those of their pristine components. However, they are usually produced by tedious and energy-intensive bottom-up growth approaches, not compatible with scalable solution-processing technologies. Here, we report a new stepwise microfluidic approach, based on defect engineering of liquid-phase exfoliated transition metal dichalcogenides (TMDs), to synthesize 2D hetero-networks. The healing of sulfur vacancies in MoS2 and WS2 is exploited to controllably bridge adjacent nanosheets of different chemical nature with dithiolated conjugated molecular linkers, yielding solution-processed nm-scale thick networks with enhanced percolation pathways for charge transport. In-plane growth and molecular-driven assembly synergistically lead to molecularly engineered heterojunctions suppressing the formation of tightly bound interlayer excitons that are typical of conventional TMD blends, promoting faster charge separation. Our strategy offers an unprecedented route to chemically assemble solution-processed heterostructures with functional complexity that can be further enhanced by exploiting stimuli-responsive molecular linkers.

Electron transport through the multiple sulfur vacancies in MoS2

We conducted a thorough investigation of the impact of sulfur vacancies on electronic structures and electron transport in MoS2, with a particular focus on neighboring sulfur vacancies by employing density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. Although pristine MoS2 exhibits a Schottky-like behavior, an intriguing behavior emerged in the presence of aligned vacancies. The current flows through interconnected in-gap states even at low bias region (0.0–0.2 V), but it does through tunneling from defect states to the conduction bands (CBs) at higher voltages. Negative differential resistance (NDR) appears in the middle range of voltage. Through systematic examination of the inter-vacancy distance at which current could flow, we found that MoS2 ribbons could exhibit a random network of vacancies capable of mediating current at feasible vacancy concentration, which means percolation emerges. This research offers valuable insights into the potential of defect-engineered MoS2 for novel electronic applications.

Covalently linked graphene oxide-transition metal disulfide quantum dots nanocomposites featuring enhanced nonlinear optical absorption

Nanocomposites composed of different nanomaterials are a promising class of optoelectronic materials, owing to their interfacial electronic interactions. Compared to van der Waals forces, covalent bond-based linkages may endow nanocomposites with promoted electronic interactions and reinforced nonlinear optical (NLO) responses. In this study, we constructed covalently linked, mixed-dimensional nanocomposites by combining graphene oxide (GO) with MoS2 and WS2 quantum dots (QDs). These nanocomposites were prepared by employing a bifunctional molecule, 4-mercaptobenzenediazonium tetrafluoroborate, which carries a reactive diazonium group and a thiol group. The reactive diazonium group allowed for the attachment of thiophenol to GO via a radical addition reaction, while the thiol group enabled the subsequent passivation of sulfur vacancies in MoS2 and WS2 QDs. The resulting nanocomposites, denoted as GO-MoS2 and GO-WS2, respectively, exhibited significant fluorescence quenching, indicating effective electron and/or energy transfer from MoS2 or WS2 QDs to GO. Importantly, these covalently linked nanocomposites demonstrated superior two-photon absorption (TPA) responses compared to the individual components (GO, MoS2 and WS2 QDs) as well as the corresponding physical blends, presumably resulting from the efficient electron and/or energy transfer within these nanocomposites. This study not only demonstrates the significant promise of covalently linked GO-MoS2 and GO-WS2 nanocomposites in optical limiting applications, but also opens up new opportunities for the advancement of nanocomposites in ultrafast photonic devices.

Reduced Defect Density in MOCVD-Grown MoS2 by Manipulating the Precursor Phase.

Advancements in the synthesis of large-area, high-quality two-dimensional transition metal dichalcogenides such as MoS2 play a crucial role in the development of future electronic and optoelectronic devices. The presence of defects formed by sulfur vacancies in MoS2 results in low photoluminescence emission and imparts high n-type doping behavior, thus substantially affecting material quality. Herein, we report a new method in which single-phase (liquid) precursors are used for the metal-organic chemical vapor deposition (MOCVD) growth of a MoS2 film. Furthermore, we fabricated a high-performance photodetector (PD) and achieved improved photoresponsivity and faster photoresponse in the spectral range 405-637 nm compared to those of PDs fabricated by the conventional MOCVD method. In addition, the fabricated MoS2 thin film showed a threshold voltage shift in the positive gate bias direction owing to the reduced number of S vacancy defects in the MoS2 lattice. Thus, our method significantly improved the synthesis of monolayer MoS2 and can expand the application scope of high-quality, atomically thin materials in large-scale electronic and optoelectronic devices.

Insights into the electron activation mechanisms at the micro level by nano zero-valent iron supported by molybdenum disulfide (nZVI@MD) from preparation to application

Both molybdenum disulfide (MoS2) and nano zero-valent iron (nZVI) exhibit excellent adsorption abilities. However, the constrained conductivity of MoS2 and the lack of selectivity of nZVI for electron transfer still pose challenges. In this study, we designed a series of novel nano zero-valent iron supported by molybdenum disulfide composites (nZVI@MD) with multiple electron-rich active sites, including iron dopant replacement, iron atom intercalation and exposed Mo4+, for effective removal of Cr(VI). Results showed that preparation temperature and the amount of MoS2 added were identified as the two most significant factors affecting the reduction properties of nZVI@MD. Systematic experiments revealed that the nZVI@MD exhibited good anti-interference performance, stability and reusability due to its excellent electron selectivity. Characterization results exhibited that iron atoms replaced the sulfur vacancies in MoS2 and inserted into an intercalation of MoS2 during the preparation process. The mechanisms underlying the uptake of Cr(VI) by nZVI@MD can be proposed as follows: (i) electrostatic interactions, (ii) reduction reaction, and (iii) co-precipitation involving Fe-O-Cr. Furthermore, nZVI@MD exhibited excellent electron activity, hydrophilicity and oxidation resistance, confirmed by density functional theory (DFT) calculations. This work provided new strategies and mechanistic insights for the rational design of adsorbents.

Solvent-assisted sulfur vacancy engineering method in MoS2 for a neuromorphic synaptic memristor.

Recently, two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) have attracted great attention due to their unique properties. To modulate the electronic properties and structure of TMDs, it is crucial to precisely control chalcogenide vacancies and several methods have already been suggested. However, they have several limitations such as plasma damage by ion bombardment. Herein, we introduced a novel solvent-assisted vacancy engineering (SAVE) method to modulate sulfur vacancies in MoS2. Considering polarity and the Hansen solubility parameter (HSP), three solvents were selected. Sulfur vacancies can be modulated by immersing MoS2 in each solvent, supported by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses. The SAVE method can further expand its application in memory devices representing memristive performance and synaptic behaviors. We represented the charge transport mechanism of sulfur vacancy migration in MoS2. The non-destructive, scalable, and novel SAVE method controlling sulfur vacancies is expected to be a guideline for constructing a vacancy engineering system of TMDs.

Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy.

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

Electrical role of sulfur vacancies in MoS2: Transient current approach

The electrical role of sulfur vacancies in MoS2 has attracted considerable attention, and numerous studies have attempted to reveal their characteristics in a donor state, an acceptor state, or a neutral state. However, there are still no definitive conclusions, and the debate continues. In this study, we investigated the effect of sulfur vacancies on the electrical properties of MoS2 using transient current measurements. After treatment with hydrazine to generate sulfur vacancies, the density of shallow traps increased by a factor of 4, whereas that of deep-level traps increased by a factor of 200. These results indicate that sulfur vacancies induce both deep- and shallow-level traps, but the trap density is higher at the deep level.

Design of ruthenium-doped MoS2 microsphere with optimal electronic structure for enhanced hydrogen evolution

Engineering the electronic structure is significant to design of high active electrocatalysts, while it is challenging to create an optimal configuration and understand the enhanced mechanism. Here, we reported the design of molybdenum disulfide (MoS2) with an optimal electronic structure via introducing ruthenium (Ru) and sulfur vacancy to boost electrochemical hydrogen evolution. Specially, the uniform and monodispersed MoS2 microspheres with an average diameter of 567 nm were prepared through a microemulsion technology. The sulfur vacancies in MoS2 and 2Ru-MoS2 were demonstrated by EPS. The optimized 2Ru-MoS2 delivered a much lower overpotential of 70 mV (10 mA cm−2), compared with MoS2, 1Ru-MoS2, and 3Ru-MoS2. The parabolic enhancement mechanism related to the reduced band gap, the appropriate electron density, and the lowered barrier was revealed by density functional theory calculations. Our work shed light to rational design of highly efficient electrocatalysts via efficiently utilization of metal doping and vacancy.

Flower-Like Interlayer-Expanded MoS2- x Nanosheets Confined in Hollow Carbon Spheres with High-Efficiency Electrocatalysis Sites for Advanced Sodium-Sulfur Battery.

The room-temperature sodium-sulfur (RT-Na/S) battery is one of the most promising technologies for low-cost energy storage. However, application of RT-Na/S batteries is currently impeded by severe shuttle effects and volume expansion that limits both energy density and cycling stability. Herein, first, the first-principal calculation is used to find that the introduction of sulfur vacancies in MoS2 can effectively enhance polysulfide adsorption and catalytic ability as well as both the ion and electron conductivities. Then, unique MoS2- x /C composite spheres are further designed and synthesized with flower-like few-layer and interlayer-enlarged MoS2- x nanosheets space-confined in hollow carbon nanospheres by a "ship-in-a-bottle" strategy. With this novel design, the mass loading of S in the MoS2- x /C composite can be reached to as high as 75wt%. Owing to the synergetic effect of interlayer-expanded and few-layer MoS2- x nanosheets and hollow carbon spheres matrix with high electronic/Na+ conductivity, the RT-Na/S batteries deliver highly stable cycle durability (capacity retention of 85.2% after 100 cycles at 0.1 A g-1 ) and remarkable rate capability (415.7 mAh g-1 at 2 A g-1 ) along with high energy density. This design strategy of defect- and interlayer-engineering may find wide applications in synthesizing electrode materials for high-performance RT-Na/S batteries.

Cobalt-Doping of Molybdenum Disulfide for Enhanced Catalytic Polysulfide Conversion in Lithium-Sulfur Batteries.

Metal sulfides, such as MoS2, are widely investigated in lithium-sulfur (Li-S) batteries to suppress the shuttling of lithium polysulfides (LiPSs) due to their chemical adsorption ability and catalytic activity. However, their relatively low conductivity and activity limit the LiPS conversion kinetics. Herein, the Co-doped MoS2 is proposed to accelerate the catalytic conversion of LiPS as the Co doping can promote the transition from semiconducting 2H phase to metallic 1T phase and introduce the sulfur vacancies in MoS2. A one-step hydrothermal process is used to prepare such a Co-doped MoS2 with more 1T phase and rich sulfur vacancies, which enhances the electron transfer and catalytic activity, thus effectively improving the LiPS adsorption and conversion kinetics. The cathode using the three-dimensional graphene monolith loaded with Co-doped MoS2 catalyst as the sulfur host shows a high rate capability and long cycling stability. A high capacity of 941 mAh g-1 at 2 C and a low capacity fading of 0.029% per cycle at 1 C over 1000 cycles are achieved, suggesting the effectively suppressed LiPS shuttling and improved sulfur utilization. Good cyclic stability is also maintained under a high sulfur loading indicating the doping is an effective way to optimize the metal sulfide catalysts in Li-S batteries.

Surface effects on temperature-driven spin crossover in Fe(phen)2(NCS)2.

Despite their importance in molecular spintronics, the surface effects on spin crossover (SCO) behaviors are still poorly understood. Here, we report the impact of substrates on thermal SCO in Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline) deposited on metallic surfaces and monolayer two-dimensional materials. By first-principles calculations, we show that temperature-driven SCO is preserved on both hexagonal boron nitride and molybdenum disulfide (MoS2), while low-spin ground states are locked on metal surfaces, including Cu(111), Ag(111), and Au(111). On the contrary, the molecule in contact with graphene exhibits a high-spin ground state. We demonstrate that the spin transition temperature Tc depends critically on surface environments, and we correlate this effect with the modification of electronic structures and molecular vibrations upon adsorption. In particular, a sulfur vacancy in MoS2 considerably increases Tc. These findings open a way to nanoscale applications related to spin state bistability.

Synthesis of Ultrahigh-Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules.

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD-grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS2 . Atomic-resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS2 grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS2 . Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS2 and repair these defects during the growth of MoS2 , resulting in high-quality MoS2 . This work provides a facile and controllable method for the growth of high-quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.

First-principle study of sulfur vacancy and O2 adsorption on the electronic and optical properties of ferroelectric CuInP2S6 monolayer

Sulfur vacancy in MoS2 has been found to have an important influence on the performance of optoelectronic devices. Here, we study the effect of sulfur vacancy and O2 adsorption on the electronic and optical properties in the two-dimensional ferroelectric CuInP2S6. It is revealed that a defect state appears at the top of valence band with the presence of sulfur vacancy. However, when O2 is chemisorbed at sulfur vacancy, the defect state disappears. The variation of charge state and charge transfer are calculated and discussed. Although the ferroelectricity is greatly suppressed with the presence of sulfur vacancy, the ferroelectric state can be recovered when the O2 is adsorbed. Within the framework of GW + BSE method, the optical absorption edge of CuInP2S6 monolayer exhibits a red-shift for the presence of sulfur vacancy and further O2 adsorption gives rise to a blue-shift of the spectrum. Our findings have shown an effective way to improve the functionality of two-dimensional ferroelectrics via defect engineering.

Ultrastable molybdenum disulfide-based electrocatalyst for hydrogen evolution in acidic media

Despite the incredible success in reducing the overpotential of nonprecious catalysts for acidic hydrogen evolution reaction (HER) in the past few years, the stability of most platinum-free electrocatalysts is still poor. Here, we report an ultrastable electrocatalyst for acidic HER based on two-dimensional (2D) molybdenum disulfide (MoS2) doped with trace amount of palladium (<5 μg cm−2), which creates sulfur vacancies (S-vacancies). The optimized catalyst shows stable operation over 1000 h at 10 mA cm−2 with overpotential of 106 mV. The MoS2 catalyst is stabilized on a defective vertical graphene support, where the strong interaction at the 2D-2D interface increases the adhesion between the catalyst and the support. Palladium (Pd) doping generates rich sulfur vacancies in MoS2 that have a twofold role: (1) increasing hydrogen adsorption energy, which enhances activity; and (2) further increasing the adhesion between graphene support and defective MoS2, and thus enhancing stability. Complementary theoretical studies reveal the reaction pathways for substitutional doping, where the Mo-vacancy sites are prior to be doped by Pd. Our work thus offers a strategy for making stable, efficient, and earth-abundant HER catalysts with strong potential to replace platinum for PEM electrolysis.

Unravelling the effect of sulfur vacancies on the electronic structure of the MoS2 crystal.

Molybdenum disulfide (MoS2) is one of the two-dimensional layered semiconductor transition metal dichalcogenides (TMDCs) with great potential in electronics, optoelectronics, and spintronic devices. Sulfur vacancies in MoS2 are the most prevalent defects. However, the effect of sulfur vacancies on the electronic structure of MoS2 is still in dispute. Here we experimentally and theoretically investigated the effect of sulfur vacancies in MoS2. The vacancies were intentionally introduced by thermal annealing of MoS2 crystals in a vacuum environment. Angle-resolved photoemission spectroscopy (ARPES) was used directly to observe the electronic structure of the MoS2 single crystals. The experimental result distinctly revealed the appearance of an occupied defect state just above the valence band maximum (VBM) and an upward shift of the VBM after creating sulfur vacancies. In addition, density functional theory (DFT) calculations also confirmed the existence of the occupied defect state close to the VBM as well as two deep unoccupied states induced by the sulfur vacancies. Our results provide evidence to contradict that sulfur vacancies indicate the origin of n-type behaviour in MoS2. This work provides a rational strategy for tuning the electronic structures of MoS2.

Unique-Structure MoS2 Grow on Nickel Foam As Highly Efficient Self-Supported Electrode for Oxygen Evolution Reaction

Growing demand for sustainable, clean, efficient energy conversion system has derived tremendous interest to develop cost-effective and highly efficient electrocatalysts for oxygen evolution reaction (OER). Sulfides have recently attracted great attention due to their highly efficient electrocatalytic activity. MoS2 has been verified as an efficient catalyst toward OER because of its durability, cost-efficiency and high activity. However, binder and extra conductive materials are required to attach electrocatalyst on a conductive substrate which will result in restricted active surface areas, undesirable interfaces and lots of dead volumes. Binder-free self-supported material can eliminate these problems. Herein, we synthesize self-supported binder-free MoS2 on 3D porous nickel foam (NF) with excellent activity and stability via a facile hydrothermal method. The MoS2@NF electrode is tested in 0.1mol KOH by linear sweep voltammetry (LSV), electrochemical surface area (ECSA) and electrochemical impedance spectroscopy (EIS). Compared with different substrates (Cu foam, NF, Cu paper and carbon paper), MoS2@NF electrode where NF as substrate shows the lowest overpotential at 10mA cm-2 of 359mV for OER. Furthermore, the overpotential at 10mA cm-2 after 10hr is 419mV, which demonstrates high stability and activity. Such high electrochemical performance may attribute to the following reasons: 1) the large ECSA and good conductivity, that improves utilization of electroactive sites and electron transfer; 2) the unique 3D porous structure, that facilitates electrolyte penetration and reactant/product diffusion; and 3) the sulfur vacancy in MoS2, that accelerates the O-H bond breaking.

Enhancing hydrogen evolution on the basal plane of transition metal dichacolgenide van der Waals heterostructures

Recent years have seen a surge in the use of low-dimensional transition metal dichacolgenides, such as MoS2, as catalysts for the electrochemical hydrogen evolution reaction. In particular, sulfur vacancies in MoS2 can activate the inert basal plane, but that requires an unrealistically high defect concentration (~9%) to achieve optimal activity. In this work, we demonstrate by first-principles calculations that assembling van der Waals heterostructures can enhance the catalytic activity of MoS2 with low concentrations of sulfur vacancies. We integrate MoS2 with various two-dimensional nanostructures, including graphene, h-BN, phosphorene, transition metal dichacolgenides, MXenes, and their derivatives, aiming to fine-tune the free energy of atomic hydrogen adsorption. Remarkably, an optimal free energy can be achieved for a low sulfur vacancy concentration of ~2.5% in the MoS2/MXene-OH heterostructure, as well as high porosity and tunability. These results demonstrate the potential of combining two-dimensional van der Waals assembly with defect engineering for efficient hydrogen production.