MoS2 Substrate Research Articles

Anthracite-Derived Porous Carbon@MoS2 Heterostructure for Elevated Lithium Storage Regulated by the Middle TiO2 Layer.

The rational design of MoS2/carbon composites have been widely used to improve the lithium storage capability. However, their deep applications remain a big challenge due to the slow electrochemical reaction kinetics of MoS2 and weak bonding between MoS2 and carbon substrates. In this work, anthracite-derived porous carbon (APC) is sequential coated by TiO2 nanoparticles and MoS2 nanosheets via a chemical activation and two-step hydrothermal method, forming the unique APC@TiO2@MoS2 ternary composite. The dynamic analysis, in-situ electrochemical impedance spectroscopy as well as theoretical calculation together demonstrate that this innovative design effectively improves the ion/electron transport behavior and alleviates the large volume expansion during cycles. Furthermore, the introduction of middle TiO2 layer in the composite significantly strengthens the mechanical stability of the entire electrode. As expected, the as-prepared APC@TiO2@MoS2 anode displays a high lithium storage capacity with a reversible capacity of 655.8 mAh g-1 after 150 cycles at 200 mA g-1, and robust cycle stability. Impressively, even at a high current density of 2 A g-1, the electrode maintains a superior reversible capacity of 597.7 mAh g-1 after 1100 cycles. This design highlights a feasibility for the development of low-cost anthracite-derived porous carbon-based electrodes.

Role of Trapped Molecules at Sliding Contacts in Lattice-Resolved Friction.

Understanding atomic friction within a liquid environment is crucial for engineering friction mechanisms and characterizing surfaces. It has been suggested that the lattice resolution of friction force microscope in liquid environments stems from a dry contact state, with all liquid molecules expelled from the area of closest approach between the tip and substrate. Here, we revisit this assertion by performing in-depth friction force microscopy experiments and molecular dynamics simulations of the influence of surrounding water molecules on the dynamic behavior of the nanotribological contact between an amorphous SiO2 probe and a monolayer MoS2 substrate. An analysis of simulation and experimental stick-slip patterns demonstrates the entrapment of water molecules at the contact interface. These trapped water molecules behave as an integral component of the probe and participate in its interaction with the substrate, affecting the dynamics of the probe and preventing long slips. Significantly, surrounding water from the capillary or layer exhibits a replenishing effect, acting as a water reservoir during sliding. This phenomenon facilitates the preservation of lattice-scale resolution across a range of applied normal loads.

On-Surface Self-Assembly Kinetic Study of Cu-Hexaazatriphenylene 2D Conjugated Metal-Organic Frameworks on Coinage Metals and MoS2 Substrates.

Supramolecular coordination self-assembly on solid surfaces provides an effective route to form two-dimensional (2D) metal-organic frameworks (MOFs). In such processes, surface-adsorbate interaction plays a key role in determining the MOFs' structural and chemical properties. Here, we conduct a systematic study of Cu-HAT (HAT = 1,4,5,8,9,12-hexaazatriphenylene) 2D conjugated MOFs (c-MOFs) self-assembled on Cu(111), Au(111), Ag(111), and MoS2 substrates. Using scanning tunneling microscopy and density functional theory calculations, we found that the as-formed Cu3HAT2 c-MOFs on the four substrates exhibit distinctive structural features including lattice constant and molecular conformation. The structural variations can be attributed to the differentiated substrate effects on the 2D c-MOFs, including adsorption energy, lattice commensurability, and surface reactivity. Specifically, the framework grown on MoS2 is nearly identical to its free-standing counterpart. This suggests that the 2D van der Waals (vdW) materials are good candidate substrates for building intrinsic 2D MOFs, which hold promise for next-generation electronic devices.

Atomic modulation and phase engineering of MoS2 for boosting N2 reduction

Electrochemical nitrogen reduction reaction (ENRR) has emerged as a potential alternative to the conventional Haber-Bosch process for ammonia production. However, ENRR technology is still restricted by the limited Faradaic efficiency due to the hard-to-break N-N triple bond. Herein, inspired by the biomimetic catalyst, we developed a Fe-modulated MoS2 catalyst (named Fe@MoS2) as an efficient ENRR catalyst. Raman spectra, coupled with the X-ray absorption spectroscopy, demonstrate the introduction of Fe into the MoS2 lattice and achieve partial 2H to 1T phase conversion. The presence of S-vacancies on MoS2 substrates was observed on scanning transmission electron microscopy images. Operando infrared absorption spectroscopy confirms that the constructed catalytic site significantly reduces barriers to nitrogen activation. The synthesized Fe@MoS2, with its superior geometric and electronic structures, exhibits a remarkable Faradaic efficiency of 19.7 ± 5.5% at -0.2 V vs. Reversible Hydrogen Electrode and a high yield rate of 20.2 ± 5.3 μg h-1 mg-1 at -0.8 V vs. Reversible Hydrogen Electrode. Therefore, this work provides a fresh direction for designing novel catalysts, eventually boosting the nitrogen reduction reaction kinetics and accelerating the ENRR application.

Detailed work function and structural investigations of layered MoO3 onto SiO2 and MoS2 in air

MoS2, its oxides and heterostructures are increasingly utilized in nanoscale opto-electronics and catalysis, where electrical properties such as work function (WF) affect their performance. Herein, a detailed study of the thickness-dependent work function in the case of thermally produced layered Mo oxides onto SiO2 and MoS2 substrates in air is presented. First, the effects of typical contaminants on the mechanically exfoliated MoS2 substrates are investigated. Then, high resolution structural characterization by atomic force microscopy supported by X-ray photoelectron spectroscopy shows that thermal MoS2 oxidation in humid air produces layered amorphous MoO3 with nano-crystallites. Kelvin-probe force microscopy analysis shows that the first MoO3 layer on both substrates is negatively charged and that the MoO3 work function depends on the oxide thickness. The thickness-dependent WF is explained by presence of screened electric field at the MoO3/substrate interface. The negative doping to MoO3 by the studied substrates is confirmed through density functional theory simulations. Moreover, an electrical device from amorphous MoO3 monolayer onto MoS2 is built and shows rectification behavior due to p-type doping of the MoS2 by the oxide layer. Overall, this study provides an insight to understand and manipulate electrical properties of MoO3 and MoO3/MoS2 heterostructures in various devices.

Gold Nano-colloids impregnated in Langmuir-Blodgett Film of MoS2 flakes as SERS active platform: Fabrication and its application in Malathion detection

The current work focuses on fabrication of Langmuir-Blodgett (LB) films of MoS2 flakes with impregnated gold nano particles (AuNPs) as a noble Surface enhanced Raman Scattering (SERS) active hetero structure. The hot spots created over the heterostructures work as a potent agent for localization of electromagnetic field resulting in high enhancement of Raman Signals. The LB film of MoS2 flakes with and without the AuNPs were thoroughly characterized in our present work. The efficacy of Au–MoS2 substrate as a SERS sensing platform was investigated up to ultrasensitive concentrations using Raman probe molecules. Moreover, the reproducibility and homogeneity of the substrate was also tested with Raman mapping. The as prepared SERS sensing platform was engaged further for detection of Malathion at trace concentrations. The tests to check on service time was also performed. In the contemporary exploration the fabricated substrate reveals its accuracy and effectivity as a SERS sensor. Thus, in future this substrate can be a novel “Nano-Lab on Chip” device for ultrasensitive detections of chemical and bio-chemical composites.

Modification of mono-layer MoS2 through post-deposition treatment and oxidation for enhanced optoelectronic properties

Precise control of the optical and electrical properties of mono-layer (ML) thin MoS2 is crucial for future applications in functional devices. Depending on the synthesis route and the post-deposition annealing protocols, the number of sulfur vacancies in the material is different, which has a profound impact on the properties of the 2D layer. Here, we show that the sulfur vacancy-rich ML MoS2 films oxidize already at room temperature, which changes the photoluminescence (PL) yield, the MoS2–Al2O3 substrate interaction, and the structural integrity of the films. We used x-ray photoelectron spectroscopy to monitor the formation of MoO3 and possibly MoS3−xOx after exposure to air and to quantify the number of sulfur defects in the films. Atomic force microscopy measurements allow us to pinpoint the exact regions of oxidation and develop a dedicated low temperature heating procedure to remove oxidized species, leading to MoO3-free MoS2 films. AFM and Kelvin probe force microscopy show that the MoS2–Al2O3 substrate coupling is changed. The reduction in the MoS2–substrate coupling, combined with a preferential oxidation of sulfur vacancies, leads to a sevenfold increase in the PL intensity, and the ratio between trions and neutral excitons is changed. Our work highlights the importance of oxidized sulfur vacancies and provides useful methods to measure and manipulate their number in MoS2. Furthermore, changes in the MoS2–substrate interaction via sulfur vacancies and oxidation offer an elegant pathway to tune the optoelectronic properties of the two-dimensional films.

Adsorption behavior of formaldehyde gas on two-dimensional semiconductor monolayers MS2 (M=W, Mo)

Detecting methanal molecule, an indoor air pollutant and potential carcinogen, is crucial for safeguarding human health, ensuring occupational safety, and maintaining environmental quality. In this study, density functional theory calculations have been performed to explore the adsorption behavior of formaldehyde (methanal) gas on the surface of two-dimensional semiconductor monolayers MS2 (M=W, Mo). Using the Computational DFT-based Nanoscope tool, we compute binding energies and determine configurations at global minimum energy of molecule adsorbed monolayer MS2. Five nonlocal van der Waals functionals; revPBE-vdW, optPBE-vdW, vdW-DF2, optB88-vdW, and optB86b-vdW are used to compute the adsorption energy profiles. The calculated results show that: (i) the optPBE-vdW functional products the largest adsorption energy magnitude, (ii) Methanal molecule exhibits physical adsorption on both MoS2 and WS2 materials (iii) Adsorption of methanal molecules may enhance the electrical conductivity of MoS2 and WS2 upon the electron donation to molecule by substrates. The adsorption energy magnitude, bandgap reduction, and charge transfer of the methanal-MoS2 adsorption system are respectively 1.04, 1.27, and 1.47 times larger than those of the methanal-WS2 adsorption system, while the diffusion barrier energy is 0.25 times smaller. These characteristic adsorption parameters imply that methanal exhibits higher sensitivity to the MoS2 substrate. This study also provides an in-depth discussion regarding the interaction between methanal and the MS2 substrate, focusing on aspects of relaxed geometrical structures, potential energy surface, adsorption energy, response length, recovery time, work function, charge transfer, density of states, and energy band structure.

Thermal investigation of Pd interface with molybdenum disulfide

The thermal stability of molybdenum disulfide (MoS2) films, and particularly of the 1T-MoS2 metallic phase, plays a crucial role in their applicability across diverse technological fields. In this research, we successfully achieved a phase transition of multilayer MoS2 nanosheets from the semiconducting 2H phase to the metallic 1T phase through Pd substitution of Mo atoms on non-stoichiometric (sulfur-rich) MoS2 substrates and MoS2 hybrids with reduced graphene oxide (MoS2-rGO). This investigation involved studying the thermal stability of MoS2 thin films with mixed 1T-2H phases, up to 500 °C, in an ultra-high vacuum (UHV) environment. Our findings indicate that the 1T phase transforms to the 2H semiconducting stable phase at different temperatures, depending on the surface chemistry. In sulfur-rich substrates, the 1T phase remains present up to approximately 400 °C, while it vanishes on the stoichiometric MoS2-rGO films. Notably, enhanced thermal stability of the 1T phase is achieved with a sulfur surplus, regardless of the presence of rGO. Furthermore, during annealing, we observed Pd sintering, accompanied by a chemical state change from metallic Pd0 to Pd2+. These results elucidate the stability of 1T-MoS2 within an acceptable temperature range for the catalytic process and energy applications involving MoS2.

The Stability Prediction and Epitaxial Growth of Boron Nitride Nanodots on Different Substrates.

Boron nitride (BN) is a wide-bandgap material for various applications in modern nanotechnologies. In the technology of material science, computational calculations are prerequisites for experimental works, enabling precise property prediction and guidance. First-principles methods such as density functional theory (DFT) are capable of capturing the accurate physical properties of materials. However, they are limited to very small nanoparticle sizes (<2 nm in diameter) due to their computational costs. In this study, we present, for the first time, an important computational approach to DFT calculations for BN materials deposited on different substrates. In particular, we predict the total energy and cohesive energy of a variety of face-centered cubic (FCC) and hexagonal close-packed (HCP) boron nitrides on different substrates (Ni, MoS2, and Al2O3). Hexagonal boron nitride (h-BN) is the most stable phase according to our DFT calculation of cohesive energy. Moreover, an experimental validation equipped with a molecular beam epitaxy system for the epitaxial growth of h-BN nanodots on Ni and MoS2 substrates is proposed to confirm the results of the DFT calculations in this report.

Correlation-Induced Symmetry-Broken States in Large-Angle Twisted Bilayer Graphene on MoS2.

Strongly correlated states commonly emerge in twisted bilayer graphene (TBG) with "magic-angle" (1.1°), where the electron-electron (e-e) interaction U becomes prominent relative to the small bandwidth W of the nearly flat band. However, the stringent requirement of this magic angle makes the sample preparation and the further application facing great challenges. Here, using scanning tunneling microscopy (STM) and spectroscopy (STS), we demonstrate that the correlation-induced symmetry-broken states can also be achieved in a 3.45° TBG, via engineering this nonmagic-angle TBG into regimes of U/W > 1. We enhance the e-e interaction through controlling the microscopic dielectric environment by using a MoS2 substrate. Simultaneously, the width of the low-energy van Hove singularity (VHS) peak is reduced by enhancing the interlayer coupling via STM tip modulation. When partially filled, the VHS peak exhibits a giant splitting into two states flanked by the Fermi level and shows a symmetry-broken LDOS distribution with a stripy charge order, which confirms the existence of strong correlation effect in our 3.45° TBG. Our result demonstrates the feasibility of the study and application of the correlation physics in TBGs with a wider range of twist angle.

Magnetism of MoS2 bilayers with intercalated and surface adsorbed Fe

Magnetic thin films composed of two-dimensional van der Waals materials will play an important role in future spintronics devices. Recently, Tuli et al. performed in-situ MOKE measurements of Fe deposited on an MoS2 substrate and found that a magnetic dead layer (MDL) exists up to approximately 2 Å (Tuli et al. (2021)). To understand the origin of the MDL, we investigate MoS2 bilayers with intercalated and surface adsorbed Fe using first-principles calculations. The results show that the Fe-intercalated bilayer MoS2 is more stable than the bilayer MoS2 with surface adsorbed Fe. The Fe-intercalated bilayer MoS2 is non-magnetic and behaves as the MDL. The systematic investigation of variation in the inter-layer distance between the intercalated Fe layer and surface of MoS2 layer reveals that the strong hybridization between Fe and S is the origin of the MDL. We also show that further intercalation of Fe makes the film magnetic. The results provide a deep understanding of the MDL in two-dimensional van der Waals magnetic materials.

Dual-vacancies modulation of 1T/2H heterostructured MoS2-CdS nanoflowers via radiolytic radical chemistry for efficient photocatalytic H2 evolution

Precise defect engineering of photocatalysts is highly demanding but remains a challenge. Here, we developed a facile and controllable γ-ray radiation strategy to assemble dual-vacancies confined MoS2-CdS-γ nanocomposite photocatalyst. We showed the solvated electron induced homogeneous growth of defects-rich CdS nanoparticles, while the symbiotic •OH radicals etched flower-like 1T/2H MoS2 substrate surfaces. The optimal MoS2-CdS-γ exhibited a H2 evolution rate of up to 37.80 mmol/h/g under visible light irradiation, which was 36.7 times higher than that of bare CdS-γ, and far superior to those synthesized by hydrothermal method. The microscopic characterizations and theoretical calculations revealed the formation of such unprecedented dual-sulfur-vacancies ensured the tight interfacial contact for fast charge separation. Besides, the existence of 1T-MoS2 phase further improved the conductivity and strengthened the adsorption interaction with H+ intermediate. Therefore, the radiolytic radical chemistry offered a facile, ambient and effective synthetic strategy to improve the catalytic performances of photocatalytic materials.

Interfacial coupling of NiSe in heterostructures promotes electrocatalytic hydrolysis of MoS2.

Molybdenum sulfide (MoS2) has attracted much attention as a potential catalyst for the oxygen evolution reaction (OER), but its unique low activity and low edge active centers limit its electrocatalytic activity. In this study, catalysts were prepared by growing NiSe nanoclusters in situ onto MoS2 substrates via electrodeposition; ultrathin MoS2 nanosheets and NiSe nanoclusters were cross-linked with each other to form a unique three-dimensional rosette structure; and MoS2@NiSe catalysts were successfully synthesised, which significantly improved bifunctional catalytic performance. The synthesised MoS2@NiSe catalysts exhibited good electrochemical performance: overpotentials required to satisfy the HER and OER processes at a current density of 10 mA cm-2 in 1 M KOH were 80 mV and 254 mV, respectively. When applied as a cathode and anode to assemble a bifunctional electrode system, the MoS2@NiSe||MoS2@NiSe electrolytic cell system required only 1.54 V to achieve 10 mA cm-2 in an alkaline electrolyte, which exceeded the value of most of the bifunctional catalysts reported in the literature to date. In addition, the catalyst maintained good surface structure and catalytic performance after a 24 h stability test. This study provides a new idea for the improvement and design of MoS2-based bifunctional catalysts and provides an important reference for research in the field of clean energy.

Insights of the efficient hydrogen evolution reaction performance in bimetallic Au4Cu2 nanoclusters.

The design of efficient electrocatalysts for improving hydrogen evolution reaction (HER) performance using atomically precise metal nanoclusters (NCs) is an emerging area of research. Here, we have studied the HER electrocatalytic performance of monometallic Cu6 and Au6 nanoclusters and bimetallic Au4Cu2 nanoclusters. A bimetallic Au4Cu2/MoS2 composite exhibits excellent HER catalytic activity with an overpotential (η10) of 155 mV vs. reversible hydrogen electrode observed at 10 mA cm-2 current density. The improved HER performance in Au4Cu2 is due to the increased electrochemically active surface area (ECSA), and Au4Cu2 NCs exhibits better stability than Cu6 and Au6 systems and bare MoS2. This augmentation offers a greater number of active sites for the favorable adsorption of reaction intermediates. Furthermore, by employing X-ray photoelectron spectroscopy (XPS) and Raman analysis, the kinetics of HER in the Au4Cu2/MoS2 composite were elucidated, attributing the favorable performance to better electronic interactions occurring at the interface between Au4Cu2 NCs and the MoS2 substrate. Theoretical analysis reveals that the inherent catalytic enhancement in Au4Cu2/MoS2 is due to favorable H atom adsorption over it and the smallest ΔGH* value. The downshift in the d-band of the Au4Cu2/MoS2 composite influences the binding energy of intermediate catalytic species. This new catalyst sheds light on the structure-property relationship for improving electrocatalytic performance at the atomic level.

Synergetic Role of Thermal Catalysis and Photocatalysis in CO2 Reduction on Cu2/MoS2.

Effective activation of CO2 is a primarily challenging issue in CO2 reduction to value-added hydrocarbon chemicals, due to the large energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (HOMO-LUMO). Here, we employ state-of-the-art first-principles calculations to explore the synergetic role of thermal catalysis and photocatalysis in CO2 reduction, on typical single-atom scale catalyst, i.e., Cu2 magic cluster on a semiconducting two-dimensional MoS2 substrate. It is identified that only about 1% of the hot electrons excited from the MoS2 substrate by at least 6.3 eV photons may be trapped by the inert CO2 molecule at the expense of 400 fs. Moreover, the physisorption-to-chemisorption transition of CO2 can be observed within 500 fs upon overcoming an about 0.05 eV energy barrier. Contrastingly, upon chemisorption, the activated CO2δ- species may trap about 7% of the hot electron excited from the MoS2 substrate by about 2.5 eV visible photons, with a cost of 140 fs.

Tuning the content of S vacancies in MoS2 by Cu doping for enhancing catalytic hydrogenation of CO2 to methanol

Hydrogenation of CO2 to methanol is one of effective ways to promote "carbon neutrality". The activity of MoS2 as an efficient catalyst for the catalytic hydrogenation of CO2 to methanol is influenced by the content of in-plane S vacancies. Theoretically, the higher the density of in-plane S vacancies is, the better the performance of the catalyst is. In this work, a series of MoS2 nanosheets with different copper (Cu) dopant concentrations were prepared via hydrothermal method. It was found that the highest selectivity of MoS2-catalyzed CO2 hydrogenation to methanol along with the spatial-temporal yield was achieved at the Cu doping of 5%. Compared to the undoped MoS2 catalyst, the increase in methanol selectivity and spatial-temporal yield after doping was 13.37% and 2.27 times, respectively. This was due to the fact that Cu doping multiplied the number of S vacancies on MoS2, which altered the microstructure and chemical properties of the catalyst. However, a further increase in Cu dopant concentration reduced the S vacancy content on the MoS2 substrate, hindering the hydrogenation of the decomposed CO and thus decreasing the methanol yield. Therefore, a new technique to enhance MoS2-catalyzed CO2 hydrogenation to methanol was proposed.

Shape control in 2D molecular nanosheets by tuning anisotropic intermolecular interactions and assembly kinetics

Since molecular materials often decompose upon exposure to radiation, lithographic patterning techniques established for inorganic materials are usually not applicable for the fabrication of organic nanostructures. Instead, molecular self-organisation must be utilised to achieve bottom-up growth of desired structures. Here, we demonstrate control over the mesoscopic shape of 2D molecular nanosheets without affecting their nanoscopic molecular packing motif, using molecules that do not form lateral covalent bonds. We show that anisotropic attractive Coulomb forces between partially fluorinated pentacenes lead to the growth of distinctly elongated nanosheets and that the direction of elongation differs between nanosheets that were grown and ones that were fabricated by partial desorption of a complete molecular monolayer. Using kinetic Monte Carlo simulations, we show that lateral intermolecular interactions alone are sufficient to rationalise the different kinetics of structure formation during nanosheet growth and desorption, without inclusion of interactions between the molecules and the supporting MoS2 substrate. By comparison of the behaviour of differently fluorinated molecules, experimentally and computationally, we can identify properties of molecules with regard to interactions and molecular packing motifs that are required for an effective utilisation of the observed effect.

Local wrinkles of van der Waals heterostructures under nanoindentation

Probing van der Waals (vdW) heterostructures to characterize their properties, especially the stability and durability is of vital importance due to the weak interactions between two-dimensional materials. However, the wrinkling behavior of vdW heterostructures under nanoindentation is still unclear, which hinders the accuracy and development of probing methods at the nanoscale. Herein, the wrinkles of vdW heterostructures composed of graphene and molybdenum disulfide (MoS2) under indentation are studied through molecular simulation and theoretical analysis. It is found that wrinkles occur when the hoop strain of the upper layer reaches the critical value for the graphene film-single layer MoS2 substrate system. By fitting the force–displacement relationship, we estimate the predicted elastic modulus of the vdW heterostructure, and uncover that the wrinkling phenomena have ignorable effect on the probing results. It is interesting to find a transition from a sinusoidal wrinkling to a period-doubling bifurcation of vdW heterostructures with the increase of indentation depth. What is more, the wrinkling morphology of vdW heterostructures can be tuned through choosing suitable indentation depth, indenter size, upper film size as well as applied pretension. The present findings should be useful for the developing probing methods of vdW heterostructures, and serve as a guide for their blooming applications.

“Nano Killers” Activation by permonosulfate enables efficient anaerobic microorganisms disinfection

The development of effective nanomaterials for killing anaerobic bacteria is essential for human health and economic development. Here, we propose a new bactericidal mechanism where theoretical calculations are in good agreement with experimental results. The “poison arrow-head” of MoS2 nanosheets enables the vigorous extraction of lipids from the cell membrane. Based on density functional calculations, oxidation active species (OAS) are generated due to the strong adsorption energy between S vacancies in MoS2 and chemical substrates (permonosulfate (PMS) and H2O). These OAS can be visualized as numerous moving “nano killers”, constantly oxidizing the lipids around MoS2; thereby, re-releasing the surface of the sharp knife. The process of physical extraction collaborated with chemical oxidation not only precisely positions the cell membrane but also allows for continuous sterilization. This work digs into the mechanism of anaerobic bacterial sterilization, which sheds significant light on biological analysis, antibacterial, cancer therapy, and anti microbiologically influenced corrosion.