Interactions of Water with Pristine and Defective MoS2

Threshold Voltage Control through Solvent Doping of Monolayer MoS2 Transistors

Two-dimensional (2D) materials are promising for beyond-silicon logic due to their ultrathin bodies for atomically thin channels. A key challenge lies in doping, to enable high-performance devices with a predictable and tunable threshold voltage (VT), while retaining switching behavior. In this work, we explore n-doping monolayer MoS2 with solvents of varying polarity to both enhance transistor performance and understand how solvents impact the VT. We find that solvent polarity predictably shifts the VT when states are available near the MoS2 conduction band. This n-doping shifts the VT, increases the maximum on-current, and is achieved without significant degradation in the subthreshold swing. We also find that solvent doping reduces the Schottky barrier width, enabling a two-fold reduction in contact resistance. These findings provide a method to tune carrier concentrations by VT shifting and offer clarity on the role solvents play in processing 2D devices.

Defect Engineering in MoS2 Monolayers on Au(111): Insights from Combined Experimental and Theoretical Approaches

In this study, we present a combined experimental and theoretical study of point defects in MoS2 monolayers supported on Au(111). By tuning the experimental conditions, we achieved selective defect formation, paving the way for advanced defect engineering. Density functional theory (DFT) simulations were performed to model both the perfect Moiré superstructure and a variety of defect configurations. This allowed us to precisely identify the experimentally created single- and multiatom vacancies, interpret their contrast in scanning tunneling microscopy (STM), and characterize their electronic properties and effects on the valence band (VB). Our results show that tuning the kinetics of ion bombardment and the chemical environment during annealing treatments can produce different combinations of simple and complex defects. Additionally, we find that the Moiré modulation has minimal impact on the geometric and electronic properties of the surface, suggesting that the defect-engineered MoS2/Au surface could serve as a rather general model system to further investigate the electronic and catalytic properties of MoS2-based nanomaterials.

Nanoconfined Solvothermal Synthesis of Defective 1T-MoS2 Monolayers with High Electrocatalytic Performance

Synthesizing 2D nanosheets in a controlled and scalable manner remains a significant challenge. Here, a nanoconfined solvothermal synthesis is presented of metallic phase MoS2 (1T-MoS2) monolayers at kilogram scale. The MoS2 nanosheets exhibit a remarkably high monolayer ratio of 97%, a 1T content of ≈89%, and a well-defined average lateral size ranging from ≈100 nm to 1.0 µm, with a narrow size distribution. Moreover, these nanosheets possesses abundant surface defects, and the defect density can be regulated in situ through changing the reaction conditions. Intriguingly, the monolayer MoS2 nanosheets demonstrate good dispersibility and high stability in various solvents, including water, ethylene glycol, dimethyl formamide and others, with a high concentration of up to 1.0 mg mL-1. They are also proven to be high-performance electrocatalysts for the hydrogen evolution reaction, exhibiting an overpotential of 315 mV at an industrial current density of 1000 mA cm-2 and maintaining constant current densities of 500 mA cm-2 for up to 100 h, surpassing the performance of the commercial 20 wt.% Pt/C. Our strategy represents a significant advancement in the controlled synthesis of monolayer MoS2 at scale, providing a promising avenue for the practical application of 2D materials.

High-Performance 2D Ambipolar MoTe2 Lateral Memristors by Mild Oxidation

2D transition metal dichalcogenides (TMDCs) have been intensively explored in memristors for brain-inspired computing. Oxidation, which is usually unavoidable and harmful in 2D TMDCs, could also be used to enhance their memristive performances. However, it is still unclear how oxidation affects the resistive switching behaviors of 2D ambipolar TMDCs. In this work, a mild oxidation strategy is developed to greatly enhance the resistive switching ratio of ambipolar 2H-MoTe2 lateral memristors by more than 10 times. Such an enhancement results from the amplified doping due to O2 and H2O adsorption and the optimization of effective gate voltage distribution by mild oxidation. Moreover, the ambipolarity of 2H-MoTe2 also enables a change of resistive switching direction, which is uncommon in 2D memristors. Consequently, as an artificial synapse, the MoTe2 device exhibits a large dynamic range (≈200) and a good linearity (1.01) in long-term potentiation and depression, as well as a high-accuracy handwritten digit recognition (>96%). This work not only provides a feasible and effective way to enhance the memristive performance of 2D ambipolar materials, but also deepens the understanding of hidden mechanisms for RS behaviors in oxidized 2D materials.

Accurate Simulation for 2D Lubricating Materials in Realistic Environments: From Classical to Quantum Mechanical Methods

2D materials such as graphene, MoS2, and hexagonal BN are the most advanced solid lubricating materials with superior friction and anti-wear performance. However, as a typical surface phenomenon, the lubricating properties of 2D materials are largely dependent on the surrounding environment, such as temperature, stress, humidity, oxygen, and other environmental substances. Given the technical challenges in experiment for real-time and in situ detection of microscopic environment-material interaction, recent years have witnessed the acceleration of computational research on the lubrication behavior of 2D materials in realistic environments. This study reviews the up-to-date computational studies for the effect of environmental factors on the lubrication performance of 2D materials, summarizes the theoretical methods in lubrication from classical to quantum-mechanics ones, and emphasizes the importance of quantum method in revealing the lubrication mechanism at atomic and electronic level. An effective simulation method based on ab initio molecular dynamics is also proposed to try to provide more ways to accurately reveal the friction mechanisms and reliably guide the lubricating material design. On the basis of current development, future prospects, and challenges for the simulation and modeling in lubrication with realistic environment are outlined.

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology

Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.

Spectroscopic Evaluation of Surface Chemical Processes Occurring in MoS2 upon Aging

Molybdenum disulfide (MoS₂) coatings have attracted widespread industrial interest owing to their excellent lubricating properties under vacuum and inert conditions. Unfortunately, the increase in MoS₂ interfacial shear strength following prolonged exposure to ambient conditions (a process referred to as "aging") has resulted in reliability issues when MoS₂ is employed as solid lubricant. While aging of MoS₂ is generally attributed to physical and chemical changes caused by adsorbed water and/or oxygen, a mechanistic understanding of the relative role of these two gaseous species in the evolution of the surface chemistry of MoS₂ is still elusive. Additionally, remarkably little is known about the effect of thermally- and tribologically-induced microstructural variations in MoS₂ on the aging processes occurring in the near-surface region of the coating. Here, we employed three analytical techniques, namely, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and grazing-incidence X-ray diffraction (GIXRD), to gain insights into the aging phenomena occurring in sputtered MoS₂ coatings before and after tribological testing, while also evaluating the impact of thermally-induced variations in the coating structure on aging. The outcomes of XPS analyses provide evidence that a substantial surface oxidation of MoS₂ only takes place under humid conditions. Furthermore, the correlation of XPS, ToF-SIMS, and GIXRD results allowed for the development of a qualitative model for the impact of shear-induced microstructural variations in MoS₂ on the transport of water in the near-surface region of this material and on the extent of surface oxidation. These results add significantly to our understanding of the aging mechanisms of MoS₂ coatings used in tribological applications and their dependence on environmental conditions.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.