Single-layer Molybdenum Disulfide Research Articles

Facile Synthesis of Semiconducting Ultrathin Layer of Molybdenum Disulfide.

In this paper, we have reported a simple and efficient method for the synthesis of uniform, highly conducting single or few layer molybdenum disulfide (MoS2) on large scale. Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) have been used for the confirmation of mono or few layered nature of the as-synthesized MoS2 sheets. X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Raman Spectroscopy have also been used to study the elemental, phase, and molecular composition of the sample. Optical properties of as-synthesized sample have been probed by measuring absorption and photoluminescence spectra which also compliment the formation of mono and few layers MoS2 Current-voltage (I-V ) characteristics of as-synthesized sample in the pellet form reveal that MoS2 sheets have an ohmic character and found to be highly conducting. Besides characterizing the as-synthesized sample, we have also proposed the mechanism and factors which play a decisive role in formation of high quality MoS2 sheets.

Atomic scale study for the structural transformation of single layered MoS2

The structural transformation of single and few layered molybdenum disulfide (MoS2) from 2H to 1T phase was studied at the atomic scale.

Controllable and reversible DNA translocation through a single-layer molybdenum disulfide nanopore.

A challenge that remains to be solved in the high-throughput and low-cost nanopore DNA sequencing is that DNA translocates through the nanopore too quickly to be sequenced with enough accuracy. Here, we present a proof of principle study of slowing down DNA translocation across the molybdenum disulfide nanopore and even reversing its translocation direction by adjusting the proportion of molybdenum atoms to sulfur atoms at the nanopore boundary. When the proportion is smaller than 0.17, the electro-osmotic flow moves in the opposite direction to the electric force exerted on the DNA molecule and the more sulfur atoms at the nanopore boundary, the stronger the electro-osmotic flow is. For the nanopore with the proportion equal to 0.17, the electro-osmotic force exerted on DNA is smaller than the electrophoretic force, DNA can be captured and its translocation speed was found to be almost three times smaller than the speed through nanopores with the proportion larger than 0.27. However, for nanopores with the proportion smaller than 0.08, DNA would even be pushed away and prevented from entering the nanopore so that its translocation direction would be reversed. The theoretical study performed here provides a new means for controlling DNA transport dynamics in both translocation velocity and direction, which would facilitate better and cheaper nanopore DNA sequencing in the future.

Point defects in the 1T′ and 2H phases of single-layer MoS2 : A comparative first-principles study

The metastable $1{T}^{\ensuremath{'}}$ phase of layered transition metal dichalcogenides has recently attracted considerable interest due to electronic properties, possible topological phases, and catalytic activity. We report a comprehensive theoretical investigation of intrinsic point defects in the $1{T}^{\ensuremath{'}}$ crystalline phase of single-layer molybdenum disulfide ($1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$) and provide comparison to the well-studied semiconducting $2H$ phase. Based on density functional theory calculations, we explore a large number of configurations of vacancy, adatom, and antisite defects and analyze their atomic structure, thermodynamic stability, and electronic and magnetic properties. The emerging picture suggests that, under thermodynamic equilibrium, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ is more prone to hosting lattice imperfections than the $2H$ phase. More specifically, our findings reveal that the S atoms that are closer to the Mo atomic plane are the most reactive sites. Similarly to the $2H$ phase, S vacancies and adatoms in $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ are very likely to occur while Mo adatoms and antisites induce local magnetic moments. Contrary to the $2H$ phase, Mo vacancies in $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ are expected to be an abundant defect due to the structural relaxation that plays a major role in lowering the defect formation energy. Overall, our study predicts that the realization of high-quality flakes of $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ should be carried out under very careful laboratory conditions but at the same time the facile defects introduction can be exploited to tailor physical and chemical properties of this polymorph.

Adsorption of DNA/RNA nucleobases onto single-layer MoS2 and Li-Doped MoS2: A dispersion-corrected DFT study

The kind of sensing platform in nano biosensor plays an important role in nucleic acid sequence detection. It has been demonstrated that graphene does not have an intrinsic band gap; therefore, transition metal dichalcogenides (TMDs) are desirable materials for electronic base detection. In the present work, a comparative study of the adsorption of the DNA/RNA nucleobases [Adenine (A), Cytosine (C) Guanine (G), Thymine (T) and Uracil (U)] onto the single-layer molybdenum disulfide (MoS2) and Li-doped MoS2 (Li-MoS2) as a sensing surfaces was investigated by using Dispersion-corrected Density Functional Theory (D-DFT) calculations and different measure of equilibrium distances, charge transfers and binding energies for the various nucleobases were calculated. The results revealed that the interactions between the nucleobases and the MoS2 can be strongly enhanced by introducing metal atom, due to significant charge transfer from the Li atom to the MoS2 when Lithium is placed on top of the MoS2. Furthermore, the binding energies of the five nucleobases were in the range of −0.734 to −0.816eV for MoS2 and −1.47 to −1.80eV for the Li-MoS2. Also, nucleobases were adsorbed onto MoS2 sheets via the van der Waals (vdW) force. This high affinity and the renewable properties of the biosensing platform demonstrated that Li-MoS2 nanosheet is biocompatible and suitable for nucleic acid analysis.

Adsorption patterns of aromatic amino acids on monolayer MoS2 and Au-modified MoS2 surfaces: A first-principles study

As a pioneering non-graphene material, two dimensional, single-layered molybdenum disulfide (MoS2) has achieved outstanding performance in biosensor research in recent years. However, either reliable exploitation of bio-applications or keeping eyes on biological safety requires clear understanding of the linkage between the MoS2 surface and the biological elementary blocks, such as amino acids. In this manuscript, first-principles density functional theory (DFT) was employed to investigate the adsorption behaviors of aromatic amino acids, phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp), on monolayer pristine MoS2 and Au-modified MoS2 surfaces. Optimal adsorption configurations of amino acids on different surfaces were predicted by adsorption energy calculation. The calculations indicate that three amino acids can be physically adsorbed on pristine MoS2 surface by parallel interaction between the aromatic ring and surface S atoms, which is consistent with previous work. On the Au-modified surface, amino acids demonstrate a coexistence of chemisorption and physisorption through covalent linkage to Au atom and no-covalent interaction to the sheet respectively. And the adsorption energies are almost twice as large as that of on a pure MoS2 surface. Yet there is a same adsorption ability order Trp>Tyr>Phe on both surfaces. By comparing changes in molecular geometry before and after adsorption, it is found that the structure influences of amino acids induced from both surfaces adsorption are limited. The DOS and PDOS analysis show that amino acids adsorption can modify the electronic properties of substrates through π-p interaction and other electronic interactions. Especially, amino acids with Au linker can transform p-type semiconductor of MoS2 into a n-type one. Our calculations clarify the sensitivity of intrinsic MoS2 and Au-modified MoS2 surfaces to Phe, Tyr and Trp in a waterless environment and demonstrate their fleshed-out interaction patterns. The results provide a first-hand theoretical reference for their potential bio-nanotechnical applications of both MoS2 and Au-modified MoS2 materials.

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS2 Device.

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS2) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on-off ratio drops by 3 orders of magnitude due to the increase in "off" current for both single and few layer MoS2 FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 103 and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS2 interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices.

Highly Sensitive MoS2 Humidity Sensors Array for Noncontact Sensation.

Recently, 2D materials exhibit great potential for humidity sensing applications due to the fact that almost all atoms are at the surface. Therefore, the quality of the material surface becomes the key point for sensitive perception. This study reports an integrated, highly sensitive humidity sensors array based on large-area, uniform single-layer molybdenum disulfide with an ultraclean surface. Device mobilities and on/off ratios decrease linearly with the relative humidity varying from 0% to 35%, leading to a high sensitivity of more than 104 . The reversible water physisorption process leads to short response and decay times. In addition, the device array on a flexible substrate shows stable performance, suggesting great potential in future noncontact interface localization applications.

Cellular proliferation and differentiation induced by single-layer molybdenum disulfide and mediation mechanisms of proteins via the Akt-mTOR-p70S6K signaling pathway

Single-layer molybdenum disulfide (SLMoS2) is a novel kind of 2D nanosheet that has attracted great attention regarding its use in biosensors, drug delivery, tissue engineering, and therapy. However, our results demonstrated that SLMoS2 accelerated proliferation and promoted myogenic differentiation and epithelial–mesenchymal transition (EMT) in human embryonic lung fibroblasts (HELFs). The abnormal proliferation and differentiation of HELFs contribute to idiopathic pulmonary fibrosis. Specifically, SLMoS2 significantly stimulated the expression of myofibroblast- and mesenchymal-associated genes and proteins. The Akt-mTOR-p70S6K signaling pathway plays a critical role in the acceleration of proliferation and promotion of myogenic differentiation and EMT in HELFs induced by SLMoS2. After cell uptake, SLMoS2 was primarily located in the cytoplasm and the perinuclear region and activated Akt-dependent signaling due to the generation of reactive oxygen species (ROS). Moreover, bovine serum albumin (BSA) binding markedly inhibited the cellular uptake of SLMoS2 and the production of intracellular ROS due to an increased thickness and reduced adhesion of HELFs. BSA binding also mitigated the SLMoS2-activated phosphorylation of Akt-dependent signaling pathways. This study is the first to illustrate the induction of cellular proliferation and differentiation by SLMoS2 and the related mediation by proteins through Akt-mTOR-p70S6K signaling pathway.

Abnormal nonlocal scale effect on static bending of single-layer MoS2

The nonlocal scale parameter of nonlocal Euler–Bernoulli beam theory is evaluated for the static bending of single-layer molybdenum disulfide (SLMoS2) without predetermined bending rigidity. The evaluation is performed by matching the fitted curve between the maximum deflection and the beam length obtained from molecular mechanics simulations. It was observed that the fitted curves have an abnormal sign in the second-order term of the maximum deflection for SLMoS2, opposite to that for graphene and regardless of the interatomic interaction potentials used. Based on the nature of ‘nonlocal’ and the phenomenological point of view, a modified nonlocal constitutive relation with a positive sign in front of the higher-order term is suggested for SLMoS2. The nonlocal parameter and the bending rigidity of SLMoS2 are finally extracted, and the effect of the nonlocal scale parameter on the bending response for SLMoS2 is found to be significant for beam length less than a critical length, depending on both the interatomic interaction potentials and the boundary conditions. Our new perspective should be useful for researchers who are interested in the engineering application of graphene-like quasi-two-dimensional nanostructures using nonlocal beam theories.

Atomic simulation of thermal fluctuation (ripples) in constrained single-layer MoS2 membranes

Molecular dynamics simulations based on reactive empirical bond order potentials are performed to investigate the attributes of the thermally agitated ripples in single-layer molybdenum disulfide (SLMoS2) membranes, including the vibrational mode and the height fluctuation. The effects of aspect ratio, type and shape of boundary conditions, and chirality, on the attributes of the ripples are mainly concerned. The aspect ratio has a major influence on the vibrational mode of the ripples for rectangular SLMoS2 membranes. Most of the effect of chirality is concentrated on the height fluctuation for the slender SLMoS2 nanoribbons due to inducing different number of ripples. The types of boundary conditions is shown to have minor influence on the vibrational mode and height fluctuation for the rectangular SLMoS2 membranes. Only high-order resonant mode is thermally agitated in SLMoS2 nanosheets with circular boundary conditions. The height fluctuation in most of SLMoS2 membranes increases with increasing the temperature, which is consistent with the elastic theory of membrane. The work can be useful for the application of strain engineering in SLMoS2 nanodevices.

Tunable Doping in Hydrogenated Single Layered Molybdenum Disulfide

Structural defects in the molybdenum disulfide (MoS2) monolayer are widely known for strongly altering its properties. Therefore, a deep understanding of these structural defects and how they affect MoS2 electronic properties is of fundamental importance. Here, we report on the incorporation of atomic hydrogen in monolayered MoS2 to tune its structural defects. We demonstrate that the electronic properties of single layer MoS2 can be tuned from the intrinsic electron (n) to hole (p) doping via controlled exposure to atomic hydrogen at room temperature. Moreover, this hydrogenation process represents a viable technique to completely saturate the sulfur vacancies present in the MoS2 flakes. The successful incorporation of hydrogen in MoS2 leads to the modification of the electronic properties as evidenced by high resolution X-ray photoemission spectroscopy and density functional theory calculations. Micro-Raman spectroscopy and angle resolved photoemission spectroscopy measurements show the high quality of the hydrogenated MoS2 confirming the efficiency of our hydrogenation process. These results demonstrate that the MoS2 hydrogenation could be a significant and efficient way to achieve tunable doping of transition metal dichalcogenides (TMD) materials with non-TMD elements.

Electron screening lengths and plasma spectrum in single layer MoS2

We obtain the energy eigenvalues and wave functions of the single layer molybdenum disulfide by using an effective Hamiltonian. Moreover, the density of states and high electron-electron screening length up to 108 cm-1 are also evaluated based on the dielectric function of MoS2. It is shown that the quasi-linear energy bands split off due to the spin-orbit couplings. Plasmons in such a system are investigated theoretically within diagrammatic self-consistent field theory. In the random phase approximation, it is found that two plasma spectra can be produced via intra band transitions induced in conduction bands in monolayer MoS2 because of splitting off. The plasma spectrum frequency increases with increasing wave-vector q and electron density. It is found that the two plasmon modes induced by the spin intra-subband transitions are acoustic-like and depend strongly on wave-vector q. We find that the plasma spectrum is very different from those of graphene and two-dimensional electron gas due to the quasi-linear dispersion and spin-orbit couplings in single layer MoS2. Moreover, the plasmon frequency can be effectively controlled through changing the doping electron density. Our results exhibit some interesting features which can be utilized to realize the plasmonic devices based on the single layer MoS2.

The Effect of Substrate on Vibrational Properties of Single-Layer MoS_2

We report on the Raman scattering from single-layer molybdenum disulfide (MoS2) deposited on various substrates: Si/SiO2, hexagonal boron nitride (h-BN), sapphire, as well as suspended. Room temperature Raman scattering spectra are investigated under both resonant (632.8 nm) and non-resonant (514.5 nm) excitations. A rather weak influence of the substrate on the Raman scattering signal is observed. The most pronounced, although still small, is the effect of h-BN, which manifests itself in the change of energy positions of the E′ and A1 Raman modes of single-layer MoS2. We interpret this modification as originating from van der Waals interaction between the MoS2 and h-BN layers.

Mutual Photoluminescence Quenching and Photovoltaic Effect in Large-Area Single-Layer MoS2-Polymer Heterojunctions.

Two-dimensional transition metal dichalcogenides (TMDCs) have recently attracted attention due to their superlative optical and electronic properties. In particular, their extraordinary optical absorption and semiconducting band gap have enabled demonstrations of photovoltaic response from heterostructures composed of TMDCs and other organic or inorganic materials. However, these early studies were limited to devices at the micrometer scale and/or failed to exploit the unique optical absorption properties of single-layer TMDCs. Here we present an experimental realization of a large-area type-II photovoltaic heterojunction using single-layer molybdenum disulfide (MoS2) as the primary absorber, by coupling it to the organic π-donor polymer PTB7. This TMDC-polymer heterojunction exhibits photoluminescence intensity that is tunable as a function of the thickness of the polymer layer, ultimately enabling complete quenching of the TMDC photoluminescence. The strong optical absorption in the TMDC-polymer heterojunction produces an internal quantum efficiency exceeding 40% for an overall cell thickness of less than 20 nm, resulting in exceptional current density per absorbing thickness in comparison to other organic and inorganic solar cells. Furthermore, this work provides insight into the recombination processes in type-II TMDC-polymer heterojunctions and thus provides quantitative guidance to ongoing efforts to realize efficient TMDC-based solar cells.

Mechanical Transfer of Large-Scale Two-Dimensional Materials Onto Arbitrary Substrates Via Water-Penetration-Assisted Method

Recently, two-dimensional (2D) materials have emerged as an important research topic. Single layer molybdenum disulfide (MoS2) is one of the most widely investigated 2D materials, and possesses outstanding properties with applications in band gap engineering, optical and electrical devices, and spin-orbit research. Large-area and uniform single layer MoS2 can be synthesized by performing chemical vapor deposition (CVD) on various substrates. The full exploitation of the potential of CVD MoS2 requires a method for the clean transfer of the as-grown MoS2 from the growth substrate to the target substrate without degeneration of its properties. The current approach to the transfer of CVD MoS2 usually involves coating a polymer carrier film on top of the CVD MoS2 as a supporting layer and the chemical etching of the growth substrate to separate MoS2 from its surface. However, this etching process causes serious damage to the 2D material due to the harsh corrosivity of the etchant, usually hydrogen fluoride (HF) or a strong base (NaOH or KOH), and the removal of polymer carrier film leaves a residue. Thus the development of a gentle transfer approach that is highly efficient, repeatable, and environmentally friendly is required. In this work, we report a method that with the assistance of water penetration enables the mechanical transfer of MoS2 from the growth substrate to a target substrate without any etching of the growth substrate or leaving any polymer residue on the MoS2 layer. The whole transfer process is as below: After CVD growth of MoS2 on SiO2, a thin Cu carrier film (50–100 nm) is prepared with e-beam deposition on top of the MoS2 layer. Then a layer of polymethyl methacrylate (PMMA) is spin-coated onto Cu/MoS2/SiO2, then baking process is performed, and a layer of polydimethylsiloxane (PDMS) is attached onto PMMA/Cu/MoS2/SiO2. Next, PDMS/PMMA/Cu/MoS2/SiO2 is immersed in deionized (DI) water (We also carried out peeling off out of water under ambient conditions as in other mechanical transfer processes as a contrast experiment). Once the peeling off process starts, water will immediately flood into the space generated by the separation of PDMS/PMMA/Cu/MoS2 because of this natural tendency and the hydraulic pressure. The penetrated water mechanically supports the upper PDMS/PMMA/Cu/MoS2 with buoyancy force, which prevents physical damage. After separation of PDMS/PMMA/Cu/MoS2, it is dried under ambient conditions, and then PDMS/PMMA/Cu/MoS2 is placed and pressured onto a target substrate. The whole sample is heated at 130°C on a hotplate. Once PDMS has lost adhesion, it can easily be peeled off leaving PMMA/Cu/MoS2 on top of the target substrate. In the next step, the PMMA/Cu/MoS2/target substrate is dipped in acetone to remove PMMA and then in Cu etchant to remove Cu. In contrast to the residue left on the MoS2 layer after the removal of the polymer contact material in previous studies, the Cu layer is completely removed by the Cu etchant due to the de-wetting of metals on 2D materials. We also experimentally demonstrated that Cu, which has a strong adhesion force with MoS2, is an ideal contact material for the clean peeling off of the carrier film and MoS2. Finally, perfectly uniform, large-area CVD MoS2 with a clean surface is obtained on the target substrate with our transfer process. The morphology of MoS2 before and after transfer under or out of water were observed with optical microscopy (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The MoS2 before and after transfer under water both appear to be uniform, continuous, and clean without any wrinkles, cracks or polymer residue. However, many holes were observed in the sample transferred out of water which were generated by the abrupt forces generated by the peeling off process; the absence of support by water means that the MoS2 layer, which is just one atomic layer thick, can easily be damaged. Samples were also studied with Raman and photoluminescence (PL) spectroscopy. Raman spectra and PL peak positions are same for all three samples. But the PL peak intensity of the MoS2 sample transferred out of water is much weaker than the other two samples due to the physical damage generated by the transfer process without water support. All above results confirmed the important role of water penetration in our mechanical transfer process. Our transfer method protects the original quality and morphology of large area MoS2 without leaving any polymer residue, and enables the reuse of the growth substrate. This clean transfer approach is expected to facilitate the realization of industrial applications of MoS2 and other 2D materials.

Single-layer MoS2 nanopores as nanopower generators.

Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy'. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10(6) watts per square metre--a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment--for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.

Large-scale chemical assembly of atomically thin transistors and circuits.

Next-generation electronics calls for new materials beyond silicon, aiming at increased functionality, performance and scaling in integrated circuits. In this respect, two-dimensional gapless graphene and semiconducting transition-metal dichalcogenides have emerged as promising candidates due to their atomic thickness and chemical stability. However, difficulties with precise spatial control during their assembly currently impede actual integration into devices. Here, we report on the large-scale, spatially controlled synthesis of heterostructures made of single-layer semiconducting molybdenum disulfide contacting conductive graphene. Transmission electron microscopy studies reveal that the single-layer molybdenum disulfide nucleates at the graphene edges. We demonstrate that such chemically assembled atomic transistors exhibit high transconductance (10 µS), on-off ratio (∼106) and mobility (∼17 cm2 V-1 s-1). The precise site selectivity from atomically thin conducting and semiconducting crystals enables us to exploit these heterostructures to assemble two-dimensional logic circuits, such as an NMOS inverter with high voltage gain (up to 70).

Giant magnetic anisotropy in doped single layer molybdenum disulfide and fluorographene

Stable monolayer materials based on existing, well known and stable two-dimensional crystal fluorographene and molybdenum disulfide are predicted to exhibit a huge magnetocrystalline anisotropy when functionalized with adsorbed transition metal atoms at vacant sides. Ab initio calculations within the density-functional theory formalism were performed to investigate the adsorption of the transitional metals in a single S (or F) vacancy of monolayer molybdenum disulfide (or fluorographene). We found strong bonding of the transitional metal atoms to the vacant sites with binding energies ranging from 2.5 to 5.2 eV. Our calculations revealed that these systems with adsorbed metal atoms exhibit a magnetic anisotropy, specifically the structures including Os and Ir show a giant magnetocrystalline anisotropy energy of 31–101 meV. Our results demonstrate the possibility of obtaining stable monolayer materials with huge magnetocrystalline anisotropy based on preexisting, well known and stable two-dimensional crystals: fluorographene and molybdenum disulfide. We believe that the results obtained here are useful not only for deeper understanding of the origin of magnetocrystalline anisotropy but also for the design of monolayer optoelectronic devices with novel functionalities.

Simulations show that a nanometre-thick sheet of MoS2 removes salt from sea water

Engineers in the USA at the University of Illinois are developing an energy-efficient material that is capable of removing salt from sea water. By performing molecular dynamics simulations they have shown that nanopores in single-layer molybdenum disulfide can effectively reject ions and allow transport of water at a high rate.