MoS2 Membranes Research Articles

(Invited, Digital Presentation) Heterogeneous Integration of Ultrahigh-Κ Single-Crystalline SrTiO3 Membranes for Two-Dimensional Electronics

Even though competitive 2D field-effect transistors (FETs) with the scaled channel dimensions have been demonstrated, it still is a challenge to integrate 2D semiconductors and high-κ dielectrics without deteriorating their interfaces, while decreasing capacitance equivalent thickness (CET) of dielectrics to maintain the gate controllability. In particular, the dielectric/channel interface is one of the predominant factors to affect device performance, including carrier mobility, switching behavior, and drifts of device parameters. This is often caused by the nature of dielectrics used, and the integrating methodologies applied.Typically, high-κ dielectrics used in silicon technology (i.e., Al2O3 and HfO2) are inherited by 2D FETs. The amorphous nature of these oxides makes the elimination of charge scattering and trapping sites at the dielectric/channel interfaces extremely difficult, not to mention that the direct deposition of dielectrics usually damages the 2D channel and results in poor uniformity in sheer thickness. Some interfacial passivation layers and processes have been developed, but they lead to thickening overall CET instead. Alternative approaches with the crystalline dielectric materials such as multilayer hBN and epitaxial calcium fluoride (CaF2), whose surfaces are well-constructed and atomic-flat, have demonstrated the competitive advantages of crystalline dielectric over the conventional amorphous oxides in spite of relatively lower κ values.Here, we demonstrate ultra-scaled 2D FETs with desirable sub-1 nm CET through heterogeneous integration of monolayer CVD MoS2 and quasi-2D single-crystalline SrTiO3 membranes, where the optimized SrTiO3 gate dielectrics exhibit a low gate leakage (J leak < 10-2 A/cm2 at 2.5 MV/cm). Typical transistors manifest good reliability and competitive performance characteristics, including steep subthreshold swings (SS) down to ~70 mV dec-1 and ON/OFF current ratios up to 107, matching low-power specifications suggested by the latest International Roadmap for Devices and Systems (IRDS). In addition, the van der Waals (vdW) interface between quasi-2D dielectrics and 2D semiconductors moderates the unfavorable fringing-induced barrier lowering (FIBL) effect occurring in ultra-scaled Si transistors with very high-κ dielectrics, which broadens the dielectric selection in 2D electronics for future technology node.

Design and implementation of sequential excitation module for high fidelity piezoresponse force microscopy

The acquisition of accurate information through a contact resonance mode is critical for mapping weak electromechanical effect reliably by using piezoresponse force microscopy (PFM). However, it is very challenging to track resonance frequency shifting when the contact stiffness from the sample varies significantly. In this work, we have developed a sequential excitation (SE) module to enable high fidelity PFM. A customized discrete frequency sweep signal from an arbitrary waveform generator is used for drive excitation so that resonance frequency tracking is no longer necessary. Furthermore, the AC component of the piezoresponse is sampled by using an oscilloscope instead of using lock-in amplifiers. To accommodate high volume of data acquisition, a fast analysis method is also developed to fit the transfer function of the cantilever efficiently on the fly during scanning. Hardware implementation and data processing are described in detail. The capability of our SE module has been demonstrated on an ordinary PMN-PT film via first and second harmonic PFM, as well as a suspended freestanding MoS2 membrane that is very challenging to probe due to its substantial variation in contact stiffness.

Alcohols assisted in-situ growth of MoS2 membrane on tubular ceramic substrate for nanofiltration

Molybdenum disulfide-based membranes have received wide attention for their excellent performance in chemical separation field. Yet, manipulating the structure of MoS2 membranes still remains an unprecedented challenge. In this work, the tubular MoS2 ceramic membranes were prepared by in-situ hydrothermal method using aqueous solution of thiourea and ammonium molybdate as precursor solution. The structure and morphology of the membranes were manipulated by introducing different alcohols into the precursor solution. The results indicated that the presence of alcohols drove the transformation of MoS2 morphology from nanosheets to spherical aggregates. As the molecular weight of the alcohols decreased, more small-sized molybdenum disulfide spheres were formed on the tubular ceramic substrate, which resulted in the formation of a great number of large transportation channels and promotion of surface area, and thereby improved the permeability of the membrane. Moreover, the addition of different alcohols enhanced the negative charge of the membrane surface, which ensured an effective rejection while enhancing the membrane permeability. The results demonstrate that the membranes prepared in a water-ethanol mixed solvent possess an excellent permeability for water (33.2 L/(m2 h bar)) and methanol (52.2 L/(m2 h bar)), with a rejection of more than 97% for Eriochrome black T, and an excellent pressure resistance as well as long-term running stability under cross-flow filtration. In addition, such membrane exhibited a below 30% rejection for salts, which can be utilized for dye desalination.

Large-scale few-layered MoS2 as a saturable absorber for Q-switching operation at 2.3 µm.

In this Letter, the fabrication of large-scale (50.8 mm in diameter) few-layered MoS2 with physical vapor deposition on sapphire is described. Open-aperture Z-scan technology with a home-made excitation source at 2275 nm was applied to explore its nonlinear saturable absorption properties. The as-grown few-layered MoS2 membrane possessed a modulation depth of 17% and a saturable intensity of 1.185 MW cm-2. As a consequence, the deposited MoS2 membrane was exploited as a saturable absorber to realize a passively Q-switched Tm:YAP laser for the first time, to the best of our knowledge. Pulses as short as 316 ns were generated with a repetition rate of 228 kHz, corresponding to a peak power of 5.53 W. Results confirmed that the two-dimensional layered MoS2 could be beneficial for mid-infrared photonic applications.

Simultaneous Electrochemical Exfoliation and Covalent Functionalization of MoS2 Membrane for Ion Sieving.

Transition metal dichalcogenide membranes exhibit good antiswelling properties but poor water desalination property. Here, a one-step covalent functionalization of MoS2 nanosheets for membrane fabrication is reported, which is accomplished by simultaneous exfoliating and grafting the lithium-ion-intercalated MoS2 in organic iodide water solution. The lithium intercalation amount in MoS2 is optimized so that the quality of the produced 2D nanosheets is improved with homogeneous size distribution. The lamellar MoS2 membranes are tested in reverse osmosis (RO), and the functionalized MoS2 membrane exhibits rejection rates of >90% and >80% for various dyes (Rhodamine B, Crystal Violet, Acid Fuchsin, Methyl Orange, and Evans Blue) and NaCl, respectively. The excellent ion-sieving performance and good water permeability of the functionalized MoS2 membranes are attributed to the suitable channel widths that are tuned by iodoacetamide. Furthermore, the stability of the functionalized MoS2 membranes in NaCl and dye solutions is also confirmed by RO tests. Molecular dynamics simulation shows that water molecules tend to form a single layer between the amide-functionalized MoS2 layers but a double layer between the ethanol-functionalized MoS2 (MoS2 -ethanol) layers, which indicates that a less packed structure of water between the MoS2 -ethanol layers leads to lower hydrodynamic resistance and higher permeation.

Thermal and mechanical characterization of nanoporous two-dimensional MoS2 membranes

For practical application, determining the thermal and mechanical characterization of nanoporous two-dimensional MoS2 membranes is critical. To understand the influences of the temperature and porosity on the mechanical properties of single-layer MoS2 membrane, uniaxial and biaxial tensions were conducted using molecular dynamics simulations. It was found that Young’s modulus, ultimate strength, and fracture strain reduce with the temperature increases. At the same time, porosity effects were found to cause a decrease in the ultimate strength, fracture strain, and Young’s modulus of MoS2 membranes. Because the pore exists, the most considerable stresses will be concentrated around the pore site throughout uniaxial and biaxial tensile tests, increasing the possibility of fracture compared to tensing the pristine membrane. Moreover, this article investigates the impacts of temperature, porosity, and length size on the thermal conductivity of MoS2 membrane using the non-equilibrium molecular dynamics (NEMD) method. The results show that the thermal conductivity of the MoS2 membrane is strongly dependent on the temperature, porosity, and length size. Specifically, the thermal conductivity decreases as the temperature increases, and the thermal conductivity reduces as the porosity density increases. Interestingly, the thermal and mechanical properties of the pristine MoS2 membrane are similar in armchair and zigzag directions.

The electronic properties and water desalination performance of a photocatalytic TiO2/MoS2 nanocomposites bilayer membrane: a molecular dynamic simulation.

Due to the rapid depletion of water resources, more interest is paid for the efficient desalination process in recent years. MoS2 membrane aroused attention due to its high mechanical stability and electronic properties, which can sustain extra-large strains. In this study, the electronic properties and water desalination performance of TiO2/MoS2-hexagonal, and TiO2/MoS2-rhombohedral nanocomposites bilayer membranes were studied and simulated for the first time. The effect of TiO2 in the performance of MoS2 was observed in water desalination under the defined applied pressure ranging from 50 to 250MPa with a 6.4Å pore diameter. The membrane structure is created and optimized. The energy minimized for TiO2 from - 19,596.4282kcal/mol for the initial structure to - 19,605.1611kcal/mol for the final structure. For TiO2/MoS2-hexagonal, the energy minimized from - 4955.54271eV) to - 4955.62091eV and TiO2/MoS2-rhombohedral from - 6042.26925eV to - 6046.91835eV. A molecular dynamic (MD) simulation was performed using Material Studio 2019 to study the electronic properties under 0-1eV electric field using the CASTEP code. The results showed a better photocatalytic performance under the external electric field. The effect of external electric field significantly intensifies absorption in the visible range and achieved a high photocatalytic activity on TiO2/MoS2. TiO2, TiO2/MoS2-hexagonal and TiO2/MoS2-rhombohedral nanocomposites bilayer membranes are simulated and evaluated for the water desalination using ReaxFF software. Both MoS2 phases with TiO2 have achieved a high salt rejection up to 97% (P-value = 0.0036, R2 = 0.958), while TiO2/MoS2-rhombohedral achieved the highest permeability (6.0*10-8mmgcm-2s-1bar-1) (P-value = 0.000296, R2 = 0.972) under 250MPa applied pressure.

Can MoS2 membrane be used for removal of mineral pollutants from water? First-principle study

In this paper, first-principle calculations were used to show that a single-layer MoS2 with pores can serve as a filter for removing inorganic pollutants from water. Two types of pores (Mo and S) were considered on the MoS2 surface and their chemistry, structure, and stability were assessed. Under strain application, the porous MoS2 filter can practically tolerate high pressures which is an essential feature in filter applications. Our results indicated that pores in MoS2 can effectively reject Ca, S, Mg, and As atoms that are commonly found in water resources. First-principle molecular dynamics simulations indicated a high flux of H2O through the Mo pore in the MoS2 membrane. These findings indicate that MoS2 with an appropriate pore on its center may have a valuable role in the design of highly efficient filter architectures and other applications.

Molecular understanding of charge effect on desalination performance in lamellar MoS2 membranes.

Due to its advantages of superior oxidation resistance, excellent chemical stability and non-toxicity, molybdenum disulfide (MoS2) has shown prospects in seawater desalination applications. In this manuscript, molecular dynamics (MD) simulation has been employed to explore the effect of charge distribution in MoS2 nanosheets on the desalination performance of the lamellar MoS2 membrane. It is found that the model considering the atomic charge better describes the transport behavior of salt solution in the membranes. The water flux passing through the lamellar MoS2 membrane would be influenced little by the atomic charges in the MoS2 nanosheet. The lamellar MoS2 membrane considering the atomic charge distribution shows a screening effect between Na+ and Cl- ions.

Stress induced delamination of suspended MoS2 in aqueous environments.

Applying hydrostatic pressure with suspended 2D material thin membranes allows probing the effects of lateral strain on the ion and fluid transport through nanopores. We demonstrate how both permanent and temporary delamination of 2D materials can be induced by pressure and potential differences between the membrane, causing a strong mechanosensitive modulation of ion transport. Our methodology is based on in situ measurements of ionic current and streaming modulation as the supporting membrane is indented or bulged with pressure. We demonstrate how indirect measurements of fluid transport through delaminated MoS2 membranes is achieved through monitoring streaming current and potential. This is combined with TEM images of 2D material membranes before and after aqueous measurements, showing temporary delamination during mechanical or electrical stress. The obtained results allow a better understanding of measurements with supported 2D materials, i.e. avoiding misinterpreting measured data, and could be used to probe how the electrical field and fluid flow at the nanoscale influence the adhesion of supported 2D materials.

Single-layer membranes for organic solvent nanofiltration: a molecular dynamics simulation and comparative experimental study.

Organic solvents are widely used in pharmaceutical and chemical industries. Their separation and recovery account for a large part of energy consumption and capital cost in many industrial processes. MoS2 membranes with varying pore sizes (0.6 nm pore with S atoms, 0.7 nm pore with Mo atoms, 1.3 nm pore with S atoms, 1.4 nm pore with Mo atoms) were investigated as organic solvent nanofiltration (OSN) membranes using molecular simulation in this study. The fluxes of five polar solvents (methanol, ethanol, propanol, acetonitrile and acetone) and a nonpolar solvent (n-hexane) were predicted. Although the 0.6 nm S pore has a smaller pore size, it has a better flux for some organic solvents than the 0.7 nm Mo pore. This selective behavior of molybdenum disulfide was confirmed by calculating the potential of mean force (PMF) of each solvent molecule. The PMFs show that polar solvents face a higher energy barrier through the pore, and greater resistance needs to be overcome. After testing the permeability of solvent by experiment and simulation, the flux changes of different solvents have the same trend in experiment and simulation. The solvent permeability was slightly affected in the presence of solute (acetaminophen), and MoS2 membranes with small pores demonstrated 100% rejection rate for acetaminophen. This study confirmed that pore chemistry and pore size play important roles in OSN, and MoS2 is a promising OSN membrane for the recovery of organic solvents.

Exceptional Elasticity of Microscale Constrained MoS2 Domes.

The outstanding mechanical performances of two-dimensional (2D) materials make them appealing for the emerging fields of flextronics and straintronics. However, their manufacturing and integration in 2D crystal-based devices rely on a thorough knowledge of their hardness, elasticity, and interface mechanics. Here, we investigate the elasticity of highly strained monolayer-thick MoS2 membranes, in the shape of micrometer-sized domes, by atomic force microscopy (AFM)-based nanoindentation experiments. A dome’s crushing procedure is performed to induce a local re-adhesion of the dome’s membrane to the bulk substrate under the AFM tip’s load. It is worth noting that no breakage, damage, or variation in size and shape are recorded in 95% of the crushed domes upon unloading. Furthermore, such a procedure paves the way to address quantitatively the extent of the van der Waals interlayer interaction and adhesion of MoS2 by studying pull-in instabilities and hysteresis of the loading–unloading cycles. The fundamental role and advantage of using a superimposed dome’s constraint are also discussed.

Light-Controlled Ionic Transport through Molybdenum Disulfide Membranes.

In recent years, two-dimensional (2D) nanomaterials have been extensively explored in the field of nanofluidics due to their interconnected and well-controlled nanochannels. In particular, the investigation of 2D nanomaterials using their intrinsic properties for smart nanofluidics is receiving increased interest. Here, we report that MoS2 membranes can be used for light-controlled nanofluidic applications based on their photoelectrical properties. We show that the MoS2 membranes exhibit surface charge-governed ionic transport in NaCl and KCl solution without light illumination, while the ionic conductivity of the MoS2 membranes is up to 2 orders of magnitude higher at low concentration solution than that in bulk solution. We also show that the ionic conductivity of the membranes is enhanced under light illumination at 405 and 635 nm and reversible and stable switching of ionic current upon light illumination is observed. In addition, ionic current through membranes is enhanced by increasing light intensity. Therefore, our findings demonstrate that MoS2 membranes can be a potential platform for light-controlled nanofluidic applications.

Influence of the Exclusion-Enrichment Effect on Ion Transport in Two-Dimensional Molybdenum Disulfide Membranes.

Two-dimensional (2D) nanosheet membranes have been widely studied for water and wastewater treatment. However, mass transport inside 2D nanosheet membranes is far from being fully understood, and suitable applications of these membranes are yet to be identified. In this study, we investigate ion transport inside a 2D molybdenum disulfide (MoS2) membrane by combining experimental results with numerical modeling. Specifically, we analyze the influence of the electrical double layer (EDL) extension on ion diffusion in the MoS2 membrane, and a parameter called the exclusion-enrichment coefficient (β) is introduced to quantify how the electrostatic interaction between the coions and the EDL can affect the ion diffusion. Using the model developed in this study, the β values under different experimental conditions (feed solution concentration and applied hydraulic pressure) are calculated. The results show that coion diffusion inside the membrane can be retarded since β is smaller than one. Furthermore, the underlying mechanism is explored by theoretically estimating the radial ion concentration and electrical potential distributions across the membrane nanochannel. In addition, we find that convective mass transport can weaken the exclusion-enrichment effect by increasing β. Based on the results in this study, the potential applications and feasible membrane design strategies of 2D nanosheet membranes are discussed.

Nanoporous MoS2 Membrane for Water Desalination: A Molecular Dynamics Study.

Molybdenum disulfide (MoS2), a two-dimensional (2D) material, promises better desalination efficiency, benefiting from the small diffusion length. While the monolayer nanoporous MoS2 membrane has great potential in the reverse osmosis (RO) desalination membrane, multilayer MoS2 membranes are more feasible to synthesize and economical than the monolayer MoS2 membrane. Building on the monolayer MoS2 membrane knowledge, the effects of the multilayer MoS2 membrane in water desalination were explored, and the results showed that increasing the pore size from 3 to 6 Å resulted in higher permeability but with lower salt rejection. The salt rejection increases from 85% in a monolayer MoS2 membrane to about 98% in a trilayer MoS2 membrane. When averaged over all three types of membranes studied, the ions rejection follows the trend of trilayer > bilayer > monolayer. Besides, a narrow layer separation was found to play an important role in the successful rejection of salt ions in bilayer and trilayer membranes. This study aims to provide a collective understanding of this high permiselective MoS2 membrane's realization for water desalination, and the findings showed that the water permeability of the MoS2 monolayer membrane was in the order of magnitude greater than that of the conventional RO membrane and the nanoporous MoS2 membrane can have an important place in the purification of water.

Effect of composition of few-layered transition metal dichalcogenide nanosheets on separation mechanism of hydrogen selective membranes

The chemical composition of two-dimensional transition metal dichalcogenides (TMDs) affects the stacking of the nanosheets and changes the gas separation behavior of the stacked layer; however, this phenomenon has not been discussed in detail so far. In this work, a simple, environmental-friendly aqueous exfoliation of TMDs is applied using tannic acid (TA) for membrane synthesis. To efficiently delaminate bulk TMD, TA not only serves as a negatively charged surfactant but also adsorbs to the delaminated nanosheets of TMD owing to its amphiphilic nature. The exfoliated nanosheets and prepared membranes were characterized using spectroscopic and microscopic techniques. The microstructure of WSe2, MoSe2, and MoS2 membranes were explored using positron annihilation spectroscopy and X-ray diffraction to correlate membrane microstructure and interlayer spacing with the gas transport mechanism. The H2/CO2 separation performance of the resultant membranes surpasses the Robeson's upper bound. On comparison of WSe2 with MoSe2, replacing W with Mo changes the dominative gas transport mechanism of the membrane from molecular sieving to Knudsen diffusion. However, a change in the chalcogen atom will not affect the gas transport mechanism. Our findings are an important step towards the industrial application of membranes, such as the refinement of H2 and removal of CO2.

Black Phosphorus-Molybdenum Disulfide Hetero-Junctions Formed with Ink-Jet Printing for Potential Solar Cell Applications with Indium-Tin-Oxide

In this work, we implemented liquid exfoliation to inkjet-print two-dimensional (2D) black phosphorous (BP) and molybdenum disulfide (MoS2) p–n heterojunctions on a standard indium tin oxide (ITO) glass substrate in a vertical architecture. We also compared the optical and electrical properties of the inkjet-printed BP layer with that of the MoS2 and the electrical properties of the mechanically exfoliated MoS2 with that of the inkjet-printed MoS2. We found significant differences in the optical characteristics of the inkjet-printed BP and MoS2 layers attributed to the differences in their underlying crystal structure. The newly demonstrated liquid exfoliated and inkjet-printed BP–MoS2 2D p–n junction was also compared with previous reports where mechanically exfoliated BP–MoS2 2D p–n junction were used. The electronic transport properties of mechanically exfoliated MoS2 membranes are typically better compared to inkjet-printed structures but inkjet printing offers a cost-effective and quicker way to fabricate heterostructures easily. In the future, the performance of inkjet-printed structures can be further improved by employing suitable contact materials, amongst other factors such as modifying the solvent chemistries. The architecture reported in this work has potential applications towards building solar cells with solution processed 2D materials in the future.

Molecular fluid flow in MoS2 nanoporous membranes and hydrodynamics interactions.

We study the impact of the induced pressure fields on the water flow and salt rejection in nanopores produced in MoS2 membranes. We observe that the water permeability and the salt rejection are not impacted by the distance between the pores. This result contradicts the continuous fluid mechanics calculations in microfilters, which indicates the existence of hydrodynamic interactions between adjacent pores that increase the water mobility. Our results suggest that at this nanoscale, the hydrodynamic interactions do not affect the water mobility through nanopores.

In-situ growth of graphene quantum dots modified MoS2 membrane on tubular ceramic substrate with high permeability for both water and organic solvent

Molybdenum disulfide (MoS2) is widely used in membrane separation because of its outstanding chemical and structural stability. In this study, MoS2 nanosheets were in-situ grown on a tubular ceramic substrate through one-step hydrothermal synthesis with the graphene quantum dots (GQDs) incorporated in the precursor solution. The growth of the MoS2 nanosheets was inhibited due to the combination with GQDs through the Mo-O bond. Therefore, the number of transport channels in the MoS2/GQDs composite membrane were increased to improve the permeability. The results indicated that the MoS2/GQDs composite membrane have excellent permeances for both the aqueous solution and organic solvent. Moreover, the MoS2/GQDs composite membrane can be continuously operated in cross-flow filtration device for 100 h because the in-situ hydrothermal method has greatly improved the stability of membrane. The results indicated that the MoS2/GQDs composite membrane exhibits great potential in nanofiltration.

High-Throughput Zwitterion-Modified MoS2 Membranes: Preparation and Application in Dye Desalination.

Although loose nanofiltration membranes have been extensively studied for dye desalination, high-throughput membranes with antifouling and antibacterial properties are still highly needed. In this study, a zwitterion-modified molybdenum disulfide (MoS2) dual-layer loose nanofiltration membrane was prepared with the integration of antibacterial, antifouling, and high-flux properties. To be specific, MoS2 nanosheets were loaded on a polyacrylonitrile ultrafiltration membrane through pressure-assisted self-assembly. Then, poly (sulfobetaine methacrylate) (PSBMA) was coated on the surface of the MoS2 membrane via a simple polydopamine (PDA)-assisted one-step codeposition to prepare PSBMA/PDA/MoS2 nanofiltration membranes. Elemental and morphological analyses confirmed the formation of the MoS2 layer and PSBMA/PDA coating. In addition, the effect of the PSBMA amount and codeposition time on surface properties and membrane performances was investigated. Under optimum conditions, the as-prepared membrane showed excellent water permeance of 262 LMH/bar with good dye rejection (99.8% for methylene blue) and salt permeability, as well as excellent antifouling and antibacterial properties benefiting from the synergy of PSBMA/PDA coating layers and MoS2 layers.