Few-layer MoS2 Research Articles

Low resistance electrical contacts to few-layered MoS2 by local pressurization

The performance of electronic and optoelectronic devices is dominated by charge carrier injection through the metal–semiconductor contacts. Therefore, creating low-resistance electrical contacts is one of the most critical challenges in the development of devices based on new materials, particularly in the case of two-dimensional semiconductors. Herein, we report a strategy to reduce the contact resistance of MoS2 via local pressurization. We fabricated electrical contacts using an atomic force microscopy tip and applied variable pressure ranging from 0 to 25 GPa. By measuring the transverse electronic transport properties, we show that MoS2 undergoes a reversible semiconducting-metallic transition under pressure. Planar devices in field effect configuration with electrical contacts performed at pressures above ∼15 GPa show up to 30-fold reduced contact resistance and up to 25-fold improved field-effect mobility when compared to those measured at low pressure. Theoretical simulations show that this enhanced performance is due to improved charge injection to the MoS2 semiconductor channel through the metallic MoS2 phase obtained by pressurization. Our results suggest a novel strategy for realizing improved contacts to MoS2 devices by local pressurization and for exploring emergent phenomena under mechano-electric modulation.

Petroleum-Pitch-Based Carbon Nanocages Encapsulated Few-Layer MoS2 with S Vacancies for a High-Performance Sodium-Ion Battery

Molybdenum disulfide (MoS2), one of the promising anode materials for high-energy-density sodium-ion batteries (SIBs), is subjected to poor electronic conductivity and severe volume expansion during the charge–discharge cycle. Herein, cage-like carbon was prepared from petroleum pitch to composite with few-layer MoS2 as MoS2/C anodes for SIBs, giving an excellent capacitive performance (89.18% at 1.4 mV s–1) and nearly 100% capacity retention after 300 cycles at 1 A g–1. The unique cage-like structures relieve the volume expansion of MoS2 during the charge–discharge cycles. Meanwhile, MoS2 has an expanded interlayer distance (0.685 nm) and abundant S vacancies, affording rich active sites for the adsorption diffusion of sodium ions. This work provides a convenient method to solve the inherent defects of MoS2 by introducing cage-like carbon from petroleum pitch.

Synthesis of Large-Area Single- to Few-Layered MoS2 on an Ionic Liquid Surface.

Large-area fabrication of transition metal dichalcogenides via environmentally friendly and efficient processes has been a long-standing issue in the field of two-dimensional (2D) materials. Here, we report that single- to few-layered MoS2 sheets with an average size of the order of micrometers have been successfully synthesized on an ionic liquid surface by a modified low-pressure chemical vapor deposition (LP-CVD) method without the assistance of catalysts. It is found that the MoS2 sheets grown on the liquid substrate exhibit a complete molecular crystal structure, which is confirmed by transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy measurements. The interlayer spacing does not change significantly with the increase of the MoS2 layers, corresponding to a layer-by-layer growth pattern. The growth mechanism of the MoS2 sheets is presented according to the experimental results. The work provides a new and simple method of preparing more molecular crystals on liquid substrates and will contribute to further research in this field.

Measuring Band Modulation of MoS2 with Ferroelectric Gates.

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS2 modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS2 layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Assessing the Optical Properties of 2D MoS2 Nanosheets Synthesized Using Facile Two-Step Sequential Process

Among 2D Transition Metal Dichalcogenides (TMDs), ultrathin molybdenum disulfide (MoS2) nanosheets are well researched in terms of synthesis, characterization and applications owing to its unique properties in contrast to the bulk material. Here, 2D MoS2 suspensions are prepared via grinding-assisted liquid phase exfoliation. The processing parameters like initial concentration of MoS2 powder, grinding hours, solvent and high-power sonication are optimized for efficient and scalable production of MoS2 nanosheets. The suspensions are characterized for their optical and structural properties and are compared to analyze the effect of synthesis conditions on the properties of the obtained samples. The bandgap of the synthesized MoS2 lies above 2.0 eV in contrast with a bandgap of 1.57 eV for bulk sample. The difference between the peaks corresponding to in-plane and out-of-plane vibration modes is lower than bulk sample depicting the formation of mono to few-layered MoS2. It is found that grinding-assisted sonication in NMP solvent is the most efficient method to produce low-dimensional nanosheets. The well dispersed MoS2 nanoflakes can be blended with other nanomaterials to prepare hybrid dispersions and can thus be explored for optoelectronic applications.

Liquid Phase Exfoliated 2-D MoS2-Based Broadband Heterojunction Low-Powered Photosensor

Researchers are facing a strong reverse thrust toward the exfoliation of 2-D materials at a large scale. These limitations restrict so many intensively needed applications. We have demonstrated chemically exfoliated MoS2 nanosheets-assisted photosensor examined for low-incident optical power ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.78 ~\mu \text{W}$ </tex-math></inline-formula> ) and bias voltage −0.5 to +0.5 V.X-ray diffraction (XRD) results show no secondary peaks that neglect the presence of any impurity in the synthesized MoS2. The high-resolution scanning electron microscopy (HR-SEM) shows synthesized flower-like MoS2 nanosheets. Exfoliation of MoS2 nanosheets was carried out in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> -methyl di-pyridine (NMP), a suitable solvent to develop single to few-layer MoS2 nanoflakes. During probe sonication, a low-temperature outer atmosphere arrangement has been ensured by using an ice bath. Sonicated solution has been centrifuged at 12 000 rpm, and the supernatant was collected and drop cast over a p-type (1,0,0) rectangular silicon wafer. Ohmic aluminum (Al) contact has been established. Current and voltage characteristics were performed to study the electrical behavior of fabricated p-Si and synthesized MoS2 heterostructure. The rectification ratio was ≈5800 achieved at ±2 V and the ideality factor was obtained 2.06 in the best-fitted region. The maximum responsivity and detectivity of 14.7 A/W and 2.25 jones were obtained at 300-nm wavelengths corresponding at low bias voltage of −0.5 V.

Enhancing photocatalytic properties of continuous few-layer MoS2 thin films for hydrogen production by water splitting through defect engineering with Ar plasma treatment

Continuous few-layer MoS2 thin films composed of the stacked MoS2 nanoflakes were fabricated with a facile thermal decomposition method. These nanoflakes possess exposed active sites which are beneficial to the photocatalytic properties. Defect engineering with the aim to generate more exposed edges accompanied with abundant excited carriers by variation of Ar plasma power has been carried out. Positive results were indeed obtained. On the other hand, increasing exposed active edges/defects induced by Ar plasma was found to lead to the occurrence of oxidation in air with time. The variations of crystallinity, S/Mo ratio, and oxygen adsorption after Ar plasma treatment were investigated by high resolution transmission electron microscopy, high resolution x-ray analysis of photoemission spectroscopy as well as Raman spectroscopy. The few-layer MoS2 bombarded by Ar plasma with 30 W power provides the optimal photocatalytic enhancement in hydrogen production about 1.4 times reaching 246 mmolg−1h−1and the photocurrent density was increased by about 15 times to the pristine one. In addition, MoS2/Au system from our previous survey is also utilized this method can generate 338 mmol/g−1h−1, as high as 188% enhancement compared to pristine one. The increase in conductivity for Ar plasma treated MoS2 thin films is attributed to the formation of 2D phase of MoOx with higher electron mobility. The investigation has clarified the influence of defects on the photocatalytic performance. Nevertheless, the defects were found to be unstable when the samples were stored in the air. It is highly desired to pursue a scheme to stabilized the defects for practical applications.

Decoupling Effects of Electrostatic Gating on Electronic Transport and Interfacial Charge-Transfer Kinetics at Few-Layer Molybdenum Disulfide.

The electronic properties of electrode materials play a crucial role in defining their electrochemical behavior in energy conversion and storage devices. The assembly of van der Waals heterostructures and fabrication into mesoscopic devices enable the dependence of an electrochemical response on electronic properties to be systematically interrogated. Here, we evaluate the effect of charge carrier concentration on heterogeneous electron transfer at few-layer MoS2 electrodes by combining spatially resolved electrochemical measurements with field-effect electrostatic manipulation of band alignment. Steady-state cyclic voltammograms and finite-element simulations reveal a strong modulation of the measured electrochemical response for outer-sphere charge transfer at the electrostatic gate voltage. In addition, spatially resolved voltammetric responses, obtained at a series of locations at the surface of few-layer MoS2, reveal the governing role of in-plane charge transport on the electrochemical behavior of 2D electrodes, especially under conditions of low carrier densities.

An Improved Theoretical Model to Extract the Optical Conductivity of Two-Dimensional Material from Terahertz Transmission or Reflection Spectroscopy

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS2 on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy.

Charges-Enhanced Molybdenum Disulfide Nanozyme Activity for Ultrasound-Mediated Cascade-Catalytic Tumor Ferroptosis.

Deficient catalytic activity of nanozyme and insufficient endogenous H2O2 in tumor microenvironment (TME) are major obstacles for nanozyme-mediated catalytic tumor therapy. Herein, since electron transfer is the basic essence of catalysis-mediated redox reaction, we explored the contributing factors of enzymatic activity based on positive and negative charges, which are experimentally and theoretically demonstrated to enhance the peroxidase (POD)-like activity of MoS2 nanozyme. Hence, an acidic TME-responsive and ultrasound-mediated cascade nanocatalyst (BTO/MoS2@CA) is presented by making few-layer MoS2 nanosheets grown on the surface of piezoelectric tetragonal barium titanate (T-BTO) and modified with pH-responsive cinnamaldehyde (CA). The integration of pH-responsive CA-mediated H2O2 self-supply, ultrasound-mediated charges enhanced enzymatic activity and GSH depletion enables out-of-balance redox homeostasis, leading to effective tumor ferroptosis with minimal side effects.

Atomic-scale characterization of contact interfaces between thermally self-assembled Au islands and few-layer MoS2 surfaces on SiO2

The interaction between metallic nanoparticles and transition metal chalcogenides (TMDs) can realize new functionalities in thriving technologies such as optoelectronics and nanoengineering. Here we have investigated the self-assembly of triangular-shaped crystalline Au nanoislands on MoS2 flakes mechanically exfoliated or grown by chemical vapor deposition (CVD). The density and size of the islands are determined by substrate temperature, deposition flux, and subsurface morphology. The thickness of the MoS2 layers is measured by Raman spectroscopy, which also enables the evaluation of the strain and doping distributions induced by the Au islands. Top and cross-sectional images of the Au-MoS2 interface are obtained by scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Sub-nanometer resolution of the Au, Mo and S layers reveals that the MoS2 flakes follow the corrugation of the SiO2 substrate, with flattening and wrinkling effects induced by the growth of the Au islands on top.

Thermal Atomic Layer Etching of MoS2 Using MoF6 and H2O.

Two-dimensional (2D) layered materials offer unique properties that make them attractive for continued scaling in electronic and optoelectronic device applications. Successful integration of 2D materials into semiconductor manufacturing requires high-volume and high-precision processes for deposition and etching. Several promising large-scale deposition approaches have been reported for a range of 2D materials, but fewer studies have reported removal processes. Thermal atomic layer etching (ALE) is a scalable processing technique that offers precise control over isotropic material removal. In this work, we report a thermal ALE process for molybdenum disulfide (MoS2). We show that MoF6 can be used as a fluorination source, which, when combined with alternating exposures of H2O, etches both amorphous and crystalline MoS2 films deposited by atomic layer deposition. To characterize the ALE process and understand the etching reaction mechanism, in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR), and quadrupole mass spectrometry (QMS) experiments were performed. From temperature-dependent in situ QCM experiments, the mass change per cycle was -5.7 ng/cm2 at 150 °C and reached -270.6 ng/cm2 at 300 °C, nearly 50× greater. The temperature dependence followed Arrhenius behavior with an activation energy of 13 ± 1 kcal/mol. At 200 °C, QCM revealed a mass gain following exposure to MoF6 and a net mass loss after exposure to H2O. FTIR revealed the consumption of Mo-O species and formation of Mo-F and MoF x =O species following exposures of MoF6 and the reverse behavior following H2O exposures. QMS measurements, combined with thermodynamic calculations, supported the removal of Mo and S through the formation of volatile MoF2O2 and H2S byproducts. The proposed etching mechanism involves a two-stage oxidation of Mo through the ALE half-reactions. Etch rates of 0.5 Å/cycle for amorphous films and 0.2 Å/cycle for annealed films were measured by ex situ ellipsometry, X-ray reflectivity, and transmission electron microscopy. Precisely etching amorphous films and subsequently annealing them yielded crystalline, few-layer MoS2 thin films. This thermal MoS2 ALE process provides a new mechanism for fluorination-based ALE and offers a low-temperature approach for integrating amorphous and crystalline 2D MoS2 films into high-volume device manufacturing with tight thermal budgets.

Inkjet-printed MoS2-based 3D-structured electrocatalysts on Cu films for ultra-efficient hydrogen evolution reaction

2D transition metal dichalcogenides (TMDs) are emerging as promising alternative electrocatalysts for hydrogen evolution reaction (HER). However, key challenges are the limited density of available activity sites and poor electron accessibility. Herein, Cu film supported MoS2-based electrocatalyst with partial 1T phase and 3D architecture is constructed by inkjet printing. We aim to investigate if this configuration could provide largely exposed edge sites and enhance electron transfer for ultra-efficient hydrogen production. Towards this end, an ink containing few-layer MoS2 nanosheets and reduced graphene oxide (RGO) is firstly prepared by liquid-phase exfoliation, in which semiconducting 2H-MoS2 is partially transformed to metallic 1 T-MoS2. We demonstrate that 1) nanosized few-layer MoS2 spatially patterned by inkjet-printing provides sufficient active site exposure, 2) the 1T-MoS2 and RGO conductive network reduce the charge-transfer impedance, and 3) the Cu support enhances the catalyst-electrode charge injection. Our all-in-one structure exhibits outstanding HER activities with low overpotential (51 mV at 10 mA cm−2 and 126 mV at 100 mA cm−2) and very low Tafel slope (32 mV dec-1). Also, the as-engineered electrode demonstrates ultrahigh cathodic current density. This work provides a facile, inexpensive, and scalable way for designing and fabricating TMDs-based electrocatalysts with excellent HER activity.

Few-layer MoS2 nanosheets with and without silicon nanoparticles as anodes for lithium-ion batteries

NaCl crystals are employed as a substrate to grow MoS2 nanosheets. On dissolving NaCl, the nanosheets are introduced to silicon nanoparticles using layer-by-layer assembly forming robust MoS2@Si anodes for lithium-ion batteries.

Insulator-to-metal phase transition in a few-layered MoSe2 field effect transistor.

The metal-to-insulator phase transition (MIT) in low-dimensional materials and particularly two-dimensional layered semiconductors is exciting to explore due to the fact that it challenges the prediction that a two-dimensional system must be insulating at low temperatures. Thus, the exploration of MITs in 2D layered semiconductors expands the understanding of the underlying physics. Here we report the MIT of a few-layered MoSe2 field effect transistor under a gate bias (electric field) applied perpendicular to the MoSe2 layers. With low applied gate voltage, the conductivity as a function of temperature from 150 K to 4 K shows typical semiconducting to insulating character. Above a critical applied gate voltage, Vc, the conductivity becomes metallic (i.e., the conductivity increases continuously as a function of decreasing temperature). Evidence of a metallic state was observed using an applied gate voltage or, equivalently, increasing the density of charge carriers within the 2D channel. We analyzed the nature of the phase transition using percolation theory, where conductivity scales with the density of charge carriers as σ ∝ (n - nc)δ. The critical exponent for a percolative phase transition, δ(T), has values ranging from 1.34 (at T = 150 K) to 2 (T = 20 K), which is close to the theoretical value of 1.33 for percolation to occur. Thus we conclude that the MIT in few-layered MoSe2 is driven by charge carrier percolation. Furthermore, the conductivity does not scale with temperature, which is a hallmark of a quantum critical phase transition.

Few-layer MoS2 nanosheets vertically supported on Ti3C2-MXene sheets promoting lithium storage performance.

2H phase MoS2 with a two-dimensional nanostructure, high chemical stability and large theoretical capacity has been considered as a potential anode material for lithium-ion batteries. However, some practical problems hinder the direct use of 2H-MoS2 for lithium storage, such as its volume expansion effect that leads to capacity loss and its semiconductor properties that cannot provide sufficient conductivity. Herein, the surface of an MXene with abundant surface groups was modified with CTAB to promote its ability to adsorb MoO42- anions, and then 2H-MoS2 with a few layers was directly grown on the surface of MXene sheets vertically. Thanks to the conductive MXene sheets and the vertically-supported high-capacity MoS2 on them, the as-obtained composite MXene@MoS2 offers enhanced performance in specific capacity, long cycling stability and high rate capability. A reversible specific capacity of 1198 mA h g-1 was retained after 100 cycles at 200 mA g-1 and a specific capacity of 717 mA h g-1 was exhibited at 8000 mA g-1.

Template-free scalable growth of vertically-aligned MoS2 nanowire array meta-structural films towards robust superlubricity.

Two-dimensional (2D) molybdenum disulfide exhibits a variety of intriguing behaviors depending on its orientation layers. Therefore, developing a template-free atomic layer orientation controllable growth approach is of great importance. Here, we demonstrate scalable, template-free, well-ordered vertically-oriented MoS2 nanowire arrays (VO-MoS2 NWAs) embedded in an Ag-MoS2 matrix, directly grown on various substrates (Si, Al, and stainless steel) via one-step sputtering. In the meta-structured film, vertically-standing few-layered MoS2 NWAs of almost micron length (∼720 nm) throughout the entire film bulk. While near the surface, MoS2 lamellae are oriented in parallel, which are beneficial for caging the bonds dangling from the basal planes. Owing to the unique T-type topological characteristics, chemically inert Ag@MoS2 nano-scrolls (NSCs) and nano-crystalline Ag (nc-Ag) nanoparticles (NPs) are in situ formed under the sliding shear force. Thus, incommensurate contact between (002) basal planes and nc-Ag NPs is observed. As a result, robust superlubricity (friction coefficient μ = 0.0039) under humid ambient conditions is reached. This study offers an unprecedented strategy for controlling the basal plane orientation of 2D transition metal dichalcogenides (TMDCs) via substrate independence, using a one-step solution-free easily scalable process without the need for a template, which promotes the potential applications of 2D TMDCs in solid superlubricity.

Photogating induced high sensitivity and speed from heterostructure of few-layer MoS2 and reduced graphene oxide-based photodetector.

Over the past few years, two-dimensional transition metal dichalcogenides (2D-TMDC) have attracted huge attention due to their high mobility, high absorbance, and high performance in generating excitons (electron and hole pairs). Especially, 2D molybdenum disulfide (MoS2) has been extensively used in optoelectronic and photovoltaic applications. Due to the low photo-to-dark current ratio (Iphoto/dark) and low speed, pristine MoS2-based devices are unsuitable for these applications. So, they need some improvements, i.e., by adding layers or decorating with materials of complementary majority charges. In this work, we decorated pristine MoS2 with reduced graphene oxide (rGO) and got improved dark current, Iphoto/dark, and response time. When we compared the performance of pristine MoS2 based device and rGO decorated MoS2 based device, the rGO/MoS2-based device showed an improved performance of responsivity of 3.36 A W-1, along with an Iphoto/dark of about 154. The heterojunction device exhibited a detectivity of 4.75 × 1012 Jones, along with a very low response time of 0.184 ms. The stability is also outstanding having the same device performance even after six months.

Atomic-level understanding on progressive lithiation of few-layer MoS2 with surface vacancies

Vacancy engineering is a powerful modification strategy to elevate the electrochemical performance of electrode materials. In this work, the impacts of various surface vacancies in few-layer molybdenum disulfide (MoS2) on Li adsorption/intercalation and diffusion at both MoS2 surface and interlayers are systematically investigated to advance the material performance using first-principles calculations. The results show that the surface vacancy in few-layer MoS2 is easier to form than internal ones, and surface single S vacancy (VS) is the most stable. All seven surface vacancies can promote Li adsorption and intercalation at MoS2 surface and interlayer, and have a much more beneficial role on Li surface adsorption than interlayer intercalation. Surface single Mo vacancy (VMo) can upgrade the diffusion rate of Li at the few-layer MoS2 surface and nearby interlayer, and VS has a neglected impact on the rate capability. VMo is found to be the most propitious vacancy type for Li binding and diffusion at the few-layer MoS2. This theoretical exploration can help the comprehension of the impact of vacancies on the Li adsorption, intercalation, and transportation at the surface and interlayers of the dichalcogenides as high-performance electrode materials for rechargeable batteries.

Large Size Few-Layer Ambipolar MoS&lt;sub&gt;2&lt;/sub&gt; Metal-Oxide-Semiconductor Field Effect Transistors by Nitrogen Plasma Doping

Molybdenum disulfide (MoS2), a typical two-dimensional layered semiconductor material, is widely studied due its excellent electronic properties in atomic scale. In this study, we achieved the growth of large size few-layer MoS2 films by using molybdenum boat with vertical shield at the end to carry MoO3 precursor in the chemical vapor deposition (CVD) system. The optical microscopy reveals the morphology and lateral size of as-grown films. The Raman spectrum testified that the synthesized films are few-layer MoS2 with defects. Metal-oxide-semiconductor field effect transistors (MOSFETs) based on CVD-grown MoS2 are fabricated, presenting n-type transportation with ION/IOFF ratio about 103. The transportation behaviour of MoS2 MOSFETs is changed from n-type to ambipolar by introducing nitrogen plasma into MoS2 films. The electron or hole transportation in MoS2 is controlled by gate-source voltage. The ambipolar MOSFETs show ION/IOFF ratio about 103.