Few-layer Molybdenum Disulfide Research Articles

New insight to piezocatalytic peroxymonosulfate activation: The critical role of dissolved oxygen in mediating radical and nonradical pathways

This study leverages piezocatalysis to accelerate redox reactions in heterogeneous peroxymonosulfate (PMS) activation. We syntheized few-layered molybdenum disulfide (MoS2) nanosheets as the piezoelectric catalyst for ultrasonic vibration (US)-coupled PMS system toward enhanced pollutant abatement. Scavenger tests, dissolved oxygen (DO) exclusion experiment and electron paramagnetic resonance (EPR) identify that sulfate radical (SO4•−) and singlet oxygen (1O2) are the primary ROS in the US/MoS2/PMS system. Particularly, O2•− is a vital intermediate for 1O2 generation, and multiple formation pathways of 1O2 were proposed in the US/MoS2/PMS system. With the assistance of DO and ultrasound, the utilization efficiency and activity of PMS will be remarkably increased because the majority of PMS evolves into more reactive SO4•− and 1O2 for pollutant degradation. This work not only provides mechanistic insights into the interconnected regimes of piezocatalysis and heterogeneous Fenton-like reactions, but also achieves high chemical efficiency for sustainable water remediation.

Ultrastrong Coupling Leads to Slowed Cooling of Hot Excitons in Few-Layer Transition-Metal Dichalcogenides

The excited-state dynamics of few-layer molybdenum disulfide (FL-MoS2) are studied in conditions of strong light–matter coupling to plasmon polaritons. Hot carrier extraction in these systems has been proposed due to the observation of slow cooling of the high-energy C exciton relative to the bandgap A exciton. Here, we show that in conditions of ultrastrong coupling to plasmon polaritons, the lifetimes of the two slowest C exciton decay processes are extended by factors of 1.5 and 5.8 in FL-MoS2. We hypothesize that strong coupling delocalization suppresses multiple decay mechanisms throughout the visible spectrum in MoS2 such as defect scattering, intervalley scattering of band-nested excitations to the Κ-valley band edges, and exciton–exciton annihilation. We also find that decay from the upper to the lower hybrid mode is not ultrafast as seen in organic systems but in fact introduces an additional delay of ∼20 ps. Our observations show that strong coupling can be used to extend the lifetimes of hot excitons in FL-MoS2 and potentially in other two-dimensional transition-metal dichalcogenides, with potential for above-bandgap photophysics and photochemistry applications such as hot carrier extraction, which could lead to more efficient solar energy conversion.

An electrochemical biosensor based on few-layer MoS2 nanosheets for highly sensitive detection of tumor marker ctDNA.

An electrochemical biosensor based on few-layer molybdenum disulfide (MoS2) nanosheets was fabricated for the highly sensitive detection of tumor marker circulating tumor DNA (ctDNA) in this paper. The MoS2 nanosheets with few layers were prepared by the shear stripping. Compared with the mechanical stripping method and the lithium ion intercalation method, this method is simpler to operate, and the prepared MoS2 nanosheets had good electrochemical activity. The biosensing platform was fabricated based on the discriminative affinity of MoS2 nanosheets towards single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). Methylene blue (MB) was used as the signal molecule. The results showed that the detection of ctDNA by this sensor showed an excellent linear relationship in the concentration range of 1.0 × 10-7 M to 1.0 × 10-16 M, and the detection limit was 2.5 × 10-18 M. In addition, this sensor exhibited outstanding stability and specificity. This strategy provides an alternative approach for ctDNA detection and an effective sensing strategy for future in vitro cancer diagnosis by label-free detection.

Size selection and thin-film assembly of MoS2 elucidates thousandfold conductivity enhancement in few-layer nanosheet networks.

Printed electronics based on liquid-exfoliated nanosheet networks are limited by inter-nanosheet junctions and thick films which hinder field-effect gating. Here, few-layer molybdenum disulfide nanosheets are assembled by Langmuir deposition into thin films, and size selection is shown to lead to a thousandfold conductivity enhancement with potential applicability to all nanosheet networks.

Charge transport mechanisms in inkjet-printed thin-film transistors based on two-dimensional materials

Printed electronics using inks based on graphene and other two-dimensional materials can be used to create large-scale, flexible and wearable devices. However, the complexity of ink formulations and the polycrystalline nature of the resulting thin films have made it difficult to examine charge transport in such devices. Here we report the charge transport mechanisms of surfactant- and solvent-free inkjet-printed thin-film devices based on few-layer graphene (semimetal), molybdenum disulfide (MoS2, semiconductor) and titanium carbide MXene (Ti3C2, metal) by investigating the temperature, gate and magnetic-field dependencies of their electrical conductivity. We find that charge transport in printed few-layer MXene and MoS2 devices is dominated by the intrinsic transport mechanism of the constituent flakes: MXene exhibits a weakly localized 2D metallic behaviour at any temperature, whereas MoS2 behaves as an insulator with a crossover from 3D Mott variable-range hopping to nearest-neighbour hopping around 200 K. Charge transport in printed few-layer graphene devices is dominated by the transport mechanism between different flakes, which exhibit 3D Mott variable-range hopping conduction at any temperature.

Multiplexing implementation of rubbing-induced site-selective growth of MoS2 feature arrays

Two-dimensional layered transition-metal dichalcogenides have drawn enormous interest because of their desired electrical and mechanical properties for making various devices with attractive functions. However, the device fabrication process typically introduces lithography-induced contamination and damage to such fragile and sensitive atomically layered materials. Here, we present a multiplexing lithography process system capable of directly generating few-layer molybdenum disulfide (MoS2) feature arrays with no need of additional lithographic or etching steps. This process combines a site-selective growth scheme based on mechanically generated triboelectric charge patterns and programmable actuation of rubbing templates bearing 2D feature arrays. To achieve a good processing uniformity, we have systematically investigated the effects of implementation of an air cushion on the rubbing template, various interfacing layers on the rubbing features, as well as mechanical load on rubbing templates and substrates. Using this process, we have demonstrated the growth of “L” shaped few-layer MoS2 arrays on SiO2/Si substrates with a good yield.

In-situ investigation of the elastic behavior of two-dimensional MoS2 on flexible substrate by nanoindentation

Mechanical properties are a crucial factor for the application of two-dimensional (2D) materials in flexible electronic and optoelectronic devices. Until now, mechanical properties of 2D materials have been primarily investigated from the load–deformation relationship of freely suspended 2D nanofilms by in-situ atomic force microscope probe nanoindentation measurements. Herein, we acquired the mechanical behavior of monolayer and few-layer molybdenum disulfide (MoS2) via applying a point load on 2D MoS2 on a soft poly-dimethylsiloxane (PDMS) substrate. The primary objective of this work is to study the elastic behavior of 2D MoS2 based on the indentation deformation of the 2D MoS2/PDMS substrate. Nanoindentation behavior of 2D MoS2 mounted on PDMS substrate is also simulated by finite element method. Combining nanoindentation experiments with theoretical simulations, we can understand the mechanical behavior of 2D materials on flexible substrates perfectly well. This work provides a new method to characterize the mechanical response of atomically ultrathin 2D nanofilms with substrate-binding effect, which is essential to promote flexible devices based on functional 2D materials.

Electromagnetic interference shielding properties of hierarchical core-shell palladium-doped MoS2/CNT nanohybrid materials

A simple hydrothermal method was used to prepare palladium-doped few-layer molybdenum disulfide nanosheets grown on the surface of a multi-walled carbon nanotube (Pd–MoS2/CNT) with forming a three-dimensional (3D) hierarchical core-shell heterostructure. This paper provides first-hand information on electromagnetic interference (EMI) scattering and electromagnetic (EM) parameters in the X-band (8–12 GHz) microwave frequency range. In this hierarchical core-shell heterostructure, SEM and TEM photos clearly show that CNT forms the core, while Pd-doped few layers MoS2 grown on the surface of CNT is the shell. The Pd-doped MoS2/CNT nanohybrids can greatly improve electrical conductivity, defect dipole polarization, interfacial polarization, dielectric relaxation loss, and mobile charge carriers between core and shell. These are the reason for the substantial enrichment in EMI shielding effectiveness (SE) and dielectric performance. The 10% Pd–MoS2/CNT nanohybrid has a higher EMI SE and dielectric loss tangent (SET ~ 26.50 dB, tanδε ~2.37) than the 5% Pd–MoS2/CNT (SET ~ 22.50 dB, tanδε ~1.90) and undoped MoS2/CNT (SET ~ 21.55 dB, tanδε ~0.92) at the same thickness of 1 mm. The strong EMI SE and dielectric loss tangent are found due to their higher dipole polarization, conduction, and relaxation losses. The doping concentration of Pd-atoms has a significant impact on EM parameters (ε′, ε″, μ′ & μ″) and EMI scattering parameters (S11, S21, S12 & S22). The experimental results indicated that the 10% Pd–MoS2/CNT nanohybrid might be considered a good EMI shielding and dielectric material for large scale use in intelligence wireless information and communication devices in the future.

Polarization-insensitive broadband absorption enhancement with few-layer MoS2 film

A polarization-insensitive broadband absorber with few-layer molybdenum disulfide (MoS2) films is designed. The proposed absorber is composed of a dielectric antireflective coating and an ultrathin MoS2 film deposited on a metal square resonator array backed with a metallic mirror. It is shown that the polarization-insensitive absorption enhancement in the ultrathin MoS2 film is realized in the visible region for normally incident light. The distributions of electric field intensity at several resonant wavelengths are investigated to intuitively disclose the physical origin of such phenomenon. Moreover, the optical performances, i.e. the absorption spectra versus the change of incident angle for both polarizations, the angular sensitivity of the integrated absorption and the short-circuit current density, are simulated and examined, all of which outperforms the referenced planar structure. It is believed that the conclusions provide a promising way to design and fabricate ultrathin MoS2 solar cells and other novel optoelectronic devices.

Establishment of anti-oxidation platform based on few-layer molybdenum disulfide nanosheet-coated titanium dioxide nanobelt nanocomposite

The nanozyme-based antioxidant system could protect normal cells from oxidative stress due to their reactive oxygen species (ROS) scavenging activity and good chemical stability. However, its use is limited for practical in vivo applications due to the high cost and poor biocompatibility (low catalytic efficiency). Herein, MoS2 decorated on TiO2 nanobelts (MoS2@TiO2) was prepared for antioxidation applications. The as-prepared MoS2@TiO2 heterostructure with 50 wt% MoS2 showed the highest efficient catalase activity and superoxide dismutase (SOD) activity under normal physiological conditions. The composite was superior to its single component in terms of enhanced dispersibility and catalytic performance resulting from the higher surface specific area and more exposed active sites. MoS2@TiO2 was not only confirmed to have good in vitro and in vivo biocompatibility but can also effectively eliminate the endogenous excessive accumulation of ROS caused by oxidative stress using the fibroblast cell (L929) line as a model. Further experiments confirmed that in the established mouse oxidative stress model, MoS2@TiO2 can quickly restore the ROS to a normal level in the oxidative stress site of the mouse. These results indicated that MoS2@TiO2 enzyme-like nanomaterials can provide a huge therapeutic potential in future antioxidant defence applications.

Early-stage growth observations of orientation-controlled vacuum-deposited naphthyl end-capped oligothiophenes

We report on the real-time structure formation and growth of two thiophene-based organic semiconductors, $5,{5}^{\ensuremath{'}}$-bis(naphth-2-yl)-$2,{2}^{\ensuremath{'}}$-bi- and $5,{5}^{\ensuremath{'}\ensuremath{'}}$-bis(naphth-2-yl)-$2,{2}^{\ensuremath{'}}$:${5}^{\ensuremath{'}},{2}^{\ensuremath{'}\ensuremath{'}}$-terthiophene (NaT2 and NaT3), studied in situ during vacuum deposition by grazing-incidence x-ray diffraction and supported by atomic force microscopy and photoabsorption spectroscopy measurements on corresponding ex situ samples. On device-relevant silicon dioxide substrates, for both molecules the growth is observed to transition from two-dimensional (2D) layer-by-layer growth to three-dimensional (3D) growth after the formation of a few-molecule-thick wetting layer. The crystal structure of the NaT2 film is considerably more ordered than the NaT3 counterpart, and there is a significant collective change in the unit cell during the initial stage of growth, indicating strain relief from substrate induced strain as the growth transitions from two to three dimensions. In addition, the orientation of the film molecules are controlled by employing substrates of horizontally and vertically oriented few-layer molybdenum disulfide. Both molecules form needle-like crystals on horizontally oriented ${\mathrm{MoS}}_{2}$ layers, while the NaT3 molecules form tall, isolated islands on vertically oriented ${\mathrm{MoS}}_{2}$ layers. The molecules are standing on silicon dioxide and on vertically oriented ${\mathrm{MoS}}_{2}$, but lying flat on horizontally oriented ${\mathrm{MoS}}_{2}$. These results demonstrate the importance of film-substrate interactions on the thin-film growth and microstructure formation in naphthyl-terminated oligothiophenes.

Phosphorescent MoS2 quantum dots as a temperature sensor and security ink.

Currently, few phosphorescent materials (PMs) possess a long phosphorescence lasting time and have potential for application in chemical sensors. Herein, we disclose that the incorporation of few-layer molybdenum disulfide quantum dots (FL-MoS2 QDs) into poly(vinyl alcohol) (PVA) matrices leads to the emission of bright green phosphorescence with a long lasting time of 3.0 s and a phosphorescence quantum yield of 20%. This enhanced phosphorescence originates from the formation of O–H⋯S hydrogen bonding networks between the rich sulfur sites of the FL-MoS2 QDs and the hydroxyl groups of the PVA molecules, which not only rigidifies the vibration modes of the FL-MoS2 QDs but also provides an oxygen barrier. Further investigations reveal that the FL-MoS2 QD/PVA composites exhibit a longer phosphorescence lasting time than N,S-doped carbon dots, few layer tungsten disulfide quantum dots, Rhodamine 6G, and Rhodamine B in PVA matrices. Since heat efficiently induced the removal of water moisture from PVA matrices, the FL-MoS2 QD/PVA composites could be implemented for phosphorescence turn-on and naked-eye detection of temperature variations ranging from 30 to 70 °C. By contrast, the carbon dot/PVA composites were incapable of sensing environmental temperature due to their weak hydrogen bonding with the hydroxyl groups of PVA matrices. Additionally, this study reveals the potential of the FL-MoS2 QD/PVA composites as an advanced security ink for anti-counterfeiting and encryption applications. The given results could open a new direction for potential application of two-dimensional quantum dots in phosphorescence-based sensors and security inks.

One-pot synthesis of few-layered molybdenum disulfide anchored on N, S-codoped carbon for enhanced hydrogen generation

Transition metal dichalcogenides (TMDs) are regarded as the inexpensive and stable electrocatalysts for hydrogen evolution reaction (HER). Designing an electrocatalyst with few-layered molybdenum disulfide (MoS2) is one of the most effective methods to enhance the activity and durability for HER due to its abundant exposed rim-sites, fast electron transfer and high chemical stability. Herein, a green and facile one-pot approach is developed to synthesize few-layered MoS2/nitrogen, sulfur codoped porous carbon (MoS2/NSPC) electrocatalyst via pyrolyzing the in-situ formed molybdate-melamine complex (MoOx-MEL) within the self-assembled networks of melamine-trithiocyanuric acid (MT). The optimal MoS2/NSPC electrocatalyst exhibits excellent HER stability and activity with overpotential of 114 mV at 10 mA cm−2 in 0.5 M H2SO4. Furthermore, few-layered tungsten disulfide anchored on N, S-codoped porous carbon (WS2/NSPC) with high HER activity can also be successfully synthesized by this strategy. It paves a universal way to design active sites–enriched few-layered dichalcogenides with high performance in the applications from energy conversion to storages.

Role of Electrostatics in the Heterogeneous Interaction of Two-Dimensional Engineered MoS2 Nanosheets and Natural Clay Colloids: Influence of pH and Natural Organic Matter.

Few-layered molybdenum disulfide (MoS2) nanosheets are poised to be at the core of low-voltage electronic device development. Upon environmental release, these two-dimensional (2D) structures can interact with abundant natural geocolloids. This study probes the role of dimensionality in modulating the aggregation behavior of 2D MoS2 nanosheets with plate-like geocolloids (i.e., homoionized kaolinite and montmorillonite clays). MoS2 nanosheets were exfoliated using an ethanol/water mixture, and aggregation kinetics were investigated with time-resolved dynamic light scattering at low monovalent salt concentrations and at three pH levels, in the presence and absence of Suwannee River humic acid (SRHA). Results indicate that pH and particle ratios are key to modulating the stability of MoS2/clay systems. At pH 4, aggregation of MoS2 increased with increasing MoS2/clay ratios and approached maximum values of 0.09 and 0.06 nm/s in the binary systems with montmorillonite and kaolinite, respectively. Electrostatic attraction facilitates heteroaggregation at pH values of 4 and 6; differences in the clay structures (i.e., face-face or face-edge aggregates) might explain the resulting MoS2/clay aggregate configurations, which were probed via the evolution of particle size distribution. The presence of only 0.1 mg/L SRHA drastically suppresses the heteroaggregation propensity of MoS2 nanosheets with geocolloids (to less than 0.01 nm/s at all pH values tested). The high stability of these heterogeneous systems under environmentally relevant conditions can increase the likelihood for cellular uptake and long-distance transport of MoS2.

Spin valve effect in sputtered FL-MoS2 and ferromagnetic shape memory alloy based magnetic tunnel junction

In the last decade, two-dimensional (2D) transition metal dichalcogenides have been introduced with great significance in the spintronic devices for their extraordinary electrical, optical, and spin-dependent properties. In this work, we have fabricated a few-layer molybdenum disulfide (FL-MoS2) (~6 nm) as a non-magnetic spacer layer in Ni–Mn–In/FL-MoS2/Ni–Mn–In magnetic tunnel junction (MTJ) using DC magnetron sputtering. FL-MoS2 thin film sandwiched between two ferromagnetic shape memory alloy based electrodes exhibit semiconducting behavior, confirmed by current-voltage (I–V) characteristics and temperature dependent resistance measurement. The fabricated MTJ shows spin valve effect in the presence of an external magnetic field. The tunneling magnetoresistance (TMR) has been recorded in 10 K–300 K temperature range. The highest TMR ratio of 0.51% was obtained at a low temperature ~10 K, corresponding to the spin polarization of ~5%. This TMR ratio reduces to a value of 0.032% as the temperature of the device increases up to 300 K, displaying a finite TMR at room temperature. A detailed study of thickness and temperature-dependent magnetization versus magnetic field (M − H) hysteresis loops of Ni–Mn–In thin films has been performed to understand the complex TMR behavior. The present study paves the way for the use of sputtered FL-MoS2 and ferromagnetic shape memory alloy in ultrafast spintronics for advanced magnetic devices application.

Direct imaging, three-dimensional interaction spectroscopy, and friction anisotropy of atomic-scale ripples on MoS2

Theory predicts that two-dimensional (2D) materials may only exist in the presence of out-of-plane deformations on atomic length scales, frequently referred to as ripples. While such ripples can be detected via electron microscopy, their direct observation via surface-based techniques and characterization in terms of interaction forces and energies remain limited, preventing an unambiguous study of their effect on mechanical characteristics, including but not limited to friction anisotropy. Here, we employ high-resolution atomic force microscopy to demonstrate the presence of atomic-scale ripples on supported samples of few-layer molybdenum disulfide (MoS2). Three-dimensional force/energy spectroscopy is utilized to study the effect of ripples on the interaction landscape. Friction force microscopy reveals multiple symmetries for friction anisotropy, explained by studying rippled sample areas as a function of scan size. Our experiments contribute to the continuing development of a rigorous understanding of the nanoscale mechanics of 2D materials.

34.7 W passive peak power Q-switch Ho:Sc2SiO5 laser operating at 2 μm with a few-layer molybdenum disulfide saturable absorber

Laser emission performances at 2 μm obtained from a Ho:Sc2SiO5 crystal was reported in this work. A few-layer MoS2 saturable absorber was used to obtain passively Q-switched regime. The laser beam quality was characterized by an M2 factor of 1.13 × 1.11. In continuous-wave mode the laser delivered 3.28 W output power for the pump with 12.58 W at 1.94 μm, which corresponds to an overall optical-to-optical efficiency of 0.26. The slope efficiency was 0.50. The maximum average power was 1.11 W and the laser run at 128.4 kHz repetition rate, indicating pulse energy of 8.62 μJ. The shortest laser pulse duration was measured to be 248 ns from which pulse peak power was determined as 34.77 W.

Few-Layered MoS2 Field-Effect Transistors with a Vertical Channel of Sub-10 nm.

Few-layered molybdenum disulfide (MoS2) has demonstrated promising advantages for the integration of next-generation electronic devices. A vertical short-channel MoS2 transistor with a channel length of sub-10 nm can be realized using mica as the insulated mesa and MoS2 flake dry-transferred onto the mica as the channel. A near-perfect symmetrical and fully saturated output characteristic can be obtained for the positive or negative drain-source voltage. This result is attributed to an effective transformation of the drain-source electrode contact from Schottky contact to Ohmic contact via forming gas annealing. The vertical-channel MoS2 transistor with a channel length of 8.7 nm exhibits excellent electrical characteristics, for example, a negligible hysteresis voltage of 60 mV, an extraordinarily small subthreshold swing of 73 mV/dec, a considerably weakened drain-induced barrier-lowering effect (100 mV/V), and the first-reported intrinsic delay time of 2.85 ps. Moreover, a logic inverter can be realized using the two vertical-channel MoS2 transistors, with a high voltage gain of 33. Experimental results indicate that the developed method is a potential approach for fabricating MoS2 transistors with an ultrashort channel and high performance, and consequently, manufacturing MoS2-based integrated circuits.

MoS2-based ferroelectric field-effect transistor with atomic layer deposited Hf0.5Zr0.5O2 films toward memory applications

Recently, hafnium oxide (HfO2)-based ferroelectric materials have achieved phenomenal success in next-generation nonvolatile memory applications. In this study, we fabricated Hf0.5Zr0.5O2 (HZO) ferroelectric capacitors and back-gate field-effect transistors (FETs) with few-layered molybdenum disulfide (MoS2) nanosheets as the channel and a ferroelectric Hf0.5Zr0.5O2 film as the gate dielectric. Good dielectric and ferroelectric properties have been observed from the HZO film fabricated with atomic layer deposition-based techniques. The MoS2–HZO ferroelectric FETs (FeFETs) have exhibited excellent performance including ferroelectric polarization switching with a high on/off ratio and negligible degradation in endurance and retention properties. Our results shown here suggest that MoS2–HZO FeFETs can be a promising alternative for next-generation nonvolatile memories.

Highly efficient solution exfoliation of few-layer molybdenum disulfide nanosheets for photocatalytic hydrogen evolution

Two-dimensional (2D) molybdenum disulfide (MoS2) has drawn increasing attention due to its great potential for advanced electronics, photonics, optoelectronics, energy storage, catalysis and solar cell. However, high effective exfoliation for the mass production of 2D MoS2 is still a challenge. To improve the exfoliation efficiency of MoS2 nanosheets, a facile liquid-phase exfoliation strategy has been developed by using organic electrolyte solutions in this work. It is shown that the dispersion concentration of MoS2 nanosheets has been significantly enhanced in organic electrolyte solutions. For example, by adding sodium tartrate into dimethyl sulphoxide, the dispersion concentration of MoS2 nanosheets is 85 times that in neat dimethyl sulphoxide. About 80% of MoS2 nanosheets obtained in such a strategy is 1–4 layers and the nanosheets remain 2H phase structure. In addition, the photocatalytic hydrogen evolution performance of the as-prepared few-layer MoS2 nanosheets is evaluated. The photocatalytic hydrogen evolution rate is as high as 0.5 mmolg−1 h−1, which is 71 and 10 times higher than that of bulk MoS2 and the 2H-MoS2 nanosheets, respectively, reported to date. Exfoliation mechanism investigation suggests that the size of cation and the intensity of Lewis basicity of anion in the salts have played an important role in the exfoliation of MoS2. The liquid exfoliation strategy reported here has great potential in the large-scale and high quality production of few-layer MoS2 nanosheets for efficient photocatalytic hydrogen evolution.