Few-layer Molybdenum Disulfide Research Articles

Stepwise Sulfurization from MoO3 to MoS2 via Chemical Vapor Deposition

Chemical vapor deposition (CVD) is used widely to synthesize monolayer and few-layer transition metal dichalcogenide molybdenum disulfide (MoS2), a two-dimensional (2D) material with various applications in nanoelectronics, catalysis, and optoelectronics. However, the CVD synthesis of 2D MoS2 is highly sensitive to small changes in growth parameters and the growth mechanism has not been extensively studied. This work systematically investigates the effect of sulfur concentration on CVD synthesis of MoS2 using molybdenum trioxide (MoO3) and sulfur precursors. We find that with increasing concentration of sulfur vapor, intermediate products of molybdenum dioxide (MoO2) and molybdenum oxysulfide (MoOS2) can form during a stepwise sulfurization of MoO3 to the final product of MoS2. The intermediate MoOS2, formed due to sulfur vapor deficiency, can be fully converted to MoS2 with further sulfurization. We show that the local sulfur to molybdenum vapor ratio at the growth substrate critically determines the growth products. This study thus highlights the importance of keeping the molar ratio of sulfur to molybdenum vapor well in excess of the stoichiometrically required ratio of 3.5:1 in order to grow 2D MoS2.

Few-layer molybdenum disulfide nanosheets functionalized with l-cysteine for selective capture of Cd2+ ions

Water purification and sensing using nanostructured thin films as filtration membranes is attractive due to their high surface area, which leads to the potential for high impurity retention rate, and superior sensitivity to dilute contaminants. Molybdenum disulfide (MoS2) is a widely investigated layered material that can be exfoliated and suspended in water at high concentrations. It can also be conveniently oxidized and functionalized without significantly affecting its electrical properties. All these characteristics indicate that MoS2 has the potential to be competitive with graphene in water purification and sensing applications. Here is presented a new protocol for the fabrication of MoS2 membranes functionalized with l-cysteine, an amino acid with the capability of coordinating to divalent metal ions including Cd and Hg in water. MoS2 nanosheets were exfoliated and functionalized with l-cysteine using a one-pot aqueous amide cross-linking process. These membranes preferentially absorb Cd2+ ions from water at concentration up to 76 ± 15 mg Cd per gram of functionalized MoS2. The selectivity for Cd2+ makes l-cysteine functionalized MoS2 an excellent candidate material for selective heavy metal capture and detection.

Highly-sensitive gas sensor based on two-dimensional material field effect transistor

Atomically thin two-dimensional (2D) materials are ideal gas sensing materials for achieving an ultra-low detection limit, due to the high surface-to-volume ratio, low electronic noise and sensitively tunable Fermi level. However, the sensitivity of 2D materials to their surrounding environment may also severely degrade the long-term stability of sensing devices, since most of them use the same 2D material flake as both the sensing and conduction material. In this work, we report a gas sensor based on a 2D material field effect transistor (FET) which uses few-layer black phosphorus (BP), boron nitride (BN) and molybdenum disulfide (MoS2) as the top-gate, dielectric layer and conduction channel, respectively. In this device configuration, the top-gate of BP with a superior gas adsorption capability serves as the sensing material, while the conduction channel of MoS2 is isolated from ambient environment by the coverage of the BN dielectric layer. The separation of the sensing and conduction materials not only improves the long-term stability of the device, but also enables us to use different materials for gas adsorption and conduction purposes to achieve optimum sensing performances. In addition, the adsorption kinetics of the gas molecules on the sensing channel can be sensitively detected by the current/resistance variation of the conduction channel, since the adsorbed gas molecules can effectively tune the Fermi level of sensing and conduction materials (BP and MoS2, respectively) through band alignment. We experimentally demonstrated that the proposed 2D material FET not only achieved a detection limit of 3.3 ppb to NO2, but was also capable to differentiate oxidizing and reducing gases.

Improving photoelectric performance of MoS2 photoelectrodes by annealing

Semiconductor photoelectrochemistry (PEC) technology has attracted wide interest as it not only reveals the photoelectric properties of advanced materials, but also promotes energy conversion from the sunlight based on these materials. Herein, we investigated photoelectric properties of annealed and unannealed liquid phase exfoliated few-layer molybdenum disulfide (MoS2) photoelectrodes by PEC measurement. The linear sweep voltammograms and photoelectric response I-t curves demonstrate enhanced performance of MoS2 films after annealing. Nyquist impedance and Bode phase plots demonstrate a higher charge transfer property and a longer charge lifetime after annealing. The enhancement due to annealing could come from the increase of the crystalline and compact density of MoS2 nano-sheets, which is proved by X-ray diffraction and Raman spectroscopy in line with the absorption spectroscopy. Our results pave the way for the improvement of two-dimensional material based photoanodes for high performance of PEC applications by a simple method.

Optical-resonance-enhanced nonlinearities in a MoS2-coated single-mode fiber.

Few-layer molybdenum disulfide (MoS2) has an electronic band structure that is dependent on the number of layers and, therefore, is a very promising material for an array of optoelectronic, photonic, and lasing applications. In this Letter, we make use of a side-polished optical fiber platform to gain access to the nonlinear optical properties of the MoS2 material. We show that the nonlinear response can be significantly enhanced via resonant coupling to the thin film material, allowing for the observation of optical modulation and spectral broadening in the telecom band. This route to access the nonlinear properties of two-dimensional materials promises to yield new insights into their photonic properties.

Atomic Layer GaSe/MoS2 van der Waals Heterostructure Photodiodes with Low Noise and Large Dynamic Range

We report on the demonstration of atomic layer van der Waals (vdW) heterostructure photodiodes operating in the visible regime, enabled by stacking single- to few-layer n-type molybdenum disulfide (MoS2) on top of few-layer p-type gallium selenide (GaSe) crystals. The atomic layer vdW photodiode exhibits an excellent photoresponsivity of ∼3A/W at the wavelength of 532 nm when symmetric few-layer graphene (FLG) contacts with low contact resistance are used. On the other hand, for a GaSe/MoS2 photodiode with asymmetric GaSe/FLG and MoS2/gold (Au) contacts, a very low noise equivalent power of NEP ∼ 10–14 W/Hz is obtained due to dark current reduction, which demonstrates the feasibility of detecting sub-pW (<10–12 W) level optical illumination. Further, the same photodiode exhibits a large linear dynamic range of DR ≈ 70 dB due to the remarkable photocurrent to dark current ratio. These results show that not only the p–n junction formed at the interface between p-type GaSe and n-type MoS2 but also the metal–...

Temperature-dependent layer breathing modes in two-dimensional materials

Relative out-of-plane displacements of the constituent layers of two-dimensional materials give rise to unique low-frequency breathing modes. By computing the height-height correlation functions from molecular dynamics simulations, we show that the layer breathing modes (LBMs) can be mapped consistently to vibrations of a simple linear chain model. Our calculated thickness dependence of LBM frequencies for few-layer (FL) graphene and molybdenum disulfide $({\mathrm{MoS}}_{2})$ are in excellent agreement with available experiments. Our results show a redshift of LBM frequency with an increase in temperature, which is a direct consequence of anharmonicities present in the interlayer interaction. We also predict the thickness and temperature dependence of LBM frequencies for FL hexagonal boron nitride. Our Rapid Communication provides a simple and efficient way to probe the interlayer interaction for layered materials and their heterostructures with the inclusion of anharmonic effects.

Density functional theory study of electronic structure of defects and the role on the strain relaxation behavior of MoS2 bilayer structures

Recent capability of chemical vapor deposition (CVD) to grow large-area and high-quality monolayer and few-layered molybdenum disulfide (MoS2) structures renders intrinsic defects such as vacancies that alter the electronic properties of these structures. As a result, density functional theory (DFT) calculations are carried out to investigate the electronic structure of various types of CVD-grown vacancy defects and the role on the strain relaxation behavior of bilayer MoS2. DFT calculations suggest that additional charge states are activated in the gap between the valence band and conduction band for the atoms neighboring the defects in the layer and in the layer above the defects. In addition, the DFT results indicate that the presence of local defects lower energy barriers for strain relaxation of bilayer MoS2 attributed to sliding between the layers. These results demonstrate the modifications of the electronic properties of 2D structures and the strain due to the presence of defects.

High Repetition Rate QML YVO4/Nd:YVO4/YVO4 Laser With a Reflective MoS2-SA

Two-dimensional (2D) materials have been proved to be excellent saturable absorbers. However, the study in Q-switched and mode-locked (QML) lasers has not shown good performances. In this letter, a reflective few-layer molybdenum disulfide saturable absorber (MoS 2 -SA) is fabricated. Using the reflective MoS 2 -SA, a nanosecond passively QML YVO 4 /Nd:YVO 4 /YVO 4 laser is experimentally demonstrated. At the pump power of 11 W, a stable QML laser operation can be achieved, corresponding to a repetition rate of 11 MHz, an envelope pulse width of 72 ns, and a modulation depth of 100%. So far as we know, it is the highest envelope repetition rate and the shortest envelope pulse duration in the reported QML lasers with 2D materials, which prove the MoS 2 -SA has excellent optical properties for ultrafast solid-state laser applications.

Quantitative analysis of trap states through the behavior of the sulfur ions in MoS2 FETs following high vacuum annealing

Few-layer molybdenum disulfide (MoS2) has attracted a great deal of attention as a semiconductor material for electronic and optoelectronic devices. However, the presence of localized states inside the bandgap is a critical issue that must be addressed to improve the applicability of MoS2 technology. In this work, we investigated the density of states (DOS: g(E)) inside the bandgap of MoS2 FET by using a current–voltage (I–V) analysis technique with the aid of high vacuum annealing (HVA). The g(E) can be obtained by combining the trap density and surface potential (ψS) extracted from a consistent subthreshold current (ID-sub). The electrical performance of MoS2 FETs is strongly dependent on the inherent defects, which are closely related to the g(E) in the MoS2 active layer. By applying the proposed technique to the MoS2 FETs, we were able to successfully characterize the g(E) after stabilization of the traps by the HVA, which reduces the hysteresis distorting the intrinsic g(E). Also, the change of sulfur ions in MoS2 film before and after the HVA treatment is investigated directly by Auger electron spectroscopy analysis. The proposed technique provides a new methodology for active channel engineering of 2D channel based FETs such as MoS2, MoTe2, WSe2, and WS2.

Scalable production of few-layer molybdenum disulfide nanosheets by supercritical carbon dioxide

Molybdenum disulfide (MoS2) has attracted a great deal of attention to its unique properties in recent years. In this work, a scalable and green method was developed for the production of high-quality MoS2 nanosheets using shear-assisted supercritical CO2 exfoliation. Our experiments indicate that high temperature, pressure, and shearing speed are favorable to the exfoliation of MoS2. It is found that over 95% of the exfoliated MoS2 nanosheets are less than 10 layers, among which about 50% are 1–4 layers. Raman spectra and XRD patterns reveal that few-layer MoS2 nanosheets with high planar crystal structure are successfully prepared. Moreover, the produced MoS2 has a better stability in N-methyl-pyrrolidone (NMP) than the bulk MoS2. The concentration of the exfoliated MoS2 in NMP after sedimentation for 7 days is as high as 0.97 mg/ml.

Fabrication of polyaniline–few-layer MoS2 nanocomposite for high energy density supercapacitors

Polyaniline–few-layer molybdenum disulfide nanocomposite (PANI–MoS2) has been synthesized by in situ polymerization of aniline over MoS2 using HCl as the dopant. X-ray crystallographic studies show the characteristic peaks of few-layered MoS2 in the PANI matrix. The changes in Raman and UV–Vis spectra have proved that there are definite interactions between PANI and few-layer MoS2.PANI–MoS2 nanocomposite as an electrode material for supercapacitor exhibits a maximum specific capacitance of 687 Fg−1 at a scan rate of 5 mVs−1 using cyclic voltammetry (CV) and a Csp of 612 Fg−1 at a current density of 0.2 Ag−1 using chronopotentiometry (CP). Further, the material also exhibits a high energy density of 128 Wh kg−1with a maximum power density of 9.8 kW kg−1 and a tremendous cyclic stability with 93% capacitance retention after 2000 cycles. These superior electrochemical results over bare PANI and MoS2 showed the PANI–MoS2 nanocomposites to be a capable electrode material for energy storage systems.

All-dry transferred single- and few-layer MoS2 field effect transistor with enhanced performance by thermal annealing

We report on the experimental demonstration of all-dry stamp transferred single- and few-layer (1L to 3L) molybdenum disulfide (MoS2) field effect transistors (FETs), with a significant enhancement of device performance by employing thermal annealing in moderate vacuum. Three orders of magnitude reduction in both contact and channel resistances have been attained via thermal annealing. We obtain a low contact resistance of 22 kΩ μm after thermal annealing of 1L MoS2 FETs stamp-transferred onto gold (Au) contact electrodes. Furthermore, nearly two orders of magnitude enhancement of field effect mobility are also observed after thermal annealing. Finally, we employ Raman and photoluminescence measurements to reveal the phenomena of alloying or hybridization between 1L MoS2 and its contacting electrodes during annealing, which is responsible for attaining the low contact resistance.

All-optical modulator based on MoS2-PVA thin film

The all-optical approach plays an important role in ultrafast all-optical signal processing, and the all-fiber scheme has a wide application in optical communications. In this letter, we investigate an all-optical modulator using few-layer molybdenum disulfide (MoS2)-polyvinyl alcohol (PVA) thin films based on the thermo-optic effect and obtain a long-time stable modulated output by applying polarization interference. By absorbing the injected 980 nm pump (control light), MoS2 generates heat, changes the refractive index of MoS2, and modulates the polarization of light. The obtained thermal all-optical modulator has a rise time of 526 μs.

Synthesis of MoS2 nanosheets for mercury speciation analysis by HPLC-UV-HG-AFS.

Mercury species have aroused wide concern in the past several decades due to their high toxicity. However, it is still difficult to detect ultra-trace mercury species due to their biochemical transformation in complex samples. To establish a simpler and more sensitive method for pre-concentration and determination of trace mercury species, molybdenum disulfide (MoS2) nanosheets with sulfur-rich characteristics and enlarged interlayer spacing were prepared by a hydrothermal method coupled with a sonication-assisted liquid exfoliation method and acted as solid-phase extraction adsorbent. The nano-MoS2 had high adsorption capacity, fast adsorption rate and excellent selectivity towards mercury ions (Hg2+), methyl mercury (MeHg+) and ethyl mercury (EtHg+) in a wide pH range and complex matrices. And it could be easily regenerated by 4 mol L−1 HCl and reused several times. After optimizing HPLC-UV-HG-AFS conditions, a great linearity (1.0–10.0 μg L−1, R2 = 0.999 for Hg2+, MeHg+ and EtHg+), lower detection limits (0.017, 0.037 and 0.021 ng mL−1 for Hg2+, MeHg+ and EtHg+, respectively), relative standard deviations (<5%) and addition recoveries of the samples within 82.75–113.38% were observed. In summary, trace inorganic and organic mercury species in environmental and biological samples could be selectively enriched by the prepared nano-MoS2 and efficiently seperated and detected by HPLC-UV-HG-AFS. The present study will help provide a better strategy for environmental monitoring and health assessment of mercury pollutants.

Out-of-Plane Electromechanical Response of Monolayer Molybdenum Disulfide Measured by Piezoresponse Force Microscopy.

Two-dimensional (2D) materials have recently been theoretically predicted and experimentally confirmed to exhibit electromechanical coupling. Specifically, monolayer and few-layer molybdenum disulfide (MoS2) have been measured to be piezoelectric within the plane of their atoms. This work demonstrates and quantifies a nonzero out-of-plane electromechanical response of monolayer MoS2 and discusses its possible origins. A piezoresponse force microscope was used to measure the out-of-plane deformation of monolayer MoS2 on Au/Si and Al2O3/Si substrates. Using a vectorial background subtraction technique, we estimate the effective out-of-plane piezoelectric coefficient, d33eff, for monolayer MoS2 to be 1.03 ± 0.22 pm/V when measured on the Au/Si substrate and 1.35 ± 0.24 pm/V when measured on Al2O3/Si. This is on the same order as the in-plane coefficient d11 reported for monolayer MoS2. Interpreting the out-of-plane response as a flexoelectric response, the effective flexoelectric coefficient, μeff*, is estimated to be 0.10 nC/m. Analysis has ruled out the possibility of elastic and electrostatic forces contributing to the measured electromechanical response. X-ray photoelectron spectroscopy detected some contaminants on both MoS2 and its substrate, but the background subtraction technique is expected to remove major contributions from the unwanted contaminants. These measurements provide evidence that monolayer MoS2 exhibits an out-of-plane electromechanical response and our analysis offers estimates of the effective piezoelectric and flexoelectric coefficients.

Effect of ultrathin Fe dusting layer on electrical transport properties of few-layer MoS2 field-effect transistors

Transition-metal-doped MoS2 has been predicted to be a potential candidate for a two dimensional dilute magnetic semiconductor, while the effect of transition metal dopants on the electrical properties of MoS2-based devices has received relatively less attention so far. Here, the authors report on a systematic electrical transport study of Fe-dusted few-layer molybdenum disulfide (MoS2) field-effect transistors via repeated in situ Fe deposition (total nominal thickness ≤ 2 nm) and electrical measurements in ultrahigh vacuum. It is found that an ultrathin Fe adsorption layer (≪0.5 nm) n-dopes MoS2 without noticeably affecting the electrical characteristics of the device. In contrast, a thicker Fe layer (0.5–2 nm) results in the loss of the carrier tunability and a nonlinear current-voltage characteristic with the differential conductance approximately linear to drain voltage. The authors show through result analysis and qualitative modeling that Fermi level pinning in MoS2 directly underneath Fe nanoclusters plays an important role in causing these degradations. The pinning effect can be partially removed by in situ oxidizing the Fe nanoclusters for a short duration of 16 min. The mechanism of Fermi level pinning is discussed.

Biotunable Nanoplasmonic Filter on Few-Layer MoS2 for Rapid and Highly Sensitive Cytokine Optoelectronic Immunosensing.

Monitoring of the time-varying immune status of a diseased host often requires rapid and sensitive detection of cytokines. Metallic nanoparticle-based localized surface plasmon resonance (LSPR) biosensors hold promise to meet this clinical need by permitting label-free detection of target biomolecules. These biosensors, however, continue to suffer from relatively low sensitivity as compared to conventional immunoassay methods that involve labeling processes. Their response speeds also need to be further improved to enable rapid cytokine quantification for critical care in a timely manner. In this paper, we report an immunobiosensing device integrating a biotunable nanoplasmonic optical filter and a highly sensitive few-layer molybdenum disulfide (MoS2) photoconductive component, which can serve as a generic device platform to meet the need of rapid cytokine detection with high sensitivity. The nanoplasmonic filter consists of anticytokine antibody-conjugated gold nanoparticles on a SiO2 thin layer that is placed 170 μm above a few-layer MoS2 photoconductive flake device. The principle of the biosensor operation is based on tuning the delivery of incident light to the few-layer MoS2 photoconductive flake thorough the nanoplasmonic filter by means of biomolecular surface binding-induced LSPR shifts. The tuning is dependent on cytokine concentration on the nanoplasmonic filter and optoelectronically detected by the few-layer MoS2 device. Using the developed optoelectronic biosensor, we have demonstrated label-free detection of IL-1β, a pro-inflammatory cytokine, with a detection limit as low as 250 fg/mL (14 fM), a large dynamic range of 106, and a short assay time of 10 min. The presented biosensing approach could be further developed and generalized for point-of-care diagnosis, wearable bio/chemical sensing, and environmental monitoring.

Acceleration of Hen Egg White Lysozyme Amyloid Fibrillation by Single- or Few-Layer Molybdenum Disulfide Nanosheets

Acceleration of Hen Egg White Lysozyme Amyloid Fibrillation by Single- or Few-Layer Molybdenum Disulfide Nanosheets

Plasmon-Induced Energy Transfer and Photoluminescence Manipulation in MoS2 with a Different Number of Layers

Monolayer or few-layer molybdenum disulfide (MoS2) with intriguing physical properties enables a wide range of applications such as photocatalysis and photodetection. The controllable light–matter interaction in MoS2 nanoflakes with different numbers of layers is critical for developing new optoelectronic functionalities. Recently, plasmonic nanostructures have been used to obtain strong near-field enhancement for the effective photoluminescence (PL) manipulation of MoS2. However, it is still unclear whether the PL manipulation is dominated by electron injection processes or the strong field induced Purcell effect so far. Here, we investigate the PL manipulation of MoS2 nanoflakes with different numbers of layers using Au nanoparticles with different aggregate states. Combining the measured PL and scattering spectra, the Kelvin probe force microscopy images at the single-particle level, and the numerical simulations, we figure out how the electron injection and strong field enhancement contribute to the PL manipulation and why PL quenching occurs in few-layer flakes but PL enhancement occurs in thicker flakes. These findings would give us a greater understanding of the interaction between two-dimensional materials and plasmonic nanostructures.