Few-layer Molybdenum Disulfide Research Articles

Faradaic deionization of brackish and sea water via pseudocapacitive cation and anion intercalation into few-layered molybdenum disulfide

This work establishes molybdenum disulfide/carbon nanotube electrodes for the electrochemical desalination of high molar saline water.

Towards an all-in fiber photodetector by directly bonding few-layer molybdenum disulfide to a fiber facet.

Although photodetectors based on two dimensional (2D) materials have been intensively studied, there are few reports of optical fiber compatible devices. Herein we successfully fabricated an all-in fiber photodetector (FPD) based on an end-face bonded with few-layer molybdenum disulfide (MoS2). Our FPD has a considerably high photo-responsivity of ∼0.6 A W-1 at a bias voltage of 4 V and 0.01 A W-1 under the bias-free conditions. We believe that the proposed platform may provide a new strategy for the integration of 2D materials in fibers and realization of optoelectronic and sensing applications.

Synthesis and electrochemical property of few-layer molybdenum disulfide nanosheets

Large-scale few-layer MoS2 nanosheets have been fabricated via a simple hydrothermal route using molybdenum powder as precursors. The as-prepared MoS2 samples were characterized by X-ray powder diffraction (XRD) analysis, transmission electron microscopy (TEM), and Raman and photoluminescence (PL) spectral analyses at room temperature. The results confirm that the as-prepared MoS2 displays a sheet-like morphology with a thickness of few (bi- to tri-) layers. Electrochemical measurements showed that the as-prepared few-layer MoS2 exhibited the highest reversible capacity of 1127 mAh g−1 and a stable reversible capacity of 1057 mAh g−1 after 30 cycles.

Theory of edge-state optical absorption in two-dimensional transition metal dichalcogenide flakes

We develop an analytical model to describe sub-bandgap optical absorption in two-dimensional semiconducting transition metal dichalcogenide (s-TMD) nanoflakes. The material system represents an array of few-layer molybdenum disulfide crystals, randomly orientated in a polymer matrix. We propose that optical absorption involves direct transitions between electronic edge-states and bulk-bands, depends strongly on the carrier population, and is saturable with sufficient fluence. For excitation energies above half the bandgap, the excess energy is absorbed by the edge-state electrons, elevating their effective temperature. Our analytical expressions for the linear and nonlinear absorption could prove useful tools in the design of practical photonic devices based on s-TMDs.

Electro-response of MoS2 Nanosheets-Based Smart Fluid with Tailorable Electrical Conductivity.

The correlation between electrical conductivity and electro-responsive behavior is identified by introducing few-layer molybdenum disulfide (MoS2) nanosheets to electrorheological (ER) fluid. Few-layer MoS2 nanosheets are successfully fabricated, with a high yield of above 60%, using a straightforward method, and applied to an electro-responsive smart fluid. The electrical conductivity of MoS2 is easily tunable by adjusting the annealing temperature because of its semiconducting behavior. From an in-depth study on the conductivity-dependent ER behavior of few-layer MoS2 nanosheets, it can be verified that an optimum value of the electrical conductivity exists for the electro-responsive material, corresponding to the Wagner model. To the best of our knowledge, this is the first report on the potential of a transition-metal dichalcogenide as a candidate material for an ER fluid. This study may provide promising approaches for the performance improvement of electro-responsive smart fluids.

Mode-locking pulse generation with MoS2-PVA saturable absorber in both anomalous and ultra-long normal dispersion regimes.

We experimentally demonstrate a stable and simple mode locked erbium doped fiber laser (EDFL) utilizing passive few-layer molybdenum disulfide (MoS2) as a saturable absorber. The MoS2 is obtained by liquid phase exfoliation before it is embedded in a polymer composite film and then inserted in the laser cavity. A stable soliton pulse train started at a low threshold pump power of 20mW in the anomalous dispersion regime after fine-tuning the rotation of the polarization controller. The central wavelength, 3dB bandwidth, pulse width, and repetition rate of the soliton pulses are 1574.6nm, 9.5nm, 790fs, and 29.5MHz, respectively. By inserting a 850m long dispersion shifted fiber (DSF) in the cavity, a dissipative soliton with square pulse train is obtained in the normal dispersion regime where the operating wavelength is centered at 1567.44nm with a 3dB bandwidth of 19.68nm. The dissipative soliton pulse has a pulse width of 90ns at a low repetition rate of 231.5kHz due to the long DSF used. These results are a contribution to the pool of knowledge in nonlinear optical properties of two-dimensional nanomaterials especially for ultrafast photonic applications.

Exploring biological effects of MoS2 nanosheets on native structures of α-helical peptides.

Recent reports of mono- and few-layer molybdenum disulfide (MoS2), a representative transition metal dichacogenide (TMD), as antibacterial and anticancer agents have shed light on their potential in biomedical applications. To better facilitate these promising applications, one needs to understand the biological effects of these TMDs as well, such as their potential adverse effects on protein structure and function. Here, we sought to understand the interaction of MoS2 nanosheets with peptides using molecular dynamics simulations and a simple model polyalanine with various lengths (PAn, n = 10, 20, 30, and 40; mainly α - helices). Our results demonstrated that MoS2 monolayer has an exceptional capability to bind all peptides in a fast and strong manner. The strong attraction from the MoS2 nanosheet is more than enough to compensate the energy needed to unfold the peptide, regardless of the length, which induces drastic disruptions to the intra-peptide hydrogen bonds and subsequent secondary structures of α - helices. This universal phenomenon may point to the potential nanotoxicity of MoS2 when used in biological systems. Moreover, these results aligned well with previous findings on the potential cytotoxicity of TMD nanomaterials.

All-normal dispersion passively mode-locked Yb-doped fiber laser using MoS2–PVA saturable absorber

We demonstrate the generation of a dissipative soliton in an all-normal dispersion ytterbium (Yb)-doped fiber laser using few-layer molybdenum disulfide (MoS2) as a saturable absorber. The saturable absorber is prepared by mixing few-layer MoS2 solution with polyvinyl alcohol (PVA) to form a free-standing composite film. The modulation depth and saturation intensity of the MoS2–PVA film are 11 and 5.86 MW cm−2, respectively. By incorporating the MoS2 saturable absorber in the fiber laser cavity, the mode-locked pulses are generated with a pulse width of 1.55 ns and a 3 dB spectral bandwidth of 0.9 nm centered at 1037.5 nm. The fundamental repetition rate and the average power are measured as 15.43 MHz and 1.5 mW, respectively. These results reveal the feasibility of deploying liquid-phase exfoliated few-layer MoS2 nanosheets for dissipative soliton generation in the near-IR region.

Near-Infrared Photodetector Based on MoS2/Black Phosphorus Heterojunction

Two-dimensional (2D) materials present their excellent properties in electronic and optoelectronic applications, including in ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, and the ability to confine electromagnetic energy to extremely small volumes. Herein, we demonstrate a photodetector with visible to near-infrared detection range, based on the heterojunction fabricated by van der Waals assembly between few-layer black phosphorus (BP) and few-layer molybdenum disulfide (MoS2). The heterojunction with electrical characteristics which can be electrically tuned by a gate voltage achieves a wide range of current-rectifying behavior with a forward-to-reverse bias current ratio exceeding 103. The photoresponsivity (R) of the photodetector is about 22.3 A W–1 measured at λ = 532 nm and 153.4 mA W–1 at λ = 1.55 μm with a microsecond response speed (15 μs). In addition, its specific detectivity D* is calcul...

Femtosecond mode-locked erbium-doped fiber laser based on MoS2–PVA saturable absorber

We fabricate a free-standing few-layer molybdenum disulfide (MoS2)–polymer composite by liquid phase exfoliation of chemically pristine MoS2 crystals and use this to demonstrate a soliton mode-locked Erbium-doped fiber laser (EDFL). A stable self-started mode-locked soliton pulse is generated by fine-tuning the rotation of the polarization controller at a low threshold pump power of 25mW. Its solitonic behavior is verified by the presence of Kelly sidebands in the output spectrum. The central wavelength, pulse width, and repetition rate of the laser are 1573.7nm, 630fs, and 27.1MHz, respectively. The maximum pulse energy is 0.141nJ with peak power of 210W at pump power of 170mW. This result contributes to the growing body of work studying the nonlinear optical properties of transition metal dichalcogenides that present new opportunities for ultrafast photonic applications.

Swift-heavy ion irradiation-induced latent tracks in few- and mono-layer MoS2

The latent tracks in mono- and few-layer molybdenum disulfide (MoS2) induced by 209Bi ions with energies of 0.45–1.23 GeV were characterized by atomic force microscopy (AFM). The hillock-like latent tracks were observed on the surface of irradiated monolayer MoS2. The diameter of the hillock after deconvolution procedure is 15.8 ± 1.7 nm and the height is 1.0 ± 0.3 nm. Hillock-like tracks are induced by energy transfer from energetic 209Bi ions to electron system of MoS2, resulting in the ionization and excitation and then the displacement of target atoms. Since Raman spectroscopy is sensitive to damages induced by swift-heavy ion irradiation, the in-plane E 2 1 mode (~385 cm−1) and the out-of-plane A 1g mode (~408 cm−1) of MoS2 were investigated. With increasing ion fluence, the A 1g peak shifts to higher frequencies, and the intensity ratio between A 1g and E 2 1 peak increases. Besides, the A 1g peak narrows. The evolution of the structural and vibrational properties of MoS2 with fluence is discussed. It can be concluded that the blue shift and narrowing of A 1g peak in irradiated MoS2 is due to the adsorption of oxygen molecules at latent tracks. With decreasing thickness of MoS2, the irradiation resistance decreases.

Spatially resolved optical absorption spectroscopy of single- and few-layer MoS2 by hyperspectral imaging

The possibility of spatially resolving the optical properties of atomically thin materials is especially appealing as they can be modulated at the micro- and nanoscale by reducing their thickness, changing the doping level or applying a mechanical deformation. Therefore, optical spectroscopy techniques with high spatial resolution are necessary to get a deeper insight into the properties of two-dimensional (2D) materials. Here we study the optical absorption of single- and few-layer molybdenum disulfide (MoS2) in the spectral range from 1.24 eV to 3.22 eV (385 nm to 1000 nm) by developing a hyperspectral imaging technique that allows one to probe the optical properties with diffraction limited spatial resolution. We find hyperspectral imaging very suited to study indirect bandgap semiconductors, unlike photoluminescence which only provides high luminescence yield for direct gap semiconductors. Moreover, this work opens the door to study the spatial variation of the optical properties of other 2D systems, including non-semiconducting materials where scanning photoluminescence cannot be employed.

Pulsed erbium-doped fiber laser by a few-layer molybdenum disulfide saturable absorber: from Q-switching to mode-locking

Abstract. Wedemonstrated a pulsederbium-doped fiber laser (EDFL)based on a few-layer molybdenum disul-fide saturableabsorber(MoS 2 -SA). The MoS 2 -SA wasfabricated intoa film structure by evaporatingthe mixtureof MoS 2 nanosheets and polyvinyl alcohol. The Raman spectra and nonlinear optical characteristics were mea-sured to confirm the quality of the as-prepared MoS 2 -SA. By inserting the MoS 2 -SA into an EDFL, the pulsedoperation from the stable Q-switched to mode-locking regime could be achieved by simply increasing the pumppower level. The Q-switched pulse duration and repetition rate could vary from 5.18 to 3.53 μsand from 72.74to86.39 kHz, respectively. The maximum pulse energy was 74.93 nJ. After the achievement of Q-switched oper-ation, the ultrashort mode-locking pulse was obtained by further increasing the pump power. The results furtherdemonstrate the excellent saturable absorption ability of the few-layer MoS 2 at telecommunication waveband. ©2016 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.55.8.081308]

Onset of two-dimensional superconductivity in space charge doped few-layer molybdenum disulfide.

Atomically thin films of layered materials such as molybdenum disulfide (MoS2) are of growing interest for the study of phase transitions in two-dimensions through electrostatic doping. Electrostatic doping techniques giving access to high carrier densities are needed to achieve such phase transitions. Here we develop a method of electrostatic doping which allows us to reach a maximum n-doping density of 4 × 1014 cm−2 in few-layer MoS2 on glass substrates. With increasing carrier density we first induce an insulator to metal transition and subsequently an incomplete metal to superconductor transition in MoS2 with critical temperature ≈10 K. Contrary to earlier reports, after the onset of superconductivity, the superconducting transition temperature does not depend on the carrier density. Our doping method and the results we obtain in MoS2 for samples as thin as bilayers indicates the potential of this approach.

In-Situ Growth of Few-Layered MoS2 Nanosheets on Highly Porous Carbon Aerogel as Advanced Electrocatalysts for Hydrogen Evolution Reaction

Molybdenum disulfide-based hybrids, acting as cost-effective and acid-stable electrocatalysts for hydrogen evolution reaction (HER), have been developed fast for providing sustainable hydrogen energy in recent years. Herein, few-layered molybdenum disulfide (MoS2) nanosheets/carbon aerogel (CA) hybrids were successfully obtained through the combination of sol–gel process, aging, freeze-drying, high temperature carbonization, and solvothermal reaction. CA with highly continuous porosity and high specific surface area is used as a matrix material for construction of hierarchical MoS2/CA hybrids where few-layered MoS2 nanosheets are uniformly covered on a CA surface. In this heterostructured system, CAs not only provide three-dimensional (3D) conductive pathway for fast transportation of electrons and ions, but also offer highly active regions for the growth of MoS2, greatly preventing the aggregation of MoS2 nanosheets. Due to the rationally designed hybrids with 3D porous nanostructures, the as-prepared Mo...

Photocurrent Response in Multiwalled Carbon Nanotube Core–Molybdenum Disulfide Shell Heterostructures

In this report, a few-layer molybdenum disulfide (MoS2) shell was coated on core multiwalled carbon nanotube (CNT) by a facile solvothermal method. The morphology and high crystallinity of this structure were demonstrated and verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). After being integrated into a planar device, the CNT–MoS2 core–shell structure exhibits clear photoresponse and a wide response range upon laser illumination. In addition, the device shows a bias-dependent and position-sensitive photocurrent effect. Further experiments show that larger photocurrent was obtained under laser illumination with longer wavelength. Both the photocurrent and response speed are enhanced when the device is placed under vacuum condition. The simple material synthesis and device fabrication method used in this work may provide a practical strategy for low-cost and large-scale optical applications.

Controlled synthesis and optical properties of polycrystalline molybdenum disulfide atomic layers grown by chemical vapor deposition

We report a scalable growth of mono to few-layer molybdenum disulfide (MoS2) atomic layers on different substrates by chemical vapor deposition (CVD). The effects of the source material (sulfur and MoO3 powder) and the growth temperature were systematically optimized for the growth of both highly crystalline and large area MoS2. The deposited film thickness could be precisely controlled by varying the growth temperatures, and this was confirmed by Raman and AFM results. The monolayer, bilayer, and multilayer MoS2 could be obtained at 650 °C, 700 °C, and 750–800 °C, respectively. The mobility value of ∼0.89 cm2/V s and current on/off ratio in the order of ∼104 was estimated for monolayer MoS2. The mobility value increased to ∼7.6 cm2/V s for the bilayer MoS2. Our results pave the way for the controlled synthesis of high-quality transition metal dichalcogenide materials, which are an attractive option for applications in electronic and optoelectronic devices.

Controlled MoS2 layer etching using CF4 plasma

A few-layered molybdenum disulfide (MoS2) thin film grown by plasma enhanced chemical vapor deposition was etched using a CF4 inductively coupled plasma, and the possibility of controlling the MoS2 layer thickness to a monolayer of MoS2 over a large area substrate was investigated. In addition, damage and contamination of the remaining MoS2 layer surface after etching and a possible method for film recovery was also investigated. The results from Raman spectroscopy and atomic force microscopy showed that one monolayer of MoS2 was etched by exposure to a CF4 plasma for 20 s after an initial incubation time of 20 s, i.e., the number of MoS2 layers could be controlled by exposure to the CF4 plasma for a certain processing time. However, XPS data showed that exposure to CF4 plasma induced a certain amount of damage and contamination by fluorine of the remaining MoS2 surface. After exposure to a H2S plasma for more than 10 min, the damage and fluorine contamination of the etched MoS2 surface could be effectively removed.

Optoelectrical Molybdenum Disulfide (MoS2)--Ferroelectric Memories.

In this study, we fabricated and tested electronic and memory properties of field-effect transistors (FETs) based on monolayer or few-layer molybdenum disulfide (MoS2) on a lead zirconium titanate (Pb(Zr,Ti)O3, PZT) substrate that was used as a gate dielectric. MoS2-PZT FETs exhibit a large hysteresis of electronic transport with high ON/OFF ratios. We demonstrate that the interplay of polarization and interfacial phenomena strongly affects the electronic behavior and memory characteristics of MoS2-PZT FETs. We further demonstrate that MoS2-PZT memories have a number of advantages and unique features compared to their graphene-based counterparts as well as commercial ferroelectric random-access memories (FeRAMs), such as nondestructive data readout, low operation voltage, wide memory window and the possibility to write and erase them both electrically and optically. This dual optoelectrical operation of these memories can simplify the device architecture and offer additional practical functionalities, such as an instant optical erase of large data arrays that is unavailable for many conventional memories.

Microfiber-coupler-assisted control of wavelength tuning for Q-switched fiber laser with few-layer molybdenum disulfide nanoplates.

Based on the liquid exfoliated method, we obtained the few-layer molybdenum disulfide (MoS2) nanoplates solution. By thermal evaporation method, we directly deposited MoS2 thin film onto the facet of a fiber patch cord. The modulation depth of the film is as high as 29%, and a Q-switched fiber laser was achieved. We also provided a new method to continuously tune the output laser with a tuning sensitivity of ∼5.5 nm/(1% strain) by controlling the cavity loss with a strained microfiber coupler (MFC).