MoS2 Nanofilms Research Articles

Solar‐Driven Hydrogen Evolution with Superior Efficiency by a Low‐Cost, Large‐Scale Synergetic Hybrid of 1D‐Si Nanowires/0D‐Au Nanoparticles/2D‐MoS2 Nanofilms

The technology of semiconductor photocatalystshas become an important research area. 2D molybdenum disulfide (MoS2) is recognized as a promising catalyst for photoelectrocatalytic and photocatalytic (PC) hydrogen evolution reaction (HER). However, owing to its 2D nature of low optical cross‐section, the effective light absorption needs to be further boosted. Herein, we dramatically amplify the light‐matter interaction by hybridizing these photocatalysts with plasmonic materials that present strong electromagnetic field confinement and with nanowires that offer efficient light management. Such a system is designed in the heterostructure of silicon nanowires (SiNW)/ gold nanoparticles (AuNP)/ MoS2 nanofilms (SiNW/AuNP/MoS2), which demonstrates excellent PC HER. The absorption frequency of 2D‐MoS2, the resonance frequency of the 0D‐AuNP, and the antireflection frequency of 1D‐SiNW match with the visible range, enabling this heterostructure to effectively utilize solar energy. Additionally, an optimal MoS2 nanofilm that is a mixture of 1 T and 2 H phases was prepared with high reproducibility using facile pyrolysis. Moreover, the SiNW substrate validates high antireflection properties, achieving 95% visible light absorption. In addition, SiNW forms a p–n junction with the MoS2 to facilitate charge separation. The synergetic hybrid of 1D‐SiNW/0D‐AuNP/2D‐MoS2 nanofilms exhibits the highest hydrogen generation rate of 246 mmol g−1 h−1.

Spreading resistance and conductance anisotropy in multilayer MoS2

The increasing interest in realizing the full potential of two-dimensional (2D) layered materials for developing electronic components strongly relies on quantitative understanding of their anisotropic electronic properties. Herein, we use conductive atomic force microscopy to study the anisotropic electrical conductance of multilayer MoS2 by measuring the spreading resistance of circular structures of different radii ranging from 150 to 400 nm. The observed inverse scaling of the spreading resistance with contact radius, with an effective resistivity of ρeff = 2.89 Ω cm, is compatible with a diffusive transport model. A successive etch of the MoS2 nanofilms was used to directly measure the out-of-plane resistivity, i.e., 29.43 ± 7.78 Ω cm. Based on the scaling theory for conduction in anisotropic materials, the model yields an in-plane resistivity of 0.28 ± 0.07 Ω cm and an anisotropy of ∼100 for the ratio between the in-plane and out-of-plane resistivities. The obtained anisotropy indicates that the probed surface area can extend up to 400 times the metal contact area, whereas the penetration depth is limited to roughly 20% of the contact radius. Hence, for contact radius less than 3 nm, the conduction will be limited to the surface. Our investigation offers important insight into the anisotropic transport behavior of MoS2, a pivotal factor enabling the design optimization of miniaturized devices based on 2D materials.

Facile fabrication of MoS2 and MoSe2 layered structures on Mo foil for the efficient photocatalytic dye degradation and electrocatalytic hydrogen evolution reaction

The rapidly growing necessity for sustainable materials in fields such as energy, environmental science, and medicine, two dimensional-based photo- and electro-catalysts has obtained mounting consideration. A simple methodology was employed to fabricate MoSe2 and MoS2 layered structures on flexible Mo foil as heterogeneous catalysts for the electrocatalytic hydrogen evolution reaction (HER) and the photocatalytic decolorization of chromic red (CR) and methylene blue (MB) dyes. The structural characteristics confirm the formation of the multi-layer thick MoSe2 and MoS2 nanofilms. The toxic MB and CR dyes were rapidly and effectively decolorized in polluted water by the photocatalysts within 30 min of UV-light irradiation. The fabricated layered structures exhibited favorable HER electrocatalytic activities along with low overpotentials, small Tafel slopes, and excellent 24-h stability performances. The strong catalytic behavior of these structures fabricated on Mo foil in relation to the HER reaction and the dye decolorization process make them suitable candidates for the catalyst industry.

Flash phase engineering of MoS2 nanofilms for enhanced photoelectrochemical performance.

A heterophase structure combining semiconducting 2H- and metallic 1T-MoS2 exhibits significantly enhanced photoelectrochemical performance due to the electrical coupling and synergistic effect between the phases. Therefore, site-selective effective phase engineering is crucial for the fabrication of MoS2-based photoelectrochemical devices. Here, we employed a flash phase engineering (FPE) strategy to precisely fabricate a 2H-1T heterophase structure. This technique allows simple, efficient, and precise control over the micropatterning of MoS2 nanofilms while enabling site-selective phase transition from the 1T to the 2H phase. The detection of reduced glutathione (GSH) showed an approximately 5-fold increase in sensitivity when using the electrode fabricated by FPE.

Ultrafast nonlinear absorption in MoTe2 and MoTe2/MoS2 nanocomposite films and its application to all-optical logic gates

We present a detailed theoretical analysis of ultrafast saturable absorption (SA) and reverse SA (RSA) in MoTe2 nano-films with femtosecond (fs) laser pulses at 800 nm. A transition from RSA to SA occurs on increasing the thickness from 30 nm to 80 nm at a constant pump intensity of 141 GW cm−2. On the other hand, a transition from SA to RSA occurs upon increasing the pump intensity in an 80 nm thick MoTe2 nano-film. Theoretical results are in good agreement with reported experimental results. The effect of pump pulse intensity, pulse width, nonlinear absorption coefficient and sample thickness has been studied to optimize the SA ↔ RSA transition. The results for low-power and high contrast all-optical switching in MoTe2 nano-films have been used to design all-optical fs NOT, OR, AND, as well as the universal all-optical NOR and NAND logic gates. The SA behavior of MoTe2/MoS2 nanocomposite films has been used to design all-optical AND and OR logic gates. The nanocomposite films of MoTe2/MoS2 possess a larger nonlinear optical response in comparison to MoTe2 and MoS2 nano-films and, therefore, all-optical logic gates designed using nanocomposite films result in a good switching contrast compared to pure MoTe2 nano-films. Ultrafast operation at relatively low pump intensities demonstrates the applicability of MoTe2 and MoTe2/MoS2 nano-films for ultrafast all-optical information processing.

Ultrasmall CoSe2 Nanoparticles Grown on MoS2 Nanofilms: A New Catalyst for Hydrogen Evolution Reaction

Non‐noble‐metal‐based and earth‐abundant electrocatalysts have been extensively investigated for the hydrogen evolution reaction (HER). However, they still suffer from large Tafel slopes and high overpotentials, resulting in inferior HER catalytic activity. Herein, a novel, cost‐effective, and electrochemically stable catalyst is presented by growing ultrasmall CoSe2 nanoparticles on MoS2 nanofilms using a facile hydrothermal method. The CoSe2/MoS2 hybrid catalyst shows superior catalytic activity in HER to both CoSe2 or MoS2 alone and their physical mixture. With the Tafel slope as small as 37.8 mV dec−1, the CoSe2/MoS2 shows excellent catalytic properties for HER in an acidic solution. The promising performance can be attributed to the synergistic effect of CoSe2 nanoparticles and MoS2 nanofilms. In addition, the catalytic activity of the CoSe2/MoS2 catalyst is almost unchanged after 1000 cyclic voltammetric sweeps, indicating its long‐term durability under acidic conditions.

Quantitative Characterization of the Anisotropic Thermal Properties of Encapsulated Two-Dimensional MoS2 Nanofilms.

Two-dimensional (2D) semiconductors exhibit unique physical properties at the limit of a few atomic layers that are desirable for optoelectronic, spintronic, and electronic applications. Some of these materials require ambient encapsulation to preserve their properties from environmental degradation. While encapsulating 2D semiconductors is essential to device functionality, they also impact heat management due to the reduced thermal conductivity of the 2D material. There are limited experimental reports on in-plane thermal conductivity measurements in encapsulated 2D semiconductors. These measurements are particularly challenging in ultrathin films with a lower thermal conductivity than graphene since it may be difficult to separate the thermal effects of the sample from the encapsulating layers. To address this challenge, we integrated the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques in the same experimental platform. First, we use the FDTR technique to characterize the cross-plane thermal conductivity and thermal boundary conductance. Next, we measure the in-plane thermal conductivity by model-based analysis of the OTRS measurements, using the cross-plane properties obtained from the FDTR measurements as input parameters. We provide experimental data for the first time on the thickness-dependent in-plane thermal conductivity of ultrathin MoS2 nanofilms encapsulated by alumina (Al2O3) and silica (SiO2) thin films. The measured thermal conductivity increased from 26.0 ± 10.0 W m-1 K-1 for monolayer MoS2 to 39.8 ± 10.8 W m-1 K-1 for the six-layer films. We also show that the thickness-dependent cross-plane thermal boundary conductance of the Al2O3/MoS2/SiO2 interface is limited by the low thermal conductance (18.5 MW m-2 K-1) of the MoS2/SiO2 interface, which has important implications on heat management in SiO2-supported and encased MoS2 devices. The measurement methods can be generalized to other 2D materials to study their anisotropic thermal properties.

Synthesis of vertically-aligned large-area MoS2 nanofilm and its application in MoS2/Si heterostructure photodetector

Van der Waals heterostructures based on the combination of 2D transition metal dichalcogenides and conventional semiconductors offer new opportunities for the next generation of optoelectronics. In this work, the sulfurization of Mo film is used to synthesize vertically-aligned MoS2 nanofilm (V-MoS2) with wafer-size and layer controllability. The V-MoS2/n-Si heterojunction was fabricated by using a 20 nm thickness V-MoS2, and the self-powered broadband photodetectors covering from deep ultraviolet to near infrared is achieved. The device shows superior responsivity (5.06 mA W−1), good photodetectivity (5.36 × 1011 Jones) and high on/off ratio I on/I off (8.31 × 103 at 254 nm). Furthermore, the V-MoS2/n-Si heterojunction device presents a fast response speed with the rise time and fall time being 54.53 ms and 97.83 ms, respectively. The high photoelectric performances could be attributed to the high-quality heterojunction between the V-MoS2 and n-Si. These findings suggest that the V-MoS2/n-Si heterojunction has great potential applications in the deep ultraviolet-near infrared detection field, and might be used as a part of the construction of integrated optoelectronic systems.

Unusually High Ion Conductivity in Large-Scale Patternable Two-Dimensional MoS2 Film

The advancement of ion transport applications will require the development of functional materials with a high ionic conductivity that is stable, scalable, and micro-patternable. We report unusually high ionic conductivity of Li+, Na+, and K+ in 2D MoS2 nanofilm exceeding 1 S/cm, which is more than 2 orders of magnitude higher when compared to that of conventional solid ionic materials. The high ion conductivity of different cations can be explained by the mitigated activation energy via percolative ion channels in 2H-MoS2, including the 1D ion channel at the grain boundary, as confirmed by modeling and analysis. We obtain field-effect modulation of ion transport with a high on/off ratio. The ion channel is large-scale patternable by conventional lithography, and the thickness can be tuned down to a single atomic layer. The findings yield insight into the ion transport mechanism of van der Waals solid materials and guide the development of future ionic devices owing to the facile and scalable device fabrication with superionic conductivity.

Wafer-scale MoS2 for P-type field effect transistor arrays and defects-related electrical characteristics

Because of their fascinating electrical/optoelectrical characteristics, two-dimensional molybdenum disulphide (MoS2) attracts much attention recently. However, it is still a challenge to fabricate wafer-scale MoS2 nanofilms of controlled thickness and high quality and that, limits their application. Here, a two-step thermal vulcanization method is proposed to fabricate wafer-scale MoS2 nanofilms with different thicknesses (3-10 layers). The synthesized MoS2 nanofilms have high uniformity and good quality. Furthermore, back gate field effect transistor (FET) arrays were fabricated based on these wafer-scale MoS2 nanofilms. The FET devices showed good electrical performance, mobility in the range of 0.44 ~ 0.96 cm2V−1s−1 and on/off ratio of ~103. Here, first-principle calculation was used to analyse the different types of defects observed in the tri-layer MoS2, the doping effects induced by the defects were also discussed. This work provides a reliable way to fabricate wafer-scale MoS2 nanofilms and give a detailed study of the electrical properties of multilayers MoS2 nanofilms paving the way for the manufacture of two-dimensional semiconductor materials.

Free‐standing molybdenum disulfides on porous carbon cloth for lithium‐ion battery anodes

In this study, a free-standing MoS2 nanofilm on a porous carbon cloth (MoS2@PCC) was prepared for application as an anode in Li-ion batteries. Uniform, non-aggregated MoS2@PCC electrodes were synthesized via facile electric-wire-explosion, dip-coating, and thermal sulfidation processes. The phase and morphologies were controlled using a variety of explosion media that had different carbon contents. The dip-coating of PCC into a colloidal solution prepared by underwater explosion of Mo metallic wire and the thermal sulfidation process provided higher uniformity of MoS2 nanoparticles with no particle aggregation. This facilitated the charge transfer and accommodation of volume expansion of Li-active MoS2 upon cycling. Consequently, the free-standing MoS2@PCC electrodes exhibited enhanced lithium reactivity, high rate capability, and cycle durability, compared with the conventional MoS2 nanoparticle electrode.

High-mobility patternable MoS2 percolating nanofilms

Fabrication of large-area and uniform semiconducting thin films of two-dimensional (2D) materials is paramount for the full exploitation of their atomic thicknesses and smooth surfaces in integrated circuits. In addition to elaborate vapor-based synthesis techniques for the wafer-scale growth of 2D films, solution-based approaches for high-quality thin films from the liquid dispersions of 2D flakes, despite underdeveloped, are alternative cost-effective tactics. Here, we present layer-by-layer (LbL) assembly as an effective approach to obtaining scalable semiconducting films of molybdenum disulfide (MoS2) for field-effect transistors (FETs). LbL assembly is achieved by coordinating electrochemically exfoliated MoS2 with cationic poly (diallyldimethylammonium chloride) (PDDA) through electrostatic interactions. The PDDA/MoS2 percolating nanofilms show controlled and self-limited growth on a variety of substrates, and are easily patterned through lift-off processes. Ion gel gated FETs are fabricated on these MoS2 nanofilms, and they show mobilities of 9.8 cm2·V−1·s−1, on/off ratios of 2.1 × 105 with operating voltages less than 2 V. The annealing temperature in the fabrication process can be as low as 200 °C, thereby permitting the fabrication of flexible FETs on polyethylene terephthalate substrates. The LbL assembly technique holds great promise for the large-scale fabrication of flexible electronics based on solution-processed 2D semiconductors.

Electrophysical and Photo-Electrocatalytic Properties of MoS2 Nanofilms

Electrophysical and Photo-Electrocatalytic Properties of MoS2 Nanofilms

Structural analysis of the chemical vapour deposition grown molybdenum disulphide nanofilms for multifaceted applications

Background/Objectives: In recent years, the research on molybdenum disulphide (MoS2) has gained significance because of its unique properties and ease of incorporation in hybrid structures, which makes it one of the most suitable materials for devices and multifaceted Applications. The objective of the study is to synthesize MoS2 nanofilms and then to characterize them through X-ray diffraction (XRD) technique. Methods: In this study, MoS2 nanofilms are synthesized on silicon dioxide substrates by the thermal Chemical Vapour Deposition (CVD) technique, where molybdenum trioxide (MoO3) (VI) powder and sulphur (S) flakes are used as precursors. Findings:X-ray diffraction (XRD) measurements have been carried out for the thermal CVD grown MoS2 nanofilm samples. Further, the observed XRD data has been analyzed and the structural analysis of synthesized MoS2 nanofilms is presented in this report. Furthermore, the experimentally observed findings are compared with the standard findings and shown that they are resembling closely. Novelty/Applications: In order to highlight the scope of our work, the important applications, of the molybdenum disulphide nanostructures are also discussed, that make MoS2 nanostructures attractive candidates in fields as diverse as energy, environmental, biomedical and semiconductors. Keywords: Chemical vapour deposition; molybdenum disulphide; nanofilms;XRD; transition metal dichalcogenide

Sucrose concentration sensor based on MoS2 nanofilm and Au nanowires array enhanced SPR with graphene oxide nanoshee

Sucrose concentration sensor based on MoS2 nanofilm and Au nanowires array enhanced SPR with graphene oxide nanoshee

Transparent 3 nm-thick MoS2 counter electrodes for bifacial dye-sensitized solar cells

Molybdenum disulfide (MoS2) counter electrode (CE) is considered one of the most viable alternatives to Pt CE in dye-sensitized solar cells (DSSCs) owing to its abundance, low cost, and superior electrocatalytic activity. However, mostly, MoS2 CEs for DSSCs are prepared by conventional chemical reactions and annealing at a high temperature. By these conventional processes, deposition of sufficiently thin and transparent MoS2 layers is challenging; therefore, bifacial DSSCs employing transparent MoS2 CEs have not been studied. Here, we report transparent few-nanometer-thick MoS2 CEs prepared by atomic layer deposition at a relatively low temperature (98°C) for bifacial DSSC applications. MoS2 nanofilms with precisely controlled thicknesses of 3–16nm are conformally coated on transparent conducting oxide glass substrates. With increase in the MoS2 nanofilm thickness, the MoS2 CE electrocatalytic activity for the iodide/triiodide redox couple enhances, but its transparency decreases. Notably, the application of a thinner MoS2 nanofilm in a bifacial DSSC leads to lower conversion efficiency under front-illumination, but higher conversion efficiency under back-illumination. In particular, only the 3nm-thick MoS2 nanofilm shows reasonable photovoltaic performances under both front- and back-illumination conditions.

Improving Electrochemical Pb2+ Detection Using a Vertically Aligned 2D MoS2 Nanofilm.

Recent advancements in MoS2 nanofilms have aided in the development for important water-related environmental applications. However, a MoS2 nanofilm-coated sensor has yet to have been applied for heavy metal detection in water-related environmental samples. In this study, a novel vertically aligned two-dimensional (2D) MoS2 (edge exposed) nanofilm was applied for in situ lead ion (Pb2+) detection. The developed sensor showed an excellent linear relationship toward Pb2+ between 0 and 20 ppb at -0.45 V vs Ag/AgCl using square wave anodic stripping voltammetry (SWASV) with the improved limit of detection (LOD) of 0.3 ppb in a tap water environment. The vertically aligned 2D MoS2 sensor exhibited improved detection sensitivity (2.8 folds greater than a previous metallic [Bi] composite electrode) with lower relative standard deviation for repetitive measurements (n = 11), indicating enhanced reproducibility for Pb2+ detection. The vertically aligned 2D MoS2 layers exhibited 2.6 times higher sensitivity than horizontally aligned 2D MoS2 (basal plane exposed). Density functional theory calculations demonstrated that adsorption energy of Pb on the MoS2 side edge was much higher (4.11 eV) than those on the basal plane (0.36 and 0.07 eV). In addition, the band gap center of vertical MoS2 was found to be higher than the Pb2+ → Pb reduction potential level and capable of reducing Pb2+. Overall, the newly developed vertically aligned 2D MoS2 sensor showed excellent performance for detecting Pb2+ in a real drinking water environment with good reliability.

Effect of H2 Plasma Modification on the Electrocatalytic Hydrogen Evolution Performance of MoS2

The hydrogen evolution reaction (HER) is the cathodic half reaction of electrocatalytic water splitting and is thus highly desirable to power the "hydrogen-based economy". Earth-abundant MoS2 has emerged as a promising HER catalyst with high electrocatalytic activity and stability due to its unique physical and chemical properties (1, 2). In the electronic structure of MoS2, the orbitals of HOMO states are mainly localized at the edged S sites, so that the localized electrons are mainly at the edge of MoS2 for charge exchange of proton (3). Extensive efforts have been devoted to developing MoS2 nanostructures to maximize the number of active edge sites, but little attention has been paid to activating the MoS2 basal planar structure. In this work, we used the H2 plasma modification to engineer the planar S-vacancies of MoS2 for enhancing the HER kinetics. Moreover, controlled plasma exposure time has been designed to explore the influence of H2 plasma on the HER performance of MoS2 catalysts. Our experimental results show that the S/Mo atomic ratios decrease with increase the H2 plasma exposure time (Figure 1a). More importantly, the HER performance is becoming better after H2 plasma modification (Figure 1b). the MoS2 H2-plasma 1200s catalyst exhibits a boosted HER activity with the overpotential as low as 92 mV at the current density of 10 mA/cm2 and striking turnover frequency per surface Mo atom (TOF) as 0.0044 s-1, better than most reported MoS2-based catalysts (Figure 1c and d). Figure Caption Figure 1. (a) Atomic ratio of S/Mo for MoS2 samples as the function of H2 plasma exposure time. (b) Polarization curves of H2 plasma treated MoS2 catalysts in 0.5 M H2SO4 solution at a scan rate of 5 mV/s. (c) Overpotential at 10 mA/cm2 and (d) TOF of the H2 plasma treated MoS2 catalysts. References X. Huang, Z. Zeng, H. Zhang, Metal dichalcogenide nanosheets: preparation, properties and applications. Chem. Soc. Rev. 42, 1934-1946 (2013).H. T. Wang et al., Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction. P. Natl. Acad. Sci. USA 110, 19701-19706 (2013).J. Hu et al., Engineering stepped edge surface structures of MoS2 sheet stacks to accelerate the hydrogen evolution reaction. Energy Environ. Sci. 10, 593-603 (2017). Figure 1

High-performance FET arrays enabled by improved uniformity of wafer-scale MoS2 synthesized via thermal vapor sulfurization

The fabrication of wafer-scale multilayers molybdenum disulfide (MoS2) induced by sulfurizing molybdenum (Mo) seed layer could compatible with traditional semiconductor process but the random growth orientations especially the vertical aligned structures lead to the unreliability for electronic devices thus hindered their wide application. Here, we utilize multi-step heating (MH) sulfurization method to regulate the sulfurization rate and diffusion rate of thermal sulfur vapor, thus realize the fabrication of MoS2 nanofilms with wafer-level uniformity and based on this, we integrate back-gate field-effect transistor (FET) arrays with an average hole mobility of 0.9cm2 V−1 s−1 and high device yield (~98%) which illustrate the reliable electrical-performance among the device arrays and the high quality of the as-fabricated MoS2 nanofilms. Therefore, we believe our efforts could pave the way for fabricating wafer-scale transition metal dichalcogenides (TMDCs) nanofilms with high uniformity and promoting their application for the integration of electronics/optoelectronics.

Band Alignment of MoTe2/MoS2 Nanocomposite Films for Enhanced Nonlinear Optical Performance

AbstractBand alignment is a key issue for the optoelectronics based on 2D layered transition metal dichalcogenides (TMDs) heterostructures. Herein, band alignment of MoTe2/MoS2 mixed heterostructure is measured with high‐resolution X‐ray photoelectron spectroscopy. The MoTe2/MoS2 heterostructure belongs to type‐II heterostructure with the conduction band offset of 0.46 eV and the valence band offset of 0.9 eV. The stronger saturable absorption is observed in MoTe2/MoS2 heterostructure film compared with that of pure MoTe2 and MoS2 nanofilms at the same condition. An energy‐level model combined with Runge–Kutta algorithm is used to understand the enhancement mechanism. It is found that the interlayer transition from MoTe2/MoS2 heterojunction plays an important role in the nonlinear optical enhancement. Meanwhile, band structure of MoTe2/MoS2 heterostructure is calculated by the first principles. The contributions of the MoTe2 and MoS2 to the heterojunction are almost equal and the valence band maximum and conduction band minimum exist in MoTe2 and MoS2 separately. This structure can form the interlayer carriers easily. The results suggest that the band alignment of TMDs paves the way for the type‐II heterostructure for enhanced nonlinear response in the development of optical modulator, ultrafast laser mode lockers, saturable absorbers, and optical switches.