Electronic and optical properties of single-layer MoS<Subscript>2</Subscript>

The electronic structures of a MoS₂ monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS₂ is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS₂. Moreover, the strong optical absorption up to 10⁵ cm^-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS₂ as an advanced photoelectronic device.     

The interfacial properties of SrRuO<Subscript>3</Subscript>/MoS<Subscript>2</Subscript> heterojunction: a first-principles study

First-principles calculation was used to study the interfacial properties of theSrRuO₃ (1 1 1)/MoS₂(√3 × √3) heterojunction. It is found that the huge magneticmoments in of monolayer MoS₂ largely originate from the Ru-S hybridization for theRu-terminated interface. Moreover, for the SrO-terminated interface, we studied mainly themetal and semiconductor contact characteristic. The calculated results show that theSchottky barrier height can be significantly reduced to zero for the SrO-terminatedinterface. Schottky barrier heights dominate the transport behavior of theSrRuO₃/MoS₂ interface. Our results not only have potentialapplications in spintronics devices, but also are in favour of the scaling of field effecttransistors.     

Structural transformation of MoO<Subscript>3</Subscript> nanobelts into MoS<Subscript>2</Subscript> nanotubes

The structural transformation of MoO₃ nanobelts into MoS₂ nanotubes using a simple sulfur source has been reported. This transformation has been extensively investigated using electron microscopic and spectroscopic techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), and energy-dispersive X-ray analysis (SEM-EDAX and TEM-EDX). The method described in this report will serve as a generic route for the transformation of other oxide nanostructures into the chalcogenide nanostructures.     

Flower-like MoS<Subscript>2</Subscript> supported on three-dimensional graphene aerogels as high-performance anode materials for sodium-ion batteries

Flower-like MoS₂ supported on three-dimensional graphene aerogel (MoS₂/GA) composite has been prepared by a facile hydrothermal method followed by subsequent heat-treatment process. Each of MoS₂ microflowers is surrounded by the three-dimensional graphene nanosheets. The MoS₂/GA composite is applied as an anode material of sodium-ion batteries (SIBs) and it exhibits high initial discharge/charge capacities of 562.7 and 460 mAh g^?1 at a current density of 0.1 A g^?1 and good cycling performance (348.6 mAh g^?1 after 30 cycles at 0.1 A g^?1). The good Na⁺ storage properties of the MoS₂/GA composite could be attributed to the unique structure which flower-like MoS₂ are homogeneously and tightly decorated on the surface of three-dimensional graphene aerogel. Our results demonstrate that as-prepared MoS₂/GA composite has a great potential prospect as anodes for SIBs.     

Generation of MoS<Subscript>2</Subscript> quantum dots by laser ablation of MoS<Subscript>2</Subscript> particles in suspension and their photocatalytic activity for H<Subscript>2</Subscript> generation

MoS₂ quantum dots (QDs) have been obtained in colloidal suspensions by 532 nm laser ablation (7 ns fwhp/pulse, 50 mJ/pulse) of commercial MoS₂ particles in acetonitrile. High-resolution transmission electron microscopy images show a lateral size distribution from 5 to 20 nm, but a more homogeneous particle size of 20 nm can be obtained by silica gel chromatography purification in acetonitrile. MoS₂ QDs obtained by laser ablation are constituted by 3–6 MoS₂ layers (1.8–4 nm thickness) and exhibit photoluminescence whose λ_PL varies from 430 to 530 nm depending on the excitation wavelength. As predicted by theory, the confinement effect and the larger periphery in MoS₂ QDs increasing the bandgap and having catalytically active edges are reflected in an enhancement of the photocatalytic activity for H₂ generation upon UV–Vis irradiation using CH₃OH as sacrificial electron donor due to the increase in the reduction potential of conduction band electrons and the electron transfer kinetics.     

High-efficiency preparation of oil-dispersible MoS<Subscript>2</Subscript> nanosheets with superior anti-wear property in ultralow concentration

Molybdenum disulfide (MoS₂) nanosheets are a promising lubricant additive for enhanced engine efficiency in cars. However, high-cost production methods and poor dispersion have limited their application in industry. In this study, the ball milling process is demonstrated as a low-cost and high-efficient method for fabrication of oil-dispersible MoS₂ nanosheet, and the ball milling parameters are optimized. Moreover, the lubrication effectiveness of ball-milled MoS₂ nanosheet was also evaluated. Results indicated that well-dispersed MoS₂ nanosheets with a size of 250 nm can be manufactured with optimized surfactants of zinc dialkyldithiphosphates (ZDDP) and polyisobutylene succinimide (PIBS) after being ball milled for 36 h. Tribological results revealed that a friction coefficient of white oil with 0.25% MoS₂ nanosheets reached 0.075, much lower than that of lubricant without nanosheets (0.16). The wear scar radius of 0.015% MoS₂ nanosheets was similar with that of Hertz contact, and the wear scar radius reduction reached 20% compared with that of 1% ZDDP. In addition, EDS and XPS results indicated the formation of a MoS₂ and FeS tribofilm on the wear surface.     

Perfect valley polarization in MoS<Subscript>2</Subscript>

We study perfect valley polarization in a molybdenum disulfide (MoS₂) nanoribbon monolayer using two bands Hamiltonian model and non-equilibrium Green’s function method. The device consists of a one-dimensional quantum wire of MoS₂ monolayer sandwiched between two zigzag MoS₂ nanoribbons such that the sites A and B of the honeycomb lattice are constructed by the molecular orbital of Mo atoms, only. Spin-valley coupling is seen in energy dispersion curve due to the inversion asymmetry and time-reversal symmetry. Although, the time reversal symmetry is broken by applying an external magnetic field, the valley polarization is very small. A valley polarization equal to 46% can be achieved using an exchange field of 0.13 eV. It is shown that a particular spin-valley combination with perfect valley polarization can be selected based on a given set of exchange field and gate voltage as input parameters. Therefore, the valley polarization can be detected by detecting the spin degree of freedom.     

Transport properties of MoS<Subscript>2</Subscript> nanoribbons: edge priority

We report about results from density functional based calculations on structural, electronic and transport properties of one-dimensional MoS₂ nanoribbons with different widths and passivation of their edges. The edge passivation influences the electronic and transport properties of the nanoribbons. This holds especially for nanoribbons with zigzag edges. Nearly independent from the passivation the armchair MoS₂ nanoribbons are semiconductors and their band gaps exhibit an almost constant value of 0.42 eV. Our results illustrate clearly the edge priority on the electronic properties of MoS₂ nanoribbons and indicate problems for doping of MoS₂ nanoribbons.     

Optically probing the interaction between monolayer MoS<Subscript>2</Subscript> and single-wall carbon nanotube

Owing to outstandingly tunable optoelectronic properties, hybrid materials consisting of atomic scale thickness of two dimensional (2D) transition metal dichalcogenides (TMDs) and one dimensional (1D) nanowires have been attracting steady interests over the last several years. In this research for the first time we report optically probing the interaction between monolayer MoS₂ and single-wall carbon nanotube (SWCNT). By using Raman and photoluminescence measurements, we found the charge transfer between MoS₂ and SWCNT is sensitive to the intensity of light field. We also demonstrate that SWCNT acts as p-type dopants at physical contact with monolayer MoS₂. Our study gives new insight into the interaction between monolayer MoS₂ and SWCNT, which may allow new phenomena and ideas for novel low dimensional hybrid materials.     

Alkali-metal-embedded in monolayer MoS<Subscript>2</Subscript>: optical properties and work functions

Embedding alkali-metal in monolayer MoS₂ has been investigated by using first principles with density functional theory. The calculation of the electronic and optical properties indicates that alkali-metal was embedded in monolayer MoS₂ appearing almost metallic behavior, and the MoS₂ layer shows clear p-type doping behavior. The covalent bonding appears between the alkali-metal atoms and defective MoS₂. More importantly, embedding alkali-metal can increase the work function for monolayer MoS₂. Furthermore, the absorption spectrum of monolayer MoS₂ is red shifted because of alkali metal embedding. Accordingly, this study will provide the theoretical basis for producing the alkali-metal-doped monolayer MoS₂ radiation shielding and photoelectric devices.     

Hydrogen on hybrid MoS2/graphene nanostructures

Transition metal dichalcogenides are rising candidates for the replacement of Pt catalysts in water splitting. In this theoretical study we focus on the hydrogen evolution reaction part of this process and on how hydrogen (H) interacts with MoS₂ nanostructures, free‐standing or positioned on a graphene substrate. Density functional theory calculations confirm the stability of such nanostructures and our results for H on several configurations, from 2D infinite monolayers to quasi‐1D MoS₂ ribbons and quasi‐0D MoS₂ flakes, are presented. We calculate the adsorption energy of H atoms on various sites of the MoS₂ nanostructures, notably at Mo and S active edges. Comparing free‐standing and MoS₂/graphene hybrid systems we find that the effect of the support on the adsorption of H on MoS₂ nanostructures is quite significant when the substrate induces strain. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The rise of two-dimensional MoS<Subscript>2</Subscript> for catalysis

Two-dimensional (2D) MoS₂ is used as a catalyst or support and has received increased research interest because of its superior structural and electronic properties compared with those of bulk structures. In this article, we illustrate the active sites of 2D MoS₂ and various strategies for enhancing its intrinsic catalytic activity. The recent advances in the use of 2D MoS₂-based materials for applications such as thermocatalysis, electrocatalysis, and photocatalysis are discussed. We also discuss the future opportunities and challenges for 2D MoS₂-based materials, in both fundamental research and industrial applications.     

Photodeposition of ultrathin MoS<Subscript>2</Subscript> nanosheets onto cubic CdS for efficient photocatalytic H<Subscript>2</Subscript> evolution

In this work, the photocatalyst composed of ultrathin MoS₂ nanosheets onto the surface of cubic CdS nanoparticles with an average diameter of 7~10 nm has been successfully fabricated through a facile and mild photodeposition route. The ultrathin MoS₂ nanosheets as a cocatalyst were demonstrated to greatly boost photocatalytic H₂ evolution over cubic CdS upon visible light irradiation. It was clearly revealed that both the cubic CdS substrate and structure of ultrathin MoS₂ nanosheets play critical roles in the observed efficient H₂ evolution. The cubic CdS offers a strong adherence for ultrathin MoS₂ nanosheets to form a well contact interface, across which the photogenerated charge transfer and charge separation are achieved. The ultrathin MoS₂ nanosheets introduce a high density of unsaturated active S atoms for H₂ evolution.     

Templated synthesis of plate-like MoS<Subscript>2</Subscript> nanosheets assisted with HNTs and their tribological performance in oil

Two-dimensional MoS₂ nanosheets were synthesized by using halloysite nanotubes (HNTs) as template under the hydrothermal synthesis. The structure and morphology of the as-synthesized MoS₂ nanosheets were determined by a series of characterizations. The results showed that the as-synthesized MoS₂ nanosheets were of the plate-like structure with about five layers, and the basal spacing was about 0.63 nm. It was demonstrated that HNTs played a crucial template role in the formation of the plate-like MoS₂ nanosheets. The formation mechanism was proposed. Furthermore, the tribological performance of the as-prepared MoS₂ nanosheets in oil was intensively examined on the ball-on-ball wear tester. The testing results verified that the as-prepared MoS₂ nanosheets as additive could significantly improve the friction performance of oil, which exhibited the good antifriction, antiwear, and load-carrying properties.     

Fabrication of Fe/Fe<Subscript>3</Subscript>C-functionalized carbon nanotubes and their electromagnetic and microwave absorbing properties

Fe/Fe₃C-functionalized carbon nanotubes (CNTs) have been prepared by the floating catalyst chemical vapor-deposition method. It is demonstrated that the Fe and Fe₃C nanostructures are both encapsulated in the CNTs or decorated on the surface of CNTs. The Fe/Fe₃C content in the composites can easily be adjusted by changing the ferrocene concentration in the preparation. The electromagnetic properties of the CNTs have been evaluated in the frequency range of 2–18 GHz, and the nanocomposites exhibit excellent microwave absorbing performance. The CNT composites with higher Fe/Fe₃C content show enhanced microwave reflection losses. The significant influence of the Fe/Fe₃C nanostructures on the microwave absorption is realized by tuning the characteristic impedance of the nanocomposites. With increasing thickness, the maximum reflection loss peak shifts to lower frequency. The microwave absorbing performance of the composites is mainly caused by dielectric loss, resulting from the continuous CNT networks with excellent electrical conductivity.     

From layers to nanotubes: Transition metal disulfides TMS<Subscript>2</Subscript>

MoS₂ and WS₂ layered transition-metal dichalcogenides are indirect band gap semiconductors in their bulk forms. Thinned to a monolayer, they undergo a transition and become direct band gap materials. Layered structures of that kind can be folded to form nanotubes. We present here the electronic structure comparison between bulk, monolayered and tubular forms of transition metal disulfides using first-principle calculations. Our results show that armchair nanotubes remain indirect gap semiconductors, similar to the bulk system, while the zigzag nanotubes, like monolayers, are direct gap materials, what suggests interesting potential applications in optoelectronics.     

Enhanced lithium-ion storage and hydrogen evolution reaction catalysis of MoS<Subscript>2</Subscript>/graphene nanoribbons hybrids with loose interlaced three-dimension structure

Molybdenum disulfide hybridized with graphene nanoribbon (MoS₂/GNR) was prepared by mild method. MoS₂/GNR hybrids interlace loosely into a three-dimension structure. GNR hybridization can improve the dispersity of MoS₂, reduce the grain size of MoS₂ to 3–6 nm, increase the specific surface area, and broaden the interlamellar spacing of MoS₂ (002) plane to 0.67–0.73 nm, which facilitates the transportation of Li⁺ ions for lithium-ion battery. MoS₂/GNR hybrids have better cyclic durability, higher specific discharge capacity, and superior rate performance than MoS₂. The electrocatalytic activity in hydrogen evolution reaction shows that MoS₂/GNR hybrids have the lower overpotential and the larger current density with a negligible current loss after 2000 cycles. Hybridizing with GNRs enhances both the lithium-ion electrochemical storage and the electrocatalytic activity of MoS₂.

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Graphical abstract MoS₂/GNR hybrid prepared by a mild method is interlaced loosely into a three-dimension structure. Superior electrochemical performances of MoS₂/GNR hybrids than MoS₂ have been highlighted for the potential application for long- term durability energy-storage devices and HER electrocatalytic materials.

     

Regular hexagonal MoS<Subscript>2</Subscript> microflakes grown from MoO<Subscript>3</Subscript> precursor

Regular hexagonal MoS₂ microflakes with high yield were grown from MoO₃ precursor by a sulfurization process using S powders as sulfuration reducer. The precursors, long and smooth MoO₃ microbelts, were synthesized through a direct oxidation reaction of Mo plates in air. X-ray powder diffraction and scanning electron microscopy revealed that the sulfurized products were hexagonal MoS₂ with regular hexagonal flake-like morphology. The results of transmission electron microscopy examinations demonstrated that the microflakes were single crystalline MoS₂. Elemental analysis by EDAX and XPS showed that the microflakes consist of Mo and S with the atomic ratio near to 0.5. Factors influencing the formation of the product were systematically studied. PACS 81.15.Gh; 81.15.Kk; 81.05.Hd; 78.67.Pt; 82.40.Ck     

The effect of carrier gas flow rate on the growth of MoS<Subscript>2</Subscript> nanoflakes prepared by thermal chemical vapor deposition

Molybdenum disulfide nanoflakes (MoS₂) are superior material for their semiconducting properties. For bulk and monolayer MoS₂ the band gap changes from indirect-to-direct, respectively. So, it exhibits promising prospects in the applications of optoelectronics and valleytronics, such as solar cells, transistors, photodetectors, etc. In this research, the influence of different Ar flow rates as the carrier gas, is investigated for growing MoS₂ nanoflakes on silicon substrates using one-step thermal chemical vapor deposition by simultaneously evaporating of solid sources like sulfur and molybdenum trioxide powders. The structural and optical properties of the obtained nanoflakes are assessed by using X-ray diffraction pattern, scanning electron microscopy, UV–visible absorption, photoluminescence and Raman spectroscopy. It is shown that, Ar gas flow rate is strongly affects on the final products as few-layer MoS₂ structures. Moreover, the abundance of MoS₂ in comparison to MoO₂ and MoO₃ structures, in the obtained nanoflakes, is influenced by the Ar flow rate.     

Structural and catalytic properties of ZnO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanostructures loaded with metal nanoparticles

Nanostructured zinc oxide (ZnO) nanobelts and aluminum oxide (Al₂O₃) nanoribbons have been grown successfully from the vapor phase. XRD results confirmed the purity and the high quality of the formed crystalline materials. TEM images showed that ZnO nanostructures grew in the commonly known tetrapod structure with nanobelts separated from the tetrapods with an average width range of 10–30 nm and a length of about 500 nm. Al₂O₃ nanostructures grew in the form of nanoribbons with an average width range of 20–30 nm and a length of up to 1 μm. The catalytic oxidation of CO gas into CO₂ gas over the synthesized nanostructures is also reported. Higher catalytic activity was observed for Pd nanoparticles loaded on the ZnO nanobelts (100% conversion at 270 °C) and Al₂O₃ nanoribbons (100% conversion at 250 °C). The catalytic activity increased in the order Cu < Co < Au < Pd for the metal-loaded nanostructures. The preparation methods could be applied for the synthesis of novel nanostructures of various materials with novel properties resulting from the different shapes and morphologies.