Narrow Results

We study Hall and Nernst transports in monolayer MoS₂ based on Green’s function formalism. We have derived analytical results for spin and valley Hall conductivities in the zero temperature and spin and valley Nernst conductivities in the low temperature. We found that tuning of the band gap and spin-orbit splitting can drive system transition from spin Hall insulator (SHI) to valley Hall insulator (VHI). When the system is subjected to a temperature gradient, the spin and valley Nernst conductivities are dependent on Berry curvature.

The ability of MoS₂ nanomaterials as adsorbent and photocatalytic materials of methylene blue (MB) dye after $\gamma$-ray irradiation is investigated. The MoS₂ nanomaterials are prepared by a simple hydrothermal route, and then irradiated with Co-60 gamma radioisotope at different doses of 1, 10, 100, 1000 kGy. All the samples are characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Diffusivity Reflectance Spectroscopy (DRS) and Brunauer–Emmet–Teller (BET). The XRD analysis shows no change in crystal structure of MoS₂ nanomaterials after irradiation. The DRS analysis indicates the optical bandgap increases from 1.73 to 1.82, 1.86, 1.94 and 2.00 eV, respectively. The performance of the dye-absorbing solutions and the photocatalytic dye solutions before and after irradiation is compared. After $\gamma$-ray irradiation, the adsorption capacity of the MoS₂ nanomaterials degrades, which can be attributed to the decreased specific surface area, from 77.060 to 48.812, 35.855, 38.789 and 27.137 m²/g, respectively. The photocatalytic degradation ability for the MB solution also decreases due to the increase of optical bandgap of the samples after $\gamma$-ray irradiation.

MoS₂ self-lubricating films were prepared on long-range ordered porous anodic alumina (PAA) by an electrophoretic deposition (EPD) method. The PAA was prepared by two-step anodization of aluminum plates. Oxalic-acid-based electrolytes were used in the first step and phosphoric-acid-based electrolytes were used in the second step. This process offers a new approach to preparing PAA with wide adjustable boundary distances (43.5–21 nm) by increasing the voltage from 80 V to 100 V. The boundary distances were decreased from 78 nm to 42.2 nm by increasing the solution concentration, which increased the interpore distance. The coefficient of friction and hard-wearing of the MoS₂ lubrication film on the PAA were studied by a ball-on-disk friction and wear tester. The results showed that the nanotubes stored MoS₂ particles, which provided continuous lubrication.

We investigate the effects of biaxial tensile and compressive strains on the electronic structure of O-doped monolayer MoS₂ by density functional theory (DFT) in this paper. O-doped monolayer MoS₂ is an exothermic reaction. The doping of O leads to the transformation of the system from direct bandgap to indirect, and the bonding of Mo and O causes a large amount of charge transfer. The application of tensile strain leads to a decrease in the stability of the doped system, and the system always maintains the nature of indirect bandgap. The degree of interatomic charge transfer and bandgap value gradually decrease with the increase of tensile strain. The application of compression strain improves the stability of the doped system, and as the compressive strain increases, the bandgap of the doped system completes the indirect–direct–indirect transformation. The bandgap value shows a trend of increasing and then decreasing. Additionally, the degree of charge transfer between atoms is strengthened.

Functionalization of ${\mathrm{\text{MoS}}}_2$ monolayer doped by the transition-metal Fe adatom $(\mathrm{\text{Fe}}$–${\mathrm{\text{MoS}}}_2)$ and NO absorption on $\mathrm{\text{Fe}}$–${\mathrm{\text{MoS}}}_2$ has been investigated computationally using first-principles calculations based on the density functional theory. We found that the system of $\mathrm{\text{Fe}}$–${\mathrm{\text{MoS}}}_2$ remains a semiconductor, with spin polarization at the Fermi level. However, for the system with absorption of NO molecule on the surface of $\mathrm{\text{Fe}}$–${\mathrm{\text{MoS}}}_2$ monolayer, its spin polarization is turned over at the Femi level, which provides a promising material for spintronic sensors.

n-MoS₂/p-Si heterojunction solar cells were simulated by using Analysis of Microelectronic and Photonic Structures (AMPS-1D) software. In order to fundamentally understand the mechanism of such kind of cells, the effects of electron affinity, band gap and thickness for MoS₂, as well as the donor concentration in Si layer on the devices performance were simulated and discussed in detail. The effects of defect states in Si layer and at n-MoS₂/p-Si interface on the performance of devices were also simulated. It is demonstrated that two-dimensional monolayer MoS₂ with the highest band gap of 1.8 eV is the optimized option for ideal devices which can give out the highest efficiency over 19.0%. Si layer with higher acceptor concentration is more likely to be recommended in achieving higher power conversion efficiency if defect level can be effectively controlled. The defect states in Si layer and at MoS₂/Si interface were identified to influence the performance of the devices significantly.

Half-metallic ferromagnetism (HMFM) has great potential application in spin filter. However, it is extremely rare, especially in two-dimensional (2D) materials. At present, 2D materials have drawn international interest in spintronic devices. Here, we use ab initio density functional theory (DFT) calculations to study the structural stability and electrical and magnetic properties of the MoS₂-based 2D superlattice formed by inserting graphene hexagonal ring in $6\times 6\times 1$ MoS₂ supercell. Two kinds of structures with hexagonal carbon ring were predicted with structural stability and were shown HMFM. The two structures combine the spin transport capacity of graphene with the magnetism of the defective 2D MoS₂. And they have strong covalent bonding between the C and S or Mo atoms near the interface. This work is very useful to help us to design reasonable MoS₂-based spin filter.

The electronic plasmons of single layer MoS₂ induced by different spin subbands owing to spin-orbit couplings (SOCs) are theoretically investigated. The study shows that two new and anomalous plasmonic modes can be achieved via inter-spin subband transitions around the Fermi level due to the SOCs. The plasmon modes are optic-like, which are very different from the plasmons reported recently in single-layer (SL) MoS₂, and the other two-dimensional systems. The frequency of such plasmons ascends with the increasing of electron density or spin polarizability, and decreases with the increasing of wave vector. The promising plasmonic properties of SL MoS₂ make it interesting for future applications in plasmonic and terahertz devices.

In this paper, we study Hall effects of the monolayer MoS₂ with Rashba and Ising spin-orbit coupling (SOC) under the application of a circularly polarized light. The Chern number and spin textures at high frequency regime are studied based on the Floquet theory. We found that the SOCs induced valley Hall effect. The sign of Chern numbers at high frequency regime can be reversed by engineering interplay between Ising SOC and light intensity. The system undergoes a topological phase transition from valley Hall state to anomalous Hall state. By analyzing the spin texture, we study the origin of the Hall effects.

A broadband MoS₂-based absorber composed of Ag rod/MoS₂/dielectric/Ag is proposed in the visible band. The relative bandwidth is 65% for the absorption above 80%. The absorber also has the properties of polarization-independence and wide-angle absorption. Impedance matching theory is used to analyze the physical mechanism of the broadband absorption. By investigating the absorption property of each part of the absorber, it is found that the absorption is enhanced by introducing the two-dimensional material MoS₂. The broadband absorber can be changed to be multiband absorber by changing the thickness of dielectric substrate. This structure provides a new perspective to enhance absorption in the visible band and has promising applications in solar cells.

Cu₂ZnSnS₄ (CZTS) materials have been widely investigated due to their excellent properties in solar cell applications. The common reference structure for CZTS cells is Al:ZnO(AZO)/i-ZnO/CdS/CZTS, but it is critical to find a suitable buffer layer material to replace toxic cadmium (Cd). In addition, the efficiency of CZTS cells is improved by improving the doping type (n or p) and doping concentration of MoS₂ generated during the manufacturing process. wxAMPS was used to simulate the performance of a CZTS battery with an Al:ZnO/i-ZnO/Zn(O,S)/CZTS/(MoS₂) structure. The performance of batteries using Zn(O,S) and CdS as buffer layers was compared. The optimal thickness of CZTS layer and the doping concentration of CZTS layer were calculated, and the doping type and concentration of MoS₂ layer were analyzed and the performance of the battery was improved by optimizing the solar cell parameters. This work provides novel ideas for designing and manufacturing higher performance solar cells.

Different from the traditional tunable Smith–Purcell (SP) radiation in the graphene-based gratings in the terahertz band, we propose a tunable SP radiation generated from an electron beam passing through a single-layer molybdenum disulfide (MoS₂) based grating in the visible band. The comparison between the simulation and the theoretical results shows good agreement. By varying the Fermi energy of MoS₂ from 0.025 eV to 0.125 eV for the MoS₂-based grating, we can not only control the radiation frequency but also can change the radiation magnitude. The radiation frequency, angle, and magnitude varying with the Fermi energy are also discussed, respectively. These properties would have potential applications in developing tunable visible SP radiation.

In this study, the surface acoustic wave device combined with an oscillator circuit was used as the excitation platform to explore the photoluminescence responses of the two-dimensional atomic layers of MoS₂. The MoS₂ layers were prepared using the mechanical exfoliation method, and then transferred onto the SAW delay line area of the platform. Under the varied servo driving voltages of the excitation platform, various power intensities of the oscillator were obtained. It represented that the various energy levels of acoustic waves were propagated underneath the MoS₂ layers. From the observation by multiphoton excitation microscopy, the excited fluorescent intensities of the MoS₂ species were detected at various levels. Studies have proved that as the driving voltage of the oscillator increases, the interaction between the MoS₂ species and the acoustic wave is also enhanced.

The effect of biaxial strain on O-doped monolayers MoS₂ has been systematically studied by the first-principles calculations. It is shown that the strain decreases the structural stability of O-doped monolayer MoS₂. Between 0% and 12% tensile strains, the bandgap steadily narrows. At different compression strains, the bandgap increases and then decreases. The optical properties analysis shows that the strain causes the peaks of both the real and imaginary parts of the dielectric function to appear in the low energy region. And it affects the absorption and reflection peaks of the doping system so that it has a strong absorption of photons in the ultraviolet region. The doping system shows resonance in the range of 0–10eV. The results of this study verify that strain can properly regulate the electronic and optical properties of O-doped monolayer MoS₂, and provide a theoretical reference for the implementation of MoS₂ in optoelectronic devices.

In this paper, we present a novel hybrid material consisting of multiwalled carbon nanotubes (MWCNTs) and molybdenum disulfide (MoS${}_2)$ with enhanced photoresponsivity in the near-infrared (NIR) region. MoS₂ nanosheets are obtained through chemical exfoliation in NMP solvent, and MWCNTs are grown on these nanosheets using the chemical vapor deposition (CVD) technique. The combination of the NIR transparency of MWCNTs and the high UV light absorption of MoS₂ leads to a substantial increase in the photoresponsivity ($R)$ of the MoS₂@MWCNTs hybrid compared to bare MoS₂ specifically in the NIR region. Experimental results demonstrate a remarkable enhancement of $R$ from 18.6 $\mu$A/W to 155.7 $\mu$A/W in the hybrid material, whereas the opposite trend is observed in the case of bare MoS₂.

In the quest of extending isostructural hybridization approach to organic–inorganic nanocomposite-based photocatalytic systems, a unique strategy of replacing the traditional inorganic semiconductors with naturally produced mycosporine-like amino acids (MAA) is proposed. The main motivation of incorporating MAA in symbiotically configured nanocomposites is with regard to MAA green, nontoxic nature, UV absorption and photostability. Our facile one-pot solvothermal method is to facilitate the amalgamation of MAA and molybdenum disulfide-graphene (MG) composite at the molecular/nanoscale level to endow better photocatalytic functionality. It is observed that the rate of photocatalytic dye degradation of Rhodamine 6G (R6G) becomes consistently enhanced with an incremental increase in the concentration of MAA in MG. The combination of MG-MAA leads up to 81.2% quenching of the PL emission, as compared with MG. Noticeable decrease in PL lifetime from 280 ps (MG) to 77ps (MG-MAA) explicitly implies fast charge extraction and transport of the charge carriers.

Nanoparticles (NPs) with high uniformity have been extensively investigated for their excellent chemical stability. Near-monodisperse globular MoS₂ NPs were prepared with sulphur powders (SPs) as a sulphur source by a one-pot polyol-mediated process without surfactants, transfer agents and toxic agents at 170–190${}^{\circ }$C. The as-processed SPs greatly affected the formation of the MoS₂ NPs after low-activity sulphur (S₈)_n was reassembled from common SPs (S${}_8)$. The average size of MoS₂ NPs can be reduced remarkably from 100–200nm to 50nm by introducing low amounts of MnCl₂. A preliminary four-step growth mechanism based on the aggregation-coalescence model was also proposed. This green and simple method may be an alternative to the common hot-injection and heating-up methods for the preparation of monodisperse NPs, particularly transition metal dichalogenides.

To improve the high charge carrier recombination rate and low visible light absorption of {001} facets exposed TiO₂ [TiO₂(001)] nanosheets, few-layered MoS₂ nanoparticles were loaded on the surfaces of TiO₂(001) nanosheets by a simple photodeposition method. The photocatalytic activities towards Rhodamine B (RhB) were investigated. The results showed that the MoS₂–TiO₂(001) nanocomposites exhibited much enhanced photocatalytic activities compared with the pure TiO₂(001) nanosheets. At an optimal Mo/Ti molar ratio of 25%, the MoS₂–TiO₂(001) nanocomposites displayed the highest photocatalytic activity, which took only 30min to degrade 50mL of RhB (50mg/L). The active species in the degradation reaction were determined to be h${}^{+}$ and ${}^{•}$OH according to the free radical trapping experiments. The reduced charge carrier recombination rate, enhanced visible light utilization and increased surface areas contributed to the enhanced photocatalytic performances of the 25% MoS₂–TiO₂(001) nanocomposites.

Three-dimensional reduced graphene oxide (RGO) matrix decorated with nanoflowers of layered MoS₂ (denoted as 3D MoS₂/RGO) have been synthesized via a facile one-pot stepwise hydrothermal method. Graphene oxide (GO) is used as precursor of RGO and a 3D GO network is formed in the first-step of hydrothermal treatment. At the second stage of hydrothermal treatment, nanoflowers of layered MoS₂ form and anchor on the surface of previously formed 3D RGO network. In this preparation, thiourea not only induces the formation of the 3D architecture at a relatively low temperature, but also works as sulfur precursor of MoS₂. The synthesized composites have been investigated with XRD, SEM, TEM, Raman spectra, TGA, N₂ sorption technique and electrochemical measurements. In comparison with normal MoS₂/RGO composites, the 3D MoS₂/RGO composite shows improved electrochemical performance as anode material for lithium-ion batteries. A high reversible capacity of 930mAh$\cdot$g${}^{-1}$ after 130 cycles under a current density of 200mA$\cdot$g${}^{-1}$ as well as good rate capability and superior cyclic stability have been observed. The superior electrochemical performance of the 3D MoS₂/RGO composite as anode active material for lithium-ion battery is ascribed to its robust 3D structures, enhanced surface area and the synergistic effect between graphene matrix and the MoS₂ nanoflowers subunit.

Based on U-g-C₃N₄ (U-gCN) and T-g-C₃N₄ (T-gCN) prepared with urea and thiourea as raw materials, respectively, a visible-light-driven MoS₂-modified U-gCN/T-gCN/MoS₂ (UTM) ternary heterojunction photocatalyst was successfully prepared using a sonication and bathing method. The photocatalytic activity of as-prepared photocatalyst was evaluated through the degradation of tetracycline hydrochloride (TC) and Rhodamine B (RhB) under the visible light irradiation. The UTM ternary heterojunction showed remarkably enhanced photocatalytic activity. For the degradation of TC and RhB, the degradation rates of 93.9% and 99.9% have been achieved after being irradiated under visible light for 2h and 1h, respectively. The enhanced photocatalytic performance can be ascribed to the role of loaded MoS₂ cocatalyst and the well-formed interfaces between U-gCN and T-gCN, which not only enhance the light absorption, but also accelerate the separation and transfer of photogenerated electron–hole pairs. Furthermore, UTM ternary heterojunction has excellent recyclability and chemical stability. The photodegradation rates of 89.9% and 96.78% of TC and RhB have been obtained, respectively, after being reused for five times. Sacrificial agent tests demonstrate that $•$${\mathrm{\text{O}}}_2^{-}$ is the major reactive species in the photocatalytic reaction system.