Single-layer MoS2 Research Articles

Cu2O induced the enhancement of nonlinear absorption of MoS2 thin film

In this paper, Cu2O/MoS2 composite films with excellent nonlinear properties were fabricated. X-ray diffraction, Raman scattering and scanning electron microscope results show that Cu2O buffer layer has the effects on promoting the multiphase growth and improving both the particle size and the uniformity of MoS2 film. The nonlinear optical response of the samples was investigated by Z-scan and ultrafast carrier dynamics, respectively. The nonlinear absorption coefficient β of Cu2O/MoS2 composite sample was observed to reach the maximum value of −23.08 × 10-6 cm/W, which was 24.34 and 1.76 times than that of single-layer Cu2O and MoS2, respectively. In addition, the nonlinear enhancement mechanism of composite samples was confirmed by the results of both transient absorption spectroscopy measurements and finite difference time domain simulation calculation.

Light-induced correlated ion transport

An "on/off" functionality of a nanopore or channel is critical to control ion transport. In this issue, Chen et al.² report the elegant use of light as a trigger to induce ionic Coulomb blockade in single-layer sub-nanoporous MoS_2. Light-induced surface charge phenomenon is fast and reversible making it a promising technique to probe charged transport.

Decoding the DC and optical conductivities of disordered MoS2 films: an inverse problem

To calculate the conductivity of a material having full knowledge of its composition is a reasonably simple task. To do the same in reverse, i.e. to find information about the composition of a device from its conductivity response alone, is very challenging and even more so in the presence of disorder. An inversion methodology capable of decoding the information contained in the conductivity response of disordered structures has been recently proposed but despite claims of generality and robustness, the method has only been used with 2D systems possessing relatively simple electronic structures. Here we put these claims to the test and generalise the inversion method to the case of monolayer MoS2, a material whose electronic structure is far more complex and elaborate. Starting from the spectral function that describes the DC conductivity of a disordered sample of a single layered MoS2 containing a small concentration of randomly dispersed vacancies, we are able to invert the signal and find the exact composition of defects with an impressive degree of accuracy. Remarkably, equally accurate results are obtained with the optical conductivity. This is indicative of a methodology that is indeed suitable to extract composition information from different 2D materials, regardless of their electronic structure complexity. Calculated conductivity results were used as a proxy for their experimental counterpart and were obtained with an efficient quantum transport code (KITE) based on a real-space multi-orbital tight-binding model with parameters generated by density functional theory.

The Transition From MoS2 Single-Layer to Bilayer Growth on the Au(111) Surface

The transition from single-layer to bilayer growth of molybdenum disulfide on the Au(111) surface is investigated by in situ low-energy electron and photoemission microscopy. By mapping the film morphology with nanometer resolution, we show that a MoS2 bilayer forms at the boundaries of single-layer single-domain MoS2 islands and next to merging islands whereas bilayer nucleation at the island centers is found to be suppressed, which may be related to the usage of dimethyl disulfide as sulfur precursor in the growth process. This approach, which may open up the possibility of growing continuous films over large areas while delaying bilayer formation, is likely transferable to other transition metal dichalcogenide model systems.

Structural, Electronic and Magnetic Properties of Defected Water Adsorbed Single-Layer MoS2

Water adsorbed in MoS2 (wad-MoS2), 1S atom vacancy defect in wad-MoS2 (1S-wad-MoS2), 2S atoms vacancy defects in wad-MoS2 (2S-wad-MoS2), and 1Mo atom vacancy defect in wad-MoS2 (Mo-wad-MoS2) materials were constructed, and their structural, electronic, and magnetic properties were studied by spin-polarized density functional theory (DFT) based first-principles calculations. The wad-MoS2, 1S-wad-MoS2, 2S-wad-MoS2, and Mo-wad-MoS2 materials were found stable. From band structure calculations, wad-MoS2, 1S-wad-MoS2 and 2S-wad-MoS2 materials open energy bandgap of values 1.19 eV, 0.65 eV and 0.38 eV respectively. Also, it was found that the conductivity strength of the material increases with an increase in the concentration of S atom vacancy defects in the structure. On the other hand, the Mo-wad-MoS2 material has metallic properties because energy bands of electrons crossed the Fermi energy level in the band structure. For the investigation of magnetic properties, the density of states (DoS) and partial density of states (PDoS) calculations were used and found that wad-MoS2, 1S-wad-MoS2, and 2S-wad-MoS2 are non-magnetic materials, while Mo-wad-MoS2 is a magnetic material. The total magnetic moment of Mo-wad-MoS2 has a value of 2.66 µB/cell, due to the arrangement of unpaired up-spin and down-spin of electrons in 3s & 3p orbitals of S atoms; and 4p, 4d & 5s orbitals of Mo atoms in the material.

Large-Area, Two-Dimensional MoS2 Exfoliated on Gold: Direct Experimental Access to the Metal-Semiconductor Interface.

Two-dimensional semiconductors such as MoS2 are promising for future electrical devices. The interface to metals is a crucial and critical aspect for these devices because undesirably high resistances due to Fermi level pinning are present, resulting in unwanted energy losses. To date, experimental information on such junctions has been obtained mainly indirectly by evaluating transistor characteristics. The fact that the metal–semiconductor interface is typically embedded, further complicates the investigation of the underlying physical mechanisms at the interface. Here, we present a method to provide access to a realistic metal–semiconductor interface by large-area exfoliation of single-layer MoS2 on clean polycrystalline gold surfaces. This approach allows us to measure the relative charge neutrality level at the MoS2–gold interface and its spatial variation almost directly using Kelvin probe force microscopy even under ambient conditions. By bringing together hitherto unconnected findings about the MoS2–gold interface, we can explain the anomalous Raman signature of MoS2 in contact to metals [ACS Nano. 7, 2013, 11350] which has been the subject of intense recent discussions. In detail, we identify the unusual Raman mode as the A1g mode with a reduced Raman shift (397 cm–1) due to the weakening of the Mo–S bond. Combined with our X-ray photoelectron spectroscopy data and the measured charge neutrality level, this is in good agreement with a previously predicted mechanism for Fermi level pinning at the MoS2–gold interface [Nano Lett.14, 2014, 1714]. As a consequence, the strength of the MoS2–gold contact can be determined from the intensity ratio between the reduced A1greduced mode and the unperturbed A1g mode.

Optical Constants and Structural Properties of Epitaxial MoS2 Monolayers.

Two-dimensional layers of transition-metal dichalcogenides (TMDs) have been widely studied owing to their exciting potential for applications in advanced electronic and optoelectronic devices. Typically, monolayers of TMDs are produced either by mechanical exfoliation or chemical vapor deposition (CVD). While the former produces high-quality flakes with a size limited to a few micrometers, the latter gives large-area layers but with a nonuniform surface resulting from multiple defects and randomly oriented domains. The use of epitaxy growth can produce continuous, crystalline and uniform films with fewer defects. Here, we present a comprehensive study of the optical and structural properties of a single layer of MoS2 synthesized by molecular beam epitaxy (MBE) on a sapphire substrate. For optical characterization, we performed spectroscopic ellipsometry over a broad spectral range (from 250 to 1700 nm) under variable incident angles. The structural quality was assessed by optical microscopy, atomic force microscopy, scanning electron microscopy, and Raman spectroscopy through which we were able to confirm that our sample contains a single-atomic layer of MoS2 with a low number of defects. Raman and photoluminescence spectroscopies revealed that MBE-synthesized MoS2 layers exhibit a two-times higher quantum yield of photoluminescence along with lower photobleaching compared to CVD-grown MoS2, thus making it an attractive candidate for photonic applications.

Two-dimensional plasmonic polarons in n -doped monolayer MoS2

We report experimental and theoretical evidence of strong electron-plasmon interaction in n-doped single-layer MoS2. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the emergence of distinctive signatures of polaronic coupling in the electron spectral function. Calculations based on many-body perturbation theory illustrate that electronic coupling to two-dimensional (2D) carrier plasmons provides an exhaustive explanation of the experimental spectral features and their energies. These results constitute compelling evidence of the formation of plasmon-induced polaronic quasiparticles, suggesting that highly-doped transition-metal dichalcogenides may provide a new platform to explore strong-coupling phenomena between electrons and plasmons in 2D.

Efficient utilization of light energy based on MoS<sub>2</sub>-XS (X=Al, B, Ga) heterostructures

<p indent=0mm>In recent years, two-dimensional transition metal sulfide materials have received widespread attention due to their unique physical and chemical properties, and have been successfully applied in many fields such as microelectronic devices, photovoltaic technology and hydrogen evolution catalysis. However, high recombination rate, low mobility, and low cycle stability all limit the further improvement of the related properties of this kind of material to varying degrees. In our work, two-dimensional materials are composited to optimize the band structure and physical/chemical properties. This kind of approach has been proved to be feasible in many experiments. Not long ago, Demirci et al. studied the III-VI two-dimensional single-layer hexagonal structure MX (M = B, Al, Ga, In; X = O, S, Se, Te). A series of analyses show that all the above binary compounds are structurally stable. They believe that the III-VI group of binary monolayer materials can be widely used for high-performance optoelectronic devices. Based on density functional theory, this work proposes a new heterostructure form of MoS₂-XS (X=Al, B, Ga). Both MoS₂ and XS (M = B, Al, Ga) have the same hexagonal crystal structure (P-6m2), and there are reasons to believe that the MoS₂-XS heterogeneous structures will be fabricated under certain experimental condition. Therefore, it is meaningful to understand how the single-layer MoS₂ and XS form a stable two-dimensional heterostructure. At present, MoS₂-XS based structural characteristics and related physical/chemical properties of the heterostructures have not been reported. Therefore, the purpose of this research is to grasp the basic characteristics and potential application fields of the MoS₂-XS as far as possible. Based on the periodic boundary conditions of the two-dimensional structure, the establishment of a heterostructure requires us to strictly control the mismatch between MoS₂ and XS. We have obtained three highly reliable heterostructures, which maintain the lowest binding energies as well as the smallest mismatches after a series of preliminary calculations. The VBM and CBM of four pristine materials are composed of S atomic states to varying degrees, especially MoS₂ and AlS. Almost all of their band edges are derived from S atoms, which will greatly enhance the recombination of carriers. Therefore, the electrons and holes that can be actually used in the reaction are significantly reduced. How to suppress the recombination of carriers will be one of main goals in this work. The electronic properties of MoS₂-XS will be significantly affected by external strain. We conducted a detailed analysis and discussion on how the strain affects the band gap and band edges. From variable interlayer spacing, electric field to strain, MoS₂-AlS exhibits good catalytic activity, and has an ideal type-II band alignment. Therefore, we believe that MoS₂-AlS is the most ideal choice in terms of photocatalytic water splitting. It is proved that MoS₂-BS is unsuitable for the application of water splitting, but it has excellent efficiency in solar cell applications and its PCE can be as high as ~20.4%. It is worth noting that this value is very close to or even better than that of the reported heterostructure solar cells. In short, we believe that this work provides a meaningful reference for how to efficiently use light energy in two-dimensional composite materials.

Hydrogen evolution from MoSe2/WO3(0 0 1) heterojunction by photocatalytic water splitting: A density functional theory study

Photocatalytic water splitting for hydrogen evaluation as a green approach for energy production has received extensive attention from researchers. The strongly reducing single layer MoSe2 and oxidizing WO3(0 0 1) were constructed as a MoSe2/WO3(0 0 1) heterojunction (MWH). The electronic structure and optical properties of the MWH were calculated and analyzed using the density functional theory (DFT) to reveal the internal causes of the photogenerated carrier migration path and the photocatalytic mechanism. The electronic structure and optical absorption analysis showed that MoSe2 and WO3 contributed to the valence band maximum (VBM) and conduction band minimum (CBM) of MWH, respectively, narrowing the bandgap and broadening the optical absorption range. The energy band bending results indicated that the MWH was a Z-scheme photocatalytic heterojunction. Through the segregation of the oxidation (WO3) and reduction sites (MoSe2), photogenerated electrons and holes could be separated, and the formation of a built-in electric field inhibited the recombination of photogenerated carriers. Given that the MWH exhibited superior oxidation (3.302 eV) and reduction (−0.128 eV) properties, it has great potential for applications in the field of photocatalytic hydrogen production.

Single-Layer MoS2 Grown on Atomically Flat SrTiO3 Single Crystal for Enhanced Trionic Luminescence.

The elaborate interface interactions can be critical in determining the achievable functionality of a semiconductor heterojunction (SH), particularly when two-dimensional material is enclosed in the system and its thickness is at an atomic extreme. In this work, we have successfully constructed a SH model system composed of typical transition-metal chalcogenide (TMDs) and transition metal oxides (TMO) by directly growing molybdenum sulfide (MoS2) nanosheets on atomically flat strontium titanate (SrTiO3) single crystal substrates through a conventional chemical vapor deposition (CVD) synthetic method. Multiple measurements have demonstrated the uniform monolayer thickness and single crystallinity of the MoS2 nanosheets as well as the atomic flatness of the heterojunction surface, both characterizing an extremely high quality of the interface. Clear evidence have been obtained for the electron transfer from the MoS2 adlayer to the SrTiO3 substrate which varies against the interface conditions. More importantly, the photoluminescence of MoS2 is significantly tailored, which is correlated with both the cleanness of the interface and the crystal orientation of the SrTiO3 substrate. These results not only shed fresh lights on the structure-property relationship of the TMDs/TMO heterostructures but also manifest the importance of the ideal interface structure for a hybridized system.

Toward alcohol synthesis from CO hydrogenation on Cu(111)-supported MoS2 - predictions from DFT+KMC.

In the quest for cheap and efficient catalysts for alcohol synthesis from syngas, a material of interest is single-layer MoS2 owing to its low cost, abundancy, and flexible structure. Because of the inertness of its basal plane, however, it is essential to find ways that make it catalytically active. Herein, by means of density functional theory based calculations of reaction pathways and activation energy barriers and accompanying kinetic Monte Carlo simulations, we show that while S vacancy row structures activate the MoS2 basal plane, further enhancement of chemical activity and selectivity can be achieved by interfacing the MoS2 layer with a metallic support. When defect-laden MoS2 is grown on Cu(111), there is not only an increase in the active region (surface area of active sites) but also charge transfer from Cu to MoS2, resulting in a shift of the Fermi level such that the frontier states (d orbitals of the exposed Mo atoms) appear close to it, making the MoS2/Cu(111) system ready for catalytic activity. Our calculated thermodynamics of reaction pathways lead to the conclusion that the Cu(111) substrate promotes both methanol and ethanol as the products, while kinetic Monte Carlo simulations suggest a high selectivity toward the formation of ethanol.

Hydrophilic Character of Single-Layer MoS2 Grown on Ag(111).

The study of MoS2/metal interfaces is crucial for engineering efficient semiconductor–metal contacts in 2D MoS2-based devices. Here we investigate a MoS2/Ag heterostructure fabricated by growing a single MoS2 layer on Ag(111) by pulsed laser deposition under ultrahigh vacuum (UHV) conditions. The surface structure is observed in situ by scanning tunneling microscopy, revealing the hexagonal moiré pattern characteristic of the clean MoS2/Ag(111) interface. Ex situ Raman spectroscopy reveals an anomalous behavior of vibrational modes, induced by the strong MoS2–Ag interaction. After few-hours exposure to ambient conditions the Raman response significantly changes and the formation of molybdenum oxysulfides is revealed by X-ray photoelectron spectroscopy. These effects are due to the interplay with water vapor and can be reversed by a moderate UHV annealing. A polymeric (PMMA) capping is demonstrated to hinder water-induced modifications, preserving the original interface quality for months.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Band Structure Engineering of Black Phosphorus/Graphene/MoS2 van der Waals Heterojunctions for Photovoltaic and Optoelectronic Device Application

In this paper, we study the band structure engineering of black phosphorus/graphene/MoS2 (BP/graphene/MoS2) van der Waals heterojunctions based on ab initio simulations. The density of state and charge density of BP/MoS2, bilayer-BP/MoS2, bilayer-BP/graphene/MoS2 and graphene/bilayer-BP/MoS2 heterojunctions are also investigated. It is found that the bandgaps of these four heterojunctions are smaller than the case of single layer BP or MoS2. When graphene is inserted into or stacked upon BP and MoS2 layers, the heterojunction can obtain a minimum bandgap and a higher density of state distribution. In the differential charge density diagram, electrons over the carbon, sulfur and phosphorus atoms are lost from those corresponding parts. These investigations based on the band structure engineering of 2D van der Waals heterojunctions can provide an effective guidance to manufacture the future nanoscale high performance photovoltaic and optoelectronic devices.

Single-atom Pt promotion of industrial Co-Mo-S catalysts for ultra-deep hydrodesulfurization

We introduce a tertiary transition metal sulfide nanostructure, Pt-Co-Mo-S, for catalytic hydrodesulfurization (HDS) of sulfur-containing molecules in crude oil distillates with the aim to produce ultra-low sulfur transport fuels. The addition of ppm-levels of Pt to a standard industrial Co-Mo-S catalyst boosts the HDS activity by up to 46%. The promotional effect is examined by combining atomic-resolution Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Absorption Near Edge Spectroscopy (XANES) and Density Functional Theory (DFT). It is shown that the Pt-Co-Mo-S catalyst contains predominantly single-layer MoS2 nanocrystals with Co atoms fully covering the S-edge terminations and Pt atoms uniquely attached to corner and edge sites in a platinum(IV) sulfide-like structural motif. Platinum is suggested to reduce the sulfur binding energy and increase the abundance of coordinately undersaturated sites (CUS) and not necessarily changing the reactivity towards 4,6-DMDBT molecules, although more elaborate studies are needed to address this in detail.

2D MoS2 Encapsulated Silicon Nanopillar Array with High-Performance Light Trapping Obtained by Direct CVD Process

Weak absorption remains a vital factor that limits the application of two-dimensional (2D) materials due to the atomic thickness of those materials. In this work, a direct chemical vapor deposition (CVD) process was applied to achieve 2D MoS2 encapsulation onto the silicon nanopillar array substrate (NPAS). Single-layer 2D MoS2 monocrystal sheets were obtained, and the percentage of the encapsulated surface of NPAS was up to 80%. The reflection and transmittance of incident light of our 2D MoS2-encapsulated silicon substrate within visible to shortwave infrared were significantly reduced compared with the counterpart planar silicon substrate, leading to effective light trapping in NPAS. The proposed method provides a method of conformal deposition upon NPAS that combines the advantages of both 2D MoS2 and its substrate. Furthermore, the method is feasible and low-cost, providing a promising process for high-performance optoelectronic device development.

Exciton\u2013phonon coupling strength in single-layer MoSe2 at room temperature

Single-layer transition metal dichalcogenides are at the center of an ever increasing research effort both in terms of fundamental physics and applications. Exciton–phonon coupling plays a key role in determining the (opto)electronic properties of these materials. However, the exciton–phonon coupling strength has not been measured at room temperature. Here, we use two-dimensional micro-spectroscopy to determine exciton–phonon coupling of single-layer MoSe2. We detect beating signals as a function of waiting time induced by the coupling between A excitons and A′1 optical phonons. Analysis of beating maps combined with simulations provides the exciton–phonon coupling. We get a Huang–Rhys factor ~1, larger than in most other inorganic semiconductor nanostructures. Our technique offers a unique tool to measure exciton–phonon coupling also in other heterogeneous semiconducting systems, with a spatial resolution ~260 nm, and provides design-relevant parameters for the development of optoelectronic devices.

Tunable polarization-independent MoS2-based coherent perfect absorber within visible region

The paper suggests a new perfect absorber design based on the coherent perfect absorption (CPA) phenomenon of two-dimensional MoS2 monolayer in the visible region. Our proposed structure consists of a single MoS2 layer that is sandwiched between a SiO2 layer and a silver film with the nanocube dielectric array. According to the finite difference time domain simulations, CPA occurs at 722.2 nm in the certain structural configuration. Our computational results confirm the tunability of CPA resonant wavelength as a function of geometric and optical parameters of the nanocube dielectric array. Therefore, our CPA design may offer a solution to the light absorption enhancement of MoS2 monolayer in the visible region for optoelectronics applications.

Thermal vibration of circular single-layered MoS2 predicted by the circular Mindlin plate model

Thoroughly understanding the dynamic behavior of two-dimensional molybdenum disulfide (MoS2) is extremely important to the MoS2-based nanoelectromechanical device. In this paper, the circular Mindlin plate model (CMPM) is proposed to investigate the temperature-induced vibration of circular single-layered MoS2 (CSLMoS2). When the size of the CSLMoS2 is very small, the natural frequencies calculated by the CMPM are closer to the natural frequencies calculated by molecular dynamic (MD) simulations than those calculated by the circular Kirchhoff plate model (CKPM). The frequencies obtained by the CMPM are closer to the MD results than those obtained by the CKPM when in the higher-order frequencies. The root-mean-squared (rms) amplitude of CSLMoS2 is calculated by the CMPM, the CKPM, and MD simulations. The rms amplitude of CSLMoS2 calculated by the CMPM is much larger than that calculated by the CKPM. The comparison of the rms amplitude calculated by MD simulations shows that both CMPM and CKPM can roughly predict the temperature-induced vibrational behavior of CSLMoS2. However, the rms amplitude forecasted by the CMPM is more accurate than that calculated by the CKPM. The CMPM can forecast the thermal vibration of CSLMoS2 well.