Electronic structure and scanning-tunneling-microscopy image of molybdenum dichalcogenide surfaces

Unveiling the antibacterial strategies and mechanisms of MoS2: a comprehensive analysis and future directions

Antibiotic resistance is a growing problem that requires alternative antibacterial agents. MoS2, a two-dimensional transition metal sulfide, has gained significant attention in recent years due to its exceptional photocatalytic performance, excellent infrared photothermal effect, and impressive antibacterial properties. This review presents a detailed analysis of the antibacterial strategies and mechanism of MoS2, starting with its morphology and synthesis methods and focusing on the different interaction stages between MoS2 and bacteria. The paper summarizes the main antibacterial mechanisms of MoS2, such as photocatalytic antibacterial, enzyme-like catalytic antibacterial, physical antibacterial, and photothermal-assisted antibacterial. It offers a comprehensive discussion focus on recent research studies of photocatalytic antibacterial mechanisms and categorizes them, guiding the application of MoS2 in the antibacterial field. Overall, the review provides an in-depth understanding of the antibacterial mechanisms of MoS2 and presents the challenges and future directions for the improvement of MoS2 in the field of high-efficiency antibacterial materials.

Graphene oxide-based large-area dynamic covalent interfaces

Dynamic materials, being capable of reversible structural adaptation in response to the variation of external surroundings, have experienced significant advancements in the past several decades. In particular, dynamic covalent materials (DCMs), where the dynamic covalent bonds (DCBs) can reversibly break and reform under defined conditions, present superior dynamic characteristics, such as self-adaptivity, self-healing and shape memory. However, the dynamic characteristics of DCBs are mainly limited within the length scale of covalent bonds, due to the local position exchange or the inter-distance variation between the chemical compositions involved in the reversible covalent reactions. In this minireview, a discussion regarding the realization of long-range migration of chemical compositions along the interfaces of graphene oxide (GO)-based materials via the spatially connected and consecutive occurrence of DCB-based reversible covalent reactions is presented, and the interfaces are termed "large-area dynamic covalent interfaces (LDCIs)". The effective strategies, including water adsorption, interfacial curvature and metal-substrate support, as well as the potential applications of LDCIs in water dissociation and humidity sensing are summarized. Additionally, we also give an outlook on potential strategies to realize LDCIs on other 2D carbon-based materials, including the interfacial morphology and periodic element doping. This minireview provides insights into the realization of LDCIs on a wider range of 2D materials, and offers a theoretical perspective for advancing materials with long-range dynamic characteristics and improved performance, including controlled drug delivery/release and high-efficiency (bio)sensing.

Scalable synthesis of soluble crystalline ionic-graphdiyne by controlled ion expansion

Graphdiyne (GDY) is a promising material possessing extensive electronic tunability, high π conjugacy, and ordered porosity at a molecular level for the sp/sp2-hybridized periodic structures. Despite these advantages, the preparation of soluble and crystalline graphdiyne is limited by the relatively compact stacking interactions, mostly existing in thick-layer and insoluble solids. Herein, we proposed a strategy of "framework charge-induced intercalation (FCII)" for the synthesis of a soluble (4.3 mg ml-1) and yet interlayer-expanded (∼0.6 Å) crystalline ionic graphdiyne, named as N+-GDY, through regulating the interlayer interactions. The skeleton of such a sample is positively charged, and then the negative ions migrate to the interlayer to expand the space, endowing the N+-GDY with solution processability. The crystal structure of N+-GDY is proved through analysis of HR-TEM images under different axes of observation and theoretical simulations. The resulting N+-GDY possesses high dispersity in organic solvents to produce a pure-solution phase which is conducive to the formation of oriented N+-GDY films, accompanied by exfoliation-nanosheet restacking. The film exhibits a conductivity of 0.014 S m-1, enabling its applications in electronic devices.

On the Electronic Structure of 2H-MoS2: Correlating DFT Calculations and In-Situ Mechanical Bending on TEM

We present a systematic density functional theory study to determine the electronic structure of bending 2H-MoS₂ layers up to 75° using information from in-situ nanoindentation TEM observations. The results from HOMO/LUMO and density of states plots indicate a metallic transition from the typical semiconducting phase, near Fermi energy level (E_F) as a function of bending, which can mainly occur due to bending curvatures inducing a stretching and contracting of sulfur-sulfur chemical bonds located mostly over basal (001)-plane; furthermore, molybdenum ions play a major role in such transitions due to reallocation of their metallic d-character orbitals and the creation of "free electrons", possibly having an overlap between Mo_-dx²_-y² and Mo_dz² orbitals. This research on the metallic transition of 2H-MoS₂ allows us to understand the high catalytic activity for MoS₂ nanostructures as extensively reported in the literature.

Electron Doping of Semiconducting MoS2 Nanosheets by Silver or Gold Nanoclusters

Semiconducting two-dimensional (2D) materials have potential applications as ultrathin optoelectronic materials. Therefore, being able to precisely modulate the band gap is useful to improving their applicability. Electron doping of the semiconducting materials is one of the successful techniques used to modulate their band gap. Silver nanoclusters (AgNCs) or gold nanoclusters (AuNCs) a few nanometers in size can generate a high density of highly energetic hot electrons with relatively long lifetimes when photoexcited. The optical band gap of 2D MoS₂ nanosheets shows different responses when integrated with different amounts of AgNCs or AuNCs due to the electron doping effect. Introducing a small amount of the nanoclusters to the surface of a MoS₂ nanosheet lowered its optical band gap. Further reduction of the optical band gap of MoS₂ is obtained upon tripling the amount of integrated nanoclusters. Conversely, the optical band gap of MoS₂ was increased when integrated with 5 times the concentration of AuNCs and AgNCs. The optical band gap of the MoS₂ nanosheets was significantly increased when integrated with an even higher concentration of AuNCs or AgNCs. The magnitude of the shift of the optical band gap of MoS₂ induced by AgNCs is higher than that induced by AuNCs because the energy of LUMO of the AgNCs is higher than that of the AuNCs.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

Incommensurate transition-metal dichalcogenides via mechanochemical reshuffling of binary precursors

A new family of heterostructured transition-metal dichalcogenides (TMDCs) with incommensurate ("misfit") spatial arrangements of well-defined layers was prepared from structurally dissimilar single-phase 2H-MoS₂ and 1T-HfS₂ materials. The experimentally observed heterostructuring is energetically favorable over the formation of homogeneous multi-principle element dichalcogenides observed in related dichalcogenide systems of Mo, W, and Ta. The resulting three-dimensional (3D) heterostructures show semiconducting behavior with an indirect band gap around 1 eV, agreeing with values predicted from density functional theory. Results of this joint experimental and theoretical study open new avenues for generating unexplored metal-dichalcogenide heteroassemblies with incommensurate structures and tunable physical properties.

Transition Metal Chalcogenides as a Versatile and Tunable Platform for Catalytic CO2 and N2 Electroreduction

Group VI transition metal chalcogenides are the subject of increasing research interest for various electrochemical applications such as low-temperature water electrolysis, batteries, and supercapacitors due to their high activity, chemical stability, and the strong correlation between structure and electrochemical properties. Particularly appealing is their utilization as electrocatalysts for the synthesis of energy vectors and value-added chemicals such as C-based chemicals from the CO₂ reduction reaction (CO₂R) or ammonia from the nitrogen fixation reaction (NRR). This review discusses the role of structural and electronic properties of transition metal chalcogenides in enhancing selectivity and activity toward these two key reduction reactions. First, we discuss the morphological and electronic structure of these compounds, outlining design strategies to control and fine-tune them. Then, we discuss the role of the active sites and the strategies developed to enhance the activity of transition metal chalcogenide-based catalysts in the framework of CO₂R and NRR against the parasitic hydrogen evolution reaction (HER); leveraging on the design rules applied for HER applications, we discuss their future perspective for the applications in CO₂R and NRR. For these two reactions, we comprehensively review recent progress in unveiling reaction mechanisms at different sites and the most effective strategies for fabricating catalysts that, by exploiting the structural and electronic peculiarities of transition metal chalcogenides, can outperform many metallic compounds. Transition metal chalcogenides outperform state-of-the-art catalysts for CO₂ to CO reduction in ionic liquids due to the favorable CO₂ adsorption on the metal edge sites, whereas the basal sites, due to their conformation, represent an appealing design space for reduction of CO₂ to complex carbon products. For the NRR instead, the resemblance of transition metal chalcogenides to the active centers of nitrogenase enzymes represents a powerful nature-mimicking approach for the design of catalysts with enhanced performance, although strategies to hinder the HER must be integrated in the catalytic architecture.

Highly improved supercapacitance properties of MnFe2O4 nanoparticles by MoS2 nanosheets

Manganese ferrite (MnFe₂O₄) nanoparticles were synthesized via a hydrothermal method and combined with exfoliated MoS₂ nanosheets, and the nanocomposite was studied as a supercapacitor. X-ray diffractometry and Raman spectroscopy confirmed the crystalline structures and structural characteristics of the nanocomposite. Transmission electron microscopy images showed the uniform size distribution of MnFe₂O₄ nanoparticles (~ 13 nm) on few-layer MoS₂ nanosheets. UV-visible absorption photospectrometry indicated a decrease in the bandgap of MnFe₂O₄ by MoS₂, resulting in a higher conductivity that is suitable for capacitance. Electrochemical tests showed that the incorporation of MoS₂ nanosheets largely increased the specific capacitance of MnFe₂O₄ from 600 to 2093 F/g (with the corresponding energy density and power density of 46.51 Wh/kg and 213.64 W/kg, respectively) at 1 A/g, and led to better charge-discharge cycling stability. We also demonstrated a real-world application of the MnFe₂O₄/MoS₂ nanocomposite in a two-cell asymmetric supercapacitor setup. A density functional theory study was also performed on the MnFe₂O₄/MoS₂ interface to analyze how a MoS₂ monolayer can enhance the electronic properties of MnFe₂O₄ towards a higher specific capacitance.

Tunable Electronic Properties of Lateral Monolayer Transition Metal Dichalcogenide Superlattice Nanoribbons

The structural stability and structural and electronic properties of lateral monolayer transition metal chalcogenide superlattice zigzag and armchair nanoribbons have been studied by employing a first-principles method based on the density functional theory. The main focus is to study the effects of varying the width and periodicity of nanoribbon, varying cationic and anionic elements of superlattice parent compounds, biaxial strain, and nanoribbon edge passivation with different elements. The band gap opens up when the (MoS₂)₃/(WS₂)₃ and (MoS₂)₃/(MoTe₂)₃ armchair nanoribbons are passivated by H, S and O atoms. The H and O co-passivated (MoS₂)₃/(WS₂)₃ armchair nanoribbon exhibits higher energy band gap. The band gap with the edge S vacancy connecting to the W atom is much smaller than the S vacancy connecting to the Mo atom. Small band gaps are obtained for both edge and inside Mo vacancies. There is a clear difference in the band gap states between inside and edge Mo vacancies for symmetric nanoribbon structure, while there is only a slight difference for asymmetric structure. The electronic orbitals of atoms around Mo vacancy play an important role in determining the valence band maximum, conduction band minimum, and impurity level in the band gap.

Kinked row-induced chirality driven by molecule-substrate interactions

Combining STM measurements on three different substrates (HOPG, MoS₂, and Au[111]) together with DFT calculations allow for analysis of the origin of the self-assembly of 4-cyano-4'-n-decylbiphenyl (10CB) molecules into kinked row structures using a previously developed phenomenological model. This molecule has an alkyl chain with 10 carbons and a cyanobiphenyl group with a particularly large dipole moment. 10CB represents a toy model that we use here to unravel the relationship between the induced kinked structure, in particular the corresponding chirality expression, and the balanced intermolecular/molecule-substrate interaction. We show that the local ordered structure is driven by the typical alkyl chain/substrate interaction for HOPG and Au[111] and the cyanobiphenyl group/substrate interaction for MoS₂. The strongest molecule/substrate interactions are observed for MoS₂ and Au[111]. These strong interactions should have led to non-kinked, commensurate adsorbed structures. However, this latter appears impossible due to steric interactions between the neighboring cyanobiphenyl groups that lead to a fan-shape structure of the cyanobiphenyl packing on the three substrates. As a result, the kink-induced chirality is particularly large on MoS₂ and Au[111]. A further breaking of symmetry is observed on Au[111] due to an asymmetry of the facing molecules in the rows induced by similar interactions with the substrate of both the alkyl chain and the cyanobiphenyl group. We calculate that the overall 10CB/Au[111] interaction is of the order of 2 eV per molecule. The close 10CB/MoS₂ interaction, in contrast, is dominated by the cyanobiphenyl group, being particularly large possibly due to dipole-dipole interactions between the cyanobiphenyl groups and the MoS₂ substrate.

An in situ grown lanthanum sulfide/molybdenum sulfide hybrid catalyst for electrochemical hydrogen evolution

An La₂S₃–MoS₂ catalyst with expanded interlayer spacing and engineered nano-interfaces was facilely synthesized, demonstrating enhanced catalytic activity for electrochemical hydrogen evolution.

Beneficial restacking of 2D nanomaterials for electrocatalysis: a case of MoS2 membranes

The tight stacking of 2D MoS₂ nanosheets is demonstrated to be beneficial for electrochemical hydrogen evolution reaction activity, challenging the traditional views.

Engineering Point-Defect States in Monolayer WSe2

Defect engineering is a key approach for tailoring the properties of the emerging two-dimensional semiconductors. Here, we report an atomic engineering of the W vacancy in monolayer WSe₂ by single potassium atom decoration. The K decoration alters the energy states and reshapes the wave function such that previously hidden midgap states become visible with well-resolved multiplets in scanning tunneling spectroscopy. Their energy levels are in good agreement with first-principle calculations. More interestingly, the calculations show that an unpaired electron donated by the K atom can lead to a local magnetic moment, exhibiting an on-off switching by the odd-even number of electron filling. Experimentally the Fermi level is pinned above all defect states due to the graphite substrate, corresponding to an off state. The close agreement between theory and experiment in the off state, on the other hand, suggests the possibility of gate-programmable magnetic moments at the defects.

Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers

Atom-thick two-dimensional materials usually possess unique properties compared to their bulk counterparts. Their properties are significantly affected by defects, which could be uncontrollably introduced by irradiation. The effects of electromagnetic irradiation and particle irradiation on 2H MoS 2 two-dimensional nanolayers are reviewed in this paper, covering heavy ions, protons, electrons, gamma rays, X-rays, ultraviolet light, terahertz, and infrared irradiation. Various defects in MoS 2 layers were created by the defect engineering. Here we focus on their influence on the structural, electronic, catalytic, and magnetic performance of the 2D materials. Additionally, irradiation-induced doping is discussed and involved.

Pulsed laser deposition of single-layer MoS2 on Au(111): from nanosized crystals to large-area films

Molybdenum disulphide (MoS₂) is a promising material for heterogeneous catalysis and novel two-dimensional (2D) optoelectronic devices. In this work, we synthesized single-layer (SL) MoS₂ structures on Au(111) by pulsed laser deposition (PLD) under ultra-high vacuum (UHV) conditions. By controlling the PLD process, we were able to tune the sample morphology from low-coverage SL nanocrystals to large-area SL films uniformly wetting the whole substrate surface. We investigated the obtained MoS₂ structures at the nanometer and atomic scales by means of in situ scanning tunneling microscopy/spectroscopy (STM/STS) measurements, to study the interaction between SL MoS₂ and Au(111)-which for example influences MoS₂ lattice orientation-the structure of point defects and the formation of in-plane MoS₂/Au heterojunctions. Raman spectroscopy, performed ex situ on large-area SL MoS₂, revealed significant modifications of the in-plane E12g and out-of-plane A_1g vibrational modes, possibly related to strain and doping effects. Charge transfer between SL MoS₂ and Au is also likely responsible for the total suppression of excitonic emission, observed by photoluminescence (PL) spectroscopy.

Interlayer expansion of 2D MoS2 nanosheets for highly improved photothermal therapy of tumors in vitro and in vivo

Interlayer-expanded MoS2 (E-MoS2) nanosheets with an interlayer spacing of 0.94 nm are demonstrated to show an high photothermal conversion efficiency of ∼62%. More importantly, such biocompatible E-MoS2 nanosheets show highly improved photothermal therapy (PTT) of tumors in vitro and in vivo under near-infrared light irradiation.

Atomic-scale defects and electronic properties of a transferred synthesized MoS2 monolayer

MoS₂ monolayer samples were synthesized on a SiO₂/Si wafer and transferred to Ir(111) for nano-scale characterization. The samples were extensively characterized during every step of the transfer process, and MoS₂ on the final substrate was examined down to the atomic level by scanning tunneling microscopy (STM). The procedures conducted yielded high-quality monolayer MoS₂ of milimeter-scale size with an average defect density of 2 × 10¹³ cm^-2. The lift-off from the growth substrate was followed by a release of the tensile strain, visible in a widening of the optical band gap measured by photoluminescence. Subsequent transfer to the Ir(111) surface led to a strong drop of this optical signal but without further shifts of characteristic peaks. The electronic band gap was measured by scanning tunneling spectroscopy (STS), revealing n-doping and lateral nano-scale variations. The combined use of STM imaging and density functional theory (DFT) calculations allows us to identify the most recurring point-like defects as S vacancies.

Compressive strain induced enhancement in thermoelectric-power-factor in monolayer MoS2 nanosheet

Strain and temperature induced tunability in the thermoelectric properties in monolayer MoS₂ (ML-MoS₂) has been demonstrated using density functional theory coupled to semi-classical Boltzmann transport theory. Compressive strain, in general and uniaxial compressive strain (along the zig-zag direction), in particular, is found to be most effective in enhancing the thermoelectric power factor, owing to the higher electronic mobility and its sensitivity to lattice compression along this direction. Variation in the Seebeck coefficient and electronic band gap with strain is found to follow the Goldsmid-Sharp relation. n-type doping is found to raise the relaxation time-scaled thermoelectric power factor higher than p-type doping and this divide widens with increasing temperature. The relaxation time-scaled thermoelectric power factor in optimally n-doped ML-MoS₂ is found to undergo maximal enhancement under the application of 3% uniaxial compressive strain along the zig-zag direction, when both the (direct) electronic band gap and the Seebeck coefficient reach their maximum, while the electron mobility drops down drastically from 73.08 to 44.15 cm² V^-1 s^-1. Such strain sensitive thermoelectric responses in ML-MoS₂ could open doorways for a variety of applications in emerging areas in 2D-thermoelectrics, such as on-chip thermoelectric power generation and waste thermal energy harvesting.

First step to investigate nature of electronic states and transport in flower-like MoS2: Combining experimental studies with computational calculations

In the present paper, the nature of electronic states and transport properties of nanostructured flower-like molybdenum disulphide grown by hydrothermal route has been studied. The band structure, electronic nature of charge, thermodynamics and the limit of phonon scattering through density functional theory (DFT) has also been studied. The band tail states, dynamics of trap states and transport of carriers was investigated through intensive impedance spectroscopy analysis. The direct fingerprint of density and band tail state is analyzed from the capacitance plot as capacitance reflects the capability of a semiconductor to accept or release the charge carriers with a corresponding change in its Fermi potential levels. A recently introduced infrared photo-carrier radiometry and density functional perturbation theory (DFPT) techniques have been used to determine the temperature dependence of carrier mobility in flower type-MoS₂. The present study illustrates that a large amount of trapped charges leads to an underestimation of the measured effective mobility and the potential of the material. Thus, a continuous engineering effort is required to improve the quality of fabricated nanostructures for its potential applications.

Pressure coefficients for direct optical transitions in MoS2, MoSe2, WS2, and WSe2 crystals and semiconductor to metal transitions

The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: αA = 2.0 ± 0.1 and αB = 3.6 ± 0.1 meV/kbar for MoS2, αA = 2.3 ± 0.1 and αB = 4.0 ± 0.1 meV/kbar for MoSe2, αA = 2.6 ± 0.1 and αB = 4.1 ± 0.1 meV/kbar for WS2, αA = 3.4 ± 0.1 and αB = 5.0 ± 0.5 meV/kbar for WSe2. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be -7.9, -5.51, -6.11, and -3.79, meV/kbar for MoS2, MoSe2, WS2, and WSe2, respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS2, MoSe2, WS2, and WSe2, crystals at around 140, 180, 190, and 240 kbar, respectively.

Exfoliation of large-area transition metal chalcogenide single layers

Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically < 10 μm) lateral size of the produced single layers. Here, we report a novel mechanical exfoliation technique, based on chemically enhanced adhesion, yielding MoS2 single layers with typical lateral sizes of several hundreds of microns. The idea is to exploit the chemical affinity of the sulfur atoms that can bind more strongly to a gold surface than the neighboring layers of the bulk MoS2 crystal. Moreover, we found that our exfoliation process is not specific to MoS2, but can be generally applied for various layered chalcogenides including selenites and tellurides, providing an easy access to large-area 2D crystals for the whole class of layered transition metal chalcogenides.

Synthesis of Epitaxial Single-Layer MoS2 on Au(111)

We present a method for synthesizing large area epitaxial single-layer MoS2 on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling microscopy and low energy electron diffraction, the evolution of the growth is followed from nanoscale single-layer MoS2 islands to a continuous MoS2 layer. An exceptionally good control over the MoS2 coverage is maintained using an approach based on cycles of Mo evaporation and sulfurization to first nucleate the MoS2 nanoislands and then gradually increase their size. During this growth process the native herringbone reconstruction of Au(111) is lifted as shown by low energy electron diffraction measurements. Within the MoS2 islands, we identify domains rotated by 60° that lead to atomically sharp line defects at domain boundaries. As the MoS2 coverage approaches the limit of a complete single layer, the formation of bilayer MoS2 islands is initiated. Angle-resolved photoemission spectroscopy measurements of both single and bilayer MoS2 samples show a dramatic change in their band structure around the center of the Brillouin zone. Brief exposure to air after removing the MoS2 layer from vacuum is not found to affect its quality.

Edge-terminated molybdenum disulfide with a 9.4-Å interlayer spacing for electrochemical hydrogen production

Layered molybdenum disulfide has demonstrated great promise as a low-cost alternative to platinum-based catalysts for electrochemical hydrogen production from water. Research effort on this material has focused mainly on synthesizing highly nanostructured molybdenum disulfide that allows the exposure of a large fraction of active edge sites. Here we report a promising microwave-assisted strategy for the synthesis of narrow molybdenum disulfide nanosheets with edge-terminated structure and a significantly expanded interlayer spacing, which exhibit striking kinetic metrics with onset potential of −103 mV, Tafel slope of 49 mV per decade and exchange current density of 9.62 × 10⁻³ mA cm⁻², performing among the best of current molybdenum disulfide catalysts. Besides benefits from the edge-terminated structure, the expanded interlayer distance with modified electronic structure is also responsible for the observed catalytic improvement, which suggests a potential way to design newly advanced molybdenum disulfide catalysts through modulating the interlayer distance.

Layered molybdenum disulfide is a promising hydrogen evolution catalyst. Here, the authors report a strategy for synthesizing molybdenum disulfide nanosheets with edge-terminated structure and a significantly expanded interlayer spacing and demonstrate their enhanced catalytic activity.

Continuously tunable electronic structure of transition metal dichalcogenides superlattices

Two dimensional transition metal dichalcogenides have very exciting properties for optoelectronic applications. In this work we theoretically investigate and predict that superlattices comprised of MoS₂ and WSe₂ multilayers possess continuously tunable electronic structure with direct bandgaps. The tunability is controlled by the thickness ratio of MoS₂ versus WSe₂ of the superlattice. When this ratio goes from 1:2 to 5:1, the dominant K-K direct bandgap is continuously tuned from 0.14 eV to 0.5 eV. The gap stays direct against −0.6% to 2% in-layer strain and up to −4.3% normal-layer compressive strain. The valance and conduction bands are spatially separated. These robust properties suggest that MoS₂ and WSe₂ multilayer superlattice should be a promising material for infrared optoelectronics.

The electronic structure and optical properties of Mn and B, C, N co-doped MoS2 monolayers

The electronic structure and optical properties of Mn and B, C, N co-doped molybdenum disulfide (MoS2) monolayers have been investigated through first-principles calculations. It is shown that the MoS2 monolayer reflects magnetism with a magnetic moment of 0.87 μB when co-doped with Mn-C. However, the systems co-doped with Mn-B and Mn-N atoms exhibit semiconducting behavior and their energy bandgaps are 1.03 and 0.81 eV, respectively. The bandgaps of the co-doped systems are smaller than those of the corresponding pristine forms, due to effective charge compensation between Mn and B (N) atoms. The optical properties of Mn-B (C, N) co-doped systems all reflect the redshift phenomenon. The absorption edge of the pure molybdenum disulfide monolayer is 0.8 eV, while the absorption edges of the Mn-B, Mn-C, and Mn-N co-doped systems become 0.45, 0.5, and 0 eV, respectively. As a potential material, MoS2 is widely used in many fields such as the production of optoelectronic devices, military devices, and civil devices.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

The remarkable properties of graphene have renewed interest in inorganic, two-dimensional materials with unique electronic and optical attributes. Transition metal dichalcogenides (TMDCs) are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness. Although TMDCs have been studied for decades, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS(2), MoSe(2), WS(2) and WSe(2) have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. We review the historical development of TMDCs, methods for preparing atomically thin layers, their electronic and optical properties, and prospects for future advances in electronics and optoelectronics.

Surface adatom conductance filtering in scanning tunneling spectroscopy of co-doped BaFe2As2 iron pnictide superconductors

We establish in a combination of ab initio theory and experiments that the tunneling process in scanning tunneling microscopy or spectroscopy on the A-122 iron pnictide superconductors-in this case BaFe(2-x)Co(x)As(2)-involves a strong adatom filtering of the differential conductance from the near-E(F) Fe-3d states, which in turn originates from the topmost subsurface Fe layer of the crystal. The calculations show that the dominance of surface Ba-related tunneling pathways leaves fingerprints found in the experimental differential conductance data, including large particle-hole asymmetry and energy-dependent contrast inversion in conductance maps.

Electric field effects on armchair MoS2 nanoribbons

Ab initio density functional theory calculations are performed to investigate the electronic structure of MoS(2) armchair nanoribbons in the presence of an external static electric field. Such nanoribbons, which are nonmagnetic and semiconducting, exhibit a set of weakly interacting edge states whose energy position determines the band gap of the system. We show that, by applying an external transverse electric field, E(ext), the nanoribbon band gap can be significantly reduced, leading to a metal-insulator transition beyond a certain critical value. Moreover, the presence of a sufficiently high density of states at the Fermi level in the vicinity of the metal-insulator transition leads to the onset of Stoner ferromagnetism that can be modulated, and even extinguished, by E(ext). In the case of bilayer nanoribbons we further show that the band gap can be changed from indirect to direct by applying a transverse field, an effect that might be of significance for opto-electronics applications.

Emerging photoluminescence in monolayer MoS2

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS(2), a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS(2) crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS(2) provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Magnetic molybdenum disulfide nanosheet films

Bulk molybdenum disulfide is known to be a nonmagnetic material. We have synthesized edge-oriented MoS2 nanosheet-like films that exhibit weak magnetism ( approximately 1-2 emu/g) and 2.5% magnetoresistance effects with a Curie temperature of 685 K. The magnetization is related to the presence of edge spins on the prismatic edges of the nanosheets. Spin-polarized calculations were performed on triangular-shaped cluster models in order to provide insight into the origin of magnetism on the edges as well as the size-property correlation in these MoS2 nanosheets. Our results imply that nanostructured films with a high density of edge spins can give rise to magnetism even though the bulk material is nonmagnetic.

Scanning tunneling microscopy chemical signature of point defects on the MoS2(0001) surface

Using ab initio calculations, we have studied the modification of the electronic structure of the MoS2(0001) surface by several point defects: a surface S vacancy and different transition metal atoms substituting a S atom (Pd, Au, Fe, and V). With a S vacancy, a gap state appears with weight mostly on the Mo and S atoms surrounding the vacancy. The substitutional atoms of complete d band (Pd and Au) do not present magnetic polarization and slightly modify the DOS near the Fermi energy. On the other hand, the incomplete d band atoms (Fe and V) present spin polarization and modify significantly the states near the band edges. From calculated STM images and STS curves, we show that this chemical signature can be measured and used to characterize the surface defects of the substrate which are suitable nucleation centers for nanocluster growth.