Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

MoS2 2D materials induce spinal cord neuroinflammation and neurotoxicity affecting locomotor performance in zebrafish

MoS2 nanosheets belong to an emerging family of nanomaterials named bidimensional transition metal dichalcogenides (2D TMDCs). The use of such promising materials, featuring outstanding chemical and physical properties, is expected to increase in several fields of science and technology, with an enhanced risk of environmental dispersion and associated wildlife and human exposures. In this framework, the assessment of MoS2 nanosheets toxicity is instrumental to safe industrial developments. Currently, the impact of the nanomaterial on the nervous tissue is unexplored. In this work, we use as in vivo experimental model the early-stage zebrafish, to investigate whether mechano-chemically exfoliated MoS2 nanosheets reach and affect, when added in the behavioral ambient, the nervous system. By high throughput screening of zebrafish larvae locomotor behavioral changes upon exposure to MoS2 nanosheets and whole organism live imaging of spinal neuronal and glial cell calcium activity, we report that sub-acute and prolonged ambient exposures to MoS2 nanosheets elicit locomotor abnormalities, dependent on dose and observation time. While 25 μg mL-1 concentration treatments exerted transient effects, 50 μg mL-1 ones induced long-lasting changes, correlated to neuroinflammation-driven alterations in the spinal cord, such as astrogliosis, glial intracellular calcium dysregulation, neuronal hyperactivity and motor axons retraction. By combining integrated technological approaches to zebrafish, we described that MoS2 2D nanomaterials can reach, upon water (i.e. ambient) exposure, the nervous system of larvae, resulting in a direct neurological damage.

Adsorption of Zn atoms by monolayer WS2 doped with different atoms X (X = O, Se, N, P, F, Cl): first principles study

CONTEXT

The effect of X (X = O, Se, N, P, F, Cl) doping on the adsorption of Zn atoms by WS2 was investigated based on first principles. The electronic structure and optical properties of the adsorbed system after atomic doping were calculated. It is found that the Zn atom adsorbed on the W top (Tw) site has the most stable structure. When an S atom is replaced with an X atom based on the adsorption system, where the adsorption energy decreases after doping of O, P, F, and Cl atoms compared to the undoped system, it means that each system is more stable after doping of these atoms; charge transfer shows that the adsorption system after P-atom doping the system around the Zn atom loses electrons while S-atom gains electrons, which indicates that P-atom doping is favorable for the adsorption of Zn by WS2, N, P-atom is introduced as p-type doping and F, Cl-atom is introduced undoped by n-type doping, and the band gap of the doped system is less than that of the undoped one. With the introduction of different dopant atoms, certain impurity energy levels are introduced into the adsorption system. The prohibited bandwidth around the Fermi energy level reduces the density of states, causing the doped system's density of states to shift to lower energies, among which the shifts of N, P, F, and Cl are more pronounced. The P-doped adsorption system shows a new peak near the energy of - 11 eV. In addition, the study of optical properties showed that the peak reflections of both doped and non-doped systems adsorbing Zn atoms appeared in the ultraviolet region; the absorbance coefficient of the doped system is moved in the lower energy direction and red-shifted after atom doping; in addition, the absorption coefficients and reflectance of the P, Se doped systems are enhanced in the wavelength range of 200-300 nm compared with that before doping, the dielectric function and CBM and VBM positions were also calculated further indicating the potential of Se-doped systems in improving photocatalytic efficiency.

METHODS

In this paper, the structure optimization of X (X = O, Se, N, P, F, Cl) doping on WS2 adsorbed Zn atom model is performed based on the CASTEP module in Materials-Studio software under the first principles using GGA and PBE generalized function. The corresponding binding energies, bond lengths, bond angles, charge densities, energy band structures, densities of states, and optical properties were also analyzed. The Monkhorst-Pack particular K-point sampling method is used in the calculations; the K-point grid is 6 × 6 × 1, and the cutoff energy for the plane wave expansion is 500 eV. After geometric optimization, the iterative accuracy converges to a value of less than 1 × 10-5 eV/atom for the total energy of each atom and less than 0.03 eV/Å for all atomic forces. The thickness of the vacuum layer was set to 20 Å to avoid the effect of interlayer interaction forces. In this paper, 27 atoms were used to form a 3 × 3 × 1 supercellular tungsten disulfide system consisting of 18 S atoms and 9 W atoms.

Recent Development on Transition Metal Oxides-Based Core-Shell Structures for Boosted Energy Density Supercapacitors

In recent years, nanomaterials exploration and synthesis have played a crucial role in advancing energy storage research, particularly in supercapacitor development. Researchers have diversified materials, including metal oxides, chalcogenides, and composites, as well as carbon materials, to enhance energy and power density. Balancing energy density with electrochemical stability remains challenging, driving intensified efforts in advancing electrode materials. This review focuses on recent progress in designing and synthesizing core-shell materials tailored for supercapacitors. The core-shell architecture offers advantages such as increased surface area, redox active sites, electrical conductivity, ion diffusion kinetics, specific capacitance, and cyclability. The review explores the impact of core and shell materials, specifically transition metal oxides (TMOs), on supercapacitor electrochemical behavior. Metal oxide choices, such as cobalt oxide as a preferred core and manganese oxide as a shell, are discussed. The review also highlights characterization techniques for assessing structural, morphological, and electrochemical properties of core-shell materials. Overall, it provides a comprehensive overview of ongoing TMOs-based core-shell material research for supercapacitors, showcasing their potential to enhance energy storage for applications ranging from gadgets to electric vehicles. The review outlines existing challenges and future opportunities in evolving TMOs-based core-shell materials for supercapacitor advancements, holding promise for high-efficiency energy storage devices.

Selective CW Laser Synthesis of MoS2 and Mixture of MoS2 and MoO2 from (NH4)2MoS4 Film

Very recently, the synthesis of 2D MoS2 and WS2 through pulsed laser-directed thermolysis can achieve wafer-scale and large-area structures, in ambient conditions. In this paper, we report the synthesis of MoS2 and MoS2 oxides from (NH4)2MoS4 film using a visible continuous-wave (CW) laser at 532 nm, instead of the infrared pulsed laser for the laser-directed thermolysis. The (NH4)2MoS4 film is prepared by dissolving its crystal powder in DI water, sonicating the solution, and dip-coating onto a glass slide. We observed a laser intensity threshold for the laser synthesis of MoS2, however, it occurred in a narrow laser intensity range. Above that range, a mixture of MoS2 and MoO2 is formed, which can be used for a memristor device, as demonstrated by other research groups. We did not observe a mixture of MoS2 and MoO3 in the laser thermolysis of (NH4)2MoS4. The laser synthesis of MoS2 in a line pattern is also achieved through laser scanning. Due to of the ease of CW beam steering and the fine control of laser intensities, this study can lead toward the CW laser-directed thermolysis of (NH4)2MoS4 film for the fast, non-vacuum, patternable, and wafer-scale synthesis of 2D MoS2.

Effect of O-doping on electronic and optical properties of monolayer MoSe2 under shear deformation

CONTEXT

In this study, the electronic structures and optical properties of the pure MoSe2 and O-doped MoSe2 systems under different shear deformations are calculated based on the first-principles approach. It is hoped to provide new possibilities for the design of novel controllable optoelectronic devices and to provide guidance for the application of MoSe2 in the field of optoelectronic devices. The findings indicate that both pure MoSe2 and O-doped MoSe2 systems are somewhat impacted by shear deformation. The pure MoSe2 undergoes a transition from direct to indirect and then to direct bandgap under shear deformation, but still maintains the semiconductor properties. The bandgap of the doped system changes from a direct to an indirect bandgap at 8% shear deformation. According to the examination of the density of states, we find that the density of states of the pure MoSe2 system is mainly contributed by the Mo-d and Se-p orbitals, and the total density of states of the system after O-atom doping mainly originates from the results of the contributions of the Mo-d, Se-p, and O-p orbitals. Optical property analysis reveals that the conductivity and peak value of the pure MoSe2 system are gradually red-shifted toward the low-energy region with the increase of shear deformation. The dielectric function of the O-doped MoSe2 system is red-shifted in the region of 6~10% shear deformation, and the degree of red-shift rises with deformation amount. The findings demonstrate that the electrical structure and optical characteristics of the O-doped MoSe2 system may be modulated effectively by shear deformation, providing a theoretical foundation for expanding the usage of MoSe2 materials in the field of optoelectronic devices.

METHODS

This study is founded on the CASTEP module in the Materials-Studio software within the first-principles of the density-functional theory framework. The photoelectric properties of the intrinsic and doped systems under shear deformation are calculated using the Perdew-Burke-Ernzerh (PBE) of generalized function under the generalized gradient approximation (GGA). The Monkhorst-Pack special K-point sampling method is used in the calculations, and a 5 × 5 × 1 K-point grid is used for the calculations with a plane-wave truncation energy of 400 eV in the optimization of the structure of each model. After geometrical optimization, the energy convergence criterion for each atom is 1 × 10-5 eV/atom, the force convergence criterion is 0.05 eV/Å, and a vacuum layer of 20 Å in the c-direction is set.

Two-Dimensional Nanoarchitectonics for Two-Dimensional Materials: Interfacial Engineering of Transition-Metal Dichalcogenides

Transition-metal dichalcogenides (TMDs) have attracted increasing attention in fundamental studies and technological applications owing to their atomically thin thickness, expanded interlayer distance, motif band gap, and phase-transition ability. Even though TMDs have a wide variety of material assets from semiconductor to semimetallic to metallic, the materials with fixed features may not show excellence for precise application. As a result of exclusive crystalline polymorphs, physical and chemical assets of TMDs can be efficiently modified via various approaches of interface nanoarchitectonics, including heteroatom doping, heterostructure, phase engineering, reducing size, alloying, and hybridization. With modified properties, TMDs become interesting materials in diverse fields, including catalysis, energy, electronics, transistors, and optoelectronics.

Large Mechanosensitive Thermoelectric Enhancement in Metallo-Organic Magnetic Molecules

Organic materials are promising candidates for thermoelectric cooling and energy harvesting at room temperature. However, their electrical conductance (G) and Seebeck coefficient (S) need to be improved to make them technologically competitive. Therefore, radically new strategies need to be developed to tune their thermoelectric properties. Here, we demonstrate that G and S can be tuned mechanically in paramagnetic metallocenes, and their thermoelectric properties can be significantly enhanced by the application of mechanical forces. With a 2% junction compression, the full thermoelectric figure of merit is enhanced by more than 200 times. We demonstrate that this is because spin transport resonances in paramagnetic metallocenes are strongly sensitive to the interaction between organic ligands and the metal center, which is not the case in their diamagnetic analogue. These results open a new avenue for the development of organic thermoelectric materials for cooling future quantum computers and generating electricity from low-grade energy sources.

Trends and prospects of 2-D tungsten disulphide (WS2) hybrid nanosystems for environmental and biomedical applications

Recently, 2D layered transition metal dichalcogenides (TMDCs) with their ultrathin sheet nanostructure and diversified electronic structure have drawn attention for various advanced applications to achieve high-performance parameters. Unique 2D TMDCs mainly comprise transition metal and chalcogen element where chalcogen element layers sandwich the transition metal element layer. In such a case, various properties can be enhanced and controlled depending on the targeted application. Among manipulative 2D TMDCs, tungsten disulphide (WS2) is one of the emerging nano-system due to its fascinating properties in terms of direct band gap, higher mobility, strong photoluminescence, good thermal stability, and strong magnetic field interaction. The advancement in characterization techniques, especially scattering techniques, can help in study of opto-electronic properties of 2D TMDCs along with determination of layer variations and investigation of defect. In this review, the fabrication and applications are well summarized to optimize an appropriate WS2-TMDCs assembly according to focused field of research. Here, the scientific investigations on 2D WS2 are studied in terms of its structure, role of scattering techniques to study its properties, and synthesis routes followed by its potential applications for environmental remediation (e.g., photocatalytic degradation of pollutants, gas sensing, and wastewater treatment) and biomedical domain (e.g., drug delivery, photothermal therapy, biomedical imaging, and biosensing). Further, a special emphasis is given to the significance of 2D WS2 as a substrate for surface-enhanced Raman scattering (SERS). The discussion is further extended to commercial and industrial aspects, keeping in view major research gaps in existing research studies.

From VIB- to VB-Group Transition Metal Disulfides: Structure Engineering Modulation for Superior Electromagnetic Wave Absorption

The laminated transition metal disulfides (TMDs), which are well known as typical two-dimensional (2D) semiconductive materials, possess a unique layered structure, leading to their wide-spread applications in various fields, such as catalysis, energy storage, sensing, etc. In recent years, a lot of research work on TMDs based functional materials in the fields of electromagnetic wave absorption (EMA) has been carried out. Therefore, it is of great significance to elaborate the influence of TMDs on EMA in time to speed up the application. In this review, recent advances in the development of electromagnetic wave (EMW) absorbers based on TMDs, ranging from the VIB group to the VB group are summarized. Their compositions, microstructures, electronic properties, and synthesis methods are presented in detail. Particularly, the modulation of structure engineering from the aspects of heterostructures, defects, morphologies and phases are systematically summarized, focusing on optimizing impedance matching and increasing dielectric and magnetic losses in the EMA materials with tunable EMW absorption performance. Milestones as well as the challenges are also identified to guide the design of new TMDs based dielectric EMA materials with high performance.

Phase Engineering of W-Doped MoS2 by Magneto-Hydrothermal Synthesis for Hydrogen Evolution Reaction

Molybdenum disulfide (MoS2 ) has been proved as an excellent potential hydrogen evolution reaction (HER) catalyst. Compared with thermodynamically stable 2H-MoS2 , 1T-MoS2 exhibits higher conductivity and catalytic activity, whereas it is usually difficult to prepare since of thermodynamically metastable. Herein, a feasible method is reported to fabricate ambient-stable MoS2 with high concentration 1T phase through magnetic free energy synergistic microstrain induced by W doping under low magnetic field. The 1T phase proportion in MoS2 can be as high as 80% and is ambient-stable for more than one year. The catalyst prepared under a magnetic field of 3 T delivers an overpotential of 195 mV at a current density of 10 mA cm-2 and has a long-term stability over 50 h. This work provides a novel strategy for preparation of MoS2 with high 1T concentration and high stability.

Unusual Defect-Related Room-Temperature Emission from WS2 Monolayers Synthesized through a Potassium-Based Precursor

Alkali-metal-based synthesis of transition metal dichalcogenide (TMD) monolayers is an established strategy for both ultralarge lateral growth and promoting the metastable 1T phase. However, whether this can also lead to modified optical properties is underexplored, with reported photoluminescence (PL) spectra from semiconducting systems showing little difference from more traditional syntheses. Here, we show that the growth of WS2 monolayers from a potassium-salt precursor can lead to a pronounced low-energy emission in the PL spectrum. This is seen 200-300 meV below the A exciton and can dominate the signal at room temperature. The emission is spatially heterogeneous, and its presence is attributed to defects in the layer due to sublinear intensity power dependence, a noticeable aging effect, and insensitivity to washing in water and acetone. Interestingly, statistical analysis links the band to an increase in the width of the A1g Raman band. The emission can be controlled by altering when hydrogen is introduced into the growth process. This work demonstrates intrinsic and intense defect-related emission at room temperature and establishes further opportunities for tuning TMD properties through alkali-metal precursors.

Liquid Shear Exfoliation of MoS2: Preparation, Characterization, and NO2-Sensing Properties

2D materials possess great potential to serve as gas-sensing materials due to their large, specific surface areas and strong surface activities. Among this family, transition metal chalcogenide materials exhibit different properties and are promising candidates for a wide range of applications, including sensors, photodetectors, energy conversion, and energy storage. Herein, a high-shear mixing method has been used to produce multilayered MoS2 nanosheet dispersions. MoS2 thin films were manufactured by vacuum-assisted filtration. The structural morphology of MoS2 was studied using ς-potential, UV-visible, scanning electron microscopy (SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). The spectroscopic and microscopic analyses confirm the formation of a high-crystalline MoS2 thin film with good inter-sheet connectivity and relative thickness uniformity. The thickness of the MoS2 layer is measured to be approximately 250 nm, with a nanosheet size of 120 nm ± 40 nm and a number of layers between 6 and 9 layers. Moreover, the electrical characteristics clearly showed that the MoS2 thin film exhibits good conductivity and a linear I-V curve response, indicating good ohmic contact between the MoS2 film and the electrodes. As an example of applicability, we fabricated chemiresistive sensor devices with a MoS2 film as a sensing layer. The performance of the MoS2-chemiresistive sensor for NO2 was assessed by being exposed to different concentrations of NO2 (1 ppm to 10 ppm). This sensor shows a sensibility to low concentrations of 1 ppm, with a response time of 114 s and a recovery time of 420 s. The effect of thin-film thickness and operating temperatures on sensor response was studied. The results show that thinner film exhibits a higher response to NO2; the response decreases as the working temperature increases.

Molybdenum Disulfide as Tunable Electrochemical and Optical Biosensing Platforms for Cancer Biomarker Detection: A Review

Cancer is a common illness with a high mortality. Compared with traditional technologies, biomarker detection, with its low cost and simple operation, has a higher sensitivity and faster speed in the early screening and prognosis of cancer. Therefore, extensive research has focused on the development of biosensors and the construction of sensing interfaces. Molybdenum disulfide (MoS2) is a promising two-dimensional (2D) nanomaterial, whose unique adjustable bandgap shows excellent electronic and optical properties in the construction of biosensor interfaces. It not only has the advantages of a high catalytic activity and low manufacturing costs, but it can also further expand the application of hybrid structures through different functionalization, and it is widely used in various biosensors fields. Herein, we provide a detailed introduction to the structure and synthesis methods of MoS2, and explore the unique properties and advantages/disadvantages exhibited by different structures. Specifically, we focus on the excellent properties and application performance of MoS2 and its composite structures, and discuss the widespread application of MoS2 in cancer biomarkers detection from both electrochemical and optical dimensions. Additionally, with the cross development of emerging technologies, we have also expanded the application of other emerging sensors based on MoS2 for early cancer diagnosis. Finally, we summarized the challenges and prospects of MoS2 in the synthesis, functionalization of composite groups, and applications, and provided some insights into the potential applications of these emerging nanomaterials in a wider range of fields.

Morphology Transition with Temperature and its Effect on Optical Properties of Colloidal MoS2 Nanostructures

Morphology plays a crucial role in determining the chemical and optical properties of nanomaterials due to confinement effects. We report the morphology transition of colloidal molybdenum disulfide (MoS2) nanostructures, synthesized by a one-pot heat-up method, from a mix of quantum dots (QDs) and nanosheets to predominantly nanorods by varying the synthesis reaction temperature from 90 to 160 °C. The stoichiometry and composition of the synthesized QDs, nanosheets, and nanorods were quantified to be MoS2 using energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. A nanostructure morphology transition due to variation in the reaction temperature resulted in a photoluminescence quantum yield enhancement from 0 to 4.4% on increasing the temperature from 90 to 120 °C. On further increase in the temperature to 160 °C, a decrease in the quantum yield to 3.06% is observed. Red-shifts of ≈18 and ≈140 nm in the emission maxima and absorption edge, respectively, are observed for the synthesized nanostructures with an increase in the reaction temperature from 90 to 160 °C. The change in the quantum yield is attributed to the change in shape and hence confinement of charge carriers. To the best of our knowledge, microscopic analysis of variation in shape and optical properties of colloidal MoS2 nanostructures with temperature, explained by a nonclassical growth mechanism is presented here for the first time.

Toolbox of Advanced Atomic Layer Deposition Processes for Tailoring Large-Area MoS2 Thin Films at 150 °C

Two-dimensional MoS₂ is a promising material for applications, including electronics and electrocatalysis. However, scalable methods capable of depositing MoS₂ at low temperatures are scarce. Herein, we present a toolbox of advanced plasma-enhanced atomic layer deposition (ALD) processes, producing wafer-scale polycrystalline MoS₂ films of accurately controlled thickness. Our ALD processes are based on two individually controlled plasma exposures, one optimized for deposition and the other for modification. In this way, film properties can be tailored toward different applications at a very low deposition temperature of 150 °C. For the modification step, either H₂ or Ar plasma can be used to combat excess sulfur incorporation and crystallize the films. Using H₂ plasma, a higher degree of crystallinity compared with other reported low-temperature processes is achieved. Applying H₂ plasma steps periodically instead of every ALD cycle allows for control of the morphology and enables deposition of smooth, polycrystalline MoS₂ films. Using an Ar plasma instead, more disordered MoS₂ films are deposited, which show promise for the electrochemical hydrogen evolution reaction. For electronics, our processes enable control of the carrier density from 6 × 10¹⁶ to 2 × 10²¹ cm^-3 with Hall mobilities up to 0.3 cm² V^-1 s^-1. The process toolbox forms a basis for rational design of low-temperature transition metal dichalcogenide deposition processes compatible with a range of substrates and applications.

Impedimetric immunosensor for microalbuminuria based on a WS2/Au water-phase assembled nanocomposite

An electrochemical impedimetric biosensor for human serum albumin (HSA) determination is proposed. The biosensor is based on water-phase assembled nanocomposites made of 2D WS₂ nanoflakes and Au nanoparticles (AuNPs). The WS₂ has been produced using a liquid-phase exfoliation strategy assisted by sodium cholate, obtaining a water-stable suspension that allowed the straightforward decoration with AuNPs directly in the aqueous phase. The resulting WS₂/Au nanocomposite has been characterized by atomic force microscopy and Raman spectroscopy and, then, employed to modify screen-printed electrodes. Good electron-transfer features have been achieved. An electrochemical immunosensing platform has been assembled exploiting cysteamine-glutaraldehyde covalent chemistry for antibody (Ab) immobilization. The resulting immunosensor exhibited good sensitivity for HSA detection (LOD = 2 ng mL^-1), with extended linear range (0.005 - 100 µg mL^-1), providing a useful analytical tool for HSA determination in urine at relevant clinical ranges for microalbuminuria screening. The HSA quantification in human urine samples resulted in recoveries from 91.8 to 112.4% and was also reproducible (RSD < 7.5%, n = 3), with marked selectivity. This nanocomposite, thanks to the reliable performance and the ease of the assembling strategy, is a promising alternative for electrochemical immunosensing of health relevant markers.

An open database of computed bulk ternary transition metal dichalcogenides

We present a dataset of structural relaxations of bulk ternary transition metal dichalcogenides (TMDs) computed via plane-wave density functional theory (DFT). We examined combinations of up to two chalcogenides with seven transition metals from groups 4-6 in octahedral (1T) or trigonal prismatic (2H) coordination. The full dataset consists of 672 unique stoichiometries, with a total of 50,337 individual configurations generated during structural relaxation. Our motivations for building this dataset are (1) to develop a training set for the generation of machine and deep learning models and (2) to obtain structural minima over a range of stoichiometries to support future electronic analyses. We provide the dataset as individual VASP xml files as well as all configurations encountered during relaxations collated into an ASE database with the corresponding total energy and atomic forces. In this report, we discuss the dataset in more detail and highlight interesting structural and electronic features of the relaxed structures.

Freestanding laser-induced two dimensional heterostructures for self-contained paper-based sensors

The production of 2D/2D heterostructures (HTs) with favorable electrochemical features is challenging, particularly for semiconductor transition metal dichalcogenides (TMDs). In this studies, we introduce a CO₂ laser plotter-based technology for the realization of HT films comprising reduced graphene oxide (rGO) and 2D-TMDs (MoS₂, WS₂, MoSe₂, and WSe₂) produced via water phase exfoliation. The strategy relies on the Laser-Induced production of HeterosTructures (LIHTs), where after irradiation the nanomaterials exhibit changes in the morphological and chemical structure, becoming conductive easily transferable nanostructured films. The LIHTs were characterized in detail by SEM, XPS, Raman and electrochemical analysis. The laser treatment induces the conversion of GO into conductive highly exfoliated rGO decorated with homogeneously distributed small TMD/TM-oxide nanoflakes. The freestanding LIHT films obtained were employed to build self-contained sensors onto nitrocellulose, where the HT works both as a transducer and sensing surface. The proposed nitrocellulose-sensor manufacturing process is semi-automated and reproducible, multiple HT films may be produced in the same laser treatment and the stencil-printing allows customizable design. Excellent performance in the electroanalytical detection of different molecules such as dopamine (a neurotransmitter), catechin (a flavonol), and hydrogen peroxide was demonstrated, obtaining nanomolar limits of detection and satisfactory recovery rates in biological and agrifood samples, together with high fouling resistance. Considering the robust and rapid laser-induced production of HTs and the versatility of scribing desired patterns, the proposed approach appears as a disruptive technology for the development of electrochemical devices through sustainable and accessible strategies.

Revealing the Polaron State at the MoS2/TiO2 Interface

Interfacial polarons determine the distribution of free charges at the interface and thus play important roles in manipulating the physicochemical properties of hybridized polaronic materials. In this work, we investigated the electronic structures at the atomically flat interface of the single-layer MoS₂ (SL-MoS₂) on the rutile TiO₂ surface using high-resolution angle-resolved photoemission spectroscopy. Our experiments directly visualized both the valence band maximum and the conduction band minimum (CBM) of SL-MoS₂ at the K point, which clearly defines a direct bandgap of ∼2.0 eV. Detailed analyses corroborated by density functional theory calculations demonstrated that the CBM of MoS₂ is formed by the trapped electrons at the MoS₂/TiO₂ interface that couple with the longitudinal optical phonons in the TiO₂ substrate through an interfacial Fröhlich polaron state. Such an interfacial coupling effect may register a new route for tuning the free charges in the hybridized systems of two-dimensional materials and functional metal oxides.

Redox exfoliated NbS2: characterization, stability, and oxidation

Niobium disulfide is a layered transition metal dichalcogenide that is being exploited as a two-dimensional material. Although it is a superconductor at low temperatures and demonstrates great potential to be applied as a catalyst or co-catalyst in hydrogen evolution reactions, only a few reports have demonstrated the synthesis of a few-layer NbS₂. However, before applications can be pursued, it is essential to understand the main characteristics of the obtained material and its stability under an atmospheric environment. In this work, we conducted a thorough characterization of redox-exfoliated NbS₂ nanoflakes regarding their structure and stability in an oxygen-rich environment. Structural, morphological, and spectroscopic characterization demonstrated different fingerprints associated with distinct oxidation processes. This led us to identify oxide species and analyse the stability of the redox exfoliated NbS₂ nanosheets in air, suggesting the most likely reaction pathways during the NbS₂ interaction with oxygen, which agrees with our density-functional theory results. The mastery over the stability of layered materials is of paramount importance to target future applications, mainly because the electronic properties of these materials are strongly affected by an oxidizing environment.

A universal growth method for high-quality phase-engineered germanium chalcogenide nanosheets

Low-dimensional group IV-VI metal chalcogenide-based semiconductors hold great promise for opto-electronic device applications owing to their diverse crystalline phases and intriguing properties related to thermoelectric and ferroelectric effects. Herein, we demonstrate a universal chemical vapor deposition (CVD) growth method to synthesize stable germanium chalcogenide-based (GeS, GeS₂, GeSe, GeSe₂) nanosheets, which increases the library of the p-type semiconductor. The phase transition between different crystalline polytypes can be deterministically controlled by hydrogen concentration in the reaction chamber. Structural characterization and synthesis experiments identify the behavior, where the higher hydrogen concentration promotes the transiton from germanium dichalcogenides to germanium monochalcogenides. The angle-polarized and temperature-dependent Raman spectra demonstrate the strong interlayer coupling and lattice orientation. Based on the optimized growth scheme and systematic comparison of electrical properties, GeSe nanosheet photodetectors were demonstrated, which exhibit superior device performance on SiO₂/Si and HfO₂/Si substrate with a high photoresponsivity up to 10⁴ A W^-1, fast response time less than 15 ms, and high mobility of 3.2 cm² V^-1 s^-1, which is comparable to the mechanically exfoliated crystals. Our results manifest the hydrogen-mediated deposition strategy as a facile control knob to engineer crystalline phases of germanium chalcogenides for high performance optoelectronic devices.

Catechin versus MoS2 Nanoflakes Functionalized with Catechin: Improving the Sperm Fertilizing Ability-An In Vitro Study in a Swine Model

Nowadays, the adoption of In Vitro Fertilization (IVF) techniques is undergoing an impressive increase. In light of this, one of the most promising strategies is the novel use of non-physiological materials and naturally derived compounds for advanced sperm preparation methods. Here, sperm cells were exposed during capacitation to MoS₂/Catechin nanoflakes and catechin (CT), a flavonoid with antioxidant properties, at concentrations of 10, 1, 0.1 ppm. The results showed no significant differences in terms of sperm membrane modifications or biochemical pathways among the groups, allowing the hypothesis that MoS₂/CT nanoflakes do not induce any negative effect on the parameters evaluated related to sperm capacitation. Moreover, the addition of CT alone at a specific concentration (0.1 ppm) increased the spermatozoa fertilizing ability in an IVF assay by increasing the number of fertilized oocytes with respect to the control group. Our findings open interesting new perspectives regarding the use of catechins and new materials obtained using natural or bio compounds, which could be used to implement the current strategies for sperm capacitation.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Understanding the Photoluminescence Quenching of Liquid Exfoliated WS2 Monolayers

Monolayer transition metal dichalcogenides (TMDs) are being investigated as active materials in optoelectronic devices due to their strong excitonic effects. While mechanical exfoliation (ME) of monolayer TMDs is limited to small areas, these materials can also be exfoliated from their parent layered materials via high-volume liquid phase exfoliation (LPE). However, it is currently considered that LPE-synthesized materials show poor optoelectronic performance compared to ME materials, such as poor photoluminescence quantum efficiencies (PLQEs). Here we evaluate the photophysical properties of monolayer-enriched LPE WS₂ dispersions via steady-state and time-resolved optical spectroscopy and benchmark these materials against untreated and chemically treated ME WS₂ monolayers. We show that the LPE materials show features of high-quality semiconducting materials such as very small Stokes shift, smaller photoluminescence line widths, and longer exciton lifetimes than ME WS₂. We reveal that the energy transfer between the direct-gap monolayers and in-direct gap few-layers in LPE WS₂ dispersions is a major reason for their quenched PL. Our results suggest that LPE TMDs are not inherently highly defective and could have a high potential for optoelectronic device applications if improved strategies to purify the LPE materials and reduce aggregation could be implemented.

Synthesis of mono- and few-layered n-type WSe2 from solid state inorganic precursors

Tuning the charge transport properties of two-dimensional transition metal dichalcogenides (TMDs) is pivotal to their future device integration in post-silicon technologies. To date, co-doping of TMDs during growth still proves to be challenging, and the synthesis of doped WSe2, an otherwise ambipolar material, has been mainly limited to p-doping. Here, we demonstrate the synthesis of high-quality n-type monolayered WSe2 flakes using a solid-state precursor for Se, zinc selenide. n-Type transport has been reported with prime electron mobilities of up to 10 cm2 V-1 s-1. We also demonstrate the tuneability of doping to p-type transport with hole mobilities of 50 cm2 V-1 s-1 after annealing in air. n-Doping has been attributed to the presence of Zn adatoms on the WSe2 flakes as revealed by X-ray photoelectron spectroscopy (XPS), spatially resolved time of flight secondary ion mass spectroscopy (SIMS) and angular dark-field scanning transmission electron microscopy (AD-STEM) characterization of WSe2 flakes. Monolayer WSe2 flakes exhibit a sharp photoluminescence (PL) peak at room temperature and highly uniform emission across the entire flake area, indicating a high degree of crystallinity of the material. This work provides new insight into the synthesis of TMDs with charge carrier control, to pave the way towards post-silicon electronics.

Efficient Optical Modulation of Exciton State Population in Monolayer MoS2 at Room Temperature

The modulation of exciton energy and state density of layer-structured transition metal dichalcogenides (TMDs) is required for diverse optoelectronic device applications. Here, the spontaneous inversion of exciton state population in monolayer MoS₂ is observed by turning the pump light power. The excitons prefer to exist in low energy state under low pump power, but reverse under high pump power. To discuss the mechanism in depth, we propose a semiclassical model by combining the rate equation and photo-exciton interaction. Considering the modifying of exciton-exciton annihilation, the spontaneous inversion of exciton state population is phenomenologically described.

Atomic Layer Deposition of Large-Area Polycrystalline Transition Metal Dichalcogenides from 100 °C through Control of Plasma Chemistry

Two-dimensional transition metal dichalcogenides, such as MoS₂, are intensely studied for applications in electronics. However, the difficulty of depositing large-area films of sufficient quality under application-relevant conditions remains a major challenge. Herein, we demonstrate deposition of polycrystalline, wafer-scale MoS₂, TiS₂, and WS₂ films of controlled thickness at record-low temperatures down to 100 °C using plasma-enhanced atomic layer deposition. We show that preventing excess sulfur incorporation from H₂S-based plasma is the key to deposition of crystalline films, which can be achieved by adding H₂ to the plasma feed gas. Film composition, crystallinity, growth, morphology, and electrical properties of MoS _x films prepared within a broad range of deposition conditions have been systematically characterized. Film characteristics are correlated with results of field-effect transistors based on MoS₂ films deposited at 100 °C. The capability to deposit MoS₂ on poly(ethylene terephthalate) substrates showcases the potential of our process for flexible devices. Furthermore, the composition control achieved by tailoring plasma chemistry is relevant for all low-temperature plasma-enhanced deposition processes of metal chalcogenides.

GaN/MgI2 van der Waals heterostructure: a two-factor tunable photocatalyst for hydrogen evolution

With the increasing environmental pollution and energy crisis, it is significant to develop environmentally friendly and adjustable photocatalysts for water splitting. Here we explored the optoelectronic properties of several H-GaN/MgI₂ vdW heterostructures by regulating different polarization surfaces and numbers of GaN layers. Our results demonstrate that all structures, except 2L-Ga-GaN/MgI₂, exhibit excellent physical stability. Moreover, the band structures and band edge positions demonstrate that only the heterostructure of 3L-Ga-GaN/MgI₂ with both suitable band alignment (type-II) and an acceptable band gap (∼1.92 eV) is most satisfactory for water splitting. Additionally, the absorption coefficient of the 3L-Ga-GaN/MgI₂ heterostructure can reach over ∼10⁵ cm^-1, which has further confirmed its excellent advantage in photocatalysis. Finally, in the case of 6% external strain for the 3L-Ga-GaN/MgI₂ heterostructure, a rollover in band alignment (from type-II to type-I) is exhibited. These promising features of the GaN/MgI₂ vdW heterostructure give a new paradigm for developing novel efficient and adjustable photocatalytic water-splitting materials.

Constructing MoSe2/MoS2 and MoS2/MoSe2 inner and outer-interchangeable flower-like heterojunctions: A combined strategy of interface polarization and morphology configuration to optimize microwave absorption performance

Interfacial polarization and geometrical morphology play a significant role in the attenuation of electromagnetic (EM) wave. Herein, the two-dimensional (2D)/2D heterojunction with flower-like geometrical morphology is proposed and produced, which may simultaneously provide a large contact area for achieving strong interfacial polarization and activates more sites for the possible multiple EM wave reflection and scattering. By adopting a simple two-step hydrothermal method, MoSe₂/MoS₂and MoS₂/MoSe₂ inner and outer-interchangeable heterojunctions consisting of 2D MoSe₂ and MoS₂ nanosheets with flower-like geometrical morphology were successfully synthesized. The results revealed that the hydrothermal temperatures significantly impacted the flower-like geometrical morphology and MoS₂ content. By optimizing the microstructures, the designed MoSe₂/MoS₂ and MoS₂/MoSe₂ heterojunctions presented enhanced comprehensive EM wave absorption properties (EMWAPs), possessing strong absorption capability, wide absorption bandwidth and thin matching thicknesses. Generally, this work demonstrates that the optimized EMWAPs of designed heterojunctions mainly originate from the special interfaces and morphology configuration, which also paves a new way for the designing and synthesis of transition metal dichalcogenides-based heterojunction as a novel and desirable candidate for high-performance microwave absorbers.

Enhanced Optical Response of Zinc-Doped Tin Disulfide Layered Crystals Grown with the Chemical Vapor Transport Method

Tin disulfide (SnS₂) is a promising semiconductor for use in nanoelectronics and optoelectronics. Doping plays an essential role in SnS₂ applications, because it can increase the functionality of SnS₂ by tuning its original properties. In this study, the effect of zinc (Zn) doping on the photoelectric characteristics of SnS₂ crystals was explored. The chemical vapor transport method was adopted to grow pristine and Zn-doped SnS₂ crystals. Scanning electron microscopy images indicated that the grown SnS₂ crystals were layered materials. The ratio of the normalized photocurrent of the Zn-doped specimen to that of the pristine specimen increased with an increasing illumination frequency, reaching approximately five at 10⁴ Hz. Time-resolved photocurrent measurements revealed that the Zn-doped specimen had shorter rise and fall times and a higher current amplitude than the pristine specimen. The photoresponsivity of the specimens increased with an increasing bias voltage or decreasing laser power. The Zn-doped SnS₂ crystals had 7.18 and 3.44 times higher photoresponsivity, respectively, than the pristine crystals at a bias voltage of 20 V and a laser power of 4 × 10⁻⁸ W. The experimental results of this study indicate that Zn doping markedly enhances the optical response of SnS₂ layered crystals.

Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope

We present the design, implementation, and illustrative results of a light collection/injection strategy based on an off-axis parabolic mirror collector for a low-temperature Scanning Tunneling Microscope (STM). This device allows us to perform STM induced Light Emission (STM-LE) and Cathodoluminescence (STM-CL) experiments and in situ Photoluminescence (PL) and Raman spectroscopy as complementary techniques. Considering the Étendue conservation and using an off-axis parabolic mirror, it is possible to design a light collection and injection system that displays 72% of collection efficiency (considering the hemisphere above the sample surface) while maintaining high spectral resolution and minimizing signal loss. The performance of the STM is tested by atomically resolved images and scanning tunneling spectroscopy results on standard sample surfaces. The capabilities of our system are demonstrated by performing STM-LE on metallic surfaces and two-dimensional semiconducting samples, observing both plasmonic and excitonic emissions. In addition, we carried out in situ PL measurements on semiconducting monolayers and quantum dots and in situ Raman on graphite and hexagonal boron nitride (h-BN) samples. Additionally, STM-CL and PL were obtained on monolayer h-BN gathering luminescence spectra that are typically associated with intragap states related to carbon defects. The results show that the flexible and efficient light injection and collection device based on an off-axis parabolic mirror is a powerful tool to study several types of nanostructures with multiple spectroscopic techniques in correlation with their morphology at the atomic scale and electronic structure.

Optical Response of CVD-Grown ML-WS2 Flakes on an Ultra-Dense Au NP Plasmonic Array

The combination of metallic nanostructures with two-dimensional transition metal dichalcogenides is an efficient way to make the optical properties of the latter more appealing for opto-electronic applications. In this work, we investigate the optical properties of monolayer WS2 flakes grown by chemical vapour deposition and transferred onto a densely-packed array of plasmonic Au nanoparticles (NPs). The optical response was measured as a function of the thickness of a dielectric spacer intercalated between the two materials and of the system temperature, in the 75–350 K range. We show that a weak interaction is established between WS2 and Au NPs, leading to temperature- and spacer-thickness-dependent coupling between the localized surface plasmon resonance of Au NPs and the WS2 exciton. We suggest that the closely-packed morphology of the plasmonic array promotes a high confinement of the electromagnetic field in regions inaccessible by the WS2 deposited on top. This allows the achievement of direct contact between WS2 and Au while preserving a strong connotation of the properties of the two materials also in the hybrid system.

Atomistic insights into highly active reconstructed edges of monolayer 2H-WSe2 photocatalyst

Ascertaining the function of in-plane intrinsic defects and edge atoms is necessary for developing efficient low-dimensional photocatalysts. We report the wireless photocatalytic CO₂ reduction to CH₄ over reconstructed edge atoms of monolayer 2H-WSe₂ artificial leaves. Our first-principles calculations demonstrate that reconstructed and imperfect edge configurations enable CO₂ binding to form linear and bent molecules. Experimental results show that the solar-to-fuel quantum efficiency is a reciprocal function of the flake size. It also indicates that the consumed electron rate per edge atom is two orders of magnitude larger than the in-plane intrinsic defects. Further, nanoscale redox mapping at the monolayer WSe₂–liquid interface confirms that the edge is the most preferred region for charge transfer. Our results pave the way for designing a new class of monolayer transition metal dichalcogenides with reconstructed edges as a non-precious co-catalyst for wired or wireless hydrogen evolution or CO₂ reduction reactions.

Systematically study of in-plane intrinsic defects and edge atoms is important to guide the design of low-dimensional photocatalysts. Here the authors investigate photocatalytic CO₂ reduction to CH₄ over reconstructed edge atoms of monolayer semiconducting WSe₂.

Atomic Layer Deposition of Ultrathin La2O3/Al2O3 Nanolaminates on MoS2 with Ultraviolet Ozone Treatment

Due to the chemically inert surface of MoS2, uniform deposition of ultrathin high-κ dielectric using atomic layer deposition (ALD) is difficult. However, this is crucial for the fabrication of field-effect transistors (FETs). In this work, the atomic layer deposition growth of sub-5 nm La2O3/Al2O3 nanolaminates on MoS2 using different oxidants (H2O and O3) was investigated. To improve the deposition, the effects of ultraviolet ozone treatment on MoS2 surface are also evaluated. It is found that the physical properties and electrical characteristics of La2O3/Al2O3 nanolaminates change greatly for different oxidants and treatment processes. These changes are found to be associated with the residual of metal carbide caused by the insufficient interface reactions. Ultraviolet ozone pretreatment can substantially improve the initial growth of sub-5 nm H2O-based or O3-based La2O3/Al2O3 nanolaminates, resulting in a reduction of residual metal carbide. All results indicate that O3-based La2O3/Al2O3 nanolaminates on MoS2 with ultraviolet ozone treatment yielded good electrical performance with low leakage current and no leakage dot, revealing a straightforward approach for realizing sub-5 nm uniform La2O3/Al2O3 nanolaminates on MoS2.

Recent progress on transition metal diselenides from formation and modification to applications

The development of graphene promotes the research of similar two-dimensional (2D) materials, especially 2D transition metal dichalcogenides (TMDCs) with semiconductor properties. Monolayer or few-layer TMDCs have several advantages, such as direct band gap, weak interlayer van der Waals force, large interlayer spacing, and abundant marginal active sites, which make them widely used in catalysis, optoelectronics, as well as energy conversion and storage devices. In addition, transition metal diselenides (TMDSs) also possess many intriguing characteristics. For instance, transition metal diselenides (e.g., MoSe₂) have a more stable 1T phase, larger interlayer spacing, smaller band gap, and more obvious metallic property of Se than TMDCs (e.g., MoS₂). Thus, it has become one of the most attractive research topics branching out from TMDCs. Herein, this review unveils the structures, synthesis, properties, modifications, applications, and perspectives for TMDSs.

Influence of Mn, Fe, Co, and Cu Doping on the Photoelectric Properties of 1T HfS2 Crystals

Doping plays a vital role in the application of transition-metal dichalcogenides (TMDCs) because it can increase the functionality of TMDCs by tuning their native characteristics. In this study, the influence of Mn, Fe, Co, and Cu doping on the photoelectric properties of HfS2 was investigated. Pristine, Mn-, Fe-, Co-, and Cu-doped HfS2 crystals were grown using the chemical vapor transport method. Scanning electron microscopy images showed that the crystals were layered and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy measurements confirmed that the crystals were in the 1T-phase with a CdI2-like structure. The bandgap of pristine HfS2 obtained from the absorption and photoconductivity spectra was approximately 1.99 eV. As the dopant changed from Mn, Fe, and Co, to Cu, the bandgap gradually increased. The activation energies of the samples were determined using temperature-dependent current-voltage curves. After doping, the activation energy decreased, and the Co-doped HfS2 exhibited the smallest activation energy. Time-resolved photoresponse measurements showed that doping improved the response of HfS2 to light; the Co-doped HfS2 exhibited the best response. The photoresponsivity of HfS2 as a function of the laser power and bias voltage was measured. After doping, the photoresponsivity increased markedly; the Co-doped HfS2 exhibited the highest photoresponsivity. All the experimental results indicated that doping with Mn, Fe, Co, and Cu significantly improved the photoresponsive performance of HfS2, of which Co-doped HfS2 had the best performance.

Construction of a Au@MoS2 composite nanosheet biosensor for the ultrasensitive detection of a neurotransmitter and understanding of its mechanism based on DFT calculations

MoS₂ nanosheets can be applied as electrochemical biosensors to selectively and sensitively respond to the surrounding environment and detect various biomolecules due to their large specific surface area and unique physicochemical properties. In this paper, single-layer or few-layer MoS₂ nanosheets were prepared by an improved liquid phase stripping method, and then combining the unique material characteristics of MoS₂ and the metallic property of Au nanoparticles (AuNPs), Au@MoS₂ composite nanosheets were synthesized based on MoS₂ nanosheets. Then, the structure and properties of MoS₂ nanosheets and Au@MoS₂ composite nanosheets were comprehensively characterized. The results proved that AuNPs were successfully loaded on MoS₂ nanosheets. At the same time, on the basis of the successful preparation of Au@MoS₂ composite nanosheets, an electrochemical biosensor targeting dopamine was successfully constructed by cyclic voltammetry. The linear detection range was 0.5–350 μM, and the detection limit was 0.2 μM. The high-sensitive electrochemical detection of dopamine has been achieved, which provides a new idea for the application of MoS₂-based nanomaterials in the biosensing of neurotransmitters. In addition, density functional theory (DFT) was used to explore the electrochemical performance of Au@MoS₂ composite nanosheets. The results show that the adsorption of Au atoms on the MoS₂ 2D structure improves the conductivity of MoS₂ nanosheets, which theoretically supports the possibilities of its application as a platform for the ultrasensitive detection of neurotransmitters or other biomolecules in the field of disease diagnosis.

An electrochemical biosensor based on Au@MoS₂ composite nanosheets was successfully prepared for the high-sensitivity detection of dopamine.

In-situ fabrication of few-layered MoS2 wrapped on TiO2-decorated MXene as anode material for durable lithium-ion storage

Rational construction of hybrid materials integrating the collective virtues of individual building blocks has spurred significant interest in electrode materials for energy storage. Herein, a smart hybrid was fabricated via in-situ assembling of the few-layered MoS₂ (f-MoS₂) coated on the multi-layered Ti₃C₂ MXene decorated with the TiO₂ nanoparticles by the scalable hydrothermal and annealing approaches. In the unique architecture, the multi-layered Ti₃C₂ with the expanded interspaces as the conductive backbone can facilitate the electron transport, provide adequate space to facilitate the infiltration of organic electrolyte into the interior of electrode, and inhibit the aggregation of MoS₂ nanosheets, while the f-MoS₂ with enlarged interlayer can be beneficial for the lithium-ion diffusion and prevent the multi-layered Ti₃C₂from restacking. Moreover, the TiO₂ decorated on the Ti₃C₂ can effectively inhibit the instability of long-chain lithium polysulfides dissolved in organic electrolyte to improve the cycling stability. Thanks to the synergistic effect of the building blocks, the Ti₃C₂/TiO₂@f-MoS₂ hybrid employed as lithium storage anode delivers an extraordinary endurable ability with a high storage capacity of 403.1 mA h g^-1 after 1200 cycles at 2 A g^-1. Importantly, the smart hybridization strategy in this work paves an efficient way to explore the high-performance MXene-based hybrid materials in energy storage fields.

Impact of graphitic carbon nitrides synthesized from different precursors on Schottky junction characteristics

Graphitic carbon nitride (g-CN) has gained wide interest in many areas, such as energy and the environmental remediation as a layered polymeric semiconductor that allows the formation of catalytically active Schottky junctions due to its proper electronic band structure. Interestingly, although it is known that the precursors used in the synthesis, can influence the properties of the g-CN, no detailed study on these effects on Schottky junctions could be found in the literature. In this research, the effects of g-CNs synthesized by thermal polycondensation of different precursors on the photocatalytic efficiency of Schottky junctions were investigated. For this purpose, urea, thiourea, melamine, and guanidine hydrochloride were used as different precursors, while the photocatalytic dehydrogenation of formic acid was used as a test reaction. The Schottky junctions were formed by decorating the as-prepared g-CNs with AgPd alloy nanoparticles (NP), which were synthesized by reduction of Ag and Pd salts with NaBH₄. The structural, electronic and charge carrier dynamics of all prepared structures have been fully characterized by TEM, XRD, BET, XPS, UV-Vis DRS, PL, and PL life measurements. The results showed that the charge transfer dynamics of g-CNs surface defects are more effective in the photocatalytic performance of Schottky junctions than in structural features such as the size of the metal NPs or the surface area of the catalysts.

MoS2 flake as a van der Waals homostructure: luminescence properties and optical anisotropy

We investigated multilayer plates prepared by exfoliation from a high-quality MoS₂ crystal and revealed that they represent a new object - a van der Waals homostructure consisting of a bulk core and a few detached monolayers on its surface. This architecture comprising elements with different electron band structures leads to specific luminescence, when the broad emission band from the core is cut by the absorption peaks of strong exciton resonances in the surface monolayers. The exfoliated flakes exhibit strong optical anisotropy. We have observed linear to circular polarization conversion that reaches 15% for normally incident light in transmission geometry. This background effect is due to the fluctuations of the c axis relative to the normal, whereas the pronounced resonance contribution is explained by the polarization anisotropy of the excitons localized in the stripes of the dissected surface monolayers.

A comparative study on the electrochemical capacitor performance of 1T/2H hybridized phase and 2H pure phase of MoS2nanoflowers

The 1T/2H hybridized and 2H pure phases of MoS₂nanoflowers were synthesized in a one-step hydrothermal process with the molybdenum source as sodium molybdate dihydrate and the sulfur source as thiourea. The as-prepared 1T/2H hybridized and 2H pure phases of MoS₂were investigated using a thermogravimetry\differential thermal analysis, powder x-ray diffraction, field emission scanning electron microscopy, and energy-dispersive x-ray spectroscopy. The obtained 1T/2H hybridized phases of MoS₂were confirmed by the Raman spectroscopy. The electrochemical characteristics of MoS₂electrodes were examined using cycle voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The electrodes are based on the 1T/2H hybridized phases MoS₂with specific capacitance (C_p) of 555.4 F g^-1at current densities (C_d) of 0.5 A g^-1, capacity retention ratio of 85% after 10 000 cycles were observed that could be a strong potential electrode material for supercapacitors application.

Band alignment tuning of heptazine-g-C3N4/g-ZnO vdW heterostructure as a promising water-splitting photocatalyst

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C₃N₄) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C₃N₄/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (E_g = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C₃N₄ to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C₃N₄, leading to a decrease in the electron-hole recombination rate. We also found that both g-ZnO and hg-C₃N₄ synergistically lead to higher light absorption of the heterostructure (λ_max = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (E_g = 2.34 eV, direct) and an electric field of 1 V Å^-1 leads to a type-II heterostructure (E_g = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.

Load-dependent energy dissipation induced by the tip-membrane friction on suspended 2D materials

The tip-membrane interface plays a critical role in characterizing the mechanical properties of ultrathin 2D materials by commonly employed nanoindentation based on atomic force microscopy (AFM). However, the reliability of the assumption that the tip-membrane interface remains pinned during nanoindentation remains unclear, which may introduce unignorable uncertainty in evaluating their true mechanical properties. In this work, it is reported that load-dependent frictional behavior would occur on the tip-membrane interface during nanoindentation tests on monolayer and multilayer suspended WS₂ and graphene, and the curve hysteresis could be well explained by the stick-slip behavior. Further analyses and finite element simulations demonstrated that the frictional energy dissipation should be mainly attributed to the frictional behavior along the direction parallel to the cantilever beam. Meanwhile, the in-plane membrane stiffness was mainly responsible for the different frictional behavior on monolayer and multilayer 2D materials. Based on these analyses, some suggestions were proposed to help reduce the uncertainty when extracting the mechanical properties of 2D materials. These findings not only facilitate the deep understanding of the origin of the curve hysteresis during nanoindentation, but also help to evaluate the mechanical properties of 2D materials in a more reliable way.