Nanostructured Transition Metal Dichalcogenides Research Articles

Enhanced NO2 Gas Sensing in Nanocrystalline MoS2 via Swift Heavy Ion Irradiation: An Experimental and DFT Study.

Nanostructured transition metal dichalcogenides (TMDs) like MoS2 hold promise for gas sensing applications due to their exceptional properties. However, limitations exist in maximizing sensor performance, such as limited active sites for gas interaction and sluggish response/recovery times. This study explores swift heavy ion (SHI) irradiation as a strategy to address these challenges in MoS2-based NO2 gas sensors. MoS2 nanoflakes were fabricated and subsequently irradiated with 120 MeV silver (Ag) ions to induce structural and morphological modifications. Characterization techniques confirmed the formation of Mo and S vacancies within the MoS2 lattice due to irradiation. Significantly, SHI irradiation resulted in a remarkable enhancement of approximately 3 times improvement in sensing response compared to pristine MoS2 sensors. Additionally, the irradiated sensors exhibit substantial improvements in both response and recovery times for NO2 detection. SHI irradiation resulted in the formation of self-affine nanostructures and increased grain fragmentation as fluence rises. This enhanced surface area is hypothesized to promote gas-sensor response. To gain deeper insights into the underlying mechanism, first-principles calculations were employed. These calculations suggest that electron transfer occurs from the MoS2 surface to the NO2 molecule during interaction. Furthermore, the irradiation-induced vacancies facilitate stronger NO2 adsorption on the MoS2 surface compared to the pristine sample. This work demonstrates the effectiveness of SHI irradiation in engineering defects within MoS2 nanoflakes, leading to significantly improved NO2 gas-sensing performance. This approach offers a promising avenue for developing next-generation TMD-based gas sensors with enhanced sensitivity, response times, and stability.

1D Crystallographic Etching of Few‐Layer WS2

AbstractLayer number‐dependent band structures and symmetry are vital for the electrical and optical characteristics of 2D transition metal dichalcogenides (TMDCs). Harvesting 2D TMDCs with tunable thickness and properties can be achieved through top‐down etching and bottom‐up growth strategies. In this study, a pioneering technique that utilizes the migration of in situ generated Na‐W‐S‐O droplets to etch out 1D nanotrenches in few‐layer WS2 is reported. 1D WS2 nanotrenches are successfully fabricated on the optically inert bilayer WS2, showing pronounced photoluminescence and second harmonic generation signals. Additionally, the modulation of inkjet‐printed Na2WO4‐Na2SO4 particles to switch between the etching and growth modes by manipulating the sulfur supply is demonstrated. This versatile approach enables the creation of 1D nanochannels on 2D TMDCs. The research presents exciting prospects for the top‐down and bottom‐up fabrication of 1D‐2D mixed‐dimensional TMDC nanostructures, expanding their use for electronic and optoelectronic applications.

High-Yield WS2 Synthesis through Sulfurization in Custom-Modified Atmospheric Pressure Chemical Vapor Deposition Reactor, Paving the Way for Selective NH3 Vapor Detection.

Nanostructured transition metal dichalcogenides have garnered significant research interest for physical and chemical sensing applications due to their unique crystal structure and large effective surface area. However, the high-yield synthesis of these materials on different substrates and in nanostructured films remains a challenge that hinders their real-world applications. In this work, we demonstrate the synthesis of two-dimensional (2D) tungsten disulfide (WS2) sheets on a hundred-milligram scale by sulfurization of tungsten trioxide (WO3) powder in an atmospheric pressure chemical vapor deposition reactor. The as-synthesized WS2 powders can be formulated into inks and deposited on a broad range of substrates using techniques like screen or inkjet printing, spin-coating, drop-casting, or airbrushing. Structural, morphological, and chemical composition analysis confirm the successful synthesis of edge-enriched WS2 sheets. The sensing performance of the WS2 films prepared with the synthesized 2D material was evaluated for ammonia (NH3) detection at different operating temperatures. The results reveal exceptional gas sensing responses, with the sensors showing a 100% response toward 5 ppm of NH3 at 150 °C. The sensor detection limit was experimentally verified to be below 1 ppm of NH3 at 150 °C. Selectivity tests demonstrated the high selectivity of the edge-enriched WS2 films toward NH3 in the presence of interfering gases like CO, benzene, H2, and NO2. Furthermore, the sensors displayed remarkable stability against high levels of humidity, with only a slight decrease in response from 100% in dry air to 93% in humid environments. Density functional theory and Bayesian optimization simulations were performed, and the theoretical results agree with the experimental findings, revealing that the interaction between gas molecules and WS2 is primarily based on physisorption.

Controlled epitaxial growth of strain-induced large-area bilayer MoS2 by chemical vapor deposition based on two-stage strategy

Two-dimensional (2D) bilayer transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their promising applications in the fields of electronics, optoelectronics, valleytronics and nonlinear optics. However, the precise synthesis of large-area, high-yield and uniform bilayer MoS2 semiconductors remains a significant challenge. Herein, we have developed one two-stage chemical vapor deposition strategy based on strain engineering, enabling the controlled preparation of large-area (4 × 6 cm2) bilayer MoS2 nanostructures. Systematic characterizations indicate that compressive strain was introduced during the growth of first layer MoS2, which effectively induces the synthesis of the second layer MoS2. First-principles calculations based on density functional theory further reveal the mechanism of strain induced controllable growth of bilayer MoS2. Field-effect transistors based on AA and AB stacking bilayer MoS2 have been fabricated exhibiting excellent electronic properties. Our work provides a new pathway for the precise preparation of bilayer TMDCs nanostructures, offering experimental support for their application in the field of electronic and optoelectronic devices.

Nanostructured transition metal dichalcogenides-based colorimetric sensors: Synthesis, characterization, and emerging applications.

Nanostructured transition metal dichalcogenides (TMDCs) have garnered significant attention as prospective materials for the development of highly sensitive and versatile colorimetric sensors. This work explores the synthesis, characterization, and emerging applications of TMDC-based sensors, focusing on their unique structural aspects and inherent properties. The synthesis methods involve tailored fabrication techniques, such as chemical vapor deposition and hydrothermal processes, aimed at producing well-defined nanostructures that enhance sensor performance. Characterization techniques, including microscopy, spectroscopy, and surface analysis, are employed to elucidate the structural and chemical features of the nanostructured TMDCs. These analyses provide insights into the correlation between the material's morphology and its sensing capabilities. The colorimetric sensing mechanism relies on the modulation of optical properties in response to specific analytes, enabling rapid and visual detection. The emerging applications of TMDC-based colorimetric sensors span diverse fields, including environmental monitoring, healthcare, and industrial processes. The sensors exhibit high sensitivity, selectivity, and real-time response, making them ideal candidates for detecting various target analytes. Furthermore, their integration with complementary technologies such as microfluidics, can facilitate the development of on-site and point-of-care applications. This work highlights the interdisciplinary significance of nanostructured TMDC-based colorimetric sensors and underscores their potential contributions to addressing contemporary challenges in sensing technology.

Augmented ammonia sensing of ion-beam modified MoSe2

Gas sensors based on nanostructured transition metal dichalcogenides, such as MoSe2, are increasingly becoming important due to their low band gap, good electrical conductivity and selectivity. However, some of the parameters related to the gas adsorption can be tuned to enhance the overall sensing performance. The change of response sometimes remains low because of small number of active interaction sites on the surface. There are challenges related to slow response and slow recovery of the sensor. Furthermore, if the surface is hydrophilic, it can attract moisture, which may reduce interactions with the target gas and may also reduce the lifetime of the sensor. In this study we have used ion irradiation technique to overcome these challenges and exploit theoretical models to explain the physics behind enhanced sensing performance of 3D nanostructured MoSe2. A low energy (5 keV) argon ions have been used to induce structural and morphological modification of MoSe2. The irradiation leads to formation of Mo and Se vacancies, which is predicted by ion-solid interaction simulation and corroborated with various characterization techniques. The surface becomes superhydrophobic after irradiation and a significant increase of electrical conductivity takes place. We found that the ammonia sensing response increases by about 60 % due to the irradiation. There are further significant gains of response and recovery time of the sensor, post-irradiation. The response of irradiated samples shows more stability as compared to the pristine sample over a period of 180 days. First principles-based calculation points out that the charge transfer happens from surface to ammonia during the interaction and ion irradiation induced vacancies lead to better adsorption of ammonia in case of irradiated sample than that of the pristine sample.

PVD growth of spiral pyramid-shaped WS2 on SiO2/Si driven by screw dislocations.

Atomically thin layered transition metal dichalcogenides (TMDs), such as MoS2 and WS2, have been getting much attention recently due to their interesting electronic and optoelectronic properties. Especially, spiral TMDs provide a variety of candidates for examining the light-matter interaction resulting from the broken inversion symmetry, as well as the potential new utilization in functional optoelectronic, electromagnetic and nanoelectronics devices. To realize their potential device applications, it is desirable to achieve controlled growth of these layered nanomaterials with a tunable stacking. Here, we demonstrate the Physical Vapor Deposition (PVD) growth of spiral pyramid-shaped WS2 with ∼200 in size and the interesting optical properties via AFM and Raman spectroscopy. By controlling the precursors concentration and changing the initial nucleation rates in PVD growth, WS2 in different nanoarchitectures can be obtained. We discuss the growth mechanism for these spiral-patterned WS2 nanostructures based on the screw dislocations. This study provides a simple, scalable approach of screw dislocation-driven (SDD) growth of distinct TMD nanostructures with varying morphologies, and stacking.

Photonic bound states in the continuum in nanostructured transition metal dichalcogenides for strong photon-exciton coupling.

Optical cavities supporting the optical bound states in the continuum (BICs) with infinite radiative lifetime have recently been widely reported. Here, we theoretically investigate BICs in an ultrathin photonic structure consisting of a multilayer, nanostructured transition metal dichalcogenide (TMD). Using finite element simulations, we demonstrate two distinct groups of BICs, namely symmetry-protected BICs and interference-based BICs, in the photonic system. Under normal incidence, their dispersion can be mediated by altering the grating pitch, which makes it possible to explore the strong coupling of these two photonic modes with the TMD exciton band in the same structure. This work expands not only the library of traditional nanophotonic approaches, but also provides more possibilities for optoelectronic devices toward miniaturization.

MoS2 Nanodonuts for High-Sensitivity Surface-Enhanced Raman Spectroscopy.

Nanohybrids of graphene and two-dimensional (2D) layered transition metal dichalcogenides (TMD) nanostructures can provide a promising substrate for extraordinary surface-enhanced Raman spectroscopy (SERS) due to the combined electromagnetic enhancement on TMD nanostructures via localized surface plasmonic resonance (LSPR) and chemical enhancement on graphene. In these nanohybrid SERS substrates, the LSPR on TMD nanostructures is affected by the TMD morphology. Herein, we report the first successful growth of MoS2 nanodonuts (N-donuts) on graphene using a vapor transport process on graphene. Using Rhodamine 6G (R6G) as a probe, SERS spectra were compared on MoS2 N-donuts/graphene nanohybrids substrates. A remarkably high R6G SERS sensitivity up to 2 × 10−12 M has been obtained, which can be attributed to the more robust LSPR effect than in other TMD nanostructures such as nanodiscs as suggested by the finite-difference time-domain simulation. This result demonstrates that non-metallic TMD/graphene nanohybrids substrates can have SERS sensitivity up to one order of magnitude higher than that reported on the plasmonic metal nanostructures/2D materials SERS substrates, providing a promising scheme for high-sensitivity, low-cost applications for biosensing.

Position‐Controlled Fabrication of Vertically Aligned Mo/MoS2 Core–Shell Nanopillar Arrays

AbstractThe fabrication of 2D materials, such as transition metal dichalcogenides (TMDs), in geometries beyond the standard platelet‐like configuration exhibits significant challenges which severely limit the range of available morphologies. These challenges arise due to the anisotropic character of their bonding van der Waals out‐of‐plane while covalent in‐plane. Furthermore, industrial applications based on TMD nanostructures with non‐standard morphologies require full control on the size‐, morphology‐, and position on the wafer scale. Such a precise control remains an open problem of which solution would lead to the opening of novel directions in terms of optoelectronic applications. Here, a novel strategy to fabricate position‐controlled Mo/MoS2 core–shell nanopillars (NPs) is reported on. Metal‐Mo NPs are first patterned on a silicon wafer. These Mo NPs are then used as scaffolds for the synthesis of Mo/MoS2 core/shell NPs by exposing them to a rich sulfur environment. Transmission electron microscopy analysis reveals the core/shell nature of the NPs. It is demonstrated that individual Mo/MoS2 NPs exhibits significant nonlinear optical processes driven by the MoS2 shell, realizing a precise localization of the nonlinear signal. These results represent an important step towards realizing 1D TMD‐based nanostructures as building blocks of a new generation of nanophotonic devices.

Colloidal Nanostructures of Transition-Metal Dichalcogenides.

ConspectusLayered transition-metal dichalcogenides (TMDs) are intriguing two-dimensional (2D) compounds where metal and chalcogen atoms are covalently bonded in each monolayer, and the monolayers are held together by weak van der Waals forces. Distinct from graphene, which is chemically inert, layered TMDs exhibit a wide range of electronic, optical, catalytic, and magnetic properties dependent upon their compositions, crystal structures, and thicknesses, which make them fundamentally and technologically important. TMD nanostructures are traditionally synthesized using gas-phase chemical deposition methods, which are typically limited to small-scale samples of substrate-bound planar materials. Colloidal synthesis has emerged as an alternative synthesis approach to enable the scalable synthesis of free-standing TMDs. The judicious selection of precursors, solvents, and capping ligands together with the optimization of synthesis parameters such as concentrations and temperatures leads to the fabrication of colloidal TMD nanostructures exhibiting tunable properties. In addition, understanding the formation and transformation of TMD nanostructures in solution contributes to the discovery of important structure-function relationships, which may be extendable to other anisotropic systems.In this Account, we summarize recent progress in the colloidal synthesis, characterization, and applications of TMD nanostructures with tunable compositions, structures, and thicknesses. On the basis of the preparation of Mo- and W-based disulfide, diselenide, and ditelluride nanostructures, we discuss examples of phase engineering where various metastable TMD compounds can be directly accessed at low temperatures in solution. We also analyze the chemistry involved in broadly tuning the composition across the MoSe2-WSe2, WS2-WSe2, and MoTe2-WTe2 solid solutions as well as atomic-level microscopic characterization and the resulting composition-tunable properties. We then highlight how the high densities of defects in the colloidally synthesized TMD nanostructures enable unique catalytic properties, including their ability to facilitate the selective hydrogenation of substituted nitroarenes using molecular hydrogen. Finally, using this library of colloidal TMD nanostructures as substrates, we discuss the pathways by which noble metals deposit onto them in solution. We highlight the importance of the relative strengths of the interfacial metal-chalcogen bonds in determining the sizes and morphologies of the deposited noble metal components. These synthesis capabilities for colloidal TMD nanostructures, which have been generalized to a library of composition-tunable phases, enable new systematic studies of structure-property relationships and chemical reactivity in this important class of 2D materials.

Illuminating the Electronic Properties of WS2 Polytypism with Electron Microscopy

AbstractTailoring the specific stacking sequence (polytypes) of layered materials represents a powerful strategy to identify and design novel physical properties. While nanostructures built upon transition‐metal dichalcogenides (TMDs) with either the 2H or 3R crystalline phases have been routinely studied, knowledge of TMD nanomaterials based on mixed 2H/3R polytypes is far more limited. In this work, mixed 2H/3R free‐standing WS2 nanostructures displaying a flower‐like configuration are fingerprinted by means of state‐of‐the‐art transmission electron microscopy. Their rich variety of shape‐morphology configurations is correlated with relevant local electronic properties such as edge, surface, and bulk plasmons. Machine learning is deployed to establish that the 2H/3R polytype displays an indirect band gap of . Further, high resolution electron energy‐loss spectroscopy reveals energy‐gain peaks exhibiting a gain‐to‐loss ratio greater than unity, a property that can be exploited for cooling strategies of atomically‐thin TMD nanostructures and devices built upon them. The findings of this work represent a stepping stone towards an improved understanding of TMD nanomaterials based on mixed crystalline phases.

Size-dependent phase stability in transition metal dichalcogenide nanoparticles controlled by metal substrates.

Nanomaterials based on MoS2 and related transition metal dichalcogenides (TMDCs) are remarkably versatile; MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and the hydrogen evolution reaction, and transition metal dichalcogenides in general have recently emerged as novel 2D components for nanoscale electronics and optoelectronics. The properties of such materials are intimately related to their structure and dimensionality. For example, only the edges exposed by MoS2 nanoparticles (NPs) are catalytically active, and extended MoS2 systems show different character (direct or indirect gap semiconducting, or metallic) depending on their thickness and crystallographic phase. In this work, we show how particle size and interaction with a metal surface affect the stability and properties of different MoS2 NPs and the resulting phase diagrams. By means of calculations based on the Density Functional Theory (DFT), we address how support interactions affect MoS2 nanoparticles of varying size, composition, and structure. We demonstrate that interaction with Au modifies the relative stability of the different nanoparticle types so that edge terminations and crystallographic phases that are metastable for free-standing nanoparticles and monolayers are expressed in the supported system. These support-effects are strongly size-dependent due to the mismatch between Au and MoS2 lattices, which explains experimentally observed transitions in the structural phases for supported MoS2 NPs. Accounting for vdW interactions and the contraction of the Au(111) surface underneath the MoS2 is further found to be necessary for quantitatively reproducing experimental results. We finally find that support-induced effects on the stability of nanoparticle structures are general to TMDC nanoparticles on metal surfaces, which we demonstrate also for MoS2 on Au(111), WS2 on Au(111), and WS2 on Ag(111). This work demonstrates how the properties of nanostructured transition metal dichalcogenides and similar layered systems can be modified by the choice of supporting metal.

Dendritic Ferroselite (FeSe2) with 2D Carbon-Based Nanosheets of rGO and g-C3N4 as Efficient Catalysts for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides are demonstrated to be potential catalysts to produce molecular hydrogen through electroreduction of water. Finding an efficient and cost-effective c...

Environment-dependent edge reconstruction of transition metal dichalcogenides: a global search

The properties of an edge of a transition metal dichalcogenide (TMDC) sensitively depend on its atomic structure; therefore, knowing the exact structure is a precondition for predicting the applications of TMDC, such as in catalysis. Although some edge reconstructions of TMDCs have been reported in previous studies, a global search of TMDC edge structures and calculations of their stabilities are still absent. Here, we propose an approach to explore all possible edge reconstructions of a monolayer TMDC by employing the particle swarm optimization algorithm and first-principles calculations. Taking the most studied TMDC material, MoS2, as a representative, we have built a database of the edge structures of TMDCs. Including the five experimentally observed edges, the thirty-four most stable edges with various sulfur concentrations are predicted for the first time. The most stable edge structures of 1H–MoS2 in different environments along both the armchair and zigzag directions have been predicted and agree well with experimental observations. A specific edge of 1H–MoS2 in the database can be stabilized by modulating the chemical composition of the atoms, which offers an efficient way to tune the properties of TMDC nanostructures, such as TMDC quantum dots and/or nanoribbons.

Regulation of Autophagy Orchestrates Pyroptotic Cell Death in Molybdenum Disulfide Quantum Dot-Induced Microglial Toxicity.

Molybdenum disulfide quantum dots (MoS2 QDs) represent an emerging class of two-dimensional (2D) atomically layered transition metal dichalcogenide nanostructures with few nanometers in lateral size, which show attractive potential as versatile platforms for theranostic applications in various neurological disorders. However, the potential impacts of MoS2 QDs on microglia remain unclear. In this report, we showed that exposure of microglia to MoS2 QDs triggered NLRP3 inflammasome activation as revealed by the cleavage of the inactive precursor of caspase-1 to its active form and the increased release of downstream pro-inflammatory cytokines, resulting in microglia cell death that occurred through caspase-1-dependent pyroptosis. We also found that MoS2 QDs activated autophagy, and suppression of autophagy by specific inhibitors potentiated MoS2 QD-induced pyroptosis. Additionally, MoS2 QDs stimulated mitochondria-derived reactive oxygen species (mtROS) generation in BV-2 cells. However, ROS scavengers could diminish the MoS2 QD-mediated NLRP3 inflammasome activation and pyroptotic cell death in microglia. Overall, our findings identified pyroptosis as a cellular response to MoS2 QD exposure in microglial cells, affording novel insights into the neurotoxicity of MoS2 QDs and facilitating the rational design and application of functional MoS2 QDs in neuroscience.

Sulfur-Vacancy-Enriched MoS2 Nanosheets Based Heterostructures for Near-Infrared Optoelectronic NO2 Sensing

Transition metal dichalcogenide nanostructures especially MoS2 have emerged as novel gas sensing materials. Particularly, MoS2-based optoelectronic devices activated by UV or visible light have bee...

Ultrafast carrier dynamics in colloidal WS2 nanosheets obtained through a hot injection synthesis.

In recent years, hot injection synthesis has emerged as a promising route for the production of nanostructured transition metal dichalcogenides, in large due to its better control over the crystallinity and monodispersity compared to other solution based methods. Understanding the photophysics of excitons in the thus obtained colloidal nanosheets is of great importance to explore their potential for applications in optoelectronics. Here, we study the carrier dynamics in these few-layer colloidal WS2 nanosheets by use of broadband transient absorption spectroscopy. The dynamics of both the bleach, linewidth broadening and energy shift across the entire visible and near-infrared spectrum, allows us to identify subpicosecond electron trapping as the main carrier loss channel. A more quantitative analysis shows that the intrinsic properties of colloidally synthesized nanosheets are on par with other synthesis methods, paving the way for this method to produce high quality nanosheets.

Temperature profiles and thermal conductivities of nanostructured transition metal dichalcogenides

Thermal transport in nanostructured transition metal dichalcogenides has been examined using molecular dynamics simulations. The study is focused on the calculation of the thermal conductivities of pristine and periodically patterned MX2 crystals, assessing the effects of patterning these materials with stripes of substitutional defects. The influence of several patterning parameters has been examined, such as the number and the width of defect stripes and the separation distance between consecutive stripes. The effect of a single defect stripe is also discussed for different compositions of the MX2 crystals and atomic types of substitutional defects. The thermal conductivity of pristine MX2 materials is reversely related to the width of the phononic band gap. Nanostructured materials with stripes of substitutional defects exhibit discontinuous temperature profiles with step-like characteristics, whilst the effect becomes weaker with increasing defect density. The thermal conductivity of periodically patterned crystals is much lower than the one of the pristine materials and increases by increasing the number of defect stripes.

CVD Grown MoS2 Nanoribbons on MoS2 Covered Sapphire(0001) Without Catalysts

One‐dimensional (1D) atomically thin crystals have attracted considerable interest due to their unique properties and potential applications. Herein, the chemical vapor deposition (CVD) growth of molybdenum disulfide (MoS2) nanoribbons on sapphire(0001) supported monolayer MoS2 flakes without catalysts as well as the characterizations by means of Raman and photoluminescence (PL) spectroscopy, optical microscopy (OM), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM). The Raman and PL results is reported, as well as OM images reveal that such MoS2 nanoribbons grow on monolayer MoS2 flakes. STEM and selected area electron diffraction (SAED) results reveal that they are grown along the direction with atomically sharp edges. Controlled experiments show that the deficiency of sulfur vapor at the beginning of growth induced 1D MoOxS2−x crystals as precursors. The subsequent sulfurization changes them into MoS2 nanoribbons. These findings provide a facile approach to MoS2 nanoribbons and enrich the horizon of the preparation of nanostructured transition metal dichalcogenides.