Applications Of Transition Metal Dichalcogenides Research Articles

Influence of Substrate-Induced Charge Doping on Defect-Related Excitonic Emission in Monolayer MoS2.

Many applications of transition metal dichalcogenides (TMDs) involve transfer to functional substrates that can strongly impact their optical and electronic properties. We investigate the impact that substrate interactions have on free carrier densities and defect-related excitonic (XD) emission from MoS2 monolayers grown by metal-organic chemical vapor deposition. C-plane sapphire substrates mimic common hydroxyl-terminated substrates. We demonstrate that transferring MoS2 monolayers to pristine c-plane sapphire dramatically increases the free electron density within MoS2 layers, quenches XD emission, and accelerates exciton recombination at the optical band edge. In contrast, transferring MoS2 monolayers onto inert hexagonal boron nitride (h-BN) has no measurable influence on these properties. Our findings demonstrate the promise of utilizing substrate engineering to control charge doping interactions and to quench broad XD background emission features that can influence the purity of single photon emitters in TMDs being developed for quantum photonic applications.

The futuristic applications of transition metal dichalcogenides for cancer therapy.

The second-most common cause of death resulting from genetic mutations in DNA sequences is cancer. The difficulty in the field of anticancer research is the application of the traditional methods, which also affects normal cells. Mutations, genetic replication alterations, and chromosomal abnormalities have a direct impact on the effectiveness of anticancer drugs at different stages. Presently, therapeutic techniques utilize nanotechnology, transition metal dichalcogenides (TMDCs), and robotics. TMDCs are being increasingly employed in tumor therapy and biosensing applications due to their biocompatibility, adjustable bandgap, versatile functionality, exceptional photoelectric properties, and wide range of applications. This study reports the advancement of nanoplatforms based on TMDCs that are specifically engineered for responsive and intelligent cancer therapy. This article offers a thorough examination of the current challenges, future possibilities for theranostic applications using TMDCs, and recent progress in employing TMDCs for cancer therapy. Currently, there is significant interest in two-dimensional (2D) TMDCs nanomaterials as ultrathin unique physicochemical properties. These materials have attracted attention in various fields, including biomedicine. Due to their inherent ability to absorb near-infrared light and their exceptionally large surface area, significant efforts are being made to prepare multifunctional nanoplatforms based on 2D TMDCs.

(Invited) Metal Oxide Mixed Sulphide of Controlled Crystal Structure As Catalysts for Two-Electron Water Oxidation

Conventional catalysts based on noble metals (Pt, Ru) are often used for production of oxygen during water electrolysis in four-electron pathway as they can decrease gas evolution overpotential. Another possible valuable product from water oxidation is hydrogen peroxide obtained via two-electron process. Currently, transition metal oxides, are readily employed to catalyse this reaction. Although they display certain activity, it is still not satisfactory for mass production of H2O2, therefore further optimisations are needed. We suggest in this work, in addition to typical nanostructure modifications of metal oxides (MoO2) such as doping or structural engineering, an application of transition metal dichalcogenides (MoS2). Consequently, the oxide is partially replaced by sulphur. The selection of molybdenum as a centre atom ensures sufficient electrical conductivity of material as an electrode active layer. This combination is intended to provide the benefits thanks to layered materials of tuned interlayer spacing as well as controlled number of active sites. It enables selective adsorption of intermediate products of water oxidation (O∗, OH∗, and OOH∗), and therefore increased preference towards two-electron pathway.Various chemical approaches are considered to synthesize this material, especially hydrothermal treatment in autoclave and high-temperature sintering of precursors. The catalytic activity of the nanostructured metal oxide is further enhanced by its deposition on conductive high surface area support made of carbon/graphite particles of strictly defined textural parameters and surface chemistry. The final composite is obtained using hydrophobic binders (PVDF and PTFE) to form the electrode and improve the catalyst stability. The measurements are carried out in flow electrolyser unit with an alkaline electrolyte. The amount of produced H2O2, H2 as well as quantity and quality of evolved gases are assessed in terms of faradaic efficiency, pressure and overvoltage measurements.

2D layered double hydroxides and transition metal dichalcogenides for applications in the electrochemical production of renewable hydrogen

Layered double hydroxides (LDHs) and transition metal dichalcogenides (TMDs) are promising electrocatalysts in the splitting of water and the production of hydrogen.

Interfacial charge transfers and carrier regulation characteristics of narrow/wide band gap TMDs@Ga2O3 n-n heterojunction film

The application of transition metal dichalcogenides (TMDs) in many optical devices has been seriously limited by its narrow band-gap and high electron-hole recombination rate. Therefore, more studied are desired for exploring the novel TMDs/Ga2O3 heterostructures with excellent photoexcitation properties and interfacial dynamics. Herein, the WSe2/Ga2O3 and MoS2/Ga2O3 heterostructures were successfully prepared by a two-step radio frequency magnetron sputtering method. The growth mechanism and interfacial charge transfer of the wide and narrow TMDs/Ga2O3 heterostructures are considered. The X-ray diffraction results show that the (002) orientation of the WSe2 and MoS2 was suppressed, which can be attributed to interfacial interaction. Compared to pure TMDs, the A1 g was blue-shifted in the Raman spectrum for the proposed heterostructures, which is related to the decrease of the electron density. The photoluminescence emission of TMDs/Ga2O3 heterostructures was enhanced mainly due to the transition from trions recombination to exciton recombination. Meanwhile, the transmission properties of electrons under the built-in electric field are further confirmed by the transient absorption spectroscopy. Effective charge separation not only reduces radiation emission rate but also significantly enhances the nonlinear optical response of TMDs. An open-aperture Z-scan measurement was carried out to confirm that Ga2O3 is a promising candidate for enhancing the saturable absorption or reverse saturable absorption of TMDs.

Bimetallic doping-derived heterostructures in NiCo-WSe2 to promote hydrogen evolution reaction

The application of transition metal dichalcogenides (TMDs) in hydrogen evolution reaction (HER) is limited due to fewer edge active sites and poor conductivity. Appropriate transition metal dopants on TMDs are an important strategy. Herein, the Fe, Cu, Mo, Ni, and Co doped WSe2 have been synthesized. Compared with pristine WSe2, the results show that the overpotential of WSe2 was significantly reduced after doping, and the order of activity was ranked as follows: Co-WSe2> Ni-WSe2> Mo-WSe2> Fe-WSe2> Cu-WSe2. Among them, Co-WSe2 showed a smaller overpotential and charge transfer resistance, and Ni-WSe2 exhibited a larger electrochemically active surface area and a smaller Tafel slope. Further, NiCo-WSe2 was also prepared, which has an overpotential of 205 mV at 10 mA cm−2 and a tafel slope of 118.6 mV dec−1, respectively. Crystal structure analysis shows that CoSe2 is agglomerated in Co-WSe2, but it is uniformly distributed on the surface of NiCo-WSe2 because the competitive substitution effect between Ni and Co slows down the growth rate of CoSe2, and thus providing more efficient interfacial active site. In addition, the higher electronegativity of Ni and Co reduces the electron density around W and Se, which changes the electronic structure of WSe2 and accelerates the mass transfer efficiency between charges, thereby promoting the HER performance of NiCo-WSe2. This work provides a technological pathway for the development of inexpensive and commercializable HER electrocatalysts.

Transition Metal Dichalcogenides [MX2] in Photocatalytic Water Splitting

The quest for a clean, renewable and sustainable energy future has been highly sought for by the scientific community over the last four decades. Photocatalytic water splitting is a very promising technology to proffer a solution to present day environmental pollution and energy crises by generating hydrogen fuel through a “green route” without environmental pollution. Transition metal dichalcogenides (TMDCs) have outstanding properties which make them show great potential as effective co-catalysts with photocatalytic materials such as TiO2, ZnO and CdS for photocatalytic water splitting. Integration of TMDCs with a photocatalyst such as TiO2 provides novel nanohybrid composite materials with outstanding characteristics. In this review, we present the current state of research in the application of TMDCs in photocatalytic water splitting. Three main aspects which consider their properties, advances in the synthesis routes of layered TMDCs and their composites as well as their photocatalytic performances in the water splitting reaction are discussed. Finally, we raise some challenges and perspectives in their future application as materials for water-splitting photocatalysts.

Probing the long-lived photo-generated charge carriers in transition metal dichalcogenides by time-resolved microwave photoconductivity

Abstract Understanding the dissociation of excitons into long-lived free charge carriers is a crucial issue when considering the applications of transition metal dichalcogenides (excitonic semiconductors) oriented toward the use of solar energy (such as photovoltaics or photocatalysis). In our work, long-lived carriers have been observed by time-resolved microwave photoconductivity (TRMC) for the first time in both atomically thin and bulk MoS2, MoSe2, WS2, and WSe2 crystals. The lifetime of majority carriers is close to microseconds and can even reach several microseconds due to different contribution of surface and defect states, as well as surface band bending (bulk). The three components depend on the material and vary from sample to sample, therefore determining the dynamics of the TRMC signal. The rise time of TRMC signal was found to be in the range of 0.1–0.2 μs and as it depends on the studied material it can be speculated that it is related to the dissociation time of excitons captured by traps.

First-principles investigation of in-plane anisotropies in XYTe4 monolayers with X = Hf, Zr, Ti and Y = Si, Ge.

In-plane anisotropic materials can introduce additional degrees of freedom while tuning their physical properties, which expand the range of opportunities for designing novel semiconductor devices and exploring distinct applications. In this work, we investigate the in-plane anisotropic electronic, elastic, transport and piezoelectric properties in a family of isostructural telluride XYTe4 (X = Hf, Zr and Ti, Y = Si and Ge) monolayers based on first-principles calculations. Six types of structures are verified to harbor direct bandgaps at the Γ point ranging between 0.98 and 1.36 eV. The orientation-dependent in-plane elastic stiffness of XYTe4 reveals the anisotropic and ultrasoft nature. Superior dielectric constants and giant switching effects are found in TiGeTe4 monolayers because of giant in-plane anisotropy. Strikingly, the piezoelectric coefficients of XSiTe4 differ by an order of magnitude along the two main directions. The strong in-plane anisotropic elastic properties of XYTe4 monolayers together with outstanding piezoelectric responses show that these structures can compete with that of transition metal dichalcogenides for applications in the field of flexible electronic devices.

Tip-enhanced nanoscopy of two-dimensional transition metal dichalcogenides: progress and perspectives.

The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMD) thin layers prepared by exfoliation or chemical vapour deposition are strongly modulated by defects at the nanoscale. The mediated electronic and optical properties are expected to be spatially localised in a nanoscale width neighbouring the defects. Characterising such localised properties requires an analytical tool with nanoscale spatial resolution and high optical sensitivity. In recent years, tip-enhanced nanoscopy, represented by tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL), has emerged as a powerful tool to characterise the localised phonon and exciton behaviours of 2D TMDs and heterojunctions (HJs) at the nanoscale. Herein, we first summarise the recent progress of TERS and TEPL in the characterisation of several typical defects in TMDs, such as edges, wrinkles, grain boundaries and other defects generated in transfer and growth processes. Then the local strain and its dynamic control of phonon and exciton behaviours characterised by TERS and TEPL will be reviewed. The recent progress in characterising TMD HJs using TERS and TEPL will be subsequently summarised. Finally, the progress of TERS and TEPL combined with optoelectronic sensitive electronic scanning probe microscopy (SPM) in the applications of TMDs will be reviewed.

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.

Boosting potassium storage performance via construction of NbSe2–based misfit layered chalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have become promising anode materials for potassium-ion batteries (KIBs) due to their high theoretical capacity and large interlayer spacing. The application of TMDCs is prevented by their volume expansion during cycling and relatively poor conductivity. Herein, we developed a general strategy to prepare a series of NbSe2-based misfit layered compounds MLCs ((SnSe)1.16NbSe2, (PbSe)1.14NbSe2) by a facile top-down process. The unique sandwich structure of MLCs combines the excellent structure stability and conductivity of NbSe2 with the high reactivity of selenides (SnSe and PbSe), which greatly improves the potassium storage performance. The high-strength lamella and weak interlayer bonding of NbSe2 improve the K+ diffusion rate, leading to high rate-capability of the MLCs. This work provides a new avenue for designing novel types of sandwich structured electrode materials for energy storage systems.

Design of earth-abundant ternary Fe1−xCoxS2 on RGO as efficient electrocatalysts for hydrogen evolution reaction

Developing inexpensive and earth-abundant materials instead of precious metal Pt has broad applications in the production of hydrogen through electrolysis of water. Herein, a series of ternary Fe1−xCoxS2 nanoparticles hybridized with reduced graphene oxide (RGO) (Fe1−xCoxS2/RGO) have been synthesized by a facile hot-injection method for hydrogen evolution reaction (HER) in acidic solutions. The electrochemical measurements show that the optimal HER activity is obtained when the ratio of Co is 61%, exhibiting a low overpotential of 198 mV at 10 mA cm−2 and a small Tafel slope of 94 mV dec−1. Moreover, density-functional theoretical calculations indicate that the incorporation of Co atoms can activate the electrocatalytic performance of FeS2 with a smaller Gibbs free energy for hydrogen adsorption (ΔGH*). The enhanced HER activity might be owing to the synergistic effects between the highly conductive RGO supports and the electroactive Fe0.39Co0.61S2. This study gives an insight into the design and development of the HER electrocatalysts based on earth-abundant Fe1−xCoxS2 and promotes the applications of transition metal dichalcogenides for hydrogen generation in the future.

Wrinkle facilitated hydrogen evolution reaction of vacancy-defected transition metal dichalcogenide monolayers

Utilizing transition metal dichalcogenides (TMDs) as catalysts in the hydrogen evolution reaction (HER) is a promising prospect for hydrogen production. Here, by first-principles calculations we reveal that the catalytic activities of vacancy-defected TMD MX2 (M = Mo or W and X = S, Se or Te) monolayers for the HER can be significantly improved by wrinkle engineering. The hydrogen adsorption Gibbs free energies of defected TMDs decrease with decreasing wrinkle length. By appropriately controlling and adjusting the wrinkle size and vacancy number, the hydrogen adsorption Gibbs free energy will be close to zero, allowing the wrinkled TMDs to reach their optimum catalytic capability. The improvement of the catalytic activity of TMDs is mainly attributed to the charge transfer and polarization enhancement of metal atoms at the vacancy sites, which are caused by the coupling effect of vacancy defects and wrinkling deformation induced flexoelectricity. These results provide an attractive route for the application of TMDs in hydrogen production by combining wrinkle engineering and defect engineering.

Colloidal MoS2 van der Waals Template for Growing Highly Uniform Nanomaterials.

The van der Waals (vdW) structures of transition metal dichalcogenides (TMDCs) have been studied extensively owing to the excellent electronic, optoelectronic, and catalytic performance with the atomic-scale sharpness of the interfaces. In addition, the presence of substitution and redox reactions in the vdW gaps also provides huge potential to be employed as a nanoscale reactor. Herein, the vdW gap of colloidal MoS2 is reported as a natural template for the formation of CoMo bimetallic oxide nanoparticles (BMONPs) with highly uniform size (∼4 nm), which resulted in further synthesis of the thin multiwalled carbon nanotubes with extremely narrow diameter distribution. This study not only pioneers a new application of TMDCs but also provides an efficient means to control the size of nanomaterials, which directly affect material properties.

Bimetallic Sulfide/Sulfur Doped T3C2Tx MXene Nanocomposites as High-performance Anode Materials for Sodium-ion Batteries

The application of transition metal dichalcogenides(TMDs) as anode materials in sodium-ion batteries (SIBs) has been hindered by low conductivity and poor cyclability. Herein, we report the synthesis of CoxFe1−xS2 bimetallic sulfide/sulfur-doped Ti3C2 MXene nanocomposites(CoxFe1−xS2@S-Ti3C2) by a facile co-precipitation process and thermal-sulfurization reaction. The interconnected 3D frameworks consisting of MXene nanosheets can effectively buffer the volume change and enhance the charge transfer. In particular, sulfur-doped MXene nanosheets provide rich active sites for sodium storage and restrain sulfur loss during charging/discharging processes, leading the increase of specific capacity and cycling the stability of anode materials. As a result, CoxFe1−xS2@S-Ti3C2 anodes exhibited high capacity, high rate capability and long cycle life(399 m·Ah/g at 5 A/g with an 94% capacity retention after 600 cycles).

Strain-Controllable Phase and Magnetism Transitions in Re-Doped MoTe2 Monolayer

Developing approaches to effectively manipulate the magnetic states and phase transition is critical to the application of transition metal dichalcogenides (TMDs) in spintronic or phase-switching d...

Pulsed thermal deposition of binary and ternary transition metal dichalcogenide monolayers and heterostructures

The application of transition metal dichalcogenides in optoelectronic, photonic, or valleytronic devices requires the growth of continuous monolayers, heterostructures, and alloys of different materials in a single process. We present a facile pulsed thermal deposition method which provides precise control over the number of layers and the composition of two-dimensional systems. The versatility of the method is demonstrated on ternary monolayers of Mo1−xWxS2 and on heterostructures combining metallic TaS2 and semiconducting MoS2 layers. The fabricated ternary monolayers cover the entire composition range of x = 0…1 without phase separation. Bandgap engineering and control over the spin–orbit coupling strength are demonstrated by absorption and photoluminescence spectroscopy. Vertical heterostructures are grown without intermixing. The formation of clean and atomically abrupt interfaces is evidenced by high-resolution transmission electron microscopy. Since both the metal components and the chalcogen are thermally evaporated, complex alloys and heterostructures can thus be prepared.

Epitaxial growth of non-layered PbSe nanoplates on MoS2 monolayer for infrared photoresponse

The epitaxial growth of non-layered PbSe nanoplates on monolayer MoS2 is demonstrated. Scanning electron microscopy characterization reveals a position-independent growth of PbSe plates on a MoS2 sheet with large coverage. The epitaxial relationship between the PbSe plates and MoS2 sheet was found to be ∥ in-plane and ∥ in the thickness direction. Photodetectors based on these heterostructures exhibit a responsivity and detectivity of 51 mA W–1 and 4.4 × 106 Jones, respectively, under 1550 nm light illumination at room temperature. The realization of these PbSe/MoS2 heterostructures paves the way for the application of transition metal dichalcogenides in the IR light region.

The Phases of WS2 Nanosheets Influence Uptake, Oxidative Stress, Lipid Peroxidation, Membrane Damage, and Metabolism in Algae.

Application of transition metal dichalcogenide (TMDC) nanosheets with different phases have attracted much attention in various fields. However, the effects of TMDC phases on environmental biology remain largely unknown. In this study, chemically exfoliated WS2 nanosheets (Ce-WS2, mainly the 1T phase) and annealed exfoliated WS2 nanosheets (Ae-WS2, 2H phase) were fabricated to serve as representative TMDC nanomaterials. Ce-WS2 showed higher levels of cellular uptake, oxidative stress, lipid peroxidation, membrane damage, and inhibition of photosynthesis than Ae-WS2 in Chlorella vulgaris. These differences were attributed to the higher electron conductivity and higher separation efficiency of electrons and holes in the 1T phase, a typical feature of Ce-WS2. Correspondingly, 2H-phase Ae-WS2 exhibited lower photooxidation/reduction activity and a lower ability to generate reactive oxygen species (mainly •OH) under visible-light irradiation. 1T-phase Ce-WS2 dissolved more readily than Ae-WS2 and released more W ions into aqueous environments, but the W ions exhibited negligible toxicity. Metabolomic analysis revealed that Ce-WS2 induced more obvious alterations in metabolites (e.g., amino acids and fatty acids) and metabolic pathways (e.g., starch and sucrose metabolism) than Ae-WS2. These alterations correlated with cell membrane damage, oxidative stress and photosynthesis inhibition. The present work provides insights into the environmentally friendly design of two-dimensional TMDCs.