Transition Metal Dichalcogenides Family Research Articles

Study on pH dependent ON/OFF fluorescent switching behaviour of hydrothermally synthesised vanadium disulphide quantum dots (VS2-QDs)

It is fascinating to design and develop an inorganic quantum dot (QDs) which exhibit luminescence features with ion concentration, temperature, and pH sensitivity dependence. Vanadium disulphide (VS2) belongs to the family of transition metal dichalcogenides (TMDs), and is extensively studied owing to its layered characteristics features in sensing and filtrations technology, however, its fluorescence features have not been well studied so far. The current work describes a widely adopted eco-friendly hydrothermal approach for the synthesis of VS2-QDs. The average size of particles was determined to be ∼10 nm by transmission electron microscopy (TEM) and Zetasizer analysis. The synthetic approach adopted here facilitates in situ functionalization of QDs, which results in their high stability in aqueous medium and enhanced sensitivity towards their surroundings. As most of the QDs show the sensitivity towards pH, a thorough investigation of VS2 QDs for their pH dependent photoluminescence has been carried out. The VS2-QDs were discovered to exhibit a remarkable luminescence intensity that was found to be about 11 times higher in basic conditions (pH ∼ 13, QY = ∼4.80%) as compared to that in acidic conditions (pH ∼ 1, QY = ∼1.10%). The root cause of this ON/OFF florescence flipping may be related to the functional groups (such as −SO42−, −NH2, OH−, etc.) attached over the surface of QDs, acid etching, and protonation–deprotonation process. This paper highlights the pH dependent switching behaviour of VS2-QDs that could offer valuable information for designing a futuristic pH-based device for biomedical applications.

Resistive gas sensors for the detection of NH3 gas based on 2D WS2, WSe2, MoS2, and MoSe2: A review.

Transition metal dichalcogenides (TMDs) with a two-dimensional (2D) structure and semiconducting features are highly favorable for the production of NH3 gas sensors. Among the TMD family, WS2, WSe2, MoS2, and MoSe2 exhibit high conductivity and a high surface area, along with high availability, reasons for which they are favored in gas-sensing studies. In this review, we have discussed the structure, synthesis, and NH3 sensing characteristics of pristine, decorated, doped, and composite-based WS2, WSe2, MoS2, and MoSe2 gas sensors. Both experimental and theoretical studies are considered. Furthermore, both room temperature and higher temperature gas sensors are discussed. We also emphasized the gas-sensing mechanism. Thus, this review provides a reference for researchers working in the field of 2D TMD gas sensors.

Interface engineering of tungsten disulfide by incorporating chromium nitride interfacial layer for hybrid supercapacitors

By confronting the ongoing challenges of energy storage after generation, a unique technology referred to as hybrid energy storage systems appears as a potential solution for energy storage applications. In hybrid supercapacitors, optimizing the electrode characteristics by precise interface engineering greatly improves performance, supporting the device's stability and ability to store charge. The focus of this research is on the interface engineering of tungsten disulfide from the family of transition metal dichalcogenides by the incorporation of chromium nitride interfacial layer of 100 nm on 1 cm2 substrate via magnetron sputtering. The structural characteristics of the resulting samples were acquired using SEM, XRD, Raman, and EDX. The study further employed the half-cell assembly for exploring the electrochemical performance of pure WS2 and WS2 with CrN interfacial layer. The enhanced composition of WS2/CrN was then used as a working electrode alongside activated carbon in a hybrid supercapacitor arrangement. The device reveals a specific capacity, capacitance, power and energy density of 348 C/g, 481.3 F/g, 4250 W/kg and 82.6 Wh/kg, respectively along with capacity retention of 89 % after 10,000 GCD cycles. Following this, the device's performance was evaluated using a semi-empirical approach, comparing capacitive and diffusive mechanisms from linear and quadratic models to study battery-supercapacitor hybrids.

Revealing 1T-MoS2 with 76 % purity induced by various saccharides for supercapacitor performance

The preparation of metallic phase molybdenum disulfide (1T-MoS2) through an implementable method has become a top priority in research due to its excellent electrochemical performance. In this study, 1T-MoS2 using a facile one-pot hydrothermal method is successfully synthesized by incorporating saccharides. Moreover, a comprehensive understanding of the synthesis mechanism is induced by various saccharides. Glucose, a monosaccharide, not only possesses reducibility but also plays a crucial role in preparation of sulfur vacancies. Maltose is a disaccharide that can hydrolyze into glucose and fructose, both of which exhibit reducibility. On the contrary, dextrin is difficult to break down into fructose or glucose, and faces certain challenges in promoting the formation of 1T-MoS2. Importantly, 1T-MoS2 with 76 % purity obtained in our study exhibits exceptional electrochemical performance, demonstrating a high specific capacitance of 228.4 F g−1 at 2 A g−1 and maintaining a cycling stability of 97 % after 10,000 cycles. Furthermore, an asymmetric flexible supercapacitor device is assembled by 1T-MoS2 and active carbon on carbon cloths, which shows superior flexibility, high power density, and energy density. Notably, this work not only unveils the mechanism by which saccharides influence the synthesis of 1T-MoS2 but also provides support for the preparation of other metallic members in the transition metal dichalcogenides (TMDs) family.

Nano tumbleweed-like tungsten disulfide as a viable surrogate for platinum counter electrodes in dye-sensitized solar cells

Tungsten disulfide (WS2), a notable member of the transition metal dichalcogenides family, has emerged as a promising candidate for energy storage and conversion applications. In this study, WS2 was synthesized via a hydrothermal method using sodium tungstate and thiourea as precursor materials, with hydroxylamine hydrochloride as the reducing agent and cetyl trimethyl ammonium bromide as a surfactant. The primary objective was to develop a cost-effective, easily synthesized, and scalable material capable of replacing platinum (Pt) in dye-sensitized solar cells (DSSCs), where Pt is both prohibitively expensive and scarce. Characterization of the synthesized WS2 was conducted through a range of physical analyses, including SEM, TEM, XRD, EDAX, FTIR, and Raman spectroscopy, which provided compelling evidence for the formation of a unique nano-tumbleweed-like structure of WS2. When used as a counter electrode (CE), the synthesized WS2 exhibited exceptional photocatalytic activity, resulting in a commendable power conversion efficiency (PCE) of 12.06 % when coupled with fluorine-doped tin oxide (FTO) plates coated with TiO2 as the photoanode, N719 as the dye, and Iˉ/I3ˉ as the liquid electrolyte. Additionally, the fabricated device underwent comprehensive electrical characterization, including cyclic voltammetry (CV), current-voltage (IV) measurements, electrochemical impedance spectroscopy (EIS), and Tafel analysis, revealing properties similar to those of Pt counter electrodes. Notably, WS2 demonstrated an open-circuit potential of 852 mV and a short-circuit current density of 28 mA/cm2, accompanied by a fill factor of 0.43 and a commendable lifetime of 15.92 ms.

Electronic phase transformations and energy gap variations in uniaxial and biaxial strained monolayer VS[formula omitted] TMDs: A comprehensive DFT and beyond-DFT study

In the rapidly evolving field of 2D materials, transition metal dichalcogenides (TMDs) have emerged as compelling candidates for electronic applications. This study investigates the electronic structure of the H-phase monolayer VS2 belonging to TMD family and the influence of strain on its band structure through Density Functional Theory (DFT). We employ two different pseudopotential approximations and a suite of computational methods including DFT+U, GAUPBE, G0W0, and self-GW to provide a nuanced understanding of its electronic band structure. A highlight of the study is its focus on how both uniaxial and biaxial strains, ranging from -5% to +5%, affect the electronic properties of the H-phase monolayer VS2. Our comprehensive analysis reveals that these tensile strains significantly widen the energy gap, with uniaxial strains having a more pronounced effect than their biaxial counterparts. Additionally, we identify an intriguing phase transition from a semiconducting to a metallic state under compressive strains, this transition is attributed to both symmetry breaking and bond length variation in the uniaxial case, the bond length in biaxial. These key findings not only enrich our understanding of the intricate electronic behavior of monolayer VS2 under different strains but also pave the way for the design of innovative electronic devices using strain engineering.

A comparative study on 2D materials with native high-[formula omitted] oxides for sub-10 nm transistors

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with native high-κ oxides have presented a new avenue towards the development of next-generation ultra-scaled field-effect transistors (FETs). These materials have been experimentally shown to form a natively compatible oxide layer with a high dielectric constant, which can help scale down both the transistor size and the supply voltage. We present a material and device performance study into the use of several of these materials – namely HfS2, HfSe2, ZrS2, ZrSe2 – as channels in sub-10 nm FETs. All four materials exhibit isotropic transport at 10 nm channel length with ON currents over 1000 μA/μm but show anisotropic transport and degraded ON currents at 5 nm channel length. In general, the sulfide family excels in terms of subthreshold characteristics at sub-10 nm channel lengths. HfS2, in particular, surpasses all the other materials in terms of ON currents and subthreshold swing (SS), allowing it to also achieve excellent intrinsic performance. We have identified HfS2 as a superior material within this TMD family for sub-10 nm FETs.

Room-Temperature Strong Coupling of Few-Exciton in a Monolayer WS2 with Plasmon and Dispersion Deviation.

Engineering room-temperature strong coupling of few-exciton in transition-metal dichalcogenides (TMDCs) with plasmons promises to construct compact and high-performance quantum optical devices. But it remains unimplemented due to their in-plane excitons. Here, we demonstrate the strong coupling of few-exciton within 10 in monolayer WS2 with the plasmonic mode with a large tangential component of the electric field tightly trapped around the sharp corners of an Au@Ag nanocuboid, the fewest number of excitons observed in the TMDC family so far. Furthermore, we for the first time report a significant deviation with a relative difference of up to 100.6% between the spectrum and eigenlevel splitting dispersions, which increases with decreasing coupling strength. It is also shown that the coupling strength obtained by the conventional concept of both being equal to the measured spectrum splitting is markedly overestimated. Our work enriches the understanding of strong light-matter interactions at room temperature.

Enhanced photocatalytic performance of SnS2 under visible light irradiation: strategies and future perspectives.

Tin(II) sulfide (SnS2) has emerged as a promising candidate for visible light photocatalytic materials. As a member of the transition metal dichalcogenides (TMDs) family, SnS2 features a band gap of approximately 2.20 eV and a layered structure, rendering it suitable for visible light activation with a high specific surface area. However, the application of SnS2 as a visible light photocatalyst still requires improvement, particularly in addressing the high recombination of electrons and holes, as well as the poor selectivity inherent in its perfect crystal structure. Therefore, ongoing research focuses on strategies to enhance the photocatalytic performance of SnS2. In this comprehensive review, we analyze recent advances and promising strategies for improving the photocatalytic performance of SnS2. Various successful approaches have been reported, including controlling the reactive facets of SnS2, inducing defects in the crystal structure, manipulating morphologies, depositing noble metals, and forming heterostructures. We provide a detailed understanding of these phenomena and the preparation techniques involved, as well as future considerations for exploring new science in SnS2 photocatalysis and optimizing performance.

Vacancy-mediated inelasticity in two-dimensional vanadium-based dichalcogenides.

Amongst the two-dimensional (2D) transition metal dichalcogenide (TMDC) family, VX2 (X = S, and Se) are key members for next-generation electronic, spintronic, and energy storage applications. Structural defects may be introduced in these emerging atomically thin materials during synthesis, which can lead to both desirable and detrimental impacts on the physical and chemical properties. Owing to their brittle behaviour, defects may deteriorate the mechanical properties of 2D TMDCs drastically, causing reliability-related issues critical for long-term device-scale applications. In this context, more than 1600 classical molecular dynamics simulations were performed to obtain estimates of chirality-dependent failure stresses of 2D VX2 under technologically relevant conditions that are encountered in typical industrial and lab-scale experiments. The energy barriers associated with the fracture of defective samples were calculated using a thermal activation-based analytical model. Furthermore, the effect of various concentrations of randomly distributed defects on the mechanical reliability of VX2 samples was explored using two-parameter Weibull statistics. These monolayers were found to possess strong flaw tolerance and high Weibull modulus values, making them promising for flexible electronics and energy storage.

Fourier‐Transform Infrared Spectroscopy of MoTe2 Thin Films

Molybdenum ditelluride (MoTe2), a member of the transition metal dichalcogenide family, holds significant promise in optoelectronic applications. To harness its full potential, it is imperative to understand its fundamental optical properties, including refractive indices. Herein, measurements of the transmittance and reflectance spectra of thin MoTe2 layers are conducted featuring hexagonal and monoclinic structures across the far‐infrared (IR) and mid‐IR spectral regions. These spectra are fitted, allowing to extract the complex refractive index and optical conductance. In the semiconducting hexagonal phase, both the conductance and extinction coefficient remain negligible below 3000 cm−1 and increase at higher energies. Conversely, the monoclinic phase exhibits a conductance peak in the far‐IR region and a continuous increase from 1000 to 6000 cm−1. The findings are compared with existing theoretical and experimental results to provide further insights into MoTe2's optical behavior.

Machine learning driven prediction of lattice constants in transition metal dichalcogenides

Machine learning represents an emerging branch of artificial intelligence, centering on the enhancement of algorithms in computer programs through the utilization of data and the accumulation of research-driven knowledge. The requirement for artificial intelligence in materials science is essential due to the significant need for innovative high-performance materials on a large scale. In this report, the gradient boosting regression tree model of machine learning was applied to predict the lattice constants of cubic and trigonal MX2 systems (M=transition metal and X=chalcogen atoms). The theoretical/experimental values of the materials were compared to the predicted values to calculate the standard errors such as RMSE (root mean square error) and MAE (mean absolute error). The features used to predict lattice constants were ionic radius, lattice angles, bandgap, formation energy, total magnetic moment, density and oxidation states. The features versus contribution barplot has been drawn to reveal the contribution level of each parameter in the degree of [0,1] to obtain the predictions. This report provides a precise account of the prediction methodology for lattice parameters of the transition metal dichalcogenides family, a process that was previously not reported.

Ultrafast Fiber Laser Based on Tungsten Sulphoselenide Materials

Tungsten sulphoselenide (WSSe) alloys, belonging to the transition metal dichalcogenide family, have attracted significant interest in the area of optoelectronics because of their unique optical and electronic properties. However, there has been a dearth of sufficient research on the saturable absorption features and ultrafast lasers applications. Herein, we fabricated a WSSe-microfiber saturable absorber (SA) based on WSSe nanosheets prepared by liquid exfoliation technique. The SA provided a saturation intensity of a modulation depth of 27.95% and a nonsaturable loss of 21.34%. To investigate the potential applications of WSSe in ultrafast photonics, the prepared WSSe-microfiber was incorporated into an Er-doped fiber laser ring cavity. The results demonstrated that the WSSe-based SA successfully generated mode-locking laser pulses with a remarkably short pulse width of 231 fs. Furthermore, the output power of this ultrafast fiber laser reached an impressive value of 15.68 mW. These findings provide valuable views into the unique features of WSSe alloys in the areas of ultrafast optics and develop recipes for SA in ultrafast fiber lasers.

HfSe2: Unraveling the microscopic reason for experimental low mobility.

Monolayer HfSe2, in the family of transition metal dichalcogenides (TMDCs), is a potential thermoelectric candidate due to its low thermal conductivity. While its mobility remains low as in other 2D TMDCs is inconceivable for electronic and thermoelectric applications. Earlier theoretical attempts have failed to give justification for the orders of low experimental mobility obtained for monolayer HfSe2. We calculate the carrier mobility in the framework of the density functional perturbation theory in conjunction with the Boltzmann transport equation and correctly ascertain the experimental value. We also calculate the carrier mobility with the previously employed method, the deformation potential (DP) model, to figure out the reason for its failure. We found that it is the strong electron-optical phonon interaction that is causing the low mobility. As the DP model does not account for the optical phonons, it overestimates the relaxation time by an order of two and also underestimates the temperature dependence of mobility. A strong polar type interaction is evidenced as a manifestation of a discontinuity in the first derivative of the optical-phonons at the K and Γ points as well as a dispersive optical phonon at the K point. We also included the spin-orbit coupling which leads to an energy splitting of ∼330meV and significantly affects mobility and scattering rates.

Direct Solution Phase Synthesis and Functionalization of 2D WSe2 for Ambient Stability.

2H phase tungsten diselenide (WSe2) is a p-type 2D semiconductor from the transition metal dichalcogenides (TMDs) family with unique opto-electrical properties. Solution phase production of atomically thin WSe2is challenging due to its instability under ambient conditions. We present a highly efficient and scalable solution method for simultaneously exfoliating and functionalizingWSe2by leveraging the noncovalent interaction between mercapto-group and bulk WSe2.Single and few-layer 2H phase pure WSe2sheets of lateral size up to 5µm with minimal basal plane defects, as revealed by XPS, Raman and FTIR spectroscopy, are produced in water-ethanol mixture. Remarkably, WSe2dispersion remains stable even at high concentrations (10 mg/ml) and exhibited high colloidal stability with shelf-life exceeding a year. The findings from our study suggest that through precise manipulation of intercalation chemistry, mass production of solution processable phase-sensitive 2D materials such as WSe2can be achieved. This advancement holds great potential for facilitating their practical utilization in various real-world applications.

Optical Properties of Sputtered Iron Sulfide for Photonic Device Applications

Iron Pyrite is an earth-abundant, non-toxic, low-cost material commonly referred to as fool’s gold due to its golden-yellow appearance. It belongs to the family of transition metal dichalcogenides (TMDs) and gained fame as a potential solar material during the 70s and 80s. The reason was its suitable band gap and excellent absorption coefficient. However, the performance of pyrite never achieved a beneficial place among other materials and remained a mysterious path of research so far. With advanced technology and better characterization techniques and equipment available, the focus is again shifting towards exploring pyrite as a potential photovoltaic material which can compete with other materials, such as silicon, primarily in terms of cost. However, the true nature of pyrite and its application in photovoltaics cannot be adequately defined without looking deeply into its optical properties and interaction with light [1-3].Here in this work, we explored the photosensitivity of pyrite thin films at different thicknesses. These thin films were fabricated using an RF-powered sputtering method using a 99.9% FeS2 target. P-doped silicon and fused silica were used as substrates. The pressure and RF power were kept at 03 mTorrs and 70 watts, respectively, and the procedure took place at room temperature with an argon gas flow rate of ~24 cm3/min. The thickness was controlled by varying the sputtering time from 20 to 90 mins. As-prepared pyrite films were characterized through variable angle spectroscopic ellipsometry (VASE) and UV-vis spectrophotometer. An optical model was first developed for VASE in which the thin pyrite film showed light-absorbing characteristics (B-spline). Based on that model, the film thickness was determined. It was observed that the thickness of pyrite increased linearly with time, having grown to ~100 nm after 90 minutes of sputtering.UV-vis spectrophotometry was used to collect the transmission, reflectance and absorbance data for the films deposited onto a fused silica substrate. It was observed that the films showed very high absorbance in the UV range, and the absorbance tended to increase with the increase in thickness. The direct band gap was then calculated using absorbance data and building Tauc’s plot, which was observed to be around 2.25 eV.In conclusion, owing to its suitable properties and optical characteristics, iron pyrite opens many paths to scientists and researchers to explore its utilization not only in photovoltaics but also in other opto-electronic fields. However, further studies are also required to deeply understand its mechanism and capability. Nonetheless, high absorption of light in UV and visible range at very thin layers, suitable band gap and easy accessibility could be breakthrough parameters in near future for pyrite.

Stacking-Order-Dependent Excitonic Properties Reveal Interlayer Interactions in Bulk ReS2.

Rhenium disulfide, a member of the transition metal dichalcogenide family of semiconducting materials, is unique among 2D van der Waals materials due to its anisotropy and, albeit weak, interlayer interactions, confining excitons within single atomic layers and leading to monolayer-like excitonic properties even in bulk crystals. While recent work has established the existence of two stacking modes in bulk, AA and AB, the influence of the different interlayer coupling on the excitonic properties has been poorly explored. Here, we use polarization-dependent optical measurements to elucidate the nature of excitons in AA and AB-stacked rhenium disulfide to obtain insight into the effect of interlayer interactions. We combine polarization-dependent Raman with low-temperature photoluminescence and reflection spectroscopy to show that, while the similar polarization dependence of both stacking orders indicates similar excitonic alignments within the crystal planes, differences in peak width, position, and degree of anisotropy reveal a different degree of interlayer coupling. DFT calculations confirm the very similar band structure of the two stacking orders while revealing a change of the spin-split states at the top of the valence band to possibly underlie their different exciton binding energies. These results suggest that the excitonic properties are largely determined by in-plane interactions, however, strongly modified by the interlayer coupling. These modifications are stronger than those in other 2D semiconductors, making ReS2 an excellent platform for investigating stacking as a tuning parameter for 2D materials. Furthermore, the optical anisotropy makes this material an interesting candidate for polarization-sensitive applications such as photodetectors and polarimetry.

Nano-infrared imaging of metal insulator transition in few-layer 1T-TaS2

Abstract Among the family of transition metal dichalcogenides, 1T-TaS2 stands out for several peculiar physical properties including a rich charge density wave phase diagram, quantum spin liquid candidacy and low temperature Mott insulator phase. As 1T-TaS2 is thinned down to the few-layer limit, interesting physics emerges in this quasi 2D material. Here, using scanning near-field optical microscopy, we perform a spatial- and temperature-dependent study on the phase transitions of a few-layer thick microcrystal of 1T-TaS2. We investigate encapsulated air-sensitive 1T-TaS2 prepared under inert conditions down to cryogenic temperatures. We find an abrupt metal-to-insulator transition in this few-layer limit. Our results provide new insight in contrast to previous transport studies on thin 1T-TaS2 where the resistivity jump became undetectable, and to spatially resolved studies on non-encapsulated samples which found a gradual, spatially inhomogeneous transition. A statistical analysis suggests bimodal high and low temperature phases, and that the characteristic phase transition hysteresis is preserved down to a few-layer limit.

The structure analysis and chemical properties probing during recycling processes of transition metal dichalcogenides exfoliation

Transition metal dichalcogenides (TMDs) have attracted much attention in electrochemistry due to their outstanding properties; however, there is a limit to the production of TMDs samples on a large scale. In this work, we have demonstrated the recycling process of TMDs materials using WSe2 as a representative of the TMDs family. WSe2 nanoflakes were synthesised by solvent-assisted liquid phase exfoliation through sonication of “bulk” WSe2. It has been found that the unexfoliated WSe2 can be recycled up to 6 times, this improve the production yield. The physical and chemical properties of the exfoliated WSe2 nanoflakes were investigated during the recycling processes. Interestingly, the recycling processes (number of recycling times) directly affect the flake sizes, and surface chemistry of the exfoliated samples. The obtained flake size decreases from about 1 μm to less than 200 nm (from 1st to 7th exfoliation), the diminished of sample thickness has also been found. Besides, the local structure of WSe2 remains unchanged, showing the W4+ oxidation state of WSe2, which to the best of our knowledge. The applicability of the as-recycled samples were then demonstrated as bi-functional electrode materials: (i) supercapacitor, (ii) and electrocatalyst (for hydrogen evolution reaction). From 1st to 7th exfoliation, the electrochemical performance improves about 1-fold (capacitance increase from 3 to 6 F g−1, and the overpotential at 10 mA cm−2 improves from -0.67 to -0.42 vs. RHE). This study provides an alternative way to improve the synthesis method, which can not only utilise “waste” but also recover the production yield of TMDs leading to the continued development of 2D materials, and related families.

Substitutional p-Type Doping in NbS2 -MoS2 Lateral Heterostructures Grown by MOCVD.

Monolayer MoS2 has attracted significant attention owing to its excellent performance as an n-type semiconductor from the transition metal dichalcogenide (TMDC) family. It is however strongly desired to develop controllable synthesis methods for 2D p-type MoS2 , which is crucial for complementary logic applications but remains difficult. In this work, high-quality NbS2 -MoS2 lateral heterostructures are synthesized by one-step metal-organic chemical vapor deposition (MOCVD) together with monolayer MoS2 substitutionally doped by Nb, resulting in a p-type doped behavior. The heterojunction shows a p-type transfer characteristic with a high on/off current ratio of ≈104 , exceeding previously reported values. The band structure through the NbS2 -MoS2 heterojunction is investigated by density functional theory (DFT) and quantum transport simulations. This work provides a scalable approach to synthesize substitutionally doped TMDC materials and provides an insight into the interface between 2D metals and semiconductors in lateral heterostructures, which is imperative for the development of next-generation nanoelectronics and highly integrated devices.