Janus WSSe Research Articles

First-Principles Investigation on Ru-Doped Janus WSSe Monolayer for Adsorption of Dissolved Gases in Transformer Oil: A Novel Sensing Candidate Exploration.

Using first-principles theory, this work purposes Ru-doped Janus WSSe (Ru-WSSe) monolayer as a potential gas sensor for detection of three typical gas species (CO, C2H2, and C2H4), in order to evaluate the operation status of the oil-immersed transformers. The Ru-doping behavior on the WSSe surface is analyzed, giving rise to the preferred doping site by the replacement of a Se atom with the formation energy of 0.01 eV. The gas adsorption of three gas species onto the Ru-WSSe monolayer is conducted, and chemisorption is identified for all three gas systems with the adsorption energy following the order: CO (-2.22 eV) > C2H2 (-2.01 eV) > C2H4 (-1.70 eV). Also, the modulated electronic properties and the frontier molecular orbital are investigated to uncover the sensing mechanism of Ru-WSSe monolayer upon three typical gases. Results reveal that the sensing responses of the Ru-WSSe monolayer, based on the variation of energy gap, to CO, C2H2, and C2H4 molecules are calculated to be 1.67 × 106, 2.10 × 105, and 9.61 × 103, respectively. Finally, the impact of the existence of O2 molecule for gas adsorption and sensing is also analyzed to uncover the potential of Ru-WSSe monolayer for practical application in the air atmosphere. The obtained high electrical responses manifest strong potential as a resistive sensor for detection of three gases. The findings hold practical implications for the development of novel gas sensing materials based on Janus WSSe monolayer. We anticipate that our results will inspire further research in this domain, particularly for applications in electrical engineering where the reliable detection of fault gases is paramount for maintaining the integrity and safety of power systems.

Schottky-barrier-free contacts with Janus WSSe 2D semiconductor using surface-engineered MXenes

Forming low-barrier contacts at metal/semiconductor interfaces is an important research area. The Fermi level pinning (FLP) effect is induced by strong interactions between 3D metal/semiconductor interfaces. By using a 2D metal in contact with a 2D semiconductor, the FLP effect can be weakened considerably, resulting in a reduction of the Schottky barrier height (SBH). Surface-engineered MXenes are the best electrode materials available presently. Therefore, herein, the interfacial properties of surface-engineered MXenes (M2CT2) (M=Hf, Mo, Ta, Zr, Nb; T = F, OH, O)/Janus WSSe (WSSe) heterostructures are systematically investigated using first-principles calculations. OH-terminated MXenes [M2C(OH)2]/WSSe heterostructures are found to form n-type ohmic contacts, and the tunnelling probabilities are high. O-terminated MXenes (M2CO2)/WSSe heterostructures tend to form p-type ohmic contacts and p-type Schottky contacts with a very low SBH, and the tunnelling probability is low. Conversely, F-terminated MXenes (M2CF2)/WSSe heterostructures form n-type and p-type Schottky contacts readily and n-type ohmic contacts (Hf2CF2/WSSe and Zr2CF2/SWSe heterostructures) partially, and the tunnelling probability is found to be low. In addition, the synergistic action of interface and intrinsic dipoles can effectively tune the contact type and SBH of the M2CT2/WSSe heterostructures, transforming the Mo2CF2/WSSe and Nb2CF2/WSSe heterostructures from n-type to p-type Schottky contacts. Through this novel strategy, an SB-free contact can be readily obtained, especially for OH-terminated MXenes (which are the most ideal electrode materials), followed by O-terminated MXenes. This work provides an important reference for the selection of suitable electrode materials and a novel approach for the development of high-performance electronic devices based on Janus WSSe.

Structural origin of spin-splitting anisotropy in janus dichalcogenides monolayers under pressure

Abstract Janus transition-metal dichalcogenides (TMDs) have drawn a great deal of attention because of their mirror plane symmetry breaking that allows the emergence of a built-in out-of-plane dipole which determine superior piezoelectric and spin-related properties. Furthermore, it has been shown in the recent literature that pressure application is capable of modulating spin-related phenomena in this class of materials. Generally, the spin-splitting presence in real systems is explored in terms of point group symmetry reduction using solely group theory arguments. However, we seek to associate the enhancement of spin-splitting in Janus TMDs monolayers by searching the most important local asymmetries responsible for the symmetry lowering that leads the monolayer larger spin-splitting energies. In this sense, we seek to unveil a possible structural descriptor that correlates with subbands splitting magnitude in Janus TMDs. To accomplish this, we performed a detailed first-principles investigation into WSSe Janus monolayers under biaxial in-plane strain to find that pressure induces a symmetry lowering from the C 3v to the C s point group. From these observations, we found that in-plane angle asymmetries between the chalcogens yield a distortion metric that can serve as a descriptor for enhanced spin-splitting in Janus WSSe since it strongly correlates with spin-splitting energies. Hence, our work establishes that, rather than solely global symmetry analysis, specific local distortions provide a key design principle to achieve strong spin-splitting in 2D Janus TMDs.

Strain-induced scrolling of Janus WSSe

Strain-induced scrolling of Janus WSSe

Energetics and electronic structure of Janus WSSe formation by continuous chalcogen substitutions

We investigated the energetics and the electronic structure of Janus WSSe when formed by continuous sulfurization and selenization of WSe2 and WS2, respectively, using density functional theory combined with the effective screening medium method. The total energy of WS2−x Se x is sensitive to the stoichiometry. The total energy increases monotonically as the substitutional surface selenization of WS2 increases. The sulfurization of WSe2 causes complex energetics with respect to the number of S atoms involved. Competition between the energy cost of polarization and the energy gain through S–W bond formation produces the compound WS0.445Se1.445, which is a metastable structure that gives the local minimum in the energy landscape. The electronic structures of the partially substituted structures are interpolated smoothly from those of Janus WS2 and WSe2.

Pt decorated Janus WSSe monolayer: A gas-sensitive material candidate for SF6 decomposition gases based on the first-principles

In gas insulated switchgear, unavoidable partial discharge faults cause SF6 to decompose various low-fluorine compounds. These compounds react with micro-water to form acids, leading to gas insulated switchgear corrosion as well as reduced insulation properties. For this reason, on-line monitoring of SF6 decomposition gases is essential. In this work, based on density functional theory, the structures of pristine WSSe and platinum doped WSSe monolayer are established, and eight adsorption systems of SO2, SO2F2, SOF2 and HF gas molecules on pristine WSSe and Pt-WSSe monolayer are constructed by geometrical optimization. By analyzing parameters such as adsorption energy, charge density, density of states, orbital theory, conductivity, sensing response and recovery time, the adsorption performance and gas-sensing mechanism of each adsorption system were investigated. The analytical results showed that both intrinsic WSSe and Pt-WSSe were unsuitable for HF gas detection and exhibited physical adsorption. The adsorption energies of Pt-WSSe for SO2, SO2F2, and SOF2 were −1.596 eV, −1.702 eV, and −2.288 eV, respectively, which showed strong chemisorption. At the optimum operating temperature, the Pt-WSSe/SO2F2 system has a fast recovery time of 1.928 s, a high response of 28.436 and a strong resistivity change, and this gas-sensitive material is well suited for the detection of this gas molecule. The Pt-WSSe/SO2 system has a small response value of 1.928 and the Pt-WSSe/SOF2 system has a strong adsorption energy of −2.288 eV. Therefore, the Pt-WSSe monolayer is not suitable as a gas-sensitive material for SO2 and SOF2 detection. This work provides theoretical guidance for the future development of low power, highly sensitive, fast response gas sensors for gas insulated switchgear fault detection.

Effect of van der Waals homogeneous interface on lattice thermal conductivity of Janus WSSe bilayer

Vertically stacked 2D materials have provided an unprecedented platform to identify various physical properties and discover novel interfacial emergent phenomena. In this study, the lattice thermal conductivity of Janus WSSe bilayer, involving diverse interfacial stacking configurations, has been clarified by solving the phonon Boltzmann transport equation based on first-principles calculations. The effect of homogeneously stacking on thermal conductivity and potential modulation approaches has also been revealed for Janus WSSe. Our results indicate that the thermal transport in Janus WSSe bilayer can be suppressed by van der Waals interface, and depends on both the stacking patterns and interfacial compositions (S-S, Se-Se and Se-S interfaces). Moreover, the interface effect can be further modulated by an applied vertical pressure. A further reduction in thermal conductivity can be achieved for Janus WSSe bilayer under vertical pressure, implying great potential for efficient thermal management and sensing applications.

Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Emergence of superconductivity by intercalation of alkali metals and alkaline earth metals in Janus transition-metal dichalcogenide heterostructures.

Superconductivity in two-dimensional materials has attracted considerable attention. A new material belonging to the family of Janus transition metal dichalcogenides with out-of-plane structural asymmetry has been recently found to show interesting physical and chemical properties. Using density functional theory and density functional perturbation theory, within the generalized gradient approximation with van der Waals correction, we performed a detailed investigation of the electronic structure, phonon dispersion, Eliashberg spectral function, and electron-phonon coupling of Janus MSSe bilayers (M = Mo or W) and the Janus MoSSe/WSSe heterostructure intercalated with alkali metals (Li, Na, and K) or alkaline earth metals (Mg, Ca, and Sr). We found that the Janus MoSSe bilayer, Janus WSSe bilayer, and Janus MoSSe/WSSe heterostructure transform from a direct band gap semiconductor to a metal or semimetal due to charge transfer from intercalated atoms to the Se and S planes. We showed that all compounds are dynamically stable conventional superconductors. In addition, the Janus heterostructure MoSSe/WSSe intercalated with K exhibits the highest electron-phonon coupling of about 2.12 and the highest superconducting transition temperature of about 14.77 K. Our results indicate the potential application of the Janus materials we investigated in superconductivity.

A DFT screening of transition metal single-atom catalyst anchored on Janus WSSe basal plane for highly efficient carbon dioxide reduction reaction

Electrochemical CO2 reduction reaction (CO2RR) can efficiently and selectively reduce carbon dioxide into chemical fuel. Here, we designed the novel catalysts of transition-metal atoms anchored on Janus WSSe basal plane (TM@SSe) monolayer by first-principles calculations. The results show that Zn@WSSe has the best CO2RR catalytic activity with limit potential of -0.32 V and a high selectivity for the competitive hydrogen precipitation reaction (HER). Moreover, ab initio molecular dynamics simulation (AIMD) calculation at 300 K indicated high thermodynamic stability of Zn@WSSe. The study of the rate limiting step (CO*+H++e−→ *CHO) of the Zn@WSSe structure illustrates that the CO2RR catalytic activity of these candidate catalysts largely depends on the better interaction between Zn atoms and surrounding W atoms, which H atom is more easily adsorbed to C atom, making the reaction process easier to achieve. The above research provides a new way to design efficient CO2RR electrocatalyst under ambient conditions.

First-Principles Studies of Strain-Adjustable Janus WSSe Monolayer/g-GeC Monolayer Heterojunctions: Implications for Photoelectric and Switching Devices

First-Principles Studies of Strain-Adjustable Janus WSSe Monolayer/g-GeC Monolayer Heterojunctions: Implications for Photoelectric and Switching Devices

Exploration of Pt-doped Janus WSSe monolayer as a typical gas sensor for condition assessment in XLPE cables

To evaluate the operation status of the XLPE cables in the power system, we purpose Pt-doped WSSe (Pt-WSSe) monolayer as a novel gas sensor for detections of the XPLE typical gases under the insulation defects. The adsorption and sensing mechanisms of the Pt-WSSe monolayer upon five typical gases, namely CO, C2H6, C2H4O, C4H6O and C8H8O are uncovered in this work, using the first-principles theory. We find that the for Pt-doping on Se-surface of the Janus WSSe monolayer is more energy-favorable and chemical stable, with the formation energy of −0.29 eV, and the bandgap of the identified Pt-WSSe monolayer is obtained as 1.34 eV. Besides, the Pt-WSSe monolayer performs chemisorption upon CO, C2H4O, C4H6O and C8H8O molecules, while performs physisorption upon C2H6 molecule, and the adsorption energies are calculated as −1.88, −0.97, −0.77, −1.55 and −1.62 eV, respectively. These gas interactions deform the electronic property of the Pt-WSSe monolayer, causing the remarkably change in its bandgap, and the caused sensing responses indicate the strong potential to be a potential resistance-type gas sensor upon CO, C2H4O and C4H6O detections, with the sensing response of −93.4 %, −97.5 % and −99.7 %, respectively. Moreover, the limited change in the work function (WF) of Pt-WSSe monolayer suggests its unsuitability to be a WF-type gas sensor for detection of XLPE typical gas species. All these findings not only illustrate the Pt-doping behavior on the physicochemical properties of Janus WSSe monolayer, but also uncover the gas sensing mechanism of metal-doped Janus TMDs for explorations of novel sensing materials.

First-principles investigations upon Pd-decorated Janus WSSe monolayer for sensing typical gases in overload dry-type transformers

In a long-running dry-type transformer, the insulation defects, such as partial discharge and overload, can decompose the inside insulation medium, epoxy resin, into several toxic gases, including HCHO and C2H3Cl, called typical gases spreading around the transformer substations. In this work, we purpose Pd-doped Janus WSSe (Pd-WSSe) monolayer as a novel sensing material for detections of HCHO and C2H3Cl, based on the first-principles theory, in order to evaluate the operation status of the dry-type transformers. The Pd-doping behavior is studied and we find that the Pd dopant prefers to be adsorbed on the S-surface through the TW1 site with the binding energy of −2.54 eV. Besides, the Pd-WSSe monolayer performs chemisorption upon HCHO and C2H3Cl molecules with Ead of −0.83 and −1.21 eV, resulting in the sensing response of −93.2 % and −99.2 % in two systems, respectively. Moreover, it is proved that the existence of H2O molecule impact little to the adsorption and sensing performance of Pd-WSSe monolayer. The findings herein are meaningful to explore more sensing materials for applications in the power system.

Controllable structure-engineered janus and alloy polymorphic monolayer transition metal dichalcogenides by plasma-assisted selenization process toward high-yield and wafer-scale production

Two-dimensional (2D) materials with an asymmetric structure like Janus transition metal dichalcogenides (TMDCs) exhibit intriguing electronic and optical properties, which can be used for numerous novel electronic and optoelectronic devices application. However, despite tremendous efforts, the synthesis technology of Janus TMDs has not been fully established, especially for high-yield and wafer-scale production. Here, we demonstrate the synthesis of Janus MoSSe by a top-down method for high-yield and wafer-scale production. In particular, we replace the top S atom of monolayer (ML) molybdenum disulfide (MoS2) with Se by controlled vertical plasma-assisted selenization process (PASP). We first synthesized ML Janus MoSSe flakes through PASP. A set of microscopies, spectroscopies and electrical measurements showed that low synthesis temperature (200 °C) leads to the formation of ML Janus MoSSe flake while high temperatures (400 °C and 600 °C) lead to the formation of polymorphic alloy MoSSe flakes. Furthermore, we show that Janus WSSe can likewise be obtained by PASP, indicating the universality of the process. To demonstrate the feasibility of PASP for wafer-scale production of Janus MoSSe, we perform the exact same process but with continuous ML MoS2 on 2-in. sapphire. Our synthesis approach provides a new scalable route to synthesize Janus TMDs toward future electronics.

Investigating the performance of N-type janus 2D WSSe monolayer photo-detectors by enhancing its optoelectronic properties

Photo-detector based on the (P, Mn, As)-doped janus WSSe monolayer is the area of interest in this work. Internal (IQE), External (EQE) and total quantum efficiency (ηQE) are analyzed as figure of merit of the WSSe ML based photo-detector. The quantum efficiencies are dependent on the absorption coefficient of the 2D material. In our work, properties such as optical and electronic of janus WSSe pristine and n-doped–(P, Mn, As) WSSe are discussed in depth. The optical properties include the study of absorption coefficient (α), dielectric function(real (Ɛ1), imaginary (Ɛ2)). Next, the formation and binding energy of both the system i.e. pristine and doped have been calculated and is found to be stable. Further total density of state (TDOS) and band structure is extracted which are the part of electronic properties. Moreover, the asymmetricity in the TDOS behavior is due to the presence of n-type doping impurities in the WSSe ML. TDOS calculation concludes the introduction of magnetic attributes in the n-doped system which was absent in the pristine system. Also, the band gaps of the pristine and n-doped systems are in close agreement to the TDOS behavior. On the other side the reduce band gap of the n-doped system marks the presence of the defect states in the lower region of the conduction band. Lastly, the device parameters namely, IQE, EQE and ηQE of WSSe ML photodetectors are reported with higher values for Mn-doped WSSe ML separately.

Nonlinear Photocurrent Responses in Janus WSSe Monolayer

Janus WSSe monolayer is a novel two-dimensional (2D) material that breaks the out-of-plane mirror symmetry and has a large built-in electric field. These features lead to sizable Rashba spin-orbit coupling and enhanced nonlinear optical properties, making it a promising material platform for various spintronic and optoelectronic device applications. In recent years, nonlinear photocurrent responses such as shift and injection currents were found to be closely related to the quantum geometry and Berry curvature of materials, indicating that these responses can serve as powerful tools for probing the novel quantum properties of materials. In this work, we investigate the second-order nonlinear photocurrent responses in a Janus WSSe monolayer theoretically based on first-principles calculations and the Wannier interpolation method. It is demonstrated that the Janus WSSe monolayer exhibits significant out-of-plane nonlinear photocurrent coefficients, which is distinct from the non-Janus structures. Our results also suggest that the second-order nonlinear photocurrent response in the Janus WSSe monolayer can be effectively tuned by biaxial strain or an external electric field. Thus, the Janus WSSe monolayer offers a unique opportunity for both exploring nonlinear optical phenomena and realizing flexible 2D optoelectronic nanodevices.

Energetics and electronic structure of bilayer Janus WSSe

Employing density functional theory along with the effective screening medium method, we investigated the energetics and electronic structure of bilayer Janus WSSe in terms of their interlayer stacking arrangement. Through the orbital hybridization between chalcogen atoms at interfaces, the energetics are sensitive to the interlayer stacking orientation and interface atomic arrangements. This interface atomic arrangement creates the unique electronic structure of bilayer Janus WSSe determined by the dipole moment arrangement of the constituent WSSe layers. The net polarity of thin films of Janus transition-metal dichalcogenides is a simple superposition of the dipole moments of the constituent layers.

Armchair Janus WSSe Nanotube Designed with Selenium Vacancy as a Promising Photocatalyst for CO2 Reduction.

Photocatalytic conversion of carbon dioxide into chemical fuels offers a promising way to not only settle growing environmental problems but also provide a renewable energy source. In this study, through first-principles calculation, we found that the Se vacancy introduction can lead to the transition of physical-to-chemical CO2 adsorption on Janus WSSe nanotube. Se vacancies work at the adsorption site, which significantly improves the amount of transferred electrons at the interface, resulting in the enhanced electron orbital hybridization between adsorbents and substrates, and promising the high activity and selectivity for carbon dioxide reduction reaction (CO2RR). Under the condition of illumination, due to the adequate driving forces of photoexcited holes and electrons, oxygen generation reaction (OER) and CO2RR can occur spontaneously on the S and Se sides of the defective WSSe nanotube, respectively. The CO2 could be reduced into CH4, meanwhile, the O2 is produced by the water oxidation, which also provides the hydrogen and electron source for the CO2RR. Our finding reveals a candidate photocatalyst for obtaining efficient photocatalytic CO2 conversion.

First principles study of the adsorption of alkali metal ions (Li, Na, and K) on Janus WSSe monolayer for rechargeable metal-ion batteries

Two-dimensional (2D) materials with high specific capacities and superior physical properties are essential for designing rechargeable metal-ion batteries. In this study, first-principles calculations were performed to evaluate the potential of a WSSe monolayer as an electrode material for rechargeable lithium (Li), sodium (Na), and potassium (K) ion batteries. Our results showed that all alkali adsorptions were energetically stable and caused a semiconductor-to-metal transition, improving electronic conductivity. The calculated open-circuit voltage (OCV) for Li ions (0.48 V), Na ions (0.57 V), and K ions (0.37 V) was less than 1 V, which is critical for high charge and discharge rates. The maximum theoretical capacities for Li, Na, and K atoms adsorbed on the Janus WSSe monolayer were 477.8, 371.5, and 156.0 mAh/g, respectively. Our calculated migration energy barriers for Li, Na and K on S layer (Se layer) are (0.25, 0.07 and 0.07 (0.18, 0.04 and 0.038) eV, respectively, suggesting that the Se layer experiences faster Li-ion diffusion than the S layer. The ion diffusion potential for Li, Na, and K on the S layer (Se layer) for path 1 was considerably lower than paths 2 and 3, suggesting that the Se layer has faster Li-ion diffusion than the S layer. These findings provide a promising avenue for designing high-performing anode materials for rechargeable metal-ion batteries.

Electronic properties and photocatalytic water splitting with high solar-to-hydrogen efficiency in a hBNC/Janus WSSe heterojunction: First-principles calculations

Currently, designing and exploring a photocatalyst with an interfacial electric field for hydrogen production from water splitting is crucial. Hence, based on first-principles calculations, we first calculate the electronic properties of the two-dimensional hexagonal boron nitride (hBN)/Janus WSSe heterojunction. However, the inherent energy band conformational defects of the hBN/Janus WSSe heterojunction inhibit the efficiency of photocatalytic. To solve this problem, we doped $\mathrm{C}=\mathrm{C}$ bonds on the monolayer hBN to tune the band gap. The research shows that the transition from the traditional type-I band alignment to the type-II band alignment can be achieved through different contracting methods between monolayer hexagonal boron nitride carbon (hBNC) and monolayer Janus WSSe. Notably, the hBNC/SWSe heterojunction is a direct Z-scheme photocatalyst and has high carrier mobility. Compared with the hBNC/SeWS heterojunction, the hBNC/SWSe heterojunction shows higher reduction reaction overpotential (${\ensuremath{\chi}}_{{\mathrm{H}}_{2}}=2.56\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) and oxidation reaction overpotential (${\ensuremath{\chi}}_{{\mathrm{O}}_{2}}=0.83\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) under acidic conditions. Furthermore, the hBNC/SWSe heterojunction has a high quantum efficiency and solar hydrogen production efficiency (33.31%). Gibbs free energy calculations confirm that both oxidation evolution reaction and hydrogen evolution reaction are exothermic reactions and can proceed spontaneously under the external potential provided by photogenerated carriers. These results suggest that the hBNC/SWSe heterojunction has potential applications in visible-light-driven water splitting.