Janus Transition Metal Dichalcogenides Research Articles

Harnessing intrinsic electric fields in 2D Janus MoOX (X=S, Se, and Te) monolayers for enhanced photocatalytic hydrogen evolution

Two-dimensional (2D) Janus monolayers, distinguished by their intrinsic vertical electric fields, emerge as highly efficient and eco-friendly materials for advancing the field of hydrogen evolution reactions (HER). In this study, we explore, for the first time, the potential viability of the oxygenation phase of two-dimensional Janus transition metal dichalcogenides MoOX (X = S, Se, and Te) monolayers as an exceptionally efficient photocatalyst for hydrogen production. Based on first-principles computations, we demonstrate that all three monolayers exhibit semiconductor behavior, characterized by a band gap ranging from 0.66 to 1.55 eV. This narrow band gap renders the proposed materials highly efficient at absorbing light within the visible region. Excitingly, the introduction of an electrostatic potential difference ΔΦ has granted us the ability to surpass the conventional bandgap limit (Eg≥1.23). Consequently, all monolayers exhibit favorable band alignment with respect to the vacuum level. Moreover, the calculated solar-to-hydrogen efficiency for the envisaged monolayer exceeds the established theoretical limit. Particularly, the MoOTe monolayer emerges as an infrared-light-driven photocatalyst, demonstrating a remarkable solar-to-hydrogen efficiency limit of up to 25,21% when considering the entire solar spectrum. A thorough examination of the Gibbs free energy differences across these monolayers has revealed that the values during the oxygenation phase are significantly smaller and approach the optimum, in contrast to the parental two-dimensional Janus transition metal dichalcogenides. Our results conclusively establish that the proposed materials exhibit exceptional efficiency as photocatalysts for hydrogen evolution reactions. Notably, their efficacy is demonstrated even in the lack of co-catalysts or sacrificial agents.

2D Janus ZrSSe/SnSSe Heterostructure: A Promising Candidate for Photocatalytic Water Splitting.

The distinctive physical characteristics and wide range of potential applications in optoelectronic and photovoltaic devices have ignited significant interest in two-dimensional materials. Intensive research attention has been focused on Janus transition metal dichalcogenides due to their unique properties resulting from symmetry disruption and their potential in photocatalysis applications. Motivated by the current fascination with Janus TMD heterostructures, we conducted first-principles calculations to examine the stability, electronic, and optical properties of monolayers consisting of ZrSSe, SnSSe, and the ZrSSe/SnSSe heterostructure. The results indicate that the Janus ZrSSe/SnSSe heterostructure exhibits a structural and mechanical stability. Using the HSE06 functional, the ZrSSe/SnSSe heterostructure shows an indirect band gap of 1.20 eV, and band edge analysis reveals a type-II band alignment. The potential for photo/electrocatalysis in the ZrSSe/SnSSe heterostructure for water splitting or generating reactive oxygen species (ROS) has been explored, and it was found that the oxygen evolution reaction (OER) can spontaneously activate in acidic (pH = 0) media under light irradiation, with a potential of U = 1.82 eV. Additionally, the ZrSSe/SnSSe heterostructure exhibits strong light absorption across a wide range, from visible light to the ultraviolet region, at various levels. These findings open up possibilities for the application of ZrSSe/SnSSe-based materials in optoelectronic devices.

Impact of the Interface Stacking and Composition of Janus Transition Metal Dichalcogenide Bilayers on the Solar Power Conversion Efficiency

Janus transition metal dichalcogenides hold great promise for achieving highly efficient solar cells. Therefore, the MoXY and WXY (X, Y = S, Se, Te; X ≠ Y) monolayers are examined using first‐principles calculations to identify candidates with direct bandgap. A systematic analysis in terms of the interface stacking and composition of bilayers constructed of these monolayers is performed. Out of the 40 studied cases, 12 cases exhibit a direct bandgap with type‐II band alignment and therefore are explored in detail. It is discovered that the composition (for the same stacking) can strongly enhance the power conversion efficiency, while the stacking (for the same composition) has minor effects.

Enhanced magnetic anisotropy of iridium dimers on antisite defects of two-dimensional transition-metal dichalcogenides.

The combination of transition-metal (TM) elements with two-dimensional (2D) transition-metal dichalcogenides (TMDs) provides an effective route to realizing a 2D controllable magnetic order, leading to significant applications in multifunctional nanospintronics. However, in most TM atoms@TMDs nanostructures, it is challenging for the magnetic anisotropy energy (MAE) to exceed 30 meV when affected by the crystal field. Hence, the stronger magnetic anisotropy of TMDs has yet to be developed. Here, utilizing first-principle calculations based on density functional theory (DFT), a feasible method to enhance the MAEs of TMDs via configurating iridium dimers (Ir2) on 2D traditional and Janus TMDs with antisite defects is reported. Calculations revealed that 28 of the 54 configurations considered possessed structure-dependent MAEs of >60 meV per Ir2 in the out-of-plane direction, suggesting the potential for applications at room temperature. We also showed the ability to tune the MAE further massively by applying a biaxial strain as well as the surface asymmetric polarization reversal of Janus-type substrates. This approach led to changes to >80 meV per Ir2. This work provides a novel strategy to achieve tunable large magnetic anisotropy in 2D TMDs. It also extends the functionality of antisite-defective TMDs, thereby providing theoretical support for the development of magnetic nanodevices.

Pd doped Janus HfSeS monolayer: Ultrahigh sensitive gas sensing material for reversible detection of NO

Recent studies on two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) show that these materials have excellent electronic property and raised very promising prospects for applying them as gas sensor materials. Here we consider the potential application of replacement doping with Pd on the monolayer HfSeS as well as its pristine structure as gas sensor materials for detecting CO, CO2, NH3 and NO through first principles calculations. The calculated results of Ab initio molecule dynamics (AIMD) simulations as well as phonon spectrum which shows no negative frequencies confirmed the good thermodynamical stability of pristine and doped HfSeS. The formation energies of the doped structures with S and Se atoms substituted by Pd atom are − 5.174 and − 5.168 eV which further proves the stability of the structures. The pristine HfSeS cannot well distinguish these four gas molecules due to very close adsorption energies (range from −0.142 eV to −0.477 eV). However, after the introduction of Pd, the doped structure could effectively achieve the distinguish of these four gas molecules (with adsorption energy ranges from −0.229 eV to −0.933 eV). Furthermore, the recovery time of these gases is within 500 s, which shows that the adsorption is reversible both in pristine and Pd-doped monolayer HfSeS. Most interestingly, Pd-doped monolayer HfSeS exhibits ultrahigh sensitive to NO, as the systems become metallic after NO adsorption. Due to the change in conductivity, enhanced adsorption energy, and together with a relatively lower recovery time, Pd-doped monolayer HfSeS exhibits promising application as gas sensors for reversible detection of NO.

Investigation of Atomic‐Scale Mechanical Behavior by Bias‐Induced Degradation in Janus and Alloy Polymorphic Monolayer TMDs via In Situ TEM

The 2D Janus transition‐metal dichalcogenides (TMDs) and alloyed TMDs are a widely studied emerging class of 2D materials that have been extensively used in electronic devices because of their excellent electronic, optical, and mechanical properties. The properties and behaviors of 2D‐materials‐based devices, such as the electrical breakdown caused by structural failure, are significant issues that have drawn considerable attention. In this study, the electrical behavior of polymorphic molybdenum sulfide selenide (MoSSe) devices is studied via in situ biasing experiments and recorded using transmission electron microscopy (TEM) at the atomic scale. The selenization temperature is a key factor in the phase transition of the material, which further affects the electrical and mechanical properties of MoSSe. The effects of electron‐beam irradiation and bias voltage are also discussed through a combination of experiments and theory. Quantifying the defect coverage and defect size also helps us to understand the behavior of material degradation. Furthermore, Cs‐corrected scanning TEM is utilized to identify the evolution of the morphology. The fracture morphology of the synthesized structure also varies with the application of high voltage. The cracks and defects caused by Joule heating are studied in terms of fracture type and size.

Understanding Solvent‐Induced Delamination and Intense Water Adsorption in Janus Transition Metal Dichalcogenides for Enhanced Device Performance

AbstractRecently, there has been considerable interest in 2D Janus transition metal dichalcogenides owing to their unique structure that exhibits broken mirror symmetry along the out‐of‐plane direction, which offers fascinating properties that are applicable in various fields. This study investigates the issue of process instability in Janus MoSSe, which is mainly caused by its nonzero net dipole moments. It systematically investigates whether the built‐in dipole moments in Janus MoSSe make it susceptible to delamination by most polar solvents and increase its vulnerability to intense moisture adsorption, which leads to the deterioration of its semiconducting properties. To address these issues, as an example of device applications, field‐effect transistors (FETs) based on a van der Waals heterostructure are devised, where the bottom h‐BN (top h‐BN) insulating material is employed to prevent delamination (adsorption of moisture). The fabricated FETs exhibit improved electron mobility and excellent stability under ambient conditions.

Design of Electrocatalytic Janus WSeS/WSe2 Heterostructure Nanowall Electrodes with High Selectivity and Faradaic Efficiency for Nitrogen Reduction

AbstractThe electrochemical nitrogen reduction reaction (NRR) is an attractive process for next‐generation ammonia synthesis; therefore, identifying a suitable catalyst for this reaction is critical. In recent years, transition‐metal dichalcogenides (TMDs) and their Janus structures have gained significant attention because of their outstanding catalytic properties. However, the synthesis of Janus TMDs remains challenging, and exposing their active sites is difficult when using a low‐dimensional structure to improve the catalytic activity. To date, relatively little research has been conducted in this area. Herein, emerging Janus WSeS/WSe2 heterostructure nanowalls are systematically explored. These nanowalls are used as a nitrogen fixation catalyst in electrolytes. The nanowalls demonstrate a significant NH3 yield rate and Faradaic efficiency of 13.97 µg h‐mgcat−1 and 35.24% at −0.3 V in 0.1 m HCl, as well as 15.96 µg h‐mgcat−1 and 40.2% in 0.1 M Na2SO4. This study presents an in‐depth analysis of the properties of Janus WSeS/WSe2 heterostructure nanowalls and a conceptual framework for linking TMD‐based catalysts and the NRR.

Pressure-Driven Structural and Electronic Transitions in a Two-Dimensional Janus WSSe Crystal.

In this work, we presented the first report on the high-pressure structural stability and electrical transport characteristics in WSSe under different hydrostatic environments through Raman spectroscopy, electrical conductivity, and high-resolution transmission electron microscopy (HRTEM) coupled with first-principles theoretical calculations. For nonhydrostatic conditions, WSSe endured a phase transition at 15.2 GPa, followed by a semiconductor-to-metal crossover at 25.3 GPa. Furthermore, the bandgap closure was accounted for the metallization of WSSe as derived from theoretical calculations. Under hydrostatic conditions, ∼ 2.0 GPa pressure hysteresis was detected for the emergence of phase transition and metallization in WSSe because of the feeble deviatoric stress. Upon depressurization, the reversibility of the phase transition was substantiated by those of microscopic HRTEM observations under different hydrostatic environments. Our high-pressure investigation on WSSe advances the insightful understanding of the crystalline structure and electronic properties for the Janus transition-metal dichalcogenide (TMD) family and boosts prospective developments in functional devices.

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.

Single-atom W anchored Janus transition metal dichalcogenides as a promising catalyst for the ammonia synthesis

Electrochemical nitrogen reduction reaction (NRR) has emerged as an eco-friendly and economical tactic for ammonia synthesis recently. However, for most NRR catalysts, high overpotential and low Faraday efficiency are the main bottlenecks for their practical applications. Herein, a sequence of transition metals anchored on a novel category of transition metal dichalcogenides monolayers, Janus TMDs (TM@MoSSe, TM = Sc-Zn, Mo, W, Ru-Ag, and Os-Au), as NRR catalysts were investigated through a three-step detailed screening based on the density function theory (DFT) calculations. According to the estimated stability, high catalytic activity, and unexceptionable selectivity of TMDs, W@MoSSe and Os@MoSSe were identified as effective candidate catalysts for NRR, of which W@MoSSe was screened as the optimal NRR catalyst, and the limiting potential is −0.15 V. Importantly, W@MoSSe exhibits remarkable inhibition on the hydrogen evolution reaction (HER), due to the ultra-high Faraday efficiency (100%). Notably, more density of states in W@MoSSe, apparent charge transfer, and strong orbital hybridization between W@MoSSe and NRR intermediates may contribute to its observed high NRR catalytic activity. These findings showed the feasible applications of Janus TMDs in the single-atom catalysis of the NH3 synthesis, which could open a new bright prospect for exploring efficient NRR catalysts experimentally.

Adsorption and Sensing of NO2, SO2, and NH3 on a Janus MoSeTe Monolayer Decorated with Transition Metals (Fe, Co, and Ni): A First-Principles Study.

This paper reports the adsorption of toxic gases (NO2, SO2, and NH3) on a MoSeTe structure based on first principles. It was found that the gas (NO2, SO2, and NH3) adsorption on a pure MoSeTe monolayer was weak; however, the adsorption performance of these gas molecules on transition-metal-atom-supported MoSeTe monolayers (TM-MoSeTe) was better than that on pure MoSeTe monolayers. In addition, there was more charge transfer between gas molecules and TM-MoSeTe. By comparing the adsorption energy and charge transfer values, the trend of adsorption energy and charge transfer in the adsorption of NO2 and SO2 was determined to be Fe-MoSeTe > Co-MoSeTe > Ni-MoSeTe. For the adsorption of NH3, the effect trend was as follows: Co-MoSeTe > Ni-MoSeTe > Fe-MoSeTe. Finally, by comparing their response times, the better gas sensor was selected. The Ni-MoSeTe system is suitable for NO2 gas sensors, and the Fe-MoSeTe and Co-MoSeTe systems are suitable for SO2 gas sensors. The Fe-MoSeTe, Co-MoSeTe, and Ni-MoSeTe systems are all suitable for NH3 gas sensors. Janus transition-metal dichalcogenides have the potential to be used as gas-sensing and scavenging materials.

Giant room-temperature nonlinearities in a monolayer Janus topological semiconductor

Nonlinear optical materials possess wide applications, ranging from terahertz and mid-infrared detection to energy harvesting. Recently, the correlations between nonlinear optical responses and certain topological properties, such as the Berry curvature and the quantum metric tensor, have attracted considerable interest. Here, we report giant room-temperature nonlinearities in non-centrosymmetric two-dimensional topological materials—the Janus transition metal dichalcogenides in the 1 T’ phase, synthesized by an advanced atomic-layer substitution method. High harmonic generation, terahertz emission spectroscopy, and second harmonic generation measurements consistently show orders-of-the-magnitude enhancement in terahertz-frequency nonlinearities in 1 T’ MoSSe (e.g., > 50 times higher than 2H MoS2 for 18th order harmonic generation; > 20 times higher than 2H MoS2 for terahertz emission). We link this giant nonlinear optical response to topological band mixing and strong inversion symmetry breaking due to the Janus structure. Our work defines general protocols for designing materials with large nonlinearities and heralds the applications of topological materials in optoelectronics down to the monolayer limit.

Nonlinear Dispersion Relation and Out‐of‐Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers

AbstractJanus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long‐lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, second and third harmonic generation in MoSSe and WSSe Janus monolayers is studied. Polarization‐resolved spectroscopy is used to map the full second‐order susceptibility tensor of MoSSe, including its out‐of‐plane components. In addition, the effective third‐order susceptibility and the second‐order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe are measured at room and cryogenic temperatures. This work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next‐generation on‐chip multifaceted photonic devices.

Rashba spin-splitting in Janus SnXY/WXY (X, Y = S, Se, Te; X ≠ Y) heterostructures.

Janus transition metal dichalcogenide monolayers have shown a lack of mirror symmetry perpendicular to the 2D plane. The breaking of out-of-plane symmetry, along with the spin-orbit coupling, induces Rashba spin-splitting in these materials. In this work, Rashba spin splitting in Janus tin dichalcogenide monolayers are studied. In addition, the heterostructures of Janus SnXY and WXY (X, Y = S, Se, Te; X 6 = Y) monolayers are discussed. A Rashba spin-splitting energy of about 43 meV, more significant than the room temperature energy, is observed in the Janus SnSSe/WSSe heterostructure. The consequences of vertical strain on the semiconducting heterostructure are examined. Compressive vertical strain enhances the Rashba splitting, and tensile strain reduces the spin-splitting. For the compressive strain of 10.4%, Janus SnSSe/WSSe heterostructure remains semiconductor with only Rashba bands surrounding near the Fermi level. Enhanced Rashba parameter of about 0.96 eVÅ and splitting energy of about 72 meV are observed. These findings confirm that Janus SnSSe/WSSe heterostructure is a productive Rashba material for spintronic device applications.

On the excitonic effects of the 1T and 1O[formula omitted] phases of PdS2, PdSe2, and PdSSe monolayers

We report the excitonic effects in the 1T and 1OT phases of PdS2, PdSe2, and PdSSe monolayers. Using fully relativistic norm-conserving non-local PBE pseudopotentials and including the spin–orbit (SOC) coupling, the 1T geometries are indirect band gap semiconductors; conversely, the 1OT structures are semimetallic. The band gaps were corrected using the HSE06 hybrid functional, making all structures semiconductors. Maximally localized Wannier functions were utilized to fit the bands of all six structures. The excitonic band structures were obtained using the Bethe–Salpeter equation (BSE), and the absorption coefficients were estimated at both the BSE and the independent particle approximation (IPA) levels. While the 1OT monolayers are more inclined to absorb light polarized in the y-direction, the 1T structures absorb light equally, independent of the polarization direction. We perceive that the 1T phase is the most promising candidate for photovoltaic devices; in particular, the 1T PdSSe system possesses a maximum power conversion efficiency (PCE) of 29.31% if light trapping techniques are used to enhance absorbance. Amongst the 1OT materials, PdS2 features the highest PCE of 8.38%. Our results constitute the first step to encourage experimental studies towards the fabrication of Pd-based regular and Janus transition metal dichalcogenides photoabsorbers and give hints of how to increase the PCE of such materials.

Construction of optimized tight-binding models using ab initio Hamiltonian: application to monolayer 2H-transition metal dichalcogenides

We present optimized tight-binding (TB) models with atomic orbitals to improve ab initio TB models constructed by truncating full density functional theory (DFT) Hamiltonian based on localized orbitals. Retaining qualitative features of the original Hamiltonian, the optimization reduces quantitative deviations in overall band structures between the ab initio TB model and the full DFT Hamiltonian. The optimization procedure and related details are demonstrated by using semiconducting and metallic Janus transition metal dichalcogenides monolayers in the 2 H configuration. Varying the truncation range from partial second neighbors to third ones, we show differences in electronic structures between the truncated TB model and the original full Hamiltonian, and how much the optimization can remedy the quantitative loss induced by truncation. We further elaborate the optimization process so that local electronic properties such as valence and conduction band edges and Fermi surfaces are precisely reproduced by the optimized TB model. We also extend our discussions to TB models including spin-orbit interactions, so we provide the optimized TB model replicating spin-related properties of the original Hamiltonian such as spin textures. The optimization process described here can be readily applied to construct the fine-tuned TB model based on various DFT calculations.

Strain Anisotropy Driven Spontaneous Formation of Nanoscrolls from 2D Janus Layers

Abstract2D Janus transition metal dichalcogenides (TMDs) have attracted attention due to their emergent properties arising from broken mirror symmetry and self‐driven polarization fields. While it has been proposed that their vdW superlattices hold the key to achieving superior properties in piezoelectricity and photovoltaic, available synthesis has ultimately limited their realization. Here, the first packed vdW nanoscrolls made from Janus TMDs through a simple one‐drop solution technique are reported. The results, including ab initio simulations, show that the Bohr radius difference between the top sulfur and the bottom selenium atoms within Janus (M = Mo, W) results in a permanent compressive surface strain that acts as a nanoscroll formation catalyst after small liquid interaction. Unlike classical 2D layers, the surface strain in Janus TMDs can be engineered from compressive to tensile by placing larger Bohr radius atoms on top (to yield inverted C scrolls. Detailed microscopy studies offer the first insights into their morphology and readily formed Moiré lattices. In contrast, spectroscopy and FETs studies establish their excitonic and device properties and highlight significant differences compared to 2D flat Janus TMDs. These results introduce the first polar Janus TMD nanoscrolls and introduce inherent strain‐driven scrolling dynamics as a catalyst to create superlattices.

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.

Intermediate State between MoSe2 and Janus MoSeS during Atomic Substitution Process

Janus transition metal dichalcogenides (TMDCs), with dissimilar chalcogen atoms on each side of TMDCs, have garnered considerable research attention because of the out-of-plane intrinsic polarization in monolayer TMDCs. Although a plasma process has been proposed for synthesizing Janus TMDCs based on the atomic substitution of surface atoms at room temperature, the formation dynamics and intermediate electronic states have not been completely examined. In this study, we investigated the intermediate state between MoSe2 and Janus MoSeS during plasma processing. Atomic composition analysis and atomic-scale structural observations revealed the intermediate partially substituted Janus (PSJ) structure. Combined with theoretical calculations, we successfully clarified the characteristic Raman modes in the intermediate PSJ structure. The PL exhibited discontinuous transitions that could not be explained by the theoretical calculations. These findings will contribute toward understanding the formation process and electronic-state modulation of Janus TMDCs.