Janus Transition Metal Dichalcogenides Research Articles

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.

Controllable and enormous spin splitting in antiferromagnetic MnPSe3 through interfacial coupling with Janus MoSSe

The manipulation of spin and valley degree of freedom in two-dimensional materials is desirable for emerging applications in next-generation electronics. Here, we report a regulation of the spin splitting in monolayer antiferromagnetic MnPSe3 through stacking van der Waals (vdW) heterostructures with transition-metal dichalcogenides (TMDs). Different TMDs including Janus MoSSe, MoSe2, and MoS2 are engaged and the spin and valley dependences on stacking configurations, interfacial atoms, and interlayer spacings are comprehensively investigated based on first-principles calculations. The results show that spin degeneracy of MnPSe3 can be lifted through the interfacial interaction in the vdW heterostructures. Compared with the MnPSe3 interfacing with traditional TMDs (MoSe2 and MoS2), those with Janus TMDs (MoSSe or MoSeS) have larger spin splitting. A maximal spin splitting of 50.8 meV at valence band maximum is achieved in MoSSe/MnPSe3 heterostructure, and such splitting is further enhanced to 243.0 meV when the interlayer spacing is reduced to 2.9 Å. It is found that the different hybridizations between the d-orbitals of the two inequivalent Mn atoms in MnPSe3 and the above TMDs lead to the occurrence of net magnetic moments, and thus induce spin splitting. This work paves a novel way to modify the spin-valley properties of two-dimensional semiconductors.

Identification of Exciton Complexes in Charge-Tunable Janus WSeS Monolayers.

Janus transition-metal dichalcogenide monolayers are artificial materials, where one plane of chalcogen atoms is replaced by chalcogen atoms of a different type. Theory predicts an in-built out-of-plane electric field, giving rise to long-lived, dipolar excitons, while preserving direct-bandgap optical transitions in a uniform potential landscape. Previous Janus studies had broad photoluminescence (>18 meV) spectra obfuscating their specific excitonic origin. Here, we identify the neutral and the negatively charged inter- and intravalley exciton transitions in Janus WSeS monolayers with ∼6 meV optical line widths. We integrate Janus monolayers into vertical heterostructures, allowing doping control. Magneto-optic measurements indicate that monolayer WSeS has a direct bandgap at the K points. Our results pave the way for applications such as nanoscale sensing, which relies on resolving excitonic energy shifts, and the development of Janus-based optoelectronic devices, which requires charge-state control and integration into vertical heterostructures.

First-principles study of electronic, mechanical and piezoelectric properties of Janus MoSH

ABSTRACT Janus transition metal dichalcogenides (TMDCs) type two-dimensional materials have been widely studied because of their unique asymmetrical structures and properties. In the preparation of Janus TMDCs, replacing the top layer S atoms of MoS2 with H atoms could yield unique Janus MoSH, which has been scarcely reported. In this work, the electronic, mechanical and piezoelectric properties of Janus MoSH are systematically studied using first principles calculations for the first time. The band structure of Janus MoSH is of metallic nature with two bands crossing the Fermi level. This material has a stable structure, and the Poison ratio ν > 1/3 indicates the good ductility. The relatively higher piezoelectric coefficients, e11 of −4.27 × 10−10 C/m and d11 of −5.09 pm/V for Janus MoSH, compared with the typical piezoelectric semiconductor materials MoS2 (3.64 × 10−10 C/m and 3.73 pm/V, respectively) may reflect suitable piezoelectric effect in the former. The above calculations may show that Janus MoSH would be potentially adopted as efficient sensors and piezoelectric components.

Prediction of photogalvanic effect enhancement in Janus transition metal dichalcogenide monolayers induced by spontaneous curling

Enhancement of photogalvanic effect (PGE) in two-dimensional materials is of great importance for nanoscale self-powered optoelectronic devices. In this paper, we systematically investigate the generation of PGE photocurrent in six Janus transition metal dichalcogenides (MXY, M = Mo,W, X,Y = S,Se,Te) based on the quantum transport simulations. The proposed photodetectors along zigzag and armchair directions with non-collinear electrodes are built based on spontaneous curvatures of Janus TMDs. The spontaneous curling is induced by the relaxation of intrinsic strain in Janus TMDs and the curvatures are determined by molecular simulations. Vertically illuminated by linearly polarized light, PGE photocurrents can be observed in Janus TMDs with both curled and planar structure. The influences of polarization angle and photon energy on the photocurrents are quantitatively demonstrated. The photocurrents are largely amplified by more than 1 order of magnitude on average in the curled photodetector. This enhancement can be attributed to the device symmetry reduction from the C3v to Cs symmetry. We expect the results can help develop recipes for flexible two-dimensional optoelectronic devices.

Anisotropic Rashba effect in two-dimensional non-Janus transition-metal dichalcogenide, MSSe (M=Mo,W) alloys

The breaking of the out of plane mirror symmetry in the Janus transition-metal dichalcogenides (TMDs) has made them appealing candidates for optoelectronic, spintronic, valleytronic, and photocatalytic applications. In this study, we compare the structural and electronic properties of the Janus structure with the more stable TMD alloys of the form $M\mathrm{SSe} (M=\mathrm{Mo},\mathrm{W})$. The charge distribution analysis and two-dimensional electron localization function analysis indicate that the geometry of the system plays a vital role in determining the magnitude of the metal to ligand charge transfer, which in turn dictates the stability of the alloys. The arrangement of the chalcogen atoms in the more stable alloy allows for a better charge distribution, resulting in superior stability. Band structure calculations reveal that both structures are semiconducting with identical direct band gaps and exhibit Rashba splitting near the \ensuremath{\Gamma} point in the presence of the spin-orbit coupling (SOC) effects. While in the Janus structures, the broken mirror symmetry gives rise to an out of plane uniform electric field, the reduced symmetry of the more stable alloy structure produces an anisotropic arrangement of the perpendicular dipole moments. The nonuniformity of the electric dipole moments is reflected in the Rashba effect, which has an anisotropic nature, unlike the isotropic Rashba splitting observed in the Janus structures.

Novel Janus 2D structures of XMoY (X, Y = O, S, Se, Te) composition for solar hydrogen production

The successful fabrication of H-phase Janus transition metal dichalcogenides (TMDs) has received considerable interest due to its great potential in photocatalytic applications. Here, new A′-XMoY (X/Y = O, S, Se, Te) Janus-type structures belonging to the family of TMDs were theoretically investigated for the first time in terms of photocatalytic water splitting via DFT calculations. For all compounds, the Raman spectra were calculated. The SMoO, SeMoO, SMoSe, SMoTe and SeMoTe compounds are dynamically stable and are semiconductors. Among all considered structures SMoTe is the most promising candidate for solar hydrogen production because valence and conduction bands perfectly engulf the redox potentials of water at both neutral and acidic media, opposite to SMoSe, SMoO, SeMoO suitable only in the acidic media, and SeMoTe – in the neutral media. Moreover, A′-SMoTe demonstrates the outstanding values of the solar-to-hydrogen (STH) conversion efficiencies of 54.0 and 67.1 for neutral and acidic media.