Janus Transition-Metal Dichalcogenides

• Janus TMDs are atomically thin materials with asymmetric chalcogen layers that create intrinsic dipoles and enable tunable electronic, excitonic, and mechanical properties.
• Advanced synthesis methods like plasma halogenation and CVD verify controlled symmetry breaking and structural integrity via atomic-scale imaging and spectroscopy.
• They exhibit unique phenomena such as Rashba spin–orbit coupling, enhanced piezoelectricity, and promising applications in catalysis, spintronics, and optoelectronics.

Janus transition-metal dichalcogenides (TMDs) are a class of atomically thin materials in which the two chalcogen layers sandwiching a transition metal differ in composition. This deliberate breaking of out-of-plane symmetry yields an intrinsic dipole and internal electric field, giving rise to highly tunable spin–orbit coupling, piezo-/flexoelectricity, nonlinear optics, and emergent many-body phenomena. Janus TMDs generalize the widely studied MX₂ (M = Mo, W, Nb, V, Cr; X = S, Se, Te) monolayers to forms MXY (or MXX'), where X ≠ Y, enabling designer control over electronic, phononic, and topological properties. The past decade has seen rapid advances across synthesis, structural characterization, theoretical modeling, and device realization.

1. Atomic Structure, Symmetry Breaking, and Synthesis

Janus TMDs adopt the same in-plane hexagonal lattice as conventional 2H-TMDs but substitute one chalcogen layer with a chemically distinct element, reducing the symmetry from D₃h (MX₂) to C₃v (MXY) in the 2H phase (Kowalski et al., 10 Oct 2025, Zhang et al., 2017, Bian et al., 17 Dec 2025). The absence of a horizontal mirror plane (σ_h) confers a robust out-of-plane electric dipole per unit cell, with calculated polarizations P_z ≈ 0.1–0.3 e/Å, internal fields E_int ≈ 10⁸–10⁹ V/m, and built-in Rashba fields (Sahu et al., 13 Nov 2025, Sayyad et al., 2024). The structural motif is confirmed through HAADF-STEM, TOF-SIMS, and cross-sectional EELS, showing spatially separated chalcogen species at the top and bottom surfaces (Bian et al., 17 Dec 2025, Zhang et al., 2017).

Synthesis approaches include top-layer substitution via plasma halogenation or solution-phase etching, bottom-plane replacement in nano-confined environments (using graphene/hBN capping), and direct vapor-phase CVD (Bian et al., 17 Dec 2025, Zhang et al., 2017). Reaction energetics reveal that one-sided chalcogen substitution is kinetically enabled by capping and site-selective intercalation, even when thermodynamically slightly unfavorable at ambient pressure (Bian et al., 17 Dec 2025). The Janus configuration imparts a vertical dipole (p₀ ∼ 0.5 e·Å per unit cell), imaged directly by atomic-resolution STEM (Bian et al., 17 Dec 2025).

Janus structures exist in both hexagonal 2H and orthorhombic Td (for WTe₂) as well as 1T and 1T' polymorphs, with corresponding symmetry reductions and Raman/phonon signatures (Kowalski et al., 10 Oct 2025, Petrić et al., 2020).

2. Electronic Structure, Spin–Orbit Coupling, and Rashba Effects

Breaking mirror symmetry in Janus TMDs leads to consequential changes in their electronic band structures. μ-ARPES measurements on WSSe reveal an upward shift of the valence band maximum (VBM) at Γ by ≈160 meV relative to pristine WSe₂ (ΔE_Γ ≈ +160 meV), with the K-point VBM remaining unchanged (Sakano et al., 5 Oct 2025). This shift is attributed to the orbital rehybridization induced by the internal dipole, and can drive transitions between indirect and direct band gaps, offering band alignment control (Sakano et al., 5 Oct 2025).

Spin degeneracy of the bands is lifted via Rashba-type spin–orbit coupling (SOC) allowed only when σ_h is broken. Experimental ARPES and DFT confirm Rashba splittings ΔE_Rashba ≈ 200–300 meV along Γ–M in WSSe, with estimated Rashba parameters α_R ≈ 0.3–0.4 eV·Å (Sakano et al., 5 Oct 2025, Sahu et al., 13 Nov 2025). This effect is intrinsic and grows in magnitude with increased electronegativity contrast between the chalcogens (e.g., S–Te > S–Se). SOC-induced spin textures produce orbital and spin “chirality reversals” and rich Edelstein (current-induced spin and orbital polarization) effects (Sahu et al., 13 Nov 2025).

In monolayer Janus MXY, DFT and tight-binding models establish an 11-orbital Hamiltonian capturing the mixing among metal d and chalcogen p orbitals, with internal E_int driving strong intermixing and in-plane orbital moments proportional to wave vector q (orbital Rashba effect) (Sahu et al., 13 Nov 2025). Table II of (Sahu et al., 13 Nov 2025) reports α_L^Γ(MoSSe) ≃ 0.33 μ_B·Å, α_L^Γ(MoSTe) ≃ 2.2 μ_B·Å.

3. Vibrational Properties, Raman Signatures, and Quality Assessment

Janus monolayers display four first-order Raman-active modes (two E, two A₁) at the Γ point, in contrast to parent MX₂ with only two (E', A₁') (Kowalski et al., 10 Oct 2025, Petrić et al., 2020). DFT-calculated and measured frequencies for MoSSe are: E(1) ≈ 209 cm⁻¹, E(2) ≈ 359 cm⁻¹, A₁(1) ≈ 295 cm⁻¹, A₁(2) ≈ 446 cm⁻¹ (Kowalski et al., 10 Oct 2025, Petrić et al., 2020). The appearance and evolution of these peaks as X→Y substitution proceeds enables real-time, non-destructive monitoring of the conversion process and domain purity (Kowalski et al., 10 Oct 2025, Petrić et al., 2020).

Defect-activated Raman modes (e.g., at ∼155 cm⁻¹ in MoSSe) provide a quantitative metric for chalcogen vacancy concentrations n_D; the ratio I_defect/I_A₁ scales linearly with vacancy density across 10¹¹–10¹³ cm⁻² (Petrić et al., 2020, Sayyad et al., 2024).

Temperature and resonance effects modify line widths and energies, with resonance enhancement observed near excitonic optical transitions (λ_ex ≈ 633 nm) (Petrić et al., 2020). Theoretical Raman libraries (“digital twins”) now exist for rapid identification and structural analysis (Kowalski et al., 10 Oct 2025).

4. Optical, Nonlinear, and Excitonic Responses

Janus TMDs combine strong out-of-plane polarization with tight binding of excitons due to weaker screening and quantum confinement (binding energies up to ≈0.5–0.65 eV in MoSSe/WSSe (Bian et al., 17 Dec 2025, Mao et al., 19 Jun 2025)). Exciton-phonon and defect-bound excitonic states are observed in hBN-encapsulated samples, with neutral and defect-bound emission lines at 1.84, 1.68, 1.57, and 1.54 eV (WSSe) (Sayyad et al., 2024). Cryogenic PL resolves these features with linewidths 2–4 meV and saturation powers <1 μW.

Janus 1T′-phase TMDs exhibit topological (band-inverted) electronic structures with ultra-small gaps Eg ~ 10–50 meV and colossal bulk photovoltaic (shift current) effects in the THz regime: σ_xx^SC ~ 2300 nm·μA/V², corresponding to 2800 mA/W photoresponsivity (Xu et al., 2021). The sign of the shift current flips across topological transitions (tuned via strain or out-of-plane electric field), introducing a non-volatile, electrically switchable nonlinear response (Xu et al., 2021).

In Janus 2H-TMDs, shift-current generation is strongly enhanced at the “C-exciton” resonance, where electron and hole are spatially separated on different atomic sites (e.g., S and Se), producing large real-space shift vectors and enabling bulk photovoltaic currents even in monolayer form. Calculated shift conductivity at the C-peak reaches up to 3 × 10⁻⁷ A/V² in WSSe (Mao et al., 19 Jun 2025).

5. Charge, Spin, and Orbital Transport Phenomena

The inversion symmetry breaking and large internal fields in Janus TMDs drive robust spin-charge coupling and emergent transport phenomena. Experimental and modeling studies demonstrate that, upon application of an in-plane current or optical field:

• The orbital Edelstein effect (OEE) and the spin Edelstein effect (SEE) yield current-induced orbital and spin polarization; for MoSSe χ^L_{yx} ~ 10^{–7} μ_B/(V m^{-1}) and χ^S_{yx} ~ 10^{–8} μ_B/(V m^{-1}) (Sahu et al., 13 Nov 2025).
• Optically induced spin-Hall current generation is enhanced by Rashba SOC, allowing for pure spin currents with in-plane or out-of-plane spin polarization under linearly or circularly polarized light, with spin-Hall angle up to tens of percent (Kameda et al., 10 May 2025).
• In (magnetic) Janus monolayers, such as VSeTe and CrSTe, giant intrinsic Rashba SOC, strong exchange, and large perpendicular magnetic anisotropy enable efficient spin–orbit torque (SOT) switching, with damping- and field-like torque efficiencies (η_{DL, FL}) of ≈1–2%, and switching current densities J_c^0 ~ 10⁶–10⁷ A/cm² (Smaili et al., 2020, Vojáček et al., 2024).

For bilayer stacks, symmetry and stacking dictate whether Rashba, Dirac, or Kagome-type minibands form in the moiré superlattice, with bandwidths and SOC scales tunable through composition and twist angle (Angeli et al., 2022).

6. Defect Landscape, Chemical Modifications, and Nanostructuring

Janus TMDs possess a characteristic “defect genome” dominated by single and double chalcogen vacancies (Vs, VSe), chalcogen interstitials, and metal impurities (Sayyad et al., 2024). These introduce in-gap states, modify optical emission (defect-bound excitons), and influence catalytic activity (HER, OER) (Sayyad et al., 2024, Zhang et al., 2017). The formation energies of chalcogen vacancies are ∼2–2.3 eV, with double vacancies slightly stabilized due to binding (Sayyad et al., 2024).

Quantum dots and nanoscrolls further amplify Janus-specific effects. Janus TMD QDs (MXY, MXO) exhibit large out-of-plane dipoles, curvature-induced charge separation, and high reactivity for hydrodesulfurization due to exposed basal edges and built-in polarity (Dominguez et al., 4 Apr 2025). Spontaneous nanoscrolls form due to Bohr-radius mismatch-induced surface strain; solvent intercalation triggers rolling, yielding hollow superlattices with high aspect ratio, enhanced environmental stability, unique optical and transport properties, and robust excitonic responses (Sayyad et al., 2023).

7. Functional Properties and Applications

Janus monolayers couple piezoelectric, flexoelectric, and pyroelectric responses with engineered band structure and SOC. Calculated out-of-plane piezoelectric coefficients d₃₁ can exceed 1 pm V⁻¹ (Bian et al., 17 Dec 2025), and calculated flexoelectric constants reach 0.09–0.13 nC/m (3–4x MoS₂, >30x graphene) (Javvaji et al., 2022). Bending-induced flexoelectric polarization augments piezoelectric response in devices, enabling high-output electromechanical energy conversion (Sayyad et al., 2023, Javvaji et al., 2022).

Carrier mobilities, when properly accounting for polar optical phonon scattering through Born effective charge (BEC), reach μ_e ≈ 400 cm²/(Vs) for WSSe, scaling inversely with |Z*| (cf. T phase ZrSSe: μ_e ~26 cm²/(Vs)) (Hu et al., 2022).

Janus TMDs have demonstrated enhanced basal-plane HER activity (overpotentials ~310–350 mV, approaching MoSe₂’s best values but with better environmental stability), traceable to synergistic strain, dipole, and defect effects (Zhang et al., 2017). The asymmetric chemistry accelerates H adsorption/desorption, while out-of-plane fields can tune intermediate binding energies.

Potential device platforms include:

• SOT-MRAM: all-in-one, atomically abrupt, scalable memory cells without heavy-metal underlayers (Smaili et al., 2020, Vojáček et al., 2024)
• Optoelectronics: vertical field photodiodes, nonlinear photodetectors, and switchable terahertz rectifiers (Xu et al., 2021, Mao et al., 19 Jun 2025)
• Piezo/flexoelectric nanogenerators (Javvaji et al., 2022, Sayyad et al., 2023)
• Valleytronics and spintronics: controllable valley and magnetization states via gating, strain, or optical excitation (Sahu et al., 13 Nov 2025, Kameda et al., 10 May 2025)
• Catalysts for HER/OER and hydrodesulfurization (Zhang et al., 2017, Dominguez et al., 4 Apr 2025)
• Platform for CDW manipulation and emergent many-body phases, including symmetry-tunable charge-density waves in Janus VTeSe (Xu et al., 2024)

High-throughput computational screening is feasible by using BEC and symmetry-based descriptors, with rapid estimation of key optoelectronic and transport properties (Hu et al., 2022, Kowalski et al., 10 Oct 2025).

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Janus TMDs exemplify a paradigm for symmetry and interface engineering at the atomic limit. Through compositional and geometric control, they enable the co-design of dipole, SOC, excitonic, and defect physics, unlocking applications from quantum devices to sustainable catalysis (Sahu et al., 13 Nov 2025, Bian et al., 17 Dec 2025, Mao et al., 19 Jun 2025, Xu et al., 2021, Angeli et al., 2022).