MSSe Janus Layers in 2D Materials

• MSSe Janus layers are two-dimensional materials where a transition metal is sandwiched between distinct chalcogen layers, forming an intrinsic out-of-plane dipole.
• They exhibit unique electronic structures, including tunable band gaps and robust spin-orbit effects, which are critical for applications in electronics, spintronics, and optoelectronics.
• Advanced synthesis techniques, interfacial engineering, and defect doping strategies enable precise control over their properties for enhanced device performance.

MSSe Janus Layers are a distinct subclass of two-dimensional materials in which a transition metal (M = Mo, W, etc.) is sandwiched between two chemically inequivalent chalcogen layers (S and Se), breaking out-of-plane symmetry and producing a permanent electric dipole perpendicular to the basal plane. These structures, realized primarily in the 2H and 1T polytypes, exhibit properties that are uniquely enabled by the broken mirror symmetry, with direct implications for electronic structure, device physics, and interfacial engineering.

1. Atomic and Electronic Structure

Pristine MSSe Janus monolayers feature a trigonal-prismatic metal coordination (2H, D₃h or P6₃/mmc) or an octahedral 1T stacking (P$\overline{3}$m1). The lattice constants and bond lengths are consistently reported as $a \approx 3.25$–$3.28$ Å and M–S/M–Se bond lengths of $\sim$2.41–2.53 Å. The S–M–Se stacking leads to an intrinsic dipole, measurable by the vacuum potential drop $\Delta V$ across the slab, with values $\Delta V\sim 0.77$ eV for monolayer MoSSe (Zhang et al., 2021) and dipole moments $p_{\rm Janus}\sim 3.4$–4.4 D (Boukhvalov, 2022).

Electronic structure calculations show that monolayer MoSSe is a direct gap semiconductor ($E_g=1.50$ eV at K), while monolayer g-AlN is an indirect gap semiconductor ($E_g=3.46$ eV). In MoSSe/g-AlN heterostructures, the band gap is $1.627$ eV and indirect, with VBM dominated by N $p$ states and CBM by Mo $d$ and Se $p$ states (Yelgel, 17 Jan 2024). The intrinsic out-of-plane dipole affects potential steps at the interface and facilitates interlayer charge transfer.

2. Synthesis and Thermodynamic Stability

Large-area, high-optical-quality Janus MoSSe monolayers have been synthesized via a one-pot CVD method in which a bottom Se$\rightarrow$S exchange in pre-grown MoSe₂ on Au(111) is thermodynamically and kinetically accessible, with a formation plus adsorption energy $E_{\rm form+ads}\approx 1.7~\mathrm{eV}$ for Se abstraction at 700 °C under S atmosphere (Gan et al., 2022). Structural fingerprints, including Raman (A$_{1}^{\prime}$ at $290~\mathrm{cm}^{-1}$, E$^\prime$ at $351~\mathrm{cm}^{-1}$) and ARXPS, confirm the vertical Se–Mo–S stacking.

Thermodynamic analysis underscores the role of bond-length frustration (e.g., Mo–S $2.417$ Å, Mo–Se $2.547$ Å in MoSSe). The elastic energy penalty per formula unit ($\Delta E_{\rm frust}\sim 36$ meV) is compensated in multilayer stacks or bulk by favorable dipole–dipole interactions ($E_{dd}\sim 38$ meV/f.u.), provided the layer dipole exceeds a critical value ($p_{\rm crit}\sim 3$ D) (Boukhvalov, 2022). However, entropic effects favor S/Se disorder above $T_c\sim 72$ K in monolayer MoSSe.

3. Ferroelasticity, Piezoelectricity, and Quantum Properties

MSSe layers in the distorted Haeckelite S′ phase (e.g., 1S′–MoSSe, 1S′–WSSe) simultaneously support ferroelasticity and quantum spin Hall (QSH) topology (Ma et al., 2018). Ferroelastic switching between degenerate ground states is characterized by low barriers (e.g., $\Delta E=2.4$ meV/f.u. for MoSSe) and significant spontaneous strain (4.7% in WSSe). First-principles calculations show bulk (SOC-opened) bandgaps of $E_g=17$–$63$ meV and $Z_2=1$ topological invariants, with Dirac helical edge states robust under strain or switching. Angle-dependent Young’s and Poisson’s moduli highlight pronounced mechanical anisotropy, while the Janus geometry guarantees nonzero in-plane piezoelectricity, supported by DFPT-calculated $e_{11}\sim 3.89$ C/m and $d_{11}\sim 4.24$ pm/V for MoSSe (Guo et al., 2019).

4. Interfacial Engineering and Heterostructures

The intrinsic dipole enables control over van der Waals heterostructures. In MoSSe/g-AlN, the interlayer binding energy is $E_{\rm bind}=99.3$ meV/atom, and defect engineering in the g-AlN layer allows deliberate tuning of the heterostructure’s electronic gap, Fermi level, and presence of localized midgap states (Yelgel, 17 Jan 2024). In Janus MoSSe/MoS₂ heterobilayers (Zhang et al., 2021), the interface (S/S vs Se/S) and twist angle determine the interlayer separation (6.10 vs 6.31 Å), PL quenching ($R_A^{\rm S/S}\sim0.10$, $R_A^{\rm Se/S}\sim0.20$), and built-in field (potential step $\Delta V_{\rm int}\sim 0.76$ vs $-0.65$ eV) via steered charge redistribution.

The dipole also strongly modifies band alignment in heterojunctions (e.g., MoS₂/WSSe), collapsing the type-II offset from $300$ meV to $\sim50$ meV and resonantly bringing interlayer and intralayer excitons within $40$ meV. This enables efficient phonon-mediated conversion between bright and dark excitons, crucial for solar conversion devices (Torun et al., 2022).

5. Defect Engineering, Doping, and Machine Learning Design

Substitutional doping of MSSe by metalloids (B, Si, Ge) at Mo, S or Se sites (typically 2–4%) breaks local symmetry, enhances site activity for H adsorption, and tunes hydrogen adsorption energies from strongly endothermic ($E_{\rm ad}\sim$+1.7 eV on S in pristine MoSSe) to exothermic (e.g., $E_{\rm ad}\sim-0.04$ eV for B@Mo) (Tejaswini et al., 20 Nov 2025). These modifications are relevant for hydrogen evolution and photocatalytic applications. Interstitial doping remains less effective. A ML multi-layer perceptron trained on DFT-computed features (23 descriptors, PCA-reduced to 11 principal components) predicts $E_{\rm ad}$ with test $R^2=0.90$ after data augmentation, expediting the screening of dopants, adsorption sites, and coverages.

6. Magnetism, Spintronics, and Topological States

Janus MSSe-type magnets (MnSSe, VSSe, etc.) exhibit intrinsic half-metallicity, ferromagnetism, and large spin-orbit effects (Chen et al., 2022, Smaili et al., 2020). For example, monolayer MnSSe (1T, $a=3.522$ Å) has Curie temperature $T_c\sim72$ K, direct gap $E_g^{\downarrow}=1.14$ eV in the spin-down channel, and strong out-of-plane built-in fields [$E_{\rm built}\sim0.89$ V/Å, $\Delta V\sim2.66$ eV]. Electric field and doping efficiently tune the magnetic anisotropy and charge transfer, enabling field-effect spintronic operation. In VSSe, the Rashba coefficient $\alpha_R\sim70$ meV⋅Å and spin-orbit torque efficiency $\xi_{\rm DL}\sim1.5\%$ approach those of heavy-metal systems (Smaili et al., 2020). By contrast, in MnSSe DMI is negligible, precluding intrinsic skyrmion formation, as opposed to heavier Te-based Janus materials (Yuan et al., 2019).

7. Device Concepts and Application Landscape

The Janus-built-in dipole enables intrinsic “self-doping,” forming n- and p-type Ohmic or Schottky contacts. In trilayer MoSSe, the built-in field ($E_0=1.4\times10^9$ V/m) and surface polarization density ($P_{\rm sp}=4.76\times10^{13}$ cm⁻²) engineer band bending at metal/MoSSe interfaces: Se-faced contacts yield n-type, S-faced yield p-type Ohmic behavior (Chiu et al., 2023). 2D Poisson–drift–diffusion simulations show both n- and p-MOSFETs on a single flake, with on-currents $I_{\rm on}>10^3$ A/cm at $|V_g-V_{\rm th}|=1$ V and subthreshold swings $SS\sim60$ mV/dec. No chemical doping is needed; CMOS circuits can be monolithically integrated in a single Janus sheet.

In van der Waals heterostructures (e.g., MoSSe/g-AlN, MoSSe/MoS₂), the intrinsic dipole and defect engineering enable tunable rectification ratios (R$\sim$34–85) and parity-dependent transmission (Liu et al., 2020, Yelgel, 17 Jan 2024). Scrolls and nanoscrolls formed via spontaneous strain anisotropy (surface strain from S/Se Bohr radii mismatch) yield multiwall tubes (15–45 nm diameters), presenting Moiré superlattices, robust field-effect transistor operation, and environmental stability (Sayyad et al., 2023).

On the optoelectronic frontier, hBN-encapsulated MoSSe supports narrow exciton lines ($18$ meV FWHM), valley Zeeman splitting ($g=-3.3$), and enhanced exciton–phonon coupling, with PL and magneto-optical signatures intermediate between MoS₂ and MoSe₂.

In sum, MSSe Janus layers are prototype systems for exploring the coupled physics of broken symmetry, built-in electric fields, interfacial control, and emergent quantum and device phenomena across electronics, spintronics, optoelectronics, and catalysis (Yelgel, 17 Jan 2024, Gan et al., 2022, Chiu et al., 2023, Liu et al., 2020, Tejaswini et al., 20 Nov 2025, Torun et al., 2022, Zhang et al., 2021, Chen et al., 2022, Smaili et al., 2020).