Janus In2S2Se: Tunable 2D Ferroelectric Monolayer

• Janus In2S2Se is a 2D polar monolayer with chemically distinct S and Se surfaces that create a permanent out-of-plane dipole.
• Its non-degenerate ferroelectric phases, WZ' and ZB', offer tunable band gaps and unique photovoltaic responses controlled by lateral sliding.
• Computational studies confirm robust structural and thermal stability, highlighting its potential for advanced nanoscale optoelectronic devices.

Searching arXiv for the specified paper and closely related Janus indium chalcogenide work. Janus In$_2$S$_2$Se is a two-dimensional polar monolayer derived from monolayer $\alpha$-In$_2$Se$_3$ by selectively substituting two of the three Se atomic planes with homotopic S, producing a quintuple-layer sequence with chemically dissimilar top and bottom faces and an intrinsic out-of-plane dipole. In the phase-engineered description reported for this material, the monolayer retains two sliding ferroelectric stackings, denoted WZ$'$ and ZB$'$, but the Janus asymmetry renders them non-degenerate, with distinct polarization magnitudes, band structures, and photovoltaic responses. The resulting system is presented as a monolayer platform in which reversible lateral sliding between polar phases tunes visible and near-infrared photocurrent characteristics (Li et al., 30 Jul 2025).

1. Structural definition and Janus asymmetry

Janus In$_2$S$_2$Se is constructed from $\alpha$-In$_2$0Se$_2$1, whose parent monolayer has a Se–In–Se–In–Se quintuple-layer sequence. Replacing two Se planes yields In$_2$2S$_2$3Se and creates a built-in chemical asymmetry between the S-terminated and Se-terminated surfaces. This out-of-plane asymmetry breaks inversion and mirror symmetry along $_2$4, yielding a permanent out-of-plane dipole even at zero external field (Li et al., 30 Jul 2025).

The monolayer preserves the corrugated quintuple-layer motif of $_2$5-In$_2$6Se$_2$7, in which the In layers are coordinated by chalcogen planes above and below and the polar distortion displaces sublayers along $_2$8. In the Janus derivative, the different electronegativities and polarizabilities of S and Se amplify the dipole asymmetry. Three substitution configurations, denoted $_2$9-, $\alpha$0-, and $\alpha$1-In$\alpha$2S$\alpha$3Se, were examined for each ferroelectric phase, and the most stable $\alpha$4-In$\alpha$5S$\alpha$6Se was selected for detailed study (Li et al., 30 Jul 2025).

A recurrent source of confusion is the proximity of this material to Janus In$\alpha$7SSe. Those systems are not structurally equivalent. Janus In$\alpha$8SSe, as studied in an InSe-derived context, is a Se–In–In–S quadruple layer obtained by replacing one chalcogen face of monolayer InSe and belongs to a distinct structural family with $\alpha$9 symmetry (Wan et al., 2019). By contrast, Janus In$_2$0S$_2$1Se in the phase-engineered ferroelectric work is an $_2$2-In$_2$3Se$_2$4-derived quintuple layer (Li et al., 30 Jul 2025).

2. Non-degenerate sliding ferroelectric phases

Monolayer $_2$5-In$_2$6Se$_2$7 is known to host two polar stackings, WZ$_2$8 and ZB$_2$9, corresponding to distinct registry of the quintuple-layer sublayers relative to the top plane. In Janus In$_3$0S$_3$1Se, both stackings remain polar, but they become non-degenerate because the top and bottom faces are no longer equivalent. The WZ%%%%4$\alpha$4%%%%3ZB$_3$4 transformation proceeds by lateral sliding of the middle and bottom layers with respect to the top layer across the two-dimensional honeycomb registry, and the pathway is reversible with a single barrier (Li et al., 30 Jul 2025).

The ferroelectric response was evaluated from the plane-averaged electrostatic potential drop $_3$5 across the monolayer together with Bader charge analysis. The two phases exhibit strengthened, non-degenerate out-of-plane polarization:

Property	WZ'	ZB'
$_3$6	$_3$7 pC m$_3$8	$_3$9 pC m$'$0
$'$1	$'$2 eV	$'$3 eV
Bader charge transfer magnitude	$'$4 e	$'$5 e

For comparison, monolayer $'$6-In$'$7Se$'$8 is approximately $'$9 pC m$'$0, while sliding out-of-plane polarization in bilayer BN and MoS$'$1 is approximately $'$2 and $'$3 pC m$'$4 under similar computational settings (Li et al., 30 Jul 2025).

The sliding barrier along the WZ%%%%6$'$6%%%%6ZB$'$7 path is approximately $'$8 meV per structural cell, comparable to monolayer In$'$9Se$_2$0 at approximately $_2$1 meV. This barrier is described as small enough for reversible switching by feasible stimuli such as nanoscale shearing, lateral strain, or electric-field-assisted actuation, although a coercive field is not reported (Li et al., 30 Jul 2025).

As a formal point of reference, the modern theory of polarization expresses the electronic contribution as

$_2$2

where $_2$3 are cell-periodic Bloch states and the sum runs over occupied bands. The reported numerical polarization values, however, were obtained from potential-drop and Bader-charge analysis rather than Berry-phase evaluation (Li et al., 30 Jul 2025).

3. Stability and computational characterization

The reported first-principles workflow used VASP with PBE-GGA for structural relaxation and electronic structure, HSE06 with $_2$4 exact exchange for band-gap correction, projector augmented-wave potentials, a plane-wave cutoff of $_2$5 eV, a $_2$6 $_2$7-mesh, and vacuum larger than $_2$8 Å. The self-consistent-field tolerance was $_2$9 eV and ionic relaxation continued until forces were below $_2$0 eV Å$_2$1 (Li et al., 30 Jul 2025).

Energetically, the tested In$_2$2S$_2$3Se configurations have binding energies of $_2$4–$_2$5 J m$_2$6, larger than typical values for MoS$_2$7, GaTe, and Bi$_2$8O$_2$9Se, which was taken as evidence of robust cohesion and experimental feasibility. Dynamical stability was supported by phonon dispersions of $\alpha$0-In$\alpha$1S$\alpha$2Se in both WZ$\alpha$3 and ZB$\alpha$4, which show no imaginary modes throughout the Brillouin zone. Thermal stability was assessed by ab initio molecular dynamics at $\alpha$5 K for $\alpha$6 ps in a $\alpha$7 supercell, yielding only small energy fluctuations and no bond breaking or reconstruction (Li et al., 30 Jul 2025).

These stability results are significant because Janus ordering in two-dimensional materials is not universally favored thermodynamically. A separate disorder-focused study on Janus In$\alpha$8SSe, MoSSe, SnSSe, PtSSe, and GaInSe$\alpha$9 concluded that ordered Janus arrangements in monolayers are energetically penalized relative to less ordered allotropes because of bond-length mismatch between sulfide and selenide environments (Boukhvalov, 2022). That study did not explicitly examine the $_2$00-In$_2$01Se$_2$02-derived Janus In$_2$03S$_2$04Se addressed here, but it establishes a broader caution: chemical asymmetry can enhance dipoles while also introducing structural frustration (Boukhvalov, 2022).

4. Electronic structure and carrier transport

Janus In$_2$05S$_2$06Se exhibits phase-dependent band topology. The WZ$_2$07 phase is an indirect-gap semiconductor with $_2$08 eV at the PBE level and $_2$09 eV at the HSE06 level. The ZB$_2$10 phase is a direct-gap semiconductor with $_2$11 eV at the PBE level and $_2$12 eV at the HSE06 level. The WZ$_2$13$_2$14ZB$_2$15 transition therefore moderates the band gap and induces an indirect-to-direct transition, shifting the material toward stronger near-infrared optical activity (Li et al., 30 Jul 2025).

The spatial character of the band edges is also phase selective. In WZ$_2$16, the conduction-band minimum is dominated by the bottom Se layer, whereas the valence-band maximum is dominated by the top and middle S layers. In ZB$_2$17, the conduction-band minimum is dominated by the top S layer and the valence-band maximum by the bottom Se layer. This layer-selective separation of electron and hole densities across the quintuple layer is consistent with the opposite sign of the out-of-plane polarization and electrostatic potential drop in the two phases (Li et al., 30 Jul 2025).

Carrier mobilities $_2$18 were computed using deformation-potential theory,

$_2$19

where $_2$20 is the elastic modulus along the transport direction, $_2$21 is the deformation potential of the band edge, $_2$22 is the transport effective mass, and $_2$23 is the density-of-states mass. The reported qualitative outcome is that mobilities in Janus In$_2$24S$_2$25Se exceed those of monolayer $_2$26-In$_2$27Se$_2$28, and that ZB$_2$29 has higher electron and hole mobilities than WZ$_2$30, although numerical mobilities and anisotropy are not listed in the main text excerpt (Li et al., 30 Jul 2025).

This transport trend is relevant to the optoelectronic contrast between phases. WZ$_2$31 benefits from stronger built-in field strength through larger $_2$32, while ZB$_2$33 benefits from a direct and smaller gap together with higher mobility. The material therefore does not present a single optimal phase across all wavelengths; rather, the sliding coordinate selects between different transport and conversion regimes (Li et al., 30 Jul 2025).

5. Photovoltaic response and phase-selective operation

The photovoltaic analysis was carried out with NEGF-DFT using Nanodcal under open boundary conditions, with PBE, a DZP localized basis, norm-conserving pseudopotentials, a $_2$34 $_2$35-mesh in the device center and $_2$36 in the leads, and a convergence criterion of $_2$37 eV. The device consists of a central scattering region formed by the monolayer and periodically extended leads. A small source-drain bias of $_2$38 eV is applied only to drive current, and linearly polarized light is incident normal to the plane (Li et al., 30 Jul 2025).

Within first-order Born approximation, the photocurrent into the left lead is

$_2$39

with photocurrent density $_2$40. The polarization-angle dependence is decomposed as

$_2$41

where $_2$42, $_2$43, and $_2$44 are energy-dependent coefficients (Li et al., 30 Jul 2025).

The optical absorption coefficients $_2$45 of WZ$_2$46 and ZB$_2$47 are broadly similar across infrared, visible, and ultraviolet ranges. This is an important constraint on interpretation: the different photocurrent spectra are not attributed primarily to differences in absorption magnitude. Instead, the decisive variables are polarization, band topology, and mobility (Li et al., 30 Jul 2025).

At $_2$48, the main device photocurrent peaks in the visible range are $_2$49 $_2$50A mm$_2$51 at $_2$52 eV for WZ$_2$53 and $_2$54 $_2$55A mm$_2$56 at $_2$57 eV for ZB$_2$58. Both values exceed the previously reported value for monolayer In$_2$59Se$_2$60 under the same computational framework, approximately $_2$61 $_2$62A mm$_2$63 (Li et al., 30 Jul 2025).

The phase contrast is spectrally resolved. WZ$_2$64 shows superior photoelectric conversion efficiency across the visible light region because its stronger out-of-plane polarization intensifies the built-in field and promotes more effective separation of photogenerated carriers. Conversely, the WZ$_2$65$_2$66ZB$_2$67 transition red-shifts the primary photocurrent peak and enhances the infrared response, consistent with the reduced direct band gap and higher mobility in ZB$_2$68 (Li et al., 30 Jul 2025).

Angular response is also phase dependent. At $_2$69 eV, WZ$_2$70 and ZB$_2$71 display phase-shifted angular dependences, described as cosine-like and sine-like, respectively. At $_2$72 eV, both phases show cosine-like dependence. In the reported interpretation, these changes reflect differences in symmetry-selected optical transitions and are captured by the coefficients $_2$73, $_2$74, and $_2$75 in the angular decomposition (Li et al., 30 Jul 2025).

6. Mechanistic interpretation, device concepts, and relation to adjacent Janus systems

The mechanistic picture advanced for Janus In$_2$76S$_2$77Se couples three ingredients. First, Janus asymmetry creates a strong, phase-dependent built-in field, visualized by $_2$78 eV in WZ$_2$79 and $_2$80 eV in ZB$_2$81. Second, sliding modifies the band-gap magnitude and changes the band topology from indirect to direct. Third, the carrier mobility is higher in ZB$_2$82 than in WZ$_2$83. The non-degeneracy of the two sliding ferroelectric states is therefore not merely a difference in polarization sign; it produces different magnitudes of $_2$84, different gaps, and different transport characteristics, enabling phase-selective optimization of visible and near-infrared operation (Li et al., 30 Jul 2025).

This suggests a deterministic phase knob for ultrathin optoelectronic devices. The reported device concepts include in-plane sliding-controlled photovoltaic modulators, lateral ferroelectric photodiodes with phase-patterned domains for wavelength-division multiplexing, and graphene/Janus-In$_2$85S$_2$86Se/graphene photodetectors in which interfacial sliding modulates phase and responsivity. The operational windows were summarized as WZ$_2$87 optimized for visible light, approximately $_2$88–$_2$89 eV, and ZB$_2$90 optimized for near-infrared through visible onset, approximately $_2$91–$_2$92 eV (Li et al., 30 Jul 2025).

Direct experimental synthesis of monolayer Janus In$_2$93S$_2$94Se was not reported in the phase-engineered study. Feasibility was instead argued from the calculated binding energies, phonon stability, and room-temperature ab initio molecular dynamics, together with analogy to face-selective chalcogen exchange used in Janus transition-metal dichalcogenides (Li et al., 30 Jul 2025). A plausible implication is that structural verification for this specific composition would require techniques sensitive to chemical asymmetry and sliding-controlled registry, such as Raman, TEM, XPS, and electrostatic probes, although those validations were not presented in the study.

Related Janus indium chalcogenides provide context but not direct equivalence. Janus In$_2$95SSe based on monolayer InSe has an indirect gap of approximately $_2$96 eV, electron mobility $_2$97 cm$_2$98/(V·s), thermal conductivity $_2$99 W/(m·K) at $\alpha$00 K, and Raman-active $\alpha$01 peaks at $\alpha$02, $\alpha$03, and $\alpha$04 cm$\alpha$05 because mirror-symmetry breaking activates modes that are Raman-inactive in $\alpha$06 InSe (Wan et al., 2019). Those results illustrate the broader consequences of Janus asymmetry in indium chalcogenides, but the sliding ferroelectricity and phase-tunable photovoltaic behavior discussed above are specific to the $\alpha$07-In$\alpha$08Se$\alpha$09-derived Janus In$\alpha$10S$\alpha$11Se monolayer (Li et al., 30 Jul 2025).