Janus Sb2SSeO: 2D Antimony Oxychalcogenide

• Janus Sb2SSeO is a 2D antimony-based oxychalcogenide monolayer featuring distinct sulfur and selenium layers that create an intrinsic polar structure.
• The material exhibits pronounced anisotropic structural, electronic, and mechanical properties, with effective band-gap tuning under biaxial strain.
• Its favorable band-edge alignment and high photocatalytic efficiency make it a strong candidate for direction-dependent optoelectronics and sustainable water splitting.

Janus Sb₂SSeO is a two-dimensional (2D) antimony-based oxychalcogenide monolayer characterized by intrinsic structural asymmetry and pronounced electronic and optoelectronic anisotropy. As established by first-principles calculations, this monolayer features a Janus configuration with sulfur and selenium occupying distinct out-of-plane positions, leading to unique physicochemical properties. The combination of robust thermodynamic, mechanical, and dynamical stability, substantial band anisotropy, tunable band structure, and favorable band-edge alignment makes Janus Sb₂SSeO a particularly strong candidate for direction-dependent optoelectronic devices and for photocatalytic water splitting under neutral pH conditions (Shahrokhi et al., 9 Feb 2026).

1. Structural and Mechanical Characteristics

Janus Sb₂SSeO crystallizes in the triclinic space group P1̅ (No. 2), with a primitive cell comprising 20 atoms: 8 Sb, 4 O (one face), 4 S (opposite face), and 4 Se (mid-plane). HSE06-optimized lattice parameters are $a = 11.37$ Å, $b = 8.34$ Å, and $\gamma = 68.49^\circ$. The out-of-plane asymmetry is quantified by a vertical separation $l = 3.51$ Å between the S and Se layers, producing the Janus “polarity.”

Bond distances exhibit two distinct ranges: $d_{1-2} \approx 2.05$–$2.20$ Å for Sb–S bonds, and $d_{3-6} \approx 2.57$–$2.98$ Å for Sb–Se. The formation energy ($E_\text{form} \simeq -10.80$ eV/atom) confirms thermodynamic stability, with the value being competitive with symmetric analogues Sb₂S₂O and Sb₂Se₂O.

Phonon calculations reveal an absence of imaginary frequencies throughout the Brillouin zone, affirming dynamical stability. The highest optical phonon mode appears near 18.50 THz. Ab initio molecular dynamics simulations at both 300 K and 500 K (10 ps) show negligible energy drift or structural distortion.

Cleavage energies, by analogy to symmetric counterparts, are estimated at $E_\text{cleave} \approx 0.38$ J/m², similar to graphene, suggesting mechanical exfoliation is practical.

Elastic constants are directionally dependent: $C_{11} = 24.02$ N/m, $C_{22} = 38.18$ N/m, $C_{12} = 3.53$ N/m, and $C_{66} = 15.03$ N/m. Young’s moduli are $E_{xx} = 37.67$ N/m and $E_{yy} = 23.70$ N/m, with an anisotropy ratio ~1.6. Poisson’s ratios are correspondingly $\nu_{xx} = 0.15$, $\nu_{yy} = 0.09$, and minimal and maximal shear moduli are $G_\text{min} = 10.85$ N/m and $G_\text{max} = 16.42$ N/m. Satisfaction of Born–Huang criteria confirms mechanical stability.

Quantity	Value/Description	Notes
Space group	P1̅ (No. 2)	Triclinic, Janus structure
Out-of-plane separation $l$	3.51 Å	S vs. Se layer asymmetry
Cleavage energy	$\approx$0.38 J/m²	Comparable to graphene
$C_{11}$, $C_{22}$	24.02, 38.18 N/m	Indicates in-plane anisotropy

2. Electronic Structure

Janus Sb₂SSeO is an indirect-band-gap semiconductor. PBE (GGA) calculations yield a gap of 1.67 eV (indirect), which reduces to 1.58 eV upon inclusion of SOC. The HSE06 functional gives a corrected gap of 2.44 eV (indirect, VBM along X–H₁, CBM at X); with SOC, this is 2.35 eV.

Valence-band maxima (VBM) originate primarily from $X$ p (chalcogen) and O p orbitals, while conduction-band minima (CBM) are predominantly Sb 5p with minor chalcogen p contributions. The work function, calculated via HSE06, is $\Phi = 4.82$ eV.

Effective masses, obtained via parabolic fitting, show $m_e^* = 0.75\,m_0$ (x) and $1.10\,m_0$ (y) for electrons, and $m_h^* = 1.60\,m_0$ (x) and $0.65\,m_0$ (y) for holes. Mobility-based values (with $\tau = 10$ fs) produce lower masses: $m_e^* = 0.37\,m_0$ (x), $0.71\,m_0$ (y); $m_h^* = 0.19\,m_0$ (x), $0.17\,m_0$ (y).

At 300 K and $n_\text{dop} = 1\times10^{15}$ cm⁻² (AMSET), electrons exhibit $\mu_x \approx 47$ cm²/V·s and $\mu_y \approx 25$ cm²/V·s, while holes yield $\mu_x \approx 91$ cm²/V·s and $\mu_y \approx 104$ cm²/V·s. Mobilities persist above 10 cm²/V·s up to 600 K.

Band-edge positions with respect to vacuum and water redox levels (pH 7) are: CBM $\approx -3.22$ eV, VBM $\approx -5.97$ eV, enabling overall water splitting, with driving forces of $U_e = 0.81$ V and $U_n = 1.75$ V.

3. Optoelectronic Response

The static in-plane dielectric constant $\epsilon_\parallel > 10$, with marked anisotropy ($\epsilon_{xx} \neq \epsilon_{yy} \gg \epsilon_{zz}$), indicating efficient charge screening and suppressed excitonic binding. Optical absorption demonstrates strong in-plane anisotropy, with pronounced differences between $\alpha_x(\omega)$ and $\alpha_y(\omega)$.

The complex dielectric function $$\epsilon(\omega) = \epsilon_1(\omega) + i\epsilon_2(\omega)$$, calculated with sum-over-states HSE06, yields an absorption onset near $E_g \approx 2.44$ eV (approximately 508 nm). The absorption coefficient $\alpha(\omega)$ spikes to $\sim10^5$ cm⁻¹ in the near-UV (3–4 eV) and is comparable or superior to monolayer MoS₂ in the 300–500 nm spectral window.

Optical Quantity	Value	Notes
$\epsilon_\parallel$	$>$10	Strong in-plane screening
$\alpha_\text{peak}$	$\sim10^5$ cm⁻¹	In near-UV, exceeds MoS₂
Absorption onset	2.44 eV	Visible-light region

4. Band Engineering via Strain

Biaxial strain is defined as $\varepsilon_\text{biaxial} = (a-a_0)/a_0$, where $a_0$ is the unstrained lattice parameter. HSE06 calculations predict systematic band-gap shifts under ±5% biaxial strain:

• Compressive strain (−5%): $E_g \downarrow$ to $\approx2.09$ eV.
• Tensile strain (+5%): $E_g \uparrow$ to $\approx2.61$ eV.

CBM remains relatively stable ($<0.1$ eV shift), thus $U_e$ stays nearly constant. VBM shifts upward with compression, reducing $U_n$, and downward under tension, increasing $U_n$. The indirect nature of the band gap is preserved across the entire strain range. This suggests feasible strain-tuning for application-tailored electronic and photocatalytic response.

5. Photocatalytic Water Splitting

Band-edge alignment positions CBM above the H⁺/H₂ level and VBM below O₂/H₂O, supporting overall photocatalytic water splitting at pH 7, with calculated driving forces $U_e = 0.81$ V and $U_n = 1.75$ V—both providing significant overpotential margins.

Solar-to-hydrogen (STH) efficiency, considering light absorption and carrier utilization (at pH 7, unstrained), reaches $\eta_\text{STH} = 6.49\%$ (corrected $\eta_\text{STH}' = 6.48\%$). Under −2% strain, efficiency can be boosted to approximately 7.78%. The light absorption efficiency is 19.9%, and carrier utilization efficiency is 32.6%.

A computational hydrogen electrode (CHE) model indicates that the oxygen evolution reaction (OER) follows a dual-site mechanism with photohole potentials of 1.75 V, resulting in barrierless O–O coupling steps. For the hydrogen evolution reaction (HER), H* adsorption at $U_e = 0.81$ V yields $\Delta G_\text{H*} = +1.11$ eV, comparable to other 2D photocatalysts such as Cd₆S₂ and AgBiP₂Se₆.

Janus Sb₂SSeO demonstrates structural and thermal stability up to 500 K, with the moderate indirect band gap ($\sim$2.44 eV) providing efficient visible-light absorption. In addition, its anisotropic high carrier mobilities (up to ∼100 cm²/V·s) and tunable STH efficiency underscore its potential for sustainable energy conversion.

6. Significance and Prospects

Janus Sb₂SSeO exemplifies a new class of antimony oxychalcogenide monolayers with direction-dependent optoelectronic and photocatalytic functionalities. The combination of structural integrity, robust mechanical exfoliability, tunability via biaxial strain, favorable band-edge placement for overall water splitting, and substantial optical response positions this material as a promising platform for future 2D device architectures. These results highlight sound theoretical foundations for rational design in Sb-based nanostructures, with the potential for significant impact in polarization-sensitive optoelectronics and sustainable water splitting (Shahrokhi et al., 9 Feb 2026).