Antimony Oxychalcogenide Monolayers: 2D Semiconductors

• Antimony oxychalcogenide monolayers are 2D semiconductors derived from triclinic Sb₂S₂O, noted for their tunable bandgaps, anisotropic transport, and robust thermal and dynamical stability.
• They exhibit direct or indirect bandgaps (2.15–2.74 eV) with significant carrier effective mass anisotropy, enabling efficient charge separation and enhanced optoelectronic performance.
• These materials show promising photocatalytic activity for neutral-pH water splitting with solar-to-hydrogen efficiencies up to 7.2% and strain-tunable optical absorption for advanced device applications.

Antimony oxychalcogenide monolayers represent a class of two-dimensional (2D) semiconductors with the general formula Sb₂X₂O (X = S, Se) and the Janus configuration Sb₂SSeO. These materials have emerged as candidates for optoelectronic and photocatalytic applications due to their robust thermodynamic and dynamical stability, readily achievable exfoliation, anisotropic transport and optical properties, tunable bandgaps, and favorable energetics for overall water splitting under neutral conditions (Shahrokhi et al., 9 Feb 2026).

1. Structural Characterization

1.1 Crystal Structure and Lattice Parameters

All known antimony oxychalcogenide monolayers are derived from the bulk triclinic Sb₂S₂O, crystallizing in space group $P\bar{1}$ (#2). The monolayer primitive cell contains 20 atoms: 8 Sb, 8 chalcogen (X = S or Se), and 4 O atoms. Two distinct Sb coordination environments are observed: one Sb bonded to five chalcogen atoms, and the other to three O and one X atom.

The optimized lattice parameters for these monolayers are:

System	$a$ (Å)	$b$ (Å)	$\gamma$ (°)
Sb₂S₂O	11.12	8.28	68.17
Sb₂Se₂O	11.44	8.36	68.62
Sb₂SSeO	11.37	8.34	68.49

Key bond lengths (Å) include: Sb–S in Sb₂S₂O (2.16–2.63), Sb–O (2.04–2.50); Sb–Se in Sb₂Se₂O (2.19–2.98), Sb–O (2.05–2.64); Janus Sb₂SSeO exhibits mixed Sb–S (2.20–2.65) and Sb–Se (2.66–2.98) bonds.

1.2 Cleavage Energy and Exfoliation Feasibility

Cleavage energies, calculated by separating one layer from a six-layer stack, are $E_{cl} = 0.36$ J m⁻² for Sb₂S₂O and $0.40$ J m⁻² for Sb₂Se₂O. By comparison, graphene exhibits $E_{cl} \approx 0.37$ J m⁻² and MoS₂ $\approx 0.6$ J m⁻². These low values confirm the strong experimental feasibility of mechanical exfoliation.

1.3 Thermodynamic and Dynamical Stability

The formation energies per atom are:

System	$E_f$ (eV/atom)
Sb₂S₂O	–11.00
Sb₂Se₂O	–10.64
Sb₂SSeO	–10.80

These values are more negative than corresponding Sb₂X₃ monolayers, indicating enhanced stability. Phonon dispersion analysis reveals no imaginary modes over the Brillouin zone, and optical mode frequencies reach up to ~18.7 THz (625 cm⁻¹), higher than silicene and black phosphorene. Ab initio molecular dynamics (AIMD) at 300 K and 500 K for 10 ps demonstrates negligible structural distortion, confirming thermal robustness.

2. Electronic Properties

2.1 Band Structure and Density of States

First-principles calculations yield the following bandgaps:

System	$E_g^{HSE06+SOC}$ (eV)
Sb₂S₂O	2.74
Sb₂Se₂O	2.15
Sb₂SSeO	2.35

Sb₂S₂O exhibits a direct gap at X, while Sb₂Se₂O and Sb₂SSeO feature indirect gaps with the valence band maximum (VBM) along X–H₁ and the conduction band minimum (CBM) at X. The VBM originates primarily from X–p and O–p orbitals; the CBM is mainly Sb–5p, with weak spin-orbit splitting.

2.2 Carrier Effective Masses and Anisotropy

Carrier effective masses ($m^*$, in units of $m_0$) were extracted via parabolic fitting:

System	$m^*_{e,x}$	$m^*_{e,y}$	$m^*_{h,x}$	$m^*_{h,y}$
Sb₂S₂O	0.18	0.19	0.12	0.05
Sb₂Se₂O	1.71	0.56	0.15	0.19
Sb₂SSeO	0.37	0.71	0.19	0.17

Notably, Sb₂Se₂O exhibits pronounced anisotropy in electron transport ($m^*_{e,x}/m^*_{e,y} \approx 3.05$).

3. Optoelectronic Response and Strain Tuning

3.1 Dielectric Properties and Optical Absorption

The frequency-dependent dielectric tensor $\varepsilon(\omega)$ yields in-plane static dielectric constants ($\varepsilon_{1,xx}$, $\varepsilon_{1,yy}$) exceeding 10, supporting efficient electronic screening and charge separation. The absorption coefficient $$\alpha(\omega) = (2\omega/c)\ \mathrm{Im}[\sqrt{\varepsilon(\omega)}]$$ peaks at $\sim 1 \times 10^{5}$ cm⁻¹ in the UV–visible range; monolayer absorptance reaches up to 18% at 300–500 nm, which is comparable to or exceeds MoS₂ (5–10%).

3.2 Biaxial Strain Effects

Biaxial strain within $\pm$6% was found to modulate the bandgaps nearly linearly (for $|\varepsilon| \leq 4\%$):

System	$E_g$ min (eV, $-6\%$)	$E_g$ max (eV, $+6\%$)
Sb₂S₂O	2.24	3.07
Sb₂Se₂O	1.70	2.52
Sb₂SSeO	1.94	2.67

Tensile strain lowers the VBM, increasing the hole driving force $U_h$; compressive strain raises the CBM, modulating the electron driving force $U_e$. This tunability enables material-specific optical gap engineering.

4. Photocatalytic Activity for Water Splitting

4.1 Band Edge Alignment and Overpotential

At pH 7, reference redox potentials relative to vacuum are $E(\mathrm{H}^+/\mathrm{H}_2) = -4.03$ eV and $E(\mathrm{O}_2/\mathrm{H}_2\mathrm{O}) = -5.26$ eV. HSE06+SOC calculated band alignments are:

System	CBM (eV)	$U_e$ (V)	VBM (eV)	$U_h$ (V)
Sb₂S₂O	–3.15	+0.88	–5.95	+1.92
Sb₂Se₂O	–3.33	+0.70	–5.57	+1.55
Sb₂SSeO	–3.15	+0.81	–5.59	+1.75

For all material systems, both CBM and VBM straddle the water redox levels, fulfilling the thermodynamic condition for overall water splitting.

4.2 Reaction Energetics

The electron driving force $U_e = E_{CBM} - E(\mathrm{H}^+/\mathrm{H}_2)$ and hole driving force $$U_h = E(\mathrm{O}_2/\mathrm{H}_2\mathrm{O}) - E_{VBM}$$ are favorable for both HER and OER. Free energy computations (CHE model) demonstrate OER proceeds via a dual-site mechanism without uphill steps under illumination ($U_h$ applied), but HER remains limited by the H* adsorption barrier ($\Delta G \approx 1.11–1.50$ eV).

4.3 Solar-to-Hydrogen Conversion Efficiency

The calculated STH efficiencies (pH = 7, $\varepsilon=0$) are:

System	$\eta_{STH}$ (%)
Sb₂S₂O	3.8
Sb₂Se₂O	7.2
Sb₂SSeO	6.5

Under selected strain, STH efficiency can be further enhanced to 7–8%, placing these materials in the competitive range for state-of-the-art 2D photocatalysts.

5. Direction-Dependent Device Applications

Antimony oxychalcogenide monolayers display pronounced in-plane anisotropy in carrier mobility and optical response. Hole mobilities up to 400 cm² V⁻¹ s⁻¹ (Sb₂S₂O, $y$ direction) are predicted, supporting field-effect transistor (FET) channels tailored for maximal on-state current by orientation. The pronounced optical anisotropy enables polarization-sensitive photodetection; device designers can selectively orient electrodes along $x$–$y$ to tune responsivity and spectral selectivity.

The built-in out-of-plane dipole of Janus Sb₂SSeO imparts an intrinsic normal electric field, facilitating efficient carrier separation in ultrathin solar cells or z-scheme photocatalytic assemblies without external bias. Mechanical flexibility, quantified by Young’s modulus values between 25–40 N/m, enables their use in wearable optoelectronic and strain sensor platforms.

6. Outlook and Significance

Antimony oxychalcogenide monolayers meet criteria for thermodynamic and dynamical stability, low exfoliation energy, strain-tunable electronic structure, strong visible-light absorption, and intrinsic driving forces for photocatalysis. The convergence of these properties, especially their direction-dependent electronic and optical response, positions Sb₂X₂O (X = S, Se) and Janus Sb₂SSeO as promising candidates for next-generation, direction-aware optoelectronic and neutral-pH water splitting applications (Shahrokhi et al., 9 Feb 2026). A plausible implication is that the intrinsic anisotropy and built-in electric fields of these materials will be exploited for tailored device architectures in sustainable energy conversion and advanced nanoelectronics.