Chalcogenide semiconductors such as tin disulfide (SnS₂) and copper sulfide (CuS) have gained attention for gas sensing applications due to their tunable electronic properties, high surface-to-volume ratios, and strong interactions with target gas molecules. These materials enable the development of resistive and optical gas sensors with high sensitivity and selectivity. The sensing mechanisms rely on changes in electrical resistance or optical properties upon gas adsorption, influenced by surface chemistry, defect engineering, and functionalization strategies. Key challenges include improving selectivity and mitigating humidity interference, which are critical for real-world applications.

### Resistive Gas Sensors Based on Chalcogenides
Resistive gas sensors operate by measuring changes in electrical resistance when target gases adsorb onto the chalcogenide surface. The sensing mechanism involves charge transfer between the gas molecules and the semiconductor, altering the carrier concentration and mobility.

**SnS₂-Based Sensors**
SnS₂, an n-type semiconductor, exhibits a layered structure with van der Waals gaps, providing abundant active sites for gas adsorption. When oxidizing gases like nitrogen dioxide (NO₂) interact with SnS₂, electrons are extracted from the conduction band, increasing resistance. Conversely, reducing gases such as ammonia (NH₃) donate electrons, decreasing resistance. The selectivity of SnS₂ sensors can be enhanced by controlling crystallinity, layer thickness, and defect density. Sulfur vacancies in SnS₂ act as adsorption sites, improving sensitivity but may also increase cross-sensitivity to humidity.

**CuS-Based Sensors**
CuS, a p-type semiconductor, shows high sensitivity to volatile organic compounds (VOCs) like ethanol and formaldehyde. The sensing mechanism involves hole accumulation or depletion upon gas adsorption. CuS nanostructures with high porosity exhibit faster response times due to increased gas diffusion. Selectivity is achieved by optimizing operating temperature; for example, CuS shows preferential response to ethanol at 150–200°C.

**Selectivity Mechanisms**
- **Surface Functionalization:** Modifying chalcogenide surfaces with noble metals (e.g., Au, Pt) or metal oxides (e.g., ZnO) enhances selectivity. For instance, Au-decorated SnS₂ shows improved NO₂ selectivity due to catalytic effects.
- **Defect Engineering:** Introducing sulfur vacancies or doping with transition metals (e.g., Fe, Co) tailors the electronic structure, favoring specific gas interactions.
- **Operational Temperature:** Temperature-dependent adsorption kinetics can discriminate between gases with different activation energies.

**Humidity Interference Mitigation**
Humidity affects chalcogenide sensors by competing with target gases for adsorption sites. Strategies to reduce interference include:
- **Hydrophobic Coatings:** Functionalizing surfaces with alkylthiols or fluorinated polymers reduces water adsorption.
- **Material Composites:** Combining SnS₂ or CuS with hydrophobic materials like graphene oxide minimizes humidity effects.
- **Temperature Modulation:** Operating at elevated temperatures desorbs water molecules, improving stability.

### Optical Gas Sensors Based on Chalcogenides
Optical gas sensors detect gas-induced changes in absorbance, reflectance, or photoluminescence. Chalcogenides like SnS₂ and CuS exhibit strong optical responses due to their tunable bandgaps and excitonic effects.

**SnS₂ Optical Sensors**
SnS₂ exhibits layer-dependent optical properties, with thicker films showing redshifted absorption edges. NO₂ adsorption quenches photoluminescence by introducing non-radiative recombination centers. The sensitivity can be enhanced by coupling SnS₂ with plasmonic nanoparticles (e.g., Ag), which amplify local electromagnetic fields.

**CuS Optical Sensors**
CuS shows plasmonic absorption in the near-infrared region, which shifts upon gas adsorption due to changes in free carrier density. Ethanol exposure, for example, reduces plasmonic damping, enabling detection at low concentrations.

**Selectivity Mechanisms**
- **Wavelength-Specific Detection:** Monitoring absorbance changes at specific wavelengths (e.g., NO₂ at 400–500 nm) improves selectivity.
- **Surface Plasmon Resonance (SPR):** Functionalizing CuS with gas-selective ligands (e.g., thiols for H₂S) enhances specificity.
- **Photoluminescence Quenching:** Gases like NO₂ selectively quench excitonic emissions in SnS₂ due to charge transfer.

**Humidity Interference Mitigation**
- **Encapsulation:** Embedding chalcogenides in moisture-resistant matrices (e.g., SiO₂) preserves optical properties.
- **Reference Channels:** Dual-channel sensors compensate for humidity-induced baseline drift.

### Comparative Performance
The table below summarizes key performance metrics for resistive and optical chalcogenide gas sensors:

| Material | Target Gas | Sensing Mechanism | Sensitivity | Selectivity Strategy | Humidity Mitigation |
|-----------|------------|--------------------|-------------|-----------------------|----------------------|
| SnS₂ | NO₂ | Resistive | 10–100 ppm | Au functionalization | Hydrophobic coating |
| SnS₂ | NH₃ | Optical (PL) | 5–50 ppm | Layer thickness control | SiO₂ encapsulation |
| CuS | Ethanol | Resistive | 50–200 ppm | Temperature modulation | Composite with rGO |
| CuS | H₂S | Optical (SPR) | 1–10 ppm | Thiol functionalization| Reference channel |

### Future Directions
Advancements in chalcogenide gas sensors will focus on improving selectivity through machine learning-assisted pattern recognition and developing hybrid resistive-optical systems for cross-validated detection. Further research into atomic-level surface modifications and advanced encapsulation techniques will address humidity challenges, enabling deployment in harsh environments.

Chalcogenide-based gas sensors offer a versatile platform for detecting a wide range of analytes, with resistive and optical approaches providing complementary advantages. By leveraging material engineering and innovative functionalization strategies, these sensors can achieve the reliability required for industrial, environmental, and medical applications.