Surface-enhanced Raman spectroscopy (SERS) is a powerful technique for trace-level molecular detection. Two-dimensional (2D) materials have emerged as promising SERS substrates due to their stability, uniformity, and reproducibility. However, a comprehensive understanding of the molecule-substrate coupling mechanism is crucial for developing effective 2D SERS substrates. While the Raman enhancement in 2D materials is generally attributed to chemical enhancement (increased molecular polarizability due to electronic coupling), the role of exciton resonance remains relatively unexplored. Existing theories suggest that strong Raman intensities are anticipated when excitation energy matches charge-transfer transitions, molecular absorption, or exciton resonance. The potential for improved Raman enhancement by aligning exciton resonances with charge transfer or molecular resonances has been hinted at but lacks systematic investigation. This study focuses on the co-resonance effect on Raman enhancement by carefully selecting a system where both molecular and 2D material resonances are achieved with the same excitation energy. SnS₂, a 2D material with a bandgap around 2.2-2.4 eV (exciton resonance ~2.3 eV), and Rh 6G (with an S₀-S₀ transition at 532 nm, ~2.3 eV) are chosen as a model system. Rh 6G's large Raman cross-section facilitates Raman spectral acquisition even off-resonance. The research aims to comprehensively investigate the simultaneous achievement of molecular and SnS₂ resonances using excitation-wavelength dependent Raman measurements and determine if this leads to a significant enhancement in detection sensitivity.

The discovery of the graphene-enhanced Raman scattering (GERS) effect in 2010 initiated extensive research into Raman enhancement on 2D materials. Various layered 2D materials, including boron nitride, black phosphorus, transition metal dichalcogenides (TMDs), and MXenes, have since shown Raman enhancement effects. Unlike traditional noble-metal SERS substrates, the enhancement in 2D materials is primarily due to chemical enhancement mechanisms rather than electromagnetic effects. Studies have explored charge-transfer resonances as a key mechanism for enhancement in these materials. However, the effect of dual resonances, specifically the alignment of exciton resonances with either charge transfer or molecular resonances, remains largely under-investigated. While some reports suggest potential improvements in Raman enhancement through such alignment, a systematic study exploring the impact of aligning exciton resonances with molecular resonances is lacking. The current work addresses this gap by systematically investigating Raman enhancement through exciton hybridization, examining various alignments of molecular and exciton resonances and providing insights into the design of high-sensitivity semiconductor-based SERS substrates.

The study employed several key methods: **Preparation of 2D materials:** Bulk crystals of SnS₂, MoS₂, WSe₂, and graphene were purchased and mechanically exfoliated onto SiO₂/Si substrates. **Atomic force microscopy (AFM):** AFM was used to determine the number of layers in the exfoliated SnS₂ samples. **Deposition of probe molecules:** Rhodamine 6G (Rh 6G), Rhodamine B (Rh B), and Rhodamine 123 (Rh 123) were dissolved in isopropyl alcohol (IPA) to create stock solutions. Exfoliated 2D crystals were submerged in these solutions for 2 hours, then rinsed to remove excess molecules. Concentration-dependent studies were performed using serial dilutions of Rh 6G (10⁻⁴ to 10⁻¹⁴ M). **Optical measurements:** Raman scattering was measured using a micro-Raman spectrometer with various laser lines (458-647 nm) to obtain excitation-dependent Raman spectra and Raman excitation profiles (REPs). Micro-absorption measurements were performed using a custom-built setup measuring reflectance and transmittance. UV-Vis spectrophotometry was used to measure the absorption spectra of Rh 6G, Rh B, and Rh 123 solutions. X-ray photoelectron spectroscopy (XPS) was also employed (though the results are not detailed in the main text). In summary, the researchers used a combination of material preparation techniques, microscopic characterization, and advanced optical spectroscopy to study the Raman enhancement mechanism.

The key findings of this study include: 1. **High sensitivity detection of Rh 6G on SnS₂:** The researchers achieved a limit of detection (LOD) of 10⁻¹³ M for Rh 6G on SnS₂, which is comparable to, and in some cases superior to, that achieved with plasmon-based SERS substrates and other 2D materials. This LOD is highly promising for trace-level sensing applications. 2. **Exciton hybridization in Rh 6G/SnS₂:** Excitation-dependent Raman spectroscopy revealed a strong coupling between Rh 6G and SnS₂ due to the alignment of their degenerate excitons. This coupling leads to exciton hybridization, resulting in new resonance peaks in the Raman excitation profiles (REPs). Specifically, a new resonance peak at 2.54 eV (R₂) was observed in addition to the expected peak at ~2.35 eV (R₁) that aligns with the Rh 6G S₀₋₀₋S₁₀ transition. 3. **Mode-selective enhancement:** The C-C bending mode at 613 cm⁻¹ showed unusually high intensity under resonant conditions, suggesting that Herzberg-Teller vibronic coupling plays a crucial role in the Raman enhancement mechanism. This mode-selective enhancement further underscores the importance of efficient vibronic coupling in the Rh 6G/SnS₂ system. 4. **Thickness dependence:** The Raman enhancement was found to be dependent on the number of layers of SnS₂, with fewer layers resulting in higher enhancement. This is attributed to increased light absorption and layer-dependent electronic band structures. However, the same LOD was obtained for 3-layer and 18-layer SnS₂, suggesting that trace-level detection is achievable even for multi-layered samples. 5. **Control experiments:** Experiments with Rh B and Rh 123 on SnS₂ confirmed the importance of exciton energy alignment for hybridization. These molecules, with different molecular exciton energies, did not exhibit the same hybridization and additional resonance peak (R₂), indicating that the observed R₂ peak in the Rh 6G/SnS₂ system is due to the specific energy alignment and subsequent exciton hybridization. 6. **Exclusion of alternative mechanisms:** The researchers carefully ruled out alternative explanations for the R₂ peak, such as scattered light resonance and charge-transfer resonance, by analyzing the energy positions and mode-dependent behavior of the resonance peaks in the REPs.

The results demonstrate that exciton hybridization plays a crucial role in enhancing Raman scattering in the Rh 6G/SnS₂ system. The appearance of the new resonance peak (R₂) at 2.54 eV, only observed when the molecular and substrate exciton energies are closely aligned, strongly supports this conclusion. The control experiments with Rh B and Rh 123, lacking this energy alignment, further reinforce the importance of precise exciton alignment for hybridization and subsequent Raman enhancement. The unusually intense C-C-C bending mode at 613 cm⁻¹ further supports this finding, indicating strong coupling facilitated by exciton hybridization. The high sensitivity achieved (LOD of 10⁻¹³ M) highlights the potential of this exciton hybridization mechanism for developing highly sensitive SERS substrates. The observation is significant as it moves beyond the conventional focus on charge-transfer mechanisms in semiconductor-based SERS and offers a new design principle for enhanced Raman sensors.

This study provides a comprehensive understanding of Raman enhancement in 2D material-based SERS by demonstrating the significant role of exciton hybridization. The observed strong coupling between Rh 6G and SnS₂, resulting in a high LOD of 10⁻¹³ M, highlights the potential of this mechanism for developing highly sensitive SERS substrates. Future research could explore tailoring SnS₂ crystal structures through defect, strain, and heterostructure engineering to optimize the band alignment and enhance exciton coupling strength, potentially leading to even higher detection sensitivity. This work significantly advances the rational design of SERS substrates for trace-level detection.

While the study demonstrates a significant enhancement in Raman scattering through exciton hybridization, some limitations exist. The study primarily focuses on Rh 6G and a limited number of other rhodamine derivatives. Extending the investigation to a wider range of molecules and 2D materials would strengthen the generality of the conclusions. Additionally, the exact nature of the interaction between Rh 6G and SnS₂ at the molecular level could benefit from further investigation, potentially using techniques like high-resolution microscopy or theoretical calculations.