Unraveling the Raman Enhancement Mechanism on 1T'-Phase ReS₂ Nanosheets

2D transition metal dichalcogenides materials are explored as potential surface-enhanced Raman spectroscopy substrates. Herein, a systematic study of the Raman enhancement mechanism on distorted 1T (1T') rhenium disulfide (ReS₂ ) nanosheets is demonstrated. Combined Raman and photoluminescence studies with the introduction of an Al₂ O₃ dielectric layer unambiguously reveal that Raman enhancement on ReS₂ materials is from a charge transfer process rather than from an energy transfer process, and Raman enhancement is inversely proportional while the photoluminescence quenching effect is proportional to the layer number (thickness) of ReS₂ nanosheets. On monolayer ReS₂ film, a strong resonance-enhanced Raman scattering effect dependent on the laser excitation energy is detected, and a detection limit as low as 10^-9 m can be reached from the studied dye molecules such as rhodamine 6G and methylene blue. Such a high enhancement factor achieved through enhanced charge interaction between target molecule and substrate suggests that with careful consideration of the layer-number-dependent feature and excitation-energy-related resonance effect, ReS₂ is a promising Raman enhancement platform for sensing applications.