High-Pressure Research Of Surface-Enhanced Raman Spectroscopy On Two-Dimensional Semiconductors

As one of the most promising detection and analysis technologies at present,surface-enhanced Raman spectroscopy（SERS）can effectively identify and detect trace compounds by its unique fingerprint vibration information,which is widely used in food safety,biomedical,chemical catalysis and environmental monitoring fields.The mechanisms of SERS enhancement mainly include the electromagnetic enhancement and charge transfer enhancement,the former is originated from the increase of electromagnetic field intensity induced by local surface plasma resonance（LSPR）of noble metals,while the latter is attributed to the enhancement of charge transfer（CT）effect between the substrate and probe molecules.For the semiconductor-based SERS substrates,the interaction between probe molecules and substrate,as well as the matching degree between the energy level structure of probe molecules and the energy band structure of the SERS substrates play a critical role in SERS effects.The band gap or Fermi level position of semiconductors can be controlled by doping of heterogeneous elements and introducing defects.However,these methods usually require complex procedures and can bring impurities into the SERS systems.High pressure,as a“clean”regulation technology,can effectively change the interaction between probe molecules and substrates and their energy level/band structures by combining high pressure and SERS,and can provide a new way for in-depth study of SERS effects and the corresponding enhancement mechanisms,and can be also expected to solve the problem that the Raman intensity of materials under high pressure becomes too weak to characterize the structural properties of materials.Therefore,we use the high pressure technology to regulate the band gap of SERS substrate and explore the interaction mechanism between the substrate and probe molecules.Thus,to clarify the physical mechanism involved in pressure-induced SERS（PI-SERS）enhancement is the key scientific problem that needs to be solved urgently to achieve SERS enhancement under extreme conditions.To solve this key problem,in this paper,we use molybdenum disulfide/gold （Mo S₂/Au）,monolayer Mo S₂（ML-Mo S₂）and few-layer WS₂（FL-WS₂）as the substrates for PI-SERS effects,and realize the Raman enhancement at different pressures,and the results are as following:1.Mo S₂/Au composite materials were prepared by the hydrothermal method and redox method,which were used as the SERS active substrates.The detection sensitivity was characterized and the enhanced ability was evaluated at ambient conditions by using R6G molecules as probes.The results show that the minimum concentration of R6G solution is 10^-10 mol/L and its enhancement factor（EF）is as high as 10⁷ orders of magnitude,indicating the feasibility of Mo S₂/Au as SERS active substrates.Further,we studied the enhancement mechanism of PI-SERS based on Mo S₂/Au substrates.Compared with the gradual decrease of Raman intensity in pure R6G molecules under high pressure,the abnormal Raman enhancement occurs in Mo S₂/Au/R6G system at2.39 GPa by modulating the HOMO-LUMO enegy gap of R6G molecules and the energy matching.To clarify the PI-SERS enhancement mechanism,we applied the Mo S₂substrate as a control,it is found that the introduction of Au nanoparticles not only introduces the LSPR effect,but also brings a two-step charge transfer process,this synergistic effect promotes the PI-SERS enhancement.This research not only provides a new perspective for the SERS enhancement mechanism under high pressure,but also provides new ideas to improve Raman signals under high pressure.2.Combining the theoretical calculations and experiments,SERS performance based on ML-Mo S₂was investigated in depth.We analyzed and discussed the influence of incident laser energies and molecular symmetries on PI-SERS performances,and proposed the SERS enhancement transformation mechanism.The lasers with different energies（from visible to near-infrared light）were applied to excite the ML-Mo S₂/MB system（methylene blue,MB）,the results show that the pressure corresponding to the SERS enhancement can reach up to 8.04 GPa,which is more than double the pressure value reported by previous studies,indicating that the ML-Mo S₂substrate is expected to achieve Raman signal enhancement at higher pressures.According to the results calculated by density functional theory（DFT）,high pressure can induce the rise of the Fermi level of ML-Mo S₂ substrate and the decrease of the HOMO-LUMO energy gap of MB molecule,which realizes the mechanism transition from the multiple resonances-related SERS enhancement to the CT dominant PI-SERS selective enhancement.In addition,to study the influence of the structural symmetries of probe molecules and SERS substrates on PI-SERS enhancement,three molecules with similar energy level structures but different symmetries were used as probes.The experimental and theoretical calculation results show that the symmetries of SERS substrate and the probe molecules affect each other under high pressure,and their symmetry matching can promote the charge transfer effect.Therefore,in the SERS system without resonance effect,the enhancement of electron-phonon coupling will also promote the PI-SERS selective enhancement phenomena under high pressure.These results provide us with a new understanding of the PI-SERS enhancement mechanism,and this mechanism is expected to be used for SERS enhancement of other molecules under extreme conditions.3.In order to explore whether the SERS signal can be enhanced at higher pressures,we used FL-WS₂ as the SERS-active substrate and 4-MBA molecules with big HOMO-LUMO energy gap as probes.By adjusting the increase of band gap of FL-WS₂and the decrease of HOMO-LUMO energy gap of 4-MBA molecules,the charge-transfer transition energy between FL-WS₂ and 4-MBA molecules can match the laser energy.The results indicate that the enhancement of SERS signals was achieved at a pressure as high as 18.38 GPa,which was significantly improved compared with the previous reports.It is the highest record of achieving SERS enhancement under high pressure.This research not only provides a new idea for designing an efficient SERS system to achieve SERS enhancement under higher pressure,but also deepens the understanding of the SERS enhancement mechanism.