Preparation Of Two-Dimensional Transition Metal Dichalcogenides SERS Substrate And Mechanism Research

Surface-enhanced Raman spectroscopy（SERS）is a powerful and ultra-highly sensitive analytical technique that has been widely used in environmental protection,food safety and life sciences.Since the discovery of surface enhanced Raman scattering in the 1970s,various materials（including noble metals,semiconductor materials,and nanostructured materials）have been studied as SERS substrates,which has accelerated the development of SERS active substrates.In recent years,two-dimensional materials have been widely studied in the field of SERS due to their unique layer-dependent physicochemical properties and enhanced charge transfer process,and their enhancement mechanism has been continuously improved.Noble metal substrates based on electromagnetic mechanism have good SERS enhancement,but their uniformity and stability are poor.While semiconductor substrates based on chemical mechanisms have good SERS uniformity and stability,but their SERS enhancement is relatively poor,which hinders its application prospects as active substrates.In this paper,different layers of transition metal dichalcogenides（TMDs）were prepared by electrochemical and liquid phase exfoliation methods,and the relationship between the number of layers of TMDs and their SERS activity was studied,and the application of TMDs in SERS active substrates was expanded.The main works are as follows:A simplified electrochemical exfoliation method was used to strip the transition metal sulfide titanium disulfide（Ti S₂）,and its structure was characterized by Raman,XRD,SEM,TEM,and AFM.It is proved that differential centrifugation can separate Ti S₂ samples with different layers.Subsequently,using R6G as a probe molecule,the effects of different electrolysis currents and the number of Ti S₂ layers on the SERS activity of Ti S₂ substrates were studied.The results show that when the deposition current is 0.25 m A/cm²,the SERS activity of Ti S₂ substrate increases significantly as the number of layers decreases.Compared with the bulk Ti S₂,the Raman enhancement factor of the few-layer Ti S₂ is increased by two orders of magnitude to 3.18×10⁵,and the detection limit can reach 10^-8 M R6G.The main reason for the enhancement of SERS activity was attributed to the chemical enhancement.And the reduction of the number of layers will cause the transition of the band gap of Ti S₂,which improves the efficiency of charge transfer between R6G and Ti S₂.In this work,not only the application of Ti S₂ as a SERS active substrate is expanded,but also an effective method for improving the SERS activity of TMDs is provided,making TMDs a potential SERS active substrate.The bulk materials WSe₂ and Mo S₂ were stripped off by the liquid phase stripping method to obtain TMDs samples with different layers,and TMDs samples with different layer number gradients were obtained by differential centrifugation.In the process of liquid phase exfoliation,the surfactants SDS and CTAB with different electrical properties were used to treat WSe₂ and graphene respectively,and the electrostatic adsorption composite of WSe₂-Graphene was realized under the conditions of the liquid phase.Using R6G to test the SERS activity of WSe₂-Graphene,it is found that as the number of layers decreases,the SERS activities of WSe₂ and Mo S₂ are getting stronger,and the Raman enhancement factor increases by nearly three orders of magnitude.This is mainly due to the increasing efficiency of charge transfer between TMDs and R6G.The SERS activities of WSe₂-Graphene samples after compounding are doubled compared with that before compounding.The detection limit of R6G is as low as 10^-9 M,and the Raman enhancement factor reaches 4.85×10⁵.This is mainly attributed to the free shuttle of electrons between the layers after WSe₂ and graphene recombination,which improves the efficiency of charge transfer.The experimental results prove that some TMDs have layer-dependent SERS enhancement characteristics,and the regulation of the number of layers and the combination of materials further expand the method of improving the SERS activity of TMDs materials.