The Mechanism Research Of High Sensitivity Raman Signal Detected By Molybdenum Disulphide

Surface-enhanced Raman scattering has become a non-destructive,real-time detective and powerful technique in analytical chemistry due to its high sensitivity and molecular specificity.However,the deficiencies of traditional metal SERS substrate such as poor biological compatibility,instability and poor reproducibility have restricted the development of SERS technology.Recently,enhanced Raman scattering of target molecule adsorbed on molybdenum disulfide?MoS ₂?is observed.The high Raman enhancement factor,tunable opto-electric properties and strong excitonic features make MoS₂ become a very hot substrate for Raman enhancement.However,the detailed mechanism of using two-dimensional?2D?MoS₂ as Raman active substrate is still controversial.It has been believed that the enhancement is attributed to charge transfer and dipole-dipole coupling,some works report the enhancement is due to charge transfer induced chemical enhancement,there is also report of laser-induced charge transfer and exciton resonance.Therefore,the study of detailed mechanism underlying enhancement of Raman scattering induced by MoS₂ will faciliate obtaining optimized SERS enhancement factor for specific molecules,and also promote its application in molecular detection and in other fields.Firstly,the SERS effect and the corresponding mechanisms were introduced.Secondly,the preparation and characterization processes of MoS₂ were discussed.Most important of all,the mechanism of MoS₂ for Raman detection was studied detailedly.It was found that the Raman signal of R6G probe molecule decreased with the increase of MoS₂ layers.We calculated the resonance Raman scattering cross section of adsorbed molecules on monolayer MoS₂ surface under the excitation of 532 nm incident light.The calculated average 0.53±0.23×10^-23 was lower than the value of 1.60±0.20×10^-23 in the methanol solution.Therefore,the monolayer MoS₂ can't enhance the R6G Raman scattering cross section.To obtain the Raman scattering cross section of the R6G molecule,we need to know its adsorbed concentration on the MoS₂ surface.This thesis proposed a method to determine the concentration of molecules through R-channel contrast value for optical image by connecting the theory of Fresnel formula.For example,the adsorbed concentration 6.4×10¹³/cm² was from the soaked sample whose R-contrast is 0.20,after that,the relevant molecular thickness has also been verified by atomic force microscope?AFM?.In conclusion,experimental results demonstrated that the MoS₂ Raman detection mechanism is from the effective fluorescence quenching caused by the charge transfer,which make the Raman signal for probe molecules appear under the resonance excitation.In order to further improve the Raman signal of probe molecules,we introduced different degrees of defects on MoS₂ surface by high temperature annealing which will increase the molecular adsorbed concentration.It was found that there are many different size of triangle oxidation pits on the surface of MoS ₂during annealing in the air,and the Raman signal intensity of R6G showed a trend of lower after the first increase with the defect degree of sample enhance.We believe that the number of suspended bonds on MoS₂ surface increases with the increase of defect degree,thus adding the adsorption concentration of the molecule.However,as the defect degree continues to increase,the smaller oxidation pits are gradually connected to some larger amorphous oxidation pits,and the Raman signal decreases with the decrease of the molecular adsorption concentration.In a ddition,the molecular Raman signal on the modulated MoS₂ substrate was about 7.5 times higher than that of the intrinsic MoS₂ substrate.This research provides more possibilities for 2D-materials application in the field of Raman detection technoloy.