Crystal Structure And SERS Effect Of Two-dimensional Transition Metal Dichalcogenides

Two-dimensional(2D)transition metal dichalcogenides(TMDs)materials have been recognized as active substrates with surface-enhanced Raman scattering(SERS)effect.Thanks to the advantages such as chemical inertness,high specific surface area and homogeneous surface state,2D TMDs SERS substrates can effectively overcome the drawbacks of traditional metal substrates(such as low signal reproducibility and limited repeatability)and lead to quantitative SERS detection,which is of great significance to promote the practical application of SERS technique.The SERS effect of TMDs materials is mainly explored as the chemical enhancement mechanism(CM)between the adsorbed molecules and the substrate.TMDs have been considered as an ideal model to reveal the CM effect by removing the disturbance of the electrometric enhancement effect originated from surface plasmon resonance(SPR).Studying the influence of phase state,number of layers,and defects of TMDs on its SERS effect can improve the understanding of the chemical enhancement mechanism and provide new pathways and research methods for the design of excellent SERS substrates.Monolayer WS₂ triangular film material with uneven defect distribution has been grown on a SiO₂/Si substrate by chemical vapor deposition,and the influence of surface defects on the SERS effect has been explored.The defect density of different regions of the monolayer WS₂ triangular film was quantitatively calculated by measuring the PL intensity.The relationship between the surface defect density of WS₂ film and its SERS enhancement ability was quantitatively analyzed by using dye molecule Copper(II)phthalocyanine(CuPc)as the probe molecule.The results show that the defect density in the material has a positive correlation with its SERS effect.By changing the polarization modes of the incident laser,it is proved that the azimuthal polarization mode of the incident laser is more effective to the excitation of the monolayer WS₂ triangular film and the adsorbed molecules than the radial polarization mode,which can improve the charge transfer process between the substrates and the adsorbed molecules.Monolayer 2H and 1T phase MoX₂(X=S,Se)nanosheets prepared by liquid exfoliation method were used as SERS substrate to study the effect of phase transition on its SERS effect.Converting MoX₂ from semiconductor 2H phase to metallic 1T phase can reduce the symmetry of the crystal structure and improve the conductivity of the material.The Raman enhancement effect of different substrates were compared by using the dye molecules CuPc,Rhodamine 6G(R6G)and crystal violet(CV)as probe molecules.Raman results indicate that a phase transition from 2H to 1T phase can significantly increase the Raman enhancement effect of monolayer MoX₂ nanosheets.The detection limit of dye molecule R6G on 1T phase Mo Se₂ nanosheets can be as low as 10^-8 mol L^-1.Density functional theory(DFT)calculations indicate that the significant enhancement of the Raman signals on metallic 1T phase MoX₂ can be attributed to the facilitated electron transfer from the Fermi energy level of metallic 1T phase MoX₂ to the highest occupied molecular orbital(HOMO)level of the probe molecules,which is more efficient than the process from the top of valence band of semiconducting 2H phase MoX₂ to the HOMO level of the probe molecules.The distorted 1T(1T')phase ReS₂ film grown by CVD method was used as the SERS substrate to further verify the SERS performance of metallic TMDs materials.Herein,dye molecules(CuPc,R6G,Rhodamine B(Rh B),CV and methylene blue(MB))were used as probe molecules.The Raman results show that the detection limit of the dye molecule R6G on the monolayer ReS₂ film can be as low as 10^-9 mol L^-1.The layer-number dependent feature and resonance-enhanced Raman scattering effect was ruled out by comparing the effect of material thickness and laser wavelength on the Raman signal intensity of adsorbed molecules.By comparing the difference between the Raman and PL spectrum of the R6G molecule on the ReS₂ film before and after the deposition of Al₂O₃dielectric layer,it was revealed that Raman enhancement of ReS₂ materials is from a charge transfer process rather than an energy transfer process.VO₂ nanosheets were further explored as another active substrate to expand TMDs materials for SERS technique.By comparing the SERS ability of bulk VO₂nanoparticles and VO₂ nanosheets for the enhancement of the Raman signals of dye molecules,it was shown that VO₂ nanosheets have stronger Raman enhancement ability.It was also found that the phase transitions from VO₂(B)to VO₂(M)to VO₂(R)could increase the crtstallinity and crystal symmetry of the VO₂ material and decrease the surface and internal crystal defects.This lead a dramatic decrease in the Raman enhancement effect on VO₂ nanosheets.On defect-rich VO₂(B)nanosheets,a detection limit as low as 10^-8 mol L^-1 can be reached for R6G molecule.These results show the way for designing new 2D SERS substrates via phase-transition engineering.