Study On Molybdenum Disulfide Raman Imaging With Few Layers Excited By Polarization Vector Beam

With the advance of graphene as a real two-dimensional material,the development of two-dimensional materials has attracted much attention in recent years.Compared with other two-dimensional materials,MoS₂ not only has excellent optical,electrical and mechanical properties,but also changes its band structure as the number of layers decreases,from indirect bandgap in bulk material to direct gap in singer-layer material.Direct band gap means that phonons are not involved in the electron excitation transition.Therefore,this material can be used to fabricate optoelectronic devices with high photoelectric conversion efficiency,such as lasers,photodetectors and solar panels.The fabrication process of two-dimensional materials may produce lattice defects,and lattice defects will change the physical properties of two-dimensional materials.For ultra-thin two-dimensional materials,conventional detection methods can not characterize the surface structure and lattice defects of materials.Therefore,it is necessary to develop a suitable detection method to characterize two-dimensional materials.Raman spectroscopy is an effective method to study the composition and structure of molecules and the lattice defects on the surface of materials.It is called molecular fingerprint technology because it is related to the internal vibration of molecules.MoS₂ has two main vibration modes:in-plane vibration E¹_2g mode and out-of-plane vibration A_1g mode.The corresponding Raman peaks are 382 cm^-1 and 406 cm^-1,respectively.Traditional Raman detection uses Gauss light as the excitation beam,but because the focused field of Gauss light does not have spatial symmetry and the size of the spot is large,the imaging resolution is low,and the characterization effect is not good enough to detect the defects of the lattice.Vector beam is an optical beam whose spatial distribution of polarization changes with the propagation.Vector beam has strong focusing characteristics and has wide applications in many fields.In this paper,the changes of Raman peaks corresponding to two modes of MoS₂ vibration under different tight focusing vector beams illumination are studied.It is found that the Raman peaks of different modes have different polarization selectivity.Radially polarized light and angularly polarized light are used in the experiment.The radially polarized light will have a strong longitudinal component in the optical axis after tight focusing,which has a strong enhancement effect on the out-of-plane vibration A_1g mode.After the angular polarized beam is tightly focused,the light field is still all distributed in the plane,so it will have a strong enhancement effect on the vibration mode of E¹_2g in the plane.In this paper,the tip-enhanced Raman scattering（TERS）signals of MoS₂ and 4-nitrobenzenethiol were also studied by the system of combining atomic force microscope（AFM）with Raman spectrometer.The main research work includes the preparation of TERS probe,the preparation of experimental samples and substrates,and the collection and analysis of optical signals.TERS signal of 4-nitrobenzenethiol was successfully observed in the experiment,and the experimental results of MoS₂ were also analyzed.The experimental system will be continuously optimized in the near future.