Controllable Preparation And Lattice Dynamics Of Two-Dimensional Chalcogenides

Compared with traditional bulk materials,two-dimensional materials have many unique properties.For example,their heat transfer and carrier migration are confined in a limited two-dimensional plane,the band gap can be adjusted by the thickness and the number of layers,excellent quantum confinement effect and excellent anisotropy,etc.Two-dimensional materials have broad commercial prospects in many fields.How to maximize the potential applications of two-dimensional materials in the fields of electronics and optoelectronics is inseparable from basic research on their physical properties.In this study,we successfully synthesized a variety of high-quality two-dimensional thin film materials by chemical vapor deposition,and used Raman scattering spectroscopy to study the lattice dynamics of different types of two-dimensional chalcogenide materials and the phase control under external field control.Structural dependencies.The main research results are as follows:(1)A thin and large two-dimensional chalcogenide compound of WS₂,WSe₂,MoSe₂and WTe₂was prepared on a glass substrate by chemical vapor deposition.Optical imaging shows that these two-dimensional materials are stable structures of triangles,hexagons and long strips.The intensity of the Raman scattering peak is high and the FWHM is very narrow,which indicates that the material has good crystallinity.The 3D mapping image shows that the structure of our synthesized material is uniform.Use SEM and EDS to test the chemical element composition of the samples.We successfully prepared large-size,high-quality,and uniform single-layer two-dimensional chalcogenides,which laid a good experimental foundation for the research team in the next two-dimensional material devices.(2)Polarized Raman scattering spectroscopy was used to study the change characteristics of the lattice dynamics of platinum diselenide(PtSe₂)under different pressures,and to reveal the evolution of its internal crystal structure.We found that its two characteristic phonon modes,E_gand A_1g,show similar hardening trends with increasing pressure;under high pressure,the Raman peak position and half-height width have obvious mutations.In particular,the split lattice mode at 4.3 GPa confirms the change in lattice symmetry.Through in-situ high-pressure Raman scattering spectroscopy and first-principles calculations,the effect of pressure on the structure of PtSe₂and its lattice dynamics is systematically explained,that is,PtSe₂has a new stabilizing structure C2/m under high pressure.And under high pressure,PtSe₂belongs to the first type of Dirac semi-metallic material.Not only that,the use of high-pressure in-situ Raman spectroscopy combined with theoretical prediction also opened a new window for finding and regulating new structures of two-dimensional materials.(3)Using external fields(temperature and pressure)to control the Raman scattering spectroscopy study of the ternary chalcogenide compound TlGaS₂,summarize its structural phase transition evolution process.Studies have found that as the temperature increases,almost all Raman scattering peaks gradually move to the low-frequency direction,which is due to the thermal expansion effect existing in the experiment.What needs to be pointed out is the Raman phonon mode at 40.6 cm^-1,which moves to the high frequency direction as the temperature increases.The Raman peaks at 70.6 cm^-1,173.3 cm^-1and 331.6 cm^-1disappeared between 225-250 K.Therefore,the study of phonon dynamics under the temperature field reveals the structural transformation of TlGaS₂from the paraelectric phase to the ferroelectric phase.As the temperature drops,we find that the low-temperature transition of the material exhibits a hysteresis,which also confirms the existence of the first-order phase transition.In addition,through Raman scattering spectra under different pressures,we found that at6.43-8.26 GPa,the Raman scattering phonon frequency has a sudden change with the increase of pressure,and it is located at 141.2 cm^-1and 189.3 cm^-1.The Raman peak disappeared with the increase of pressure,confirming the structural transition from P to HP-I to HP-II.The current research results will effectively promote the regulation of the crystal structure of ternary chalcogenide compounds in extreme environments,which will play an important role in exploring the basic mechanism of potential phase transitions and preparing structural and functionally stable devices.