Synchrotron Radiation-based Infrared Spectroscopic Study Of Layered Materials Under High Pressure

Layered materials(such as graphene,transition metal dichalcogenides,black phosphorous and boron nitride,etc.)possess layered crystal structure,with loose interlayer Van der Waals(vdW)bonds and strong intralayer covalent bonds.Due to their unique structure,layered materials could strongly be modulated by external stimuli,and exhibiting unusual electronic properties and potential technological applications in optoelectronics and energy technology.Pressure,being an important approach,could effectively tune the crystal and electronic structures of layered materials by regulating the weak van der Waals interactions.Therefore,investigating the evolution of the crystal and electronic structures of layered materials under pressure is of significance to understand their pressure-driven phase transition and physical properties.On the other hand,infrared and Raman spectroscopy has been a very useful technique in detecting the vibrational and electronic properties of materials.By combining with high pressure technology,infrared and Raman spectroscopy has been most commonly used to study the structure and electronic structure of materials under pressure.In particular,due to the high brightness of synchrotron radiation light source,synchrotron radiation-based infrared spectroscopy has played a vital role in detecting the optical property of micron scale layered materials.In this dissertation,we systematically studied the pressure-induced structural phase transition in the anisotropic layered materials SnSe,MoO3 and black phosphorus,as well as their anisotropic optical absorption under pressure,by synchrotron radiation-based infrared spectroscopy and high-pressure Raman spectroscopy.The main results are listed as follow:1.By combining synchrotron radiation-based infrared and Raman spectroscopy as well as electrical measurement,we systematically studied the effect of pressure on the phase transition of SnSe.Our result shows the existence of structure transition at around 8 GPa,be attributed to a first-order phase transition from a high symmetry phase(αSnSe)to a distorted phase(α’-SnSe).An electronic transition from semiconductor to semimetal was observed above 12 GPa,followed by Pnma to Bbmm structural transition.Meanwhile,the anisotropic optical property of SnSe reduces with pressure increasing and disappeared at around 12 GPa.The anisotropic-to-isotropic transition furtherly confirms that SnSe undergoes a structural transition from a low symmetry(Pnma)to a higher symmetry(Bbmm)structure around 12 GPa.2.We investigated the pressure effect on the structure transition and optical anisotropy of MoO3 by combined polarized infrared and Raman spectroscopy.We found that MoO3 undergoes a phase transition from orthorhombic(Pnma)to monoclinic MoO3-Ⅱ(P21/m)at around 11 GPa.And another phase transition from MoO3-Ⅱ(P21/m)to MoO3-Ⅲ(Pmma)was observed above 27 GPa.The polarization angle with the maximum intensity of the Mo-O2-Mo symmetric stretching mode undergo an angular rotation with compression.The polarization angle rotates moves from 90° to 105° then back to 90° with increasing pressure,in corresponding to the structural transition under pressure.3.We performed the investigation of pressure induced structural phase transition of black phosphorus by synchrotron radiation-based infrared and Raman spectroscopy.Our Raman results show that the full width at half maximum of all the three Raman modes Ag1,B2g,and Ag2 of the orthorhombic phase decrease with pressure up to 1.2 GPa and then increase,indicating a turning point at～1.2 GPa,which are related to the electronic topological transition.With increasing to 11 GPa,these modes all disappear completely and three new modes N1,N2,and N3 occurs,suggesting the phase transition from rhombohedral structure(R3m)to the simple cubic structure(sc)。Besides,the transmittance spectra of black phosphorus show a drastic change at～11 GPa,which confirm the structural phase transition at 11 GPa.In addition,we selected few-layer and bulk black phosphorus as sample for high pressure Raman measurement.And we found that the frequency shifts of the Raman active modes behave differently for 6 layers,10 layers and bulk sample.Moreover,compared with the bulk sample,the few-layer sample has higher critical transition pressure,which reflects the influence of layer thickness on the pressure-induced phase transition of black phosphorus.