Research On Tunablility Of Optical Properties Of Several Two-dimensional Chalcogenides By Spectroscopic Ellipsometry

Due to the nanoscale thickness,tunable optoelectronic properties and flexibility,two-dimensional（2D）materials have been one of the most promising material systems for next-generation microelectronics and optoelectronics,and are expected to continue the Moore’s Law.As the most representative 2D materials,2D dichalcogenides have been preliminary applied in various optoelectronic devices.The optoelectronic properties of 2D dichalcogenides have significant thickness scaling effects,and their physical properties are easily turned by external factors.Thereinto,optical functions are the optical fingerprints,which quantitatively reflect the interaction between light and matter,and are important parameters to systematically study the optoelectronic properties of materials.So,accurate calibrate the optical functions of 2D dichalcogenides,systematic and quantitative analysis of their band gap,optical transitions,excitonic effects and their tunabilities are essential to understand their novel optoelectronic phenomena and mechanism,which will further guide device optimization design.Accordingly,this dissertation will carry out research on tunablility of optical properties of several 2D chalcogenides by spectroscopic ellipsometry（SE）.The main works and innovations are as follows:External-field SE systems have been exploited,and a quantitative method for analyzing optical properties of 2D materials has been improved.External-field SE systems which can introduce temperature and electric field are built.System design,window error correction and other key problems are solved.Besides,a quantitative optical transition analysis method based on SE,critical point（CP）analysis and first-principles calculations is proposed,which constructs a quantitative relationship between the experimentally optical functions and the theoretically bandstructures,and can determine the energy and momentum spaces where optical transitions occur,as well as the energy bands and carrier types involved.The tunability of optical properties of 2D Bi₂Se₃ is intensive studied by SE.The optical functions of 2D Bi₂Se₃ with different layers are accurately calibrated,and the parameters of feature peaks are determined by CP analysis and differential spectra method.The formation mechanism of feature peaks in 2D Bi₂Se₃ is revealed by the optical transition analysis.The layer dependence is revealed,the joint density of states（JDOS）dominates the layer-dependent evolution in imaginary dielectric function.The mechanism of topological phase transition and reduced surface bandgap caused by interaction between upper and lower surfaces on the center energies of feature peaks is further elucidated.A research on the tunability of optical properties of 2D WSe₂ is carried out by variable voltage SE system.The layer-dependent optical functions,as well as the optical functions of monolayer WSe₂ at different gate voltages,are obtained.By using CP analysis and optical transition analysis,the CP parameters and optical transition positions are obtained,and the scale and electric-field tunabilities are analyzed.The competition of weakened exciton effects,reduced band gap and enhanced JDOS dominates the layer-dependent evolution trend.Combined with the electric-field regulation mechanism of variable voltage system,the electric-field tunability is explained from the perspective of the gate voltage induced carrier injection and quantum confinement Stark effect.The tunability of optical properties of 2D MoTe₂ is studied by variable temperature vacuum SE system.The temperature-dependent optical functions of 2D MoTe₂ with different thicknesses are quantitatively obtained.By using CP analysis,first-principles calculations and optical transition analysis,the center energies of CPs are acquired,the scale and temperature effects on CPs are quantitatively studied,and CPs’corresponding transition forms are obtained.Results show that the energy difference between A and B peaks in 2D MoTe_2,indicating spin-orbit splitting value,does not depend on temperature and thickness.And the"V"-shaped evolution trend of electron-phonon interaction could be attributed to the alternating effects caused by thickness and surface roughness.The research on tunablility of optical properties of several 2D chalcogenides by SE in this dissertation not only accurately obtains the optical properties of 2D Bi₂Se₃,WSe₂ and MoTe₂,but also reveals the tunability of their basic physical properties.It also provides theoretical support for the design of optoelectronic devices based on 2D chalcogenides,and provides basic data support for subsequent application fields,such as integrated circuit manufacturing,flat panel display,and photoelectric detection.