Controllable Preparation And Photoelectronic Properties Of 2D Transition Metal Sulfide

In recent years,due to the development of integrated circuit technology,the feature size of the chip has become smaller and smaller.As a result,the integration of the device has become higher and higher,and various quantum effect problems have also emerged,resulting in a huge challenge in the integrated circuit engineering of traditional semiconductor materials.Over the years,in order to solve this problem,people have been working hard to find new kinds of materials to adapt to the development of the times.Since 2004,scientists prepared the single-layer graphene using the mechanical exfoliation technology,the discovery of graphene have given birth to a new class of material known as“2D material”.The single-layer graphene has unique physical and chemical properties,however,single-layer graphene has no band gap,and there is only small band gap above the second layer,which severely limits its application in the field of optoelectronic devices.Through the unremitting efforts of researchers,we have succeeded in synthesizing new two-dimensional transition metal sulfides such as MoS₂,WS₂,and GaS through physical separation and chemical epitaxy.Compared to graphene,the novel transition metal sulfides maintain the high mobility characteristics of graphene,at the same time,this type of material has energy band-gap,which overcomes many problems caused by the absence of band-gap of grapheme.In summary,the novel transition metal sulfides material has great potential for the future applications of electronics,information and energy.In order to discover the potential applications of these materials,it is extremely important for us to further study the optical properties of these two-dimensional transition metal sulfides.The optical spectroscopy method is a powerful non-destructive detection technique,we can obtain some important optical information such as optical band gaps,electronic transitions,phonon vibrations,and optical constants through the optical characterization of these semiconductor materials.By analyzing these optical properties,the crystalline quality,electronic transition and band structure of the material can be obtained.In this master's thesis,the optical character of MoS₂,WS₂and GaSe have been investigated by spectroscopic methods.The main research contents and innovations of this article include the following points:1.The effects of S-doping on the electronic transition,band gap,and optical absorption of GaSe_1-xS_x single crystals have been systemically investigated by using spectroscopic ellipsometry and first-principle calculations.Optical properties and interband electronic transitions of GaSe_1-xS_x?x=0,0.133,and 0.439?single crystals as a function of S concentration have been investigated by comparison of using spectroscopic ellipsometry and first-principle calculations.The dielectric functions and absorption coefficient in the photon energy of 1.03-6.52 eV have been obtained.Five electronic transitions energy E₁,E₂,E₃,E₄,and E₅ were assigned,respectively,at about 3.310 eV,3.674 eV,3.931 eV,4.962 eV,and 5.500eV for solid solution crystal GaSe_1-xS_x?x=0.133?.It can be found that the transition energies of E₂,E₃,and E₄ increase with the doping of S.Furthermore,the first-principle calculation results reveal that the band gap energy increases from 2.085eV at x=0 to 2.15 eV at x=0.439,which is in good agreement with the SE experiment results?from 1.908 eV at x=0 to 2.081 eV at x=0.439?.Five inter-band electronic transitions E₁,E₂,E₃,E₄,and E₅ have been obtained by fitting the second derivatives of the complex dielectric functions and the physical origins were explained with the aid of theoretical calculations.2.The Black Phosphorus/MoS₂?BP/MoS₂?heterojunctions have been prepared using the mechanical exfoliation technology,and the temperature dependence on the lattice vibration mode and photoluminescence characteristics of BP-MoS₂heterojunctions have been obtained through the Raman scattering spectra and Photoluminescence spectra.Meanwhile,the effects of polarization angle changes and the thickness on the lattice vibration modes and photoluminescence characteristics of the BP/MoS₂ heterojunctions were investigated by the Raman scattering spectra and Photoluminescence spectra at room temperature.The effects of temperature changed on the lattice vibration modes and photoluminescence characteristics of the BP/MoS₂ heterojunctions,which were prepared by the mechanical exfoliation technology,have been analyzed by using Raman scattering and photoluminescence?PL?spectroscopy.From the Raman scatting spectra,it can be found that all the lattice vibration peaks of BP/MoS₂ appear red-shifted phenomenon as the temperature rises.For the photoluminescence spectra,it was found that the temperature increased from 200 K to 500 K,the PL peak of MoS₂ shifted from 664 nm to 686 nm,the PL peak of BP/MoS₂ changed from 664 nm to 693 nm.In addition to changing the temperature,we studied the effect of different polarization angle and sample thickness on the lattice vibration modes and photoluminescence characteristics of BP/MoS₂ heterojunction at room temperature.It can be found that when the polarizer was placed horizontally-vertically,the polarizer had a filter effect on the peak of the lattice vibration pattern of the sample,and the intensity of the photoluminescence peak was enhanced.For the phonon mode analysis of BP/MoS₂ with different thickness,we found that when the thickness is thinner,Bp and MoS₂ are better stacked.3.The single layer MoS₂ and WS₂ have been prepared by chemical vapor deposition,and the temperature dependence of lattice vibration mode and photoluminescence characteristics of Mo S₂ and WS₂ have been obtained through the Raman scattering spectra and Photoluminescence spectra.The effects of temperature changed on the lattice vibration modes and photoluminescence characteristics of single-layer MoS₂ and WS₂ which were prepared by chemical vapor deposition?CVD?method have been analyzed using Ramanscatteringandphotoluminescence?PL?spectroscopy.Fromthe temperature-variable Raman scattering spectra,we found that the intensity of the lattice vibration peaks of both the monolayers MoS₂ and WS₂ decreases,and the peak position red-shifted with the temperature increasing.For the photoluminescence spectra,the photoluminescence peak position of MoS₂ slightly shifted from 670 nm to671 nm when the temperature was changed from 175 K to 600 K.The photoluminescence peak position of WS₂ was changed from 628 nm to 686 nm.Both of them move in the direction of increasing wavelength,that is,the band gap becomes smaller.This phenomenon can be explained from the following two aspects:One is thermal expansion,that is,temperature causes a change in the lattice constant,which causes the energy band structure change or band edge movement;On the other hand,the change of the lattice vibration state caused by the temperature change,that is,the change of the phonon excitation state,leads to the coupling of the electron-phonon and a slight change in the energy band.