Study On The Mechanisms Of Property Modulations In Graphene-like Two-dimensional Materials By Temperature And Strain

In the“Post Moore’s Law”era,new materials for the next technological revolution in the field of electronics and optoelectronics have attracted great attention.Two-dimensional（2D）materials represented by graphene show a broad application prospect in the fields of micronano electronics and optoelectronics.However,there are still many problems to be solved,such as how to prepare high-quality and large-area 2D materials,how to control the interface interaction between these 2D materials and substrates,how to expand applications of 2D materials in the field of micronano devices.Based on this,this thesis studies temperature-dependent Raman spectroscopy of bismuth（Bi）and antimony（Sb）2D films,and proves that the temperature effect and temperature-strain coupling effect can effectively modulate the phonon properties of 2D materials.It also shows that the temperature-strain coupling effect determines the thermodynamic potential and dynamic potential in the growth process of 2D materials by vapor deposition（VD）.Furthermore,the preparation of 2D molybdenum telluride（Mo Te₂）with controllable structure and morphology under temperature-strain coupling and its potential applications in construction of metal-semiconductor ohmic contacts and electrocatalysts are studied.The experimental researches and conclusions are as follows:（1）Large area,continuous and high quality Bi films were prepared on silica（SiO₂）substrates by VD.Based on the Raman spectra,the temperature coefficients of the first-order Raman modes of E_g and A_1g of the as-prepared Bi film were investigated.The experimental results show that the contribution of the three-phonon process to the Raman shift is much greater than that of the four-phonon process.The width of the half height of the E_g mode increases linearly with the increase of temperature,but there is an abnormal phenomenon near the temperature 193 K for the width of the half height of the A_1g mode and the intensity ratio of the E_g mode to the A_1g mode,indicating that there may be a phase transition near 193 K.In addition,2D Sb films were synthesized on SiO₂ and PDMS substrates by VD,and the temperature dependence of the Raman spectra of the films were studied in the temperature range of 77-393 K.The experimental results show that the first-order Raman temperature coefficients of E_g and A_1g of Sb films are-0.0140 and-0.0174 cm^-1/K on SiO₂ and-0.0184 and-0.0231 cm^-1/K on PDMS,respectively.The results indicate that there is a great temperature-strain coupling between Sb film and PDMS substrate,and the biaxial strain change is about 0.225%for every 100 K temperature change.The temperature strain coupling provides an effective method to study and control the 2D materials properties.（2）2D 2H Mo Te₂ and 1T′Mo Te₂ nanoplates were synthesized by VD on SiO₂substrate,and their morphology and phase structures were characterized by optical microscopy and Raman spectroscopy.We found that the temperature-strain coupling plays an important role in modalating the phase and morphology evolution of Mo Te₂.Low Te evaporation temperature results in low Te reaction concentration.Low Te reaction concentration and low reaction temperature lead to low telluride rate,which leads to a small growth strain potential.In the process of VD crystal growth,due to the thermal expansion effect of the crystal,the low reaction temperature makes the prepared material produce a small strain,which is conducive to the nucleation of 2H Mo Te₂.On the contrary,high evaporation temperature and high reaction temperature produce large strain potential.The large strain breaks the original symmetry and thus reduces the symmetry of the prepared material,which is beneficial to the nucleation of 1T′Mo Te₂.For the morphology evolution of Mo Te₂,besides the symmetry of the space group of 2H Mo Te₂ and 1T′Mo Te₂,it is also affected by the change of precursor Mo:Te ratio（>1:2,≈1:2,<1:2）and edge growth dynamics.These results improve the understanding of the factors that affect the structure growth and morphology evolution of Mo Te₂,which is critical to the realization of controllable preparation of 2D Mo Te₂ by VD.（3）By changing the growth conditions of VD,2D Mo Te₂ films with different morphologies and structures were prepared on SiO₂ and carbon cloth（CC）substrates respectively,and their potential applications in the construction of metal-semiconductor ohmic contact and electrocatalysis were studied.Firstly,2D in-plane1T′-2H Mo Te₂homojunctions were prepared on SiO₂ substrate,and the interface electrical properties of 1T′-2H Mo Te₂ homojunction were studied.The built-in potential and the surface potential between 1T′Mo Te₂ and 2H Mo Te₂ were measured by Kelvin force microscope.We found that ohmic contact is formed in 1T′-2H Mo Te₂homojunction,which is further confirmed by DFT calculation.The contact properties of 1T′-2H Mo Te₂ homojunction were further studied by injecting charge.The results provide the electrostatic information of the one-dimensional junction between 1T′Mo Te₂ and 2H Mo Te₂.The phase engineering of 2D transition metal dichalcogenides（TMDCs）is important for the buffer contact layer,which can be used to construct ohmic contacts of 2D TMDCs.Secondly,film-like 1T′Mo Te₂,film-like 1T’/2H Mo Te₂,porous 1T′Mo Te₂,small granular 1T′Mo Te₂ and large granular 1T′Mo Te₂were prepared on CC substrate,and their electrocatalytic activity was tested and studied.The results show that the film-like 1T′Mo Te₂/CC has excellent electrocatalytic properties.This is because the film-like 1T′Mo Te₂/CC electrode has rich catalytic activity sites and excellent conductivity.Our results are important for promoting the practical application of Te compounds and TMDCs materials in electrocatalytic hydrogen evolution.