| Graphene can be exfoliated from graphite,which not only changes people’s original understanding of two-dimensional materials,but also attracts wide attention due to its excellent thermal conductivity,electrical conductivity and mechanical properties.However,it was later found that the gapless graphene was not suitable for optoelectronic devices.Since then,many exciting two-dimensional materials have been discovered one after another,the most typical ones being Mo S2 and black phosphorus.Two-dimensional materials have unique characteristics due to their unique two-dimensional structure,making them popular in many fields,such as catalysis,photovoltaics,gas sensing,transistors and other fields,involving energy,environment,technology and other issues in our daily life.Mo S2 has been a leader in two-dimensional materials since its birth,and is one of the most popular two-dimensional materials after Graphene.However,black phosphorus has hindered the commercial application of the device due to its stability in air.In recent years,group IV monochalcogenides with a black phosphorus-like structure have begun to attract people’s attention due to their stability,abundant resources,and environmental friendliness.With the increasing demands on material properties,it is difficult for a single two-dimensional material to better meet the application requirements of miniaturization and multi-functionality of devices.The van der Waals heterojunctions formed by stacking different two-dimensional materials,while retaining the excellent properties of a single material,some properties have been improved,and even the synergistic effect of the two has shown new functions.Heterojunctions have gradually become a mainstream direction of two-dimensional materials research.According to our research,the heterojunctions composed of Mo S2 and group IV monochalcogenides composed of Ge Se,Sn S,and Sn Se have been reported in related fields and show excellent performance.However,we found that the heterojunction composed of monolayer Mo S2and monolayer Ge S has not been reported either experimentally or theoretically.The band arrangement and optical properties of the Ge S/Mo S2 heterojunction are yet to be studied.In addition,Mo SSe,a peculiar material formed by replacing a layer of S atoms in Mo S2 with Se,has a vertical dipole and strong Rashba spin-orbit interaction due to the breaking of the in-plane symmetry,which has attracted a lot of research in recent years.Subsequently,Mo SO,WSSe,and WSe2/WSSe heterojunctions have been successfully fabricated experimentally,but their theoretical studies are still lacking.The arrangement of the energy band of the heterojunctions,the electron transfer between the interlayers and the light absorption coefficient of the heterojunctions are crucial to the optoelectronic properties of the device,and are the basis for analyzing its applications.In this thesis,the first-principles simulation software Quantum ATK based on density functional theory is used to calculate the electronic structure and optical properties of Ge S/Mo S2 and Ge S/Mo SX（X=O,Se,Te）heterojunctions.Among them,Ge S/Mo S2 heterojunction,the related results under different interlayer distances,strains and electric fields are calculated in detail.The main conclusions are as follows:Calculations show that the Ge S/Mo S2 heterojunction is a type-II heterojunction in which the CBM and VBM of Ge S are higher than those of Mo S2,respectively.The effects of interlayer distance,strain and external electric field on the heterojunctions were further investigated.It is found that the change of the interlayer distance mainly affects the electron transfer,and the compression of the interlayer distance is beneficial to the electron transfer.Strain mainly affects the light absorption coefficient.The tensile strain greatly increases the light absorption coefficient of the heterojunction in the visible light region due to the reduction of the band gap of Mo S2.The compressive strain is detrimental to the light absorption of the heterojunction by increasing the band gap of Mo S2.The external electric field can change the type of heterojunction into type-III heterojunction with Ge S VBM higher than Mo S2 CBM,and can also convert the heterojunction into type-II heterosjunction with Mo S2 CBM and VBM higher than Ge S CBM and VBM respectively,and play a decisive role in the direction of electron transfer between the interfaces.For the Ge S/Mo SX（X=O,Se,Te）heterojunction,we constructed two stacking models of PA and VA,denoted as Ge S/Mo SX＿PA（VA）.The research shows that:Ge S/Mo SO is a type-III heterojunction with the VBM of Ge S higher than the CBM of Mo SO.Ge S/Mo SSe is a type-II heterojunction in which the CBM and VBM of Ge S are higher than those of Mo SSe,respectively.Ge S/Mo STe＿PA is a type-I heterojunction in which the CBM of Mo STe is lower than that of Ge S and the VBM is higher than that of Ge S,and Ge S/Mo STe＿VA is a type-II heterojunction that the CBM and the VBM of Mo STe are higher than the CBM and the VBM of Ge S,respectively. |
PDF Full Text Request