Study On The Effect Of Surface Modification On The Electronic Structure And Gas Sensing Properties Of Two-dimensional SnSe₂

Industrial production promotes economic development,meanwhile it generates excessive amounts of air pollutants and pose a major threat to human health.Therefore,highly sensitive gas sensors for hazardous gases need to be developed to effectively solve this problem.Sn Se₂,as an n-type narrow-band semiconductor material,has attracted many attentions in applications such as thermoelectric,optoelectronic,and gas sensors due to its large surface-to-volume ratio,large number of active sites,and ease of processing.People use the excellent gas-sensitive behavior of thin-film materials for gas sensor research,and in order to enhance the materials’performance,the influence of defects,doping and other modification behaviors generated during the material preparation process on the material properties is studied.In this study,the effects of surface modifications such as doping and strain on the properties and gas adsorption performance of two-dimensional Sn Se₂ materials are investigated,especially the adsorption of hazardous gases by noble metal-doped two-dimensional Sn Se₂.This thesis evaluates the impact of surface modification on the electronic structure and adsorption properties using the first-principle calculation method,and presents the following main research content and findings.（1）Based on the model building function of Material Studio software,a 3×3×1supercell model of two-dimensional monolayer Sn Se₂ is constructed and its geometric structure and photoelectric properties are calculated with a band gap value of 0.785 e V and a band gap type of indirect band gap.According to the density of states,the Sn-5p and Se-4p orbitals make the primary contribution to the valence band,while the Sn-5s,Sn-5p,and Se-4p orbitals are the main contributors to the conduction band.From the optical properties,the static permittivity function of the two-dimensional monolayer Sn Se₂ is 3.7.The interband jump of the orbitals gives rise to a maximum peak of 3.93in the imaginary part of the dielectric function,which occurs at 4.61 e V.（2）Based on the optimized monolayer Sn Se₂ model calculated by package,it is doped with nonmetallic elements（O,S,Te）and noble metal elements（Au,Ag）to optimize the calculation of the electronic properties of the doped structure.The doping leads to a significant change in the electronic properties compared to the intrinsic Sn Se₂.The introduction of nonmetallic elements leads to the transformation of Sn Se₂ from an indirect bandgap to a direct bandgap semiconductor.The introduction of noble metal elements leads to the metallic nature of the material.In the density of states,the doping of the elements of the isoelectronic system makes the orbital changes small,and the doping of the noble metal elements,the orbital hybridization forms impurity energy levels,mainly due to the d-orbital effect of the noble metal atoms.（3）This thesis studied the adsorption of five hazardous gases（NO,NO₂,HCN,SO₂,and H₂S）by noble metal-doped two-dimensional Sn Se₂ systems.The adsorption energies of the ten adsorption systems are all negative and stable.The density of states shows that the variations around the Fermi energy level are mainly contributed by the d orbitals of noble metals,and the p orbitals of gas molecules.The charge transfer and differential charge density diagrams allow to derive the interaction forces and charge transfer between the gas molecules and the substrate.the best performance is obtained for the adsorption system of NO₂molecules,H₂S and HCN molecules transfer a small amount of charge on the Au-doped Sn Se₂ structure,-0.041 e and-0.043 e,respectively,and SO₂ and HCN molecules transfer a small amount of charge on the Ag-doped Sn Se₂structure.with small charges of 0.05 e and-0.004 e,respectively.（4）The properties of adsorption energy,electric charge transfer,recovery time and sensitivity of five gas adsorption systems with gold-and silver-doped monolayer Sn Se₂structures in the biaxial strain range（-8%to 6%）were analyzed.The results show that:The gas adsorption behavior is affected by biaxial strain,and the absolute value of the adsorption energy increases with either applied tensile or compressive strain.It means that the gas molecules are becoming more strongly attracted to the surface they are in contact with.Excellent gas-sensitive materials also have to have an easy desorption process.The recovery time results for ten gas adsorption systems under strain are as follows:NO₂ molecules have a shorter recovery time of 0.02 s on the surface of Ag-Sn Se₂ without strain,NO molecules have a shorter recovery time of 3.2 s at 300 K,4.2s at 400 K,and 1.2 s at 500 K.HCN has a shorter recovery time of 1.6 s at 6%strain of Au-Sn Se₂ and H₂S has a shorter recovery time of HCN has a shorter recovery time of1.6 s at 6%strain of Au-Sn Se₂,and H₂S has a shorter recovery time of 0.0008 s at 2%strain of Ag-Sn Se₂.The increase in operating temperature can also shorten the desorption time of gas molecules.In practice,strain-regulated gas sensors can improve the performance of the detected gases without replacing the raw materials,and can also target specific gases for identification.