Study Of Electronic Properties And Gas Sensing Properties Of Doped SnO₂ Materials

Nowadays,gas detection has closely relationship with our life and work.The requirements of gas sensors are getting higher and higher in military,medical,industrial and environmental protection.Due to the advantages of high sensitivity,low cost and small size,metal oxide semiconductor gas sensors has been studied widely.Wide bandgap semiconductor materials have received more and more attentions because of their unique properties and potential applications.Doping can improve material properties,single-doped tin dioxide?SnO₂?has been studied by many scholars.However,the calculation of SnO₂ doped with Sb and S is less involved.It is found that co-doping can effectively improve the solubility of the dopant,and increase the activation rate by reducing the ionization energy of the acceptor level and the donor level.It can also increase the carrier mobility at low doping concentration.Based on the density functional theory of the first principle and the plane wave pseudopotential method,this thesis studies the electronic structure and electrical properties of SnO₂ doped with Sb and S by using the generalized gradient approximation algorithm?GGA?.The geometrical optimization calculation is carried out for the doped structure.The BFGS algorithm is used to find the stable structure with the lowest energy.Plane wave cutoff energy is set to 360eV,and the internal stress is less than or equal to 0.1GPa.By analyzing the electronic structures,it is found that the material is still direct bandgap N-type semiconductor after being co-doped.The electron density is changed,and the overlap of atomic orbital is enhanced.It is conducive to the transfer of electrons.New energy levels are observed in the energy band of co-doped SnO₂,and the bandgap width is narrower than that of single doping.It makes electronic transitions become easier.Fermi level is observed into the conduction-band,which lead to the metal-like properties of the material.The electronic density of states was further investigated.The results of the density of states confirm the correctness of electron transfer.In the middle of the valence-band,the hybridization is composed of the S atomic orbital and the Sn,Sb orbitals.At the top of the valence-band,it is occupied by the S-3p orbit,which providing more hole carriers to move the top of valence-band up.With the concentration increase of S,the bandgap and the width of conduction-band continues to decrease.As a result,the conductive performance tends better.By comparing the performance of different adsorption models on the?110?surface of the material,it's observed that CO has the strongest adsorption on the five-coordinated Sn_5c.Considering that the gas sensor is inevitably affected by humidity during the working process,two models of co-adsorption of H₂O on the?110?plane is constructed.After comparison,it is found that the adsorption performance of the H atom downward model is better.It can be found that electrons in some H₂O molecules are transferred to the adsorption surface,which makes the resistance of the material becomes smaller.This explains the small resistance of the gas sensor in a humid environment.The adsorption models of CO and H₂O on Sb single doped and Sb,S co-doped?110?surface were constructed.The calculation results show that the surface structure after doping has better adsorption to CO and H₂O,and the co-doped structure is better than single-doped.The gas-sensitive property of the materials are greatly improved compared with the undoped.