Study On Formation And Transformation Of Sn:VO₂ Phase

Vanadium dioxide is known as transformation from a monoclinic semiconducting phase（M）to a rutile metallic phase（R）near room temperature.Regulating the phase transformation temperature to bring it closer to practical applications has always been the focus of vanadium dioxide research.Besides a large number of studies such as charge injection and stress induction,elemental doping is considered to be one of the most effective and simple methods to lower the metal-insulator transformation temperature.Among the many studies of doped elements,effect of Sn:VO₂are undefined due to the too large atomic radius to incorporate effectively.Amorphous Sn_xV_2-xO₅films with supersaturated quantities of Sn atoms were prepared by a non-equilibrium method.Under the decomposing process in a high vacuum environment,the Sn could reside in the VO₂lattice in situ with the synergistic effect of a large number of oxygen vacancies.Thus Sn:VO₂was obtained with Sn doped effectively.Raman as well as XRD,OM,SEM,UV spectrophotometer and electrochemical workstation were used to deeply investigate the role of Sn on VO₂phase formation and phase transformation,and the results are as follows:（1）After the introduction of Sn atoms,the melting temperature and decomposing temperature of Sn_xV_2-xO₅are lowered,so that Sn:VO₂could be obtained below the original V₂O₅decomposition temperature.The ship-like Sn:VO₂grows through the way of the nucleation in the center and evolvation towards both sides,forming the Sn:VO₂strips with M2+M1+M2 composite phase.However,after a higher overheat temperature（higher than the original V₂O₅decomposition temperature）,the VO₂without Sn is reflected as a single uniform M1 phase,and the VO₂with Sn is reflected as a single uniform M2 phase.（2）With the introduction of different Sn content,Sn:VO₂reflects four room-temperature semiconductor phases,M1,T1,M2,and T2.All of them undergo a Semiconductor-Metal Transformation（SMT）through the M2 phase.The phase transformation properties of each phase are shown as follows:There are two steps phase transformation process on M1 phase,M1-M2 and M2-R.The phase transformation temperatures in the heating process are 50℃and 54℃,and that in the cooling process is 44℃and 50℃,respectively.The SMT hysteresis width is about5℃,which is suitable in the field of optical switching.There are T2-M2 and M2-R in the phase transformation process of T1 phase.The phase transformation temperatures in the heating process are 50℃and 70℃,and that in the cooling process is 45℃and55℃.The SMT hysteresis width is about 10℃.There are also two steps phase transformation process on T2 phase,T2-M2 and M2-R.The phase transformation temperatures in the heating process are 48℃and 70℃,and that in the cooling process is 45℃and 60℃.M2 phase has only the one step phase transformation process,M2-R.The heating phase transformation temperature is 70℃and the cooling phase transformation temperature is 55℃.The SMT hysteresis width is even beyond 30℃,which is suitable for optical storage devices.（3）During the phase transformation process of multi-domain Sn:VO₂,comparison of real-time optical morphology and frequency shifts of Raman vibrations reveals that:The peak representing the V-O-V vibration mode blue-shifted towards the high frequency,the phase transformation temperature of VO₂become higher;Compared with the decay degree of different vibration modes peak value（V-V,V-O-V,V-O）in the phase transformation process,it is concluded that Sn:VO₂was taken the electron-driven Mott transformation firstly and followed the structure-driven Peierls transformation;Based on the behavior of equivalent stress（Pc）on Sn,the derivative relationship between T1-M1 and T2-M2 is proposed,and then draw the（Pc-T）phase diagram.Sn:VO₂has good thermal stability.After heating at 400℃,it still maintains a good reversible phase transformation process,but after heating at 500℃,it would be oxidized into the vanadium oxide with a higher valence.