Role Of TiO₂ Surface Defect On Conductivity Switching Of PEO/TiO₂ Electrical Bistable Devices

Nanoparticle-polymer electric bistable devices have received intense attention due to their low cost,simple fabrication,flexibility and large storage density.The devices mainly have two kinds of conduction transition mechanisms:trap charge limited current(TCLC)and Fowler-Nordheim(FN)tunneling.In both switching mechanisms,nanoparticle surface defects play a key role.However,the surface defect determinant of the switching mechanism is no clear,especially in the same material system,there are two different mechanisms explained.The charge trapping induced by the surface defects of the nanoparticle plays a decisive role in the conductivity transition.Therefore,it is crucial to study the relationship between the surface defect of the nanoparticle and the conduction switching mechanism.In this work,the titanium dioxide with different defect states is combined with PEO insulating polymer to prepare electric bistable devices to study the conduction seitching mechanism of the surface defects in electrical bistable devices.The main research content is as follows:(1)Amorphous titanium dioxide was prepared by sol-gel method,and anatase-type titanium dioxide was prepared by hydrothermal method.The optical,electrical and morphological characterizations were carried out.The results show that the light transmittance and electrical conductivity of the two types of Ti02 films are similar,and the morphology measurement reveals that the surface of anatase Ti02 film is rougher compared with amorphous one.The surface contact angle test of water shows that there is a large amount of hydroxide in the surface of anatase TiO2.(2)Combined with PEO polymer,two types of TiO2 film were prepared for the electric bistable devices with the structure of ITO/TiO2/PEO/Ag.The voltage-current scan was conducted to study the electrical bistable characteristics.The experimental results show that the device ON/OFF ratio changes with the change of PEO concentration.When the PEO is 25 mg/ml,both devices can obtain the maximum switching ratio.Comparing the two devices,it is found that the switching ratio of the amorphous TiO2 electric bistable device is significantly smaller than that of the anatase TiO2 electric bistable device,and the I-V curve fitting indicates that the two kind devices have the different conduction switching mechanisms.(3)By theoretically fitting the IV curve,it is found that the conductivity transition mechanisms of the two electrical bistable devices are different,amorphous one is dominated by TCLC and anatase is dominated by F-N tunneling.Combined with the first part of the study,it is reasonable to speculate that the surface defects of TiO2 causes the difference in the conductivity switching mechanism.Therefore,the surface hydroxyl group of anatase TiO2 was tried to greatly reduce by high-temperature heating.The anatase TiO2 films containing a small or large number of hydroxyl groups are prepared for the electric bistable devices.The conduction switching mechanism of the electric bistable device containing a large amount of surface hydroxyl is FN tunneling,while the other one containing a small amount of hydroxyl is TCLC.The above experimental results show that the difference of TiO2 surface defects can significantly affect the conduction mechanism.When the surface defect is a large number of hydroxyl groups,the electrical bistable property of the device is FN tunneling,and conversely,it is TCLC.