Effect Of Nanoparticle Surface Defects On Electrical Bistable Characteristics Of PEO:ZnO Devices

Polymer-nanoparticle hybrid thin film organic bistable devices have been studied extensively recently due to their unique advantages,such as:low cost,simple fabrication and high storage density.The electrical bistable performance of polymer-nanoparticle material system is usually attributed to elecctron trapping and de-trapping because of the existence of the surface defects which act as trap center.However,the relevant conduction switch-on mechanisms based on polymer-nanoparticle material system are still unclear and mostly explained by two models:Fowler-Nordheim(FN)tunneling and trapped charge-limited-current(TCLC),even in the same material system.It is well known that carriers can be easily captured by lots of nanoparticle induced surface defects which can be passivated in certain degrees with the help of surface ligand.However,the correlations have been ignored between the surface defects of nanoparticles and the working mechanism of electrical bistable performance in the previous studies.Therefore,in this work,two kinds of PEO:ZnO electrical bistable devices based on with/without traditional surfactant were fabricated and the resulting two different conduction mechanisms demonstrate the nanoparticle surface defect dependent conduction switch-on mechanisms.The optimization of the electrical bistable devices are investigated as well.The main contents are listed as follows:(1)The ZnO nanoparticles without surface defects with the average size of less than 10 run were synthesized by a water bath heating reaction.The PEO was blended with the resulting ZnO nanoparticles to significantly reduce the aggregation of the nanoparticles,leading to a hybrid thin film with nanoparticles dispersing uniformly.The morphology of the active layer and the distribution of nanoparticle in the hybrid film were characterized by SEM and EDS-mapping measurement.The photoluminescence(PL)spectrum of the PEO:ZnO hybrid thin films with different mass ratios show that the passivation of surface defects of ZnO nanoparticles can be to some extent improved with the increasing the PEO concentration Finally,the devices with the structure of ITO/PEO:ZnO/Al were fabricated.The current-voltage measurement indicates that too little or too much PEO content can make the low conductive state too high and the high conductive state too low.When the PEO:ZnO ratio increases to 10:1 and 20:1,both devices exhibit stable performance with the enhances current hysteresis.The I-V curve fitting implies that the conductance transfer from OFF state to ON state is mainly attributed to the TCLC current.(2)The PL spectrum showed that the ZnO nanoparticles surface defects can be fully passivated with surfactant.The device contained the different PEO:ZnO ratios were prepared and the SEM and EDS-mapping were used to characterize the micro morphology and distribution of the two materials in the films.All devices have the same conductivity transfer trend from OFF state to ON state in the measured I-V curves.The linear fitting of the I-V curve shows that the device conductance transition is based on the Fowler-Nordheim tunneling current mechanism.In addition,the optimum PEO:ZnO mass ratio,annealing temperature and film thickness were found.(3)It is demonstrated that the nanoparticle surface defects can significantly affect the electrical bistable device conduction switch-on process directly.when the surface defects were present,the charges trapped by the nanoparticle surface defects can be easily released.Otherwise,the charge would be trapped by the bulk defects of ZnO nanoparticles when the surface defects were fully passivated.Thus,the switch-on mechanism of the former device is dominated by TCLC current while the latter one is dominated by FN tunneling.This study reveals why the same polymer-nanoparticle material system showed the two different electrical bistable switch-on mechanisms in the past reports.Simultaneously,the comparison of the above discussion shows that the devices based on the ZnO nanoparticles without surface defects exhibit the better device performance than those which with surface defects due to the FN tunneling.