| Memristor is an emerging technology that enables rich interdisciplinary science and novel device functions such as non-volatile memory and nano-ion-based synaptic electronics.As a wide-bandgap semiconductor,WO3 thin film shows a resistance change ability based on oxygen ions motion and intra-membrane oxidation state transition,so that the WO3 nanostructure-based memristor exhibits regulatable synaptic plasticity and become one of the preferred materials for artificial intelligence nanodevices in neuromorphic calculations.However,the optimal morphologies of the WO3 nanostructure-based memristor are still elusive.Moreover,in order to further apply the memristor to flexible neuromorphic electronics,great efforts are also needed in switching and multi-functionality.Therefore,we prepared WO3 nanostructure-based memristor with excellent electrical properties on PET/ITO flexible substrates,which was adjusted by annealing,pH treatment and interface.Combined with the bending test under variable temperatures and ABAQUS finite element analysis,the effects of external mechanical stress on bending failure mechanism of WO3 based flexible memristor were explored.The effects of different annealing temperatures,pH and top electrodes on the morphologies and electrical properties of WO3 thin films were systematically investigated.The main conclusions are as follows:the prepared WO3 nanoparticles were monoclinic phase structure,and the number and size of crystal grains increased significantly with the increase of annealing temperature.The different pH conditions of hydrothermal precursors leaded to different nanostructures in WO3 films,corresponding to block micro-bricks、some thick tetragonal nanorods with square top surface,nanosheets,bundles composed of considerable nanorods and irregular nanostructures with smaller size,respectively.The electrical test suggested that WO3 films with different crystallinities and morphologies showed significant bipolar resistance switching characteristics and good cycle durability.The threshold voltage and the on/off ratio increased gradually with the increase of annealed temperatures.The transport of carriers was limited by the crystalline dense structure of the films with higher crystallinity,which leads to the decrease of on/off ratio.The on/off ratio of the films synthesized under the condition of pH=1.5 was up to 82.4.The Ag/WO3/ITO,Al/WO3/ITO and Cu/WO3/ITO devices were prepared with Ag,Al and Cu as top electrodes respectively.The Ag/WO3/ITO memristor was found to be a non-volatile memory device with large switches and excellent durability.The memory switching process conformed to ohmic conduction in the low resistance state,while the low bias region of high resistance state was ohmic conduction and the high bias region was induced by space-charge-limited current(SCLC).The effects of different interfaces on the electrical stability of the memristors were studied by constructing WO3 based flexible memristors with four different structures(Ag/ZnO/WO3/ITO,Ag/PEDOT:PSS/WO3/ITO,Ag/P3HT/WO3/ITO and Ag/P3HT/WO3/P3HT).The four devices showed good nonlinear transmission characteristics,which can dynamically reflect the changes caused by external signals and have the basic transmission properties of synapses.The voltage stimulus required for the second and third memory recovery was less and less after the first learning-forgetting process in the empirical learning-forgetting process,indicating that the flexible memristor with "learning experience" will make it easier to re-learn forgotten information.Temperature measurements showed that the four flexible memristors work well even at high temperatures of 100℃ and the current changes were further accelerated as the temperature increased.However,the resistance change of the Ag/P3HT/WO3/ITO memristor was the smallest,indicating that the ambient temperature has little effect on the electrical characteristics of the device.It can be used as a stable flexible synaptic analog memristor device.The influence of bending on the thermomechanical fatigue life of WO3-based flexible memristors was investigated under different temperatures.We found that the smaller the bending radius was,the faster the electrical performance of the device dropped.When the bending radius was 7mm,the largest internal stress of the film exceeded 1000Mpa,but showed the smallest change in conductivity.The flexural fatigue tests were carried out on four flexible devices.It was found that the ZnO/WO3 device showed the largest decrease in electrical conductivity after repeated bending and the crack density also increased to 9.4%,indicating that the device was deteriorated severely.The Ag/P3HT/WO3/ITO memristor showed the slowest decrease in conductivity and the lowest crack density because the stress in the organic film was small under bending conditions,and the growth and expansion of cracks in the inorganic film were suppressed,which made the device resistant to bending.Subsequently,the Ag/P3HT/WO3/ITO device with better bending fatigue characteristics was tested for bending under various temperatures.It was found that the bending increased the intergranular deformation in the film,lead to the increase of the intrinsic stress.The temperature changes caused an increasing of the thermal stress in the film.The increased of intrinsic stress and thermal stress accelerated the initiation and expansion of microcracks,which increased the sheet resistance and decreased the electrical properties of the device.In summary,our works explored the electrical properties of WO3 nanoparticles in non-volatile memory and synaptic electronics.And we deeply analyzed the effect of bending on the thermomechanical fatigue life of flexible memristors under variable temperatures.It provides experimental and theoretical analysis for the application of WO3 nanoparticles in flexible memristors and synaptic simulation studies. |
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