Controllable Preparation And Electron/Ion Coupled Transport Properties Of Tungsten Bronze Nanowires

Nonvolatile Resistance Random Access Memory occupies a more and more important status in the field of memory.The resistance switching effect of resistance random access memory is widely used in the nano-devices of various materials.However,the application prospect is restricted,because the resistance change performance is not stable and the physical mechanism is still controversial.The tunnel structures constructed from WO6 octahedra leave more space interstices in the oxygen sublattice,alkali metal ions or hydrogen ions can be embedded in to form a stable structure of the non-stoichiometric ratios.Particularly in hexagonal W03 nanwires,there is a quasi one-dimensional hexagonal and trigonal tunnels parallel to the radial direction,which makes alkali metal ions or hydrogen ions easier to reversibly and repeatedly transport.Therefore,the single crystalline h-AxWO3 nanowires might be an ideal platform for studying the effect of electron-ion-coupled transport and might be the best candidate for fabricating nanoscale resistive switching devices with stable and reliable performance.The main achievements are summarized as follows:(1)Single-crystalline hexagonal K0.26WO3 nanowires and K2OW7O21 nanaowires have been successfully synthesized via a hydrothermal method by controlling the reaction temperature.The K2OW7O21 nanowires were reduced in the hydrogen atmosphere of different temperatures,which effectively overcome the fracture of the nanowires caused by K2OW7O21 nanowires,to obtain the K0.26WO3 nanowires with controllable diameter.X-ray quantitative characterization indicates that the lattice constants and crystal lattice microstrains of these K0.26WO3 nanowires exhibit anisotropic behaviors along the radial and axis directions,which can be attributed to surface effect and aggregating or ordering of K+ ion.It also indicates that oxygen vacancies will occur in the WO3 lattice when the reduction temperature is around 650℃,which will prevent K+ ions from aggregating or ordering.Controllable synthesis and quantitative analysis of size-controllable KxWO3 nanowires will contribute to deepen our understanding on their structures and properties,and then explore the method of regulation and control of K+ ions distribution and resistance switching mechanism.(2)Size-controllable Single-crystalline hexagonal NaxWO3 nanowires have been successfully synthesized via a hydrothermal method by controlling the content of Na2SO4.X-ray quantitative characterization indicates that NaxWO3 nanowires radial/axial of the lattice parameter a/c correspond to increase/decrease with the increase of diameter,and the exitence of Na+ ions make crystal lattice microstrain increases in the direction of the nanowire.Ar+ ions bombardment with different energy produced the thermal effect,the effect of surface defect and stripping effect.It can effectively control lattice parameters and crystal lattice microstrain,and it turns out that the presence of Na+ ions make the crystal lattice microstrain increase/decrease along the nanowire direction.(3)The axial distribution of Na+ ions can be regulated by scanning Au/h-NaxWO3 nanowire/Au sandwich structure devices with large bias based on Schottky contact.When the concentration of Na+ ions is changed near the interface between Au electrode and nanowire,the two Schottky barrier heights will significantly change,resulting in obvious resistance switching effect.At the same time,the random distribution of Na+ ions leads to its disorder,which causes the localization of electronic states in the bottom of the NaxWO3 nanowire.The filling and subsequent release of the localization under a small bias will also give the device a resistance switching effect.Therefore,the drift of Na+ ions along the direction of the nanowire will endow the Au/NaxWO3 nanowire/Au devices more abundant resistance swiching properties.(4)The Au/h-NaxWO3 nanowire/Au sandwich structure devices based on ohmic contact is used to realize the insulator-metal transition in the NaxWO3 nanowire with large current scanning.By controlling the limit value and the action time(scanning range)of the current,it is found that high current,long action time and bidirectional scanning are more conducive to insulator-metal transition and metal phase maintenance.The gradual change process of metal phase to insulator can be observed by continuous scanning at the small limit of current(1 μA).The effect of thermophoresis,Na+ ions under high temperature in the channel will spread to the low temperature region along the radial diffusion,which leads to increase the resistance of the device;the magnetic effect of electric current,Na+ ions with the drift velocity along the radial direction will be subjected to the force pointing to the center of the current in the radial direction,which leads to decrease the resistance of the device.The two effects restrict each other and even lead to the periodic changes of the resistance of nanowires.Controlling the radial distribution of Na+ ions along the nanowires can achieve insulator phase transition based on Aderson localization/de localization,thus the the Au/NaxWO3 nanowire/Au devices endow more abundant resistance swiching properties.The tungsten bronze nanowires are also rich in chromic properties,,and it is expected to build up a more abundant photoelectric device combining the resistance and the chromic properties.It is also an idea platform for the study of the physical transport properties of the strong correlation system as a result of the electric/ion coupled transport in the tungsten bronze nanowires.