Research On Nanoelionics Based On Hexagonal WO₃ Nanowire Memristor

Nanoelionics is a new field study on electron-ion-coupled transport effect and related physical properties, mechanism and its applications under nanoscale. With the feature size of electronic devices entering into the nanoscale, drifting of ions and electrons under strong electric field induced by the bias voltage applied at short distance will result in the electrical transport properties of the devices change remarkable. These properties make nanoelionics ideal for realization of fascinating physical phenomena. Based on memristive effect, it is expected to achieve ultra-high density storage, computing and storage integration and artificial neural networks. However, the potential applications of memristor will be restricted because of its unclear microscopic mechanism and unstable performance. Therefore, design and fabrication of nanoscale electronic devices with new structures and new functions based on electron-ion-coupled transport have become challenges and opportunities for future information technology. Hexagonal WO3 (h-WO3) naonwires,which grew along [001] direction, contain quasi-one-dimensional channels along c axis. These channels enable us to achieve reversible drift of ions. Therefore single crystalline h-WO3 nanowires might be an ideal platform for studying the effect of electron-ion-coupled transport.H-WO3 nanowires have been controllable synthesized using a simple hydrother-mal method. The individual WO3 nanowire based devices were fabricated by using a standard photolithography technique and a lift-off process. The drift of oxygen vacancies in nanowire, water-oxidized hydrogen ions drift at the surface of nanowire and hydrogen ions drift in the quasi-one-dimensional channels were systematic stud-ied based on Au/h-WO3 nanowire/Au devices. The main achievements are summa-rized as follows:(1)H-WO3 nanowire memristor was constructed based on drifting of oxygen va-cancies, which result in the change of contact barrier. The two-terminal Au/h-WO3 nanowire/Au electronic device can be modeled as being composed of two Schottky diodes connected back to back. The electrical transport properties of the device depend on the reverse biased Schottky diode. In the devices, drifting of oxygen vacancies under strong electric field induced by the bias voltage will result in the effective width of the reverse biased Schottky barrier decreasing, and then result in the memristive effect or resistive switching phenomenon. By unidirectional bias voltage sweeping, the Au/h-WO3 Schottky contact can be turned gradually and re-versibly into Ohmic contact, and then the two-terminal resistive switching device can be reconfigured gradually and reversibly from non-rectifying state to either a forward or reverse rectifying state.(2) The memristive performance of the Au/h-WO3 nanowire/Au device is en-hanced remarkably, which might be attributed to H+drifting based on Grotthuss mechanism. When the relative humidity (RH) level is less than 51%, RH had little effect on memristive performance of the device; while the memristive performance is enhanced remarkably, when the RH level is larger than 51%. WO3 has intriguing electronic band structures, the energy level for water oxidation lies below of the valence band (for holes) and the energy level for H+ ion reduction lies above the bottom of conduction band. Water molecules can always be oxidized to produce H+ ions and oxygen gas by holes near the top of the valence band injected from the positively-charged electrode; nevertheless, when electrons injected into the reverse-biased Schottky barrier have enough energy, H+ ions will be reduced to produce H2 gas. When the device is biased at low RH level (<51%), H+ will be reduced near the positively-charged electrode while bias voltage sweep polarity is changed. H+ will drift to and accumulate near the reverse-biased Schottky barrier based on Grot-thuss mechanism when the adsorbed water film is thick enough (RH>51%, physi-cal absorption layer>2), and then N-type WO3 will cut-off near the reverse-biased Schottky barrier. With bias voltage increasing, H+ will be reduced to produce H2, which results in a decrease in the H+ concentration near the reverse biased Schottky barrier. The decrease in the H+ concentration will result in an increase in electrical current, which enhance the memristive performance of the device remarkably.(3) Modulating memristive performance of kh-WO3 nanowire by water-oxidized hydrogen ions implantation. The memristive performance of the devices are sys-tematically investigated and found to be strongly affected by relative humidity, bias voltage sweep range, bias voltage sweep times, laser illumination and testing atrno- sphere. When the bias voltage is swept repeatedly under large bias sweep range in high relative humidity level, the water-oxidized hydrogen ions can be easily im-planted into the lattice of WO3 nanowire, and the semiconducting WO3 nanowire can be converted into metallic HxWO3 nanowire gradually. The insulator-metal transition will be realized more easily after being injected electron-hole pairs ex-cited by laser illumination. The hydrogen ions will decrease when the device is exposed to oxygen atmosphere or swept with bias voltage in atmosphere with low relative humidity. By modulating the concentration of hydrogen ions, conductive hydrogen tungsten bronze filament might form or rupture near electrodes when the polarity of applied voltage changes, which will endow the device with memristive performance characterized by digital resistive switching.(4) Single-crystalline samarium hexaboride (SmB6) nanowires were successfully large-scale synthesized by chemical vapor deposition using BCI3 and SmCl3 as pre-cursors at 1070℃. Transmission electron microscopy along with selected area elec-tron diffraction indicate that the nanowires consist of single crystals with a preferred [100] growth direction. Vapor-solid mechanism has been proposed in this growth process. Sm and B species will always grow or chemically adsorb on the most acces-sible high-surface-energy crystal plane when their supply is insufficient, which results in SmB6 growing in a preferred direction and SmB6 nanowires coming into being. When Sm and B species’supply is sufficient, they will reach at and grow on all equivalent high-surface-energy crystal plane equally, which results in SmB6 growing along the< 100> directions at the same growth rate and microsized SmB6 crystals coming into being. Conventional four-terminal resistance measurements show that the resistance of SmB6 nanowire contains contributions from metallic surface elec-tronic states and semiconducting (insulated) bulk electronic states. The high quality single-crystalline SmB6 nanowires with large surface-to-bulk ratio are ideally suited for the investigation of topological properties of this material.