电触发二氧化钒纳米线发生金属-绝缘体转变的机理

Vanadium dioxide (VO₂) is well known for its metal-insulator transition (MIT) at 341 K. Normally, the VO₂ presents a metallic rutile (R) phase above the T_c, but an insulator (monoclinic, M) phase below the T_c. Besides the thermally driven mode, the phase transition can also be triggered electrically, which is common in electron devices like field effect transistors and actuators. Due to the electron correlation, the Mott transition associated with electronelectron interaction as well as the Peierls transition involving electron-lattice interaction are both believed to drive the transition of VO₂, although the actual MIT mechanism is still under debate in condensed matter physics. The Coulomb screening of the electron hopping can be broken by injecting enough carriers. However, the issue is more complicated in the electrically-triggered MIT of VO₂ due to the Joule heat of current and the carrier injection of field effect. In this work, we study the electrically induced MIT in VO₂ nanowires by in-situ transmission electron microscopy (TEM). We build a closed circuit under the TEM by using in-situ electric TEM holder to capture the changes of VO₂ in electron structure and phase structure simultaneously. An alternating bias voltage is applied to the VO₂ nanowire while the selected area electron diffraction (SAED) patterns of VO₂ nanowire are recorded using Gatan Oneview® fast camera. The current rises or drops suddenly in the current-voltage curve (I-V curve), indicating a phase transition, through which the SAED pattern of nanowire is recoded every 5 ms. By correspondence analysis between the SAED patterns and the I-V data at every moment, a transition state of insulating R phase is observed, which is obviously different from the normal state of the metallic R phase or the insulating M phase. The existence of the insulating R phase indicates that electron structure transforms prior to the phase transition. The decoupling phenomenon reveals a predominant role of electron-electron interaction. Moreover, by feedback strategy of the circuit, the current through the metallic nanowire of VO₂ remains unchanged, and thus keeping the Joule heating in the nanowire constant, the phase transition from metal to insulator does not happen until the voltage decreases to about 1 V. When phase transition to insulator happens in voltage stepdown, even stronger Joule heating is generated because of the increased resistance of VO₂ nanowire. Therefore, the VO₂ phase transition is triggered electrically by the carrier injection instead of the Joule heating. The injecting of enough carriers can break the screening effect to activate the electron hopping and initiate the phase transition. The deduction is confirmed by the decoupling phenomenon in the insulating R phase. Additionally, the polarized shift rather than the phase transition of the VO₂ nanowire is observed in the non-contact electric field mode, which also supports the cause of the carrier injection for the electric induced MIT. The results prove the electron-correlation-driven MIT mechanism, or so called Mott mechanism, and open the new way for electron microscopy used to study the electron correlated MIT.