电触发二氧化钒纳米线发生金属-绝缘体转变的机理

Mechanism of electrically driven metal-insulator phase transition in vanadium dioxide nanowires

二氧化钒(VO_2)是一种强关联相变材料,在341 K下发生金属-绝缘体转变.尽管对于VO_2相变的物理机理进行了大量研究,但科学家仍未形成统一认识.与热致VO_2相变相比,电触发VO_2相变应用前景更为广阔,但其机理也更为复杂.本文利用原位通电杆和超快相机技术,在透射电镜下原位观察了单晶VO_2纳米线通电时的相转变过程,记录了相变过程中对应的电压-电流值,并在毫秒尺度下捕捉到了VO_2的过渡相态.发现VO_2电致相变并非由焦耳热引起,推断其机理是载流子注入.同时观察到电子结构相变和晶体结构相变存在解耦现象,进一步支持了上述推断.将VO_2纳米线两端施加非接触式电场,观察到VO_2纳米线在电场中的极化偏移,而未观察到相变发生,该现象同样支持相变的载流子注入机理.研究表明VO_2的金属-绝缘体转变遵循电子-电子关联机理,即根据电子关联的Mott转变进行.

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 Tc, but an insulator（monoclinic, M） phase below the Tc. 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.