Electron Radiolytic Triggering Metal‐Insultor Transformation in
VO2
Nanowire
Authors: Zhang Zhen‐Hua , Guo Hua , Ding Wen‐Qiang , Ke Xiao‐Xiang , Chen Fu‐Rong , Sui Man‐Ling 
Published: December 2016

Vanadium dioxide (VO
2
) is a correlated‐electron materials that, in the strain‐free state, undergoes a first‐order metal–insulator phase transition (MIT) at T
c
=341 K with a change in conductivity of several orders of magnitude. The MIT is accompanied by a structural phase transition from the high‐temperature rutile phase (metallic, R) to the low‐temperature monoclinic phase (insulating, M). Although VO
2
is a promising functional material to build up micro sensors, switches and actuators, the precise localization of MIT is still unavailable, and the M/R domain wall evolution is out of control. There are big challenges for device design at micro/nano scale. In this work, we demonstrate how we locally trigger nano‐scale metal‐insulator transition of vanadium dioxide with a focused electron beam. Basically, the electron beam provides a nano‐sized probe for local oxygen vacancies doping to control the transition temperature in nano‐area of the VO
2
nanowires. Our results shed lights on understanding the mechanism of MIT in VO
2
and developing VO
2
‐based MEMS device with better flexibility.

Individual single‐crystal VO
2
nanowire cantilever is fabricated to be free from various strains and mounted in a transmission electron microscope (TEM) as shown in Fig. 1a. One end of the single crystal nanowire is attached on the copper substrate and the other end is free standing. It was originally an entire monoclinic phase (M
1
) at room temperature (25 °C). At elevated temperature of 68 °C, metallic rutile phase (R) is more stable, the entire pristine VO
2
nanowire abruptly transformed from M
1
phase into R phase. That is, no metallic‐insulator domains are observed. However, as 200 KeV electron beam in TEM was employed to radiate at the free end tip of a VO
2
nanowire of 3.7 µm long with cross sectional area of 7.1×10
‐2
μm
2
with the dose of about 7.2×10
5
Å
‐2
, R phase was found to preferentially nucleate just at the irradiated site at 62.2 °C, obviously lower than 68 °C in the pristine VO
2
nanowire (Fig. 1b). The enlarged image (Fig. 1c) shows the coexistence of the R‐phase and M‐phase domains. Fig. 1d shows that the transition temperature can be controlled with the electron dose. When the total dose is lower than a critical dose, no metallic‐insulator domains are observed, but the transition temperature decreases accordingly. The domain size and structure of metallic‐insulator can be controlled with irradiation of electron beam. Fig.1e shows a VO
2
nanowire marked the electron beam irradiation sites at room temperature. Fig. 1f shows the dark field image of the designed domain structure of the nanowire. Once the metallic R phase domain forms in the tip of VO
2
nanowire, the rest of insulator M phase will transform into the R phase gradually rather than abruptly. The growth kinetics of R phase can then be controlled by the temperature. Fig. 2a is a series of TEM images to show the temperature dependent transformation of insulator M phase to metallic R phase, while Fig. 2b shows the transformation fraction of R phase vs. temperature. The transformation kinetics and the mechanism of the radiolytic triggering MIT in VO
2
nanowire will be discussed in detail in the talk.