Nanoscale dynamics of metal–insulator transition in VO₂ thin film by IR s-SNOM

抄録

Vanadium dioxide (VO₂) thin films exhibit a metal–insulator transition (MIT) with a change in resistance of three orders of magnitude nearby room temperature (~340 K). The resistive transition in VO₂ is accompanied by a structural change from a low-temperature semiconducting monoclinic phase to a high-temperature metallic tetragonal rutile phase. Still, there is ongoing debate about the MIT's initial process and mechanism, with in-depth research employing surface scientific methods like C-AFM and Raman spectroscopy.

In this study, we grew VO₂ thin films on TiO₂(110) substrates with step and terrace structures to investigate their crystallinity and MIT dynamics in real space, by using X-Ray Diffraction (XRD) and Infrared Scanning Near-field Optical Microscopy (IR-SNOM) with Atomic Force Microscopy(AFM) simultaneously. VO₂ thin films were grown by pulsed laser deposition under the substrate temperature, the laser frequency, and the partial oxygen pressure of 723 K, 2 Hz, and 0.95 Pa, respectively. The crystallinity was characterized using temperature-dependent XRD while heating and cooling the sample between 60 ℃ ~ 120 ℃.

As a result, a noticeable shift in the VO₂(220) diffraction peak was observed (Fig.1(a)), which is attributed to the structural transition from the insulating M1 phase at lower temperatures to the metallic R phase at higher temperatures. Regarding the AFM/IR-SNOM measurements, we succeeded to visualize the dynamics of the VO₂ metal-insulator transition in real space (Fig.1(b)). During the presentation, we will delve into the process of MIT on the surface, by focusing on series of temperature-dependent IR-SNOM images, coupled with pertinent morphological information obtained through tapping-mode AFM.