Characterizing Metal-Insulator Transition dynamics in VO₂ thin film by using s-SNOM

抄録

Vanadium dioxide (VO₂) thin films exhibit a metal–insulator transition (MIT) with sensitivity to the lattice strain [1, 2]. Substrates with step and terrace structures could be an attractive platform for growing high-quality thin films using the atomically-ordered surface structures. Thus, a prominent lattice strain effect could be derived using VO₂ thin films on these substrates. 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 scattering-type Scanning Near-field Optical Microscopy (s-SNOM). 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 X-ray diffractometer (Malvern Panalytical Empyrean, Cu Kα irradiation generated at 45 kV / 40 mA) while heating and cooling the sample between 60 ℃ and 120 ℃. Throughout the procedure, the sample was gradually heated from ambient temperature to each designated temperature point (60, 65, 70, 75, 80, 85, 90 ℃) while repeatedly obtaining XRD patterns in a 2θ range at 20.00°-100.00° with an angle scanning speed of 1°/min. To ensure the complete metal-insulator transition of VO₂, the sample was further heated to 120°C and held at this temperature to attain an XRD pattern. Subsequently, the cooling process was initiated, mirroring the above-described measurements. The sample was cooled from 90°C down to 60°C, employing the same measurement parameters detailed earlier.

Regarding the s-SNOM measurements, an optical parametric oscillator (Levante, APE) underwent pumping by a 1030-nm Yb:KGW oscillator (FLINT, Light Conversion). This process resulted in the generation of near-infrared signal and idler pulses. By employing difference frequency generation (Harmonixx DFG, APE) between these signal and idler pulses, mid-infrared pulses were produced. These pulses were tunable across a range of 4 – 15 μm in the central wavelength λ₀. For the subsequent experiments, mid-infrared pulses with λ₀ = 6.1 μm (~1634 cm-1), a spectral bandwidth of 200 cm-1 FWHM, and a repetition rate of 80 MHz were utilized.

Subsequently, the vertically polarized mid-infrared pulses, attenuated to below 5 mW, were directed into an infrared scattering scanning near-field optical microscopy system (IR s-SNOM; neaSNOM microscope, neaspec GmbH). To facilitate this, the mid-infrared pulses were focused onto the apex of a metallic tip (Arrow-NCPt, NanoAndMore Japan) within a tapping-mode AFM configuration. Scattered pulses emanating from the tip apex were then captured by an HgCdTe detector (IRA-20-00103, Infrared Associate). In order to ensure the detection of signals originating from the highly localized near-field, the detector signal underwent further lock-in demodulation using harmonics of the tapping frequency ω_t.

In terms of AFM operation, a tapping frequency of approximately ω_t ~ 225 kHz and a tapping amplitude of A ~ 84 nm were employed. The lock-in demodulation was detected at the third harmonics of the tapping frequency (3ω_t), contributing to the overall sensitivity of signal detection.

Figure 1 shows the temperature dependence of X-Ray Diffraction spectra of VO₂/TiO₂(110) for the heating process from 60 ℃ to 120 ℃. A noticeable shift in the VO₂(220) diffraction peak was observed, which can be attributed to the structural transition from the insulator phase (at lower temperatures) to the metallic phase (at higher temperatures). Specifically, the pronounced (220) diffraction peak exhibited an angle of 2θ = 57.56° at 60°C, while this angle shifted to 2θ = 57.32° at 120°C.

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