Characterization of structural dynamics of ${\text{VO}}_2$ thin film on $c{\text{-Al}}_2{\text{O}}_3$ using in-air time-resolved x-ray diffraction

The lattice motion and displacement of atoms in the unit cell in vanadium dioxide $\left({\text{VO}}_2\right)$ grown on $c{\text{-Al}}_2{\text{O}}_3$ were characterized by static and time-resolved x-ray diffraction (XRD) measurements. The monoclinic-tetragonal phase transition of the ${\text{VO}}_2$ unit cell and the twist motion of vanadium atoms in the unit cell were observed. The time-resolved XRD measurements were performed in air using a tabletop high-repetition femtosecond laser. The results obtained from the time-resolved XRD measurements suggested that the unit cell of the low-temperature monoclinic ${\text{VO}}_2$ transformed into the high-temperature tetragonal phase extremely rapidly (within 25 ps); however, the atoms in the unit cell fluctuated or vibrated about the center of the tetragonal coordinates, which abated within $\sim 100\text{ps}$. Thus, the time-resolved XRD measurements of the Bragg angle, intensity, and width of the diffraction lines simultaneously revealed the phase transition of ${\text{VO}}_2$ and the atomic motion in the unit cell.

Figure 1

The lattice parameters and positions of vanadium atoms in the unit cell of the epixially grown ${\text{VO}}_2$ thin film.

Figure 2

The spectra of the static out-of-plane XRD lines from the (020) plane of the ${\text{VO}}_2$ sample at temperatures of 293 K (monoclinic phase) and 343 K (tetragonal phase). The change in the unit cell induced the change in the Bragg angle and the changes in the atomic position increased the intensity of diffraction lines.

Figure 3

The typical spectra of the time-resolved XRD lines from the (020) plane of the ${\text{VO}}_2$ sample; the vertical line shows the Bragg angle of ${\text{VO}}_2$ for a negative time delay. The solid lines are fitting curves of the double Gaussian, where the $K{\alpha }_1$ and $K{\alpha }_2$ XRD lines are represented.

Figure 4

(a) The changes in the Bragg angle, (b) the integral intensity, and (c) the FWHM of the (020) XRD lines. The dotted lines are guides to the eye.