Seminář ÚFKL: Vlastimil Křápek

Ústav fyziky kondenzovaných látek vás zve na přednášku

Vlastimil Křápek (CEITEC, Vysoké učení technické v Brně): Metal-insulator transition in vanadium dioxide inspected by analytical electron microscopy

Vanadium dioxide (VO₂) stands out among materials exhibiting metal-insulator phase transition (MIT) due to the proximity of its transition temperature to the room temperature, which makes it a promising candidate for applications in photonics including fast optical switching or tunable optical metasurfaces. The nature of the MIT in VO₂ is not yet fully understood, with the Mott transition and Peierls transition being the most considered scenarios. Here we probed the MIT in a scanning transmission electron microscope (STEM) with in-situ heating. A combination of imaging, diffraction, and spectroscopy with nanometer spatial resolution allowed us to locally correlate the optical properties of VO₂ with applied temperature and local variations in stoichiometry.

For the first study, we utilized a V-shaped ultrathin lamella fabricated from a pristine VO₂ layer deposited by evaporation. High-resolution and annular dark field STEM imaging revealed pronounced structural inhomogeneity – a porous and polycrystalline character of the examined lamella. Core-loss (Fig. 1b) and low-loss electron energy loss spectroscopy were performed at temperatures well below and above the MIT, and the experimental loss spectra were interpreted by ab-initio simulations and by comparison with recent literature [2,3]. We observed that the thin part of the lamella is dominated by reduced vanadium oxides (VO, V₂O₃) and exhibits no switching. This fact presents an important limit for the fabrication of VO₂ nanostructures. The thicker parts of the lamella are dominated by VO₂ and exhibit signatures of thermal-induced switching in both core-loss and low-loss EELS. Importantly, the local thickness was identified as the only parameter determining the stoichiometry of the vanadium oxide, while structural inhomogeneities represent no observable effect.

Subsequently, we focused on VO₂ nanocrystals grown directly on the heating chip by pulsed laser deposition combined with thermal annealing. For selected crystals, we recorded temperature dependences of low-loss EELS and dark-field contrast, proved a qualitative correlation between them, and set the dark-field contrast as a fast indication of the MIT. The results were supported by high-resolution imaging and selected-area electron diffraction. We studied a hysteresis of the MIT, showing a gradual transition of some nanocrystals, for which we observe applications in multilevel nanoscale memory devices.