Seminář ÚFKL: Vlastimil Křápek

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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 [1]. 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.

We fabricated a thin layer of VO₂ by evaporation and verified the presence of MIT by ellipsometric characterization of the refractive index. Next, we fabricated lamellas for STEM by focused ion beam milling. Energydispersive X-ray spectroscopy revealed a rather homogeneous composition with only discernible contamination by Ga ions implanted during the milling. High-resolution and annular dark field STEM imaging revealed pronounced structural inhomogeneity – a porous and polycrystalline character of the examined lamella. A thinned part of the lamella has been examined by core-loss and low-loss electron energy loss spectroscopy 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 exhibit 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.

References
[1] Tripathi, A. et al. 2021. ACS Photonics, 8, 1206–1213. https://pubs.acs.org/doi/10.1021/acsphotonics.1c00124
[2] Gloter, A. et al. 2001. Eur. Phys. J. B, 22, 179–186. https://doi.org/10.1007/PL00011142
[3] Hébert, C. et al. 2002. Eur. Phys. J. B, 28, 407–414. https://doi.org/10.1140/epjb/e2002-00244-4