Doctoral Defence: Juhan Saaring “Ultrafast relaxation processes in ternary hexafluorides studied under synchrotron radiation excitation”

On 30 August at 12:00 Juhan Saaring will defend his doctoral thesis “Ultrafast relaxation processes in ternary hexafluorides studied under synchrotron radiation excitation” for obtaining the degree of Doctor of Philosophy (in Physics).

Supervisor:
Professor Marco Kirm, University of Tartu

Oponent:
Associate Professor Mauro Fasoli, University of Milano Bicocca (Italy)

Summary
Cross-luminescence and intra-band luminescence are intrinsic emissions in solids excited by photons or particles with sufficient energy. The emissions have very short luminescence decay times and can potentially be used in applications requiring high time resolution. Unfortunately, light yield of the emissions is very low, and cross-luminescence emission bands are located in the short wavelength spectral region, where sensitivity of photodetectors is low. The aim of this doctoral work was to apply band structure engineering approach to overcome these disadvantages. Based on electronic band structure calculations, suitable elements able to form wide band gap ternary fluorides together with K, Ba cations and F anion were selected. Hexafluorides (e.g. K₂GeF₆) with complex valence band structure formed by F and additional Ge or Si states were synthesised. Valence band in these compounds is split into multiple sub-bands, favouring increased light yield of ultrafast emissions. Reduced energy gap between the valence band and outermost core level allows cross-luminescence emissions to shift towards the visible spectral range. The band structure of synthesized hexafluorides as well as electronic excitations and their radiative recombinations were studied using time-resolved luminescence spectroscopy methods at the Institute of Physics of the University of Tartu and at the MAX IV synchrotron radiation facility (Lund, Sweden). A new experimental setup with 32 ps time resolution was developed for studies of ultrafast luminescence. The experimental results revealed ultrafast emissions in the studied hexafluorides with less than 500 ps decay times. The light yield of ultrafast emissions was too low for use in applications, but they were observed in a broad spectral range from vacuum-ultraviolet to visible. As a result of this work, it was demonstrated that applying the band structure engineering approach, it is possible to achieve luminescence properties desired in materials.