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Armenian journal of physics 12(4), 316 - 324 (2019)., Absorption, emission and excitation spectra of 50 MeV electron beam irradiated and as-grown YAG single crystals were studied and compared in the 10-300 K temperature range using time-resolved luminescence spectroscopy under UV/VUV/XUV excitation by synchrotron radiation and cathodoluminescence. The emission spectra consist of intrinsic (excitonic) and defect related non-elementary bands in the VIS/UV range. It is shown that fast electrons create stable F and F$^+$ color centers with characteristic emission and absorption bands in the visible/UV range. Induced absorption caused from these defects starts at 4.2 $eV.$ Energy transfer from host to color centers is not efficient process.

Symmetry 12(6), 914 (1-12) (2020). doi:10.3390/sym12060914, Characteristic dimensions and evolution times of regions of secondary electronic excitations created by the interaction of ionizing radiation with matter cannot be measured directly. At the same time these are essential parameters both for engineering of nanostructured composite materials defining optimal layer thickness and nanoparticles radii and for the development of optimized scintillators. The paper demonstrates how such spatial and temporal data can be extracted from luminescence decay kinetics excited by vacuum ultraviolet (VUV) and X-ray photons at modern sources of synchrotron radiation MAX IV and PETRA III. Specific features of energy-band structure of self-activated crystal CeF$_3$ are discussed, and its potential for a super-fast detection of ionizing radiation evaluated. Diffusion-controlled dipole–dipole interaction of Frenkel excitons is demonstrated to account well for the luminescence non-exponential decay kinetics providing information on the scales of excited regions created by photons of different energy. For 20 eV photons the radius of excited regions is estimated to be 10 nm, and for 200 eV photons it increases to 18 nm. Effective radius of excited regions of complicated shape created by 19 keV is as large as 80 nm and the diffusion length of Frenkel excitons over radiative time is 14 nm., Published by MDPI, Basel

Väitekirja elektrooniline versioon ei sisalda publikatsioone, Kross-luminestsents ja tsooni-sisene luminestsents on tahkiste omakiirguste liigid, mis tekivad piisava energiaga footonite või osakeste voogudega ergastamisel. Neid kiirgusi iseloomustab ülilühike luminestsentsi kustumisaeg, mida on võimalik potentsiaalselt ära kasutada väga head ajalist lahutust vajavates rakendustes. Kahjuks on nende kiirguste saagis väga madal ning kross-luminestsentsi kiirgusribad paiknevad lühilainelises spektriosas, kus fotodetektorite tundlikkus on madal. Doktoritöö eesmärk oli kasutada tsoonistruktuuri modifitseerimise lähenemist nende puuduste korvamiseks. Tuginedes materjalide tsoonistruktuuri arvutustele valiti sobivad elemendid, mis koos K, Ba katioonide ja F aniooniga on võimelised moodustama laia keelutsooniga fluoriide. Täiendava elemendi (Ge, Si) lisamisel on võimalik sünteesida heksafluoriide (nt K2GeF6), mille tsoonistruktuuri täiendavad lisaks valentsitsoonile (F seisundid) alamtsoonid (Ge, Si seisundid), luues soodsad võimalused tsooni-sisene luminestsentsi saagise kasvuks. Seisundite vähenenud vahekaugus nihutab kross-luminestsentsi nähtava valguse spektriosa suunas. Sünteesitud heksafluoriidide tsoonistruktuuri, elektron ergastusi ja nende kiirguslikku lagunemist uuriti aeglahutusega luminestsentsspektroskoopia meetoditel nii Tartu Ülikooli füüsika instituudis kui ka sünkrotronkiirguse keskuses MAX IV (Lund, Rootsi). Loodi uus, 32 ps ajalise lahutusega katseseade spetsiaalselt ülikiire luminestsentsi uurimiseks, mille abil näidati, et ülikiirete omakiirguste kustumisaeg uuritud heksafluoriidides jääb alla 500 ps. Kiirguste saagis oli rakendustes kasutamiseks liiga madal, kuid näidati nende esinemist laias spektriosas vaakum-ultravioletist nähtava valguseni. Doktoritöö tulemusena näidati, et tsoonistruktuuri modifitseerimise lähenemist kasutades on võimalik materjalide luminestsentsiomadusi mõjutada rakendamiseks vajalikus suunas., 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., https://www.ester.ee/record=b5508902

Väitekirja elektrooniline versioon ei sisalda publikatsioone, Alumiiniumoksiidi leidub looduses korundina ehk suure kõvadusega mineraalina. Korund kui väärtuslik vääriskivi on tuntud ka punase rubiini ja sinise safiirina. Sellised värvused tekivad lisades alumiiniumoksiidile kroomi, raua või titaani ioone. Peale termodünaamiliselt stabiilse alumiiniumoksiidi, mida tähistatakse α-Al2O3, kristalliseerub alumiiniumoksiid ka mitmete metastabiilsete struktuuridena. Need nn üleminekufaasid on sünteesitavad vaid nanokristallidena ja püsivad vaid teatud temperatuurideni – üle 1100 °C juures on võimalik vaid α-faas. Erinevate kristallstruktuuride rohkus sama keemilise koostise korral ja nanomõõtmelisus teevad alumiiniumoksiidi üleminekufaasid atraktiivseks uurimisobjektiks ja ka oluliseks rakenduslikuks materjaliks. Antud töö eesmärgiks oli alumiiniumoksiidi üleminekufaaside ja stabiilse α-faasi elektroonsete ja optiliste omaduste võrdlev uurimine ning võimalike kristalliidi suurusest tingitud efektide selgitamine. Peamiste uurimismeetoditena kasutati töös mitmeid luminestsents-spektroskoopilisi tehnikaid, mis on paljudel juhtudel ainete elektronstruktuuri uurimises asendamatud. Töös näidati kuidas muutused kristallstruktuuris mõjutavad erinevate alumiiniumoksiidi faaside elektronstruktuuri ning omaergastustega seotud kiirguslikke protsesse. Leiti samuti, et hoolimata erinevustest kristallstruktuurides on punktdefektide luminestsentsomadused erinevates alumiiniumoksiidi faasides sarnased. Cr3+ lisanditsentri kiirguse uurimine näitas, et seda on võimalik kasutada erinevate alumiiniumoksiidi faaside tuvastamiseks. Demonstreeriti ka luminestsentsmeetodite võimekust täpsustada rakenduslikult oluliste oksiid-dispersiooniga tugevdatud teraste koostist., Aluminium oxide or alumina is found in nature as a hard mineral called corundum. Corundum as a gemstone is known as a red ruby or blue sapphire. These colours are achieved by substituting chromium, iron or titanium ions into alumina. Besides the thermodynamically stable phase of alumina called α-Al2O3 there exist several metastable alumina phases with different crystal structures. These so-called transition phases can be synthesized only as nanocrystals and are stable up to certain temperatures – over 1100 °C only α-phase exists. Highly variable crystal structure of chemically the same composition and nanocrystallinity makes transition phases of alumina an attractive objects of investigation and relevant material for applications. The aim of this work was the investigation of electronic and optical properties of transition aluminas in comparison with that of α-phase and of possible effects caused by the crystallite size. Various luminescence spectroscopy techniques were used as the main investigation methods, which are in many cases indispensable for investigation of electronic structure. It was demonstrated how electronic structure in different transition aluminas is influenced by changes of crystal structure. It was found also that despite differences in crystal structures the luminescence properties of point defects of different transition aluminas are similar. The study of Cr3+ impurity centre emission showed the possibility to use it for identifying of different alumina phases. The capability of luminescence methods was demonstrated as well to clarify the composition of oxide dispersion strengthened steels, important material for many applications.