Metalloksiidsed mesostruktuurid optilisteks rakendusteks

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Materjalide mikro- ja nanomõõtmes vormimine (shaping) on tehnoloogias väga aktuaalne teema. Soovitud kuju ja dimensionaalsuse abil saab materjalile anda uusi füüsikalisi omadusi ja avanevad uued võimalused materjalide funktsionaliseerimiseks, mis viib uudsete rakendusteni. Antud töös kasutatakse tuntud mitmekülgsete kõrgtehnoloogiliste rakendustega tsirkooniumoksiid (ZrO2) materjalidele mikrostruktuurse kuju andmiseks sool-geel meetodit, kus prekursori vedelfaasis tekkinud nanoosakesi (sool) on võimalik edasise töötluse käigus vormida (kokku kleepida) soovitud kujule (geel). Kasutatud sool-geel meetod võimaldab ka materjali homogeenset dopeerimist erinevate lisanditega, näiteks optiliselt aktiivsete haruldaste muldmetallide ioonidega, suurendades nii veelgi antud materjali rakendusvõimalusi. Samuti on antud meetodi eeliseks see, sünteesi läbiviimiseks ei ole tarvis ekstreemseid temperatuuri, rõhu ja keskkonnatingimusi. Samuti pole vaja sünteesi läbiviimiseks kallist aparatuuri. Käesoleva doktoritöö peamisteks eesmärkideks oli seatud edasi arendada sool-geel meetodi abil erineva struktuuri ja dimensionaalsusega ZrO2 materjalide valmistamist. Järgnevalt oli kavandatud saadud materjalide füüsikaliste omaduste ja struktuuri põhjalike uuringuid. Peamiselt keskendusime rakenduslikult olulistele aspektidele: a) uudne funktsionaalne morfoloogia, b) faasikoostis ja selle stabiilsus ning c) optilised ja luminestsentsi omadused. Töö käigus valmistati ja uuriti mitmes erinevas vormis ja erinevate lisanditega aktiveeritud ZrO2 materjale nagu pulbrid, rullid, fiibrid ja torud. Selgitati mitmeid aspekte kõrgete temperatuuride (>1000 °C) mõjust antud materjalide mikrostruktuurile ja haruldaste muldmetallide kiirgustsentrite iseärasusi seda tüüpi oksiidmaterjalides. Demonstreeriti mitmeid rakendusi: nanopulbrid kui järelhelenduvad nanomarkerid, fiibrid kui kvaasi ühedimensionaalsed mikroskoopilised valgusjuhid, mikrotorud kui mikromõõtmes termosensorid.In modern technology, micro- and nanoshaping of materials is a very topical trend. By obtaining certain shapes and dimensions, materials can obtain special physical properties, which open new opportunities for their functionalization and can lead to new prospective applications. In the present work, the sol-gel technique is used for micro-structuring of zirconium oxide (ZrO2) based materials, which are already known for their use in versatile high-technology applications. In this technique, nanoparticles called sol, which are formed in the liquid phase of a precursor substance, can be further treated (glued together) to form a desired shape called gel. The method also enables homogeneous doping of the material with various dopants like optically active rear-earth ions, which further broadens the range of its possible applications. Among the sol-gel technique advantages is that the material synthesis does not require any extreme temperature, pressure, or environmental conditions, nor any expensive equipment. The main goal of this PhD work was to further develop the sol-gel technique for preparation of ZrO2 materials with different structure and dimensionality. Further, the structure and physical properties of obtained materials were to be thoroughly studied. The main attention was paid to the following aspects of importance for practical applications: a) novel functional morphology, b) phase content and its stability, c) optical/luminescence properties. ZrO2 materials of different structure/morphology (as powders, fibers, rolls, and tubes) activated with various dopants have been prepared and studied in course of this work. Investigated were several aspects of the influence of high temperatures (>1000 °C) on the materials’ microstructure and the peculiarities of rare-earth emission centers in the type of oxide materials studied. Several possible applications have been demonstrated: nanopowders as afterglowing nanomarkers, fibers as 1D microscopic light guides, microtubes as micro-dimensional thermossensors

Colorimetric gas detection by the varying thickness of a thin film of ultrasmall PTSA-coated TiO2 nanoparticles on a Si substrate

Financial support from the Estonian Research Council (IUT2-25, PUT170, PUT1096, PUT748, PUTJD680), the Estonian Centre of Excellence in Research Projects “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics” TK141 (2014-2020.4.01.15-0011), “Emerging orders in quantum and nanomaterials” TK134 and the Development Fund of the University of Tartu, are all gratefully acknowledged.Colorimetric gas sensing is demonstrated by thin films based on ultrasmall TiO2 nanoparticles (NPs) on Si substrates. The NPs are bound into the film by p-toluenesulfonic acid (PTSA) and the film is made to absorb volatile organic compounds (VOCs). Since the color of the sensing element depends on the interference of reflected light from the surface of the film and from the film/silicon substrate interface, colorimetric detection is possible by the varying thickness of the NP-based film. Indeed, VOC absorption causes significant swelling of the film. Thus, the optical path length is increased, interference wavelengths are shifted and the refractive index of the film is decreased. This causes a change of color of the sensor element visible by the naked eye. The color response is rapid and changes reversibly within seconds of exposure. The sensing element is extremely simple and cheap, and can be fabricated by common coating processes.Eesti Teadusagentuur PUT748,IUT2-25,PUT170,PUT1096,PUTJD680; Estonian Centre of Excellence in Research Projects 2014-2020.4.01.15-0011,TK134,TK141; University of Tartu; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Photo-, thermo- and optically stimulated luminescence of monoclinic zirconia

We carried out a careful photoluminescence (PL) and thermoluminescence (TL) characterization of nominally pure monoclinic ZrO2 nanopowders subject to oxidative vs reductive annealing (up to 1450 \ub0C). The two kinds of studied zirconia (sol-gel-prepared vs commercial powder) exhibited virtually identical 490 nm PL emission band and 280 nm PL excitation band with slight, but clearly detectable variations in the spectral shape. The TL glow peaks, recorded over the temperature range -100 to 300 \ub0C, showed an interplay depending on the type and treatment of sample. There is a strong evidence that the -35 and 205 \ub0C glow peaks are due to oxygen vacancies whereas the 5 \ub0C glow peak may relate to oxygen interstitials and the 110 \ub0C glow peak to surface defects. Although a number of distinct glow peaks emerge, the material still seems to contain a quasi-continuous distribution of trap depths. In comparison to TL, we also demonstrate effective optically stimulated luminescence (OSL) from this polymorph of ZrO2 under red and NIR illumination at 3c1 W/cm2. All traps responsible for the principal TL peaks were also found to be OSL-active, which widens the applied importance of the materia