Liana Sirmane preliminary defense PhD thesis "Luminescence and vacuum ultraviolet excitation spectroscopy of nanocrystalline oxides"

The most ambitious and fascinating application of luminescent nanomaterials is probably related to medicine and molecular biology. Here, luminescent nanomaterials are promising tags for optical imaging and fluorescent labeling to allow for novel techniques of non-invasive diagnosis and in vivo observation of complex vital functions. The current work is a summary of a systematic spectroscopic characterization of actual nanosized luminescent oxides (LaPO₄:Ce,Tb; YVO₄:Eu; Y₃Al₅O₁₂:Ce (YAG:Ce); ZnWO₄; NiWO₄) with an attempt to analyze the electronic relaxation performed using energy- and time-resolved luminescence spectroscopy. The experiments have been carried out utilizing European synchrotron centers: i) The Superlumi line of DORIS III storage ring of DESY (Hamburg, Germany); ii) The luminescence endstation of FinEst undulator beamline of MAX III storage ring of MAX IV (Lund, Sweden). The examinations focused on the energy transfer processes within the rare-earth ions leading to the optical transitions. The influence of the different parameters such as temperature, doping concentration and initial excited state was treated to provide complementary information concerning the electronic structure as well as electronic excitations in nanophosphors. Achieved results shows that small nanoparticle size and quencher centers are related to nanoparticle’s surface are responsible for changes in luminiscence properties of nanosized oxides. The following theses are discussed:

• Tb³⁺ luminescence practically cannot be directly excited in nanosized LaPO₄:Ce,Tb but could be excited after energy transfer from Ce³⁺ only.
• Degradation of the emission as well as excitation spectra in VUV range in nanosized LaPO₄:Ce,Tb and YVO₄:Eu phosphors could be explained by charge carriers trapping during their thermalisation by surface defects with subsequent non-radiative relaxation.
• New blue-UV emission band has been discovered in YAG:Ce nanocrystals. It is suggested that Ce³⁺ ion substituting Al³⁺ is emission center of this blue-UV luminescence.
• The excitation spectra of ZnWO₄ and NiWO₄ in VUV spectral range have been obtained. The correlation between excitation spectra and electronic structure of ZnWO₄ and NiWO₄ nanocrystals is demonstrated.